Niki Baccile
email: niki.baccile@upmc.fr

Guillaume Laurent

Florence Babonneau

Franck Fayon

Maria-Magdalena Titirici

Markus Antonietti

Structural Characterization of Hydrothermal Carbon spheres by Advanced Solid-State MAS 13 C NMR Investigations

Keywords: . Carbon, Biomass, Hydrothermal Carbonization, Glucose, 13 C solid-state MAS NMR, Double quantum NMR

The local structure of carbon spheres obtained via the hydrothermal carbonization process is characterized by using a combination of advanced solid-state 13 C NMR techniques.

Glucose was chosen as the starting product because it offers the possibility of 13 C isotopic enrichment and is regarded as a model compound for more complex polysaccharides and biomass, as reported in recent studies. A number of 13 C solid-state MAS NMR techniques (single pulse, cross polarization (CP), inversion recovery cross polarization (IRCP), INEPT, 13 C-13 C proton-driven magnetization exchange and 13 C-13 C double quantum-single quantum (DQ-SQ) correlation experiments) were combined to retrieve information about binding motifs and C-C closest neighbor relations. We found that the core of the carbonaceous scaffold is composed of furan rings crosslinked by domains containing short keto-aliphatic chains instead of otherwise expected graphenetype sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (< 350°C) pyrolisis treatment.

Introduction

In a series of recent publications 1 -5 a sustainable approach to the synthesis of powders, mesoporous, or nanostructured carbon materials using biomass as starting product was presented: carbohydrates and raw biomass treated at relatively low temperatures (160 < T < 220 °C) in water in autoclave can provide a carbonaceous colloidal spheres and particles with roughly 60-70 w% carbon content. Different reasons motivated these works. Firstly, there is a need to explore new cheap and sustainable ways to obtain chemicals 6 and advanced functional carbon materials from sustainable raw materials to replace on the long run crude oil. Secondly, in case of a really cheap pathway and useful mass products, recycling the byproducts of farmed biomass would additionally be a way to sequester significant amounts of CO2, creating a material benefit at the same time. The way our group approached the topic was the use of hydrothermal synthesis between 180°C and 220°C to obtain carbon-based colloidal spheres 1,2,15 , nanofibers 7 , 8 or sponge-like mesoporous carbons being meaningful as soil conditioner, ion exchange resins or sorption coals 3,5 . The synthesis proved to be feasible when glucose 2 is used or even when raw biomass materials like oak leafs and orange peals are taken 3 . In the first case, surface chemistry could also be modified by mean of hydrophilic 5 or hydrophobic coupling agents 9 . Along similar lines, the group of Clark showed that a slightly different approach using expanded starch could provide functional materials with disordered mesoporosity, which proved to be satisfactory as catalysts in the esterification of succinic acid 10,11 .

In order to use the material for applications in catalysis, chromatography or as storage materials, it is evident that both a control of the chemistry behind the process and a satisfactory knowledge of the final material structure are of primary importance. In spite of the undoubted usefulness of the recent re-discovery of the hydrothermal process to obtain carbonaceous material, some basic work is still lacking as far as process of formation and final structure of the C-C-network are concerned. Some groups tried to investigate, directly or indirectly, the reaction mechanisms which transforms glucose first into its three times dehydrate intermediate, 5-hydroxymethyl-furfural-1-aldehyde (HMF) 12,13,14 and then subsequently to the carbonaceous scaffold 15,16 but a clear description of the final structure and, consequently, of the reaction path leading to the carbonaceous material is still missing. The amorphous nature and the intrinsic chemical complexity are the main drawbacks preventing an easy and full comprehensive structural study using common techniques, like XRD or infrared spectroscopy. Nevertheless, FT-IR and in some cases FT-Raman spectroscopy have been the most popular, easily accessible, techniques used to discriminate between various functional groups (C=O, C=C, aliphatic carbons) so far 3,14,15 . XPS was also used to identify the main carbon sites but the spectral resolution is often worse than the one obtained using vibrational spectroscopies 9 . Even 13 C solid-state NMR has already been tempted 4,14,15 but mainly to complement other techniques. As for IR data, the resulting spectra only helped in the identification of the main organic functions. However, the rather good spectral resolution obtained on those hydrothermal carbons suggested that they deserved deeper investigation with more advanced NMR techniques.

Solid-state NMR studies on a large variety of carbonaceous materials, especially in soils and sediments, have already been performed, and a number of contributions have used 13 C NMR [17][18][19][20][21][22][23][24][25][26][27] to identify chemical species and to refine carbon structures. The most common experiments use 1 H to 13 C cross polarization (CP) technique coupled to Magic Angle Spinning (MAS), which has the advantage of signal enhancement leading to reasonable acquisition times with respect to 13 C single pulse MAS NMR experiments. Unfortunately, quantification using CP is more challenging than in a single pulse experiment due to the more complicated spin dynamics. On the contrary, single pulse (SP) experiments, which are largely preferred to CP for quantification, are generally timeconsuming. Extensive signal broadening coming from the potential presence of paramagnetic centers can also be a complicating factor, but this has not really prevented deep investigations of coals or chars by solid-state NMR 28 . Cross polarization at one single contact time is in general the fastest approach to obtain preliminary information about chemical composition. For improvement, Sullivan 18 presented CP experiments at variable contact times together with acquisition under modified CP conditions and two dimensional 13 C homonuclear magnetization exchange experiments to attribute large line widths to inhomogeneous broadening effects. Dela Rosa et al. 19 studied 1 H longitudinal relaxation times in the laboratory frame Tl as well as in the rotating frame T1ρ and performed CRAMPS experiments on various coals to discriminate the presence of large amount of oxygenated sites. Finally, Tekely et al. 20 employed the 13 C transverse dephasing times T2' to differentiate, based on proton mobility, aliphatic and aromatic components in overlapping signals.

We have recently found 29 by a combination of 13 C solid state MAS NMR and SEM that whatever the complexity of hexose carbohydrates (glucose, amylopectin or starch) used in the coalification process is, the NMR response and in part the particle morphology of final carbons are very close, indicating the existence of a similar chemical coalification process in behind. Very similar 13 C NMR spectra of carbons have indeed also been reported by other authors both after hydrothermal treatment of starch 4 and fructose 15 and also from pyrolytic treatment at moderate temperatures (< 350°C) of a variety of biomass-derived sources like rice hulls 30 , pectin 25 , starch 31 or cellulose 32 . This is an important precondition to ensure that the structural study on carbon obtained from 13 Cenriched glucose reported in the present paper has broader validity. 13 C isotopic enrichment (which would be more demanding for complex biomass) is a requirement to enable a complete NMR study that combines advanced methods, as recently illustrated on the structural characterization of 13 Clabeled graphite oxide 33 .

The goal of the present contribution is to show that hydrothermal carbon from 13 C isotopically enriched D(+)-glucose is a good model to analyze the chemical connection motifs of biomassderived carbons obtained either via a hydrothermal or pyrolysis step at temperatures below 350°C. This goal is achieved by combining 13 C solid-state MAS NMR methods such as one-dimensional (single pulse, CPMAS, IRCP, through-bond C-H filtering -INEPT-), and two-dimensional experiments ( 13 C- 13 C exchange spectroscopy and through space 13 C homonuclear double-quantum single-quantum correlation).

Experimental Section

Sample Preparation. For all samples, about 15 ml of a deionized water solution containing 10 wt% in mass of D(+)-Glucose (with the diverse isotope labeling discussed below) is sealed into a glass vial inside a typical PTFE-lined autoclave system for hydrothermal reaction at 180°C for 24h.

After reaction, the autoclave is cooled down in a water bath at room temperature. The obtained black solid powder is then separated from the remaining aqueous solution by centrifugation (7000 rpm for 20 minutes) and dried at 80°C in a oven under vacuum during 12 hours. Non-enriched D(+)-glucose (Aldrich, CAS: 50-99-7), D-(+)-Glucose-1-( 13 C) labeled at the C1-position (Aldrich, CAS: 40762-22-9) and fully labeled D-Glucose-13 C6 (Aldrich, CAS:110187-42-3) have been used to prepare the samples that are listed in Table 1. NMR experiments. Solution 13 C NMR experiments (acquisition with power-gated decoupling using a 30° flip angle) were performed on a Bruker Avance 400 MHz (9.4 T) spectrometer with a BBO gradient coil for 5mm tubes probe head.

1 H and 13 C solid-state MAS NMR experiments have been acquired on a Bruker Avance 300 MHz (7 T) spectrometer using 4 mm zirconia rotors spinning at a MAS frequency of νMAS= 14 kHz. 1 H and 13 C chemical shifts were referenced relative to tetramethylsilane (TMS; δ= 0 ppm).

Single pulse MAS experiments: pulse lengths (90° flip angle) for 13 C and 1 H were 3.15 and 3.80 µs and the optimum recycle delays were 30 s and 3 s, respectively. Two-pulse phase-modulated (TPPM) proton decoupling 34 was applied during 13 C acquisition (80 kHz). Number of transients was 4 for 1 H (all samples) and 64 for 13 C experiments (HC80). Spectra have been decomposed with DMFIT2004 software 35 . Integration values should be considered with a 10% error coming from a trial and error methodology used in the fitting procedure.

