domain: chim spi

text:
Simona Bennici

Claire Courson

8 Applications in catalysis

Keywords: catalysts, catalytic reactions, catalyst support, char

The impelling necessity to reduce the human impact on the environment, push the industries to develop sustainable production paths and to switch their practices towards cleaner and less resources-demanding processes and materials. The development of new eco-friendly industrial paths and the optimization of the use of natural resources passes also through the development of new eco-compatible catalysts, to be implemented in new or existing industrial processes. Chars produced by pyrolysis of bio-sourced wastes can be used as catalyst or catalysts support. Their surface, structural, morphological and redox properties are the key parameters driving the chars' catalytic activity. Moreover, the choice of the biomass sources, the chars and catalysts preparation methods, as well as the chars' post-treatments procedures have also to be taken into account to implement chars in catalytic processes. The present chapter aims to elucidate the dependence of the catalytic performances in various reactions to the physico-chemical properties of chars based catalysts and to identify the required characteristics indispensable for the future chars' development in the heterogeneous catalysis field. Applications in the biomass valorization field, electrochemical conversion reactions, and pollution control have been also reviewed to compare the performances of char based catalysts to those of conventional heterogeneous catalysts.

Introduction

During the last century, the most part of the chemicals production processes has been characterized by an unconsidered exploitation of natural resources, very often not caring about the environmental consequences. Fortunately, nowadays a renewed consciousness on the necessity to reduce the human impact on the environment, push the industries to develop sustainable production paths and to switch their practices towards cleaner processes less resources-demanding. These advances represent the hearth of the so-called "sustainable development" that aims to reach a compromise between the nowadays production needs and the preservation of the ability of the next generations to meet their needs.

The development of new eco-friendly industrial processes and the optimization of the use of natural resources by minimizing wastes will permit to reach such a challenging objective. In particular, the need to upgrade the existing processes of the chemical and petrochemical industries, towards a greener approach, passes also through the development of new eco-compatible catalysts. In order to valorize bio-sourced wastes, chars, produced by thermal treatments, are very promising materials to be applied as catalysts or catalytic supports in various applications.

The design, the preparation, and the characterization of such materials represent very challenging fields due to the required multidisciplinary approach (physical-chemistry, analytical chemistry, material science, process engineering…) to answer to the various fundamental and applicative issues that restrain their implementation in existing or new processes.

In this context, the present chapter addresses to the use of chars issued from biomass feedstocks as environmental friendly catalysts or catalyst support. Investigation on the surface, structural, morphological and redox properties of the chars will be related to their catalytic behavior in various reactions. Moreover, the choice of biomass sources, the chars and catalysts preparation methods, as well as the chars' post-treatments will be considered. The chapter aims to elucidate the dependence of the catalytic performances in the various reactions to the physico-chemical properties of chars based catalysts and to identify the required characteristics indispensable for the future char's development in the heterogeneous catalysis field.

Chars as catalysts' supports

Among the various catalyst supports, biomass derived chars are a versatile and environmentally friendly option. Some of the char features as the large specific surface area, the high porosity, the high electron conductivity and the possibility to relatively easily tailoring their surface chemical properties, make them particularly interesting as catalyst support. Even if the main role of a catalytic support is to efficiently disperse the active phase (generally metal or metal oxides), thus improving the catalytic sites accessibility and stability, other contributions of the support have also to be considered. The catalytic support can present intrinsic catalytic activity and the capacity to interact with other chemical components of the catalysts, thus creating new active sites. Moreover, the interaction between the support and the active phase can produce synergetic effects that positively contribute to the overall catalytic activity. Different biomass sources can be used to produce chars by pyrolysis performed at different temperatures and with sub-stochiometric oxygen content. The cellulose, hemicellulose, and lignin content differs for the various biomass feedstocks thus producing chars with specific surface and morphological properties. Indeed, by modulating the thermal treatment temperature and increasing temperature rate [START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF][START_REF] Qian | Recent advances in utilization of biochar[END_REF][START_REF] Prati | Carbons from second generation biomass as sustainable supports for catalytic systems[END_REF], the three polysaccharides can be selectively decomposed giving rise to chars with different porous structure [START_REF] Keiluweit | Dynamic molecular structure of plant biomass-derived black carbon (biochar)[END_REF][START_REF] Yang | Characteristics of hemicellulose, cellulose and lignin pyrolysis[END_REF]. Moreover, the char production yield also strongly depends on the feedstock composition [START_REF] Qian | Effects of Biomass Feedstocks and Gasification Conditions on the Physiochemical Properties of Char[END_REF][START_REF] Aqsha | Catalytic pyrolysis of straw biomasses (wheat, flax, oat and barley) and the comparison of their product yields[END_REF].

Certain specific properties (that will be detailed in the next sections) derive from the parent biomass used to produce the char, and might have a strong impact on the catalytic properties of the final material. As an example, Lobos et al. [START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF] showed that chars derived from fast pyrolysis of cellulose at relatively low temperature (350 °C) present high carbon content that is a positive characteristic, when the char is used as catalyst support for electro-oxidation reactions. Similar observations have been reported [START_REF] Muley | A critical comparison of pyrolysis of cellulose, lignin, and pine sawdust using an induction heating reactor[END_REF] on samples obtained by fast pyrolysis of lignin that displayed a higher amount of ash and fixed carbon when compared to those obtained by sawdust and cellulose. The char yield diminished for all biomasses when the temperature was increased from 500 °C to 700 °C. However, the cellulose char yields decrease was much lower than those of the other two biomasses. This indicates that cellulose is more thoroughly broken down at lower reaction temperatures than lignin and sawdust. Moreover, the morphology of the obtained chars can be characterized by enhanced BET surface area and porosity, in dependence of the parent biomass source [Liu, 2015a]. Since long time, the decisive role of the carbon surface chemistry on the catalytic behavior has been recognized [START_REF] Matos | Porous carbon: A versatile material for catalysis[END_REF]Rodriguez-Reinoso, 1998]. More recently, biomass derived chars have been identified as possible substitute to conventional carbons [START_REF] Xiong | A review of biochar-based catalysts for chemical synthesis, biofuel production, and pollution control[END_REF] due to the possibility to tune their surface chemistry and porosity to match the required features for specific catalytic applications. The main properties characterizing the chars as catalyst or catalyst support are described as follows.

Acid/Base properties

Surface acid base properties are the key factors that govern the adsorption ability of solid materials towards the different molecules and that can control their performances in various applications, like separation processes and catalysis.

The surface chemistry, and consequently the acid-base properties, of chars is related to the presence of heteroatoms like oxygen, nitrogen, hydrogen, phosphor, and sulfur on the surface. These elements can be introduced from the biomass precursor, during an activation process or by posttreatment on the already formed char. Interacting with the carbon surface, the heteroatoms can then form surface complexes (showed in Fig. 1 [START_REF] Figueiredo | Functionalization of porous carbons for catalytic applications[END_REF]) that characterize the basic, acid or neutral features of the solid material. Surface functionalities as carboxylic acid, phenols, anhydrides, carboxylic, lactones, lactols, and phenols are acidic, while carbonyl and ether functionalities are neutral. On the other hand, chromene, quinone, pyrone and nitrogen-containing groups are basic functionalities [START_REF] Figueiredo | Functionalization of porous carbons for catalytic applications[END_REF][START_REF] Rivera-Utrilla | Activated carbon modifications to enhance its water treatment applications. An overview[END_REF][START_REF] Shafeeyan | A review on surface modification of activated carbon for carbon dioxide adsorption[END_REF].

The adsorption properties that are correlated to the catalyst selectivity, are strongly dependent on the surface acid-base properties of the char used as catalyst support. The surface chemistry of chars can be modified by various treatments in order to create new basic/acid sites, by modifying the nature and the quantity of the surface functionalities. Often, the acidic character is enhanced by oxidative treatments of the chars that bring to the increasing of the amount of oxygen-containing surface groups. The treatments can be carried-on in liquid [START_REF] Belhachemi | Preparation of activated carbon from date pits: Effect of the activation agent and liquid phase oxidation[END_REF][START_REF] Rodríguez-Reinoso | Effect of oxygen surface groups on the immersion enthalpy of activated carbons in liquids of different polarity[END_REF][START_REF] Moreno-Castilla | Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments[END_REF][START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF], Belhachemi 2014[START_REF] Limousy | Amoxicillin removal from aqueous solution using activated carbon prepared by chemical activation of olive stone[END_REF] or gas phase [START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF][START_REF] Chiang | Ozonation of AC and its effects on the adsorption of VOCs exemplified by methylethylketone and benzene[END_REF][START_REF] Valde´s | Effect of Ozone Treatment on Surface Properties Activated Carbon[END_REF][START_REF] Ghouma | Activated carbon prepared by physical activation of olive stones for the removal of NO2 at ambient temperature[END_REF] with various oxidizing agents. The most common reactants, used in gas phase, are oxygen, ozone, nitrogen oxides, while, in liquid phase, nitric acid and hydrogen peroxide are those more widely applied. The choice to perform a pretreatment in gas or liquid phase is connected both to the desired type of oxygen-containing functionalities and to the extent of the modification of the textural properties of the char. Indeed, the oxidation in liquid phase is generally performed at more moderate reaction conditions (lower temperature and shorter reaction time) than those carried out in gas phase. The liquid phase treatment brings to the creation of surface oxygen functionalities like carboxylic groups, without having a strong impact on the textural properties (in particular the char porosity [START_REF] Moreno-Castilla | Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments[END_REF]). Differently, during gas phase oxidation treatments (that created only low amount of hydroxyl and carbonyl groups), the reaction time and temperature are more extreme, and can deeply modify the char porous structure by opening, extending, and creating pores [START_REF] Samant | Surface activation of a polymer based carbon[END_REF]. As an example of liquid phase treatment, [START_REF] Moreno-Castilla | Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments[END_REF] reported on the treatment of a series of activated carbons issued from almond shells by using three different chemicals (HNO 3 , H 2 O 2 , and (NH 4 ) 2 S 2 O 8 ). They noticed that, if carboxyl groups were created in each case, ether and ketone groups were magnified after treatment with hydrogen peroxide. Moreover, the (NH 4 ) 2 S 2 O 8 treatment surprisingly originated stronger acidity than that observed after HNO 3 treatment.

