Mr Julien Parmentier

Mr Tomas Zelenka

Mr Martin Mucha

Arjeta Kryeziu
email: kryeziu6arjeta@gmail.com

Václav Slovák

Alžběta Parcha Ňská
email: alzbeta.parchanska@osu.cz

Vaclav Slovak
email: vaclav.slovak@osu.cz

Julien Parmentier

Tomas Zelenka

Severinne Rigolet

Liquefaction of Cellulose for Production of Advanced Porous Carbon Materials

Keywords: liquefaction, cellulose, dissolution, carbon Cellulose Porous carbon Monolith Ice-templating

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to a liquid state suitable for the preparation of porous carbons are summarized in this review. Hydrothermal liquefaction leading to liquid mixtures of low-molecular-weight organics is described in detail together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in a strong gravitation field). We also focus on dissolution of cellulose without decomposition, with special attention paid to dissolution of nonderivatized cellulose. For this purpose, cold alkalines, hot acids, ionic liquids, or alcohols are commonly used.

ABSTRACT

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to liquid state suitable for porous carbons preparation are summarized in the Chapter 1. Hydrothermal liquefaction leading to liquid mixtures of low-molecular weight organics is described in details together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in strong gravitation field). Another part is focused on dissolution of cellulose without decomposition, with special attention on dissolution of nonderivatized cellulose. For this purpose, cold alkaline, hot acids, ionic liquids or alcohols are commonly used.

A simple and environmentally friendly route for the transformation of cellulose to porous carbon monoliths was developed and described in the Chapter 2. The procedure is based on freezing the cellulose solution in aqueous NaOH (-10 °C), followed by a direct regeneration of the frozen material in an ethanol bath and washing the alkaline with water. After atmospheric drying, the obtained cellulose monoliths composed mainly of cellulose II polymorph showed considerable macroporosity related to an ice-templating effect. During carbonization (500-900 °C, carbon yield > 20 %) the macroporosity (≈1-100 µm) was preserved together with monolithic character. The resulting monolithic carbons also showed significant microporosity, with micropore volume up to 0.25 cm 3 g -1 , depending on carbonization temperature.

The influence of NaOH on the pyrolysis process of cellulose and properties of resulting carbons was investigated in Chapter 3. The mixture of microcrystalline cellulose (MCC) with NaOH and cellulosic gel (CMG) prepared by the same procedure as described in Chapter 2 (but without washing out NaOH) were studied. During carbonization from 500 up to 900 °C with increasing heating rate (from 2 up to 20 K min -1 ), carbon yield of MCC and MCC/NaOH was decreased to 18 %, whereas CMG and CMG/NaOH carbon yield was around 25 %. Kinetic calculations of MCC pyrolysis by using Starink method show fluctuations of activation energy at the end of reaction, where for CMG the Ea is stable with increasing conversion. Presence of NaOH leads to higher reactivity at low temperature. Non-linear regression calculations showed that during pyrolysis, CMG material requires less activation energy then MCC to start the decomposition reaction (from 227 to 97 kJ mol -1 ). In the case where alkalis are present, Ea is decreased at low conversion levels.

Synthesis of carbon monoliths from liquefied cellulose was developed with the additional step of adding solution of metallic salts (Cu(CH3COO)2•4H2O, Mn(CH3COO)2•4H2O and FeSO4•7H2O) into dissolved cellulose solution (Chapter 4). The procedure is based on mixing dissolved cellulose solution and solution of metallic salts with metal concentration of 2 and 5 mmol, followed by regeneration of the material in ethanol and washing out the remaining NaOH with water. After drying, TG analysis of CMG and metal-based cellulosic gels indicates that materials contain residues of possible metal oxides compounds (Fe2O3/Fe3O4, Mn3O4, and CuO/Cu2O). During carbonization (at 500 and 800 °C) the macroporosity and monolithic shape was preserved. With carbonization at 800 °C, iron (III) oxide was reduced to iron carbide and metallic iron, manganese oxide maintained its oxidized form, whereas copper was reduced at the low temperatures to metallic copper. Electrochemical study of metal-based carbons with Fe, Mn and Cu showed capacitance values of 150, 133 and 83 F g -1 . Due to higher graphitization of samples with Fe and the presence of mesopores (45 m 2 g -1 ), electrochemical performance is improved.

Key words: cellulose, liquefaction, dissolution, regeneration, porosity, monoliths, carbon, kinetic, activation energy, metals

DECLARATION OF HONOUR

I, the undersigned student, hereby affirm that the submitted text of my thesis in written form is identical to the text of the final thesis embedded in database DIPL2. I also acknowledge the moral support of my family, friends and colleagues whom were always there to support me during this study.

INTRODUCTION

Carbon is a fascinating material. It is an inert substance, chemically stable and insoluble in water, organic solvents, acids and bases. Carbon is also environmentally friendly, nontoxic and a cheap material (except of diamonds, of course). One of the most important features of carbonaceous materials is connected with the possibility of their preparation with significant porosity. Based on carbon, materials with high specific surface can be prepared. All these properties make the carbon (usually in the form of activated carbon) an optimal material for many applications, such as water and air purification, gas separation and storage, catalysis (as catalysts support), energy storage (batteries, fuel cells, supercapacitors), sensors, and many others.

Most of activated carbons are produced in powder form and contains only micropores with high surface area. However, the work with fine powder is inconvenient and possibly connected with the risk of powder toxicity. Due to those problems, there is a need for bigger particles (monoliths), which can be shaped on different forms, easily handled and separated from reaction media. One possibility of monolithic porous carbon preparation is granulation of powdered activated carbon by using binders. However, binders usually block a part of the porosity and such materials have decreased surface area. Based on this, it would be better to have monolithic carbonaceous material without any binder to ensure that all present porosity is available.

The size of particles affects the rate of processes on the surface because of diffusion limitations. Diffusion is not limiting factor in small particles (micro meter scale). But for bigger particles the reactants or products need to be transported for a longer distance. If only small pores are present, the diffusion is slow and limiting the reaction rate. Due to those difficulties, well applicable porous carbon monoliths need to be equipped with bigger transport pores (meso-, macro-) together with micropores (ensure high surface area). These are called hierarchically porous materials.

Porous carbons can be prepared from different carbon rich sources like petroleum, pitch, coal, waste polymers, proteins or biomass. Biomass based precursor are traditionally used for activated carbons production (powdered and/or granulated for most applications). Contrary, a number of works describing preparation of porous carbon monoliths from biomass or biomass-based precursors is limited and this field is still quite challenging. Relatively well described is the transformation of cellulose into monolithic form leading to monolithic carbons after carbonization.

Nevertheless, most of those methods use two steps procedures with high cost and energy consumption, environmentally non-friendly chemicals, or chemically modified cellulose. This thesis focused on the preparation of porous carbon monoliths with hierarchical porosity from biomass and more particularly cellulose for various possible applications. It is organized into four chapters according to the individual topics investigated during the study period. The first theoretical part includes information about possibilities of liquefaction of cellulose. The second part describes the original preparation of porous carbon monoliths from cellulose dissolved in precooled alkaline solution, their properties and characterization. The influence of NaOH on the cellulose pyrolysis was elaborated in the third chapter. Part of this study focuses on kinetic analysis of biomass pyrolysis for optimization of its carbon yield. These materials are interesting for catalytic applications too, the last chapter of thesis focused on preparation and characterization of carbon composite monoliths containing metal-based compounds with iron or manganese or copper.

INTRODUCTION (FRENCH)

Les matériaux carbonés sont très utilisés dans de nombreuses applications du fait de leur synthèse respectueuse de l'environnement par l'utilisation de précurseurs biosourcés, leur faible toxicité et leur faible coût. L'une de leurs caractéristiques les plus importantes est la possibilité de les préparer avec une porosité relativement bien contrôlée, en particulier dans le domaine microporeux 1 , ce qui leur confère une surface spécifique élevée (ex : les charbons actifs) avec des applications pour la purification de l'eau et de l'air, la séparation, le stockage des gaz, la catalyse (comme support de catalyseurs), l'électrochimie (piles à combustible, supercondensateurs) par exemple. La plupart des charbons actifs sont produits sous forme de poudre et ne contiennent essentiellement que des micropores. Cette porosité est importante car elle apporte de la surface au matériau et détermine le volume des espèces adsorbées. Cependant, l'utilisation de poudres fines pose des problèmes, au niveau industriel par la difficulté de sa manipulation et sa toxicité. Pour ces raisons, l'utilisation de monolithes (ou de granulats) est préférée car ils peuvent être façonnés sous différentes formes et ils sont facilement manipulables et séparables du milieu réactionnel. Ces carbones poreux monolithiques peuvent être préparés par granulation d'une poudre de charbon actif à l'aide de liants. Cependant, les liants bloquent généralement une partie de la porosité et ces matériaux ont une surface réduite. De plus, dans des monolithes, les réactifs ou les produits doivent être transportés sur une plus longue distance que dans une poudre, ce qui affecte la vitesse des processus d'adsorption à la surface en raison de limitations liées à la diffusion dans la tortuosité de la porosité. Pour ces raisons, les monolithes de carbone poreux efficaces doivent être formés sans liant et avec une structuration particulière de la porosité en particulier en associant des macropores2 à des mésopores 3 connectées elles aussi à des micropores. Cette porosité hiérarchisée facilite alors le transport des espèces chimiques vers les micropores. Ce type de carbones poreux peut être préparé à partir de différentes sources riches en carbone comme le pétrole, le brai, le charbon, les déchets polymères, les protéines ou la biomasse. Les précurseurs à base de biomasse sont traditionnellement utilisés pour la production de charbons actifs (en poudre et/ou granulés pour la plupart des applications). En revanche, le nombre de travaux décrivant la préparation de monolithes de carbone à porosité hiérarchisée à partir de biomasse ou de précurseurs à base de biomasse et par un procédé vert est limité. Par contre, les études relatant la transformation de la cellulose en carbones monolithiques, avec l'utilisation de liquides ioniques ou de diméthylformamide, solvants toxiques et coûteux, sont relativement bien décrites. La liquéfaction de la cellulose par voie hydro-ou solvo-thermale est également possible. Cette liquéfaction permet de convertir la cellulose en précurseurs chimiques du carbone et de faciliter sa mise en forme (en monolithe) ainsi que la structuration de certaines porosités. Ces procédures sont assez compliquées car elles utilisent souvent des solvants toxiques, des températures et des pressions élevées et sont multi-étapes. Cependant, de nouveaux procédés utilisant des solutions aqueuses alcalines à des températures basses (-10 °C) ont permis d'augmenter la solubilité de la cellulose et de transformer en matériaux monolithiques poreux. C'est ce procédé qui sera investigué dans le cadre de cette thèse.

Cette thèse s'est concentrée sur la préparation de monolithes de carbone poreux avec une porosité hiérarchisée à partir de la biomasse (en mettant l'accent sur la cellulose) pour diverses applications possibles. Elle est organisée en quatre chapitres. La première partie théorique comprend des informations sur les possibilités de liquéfaction de la cellulose. La deuxième partie décrit la préparation originale de monolithes de carbone poreux à partir de cellulose dissoute dans une solution alcaline refroidie, leurs propriétés et leurs caractérisations. L'influence du NaOH sur la pyrolyse de la cellulose a été élaborée dans le troisième chapitre. Une partie de cette étude se concentre sur l'analyse cinétique de la pyrolyse de la biomasse pour optimiser le rendement en carbone. Les matériaux étudiés étant également intéressants pour des applications catalytiques, le dernier chapitre de la thèse s'est concentré sur la préparation de monolithes de carbone contenant des nanoparticules à base de métal et leur caractérisation.

CHAPTER 1

Liquefaction of cellulose for production of advanced porous carbon materials 1.1. Introduction

Cellulose, the most abundant natural organic polymer [1], is the main structural component of plant cell walls, as well as many algae and oomycetes. It is the most abundant homopolysaccharide in nature (approximately 1,5 x 10 12 tons of annual production), a traditional renewable energy source [2][3][4], reserve of carbon and hydrogen, and a raw material for paper making and other industrial processes such as cosmetics, food [5], and medicine production [6,7]. Being a composed of many linear β-(1,4)-linked homopolymer chains of hundreds to thousands of anhydroglucopyranose residues, connected parallel by hydrogen bonds to form tough and inflexible structures, cellulose is a promising source of glucopyranose units and their derivatives as well as mixtures of various low-molecular compounds such as phenolic compounds, furan derivatives, carbonyl compounds or carboxyl compounds, called bio-oil [8].

These small molecules can be further used as precursors for organic syntheses or preparation of various advanced carbonaceous materials such as carbon aerogels with a certain porosity [9]. For effective preparation of the latter materials, it is important to convert the precursors to solution/liquid state since this allows higher versatility in nanoporous carbon preparation.

However, the conversion of cellulose macromolecules to this state is complicated since cellulose is insoluble in a majority of common solvents (water, organic solvents) under normal conditions and the β-glycoside bonds that connect the cellulose units can be hydrolyzed by cellulase enzymes [10] or acids [11].

Several procedures for converting solid-state cellulose into a liquid state, i.e. into solution of cellulose derivatives, solution of single glucopyranose chains, oligomers or monomers, or bio-oil (a liquid product), have been developed. The first developed procedures were based on the preparation of soluble cellulose derivatives such as cellulose nitrate, acetate [12], formate [13],

carbamate or xanthogenate, the latter being an intermediate in traditional viscose process [14].

Another traditional treatment of cellulose is its dissolving in cuprammonium hydroxide solution that forms a copper-containing cellulose complex [15], which was later replaced by dissolution in N-methylmorpholine-N-oxide (NMMO) [16,17].

In recent decades, new approaches and procedures for the dissolution of cellulose or conversion of cellulose to a liquid mixture of low-molecular compounds have been invented. Due to the practical insolubility of cellulose under normal conditions [18], procedures usually involve low [19] or high temperatures [20] or high pressure (supercritical conditions) [21], aggressive solvents (concentrated mineral acid) [22], solvent mixtures [23] or other special conditions (microwave heating [24], strong gravitation field, ultrasonic field [25]). All these parameters strongly influence the yield and composition of the obtained liquid mixture.

This chapter summarizes the procedures for the liquefaction/dissolution of cellulose, resulting in liquid mixtures with a composition appropriate for preparation of nanoporous carbon material with distinct porous properties (specific pore size distribution and specific surface area).

Processes based on cellulose decomposition

Biomass or cellulose decomposition to produce a liquid mixture of organic compounds (biooil) is in literature commonly called liquefaction. In general, it is based on the chemical degradation of cellulose chains to low molecular weight products. In addition to simple biomass pyrolysis, hydrothermal liquefaction (HTL) and some other techniques enabling the conversion of cellulose to chemicals perspective as carbon precursors are used (Fig. 1).

Biomass pyrolysis

Biomass pyrolysis is a complex process determined by biomass feedstock and reaction parameters. Basic pyrolysis of biomass is carried out at atmospheric pressure. According to the heating rate during pyrolysis, slow, fast and flash pyrolysis are used [26,27]. Three processes are considered during biomass pyrolysis, including char formation, depolymerization, and fragmentation. The major products from biomass pyrolysis are char, bio-oil, and pyrolytic gases.

Char is a solid product with high carbon content; bio-oil is an organic mixture of different compounds such as alcohols, sugars, and other organic compounds containing oxygen; pyrolytic gases contain mostly carbon dioxide, carbon monoxide, hydrogen, and light hydrocarbons [28].

Char, bio-oil, and gases are usually in the range of 13-25, 50-70, and 12-15 wt%. The composition of pyrolysis products and their proportions are influenced by the type of biomass, physical and chemical treatments, pyrolysis temperature, carrier gas, and heating rate. The heating rate is considered as the main factor affecting the yield of bio-oil. During slow pyrolysis (heating rate below 1 K s -1 ) the bio-oil yield is decreased down to about 30 wt%, while increasing the heating rate (fast pyrolysis 10 -200 K s -1 , flash pyrolysis with more than 1000 K s -1 ) enables obtaining more than 70 wt% of bio-oil [27].

The composition of pyrolysis bio-oil is related to the decomposition of the main biomass components (Fig. 2). In addition to the main constituents, it contains hundreds (thousands in some literature [29]) compounds (mostly oxygenated) and undergo significant changes in composition and physical properties during storage (evaporation of volatile components, continuous reactions in the mixture) [27].

Hydrothermal Cellulose Liquefcation (HTL)

Hydrothermal liquefaction (HTL) is a thermochemical processing technology for the production of bio-oil from agricultural and forestry wastes. Bio-oil is a dark, viscous, and energy dense liquid which has a 70-95 % energy content of petroleum fuel oil [30]. The HTL process is based on the decomposition of biomass in hot pressurized water or various organic solvents (alcohols, polyalcohols like ethyleneglycol [4] or acetone) [31] in the presence of homogeneous catalysts like sulfuric acid [4], alkali hydroxides, ammonia or inorganic salts such as sodium or potassium carbonate [32] or heterogeneous catalysts such as transition metals or zeolites [33].

Some HTL processes take place under subcritical conditions, while the others are supercritical.

As the HTL process does not require prior drying of biomass [34], it can be applied to both dry (lignocellulose) and wet biomass (algae) [35].

The reactions that occur with cellulose and hemicelluloses in the course of HTL have been thoroughly investigated. It has been found that the key reactions in HTL are hydrolysis, dehydration, isomerization, and retro-aldol condensation. Cellulose and hemicellulose (mixture of highly branched low molecular weight homo and heteropolymers comprised of anhydro-β-(1-4)-D-xylopyranose, glucopyranose, mannopyranose and galactopyranose units [36]) are initially hydrolyzed into oligosaccharides and monosaccharides that are further decomposed into small molecules such as furfural, 5-hydroxymethylfurfural, lactic acid, levulinic acid, formic acid or acetic acid [32,37] (Fig. 3). Using ethylene glycol as a solvent for liquefaction, the reaction leads to decomposition of glucosides into glucose [4], organic acids, and polymerized alcohols [38].

The yield and composition of bio-oil depend on the biomass composition: the maximum biooil yield is obtained from biomass with a high lignin content [38].

Effect of Processing Parameters on Bio-Oil Yield of HTL

The HTL process is significantly affected by many processing conditions; its appropriate setup is crucial for increasing the yield of bio-oil and its composition.

Temperature is a sensitive operating parameter in the hydrothermal liquefaction process.

Cellulose liquefaction occurs through the cleavage of C-C and C-O bonds under the action of solvent radicals produced at high temperature [39]. The temperature usually ranges between 250 and 400 °C and the pressure is set between 5 and 20 MPa. The procedure can take various times, from 10 min up to 24 h [31]. However, after reaching the maximum bio-oil yield at certain temperature, further increase in temperature leads to decrease of bio-oil yield due to repolymerization of free radicals produced from liquefaction products and due to increased gas production [38]. The biomass type [38] and the solvent type affect the optimal liquefaction temperature as well [31]. The increase in cellulose crystallinity results in a change of the optimal liquefaction temperature to approximately 380 °C [40]. Beyond a certain threshold, further increase in temperature or residential time leads to decreased bio-oil yield. This threshold is different for different biomass sources/composition [38].

In general, increasing residence time leads to the increase of bio-oil yields [41]. With the increase in reaction time from the beginning of reaction up to 30 min, the yield of solid residue yield decreased from 86.3 % to 60.9 %, while the yield of bio-oil increased from 8.7 % to 28.5 %, due to the deeper liquefaction of cellulose by prolonging the reaction time [42]. At 350 °C, the cellulose liquefaction is carried out with a residence time of 30 min or 1 h. With increasing residence time for more than 1 h, the yields of oil and aqueous phase yields decreased, and only the gas yield increased [43].

Cellulose conversion and its product distribution are significantly affected by the thermal effect [44]. However, the heating rate showed a marginal influence on the bio-oil yield in supercritical EtOH-based liquefaction [45]. The significant effect on biomass conversion due to the heating rate above 280 °C using ethanol [46,47].

The major difference between liquefaction and pyrolysis experiments lies in the effect of high pressure, which helps to condensate the reaction products and prevents their escape from the reaction vessel. Cellulose is highly oxygenated, and a reducing gas is frequently used to remove oxygen. It is preferable to eliminate oxygen autonomously either as water (which is undesirable since the end product would be carbon) or as carbon dioxide [48]. Higher temperatures will require higher pressure to maintain a liquid water phase [49]. Increasing the reaction pressure is beneficial for the hydrogenation reaction [50] and leads to slowing down of the dissociation reactions, although the effects of solvation may override it. Experiments have shown that when cellulose is heated to 350 °C under pressure, and in the presence of water, phenol and certain catalysts such as oxalic acid or sulfuric acid, almost total liquefaction can occur [51].

Biomass feedstock is one of the important parameters that affect bio-oil yield. The high cellulose content in biomass leads to high bio-oil yield [52].

Optimum size of biomass particles increases the yield of bio-oil at low grinding cost since reduction in particle size improves heat transfer and increases surface area [53]. Usually, particle diameter between 4 and 10 mm are suitable to overcome heat and mass transfer at reasonable grinding cost [52].

Solvents in HTL

A very important and key factor [54] for the efficiency of the liquefaction process and the properties of the final product is the selection of the appropriate solvent [55] (Table 1). The choice of solvent can have a strong impact on the distribution and yield [56]. The switch from lowboiling and nucleophilic solvents, such as water, to high-boiling and largely nonpolar solvents is suspected to affect the chemistry of the liquefaction process [57]. Low molecular weight cosolvents such as phenol, propanol, ethanol, methanol, and glycerol are applied and preferable in order to obtain low molecular weight product compounds. Polar protic solvents gave higher conversion rates and bio-oil/gas yields [58].

Methanol is an excellent solvent for cellulose liquefaction due to its high polarity, which allows penetration even into crystalline cellulose regions, promoting the solvolysis of polymer chains [31,59] with a maximum bio-oil yield varying from 32,21 % [31] up to 100 % [60] ,but the reaction temperature and pressure must be quite high due to its physical properties [31].

Glycerol was found to be a good solvent for cellulose liquefaction because its polar molecules can easily penetrate into cellulose structure. Furthermore, glycerol liquefaction can be performed at high temperatures at atmospheric pressure [61].

Some solvents such as formic acid, light alcohols such as 2-propanol, and hydrocarbons such as tetralin are hydrogen donor solvents. These solvents effectively stabilize liquefaction products which helps to reduce product repolymerization [56]. Compared to non-hydrogen donor solvents, hydrogen donor solvents showed significant improvement in cellulose conversion and in the biooil quality [62].

Ionic liquids are also efficient for cellulose liquefaction [63] due to their high thermal and chemical stability, high polarity, low melting points, and good cellulose solvating properties [64].

Cellulose liquefaction in ionic liquids is performed at 90-130 °C and ambient pressure, followed by addition of water and washing of the precipitate [65]. Dimethylsulfoxide 100 [75] Ethylacetate 100 [76] [Alkylmethylimidazolium]Cl 75-90 120 [60] [Bmim]Cl 20 100 [77] [Bmim]Ac 11.5 50 [77] [Amin]Cl 3.5 100 [78] [Emim]Cl 10 100

[77]

Other Decomposition Processes

Cellulose exposed to microwave irradiation (frequencies 0.3 to 300 GHz and wavelength from 1 mm to 1 m) begins to decompose due to local heating [79]. Microwave heating in cellulose liquefaction leads to a reduction in time and energy consumption [80] and allows effective dissolution and depolymerization under mild reaction conditions [61,81,82].

Microwave liquefaction of microcrystalline cellulose in water under acidic conditions produces mainly 5-hydroxymethylfurfural and levulinic acid. At 120 °C and 150 °C, almost 100 % selectivity was observed. At 180 °C less selectivity was observed and glucose appeared in the product mixture [45].

The sedimentation of particles in the liquid medium, which occurs even at normal earth gravity (1 G), can be accelerated by an ultracentrifuge machine. Strong acceleration of the gravitation field (more than 10 6 G) can induce a decomposition reaction that, in combination with hydrothermal decomposition, results in cellulose decomposition. Under a strong gravitation field, glucose and its decomposition products exist in the acidic media around the capsule [83].

Crystalline cellulose contains an ordered hydrogen bonded network within which a proton transport network is possible in the presence of an electromagnetic field [84]. Ultrasonic waves are found to intensify physical and chemical processes, such as extraction and hydrolysis, in treated materials, as cellulose, resulting from the cavitation, mechanical and thermal effects of the ultrasound [85].

Cellulose Dissolution

Cellulose dissolution is a process in which solvent molecules penetrate the cellulose structure and cause polymer to swelling to certain extent. The physical properties and volume of the cellulose are significantly altered, while the chemical structure of the polymer remains practically unchanged [86]. Since cellulose is a polar molecule with a high degree of crystallinity [87], it has good hydrogen bonding ability [88].

The dissolution of cellulose without chemical modification is difficult due to the long rigid cellulose chains and strong intermolecular and intramolecular hydrogen bonds [89] and hydrophobic interactions [90]. The common solvents applicable for the dissolution of cellulose are summarized in Fig. 4.

It is not possible to define an ideal cellulose solvent because it is highly dependent on the purpose for which the cellulose is dissolved. Even if we focus only on the preparation of porous carbons, the selection of suitable solvent is obviously affected by the expected further processing of the solution, the possible use of crosslinkers, templates, or other additives, required physical form of regenerated cellulose (powder, fiber, monolith) and its behaviour during subsequent carbonization.

Traditional processes of cellulose dissolution are based on the conversion of cellulose to water-soluble cellulose derivatives. The classic cellulose dissolution is viscose process (using carbon disulfide) that is capable to prepare 8-12 wt% cellulose xanthogenate [14]. Recently a new

Lyocell process [91], using N-methylmorpholine-N-oxide (NMMO), was implemented [92,93].

The dissolution of cellulose in NMMO can be improved by microwave heating. Cellulose solutions in NMMO can be used for the preparation of cellulose membranes with a high degree of crystallinity [94]. However, new processes that use nontoxic chemicals and less energy than conventional technologies and produce a solution of cellulose (and not the cellulose derivatives)

have to be developed.

Non-derivatizing cellulose solvents are copper hydroxide in ammonia or ethylenediamine solution and its alternatives, cadmium hydroxide or nickel oxide in the same aqueous ethylenediamine [95,96]. Although the industrial application of these systems is limited because of the heavy metals and their general environmental and health risks, conversion of cellulose dissolved together with metal ions to porous carbon containing metal or metal oxide (nano)particles can be interesting for electrochemical or catalytic applications. A similar system for the dissolution of cellulose, ferric chloride, tartaric acid dihydrate, and NaOH in cold water, has been developed [97].

The dissolution of cellulose in aqueous media can be understood as an acid-base process where cellulose can act as an acid or as a base [95].

In addition to cellulose liquefaction in a hot acid solution under high pressure, concentrated phosphoric acid (85 wt%) can be used for the dissolution of cellulose [98], however, cellulose is derivatized to cellulose phosphonate. Low temperature (100-200 °C) hydrolysis in concentrated acid leads to a glucose solution with nearly 100 % yield [31].

On the other hand, cellulose dissolves in a cold (below 268 K) 8-10 wt% aqueous NaOH solution [99]. Additives such as polyethylene glycol [START_REF] Yan | Dissolving of cellulose in PEG/NaOH aqueous solution[END_REF], urea [6,[START_REF] Cai | Rapid Dissolution of Cellulose in LiOH/Urea and NaOH/Urea Aqueous Solutions[END_REF], thiourea [START_REF] Zhang | Dissolution and regeneration of cellulose in NaOH/thiourea aqueous solution[END_REF], a combination of urea and thiourea, are frequently used [START_REF] Piltonen | Green and efficient method for preparing all-cellulose composites with NaOH/urea solvent[END_REF]. All these substances are inexpensive and practically non-toxic. The solubility of cellulose in aqueous NaOH/urea solution depends on temperature, molecular weight of cellulose, and crystallinity of cellulose [START_REF] Sarko | Packing Analysis of Carbohydrates and Polysaccharides. 7 . Crystal Structure of Cellulose IIII and Its Relationship to Other Cellulose Polymorphs[END_REF]. For its dissolution, a pre-cooled solution (-12.6 °C) of 7 wt% NaOH and 12 wt% urea was used. As a result of a fast self-assembly process between small molecules (water, urea, OH -) and cellulose macromolecules [START_REF] Yang | Role of sodium zincate on cellulose dissolution in NaOH/urea aqueous solution at low temperature[END_REF], cellulose dissolution occurs quickly (within a few minutes) at low temperatures (around or below 0 °C) [99]. The solubility increases with decreasing polymerization, but no effect of crystallinity was observed. Urea prevents the reaggregation of cellulose molecules by accumulation near hydrophobic regions [START_REF] Xiong | Dissolution of cellulose in aqueous NaOH/urea solution: role of urea[END_REF]. The cellulose solution in NaOH/urea can easily form a gel [START_REF] Egal | The dissolution of microcrystalline cellulose in sodium hydroxide-urea aqueous solutions[END_REF]. An 8 wt% solution of cellulose in aqueous NaOH can be converted to porous cellulose material by physical gelation, chemical crosslinking with epichlorohydrin, coagulation in water and lyophilisation [START_REF] Ciolacu | Physically and chemically cross-linked cellulose cryogels: Structure, properties and application for controlled release[END_REF]. At low temperature, celluloses I, III, and IV are converted to cellulose II [6]. Cellulose I and cellulose II are the most common polymorphs in regenerated cellulose. The hydrogen bonding in cellulose II is more complex than in cellulose I, leading to different mechanical properties and hydrophilicity [START_REF] Mahmud | Preparation of different polymorphs of cellulose from different acid hydrolysis medium[END_REF]. Cellulose II can be prepared by alkaline treatment and regeneration. Cellulose III can be formed from cellulose I and II by treatment with ammonia. Cellulose IV is obtained by heating cellulose III [START_REF] Mittal | Effects of alkaline or liquid-ammonia treatment on crystalline cellulose: changes in crystalline structure and effects on enzymatic digestibility[END_REF]. The solubility of cellulose in water under mild conditions is promoted by the addition of certain inorganic salt hydrates with a water to salt molar ratio equal or less than the coordination number of the cation [START_REF] Wei | Depolymerization of cellulose into high-value chemicals by using synergy of zinc chloride hydrate and sulfate ion promoted titania catalyst[END_REF]: mixture of NaSCN and KSCN with Ca(SCN)2•3 H2O or DMSO is a good cellulose solvent [START_REF] Fischer | Inorganic Molten Salts as Solvents for Cellulose[END_REF]. Cellulose dissolution was also observed in concentrated (63 % w/w) aqueous solution of zinc chloride [95].

