Niki Baccile

Cilaine V Teixeira

Heinz Amenitsch

Françoise Villain

Mika Lindén
email: mlinden@abo.fi

Florence Babonneau

Time-resolved in situ Raman and SAXS approach: New insights in the formation mechanism of SBA-3 mesostructured silica powders from TEOS hydrolysis to long-range order

Keywords: Mesostructured SBA-3, SAXS, Raman Spectroscopy, Time-resolved, In-situ, Formation mechanism

The abstract should briefly state the problem or purpose of the research, indicate the theoretical or experimental plan used, summarize the principal findings, and point out major conclusions. The optimal length is one paragraph.

INTRODUCTION

Highly organized mesostructured silica based materials are, at present, a widespread research topic.

Since their discovery in the 90's, 1,2 these materials have been largely studied mainly for their potential applications that rely on the possibility to prepare very high specific surface area powders with a range of chemical modifications on their exposed surface. Interests that they raised up were primarily focused on catalysis applications, and then extended to fields ranging from adsorption, sensing, encapsulation and even electronics. Several good quality reviews are available on this topic and we address the reader towards them for more details. [3][4] Indeed, major advantages that supported research in the field are the mild conditions employed and the ease of synthesis steps. Nevertheless, the large number of works published on the topic has not entirely clarified yet all concerns about the mechanisms of formation, which involves several sequential and/or simultaneous events, occurring at different length scales: silica polycondensation process; interaction between amphiphilic moieties and inorganic species; development of micellar structure; long-range ordering; precipitation of final product with given shape and morphology.

Table 1 lists the characterization techniques that have been so far used in the literature to investigate the formation mechanisms of various silica-surfactant mesophases, with their main structural outputs.

The molecular level is best characterized by spectroscopic techniques such as EPR, NMR, IR and Fluorescence, while SAXS together with TEM will specifically target the mesoscale.

Their performance for in-situ time resolved investigations has also been analyzed. They should allow to study in situ the reactive medium without any modification of the chemical composition and processing parameters. This is the key to ensure a good reliability to the proposed formation mechanism. We estimate that the system should be adapted as seldom as possible to the technique.

This will be discussed more in details in the next paragraph. Their suitability as in-situ techniques was thus evaluated according to three criteria: a) Need to change the chemical composition of the starting system; b) Need to introduce extra entities required by the technique to be effective; c) Need to modify processing parameters such as type of the reaction container, stirring speed…. They should also ensure measurement times compatible with the kinetics of the events that one would like to follow. Real time tracking of the evolving system is preferred to punctual snapshots or to a response that has been averaged over a period of time during which the system undergoes many changes. The time-resolved efficiency was thus estimated from the characteristic measurement times associated to the technique. It is obvious that for fast reacting systems, the possibility to run time-resolved experiments will be more delicate than for slower evolving systems.

Insert Table 1

To complete this table, we would like to add some specific comments for each technique that have been suggested by analyzing in details the experimental conditions under which the so-called in-situ studies are performed. TEM: transmission electron microscopy offers a very nice way to observe micelles, their shape evolution with time as well as their long-range arrangement to form mesostructures at different moments of the material synthesis. 5 Specimen preparation is however a key issue to ensure observations representative of the solution structures. That is why cryo-TEM, a powerful tool for investigating nanostructures in soft materials, has also been used to investigate such silica/surfactant hybrid systems. 6,7 Negative points concern the fact that TEM is not a truly in-situ approach since it only provides snapshots at chosen moments of reaction time; nevertheless, snapshots furnish quite a well representation of the reaction status at a given moment provided a quenching step in the synthesis procedure. SAXS: small angle X-ray scattering is clearly the best technique to study short and long-range ordered micellar structures and shapes. Short acquisition times and adaptable experimental setup made it the most-used technique in time-resolved [8][9] and even temperature-resolved [10][11] in-situ experiments. Nevertheless, acquisition times obtained with low energetic X-ray sources are not compatible with time-resolved requirements, as it was already shown in 1998 by Lindén et al. 12 ; hence, experiments must be performed in presence of high energetic X-ray source, like synchrotron radiation; this makes this type of experiments quite expensive and of difficult access. Energy Dispersion X-ray diffraction was used on SBA-1 materials 13 but poor resolution does not give access to detailed mesophase parameters.

EPR: electron paramagnetic resonance is used to obtain information about the silica/micelle interactions during material formation. Such technique, extensively used by the groups of Goldfarb 14 and Galarneau-Ottaviani, 15 can be considered as being truly time-resolved due to its very high sensitivity even with a low amount of paramagnetic species. As far as in-situ approach is concerned, introduction of an external probe with a paramagnetic center brings an extra parameter to take into account. In addition, experimental synthesis conditions must be adapted to meet EPR apparatus requirements: use of either flat cells or tubes imply smaller reaction volumes where no stirring is available making this setup unsuitable for a number of established experiments. In some cases 15 , this aspect does not seem to really matter since stirring is replaced by temperature effect on material formation, as it occurs in material synthesis under hydrothermal conditions. DLS: dynamic light scattering is seldom used to follow evolution of overall micellar size during self-assembly process, as ref. 16 and references therein put in evidence. Time-scale of DLS measures must be considered in the minute range (from 1 to 5) to obtain exploitable significant results. DLS was adapted to the in-situ approach but caution has to be taken since experiments take place in a smaller volume and stirring does not occur. In addition, solutions generally need dilution in order to increase reaction times and prevent powder precipitation in the very first minutes of reaction.

Fluorescence: fluorescence technique is used to observe ion exchange mechanisms at the micellar/silica palisade in order to better understand organic/inorganic interactions. 17 Even if timescale for signal detection is very fast, technique has some drawbacks as far as in-situ analysis is concerned: a well-chosen fluorescent probe must be introduced in the system while solution must be kept transparent at all times; finally, solution must be placed in a sample holder. Experiments proposed in ref. 17 were performed under pH conditions where no precipitation takes place.

NMR:

Nuclear magnetic resonance is used in liquid state to reach a wide range of objectives depending on the nucleus under study. Unfortunately, it suffers from general drawbacks: 1) Confined volumes in NMR tubes do not permit stirring of the solution. 2) Precipitation should be avoided, which implies working with either slow reacting (e.g., Pluronic-structured materials) or modified systems (different pH, dilution). 3) The low sensitivity for some nuclei requires isotopic enrichment that contributes to increase the overall cost and to introduce external species and/or synthetic steps. 4) Long relaxation times increase measurement times. However, NMR studies provided interesting results on various silica/surfactant systems:

-1 H NMR was exploited by Flodström et al. 18 to follow the decrease in line width of the signal from the methyl group of the PPO part of Pluronic F108 surfactant during the synthesis of a cubic Im3m silica/Pluronic mesophase. They attribute this behavior to lower mobility of PO group due to either large aggregates formation or interaction between Pluronic and growing silica. Study of protons gives quite reasonably short acquisition times letting time-resolved approach feasible.

-Quadrupolar nuclei like 2 H and 14 N can be used to identify the surfactant mesophase geometry in presence of silicate species by looking at quadrupolar interaction parameters. Firouzi et al. 19 used deuterium enriched quaternary ammonium salts while Egger et al. 13 studied the 14 N signal from the cationic surfactant polar head groups. Only drawback of 2 H is necessity of sample isotopic labeling.

- 29 Si can be used to follow hydrolysis-condensation of the silica precursors and formation of silica network in solution. This approach, which was tested, e.g., in ref. 13 and 19, is far from being timeresolved due to the low abundance, low sensibility and long relaxation times of 29 Si nucleus.

- 17 O was also used by Egger et al. 13 to follow water reactivity and oxygen localization in quaternary ammonium salt structured silica. Here, prize to pay is necessary 17 O isotopic enrichment, and drastic decrease in the water content of the starting solution to minimize costs induced by the use of 17 O-enriched water.

FT-IR:

Infrared spectroscopy could be a perfect technique for time-resolved in-situ experiments due to its fast response time. Unfortunately, the strong adsorption of radiation by water in several regions of far and mid IR requires the use of sensitive attenuated total reflection (ATR) sampling devices, which are not compatible with true in situ measurements. Nevertheless, ATR-FTIR was effective to provide qualitative data regarding the species present during hydrolysis and condensation of TEOS under rich water conditions. 20 The solvent spectra were subtracted from the sample spectra to better visualize the absorption bands for the species of interest. ATR-FTIR technique was also used to follow the synthesis of mesoporous silica in fatty acid/aminoalkoxysilane/water systems. 21 Strong band overlap of the multiple species present in solution made however any quantitative analysis quite unreliable.

Raman spectroscopy: this technique can be used to follow inorganic precursor hydrolysis as a function of time. Time-resolved acquisitions are possible due to the fast response of Raman signal 22 and new Raman spectrometers can be perfectly adapted to the set-up required for these experiments.

The main positive point is the no-detection of the HOH angular deformation vibration band of water 23 , but in all cases, non-fluorescing and limpid solutions are required. Raman was never adapted to time-resolved in-situ study of mesostructured silica materials.