Concerning the gas-phase treatments, and in order to improve the catalytic performances of woody chars, Ducousso et al. [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF] performed a gas-phase treatment with oxygen. Even if the process diminished the overall quantity of carboxylic groups (that are the strongest acid sites), oxygenation promoted the formation of weak acid sites, but did not have any impact on the amount of basic sites like pyrone and quinone functionalities (as shown in Fig. 2). [START_REF] Ducousso | Reactivity enhancement of gasification biochars for catalytic applications[END_REF]. Reactivity enhancement of gasification biochars for catalytic applications, Fuel, 159, 491-499. Copyright 2018, Elsevier) Among the others, Chiang et al. increased the phenolic and carboxylic groups on the surface of an activated carbon issued from Coconut shells by treatment with ozone at room temperature [START_REF] Chiang | Ozonation of AC and its effects on the adsorption of VOCs exemplified by methylethylketone and benzene[END_REF]. Ozone treatment of chars was also used by [START_REF] Kastner | Catalytic esterification of fatty acids using solid acid catalysts generated from biochar and activated carbon[END_REF] to introduce weak phenolic and lactonic acid sites. Besides the treatment with oxidizing agents, sulfonation is another effective method to create acid catalyst and catalyst supports. By treating the chars with H 2 SO 4 and SO 3 , strong surface sulfonic acid sites and weak carboxyl and phenolic hydroxyl acid groups were created, as reported by the same authors [Kastner, 2012]. More recently, the acidity of chars, used as catalysts for the transformation of cellulose to 5-hydroxymethylfurfural, was enhanced by using ionic liquids. The chlorine-based ionic liquid has been graft on sulfonated chars, to allow the substitution of the chlorine by fluorine through ionic exchange with F-containing acids [Zhang, 2017].

Fig. 2. Oxygenated groups distribution in atom% of raw chars (a) and chars oxygenated 8 h at 280 °C (b) (Reprinted with permission from

Even if less widely used, basic catalyst can be applied in specific reactions as aldolization. Two types of basic sites can be identified on chars. The first kind of basic sites is related to the presence of certain specific functionalities, as chromene, ketone, pyrone, and amines while the second is associated to the presence of oxygen-free Lewis basic sites on the graphene layers.

To improve the basic character of chars, both heat and chemical treatments can be applied. The heat treatment consists in processing the sample at high temperature under inert (i.e. helium, nitrogen) or reducing gases (i.e. hydrogen); the acidic surface groups are selectively eliminated, thus enhancing the basicity ratio. Nitrogenation to form amine groups is the chemical treatment generally applied to increase the chars' surface basicity. Once exposed to high temperature for long time, the formed amine is transformed into amide, then to imine and then to imide. Finally, imide give origin to nitrile and pyridine species (see Fig. 1) [START_REF] Mangun | Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia[END_REF][START_REF] Kapteijn | The development of nitrogen functionality in model chars during gasification in CO2 and O2[END_REF][START_REF] Pietrzak | XPS study and physico-chemical properties of nitrogen-enriched microporous activated carbon from high volatile bituminous coal[END_REF]. As an example, activated carbons issued from olive stones and date pits were treated at high temperature under helium [Silvestre-Albero, 2009] and nitrogen [START_REF] Belhachemi | Effect of Heat Treatment on the Surface Properties of Activated Carbons[END_REF] flows, respectively; in both cases the strongest carboxylic acidic groups were removed resulting in an improved basic character.

The modification of the surface acidity/basicity of biomass derived chars by metals/metal oxides addition was not yet widely investigated and put in practice as other activation methods like oxidation, sulfonation, … previously described. Indeed, to share between the carbon surface species contribution and the metal/metal oxide sites contribution to the surface acid/base character of a char-based composite is not easy task. However, some authors identified the possibility to create Lewis sites by metal impregnation. Such objective was reached by Liu et al. [Liu, 2015b] when preparing SnO 2 -Co 3 O 4 /C catalysts used in corncob hydrolysis into furfural. The authors showed that the acid properties of the char support can be modified when Sn 4+ and Co 2+ species are introduced in the material; as shown by pyridine adsorption infrared spectroscopy, the Brønsted acid sites were converted into Lewis acid sites (Fig. 3).

Fig. 3. Formation of Lewis acids: comparison of char (II) and SnO 2 -Co 3 O 4 /C catalyst (III) through

FT-IR spectra after pyridine adsorption (Reprinted with permission from Liu, Q.Y., [START_REF] Liu | Preparation of SnO2-Co3O4/C biochar catalyst as a Lewis acid for corncob hydrolysis into furfural in water medium[END_REF]. Preparation of SnO2-Co3O4/C biochar catalyst as a Lewis acid for corncob hydrolysis into furfural in water medium. J. Ind. Eng. Chem. 26, 46-54, Copyright 2018, Elsevier).

Porosity/Texture

The textural properties are very important for char reactivity. They depend on the char source and the preparation method and can be improved by chemical treatment, gas activation, and metal impregnation.

It is well known that the biomass precursor has a great influence on the texture and porosity of the elaborated char [START_REF] Guerrero | Pyrolysis of eucalyptus at different heating rates: studies of char characterization and oxidative reactivity[END_REF][START_REF] Kambo | A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications[END_REF]Xiong;2017;[START_REF] Lee | Biochar as a catalyst[END_REF]. In fact, a great variety of char sources can be considered, from agricultural residues to sewage sludge with various components (cellulose, hemicellulose and lignin) distribution. However, the thermal liability of these components being different [START_REF] Yang | Characteristics of hemicellulose, cellulose and lignin pyrolysis[END_REF], their pyrolysis creates several molecular arrangements [Abdullah, 2017] including cross-linking, depolymerisation and fragmentation [START_REF] Cha | Prododuction and utilization of biochar: A review[END_REF], leading to various crystallinities [START_REF] Keiluweit | Dynamic molecular structure of plant biomass-derived black carbon (biochar)[END_REF].

Depending on the nature of the biomass source, different pore sizes ranging from the micro-porosity to the meso-and macro-porosity can be obtained [START_REF] Dabrowski | Adsorption of phenolic compounds by activated carbon-a critical review[END_REF]. Several authors reported on the textural properties of chars [START_REF] Streat | Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons[END_REF][START_REF] Ioannidou | Agricultural residues as precursors for activated carbon production-A review[END_REF][START_REF] Hecke S | Production and characterization of slow pyrolysis biochar: influence of feedstock type and pyrolysis conditions[END_REF][START_REF] Lee | Biochar as a catalyst[END_REF] Currently, chars can be produced by several techniques. However, the selection of the suitable preparation method is depending on the feedstock nature (wet or dry), and the required properties of char for the specific application [START_REF] Kambo | A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications[END_REF]]. The more common preparation methods are slow pyrolysis, under limited oxygen or oxygen free environment (char/pyrochar) and hydrothermal carbonization (Table 2), under high pressure (hydrochar [START_REF] Liu | Characterization and application of chars produced from pinewood pyrolysis and hydrothermal treatment[END_REF]). The char preparation method has a great influence on the char porosity and morphology. In fact, conditions such as the reactive atmosphere, the temperature, the pressure, and the treatment duration can vary and lead to rough surface and porous structure, through different reactions [START_REF] Liu | Characterization and application of chars produced from pinewood pyrolysis and hydrothermal treatment[END_REF]] and the formation of different intermediate compounds [START_REF] Kambo | A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications[END_REF]].

Influence of char chemical treatment

Chemical activation can be differentiate in pre-pyrolysis and post-pyrolysis type treatments, i.e. a chemical treatment applied to the parent biomass or to the char, respectively. In the former, the raw material (generally wood or peat) is impregnated by a dehydrating reagent such as H 3 PO 4 , ZnCl 2 or KOH (or K 2 CO 3 ), then the impregnated char is pyrolyzed and washed to remove the activating agent. In the latter method, chars or hydrochars are impregnated by (or mixed with) an acid (H 3 PO 4 , H 2 SO 4 , ZnCl 2 ) or, more frequently, a base (KOH, NaOH), then washed to remove the excess of activating agent, and dried and/or pyrolyzed again.

Numerous studies have reported the interest of pre-treatment procedures able to improve the textural properties of various biomass derived chars by treatment with H 3 PO 4 (Table 3), ZnCl 2 (Table 4), KOH (Table 5), K 2 CO 3 (Table 6).

Deng

The post-treatment (on char or hydrochars instead of that performed on the biomass) is easier to put in place (smaller volumes and amount of minerals), then several researchers preferred this procedure to improve the textural properties of chars or hydrochars produced from various biomasses by acidic or basic treatments

The acidic treatments performed after pyrolysis (i.e. on char or hydrochar) are generally used to modify the char surface acidity and have rarely a positive effect on their textural properties (Table 7). The basic treatments performed after pyrolysis are used to improve the textural properties of chars and hydrochars and are associated to a second thermal treatment, generally performed under nitrogen between 600 and 850 °C (pyrolysis).

Influence of char physical (gas) activation (during post synthesis treatment)

Pyro/hydrochars can be physically activated after their preparation. Various physical activation procedures exist including steam and CO 2 activations as well as their mixture.

Steam activation was for the first time performed by using a stream of moisture laden nitrogen [START_REF] Streat | Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons[END_REF] after the carbonization step. High BET surface areas were obtained (1000-1700 m 2 g -1

for coconut shell and wood char, respectively) after an activation at high temperature (800-900 °C).

It was particularly highlighted the improvement of the micropore volume percentage [START_REF] El-Hendawy | Adsorption characteristics of activated carbons obtained from corncobs, Colloids and Surfaces A: Physicochem[END_REF]. The positive effect of steam activation (under various steam contents) was largely confirmed on various char, as sewage sludge and rice straw [START_REF] Cha | The low temperature SCR of NO over rice straw and sewage sludge derived char[END_REF], broiler litter manure [START_REF] Uchimiya | Contaminant imobilization and nutrient release by biochar soil amendment: Roles of natural organic matter[END_REF], brown alga [START_REF] Cho | Removal characteristics of copper by marine macro-algae-derived chars[END_REF], mulberry twig [START_REF] Shu | Mercury adsorption of modified mulberry twig chars in a simulated flue gas[END_REF], miscanthus [START_REF] Shim | Effect of steam activation of biochar produced from a giant Miscanthus on copper sorption and toxicity[END_REF], and burcumber and tea waste [START_REF] Rajapaksha | Steam activation of biochars facilitates kinetics and pH-resilience of sulfamethazine sorption[END_REF].

The influence of biomass type was studied [Gonzales, 2009] on walnut shell, almond tree pruning, almond shell, and olive stone (Table 8). The steam activation increases usually the BET surface area, but leads to different micropore extents. In fact, the activated char from almond tree pruning presented broader pore size distribution (greater volumes of meso-and macropores) than the activated chars derived from other residues, leading to highly microporous carbons (Table 8). This different behavior seems to be linked to the higher lignocellulosic composition of almond tree pruning and to the higher hemicellulose fraction of almond shell. The thermal decomposition rate of the biomass, depending on the lignocellulosic fraction, plays a determinant role on the porosity of the activated carbon produced. The pyrolysis temperature appears to have no effect [START_REF] Uchimiya | Contaminant imobilization and nutrient release by biochar soil amendment: Roles of natural organic matter[END_REF]] below 700 °C, but to be an important parameter [Iwasaki 2010] at temperature higher than 900 °C, leading to a decrease in the BET surface area and micropore volume percentage.