Various non-aqueous systems were tested for cellulose dissolution, such as the combination of SO2 and NH3 with suitable ammonium salt [START_REF] Hudson | The Solubility of Unmodified Cellulose: A Critique of the Literature[END_REF]; mixtures of polar organic liquid such as formamide, N,N-dimethylformamide, DMSO; N,N-dimethylacetamide, SO2 or SOCl2 and aliphatic or alicyclic amines [START_REF] Isogai | Review: Catalytic oxidation of cellulose with nitroxyl radicals under aqueous conditions[END_REF]; mixtures of amine-containing component; polar organic liquid and inorganic salt such as NH3/NaCl/DMSO or ethylenediamine/NaI/DMF [115]; twocomponent mixtures such as NH3/NH4SCN [95]; thiocyanates with hydrazine, hydrazine hydrate or ethylenediamine [START_REF] Ting | Retardation in RAFT Polymerization: Does Cross-Termination Occur with Short Radicals Only?[END_REF]; DMSO with methylamine, KSCN, CaCl2, formaldehyde or substituted ammonium fluorides [115]. The last-mentioned type of mixture is capable of dissolving cellulose with a high degree of polymerization at room temperature without any pretreatment and in a couple of minutes [START_REF] Heinze | Effective preparation of cellulose derivatives in a new simple cellulose solvent[END_REF]. Another relevant two-component cellulose solvent is the ethylenediamine/KSCN mixture [START_REF] Frey | Dissolution of cellulose in ethylene diamine/salt solvent systems[END_REF]. For faster dissolution of cellulose under mild conditions, ionic liquids such as 1-butyl-3-methylimidazolium acetate are used in dimethylsulfoxide, in N,Ndimethylformamide or in N,N-dimethylacetamide are used [START_REF] Gupta | Cellulose dissolution and regeneration in ionic liquids: A computational perspective[END_REF][START_REF] Mäki-Arvela | Dissolution of lignocellulosic materials and its constituents using ionic liquids-A review[END_REF][START_REF] Shibata | All-cellulose and all-wood composites by partial dissolution of cotton fabric and wood in ionic liquid[END_REF]. No additional electrolyte is required [START_REF] Casarano | A convenient solvent system for cellulose dissolution and derivatization: Mechanistic aspects of the acylation of the biopolymer in tetraallylammonium fluoride/dimethyl sulfoxide[END_REF]. Up to 10 % by weight of cellulose can be dissolved in 1-butyl-3methylimidazoilium chloride at 100 °C; solubility can be increased to 25 wt% by microwave heating.

When the mixture of N,N-dimethylacetamide with LiCl is used as a solvent, the cellulose solubility can be improved if the cellulose suspension in water is freeze-dried. This treatment has better results than hot drying [START_REF] Hu | Cellulose porosity improves its dissolution by facilitating solvent diffusion[END_REF]. This effect is associated with an increase in cellulose porosity after freeze-drying.

Cellulose can be regenerated from the solution in ionic liquid by adding an anti-solvent such as water, alcohol, or acetone [START_REF] Ha | Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis[END_REF], however, at lower concentrations (less than 15 % by weight), water acts as a viscosity-reducing agent in the solution of cellulose in ionic liquid [START_REF] Iguchi | Dissolution and recovery of cellulose from 1-butyl-3-methylimidazolium chloride in presence of water[END_REF].

Miscibility of ionic liquid with water or organic solvents can be tuned by length of the cation side chain and choice of anion [START_REF] Cuissinat | Swelling and dissolution of cellulose. Part IV: Free floating cotton and wood fibres in ionic liquids[END_REF].

Porous Carbons Based on Liquefied/Dissolved Cellulose

The traditional way of production of porous carbons is direct carbonization of biosources with subsequent activation (activated carbons). Direct carbonization of biomass is still a very active topic intensively reported in literature (e.g. [START_REF] Jiang | Recent progress on porous carbon and its derivatives from plants as advanced electrode materials for supercapacitors[END_REF]) because it represents a cheap and effective way of highly porous carbon production.

After the development of hydrothermal treatment of biomass for the production of biofuels, the same technique attracts attention as a tool for the form of porous carbons production either biomass [START_REF] John | Biomass-based hydrothermal carbons for catalysis and environmental cleanup: a review[END_REF] or cellulose (eg [START_REF] Lin | Hydrothermal carbonization of cellulose in aqueous phase of bio-oil: The significant impacts on properties of hydrochar[END_REF]).

The above-mentioned approaches are based on solid-solid conversion, which is in principle problematic for controlling the porosity of produced carbons and do not enable involvement of recent ways of porosity formation and control like templating.

To overcome this disadvantage, phenol-based wood liquefaction was developed under acidic catalysis was developed [START_REF] Wang | Activated carbon materials derived from liquefied bark-phenol formaldehyde resins for high performance supercapacitors[END_REF][START_REF] Ozbay | Carbon foam production from bio-based polyols of liquefied spruce tree sawdust: Effects of biomass/solvent mass ratio and pyrolytic oi laddition[END_REF] leading to a phenolic solution suitable for formaldehyde condensation with formaldehyde (in the presence of a suitable template) to gel-like solids carbonizable to porous carbons. Because of large excess of phenol used in this approach, produced carbons are mostly based on phenol-formaldehyde resin, and biomass-based phenolic compounds are only partly involved.

Surprisingly, there is a lack of information on the direct use of bio-oil for production of carbons with controlled porosity. Zhu et al. [START_REF] Zhu | Hierarchical porous carbon materials produced from heavy bio-oil for high-performance supercapacitor electrodes[END_REF] simply carbonized heavy bio-oil and after activation with NaOH they received highly porous carbon with surface area (derived from BET) greater than 2000 m 2 g -1 and total pore volume 1.7 cm 3 g -1 . Similar materials were obtained with the use of crayfish shell as a biological template during carbonization of heavy bio-oil followed by washing with acid and activation [START_REF] Luo | Synthesis of 3D-interconnected hierarchical porous carbon from heavy fraction of bio-oil using crayfish shell as the biological template for high-performance supercapacitors[END_REF]. Carbon materials with developed micro and mesoporosity were also prepared by carbonization of bio-oil in the presence of MgO [START_REF] Li | Template-synthesized hierarchical porous carbons from bio-oil with high performance for supercapacitor electrodes[END_REF] or CaO (introduced as acetate) [START_REF] Li | Preparation of hydrogen storage carbon materials using bio-oil heavy components as carbon-containing precursor[END_REF] as hard templates washable with acidic media. Another approach was described by Wang et al. [START_REF] Wang | Honeycomb-like carbon with tunable pore size from bio-oil for supercapacitor[END_REF] which used bio-oil hydrothermal treatment in steel autoclave (170 °C, 10 h) in the presence of cetyltrimethylammonium bromide and nickel nitrate, followed by carbonization and activation. They reported a positive effect of increasing the amount of CTAB on surface characteristics improvement (surface area up to 1300 m 2 g -1 , pore volume 0.84 cm 3 g -1 , mesopores with diameter up to 30 nm).

Cellulose is one of the most studied bio-precursors for production of porous carbons. In addition to cellulose itself, its derivatives such as acetyl- [START_REF] Xu | Acetylated cellulose nanocrystals with high-crystallinity obtained by one-step reaction from the traditional acetylation of cellulose[END_REF], carboxymethyl- [START_REF] Klunklin | Synthesis, Characterization, and Application of Carboxymethyl Cellulose from Asparagus Stalk End[END_REF]139], ethyl- [START_REF] Koch | Properties and Uses of Ethylcellulose[END_REF], hydroxyethyl- [START_REF] Peng | Hydroxyethyl Cellulose-Based Hydrogels with Various Pore Sizes Prepared by Freeze-Drying[END_REF], and hydroxypropyl cellulose [START_REF] Khanmirzaei | Hydroxypropyl Cellulose Based Non-Volatile Gel Polymer Electrolytes for Dye-Sensitized Solar Cell Applications using 1-methyl-3propylimidazolium iodide ionic liquid[END_REF] were used for carbon preparation.

The basic approach to conversion of cellulose into carbons with controllable porosity usually of cellulose (or its derivative) followed with solidification (gelation, regeneration, freezing) in the presence of an appropriate template (not always) and final carbonization (and possible activation).

Micropores in the porous carbons of cellulose are generated during the carbonization of the gels, and are increased by activation as with NaOH or KOH [START_REF] Klepel | Porous Carbon Monoliths Made from Cellulose and Starch[END_REF]. Mesopores usually originate from hard or soft templates, but they can also be affected by activation step [START_REF] Blankenship | Modulating the porosity of carbons for improved adsorption of hydrogen, carbon dioxide, and methane: a review[END_REF].

The porous carbons of carboxymethylcellulose were prepared by ice-templating and carbonization. The carboxymethylcellulose aqueous solution was simply frozen in a liquid nitrogen bath, dried in a freeze-dryer for some days, and carbonized [START_REF] Lee | Eco-friendly fabrication of porous carbon monoliths from water-soluble carboxymethyl cellulose for supercapacitor applications[END_REF]. With the dissolution of cellulose in the calcium thiocyanate/water/ethanol system and its carbonization, porous carbons were obtained [START_REF] Jeong | Cellulose Degradation by Calcium Thiocyanate[END_REF]. Highly porous materials were also obtained by dissolution of cellulose in ionic liquid or in lithium chloride/N,N-dimethylacetamide and its precipitation in a coagulation bath of water [START_REF] Zhang | Dissolution Mechanism of Cellulose in N,N -Dimethylacetamide/Lithium Chloride: Revisiting through Molecular Interactions[END_REF].

Another approach of cellulose gel preparation is the freeze-thaw process. In general, cellulose monoliths can be prepared by the freeze-thaw process of alkaline cellulose solution with NaOH, KOH, or in mixture with urea/thiourea [START_REF] Qin | Gelation behavior of cellulose in NaOH/urea aqueous system via cross-linking[END_REF]. The freezing and heating of dissolved cellulose solution at room temperature leads to the formation of cellulosic gels [START_REF] Korhonen | Gelation of cellulose-NaOH solutions in the presence of cellulose fibers[END_REF]. Cellulose gels were also prepared by freezing solutions of cellulose Ca(SCN)2/water solvent [START_REF] Xin | A hierarchically porous cellulose monolith: A template-free fabricated, morphology-tunable, and easily functionalizable platform[END_REF].

Cellulose porous monoliths were also prepared by controlled freezing of aqueous cellulose solution in a liquid nitrogen bath. Monolithic carbon was formed after carbonization [START_REF] Parajuli | Cellulose-based monoliths with enhanced surface area and porosity[END_REF]. The Pluronic F-127 was used as porogen agent during the hydrothermal treatment of cellulose suspension in water and the resulting char was consequently carbonized to porous monolithic carbon [START_REF] Ferry | Fused sphere carbon monoliths with honeycomb-like porosity from cellulose nanofibers for oil and water separation[END_REF].

Conclusions

Cellulose represents a valuable source of carbon for the production of porous carbonaceous materials [START_REF] Hina | Preparation and performance comparison of cellulose-based activated carbon fibres[END_REF]. Obviously, the simplest way to convert cellulose to carbon is direct carbonization, but if we want to control the properties of produced (especially porosity), more advanced approaches to carbon synthesis are required, in general based on liquid carbon precursors [START_REF] Mankar | Pretreatment of lignocellulosic biomass: A review on recent advances[END_REF].

The conversion of cellulose into the liquid state can be achieved basically in two waysdecomposition to liquid mixture of low-molecular weight organics (hydrothermal liquefaction or other decomposition techniques) or dissolution in suitable solvent (cellulose or its derivatives).

In the first case, the obtained mixtures contain a large number of chemical individuals with different properties and have a variable composition that is significantly dependent on the liquefaction condition. To find a common way of controlled conversion of such a complicated system into carbon with specific porosity would be very difficult.

However, if dissolution of cellulose (or its derivative) is successful, the obtained solution is homogeneous with more or less constant properties of dissolved cellulose given by the used solvent. Such a system is much easier to a transform to defined macrostructure (e.g. by crosslinking, gelation, and templating), consequently leading to carbons with defined porous structure.

Although many pathways of cellulose liquefaction/dissolution have been developed, this area still presents a field for intensive research, especially in connection with increasing interest in carbon-based porous materials for various advanced applications.

The challenging research topics in the near future would probably be focused on (not only):

-Transformation of industrially available bio-oils towards their chemical stability and uniformity enabling their conversion to nanostructured organic and consequently carbon matter.

-Avoiding the loss of carbon, increasing the carbon yield during biomass treatment, e.g. by suitable cellulose derivatization, more extensive involvement of lignin constituent of biomass, etc.

-Production of biomass-or cellulose-based carbon monoliths with hierarchical porosity is still a challenge even for carbons produced from petroleum-based chemicals.

Introduction

Among porous carbons, activated carbons are produced on an industrial scale from precursors based on petroleum or biomass, usually through carbonization and chemical or physical activation [1]. The industrial production of biomass-based activated carbons usually occurs through an energy-consuming step. In addition, it is difficult to shape these materials at a macroscopic scale (e.g. monolithic shape as a pellet) without the aid of additives (binders), which usually decrease their porous capacities and require an additional time and cost in their processing. On the other hand, monolithic materials are more suitable, easier to use, and less toxic than powdered activated carbons. Thus, shaping porous carbons as monoliths and as fibers or pellets, directly from their precursor state, appears to be an interesting challenge with regard to synthesis and time/energy cost. The use of carbon precursors in the form of gel or foam is an established process for controlling both the shape and extent of the porosity of monolithic porous carbons. These precursor materials are usually prepared from petroleum-based compounds (eg, phenolic resin) or bioderived precursors [2]. Among them, phenolic-based foams are interesting materials since they lead to carbon foams with good thermal insulation properties, high dimensional stability, low flammability, and low smoke toxicity. Their preparation requires costly modifiers [3,4] and the use of porogen agents (eg, for the generation of gas bubbles within the solution during the polymerization of the precursor through heating of the solution). Even if this method is simple, the size and shape of the formed pores are limited, since the morphology of the gas bubbles cannot be controlled [9] and may require an additional activation process [5].

To extend porosity control (over size and shape) of carbon materials, chemical methods (other than activation) have been developed, such as sol-gel or hydrothermal syntheses, hard-templating, and more recently soft-templating [6][7][8][9][10][11][12]. For all of these processes, the organic precursors play a key role in the synthesis and therefore in the control of the properties of the final carbon materials.

Among precursors, biosource precursors have attracted increasing attention in the research community because they are a promising alternative, compared to petroleum-based precursors, for the preparation of monolithic porous materials. A lot of biomass-based compounds such as tannin, lignin, wood, coconut, rice hubs, sugarcane bagasse or bamboo chips were used as precursors and subsequently converted by heat treatment to monolithic porous carbons.

Plant and natural fibers based on cellulose and other biopolymers have been increasingly investigated in simple and cost-effective methods, yielding foams, cryogels, and xerogels. All these biopolymer-based materials exhibit biocompatibility, good mechanical properties, and highly developed porous structures with a large surface area [13][14][15]. These materials find potential applications in fields such as biofuels and biomedical applications, such as nanobiocomposites [16], due to their wide range of properties, which can be controlled by the versatility of their preparation processes.

One of the most studied precursors for biomass-based monolithic materials production is cellulose (or its more soluble derivatives prepared by acetylation, carboxymethylation, TEMPOmediated oxidation, phosphorylation, or enzymatic pretreatment).

From cellulose acetate, monoliths were prepared by thermally induced phase separation from the solution of cellulose acetate in dimethylformamide with the addition of 1-hexanol (heated to 70 °C, after the first heating left 24 h at 20 °C) followed by the exchange of solvent with ethanol and vacuum drying. The material obtained was hydrolyzed in a methanolic solution of NaOH to obtain cellulose monoliths [17][18][19][20][21]. A widely used water-soluble cellulose-derived compound is carboxymethylcellulose (CMC). CMC porous monoliths can be prepared by controlled freezing of CMC aqueous solution in a liquid nitrogen bath and consequent freeze-drying [22,23]. Adding NaCl to the CMC solution before freezing lead after carbonization to monolithic carbons with a high specific surface area of 724 m 2 g -1 [23]. When associated with classical template-based synthesis (the use of polystyrene latex and Pluronic F-127) and hydrothermal processes, a hierarchically porous (TEMPO-oxidized) cellulose monolith could be obtained, resulting in a hierarchically porous carbon monolith with a high surface area (1563 m 2 g -1 ) [24].

The transformation of pure cellulose into monolithic form could be achieved through its dissolution in a suitable ionic liquid and consequent regeneration of cellulose (in the form of the a gel) by exchange of ionic liquid with appropriate non-solvent (e.g. ethanol) [25,26].

A commonly used approach to the preparation of cellulose monoliths, which does not require preliminary chemical cellulose modification or chemical crosslinking agents, is the "freeze-thaw" process of alkaline cellulose solution. During the freezing steps, the solvent partially crystallizes, concentrating the cellulose chains in regions between the solvent crystals. Consecutive thawing promotes chain self-assembling and stable assemblies are achieved mainly by H-bonding interactions [27]. The advantage is that pristine cellulose can be used directly and its dissolution can be easily achieved with a concentrated aqueous NaOH or NaOH/urea cold solution (-5 °C) [28]. The freezing and warming of the solution to room (or higher) temperature leads after the appropriate time (usually days) directly to the cellulose gel (in some cases, consecutive solvent exchange is required) [29][30][31][32].

The "freeze-thaw" process is closely related to the ice-templating process [33], where ice crystals formed during a freezing step lead to a macroscopic pore size with a particular lamellar shape in cellulosic materials [34,35]. To preserve the macroporous network of the gel as much as possible, supercritical or freeze-drying is usually used in this case [29][30][31]35].

Micropores in the porous carbon derived from cellulose are generated during the release of volatiles during the carbonization of the cellulose gel. The volume and surface of the micropores can be further increased by activation by either a chemical or a physical process. For example, monolithic carbon materials were prepared by drying and carbonizing a microcrystalline cellulose and starch mixture, followed by activation with KOH. Carbon materials showed very high surface characteristics with 415-1122 m 2 g -1 depending on the cellulose/starch ratio and activation parameters [36].

Mesopores are more difficult to monitor for carbons derived from cellulose gels. They usually depend on the synthesis conditions but can also be generated during the micropore activation step or with the help of a hard template used as a porogen agent [37,38].

Most of the methods mentioned above for the production of cellulose monolithic materials (and consequently carbons) use tedious procedures with high cost and energy consumption (e.g. supercritical/freeze-drying, long synthesis time), require environmentally nonfriendly chemicals (ionic liquids or dimethylformamide) or start with chemically modified cellulose. They are not very suitable for industrial applications [39].

Thus, the aim of this study project is to present a novel approach of making monolithic gels directly from microcrystalline cellulose, which can be carbonized to monolithic porous carbonaceous material. The procedure is simple, inexpensive, and eco-friendly using only water, NaOH, and ethanol as an exchange solvent. We focus on avoiding tedious and time-consuming procedures such as long heating (for gelation), freeze-and supercritical drying of the gels, and any activation step. The full characterization of the prepared cellulosic gel and carbonaceous products is provided. Surprisingly, such an easy and eco-friendly route has not been investigated as far as the authors know.

Materials and Methods

Unsuccessful experiments

This part briefly summarizes the performed but unsuccessful experiments aiming to obtain cellulosic gels carbonizable to carbon monoliths.

Cellulose dissolution through liquefaction of cellulose using autoclave

Hydrothermal liquefaction (HTL) process of cellulose dissolution was tested in autoclaves.

For the experiment different solvents, temperatures and initial weights were used. In the first attempt, it was used 1 g of cellulose/10 ml solvent (methanol) at 50, 60 80 and 100 °C. NiCl2 was used as a catalyst. The liquid/solid mixture was filtered. Solid sample was dried at 110 °C for 24 h. Evaporation of filtrate was applied to remove the solvent and to obtain a bio-oil [40]. Each of the experiments (at different temperatures) was not successful to dissolve cellulose or to obtain bio-oil. From results of this experiment, increasing of temperature and increasing of time does not have any significant effect on cellulose liquefaction.

Other attempt used 1 g of cellulose, 10 and 20 ml of K2CO3 with concentration of 1 mol dm -3 at 110, 150, 170 and 180 °C for 30 minutes (no bio-oil obtained).

Mixture of 1 g cellulose and 5 ml of deionized water at 170 and 240 °C for 30 min does not provide any useful liquid too.

Acid effect on cellulose liquefaction was also tested. 1 g of cellulose was mixed with 20 ml of HCl 1 mol dm -3 at 170 and 180 °C for 30 minutes. Still any bio oil from acidification was not formed.

Cellulose dissolution through reflux

Experiments with reflux cooler were carry on by using different amount of cellulose mixed with acids. Different amount of cellulose (1/2/3/5 g) was mixed with H2SO4/HNO3 (15 ml:15 ml) and boiled for 15 minutes. The mixture was diluted with water up to 100 ml, NH3 solution (approx.

100 ml up to pH = 7) was added. The cellulose was carbonized. After carbonization the material was as a powder.

Each of the experiments with isopropanol, cyclohexane, methanol, glycerol, diethylene glycol, ethylene glycol, ethyl alcohol, propanol, ethylene glycol, ethanol, phosphoric acid at 150 °C for 30, 60 and 120 minutes (1 up to 6 g of cellulose) were unsuccessful whatever catalyst was used.

Cellulose dissolution using cold alkaline solution/urea and acids

Cellulose dissolution was tested on NaOH/urea aqueous solution. 7/3.5 g of NaOH, 12/6 g of urea and 81 g of deionized water were precooled to -10 °C. Under vigorous stirring (at 5000 rpm) 2,5 g of cellulose was added into solution. After its dissolution, transparent solution was seen.

Stirring was applied to remove air bubbles. Since the amount of cellulose used was in small quantity, it was possible to dissolve cellulose by mixture at room temperature.

Different ways of gel preparation were tested: 1) cellulose was dissolved in NaOH/urea aqueous solution using 1 g of tartaric acid or EDTA as a cross-linker (solution was heated after adding cross linker but the dissolution of cellulose was not obtained), and 2) cellulose dissolved in NaOH/urea aqueous solution was only heated (after this step we obtained soft gel). The soft gel (obtained only from cellulose and NaOH) was mixed with tartaric acid and placed in Petri dish.

Since the gel was placed in petri dish, the material after drying at 70 °C resulted in a powder form.

Cellulose dissolution in NaOH/urea aqueous solution was tested in autoclave too. NaOH/urea aqueous solution with corresponding amounts (3.5/6/40.5 g) and 2.5/5 g of cellulose, was placed into autoclave at 150 °C for 4 hours. The solution pH was 12. To decrease pH and to polymerize the solution, 10.2 g of 38 % formaldehyde solution was added. Cellulose was not well dissolved and the experiment was not used.

Other experiment was done by precooling solution of NaOH/urea/H2O at -10 °C. In the precooled solution, 2.5 g of cellulose, HCl or H2SO4 and 10.2 g of formaldehyde was mixed (pH = 9). For this experiment we used different amount of NaOH, urea, H2O, formaldehyde and cellulose (7,12,81,20.4 and 5 g) at 60, 150 °C for 4 hours. After heating cellulose solution at corresponding temperatures, the gel was very soft and melted at room temperature.

Experiment with 3.5 g NaOH, 6 g of urea and 40.5 g of H2O was precooled at -10 °C. After precooling we added 2.5/5 g of cellulose and 6 ml of polyethylene glycol (PEG). Solution was mixed at room temperature for 30 minutes. The mixture was poured into Petri dish, heated at 50 °C for 4 hours, washed with distilled water, freeze dried and kept on desiccator. Water soluble PEG could serve as polymeric porogen and was expected to cooperate with NaOH to promote the gelation. This hypothesis was not confirmed.

To cellulose solution (prepared from NaOH aqueous solution and urea) 16 or 18 g formaldehyde was added. Mixture was heated at 60, 70, 80 °C for 8, 20, 30 minutes. During the heating 2, 3, 5 ml of H2SO4 (pH = 1,5) was added. The mixture was heated at 70, 80 °C for 20, 30 minutes (during the heating 5.25 or 6 g of urea was added). Solution was frozen at -10 °C, then soaked into H2SO4 5 % (the same experiment was tested for cellulose dissolution, but instead of adding NaOH 10 % 6 g of urea was added and after freezing step the soaking at H2SO4 5 % was not applied). The tubes with the frozen gel were heated at 30 °C. Heating the gel at 30 °C, the gel was not shaped well and was very weak during the heating. Was not possible to cut it at room temperature since the gel was very soft.

The experiments were simplified by precooling solution of only 7 g NaOH, 12 g urea and 81

g H2O at -10 °C. After precooling 8 g of cellulose was poured into alkaline solution and stirred for 1 hour. After 6 hours the gel was cut into pieces. As the gel was very weak, the same experiment was repeated again, but after the cutting step, the gel was soaked into ethanol, washed several times with deionized water (to pH = 7) and dried at room temperature. As gelation can be slowed down by utilization of additives such as urea, thiourea or ZnO, we dissolved cellulose only in NaOH aqueous precooled solution (no urea used).

After so many unsuccessful attempts, we dissolved 9 g of microcrystalline cellulose in 7 g of NaOH, 81 g H2O (precooled solution), frozen at -10 °C (for gelation), soaked into ethanol (for cellulose regeneration) and washed with deionized water to preserve the monolithic shape and remove NaOH traces.

The same experiments (with NaOH aqueous solution, with or without urea/formaldehyde) were successful to prepare monolithic carbons from wood too, but it did not work with tannin or tannin/cellulosic gels).

On attempts to increase mesoporosity into cellulosic gels, we prepared also solution of 7 g NaOH, 40.5 g of H2O, 40.5 g of ethanol and 9 g of Pluronic (F127). Solution was precooled at -10 °C. At room temperature 9 g of cellulose was added, but during the mixing the cellulose was not dissolved properly due to the presence of F127. With adding F127 into the solution in such amount, the Pluronic itself into alkaline solution created some big aggregations and even with heating was not easy to dissolve.

Dissolution and final cellulosic gel preparation

Compounds were used as received without any purification. The synthesis was based on a simple mixture of sodium hydroxide (Sigma Aldrich), microcrystalline cellulose (MCC, Sigma Aldrich), and deionized water (DI). First, NaOH (7 g) was dissolved in 81 g of DI and the solution was pre-cooled down to -10 °C. Then, 9 g of MCC was added and the mixture was stirred for 40 minutes. After cellulose dissolution, the mixture was poured into tubes which were placed at -10 °C for 12 h. After this time, the frozen solid cellulosic material was cut directly into pieces and soaked in ethanol for two days for regeneration of the cellulose, the gel-like solid pieces were obtained. The solvent was then exchanged with DI up to pH ≈ 7 for 1 day (during the monolith processing the water washed the NaOH). After the solvent exchange, color of monoliths changes from yellow to the white one. Resulting material was dried at 80 °C before further use. The pieces with size up to 1 cm were obtained. Pyrolysis was carried out at different temperatures (500, 700 and 900 °C) for 1 h (heating rate 10 K min -1 ) under an inert atmosphere (nitrogen, flow 200 ml min -1 ). To ensure the removal of sodium compounds, the obtained carbonaceous monoliths were treated with 1 mol dm -3 HNO3 solution (6 h) and then with ≈ 15 % ammonia solution (6 h) followed by a drying step at 250 °C overnight.

In the following text, cellulose samples are referred to as MCC for microcrystalline cellulose and CMG for prepared cellulose monolithic gel. CMG-uw represents a monolithic cellulose gel that was not washed with DI after regeneration in ethanol. Carbonaceous samples derived for cellulose pyrolysed at a temperature x (x = 500, 700 and 900 °C) were labeled by adding x to the name of the precursor, e.g. MCC-x and CMG-x.

The carbonaceous monoliths obtained from CMG have a well-defined cylindrical shape maintained in the carbonization step despite the significant shrinkage after drying at 80 °C and pyrolysis. Pyrolysis of CMG-uw showed us clearly that the presence of NaOH is detrimental to the preservation of the monolithic shape. Therefore, the washing step with water after regeneration in ethanol is a key step to preserve the carbonaceous monolith.

Characterization of cellulosic gels and carbonaceous monoliths

XRD analysis of the cellulose samples was performed with a Bruker powder diffractometer D8 ADVANCE A25 in the Bragg-Brentano reflection geometry theta-2theta. The diffractometer is equipped with the LynxEye XE-T high-resolution energy dispersive 1-D detector (Cu Kα1, 2), which leads to ultrafast X-ray diffraction measurements. The conditions for data collection are the following: angular range 10-90º 2θ, step size 0.014º 2θ, time per step 0.7 s, variable divergence slits, irradiated sample length 15 mm, total time for acquisition 1h 11 min. During data collection, the powder sample rotates at 15 rpm. The precursor MCC was characterized by an XRD pattern (Fig. 5a). It displays the cellulose I polymorph with typical diffraction peaks at 2θ = 14.8, 16.3, 22.6° and 35° for the diffraction planes (1-10), ( 110), ( 200), (004), respectively [41]. Since this crystalline phase is often associated with an amorphous cellulosic phase, the contribution of the latter was determined on the basis of the Segal method [42]. The crystallinity index (CI) of MCC was calculated by comparing the minimum intensity above baseline at 2θ =18º (Iam) and the maximum intensity of the plane (200) at 2θ = 22.5º (I200). CI for CMG was calculated from 2θ = 21.9º for cellulose II and 2θ = 12.1º representing the amorphous contribution [43]. ( 1 H) = 400.17 MHz). 13 C single pulse MAS NMR experiments were performed with a π/4 pulse duration of 2.65 µs and a 60 s recycling delay and a 1 H decoupling radiofrequency field strength of 69 kHz. These recording conditions ensure the quantitative determination of the proportions of the different carbon species. Typically, 2500 scans were recorded.