Most of the in-situ mechanistic studies concentrated, so far, on one type of observations and correlations between them are difficult to establish because of the differences in chemical composition of the systems and experimental conditions. However, efforts have been recently made to combine complementary techniques, which allows to get a better overview of the formation process at various length scales: NMR was combined with TEM 18 and also with XRD 13 , IR with SAXS 21 , and EPR with cryo-TEM 24 . As already pointed out in this last publication from the group of Goldfarb 24 , it is important to carry out the experiments under the same reaction conditions to allow the correlation between the two length scales. It is also essential to avoid any major modification of the synthetic conditions, such as slowing down the reaction kinetics. Indeed, most of the studies combining various techniques have been performed on slow reacting systems for which measurement times in the order of minutes are compatible with the observed reaction kinetics. This is typically the case for the formation of SBA-15. 9,25 For fast reacting systems involving, for example, cationic surfactants for which precipitation occurs within few minutes, the choice of time resolved in situ techniques is somehow limited.

Since the pioneer work of the researchers from Mobil 1 mechanisms explaining the synthesis under basic conditions were proposed. None of them were based on in-situ measurements. The first proposition was based on the existence of a pre-formed liquid crystalline phase, which revealed to be a wrong guess, as shown by Chen et al. 26 , who used 14 N NMR to prove that no hexagonal mesophase pre-exists in solution before addition of the silica source. The second proposition assumed a cooperation mechanism between preformed hexagonal micelles in solution and silicates, which eventually contributed to build up the long range ordered hexagonal mesostructure. Then, the charge density matching mechanism was proposed by the groups of Stucky and Chmelka. 27,28 They explained that electrostatic interactions first occur between surfactant polar heads and multiple charged silicate oligomers in solution. Due to high surface charge density, the organic-inorganic mesophase formed from condensing silica species, has to pass through a low curvature lamellar phase before reaching the 2D-hexagonal phase, a higher curvature surface. This picture was similar to what Yanagisawa et al. 29 already observed in 1990, when a progressive phase transformation from lamellar to hexagonal was observed in kanemite upon addition of long alkyl chain ammonium ion salts. Later in the '90s, several interesting studies put in evidence a self-assembling phenomenon between silicates and surfactant micelles and this was possible using in situ techniques, but after preventing the silicate species from extensive condensation. Firouzi et al. 28 used NMR and neutron scattering techniques to show the formation of cylindrical micelles from spherical one upon silicate addition to the starting solution; Galarneau et al. 15 used EPR in presence of well-chosen paramagnetic probes to prove that an exchange between silicate anions and surfactant counter-ion occurs at the micelle double layer interface while Frasch et al. 17 , by mean of fluorescent techniques showed that ion exchange is quite limited at the micellar double layer interface but it is rather enhanced when the surfactant molecule is alone in solution. Micelles only play a role of surfactant reservoirs rather than "building blocks" of the whole structure. It is important to repeat that these studies were only pursued on MCM-41-type materials, where direct electrostatic interaction occurs between negatively charged silica and positively charged ammonium surfactants.

On the contrary, literature is not as abundant on synthesis occurring under acidic conditions, as proposed by Huo et al., 2 and leading to the formation of the SBA-1 (cubic Pm3n), SBA-2 (3Dhexagonal P63/mmc) and SBA-3 (2D-hexagonal, p6m) surfactant/silica mesophases. In this case, interactions between silica and surfactant are of different nature since a negative counter-ion (coming from the surfactant or from the acid used) interposes in between the positively charged micelles and the quasi-neutral silica surface. 30 One can mainly cite the studies from Egger et al. 13 who investigated the formation mechanism of the cubic Pm3n SBA-1 phase obtained from TEOS and CTEAB (cetyltriethylammonium bromide). They combined very nicely in situ EDXRD as well as multinuclear NMR ( 29 Si, 17 O, 14 N) techniques; however, reactant molar ratios needed to be adapted to the employed techniques in order to optimize the experiments, and ended up to be quite far from the SBA-1 synthetic conditions. Lesaint et al. 31 investigated the formation of the 2D-hexagonal SBA-3 mesophase using DLS. However, as already mentioned for such technique, they had to dilute the system in a rather extensive manner compared to the original preparation from Huo (H2O/Si = 734 instead of 130), and the acid concentration was consequently lower (0.5-0.9 M instead of 3.9 M).

We thus decided to re-investigate the formation of SBA-3 in the same range of experimental conditions used by Huo et al. with cetyltrimetylammonium bromide (CTAB) as template, tetraethoxysilane (TEOS) as silica precusor and under highly acidic conditions. We wanted to combine techniques that could probe the molecular and the mesoscopic scale in real time on systems used to prepare well-ordered SBA-3 silica powders and for which precipitation occurs within few minutes. Looking at Table 1, Raman spectroscopy and SAXS seemed to be suitable ones. In particular, Raman spectroscopy is appropriate to follow the hydrolysis of TEOS, a fast reacting silica precursor, even if studies reported so far 22 were performed in the absence of templating agents.

Raman is not as powerful as 29 Si NMR to identify hydrolyzed and condensed species 32 , but its response time is fast enough to perform in situ time resolved study, which is absolutely impossible with 29 Si NMR. It is however clear that this technique will only be efficient prior to precipitation.

We will show that it can be elegantly combined with SAXS experiments that will track the appearance and development of long-range order in various systems. A whole picture of the material synthesis can thus be obtained, starting from time t=0 when the inorganic precursor is added to the surfactant solution, until the recover from solution of the precipitated silica-surfactant mesophase powder (Figure 1).

EXPERIMENTAL SECTION

Synthesis of silica based materials. All samples were prepared under acidic conditions using tetraethyl orthosilicate (TEOS, Aldrich), as the silica source and commercial CTAB (Aldrich) as structuring agent. Molar ratios between all reactants were adapted from original Huo's 2 synthesis though several dilution conditions were tested as indicated in Table 2. The synthesis procedure is detailed elsewhere. 30 Insert Table 2 Characterization. Raman effect was stimulated with a 750 nm laser (power: 150 mW) connected to an optical fiber while the Raman emission was collected at 180° with respect to the laser source. An optical lens system focalizes the light at the centre of the reaction beaker containing the CTAB aqueous solution. The solution was continuously stirred and the experiment was performed at room temperature. Time interval between data acquisition was set to 5 s and the experiment starts (t = 0) when TEOS is added to the solution. Data were acquired and treated with HoloGRAMS™ software package.

Small Angle X-ray Scattering (SAXS) experiments were conducted at Austrian SAXS beamline of the ELETTRA synchrotron laboratory in Trieste (Italy). 33 The X-ray energy was 8 keV (1.54 Å) and the sample-detector distance was set to 955 mm. A CCD camera (X-ray Imager, Photonic Sciences, Oxford, GB) was used as detector. The calibration was done with the silver behenate (d= 58.37 Å).

The framing rate was 10 s/frame with typical exposure times of 4 s for all experiments.

The reaction solution was circulated in a 1 mm quartz capillary using a batch reactor/flow through technique. 8 The starting time (t=0s) was determined with adding TEOS to the batch reactor solution.

All acquisitions were stopped after 25 minutes of stirring due to the fast meso-structuring process (from 3 to 5 minutes) except when powder aggregates form earlier and could cause clogging of the 1 mm quartz capillary. All 2D images have been corrected for detector artefacts (flat field, spatial corrections) and they have been integrated azimuthally by FIT2D. 34 These 1D images obtained have been normalized for intensity fluctuations and the instrument background has been subtracted.

Results and discussion

TEOS hydrolysis kinetics. Time resolved in-situ Raman spectroscopy was used to follow the hydrolysis kinetics of TEOS in highly acidic aqueous CTAB solutions (CHCl ≥ 2.5M) for three different compositions (Table 2).

The Raman spectra measured for CTAB/H2O/HCl and CTAB/EtOH/H2O/HCl solutions with CHCl = 2.5 M and CCTAB = 3.3.10 -2 M are shown in Figure 2. The respective concentrations were chosen so that they correspond to the solvent composition of system B, assuming complete TEOS hydrolysis (CTAB/EtOH = 0.12:4) for the second solution. The corresponding assignments of the Raman Stokes vibrations are given in Table 3. 35-36-37 In the absence of EtOH, the two most intense Raman bands of CTAB at 763 and 1063 cm -1 , respectively assigned to NCH3 rocking mode from N + (CH3)3 and C-C stretching mode can be hardly detected due to the low CTAB concentration. 38 In the presence of EtOH (CTAB/EtOH = 0.12/4), three very strong peaks appear at 885, 1055 and 1095 cm - 1 , assigned to (CHx), s(CO) and as(CO) vibrations respectively. 37 The contribution to CTAB bands can thus be neglected in further analysis and this feature strictly depending on our synthesis protocol makes the whole Raman spectrum clearer and of much higher legibility.