CP-MAS experiments:

recycle delay for all CP experiments was 3 s and TPPM decoupling was applied during signal acquisition. Cross-polarization transfers were performed using adiabatic tangential ramps 36,37 to enhance the transfer efficiency with respect to other known methods 38 and the CP contact time was tCP = 3 ms unless otherwise indicated. NS = 1840 (HC0), 440 (HC20), 32 (HC80). All FIDs were subjected to an exponential multiplication (EM) function with a Line Broadening value of LB = 50 Hz prior Fourier transform IRCP experiments: in order to get a complete spectral editing in CP MAS NMR, inversion recovery cross polarization (IRCP) is generally used. The principle of this sequence is reported elsewhere 39 , 40 , 41 , 42 , 43 and the only modification compared to the classical CP sequence is the introduction of a phase inversion during the contact time. 13 C transverse magnetization is firstly created during a relatively long contact time, tCP, and is then progressively inverted during an inversion time, ti. The dynamics of inversion is similar to the polarization dynamics in a standard CP sequence 44 , and thus strongly depends on the 13 C-1 H dipolar coupling. This sequence is therefore very sensitive to the local proton environment and to molecular motion and a careful analysis of the CP dynamics allows a clear distinction between the various 13 CHx sites depending on the x value and also between rigid and mobile 13 CHx sites. The main advantage of this experiment relative to the standard CP sequence is that the magnetization starts from an optimum value, then decreases and eventually becomes negative when increasing ti. It is thus easy to visualize differences in inversion dynamics of various signals especially when they overlap. On the basis of theoretical arguments, the relative rates of polarization are expected to be as follows: 46,47 to enhance the sensitivity of 13 C nuclei and to detect only protonated carbon sites and to edit them according to their specific JCH coupling constants.

Refocused INEPT experiments described in references 48 , 49 , 50 are more adapted to solid-state samples since they allow to detect in-phase line shapes, avoiding cancellation effect in the case of broad 13 C resonances. In general, they reveal to be challenging when the proton resonances are subjected to a large homogeneous broadening due to strong 1 H homonuclear dipolar couplings. As clearly shown by Elena et al. 50 , the presence of strong 1 H homonuclear dipolar interaction makes both the 1 H and 13 C transverse dephasing times very short in rigid organic solids and leads to a very weak efficiency of the refocused INEPT experiments. In such case, the use of a proton homonuclear dipolar decoupling sequence is required to minimize the transverse dephasing of coherences and to recover an efficient INEPT transfer. 50 In our case, the absence of 1 H homonuclear dipolar decoupling during the magnetization transfert and refocusing delay makes the efficiency of the experiment very low. The best INEPT efficiency obtained experimentally was below 5% of the theoretical maximum efficiency and was achieved using very short magnetization transfert and refocusing delays corresponding both to one rotor period (∆3 + ∆4 = 0.134 ms). Therefore, the number of transients to record the refocused INEPT spectrum of our 13 C-enriched sample was almost 200 times higher (number of transient is 12600) than that of the single pulse MAS acquisition. For this experiment, the recycle delay was 3 s and TPPM decoupling was applied during signal acquisition.

Two-dimensional (2D) NMR:

-CPMAS 13 C homonuclear double-quantum single-quantum (DQ-SQ) correlation experiments. 13 C homonuclear DQ-SQ correlation spectra have been recorded at a spinning frequency of 14 kHz using the SC14 pulse sequence described in reference 51. Based on double quantum (DQ) dipolar recoupling between spin-1/2 nuclei, this experiment is a robust way to evidence the C-C atomic spatial proximities. We have employed the SC14 sequence 51 which requires a lower ratio of 13 C nutation frequency to spinning frequency (3.5) than other known DQ recoupling sequences like C7 52 , POSTC7 53 or SPC5 54 . Experimentally, this allows an efficient DQ excitation and reconversion with the use of a lower 1 H decoupling radio frequency field which should match almost 3 times the 13 C radio frequency field 54 . For this experiment, enhancement of 13 C signal was obtained via a first CP step described above (tCP= 3 ms) and followed by double quantum excitation (τE) and reconversion (τR) delays. Proton decoupling was applied both during DQ excitation and reconversion (continuous wave, ν1H= 110 kHz) and signal acquisition (TPPM, ν1H= 78 kHz). The type and amplitude of proton decoupling during DQ excitation and reconversion were finely tuned on both 13 C-glycine and 13 C-D(+)-glucose and were found to be coherent with what was already proposed in terms of DQ efficiency. The DQ excitation and reconversion periods were both set to 285.7 µs corresponding to two rotor periods. 16 t1 increments with 304 scans each were collected and quadrature detection in the indirect dimension was achieved using the States method 55 .

Exponential apodization with a line broadening factor of LB= 50 Hz was applied in both dimension prior Fourier transform.

-CPMAS 1 H- 13 C Heteronuclear Correlation (HETCOR). Through-space heteronuclear correlation experiment between 1 H and 13 C have been performed using a standard 2D version of the CP sequence, detailed elsewhere 56 . Acquisition conditions for the CP experiment were given above.

Here, 16 t1 increments with 64 transients each were recorded and quadrature detection was achieved using the States method 55 .

-13 C- 13 C proton-driven magnetization exchange spectroscopy: 13 C homonuclear proton-driven magnetization exchange 2D spectra [START_REF] Meier | Advances in Magnetic and optical Resonance[END_REF] have been recorded using a standard pulse sequence [START_REF] Schmidt-Rohr | Multidimensional Solid-State NMR and Polymers[END_REF][START_REF] Szeverenyi | [END_REF] in which 13 C transverse magnetization is created using standard single pulse excitation instead of classical CP step. The mixing time for 13 C-13 C proton-driven magnetization exchange was set to 150 ms. Proton decoupling was applied during both indirect t1 and direct t2 signal. 96 t1 increments with 32 transients each were recorded and quadrature detection in the indirect dimension was realized using the States method.

Results

GC-MS was performed in the liquid phase after different times of reaction, including the aqueous phase after 24 hours reaction time. The main component in all GC curves (results not shown) is a peak whose mass and fragmentation analysis suggests an attribution to hydroxymethylfurfural (HMF), the well-known dehydration product of glucose at high temperature 14,60,61,62,63 . Besides HMF, one finds at least 10 additional organic substances, the most prominent being levulinic acid, dihydroxyacetone and formic acid 63 . This result seems to be corroborated by the 13 C NMR spectrum analysis of the liquid phase of the extracted sample HC0 shown in Figure 1. to 13 are attributed to levulinic acid (C9: 178.4, C11: 37.8, C13: 29.7, C10: 27.9 ppm) while peak 14 (C14= 166.2 ppm) is attributed to formic acid; both compounds are expected by-product of HMF dehydration, whose presence is abundantly detected in the liquors by GC-MS experiments 29 ; traces of dihydroxyacetone (or related products) can also be identified by the carbonyl and methylene peaks at, respectively, 214.5 and 65.3 ppm. Peaks in the 100-60 ppm region are attributed to unreacted glucose, even though their intensities (hence the amount of glucose) may vary from one reaction set to another. These findings support the general view that glucose first mainly dehydrates towards HMF, which on one side decomposes to form a mixture of mainly levulinic acid and formic acid 63,65 and on the other side reacts at high temperature to form the carbonaceous scaffold. Re-use of the aqueous phase in a second reaction under similar conditions resulted in further carbon phase formation and a complete elimination of the HMF peak.

The average chemical composition in weight-% of the resulting carbonaceous solids is C: 62 ± 3%, H: 4 ± 1%, O: 34 ± 3%, which corresponds to the following molar formula CH0.77O0.41. The large H content will be favorable to 1 H-13 C through-space (CP) or through-bond (INEPT) heteronuclear polarization transfer NMR experiments that will help in the identification of the various sites. Figure 2 shows the 13 C solid-state CP MAS NMR spectra of hydrothermal carbon samples (Table 1) obtained from pure glucose (HC0), a mixture of pure and fully-enriched glucose (HC80) and a mixture of pure and selectively-enriched glucose-1-13 C (HC20) recorded using a contact time of 3 ms. A primary qualitative attribution based on literature [17][18][19][20][21][22][23][24][25][26][27] To contribute to a better understanding of the reaction mechanism, which transforms glucose into functionalized carbonaceous powders, selectively 13 C-enriched glucose molecule in position 1 (sample HC20) was used to promote amplification of specific signals especially in the carbonyl region, as known from previous works 60 . However all spectra (Figure 2) look similar, which demonstrates that the 13 C, initially in position 1, was abundantly reshuffled through the complete monomer structure or reacting sub-species. Even if a closer observation of the spectra nevertheless shows some slight differences in the relative intensities of several peaks, we exclude at the moment the employment of specific isotope labeling for a more refined investigation of the chemical condensation mechanism. However, as expected, comparison between HC0 (acquisition time: 5500s) and HC80 (acquisition time: 70s) shows that isotopic enrichment is extremely useful in terms of sensitivity. In addition, despite the high level of enrichment, peak broadening due to 13 C homonuclear dipolar couplings is not observed, showing that a MAS rate of 14 kHz is sufficient to efficiently average out the 13 C homonuclear dipolar interactions, which are expected to range between 2 and 7 kHz depending on C-C distances.

Such a high quality response allowed recording quantitative single-pulse (SP) MAS experiments on HC80 sample (Figure 3a). Very few differences can be noticed when comparing the SP to the CP-MAS spectrum recorded with a long contact time of 3 ms (Figure 3b), due to the large proton content (H/C = 0.77) and the presence of strong 1 H homonuclear dipolar coupling giving rise to efficient 1 H spin diffusion processes. Slight differences are mainly concentrated in the C=O region, where the two signals around 176 and 210 ppm are clearly multi-components: 1) a sharp peak centered at 210.0 ppm is present in the SP spectrum but hardly detected by CP; 2) similarly, the signal at 176.5 ppm in the SP spectrum reveals the presence of a sharp peak overlapping a broader signal. The origin of these resonances is very peculiar and it will be explained in the discussion section.