The influence of the steam activation temperature was reported by different authors [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF][START_REF] El-Hendawy | Adsorption characteristics of activated carbons obtained from corncobs, Colloids and Surfaces A: Physicochem[END_REF][START_REF] Azargohar | Steam and KOH activation of biochar: Experimental and modeling studies[END_REF]] that identified an optimum temperature for obtaining the highest BET surface area and percentage of micropore volume. This parameter appears to be related to the mass ratio S/C and to the activation time [START_REF] Azargohar | Steam and KOH activation of biochar: Experimental and modeling studies[END_REF]] (Fig. 4). The effect of the steam activation duration is not simple to understand. Indeed, high BET surface area and total pore volume depend on both temperature and duration time of the steam activation step, and to share between the two contributions is not easy matter [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF]Fan, 2004;[START_REF] Youssef | Textural properties of activated carbons from apricot stones, Colloids and Surfaces A: Physicochem[END_REF][START_REF] Azargohar | Steam and KOH activation of biochar: Experimental and modeling studies[END_REF][START_REF] Ncibi | Preparation and characterisation of raw chars and physically activated carbons derived from marine Posidonia oceanica (L.) fibres[END_REF][START_REF] Buah | Properties of activatd carbon prepared from coconut shells in Ghana[END_REF].

Few studies examine the CO 2 activation of chars [START_REF] Muradov | Production and characterization of Lemna minor bio-char and its catalytic application for biogas reforming[END_REF] for catalysis application. The increase of activation temperature up to 900 °C was reported [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF][START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF] to be positive on the BET surface area and total pore volume.

The effect of CO 2 activation duration was also studied [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF][START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF] and reveals a negative effect in the majority of cases [START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF], but an optimum value of about 1 h was determined in other cases [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF].

With respect to the micropore volume percentage, it appears that this fundamental property also depends on the activation temperature and time [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF][START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF], on the type of biomass used (corncobs [START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF], oak, corn hulls, and corn stover [START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF]). The influence of CO 2 activation was also investigated [START_REF] Hao | Activated carbons prepared from hydrothermally carbonized waste biomass used as adsorbents for CO2[END_REF] on various hydrochar types (horse manure, grass cuttings, beer waste, and bio-sludge). Authors reported different behaviors: the grass cuttings led to the higher BET surface area and micropore volume, the horse manure to the best total pore volume and the lowest micropore volume percentage, beer waste and bio-sludge showed intermediate behaviours.

The comparison of these two procedures [Rodriguez-Reinoso, 1995;[START_REF] Chang | Effects of Burn-off and Activation Temperature on Preparation of Activated Carbon from Corn Cob Agrowaste by CO 2 and Steam[END_REF] indicates that the activation rate is higher for steam activation, whereas the specific surface area and pore size were higher for CO 2 activation. However, a difference was observed [Rodriguez Reinoso, 1995] due to the formation of a lower micropore volume with steam than with CO 2 treatment.

Redox

Biomass derived char displays higher redox properties than the parent biomass, and its redox properties may originate from both organic and inorganic constituents. Klupfel et al. [Klupfel, 2014] performed a deep study on the redox properties of various chars prepared from grass and woody biomasse, through the variation of pyrolysis thermosequence and temperature. he redox behavior of the obtained chars was then systematically investigated by means of mediated electrochemical analysis. The chars obtained at the highest pyrolysis temperature were those showing enhanced electron donor/acceptor properties. Phenolic groups were identified to be the main electron donating species, while quinones and polycondensed aromatic groups were the more oxidants (electron acceptor) [Klupfel, 2014;Yuan, 2017]. Yuan et al. [Yuan, 2017], in a review article, summarized the different chars synthesis methodologies and some specific technics for analyzing the redox properties. The redox sites present on the char prepared at different pyrolysis temperatures are schematized in Fig. 5.

As previously described, the surface functionalities present on the chars' surface can affect the synthesis of supported catalysts. The group of Loulouidi [START_REF] Simaioforidou | Activated vs. pyrolytic carbon as support matrix for chemical functionalization: Efficient heterogeneous nonheme Mn(II) catalysts for alkene oxidation with H2O2[END_REF] compared the immobilization of non-heme Mn(II) complex onto commercial activated carbons (Mn II -L 3 imid@Cox) and pyrolytic carbon char (Mn II -L 3 imid@PCox). The catalyst prepared on the pyrolytic char presented equivalent performances (in terms of selectivity towards epoxides) than that prepared on commercial activated carbon, in the alkene oxidation reaction by H 2 O 2, in presence of CH 3 COONH 4, and even higher kinetics (higher Turnover Frequency, Fig. 8), showing the promising application of chars as redox catalyst support.

Chars as catalysts

Catalysts porous properties are fundamental in catalysis, where the internal diffusion of the reactive molecules (favorited by an enhanced porosity) and the amount of accessible active site (on the catalyst surface) are crucial. In this optic, the ash content can also have a big impact on the catalytic activity. In particular, certain accumulator plants [START_REF] Lu | Effects of mutual intercropping on the cadmium accumulation in accumulator plants Stellaria media, Malachium aquaticum, and Galium aparine[END_REF], once pyrolyzed, give rise to chars that naturally contain metal oxides dispersed in the carbon structure and that can potentially act as catalyst in various reactions. More commonly, the percentage of inorganics as sodium, magnesium, calcium, potassium, silicium, phosphorus, concentrated in the chars after pyrolysis, strongly depends on the biomass source [START_REF] Carpenter | Biomass feedstock for renewable fuel production a review of the impacts of feedstock and pretreatment on the yield and produc distribution of fast pyrolysis bio-oils vapors[END_REF], and they can act as active sites in the field of biomass valorisation for chemicals production [START_REF] Xiong | A review of biochar-based catalysts for chemical synthesis, biofuel production, and pollution control[END_REF], bio-oils upgrading [START_REF] Li | Biodiesel production from waste cooking oil using a heterogeneous catalyst from pyrolyzed rice husk[END_REF], tar reforming [Anis, 2011;[START_REF] Bhandari | Synthesis and evaluation of biochar-derived catalysts for removal of toluene (model tar) from biomass-generated producer gas[END_REF][START_REF] Klinghoffer | Influence of char composition and inorganics on catalytic activity of char from biomass gasification[END_REF], hydrogen production [Yao, 2016], but also in electrochemical conversion and in a less extent in pollution control as in the Selective Catalytic Reduction of nitrogen oxides (SCR-NOx) [START_REF] Wu | Novel application of biochar from biomass pyrolysis for low temperature selective catalytic reduction[END_REF].

Preparation methods

Metal addition can be also performed before or after char synthesis (pre-synthesis and postsynthesis method, respectively) and can be achieved by impregnation, mixing or sol-gel methods.

Pre-synthesis metal addition

The simplest and widely used method for biomass upgrading as catalyst precursor is impregnation.

Ni (from nitrate) is often used as catalytic active phase, particularly for biomass pyrolysis and/or gasification [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF]Shen, 2014a;Shen, 2014b].

The pH of the solution used for impregnation is an important parameter to optimize both metal loading and dispersion on the biomass matrix [START_REF] Richardson | In situ generation of Ni metal nanoparticles as catalyst for H2-rich syngas production from biomass gasification[END_REF]. When the pH is close to the pH ZPC of char, nickel adsorption increases due to the negative charge density established on the biomass surface (wood chips) that favors the electrostatic attraction between the positive sorbate species and the adsorbent wood particles. The Ni crystallites size depends on nickel loading, and was smaller than 4 nm for Ni content lower than 10 wt% (Fig. 7). An impregnation method under reduced pressure (7 mbar) was also investigated [START_REF] Richardson | Catalytic Investigation of in Situ Generated Ni Metal Nanoparticles for Tar Conversion during Biomass Pyrolysis[END_REF]. This method permitted to generate strong biomass-metal cation interaction, thanks to the formation of inner-sphere complexes with the oxygenated functions at the lignocellulosic surface.

The same authors also investigated [START_REF] Richardson | Catalytic Investigation of in Situ Generated Ni Metal Nanoparticles for Tar Conversion during Biomass Pyrolysis[END_REF] the use of a microwave assisted polyol method to prepare Ni 0 char and to improve the catalytic properties. They succeeded to insert Ni 0 into the biomass feedstock by ultrasound treatment and then depressurization, during biomass impregnation.

Iron is another transition metal often used in tar reforming and then associated to char by impregnation method [Zhang, 2013a]. The catalytic activity for tar reforming was identified to be linked to iron concentration but not directly proportional to the surface area and pore volume.

Tin can be co-impregnated with transition metals as cobalt, copper or iron starting from their chloride or hydroxide forms [Liu, 2015b] or mixed to them [START_REF] Yu | Valorization of food waste into hydroxymethylfurfural: dual role of metal ions in successive conversion steps[END_REF] in order to catalyze the hydrolysis of biomass.

Post-synthesis metal addition

In order to obtain char based catalysts, impregnation of metal active phase is the most used method.

The applications of such catalytic materials are varied as the hydrodechlorination of chlorobenzene [START_REF] Wu | Complete hydrodechlorination of chlorobenzene and its derivatives over supported nickel catalysts under liquid phase conditions[END_REF]], the hydrogenation of benzene [START_REF] Wojcieszak | Study of nickel nanoparticles supported on activated carbon prepared by aqueous hydrazine reduction[END_REF], and, more recently, the hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) of bio-oil on microalgae char [START_REF] Nguyen | Preparation of Ni/biochar catalyst for hydritreating of bio-oil from microalgae biomass[END_REF]] or biomass gasification for H 2 production [Yao 2016]. Ni nitrate and acetate are the most common precursors [START_REF] Wojcieszak | Study of nickel nanoparticles supported on activated carbon prepared by aqueous hydrazine reduction[END_REF].

The effect of the support was studied by Wu et al. [START_REF] Wu | Complete hydrodechlorination of chlorobenzene and its derivatives over supported nickel catalysts under liquid phase conditions[END_REF] through the comparison of activated carbon derived from coconut shell (AC) with classical catalyst supports such as -Al 2 O 3 and SiO 2 in the chlorobenzene hydrodechlorination reaction. The high catalytic activity is due to the enhanced dispersion of nickel on the Ni/AC catalyst surface.

The effect of Ni salt (nitrate or acetate) was investigated in benzene hydrogenation by Wojcieszak et al. [START_REF] Wojcieszak | Study of nickel nanoparticles supported on activated carbon prepared by aqueous hydrazine reduction[END_REF]. Authors reported that the reduction of nickel species occurs at lower temperature for the catalyst prepared with nitrate (450 °C) compared to the one prepared with acetate (600 °C), indicating a better Ni-support (AC) interaction for the latter. At the contrary, the benzene conversion was largely higher for the catalyst prepared starting from the nitrate precursor (100% at 420 °C) than for the catalyst prepared from acetate (20% at the same temperature).

The interest of metal addition and the influence of biomass feedstock (wheat straw, rice husk, cotton stalk) was validated [Yao, 2016] for the production of H 2 from biomass. The addition of Ni metal appeared to be more important than the morphology and structure of the catalyst for H 2 and gas production, and also than the alkaline and alkaline earth metals (AAEM) content. In order to prepare a low cost catalyst for tar reforming, Wang et al. [START_REF] Wang | Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning[END_REF] proposed a mechanical mixing of wood char and NiO. The tar removal in biomass gasification was proportional to the Ni loading (below 20 wt%) and led to 33 vol% of H 2 in the produced gas with 15 wt% of Ni compared to 28 vol% without Ni.