Insert Figure 2 and Table 3 Time-resolved Raman spectra have been first recorded for a TEOS-containing solution in the absence (Figure 3a, 3b) of CTAB. Composition corresponds to system B (Table 2). An emulsion with rather large droplets remained all along the experiment. We should mention that stirring speed is a very important parameter that will influence the reaction kinetics in these systems. We maintained it constant within each set of experiments (around 300 rpm); however, from one set of experiments to another, it may have varied, changing the apparent kinetics. In Figure 3a, spectrum 1 is acquired 10 seconds after TEOS addition to solution. The two main bands at 658 cm -1 and 1095 cm -1 assigned to TEOS 37 are observed (Table 3). The spectral evolution with time shows the rapid decrease of the band at 658 cm -1 and the appearance of two new bands at 885 cm -1 and at 1055 cm -1 characteristic of the release of ethanol molecules produced by TEOS hydrolysis.

Insert Figure 3

The time-resolved integrated intensities of the TEOS band at 658 cm -1 and the ethanol band at 885 cm -1 are shown in Figure 3b. The amount of ethanol released during the reaction was quantified.

The intensity of measured Raman scattering of a substance A can be analyzed according to equation

1: 39 I()RA= K()A.V.CA.PL Eq. 1
where CA is the concentration of A in solution, V is the volume of irradiated sample by light source, PL is the power density of laser and K()A is the absorption constant of substance A at irradiation frequency . This linear relationship is valid when no other object scatters light and when the sample does not absorb or emit at the irradiation frequency. We consider fulfilling these conditions since the solution stays clear during the whole process for t ≤ 300 s with neither gelation, nor precipitation taking place. In addition, neither water nor TEOS and ethanol absorb or emit light at the irradiation frequency (here, = 750 nm). Though we are unable to attribute absolute values to

V and PL, we can make the hypothesis that these parameters keep the same values from one solution to another if the experimental set-up is unchanged. If this condition is fulfilled, only the product X = K()A.V.PL has to be determined in order to calculate CA from measured intensity I()RA. In this 3b. Interestingly, TEOS band vanishes after a reaction time of about 120 seconds when slightly more than 3 ethanol molecules have been produced. Indeed the band at 658 cm -1 only accounts for the symmetrical breathing mode of the SiO4 tetrahedra, which means that whenever one single -Si-OEt bond is hydrolyzed, this signal is no longer detected even if un-hydrolyzed Si-OEt bonds are still present. 40 One would expect that when the overall TEOS signal is lost, the maximum ethanol concentration in solution could correspond to 1 molecule per silicon atom if at least one and only one -Si-OEt bond per TEOS is hydrolyzed. In reality, TEOS co-exists with species that have been submitted to multiple hydrolysis. This could be surprising since in excess of water, the concentration of Si-OEt groups was reported to rapidly tend to zero. 41 Moreover, the hydrolysis rate depends strongly on the degree of substitution of the monomers, TEOS being the most reactive. But one should not forget that an emulsion remains all along the reactions, which may explain that TEOS molecules located in the core of the oil droplets have difficulty to be hydrolyzed, compared to OH-substituted species which are located at the oil-water interface.

Bands for hydrolyzed monomers Si(OEt)4-x(OH)x have been reported 40,42 around 670 and 700 cm -1 , which strongly overlap with the TEOS band resulting in the detection of only one broad band shifted to higher wavenumbers. Such a shift is not observed in Figure 3a, suggesting the absence of hydrolyzed monomers with sufficient lifetime to be detected. The formation of condensed species from OH-substituted monomeric species could thus be anticipated. 29 Si NMR spectra and Raman spectra recorded on solutions with H2O:TEOS molar ratio between 0.5 and 1.75 and mild acidic conditions (HCl 0.01-0.03 M) have been compared 35,40 ; bands at 600, 576, 554, 534 cm -1 have been assigned to dimers Si2O(OEt)6, linear trimers Si3O2(OEt)8, linear and branched tetramers Si4O3(OEt)10, respectively. These bands are of comparable intensity with respect to the TEOS band at 658 cm -1 except for the bands due to the tetramers that are much weaker. Hence, lack of detection of any band in the 500-600 cm -1 region indicates that few symmetric dimers and trimers are formed.

Such species could have formed by condensation of monomeric species, Si(OEt)3(OH), since TEOS reacts predominantly by water -producing condensation reactions. 43 We then assume that under our experimental conditions, species which are located at the surface of the TEOS droplets experienced multiple hydrolysis steps and subsequent condensation reactions forming oligomeric polysiloxanes, Si-OEt + H2O  Si-OH + EtOH kH which are released in the aqueous phase, but can not be detected by Raman scattering, this technique being sensitive to symmetric vibrations of short-chain siloxanes.

Figure 3c shows the corresponding in situ time-resolved Raman experiments with CTAB present in the aqueous solution, which corresponds to system B (Table 2). First one should mention that in contrary to the previous case, an initial emulsion is formed with smaller and much more dispersed droplets, which disappears during the process. The same bands are detected due to TEOS and EtOH but the kinetics are different. The TEOS signal at 658 cm -1 rapidly decreases, and vanishes already at t = 50 s. Simultaneously, the peak due to EtOH at 885 cm -1 rapidly grows in intensity up to 90 s, where it reaches an intensity value which corresponds to CEtOH exp = 0.95 M indicative of complete TEOS hydrolysis. After t=90 s, the intensity of the EtOH peak drops to zero within a 20 seconds time-span. As images of the reaction beakers show it in Figure 3d, powder precipitation does occur in solution at that point; scattering due to silica particles is too important and Raman signal is lost.

When CTAB is added to solution, TEOS band disappears when about 2.5 EtOH per TEOS are produced, which corresponds to a total degree of TEOS hydrolysis of ~60%. This is lower than for the CTAB-free system, for which the total degree of TEOS hydrolysis is ~70%. This tells us that the hydrolysis is more homogeneous in the presence of CTAB, and gives further support for the importance of solubilization/emulsion stabilization by CTAB. As already mentioned, in the absence of CTAB, a fair fraction of TEOS will not have hydrolyzed at all at intermediate times, while a many other TEOS molecules have gone through several hydrolysis steps within the same time.

An overall hydrolysis rate constant kH can be introduced according to eq. ( 2) that corresponds to the averaging of the different hydrolysis reactions. 44 eq. ( 2)

with d[Si-OEt]/dt = -kH[Si-OEt][H2O] eq. (3)
In our system, one can consider that the concentration of water remains constant. Eq. ( 3) thus gives : The two resulting curves can be approximated with a monoexponential function according to eq (4). kH ≈ 0.5 M -1 .h _1 for the system without CTAB and kH ≈ 1.0 M -1 .h _1 for the system with CTAB. This analysis using a "global" hydrolysis rate is of course over simplified, but confirmed results reported in literature about a first-order kinetics for hydrolysis of TEOS in water-rich medium. 20,41 In the same way, an overall rate constant can be defined for condensation reaction kC.. eq. ( 5)

which indicates that in water-rich medium, condensation reactions between silanol groups is highly favored. Under such conditions, the rate of EtOH release should only depend on the hydrolysis reactions, and be independent from the initial concentration in TEOS [TEOS]0. On the contrary, the condensation rate constant should depend on [TEOS]0.