It is clear so far that 13 C NMR spectra suggest a complex nature of the final hydrothermal carbon with the presence of a larger variety of carbon sites. In the following, the 13 We have found that this approach provides selective pieces of information mainly in region II characteristics of aromatic peaks. IRCP experiments were also performed at several inversion times (from 1 µs to 500 µs) and they were extremely useful in the spectral resolution of the complex multi-component aliphatic region. Finally, the 2D 13 C- 13 C DQ-SQ correlation spectrum was very powerful to finely resolve most of the carbon species and to establish their connectivities. Our indexation (peaks A-M in Figure 3d and Table 2) is based on this 2D DQ-SQ correlation experiment (both using the skyline projection and the cross-peaks) and all techniques cited above; the decomposition of the skyline projection is shown in Figure 1 The spectrum recorded at tCP= 35 µs mainly reveals aliphatic and β-carbons and looks very similar to the INEPT spectrum. This is expected since for short contact time magnetization transfers only occur across strong dipolar couplings corresponding to short C-H distances characteristic of C-Hx bonds. 66 The aliphatic zone (10-80 ppm) of the CPMAS spectra shows a very complex shape composed of several unresolved resonances for which more insight will be provided by the IRCP experiments in the next section. However, interesting insights can be actually gained in the aromatic region (100-160 ppm, region II). Five main peaks (F, G, H, I, J) are evidenced and the corresponding evolution of the integrated area of peaks (F), (G), (J), (I) as a function of cross polarization time is shown in Figure 4b (refer to Figure 2-ESI for some CP spectra decompositions at different tCP values). Interestingly, peaks (G), (I), (J) show the same polarization behavior, typical of a quaternary carbon (as compared to the behavior of the ketone (L)-peak at 208 ppm, reported on the graph for comparative purposes), while peak (F) shows a very fast increase in intensity, which is typical of a protonated carbon. Therefore, CPMAS experiments clearly show that peak (G), which was expected to have a similar chemical nature as peak (F) on the basis of their very close 13 C chemical shifts, could not be assigned to protonated carbon as peak (F) and is dominated by quaternary species. We will use this important piece of information to depict the exact nature of the carbon sites related to peaks (F) and (G) after the analysis of the 2D DQ-SQ map. One important remark: CP behavior of peak (H) at 131.2 ppm reveals to be typical of a quaternary carbon (not shown). This piece of information will provide valuable insights about the nature of this resonance in conjunction with the DQ-SQ experiment. Insight on spatial connectivity using 13 C homonuclear DQ-SQ correlation experiments. The experiments previously performed allowed identification of the different functions present in the carbonaceous samples but did not provide information about close spatial proximities between the various carbon groups. Indeed, several NMR methods exist to visualize short-range connectivities either via through-bond or through-space techniques: for example, the group of Emsley used double quantum excitation (INADEQUATE 67 ) to probe carbon-carbon connectivities in biomass 68 while a recent report used a fpRFDR 69 pulse sequence to study 13 C-labelled graphite oxide 33 . For our study, we strongly preferred the use of double quantum excitation; in fact, the signals lying on the diagonal in a 2D DQ-SQ experiment provides crucial pieces of information about connectivity among equivalent sites. Since we were unable to optimize a high quality INADEQUATE experiment on our samples, probably due to the low values and dispersion J-coupling values for sp 2 and sp 3 carbon sites, we performed a two-dimensional (2D) 13 C homonuclear double-quantum single-quantum (DQ-SQ) MAS experiment using the SC14 sequence 51 . This experiment allows the double quantum recoupling of dipolar-coupled spin-pairs and results in a 2D spectrum (with a direct SQ MAS dimension and an indirect DQ dimension for which the frequency corresponds to the sum of the two individual chemical shifts of the pair of coupled spins) allowing to trace out the C-C proximities between both inequivalent and equivalent sites. In the case of uniformly 13 C labeled samples, the DQ-SQ correlation spectra are dominated by the direct (one-bond) strong couplings due to dipolar truncation effects 51,70,71 and the longer-range C-C proximities are evidenced through indirectly correlated peaks of lower and negative intensities 51,54 . To avoid the presence of the indirectly correlated peaks and only observe correlation peaks reflecting the one-bond C-C strong coupling, we have used short DQ excitation and reconversion times (corresponding to two rotor periods) to record the 2D CP-filtered DQ-SQ correlation spectrum of the sample HC80, which has a high 13 C enrichment level. Cross-peaks (93.2-72.7 ppm) and (97.0-75.1 ppm) relate to the DQ excitation of C1-C2 groups in the αand β-pyranose forms of glucose (whose presence was highly abundant also in the mother liquors). This is very important as glucose acts as the internal standard proving that the SC14 sequence under the employed conditions is selective to one C-C bond length. End functions: A low amount of direct functional groups directly linked to the furanic rings are also evidenced on the 2D correlation spectrum, as depicted in Scheme 3 d-f. Typically, aldehydic (Scheme 3d), methyl (Scheme 3e) and carboxy (Scheme 3f) groups bonded to the Cα carbon are evidenced through the off-diagonal (J-L), (J-A) and (J-K) cross-correlation peaks, respectively.

Interestingly, the connection between the graphene-like (peak H) and the rest of the material is allowed by a keto group (cross-peak H-L). solution. Secondly, it evidences a K-C-D-L-C connectivity pattern which is characteristic of levulinic acid. The presence of levulinic acid was confirmed by an additional 2D 13 C homonuclear proton-driven magnetization exchange experiment realized with mixing time of 150 ms without the classical { 1 H}- 13 C CP step (result not shown). This approach allows the detection of all mobile species whose sensitivity is very low using a common CP approach due to molecular mobility. We were able to visualize the same connectivity pattern 29.6-210.0-38.3-28.7-176.5 ppm typical for the levulinic acid molecule. Meanwhile, sharps signals at 210.0 and 176.5 ppm which were lost after CP treatment were recovered and found to be coherent with peak attribution and connection related to levulinic acid. This additional experiment shows that part of LA exist in a free form within the material core.

One final remark: the four well-defined, but weak, cross-peaks at 75 ppm (marked by an asterisk) and (93, 97) ppm that can be attributed to the alpha and beta glucopyranose forms of glucose, as pointed out before and existing in spurious amounts (residual glucose was previously highlighted in mother liquors). It is clear from the 2D experiment that glucose does not correlate with any other carbon in the sample suggesting its simple entrapment in the carbonaceous matrix.

Discussion

All data support the idea that the main structural motif is the furan ring directly coming from HMF, the dehydration product of glucose. The very intense cross-peaks (J-F) provides the main spectroscopic evidence for this assumption: the amount of C=C-O and C=C is close to one-to-one.

Any interpretation including widespread graphene sheets decorated with OH groups is not consistent with our data because an intense peak at 130 ppm, in addition to the resonance above 140 ppm, would be otherwise expected. This idea was also corroborated by the fact that direct carbonization of HMF provides a material whose chemical characteristics are very close to a glucose-synthesized carbon, as shown by previous solid state NMR experiments 29 . However the final products of thermally-treated biomass is not just a simple polyfurane, and the obtained material is quite complex, as suggested by the combination of co-existing structures depicted in schemes 2-3. Clearly, HMF units merge together mainly via α-carbons causing the long-range conjugated double bonded network but ramification from β-positions is also very common, as one β-carbon out of two is actually cross-linked. Interestingly, it is commonly accepted that carbon intermediates obtained from biomass pyrolisis at T< 350°C are constituted of a long-range aromatic network with terminal hydroxyl groups, which accounts for all NMR resonances between 110 and 150 ppm 25,31,32 . This interpretation, simply based on chemical shift assumptions from cross polarization experiments alone, is too loose to fit the whole set of our experimental data, which rely on a number of complementary 1D and 2D techniques. Even if the presence of furan moieties in the structure of carbonaceous materials was very seldom highlighted, we believe that this is actually the only coherent explanation to our data. Given the very close similarity between 13 C NMR spectra of hydrothermally-treated glucose, polysaccharides 15,16 and pyrolyzed biomass 25,31,32 , it is very likely that the initial stages of carbon formation goes through a cross-linked polyfuran network.

In order to have a picture of how the internal structure of the carbonaceous powder is disposed, one should consider several points: 1) dimension of each particle ranges between 0.5 and 5 µm 29 ; 2) 13 C-13 C DQ-SQ experiments show that most of the furan rings are strongly mutually connected, suggesting their very close spatial proximity; 3) quantitative data indicate that a fairly high amount of carbon signal (> 20%, excluding free levulinic acid) comes from aliphatic carbons, and throughspace connectivities show that most of those groups play a cross-linking role between carbonyls and furans. These considerations clearly exclude a long-range furanic network whose external surface is decorated with aliphatic chains and carbonyl groups but it rather leads us towards an interpretation in which the micrometer-sized carbon particle is actually composed of many interconnected (through short keto-aliphatic spacers) nano-sized domains in which an arbitrary combination of structures proposed in Scheme 2-3 co-exist together. The model as described above is visualized in Scheme 4.