Iron was often reported to upgrade chars to catalysts for various applications as dye removal [START_REF] Yang | Synthesis and properties of magnetic Fe3O4-activated carbon nanocomposite particles for dye removal[END_REF], phenol removal [Rey, 2009, Zhang;[START_REF] Shen | Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification[END_REF], Fischer-Tropsch synthesis [START_REF] Yan | Iron nanoparticles in situ encapsulated in biochar-based carbon as an effective catalyst for the conversion of biomassderived syngas to liquid hydrocarbons[END_REF], or tar reforming [START_REF] Kastner | Catalytic decomposition of tar using iron supported biochar[END_REF]. The impregnation of iron nitrate remains the more common method, but iron pentacarbonyl [START_REF] Rey | Catalytic wet peroxide oxidation of phenol over Fe/AC catalysts: Influence of iron precursor and activated carbon surface[END_REF] was also investigated as precursor. Iron pentacarbonyl permitted to obtain a homogeneous Fe distribution, but a catalyst with lower stability than that prepared with iron nitrate. However, rice husk pyrolytic char was impregnated [Zhang, 2015] with ferric nitrate solution, dried and then activated under N 2 atmosphere at various temperatures between 400 and 850 °C to study the impact of the temperature. The optimum activation temperature was identified at 750 °C leading to the highest BET surface area (480 m 2 g -1 ), to an improved iron oxide particle (C-Fe-750-1.0) distribution in the porous structure, and enhanced dispersion on iron oxide on the surface of the activated carbon (smooth surface) during the impregnation process.

Yan et al. [START_REF] Yan | Iron nanoparticles in situ encapsulated in biochar-based carbon as an effective catalyst for the conversion of biomassderived syngas to liquid hydrocarbons[END_REF] used a char from fast pyrolysis of pine wood as the support material to develop a carbon-encapsulated iron nanoparticles catalyst for the Fischer-Tropsch synthesis of biomass-derived syngas to liquid hydrocarbons. It presented very interesting results in terms of activity and stability due to the strong metal-carbon support interaction created during the thermal treatment at 1000 °C, and leading to the formation of very stable core-shell nanocomposites. The total conversion of toluene (studied as tar model compound) was reported by Kastner et al. [START_REF] Kastner | Catalytic decomposition of tar using iron supported biochar[END_REF] at 800 °C on Fe/chars containing more than 10 wt% of Fe, while only 25% of toluene conversion was obtained on the bare char. At lower temperatures (600 and 700 °C), the toluene conversion was proportional to the iron loading (up to 19 wt%). Iron and nickel were compared and their synergetic effect studied [Shen 2014b] in tar reforming. The highest performance (93% conversion) was obtained with Ni-Fe (equimolar)/char prepared by impregnation of Ni and Fe nitrates on rice husk derived char.

Manganese was also largely used in low temperature selective catalytic reduction (SCR) of NO.

The effect of Mn impregnation was different for the various studied chars (rice straw and sludge)

and the changes in specific surface area and pore volume were not dramatic [START_REF] Cha | The low temperature SCR of NO over rice straw and sewage sludge derived char[END_REF]. Cerium can be associated to Mn in char based-catalysts for low temperature SCR of NO [START_REF] Singh | Novel application of cotton stalk as a waste derived catalyst in the low temperature SCR-deNOx process[END_REF], by co-impregnation method. Chemical activation by H 3 PO 4 and physical activation by steam were also investigated to improve the char catalytic properties [START_REF] Singh | Novel application of cotton stalk as a waste derived catalyst in the low temperature SCR-deNOx process[END_REF][START_REF] Shen | Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification[END_REF].

Other metals such as Pt, Cu and Ru were studied in applications as the electrocatalytic oxidation of methanol and ethanol [START_REF] Fan | Bifunctional porous carbon spheres derived from pectin as electrode material for supercapacitors and support material for Pt nanowires towards electrocatalytic methanol and ethanol oxidation[END_REF][START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF]. Pt chloride was impregnated on a hydrochars derived from pectin [START_REF] Fan | Bifunctional porous carbon spheres derived from pectin as electrode material for supercapacitors and support material for Pt nanowires towards electrocatalytic methanol and ethanol oxidation[END_REF]. Ru chloride and Cu sulphate were co-impregnated on chars derived from untreated and acid treated cellulose [START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF].

In the photocatalytic degradation field, the association of TiO 2 (from titanium isopropoxide [START_REF] Kim | Heterogeneous photocatalytic degradation of sulfamethoxazole in water using a biochar-supported TiO2 photocatalyst[END_REF] or tetrabutylorthotitanate [START_REF] Li | The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method[END_REF]) and char was performed by sol-gel method. The comparison with TiO 2 alone proved that the char/TiO 2 present a much higher adsorption of sulfamethoxazole in water than the commercial TiO 2 powder, due to the hydrophobic interaction between the char and the sulfamethoxazole [START_REF] Kim | Heterogeneous photocatalytic degradation of sulfamethoxazole in water using a biochar-supported TiO2 photocatalyst[END_REF]. The spherical-shaped TiO 2 particles, welldispersed in the char matrix, and the low crystallite size of TiO 2 (below 40 nm) creates anticalcination effects [START_REF] Li | The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method[END_REF] (Fig. 8). from [START_REF] Li | The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method[END_REF]. The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method. Appl. Surf. Sci. 253, 9254-9258, Copyright 2018, Elsevier.) Shen et al. [Shen, 2014a] proposed a comparison of pre-and post-synthesis addition of Ni (nitrate) by impregnation of rice husk and rice husk char. They concluded that the BET surface area of the catalyst prepared by post-synthesis was largely lower (87 m 2 g -1 ) than that prepared by pre-synthesis (135 m 2 g -1 ) for the same Ni content. Furthermore, the addition of NaBH 4 in the impregnation solution increases the BET surface area, improves the char porosity and promotes the generation of metallic nickel (Ni 0 ) from the reduction of nickel oxides. Finally, the catalyst prepared by presynthesis in presence of NaBH 4 led to the best textural properties and reactivity in tar reforming, particularly for the main organic compounds as toluene, benzene, naphthalene, phenol,…

Reactions

Biochemical conversion (valorisation) of biomass

The synthesis of various platform chemicals and biofuels (Fig. 9) is largely studied from lignocellulosic biomass (instead of fossil resources) to meet the energy, food, and materials needs for a rising global population. 246, 254-270, Copyright 2018, Elsevier.) Valorisation of biomass or its biopolymers (cellulose, hemicellulose and lignin) needs several

Solid acids, solid bases catalysts

Metal, alloys catalysts transformation reactions including hydrolysis, isomerisation, dehydration, or rehydration, hydrogenation, and oxidation that can be performed with biomass derived char-based catalysts.

Hydrolysis

Large efforts have been deployed to develop hydrolysis processes of glucose from cellulose, such as catalysis with mineral acids (sulfuric acid, hydrogen chloride and phosphoric acid), enzymatic hydrolysis, hydrolysis in supercritical water and solid catalysts. Hydrolysis with sulfuric acid is the only process developed in relatively large scale, due to the lower cost and higher reaction rate comparing to the enzymatic hydrolysis and other treatments. The replacement of strong acids by solid acidic catalysts (such as zeolites, solid superacids, sulphonated carbon nanostructures, sulphated zirconia) was largely studied due to many advantages such as the low corrosion power and the reusability [START_REF] Lazaridis | d-Glucose hydrogenation/hydrogenolysis reactions on noble metal(Ru, Pt)/activated carbon supported catalysts[END_REF].

The catalytic hydrolysis of cellulose with carbon solid acids prepared by sulfonation of partially carbonized crystalline cellulose is similar to saccharification using concentrate H 2 SO 4 [START_REF] Yamaguchi | Hydrolysis of Cellulose by a Solid Acid Catalyst under Optimal Reaction Conditions[END_REF]. The apparent activation energy for the hydrolysis of cellulose into glucose using the carbon based catalyst is smaller than for sulfuric acid [START_REF] Suganuma | Hydrolysis of Cellulose by Amorphous Carbon Bearing SO3H, COOH, and OH Groups[END_REF]. The carbon catalyst can hydrolyse not only pure crystalline cellulose, but also natural lignocellulosic reactants, such as eucalyptus or straw tips, in the same way that mineral acid catalysts [START_REF] Yamaguchi | Hydrolysis of Cellulose by a Solid Acid Catalyst under Optimal Reaction Conditions[END_REF]. Strong acidic SO 3 H groups bonded to the carbon material functions are active sites capable to break the strong hydrogen bonds and hydrolyze the -1,4 glycosidic bonds of cellulose or water soluble -1,4 glucan; a good conversion of cellulose into glucose was obtained with these catalysts [START_REF] Yamaguchi | Hydrolysis of Cellulose by a Solid Acid Catalyst under Optimal Reaction Conditions[END_REF].

Biomass derived char activated by chemical treatment (by H 2 SO 4 ) showed enhanced catalytic performances compared to sulfuric acid [Li 2013], with higher hydrolysis activity and xylose and glucose selectivity. The char solid acid also generated less soluble lignin derived products than sulfuric acid. These catalytic performances are attributed to the ability of the material to adsorb -1,4 glucan. Furthermore, the char solid acid catalyst can be readily separated from the saccharide solution after reaction to be reused in the reaction without loss in activity [START_REF] Suganuma | Hydrolysis of Cellulose by Amorphous Carbon Bearing SO3H, COOH, and OH Groups[END_REF]. A char solid acid catalyst (peanut hulls or pine wood char chemically activated by H 2 SO 4 ) was compared to a wood based activated carbon [START_REF] Ormsby | Hemicellulose hydrolysis using solid acid catalysts generated from biochar[END_REF]. The char presented a specific surface area significantly lower than the activated carbon (365 vs. 1391 m 2 .g -1 , respectively), but a hydrolysis rate significantly higher (85% conversion of xylan for the char and 57% for the activated carbon).

Compared to a commercially sulfonated styrene-divinylbenzene macroreticular resin (Amberlyst 15

Resin), with relatively low surface (38 m 2 .g -1 ) area and limited pore structure, char hydrolysis rate was significantly higher at low temperature (90-110°C), because of the lower adsorption capacity of Amberlyst 15 Resin for xylan. Regarding the hydrolysis of cellobiose [START_REF] Couto Fraga | Biomass derived solid acids as effective hydrolysis catalysts[END_REF], it was reported a relationship between the concentration of sulfonic acid groups and the higher activity of char chemically activated by H 2 SO 4. The hydrolysis of holocellulose (cellulose and hemicelluloses) conversion into sugars was investigated [Wang, 2016], using sulfonated char catalysts derived from microcrystalline cellulose, hemicellulose, lignin and lignin-rich residues. The sulfonated char catalysts, derived from lignin and residues, possess a graphene-like fused ring structures, with -SO 3 H, -COOH and phenolic -OH groups. The synergetic combination of the fused ring structures and multifunctional groups in the sulfonated char catalysts are identified to be linked to the remarkable hydrolysis performance of holocellulose.