Chemical shifts reported later are related to tetramethylsilane (TMS). The decomposition of the spectra is obtained using the Dmfit program [44]. On the basis of the NMR spectrum, the crystallinity index for cellulose I was calculated by expressing the signal area at 89 ppm as a portion of the total area assigned to C-4 [45]. Crystalline regions of cellulose were derived by line fitting (composites of Gaussian and Lorentzian line shapes) of crystalline and amorphous C4 carbon signals from 85-92 ppm. For cellulose II, the crystallinity index was calculated by C4 deconvolution at 87 ppm (crystalline) and 85 ppm (amorphous part) [46][47][48].

Infrared (FT-IR) spectra of the cellulose samples (non-carbonaceous) were measured with a NICOLET 6700 FTIR instrument. ThermoScientific. The experimental conditions for the spectra recording were: 50-100 mg of the sample, 256 scans, resolution 4 cm -1 , 64 scans for background, using DTGS KBr as detector, KBr as beam splitter, and diamond as an accessory smart performer.

The spectrum is measured in the range of 4000-400 cm -1 . For qualitative measurements, we have used 250 mg KBr and 2 mg of sample.

Raman spectra were collected using the Horiba Raman spectrometer, Labram 300, with wavelength 532 nm, 60 seconds per spectrum with an objective of the microscope 50×. The D/G ratio was calculated from a deconvoluted peak area using a Voight shape.

The thermal stability of MCC and CMG was examined by thermogravimetric analysis (TG) using the STA 449C apparatus (Netzsch), in the temperature range 30-800 °C, heating rate 10 °C min -1 , a crucible of α-alumina, inert (N2, 100 ml min -1 ) or oxidizing (N2, 80 ml min -1 + O2, 20 ml min -1 ) atmosphere.

The morphology of the carbonized cellulose monolithic gels (MCC and CMG) was investigated by scanning electron microscopy (SEM) using a Quanta 400 microscope.

Transmission electron microscopy (TEM) images of MCC-700 and CMG-700 samples (Fig. 7) were recorded using a Jeol microscope (model ARM-200F) operating at 200 kV. The powder was crushed and dispersed in chloroform with ultrasound. The drops were deposited on the surface of a copper observation plate and dried under room temperature.

The porous structure of the CMG-500 and CMG-900 samples were analyzed by a gas physisorption technique. The samples were degassed in a vacuum for 10 hours at a final temperature of 300 °C; heating was gradual with isothermal steps at 65 and 105 °C with 0.5 h soak time and 3 °C min -1 heating rate. Adsorption of N2 (77K) and CO2 (273K) (99.999 % and 99.998 % purity, respectively) was carried out using the static manometric adsorption system (Autosorb iQ-XR, Quantachrome Instruments). The pore size distribution (PSD, Fig. 14b, 15b) curves were obtained by fitting experimental isotherms (Fig. 14a,15a) by a specific kernel using ASiQwin software (Quantachrome Instruments). The experimental N2 adsorption isotherms were fitted by a hybrid carbon QSDFT adsorption kernel assuming slit-shaped micropores and cylindrical mesopores. This kernel is limited to pores of diameter below 33 nm. The CO2 adsorption isotherms were fitted by a carbon NLDFT equilibrium transition kernel assuming slit-shaped micropores.

Although the equilibrium kernel (= for desorption data) was applied, the software uses the adsorption branch data because the adsorption and desorption are reversible for the micropore range. Micropore volumes and surface areas (from N2 and CO2 physisorption experiments), mesopore volumes, and mesopore surface areas (from N2 physisorption) were calculated by integration of the microporous and mesoporous parts of differential PSDs. Despite the fact that physical adsorption of gases (N2, CO2, etc.) in micropores is based on a "volume-filling" mechanism [49] and only micropore volume should be reported, we are also reporting micropore surface area for comparative purposes. CO2 (273K) physisorption measurement provides reliable micropore characteristics for pores of diameter below ca. 1 nm [50].

To determine macroporosity of the samples, the mercury intrusion (Hg) porosimetry technique (Autopore IV 9500, Micromeritics apparatus) was selected (Fig. 9a,b). Progressive mercury intrusion between 0.004 and 20 MPa was performed assuming a constant mercury contact angle of 130°.

Results and discussion

Properties and characterization of cellulosic gel

The evolution of the MCC material to a monolithic CMG through the dissolution-freezing process in the presence of NaOH was followed by numerous techniques.

The XRD pattern of the MCC sample (Fig. 5a) clearly shows the presence of cellulose I in commercial microcrystalline cellulose. A value of 80 % for the CI was found, the apparent diffracting domain of this phase was evaluated as 53 Å using the Scherer's formula [51] for the most intense peak (200). 13 C MAS+DEC NMR spectra (Fig. 5b) confirm the presence of cellulose I (signals at 65.7 and 89 ppm) and amorphous cellulose (62.9 and 84-85 ppm) [52] for the MCC sample. The CI of the deconvolution of the NMR signal was 49 %.

After dissolution of MCC in NaOH solution and subsequent regeneration, the transformation of cellulose from its native form of cellulose I to cellulose II is expected to occur [53]. This trend was confirmed in the CMG sample XRD pattern (Fig. 5a), where typical peaks for the cellulose II polymorph are shown at 2θ = 12.1, 19.8 and 22.0° for planes (1-10), ( 110) and (020), respectively. This crystallographic evolution was followed by a decrease in the CI of the material (53 %) and of its apparent diffracting domain size (27 Å).

By NMR, the typical spectrum of regenerated cellulose (after its dissolution in NaOH) is observed with the signal at 105 ppm and overlapping resonances from 70 up to 80 ppm (Fig. 5b).

CI for cellulose II was calculated by deconvolution of C4 at 87 and 85 ppm [22,45,52]. As observed by XRD, gel CI decreased during the formation of the gel to 53 %.

FTIR spectra (Fig. 5c) of MCC show the presence of cellulose I with typical intense bands at 3426, 2919 cm -1 , 2118 cm -1 , 1368 cm -1 , and 1035 cm -1 . The band at 3426 cm -1 is attributed to the OH stretching (in intra and intermolecular H-bond) of cellulose. The band at 1631 cm -1 is related to the OH bending mode of absorbed water. For regenerated cellulose (CMG sample), the decrease of the peak intensities at 2919 cm -1 compared to MCC is due to alkaline treatment [54,55], and it corresponds to the formation of a higher portion of amorphous cellulose compared to MCC. The process it takes place during regeneration and is associated with the crystallization of cellulose II [56]. Indeed, alkali has a significant effect on morphological, molecular and supramolecular properties of cellulose. NaOH is associated with a decrease in crystallinity and a transformation of cellulose I to cellulose II. When cellulose is treated with alkaline solution, cellulose adopts a modified crystal structure, irreversibly forming cellulose II [57]. Interacting with cellulose, aqueous sodium hydroxide is able to penetrate the cellulose crystalline lattice to produce a series of well-defined crystalline complexes.

The CH stretching mode at 2919 cm -1 is shifted to a lower wavenumber compared to the MCC sample [58] due to the presence of cellulose II [55]. The Raman spectra confirm the features determined by the previous techniques (Fig. 5d). The typical bands at 1121 and 1094 cm -1 , assigned to symmetric and asymmetric β-(1,4)-glycosidic linkage stretching modes [59][60][61] are observed for both MCC and CMG samples, but with more broad and diffuse bands for the CMG samples due to its higher amorphous character. This higher amorphous contribution for the CMG sample is also validated by the increase in intensity of the peak at 1379 cm -1 . The band at 1336 cm -1 shows the presence of cellulose I polymorph for MCC whereas the bands at 1150, 1279 cm -1 and 1471 cm -1 (more intense for the CMG) are attributed to cellulose II polymorph [62]. 65

The CMG sample displays a very different morphology than the MCC sample with a lamellar structure related to the ice-templating effect that arose during the freezing step (-10 °C) performed on solubilized cellulose (Fig. 6). Indeed, under the experimental conditions used, the precooled solution allows the dissolution of cellulose slightly below 0 °C whereas the second cooling (-10 °C) required to produce a tough gel at room temperature, leads to an ice crystal growth with a concomitant increase of NaOH brine concentration and Na-cellulose precipitation [53]. Owing to the anisotropic growth of the ice taking place together with the regeneration of cellulose, a typical ice-template macroscopic lamellar structure [63] is observed in the gel (see Fig. 6). The absence of ash for the CMG sample is a good evidence of the effectiveness of the washing procedure. MCC can be burned out without residuals, as expected [64].

Carbonization process

The carbonization process of MCC and CMG was followed by TG under an inert atmosphere (Fig. 8). At low temperatures up to 200 °C the evaporation of residual water (or ethanol in the case of CMG) can be observed (mass loss 1.8 % for MCC and 3.1 % for CMG). The carbonization process starts at about 220 °C for CMG, while MCC is more stable and its thermal degradation begins at 300 °C. In general, the strong interaction of O-H groups in cellulose II requires more energy to start the degradation process. However, because the amorphous region is the most unstable and reactive one, CMG (lower crystallinity) decomposition starts earlier than MCC (higher crystallinity). Furthermore, due to the preparation procedure, the presence of residual sodium compounds can be expected, which can act as a catalyst and promote the decomposition of cellulose at a lower temperature [65].

The main step of carbonization (dehydration, dehydrogenation, evolution of volatile organic decomposition products) accompanied by a large mass loss (68 % for MCC and 62 % for CMG) is finished at approximately 350 °C for both materials, and subsequent heating leads to much lower mass changes related to the decomposition/evolution of residual hydrogen and oxygen groups [64].

The yield of carbonaceous product at 800 °C is considerably higher for the CMG sample (17 % for MCC and 23.5 % for CMG). The increase in char yield can be explained by the traces of sodium compounds (derived from NaOH), leading to an increase in char formation for CMG and a decrease in gaseous pyrolysis products (Fig. 8) [66][67][68][69]. During the water-ethanol exchange, drying, and carbonization step, a shrinkage of about 30 % in length was observed, but the material still retains its monolithic character (Fig. 9). The samples were very stable and keeping the shape after pyrolysis. Using specific molds, monoliths can be prepared in different shapes and sizes (Fig. 9). FTIR spectra (not reported) are similar for both MCC-x and CMG-x samples (x = 500 to 900) and display for x = 500 °C the presence of organic functional groups typical of low-temperature carbonized biomass with the presence of carbonyl, hydroxyl, and alkene at respectively 1577, 2660 and 1567 cm -1 .

Raman spectroscopy is a powerful method to characterize carbonaceous materials. For the studied carbonized samples, their low structural ordering, as evidenced by XRD (LC ~ 1.2 nm), is due to the nongraphitizing nature of the cellulose biochar and the low temperature of carbonization (e.g. 700 °C). This feature was confirmed by the Raman and the TEM analysis too. The Raman spectra (Fig. 10b) display relatively similar features for both MCC-700 and CMG-700 samples.

They were fitted by two bands (D and G) as recommended by Puech et al. [70] for crystallite size lower than 2 nm. The results are reported in Table 3. The ID/IG ratio, with respectively values of 1.0 and 0.9 for MCC-700 and CMG-700 could not help to discriminate the two samples.

Surface properties and morphology

TEM observations were performed for the MCC and CMG carbonaceous materials pyrolysed at 700 °C. The samples are completely disordered in structure but homogeneous (Fig. 11a,b). and (h) pyrolysis at 700 °C, and (i) pyrolysis at 500 °C with different magnification.

An analysis of the volumes of mercury that entered the sample upon initial pressurization provides qualitative information on the size of the pores. The steep rise of the intrusion curve at low pressure is due to the large cavities present in the material (Fig. 13a,b) [71]. Tested CMG-500 and CMG-900 samples show similar cumulative pore volumes ranging from 1.4 to 1.5 cm 3 g -1 (Fig. 13a). No deformation after mercury intrusion and no break of monoliths after the measurement was noticed. studies, e.g. [72]. This also shows Table 4, where increase of the pyrolysis temperature of CMG from 500 to 900 °C leads to the increase in the micropore volume from 0 to 0.192 cm 3 g -1 and micropore surface area from 0 to 548 m 2 g -1 , while mesopore characteristics are not substantially different. Both CMG samples also exhibit reduced access of micropores for N2 under cryogenic conditions, as shown by the differences in micropore volumes and surface areas by adsorption measurements of N2 (77K) and CO2 (273K). The higher CO2 adsorption measurement temperature compared to 77K for nitrogen then promotes the diffusion (and improves accessibility) of the CO2 molecules into the small micropores that are becoming detectable (Fig. 15) [49]. Table 4. Textural properties of CMG carbonaceous materials obtained by N2 (77K) and CO2 (273K). Vmicro micropore volume, Smicromicropore surface area, Vmesomesopore volume, Smesomesopore surface area.

N2 (77K) CO2 (273K) Vmicro (cm 3 g -1 ) Smicro (m 2 g -1 )
Vmeso (cm 3 g -1 ) Smeso (m 2 g -1 )

Vmicro (cm 3 g -1 )

Smicro (m 2 g -

Conclusions

Our results showed that a simple and eco-friendly way of transformation of cellulose to monolithic porous carbonaceous material is possible. Frozen cellulose solution in aqueous NaOH can be easily obtained at -10 °C and shaped in this state by simple cutting (alternatively, a suitable mold can be used before freezing). Direct regeneration in an ethanol bath, washing out excess NaOH with water, and drying lead to xerogel-like macroporous cellulosic monoliths composed mainly of cellulose II polymorph and equipped with significant macroporosity due to icetemplating. During subsequent carbonization, macroporosity and monolithic character are preserved. The obtained carbonaceous material also showed significant microporosity. Since the most of the surface area is generally occupied by micropores, their characteristics usually play a key role in the practical use of the materials (e.g. as adsorbent or catalyst).

The prepared micro-and macroporous carbonaceous monoliths can be perspective candidates for applications requiring monolithic character, high micropore volume, and good transportation/diffusion properties ensured by macropores.

In addition, the used synthetic route can be easily adapted for doping of cellulose/carbon with metal or metal oxide nanoparticles (which can be added to starting solution in the form of suspension), heteroatoms (compounds soluble in alkaline solution can be used for doping), etc.

We also suppose that the suggested procedure of conversion of cellulose to monolithic carbonaceous material can be adopted for biomass-based materials in general, enabling easy and low-cost production of advanced porous carbons for various applications from renewable resources.

La

Introduction

Biomass pyrolysis is one of the primary thermochemical methods to convert biomass into biogas, bio-oil, char and high carbon products [1]. However, pyrolysis is a very complex process due to the influence of water content, minerals, heating rates, texture and structure of biomass and additives. Adding additives as a catalyst regulates pyrolysis process [2]. Many inorganic salts (such as alkali metals compounds) are present in biomass material and have catalytic activity during the pyrolysis process [3]. The mineral compounds strongly affect the pyrolysis mechanism of biomass, causing changes in product distribution and promoting the char formation [4]. It may enhances the yield of solid product [5] with char formation [6] by inhibiting the cracking of the macromolecules.

Among biomass, pyrolysis of cellulose has been widely studied. Its thermal decomposition, accompanied with a significant weight loss, starts slowly and becomes fast at high temperatures [7]. Cellulose with partly crystalline fibre structure [8] first decomposes very rapidly into fragments of low degree of polymerization (DP) with only 5 % mass loss. This first stage of cellulose decomposition mainly occurs in amorphous regions. In this process cellulose forms volatile products which lead to opening of closed pores and development of porous structure [9,10]. Moreover, the rupture of cellulose molecules occurs at crystalline-amorphous boundaries.

Some authors as Lede et al [11], proposed another mechanism which includes fragmentation and formation of active cellulose (active cellulose originates from the induction period before any weight loss takes place during thermogravimetric analysis) together with cellobiose, glucose, and fructose. The temperature of maximal decomposition rate of cellulose is a function of the degree of crystallinity of the cellulose [64,65]. Thus, an increase in the crystallinity index and crystallite size determines the increase of the decomposition temperature of cellulose residues (activation energy remains the same) [14]. At higher temperature, further reactions with intermediate formation of levoglucosan and repolymerization of volatile material leads to char and gases [11,12,13]. During pyrolysis at 250 °C, cellulose loses linearity and form a three-dimensional network polymer [18]. In crystalline form, cellulose has limited accessibility to water and chemicals, and its degradation starts at around 300 °C to produce anhydrocellulose [19], and the primary volatiles [20]. Heating at 325 °C results in 37 % weight loss and the remaining materials has the empirical formula of C6H9O4.3, indicating the loss of H2O. Heating at 350 °C produce 67 % weight loss and changes in the composition of the residue. After complete decomposition of the glycosylic units and formation of the char, heating at 450 °C and 500 °C produces only minor changes [21,22].

In general, alkaline substances reduce the bulk cellulose decomposition temperature [23,24,25,26,27,28,29]. The alkali hydroxide may cause cross-linking in the biomass structure via dehydration, to obtain a higher char yield after adding NaOH into the biomass structure [30].

NaOH treatment is also an efficient method to dissolve non-cellulosic components. It is used as an initial step for cellulose isolation from biomass. Alkaline pre-treatment increases the solubility of biomass in general and participates in the reaction system of liquefaction [31]. Ions in NaOH solution are very small, they can penetrate into the biomass structure and break the intermolecular hydrogen bonds under heating, react with cellulose and form some low molecular compounds in the biomass [32]. As a result, volatilization occurs at lower temperature [2]. The potential change in the structure of cellulose is dependent on the amount of NaOH and exposure to the water during pretreatment [33]. The treatment with NaOH may change the organic structure due to degradation and can break/reform interaction forces between macromolecules. During NaOH treatment, the cellulose molecule brake and become shorter, cellulose lattice is deformed [34,35]. However, the treatment of cellulose with NaOH changes the cellulose modification depending on the NaOH concentration. With the increase of NaOH concentration (more than 10 wt%), the changes in structures are bigger [36]. At low temperature with 10 wt% of NaOH, the concentration of Na + ions is too low to form enough Na-cellulose I that allows swelling of the fibers [37]. Modification by alkaline solution causes the changes on electron density on C1 and C4 carbon atoms, promoting the acidic character of the hydroxyl groups. This is why the large amount of alkaline metals are introduced to the cellulose modified with hydroxide solutions [32,33,34,35]. The accessibility of OH groups on cellulose chain depends on the crystallinity of cellulose. During the dissolution, the alkali solution penetrates amorphous regions, then into crystalline regions. Depending on the NaOH concentration, cellulose I or cellulose II are formed after regeneration [42].

Since mechanism of cellulose I and cellulose II decomposition is different and complex, kinetics of cellulose pyrolysis was studied and expressed in different forms (Table 5). Kinetics of cellulose pyrolysis was described as two pseudo-homogeneous first order reactions of cellulose decomposition to glucose (k1) and other degradation products (k2). The reaction rate constants are usually expressed in the form of Arrhenius equation [43]. On different types of cellulose, high values of activation energy above 200 kJ mol -1 are often reported. Pyrolysis of small sample of pure cellulose is characterized by an endothermic reaction of the first order with activation energy of 238 kJ mol -1 . Activation energy from kinetic evaluation of avicel cellulose pyrolysis was given at 245 kJ mol -1 at 1 K min -1 , while simultaneous evaluation gave 236 kJ mol -1 (with increasing heating rate they noticed small decrease on activation energy).

But, due to the heat-transfer problems (thermal lag), evaluations by using Friedman method shifts Ea to 80 kJ mol -1 . The decrease in the activation energy is largely due to temperature inhomogeneities in the sample at high heating rates or large sample mass. At solid residence times above 800 °C, char yield increases due to the carbon deposition from secondary cracking of tar and light oxygenated volatiles [51,52,53].

Another attempt of describing the cellulose pyrolysis used model of Broido-Shafizadeh [60].

This model describes cellulose pyrolysis as two parallel reactions producing either volatiles without char, or solid intermediates. The temperature dependence of the rate constant is given by Arrhenius equation. From their study it was concluded that last 30 minutes at 500 °C determined the final char yield. Activation energy was reported as 238 kJ mol -1 [61,62].

Activation energy values appeared to be related to the changes of mass loss. The higher activation energy indicates that thermal decomposition occurs by depolymerization of cellulose with production of levoglucosan. Lower activation energy in the first step of reaction is related to the thermal degradation controlled by dehydration and resulting anhydrocellulose [19]. Higher adsorbed water, results in lower activation energies [56,57,58,59].

Kinetics of pure cellulose decomposition is well known, but not a lot of studies were done for description of cellulose decomposition in a mixture with NaOH.

The aim of this chapter was to explore the effect of NaOH  on the pyrolysis process of microcrystalline cellulose (cellulose I) and cellulosic gel (cellulose II) including its kinetics, and  on the properties of carbon obtained after pyrolysis.

Materials and Methods

Materials preparation

The synthesis of cellulosic gel was based on a simple mixture of sodium hydroxide (Sigma Aldrich), microcrystalline cellulose (MCC, Sigma Aldrich), and deionized water (DI). First, NaOH (7 g) was dissolved in 81 g of DI and the solution was pre-cooled down to -10 °C. Then, 9 g of MCC was added and the mixture was stirred for 40 minutes at room temperature. After cellulose dissolution, the mixture was poured into tubes which were placed at -10 °C for 12 h. After this time, the frozen solid cellulosic material was cut directly into pieces and soaked in ethanol for two days for regeneration of the cellulose, the gel-like solid pieces were obtained. Resulting material was dried at 80 °C before further use. The pieces with size up to 1 cm were obtained. For comparison, part of the gel was washed with water before pyrolysis (see Table 6 for the names of samples).

Characterization of cellulose-based carbons

X-Ray diffraction (XRD) analysis of the cellulose and carbon samples data were collected with a powder diffractometer D8 ADVANCE A25 from Bruker in Bragg-Brentano reflexion geometry theta-theta. The diffractometer is equipped with the LynxEye XE-T high resolution energy dispersive 1-D detector (Cu Kα1, 2), leading to ultrafast X-ray diffraction measurements.

Conditions for data collection are the following: angular range: 10-90º 2θ, step size: 0.014º 2θ, time per step: 0.7 s, variable divergence slits (irradiated sample length: 15 mm) total time for acquisition 1h11min. During the data collection, the powder sample is rotating at 15 rpm. The structural parameters (Table 12) were calculated by deconvoluting an asymmetric (002) peak near 20º and 26º. The background was removed by drawing a straight line connecting the peak observed at 10º and 15º and between 35-60º, then deconvoluted by Gaussian fitting. 2θ and FWHM were measured for each peak, and the crystallite size Lc were calculated using the Scherrer equation. In the Scherrer equation, the constant K was set to 1.84 for Lc [67]. Lc was calculated based on the (002) peak of crystalline phase. Since, the internal structure of carbonized materials is composed as a mixture of crystalline and amorphous carbon nano-clusters, the (002) peak intensity, (Itotal) is expressed as the sum of crystalline (Icrystalline) and amorphous carbon (Iamorphous) intensity contributions, the latter being based on the background-line extrapolation at the (002) peak position. The relative fractions of the crystalline and amorphous carbons were calculated by using the equations (Icrystalline/Itotal) x 100 and (Iamorphous/Itotal) x 100 respectively [67,68].

Infrared spectra (FT-IR) of the cellulose samples were measured with NICOLET 6700 FTIR Thermoscientific. Experimental conditions of spectra recording were: 50-100 mg of the sample, 256 scans, resolution 4 cm -1 , 64 scans for background, using DTGS KBr as a detector, KBr as a beam splitter and diamond ATR in the form of smart performer accessory. The spectrum is measured in the range of 4000-400 cm -1 .

Raman spectra were collected using Raman spectrometer Horiba, Labram 300, with wavelength 532 nm, 60 second per spectrum with an objective of the microscope 50x.

The porous structure of the MCC, CMG, MCC/NaOH and CMG/NaOH samples pyrolysed at 500 °C and 900 °C was analyzed by a gas physisorption technique. The samples were degassed in a vacuum for 10 hours at a final temperature of 300 °C; heating was gradual with isothermal steps at 65 and 105 °C with 0.5 h soak time and 3 °C min -1 heating rate. Adsorption of N2(77K) (99.999 % purity) was carried out using the static manometric adsorption system (Autosorb iQ-XR, Quantachrome Instruments). The pore size distribution (PSD) curves were obtained by fitting experimental isotherms by a specific kernel using ASiQwin software (Quantachrome Instruments).

The experimental N2 adsorption isotherms were fitted by a hybrid carbon QSDFT adsorption kernel assuming slit-shaped micropores and cylindrical mesopores. This kernel is limited to pores of diameter below 33 nm.

The morphology of the carbonized cellulose monolithic gels was investigated by scanning electron microscopy (SEM) using a Quanta 400 microscope.

Transmission electron microscopy (TEM) images were recorded by using Jeol microscope (model ARM-200F) working at 200 kV. The powder has been crushed and dispersed in chloroform with ultrasounds. The drops were deposited at the surface of a copper observation grinded and dried at room conditions.

Results and discussion

Characterization of the starting cellulose materials

The XRD pattern of the MCC sample (Fig. 16a) clearly shows the presence of cellulose I in commercial microcrystalline cellulose. A value of 80 % for the crystallinity index (CI) was found, the apparent diffracting domain of this phase was evaluated as 53 Å using the Scherer's formula [69] for the most intense peak (200) (Fig. 16).

For the CMG sample, XRD typical peaks for the cellulose II polymorph are shown at 2θ = 12.1, 19.8 and 22.0° for planes (1-10), ( 110) and (020), respectively. This crystallographic evolution occurred with a decrease in the CI of the material (53 %) and of its apparent diffracting domain size (27 Å) (Fig. 16).

Effect of NaOH on the carbons properties

Unwashed carbons obtained in the presence of NaOH

XRD patterns (Fig. 17a) of carbon materials MCC/NaOHuw x and CMG/NaOHuw x obtained by cellulose carbonization in the presence of NaOH (Fig. 17a) show the presence of inorganic salts (Na2CO3) and/or its hydrates (Na2CO3•H2O and Na2CO3•10H2O) produced during pyrolysis or after prolonged contact with air moisture after carbonization.

FTIR spectra (Fig. 17b) were analyzed from 500 up to 2500 cm -1 . Cellulosic materials carbonized with NaOH display stretching vibration of CO3 2-at 1478 cm -1 with their associated deformation vibration around 900 cm Raman spectra of MCC/NaOHuw x and CMG/NaOHuw x carbonized at different temperature are shown in Figure 17c. The typical D (1353 cm -1 ) and G (1593 cm -1 ) bands of graphitic carbon are observed and they overlap some of sodium carbonate [70]. The latter has Raman active band modes at 1430 cm -1 , 1485 and 1507 cm -1 , so they rule out the determination of the D band intensity of some samples such as MCC/NaOHuw x and CMG/NaOHuw x. The small bands at around 1079 cm -1 was assigned to the v1 symmetric stretching mode of the carbonate (bands of 1000-1100 cm -1 ). Its shift band from 1050 to 1070 cm -1 for MCC/NaOHuw x and CMG/NaOHuw 900 is caused by the presence of water.

Washed carbons obtained from cellulose carbonized in the presence of NaOH

XRD patterns of washed carbons obtained from the carbonization of cellulose with NaOH does not show the present of inorganic salts (Fig. 18) indicating that the washing process was Analysis of FTIR spectra (Fig. 19) shows that in MCC/NaOHw 500 and CMG/NaOHw x (x = 500 and 700) samples, the carbonate groups are moved out. Aromatic systems presence (C=C stretch vibrations) can be assumed from the presence of bands at 1600 and 1579 cm -1 . Sample of CMG/NaOHw 700 has some aromatic groups, where MCC/NaOHw 700 and CMG/NaOHw 900 are almost pure carbon. The C-O stretch vibrations and deformation of C-H bonds are present in the region from 1300-900 cm -1 (ethers, primary alcohols, etc) and are observed mainly for 500 °C.

The region from 900-750 cm -1 corresponds to -COO and aromatic C-H bands. Below this region, the difficulty to interpret any organic group bands can be assigned to skeletal vibrations. At 2000 cm -1 only noise of diamond crystal (from the ATR measurement) is present (Fig. 19). Carbon materials were analyzed by Raman spectroscopy (Fig. 20). It confirms the highly defective graphitic nature of the carbon materials. The Raman spectra of disordered graphitic carbon shows two peaks. The G peak around 1580-1600 cm -1 is related to the presence of regular hexagonal sp 2 rings present in graphitic carbon. The D peak, around 1350 cm -1 , corresponds to the breathing oscillation of full 6-membered carbon rings which is no more "forbidden" due to the presence of adjacent defects (e.g. sp 3 carbon atoms) [71,72,73].

In table 8 there are shown values of D and G band intensities for deconvoluted spectra. For deconvolutions, a Voigt shape (a mixture of Lorentzian and Gaussian shape) was used. The area of D and G bands was measured by considering a linear background [74].

The study reported from Ferrari and Robertson described the ID/IG ratio evolution between amorphous and nanocrystalline graphite [75], indicating that carbonized cellulose samples are poorly crystallized graphitic materials. The ID/IG is supposed to increase with the increase of the ordering up to 2 nm of the in-plane coherence length (La) and to decrease for higher length.

Assuming that La has the same order of magnitude as the Lc value determined by XRD, our samples are close to this limit and may have coherence lengths in the range where an increase of the ordering by enlargement of the crystallite size with the temperature could successively increase and decrease the ID/IG ratio. Therefore, no straightforward conclusion can be derived from this ID/IG ratio (see Table 8).

Based on the literature [76], the G-band position should stay at the same position where the D-band position should have a red shift with the formation of larger aromatic clusters. This trend is observed for CMG/NaOHw x samples only between 500 and 700 °C. At higher temperature (900 °C), a decrease of the ratio is observed. It could be related to the activation process of the carbon taking place in the temperature range in presence of NaOH (see TG part). At high temperature of 1300 °C we observe decrease of the ratio due to the increase of thermal treatment. °C leads to the increase in the micropore volume, while mesopore characteristics are not substantially different (Fig. 21a).

Surface and morphological properties of the obtained carbons

The increase of pyrolysis temperature of MCC from 500 °C to 900 °C leads to "disappearing" of pores smaller than ca. 4.5 nm. This is also demonstrated in Table 9, where the micropore volume decreases from 0.2 cm 3 g -1 to zero, and mesopore characteristics (Vmeso, Smeso) are reduced.