Scheme 4 -Structural model of carbon particles

Analysis of the overall intensity of the SP spectrum (Table 2) of HC80 let us attribute 13% of carbon to C=O containing groups, ca. 64% to sp 2 C=C carbons (in particular about 29% to αcarbons, 29% to β-carbons and 6% to graphene sheets) and around 23% to sp 3 carbons. When comparing these values to the carbonyl content both in glucose and HMF (16.6%), one can notice that the most carbonyl groups are kept throughout coalification. 66.6 % of sp 2 C=C carbons of HMF are practically kept unchanged during the process. This clearly proves that addition/polymerization of double bonds 72 as a leading coalification mechanism at least does not take place without consecutive elimination steps. On the other side, the 1:1 α-carbon to β-carbon ratio of HMF is nearly kept in the final material, which, once again, strongly supports the presence of the furan as main structural motif. The amount of the aliphatic part is in the end larger (23%) than in HMF (16.6%). Different mechanistic proposals can be made to account for the larger aliphatic contribution. Hydration of the aldehyde group of HMF may result in a further elimination of formic acid (this by-product is abundantly detected by GC-MS) and the consequent protonation of the Cα site as proved by INEPT experiments above (scheme 1); such a proton being very labile [START_REF] Gandini | [END_REF] , a homocondensation reaction forming a direct connection between two furan rings (see cross peak I-I in Figure 6 and Scheme 2c) may occur. Formation of the abundant CH2 groups acting as linkers between furan rings may originate from nucleophilic addition after possible elimination of the hydroxyl group on HMF. On the contrary, the nucleophilic attack on the aldehyde group of HMF from a second furanic unit may lead to hydroxyl species which can in turn be eliminated with subsequent electron rearrangement and formation of a sp 2 CH carbon between two furan moieties (scheme 2b). The formation of specific CH3 and COOH ending groups (from Cα of furan ring) was previously reported [START_REF] Gandini | [END_REF] (scheme 3 e-f) while some doubts still exist on the attribution and explanation of the CH group at 50.5 ppm (E). According to the DQ-SQ map, its direct connection to ketone groups has the highest probability (even CH-CH or CH-CH2 bonds may exist at a minor extent). In this case, a reaction path directing towards a bridging CH between three C=O groups (this could be justified by the broadness of the E band) may not be excluded. In fact, cyclic condensation of the levulinic acid (LA) can lead to cyclic diketone species having a linking CH2 site to which a third LA molecule could react via an aldol condensation step. Further insights can probably be found in the complex chemistry of furans, as largely described and commented in reference 73.

Conclusion

In this communication we tried to address the difficult problem of the structure resolution of amorphous carbonaceous materials obtained, here, from hydrothermal carbonization processing of biomass. Based on previous comparative studies, glucose was selected as a good model carbon source since it allows the preparation of 13 C enriched samples and thus the use of solid-state 13 C NMR to investigate the chemical composition and local structure of the hydrothermally treated materials. Several standard MAS NMR techniques (SP, CP, IRCP), which are mainly used in the study of carbon structures, have been used here to identify the amount and type of sp 2 and sp 3 carbon sites. In addition, we used through-bond filtering techniques based on J-coupling (INEPT), which provided a clear-cut distinction between protonated and quaternary carbons. Finally, the connectivity between carbon species was identified using a 2D 13 C- 13 C correlation experiment based on double quantum excitation of through-space dipolar-coupled nuclei.

We found that about 60% of the carbon atoms belong to a cross-linked furan-based structure.

Furan moieties are either directly linked via the alpha carbon or via sp 2 or sp 3 type carbon groups, where cross-linking can occur. Additional cross-linking sites are located at the beta carbons of the furan ring. Interestingly, we found that levulinic acid, one of the decomposition product of hydroxymethylfurfural (HMF), is highly abundant in the final material both as embedded molecule and as copolymerized compound. A clear set of CP-blind NMR signals can be related to free molecular levulinic acid (also corroborated with 13 C- 13 C exchange spectroscopy experiments) while the 2D 13 C- 13 C DQ-SQ map suggests levulinic acid copolymerizes with the polyfuran network.

We believe that such a complex structure is the result of a cascade of coexisting reactions paths of inter-furfural polymerization and aldol-condensation reactions involving both the furan motif and dehydrated glucose-based products, like levulinic acid. This more vinylic, aliphatic structure explains many of the properties of hydrothermal coal, such as rather high combustion value, high chemical reactivity, reductive properties, and high ion binding capacity and wettability. In the end, we can provide a pictorial image of the inner carbon structure obtained via hydrothermal treatment of glucose but which can be extended to depict more carbons obtained from more complex biomass both via a hydrothermal and a pyrolysis step. Interestingly, comparing these results with literature studies, it seems that the furan ring may be the actual carrying structural motif for biomass-derived carbons obtained at pyrolysis temperatures below 350°C, in contrast with many previous reports in which the existence of oxydized graphene sheets was supposed. Here, less than 6% of all carbons are involved in such a structural motif.

  experiments have been performed with an Agilent Technologies (GC= 6890N; MS= 5975) apparatus.

Figure 1 -

 1 Figure 1 -13 C solution NMR of extracted liquors after synthesis of HC0 sample (recycle delay= 2s; number of transient is 17138)

Figure 2 -

 2 Figure 2 -13 C CP-MAS NMR experiments of samples a) HC0, b) HC20, and c) HC80 obtained with contact time tCP= 3 ms.

Figure 3 -

 3 Figure 3 -13 C MAS NMR spectra of sample HC80 recorded with various sequences: a) SP, b) CP and c) INEPT. d) Skyline projection of the 2D 13 C-13 C DQ-SQ experiment shown in Figure 6.

  C resonance assignments have been refined combining several 13 C NMR techniques. INEPT technique gives a straightforward interpretation on protonated carbon sites, as carbon signals are filtered via the through-bond J-coupling. Cross-polarization experiments have been performed at different contact times ranging from 35 µs to 7 ms.

  ESI. A detailed description of each experiment used to investigate the structure of the carbonaceous material is given in the following paragraphs. Insight on protonated carbons by INEPT experiments. As shown in Figure 3c, definite proof of the identification of protonated carbon sites is provided by the refocused INEPT MAS experiment which allows probing 1 H-13 C through-bond (J-coupled) connectivities. Nevertheless, this experiment confirms the existence of protonated carbons in region I (10-50 ppm) as well as for the broad signal between 110 and 130 ppm (sp 2 sites), and demonstrates that majority of the sites contributing to the signal centered at 150 ppm are actually non-protonated. This is an important piece of information since it strongly suggests that the signals at 110-130 and 150 ppm can be mainly assigned to furanic O-C=CH-and O-C=CH-sites, respectively: indeed, similar chemical shift values are observed for α-positions (161.8 and 152.3 ppm, see Scheme 1) and β-positions (111.4 and 127.3 ppm, Scheme 1) in HMF, as shown in Figure 1b. In agreement to the known chemical steps which transform glucose into HMF, we believe that this compound reacts further so that the furan ring may constitute the majority of the carbonaceous scaffold, in agreement with CP, IRCP and DQ-SQ experiments shown later. Finally, the two low-intensity signals at 176.5 and 208ppm suggest the presence of a small amount of aldehydes with respect to typical non-protonated carbonyl sites detected in the same chemical shift range, such as ketones and carboxylic acids, and indicating the aldehyde as a preferred reaction site. Nevertheless, the presence of residual aldehyde groups was also proved by 1 H-13 C CP HETCOR experiments performed at relatively short contact time of 500 µs (result not shown) showing a correlation peak between the C=O signal at 176.5 ppm and a proton resonance at ~9.0 ppm. One should remark that no cross peak was detected at 211.0 ppm, corroborating the CP data and confirming the assignment of this signal to a ketone group.

Scheme 1 .

 1 Scheme 1 . The furan ring with the α-position (or positions 2;5) and β-positions (or positions 3;4) highlighted

Figure 4 -

 4 Figure 4 -Evolution of 13 C CP-MAS spectra recorded on HC80 for different contact times (a) and variation of peak intensities of aromatic region (b) as a function of the contact time

Figure 5 -

 5 Figure 5 -a) IRCP spectra recorded at various inversion times in the aliphatic region for sample HC80; b) evolution of IRCP peak intensity for selected decomposed peaks belonging to various CH, CH2 and CH3 groups.

Figure 6 -Scheme 2 -

 62 Figure 6 -2D 13 C DQ-SQ MAS NMR correlation spectrum recorded on sample HC80 with τE=τR= 285.3 µs. Insights on the connectivity scheme are provided in Figure 4a,b-ESI

Scheme 3 -

 3 Scheme 3 -Cross-links between furanic rings and end functions at the α-carbon identified from the 2D DQ-SQ 13 C-13 C correlation spectrum

Table 1 Molar ratios of non-enriched, fully and selectively enriched glucose samples used in this study.

 1

45

  can be proposed for each spectral domain. Region I (0-100 ppm) is characteristic of sp 3 carbon atoms indicating the presence of a broad distribution of CHx (X= 1, 2, 3) sites. Region II (100 to 160 ppm) is characteristic of sp 2 carbon atoms in C=C double bonds with signals between 140 and 160 ppm more specifically due to

oxygen bound, O-C=C, sp 2 carbons. In region III (170-225 ppm), C=O groups either in carboxylic acid moieties (175 ppm) or ketones and aldehydes (200-220 ppm) resonate. Finally, a peak at 75 ppm, whose intensity largely varies among all materials, corresponds to residual glucose, as also compared to the large amount of residual glucose in the solution phase.

Table 2 -Peak assignment for HC80 material after decompositions of: 13 C DQ-SQ experiment, CP and IRCP experiments. Percentage of each carbon group was obtained by decomposition of HC80 SP spectrum. Quantitative data only concerns carbonaceous material, not including embedded levulinic acid (see text for explanation). Remaining 4% is attributed to residual adsorbed glucose (* peak at 73.4 ppm).

 2

Acknowledgements

We thank Prof. M. Sollogoub (IPCM, Université Pierre et Marie Curie, Paris, France) for helpful discussions.