Isomerisation

Xylose/xylulose and glucose/fructose are important chemical intermediates [Zhang, 2016a]. The isomerisation of xylose or glucose was largely explored with solid Lewis acids, hydrotalcite solid bases, and metallosilicate solid bases. The use of char for this step is very little reported, but it was demonstrated [Liu, 2015b] that in sulfonated char, and mineral-impregnated (SnO 2 -Co 3 O 4 ) char catalyst, the multivalent metal (Lewis site) facilitates the isomerization of xylose, and the Brønsted acidity promotes the hydrolysis of glycosidic bond.

Dehydration

The dehydration step (for the production of furfural or derivatives products) of C5-C6 sugars is conventionally catalysed by liquid acids (such as H 2 SO 4 , HCl, H 3 PO 4 , oxalic, and maleic acid, in aqueous solution), but they cause corrosion, toxicity, and difficulties for separation and product recovery [Zhang, 2016]. Solid acid catalysts (such as zeolites, sulfonic acid derivative solid acids, sulfated metal oxides, metal phosphates, composite metal and non-metal oxides, and heteropolyacids) are then largely studied as alternative. The dehydration step was recently studied with sulfonated char for xylose dehydration to furfural [START_REF] Sairanen | Functionalized Activated Carbon Catalysts in Xylose Dehydration[END_REF], or on a liquor of prehydrolysed corncob [START_REF] Deng | A feasible process for furfural production from the pre-hydrolysis liquor of corncob via biochar catalysts in a new biphasic system[END_REF], and for the dehydration of fructose to 5-hydroxymethylfurfural (HMF) [Zhao, 2016]. Furfural selectivity level was found to be higher over the functionalised carbon catalyst (chemically activated by H 2 SO 4 and/or HNO 3 ) compared to other solid acid catalysts such as TiO 2 and H-mordenite zeolite [START_REF] Sairanen | Functionalized Activated Carbon Catalysts in Xylose Dehydration[END_REF]. The combination of oxygencontaining and sulfur-containing surface sites on carbon seems to be the reason of the highest furfural selectivity. A sulfonated carbon based solid acid catalyst bearing -SO 3 H, -OH and -COOH groups synthesized by a two-step hydrothermal method [Zhao, 2016] exhibited a high acid density that accelerated the reaction and improved the selectivity to HMF. An excessive amount of acid sites and high temperature resulted in the slight decrease in HMF selectivity due to the further condensation of HMF into by-products. The effectiveness of sulfonated char catalyst was demonstrated [START_REF] Cao | Production of 5-hydroxymethylfurfural from starch-rich food waste catalyzed by sulfonated biochar[END_REF], directly correlated to the density of strong/weak Brønsted acidity (-SO 3 H, -COOH, and -OH groups), and inversely correlated to humins content on the surface.

Rehydration

Rehydration is an important step leading to levulinic acid, a key platform chemical of polymers and fuels [START_REF] Xiong | A review of biochar-based catalysts for chemical synthesis, biofuel production, and pollution control[END_REF]. Even if char-based catalysts were not yet used for the production of levulinic acid, their modification to form acidic sites could be efficient for this application.

Hydrogenation

A metallic component (such as Raney Ni and Ru, Pt, Co, Cu, and Rh) can catalyse the hydrogenation of glucose, as well as the hydrogenolysis of glucose oligomers [START_REF] Lazaridis | d-Glucose hydrogenation/hydrogenolysis reactions on noble metal(Ru, Pt)/activated carbon supported catalysts[END_REF]Zhang, 2016a] to form sorbitol. Lazaridis et al. [START_REF] Lazaridis | d-Glucose hydrogenation/hydrogenolysis reactions on noble metal(Ru, Pt)/activated carbon supported catalysts[END_REF] studied Pt and Ru based micro/mesoporous activated carbon (Norit SAE SUPER) with various contents of metal and different metal pre-treatments (as reduction by H 2 or NaBH 4 ). Although both Ru and Pt based catalysts were very active, the Pt containing catalyst was also very selective toward sorbitol.

Ru/ZSM-5 and Pt/activated carbon appear to be the best catalysts for glucose and xylose hydrogenation [Zhang, 2016]. Pt was also associated to activated carbon cloths [START_REF] Perrard | Highly efficient metal catalysts supported on activated carbon cloths: A catalytic application for the hydrogenation of D-glucose to D-sorbitol[END_REF] by metal cation exchange. It exhibited high activity in D-glucose hydrogenation and higher yields in D-sorbitol (>99.5%) than the Pt catalysts prepared on carbon powder or extrudates.

The one-pot/direct transformation of cellulose into sugar alcohols (such as sorbitol, glycerol…) by combined hydrolysis/hydrogenolysis reactions is of great interest and needs bifunctional catalysts with acidic functionalities, for cellulose hydrolysis, and metallic active sites, for the hydrogenation of glucose and the hydrogenolysis of glucose oligomers [START_REF] Lazaridis | d-Glucose hydrogenation/hydrogenolysis reactions on noble metal(Ru, Pt)/activated carbon supported catalysts[END_REF].

Oxidation

Selective oxidation of glucose was widely reported on metal catalysts [Zhang, 2013c]. To improve the performance of Pd and Au catalysts, new supports such as metal oxides, polymers, zeolites, carbon, resin, cellulose…, supported biand tri-metallic alloys [Zhang, 2016a] have been investigated. Gold on activated carbon is recognised to be one of the more active catalyst for this reaction, showing high productivity, and easy recovery, compared to the biochemical route in presence of enzymes [START_REF] Comotti | Is the biochemical route always advantageous? The case of glucose oxidation[END_REF].

Bio-oils upgrading (esterification and transesterification)

The transesterification concerns the reaction between a triglyceride and an alcohol in the presence of a catalyst, which produces a complex mixture of fatty acid methyl esters and glycerol as follow (Eq. 1).

(Eq 1)

In the last decade, the transesterification of various oils was studied to produce biodiesel, with a benign environmental footprint. Consequently, many researches have been focused on solid base catalysts, such as calcium oxide, for transesterification of vegetable oils with methanol [START_REF] Kouzu | Active phase of calcium oxide used as solid base catalyst for transesterification of soybean oil with refluxing methanol[END_REF][START_REF] Dong | Two step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived biochar[END_REF]. During the reaction, CaO is transformed into calcium diglyceroxide, due to the combination with glycerol (a by-product), without loss in activity and stability. Calcium ethoxide was also proposed [START_REF] Liu | Calcium ethoxide as a solid base catalyst for the transesterification of soybean oil to biodiesel[END_REF] as catalyst for the transesterification of soybean oil to biodiesel with methanol and ethanol and was demonstrated to be better than CaO catalyst.

More recently, char with sulfonic acid groups was prepared as heterogeneous catalyst for transesterification of canola oil with methanol [START_REF] Yu | Development of biochar-based catalyst for transesterification of Canola oil[END_REF]. The reaction yield was dependent on both catalyst surface area and total acid density. Calcium oxide based catalyst, derived from palm kernel shell gasification residue [START_REF] Bazargan | calcium oxidebased catalyst derived from palm kernel shell gasification residues for biodiesel production[END_REF], permitted to valorise the most difficult fraction of the solid waste rich in calcium (50 wt% of Ca). The various biological sources for the preparation of CaO-catalysts and their applications in biodiesel production were reviewed by Shan et al. [START_REF] Shan | Catalytic applications of calcium rich waste materials for biodiesel: Current state and perspectives[END_REF]. A char physically activated by steam, then chemically activated by KOH, was also proposed [START_REF] Dhawane | Central composite design approach towards optimization of flamboyant pods derived steam activated carbon for its use as heterogeneous catalyst in transesterification of Hevea brasiliensis oil[END_REF] as catalyst for the transesterification of Hevea brasiliensis oil into biodiesel. The optimum conditions were identified to be 1.5 h at 350°C, for the physical activation of char, leading to the best surface and porosity properties (surface area of 812 m 2 .g -1 and total pore volume of 0.325 cm 3 .g -1 ). Then, the activated char was treated with various KOH/char ratios. The optimum catalyst loading (3.5 wt%) and methanol to oil ratio (1:15) were the most important parameters. A review of biomass-derived heterogeneous catalyst for sustainable biodiesel production was recently proposed [Abdullah, 2017]. It appears that heterogeneous catalysts, capable to overcome the problems connected to the use of homogeneous catalysts, were mostly derived from non-renewable resources and presented high cost and low stability. Recently, heterogeneous catalysts, derived from biological wastes, and that offer several advantages (renewable resources, non-toxic, reusable, high catalytic activity, stability in both acidic and basic conditions and high water tolerance properties),

were proposed to provide a greener synthesis route for biodiesel production. Thus, high biodiesel yield of 98.6% was obtained by using a peat char supported catalyst in the transesterification of palm oil [Wang, 2017]. This good result was related to the very high total basicity of the catalyst.

The transesterification of waste cooking oils was evaluated in a microwave reactor with a sulfonated char and the increase in -SO 3 H groups on the char surface were identified as the responsible for the higher FAME yield [Gonzales, 2017] as reported below (Table 9) where %FAME yield is the Fatty Acid Methyl Esters yield. The esterification of fatty acids can also be catalysed using solid acid catalysts as sulfonated chars or activated carbons [Kastner, 2012]. This study demonstrated that, even if the sulfonation of the char by H 2 SO 4 clearly increases the surface area and the porosity, the acid density is lower than that obtained by SO 3 sulfonation. Wood based activated carbon sulfonated with SO 3 leads to the highest esterification activity and stability, due to improved particle strength, enhanced hydrophobicity, high surface area and sulfonic acid group density. The activity loss detected for the sulfonated char was related to the water adsorption, the particle attrition and the strong acid site leaching. However, a sulfonated char was reported to be a stable catalyst for the pre-esterification of refined microalgae oil [START_REF] Dong | Two step microalgal biodiesel production using acidic catalyst generated from pyrolysis-derived biochar[END_REF], exhibiting higher catalytic activity than Amberlyst-15. The activities of green sulfonated carbon-based catalyst from carbonized coffee residue were also higher than that of Amberlyst-15 [START_REF] Ngaosuwan | A green sulfonated carbon-based catalyst derived from coffee residue for esterification[END_REF]. A high catalytic activity and catalyst stability for esterification were achieved with this high surface area catalyst which presented strong (-SO 3 H) and weak (carboxylic and phenolic) acid site densities.

The transesterification of vegetable oils and the esterification of free fatty acids can be catalysed on the same sulfonated catalyst, obtained with concentrated sulfuric or fuming sulfuric acid [START_REF] Dehkhoda | Biochar based solid acid catalyst for biodiesel production[END_REF]. The catalyst with the highest surface area and acid density (sulfonated for 15 h with fuming sulfuric acid) showed the highest catalytic activity in biodiesel production from canola oil and methanol. The catalyst surface area and porosity were optimised by chemical activation with KOH and sulfonation by fuming sulfuric acid [START_REF] Dehkhoda | Biochar-based catalyst for simultaneous reactions of esterification and transesterification[END_REF]. A solid acid catalyst was prepared by sulfonating pyrolyzed rice husk with concentrated sulfuric acid. It was then compared to typical solid acid catalyst (Amberlyst-15) in the simultaneous esterification and transesterification of waste cooking oil to produce biodiesel [START_REF] Li | Biodiesel production from waste cooking oil using a heterogeneous catalyst from pyrolyzed rice husk[END_REF]. The biomass based catalyst led to higher free fatty acid conversion and fatty acid methyl ester (FAME) yield with an excellent stability comparing to Amberlyst 15 (Fig. 10).