Insert Figure 4

For comparison purpose, a new set of experiments was performed with similar stirring conditions on the three systems A, B and C (Table 2). As shown in Figure 4, precipitation occurs in system A after about t = 60 s though complete hydrolysis is not fully achieved, while in system C full hydrolysis is achieved about 110 s before precipitation takes place in solution. Interestingly, the rate at which EtOH is released is very similar for the three systems, confirming that dilution has little influence if any, on hydrolysis reactions ([Si] = 0.08 M for system C and 0.43 M for system A). On the contrary, dilution plays a crucial role in the extent of TEOS condensation at the same acidic concentration (CHCl = 3.9 M): the lower TEOS concentration in solution, the longer for powder precipitation to occur. In addition, one may observe that silica precursor and acid concentration both play an important role in TEOS and derived-oligomers reactivity, as shown by faster hydrolysis/condensation occurring in system A (CHCl = 3.9 M) with respect to system B (CHCl = 2.5 M). Effect of acidity on TEOS reactivity is a well-known result, as already discussed in ref. 20, 40. However, the presence of CTAB in the starting acidic solution plays a key role regarding the TEOS hydrolysis kinetics. The most plausible reason for the observed increase in the hydrolysis kinetics in the presence of CTAB is the possibility for TEOS solubilization within micelles and/or kinetically stabilized emulsion droplets, which increases the surface/volume ratio of the TEOS droplets, enhances the TEOS-water contact, and thus leads to a higher TEOS hydrolysis rate. Hydrodynamic effects, related to the stirring speed, have previously been shown to have a pronounced influence on the hydrolysis kinetics of TEOS under alkaline conditions. 45 The importance of solubilization effects is highlighted by the fact that the observed hydrolysis kinetics is proportional to the CTAB concentration in the sol, and the solubilization power of CTAB calculated per molecule should increase when the CTAB concentration is higher than the critical micelle concentration, CMC. The high ionic strength of the sol due to the high concentrations in HCl should serve to further enhance the solubilization capacity of CTAB, as the repulsive charges between the surfactant head-groups in a surfactant aggregates would be effectively screened, which serves to stabilize surfactant aggregate of lower curvature. The influence of CTAB on the hydrolysis kinetics is then supposed to be the highest at the early stages of the reaction, as TEOS is a very hydrophobic molecule and it will be solubilized inside the interior of the micelles. As the hydrolysis proceeds, the water solubility of these partially hydrolyzed TEOS molecules will increase, and thus the preference for solubilization within surfactant aggregates is lower, and further hydrolysis can occur outside the micellar aggregates. reflection of the 2D hexagonal mesophase appears in the different cases is strongly system dependent, and it follows the order of TEOS-CTAB concentrations in the sols. For the most concentrated system, A, the (10) reflection is seen already after a reaction time of about 50 s, which corresponds to a total TEOS hydrolysis degree of about 60% (Figure 4), and this time is in good agreement with the time at which the solutions became turbid in the Raman experiments. For system B, the (10) reflection is visible about 80 s into the reaction, which corresponds to a TEOS hydrolysis degree of about 90%. For the most dilute system, system C, the first sign of a 10 reflection is seen after a reaction time of about 250 s, while complete hydrolysis of the TEOS has been reached already in less than 200 s. Taking the concentration differences in the different systems into account, (the TEOS concentrations in the starting sols have the approximate ratios 5.4:3.5:1 for system A:system B:system C) one can observe that the total hydrolysis extent at which some degree of order is observed in the diffractograms are similar for systems A and B, while the kinetics of formation of an ordered silicate-surfactant phase occurs at a lower total concentration of hydrolyzed TEOS in the sol for system C. As the TEOS hydrolysis kinetics was found to be strongly dependent on the stirring rate, the phase separation kinetics was also followed by a video camera, and the time at which the first signs of a Bragg reflection (10) appears in the respective diffractograms corresponds to the time at which the sol turns milky. This allows us to establish a direct link between the Raman measurements and the SAXS measurements, although the stirring rates may be slightly different in the two sets of experiments. For system A, the ( 11) and ( 20) reflections appear virtually at the same point in time, while the (20) reflection appears before the (11) reflection for systems B and C. This suggests that the silica wall thickness is larger for system B than for system A,as the Fourier phasing was found to be  for the 10, 11, and 20 reflections, similar to what has been observed for MCM-41 45 (results not shown). While the high order reflections appear just after the appearance of the (10) reflection in system A, there was a time difference of about 50 s for system B, and 150 s for system C. In addition to the (10), (11), and (20) reflections characteristic of a 2D hexagonal mesophase, an additional broad reflection of low intensity at lower angles (q about 0.8 (FB:1.1?) nm - 1 ) can be observed both in Figure 5c and Figure 6c after a reaction time of 800 s. The origin of the reflection is not known, but could indicate the presence of another co-existing phase of low concentration and mesoscopic order.

In

The positions of the main (10) reflection are plotted as a function of time for the different systems in Figure 7 The most striking observation is that the d-spacing of the 2D hexagonal phase is generally lower the lower the TEOS/CTAB concentration was in the starting sol at any given reaction time. The positions of the (10) reflection at the time of its first appearance are 4.22 nm, 4.29 nm, and 4.31 nm for systems A, B, and C, respectively. Furthermore, the d-spacing decreases with reaction time in all systems, at a rate which is proportional to the TEOS/CTAB concentration. Two distinctively different stages in the decrease in the d-spacing with time can be observed, the kinetics of which are also strongly dependent on the sol composition. A fairly fast decrease in the d-spacing is observed in all cases up to a reaction time of about 500 s for systems A and B, followed by a second stage of slower but almost linear decrease. For system C, on the other hand, the position of the (10) reflection remains virtually constant during the time of observation. The time at which the second stage is reached is strongly concentration dependent, and it is observed after a reaction time of about 500 s for system A, about 1400 s for system B and C, although for system C the decrease is not as evident as for system B. (See also the evolution of the position of the high order reflections with time in Figure 6). At the intermediate plateau, the corresponding distances are 4.12 nm, and 4.24 nm for systems A and B, respectively, (calculated directly as d = 1/s) which indicates an only marginal decrease in the repeat distance during the first phase. We note that this small decrease in the unit cell dimensions can not be connected to a decrease in the micellar diameters due to release of hydrolyzing TEOS from the interior of the micelles, as the decrease in unit cell dimension occurs at times well past that needed for complete hydrolysis of TEOS in system B. At a reaction time of 2000 s, i.e. well within the second stage of the process, the corresponding values are 4.03 nm and 4.17 nm for system A and B, which translates to a unit cell size of 4.65 nm and 4.81 nm, respectively. Thus, the repeat distances in the silicate-surfactant phase at any point in time follow the order of TEOS/CTAB concentration in the initial sol; the higher the initial TEOS/CTAB concentration, the smaller the repeat distance. This is also observed for the as-synthesized and dried materials isolated after a complete synthesis (time = 300 min.), for which the d-spacings are 3.71 nm, 3.82 nm, and 3.87 nm, for materials synthesized according to procedure A, B, and C, respectively (results not shown).

The integrated intensities of the (10) reflection are plotted as a function of time in Figure 8.

For all systems, an exponential increase in the integrated intensity is observed during the time window corresponding to the first stage of the decrease in the d-spacing shown in Figure 7. The rate at which the intensity increases during this stage is following the order system A > system B > concentrated phase. The different reaction steps can be partly overlapping. However, according to the fact that we carried out the measurements under different degrees of dilution, and also measured the extent of TEOS hydrolysis as a function of time, we can draw some more conclusions about the chemical events leading to the formation of the 2D hexagonal phase, and the influence of their relative rates.

From our data it is clear that the time at which phase-separation occurs is not related to the relative extent of TEOS hydrolysis, but to the absolute concentration of hydrolyzed species, as indicated by the observed kinetics in systems A and B. However, this is not the whole truth, as phase-separation occurs at absolute concentration of hydrolyzed TEOS, which is lower than those in systems A and B. (FB:??) Our results thus indicate that the condensation kinetics is also a crucial parameter in the formation of SBA-3, and that a certain level of silicate condensation has to be reached before phase-separation occurs. It is plausible that the first step observed in the s-spacing versus time plots is connected to the correlation distance in the phase separated silicate and surfactant phase, and that the nucleation and growth of the 2D hexagonal phase is very fast within this phase. The low micellar scattering intensity does not allow us to determine the shape of the micelles in the phase-separated silicate-surfactant phase, but one can assume that cylindrical micelles are present due to the short correlation distance observed, meaning the silicate-surfactant phase is fairly concentrated. The single (10) reflection observed for different times in the different systems before the high order reflections appear is normally observed for wormhole-like materials, which are formed from entangled micelles. However, we note that a single reflection has also recently been observed for mesoscopic silicate-CTAC nanoparticles synthesized under alkaline conditions with a larger repeat distance than what would be normally observed for 2D hexagonally ordered MCM-41 type materials. 46 In a recent study on MCM-41, local differences in the degree of silicate condensation was suggested to be present within growing domains with hexagonal order, based on detailed analysis of the time-dependent full-with-at-half-maximum of the reflections. It was suggested, based on earlier TEM studies, 47, that the cylinders inside the domains are more tightly packed during the stages where the silicate condensation reactions are still on-going, as compared to the rim of the particle facing the solution. The formation of larger aggregates that dry/condense faster in more concentrated sols, would work in the direction of a tighter micellar packing at least during the early stages of the formation process. This would explain the differences in the repeat distances observed between the studied systems, and it would also explain why system C does not reach the second stage of the process, where the 2D hexagonal phase nucleates and grows despite the fact that the local concentration of silicate-and surfactant is high, but the particle size may still be very small during the initial stages of the process, leading to a slower particle growth by aggregation. Actually, in a recent study domain growth by particle fusion was observed under alkaline conditions (MCM-41 type materials), 46 which would be another process strongly bound to particle concentration, which again could explain the slow kinetics of mesophase nucleation and growth in system C as compared to systems A and B. This view is supported by the development of the unit cell dimensions obtained for the three studied systems, and also by the increase in the FWHM values when the hexagonal phase appears. However, the evolution of the normalized FWHM values of the 10 reflection with time is very different as compared to that observed during the formation of MCM-41 at a pH of 10.8. 45 Here, the normalized FWHM of the (10) reflection decreases very soon after the 2D hexagonal phase appears, and remains virtually constant during the observation time. A similar evolution of the FWHM of the main reflection has also been observed for SBA-15, which is synthesized under acidic conditions using non-ionic tri-block co-polymers as the structure directing agent. 9 The normalized FWHM values of the (20) reflection of MCM-41 follows a similar evolution in time as observed for the (10) reflection in the present study, where the FWHM values initially increase, followed by a second phase of slow decrease. This very different behavior for MCM-41 and SBA-3 tells that the relative rates of the kinetic events leading to the formation of the fully developed mesophase are different in the two cases. For MCM-41, the FWHM evolution is suggesting that the mesophase formation involves a stage of second order disorder, which is compatible with a difference in the rate of silicate condensation within formed aggregates, which also can be a function of size of the ordered domains. 45-46-47 For SBA-3, the time-evolution of the FWHM seems to be controlled by a combination of several effects; an increase of the dispersion of d-spacings originating from the nucleation of a hexagonally order phase together with a co-existing less-ordered phase over extended periods of time, and an increasing FWHM in systems where the nucleation and growth of the hexagonal phase occurs before full hydrolysis of TEOS. When the hexagonal phase continues to grow, the FWHM values gradually decreases again. The most homogeneous nucleation and growth, but also the slowest growth kinetics, was observed for the most dilute system, system C, which had hydrolyzed completely before the phase-separation into the silicate-surfactant rich phase occurred.