N2 sorption of samples without NaOH (MCC x, CMG x, MCC/NaOHw x and CMG/NaOHw

x) is shown in (Fig. 21b,c,d). Washing out of the NaOH leads to the increase of surface properties (Fig. 21b,c, d, see Table 9). Increasing carbonization temperature of MCC/NaOHw x from 500 to 700 °C leads to increase of mesoporosity from 0.08 up to 0.2 cm 3 g -1 . The same effect was seen to CMG/NaOHw x samples, while mesopore volume was increased from 0.06 up to 0.3 cm 3 g -1 .

As was expected, chemical activation with alkali hydroxide as NaOH is effective to prepare highly porous carbonaceous materials with large BET surface area [77] but surprisingly direct carbonisation of CMG and MCC at 700 °C yields to a similar microporous volume without any added activated agent. MCC carbonization leads to blocking pores during the nitrogen sorption resulting in less micropores with increasing temperature from 500 up to 1300 °C. Vmicro cm 3 g -1 Vmeso cm 3 g -1 Vmicro cm 3 g -1 Vmeso cm

Effect of NaOH on the cellulose pyrolysis

Cellulose treated with aqueous solution of inorganic salt such as NaOH is not a compound but a kind of mixture consisting of free chemical, residual cellulose and cellulose modified by salt [79].

TG analysis of cellulosic samples

The carbonization process of MCC, CMG, MCC/NaOH and CMG/NaOH was followed by TG and DTG under inert atmosphere at heating rate of 10 K min -1 (see Fig. 23a,b).

At low temperature, around 100-200 °C, the evaporation of residual water and/or ethanol (for MCC, CMG, MCC/NaOH and CMG/NaOH) is observed (Fig. 23a). The carbonization process starts at 300 °C for MCC (higher crystallinity and stability) and at 225 °C for CMG (see Fig. 23a).

TG and DTG analysis show different behavior for cellulose samples under the influence of NaOH.

DTG peaks for samples with and without NaOH show the significant difference in the temperature of maximum decomposition rate. Decomposition temperature of cellulose treated with NaOH is 35 °C lower than cellulose itself. At low temperatures from 240-285 °C, biomass in reaction with alkaline as NaOH, can produce oxalic acid (as a side-product), leading to different thermal stability. Heating of cellulose with NaOH can yield to sodium oxalate, which is later decomposed (due to the exothermicity of the reaction). From TG and DTG it is seen that the main step of carbonization and the mass loss is shifted from 290 °C (for samples containing NaOH) to around 350 °C for MCC and CMG. Presence of the shoulder between 290 up to 350 °C can be associated with the cellulose degradation in the sample (Fig. 23b). Above 800 °C, the peak is associated with the decomposition of sodium carbonate, observed for both MCC/NaOH and CMG/NaOH (Fig. 23a). After the decomposition process the residual mass is higher due to the impurities during the regeneration process (Fig. 23). The yield of carbonaceous product at 800 °C is higher for CMG (cellulose II and amorphous polymorph) than for MCC (cellulose I and amorphous polymorph).

Based on literature [80], NaOH has good performance as activating agent, but it is not the same case for Na2CO3 [81]. At 760 °C, the carbonates are still present. Higher temperature must be used to decompose it.

The carbonation reaction of the NaOH in a contact with CO2, thermodynamically is possible at 25 °C and at higher temperatures:

4NaOH + 2CO2 → 2Na2CO3 + 2H2O (1)
This carbonation reaction must be avoided because if NaOH transforms into Na2CO3 during the heating up to 760 °C, no reaction and no porosity will be developed. During the activation process of NaOH with carbon, NaOH is converted into Na2CO3. Carbon-NaOH reaction is explained with the reaction where NaOH is reduced by the carbon producing metal sodium during the heat treatment by reaction:

4NaOH + C → 4Na + CO2 + 2H2O (2)
This reaction generates CO2 which can react with the remaining NaOH, producing sodium carbonate.

Another possibility of Na2CO3 formation is by reaction of NaOH and C:

6NaOH + C → 2Na + 3H2 + 2Na2CO3 (3)
This reaction is thermodynamically possible at temperature around 730 °C. During the reaction Na2O can be formed instead of metal sodium since has the same Gibbs energy (ΔG° = 0) at the same temperature.

The evolution of CO and CO2 at temperatures higher than 800 °C, is related to the decomposition of Na2CO3 formed during the activation. The bulk alkaline metal carbonate melts before decomposing at 851 °C. At high temperatures as 900 °C, CO2 can be converted to CO and oxygen through reaction (only CO2 is needed as a reactant) [81]:

2CO2 → 2CO + O2 (4)

Kinetics cellulosic materials pyrolysis

Starink method

For kinetic evaluation, cellulosic samples were measured at various heating rates (2 -5 -10 -20 K min -1 ) (Fig. 24). Thermoanalytical data were treated with isoconversional Staring method.

The method is based on the isoconversional principle where the reaction rate is only a function of temperature for a given constant value of the extent of conversion [82]. The advantage of isoconversional method is the evaluation of the effective activation energy (Ea), without assuming any particular form of the reaction model. Analysis involves evaluation of dependence on the effective activation energy on conversion or temperature, and using this dependence for making predictions and finding the mechanisms of the processes [83]. The method requires a series of experiments at different temperature program and yield the values of effective activation energy as a function of the extent of conversion α. A significant variation of Ea with α indicates that the process is kinetically complex [84].

The Starink method consists in approximating the temperature integral (assuming the preexponential factor A and Ea independent of T (temperature) [85], which leads to the equation where β is the heating rate and α corresponds to conversion degree :

𝑙𝑛 ( 𝛽 𝑇 𝛼 1.92 ) = -1.0008 𝐸𝑎 𝑅𝑇 𝛼 (1)
For a given value of alpha, the plot of ln (

𝛽 𝑇 𝛼 1.92 ) versus ( 1 𝑇 𝛼
) results in a straight line whose slope is used to calculate the corresponding Ea [86]. Alpha values were calculated from the temperature range of 180 °C up to 600 °C (conversion 0.1 to 0.8) with the equation:

α = 𝑚 0 -𝑚 𝑚 0 -𝑚 𝑒 ( 2
)
Where m0 is the weight of the sample at the beginning of the measurement, m is the actual mass of the sample and me corresponds to the mass of the sample at the end of measurements.

The results of calculations and dependency of activation energy versus conversion are summarized in Fig. 25. Activation energy of MCC is stable with some small fluctuations at the end of reaction. The value determined by this method (227 kJ mol -1 ) is in the same order as the literature (see Table 10).

A similar behavior was observed for CMG materials where the activation energy is stable in a wide range of temperature. A slight discrepancy compared to MCC is detected in the beginning of the decomposition where the Ea is lower; it may be due to the present of Cellulose II (Nacellulose) instead of cellulose I which is well stabilized. The origin of that lower Ea could not be attributed to the higher presence of amorphous cellulose since it is present in both MCC and CMG samples and should also be detected for MCC. Traces of NaOH well dispersed within CMG could also be another reason of the high reactivity at lower temperature. Indeed, as show bellow, NaOH play a major role on cellulose carbonization. In the presence of NaOH and in the low conversion regime (alpha<0.2-0.3), the Ea is greatly reduced compared with pristine MCC and washed CMG samples. As described in the introduction part, it is well-know that heat-treatment of cellulose in presence of NaOH modify its carbonization mechanism. This trend occurs probably here and the associated mechanism takes place with a lower Ea. For higher conversion regime, at the beginning of the reaction, the activation energy increases up to its maximum of conversion at 0.6 then slightly decreases. This behavior is typical of complex kinetics with more than one process (Fig. 25a).

Activation energy dependence on conversion shows fluctuations of the results at the end of reaction for CMG sample (Fig. 25b). Evaluation of Ea from isoconversional Starink methods seems to be inappropriate when the temperature is slightly above the decomposition temperature. For our samples, it looks that the mechanism is changing during the reaction (it can be due to the formation of the oxalic acid at around 240-285 °C, mentioned above) and the process is kinetically complex indicating two or more values of activation energy (Fig. 26). With increasing heating rate, the reactions are shifted to higher temperatures with various extend which leads to the intersection of α on temperature dependence with increasing heating rate (Fig. 26a,b). Such situation is not expected in the theory of isoconversional methods.

Non-linear regression

Since the process of pyrolysis and calculations by using Starink method were not enough to explain all processes happening during decomposition, calculations by non-linear regression were used.

A non-linear regression optimizes kinetic parameters inside the model, relies on the assumption that the scatter of the data around the curves, follows a normal distribution. The goal is to minimize the sum of squares of the residuals between the theoretical points and the experimental curve [87].

The analysis of solid-state decomposition kinetics using non-linear regression is based on a basic kinetic equation:

𝑑𝛼 𝑑𝑡 = 𝑘(𝑇)𝑓(𝛼) (3)
Where k(T) is the temperature dependent rate factor of the pyrolysis reaction; f(α) is the reaction model and depends on the extent of conversion. Including Arrhenius temperature function and Sestak-Berggren expression of conversion function, and assuming very small distance between measured experimental points, we can obtain equation: The parameters of kinetic equation (see Table 10) were found by non-linear regression (Solver in MS Excel), obtained under various heating rates (2 -20 K min -1 ) with residual sum of squares used as the fitting criterion.

𝛼 𝑖 = 𝛼 𝑖-1 + 𝐴𝑒 -𝐸 𝑎 𝑅𝑇 𝛼 𝑚 (1 -𝛼) 𝑛 ∆𝑡 (
Cellulose decomposition with 40 up to 70 % conversion requires Ea of 227 kJ mol -1 related to the crystalline index and volatiles (for MCC) in agreement with other reported values [88].

During decomposition of MCC and CMG, there is significant difference of Ea in the first step; it goes from an Ea of 227 kJ mol -1 down to 97 kJ mol -1 due to the high presence of amorphous cellulose and/or traces of Na-Cellulose which are less stable and decomposed easily at first stage of reaction. In presence of NaOH, there is a general initial decrease of Ea at low conversion levels compared to MCC and CMG samples probably due to reactivity of NaOH with cellulose which could lead to particular decomposition scheme with for instance formation of sodium oxalate. At the beginning of the degradation process, the Ea (84 kJ mol -1 ) for MCC/NaOH is enough to start the decomposition reaction corresponding probably mainly to amorphous cellulose decomposition in presence of NaOH, whereas for CMG/NaOH sample, surprisingly, a higher Ea is required (162 kJ mol -1 ). This feature in the low decomposition temperature range could be due to the preferential dehydration occurring with MCC/NaOH and associated with a weight loss (volatile formation), char formation and a low Ea compared to depolymerization taking place with CMG/NaOH with a higher Ea and limited weight loss and tar formation [89]. Indeed, the raw MCC sample, not treated with NaOH as CMG, could display a high DP (degree of polymerization) and therefore should display a promoted decomposition through molecular gas and char (dehydration) instead of tar formation (depolymerization) [90].

High activation energy on last steps of the reaction corresponding to 322 and 265 kJ mol

Carbon yield

The obtained kinetic equations enable also predict the char yield for different heating rates (Fig. 28). Heating rate is important parameter for the yield of different products from the pyrolysis process. With increasing heating rate, heat and mass transfer limitations are break down during the pyrolysis, reducing the molecule residence time with a decrease of char formation.

NaOH significantly decrease the carbon yield with increasing heating rate. MCC and MCC/NaOH have a maximum carbon yield of 18 % (Fig. 28a). In CMG and CMG/NaOH samples, carbon yield is higher (around 25 %) (Fig. 28b). Carbon yield is highly affected by heating rate of carbonization process.

Conclusions

NaOH during carbonization process leads to destroying of the monolithic character of CMG samples.

During carbonization of cellulose in the presence of NaOH (since cellulose was dissolved into NaOH aqueous solution), we expect that NaOH is transformed to Na2CO3. Both presence of sodium hydroxide and sodium carbonates moves the maximum mass loss to lower temperatures. Based on TG analysis, kinetics of pyrolysis for each of the samples was studied. Mechanism of pyrolysis in the presence of NaOH was more complicated (for MCC/NaOH and CMG/NaOH).

For its description, 3 reaction steps are necessary (for MCC and CMG only 2 steps are enough).

Kinetic parameters were calculated by using non-linear regression which provides better results than isoconversional Staring method.

Puisque différents profils de carbonisation ont été observés par analyse thermogravimétrique Au délà de 600 °C, la carbonisation en présence de NaOH, conduit à une activation de la microporosité associée à la perte de cohésion du monolithe et à la formation de Na2CO3 détecté par DRX et par MEB sous forme d'aiguilles et de flocons.

Introduction

Porous carbons containing metals are important for applications in gas separation, gas storage, catalysis and catalytic supports and electrochemistry [1]. Preparation of metal-based nanoparticles have attracted attention due to their high activity in various fine chemical reactions, mechanical strength, high surface area, low melting point, unique characteristics such as surface resonance and optical properties, magnetic properties and large surface energies [2,3,4]. They can be synthesized and modified to allow them to bind with ligands, antibodies or drugs [5]. But, still these nanomaterials can undergo transformation since they are thermodynamically unstable, leading to poor corrosion resistance and coalescence. Metal nanoparticles should be prepared by a method which is easily reproducible, economical, use minimum of reagents, may control the particle shape and use a reaction temperature close to room temperature. When metal nanoparticles (NP) are used as a catalyst, they should be stabilized under the catalytic condition or dispersed on a porous support. Otherwise, they could be coagulated in solution and form aggregates, which are less effective as catalysts [6].

Carbon as a support for metal nanoparticles is challenging because of the complex surfaces and the ability to change the nature of its surface with different surface treatments [7]. Carbon materials have very good electrical conductivity, heat conductivity and mechanical properties, they exhibit high stability, low toxicity and are environmentally friendly. Carbons containing metallic nanoparticles can have high surface area, regular pore dimension, thermal and mechanical steadiness. The nanoparticles can also be very stable since they are embedded in the carbon matrix [9,10,11]. They can be prepared in different ways. Carbons with metal nanoparticles usually were prepared via in situ generation of the nanoparticles by solvothermal synthesis [11], or with noncovalent interactions [12], sol-gel processes [13], covalent grafting of functionalized metal nanoparticles on oxidized carbon or graphene by amidation [14].

The association of metal with cellulose gives materials with well-defined and controllable properties [16,17,18,19,20,21]. The key aspect of combining metal nanoparticles and cellulose (or its chemically modified derivates) [21] is the dispersion of the nanoparticles within the matrix without the formation of large aggregates that may compromise the final properties of the material.

The synthesis of cellulose-metals nanocomposites includes in-situ and ex-situ synthesis.

interaction with cellulose [22]. In-situ synthesis involves addition of metal oxides to the cellulosebased materials before formation of foams or aerogels. Material is proceeded with freeze-drying to fabricate aerogels [23]. External chemicals are not added to the reacting mixture. This methodology is a green approach to the synthesis of metal nanoparticles in cellulose matrixes without stabilizers or reducing agents. Another way of cellulose metal-based nanocomposite preparation used in situ reduction of metal salts in cellulose aqueous suspension. This involves the use of a soluble metal salt as a precursor, a reducing agent and a co-stabilizer. Very often as reducing agent it was used sodium borohydride, or tri-sodium citrate [25,26]. Macromolecular chains help to improve distribution of nanoparticles inside the cellulose matrix and to prevent formation of aggregates. The presence of hydroxyl groups in cellulose enables the interaction with the positive charge of the metals, since multivalent metal ion change the conformational structure of the cellulose chains [26]. Another in situ reduction for preparation of the nanocomposites on the cellulose surface is based on the photo-activation of cellulose surface, followed by chemical reduction of metal salts. The metal nanoparticles formed by UV irradiation coat the cellulose fibers and aggregates during UV irradiation leading to nanoparticles with variable morphologies.

Microwave irradiation can be also used as a method to prepare cellulose metal-based nanocomposites [27].

Ex-situ method involves the loading of metals into cellulose form with multi-step procedures. Using this synthesis, crystals of the metal are located on the surface of cellulose which leads to high accessibility to reactants [29,30,31].

Different cellulose metal-based nanoparticles were prepared from Fe, Cu, and Mn. Cellulose based magnetic materials were prepared by incorporating iron oxides into the cellulose matrix [32,33,34,35,36].

Copper nanoparticles are good candidates as a catalyst in hydrogen production [36].

Nanocoating of copper nanoparticles on cellulose were fabricated via electrostatic assembly with a chemical pre-treatment step in order to impart a surface charge on the substrate and a chemical reduction step to yield nanostructured coatings [36]. During this process, cellulose is partly converted into lactic acid and acetic acid [38].

Cellulose pretreated with alkaline solution has an important role in the phase, shape and thermal stability of manganese containing cellulose nanocomposites. Sometimes well-crystalline manganese oxides in cellulose matrix is not seen as the cellulose treated with NaOH aqueous solution can affect the synthesis and crystallinity of manganese oxides [39].

Metal-based carbons are prepared in various ways: 1) impregnation of porous carbon with metallic salt solution, 2) impregnation of solid precursor (e.g. cellulose) with metal salt solution or 3) addition of metallic salt to the precursor solution (as cellulose solution).

The fist method (impregnation of carbon) and the effect of impregnation on the porosity is scarce. The impregnation of dry carbon is done by its moistening with some metal salt solution.

The solution contains copper, ammonia and oxalic acid. Impregnation continues with drying and heating (to remove ammonia and oxalic acid content). Another synthesis with impregnation of porous carbon only with an ammoniacal solution and a metal salt can alos be performed with an immediate drying in a fluidized bed of air at 110 °C and then packing under vacuum. An increase in temperature produces a redox reaction between the metal salts as Cu (II) and carbon. The initial species of Cu (II) are reduced to Cu (I) and to metallic copper at a later stage when carbon becomes oxidized. At higher temperatures (325-405 °C) the CuO react with the carbon to yield Cu2O and CO2 [40]. Other way of sample impregnation is done with iron, by using 5:1 carbon to iron ratio, solution was magnetically stirred, filtered and placed in the oven (samples were heated in a furnace under N2 atmosphere at different temperatures) [41]. Carbons with manganese and iron were impregnated through the mixture of carbon and solutions of Mn(NO3)2 and FeCl3, baked at 150 °C. Dried material was rinsed several times with deionized water [42]. Another route of ironimpreganted carbon preparation used wet impregnation method of ferric chloride solution and heat treatment in nitrogen [44,45,46]. Other salts such as AgCl and CuCl2 were used for impregnation of carbons by magnetically stirring in HCl aqueous solution under fry helium flow. The carbon was in contact with solution for 24 h, washed and dried for further used [46]. Solutions of chloride salts of BaO, MgO, CuO were prepared in a beaker and some amount of carbon was added into the solution. After 24 h, the solution was filtered and dried in oven. Dried samples was heated up to 750 °C [47]. After pre-heating and drying, carbon was cooled and mixed with KMnO4 solution with stirring in a magnetic stirrer. Solution was diluted with millipore water, filtered, and again mixed with another salt as ZrOCl2 • 8H2O (suspension was washed with NaHCO3 solution) [48].

The second method was used for preparation of metal-based carbons from cellulose [49],

carboxymethylcellulose [50], microcrystalline cellulose [51], bacterial cellulose [52] and consequent carbonization. Impregnation of microcrystalline cellulose with aqueous solution of nitrate salt of copper, iron or other metals to produce metal-based carbons was used [53]. Porous carboxymethyl cellulose based carbons were prepared through impregnation method by calcination process with incorporation of manganese oxide. The synthesis followed the dipping of carboxymethyl cellulose in 10 wt% manganese (II) acetate tetrahydrate aqueous solution for 150 min at room temperature. The films were dried and calcined at 300 °C for 2 h [50]. Microcrystalline cellulose was impregnated with an aqueous FeCl3 solution. The resulting mixture was placed into a quartz crucible and calcined in pure N2. At 700-800 °C, metal-oxide nanoparticles were reduced by carbon to metallic iron nanoparticles [51]. Carbon based material containing copper oxide were prepared by calcining the dried material (bacterial cellulose was added into nitric acid solution and put into solution to absorb copper ion followed by drying in an air-drying oven) in a tubular furnace. The material was heated to 550 °C and calcined for 2 h in nitrogen. The resulting material was cooled to 250 °C and kept for 1 h in air to produce the material with metal oxides [52]. Carbons containing metal nanoparticles were prepared through the mixture of TEMPO-oxidized of cellulose nanofibrils with solution of divalent metals such as copper in a beaker by mechanical stirring and enhanced by carbon nanotubes. The mixture was kept overnight to complete the reaction and carbonized [54]. Iron nitrate and cellulose was mixed with the dispersed mixture of bambo into deionized water, and stirred for 3 h at room temperature. The suspension was freezedried to obtain iron nitrate/cellulose hybrid aerogel. The pyrolysis process included five steps. The temperature were increased from 240, 400 and 800 °C, after decreased to 400 °C and decreased to room temperature. All processes were carried out under a nitrogen atmosphere [49].

The third method is usually used in preparations based on polycondensation of an hydroxylated benzene (phenol or resorcinol) with an aldehyde (formaldehyde, furfural) in a solvent (using Na2CO3 as a catalyst). Metal-containing carbon samples were prepared by adding specific amount of the metal in resorcinol-formaldehyde aqueous solution without precipitation. Metal acetates and complexing agents were first mixed with deionised water. The pH was ranging from 3 to 4 without any precipitation. The pH was again increased by adding formaldehyde solution, methanol and diluted hydroxide solution. The solutions were dried at 85 °C. The obtained gels were then dried by vacuum evaporation without any pre-treatment. After drying the gels were pyrolyzed at 800 °C [55].

Since the addition of metallic salt to the biomass precursor solution (as cellulose) is not reported a lot, our work consists on this way of material preparation. Aim of the work in this chapter was to use a novel approach of cellulose based carbon monoliths preparation developed in chapter 2 for production of metals containing porous carbon materials. Monolithic gels were prepared directly from the mixture of dissolved microcrystalline cellulose and metal solutions, which can be carbonized to metal-based porous carbonaceous material. The procedure was simple and inexpensive, using NaOH, deionized water, ethanol as exchanging solvent, and metallic salts of copper acetate, manganese acetate and iron sulfate.

Materials and Methods

Material preparation

Reactants were used as received without any purification. The synthesis was derived from the synthesis of carbon monoliths from liquefied cellulose described in the previous chapter with an additional step of adding solution of metallic salts into dissolved cellulose solution. First, sodium hydroxide (7 g of NaOH, Sigma Aldrich) was dissolved in 78 g of deionized water (DI)

and the solution was pre-cooled down to -10 °C. Then, 9 g of microcrystalline cellulose (MCC, Sigma Aldrich) was added and the mixture was stirred for 40 minutes at room temperature (RT).

Meanwhile, metallic salts (0.91 g Cu(CH3COO)2•4H2O or 1.23 g of Mn(CH3COO)2•4H2O, or 1.39 g of FeSO4•7H2O)) were dissolved in 2 g of water which corresponds to 5 mmol of the metal (samples CMG/metal 5 mmol). Another set of samples were prepared with the 2.5 time less amount of metal salt in order to have 2 mmol of the metals (samples CMG/metal 2 mmol).

Then, the metallic solution was added to the cellulose one under stirring (no precipitation of the metal was observed, the color of solution changed by adding metal solution) and the mixture was poured into tubes which were placed at -10 °C for 12 h. After this time, the frozen solid cellulosic material was cut into pieces and soaked in ethanol for two days to complete the regeneration of the cellulose. Solid gel-like pieces were obtained. The solvent was then exchanged with DI up to pH ≈ 7 for 1 day in order to remove most of the NaOH present in the gel. Xerogels were dried at 70 °C overnight, and pyrolysed at different temperatures (500 and 800 °C) for 1 h (heating rate 10 K min -1 ) under an inert atmosphere (nitrogen, flow 200 ml min -1 ).

In the following text, metal-based cellulosic gels (MCMG) or metal-based carbons (MC) are referred as CMG/Me x mmol (for xerogels), and CMG/Me x mmol y °C (for carbonized samples); where Me = Cu, Mn or Fe, x = 2 mmol or 5 mmol of the metal, and y corresponds to carbonization temperature (500 or 800 °C). The morphology of the MCMG and MC was investigated by scanning electron microscopy (SEM) using a Quanta 400 microscope without metallization.

Characterization of metal-based cellulosic gel and metal-based carbon

The thermal stability of MCMG samples was examined by thermogravimetric analysis (TG) using the STA 449C apparatus (Netzsch) with a α-alumina crucible, in the temperature range 30-1000 °C under oxidizing atmosphere (air) at 10 °C min -1 heating rate.

Raman spectra were collected using the Horiba Raman spectrometer, Labram 300, with wavelength 532 nm, 60 seconds per spectrum with an objective of the microscope 50× with a power unit surface of 0.293 mW. The D/G ratio was calculated from a deconvoluted peak area using a Voight shape.

Transmission electron microscopy (TEM) images of the samples were recorded using a Jeol microscope (model ARM-200F) operating at 200 kV. The powder was crushed and dispersed in chloroform with ultrasound. The drops were deposited on the surface of a copper observation plate and dried under room temperature.

The porous structure of the samples was analyzed by a gas physisorption technique. The samples were degassed in a vacuum for 10 hours at a final temperature of 300 °C; heating was gradual with isothermal steps at 65 and 105 °C for 0.5 h dwell time and a 3 °C min -1 heating rate.

Adsorption of N2 (77K) (99.999 %) was carried out using the static manometric adsorption system (Autosorb iQ-XR, Quantachrome Instruments). The pore size distribution curves were obtained by fitting experimental isotherms by a hybrid carbon QSDFT adsorption kernel using ASiQwin software (Quantachrome Instruments). This kernel is assuming a slit-shaped micropores and cylindrical mesopores. It is limited to pores of diameter below 33 nm. Micropore volumes and surface areas, mesopore volumes, and mesopore surface areas were calculated by integration of the microporous and mesoporous parts of differential PSDs.

The electrodes for electrochemical testing were prepared by laying the slurry of tested material in the powder form (90 % by weight) with poly methyl methacrylate (10 % by weight, as binder) in acetone over the Toray Carbon paper (TGP-H-060 type with electrical resistivity of 80 mΩ-cm) as a current collector. The carbon paper was cut into several pieces with sizes of 2 cm × 1 cm and the area of sizes 1 cm × 1 cm were slurry coated. These electrodes were dried for 12 h at 60 °C in an oven. The electrochemical behavior of resulting electrodes was analyzed using a threeelectrode setup. The bottom half of the electrode, coated over 1 cm × 1 cm was dipped in the electrolyte. A 2 mol dm -3 KOH solution served as aqueous electrolyte, with Pt foil and Ag/AgCl as counter and reference electrodes, respectively. Versastat-3 (Princeton Applied Research, U.S.A.) was used to perform cyclic voltammetry (CV), and chronopotentiometry (CP).

The capacitance in three-electrode cell was calculated from CP and CV experiments using Equations 5 and 6 respectively.

𝐶 𝑠 = 𝐼𝑡 𝑑 𝑚(𝑉 -𝐼𝑅)

(5)

𝐶 𝑠 = ∫ 𝐼𝑑𝑉 𝑣 0 2 𝑚∆𝑉 𝑆 (6)
Here, Cs is specific capacitance of the electrode (F g -1 ), I is current in A, V is applied potential window (volts), IR is voltage drop, td is discharge time (s), 𝑚 is weight of active electrode material (g), and S is scan rate (V s -1 ).

Results and discussion

Characterization of metal-based cellulosic gel

Appearance of xerogels and morphology

Prepared MCMG display a good mechanical stability despite a significant shrinkage and a specific color of the metal ion (blue for Cu, brown for Mn and reddish brown for Fe) as shown in The SEM micrographs of the MCMG (Fig. 29) display the typical ice-templated cellulose morphology observed in the previous chapter with a significant lamellar macroporosity. No effect of the metal on macroporosity could be detected compared to the synthesis without metals [33].

XRD analysis of metal-based cellulosic gels

XRD analysis of the metal-based cellulosic gels are reported in Fig. 30 for the low and high metal loadings. In all cases, the presence of cellulose II (regenerated cellulose) was detected (peaks at 12.7 and 20.9° 2θ corresponding to (200) plane). For the samples containing lower metal loading (2 mmol), no significant signals of metallic crystalline phase were found (Fig. 30a). Looking at the Fig. 30b increases in the series CMG/Mn < CMG/Cu ≈ CMG/Fe < CMG and is clearly related to the total mass loss at 1000 °C (Fig. 31).

The char is slowly burnt out in the second step for CMG, CMG/Fe, and CMG/Mn. Contrary, TG of CMG/Cu after cellulose decomposition shows (after a small mass decrease in the interval 350-550 °C) formation of more stable type of carbon, which starts to burn only at around 700 °C.

According to literature, Cu can enhance the yield of levoglucosan and char formation [57].

The total mass loss at 1000 °C clearly indicates that CMG is fully burnt, but MCMG residues contain metal oxides compounds. It can be expected that at high temperatures oxides of Fe2O3/Fe3O4, Mn3O4, and CuO/Cu2O will be formed. Weight gain at the end of reaction is related with the formed metallic oxides.

Metal-based carbons

In the Fig. 32, metal-based carbons (MC) after pyrolysis at 800 °C are shown. The samples preserve the shape after carbonization despite the appearance of some cracks.

Physico-chemical analysis of metal-based carbon 4.3.6.1. XRD analysis

XRD analysis of metal-based carbons pyrolysed at 500 and 800 °C in inert atmosphere give a specific pattern for each of the metals (see Fig. 33). While for MCMG (2 mmol, a significant amount of the metal oxides was not noticed (Fig. 30), with the carbonization, the metallic phases are very well (Fig. 33a,d,e). Comparing with the XRD of MCMG (5 mmol) where MnO2 and Fe3O4 were detected, in the MC samples carbonized at 500 and 800 °C, MnO2 is reduced to MnO and Fe3O4 is reduced to metallic Fe and iron carbide (Fe3C). In all MC samples (with 5 mmol) traces, of Na2CO3 were noticed. Generally, MC samples with Fe have higher graphitization compared to MC with Mn and Cu. Carbonization of CMG at 800 °C leads to disordered material with some traces of impurities (see Fig. 33g).

Three different behaviors were observed according to the ability of the metal ion to be reduced: a) the metal stay in an oxidized form as metallic oxide (MnO) even at 800 °C, b) the metal is still in the oxide form at 500 °C and is reduced at 800 °C. Case of Fe, where

Fe is formed and further reacts with C to produce carbide Fe3C c) metals that are easily reduced even without heating such as copper.