Thermochemical conversion (valorisation) of biomass

In order to convert biomass feedstocks into syngas, for the production of high added-value molecules (platform molecules, fine chemicals, biofuels, …), pyrolysis and gasification techniques have been developed during the last decades [START_REF] Ammendola | Effect of sulphur on the performance of Rh-LaCoO3 based catalyst for tar conversion to syngas[END_REF][START_REF] Nino | Biodiesel production from biomass gasification tar via thermal/catalytic cracking[END_REF]. During these processes, tar is also produced as by-product, and catalytic methods have been developed to convert it into valuable products or additional syngas.

Tar reforming/cracking (syngas production)

Catalytic tar reforming is used to transform the heterogeneous hydrocarbon mixture deriving from gasification and pyrolysis into syngas. In addition to conventional catalysts (as transition-metal catalysts, alkali-metal catalysts, and zeolites [START_REF] Guan | Catalytic steam reforming of biomass tar: Prospects and challenges[END_REF]Anis, 2011]), biomass derived chars have been also used as catalyst or catalyst support since it is possibe to obtain them into the same gasifier used for the reaction, thus considerably decreasing the costs [START_REF] Abu | Experimental comparison of biomass chars with other catalysts for tar reduction[END_REF][START_REF] Widayatno | Steam reforming of tar derived from Fallopia japonica stem over its own chars prepared at different conditions[END_REF]Li, 2016;[START_REF] Wang | Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning[END_REF].

Many authors have shown the high potential of bare chars in tar reforming. The minerals content and their distribution in the char are important features, contributing to the catalytic activity, as

shown by Klinghoffer et al [START_REF] Klinghoffer | Influence of char composition and inorganics on catalytic activity of char from biomass gasification[END_REF]. Zhang et al. [Zhang, 2016b] reported on the use of chars, produced by pyrolysis of mallee wood, as catalyst in tar reforming during the gasification process). The performance of two kinds of catalysts, char and acid washed char, were compared in a pilot plant. During this investigations, both samples showed a good activity in tar reforming, even if the acid-treated catalyst resulted to be slightly less performant than the raw char.

The carbon structure and morphology are also important characteristics for interpreting the char reforming catalytic activity. Song et al. [START_REF] Song | Effects of volatile-char interactions on in-situ destruction of nascent tar during the pyrolysis and gasification of biomass. Part I. Roles of nascent char[END_REF] compared the catalytic behavior of various chars, and focused their attention on the positive effect of the presence of defects and unstable chemical bonds in the chars structure that act as active sites able to anchor and transform tar.

Chars have been also successfully used as reforming catalysts' support. [START_REF] Syed-Hassan | Catalytic Steam Reforming of Biomass Tar Model Compound Using Nickel and Cobalt Catalysts Supported on Palm Kernel Shell Char[END_REF] prepared char-supported nickel and cobalt catalysts tested in the steam reforming of toluene, used as tar model molecule. The char precursor was constituted of palm kernel shell and its stability was shown to be good up to 850 °C, temperature at which the agglomeration of the metal particles, the loss of alkali and alkaline earth metal, and the coke deposition take part. Shen et al [Shen, 2014a] performed in-situ tar conversion with very high tar conversion (about 93%), during gasification by using a rice husk char as support for nickel and iron species.

Shen reported on tar removal by chars combining adsorption and catalytic cracking/reforming. A new integrated concept, in which saturated chars, modified during adsorption of heavy metals or tars, were then used as char-supported metallic catalysts, for tar conversion, was designed [Shen, 2015b].

Although the use of chars as catalyst supports is promising, many issues connected to the fast deactivation have to be urgently solved. In fact, during the reforming process the chars loose oxygen species due to ring condensation [START_REF] Min | Catalytic reforming of tar during gasification. Part IV. Changes in the structure of char in the char-supported iron catalyst during reforming[END_REF], and the metal species are then relocated and agglomerated. The metal dispersion then decreases, negatively impacting the catalytic activity. The active sites are also deactivated by the deposition of coke on the catalyst surface that poisons the metal sites, blocks the char pores, and reduces the surface area [START_REF] Syed-Hassan | Catalytic Steam Reforming of Biomass Tar Model Compound Using Nickel and Cobalt Catalysts Supported on Palm Kernel Shell Char[END_REF][START_REF] Bhandari | Synthesis and evaluation of biochar-derived catalysts for removal of toluene (model tar) from biomass-generated producer gas[END_REF]Shen, 2014a]

Hydrocarbons/oils reforming (hydrogen production)

A further step in biomass valorisation concerns the production of hydrogen, an energy vector that represents a sustainable alternative to fossil fuels. Hydrogen can be obtained by further reforming of the hydrocarbons and bio-oils produces during pyrolysis, gasification, and tar reforming processes.

Ma et al. [START_REF] Ma | Catalytic steam reforming of bio-oil model compounds for hydrogen-rich gas production using bio-char as catalyst[END_REF] reached high hydrogen concentration (up to 89 %) by using an alkali-and alkaline metallic-rich char as catalyst for the steam reforming of bio-oils, for the production of an hydrogen-rich gas. Another char type (a hydrochar obtained starting from a macroalgae, the Cladophora glomerata) was applied for the catalytic hydrothermal gasification of the same algae.

The presence of the hydrochar promoted the hydrogen production in the gas phase, when compared to the process carried on in absence of catalyst [START_REF] Safari | Hydrothermal gasification of Cladophora glomerata macroalgae over its hydrochar as a catalyst for hydrogen-rich gas production[END_REF]. Yao et al [Yao, 2016] cotton-derived char, showed the highest hydrogen production (around 64 % H 2 concentration in the outstreaming gas), while the rice-derived char was less active [Yao, 2016] (Fig. 11).

Fig. 11. Gas composition and gas yield of biomass with different Ni/char catalyst (Reprinted with

permission from [START_REF] Yao | Hydrogen production from biomass gasification using biochar as a catalyst/support[END_REF] Hydrogen production from biomass gasification using biochar as a catalyst/support, Bioresour. Technol. 216, 159-164., Copyright 2018, Elsevier.)

Electrochemical conversion

In the electrochemical conversion field, chars have been also studied as support to replace the high surface area carbon black originated from the fossil fuel industry, by a process with low efficiency (30% effective carbon conversion) and high energy consumption [START_REF] Guilminot | New nanostructured carbons based on porous cellulose: elaboration, pyrolysis and use as platinum nanoparticles substrate for oxygen reduction electrocatalysis[END_REF].

Char was used to support Pt or Fe species in proton-exchange membrane fuel cells (PEMFC) electrocatalysts [START_REF] Guilminot | New nanostructured carbons based on porous cellulose: elaboration, pyrolysis and use as platinum nanoparticles substrate for oxygen reduction electrocatalysis[END_REF], in microbial fuel cell (MFC) [Yuan, 2013;Ma, 2014;Liu, 2015c] for the oxygen reduction reaction (ORR) [Wang, 2015], and in alcohols electro-oxidation [START_REF] Zhou | Synthesis of mesoporous carbon from Okara and application as electrocatalyst support[END_REF][START_REF] Fan | Bifunctional porous carbon spheres derived from pectin as electrode material for supercapacitors and support material for Pt nanowires towards electrocatalytic methanol and ethanol oxidation[END_REF][START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF].

Oxygen reduction

The advantages of biomass derived chars as metal-support are the high surface area, the enhanced microporosity, the low cost (in particular when deriving from biomass wastes), and the eventual high nitrogen content. In this context, an ultra-light and highly porous pure cellulose material prepared from cellulose gels by drying in carbon dioxide in supercritical conditions (aerocellulose)

was investigated [START_REF] Guilminot | New nanostructured carbons based on porous cellulose: elaboration, pyrolysis and use as platinum nanoparticles substrate for oxygen reduction electrocatalysis[END_REF] to support platinum. The metallic active phase was added following various procedures and compared to a commercial Pt/Vulcan XC72R catalyst in the oxygen reduction reaction (ORR). Pt/aerocellulose materials exhibit ORR specific activities comparable to those of the commercial catalysts. Chars are also used for the development of cathode in microbial fuel cell (MFC) and associated to Fe 3 O 4 [Ma, 2014]. [START_REF] Ma | Synthesis of iron oxide/partly graphitized carbon composites as a high-efficiency and low-cost cathode catalyst for microbial fuel cells[END_REF] reported that waste cornstalks and pomelo skins can be used as carbon sources for the development of iron oxide nanocomposites/partly graphitized carbon (Fe 3 O 4 /PGC-CS and Fe 3 O 4 /PGC-PS). The improvement of the BET surface area, the electrical conductivity and the catalysts density was obtained by optimising the pyrolysis temperature and by introducing Fe species, respectively. The formulation of the microbial fuel cell led to a mixed composite (Fe 3 O 4 /PGC-CS: Fe 3 O 4 /PGC-PS = 1:1) cathode, generating the highest power density (also higher than that of a Pt/C cathode) and stable during 18 cycles [Ma, 2014].

The low cost of this type of support also motivates the investigations in the development of cathode catalysts of polymer electrolyte fuel cells (PEFCs) for the ORR [START_REF] Iwazaki | High oxygen-reduction activity of silkderived activated carbon[END_REF]. The catalytic activity of carbonized silk fibroin, which is free of metallic elements, for ORR was greatly enhanced by steam activation [START_REF] Iwazaki | Oxygen-reduction activity of silk-derived carbons[END_REF]. It was demonstrated that the introduction of nitrogen (by nitrogen-containing precursor like NH 3 ) into carbon materials enhances the catalytic activity in ORR. Sewage sludge chars were used for their abundant microporosity, and their high nitrogen and iron contents [Yuan, 2013]. In a microbial fuel cell (MFC), the maximum power density was obtained from the cathode prepared with a char pyrolyzed at 900°C. The obtained power density was comparable to that of a conventional Pt cathode. Some other researches proposed to develop nitrogen and phosphorus doped carbons by pyrolysis of cellulose, using ammonium phosphate as doping source, and to employ them as ORR catalyst in MFC cathode [Liu, 2015]. Wang et al.

[ Wang, 2015] prepared N-Carbon catalysts from Okara (a cheap, nitrogen-rich, biomass precursor) using a simple synthesis method where the addition of FeCl 3 was performed at different steps. They established a correlation between N-species (pyridinic-N and graphitic-N) and ORR activity.

Mesoporous N-Carbon catalyst showed an ORR activity comparable to that of commercial Pt/C catalyst.