The more dilute the sol, the slower should the particle-particle aggregation process be as well, which is another important step in the mesophase formation. The evolutions of the FWHM of the systems A and B also suggest that the particle-particle aggregation kinetics also play an important role, as has already been observed for MCM-41, [45][46] as particle-particle aggregation can lead to a faster drying of the particles due to a decreased particle-water contact area. 47 This process should be accompanied by a faster condensation kinetics in the hexagonally ordered phase, which is supported both by the lower d-spacings observed at higher CTAB/TEOS concentrations, as well as by the fact that the intensity ratio of the ( 11) to ( 20) reflection increases more pronounced for the slower reacting systems B and C, and by the initial absence of the ( 11) reflection. Such a behavior of the ( 11)/(20) intensity ratio is compatible with an increase in the silicate layer thickness in comparison to the radius of the micelles for systems having a Fourier phasing of -+ +. 45 Furthermore, the absence of the (11) reflection at the initial stages for systems B and C suggests that these systems are at the borderline between Fourier phases -+ +, observed for MCM-41, 45 and --+, observed for SBA-15, 9 which is another indication for a thicker silicate walls in systems B and C as compared to system A. Thus, our results suggest that the silicate layer thickness is initially larger at lower TEOS/CTAB concentrations. Furthermore, differences in the relative rates of domain growth of the 2D hexagonally order portions versus particle-particle aggregation may also contribute to the observed differences in the FWHM values between the systems. A fast particle-particle aggregation as compared to the domain growth rate may induce disorder if oriented attachment effects are not prevalent. Finally, we note that although the phase separation kinetics in the studied systems are compatible with that of MCM-41 synthesized using ammonia as the catalyst, the growth kinetics of the mesophase is slower, which can be ascribed to a slower overall silicate condensation rate under the studied conditions for the SBA-3 system as compared to MCM-41 synthesized in the presence of ammonia, and that the overall ordering kinetics is slower. In this respect, also the absence of strong electrostatic interactions between the positively charged structure directing agent and the silicate species under acidic conditions, where the silicate species carry a close to neutral or slightly positive charge, may influence the silicate condensation kinetics in the mesophase. Under alkaline conditions, where the silicate species carry a negative charge, the formation of cylindrical micelles should be faster as compared to under acidic conditions, as surfactant charge neutralization favors the transition from spherical to cylindrical micelles, 48 which should increase the rate of ordering in the mesophase.

Conclusion

This work deals with a complete time-resolved in-situ study of CTAB templated silica under highly acidic conditions. We meant to follow the silica precursor reaction kinetics as well as material ordering with time. Raman spectroscopy was employed here for the first time to follow TEOS reactivity in water under unmodified synthesis conditions. Hydrolysis reaction occurs 3 times faster when CTAB is added to solution and we have explained this behaviour to smaller size of TEOS droplets in water, that is to the highest exposed TEOS area. After excluding several hypotheses, we think that either micelles or free surfactant molecules might disperse TEOS emulsion. Both of them might be consistent with known literature data and with SAXS experiments that are presented here.

In fact, once Raman spectroscopy ceased to be useful, at 140 s from TEOS addition when a precipitate forms in solution, SAXS data started to show the beginning of a mesophase formation within the observed precipitate. A correlation distance appears directly and it evolves into a 2Dhexagonal mesophase, the one observed in final material. ED calculations of SAXS patterns at different reaction times show the direct image of the micelles within the material as well as the evolution with time of silica condensation. It is clear that at early stage of reaction, inter-micellar condensation is mostly observed while intra-micellar condensation only takes place with time. This behaviour is opposite to what it was observed in SBA-15 materials. * Each + sign corresponds to one of the following criteria : 1) no external species introduced in the reacting medium (e.g. paramagnetic or fluorescence probes, etc…); 2) no modification of chemical composition (e.g. dilution, use of D2O, use of TMOS vs. TEOS, etc…); 3) no modification in the synthesis process (e.g. no stirring, smaller reaction volumes, etc…). ** The signs correspond to the characteristic measurement time of the related technique, tm; + + +: 0 < tm < 10 s; + +: 10 s < tm < 1 min; +: 1 < tm < 10 min; -: tm > 10 min.

[

  Si-OEt]t = [Si-OEt]0.exp(-kH[H2O]0.t) eq. (4) Si-OH + HO-Si  Si-O-Si + H2O kC The evolutions of the concentration in Si-OEt species versus time, [Si-OEt]t, can be obtained from the evolution of the concentration in EtOH in Figure 3b and 3c ([Si-OEt]t = 4[TEOS]0 -[EtOH]t).

5 , 6 .

 56 -situ SAXS experiments. Time-resolved in-situ SAXS experiments were used to follow the mesophase evolution. The experimental conditions were similar to those used for the Raman experiments shown in Figure 4. Time-resolved XRD patterns for systems A-C are shown in Figure and surface plots of the same measurements are shown in Figure The time at which the(10)

Figure 1 :

 1 Figure 1 : Scheme representing our time-resolved in-situ approach to study the formation of

Figure 2 :

 2 Figure 2 : Raman spectra of CTAB/H2O = 0.12/200 and CTAB/EtOH/H2O = 0.12/4/200 solutions

Figure 3 :

 3 Figure 3 : Time-resolved in-situ Raman spectra of (a, b) TEOS/H2O (1:200) and (c, d)

Figure 4 :

 4 Figure 4 : Comparison of Raman peak intensity of ethanol (885 cm -1 ) as a function of time for

Figure 5 :

 5 Figure 5: Time-resolved XRD patterns showing the formation of SBA-3 for systems A-C. See text

Figure 6 :

 6 Figure 6 : Surface plots corresponding to the time-resolved XRD patterns for systems A-C. Data

Figure 7 :

 7 Figure 7 : Time-evolution of the position of the (10) reflection of the 2D hexagonal SBA-3 phase for

Figure 8 :

 8 Figure 8 : Time-evolution of the integrated intensity of the (10) reflection of the 2D hexagonal SBA-

Figure 9 :

 9 Figure9: Time-evolution of the full-width-at-half-maximum, FWHM, of the respective(10)

Figure 1 33 Figure 2 Figure 4 36 Figure 5 a

 3324365 Figure 1

  work, measured intensity was related to ethanol concentration in water by mean of a calibration curve; experimentally XEtOH= 6678 ± 440. We can then use this value to calculate CEtOH at any time during evolution. At t= 300 s, for instance, CEtOH exp = 0.96 M, which is comparable with the theoretical value, CEtOH th = 1.10 M that is expected if 4 EtOH are produced per TEOS. Consequently, t= 300 s is thus the minimum time needed for TEOS to fully hydrolyze under the employed

experimental conditions and in absence of surfactant. Raman theoretical intensities corresponding to the release of 2, 3 or 4 ethanol molecules per TEOS are reported in Figure

Table 1 .

 1 Evaluation of experimental techniques used in literature to perform time-resolved in-situ studies on mesostructured silica powders obtained by precipitation in solution. The number of + signs measures the suitability of the technique for in situ and/or time-resolved studies.

	Techniques	Output	In-situ*	Time-resolved**
	TEM	Micelle aggregates; Ordering	-	+ +
	SAXS	Shape of aggregates; Ordering	+ + +	+ + +
	EPR	Organic/Inorganic interaction;	+ (+)	+ + +
		Micelle aggregates
	DLS	Apparent aggregate size	+ (+)	+
	Fluorescence	Organic/Inorganic interaction;	+	+ + +
		Micelle aggregates
	NMR 1 H	Surfactant mobility	+ +	+ +
	2 H	Ordering; mesophase	+	+ +
	29 Si	Extent of hydrolysis/condensation	+ +	-
	17 O	Hydrolysis	-	+ +
	14 N	Mesophase growth	+	+
	FT-IR	Reactivity of inorganic species	+	+ + +
	Raman	Reactivity of inorganic species	+ + +	+ + +

Table 2 .

 2 List of the systems under investigation.

	System		Molar Composition		CSi(M)	CHCl (M)	CCTAB (M)
		TEOS	CTAB	HCl	H2O
	A	1	0.12	9.2	130	4.3 10 -1	3.9	5.0 . 10 -2
	B	1	0.12	9.2	200	2.8 10 -1	2.5	3.3 . 10 -2
	C	1	0.12	49.5	700	0.8 10 -1	3.9	9 . 10 -3

Table 3 .