Niki Baccile

Guillaume Laurent

Christian Bonhomme

Plinio Innocenzi

Florence Babonneau

Solid State NMR characterization of the surfactant/silica interface in templated silicas: acidic versus basic conditions

Keywords: Mesoporous materials, Silica, Solid state NMR, HETCOR, CTAB, MCM-41, SBA-3, FT-IR spectroscopy

Combination of one-dimensional and two-dimensional solid state magic angle spinning nuclear magnetic resonance (MAS NMR) experiments has been used to investigate the hybrid organicinorganic interfaces in surfactant templated silicas. Samples prepared with cetyltrimethylammonium

bromide (CTAB) under acidic (HCl) and basic (NaOH) conditions have been compared. The use of sequences based on the 29 Si-1 H heteronuclear dipolar interactions allows us to selectively filter the NMR response of the protons close to the Si surface sites showing directly the clear difference between the two systems. The basic sample is characterised by a small amount of Si-OH groups, and a short distance between the Si-O -surface groups and the surfactant polar head group, while the acidic sample exhibits a silanol-rich surface with a longer distance between the Si surface sites and the polar head groups. The nature of the interface induces consequent differences in the structure of the adsorbed water layers present at the interface, and this has been revealed by near infrared experiments, as well as 1 H MAS NMR spectra recorded on dehydrated and partially re-hydrated samples. One objective of this work was also to show that the use of standard solid state NMR conditions (magnetic field of 7 T and magic angle spinning frequency less than 15 kHz) can be largely sufficient to get extremely valuable information regarding the silica/surfactant interfaces.

Introduction

In the past fifteen years a large number of studies has been focused on long-range ordered mesoporous materials obtained in solution through the self-assembly of a variety of amphiphilic templating agents with a growing inorganic phase. 1 Their structural characteristics (high surface area; tunable geometry of the porous network; narrow pore size distribution) [START_REF] Soler-Illia | [END_REF] combined with the possibility to process them in various shapes 3 (calibrated spherical powders 4 , thin films 5 , membranes, monoliths 6 ) make them extremely attractive for a wide range of applications in the fields of catalysis 7,8,9 , chromatography 10 , drug release 11 , sensors 12 , electronics 13 or environment. 14,15 Moreover, the large versatility associated with a relative simplicity of the synthetic protocols has certainly contributed to the immediate interest of the community of chemists for this class of materials. Just by playing with experimental parameters such as the inorganic precursor, the type of surfactant, the pH of the solution, the temperature, the nature of co-solvents, mesoporous materials with a large range of nanostructures can be synthesized. Despite this abundant literature, relatively few studies have been focused on a detailed characterization of the interactions that develop between the structuring agents and the inorganic species. These interactions are of key importance to drive the self-assembly process, and to lead to the formation of a given silica/surfactant mesophase. Several research groups have focused on the understanding of the formation mechanism and reviews are dedicated to this topics, not only in the case of powders obtained through precipitation 16,17 , but also in the case of thin films prepared via evaporation-induced self-assembly. , 18 Less works have been centred on the description of the inorganic/organic interfaces in the final materials. Nevertheless, the way the templating molecules interact with the inorganic walls strongly influence important structural features. Upon removal of the surfactant species, an ordered mesoporous inorganic solid can result, whose typically amorphous framework structure is a replica of the hydrophilic mesophase regions and whose pores are vestiges of the hydrophobic components. Consequently, if hydrophilic parts of the structuring agents are embedded in the inorganic framework during the condensation step, then microporosity can be created. This has been already illustrated with the synthesis of mesoporous SBA-15 silica that uses the P123 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (EO20-PO70-EO20).

Galarneau et al. have shown that for a synthesis temperature below 95°C, the SBA-15 sample presents microporosity. This was explained by the presence of polyethylene oxide PEO chains at the surface of the micellar aggregates that are lately embedded in the silica network, leading to the formation of micropores after removal of the template. 19,20 A key technique to investigate the organic/inorganic interfaces is high-resolution solid state NMR.

The cross-polarisation (CP) sequence, based on through-space heteronuclear dipolar interactions, allows probing spatial proximities between the protons of the templating molecules and the selected nuclei present at the pore surface. Chmelka et al. very nicely illustrated this by using two-dimensional (2D) 29 Si{ 1 H} as well as 27 Al{ 1 H} heteronuclear correlation (HETCOR) experiments to investigate the distribution of PEO and polypropylene oxide PPO copolymer blocks within the silica matrix 21 , the aluminum incorporation in MCM-41 mesophases 6,22 as well as the molecular proximities between the structure-directing surfactant molecules and the crystal-like silicate sheets in layered silicate surfactant mesophases. 23 One can also use high resolution 1 H MAS NMR and the associated Double Quantum (DQ) NMR spectroscopy to look for proximities between the Si-OH surface sites and the template molecules. Due to the high sensitivity of such NMR experiments, Alam et al. applied them to silicate thin films templated with the non ionic surfactant Brij 56, a polyoxyethylene (10) cetyl ether, and demonstrated a close spatial contact between the surfactant and the silica framework, through Si-O-H….O-CH2 proximities. 24 Additionally, some studies have been focused on the behaviour of the surfactant molecules using 13 C NMR experiments. 25,26 Experiments that look at the relative spatial arrangement of the amphiphilic templates with respect to the silica framework are thus quite scarce, since most of the solid state NMR characterisation concerns the porous samples after removal of the templating agents. 27 In this work, we have chosen to investigate the solid state NMR responses of two silica-surfactant mesophases characterised by the same 2D-hexagonal (p6m) symmetry, prepared with the same templating agent, cetyltrimethylammonium bromide (CTAB), but one under acidic (HCl) conditions and one under basic (NaOH) conditions. They correspond to the so-called SBA-3 28 and MCM-41 29 -type silicas, respectively. Because of different synthesis pathways, the proposed interaction mechanisms are different. 30 In basic medium, a direct S + I -interaction is proposed between the positively charged surfactant (S + ) and the negatively charged silica matrix (I -). In acidic medium, interactions with surfactant micelles are mediated by a negatively charged counter-ion (X -) whose presence is demonstrated by elemental analysis. 30 The interaction would be of type S 0 X -I + , which accounts for a direct charge neutralisation between the counter-ion and the surfactant. Indeed the weak interaction that is assumed between the template and the silica framework in these samples is in agreement with the mild conditions required to remove the surfactant (reflux in ethanol) compared to the necessity to use acidic conditions when working with MCM-41 type systems. 30 Once again, this example illustrates the importance of a good understanding of the silica/surfactant interfaces in the as-prepared samples. Our objective was thus to see whether NMR techniques could be sensitive to the different silica-surfactant interaction mechanisms, that have been proposed in the literature, and that assume longer distances between the surfactant polar head group and the silica surface when acidic conditions are used instead of basic conditions. Indeed, to our knowledge no detailed study has been published concerning interactions in the final material, between silica and cationic surfactants, like quaternary ammonium salts, though such templating agents have been widely used to prepare ordered mesoporous materials.

Preliminary 2D-29 Si{ 1 H} HETCOR experiments have been recorded on both types of samples and different NMR responses were obtained. 31 Two contact times -time during which the magnetisation transfer occurs between the 1 H and 29 Si spin systems -were used to explore increasing inter-nuclei distances; these 2D-experiments are quite time-consuming especially with a moderate static magnetic field, which limits the number of possible contact times. Indeed, in most of the previous publications, only one experiment recorded at only one contact time has been presented. Considering the valuable information that can be extracted from these experiments with a variation of contact times, we have developed a new sequence based on a double CP transfer, a first transfer from 1 H to 29 Si, to select the Si sites close to protons, and essentially the surface sites, and then a second transfer back, from 29 Si to 1 H to detect only the signal of the protons which are dipolar coupled to these surface sites. Since it is a onedimensional sequence, a complete variation of contact times can be recorded that allows exploring in a quasi-continuous way, the proximities between 1 H and 29 Si nuclei. Additional 1 H MAS-NMR experiments have been recorded with high spinning frequency on dehydrated and partially re-hydrated samples to look at the adsorbed water layers, while near infrared experiments, which are very sensitive the extent of hydrogen bonding involving water molecules, have completed the series of characterisations of the silica-surfactant interfaces of these two samples.

Experimental methods

Material synthesis

The CTAB/OH sample is prepared with the following conditions: 0.15 g of CTAB (C16H33N + (CH3)3Br -; Aldrich) are dissolved in a 3.85 wt% NaOH aqueous solution (8 g of NaOH in 200 ml of water). The solution was stirred until all surfactant was dissolved. 0.72 g of TEOS (Tetraethoxysilane; Fluka) are then added to the mixture under stirring. The final molar ratios are TEOS:CTAB:NaOH:H2O = 1:0.12:0.5:120. The stirring of the solution (~ 500 rpm) was continued for 3 hrs, then filtered and dried during one day at ambient conditions and finally in the oven overnight at 100°C.

The synthesis of the CTAB/HCl sample follows the same steps. 0.10 g of CTAB are dissolved in an acidic aqueous solution composed of 0.12 g of water and 6.27 g of a 12.63 wt% (4 mol . L -1 ) HCl aqueous solution. The mixture is stirred for a few minutes before adding 0.5 g of TEOS. The final molar ratios are TEOS:CTAB:HCl:H2O = 1:0.12:9.2:130. The filtering and the double drying processes are the same as in the CTAB/OH case.