Based on the Ellingham diagram [58], it was possible to estimate the changes in Gibbs energies of possible reactions at 500 and 800 °C to understand better reduction conditions of metal oxides with carbon. The equation for Gibbs free energy used is:

ΔG = ΔH -TΔS ( 7
)
Where ΔH is the change of enthalpy, T is absolute temperature, and ΔS is the change of entropy.

The oxygen partial pressure is taken as 1 atmosphere. Reactions are normalized to consume one mol of O2. Using the diagram, we determined the reduction of a metallic oxide to metal based on the carbonization temperature used.

In our materials, the reduction of manganese oxide and iron oxide can be expected (copper is present in metallic state even before carbonization. Reduction of metal oxides and they reaction with carbon is shown on equations below.

For manganese oxide, following reactions were considered:

2MnO → 2Mn + O2 ΔG = +600 kJ mol -1 (5) C + O2 → CO2 ΔG = -400 kJ mol -1 (6) 2MnO + C → 2Mn + CO2 ΔG = +200 kJ mol -1 (7)
Corresponding to general equation of MnO reaction with C and its ΔG (+200 kJ mol -1 O2), we assume that Mn stay in the oxide form at 800 °C.

For iron oxide, following reactions were considered:

2Fe3O4 → 6FeO + O2 ΔG = +350 kJ mol -1 (8) 6FeO → 6Fe + 3O2 ΔG = +1170 kJ mol -1 (9)
4C + 4O2 → 4CO2 ΔG = -1600 kJ mol -1 (10) 2Fe3O4 + 4C → 6Fe + 4CO2 ΔG = -80 kJ mol -1 (11) From general reaction of Fe3O4 with C and its ΔG (-80 kJ mol -1 O2), we assume that iron oxide is reduced to metallic iron at 800 °C.

Raman analysis

Raman spectroscopy was used to characterize carbonaceous materials. The studied carbonized samples are not well graphitized due to the low carbonization temperature and the nature of the precursor of cellulose which is known as a low graphitizable material (hard carbon)

[59] (Fig. 34). The deconvolution was done to better quantify the graphitization of each material with 5 mmol with temperature increase. They were fitted by two bands (D and G), a recommended procedure for not well graphitized materials [60]. The ID/IG ratio values are reported in the Table 11. Raman spectra are similar for samples containing copper and manganese, carbonized at 500 and 800 °C. For samples carbonized at 500 °C, D and G bands have not significant differences. A better graphitization is shown for MC with Fe (compared to MC with Cu and Mn) treated at 500 and 800 °C. At 500 °C, there is no real difference between the samples (ID/IG ratio value around 2), materials are highly disordered as evidenced by XRD. At 800 °C, the intensity of ID/IG ratio of samples is decreased from around 3.3 (for samples containing copper and manganese) down to 0.64 (for sample containing iron). With iron, the decrease of the ratio is related with the graphitization observed by XRD (catalytic effect) [61].

Morphological analysis of metal-based carbons 4.3.7.1. SEM and TEM analysis

SEM micrographs, previously reported (in Chapter 2), showed ice-template morphology, but if we look on the Fig. 35, ice-template morphology is less obvious (due to grinding of the sample and presence of the metals which maybe had an effect to remove the ice template). The surface morphologies show a smooth surface with some cracks (looking as leaves) on the surface and macroporosity is lost (see Fig. 35).

Surface properties of metal-based carbons

Figure 37 shows the nitrogen adsorption on the metal-based carbons (MC). All tested MC materials are nearly non-porous materials. Surface areas of MC containing iron and manganese were increased with increasing temperature pyrolysis from 500 to 800 °C (at the difference of the copper-based sample). With increasing pyrolysis temperature, the surface area of the Cu-based material is decreased from 23 to 11 m 2 g -1 . Metal-based samples surface properties were compared with CMG 800 (see Fig. 37d). Compared to MC with metals, CMG itself has high micro-and mesopore volume with surface area corresponding to 907 m 2 g -1 and of 131 m 2 g -1 (see Table 12). Vmeso (cm 3 g -1 )

S meso (m 2 g -1 )

CMG/Fe 5 mmol 500 0.000 0.04

Electrochemical performance of metal-based carbon nanocomposites for use in supercapacitors

The synthesized MC are preliminary investigated in three-electrode cell for possible use in an electrochemical capacitor. Figure 38 shows the CV plots obtained in the potential range from 0 to -0.8 V using aqueous 2 M KOH electrolyte. Generally, the double layer charge storage behavior in carbon electrode is specified by rectangular shape of the CV plot. However, carbon electrode from pure cellulose monolith (Figure 38) shows non-uniformity in shape because of the presence of oxygen functionalities arose in final form (Chapter 2). Apart from functionalities, the presence of impurities from Na content resulted in the carbonate form that deteriorate the double layer behaviors. The comparison of amount of metal doping in final electrodes (Figure 38a, b, c) shows higher CV area for higher doping of metals in liquefied cellulose, representing improved capacitance in these electrodes. This higher capacitance arises from reversible faradic redox reactions between Me 2+ to Me 3+ (e.g., Mn 2+ to Mn 3+ ) metal species present in these electrodes. This is well understood from the oxidation and reduction peaks in the CV plots (Figure 38), indicating higher performance compared to electrodes obtained with lower metal doping in liquified cellulose.

On the other hand, the comparison of three different metal-based carbon nanocomposite electrodes shows higher performance for Fe-based carbon electrode (Figure 38d). The higher mesoporosity arising in this electrode compared to Mn-and Cu-based electrodes (as confirmed from pore size distribution) provides better ion transport towards the active sites. Furthermore, the enhanced electron transport can be very well understood from the graphitization degree in Febased carbons. This observation is more evident from the TEM images and ID/IG ratios obtained from Raman spectroscopy. The capacitance calculated from CV plot for Fe-based electrode shows a promising value of 126 F g -1 at a scan rate of 10 mV s -1 .

a) b)

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 -

CMG 800 CMG/Cu 2 mmol 800 CMG/Cu 5 mmol 800 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

CMG 800 CMG/Fe 2 mmol 800 CMG/Fe 5 mmol 800

c) d)

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Potential (V)

CMG 800 CMG/Mn 2 mmol 800 CMG/Mn 5 mmol 800 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Potential (V)

CMG 800 CMG/Cu 5 mmol 800 CMG/Fe 5 mmol 800 CMG/Mn 5 mmol 800 All samples were carbonized at 800 °C.

The charge-discharge experiments (Figure 39) are carried out in the same potential window to confirm the results from CV experiment. The deviation from regular triangular shape shows pseudocapacitive behaviors of these electrodes from reversible redox reactions. The higher electrochemical performance for Fe-based MC electrode is evident from CP plot. The values of capacitance each at 1 A g -1 , as estimated from Equation 6 are found to be 150 F g -1 , 133 F g -1 , 83 F g -1 and 42 F g -1 for Fe-, Mn-, Cu-based MC, and cellulose based carbon electrodes respectively.

Thus, Fe-based MC electrode shows promise for use as electrode material in an electrochemical capacitor.

Conclusions

Xerogels of MCMG display good mechanical stability and macroporosity. At different metal concentration presence of cellulose II from regeneration of cellulose was verified. No significant metal species were noticed at 2 mmol, but with higher metal concentration formation of metallic Cu, iron oxide (Fe3O4) and MnO2 was shown. The mass loss during the first decomposition step increases in the series CMG/Mn < CMG/Cu ≈ CMG/Fe < CMG. In oxidized atmosphere, CMG is fully burnt but MCMG contains some metal oxides at the end of reaction.

In carbonized samples the presence of C disordered (for metal-based carbon with Mn and Cu) and graphite (for Mc with Fe) was verified. Sharper peak of graphitization was noticed for MC with Fe (2 mmol). During carbonization of MCMG samples Fe3O4 was reduced to Fe and Fe3C, and MnO2 was reduced to MnO. Cu remained in the metallic form in all stages of reaction.

At MC samples with 5 mmol metal concentration removal of Na2CO3 was not completed (the washing out was not so effective). Samples display well dispersed nanoparticles but with small size. No significant mesoporosity was shown.

Les xerogels de MCMG présentent une bonne stabilité mécanique et une macroporosité. A différentes concentrations de métal, la présence de cellulose II provenant de la régénération de la cellulose a été vérifiée. Aucune espèce métallique significative n'a été remarquée à 2 mmol, mais avec une concentration en métal plus élevée, la formation de Cu métallique, d'oxyde de fer (Fe3O4) et de MnO2 a été montrée. La perte de masse lors de la première étape de décomposition augmente dans la série CMG/Mn < CMG/Cu ≈ CMG/Fe < CMG. En atmosphère oxydée, le CMG est entièrement brûlé mais le MCMG contient des oxydes métalliques en fin de réaction.

CONCLUSION

Our results showed that a simple and ecological method of transforming cellulose into monolithic porous carbonaceous material is possible. A cooled solution of cellulose in aqueous NaOH can be easily shaped into a monolith at -10 °C in the chosen mold. Direct regeneration of cellulose II by freezing and then by solvent exchange in ethanol, washing of the excess NaOH with water and drying leads to microporous cellulosic monolithic xerogels composed mainly of cellulose II.

These materials have significant macroporosity linked to the transformation of ice. During subsequent carbonization, the macroporosity and monolithic character are preserved. The carbonaceous material obtained also exhibits significant microporosity. The micro-and microporous carbonaceous monoliths prepared can be potential candidates for applications requiring monolithic character, a high volume of micropores and good transport/diffusion properties ensured by the macropores and the possible mesopores.

Study of cellulose carbonization under influence of NaOH leads to the destroy of monolithic character and to the decrease of thermal stability and its carbon yield. Kinetic of pyrolysis for samples containing NaOH was complex, requiring 3 reactions steps for its description.

Furthermore, the synthetic route used can be easily adapted for the preparation of carbon monoliths with transition metal-based nanoparticles. Xerogel materials have good mechanical stability and macropores are preserved. Carbonized materials with Fe show higher mesopore volume and well graphitized material, resulting to high electrochemical performance compared with other metals used.

The suggested procedure of cellulose conversion to monolithic shape, can be adopted for biomass-based materials as wood too, preserving its properties.

CONCLUSION (FRENCH)

Nos résultats ont montré qu'une méthode simple et écologique de transformation de la cellulose en matériau carboné poreux monolithique est possible. Une solution refroidie de cellulose dans du NaOH aqueux peut être facilement mise en forme à l'état de monolithe par congélation à -10 °C dans le moule choisi. La régénération directe de cellulose II par congélation et puis par échange de solvant dans d'éthanol, le lavage de l'excès de NaOH avec de l'eau et le séchage conduisent à des xérogels monolithiques cellulosiques macroporeux composés principalement de cellulose II.

Ces matériaux sont dotés d'une macroporosité importante liée à la formation de glace. Au cours de la carbonisation ultérieure, la macroporosité et le caractère monolithique sont préservés.

Le matériau carboné obtenu présente également une microporosité significative. Les monolithes carbonés micro-et macroporeux préparés peuvent être des candidats potentiels pour des applications nécessitant un caractère monolithique, un volume élevé de micropores et de bonnes propriétés de transport/diffusion assurées par les macropores et les éventuelles mésopores. En outre, la voie de synthèse utilisée peut être facilement adaptée pour la préparation de monolithes de carbone avec des nanoparticules à base de métal de transition.

Introduction

Cellulose, the most abundant natural organic polymer [1], is the main structural component of plant cell walls, as well as many algae and oomycetes. It is the most abundant homopolysaccharide in nature (approximately 1.5 × 10 12 tons of annual production), a traditional renewable energy source [2][3][4], reserve of carbon and hydrogen, and a raw material for paper making and other industrial processes such as cosmetics, food [5], and medicine production [6,7]. Being a composed of many linear β-(1,4)-linked homopolymer chains of hundreds to thousands of anhydroglucopyranose residues, connected parallel by hydrogen bonds to form tough and inflexible structures, cellulose is a promising source of glucopyranose units and their derivatives as well as mixtures of various low-molecular-weight compounds such as phenolic compounds, furan derivatives, carbonyl compounds or carboxyl compounds, called bio-oil [8]. These small molecules can be further used as precursors for organic syntheses or preparation of various advanced carbonaceous materials such as carbon aerogels with a certain porosity [9]. For effective preparation of the latter materials, it is important to convert the precursors to solution/liquid state since this allows higher versatility in nanoporous carbon preparation. However, the conversion of cellulose macromolecules to this state is complicated since cellulose is insoluble in a majority of common solvents (water, organic solvents) under normal conditions and the β-glycoside bonds that connect the cellulose units can be hydrolyzed by cellulase enzymes [10] or acids [11].

Several procedures for converting solid-state cellulose into a liquid state (i.e., into a solution of cellulose derivatives, a solution of single glucopyranose chains, oligomers or monomers, or bio-oil (a liquid product)) have been developed. The first developed procedures were based on the preparation of soluble cellulose derivatives such as cellulose nitrate, acetate [12], formate [13], carbamate or xanthogenate, the latter being an intermediate in traditional viscose processes [14]. Another traditional treatment of cellulose is its dissolving in cuprammonium hydroxide solution that forms a copper-containing cellulose complex [15], which was later replaced by dissolution in N-methylmorpholine-N-oxide (NMMO) [16,17].

In recent decades, new approaches and procedures for the dissolution of cellulose or conversion of cellulose to a liquid mixture of low-molecular-weight compounds have been invented. Due to the practical insolubility of cellulose under normal conditions [18], procedures usually involve low [19] or high temperatures [20] or high pressure (supercritical conditions) [21], aggressive solvents (concentrated mineral acid) [22], solvent mixtures [23] or other special conditions (microwave heating [24], strong gravitation field, ultrasonic field [25]). All these parameters strongly influence the yield and composition of the obtained liquid mixture.

This review summarizes the procedures for the liquefaction/dissolution of cellulose, resulting in liquid mixtures with a composition appropriate for preparation of nanoporous carbon material with distinct porous properties (specific pore size distribution and specific surface area).

Processes Based on Cellulose Decomposition

Biomass or cellulose decomposition to produce a liquid mixture of organic compounds (bio-oil) is in the literature commonly called liquefaction. In general, it is based on the chemical degradation of cellulose chains to low-molecular-weight products. In addition to simple biomass pyrolysis, hydrothermal liquefaction (HTL) and some other techniques enabling the conversion of cellulose to chemicals as carbon precursors are used (Figure 1). trate, acetate [12], formate [13], carbamate or xanthogenate, the latter being an intermediate in traditional viscose processes [14]. Another traditional treatment of cellulose is its dissolving in cuprammonium hydroxide solution that forms a copper-containing cellulose complex [15], which was later replaced by dissolution in N-methylmorpholine-Noxide (NMMO) [16,17].

In recent decades, new approaches and procedures for the dissolution of cellulose or conversion of cellulose to a liquid mixture of low-molecular-weight compounds have been invented. Due to the practical insolubility of cellulose under normal conditions [18], procedures usually involve low [19] or high temperatures [20] or high pressure (supercritical conditions) [21], aggressive solvents (concentrated mineral acid) [22], solvent mixtures [23] or other special conditions (microwave heating [24], strong gravitation field, ultrasonic field [25]). All these parameters strongly influence the yield and composition of the obtained liquid mixture.

This review summarizes the procedures for the liquefaction/dissolution of cellulose, resulting in liquid mixtures with a composition appropriate for preparation of nanoporous carbon material with distinct porous properties (specific pore size distribution and specific surface area).

Processes Based on Cellulose Decomposition

Biomass or cellulose decomposition to produce a liquid mixture of organic compounds (bio-oil) is in the literature commonly called liquefaction. In general, it is based on the chemical degradation of cellulose chains to low-molecular-weight products. In addition to simple biomass pyrolysis, hydrothermal liquefaction (HTL) and some other techniques enabling the conversion of cellulose to chemicals as carbon precursors are used (Figure 1).

Biomass Pyrolysis

Biomass pyrolysis is a complex process determined by biomass feedstock and reaction parameters. Basic pyrolysis of biomass is carried out at atmospheric pressure. According to the heating rate during pyrolysis, slow, fast and flash pyrolysis are used [26,27]. Three processes are considered during biomass pyrolysis, including char formation, depolymerization, and fragmentation. The major products from biomass pyrolysis are char, bio-oil, and pyrolytic gases.

Char is a solid product with high carbon content; bio-oil is an organic mixture of different compounds such as alcohols, sugars, and other organic compounds containing

Biomass Pyrolysis

Biomass pyrolysis is a complex process determined by biomass feedstock and reaction parameters. Basic pyrolysis of biomass is carried out at atmospheric pressure. According to the heating rate during pyrolysis, slow, fast and flash pyrolysis are used [26,27]. Three processes are considered during biomass pyrolysis, including char formation, depolymerization, and fragmentation. The major products from biomass pyrolysis are char, bio-oil, and pyrolytic gases.

Char is a solid product with high carbon content; bio-oil is an organic mixture of different compounds such as alcohols, sugars, and other organic compounds containing oxygen; pyrolytic gases contain mostly carbon dioxide, carbon monoxide, hydrogen, and light hydrocarbons [28]. Char, bio-oil, and gases are usually in the range of 13-25, 50-70, and 12-15 wt%. The composition of pyrolysis products and their proportions are influenced by the type of biomass, physical and chemical treatments, pyrolysis temperature, carrier gas, and heating rate. The heating rate is considered as the main factor affecting the yield Polymers 2022, 14, 1621 3 of 15 of bio-oil. During slow pyrolysis (heating rate below 1 K/s) the bio-oil yield is decreased down to about 30 wt%, while increasing the heating rate (fast pyrolysis 10-200 K/s, flash pyrolysis with more than 1000 K/s) enables obtaining more than 70 wt% of bio-oil [27].

The composition of pyrolysis bio-oil is related to the decomposition of the main biomass components (Figure 2). In addition to the main constituents, bio-oil contains hundreds (thousands in some of the literature [29]) of compounds (mostly oxygenated) and undergoes significant changes in composition and physical properties during storage (evaporation of volatile components, continuous reactions in the mixture) [27].

oxygen; pyrolytic gases contain mostly carbon dioxide, carbon monoxide, hydrogen, and light hydrocarbons [28]. Char, bio-oil, and gases are usually in the range of 13-25, 50-70, and 12-15 wt%. The composition of pyrolysis products and their proportions are influenced by the type of biomass, physical and chemical treatments, pyrolysis temperature, carrier gas, and heating rate. The heating rate is considered as the main factor affecting the yield of bio-oil. During slow pyrolysis (heating rate below 1 K/s) the bio-oil yield is decreased down to about 30 wt%, while increasing the heating rate (fast pyrolysis 10-200 K/s, flash pyrolysis with more than 1000 K/s) enables obtaining more than 70 wt% of biooil [27].

The composition of pyrolysis bio-oil is related to the decomposition of the main biomass components (Figure 2). In addition to the main constituents, bio-oil contains hundreds (thousands in some of the literature [29]) of compounds (mostly oxygenated) and undergoes significant changes in composition and physical properties during storage (evaporation of volatile components, continuous reactions in the mixture) [27].

Hydrothermal Cellulose Liquefaction (HTL)

Hydrothermal liquefaction (HTL) is a thermochemical processing technology for the production of bio-oil from agricultural and forestry wastes. Bio-oil is a dark, viscous, and energy-dense liquid which has a 70-95% energy content of petroleum fuel oil [30]. The HTL process is based on the decomposition of biomass in hot pressurized water or various organic solvents (alcohols, polyalcohols such as ethyleneglycol [4], or acetone) [31] in the presence of homogeneous catalysts such as sulfuric acid [4], alkali hydroxides, ammonia or inorganic salts such as sodium or potassium carbonate [32], or heterogeneous catalysts such as transition metals or zeolites [33]. Some HTL processes take place under subcritical conditions, while the others are supercritical. As the HTL process does not require prior drying of biomass [34], it can be applied to both dry (lignocellulose) and wet biomass (algae) [35].

The reactions that occur with cellulose and hemicelluloses in the course of HTL have been thoroughly investigated. It has been found that the key reactions in HTL are hydrolysis, dehydration, isomerization, and retro-aldol condensation. Cellulose and hemicellulose (a mixture of highly branched low-molecular-weight homo and heteropolymers comprised of anhydro-β-(1-4)-D-xylopyranose, glucopyranose, mannopyranose and galactopyranose units [36]) are initially hydrolyzed into oligosaccharides and monosaccharides

Hydrothermal Cellulose Liquefaction (HTL)

Hydrothermal liquefaction (HTL) is a thermochemical processing technology for the production of bio-oil from agricultural and forestry wastes. Bio-oil is a dark, viscous, and energy-dense liquid which has a 70-95% energy content of petroleum fuel oil [30]. The HTL process is based on the decomposition of biomass in hot pressurized water or various organic solvents (alcohols, polyalcohols such as ethyleneglycol [4], or acetone) [31] in the presence of homogeneous catalysts such as sulfuric acid [4], alkali hydroxides, ammonia or inorganic salts such as sodium or potassium carbonate [32], or heterogeneous catalysts such as transition metals or zeolites [33]. Some HTL processes take place under subcritical conditions, while the others are supercritical. As the HTL process does not require prior drying of biomass [34], it can be applied to both dry (lignocellulose) and wet biomass (algae) [35].

The reactions that occur with cellulose and hemicelluloses in the course of HTL have been thoroughly investigated. It has been found that the key reactions in HTL are hydrolysis, dehydration, isomerization, and retro-aldol condensation. Cellulose and hemicellulose (a mixture of highly branched low-molecular-weight homo and heteropolymers comprised of anhydro-β-(1-4)-D-xylopyranose, glucopyranose, mannopyranose and galactopyranose units [36]) are initially hydrolyzed into oligosaccharides and monosaccharides that are further decomposed into small molecules such as furfural, 5-hydroxymethylfurfural, lactic acid, levulinic acid, formic acid or acetic acid [32,37] (Figure 3). Using ethylene glycol as a solvent for liquefaction, the reaction leads to decomposition of glucosides into glucose [4], organic acids, and polymerized alcohols [38].

that are further decomposed into small molecules such as furfural, 5-hydroxymethylfurfural, lactic acid, levulinic acid, formic acid or acetic acid [32,37] (Figure 3). Using ethylene glycol as a solvent for liquefaction, the reaction leads to decomposition of glucosides into glucose [4], organic acids, and polymerized alcohols [38].

The yield and composition of bio-oil depend on the biomass composition: the maximum bio-oil yield is obtained from biomass with a high lignin content [38].

Effect of Processing Parameters on Bio-Oil Yield of HTL

The HTL process is significantly affected by many processing conditions; its appropriate setup is crucial for increasing the yield of bio-oil and its composition.

Temperature is a sensitive operating parameter in the hydrothermal liquefaction process. Cellulose liquefaction occurs through the cleavage of C-C and C-O bonds under the action of solvent radicals produced at high temperature [39]. The temperature usually ranges between 250 and 400 °C and the pressure is set between 5 and 20 MPa. The procedure can take various times, from 10 min up to 24 h [31]. However, after reaching the maximum bio-oil yield at certain temperatures, further increases in temperature lead to decrease of bio-oil yield due to repolymerization of free radicals produced from liquefaction products and due to increased gas production [38]. The biomass type [38] and the solvent type also affect the optimal liquefaction temperature [31]. The increase in cellulose crystallinity results in a change of the optimal liquefaction temperature to approximately 380 °C [40]. Beyond a certain threshold, further increase in temperature or residential time leads to decreased bio-oil yield. This threshold is different for different biomass sources/composition [38].

In general, increasing residence time leads to the increase of bio-oil yields [41]. With the increase in reaction time from the beginning of reaction up to 30 min, the yield of solid residue yield decreased from 86.3% to 60.9%, while the yield of bio-oil increased from 8.7% to 28.5%, due to the deeper liquefaction of cellulose by prolonging the reaction time [42]. At 350 °C, the cellulose liquefaction is carried out with a residence time of 30 min or 1 h. With increasing residence time for more than 1 h, the yields of oil and aqueous phase yields decreased, and only the gas yield increased [43].

Cellulose conversion and its product distribution are significantly affected by the thermal effect [44]. However, the heating rate showed a marginal influence on the bio-oil yield in supercritical EtOH-based liquefaction [45]. The significant effect on biomass conversion is due to the heating rate above 280 °C using ethanol [46,47]. The yield and composition of bio-oil depend on the biomass composition: the maximum bio-oil yield is obtained from biomass with a high lignin content [38].

Effect of Processing Parameters on Bio-Oil Yield of HTL

The HTL process is significantly affected by many processing conditions; its appropriate setup is crucial for increasing the yield of bio-oil and its composition.

Temperature is a sensitive operating parameter in the hydrothermal liquefaction process. Cellulose liquefaction occurs through the cleavage of C-C and C-O bonds under the action of solvent radicals produced at high temperature [39]. The temperature usually ranges between 250 and 400 • C and the pressure is set between 5 and 20 MPa. The procedure can take various times, from 10 min up to 24 h [31]. However, after reaching the maximum bio-oil yield at certain temperatures, further increases in temperature lead to decrease of biooil yield due to repolymerization of free radicals produced from liquefaction products and due to increased gas production [38]. The biomass type [38] and the solvent type also affect the optimal liquefaction temperature [31]. The increase in cellulose crystallinity results in a change of the optimal liquefaction temperature to approximately 380 • C [40]. Beyond a certain threshold, further increase in temperature or residential time leads to decreased bio-oil yield. This threshold is different for different biomass sources/composition [38].

In general, increasing residence time leads to the increase of bio-oil yields [41]. With the increase in reaction time from the beginning of reaction up to 30 min, the yield of solid residue yield decreased from 86.3% to 60.9%, while the yield of bio-oil increased from 8.7% to 28.5%, due to the deeper liquefaction of cellulose by prolonging the reaction time [42]. At 350 • C, the cellulose liquefaction is carried out with a residence time of 30 min or 1 h. With increasing residence time for more than 1 h, the yields of oil and aqueous phase yields decreased, and only the gas yield increased [43].

Cellulose conversion and its product distribution are significantly affected by the thermal effect [44]. However, the heating rate showed a marginal influence on the bio-oil yield in supercritical EtOH-based liquefaction [45]. The significant effect on biomass conversion is due to the heating rate above 280 • C using ethanol [46,47].

The major difference between liquefaction and pyrolysis experiments lies in the effect of high pressure, which helps to condensate the reaction products and prevents their escape from the reaction vessel. Cellulose is highly oxygenated, and a reducing gas is frequently used to remove oxygen. It is preferable to eliminate oxygen autonomously either as water (which is undesirable since the end product would be carbon) or as carbon dioxide [48]. Higher temperatures will require higher pressure to maintain a liquid water phase [49]. Increasing the reaction pressure is beneficial for the hydrogenation reaction [50] and leads to slowing down of the dissociation reactions, although the effects of solvation may override it. Experiments have shown that when cellulose is heated to 350 • C under pressure, and in the presence of water, phenol and certain catalysts such as oxalic acid or sulfuric acid, almost total liquefaction can occur [51].

Biomass feedstock is one of the important parameters that affect bio-oil yield. The high cellulose content in biomass leads to high bio-oil yield [52].

Optimum size of biomass particles increases the yield of bio-oil at low grinding cost since reduction in particle size improves heat transfer and increases surface area [53]. Usually, particle diameters between 4 and 10 mm are suitable to overcome heat and mass transfer at reasonable grinding cost [52].

Solvents in HTL

A very important and key factor [54] for the efficiency of the liquefaction process and the properties of the final product is the selection of the appropriate solvent [55] (Table 1). The choice of solvent can have a strong impact on the distribution and yield [56]. The switch from low-boiling and nucleophilic solvents, such as water, to high-boiling and largely nonpolar solvents is suspected to affect the chemistry of the liquefaction process [57]. Low-molecular-weight cosolvents such as phenol, propanol, ethanol, methanol, and glycerol are applied and preferable in order to obtain low-molecular-weight product compounds. Polar protic solvents gave higher conversion rates and bio-oil/gas yields [58].

Methanol is an excellent solvent for cellulose liquefaction due to its high polarity, which allows penetration even into crystalline cellulose regions, promoting the solvolysis of polymer chains [31,59] with a maximum bio-oil yield varying from 32.21% [31] up to 100% [60], but the reaction temperature and pressure must be quite high due to its physical properties [31].

Glycerol was found to be a good solvent for cellulose liquefaction because its polar molecules can easily penetrate into the cellulose structure. Furthermore, glycerol liquefaction can be performed at high temperatures at atmospheric pressure [61].

Some solvents such as formic acid, light alcohols such as 2-propanol, and hydrocarbons such as tetralin are hydrogen donor solvents. These solvents effectively stabilize liquefaction products which helps to reduce product repolymerization [56]. Compared to non-hydrogen donor solvents, hydrogen donor solvents show significant improvement in cellulose conversion and in the bio-oil quality [62].

Ionic liquids are also efficient for cellulose liquefaction [63] due to their high thermal and chemical stability, high polarity, low melting points, and good cellulose solvating properties [64]. Cellulose liquefaction in ionic liquids is performed at 90-130 • C and ambient pressure, followed by addition of water and washing of the precipitate [65].

Other Decomposition Processes

Cellulose exposed to microwave irradiation (frequencies 0.3 to 300 GHz and wavelength from 1 mm to 1 m) begins to decompose due to local heating [79]. Microwave heating in cellulose liquefaction leads to a reduction in time and energy consumption [80] and allows effective dissolution and depolymerization under mild reaction conditions [61,81,82].

Microwave liquefaction of microcrystalline cellulose in water under acidic conditions produces mainly 5-hydroxymethylfurfural and levulinic acid. At 120 • C and 150 • C, almost 100% selectivity was observed. At 180 • C, less selectivity was observed and glucose appeared in the product mixture [45].

The sedimentation of particles in the liquid medium, which occurs even at normal earth gravity (1 G), can be accelerated by an ultracentrifuge machine. Strong acceleration of the gravitation field (more than 10 6 G) can induce a decomposition reaction that, in combination with hydrothermal decomposition, results in cellulose decomposition. Under a strong gravitation field, glucose and its decomposition products exist in the acidic media around the capsule [83].

Crystalline cellulose contains an ordered hydrogen bonded network within which a proton transport network is possible in the presence of an electromagnetic field [84]. Ultrasonic waves are found to intensify physical and chemical processes, such as extraction and hydrolysis, in treated materials, such as cellulose, resulting from the cavitation, mechanical, and thermal effects of the ultrasound [85].