 3 Raman bands of CTAB, Ethanol, TEOS and silica gel reported in lietrature.

	Species	Wavenumber (cm -1 )	Assignments
	CTAB 38	763	NCH rock. from N + (CH3)3
		1063	CC stretch + CH2 wag.
	Ethanol 37	436	OC def.
		885	CCO sym. Vibr.
		1055	C-O sym. stretch.
		1095

C-O asym. stretch.

system C, i.e. the order of increasing TEOS/CTAB concentration in the starting sol. For systems B and C, the intensity then reached a plateau, while the intensity increases almost linearly at reaction times corresponding to the second stage of the decrease in the d-spacing shown in Figure 7.

Furthermore, it is interesting to note, that the ratio of the integrated intensities after the first stage, i.e. after a reaction time of 500 s (system A), 1000 s (system B), and 1500 s (system C), follows virtually exactly the corresponding ratios of the initial CTAB/TEOS concentrations in the different sols. This suggests that the amount of phase-separated silicate-surfactant is virtually the same in all cases, if normalized against the initial concentrations of CTAB and TEOS.

Finally, the evolutions of the full-width-at-half-maximum, FWHM, of the respective (10) reflections are shown in Figure 9. The FWHM values are normalized against the position of the (10) reflection in order to be able to directly compare the different FWHM values with each other. There is a huge difference in the values, with system A initially having by far the largest FWHM values followed by system B. The normalized FWHM values of system C are more than 4 times lower than those observed for systems A and B. Generally, a pronounced increase in the normalized FWHM values are observed after the 2D hexagonal mesophase has formed, and this effect follows the order system C < system B < system A. For systems A and B, a slow decrease in the normalized FWHM values is observed at times corresponding to the initiation of the second reaction stage observed in the d-spacing vs. time plots, corresponding to a reaction time of about 500 s for system A, and about 1400 s for system B.

General mechanistic discussion

Generally seen, our data is in full agreement with the common understanding that the formation of silicate-CTAB mesophases in dilute sols using alkoxides as the precursor for silica involves the following steps: 1) Hydrolysis of the silica precursor followed by condensation leading to the formation of oligomeric siliceous species. Here the hydrolysis kinetics of the silicon alkoxide is highly enhanced in the presence of the surfactant due to micellar solubilization and surfactant stabilization of emulsion droplets, 2) Phase separation into one phase rich in silicate and surfactant and another solvent-rich phase, 3) Nucleation and growth of a surfactant-silica mesophase in this


Camillo Falco

Niki Baccile

Maria-Magdalena Titirici
email: magdalena.titirici@mpikg.mpg.de

Morphological and Structural Differences between Glucose, Cellulose and Lignocellulosic Biomass Derived Hydrothermal Carbons

Keywords: Hydrothermal Carbonization, Lignocellulosic Biomass, Solid-State MAS 13 C NMR, HTC Temperature, Carbonization mechanism

come

Morphological and structural differences between glucose, cellulose and lignocellulosic biomass derived hydrothermal carbons

Introduction:

Rising environmental concern and the need for pricewise market competitive products are becoming two major guidelines in modern material research. As a consequence the main effort in this field is primarily directed towards the production of new and effective materials, which can be synthesised from cheap natural precursors and through environmentally friendly and efficient processes.

Due to their unique properties, functionalised carbon materials have attracted a great deal of attention in the past and represent a hot topic in nowadays material research [1][2] [3]. Even though many synthetic methods are known for carbon materials, most of them still require harsh and rather expensive processing conditions such as very high temperatures, arc discharge, or decomposition from the plasma or gas phase [4][5] [6].

In recent years Hydrothermal Carbonisation (HTC) has instead demonstrated its capability of converting biomass into highly functionalised carbon materials under very mild processing conditions [7] [8]. Even though this methodology was developed almost 100 years ago [9], its full potential, as a synthetic route for carbon materials having potential applications in several fields such as catalysis, energy storage, CO2 sequestration, water purification, agriculture, has been fully understood only in the last decade [10] [11]. For this reason Hydrothermal Carbonisation has been lately indicated as the leading pathway towards the so-called ''chimie douce'' of carbon materials [12].

One of the main advantages of HTC is that it can successfully exploit cheap and renewable biomasses as carbon precursors. Till now, simple monosaccharide and oligosaccharides have been effectively employed as starting HTC materials [13]. Glucose especially, being one of the cheapest and most abundant carbohydrates, has been extensively studied. A detailed investigation of its transformation mechanism into HTC carbon has been carried out by using advanced 13 C solid-state NMR experiments [14]. This characterisation technique has proved to be a very powerful tool for the analysis of carbon materials [15], revealing important hints regarding the final structure of the HTC materials.

The next challenging step in HTC is the effective exploitation of lignocellulosic biomasses as carbon precursors, which would guarantee a readily accessible and carbon negative feedstock supply. Cellulose is a biopolymer made from glucose monomers and the main component of such raw biomasses. Therefore, there are clear expectations that HTC could represent an effective method to convert lignocellulosic biomasses into high value carbonaceous materials. Their use, as carbon precursor, is however definitely more complex than that of simple monosaccharides due to a higher degree of structural complexity [16], which is expected to affect the HTC mechanism.

The aim of this study is to understand the transformation mechanism involved in the hydrothermal carbonization of lignocellulosic biomasses. For the purpose of this investigation a relatively low lignin content biomass was chosen: rye straw (composition in table 1). This raw material is an agricultural waste; therefore it is available for costs well below petrochemical based chemicals.

Table 1: Rye Straw wt% composition

In order to get useful insights on the transformation mechanism of rye straw into HTC materials, we will also study individually the HTC mechanism of pure cellulose and alcell lignin by analyzing how the chemical nature and morphology of the produced HTC carbon change in relation to the processing temperature and time. The obtained results will then be compared to glucose, which will be used as a reference model. This study will allow us to highlight and discuss the probable differences in the HTC mechanism of each raw material and to determine the role of lignin in the hydrothermal carbonization of lignocellulosic biomasses.

Cellulose

Experimental procedures:

D(+)-glucose and cellulose powder were used as received (Sigma Aldrich). The raw Rye Straw as received (TU Hamburg) was grinded to a maximum size limit of 0.8 µm for an easier processing.

For each experiment 10 wt/v biomass in water was prepared and placed into 50 ml Teflon lined stainless steal autoclaves. After sealing the autoclaves were heated into a programmable oven for 24hr

(if a different time is not specified in the text) between 120 and 280°C. The hydrothermal carbon, obtained from each experiment, was abundantly washed with deionised water and then dried in a vacuum oven at 80 o C.

Characterisation:

Elemental chemical analysis was performed on a (C, N, O, S, H) Elementar Vario Micro Cube. SEM images were acquired on a LEO 1550/LEO GmbH Oberkochen provided with a Everhard Thornley secondary electron and In-lens detectors. FT-IR spectra were recorded using a Varian 1000 FT-IR apparatus in ATR (attenuated Total Reflectance) mode. Each sample was investigated in the wavenumber range of 4000-500 cm -1 using 32 scans at a spectral resolution of 4 cm -1 .

Results and discussions:

The first section of this study will focus on the effects of processing temperature upon the morphology of the HTC carbon obtained from glucose, cellulose and rye straw. Then, an analysis of the HTC yield and elemental composition of the hydrothermal carbon obtained from these three different precursors will be presented. Finally, we will discuss the results of 13 C solid state NMR experiments, which have already proved to be extremely helpful to determine the final chemical structure of our HTC materials [13][14].

Morphology of the hydrothermal materials obtained from different biomass precursors:

Fig 1 shows some scanning microscopy micrographs of hydrothermal carbon spheres obtained from pure glucose. It can be observed that the temperature affects both the carbonaceous particle average diameter as well as the size distribution: higher temperatures (260°C) lead to larger particles and a more homogeneous average size. This observation was also confirmed by DLS measurements. In the case of the material prepared at 160 o C, an average particle hydrodynamic radius of 474 nm was measured, while for the 260 o C samples the mean value was 685 nm. For samples prepared at higher temperature, the correlation between DLS and SEM measurements is reasonable. On the other hand, for the materials prepared at 160 o C, the measured values using DLS is higher than the ones observed in the SEM pictures. This might be due to the wider size distribution of this sample and therefore to the presence of very large particles, which could affect the overall diameter obtained from light scattering and therefore increase the average error. Another factor that can possibly contribute to the value mismatch might be the presence of particles aggregates in the 160 o C sample, which could increase the value of the measured hydrodynamic particle radius. However, although the DLS measurements do not allow determining a precise estimate of the particle size for the sample processed at lower temperature, it is important to underline that they quantitatively confirm the increase in the average particle diameter as the HTC processing temperature becomes higher, which could be observed from SEM measurements. A possible explanation for this finding might be that water properties are known to change substantially upon increasing the temperature under self-generated pressures; therefore its solvent ability and its reacting behavior are highly affected over a narrow temperature range. This probably can play a significant role in affecting the particles' surrounding environment, which determines their colloidal stability and the coupled size features [17][18]. of hydroxymethylfurfural [19]. Initially, their diameter is approximately in the 100-200 nm range (Fig. 3a). As the residence time increases, they keep on growing until they reach their final size, which depends upon the HTC processing temperature (Fig. 1). On the other hand, in the case of cellulose upon increasing the temperature the fibrous network starts disrupting in several points (Fig 2 ,3a), leading to the formation of nano\micro-sized cellulose fragments, which, being not soluble in water, adopt the spherical shape to minimize their contacting interface with the surrounding.