2.2 Solid state NMR spectroscopy 29 Si MAS NMR spectra have been recorded on a Bruker AVANCE 400 (9.40 T) spectrometer. 7 mm zirconia rotors spinning at 5kHz have been employed. 324 transients have been collected with a 90° pulse of 6.5µs and recycle delays of 200s (CTAB/HCl) and 500s (CTAB/OH). These values have been chosen according to previous works found in the literature. 32,[START_REF] Goletto | Synthèse et Caractérisation d'Organosilices Mésostructurées à Porosité Périodique[END_REF] Spectra have been fitted with DMFIT2004 software. [START_REF] Massiot | [END_REF] 1 H MAS NMR spectra were recorded on a Bruker AVANCE 400 spectrometer with 2.5 mm zirconia rotor and with spinning frequency of 30 kHz. 8 transients have been collected with a 90° pulse of 3 µs and a recycling delay of 5 s. The chemical shift reference for the 1 H and 29 Si NMR experiments is tetramethylsilane (TMS; δ= 0 ppm). Two-dimensional 29 Si{ 1 H} heteronuclear correlation (HETCOR) experiments and double Cross-Polarization 1 H→ 29 Si→ 1 H experiments have been recorded on a Bruker Avance 300 spectrometer with a CP-MAS probe and a 4 mm rotor spinning at 14 kHz. For the CP-HETCOR experiments, polarization transfer has been achieved by applying a constant radio-frequency (RF) field on the 29 Si channel (νRF( 29 Si) = 50 kHz) while the RF field on the 1 H channel was ramped (νRF( 1 H) = 50 ± 14 kHz). For the double CP experiments (sequence described in the text), cross-polarization transfers have been performed under adiabatic tangential ramps 35,36 in order to enhance the signal with respect to other known methods. 37 The pulse phasing was: φ1= (x2, -x2), φ2= (-x2, x2), φ3= (x, -x), φ4= (x4, -x4, y4, -y4), φ5= ((x, -x)2, (y, -x)2, (-y, x)2, (-y, y)2). For all experiments, two-pulse phase-modulated (TPPM) proton decoupling was applied during acquisition. 38 Quadrature detection in t1 was realized using the STATES method. 39 Experimental conditions for the individual NMR experiments varied and are presented in detail in the figure captions accompanying the respective spectra.

Other analysis

X-ray diffraction patterns have been recorded with a Philips PW 1830 diffractometer equipped with a Cu-Kα source (λ = 1.54 Å), with a 0.02° step and an acquisition time of 5 s per point.

Elemental analysis was performed at the CNRS "Centre d'analyse élémentaire", Vernaison (France).

Infrared absorption spectra were measured by a Fourier transform infrared (FTIR) spectrophotometer (Nicolet Nexus) mounted with a KBr-DTGS detector, in the 4000-400 cm -1 range; 256 scans with 4 cm -1 resolution. Near Infrared (NIR) spectra were measured in the 9000-4500 cm -1 range, using 256 scans with 4 cm -1 resolution. The spectra were recorded in absorption using anhydrous KBr powder, background was recorded using a KBr pellet prepared in the same condition. These experimental conditions assured that the effect of water absorption in KBr pellets from the external environment is not appearing in the infrared spectra. The beam splitter was CaF2 and the detector was made of InGaAs.

Experimental results

-Characterization of the silica/surfactant bulk powders

The two samples CTAB/HCl and CTAB/NaOH have been characterized by X-ray diffraction, elemental analysis and 29 Si MAS NMR. The results are summarized in Table 1. Both samples have a 2dimensional hexagonal structure (p6m) with very similar cell parameters (42.4 Å for CTAB/HCl and 41.4 Å for CTAB/NaOH). Elemental analysis reveals a higher surfactant/Si molar ratio in the final powders (0.18 for CTAB/HCl and 0.21 for CTAB/OH) compared to the starting nominal composition (0.12). Under acidic conditions, an almost complete anionic exchange took place with the incorporation of 0.86 Cl -, and only 0.05 Br -in CTAB/HCl. The amount of anions ((Br+Cl)/N = 0.91) almost compensates the positive charge of the surfactant polar head group, which results in a fairly low expected charge on the silica network. This has already been noticed in the literature, as mentioned in the introduction. 30 On the contrary, the amount of residual anions in CTAB/OH is extremely low (Br/N = 0.08) in good agreement with a direct electrostatic interaction between the positively charged polar head group and the negatively charged silicate species.

The 29 Si MAS NMR spectra present peaks due to the three different silicon sites : Q4= SiO2, Q3 = SiO1,5Ot, Q2 = SiO(Ot)2 (Ot : Non-bridging oxygen). 31 The number of Q3 sites is high in both samples, because of the rather short aging time of the powder after precipitation (3 h); 13 C MAS-NMR spectra (not shown) have confirmed the absence of any residual ethoxy groups. One might expect that under acidic conditions, the Q3 units thus correspond to the presence of Si-OH groups, 0.64 per Si (including Q3 and Q2 contributions). Under basic conditions, a majority of Si-O -groups are expected but some Si-OH groups might also be present in the silica network. The percentage of each Si site (Table 1) shows that in basic medium (CTAB/OH) the network is slightly more condensed than in acidic medium (CTAB/HCl), a well-known result in sol-gel chemistry. 40 The 1 H MAS NMR spectra of CTAB/HCl and CTAB/NaOH (see supporting information) show three main peaks at δiso = 0.9 ppm, 1.3 ppm and 3.3 ppm, respectively attributed to alkyl chain CH3 protons, alkyl chain CH2 protons (exception made for N-CH2 in α-position) and polar head N-CH3 and N-CH2 protons. [START_REF] Alonso | [END_REF] Two additional peaks of lower intensity are detected around 4.5 ppm and 7.2 ppm for CTAB/HCl and 6.0 ppm for CTAB/OH. They certainly belong to surface silanols and/or surface water species. This will be discussed later on in the text.

3.2. Silica/Surfactant interfaces 3.2.1. 1 H- 29 Si HETCOR experiments Two-dimensional (2D) 29 Si{ 1 H} CP heteronuclear correlation (HETCOR) experiments have been recorded at two different contact times (tCP = 500 µs and 5 ms) to probe the through-space proximities between 1 H and 29 Si sites (Figure 1). No specific thermal treatment was applied to the samples prior to these experiments.

CTAB/HCl sample

The experiment performed at tCP = 500 µs is shown in Figure 1a. The 1D projection of the 29 Si MAS NMR signal along the F2 dimension shows that almost only Q3 sites are detected at short contact time.

Interestingly, the main cross peak (A) represents a correlation between Q3 silicon sites (δ = -100.9 ppm) and protons at 7.2 ppm that confirms its assignment to Si-OH protons. The other cross peak (B) corresponds to a correlation between Q3 sites and N-CHx (x=2,3) protons belonging to the surfactant polar head group (δ = 3.3 ppm). Its lower intensity, compared to (A) can be explained by a longer Si•••H distance and by the higher mobility of polar head groups, two factors that contribute to decrease the efficiency of the polarization transfer between 1 H and 29 Si nuclei. The presence of a weak cross peak (C) between Q4 sites (δ = -110.0 ppm) and N-CHx protons is the signature of a longer distance between those species.

The experiment was repeated with tCP = 5ms (Figure 1b). Here, the 1D projection of the 29 Si signal shows Q3 as well as Q4, and even Q2 sites. New cross-peaks are now present. A weak one (E) is due to Si(OH)2 species. Its absence in the previous experiment, despite short Si… H distances, is certainly related to a lack of sensitivity of the experiment to detect minor species. More interesting is the presence of the cross-peak (F) between Q3 and protons in the alkyl chain at 1.3 ppm (presumably close to the polar head groups (β or γ positions)), together with the strong increase in intensity of the cross-peak (C).

These observations demonstrate that a contact time of 5 ms allows exploring longer Si…H distances.

Cross-peaks (A) and (B) are still observed, but with reverse intensity. Additionally, a cross peak appears between Q3 and protons at 4.6 ppm, value that will be related later on to the presence of mobile water molecules in weak interaction with Si-OH sites. 29 Si{ 1 H} CP-HETCOR experiments were recorded with 1 H Lee-Goldburg homonuclear decoupling during contact time (CP-LG). 42 The use of 1 H pulses under Lee-Goldburg conditions (proton irradiation is shifted in frequency in order to average out the 1 H homonuclear dipolar interaction to first order 43 ) eliminates most of the 1 H spin diffusion process that takes place during classical CP. The intensities of the cross peaks between Si-OH and Q3 and Q4 are slightly higher for the CP-LG HETCOR (see supporting information), which suggests that for a contact time of 5 ms, spin diffusion might occur between the protons of the Si-OH groups and the large reservoir of protons from the surfactant molecules. This will be confirmed later on in this work by 2D-1 H{ 1 H} homonuclear correlation experiments.

CTAB/OH sample

The 29 Si{ 1 H} CP HETCOR experiment performed at tCP = 500µs is shown in Figure 1c. The HETCOR spectrum only shows one cross peak (B) that represents the correlation between the Q3 sites (δ= -99.1 ppm) and the N-CHx protons of the polar head groups (δ= 3.3 ppm). In contrast to the previous sample, no cross-peak (A) between Q3 sites and Si-OH protons of significant intensity is observed: this strongly suggests that Q3 sites are primarily of Si-O -type, rather than Si-OH one, in agreement with the pH conditions used during the synthesis.

For a contact time tCP = 5ms (Figure 1d), the cross peak (B) is still of highest intensity, but two new cross-peaks between the Q3 sites and protons are now observed. They are related to 1 H at δ= 5.7 ppm (A) and at δ= 1.3 ppm (F) respectively. The latter signal corresponds to protons belonging to the surfactant alkyl chain, CH2 groups in β or γ positions with respect to the polar head group, and it was already observed for CTAB/HCl sample. More interesting is the cross peak (A) that can be assigned to water molecules hydrogen-bonded to Si-OH groups. Its non-observation at lower contact time is in agreement with high mobility, like protons in rapid exchange between surface Si-O -or Si-OH groups.