Methanol and ethanol oxidation

Recent researches were reported on carbon derived materials, prepared by carbonization of Okara (ODC), and used as metal support for Pt and applied in alcohols electro-oxidation, due to their mesoporous structure and large specific area [START_REF] Zhou | Synthesis of mesoporous carbon from Okara and application as electrocatalyst support[END_REF]. Higher electro-catalytic activity and resistance to poisoning of ODC than Pt/Carbon catalyst were reported in methanol electro-oxidation [START_REF] Zhou | Synthesis of mesoporous carbon from Okara and application as electrocatalyst support[END_REF]. Carbonaceous materials prepared by fast pyrolysis of cellulose (untreated and acid treated) were also studied as catalyst support for producing Cu-Ru@Pt core-shell nanoparticles applied in the electro-oxidation of methanol [START_REF] Lobos | Biochar from pyrolysis of cellulose: an alternative catalyst support for the electro-oxidation of methanol[END_REF] in acid media. The electro-catalytic performances observed for trimetallic catalysts supported on acid treated cellulose showed to be promising when compared to other electro-catalysts, with similar or higher Pt loading, on conventional carbon supports. Porous carbon spheres (PCSs), prepared via hydrothermal carbonization and chemical activation route of pectin, were reported [START_REF] Fan | Bifunctional porous carbon spheres derived from pectin as electrode material for supercapacitors and support material for Pt nanowires towards electrocatalytic methanol and ethanol oxidation[END_REF] to present a large specific surface area, a well-balanced micro/mesoporosity with narrow pore size distribution, and a high content of oxygen-containing functional groups. These properties are indeed very important for Pt electrocatalyst supports. A three-dimensional (3D) hybrid catalyst material exhibited high catalytic activity and stability towards both methanol and ethanol electro-oxidation in acidic media.

Such behaviour was attributed to the PtNWs network growing on PCSs. Furthermore, COpoisoning tolerance was promoted by PCSs surface oxygen groups.

Pollution control (NOx, ammonia, organics)

Only little research has been published on the possible applications of char as catalyst or catalyst support for the reduction, oxidation and removal of gaseous pollutants.

Biomass derived char can be applied in the Selective Catalytic Reduction of nitrogen oxides (SCR-NO x ) as reported by Wu et al. [START_REF] Wu | Novel application of biochar from biomass pyrolysis for low temperature selective catalytic reduction[END_REF] that used the residue of the pyrolysis of cotton stalk waste, once steam activated, as support for Ce and Mn. Various catalysts, with different porous distributions, were obtained. Authors observed that the extent of mesoporosity influenced the temperature at which the catalyst is active. The catalyst with the lower amount of mesopores showed the highest NO conversion at low temperature (180-240 °C), while a bigger amount of mesopores corresponded to enhanced activity (55% NO conversion) at temperature higher than 260 °C. Cotton stalk-derived chars were also used by Singh et al. [START_REF] Singh | Novel application of cotton stalk as a waste derived catalyst in the low temperature SCR-deNOx process[END_REF] as support for Ce and Mn. Low catalytic activity was observed. It was attributed to both the low metal loading and the presence of inorganics (Na, K, Ca, and Mg) that neutralize the Brønsted acid active sites of the catalyst, involved in the activation (by protonation) of the reducing agent (NH 3 ). Biomass derived char seems to be more adapted than commercial activated carbons for application at low temperature; the higher presence of oxygen functional groups in Mn/Ce deposited on cotton char showed to have a key role in SCR of NO at low temperature [Shen, 2015a;[START_REF] Cha | The low temperature SCR of NO over rice straw and sewage sludge derived char[END_REF]. The influence of the specific surface area, the amount of oxygen functional groups, the porosity, and the

Fig. 1 .

 1 Fig. 1. Nitrogen and oxygen surface functional groups on activated carbon (Reprinted with permission

Figure 4 .

 4 Figure 4. Three-dimensional plot of BET surface area model of physically activated carbon

Fig. 5 .

 5 Fig. 5. Chars redox behavior (Reprinted with permission fromYuan Y., Bolan N., Prevoteau A., Vithanage M.,

Fig. 6 .

 6 Fig.6. Total turnover numbers for alkene epoxidations catalyzed by Mn II -L 3 imid@Cox and Mn II -

Fig. 7 .

 7 Fig.7. STEM observations of (A) wood-Ni1 (9.9 wt% Nickel content), (B) wood-Ni0.5(8.5 wt%

Figure 8 .

 8 Figure 8. SEM micrographs of samples prepared at different treatment temperatures: (a) TiO 2 /AC at 400 °C, (b) TiO 2 at 400 °C, (c) TiO 2 /AC at 500 °C, and (d) TiO 2 at 500 °C (Reprinted with permission

Figure 9 .

 9 Figure 9. Schematic presentation of biochemical processes in lignocellulosic biomass conversion C

Fig. 10 .

 10 Fig.10. Esterification activity of sulfonated pyrolyzed rice husk (RHC-SO 3 H) and Amberlyst-15 (a);

  used different chars as catalyst and catalyst support (Ni-based char) for biomass gasification and noticed the influence of the char parent biomass (AC, commercial active carbon; WC, weat straw char; RC, rise husk char; CC, cotton stalk char) on hydrogen production; the Ni-based catalyst, prepared on

Table 1 .

 1 derived from various biomass, or after different treatments (chemical or physical). Table1resumes the influence of the biomass origin on these properties, before any treatment. The values reported in Specific Surface Area and Porosity of chars derived from pyrolysis of various biomass sources.

	Table 1 are those measured by N 2 adsorption that even if known not to correctly evaluate the
	microporosity is still largely used. The values of BET surface area obtained from the CO 2
	adsorption	isotherms	are	indicated	between	brackets,	when	available.

Raw material Char preparation BET surface area (m 2 g -1 )* Pore volume (cm 3 g -1 ) Micropore volume (cm 3 g -1 )* Average pore diameter (nm)

												Reference
	rown macro-alga	1 h at 500 °C, N 2 (50 mL min -1 )			Under	-	-		-	Cho, 2013
							detection limit
	ood chips	1 h at 400 °C, N 2 (1-2 L min -1 )			Under	-	-		-	Kastner, 2012
							detection limit
	owdered peanuts hulls 1 h at 400 °C, N 2 (1-2 L min -1 )			1.1	0.001	-		1.2	Kastner, 2012
	elletized peanut hulls 1 h at 400 °C, N 2 (1-2 L min -1 )			1 to 4	-	-		-	Kastner, 2012
	urcucumber plant	2 h at 700 °C, air limited supply			2.3	0.008	-		6.78	Rajapaksha, 2016
	afflower seed	10 min at 600 °C (100 °C min -1 ), N 2 (100 mL min -1 )	4.9	0.91	-		-	Onay, 2007
	afflower seed	10 min at 600 °C (800 °C min -1 ), N 2 (100 mL min -1 )	6.7	1.73	-		-	Onay, 2007
	elletized pine residues 1 h at 500 °C, N 2 (1-2 L min -1 )			10	0.005	-		0.96	Kastner, 2012
	ewage sludge	1 h at 500 °C, N 2 (50 mL min -1 )			17.9	0.018	-		4.06	Cha, 2010
	inewood	2 h at 700 °C (10 °C min -1 ), N 2 (10 mL min -1 )		29	1.020	0.13		1.48	Liu, 2010
	ewage sludge	2 h at 900 °C (5 °C min -1 ), N 2			44	-	-		-	Yuan, 2013
	lmond shell	1 h at 600 °C, N 2 (150 mL min -1 )			42	0.094	0.023 (0.305)	-	Gonzalez, 2009
	live stone	1 h at 600 °C, N 2 (150 mL min -1 )			53	0.036	0.028 (0.399)	-	Gonzalez, 2009
	adiata pine	5 min at 950 °C (20 °C s -1 ), N 2			57 (290)	-	-		-	Cetin, 2005
	ectin	10 min at 450 °C, N 2 (220 mL min -1 )			70	-	-		-	Sharma, 2001
	orn cob	800 °C (10 °C min -1 ), N 2 (200 mL min -1 )			91	0.022	-		0.96	Chang, 2000
	ice straw	1 h at 500 °C, N 2 (50 mL min -1 )			139.5	0.092	-		2.64	Cha, 2010
	iant Miscanthus	1 h at 500 °C (10 °C min -1 ), N 2 (1.2 L min -1 )		181	-	-		-	Shim, 2015
	icrocrystalline	20 min at 500 °C, N 2 (60 mL min -1 )			199	0.026	-		3.0	Lobos, 2016
	ellulose from cotton
	lmond tree pruning	1 h at 600 °C, N 2 (150 mL min -1 )			204	0.118	0.097 (0.426)	-	Gonzalez, 2009
	alnut shell	1 h at 600 °C, N 2 (150 mL min -1 )			280	0.160	0.138 (0.458)	-	Gonzalez, 2009
	ea waste	2 h at 700 °C, air limited supply			342.2	0.022	-		1.76	Rajapaksha, 2016
	orn cob	900 °C (10 °C min -1 ), N 2 (200 mL min -1 )			549	0.251	0.216		1.44	Chang, 2000
	ucalyptus	1 h at 600 °C (10 °C min -1 ), N 2 (1000 mL min -1 )		(570)	-	(0.218)	-	Guerrero, 2005
	*Measured	by	N 2	adsorption	and	by	CO 2	adsorption	(between	brackets	when	available)

Table 2 .

 2 Specific Surface Area and Porosity of hydrochars derived from hydrothermal carbonization of various biomass sources.

	Raw material	Hydrochar preparation				BET surface	Pore volume	Micropore	Average pore	Reference
						area (m 2 g -1 )	(cm 3 g -1 )	volume (cm 3 g -1 )	diameter (nm)
	Walnut shells	20 h at 220 °C, 50 g biomass L -1	water		31	0.072	0.011	-	Román,
	Cellulose	4 h at 250 °C, 40 g biomass L -1	water		30	-	-	-	Sevilla, 2009
	Cellulose	6 h at 230 °C, N 2 (50 mL min -1 )		28	0.369	-	-	Mumme, 2011
	Sunflower stem	20 h at 220 °C, 50 g biomass L -1	water		27	0.051	0.016	-	Román,
	Olive stone	20 h at 220 °C, 50 g biomass L -1	water		22	0.025	0.013	-	Román,
	Pinewood	20 min at 300 °C (10 °C min -1 ), 40 g biomass L -1	water	21	0.140	0.090	0.86	Liu, 2010
	Digestate	6 h at 190 °C, N 2 (50 mL min -1 ), 42.3 g C L -1		12	0.096	-	-	Mumme, 2011
	Solid waste of palm oil	20 min at 250 °C				8	0.043	-	3.77	Parshetti, 2013
	mill processing

Table 3 .