Yields and elemental composition:

The yields of the final solid residues obtained upon hydrothermal carbonization were determined using the following formula: lignin and rye straw), the HTC yield gradually decreases from an initial value of 100% due to the increasing extents of liquefaction and gasification of the biomass at higher temperatures values, which are parallel reactions to the process of HTC formation [20][21].

HTC _ yield% = amount _ of _ recovered _ solid _ after _ HTC(g) initial _ amount _ of _ biomass(g) (1)
Two important conclusions can be withdrawn looking at the yield versus temperature plot (fig. 5a) in the case of hydrothermally treated glucose:

1. The minimum temperature for the HTC formation from glucose is 160°C. Below this value no measurable quantity of solid residue can be recovered. This means that under these processing conditions the overall extent of hydrothermal carbonization reaction is practically zero.

2. The process can be tuned to optimize hydrothermal carbon yield. The maximum yield is achieved at 200 o C, after which it starts to gradually decline. An explanation for this downward trend is that the increase in temperature favors gasification reactions [20]. As a consequence, part of the HTC carbon is lost in the form of volatile compounds [START_REF] Berge | Hydrothermal Carbonization of Municipal Waste Streams[END_REF]. However it must be kept in mind that the theoretical HTC yield, that is achievable when glucose is used as a starting material, is 60% (grey dotted line in Fig. 5a).

This value can be easily derived from the mechanism of HTC formation, since from each molecule of glucose, which reacts to form HTC carbon, four water molecules (40% of the starting mass) are respectively eliminated[23] [24]. These represent an unavoidable yield loss, which must be taken into account. From figure 5b, it is possible to observe how the carbon content of the glucose-derived HTC material changes as a function of temperature. This quantity can be observed to undergo a constant and gradual increase as the reaction temperature is set at higher values. The only sharp change, a difference of approx. 20%, is detectable between the carbon content of raw glucose and the one of the HTC carbon produced at the lowest possible temperature (160 o C). The explanation for this abrupt change is the dehydration step that glucose molecules undergo in the early stages of hydrothermal carbonization [13][14][23] [24].

By combining the calculated yields at different temperatures (Fig. 5a) with the respective elemental analysis (Fig. 5b), it is possible to investigate the mass loss due to increasing temperature as a function of the different composing elements (i.e. carbon, oxygen, and hydrogen). As it can be observed from Similarly to cellulose, abrupt changes in both the HTC yield and the carbon content are observed during hydrothermal carbonization of rye straw at different temperatures. However there are two main differences that need to be underlined:

1. Rye straw is more sensitive to hydrothermal treatment in the low temperature range (120-180 o C). A probable explanation for this observation is the presence of hemicellulose and xylose-based polysaccharides within the rye straw. These biopolymers are known to be less stable than cellulose when hydrothermally treated; as a consequence they can undergo hydrothermal carbonization in a lower temperature range with a consequential yield loss due to formation of liquid and volatile side products [26].

2. The changes in HTC yield and carbon content are less sharp than in the case of cellulose in the region immediately above 180 o C and they appear to be slightly shifted to higher temperature values. The lignin, present within the lignocellulosic biomass, could be the cause of such differences. As it can be observed in figures 5a-b, this compound is affected by hydrothermal treatment to a very limited extent. As a consequence its presence might cause the HTC yield of rye straw to decrease less abruptly as temperature increases. Furthermore, since lignin acts as support within the plant wall [27], it might stabilize the cellulose and prevent its crystalline structure disruption at lower temperatures.

The data regarding yields and elemental analysis highlight an interesting feature, which is common to all the investigated precursors: they all follow similar trends for both the HTC yield and carbon content for temperature values, which are above 220 o C. This might be due to the fact that above this temperature value all substrates follow similar chemical reaction pathways. This hypothesis, as it will be seen in the next section, is supported as well by 13 C solid state NMR studies.

13 C solid state NMR analysis

Valuable information concerning the final chemical structure of hydrothermal carbon can be obtained using 13 C solid state NMR Figure 7a shows a comparison of the 13 C CP NMR spectra of glucose after hydrothermal treatment at different temperatures. At 180 o C, the hydrothermal carbon obtained from glucose has a polymer-like structure composed of polyfuranic chains domains, as Baccile et al. have demonstrated [14].

Increasing the HTC temperature leads to a hydrothermal carbon with a higher degree of aromatization, very close to what normally happens after a pyrolysis treatment [28]. Two main evidences support this finding. First of all, there is an obvious decrease in the relative intensity of the peaks at 150 and 110-118 ppm, which are known to relate to O-C=CH-and O-C=CH-carbons composing the furanic ring [13]. The second reason is the increase in relative intensity of the peak at 125-129 ppm, which can be assigned to carbon atoms belonging to aromatic rings [13].

The evolution of the peak at 208 ppm with temperature also shows that the HTC carbon produced at 180 o C is rich in carbonyl functionalities. However, increasing the HTC temperature leads to a decrease in the oxygen content and to the loss of such functionalities, which in 13 C-NMR translates into a reduced relative intensity of the 208 ppm peak. This last observation is in agreement with the observation of the previous sections.

In addition, the relative intensity of the aliphatic region peak can be observed to be fairly constant for the NMR spectra of the samples treated at 180, 200 and 240 o C while a sharp decrease takes place at 280 o C. This observation can be correlated with the carbon loss, which was observed in the previous section during HTC of glucose at temperatures higher than 250°C (fig 6). This loss is most probably due to the less thermally stable aliphatic carbon chains that are normally present in hydrothermal carbon produced at lower temperatures [14].

Figure 8a shows the time evolution of the 110-150 ppm region during hydrothermal carbonization of glucose at T= 240°C. In the early stages of the reaction, the central peak at 125-129 ppm is practically absent. The whole region is characterized only by the presence of the two peaks, which are due to the furanic rings (150 and 110-118 ppm). As the HTC residence time increases, the relative intensity of these latter peaks diminishes while the central peak at 125-129 ppm starts forming and becomes more and more prominent. From these observations it is possible to deduce that if the temperature is high enough (above 200 o C) and the residence time increases, the polyfuranic chains, which normally compose the HTC carbon obtained at 180 o C, react further via intramolecular condensation, dehydration and decarboxylation reactions to create more condensed sp 2 hybridized-aromatic chemical species. A more detailed structural scheme and an analysis, which estimates the ratio of furanic species to aromatic species can be found in 28 Erreur ! Signet non défini.. This finding is important as it suggests that the hydrothermal carbonization of cellulose does not proceed solely through a polyfuranic intermediate, as in the case of glucose. The major mechanism route is instead presumed to be the direct transformation of the cellulosic substrate into a final carbonaceous material composed of extensive aromatic networks.

In the case of hydrothermally carbonized rye straw, looking at the temperature dependent 13 C NMR spectra in Fig 7c, it can be observed that cellulose is still present up to 200°C, while one would expect it to be decomposed at this temperature based on the data shown in fig Fig . 7b. This observation is in good agreement with SEM and yield analysis observations, where we attributed the higher temperatureresistance to the presence of lignin fractions within the biomass. Furthermore, according to 13 C NMR data, the chemical structure of the HTC carbon obtained from rye straw is practically identical to the one obtained from cellulose. The only difference between the 13 C NMR spectra of the figures 7b and 7c is the presence of two additional peaks (56 and 145-148ppm) in the case of the lignocellulosic biomass. 13 C CP NMR investigations of an alcell lignin sample have shown that these peaks can be respectively assigned to the methoxy functional groups (-OCH3) and to aromatic carbons bounded to such moieties [30][31] [32], which are present in the lignin fraction of the biomass (Fig. 7c).

Structural differences between hydrothermal carbons from glucose and cellulose: from simple carbohydrates to raw biomass

During the initial stages of the hydrothermal treatment of glucose 5-hydroxymethylfurfural (HMF) is abundantly formed as well as some major degradation products such as levulinic acid, dihydroxyacetone and formic acid [14][21][23]. The acidic compounds act as an in situ catalyst promoting further dehydration of glucose into HMF, which subsequently undergoes a series of polymerizationpolycondensation reactions leading to the formation of polyfuranic type compounds as confirmed by solid state NMR [13]. During this polymerization step, nucleation occurs upon segregating the newly formed polymer phase out of the aqueous solution. Depending upon the processing temperature, the HTC carbon particles can then achieve various final diameter sizes. In addition, as it has been observed from the NMR experiments, if the processing HTC temperature is set at values higher than 180 o C, the polyfuranic chains further react via intramolecular condensation, dehydration and decarboxylation reactions generating an HTC material with an increasing aromatic nature and with a lower fraction of oxygen containing functional groups (carboxyl and carbonyl groups). The final degree of aromatization of the produced carbon is highly dependent upon the HTC processing temperature, since this parameter determines the ratio of furanic-to-aromatic species. This latter quantity can be roughly estimated between 5 and 0.5 from studying the ratio of the relative integrated intensities of the corresponding peaks in 13 C solid state NMR [28].