Similar results have already been shown in literature. 44 As for the Q4 sites (δ= -109,3 ppm), one main cross peak is observed (C) that represents correlation with the N-CHx protons of the polar head groups.

In contrast with CTAB/HCl sample, one cross peak (G) is also observed between Q4 and CH2 groups of the surfactant alkyl chain, suggesting a shorter Si…. H distance between the corresponding species in CTAB/OH compared to CTAB/HCl.

1 H-29 Si-1 H double CP experiments

The 2D HETCOR spectra are very informative, but very time-consuming (1 to 3 days) to record especially with a moderate magnetic field (here 7.05 T). That is the reason why we have implemented a double cross-polarization sequence 1 H- 29 Si-1 H (Figure 2) to record in a one-dimensional experiment the 1 H NMR spectrum, equivalent to the 1 H dimension of the HETCOR experiment, but with enhanced resolution. The acquisition of each 1D 1 H spectrum takes 1 hour. The sequence consists in a first polarization transfer from 1 H to 29 Si (characterized by a contact time tCP1), followed by a second polarization transfer back to the nearby protons (with a second contact time tCP2). The first contact time will be chosen in order to optimise the 29 Si signal intensity, while the second contact time will be incremented to reveal progressively the type of protons characterized with increasing internuclear distances from Si sites. Such an approach is not new in literature since a number of pulse sequences exploiting double or multiple cross-polarization steps among two or three nuclei already exist. They are mainly used to select and enhance signals from specific nuclei in biochemical compounds 45 Despite the large number of published sequences, we have used a different approach that is more suitable for our studied system and the desired information. The main difference between our sequence and those found in literature concerns the way of eliminating residual proton magnetization after the first CP transfer and the consequent X nucleus protection (block B in Figure 2b). After the first CP transfer, magnetization of X nucleus is flipped back to the +z direction in order to minimize self-relaxation effects. Then, saturation of residual 1 H signals is achieved by a combination of a time-decreasing loop and π/2 pulses at the end of each loop, which reveals to be very efficient even in case of different 1 H transversal relaxation times, T2( 1 H). This saturation step has two goals: suppression of all unwanted 1 H signals and, within the model of thermal reservoirs 46 , transformation of the proton bath into a hot reservoir into which magnetization can be back-transferred from the cold reservoir composed of X nuclei. After that, X magnetization is flipped back into the transversal plane before the second CP transfer. As we will see, the sequence reveals to be quite sensitive for our systems with results comparable and complementary to the previously used two-dimensional HETCOR techniques. The main drawback is the need for long relaxation times T1 for X nucleus, but this is fully satisfied in our systems for which 30s ≤ T1( 29 Si) ≤ 100s. Figure 3 demonstrates the efficiency of the technique to detect the protons, close to the silica surface through the comparison of the 1 H MAS NMR spectra of CTAB/HCl acquired either via direct 1 H signal detection (Figure 3a) or through the 1 H- 29 Si-1 H double CP sequence (Figure 3b). The protons at 7.2 ppm due to Si-OH are clearly visible with the new sequence as well as the N(CHx) protons due to the polar head groups, while those due the surfactant chains, which constitute indeed the major part of the proton bath are hardly detected.

Figure 4 shows a comparison between the 1 H- 29 Si-1 H double CP experiments recorded on CTAB/HCl and CTAB/OH for a series of the second contact time tCP2. These experiments were recorded after a thermal treatment of the samples overnight at 100°C to minimize the presence of adsorbed water. The value of tCP1 (3 ms) was chosen in order to optimize the polarization of the 29 Si nuclei after the first transfer, and it primarily concerns the Q3 sites. The spectra for CTAB/HCl (Figure 4a) show a rather broad peak around 7 ppm due to Si-OH groups, for short tCP2 (100 µs). Its intensity increases with tCP2 reaching a maximum value for 5 ms, before a decrease in intensity due to T1ρ( 1 Η) relaxation effects. On the contrary, the spectra of CTAB/OH (Figure 4b) show no similar signal; one can only notice an undefined broad signal ranging from 5 to 15 ppm that may be related to some type of OH groups. This signal can be related to the peak observed at ≈ 10.5 ppm in high silica zeolites templated with trimethylalkylammonium compounds. 47 It was assigned to silanols hydrogen bonded to a siloxy goup, SiO -...HOSi, with an O....O distance of ≈ 2.7 Å. This weak signal in the CTAB/OH sample confirms the low content in Si-OH groups. One remarkable feature in the spectral evolution is the appearance of the signal at 3.3 ppm due the N(CHx) polar head protons. It is already detected at tCP2 = 100 µs for CTAB/OH, while for CTAB/HCl, it appears only for tCP2 ≥ 300 µs. Then for both samples, the detection of a signal at 1.3 ppm related to the CH2 groups of the surfactant chain occurs at much longer time (≥ 7 ms).

The 1 H- 29 Si-1 H double CP sequence proves thus to be very efficient to discriminate the various 1 H families depending on their distance from the Si sites. The possibility to record in a reasonable time a series of spectra at increasing contact times leads to valuable information regarding the spatial distribution of the species at the silica/surfactant interface. However, the one-dimensional nature of the experiment prevents a neat identification of the Si sites involved in the polarization transfer. We have first investigated the possibility to extend this sequence to a two-dimensional 1 H-{ 29 Si} CP HETCOR correlation experiment by insertion of a time increment, t1, after the block B. This new sequence is analogous to the 29 Si-{ 1 H} CP HETCOR experiment shown previously, except that the detection channel is now 1 H, and no longer 29 Si. Consequently, we could expect to gain in resolution on the 1 H dimension, and this could be of great interest since the resolution on the indirect F1 dimension is always limited by the number of acquirable spectra. A comparison between the two sequences recorded with similar experimental parameters shows very similar results. 48 The sequence has also been modified (Figure 5) by coupling the 1 H- 29 Si-1 H double CP block with a 1 H magnetization exchange experiment. This allows to see how the 1 H magnetization that has been selected via the 1 H- 29 Si-1 H double CP, can be exchanged during a mixing time τm within the proton spin systems. Figure 6 compares experiments recorded for three different mixing times on CTAB/HCl. For τm = 140µs (Figure 6a), only diagonal peaks are present showing no magnetization exchange between the three 1 H spin systems : Si-OH groups, polar head groups and surfactant chains. For τm = 1 ms (Figure 6b), two cross-peaks appear : (A) shows a correlation between Si-OH and the NCHx groups, while (B) corresponds to a correlation between the NCHx groups and the protons of the chain. For τm = 10 ms (Figure 6c), the diagonal peak due to Si-OH groups has disappeared, and now a new cross-peak (C) is present between these Si-OH groups and the protons of the chain. By increasing the mixing time, the magnetization is progressively transferred from the surface silanols to the interior of the micelle. The fact that the transfer is going one-way is a consequence of the filtering effect of the 1 H- 29 Si-1 H double CP block, which polarized primarily the Si-OH and polar head protons, with almost nothing on the protons of the surfactant chain. The magnetization will thus be transferred along the following pathway : Si-OH  Polar head Chain.

-Characterization of the surface species

1 H MAS-NMR

In order to better identify the surface species, the samples have been dehydrated during 24 h under vacuum (~10 -2 mbar) at 100°C. The rotors were filled in a dry glove, and then water was added to the rotor to follow the re-hydration process of the powders. These experiments have been performed with MAS spinning rate of 30 kHz to get better spectral resolution. CTAB/HCl sample : besides the three peaks at δiso = 0.9 ppm, 1.3 ppm and 3.3 ppm, assigned to the surfactant molecules, and that have been already described, the spectrum of the dehydrated CTAB/HCl sample (Figure 7a) shows an additional broad signal of low intensity centered around 7.2 ppm. Similar chemical shift values have been reported for fairly acidic protons or silanols in strong H-bonding 24,49,50 in contrast to isolated silanols that are characterized by resonance peaks around 1-2 ppm. 51 Indeed, this value is in perfect agreement with the 1 H chemical shift value found in the 29 Si{ 1 H} HETCOR spectrum, for the cross-peak assigned to the Si-OH groups of Q3 sites. When 1 mg of water is added to the powder, a new signal appears that can be decomposed into two peaks: a broad one at 4.8 ppm and a sharper one at 4.5 ppm. After a second addition of water (0.8 mg), the area of the 4.8 ppm peak does not change much while the area of the 4.5 ppm peak increases 3 times as much. The peak at 4.8 ppm can be assigned to H2O molecules in strong H-bonding with surface Si-OH groups. 27 The second one could be due to H2O molecules as well, but in weaker interaction. Interestingly, with water addition, the signals due to the surfactant sharpen especially those due to the protons of the polar head. This is a result of an increasing mobility, and suggests that water molecules reached the interfacial silica/surfactant layer. The rotor was then placed under a IR lamp during 15 minutes for drying. The resulting 1 H spectrum is quite similar to the starting one obtained after dehydration, with even the re-appearance of the peak at 7.2 ppm that progressively broadened and shifted during water addition. CTAB/OH sample : In contrast to the CTAB/HCl sample, the 1 H MAS NMR spectrum of dehydrated CTAB/OH (Figure 7b) hardly shows any detectable signal around 6-8 ppm, due to surface silanols in perfect consistency with the double CP results. After water addition (2.4 mg), two new signals can be identified, one rather broad at 5.3 ppm and one sharp at 4.6 ppm. Further water addition (1.4 mg) does not cause any important modification of the spectrum, besides the increase in intensity of the peak at 4.6 ppm, which is clearly due to highly mobile H2O molecules, while the peak at 5.3 ppm can be assigned to H-bonded molecules. Further drying of the powder causes the total disappearance of the sharp peak, consistent with its assignment to free water molecules. But the signal around 5.3 ppm is still present and presents several components. In contrast to CTAB/HCl sample, a rather intense peak due to H-bonded H2O is still present, which could suggest a certain difficulty for the water molecules to escape from the interfacial zone if we consider a strong interaction between water molecules and surface silanols.