Cellulose Dissolution

Cellulose dissolution is a process in which solvent molecules penetrate the cellulose structure and cause polymer swelling to certain extent. The physical properties and volume of the cellulose are significantly altered, while the chemical structure of the polymer remains practically unchanged [86]. Since cellulose is a polar molecule with a high degree of crystallinity [87], it has good hydrogen bonding ability [88].

The dissolution of cellulose without chemical modification is difficult due to the long rigid cellulose chains and strong intermolecular and intramolecular hydrogen bonds [89] and hydrophobic interactions [90]. The common solvents applicable for the dissolution of cellulose are summarized in Figure 4.

It is not possible to define an ideal cellulose solvent because it is highly dependent on the purpose for which the cellulose is dissolved. Even if we focus only on the preparation of porous carbons, the selection of suitable solvent is obviously affected by the expected further processing of the solution, the possible use of crosslinkers, templates, or other additives, required physical form of regenerated cellulose (powder, fiber, monolith) and its behavior during subsequent carbonization. The solubility of cellulose in water under mild conditions is promoted by the addition of certain inorganic salt hydrates with a water to salt molar ratio equal or less than the coordination number of the cation [START_REF] Wei | Depolymerization of cellulose into high-value chemicals by using synergy of zinc chloride hydrate and sulfate ion promoted titania catalyst[END_REF]: a mixture of NaSCN and KSCN with Ca(SCN)2•3 H2O or DMSO is a good cellulose solvent [START_REF] Fischer | Inorganic Molten Salts as Solvents for Cellulose[END_REF]. Cellulose dissolution was also observed in concentrated (63% w/w) aqueous solution of zinc chloride [95].

Various non-aqueous systems have been tested for cellulose dissolution, such as the combination of SO2 and NH3 with suitable ammonium salt [START_REF] Hudson | The Solubility of Unmodified Cellulose: A Critique of the Literature[END_REF]; mixtures of polar organic liquid such as formamide, N,N-dimethylformamide, DMSO; N,N-dimethylacetamide, SO2 or SOCl2 and aliphatic or alicyclic amines [START_REF] Isogai | Review: Catalytic oxidation of cellulose with nitroxyl radicals under aqueous conditions[END_REF]; mixtures of amine-containing component; polar organic liquid and inorganic salt such as NH3/NaCl/DMSO or ethylenediamine/NaI/DMF [115]; two-component mixtures such as NH3/NH4SCN [95]; thiocyanates with hydrazine, hydrazine hydrate or ethylenediamine [START_REF] Ting | Retardation in RAFT Polymerization: Does Cross-Termination Occur with Short Radicals Only?[END_REF]; DMSO with methylamine, KSCN, CaCl2, formaldehyde or substituted ammonium fluorides [115]. The last-mentioned type of mixture is capable of dissolving cellulose with a high degree of polymerization at room temperature without any pretreatment and in a couple of minutes [START_REF] Heinze | Effective preparation of cellulose derivatives in a new simple cellulose solvent[END_REF]. Another relevant two-component cellulose solvent is the ethylenediamine/KSCN mixture [START_REF] Frey | Dissolution of cellulose in ethylene diamine/salt solvent systems[END_REF]. For faster dissolution of cellulose under mild conditions, ionic liquids such as 1butyl-3-methylimidazolium acetate are used in dimethylsulfoxide, in N,N-dimethylformamide or in N,N-dimethylacetamide [START_REF] Gupta | Cellulose dissolution and regeneration in ionic liquids: A computational perspective[END_REF][START_REF] Mäki-Arvela | Dissolution of lignocellulosic materials and its constituents using ionic liquids-A review[END_REF][START_REF] Shibata | All-cellulose and all-wood composites by partial dissolution of cotton fabric and wood in ionic liquid[END_REF]. No additional electrolyte is required [START_REF] Casarano | A convenient solvent system for cellulose dissolution and derivatization: Mechanistic aspects of the acylation of the biopolymer in tetraallylammonium fluoride/dimethyl sulfoxide[END_REF]. Up to 10% by weight of cellulose can be dissolved in 1-butyl-3-methylimidazoilium chloride at 100 °C; solubility can be increased to 25 wt% by microwave heating.

When the mixture of N,N-dimethylacetamide with LiCl is used as a solvent, the cellulose solubility can be improved if the cellulose suspension in water is freeze-dried. This treatment has better results than hot drying [START_REF] Hu | Cellulose porosity improves its dissolution by facilitating solvent diffusion[END_REF]. This effect is associated with an increase in cellulose porosity after freeze-drying.

Cellulose can be regenerated from the solution in ionic liquid by adding an anti-solvent such as water, alcohol, or acetone [START_REF] Ha | Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis[END_REF], however, at lower concentrations (less than 15% by weight), water acts as a viscosity-reducing agent in the solution of cellulose in ionic liquid [START_REF] Iguchi | Dissolution and recovery of cellulose from 1-butyl-3-methylimidazolium chloride in presence of water[END_REF]. Miscibility of ionic liquid with water or organic solvents can be tuned by length of the cation side chain and choice of anion [START_REF] Cuissinat | Swelling and dissolution of cellulose. Part IV: Free floating cotton and wood fibres in ionic liquids[END_REF]. Traditional processes of cellulose dissolution are based on the conversion of cellulose to water-soluble cellulose derivatives. The classic cellulose dissolution is a viscose process (using carbon disulfide) that is capable of preparing 8-12 wt% cellulose xanthogenate [14]. Recently, a new Lyocell process [91], using N-methylmorpholine-N-oxide (NMMO), was implemented [92,93]. The dissolution of cellulose in NMMO can be improved by microwave heating. Cellulose solutions in NMMO can be used for the preparation of cellulose membranes with a high degree of crystallinity [94]. However, new processes that use nontoxic chemicals and less energy than conventional technologies and produce a solution of cellulose (and not the cellulose derivatives) have to be developed.

Non-derivatizing cellulose solvents are copper hydroxide in ammonia or ethylenediamine solution and its alternatives, cadmium hydroxide or nickel oxide in the same aqueous ethylenediamine [95,96]. Although the industrial application of these systems is limited because of the heavy metals and their general environmental and health risks, conversion of cellulose dissolved together with metal ions to porous carbon containing metal or metal oxide (nano)particles can be interesting for electrochemical or catalytic applications. A similar system for the dissolution of cellulose, ferric chloride, tartaric acid dihydrate, and NaOH in cold water has been developed [97].

The dissolution of cellulose in aqueous media can be understood as an acid-base process where cellulose can act as an acid or as a base [95].

In addition to cellulose liquefaction in a hot acid solution under high pressure, concentrated phosphoric acid (85 wt%) can be used for the dissolution of cellulose [98]; however, cellulose is derivatized to cellulose phosphonate. Low temperature (100-200 • C) hydrolysis in concentrated acid leads to a glucose solution with nearly 100% yield [31].

On the other hand, cellulose dissolves in a cold (below 268 K) 8-10 wt% aqueous NaOH solution [99]. Additives such as polyethylene glycol [START_REF] Yan | Dissolving of cellulose in PEG/NaOH aqueous solution[END_REF], urea [6,[START_REF] Cai | Rapid Dissolution of Cellulose in LiOH/Urea and NaOH/Urea Aqueous Solutions[END_REF], thiourea [START_REF] Zhang | Dissolution and regeneration of cellulose in NaOH/thiourea aqueous solution[END_REF], or a combination of urea and thiourea are frequently used [START_REF] Piltonen | Green and efficient method for preparing all-cellulose composites with NaOH/urea solvent[END_REF]. All these substances are inexpensive and practically non-toxic. The solubility of cellulose in aqueous NaOH/urea solution depends on temperature, molecular weight of cellulose, and crystallinity of cellulose [START_REF] Sarko | Packing Analysis of Carbohydrates and Polysaccharides. 7 . Crystal Structure of Cellulose IIII and Its Relationship to Other Cellulose Polymorphs[END_REF]. For its dissolution, a pre-cooled solution (-12.6 • C) of 7 wt% NaOH and 12 wt% urea was used. As a result of a fast self-assembly process between small molecules (water, urea, OH -) and cellulose macromolecules [START_REF] Yang | Role of sodium zincate on cellulose dissolution in NaOH/urea aqueous solution at low temperature[END_REF], cellulose dissolution occurs quickly (within a few minutes) at low temperatures (around or below 0 • C) [99]. The solubility increases with decreasing polymerization, but no effect of crystallinity was observed. Urea prevents the reaggregation of cellulose molecules by accumulation near hydrophobic regions [START_REF] Xiong | Dissolution of cellulose in aqueous NaOH/urea solution: role of urea[END_REF]. The cellulose solution in NaOH/urea can easily form a gel [START_REF] Egal | The dissolution of microcrystalline cellulose in sodium hydroxide-urea aqueous solutions[END_REF]. An 8 wt% solution of cellulose in aqueous NaOH can be converted to porous cellulose material by physical gelation, chemical crosslinking with epichlorohydrin, coagulation in water, and lyophilisation [START_REF] Ciolacu | Physically and chemically cross-linked cellulose cryogels: Structure, properties and application for controlled release[END_REF]. At low temperature, celluloses I, III, and IV are converted to cellulose II [6]. Cellulose I and cellulose II are the most common polymorphs in regenerated cellulose. The hydrogen bonding in cellulose II is more complex than in cellulose I, leading to different mechanical properties and hydrophilicity [START_REF] Mahmud | Preparation of different polymorphs of cellulose from different acid hydrolysis medium[END_REF]. Cellulose II can be prepared by alkaline treatment and regeneration. Cellulose III can be formed from cellulose I and II by treatment with ammonia. Cellulose IV is obtained by heating cellulose III [START_REF] Mittal | Effects of alkaline or liquid-ammonia treatment on crystalline cellulose: changes in crystalline structure and effects on enzymatic digestibility[END_REF].

The solubility of cellulose in water under mild conditions is promoted by the addition of certain inorganic salt hydrates with a water to salt molar ratio equal or less than the coordination number of the cation [START_REF] Wei | Depolymerization of cellulose into high-value chemicals by using synergy of zinc chloride hydrate and sulfate ion promoted titania catalyst[END_REF]: a mixture of NaSCN and KSCN with Ca(SCN) 2 •3 H 2 O or DMSO is a good cellulose solvent [START_REF] Fischer | Inorganic Molten Salts as Solvents for Cellulose[END_REF]. Cellulose dissolution was also observed in concentrated (63% w/w) aqueous solution of zinc chloride [95].

Various non-aqueous systems have been tested for cellulose dissolution, such as the combination of SO 2 and NH 3 with suitable ammonium salt [START_REF] Hudson | The Solubility of Unmodified Cellulose: A Critique of the Literature[END_REF]; mixtures of polar organic liquid such as formamide, N,N-dimethylformamide, DMSO; N,N-dimethylacetamide, SO 2 or SOCl 2 and aliphatic or alicyclic amines [START_REF] Isogai | Review: Catalytic oxidation of cellulose with nitroxyl radicals under aqueous conditions[END_REF]; mixtures of amine-containing component; polar organic liquid and inorganic salt such as NH 3 /NaCl/DMSO or ethylenediamine/NaI/DMF [115]; two-component mixtures such as NH 3 /NH 4 SCN [95]; thiocyanates with hydrazine, hydrazine hydrate or ethylenediamine [START_REF] Ting | Retardation in RAFT Polymerization: Does Cross-Termination Occur with Short Radicals Only?[END_REF]; DMSO with methylamine, KSCN, CaCl 2 , formaldehyde or substituted ammonium fluorides [115]. The last-mentioned type of mixture is capable of dissolving cellulose with a high degree of polymerization at room temperature without any pretreatment and in a couple of minutes [START_REF] Heinze | Effective preparation of cellulose derivatives in a new simple cellulose solvent[END_REF]. Another relevant two-component cellulose solvent is the ethylenediamine/KSCN mixture [START_REF] Frey | Dissolution of cellulose in ethylene diamine/salt solvent systems[END_REF]. For faster dissolution of cellulose under mild conditions, ionic liquids such as 1-butyl-3-methylimidazolium acetate are used in dimethylsulfoxide, in N,N-dimethylformamide or in N,N-dimethylacetamide [START_REF] Gupta | Cellulose dissolution and regeneration in ionic liquids: A computational perspective[END_REF][START_REF] Mäki-Arvela | Dissolution of lignocellulosic materials and its constituents using ionic liquids-A review[END_REF][START_REF] Shibata | All-cellulose and all-wood composites by partial dissolution of cotton fabric and wood in ionic liquid[END_REF]. No additional electrolyte is required [START_REF] Casarano | A convenient solvent system for cellulose dissolution and derivatization: Mechanistic aspects of the acylation of the biopolymer in tetraallylammonium fluoride/dimethyl sulfoxide[END_REF]. Up to 10% by weight of cellulose can be dissolved in 1-butyl-3-methylimidazoilium chloride at 100 • C; solubility can be increased to 25 wt% by microwave heating.

When the mixture of N,N-dimethylacetamide with LiCl is used as a solvent, the cellulose solubility can be improved if the cellulose suspension in water is freeze-dried. This treatment has better results than hot drying [START_REF] Hu | Cellulose porosity improves its dissolution by facilitating solvent diffusion[END_REF]. This effect is associated with an increase in cellulose porosity after freeze-drying.

Cellulose can be regenerated from the solution in ionic liquid by adding an antisolvent such as water, alcohol, or acetone [START_REF] Ha | Microwave-assisted pretreatment of cellulose in ionic liquid for accelerated enzymatic hydrolysis[END_REF], however, at lower concentrations (less than 15% by weight), water acts as a viscosity-reducing agent in the solution of cellulose in ionic liquid [START_REF] Iguchi | Dissolution and recovery of cellulose from 1-butyl-3-methylimidazolium chloride in presence of water[END_REF]. Miscibility of ionic liquid with water or organic solvents can be tuned by length of the cation side chain and choice of anion [START_REF] Cuissinat | Swelling and dissolution of cellulose. Part IV: Free floating cotton and wood fibres in ionic liquids[END_REF].

Porous Carbons Based on Liquefied/Dissolved Cellulose

The traditional way of production of porous carbons is direct carbonization of biosources with subsequent activation (activated carbons). Direct carbonization of biomass is still a very active topic intensively reported in the literature (e.g., [START_REF] Jiang | Recent progress on porous carbon and its derivatives from plants as advanced electrode materials for supercapacitors[END_REF]) because it represents a cheap and effective method of highly porous carbon production.

After the development of hydrothermal treatment of biomass for the production of biofuels, the same technique attracts attention as a tool for the porous carbons production of either biomass [START_REF] John | Biomass-based hydrothermal carbons for catalysis and environmental cleanup: a review[END_REF] or cellulose (e.g., [START_REF] Lin | Hydrothermal carbonization of cellulose in aqueous phase of bio-oil: The significant impacts on properties of hydrochar[END_REF]).

The above-mentioned approaches are based on solid-solid conversion, which is in principle problematic for controlling the porosity of produced carbons and does not enable involvement of recent ways of porosity formation and control like templating.

To overcome this disadvantage, phenol-based wood liquefaction under acidic catalysis was developed [START_REF] Wang | Activated carbon materials derived from liquefied bark-phenol formaldehyde resins for high performance supercapacitors[END_REF][START_REF] Ozbay | Carbon foam production from bio-based polyols of liquefied spruce tree sawdust: Effects of biomass/solvent mass ratio and pyrolytic oi laddition[END_REF], leading to a phenolic solution suitable for formaldehyde condensation with formaldehyde (in the presence of a suitable template) to gel-like solids carbonizable to porous carbons. Because of the large excess of phenol used in this approach, produced carbons are mostly based on phenol-formaldehyde resin, and biomass-based phenolic compounds are only partly involved.

Polymers 2022, 14, 1621 9 of 15 Surprisingly, there is a lack of information on the direct use of bio-oil for production of carbons with controlled porosity. Zhu et al. [START_REF] Zhu | Hierarchical porous carbon materials produced from heavy bio-oil for high-performance supercapacitor electrodes[END_REF] simply carbonized heavy bio-oil and after activation with NaOH they received highly porous carbon with surface area (derived from BET) greater than 2000 m 2 /g and total pore volume 1.7 cm 3 /g. Similar materials were obtained with the use of crayfish shell as a biological template during carbonization of heavy bio-oil followed by washing with acid and activation [START_REF] Luo | Synthesis of 3D-interconnected hierarchical porous carbon from heavy fraction of bio-oil using crayfish shell as the biological template for high-performance supercapacitors[END_REF]. Carbon materials with developed micro and mesoporosity were also prepared by carbonization of bio-oil in the presence of MgO [START_REF] Li | Template-synthesized hierarchical porous carbons from bio-oil with high performance for supercapacitor electrodes[END_REF] or CaO (introduced as acetate) [START_REF] Li | Preparation of hydrogen storage carbon materials using bio-oil heavy components as carbon-containing precursor[END_REF] as hard templates washable with acidic media. Another approach was described by Wang et al. [START_REF] Wang | Honeycomb-like carbon with tunable pore size from bio-oil for supercapacitor[END_REF] which used bio-oil hydrothermal treatment in steel autoclave (170 • C, 10 h) in the presence of cetyltrimethylamoniumbromide and nickel nitrate, followed by carbonization and activation. They reported a positive effect of increasing the amount of CTAB on surface characteristics improvement (surface area up to 1300 m 2 /g, pore volume 0.84 cm 3 /g, mesopores with diameter up to 30 nm).

Cellulose is one of the most studied bioprecursors for production of porous carbons. In addition to cellulose itself, its derivatives such as acetyl- [START_REF] Xu | Acetylated cellulose nanocrystals with high-crystallinity obtained by one-step reaction from the traditional acetylation of cellulose[END_REF], carboxymethyl- [START_REF] Klunklin | Synthesis, Characterization, and Application of Carboxymethyl Cellulose from Asparagus Stalk End[END_REF]139], ethyl- [START_REF] Koch | Properties and Uses of Ethylcellulose[END_REF], hydroxyethyl- [START_REF] Peng | Hydroxyethyl Cellulose-Based Hydrogels with Various Pore Sizes Prepared by Freeze-Drying[END_REF], and hydroxypropylcellulose [START_REF] Khanmirzaei | Hydroxypropyl Cellulose Based Non-Volatile Gel Polymer Electrolytes for Dye-Sensitized Solar Cell Applications using 1-methyl-3propylimidazolium iodide ionic liquid[END_REF] were used for carbon preparation.

The basic approach to conversion of cellulose into carbons with controllable porosity is usually of cellulose (or its derivative) followed with solidification (gelation, regeneration, freezing) in the presence of an appropriate template (not always) and final carbonization (and possible activation). Micropores in the porous carbons of cellulose are generated during the carbonization of the gels, and are increased by activation as with NaOH or KOH [START_REF] Klepel | Porous Carbon Monoliths Made from Cellulose and Starch[END_REF]. Mesopores usually originate from hard or soft templates, but they can also be affected by activation step [START_REF] Blankenship | Modulating the porosity of carbons for improved adsorption of hydrogen, carbon dioxide, and methane: a review[END_REF].

The porous carbons of carboxymethylcellulose were prepared by ice-templating and carbonization. The carboxymethylcellulose aqueous solution was simply frozen in a liquid nitrogen bath, dried in a freeze-dryer for some days, and carbonized [START_REF] Lee | Eco-friendly fabrication of porous carbon monoliths from water-soluble carboxymethyl cellulose for supercapacitor applications[END_REF]. With the dissolution of cellulose in the calcium thiocyanate/water/ethanol system and its carbonization, porous carbons were obtained [START_REF] Jeong | Cellulose Degradation by Calcium Thiocyanate[END_REF]. Highly porous materials were also obtained by dissolution of cellulose in ionic liquid or in lithium chloride/N,N-dimethylacetamide and its precipitation in a coagulation bath of water [START_REF] Zhang | Dissolution Mechanism of Cellulose in N,N -Dimethylacetamide/Lithium Chloride: Revisiting through Molecular Interactions[END_REF].

Another approach of cellulose gel preparation is the freeze-thaw process. In general, cellulose monoliths can be prepared by the freeze-thaw process of alkaline cellulose solution with NaOH, KOH, or in mixture with urea/thiourea [START_REF] Qin | Gelation behavior of cellulose in NaOH/urea aqueous system via cross-linking[END_REF]. The freezing and heating of dissolved cellulose solution at room temperature leads to the formation of cellulosic gels [START_REF] Korhonen | Gelation of cellulose-NaOH solutions in the presence of cellulose fibers[END_REF]. Cellulose gels have also been prepared by freezing solutions of cellulose Ca(SCN) 2 /water solvent [START_REF] Xin | A hierarchically porous cellulose monolith: A template-free fabricated, morphology-tunable, and easily functionalizable platform[END_REF].

Cellulose porous monoliths have also been prepared by controlled freezing of aqueous cellulose solution in a liquid nitrogen bath. Monolithic carbon was formed after carbonization [START_REF] Parajuli | Cellulose-based monoliths with enhanced surface area and porosity[END_REF]. Pluronic F-127 has been used as a porogen agent during the hydrothermal treatment of cellulose suspension in water and the resulting char was consequently carbonized to porous monolithic carbon [START_REF] Ferry | Fused sphere carbon monoliths with honeycomb-like porosity from cellulose nanofibers for oil and water separation[END_REF].

Conclusions

Cellulose represents a valuable source of carbon for the production of porous carbonaceous materials [START_REF] Hina | Preparation and performance comparison of cellulose-based activated carbon fibres[END_REF]. Obviously, the simplest way to convert cellulose to carbon is direct carbonization, but if we want to control the properties of the products (especially porosity), more advanced approaches to carbon synthesis are required, in general based on liquid carbon precursors [START_REF] Mankar | Pretreatment of lignocellulosic biomass: A review on recent advances[END_REF].

The conversion of cellulose into the liquid state can be achieved in two ways: decomposition to liquid mixture of low-molecular-weight organics (hydrothermal liquefaction or other decomposition techniques) or dissolution in suitable solvent (cellulose or its derivatives).

In the first case, the obtained mixtures contain a large number of chemical individuals with different properties and have a variable composition that is significantly dependent on the liquefaction condition. To find a common method of controlled conversion of such a complicated system into carbon with specific porosity would be very difficult.

However, if dissolution of cellulose (or its derivative) is successful, the obtained solution is homogeneous with more or less constant properties of dissolved cellulose given by the used solvent. Such a system is much easier to a transform to defined macrostructure (e.g., by cross-linking, gelation, and templating), consequently leading to carbons with defined porous structure.

Although many pathways of cellulose liquefaction/dissolution have been developed, this area still presents a field for intensive research, especially in connection with increasing interest in carbon-based porous materials for various advanced applications.

The challenging research topics in the near future could be focused on (among other areas):

-Transformation of industrially available bio-oils towards their chemical stability and uniformity, enabling their conversion to nanostructured organic and consequently carbon matter. -Avoiding the loss of carbon, increasing the carbon yield during biomass treatment, e.g., by suitable cellulose derivatization, more extensive involvement of lignin constituent of biomass, etc. -Production of biomass-or cellulose-based carbon monoliths with hierarchical porosity is still a challenge even for carbons produced from petroleum-based chemicals.

Introduction

Porous carbon materials have a large surface area, a large pore volume, and tunable pore sizes [START_REF] Sun | Hierarchically porous materials: synthesis strategies and emerging applications[END_REF]. They are commonly used for environmental protection, liquid/gas depollution, gas separation, as catalyst supports, and in energy-related applications (supercapacitors). These materials show many advantages, such as low cost, widely available biosourced precursors, high chemical stability (necessary for long-term use) with limited toxicity [START_REF] Ma | Direct synthesis of ordered mesoporous carbons[END_REF], relatively good electronic conductivity, and practically unlimited possibility of surface chemical modification [START_REF] Liu | Facile synthesis of ordered mesoporous carbon monolith with close-packed microspheres structure through emulsion templating[END_REF][START_REF] Bandosz | Nanoporous carbons: looking beyond their perception as adsorbents, catalyst supports and supercapacitors[END_REF]. Among porous carbons, activated carbons are produced on an industrial scale from precursors based on petroleum or biomass, usually through carbonization and chemical or physical activation [START_REF] Thomas | Production of activated carbon from natural sources[END_REF]. The industrial production of biomass-based activated carbons usually occurs through an energy-consuming step. In addition, it is difficult to shape these materials at a macroscopic scale (e.g. monolithic shape as a pellet) without the aid of additives (binders), which usually decrease their porous capacities and require an additional time and cost in their processing. On the other hand, monolithic materials are more suitable, easier to use, and less toxic than powdered activated carbons. Thus, shaping porous carbons as monoliths and as fibers or pellets, directly from their precursor state, appears to be an interesting challenge with regard to synthesis and time/energy cost. The use of carbon precursors in the form of gel or foam is an established process for controlling both the shape and extent of the porosity of monolithic porous carbons. These precursor materials are usually prepared from petroleum-based compounds (eg, phenolic resin) or bio-derived precursors [START_REF] Bulavová | Facile synthesis of softtemplated carbon monoliths with hierarchical porosity for fast adsorption from liquid media[END_REF]. Among them, phenolic-based foams are interesting materials since they lead to carbon foams with good thermal insulation properties, high dimensional stability, low flammability, and low smoke toxicity. Their preparation requires costly modifiers (Yu et al., 2018;[START_REF] Mougel | Phenolic foams: a review of mechanical properties, fire resistance and new trends in phenol substitution[END_REF] and the use of porogen agents (eg, for the generation of gas bubbles within the solution during the polymerization of the precursor through heating of the solution). Even if this method is simple, the size and shape of the formed pores are limited, since the morphology of the gas bubbles cannot be controlled [9] and may require an additional activation process [START_REF] Tondi | Chemical activation of tannin-furanic carbon foams[END_REF].

To extend porosity control (over size and shape) of carbon materials, chemical methods (other than activation) have been developed, such as sol-gel or hydrothermal syntheses, hard-templating, and more recently soft-templating (Xie et , 2014). For all of these processes, the organic precursors play a key role in the synthesis and therefore in the control of the properties of the final carbon materials. Among precursors, biosourced precursors have attracted increasing attention in the research community because they are a promising alternative, compared to petroleum-based precursors, for the preparation of monolithic porous materials. A lot of biomass-based compounds such as tannin, lignin, wood, coconut, rice hubs, sugarcane bagasse or bamboo chips were used as precursors and subsequently converted by heat treatment to monolithic porous carbons.

Plant and natural fibers based on cellulose and other biopolymers have been increasingly investigated in simple and cost-effective methods, yielding foams, cryogels, and xerogels. All these biopolymerbased materials exhibit biocompatibility, good mechanical properties, and highly developed porous structures with a large surface area [START_REF] Li | Physical properties of tannin/furanic resin foamed with different blowing agents[END_REF][START_REF] Tondi | Tannin-based rigid foams: a survey of chemical and physical properties[END_REF][START_REF] Celzard | Latest progresses in the preparation of tannin-based cellular solids[END_REF]. These materials find potential applications in fields such as biofuels and biomedical applications, such as nanobiocomposites [START_REF] Adsul | Polymers from biomass: characterization, modification, degradation, and applications[END_REF], due to their wide range of properties, which can be controlled by the versatility of their preparation processes.

One of the most studied precursors for biomass-based monolithic materials production is cellulose (or its more soluble derivatives prepared by acetylation, carboxymethylation, TEMPO-mediated oxidation, phosphorylation, or enzymatic pretreatment).

From cellulose acetate, monoliths were prepared by thermally induced phase separation from the solution of cellulose acetate in dimethylformamide with the addition of 1-hexanol (heated to 70 • C, after the first heating left 24 h at 20 et al., 2020). Adding NaCl to the CMC solution before freezing lead after carbonization to monolithic carbons with a high specific surface area of 724 m 2 g -1 (Lee et al., 2020). When associated with classical template-based synthesis (the use of polystyrene latex and Pluronic F-127) and hydrothermal processes, a hierarchically porous (TEM-PO-oxidized) cellulose monolith could be obtained, resulting in a hierarchically porous carbon monolith with a high surface area (1563 m 2 g -1 ) (Ferry et al., 2021).

The transformation of pure cellulose into monolithic form could be achieved through its dissolution in a suitable ionic liquid and consequent regeneration of cellulose (in the form of the a gel) by exchange of ionic liquid with appropriate non-solvent (e.g. ethanol) [START_REF] Lin | Analysis of porous structures of cellulose aerogel monoliths and microparticles[END_REF]Du, 2017).

A commonly used approach to the preparation of cellulose monoliths, which does not require preliminary chemical cellulose modification or chemical crosslinking agents, is the "freeze-thaw" process of alkaline cellulose solution. During the freezing steps, the solvent partially crystallizes, concentrating the cellulose chains in regions between the solvent crystals. Consecutive thawing promotes chain selfassembling and stable assemblies are achieved mainly by H-bonding interactions [START_REF] Souza | Polysaccharide-based materials created by physical processes: from preparation to biomedical applications[END_REF]. The advantage is that pristine cellulose can be used directly and its dissolution can be easily achieved with a concentrated aqueous NaOH or NaOH/urea cold solution (-5 • C) [START_REF] Hagman | On the dissolution state of cellulose in cold alkali solutions[END_REF]. The freezing and warming of the solution to room (or higher) temperature leads after the appropriate time (usually days) directly to the cellulose gel (in some cases, consecutive solvent exchange is required) (Hu et al., 2016;[START_REF] Zu | Nanocellulose-derived highly porous carbon aerogels for supercapacitors[END_REF][START_REF] Zhuo | Sustainable hierarchical porous carbon aerogel from cellulose for high-performance supercapacitor and CO 2 capture[END_REF]Korhonen and Budtova, 2019).

The "freeze-thaw" process is closely related to the ice-templating process [START_REF] Bouville | Templated Grain Growth in Macroporous Materials[END_REF], where ice crystals formed during a freezing step lead to a macroscopic pore size with a particular lamellar shape in cellulosic materials [START_REF] Erlandsson | On the mechanism behind freezing-induced chemical crosslinking in ice-templated cellulose nanofibril aerogels[END_REF][START_REF] Ferreira | Porous nanocellulose gels and foams: breakthrough status in the development of scaffolds for tissue engineering[END_REF]. To preserve the macroporous network of the gel as much as possible, supercritical or freeze-drying is usually used in this case (Hu et al., 2016;[START_REF] Zu | Nanocellulose-derived highly porous carbon aerogels for supercapacitors[END_REF][START_REF] Zhuo | Sustainable hierarchical porous carbon aerogel from cellulose for high-performance supercapacitor and CO 2 capture[END_REF][START_REF] Ferreira | Porous nanocellulose gels and foams: breakthrough status in the development of scaffolds for tissue engineering[END_REF].