The postulated cellulose HTC mechanism is believed to be hydrolysis into individual monosaccharides (glucose units) [33], which then should follow the same reaction pathway as glucose during hydrothermal treatment. However, such a process involves the presence of strong acid catalysts or harsher processing conditions [27] [34][35][36][37]. When cellulose is hydrothermally treated in the absence of any additional catalysts and under mild processing conditions, its structure stays practically unaffected below 180 o C. Above this temperature the cellulosic substrate generates an HTC material that possesses a well-developed aromatic nature (peak at 125-129 ppm) since the early stages of the reaction. As previously explained, this evidence underlines the fact that the HTC mechanism for cellulose does not exclusively go through a polyfuranic intermediate, as it happens for glucose, but it might majorly involve reactions that are normally characteristic of the pyrolysis process. This idea is well supported by the similar 13 C NMR profiles of cellulose derived HTC carbon and char obtained from lignocellulosic biomass pyrolysis [31] [38]. During cellulose pyrolysis, the char formation is attributed to a series of reactions, which lead to cellulose intramolecular rearrangement and formation of an intermediate cellulose derived polymeric compound, that some authors refer to as intermediate cellulose and which then converts to aromatic network structures [39][40][41][42].

As shown by SEM, when cellulose is treated under hydrothermal conditions above 180 o C, its fibrous network is disrupted leading to the formation of nano/micro sized cellulose fragments, which forms spherical envelopes to minimize the contacting interface with the surrounding water, hence reducing the chances of hydrolysis of the glycosidic bond. The cellulose present in the bulk instead is likely to be exposed to a homogeneous thermal environment that resembles the one of pyrolytic processes. Under these conditions the cellulosic substrate undergoes intramolecular condensation, dehydration and decarboxylation reactions, which lead to the production of a hydrothermal carbon structurally composed of extensive aromatic networks. A very limited degree of hydrolysis and consequential glucose formation most likely takes simultaneously place at the envelopes' interface where cellulose is in contact with water. However this reaction mechanism accounts only for a minor part of cellulose conversion (Scheme 1). This mechanistic scheme is well supported by the fact that other non-reducing carbohydrates, such as starch and amylopectin, contrarily to cellulose do yield furan-rich HTC carbon [13]. The explanation for this evidence is that the polysaccharide chains of these other biopolymers are not as tightly bound to each other as in the cellulose case. As a consequence their hydrolysis into the monosaccharide building blocks can take place at lower reaction temperatures (<180 o C) allowing the subsequent generation of a polyfuranes rich HTC carbon. This proposed model is in good agreement with the findings of Kersten et al [41] that highlight that formation of HTC carbon from lignocellulosic biomass has a primary and secondary contribution. The former corresponds to the pyrolysis-like mechanism that we have related to cellulose conversion in the bulk. The latter, comprises instead the polymerization reactions of the intermediate liquid products derived from cellulose dissolution. According to Kersten et al the secondary contribution is the major route of cellulose conversion into HTC carbon. This last observation is in contrast with our model that highlights instead that the primary or pyrolysis-like conversion mechanism is the predominant one. This disagreement might be simply due to different operating conditions, which have been employed in the two sets of experiments. Kersten et al have in fact used higher temperatures (350 o C) for the hydrothermal treatment, which might have increased the extent of cellulose dissolution and of its subsequent conversion through the secondary route.

The developed model also applies to lignocellulosic biomass. As previously observed, the hydrothermal carbonization of rye straw is majorly governed by the reactivity of its cellulosic fraction, which is shifted to higher temperatures (240 o C) compared to pure cellulose (200 o C). Despite this increase, it is interesting to notice how under hydrothermal conditions the cellulose present in lignocellulosic biomass fully reacts in a lower temperature range than during pyrolysis [31] [38]. A possible explanation for such a difference might be the relative higher pressure (25-30 bars) present during hydrothermal treatment, which might destabilize the cellulose structure in a lower temperature range. This evidence is further supported by the loss of lignin methoxy groups, which under hydrothermal conditions takes place between 240-280 o C while during pyrolysis it does not happen before 350 o C [38][43].

Conclusions

This work unveils several features of the carbonization mechanism under hydrothermal conditions of a wide range of carbon precursors (simple carbohydrates to lignocellulosic biomass). It demonstrates that temperature, even at mild processing conditions, is a powerful switch, which allows tuning both the chemical nature and the morphology of the produced hydrothermal carbon.

In the case of glucose, changing the processing temperature allows controlling both the particle diameter and particle size distribution. The final carbon chemical structure can be switched from carbonaceous polyfuran rich in oxygen containing functional groups type to a carbon network of extensive aromatic domains.

On the other hand in the case of cellulose and generally of lignocellulosic biomasses a fundamental difference in the HTC mechanism is observed. The polyfuranic intermediate, which is characteristic for glucose derived HTC materials at either low processing temperature or short reaction times, cannot be isolated in cellulose derived HTC carbons. Contrarily these latter materials show a well-developed aromatic nature since the early stages of reaction. This finding leads to the conclusion that under mild hydrothermal operating conditions, such as the ones employed in this investigation, the cellulose present in the bulk reacts according to a reaction scheme that can be associated to classical pyrolysis process, even if the exact chemical paths are not clear yet.

This study also shows how lignin is only mildly affected by HTC and how its presence influences the hydrothermal carbonization of lignocellulosic biomasses. For rye straw the temperature, the formation of hydrothermal carbon starts at, shifts to higher values than in the case of pure cellulose. In spite of this increase, the results of this investigation show that the full conversion of cellulose into a final carbonaceous material takes place in a lower temperature range under hydrothermal conditions than during pyrolytic treatment.

Furthermore experimental evidences highlight that lignin also contributes to maintain the natural macrostructure of the initial biomass into the final HTC carbon products. This feature could represent a powerful tool to produce structured carbon materials directly from natural templates.

Overall this work further underpins the strengths of hydrothermal carbonization as a green synthetic method to produce functionalized carbon materials. It extends the range of the possible carbon precursors to lignocellulosic biomasses by proposing a model that explains their transformation to carbonaceous materials. Lastly it shows the first promising results for real waste biomass usage as carbon precursors opening thus a new door towards novel and exciting applications of natural HTC materials.

Figure 1 :

 1 Figure 1: Scanning electron micrographs of hydrothermal carbon materials obtained from glucose at

8 Figure 2 :

 82 Figure 2: Scanning electron micrographs of hydrothermal carbons obtained from cellulose at 160 o C (a,

Figure 3 :Figure 4 :

 34 Figure 3: Scanning electron micrographs of hydrothermal carbons obtained from a) glucose at 180 o C,

Fig 5a compares the

  Fig 5a compares the yields obtained upon hydrothermal carbonization of glucose, cellulose,

Figure 5 :

 5 Figure 5: a) Calculated hydrothermal carbon yields and b) carbon content obtained from chemical

figure 6 ,

 6 figure 6, in the temperature range 200-250 o C the decrease of the HTC yield is majorly due to oxygen

Figure 6 :

 6 Figure 6: HTC yield normalized elemental composition of hydrothermal carbon obtained from glucose

Figure 7 :

 7 Figure 7: Solid state 13 C CP NMR spectra of hydrothermal carbon obtained from a)glucose, b) cellulose

Figure 8 :

 8 Figure 8: Solid state 13 C CP NMR spectra of hydrothermal carbon obtained from a) glucose at 240 o C, b)

Scheme 1 :

 1 Scheme 1: Proposed model for cellulose hydrothermal carbonization under mild processing conditions (180 o C < T < 280 o C). Dotted lines represent minor reaction routes. A more detailed structural description of the aromatic networks composing the final HTC carbon can be found in reference Erreur ! Signet non défini..

Figure 1 : 28 Figure 2 :

 1282 Figure 1: FTIR spectra of hydrothermal carbon obtained from glucose at different temperatures.

  Kα1 radiation (λ = 1.54 Å). Samples were grinded and measured on silicon or deepened plastic samples holders. Each sample was scanned in the range of 2 o < 2θ < 55 o with a step size of 0.05 o at a scan rate of 4 s per step.

	Proton-to-carbon CP MAS was used to enhance carbon sensitivity with a cross polarisation time equal
	to 1 ms.
	Solutions were investigated by dynamic light scattering (DLS) using ALV-7004 Multiple Tau Digital

13

C solid-state MAS NMR experiments were acquired on a Bruker Avance 300 MHZ

(7 T)

spectrometer using 4 mm zirconia rotors as sample holders spinning at MAS rate νMAS= 14 kHz. The chemical shift reference was tetramethylsilane (TMS; δ=0 ppm). 1 H t1 relaxation time was set to 3s.

Correlator equipped with CGS-3 Compact Goniometer system, 22 mW He-Ne laser (wavelength λ= 632.8 nm) and pair of avalanche photodiodes operated in a pseudo-cross-correlation mode. All published measurements were made at a 90° angle. XRD patterns were recorded with a Bruker D8 apparatus (θθ reflection mode), operating with Cu-