 3 H 3 PO 4 pre-treatment effect on textural properties measured by N 2 adsorption.

	Raw material	Conditions	BET surface	Pore volume	Micropore	Average pore	Reference
			area (m 2 g -1 )	(cm 3 g -1 )	volume (cm 3 g -1 )	diameter (nm)
	Effect of weight acid/biomass ratio
	Corncob	50% solution of H 3 PO 4	From 39 to	From 0.075 to	-	From 4.88 to	El-Hendawy
		Pyrolysis at T = 500°C	960	0.629		1.31	2001
	Cellulose	Weight acid/biomass ratio = 25%	From 199 to	From 0.0264	-	From 3.0 to	Lobos 2016
		Pyrolysis at T = 500°C	557	to 0.2200		2.4
	Lignin	Weight acid/lignin ratio ranging from 0.7	From 600 to	From 0.3 to	From 0.25 to 0.40	-	Fierro 2006
		to 1.4	1100	0.5
	Effect of activation temperature
	Lignin	Activation temperature ranging from 400	From 1000 to	From 0.5 to	From 0.4 to 0.5	-	Fierro 2006
		to 600°C	1300	0.7
	Effect of steam activation
	Cotton stalks	Weight acid/biomass ratio = 1.5/1	From 516 to	From 0.08 to	-	-	Shen 2015
		Pyrolysis at T = 600°C	1955	0.48
		Steam activation at 800°C / 1h
	Effect of impregnation time
	Lignin	Impregnation time ranging from 1h to 48 h From 1300 to	-	-	-	Fierro 2006
			956

Table 4 .

 4 ZnCl 2 pre-treatment effect on textural properties measured by N 2 adsorption.

	Raw material	Conditions	BET surface area	Pore volume
			(m 2 g -1 )	(cm 3 g -1 )

Micropore volume (cm 3 g -1 ) Reference Effect of ZnCl 2 /biomass weight ratio

	Cherry stones	Pyrolysis T = 500 °C	From 330 to 1566	-	-	Olivares-Marin 2006
	Paulownia wood Impregnated ratio = 4	From 257 to 2620	-	-	Yorgun 2009
		Pyrolysis T = 400 °C
	Corn cob	From 0.75 to 1.75	From 651 to 1410	From 0.33 to 0.70	-	Tsai 1997
		Pyrolysis T = 500 °C
		Imgregnation time = 0.5h
	Cattle-manure	From 0.25 to 1.50	From 600 to 2170	From 0.38 to 1.2	-	Qian 2007
	compost	Pyrolysis T = 400 °C
		Imgregnation time = 0.5h
	Cattle-manure	From 1.5 to 2.5	From 2170 to 2048	From 1.2 to 1.7	-	Qian 2007
	compost	Pyrolysis T = 400°C
		Imgregnation time = 0.5h
	Paulownia wood From 1 to 4	From 977 to 2736	From 0.516 to 1.387 From 0.27 to 0.69 Yorgun 2009
		Pyrolysis T = 400 °C
		Imgregnation time = 0.5h

Effect of pyrolysis temperature (ZnCl 2

	Corn cob	Pyrolysis T from 400 to 600 °C	From 535 to 750	From 0.28 to 0.37	-	Tsai 1997
	Corn cob	Pyrolysis T from 600 to 800 °C	From 750 to 400	From 0.37 to 0.19	-	Tsai 1997
	Effect of pyrolysis time (ZnCl 2 /biomass weight ratio = 1)
	Corn cob	Imgregnation time from 0.5h to 1h	From 535 to 613	From 0.28 to 0.31	-	Tsai 1997
		Pyrolysis T = 400 °C
	Corn cob	Imgregnation time from 1h to 4h	From 820 to 744	From 0.40 to 0.38	-	Tsai 1997
		Pyrolysis T = 600 °C

/biomass weight ratio = 1, Imgregnation time = 0.5h)

Table 5 .

 5 KOH pre-treatment effect on textural properties measured by N 2 adsorption.

	Raw material	Conditions			BET surface area	Pore volume	Micropore	Reference
					(m 2 g -1 )	(cm 3 g -1 )	volume (cm 3 g -1 )
	Effect of KOH/precursor ratio
	Cassava peel	Impregnation	weight	ratio	From 1027 to 1605	From 0.434 to 0.691	From 0.390 to	Sudaryanto
		KOH/precursor from ½ to 5/2				0.513
		Pyrolysis T = 750 °C
		Pyrolysis time = 1h
	Effect of pyrolysis temperature
	Rice straw	Impregnation	weight	ratio	From 370 to 900	From 0.25 to 0.46	From 0.12 to 0.30 Oh 2002
		KOH/precursor = 4/1
		Pyrolysis T from 500 to 800 °C
	Cassava peel	Impregnation ration KOH/precursor	From 972 to 1378	From 0.421 to 0.583	From 0.383 to	Sudaryanto
		= 1/1					0.393
		Pyrolysis T from 450 to 750 °C
		Pyrolysis time = 1h
	Effect of pyrolysis time
	Cassava peel	Impregnation	weight	ratio	From 1154 to 1183	From 0.519 to 0.520	-	Sudaryanto
		KOH/precursor = 1/1
		Pyrolysis T = 650 °C
		Pyrolysis time from 1 to 3h

Table 6 .

 6 K 2 CO 3 pre-treatment effect on textural properties measured by N 2 adsorption.

	Raw material Conditions

Micropore volume (cm 3 g -1 ) Reference Effect of K 2 CO 3 /precursor weight ratio

	Almond shell	Impregnation K 2 CO 3 /precursor weight ratio from 0 to	From 104 to	-	-	Bevia 1984
		2	2111
		Pyrolysis T = 600 °C
		Pyrolysis time = 2h
	Black	Impregnation K 2 CO 3 /precursor weight ratio from 0.75	From 776 to	From 0.469	From 0.244 to	Deng
	elutrilithe (coal	to 1.5	856	to 0.523	0.234
	waste)	Pyrolysis T = 850 °C
		Pyrolysis time = 3h
	Effect of pyrolysis temperature
	Almond shell	Impregnation K 2 CO 3 /precursor weight ratio = 0.25	From 367 to	-	-	Bevia 1984
		Pyrolysis T from 500 to 800 °C	746
		Pyrolysis time = 2h
	Black	Impregnation K 2 CO 3 /precursor weight ratio = 1	From 59 to	From 0.074	From 0 to 0.159 Deng
	elutrilithe (coal	Pyrolysis T from 500 to 1000 °C	423	to 0.219
	waste)	Pyrolysis time = 3h
	Almond shell	Impregnation K 2 CO 3 /precursor weight ratio = 1	From 250 to	-	-	Hayashi
		Pyrolysis T from 500 to 800 °C	1200			2002
		Pyrolysis time = 1h
	Coconut shell	Impregnation K 2 CO 3 /precursor weight ratio = 1	From 700 to	-	-	Hayashi
		Pyrolysis T from 500 to 800 °C	1700			2002
		Pyrolysis time = 1h
	Effect of pyrolysis time
	Almond shell	Impregnation K 2 CO 3 /precursor weight ratio of 0.25	From 664 to	-	-	Bevia 1984
		Pyrolysis T = 600 °C	796
		Pyrolysis time from 80 to 480 min
	Black	Impregnation K 2 CO 3 /precursor weight ratio = 1	From 726 to	From 0.433	From 0.224 to
	elutrilithe (coal	Pyrolysis T = 850 °C	858	to 0.512	0.258
	waste)	Pyrolysis time from 1 to 3h

Table 7 .

 7 Acidic chemical post-treatment effect on textural properties measured by N 2 adsorption.

	Raw material		Conditions	BET surface	Pore volume	Micropore	Average pore	Reference
				area (m 2 g -1 )	(cm 3 g -1 )	volume (cm 3 g -1 )	diameter (nm)
	Chemical activation with H 3 PO 4
	Sewage sludge	H 3 PO 4 /char = 10/1	 5	-	-	-	Ros 2006
	char		Following thermal treatment:
			T = 450 °C, 4 h, N 2 = 100
			mL/min,
	Beer waste HTC H 3 PO 4 /HTC = unspecified	1073	0.978	0.101	-	Hao 2013
			Following thermal treatment:
			T = 600 °C, 1 h, N 2
	Chemical activation with H 2 SO 4
	Hardwood char H 2 SO 4 /char = 18.4	5.8	-	-	-	Dehkhoda 2010
			Following thermal treatment:
			T = 70° C, 1 h, air
	Peanut hull char H 2 SO 4 /char = 3	242	-	-	-	Kastner 2012
			Following thermal treatment:	(1-4 before
			T = 110 °C, 12 h, air	activation)
	Peanut hull char H 2 SO 4 /char = 3	338	0.18	-	1.1	Kastner 2012
			Following thermal treatment:
			T = 110 °C, 12 h, air
	Rice husk char	H 2 SO 4 /char = 18	4	-	-	7.7	Li 2014
			Following thermal treatment:
			T = 80 °C, 24 h, air
	Douglas	fir	H 2 SO 4 /char = 27.6	3.5	-	-	-	Dong 2015
	wood chips char	Following thermal treatment:
			T = 105 °C, 4 h, air
	Chemical activation with ZnCl 2
	Groundnut shell	ZnCl 2 /char = 1.75	1200	-	-	-	Malik 2007
	char (Pyrolysis	Following thermal treatment:
	T = 400°C)		T = 650°C, 15 min, N 2
	Effect of weight acid/biomass ratio

Table 8

 8

. Textural characteristics of chars from various biomasses measured by N 2 or CO 2 (between brackets) adsorption.

Raw biomass BET surface area (m 2 g -1 ) Micropore volume (cm 3 g -1 ) Mesopore volume (cm 3 g -1 )

	Walnut shell	280	0.138 (0.458)	0.022
	Almond tree pruning	204	0.097 (0.426)	0.021
	Almond shell	42	0.023 (0.305)	0.071
	Olive stone	53	0.028 (0.399)	0.008

Table 9 .

 9 Biodiesel production versus total acidity of different catalysts from [Gonzales 2017].

	Catalyst

NO and NH 3 adsorption capacities have been taken into account in the SCR reaction performed on Mn-based catalysts supported on rice straw char [START_REF] Cha | The low temperature SCR of NO over rice straw and sewage sludge derived char[END_REF].

Attempts in the use of chars as catalyst in the catalytic ozonation of ammonia are rarely reported [START_REF] Kastner | Catalytic ozonation of ammonia using biomass char and wood fly ash[END_REF]. The use of Chars in this process is potentially very profitable due to the low cost of biomass derived catalysts, and the possibility to simultaneously reuse agriculture waste materials in agricultural operations that involve ammonia release.

Conclusions and Perspectives

In the last years, char has been identified as an environmental friendly and low-cost catalyst, and catalyst support, in various reactions for syngas and bio-oil production, chemical synthesis and pollutants treatments.

Many researches report on the performances of such relatively new catalytic materials and compare them to more conventional catalysts. Unfortunately, often the selection of a certain char for a target reaction is not based on objective correlations between char properties and reactivity, but just on the accessibility of a certain char, due to the availability of the parent biomass. As a consequence, it is difficult to identify the properties directing a specific reaction and to produce more active char based catalysts by applying a bottom-up approach. The char catalytic properties amelioration by designing and producing appropriate chars (in terms of surface functionalities, hydrophobic/ hydrophilic features, and pores distribution) will facilitate to apply chars to a larger extent and in a larger variety of reactions.