Micropores in the porous carbon derived from cellulose are generated during the release of volatiles during the carbonization of the cellulose gel. The volume and surface of the micropores can be further increased by activation by either a chemical or a physical process. For example, monolithic carbon materials were prepared by drying and carbonizing a microcrystalline cellulose and starch mixture, followed by activation with KOH. Carbon materials showed very high surface characteristics with 415-1122 m 2 g -1 depending on the cellulose/starch ratio and activation parameters (Klepel and Danneberg, 2020).

Mesopores are more difficult to monitor for carbons derived from cellulose gels. They usually depend on the synthesis conditions but can also be generated during the micropore activation step or with the help of a hard template used as a porogen agent [START_REF] Hong | Porogen templating processes: an overview[END_REF][START_REF] Cui | Synthesis and characterization of mesoporous MgO by template-free hydrothermal method[END_REF].

Most of the methods mentioned above for the production of cellulose monolithic materials (and consequently carbons) use tedious procedures with high cost and energy consumption (e.g. supercritical/freezedrying, long synthesis time), require environmentally nonfriendly chemicals (ionic liquids or dimethylformamide) or start with chemically modified cellulose. They are not very suitable for industrial applications [START_REF] Taubert | Attempts to design porous carbon monoliths using porous concrete as a template[END_REF].

Thus, the aim of this communication is to present a novel approach to making monolithic gels directly from microcrystalline cellulose, which can be carbonized to monolithic porous carbonaceous material. The procedure is simple, inexpensive, and eco-friendly using only water, NaOH, and ethanol as an exchange solvent. We focus on avoiding tedious and time-consuming procedures such as long heating (for gelation), freeze-and supercritical drying of the gels, and any activation step. The full characterization of the prepared cellulosic gel and carbonaceous products is provided. Surprisingly, such an easy and ecofriendly route has not been investigated as far as the authors know.

Experimental part

Materials preparation

Compounds were used as received without any purification. The synthesis was based on a simple mixture of sodium hydroxide (Sigma Aldrich), microcrystalline cellulose (MCC, Sigma Aldrich), and deionized water (DI). First, NaOH (7 g) was dissolved in 81 g of DI and the solution was pre-cooled down to -10 • C. Then, 9 g of MCC was added and the mixture was stirred for 40 min. After cellulose dissolution, the mixture was poured into tubes which were placed at -10 • C for 12 h. After this time, the frozen solid cellulosic material was cut directly into pieces and soaked in ethanol for two days for regeneration of the cellulose, the gel-like solid pieces were obtained. The solvent was then exchanged with DI up to pH ≈ 7 for 1 day (during the monolith processing the water washed the NaOH). After the solvent exchange, color of monoliths changes from yellow to the white one. Resulting material was dried at 80 • C before further use. The pieces with size up to 1 cm were obtained. Pyrolysis was carried out at different temperatures (500, 700 and 900 • C) for 1 h (heating rate 10 K min -1 ) under an inert atmosphere (nitrogen, flow 200 ml min -1 ). To ensure the removal of sodium compounds, the obtained carbonaceous monoliths were treated with 1 mol dm -3 HNO 3 solution (6 h) and then with ≈ 15% ammonia solution (6 h) followed by a drying step at 250 • C overnight.

In the following text, cellulose samples are referred to as MCC for microcrystalline cellulose and CMG for prepared cellulose monolithic gel. CMG-uw represents a monolithic cellulose gel that was not washed with DI after regeneration in ethanol. Carbonaceous samples derived for cellulose pyrolysed at a temperature x (x = 500, 700 and 900 • C) were labeled by adding x to the name of the precursor, e.g. MCC-x and CMG-x.

The carbonaceous monoliths obtained from CMG have a well-defined cylindrical shape maintained in the carbonization step despite the significant shrinkage after drying at 80 • C and pyrolysis. Pyrolysis of CMGuw showed us clearly that the presence of NaOH is detrimental to the preservation of the monolithic shape. Therefore, the washing step with water after regeneration in ethanol is a key step to preserve the carbonaceous monolith.

A. Kryeziu

Characterization of cellulosic gels and carbonaceous monoliths

XRD analysis of the cellulose samples was performed with a Bruker powder diffractometer D8 ADVANCE A25 in the Bragg-Brentano reflection geometry theta-2theta. The diffractometer is equipped with the LynxEye XE-T high-resolution energy dispersive 1-D detector (Cu Kα1, 2), which leads to ultrafast X-ray diffraction measurements. The conditions for data collection are the following: angular range 10-90º 2θ, step size 0.014º 2θ, time per step 0.7 s, variable divergence slits, irradiated sample length 15 mm, total time for acquisition 1 h 11 min. During data collection, the powder sample rotates at 15 rpm. The precursor MCC was characterized by an XRD pattern (Fig. 1a). It displays the cellulose I polymorph with typical diffraction peaks at 2θ = 14.8, 16.3, 22.6 • and 35 • for the diffraction planes (1-10), ( 110), ( 200), (004), respectively [START_REF] Gong | Research on cellulose nanocrystals produced from cellulose sources with various polymorphs[END_REF]. Since this crystalline phase is often associated with an amorphous cellulosic phase, the contribution of the latter was determined on the basis of the Segal method [START_REF] Ioelovich | Determination of cellulose crystallinity by x-ray diffraction method[END_REF]. The crystallinity index (CI) of MCC was calculated by comparing the minimum intensity above baseline at 2θ = 18º (I am ) and the maximum intensity of the plane (200) at 2θ = 22.5º (I 200 ). CI for CMG was calculated from 2θ = 21.9º for cellulose II and 2θ = 12.1º representing the amorphous contribution [START_REF] Nam | Segal crystallinity index revisited by the simulation of x-ray diffraction patterns of cotton cellulose Iβ and cellulose II[END_REF]. 13 C (I = 1/2) Magic angle spinning (MAS) spectra were recorded at room temperature with a 4 mm Bruker double channel probe with a spinning frequency of 12 kHz, on a Bruker Avance NEO 400 spectrometer operating at B 0 = 9.4 T (Larmor frequency ν 0 ( 13 C) = 100.63 MHz and ν 0 ( 1 H) = 400.17 MHz). 13 C single pulse MAS NMR experiments were performed with a π/4 pulse duration of 2.65 μs and a 60 s recycling delay and a 1 H decoupling radiofrequency field strength of 69 kHz. These recording conditions ensure the quantitative determination of the proportions of the different carbon species. Typically, 2500 scans were recorded. Chemical shifts reported later are related to tetramethylsilane (TMS). The decomposition of the spectra is obtained using the Dmfit program [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra: modelling 1D and 2D solid-state NMR spectra[END_REF]. On the basis of the NMR spectrum, the crystallinity index for cellulose I was calculated by expressing the signal area at 89 ppm as a portion of the total area assigned to C-4 [START_REF] Ahvenainen | Comparison of sample crystallinity determination methods by x-ray diffraction for challenging cellulose i materials[END_REF]. Crystalline regions of cellulose were derived by line fitting (composites of Gaussian and Lorentzian line shapes) of crystalline and amorphous C4 carbon signals from 85 to 92 ppm. For cellulose II, the crystallinity index was calculated by C4 deconvolution at 87 ppm (crystalline) and 85 ppm (amorphous part) [START_REF] Foston | Cellulose isolation methodology for NMR analysis of cellulose ultrastructure[END_REF]Isogai, 1997;[START_REF] Trilokesh | Isolation and characterization of cellulose nanocrystals from jackfruit peel[END_REF].

Infrared (FT-IR) spectra of the cellulose samples (non-carbonaceous) were measured with a NICOLET 6700 FTIR instrument. Thermo-Scientific. The experimental conditions for the spectra recording were: 50-100 mg of the sample, 256 scans, resolution 4 cm -1 , 64 scans for background, using DTGS KBr as detector, KBr as beam splitter, and diamond as an accessory smart performer. The spectrum is measured in the range of 4000 -400 cm -1 . For qualitative measurements, we have used 250 mg KBr and 2 mg of sample.

Raman spectra were collected using the Horiba Raman spectrometer, Labram 300, with wavelength 532 nm, 60 s per spectrum with an objective of the microscope 50 × . The D/G ratio was calculated from a deconvoluted peak area using a Voight shape.

The thermal stability of MCC and CMG was examined by The morphology of the carbonized cellulose monolithic gels (MCC and CMG) was investigated by scanning electron microscopy (SEM) using a Quanta 400 microscope.

Transmission electron microscopy (TEM) images of MCC-700 and CMG-700 samples (Fig. 7) were recorded using a Jeol microscope (model ARM -200F) operating at 200 kV. The powder was crushed and dispersed in chloroform with ultrasound. The drops were deposited on the surface of a copper observation plate and dried under room temperature.

The porous structure of the CMG-500 and CMG-900 samples were analyzed by a gas physisorption technique. The samples were degassed in a vacuum for 10 h at a final temperature of 30 • C; heating was gradual with isothermal steps at 65 and 105 • C with 0.5 h soak time and 3 • C/ min heating rate. Adsorption of N 2 (77 K) and CO 2 (273 K) (99.999% and 99.998% purity, respectively) was carried out using the static manometric adsorption system (Autosorb iQ-XR, Quantachrome Instruments). The pore size distribution (PSD, Figs. 10b, 11b) curves were obtained by fitting experimental isotherms (Figs. 10a,11a) by a specific kernel using ASiQwin software (Quantachrome Instruments). The experimental N 2 adsorption isotherms were fitted by a hybrid carbon QSDFT adsorption kernel assuming slit-shaped micropores and cylindrical mesopores. This kernel is limited to pores of diameter below 33 nm. The CO 2 adsorption isotherms were fitted by a carbon NLDFT equilibrium transition kernel assuming slit-shaped micropores. Although the equilibrium kernel (= for desorption data) was applied, the software uses the adsorption branch data because the adsorption and desorption are reversible for the micropore range. Micropore volumes and surface areas (from N 2 and CO 2 physisorption experiments), mesopore volumes, and mesopore surface areas (from N 2 physisorption) were calculated by integration of the microporous and mesoporous parts of differential PSDs. Despite the fact that physical adsorption of gases (N 2 , CO 2 , etc.) in micropores is based on a "volume-filling" mechanism [START_REF] Lowell | Characterization of Porous Solids and Powders: Surface Area[END_REF]) and only micropore volume should be reported, we are also reporting micropore surface area for comparative purposes. CO 2 (273 K) physisorption measurement provides reliable micropore characteristics for pores of diameter below ca. 1 nm [START_REF] Ravikovitch | Unified approach to pore size characterization of microporous carbonaceous materials from N 2 , Ar, and CO 2 adsorption isotherms †[END_REF].

To determine macroporosity of the samples, the mercury intrusion (Hg) porosimetry technique (Autopore IV 9500, Micromeritics apparatus) was selected (Fig. 9a,b). Progressive mercury intrusion between 0.004 and 20 MPa was performed assuming a constant mercury contact angle of 130 • .

Results and discussion

Properties and characterization of cellulosic gel

The evolution of the MCC material to a monolithic CMG through the dissolution-freezing process in the presence of NaOH was followed by numerous techniques.

The XRD pattern of the MCC sample (Fig. 1a) clearly shows the presence of cellulose I in commercial microcrystalline cellulose. A value of 80% for the CI was found, the apparent diffracting domain of this phase was evaluated as 53 Å using the Scherer's formula [START_REF] Shiraishi | X-ray diffraction methods to study crystallite size and lattice constants of carbon materials[END_REF] for the most intense peak (200). 13 C MAS+DEC NMR spectra (Fig. 1b) confirm the presence of cellulose I (signals at 65.7 and 89 ppm) and amorphous cellulose (62.9 and 84-85 ppm) [START_REF] Dinand | Mercerization of primary wall cellulose and its implication for the conversion of cellulose I → cellulose II[END_REF] for the MCC sample. The CI of the deconvolution of the NMR signal was 49%.

After dissolution of MCC in NaOH solution and subsequent regeneration, the transformation of cellulose from its native form of cellulose I to cellulose II is expected to occur [START_REF] Budtova | Cellulose in NaOH-water based solvents: a review[END_REF]. This trend was confirmed in the CMG sample XRD pattern (Fig. 1a), where typical peaks for the cellulose II polymorph are shown at 2θ = 12.1, 19.8 and 22.0 • for planes (1-10), ( 110) and (020), respectively. This crystallographic evolution was followed by a decrease in the CI of the material (53%) and of its apparent diffracting domain size (27 Å).

By NMR, the typical spectrum of regenerated cellulose (after its dissolution in NaOH) is observed with the signal at 105 ppm and overlapping resonances from 70 up to 80 ppm (Fig. 1b). CI for cellulose II was calculated by deconvolution of C4 at 87 and 85 ppm (Zhang et al., 2014;[START_REF] Ahvenainen | Comparison of sample crystallinity determination methods by x-ray diffraction for challenging cellulose i materials[END_REF][START_REF] Dinand | Mercerization of primary wall cellulose and its implication for the conversion of cellulose I → cellulose II[END_REF]. As observed by XRD, gel CI decreased during the formation of the gel to 38%.

FTIR spectra (Fig. 1c) of MCC show the presence of cellulose I with typical intense bands at 3426, 2919 cm -1 , 2118 cm -1 , 1368 cm -1 , and 1035 cm -1 . The band at 3426 cm -1 is attributed to the OH stretching (in intra and intermolecular H-bond) of cellulose. The band at 1631 cm -1 is related to the OH bending mode of absorbed water. For regenerated cellulose (CMG sample), the decrease of the peak intensities at 2919 cm -1 compared to MCC is due to alkaline treatment (Oh et al., 2005a;[START_REF] Oudiani | Crystal transition from cellulose I to cellulose II in NaOH treated Agave americana L. fibre[END_REF], and it corresponds to the formation of a higher portion of amorphous cellulose compared to MCC. The process it takes place during regeneration and is associated with the crystallization of cellulose II (Mittal et al., 2011). Indeed, alkali has a significant effect on morphological, molecular and supramolecular properties of cellulose. NaOH is associated with a decrease in crystallinity and a transformation of cellulose I to cellulose II. When cellulose is treated with alkaline solution, cellulose adopts a modified crystal structure, irreversibly forming cellulose II (Wan Ishak et al., 2020). Interacting with cellulose, aqueous sodium hydroxide is able to penetrate the cellulose crystalline lattice to produce a series of well-defined crystalline complexes.

The CH stretching mode at 2919 cm -1 is shifted to a lower wavenumber compared to the MCC sample (Oh et al., 2005b) due to the presence of cellulose II [START_REF] Oudiani | Crystal transition from cellulose I to cellulose II in NaOH treated Agave americana L. fibre[END_REF]. The Raman spectra confirm the features determined by the previous techniques (Fig. 1d). The typical bands at 1121 and 1094 cm -1 , assigned to symmetric and asymmetric β-(1,4)-glycosidic linkage stretching modes (Alves et al., 2016;[START_REF] Wanasekara | Deformation mechanisms in ionic liquid spun cellulose fibers[END_REF]Agarwal, 2017) are observed for both MCC and CMG samples, but with more broad and diffuse bands for the CMG samples due to its higher amorphous character. This higher amorphous contribution for the CMG sample is also validated by the increase in intensity of the peak at 1379 cm -1 . The band at 1336 cm -1 shows the presence of cellulose I polymorph for MCC whereas the bands at 1150, 1279 cm -1 and 1471 cm -1 (more intense for the CMG) are attributed to cellulose II polymorph [START_REF] Wiley | Band assignments in the Raman spectra of celluloses[END_REF].

The CMG sample displays a very different morphology than the MCC sample with a lamellar structure related to the ice-templating effect that arose during the freezing step (-10 • C) performed on solubilized cellulose (Fig. 2). Indeed, under the experimental conditions used, the precooled solution allows the dissolution of cellulose slightly below 0 • C whereas the second cooling (-10 • C) required to produce a tough gel at room temperature, leads to an ice crystal growth with a concomitant increase of NaOH brine concentration and Na-cellulose precipitation [START_REF] Budtova | Cellulose in NaOH-water based solvents: a review[END_REF]. Owing to the anisotropic growth of the ice taking place together with the regeneration of cellulose, a typical ice-template macroscopic lamellar structure (Deville, 2013) is observed in the gel (see Fig. 2).

TG measurements for samples of MCC, CMG, and CMG-uw (unwashed CMG sample still containing NaOH) performed in the air are shown in Fig. 3. After the initial removal of solvent residues (up to 200 • C), the materials degrade in two separate steps. The first step (250-350 • C) is related primarily to the dehydroxylation/dehydration of the cellulose structure and the formation of a char. The amount of char is significantly higher for CMG (≈40%) compared to MCC (≈25%), probably due to presence of traces of sodium salts in CMG sample, which positively affects char formation. The char is burnt out in the second step between 350 and 600 • C. The total mass loss at 800 • C clearly indicates that CMGuw contains a considerable amount of incombustible residual (e.g. sodium salts originated from NaOH used during synthesis), while CMG is fully burnt (present traces of sodium salts are below detection limits of used thermogravimetry technique). The absence of ash for the CMG sample is a good evidence of the effectiveness of the washing procedure. MCC can be burned out without residuals, as expected [START_REF] Shafizadeh | Thermal degradation of cellulose in air and nitrogen at low temperatures[END_REF].

Carbonization process

The carbonization process of MCC and CMG was followed by TG under an inert atmosphere (Fig. 4). At low temperatures up to 200 • C the evaporation of residual water (or ethanol in the case of CMG) can be observed (mass loss 1.8% for MCC and 3.1% for CMG). The carbonization process starts at about 220 • C for CMG, while MCC is more stable and its thermal degradation begins at 300 • C. In general, the strong interaction of O-H groups in cellulose II requires more energy to start the degradation process. However, because the amorphous region is the most unstable and reactive one, CMG (lower crystallinity) decomposition starts earlier than MCC (higher crystallinity). Furthermore, due to the preparation procedure, the presence of residual sodium compounds can be expected, which can act as a catalyst and promote the decomposition of cellulose at a lower temperature (Modenbach et al., 2014).

The main step of carbonization (dehydration, dehydrogenation, evolution of volatile organic decomposition products) accompanied by a large mass loss (68% for MCC and 62% for CMG) is finished at approximately 350 • C for both materials, and subsequent heating leads to much lower mass changes related to the decomposition/evolution of residual hydrogen and oxygen groups [START_REF] Shafizadeh | Thermal degradation of cellulose in air and nitrogen at low temperatures[END_REF]. The yield of carbonaceous product at 800 • C is considerably higher for the CMG sample (17% for MCC and 23.5% for CMG). The increase in char yield can be explained by the traces of sodium compounds (derived from NaOH), leading to an increase in char formation for CMG and a decrease in gaseous pyrolysis products (Fig. 4) [START_REF] Tsuchiya | Thermal decomposition products of cellulose[END_REF][START_REF] Liu | X-ray diffraction study of bamboo fibers treated with NaOH[END_REF][START_REF] Abbott | Self-reinforced cellulose nanocomposites[END_REF][START_REF] Wang | Thermal degradation behaviors of spherical cellulose nanocrystals with sulfate groups[END_REF].

During the water-ethanol exchange, drying, and carbonization step, a shrinkage of about 30% in length was observed, but the material still retains its monolithic character (Fig. 5). The samples were very stable and keeping the shape after pyrolysis. Using specific molds, monoliths can be prepared in different shapes and sizes (Fig. 5).

Carbonaceous monoliths properties

Chemical composition and structure

XRD patterns of carbonized MCC and CMG samples show the presence of poorly graphitized carbon with (002) and (101) plane diffraction (COD9011577) as shown in Fig. 6a. The peak positions (002) in Table 1 indicate that the planes distances between the (002) planes are higher than the typical value of graphite d = 3.35 Å as expected for poorly ordered carbonaceous materials. The materials prepared in this work have a spacing between graphitic-type layers from 4.02 up to 4.10 Å. These features show the very low graphitization ability of these carbon materials as expected for hard carbon derived from biomass. This fact is confirmed by the stability and extremely low values out-of-plane coherence length of graphitic nanodomains (e.g. L c around 11 Å). The absence of a secondary phase for CMG-x samples confirmed the efficiency of the washing treatment. FTIR spectra (not reported) are similar for both MCC-x and CMG-x samples (x = 500-900) and display for x = 500 • C the presence of organic functional groups typical of low-temperature carbonized biomass with the presence of carbonyl, hydroxyl, and alkene at respectively 1577, 2660 and 1567 cm -1 .

Raman spectroscopy is a powerful method to characterize carbonaceous materials. For the studied carbonized samples, their low structural ordering, as evidenced by XRD (L C ~1.2 nm), is due to the nongraphitizing nature of the cellulose biochars and the low temperature of carbonization (e.g. 700 • C). This feature was confirmed by the Raman and the TEM analysis too. The Raman spectra (Fig. 6b 2. The I D /I G ratio, with respectively values of 1.0 and 0.9 for MCC-700 and CMG-700 could not help to discriminate the two samples.

Surface properties and morphology

TEM observations were performed for the MCC and CMG carbonaceous materials pyrolysed at 700 • C. The samples are completely disordered in structure but homogeneous (Fig. 7a,b). The surface morphology of carbonized MCC and CMG has been investigated by SEM and the images are presented in Fig. 8. While the MCC-based carbons are without any macropores (not reported), the CMG-based carbons contain macropores of 50-100 µm with typical icetemplate morphology (Fig. 8a,b,c) as previously evidenced in the dry gel state. No significant effect of pyrolysis temperature on macroporosity and overall morphology was observed.

An analysis of the volumes of mercury that entered the sample upon initial pressurization provides qualitative information on the size of the pores. The steep rise of the intrusion curve at low pressure is due to the large cavities present in the material (Fig. 9a,b) [START_REF] Njiekak | Pore systems in carbonate formations, weyburn field, Saskatchewan, Canada: micro-tomography, helium porosimetry and mercury intrusion porosimetry characterization[END_REF]. Tested CMG-500 and CMG-900 samples show similar cumulative pore volumes ranging from 1.4 to 1.5 cm 3 g -1 (Fig. 9a). No deformation after mercury intrusion and no break of monoliths after the measurement was noticed.

Fig. 10a illustrates the adsorption and desorption isotherms of N 2 (77 K) of the CMG samples pyrolysed at 500 • C and 900 • C. A lowpressure hysteresis (open isotherm) of sample CMG-500 indicates a possible diffusional restrictions of nitrogen molecules in its porous system. Increase of pyrolysis temperature of the sample to 900 • C probably results in the elimination of the restrictions as the low-pressure hysteresis disappears. Differential PSDs derived from the isotherms of these samples by the QSDFT kernel are shown in Fig. 10b. Results show that pyrolysis at 900 • C leads to the development of smaller pores below ca. 4.5 nm, including microporosity (Fig. 10b), which is in line with other studies, e.g. ( Štefelová et al., 2017). This also shows Table 3, where increase of the pyrolysis temperature of CMG from 500 to 900 • C leads to the increase in the micropore volume from 0 to 0.192 cm 3 g -1 and micropore surface area from 0 to 548 m 2 g -1 , while mesopore characteristics are not substantially different. Both CMG samples also exhibit reduced access of micropores for N 2 under cryogenic conditions, as shown by the differences in micropore volumes and surface areas by adsorption measurements of N 2 (77 K) and CO 2 (273 K). The higher CO 2

Table 3

Textural properties of CMG carbonaceous materials obtained by N 2 (77 K) and CO 2 (273 K). V micro -micropore volume, S micro -micropore surface area, V mesomesopore volume, S meso -mesopore surface area. S micro (m 2 g -1 )

V meso (cm 3 g -1 )

S meso (m 2 g -1 )

V micro (cm 3 g -1 )

S micro (m 2 g - adsorption measurement temperature compared to 77 K for nitrogen then promotes the diffusion (and improves accessibility) of the CO 2 molecules into the small micropores that are becoming detectable [START_REF] Lowell | Characterization of Porous Solids and Powders: Surface Area[END_REF].

Conclusions

Our results showed that a simple and eco-friendly way of transformation of cellulose to monolithic porous carbonaceous material is possible. Frozen cellulose solution in aqueous NaOH can be easily obtained at -10 • C and shaped in this state by simple cutting (alternatively, a suitable mold can be used before freezing). Direct regeneration in an ethanol bath, washing out excess NaOH with water, and drying lead to xerogel-like macroporous cellulosic monoliths composed mainly of cellulose II polymorph and equipped with significant macroporosity due to ice-templating. During subsequent carbonization, macroporosity and monolithic character are preserved. The obtained carbonaceous material also showed significant microporosity. Since the most of the surface area is generally occupied by micropores, their characteristics usually play a key role in the practical use of the materials (e.g. as adsorbent or catalyst).

The prepared micro-and macroporous carbonaceous monoliths can be perspective candidates for applications requiring monolithic character, high micropore volume, and good transportation/diffusion properties ensured by macropores.

In addition, the used synthetic route can be easily adapted for doping of cellulose/carbon with metal or metal oxide nanoparticles (which can be added to starting solution in the form of suspension), heteroatoms (compounds soluble in alkaline solution can be used for doping), etc.

We also suppose that the suggested procedure of conversion of cellulose to monolithic carbonaceous material can be adopted for biomassbased materials in general, enabling easy and low-cost production of advanced porous carbons for various applications from renewable resources.

CRediT authorship contribution statement

  Ostrava . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arjeta Kryeziu I hereby state that this submitted thesis is my original author work and that I elaborated it myself. I properly cite all references and other sources that I used to work up the thesis. Those references and other sources are given in the list of references. Ostrava . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arjeta Kryeziu 26/08/2022 26/08/2022 DEDICATION I dedicate all my work and effort to my parents and family for always supporting me during my studies. When I felt myself weak or with not strong motivation, they were my force to go further and to succeed. They were my motivation and my support through all my life. characterization and corresponding discussion. I am thankful to Madhav Chavhan for analyses of electrochemical measurements and its discussion. I acknowledge carbon team at IS2M, Mulhouse, for pyrolysis by Benedicte Rety, Joseph Dentzer and Adrian Beda, Gautier Schrodj for TG analysis, 13 C NMR by Ms. Severine Rigolet, Hg intrusion by Ms. Habiba Nouali, Raman analysis by Simon Gree and Nemanja Drazevicanin, XRD analysis by Mr. Jean Marc Le Meins, XPS analysis, Loic Vidal for SEM and TEM images and Cyril Vaulot for N2 sorption. I am thankful to University of Ostrava, Department of Chemistry, Ostrava, Czech Republic for providing financial support during four years of PhD study and for the financial support to carry out the research work in France. I acknowledge the financial support provided by Moravian Silesian Region, Ostrava, Czech Republic in this study. This funding was useful to complete the research work.

Figure 1 .

 1 Figure 1. Schematic representation of biomass/cellulose liquefactions.

Figure 2 .

 2 Figure 2. Major components of pyrolytic bio-oil [29].

Figure 3 .

 3 Figure 3. Decomposition products during cellulose liquefaction.

Figure 4 .

 4 Figure 4. Cellulose solvents.

13 C (I = 1 / 2 )

 1312 Magic angle spinning (MAS) spectra were recorded at room temperature with a 4 mm Bruker double channel probe with a spinning frequency of 12 kHz, on a Bruker Avance NEO 400 spectrometer operating at B0 = 9.4 T (Larmor frequency υ0 ( 13 C) = 100.63 MHz and υ0

Figure 5 .

 5 Figure 5. X-ray diffraction profiles of (a) microcrystalline cellulose powder (MCC), and dried cellulose gel synthesized from precooled alkali solution (CMG), (b) 13 C NMR spectra of MCC and CMG (c) normalized FTIR spectrum of MCC and CMG, and (d) Raman spectra for MCC and CMG.

Figure 6 .Figure 7 .

 67 Figure 6. SEM micrographs of the (a) MCC and (b, c) CMG dried gel at different magnification showing the lamellar structure due to the ice-templating effect.

68 Figure 8 .

 688 Figure 8. TG analysis of MCC and CMG in inert atmosphere.

Figure 9 . 2 . 3 . 3 . Carbonaceous monoliths properties 2 . 3 . 3 . 1 .

 92332331 Figure 9. Pieces of cellulose monolithic dried gel CMG (a) and derived carbonaceous monoliths pyrolysed at 500 (b), 700 (c), and 900 (d) °C; CMG with different size and shape before (e) and after (f) pyrolysis.

Figure 10 .

 10 Figure 10. XRD profiles (a) and Raman spectra (b) for MCC and CMG pyrolysed at 700 °C.

Figure 11 .Figure 12 .

 1112 Figure 11. TEM images of MCC-700 (a) and CMG-700 (b) samples.

Figure 13 .

 13 Figure 13. Hg intrusion porosimetry intrusion curves (a) and pore size distribution (PSD) (b, c) for carbonaceous CMG samples.

Fig. 14a illustrates

  Fig.14aillustrates the adsorption isotherms of N2 (77K) of the CMG samples pyrolysed at 500 °C and 900 °C. Differential PSDs derived from the isotherms of these samples by the QSDFT kernel are shown in Fig.14b. Results show that pyrolysis at 900 °C leads to the development of smaller pores below ca. 4.5 nm, including microporosity (Fig.14b), which is in line with other

Figure 14 .

 14 Figure 14. N2 (77K) adsorption isotherms (a) and derived differential PSDs (b) for monolithic carbonaceous CMG samples. Only relevant part of PSD is shown.

Figure 15 .

 15 Figure 15. CO2 (273K) adsorption isotherms of carbonaceous CMG materials (a) and derived differential

98

 98

Figure 16 .

 16 Figure 16. X-ray diffraction patterns of microcrystalline cellulose powder (MCC), and dried cellulose gel (CMG).

Figure 17 .

 17 Figure 17. (a) XRD pattern (defined peaks in the graph correspond to Na2CO3; undefined small peaks correspond to Na2CO3•H2O and Na2CO3•10H2O, (b) FTIR analysis, and (c) Raman spectra of samples carbonized at different temperatures.

Figure 18 .

 18 Figure 18. XRD of carbons derived from the carbonization of MCC, CMG, MCC/NaOHw, CMG/NaOHw.

Figure 19 .Figure 20 .

 1920 Figure 19. FTIR spectra of of MCCx, MCC/NaOHw x (a) and CMGx and CMG/NaOHw x (b) samples series.

Fig. 21

 21 Fig. 21 illustrates N2 (77 K) adsorption isotherms of studied samples pyrolysed at 500, 700, 900, and 1300 °C. It can be seen that the porous properties of the samples could be determined by both the pyrolysis temperature and the type of precursor. Samples containing NaOH (MCC/NaOHuw x and CMG/NaOHuw x) show only negligible porosity, probably because of presence of sodium salts which blocks the pores. Increasing pyrolysis temperature from 500 to 900

Figure 21 .Figure 22 .

 2122 Figure 21. N2 physisorption of (a) CMG/NaOHuw-x and MCC/NaOHuw-x, and (b) CMG/NaOHw-x (c) CMGx, and (d) MCC/NaOHw x and MCCx.

Figure 23 .

 23 Figure 23. TG (a) and DTG (b) curve of MCC, CMG, CMG/NaOH and MCC/NaOH (heating rate 10 K min -1 , inert atmosphere.

1 Figure 24 .

 124 Figure 24. Effect of heating rate for MCC, CMG, CMG/NaOH and MCC/NaOH samples on the TG curves.

Figure 25 .

 25 Figure 25. Activation energy of MCC, CMG, CMG/NaOH and MCC/NaOH.

1 Figure 26 .

 126 Figure 26. Dependence of conversion on temperature for (a) CMG/NaOH and (b) MCC/NaOH.

4 )

 4 which enables reconstruction of thermoanalytical curve and consequently non-linear optimization of kinetic parameters A, Ea, m and n. Equation (3) describes one single reaction, but it can be combined (because of additivity) to express more complicated processes. For cellulosic samples pyrolysis we used model of two (for MCC and CMG) or three (for NaOH containing samples) parallel reactions (Fig.27):

Figure 27 .

 27 Figure 27. Reaction model used for cellulosic samples pyrolysis.

- 1 (

 1 for MCC/NaOH and CMG/NaOH samples) can be characteristic for char formation (the values agrees with most literature data). Comparing the pre-exponential factor (A) values, the high values indicates mass transfer limitations.

Figure 28 .

 28 Figure 28. Carbon yield of MCC, CMG, CMG/NaOH and MCC/NaOH with increasing heating rate.

  NaOH presence decreases the carbon yield during decomposition reaction. After washing of CMG/NaOHuw x with nitric acid/ammonia solution, N2 sorption shows good effect of NaOH on the porosity of carbon, increasing its mesoporosity and surface area, otherwise the pores are blocked (only CMG samples keeps a macroporosity). During pyrolysis, prepared carbon materials are less graphitized. NaOH effects the process of pyrolysis and thermal stability of the samples based on the crystalline form present in the cellulosic materials. This effect was studied for MCC containing cellulose I and amorphous polymorph; CMG material synthesis in alkaline solution resulted in decreased crystallinity and formation of cellulose II and amorphous part. CMG decomposition reaction starts at lower temperature, with lower activation energy compared to cellulose I. Activation energy (Ea) of decomposition of MCC correspond to cellulose I crystallinity. NaOH presence decreases the thermal stability of all forms.

  XRD analysis of the of the metal-based cellulosic gels (MCMG) and metal-based carbons (MC) were performed with a Bruker powder diffractometer D8 ADVANCE A25 in the Bragg-Brentano reflection geometry theta-2θ. The diffractometer is equipped with the LynxEye XE-T high-resolution energy dispersive 1-D detector for (Cu Kα1, 2 radiations which leads to ultrafast Xray diffraction measurements. The conditions for data collection are the following: angular range 10-90º 2θ, step size 0.014º 2θ, time per step 0.7 s, variable divergence slits, irradiated sample length 15 mm, total time for acquisition 1h 11 min. During data collection, the powder sample rotates at 15 rpm.

Figure 28 Figure 28 .

 2828 Figure 28.

Figure 29 .

 29 Figure 29. SEM image of metal-based cellulosic gel of (a) CMG/Cu 2+ 5 mmol, (b) CMG/Mn 2+ 5 mmol and (c) CMG/Fe 2+ 5 mmol. Samples were observed in powder.

Figure 30 . 4 . 3 . 4 .

 30434 Figure 30. XRD of (a) CMG/Cu 2 mmol, CMG/Mn 2 mmol and CMG/Fe 2 mmol, and (b) CMG/Cu 5 mmol, CMG/Mn 5 mmol and CMG/Fe 5 mmol.

Figure 31 .

 31 Figure 31. TGA of metal-based cellulosic gels with CMG/Cu 5 mmol, CMG/Mn 5 mmol, and CMG/Fe 5 mmol. Samples were measured under oxidized atmosphere (air) at 10 K min -1 . CMG is cellulose monolithic gel prepared without additional of metal salts within the dissolved cellulose solution.

Figure 32 .

 32 Figure 32. Metal-based carbons carbonized at 800 °C.

148 Figure 34 .

 14834 Figure 34. Raman analysis of metal-based carbons (a) CMG/Cu 5 mmol 500, CMG/Mn 5 mmol 500 and CMG/Fe 5 mmol 500, (b) CMG/Cu 5 mmol 800, CMG/Mn 5 mmol 800 and CMG/Fe 5 mmol 800.

Figure 35 .

 35 Figure 35. SEM micrographs of (a) CMG/Cu 5 mmol 800, (b) CMG/Mn 5 mmol 800, (c) CMG/Fe 5 mmol 800, observed on grinded samples.

Figure 36 .

 36 Figure 36. TEM micrographs of (a) CMG/Cu 2 mmol 800, (b) CMG/Cu 5 mmol 800 (c) CMG/Mn 2 mmol 800, (d) CMG/Mn 5 mmol 800, (e) of CMG/Fe 2 mmol 800, (f) of CMG/Fe 5 mmol 800.

11 ) 1 ) 2 )Figure 37 .

 111237 Figure 37. N2 physisorption of metal-based carbons on (a) CMG/Fe 5 mmol 500, CMG/Fe 5 mmol 800, (b) CMG/Cu 5 mmol 500, CMG/Cu 5 mmol 800, (c), CMG/Mn 5 mmol 500, CMG/Mn 5 mmol 800, and (d) comparison of MC with metals and CMG 800.

23 CMG

 23

Figure 38 .

 38 Figure 38. CV plots at 10 mV s -1 for (a) CMG/Cu with 2 and 5 mmol, (b) CMG/Fe with 2 and 5 mmol, c) CMG/Mn with 2 and 5 mmol, and (d) comparison of CMG carbonized at 800 °C and metal-based carbon.

Figure 39 .

 39 Figure39. CP plots at 1 A g -1 for CMG carbonized at 800 °C and metal-based carbon.

  Dans les échantillons carbonisés, la présence de C désordonné (pour le carbone à base de métal avec Mn et Cu) et de graphite (pour Mc avec Fe) a été vérifiée. Un pic de graphitisation plus net a été remarqué pour MC avec Fe (2 mmol). Au cours de la carbonisation des échantillons MCMG, Fe3O4 a été réduit en Fe et Fe3C, et MnO2 a été réduit en MnO. Cu est resté sous forme métallique à toutes les étapes de la réaction. À MC, les échantillons avec une concentration de métal de 5 mmol n'ont pas été complètement éliminés du Na2CO3 (le lavage n'a pas été aussi efficace). Les échantillons présentent des nanoparticules bien dispersées mais de petite taille. Aucune mésoporosité significative n'a été mise en évidence. Les résultats électrochimiques montrent que les électrodes en carbone à base de Fe offrent des performances électrochimiques supérieures à celles des électrodes en carbone à base de Cu et Mn. La raison principale est que le volume de mésopores plus élevé dans les électrodes de carbone à base de Fe facilite l'accès des ions électrolytes pour introduire un stockage faradique. La graphitisation plus élevée due au dopage Fe améliorant le transport d'électrons dans une telle électrode est la principale raison de l'amélioration des performances électrochimiques.

Figure 1 .

 1 Figure 1. Schematic representation of biomass/cellulose liquefactions.

Figure 1 .

 1 Figure 1. Schematic representation of biomass/cellulose liquefactions.

Figure 2 .

 2 Figure 2. Major components of pyrolytic bio-oil [29].

Figure 2 .

 2 Figure 2. Major components of pyrolytic bio-oil [29].

Figure 3 .

 3 Figure 3. Decomposition products during cellulose liquefaction.

Figure 3 .

 3 Figure 3. Decomposition products during cellulose liquefaction.

Figure 4 .

 4 Figure 4. Cellulose solvents.

Figure 4 .

 4 Figure 4. Cellulose solvents.

Fig. 1 .

 1 Fig. 1. X-ray diffraction profiles of (a) microcrystalline cellulose powder (MCC), and dried cellulose gel synthesized from precooled alkali solution (CMG), (b) 13 C NMR spectra of MCC and CMG (c) normalized FTIR spectrum of MCC and CMG, and (d) Raman spectra for MCC and CMG.

Fig. 2 .

 2 Fig. 2. SEM micrographs of the CMG dried gel at different magnification showing the lamellar structure due to the ice-templating effect.

Fig. 3 .Fig. 4 .

 34 Fig. 3. TG analysis of MCC, CMG and CMGuw in air. Fig. 4. TG analysis of MCC and CMG in inert atmosphere.

  ) display relatively similar features for both MCC-700 and CMG-700 samples. They were fitted by two bands (D and G) as recommended by Puech et al.(Puech

Fig. 5 .

 5 Fig. 5. Pieces of cellulose monolithic dried gel CMG (a) and derived carbonaceous monoliths pyrolysed at 500 (b), 700 (c), and 900 (d) • C; CMG with different size and shape before (e) and after (f) pyrolysis.

Fig. 6 .

 6 Fig. 6. XRD profiles (a) and Raman spectra (b) for MCC and CMG pyrolysed at 700 0 C.

Tablee 2 Fig. 7 .

 27 Fig. 7. TEM images of MCC-700 (a) and CMG-700 (b) samples.

Fig. 8 .Fig. 9 .

 89 Fig. 8. SEM images of carbonaceous CMG-700 sample with different magnification (a-c).

Fig. 10 .

 10 Fig. 10. N 2 (77 K) adsorption and desorption isotherms (a) and derived differential PSDs (b) for monolithic carbonaceous CMG samples. Only relevant part of PSD is shown.

Fig. 11 .

 11 Fig. 11. CO 2 (273 K) adsorption isotherms of carbonaceous CMG materials (a) and derived differential PSDs (b).

Arjeta Kryeziu :

 Kryeziu Writing -Original Draft, Investigation, Validation. Vaclav Slovak: Supervision, Conceptualization, Writing -Review & Editing. Julien Parmentier: Supervision, Conceptualization, Writing -Review & Editing. Tomas Zelenka: Investigation, Formal analysis. Severinne Rigolet: Investigation, Formal analysis.

Table 1 .

 1 Solvent used for biomass/cellulose HTL.

	Solvent	Conversion (%) Temperature (°C)	Ref.
	NaOH/H2O	28-35	350	[66]
	NaOH/H2O	77	120-250	[67]
	Urea/H2O		100	[68]
	H2SO4/H2O	85	150	[42]
	H3PO4/H2O	85	150	[69]
	Citric acid/H2O	65	100	[68]
	Oxalic acid/H2O	65	100	[68]
	Glycerol	60	100	[68]
	Iso-propanol		380	[70]
	2-propanol	32-49	240-320	[71]
	2-butanol	27-53	240-320	[71]
	Ethanol	100	100-250	[60]
	Methanol	100	100-250	[60]
	1-octanol		270	[72]
	Acetone	60.5	299	[73]
	Polyethylene glycol		150	[74]
	Phenol		130-150	[74]

Table 2 .

 2 Values

		d(002) Å	Lc Å
	MCC 700	4.10	11
	CMG 500	4.07	13
	CMG 700	4.10	11
	CMG 900	4.02	11

of XRD interlayer spacing d(002) and diffracting domain size (Lc).

Table 3 .

 3 Band position, full width half maximum (FWHM) and intensity values of MCC 700 and CMG 700 from Raman analysis.

			CMG 700			MCC 700
	Band	Position (cm -1 )	FWHM	ID/IG	Position (cm -1 )	FWHM	ID/IG
	D	1345	209	0.95	1340	189	1.01
	G	1595	80	-	1588	63	-

  procédure développée dans ce travail est simple, peu coûteuse et écologique puisqu'elle ne nécessite, pour produire un monolithe de cellulose présentant une macroporosité, que de la cellulose, de l'eau, de la soude NaOH puis de l'éthanol comme solvant d'échange. Nous nous sommes attachés à éviter les procédures fastidieuses et chronophages telles que le chauffage prolongé (pour la gélification), la congélation et le séchage supercritique des gels, ainsi que toute étape d'activation. Brièvement, une solution de NaOH a été mélangée avec une quantité donnée de cellulose microcristalline (MCC) commerciale autour de 0 °C pour assurer sa dissolution comme indiqué par le diagramme de phase cellulose-eau-NaOH. Ce mélange est placé dans des tubes cylindrique pour donner la forme désirée au monolithe. Un second refroidissement est ensuite effectué vers -10 °C pour assurer une congélation de l'eau et également permettre la régénération système, cristallise alors sous forme de Na-cellulose de type II. On constate également une augmentation de la part de la cellulose amorphe suite à la régénération de la cellulose.Une étude fine par analyse thermique a été réalisée sur les échantillons MCC et CMG afin de mieux appréhender l'influence de la structuration de la cellulose (cellulose I ou II associée à une phase amorphe) sur les mécanismes de carbonisation. Pour cela, des mesures par ATG à différentes vitesses de chauffe ont été menées de manière à déterminer les variations des énergies d'activation des processus de carbonisation en fonction du taux de conversion des échantillons entre 200 et 600 °C. Un seul processus semble gouverner la carbonisation de CMG et MCC avec une énergie d'activation autour de 250-280 kJ mol -1 .

	La pyrolyse a été réalisée à différentes températures T (T = 500, 700 ou 900 °C) sous
	atmosphère inerte. Les échantillons ont alors été dénommés MCC-T et CMG-T. Le processus de
	carbonisation du MCC et du CMG a été suivi par analyse thermique gravimétrique (ATG) sous
	atmosphère inerte. La carbonisation commence à environ 220 °C pour CMG et seulement à 300
	°C pour MCC. Il est attendu une plus forte interaction des groupements O-H dans la cellulose II et
	donc une meilleure stabilité thermique. Cependant, la présence plus importante de phase amorphe,
	moins stable thermiquement, dans l'échantillon GMC, pourrait expliquer la plus faible stabilité
	La manométrie d'adsorption d'azote à 77K a révélé que la pyrolyse à 900 °C du gel CMG
	conduit au développement de micropores alors que cette porosité n'est pas observée à 500 °C, ce
	qui est en accord avec d'autres études [4]. Le volume de micropore de 0 (T = 500 °C) à 0.19 cm 3
	g -1 (T = 900 °C). Cette microporosité est confirmée par adsorption du CO2 à 273K pour les
	échantillons CMG-500 et CMG-900; leur taille est inférieure à 1 nm. La mésoporosité n'est pas
	significativement modifiée entre ces deux températures pour les deux échantillons.

de la cellulose sous forme solide. Cette étape conduit à la formation d'une structuration particulière du gel de cellulose du fait de la cristallisation de cristaux de glace, sous forme lamellaire, en son sein (obtenue par microscopie électronique à balayage (MEB)). Grace à cette étape, associée à un échange ultérieur de la solution aqueuse avec de l'éthanol (suite au réchauffage du gel à température ambiante), des gels résistants à la manipulation peuvent être obtenus. Après un lavage à l'eau désionisée pour éliminer le sel NaOH, les gels ont été séchés à 80 °C. Des morceaux d'une taille de 1 à 5 cm ont été obtenus. Les xérogels monolithiques de cellulose ont été notés CMG (cellulose monolithic gel).

L'évolution du matériau de la cellulose microcristalline (MCC) en un gel monolithique de cellulose (CMG) par le processus de dissolution-congélation en présence de NaOH a été suivie par de nombreuses techniques.

Le diagramme de diffraction de rayons X (DRX) de l'échantillon de MCC montre clairement la présence de cellulose I dans la cellulose microcristalline commerciale utilisée comme source de cellulose. Après dissolution à froid de la MCC dans une solution de NaOH et sa régénération sous forme solide lors du second refroidissement à -10 °C, la transformation de la cellulose native de type I en cellulose II est observée. Cette observation a été confirmée par RMN du solide du

13

C.

Cette régénération en cellulose de type II intervient lors de la cristallisation de la glace. Ce phénomène s'accompagne d'une augmentation de la concentration en saumure de NaOH et en chaînes de cellulose dans la solution résiduelle. Cette dernière, en accord avec le diagramme de phase de ce thermique de cet échantillon. En outre, en raison de la procédure de préparation, on peut s'attendre à la présence de sodium résiduel dans la cellulose CMG. Elle peut agir comme un catalyseur et promouvoir la décomposition de la cellulose à plus basse température. Ce phénomène a été mis en évidence sur la figure en mélangeant MCC avec NaOH (échantillon MCC/NaOH) et en ne lavant pas le gel avec de l'eau distillée (échantillon CMG/NaOH). Si la stabilité thermique de l'échantillon CMG est plus faible, son rendement de carbonisation à 800 °C est significativement plus élevé (≈40 %) par rapport à celui de l'échantillon MCC (≈25 %) probablement en raison de la présence de traces de sels de sodium dans l'échantillon de CMG, qui affectent positivement la formation de charbon.

La microstructure du xérogel avec sa macroporosité lamellaire est conservée après carbonisation comme le montre la micrographie obtenue au MEB. Cette macroporosité est également mise en évidence par porosimétrie d'intrusion de mercure. On mesure un volume macroporeux important (1,4-1,6 cm 3 g -1 ) et une macroporosité très largement distribuée entre 100 et 0,1 m.

Table 5 .

 5 Kinetic parameters of microcrystalline cellulose (MCC) and microcrystalline cellulose mixed with sodium hydroxide (MCC/NaOH) pyrolysis.

		Reaction steps Ea (kJ mol -1 )	Pre-	Reaction order	Ref.
				exponential	(n)
				factor (s -1 )
	MCC	2	147	6.5×10 10	1	[44]
			234	4×10 17
		1	209.2		1	[45]
		1	165	2.25×10 7	1	[46]
		1	227	3×10 16		[47]
		1	243	2.8×10 19		[48]
		2	198	3.2×10 14		[49]
			153	1.3×10 10
		1	257	1.58×10 20		[50]
		3	249	3.18×10 18	1	[51]
			82.7	4.69×10 5	1
			282	1.33×10 23	2
		1	108.8			[52]
		1	199.66	1.68×10 17	1.2	[53]
	MCC/NaOH	1	56-60			[54]
		3	47.9		0.86	[55]
			144.5		1.88	[56]
			260.7

Table 7 .

 7 Structural parameters of cellulosic materials obtained from XRD.

	sample	Amorphous fraction % Crystalline fraction (%) Lc (002) Å
	MCC 500	28	72
	MCC 700	26	74
	MCC 1300	25	77
	MCC/NaOHw 500 46	54
	MCC/NaOHw 700 49	51
	CMG 500	29	71
	CMG 700	26	74
	CMG 900	23	77
	CMG 1300	20	80
	CMG/NaOHw 500 48	52
	CMG/NaOHw 700 47	53
	CMG/NaOHw 900 29	71

Table 8 .

 8 Band position, full width half maximum (FWHM) and intensity values of carbonized samples from Raman analysis.

	sample	Band	Position (cm -1 )	FWHM	ID/IG
	MCC 1300	D	1351	108	1.3
		G	1587	79	-
	CMG 1300	D	1391	108	1.3
		G	1587	79	-
	MCC/NaOHw D	1369	236	2
		G	1591	86	-
	MCC/NaOHw D	1370	268	2
		G	1584	160	-
	CMG/NaOHw D	1358	199	2.1
		G	1585	85	-
	CMG/NaOHw D	1370	267	1.8
		G	1584	119	-
	CMG/NaOHw D	1359	160	1.2
		G	1590	107	-

Table 9 .

 9 N2 physisorption parameters (Vmicro, Vmeso) of prepared carbon materials (calculated with DFT method).

Table 10 .

 10 Activation

	sample	step Ea (kJ mol -1 )	A (min -1 )	n	m
	MCC	A	227	3.77×10 19	0.93	0.42
		B	170	1.02×10 14	6.16	0
	MCC/NaOH A	84	3.79×10 7	1.41	0
		B	79	7.77×10 5	2.08	0
		C	322	4.68×10 23	4.02	0.50
	CMG	A	97	1.83×10 8	0.82	0
		B	156	8.73×10 13	5.28	0.82
	CMG/NaOH A	162	1.80×10 15	2.28	0
		B	138	9.89×10 9	1.58	0
		C	265	8.29×10 18	3.38	0

energy (Ea), frequency factor (A), n and m factors for different processes of MCC, MCC/NaOH, CMG and CMG/NaOH.

  (TG) pour les gels de MCC et de CMG, une étude plus complète a été réalisée afin d'examiner l'influence de NaOH et de la cristallinité de la cellulose sur sa carbonisation. Dans ce but, les

	échantillons MCC et CMG mélangés avec NaOH ont été étudiés par ATG à différentes vitesses de
	chauffage (2, 5, 10 et 20 K min -1 ), et les énergies d'activation ont été déterminées.
	Il apparaît que la perte de masse et la température maximale de décomposition de la cellulose
	sont fonction de la teneur en NaOH de l'échantillon de cellulose. Pour le MCC, la perte de poids
	principale commence à 305 °C et se produit en une seule étape alors qu'en présence de NaOH
	(échantillon MCC/NaOH), ces événements commencent à une température plus basse et en quatre

étapes. La première perte de poids à environ 200 °C est attribuée à la désorption de l'humidité. La deuxième perte de poids à environ 300-400 °C pourrait être due à la décomposition d'une partie amorphe de la cellulose, moins stable thermiquement et formée au contact du NaOH. La troisième perte de poids (environ 450-480 °C) est attribuée à la décomposition de la partie restante de la fraction cristalline. La quatrième étape est liée à l'activation thermique par NaOH au-delà de 800 °C. Un comportement similaire est observé pour l'échantillon CMG/NaOH (échantillon CMG non lavé). L'étude des rendements de carbonisation entre 200 (ou 100) et 600 °C en présence ou l'absence de NaOH pour différentes vitesse de chauffe est reportée. Il apparait que le rendement est significativement diminué en présence de NaOH et dans une moindre mesure avec la vitesse de chauffe. Une analyse approfondie des courbes d'ATG (méthode Starink) pour extraire les énergies d'activation suggère que la carbonisation ait lieu par une réaction en deux étapes pour les échantillons de MCC et CMG et par une réaction en trois étapes pour le MCC mélangé avec NaOH et le CMG non lavé (CMG/NaOH).

  XRD analysis of metal-based carbon with (a, b) iron, (c, d) copper, (e, f) manganese and (g) normalized MC of metals (with 5 mmol, carbonized at 800 °C) and CMG 800.

	e) Figure 33.								f)
					CMG/Mn 2 mmol 800
														CMG/Mn	5	mmol	800
					CMG/Mn 2 mmol 500						CMG/Mn	5	mmol	500
	Intensity (a.u.)	C disordered C disordered	MnO MnO MnO MnO			MnO	(a.u.) Intensity	C C	disordered disordered		MnO MnO	MnO Na MnO	2 CO	3	MnO MnO	MnO
														Na	2 CO	3
														MnO
	10	20	30	40	50		60		10	20		30	40	50	60	70
		Angle (2Theta)							Angle	(2Theta)
					g)
										CMG	800
				C	disordered	+	impurities		CMG/Cu	5	mmol	800
										CMG/Fe	5	mmol	800
										CMG/Mn	5	mmol	800
			Intensity (a.u.)	Graphitic C disordered carbon	Fe Fe 3 C Cu		Fe		Fe
							MnO
				C	disordered
				10	20	30	40	50	60	70	80	90
								Angle	(2Theta)
										CMG/Cu 5 mmol 800 CMG/Cu 5 mmol 500 Na 2 CO 3 Na 2 CO 3 C disordered	Cu Cu	Cu Cu	Cu Cu

Table 11 .

 11 Band position, full width half maximum (FWHM) and intensity values of metal-based carbon from Raman analysis.

	sample	Band	Position (cm -1 )	FWHM	ID/IG
	CMG/Fe 5 mmol 500	D	1380	340	2.1
		G	1590	78	-
	CMG/Fe 5 mmol 800	D	1354	118	0.64
		G	1582	99	-
	CMG/Cu 5 mmol 500	D	1372	334	1.9
		G	1589	80	-
	CMG/Cu 5 mmol 800	D	1366	295	3.2
		G	1592	48	-
	CMG/Mn 5 mmol 500	D	1370	325	1.8
		G	1590	79	-
	CMG/Mn 5 mmol 800	D	1369	296	3.4
		G	1587	92

Table 12 .

 12 N2 physisorption parameters (Vmicro, Vmeso) of metal-based carbons using DFT model.

		Without NaOH
	Vmicro (cm 3	g -1	)

Table 1 .

 1 Solvent used for biomass/cellulose HTL.

	Solvent	Conversion (%)	Temperature ( • C)	Ref.
	NaOH/H 2 O	28-35	350	[66]
	NaOH/H 2 O	77	120-250	[67]
	Urea/H 2 O		100	[68]
	H 2 SO 4 /H 2 O	85	150	[42]
	H 3 PO 4 /H 2 O	85	150	[69]
	Citric acid/H 2 O	65	100	[68]
	Oxalic acid/H 2 O	65	100	[68]
	Glycerol	60	100	[68]
	Iso-propanol		380	[70]
	2-propanol	32-49	240-320	[71]
	2-butanol	27-53	240-320	[71]
	Ethanol	100	100-250	[60]
	Methanol	100	100-250	[60]

Table 1 .

 1 Cont.

	Solvent	Conversion (%)	Temperature ( • C)	Ref.
	1-octanol		270	[72]
	Acetone	60.5	299	[73]
	Polyethylene glycol		150	[74]
	Phenol		130-150	[74]
	Dimethylsulfoxide	100		[75]
	Ethylacetate	100		[76]
	[Alkylmethylimidazolium]Cl	75-90	120	[60]
	[Bmim]Cl	20	100	[77]
	[Bmim]Ac	11.5	50	[77]
	[Amin]Cl	3.5	100	[78]
	[Emim]Cl	10	100	[77]

  al., 2016; Rodriguez-Abreu et al., 2011; Zhang et al., 2003; Schlienger et al., 2012; Braghiroli et al., 2016, 2015; Parmentier et al.

  • C) followed by the exchange of solvent with ethanol and vacuum drying. The material obtained was hydrolyzed in a methanolic solution of NaOH to obtain cellulose monoliths (Lyu et al., 2020; Xin et al., 2017; Zhang et al., 2020; Xie et al., 2020; Bai et al., 2017). A widely used water-soluble cellulose-derived compound is carboxymethylcellulose (CMC). CMC porous monoliths can be prepared by controlled freezing of CMC aqueous solution in a liquid nitrogen bath and consequent freeze-drying (Zhang et al., 2014; Lee

  et al.

	Industrial Crops & Products 183 (2022) 114961
	3

Table 1

 1 Values of XRD interlayer spacing d (002) and diffracting domain size (L c ). al., 2019) for crystallite size lower than 2 nm. The results are reported in Table

		d (002) Å	L c Å
	MCC 700	4.10	11
	CMG 500	4.07	13
	CMG 700	4.10	11
	CMG 900	4.02	11

et

Micropores : tailles de pores inférieures à

nm2 Macropores : tailles de pores supérieures à 50 nm

Mésopores : tailles de pores comprises entre 2 et 50 nm

Acknowledgments

This work was partially (A. Kryeziu) supported by the grant program "Support for science and research in the Moravian-Silesian Region 2019 ′′ , code RRC/10/2019, in the framework of the project "Support for talented doctoral students at the University of Ostrava III". We also thank Jean-Marc Le Meins and Habiba Nouali, for XRD and Hg intrusion analyses performed on IS2M Mulhouse technical platform.

Data Availability Statement:

The data presented in this study are available on request from the corresponding author.

Effect of NaOH on cellulose pyrolysis

Pyrolysis was carried out at different temperatures (500, 700, 900, and 1300 °C) for 1 h (heating rate 10 K min -1 ) under an inert atmosphere (nitrogen, flow 200 ml min -1 ). Carbonized samples due to the presence of NaOH does not preserve the monolithic shape of the pristine materials (a powder is obtained) at the difference of carbonization performed on washed samples

For the removal of sodium compounds, part of the obtained carbonaceous materials was treated with 1 mol dm -3 HNO3 solution (6 h) and then with ≈15 wt% ammonia solution (6 h) followed by a drying step at 250 °C overnight.

MCC/NaOH sample was prepared by simple mixing of cellulose and sodium hydroxide powders together in mass ratio 1:1.

Samples notation used in this chapter is summarized in Table 6.

Table 6. Notation of samples used in this chapter.

Sample name Description

MCC

Thermoanalytical experiments

The thermal stability of cellulosic samples was examined by thermogravimetric analysis (TG) using the STA 449C apparatus (Netzsch), in the temperature range 30-800 °C, heating rate 2, 5, 10 and 20 °C min -1 , a crucible of α-alumina, inert (N2, 100 ml min -1 ) atmosphere.

SUMMARY of CHAPTER 3 (FRENCH) CHAPTER 4 Metal-based carbon nanocomposites derived from liquefied cellulose

The electrochemical results show that Fe-based carbon electrodes deliver higher electrochemical performance compared to Cu-and Mn-based carbon electrodes. The primary reason is that the higher mesopore volume arising in Fe-based carbon electrodes provides easier access for electrolyte ions to introduce faradic storage. The higher graphitization from Fe-doping enhancing electron transport in such electrode is major reason for improved electrochemical performance.

SUMMARY of CHAPTER 4 (FRENCH) APPENDIX 1: A list of original publications

Conflicts of Interest:

The authors declare no conflict of interest.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.