FTIR and FT-NIR experiments

Figure 8a shows the FTIR absorption spectra, in the 4000-450 cm -1 range, of CTAB/HCl (dot line) and CTAB/OH (full line) samples. The spectra have been normalized with respect to the intensity of the 1050 cm -1 band. This main intense band is assigned to Si-O-Si antisymmetric stretching modes 52 , and is accompanied with the symmetric stretching and rocking bands around 800 and 450 cm -1 , respectively, which represent the signature of the silica network. The 1050 cm -1 vibrational mode is accompanied by another band around 1230 cm -1 that is assigned to the antisymmetric longitudinal optical mode (LO3) 52 .

A sharp intense band is present at 1160 cm -1 , this band is overlapped with the silica stretching band at 1050 cm -1 and can be clearly observed in the CTAB/HCl sample but not in CTAB/OH. The presence of this band can be correlated to the presence of cyclic or cage-like silica-based species or disordered induced modes 53 . Even if a specific attribution requires a detailed investigation that is beyond the purpose of this work we can, however, deduce that this mode is an indication of different structural features present in the silica backbone. The basic and acidic synthesis produced structural differences, as also indicated by the band at 580 cm -1 , which is due to four-fold silica rings 54 , and is more intense in the CTAB/OH sample. The presence of the 1160 cm -1 band in CTAB/HCl certainly accounts for a different nature in the silica structure with respect to CTAB/OH. Another informative band is observed at 1640 cm -1 (νb(H2O)), this band is assigned to molecular water and clearly shows that the water content in the CTAB/OH is higher than in CTAB/HCl. The bands between 2700 and 3000 cm -1 are due to the C-H stretching vibrations from the surfactant molecules and are related with the corresponding deformation bands between 1400 and 1500 cm -1 . 55 The region between 3000 and 3600 cm -1 corresponds to the O-H stretching vibrations from silanols and water species. The wide band is formed by several overlapped vibrational modes and, in general, a detailed attribution requires complex deconvolution methods. The spectra show, however, some distinctive features that can give some valuable information. The first one is related to the pore surface, the spectra suggest, in fact, the presence of longer H-bonded chains due to adsorbed water and surface silanols in the CTAB/HCl with respect to CTAB/OH. The maximum falls, in fact, around 3326 cm -1 for CTAB/HCl instead of 3468 cm -1 for CTAB/OH, furthermore the band appears sharper, which is a supporting indication of shorter silanol chains. The shoulder appearing at 3632 cm -1 mainly in CTAB/OH sample indicates the presence of terminal silanols with a weaker H-bonding character. Their presence can be associated to a higher degree of condensation in CTAB/OH. In both samples, we can note the absence of a peak due to isolated surface silanols around 3750 cm -1 .

In the OH stretching region the presence of molecular water can be followed using the first overtone of the bending mode of H-bonded water (2νb), which falls around 3200 cm -1 . The broadness of the HCl band, which overlaps the 2νb, band of water, does not allow, however, a direct comparison between the two samples.

Bands in the NIR region (Figure 8b) account for the first overtones of ν(OH) stretching vibrations between 6800 and 7300 cm -1 and combinations of ν(OH) and δ(HOH) vibrations between 5000 and 5300 cm -1 . CH groups from CTAB resonate in the 5500-6000 cm -1 and 4000-4500 cm -1 regions and they will not be discussed here. The various types of water molecules interacting with the surface of silica have been discussed in a publication from Perry et al. 56 in terms of S'0, S'1, S'2 species. S'0 corresponds to a water molecule bonded to a silanol group through an oxygen atom; S'1 to a water molecule bound to SiOH group via oxygen and hydrogen-bonded to one other water molecule; S'2 to a water molecule bound to SiOH group via oxygen and hydrogen-bonded to two water molecules. Bands due to combinations of ν(OH) and δ(HOH) vibrations are expected at 5285 cm -1 and 5128 cm -1 for S'0 and (S'1,S'2) respectively. A main difference is observed between CTAB/HCl and CTAB/NaOH samples, indicating that water molecules exhibit weaker interactions with the surface in CTAB/NaOH (dominant band due to S'0) in agreement with data in the middle infrared. In CTAB/HCl material, water tends to strongly hydrogen-bond to surface silanols (bands at 5245 cm -1 and 5134 cm -1 assigned to S'1 and S'2).

Presence of S'2 water species should account for the formation of long range H-bonded chains and multilayers at silica surface.

Discussion

The combination of two-dimensional 29 Si-{ 1 H} HETCOR and one-dimensional 1 H- 29 Si-1 H double CP spectra clearly show different responses for the CTAB/HCl and CTAB/OH. At short contact times (500 µs), only 1 H nearest neighbors to Q3 sites are selected. In CTAB/HCl material, the related high chemical shift value (δ= 7.2 ppm) suggests the high acidic character of the protons as previously discussed. The presence of some Si-OH2 + species along with Si-OH ones probably completes the picture of the silica surface. The only fact against a massive presence of Si-OH2 + species, though in perfect agreement with the synthesis conditions used for the CTAB/HCl material, is the almost 1:1 (Cl -+Br -)/N + molar ratio (Table 1), indicating that charge matching with the positive charge of the polar head group, CTA + , is almost completely accomplished through the presence of the Cl -counter-ions. Indeed, a correlation between Q3 sites and the protons of the surfactant polar head exists, demonstrating the very close proximity between these two species despite the presence of the counter-ion. In the CTAB/OH sample, the absence of correlation between Q3 sites and OH-type protons for 500 µs contact time, suggests that the majority of Q3 sites is composed of Si-O -species as previously discussed in perfect agreement with initial basic conditions. Consequently, Q3 sites are in strong interaction with the protons of the CTA + polar head groups, as shown by the intense and lone correlation present in the corresponding HETCOR spectrum.

For longer contact times (5ms), the two systems still present different behaviors, especially for the OH surface species (Si-OH as well as H2O). In both samples, Q3 sites strongly interact with the CTA + polar head protons (N-CHx) as well as with the CH2 groups of the surfactant chain certainly in β or γ positions with respect to the polar head group. The main difference regarding the correlations involving the protons of the surfactant molecules is the intense cross peak between the CTA + polar head protons and Q4 sites that is detected only in the CTAB/OH sample. It corresponds to a stronger 29 Si-1 H heteronuclear dipolar interaction between the N + (CH3)3 protons and the Si surface sites, that could be attributed to shorter distances existing between the positively charged polar head of surfactant and the silica walls. However, one may keep in mind that these systems can not be considered as rigid ones.

Indeed, the relatively good resolution of the 1 H MAS-NMR spectra recorded at relatively low spinning frequency (14 kHz; see supplementary information) is clearly due to internal mobility of the constitutive species that lead to a partial averaging of the strong 1 H homonuclear dipolar couplings. This prevents a direct correlation between the CP responses and the 29 Si-1 H internuclear distances.

Important structural information that can be extracted from the 29 Si-1 H CP-based NMR experiments is the apparent differences in the OH species present at the silica-surfactant interfaces. At longer contact times (5 ms), the Q3 sites show correlation with OH-type protons, characterized by different chemical shift values depending on the sample. In CTAB/HCl, the peak at 7.2 ppm (already detected at short contact time) is assigned to primarily Si-OH groups and possibly some Si-OH2 + species. Additionally a signal is now detected at 4.6 ppm assigned to H2O molecules in interaction with the surface. In CTAB/OH, a 1 H resonance signal at 5.7 ppm appears. As previously hypothesized, the associated OH groups could be part of water molecules in interaction with the Si-O -surface sites, since double-CP experiments perfomed on the dried sample does not show any explicit silanol peak.

To get a better description about these OH surface species, one can compare the 1 H MAS NMR experiments performed on dehydrated and partially re-hydrated samples. The spectra of the de-hydrated samples totally confirm the results extracted from the 29 Si-1 H CP-MAS NMR experiments with no evidence for any OH in the CTAB/OH sample and a signal at 7.2 ppm for CTAB/HCl that can be safely assigned to Si-OH and/or SiOH2 + groups. Due to the de-hydrated nature of the sample, one could rule out extensive H-bonding with H2O molecules. But the high chemical shift value could be explained by the presence of short hydrogen-bond distances between the H-atom of a Si-OH group and the O-atom of a neighboring Si-O-Si surface site. 57 When water is added to the samples, the 1 H NMR responses are completely different for both systems. In CTAB/OH, a broad (δ = 5.3 ppm) and a sharp (δ = 4.6 ppm) distinct 1 H resonance signals are detected. The sharp one is undoubtedly due to free H2O molecules that do not interact with the surface, if one considers the small linewidth of the peak. The broader one can be related to the peak at 5.7 ppm observed in the HETCOR experiment, and assigned to H-bonded H2O molecules, certainly in interaction with Si-O -surface sites. Interestingly, after the second water addition, only the peak due to free water increases indicating that water is no longer interacting with the surface.

One possible scenario is that in CTAB/NaOH sample, the interfacial space is limited to accommodate a large amount of water molecules, and this is in agreement with the strong electrostatic interaction between the CTA + polar head groups and the negatively charge silica walls. In CTAB/HCl sample, no sharp signal due to free water is detected even after the second water addition, indicating that all water molecules are able to interact with the silica surface. Two overlapped broad resonance signals at 4.8 and 4.5 ppm are now present. As previously seen, only the intensity of the peak at 4.5 ppm increases with the amount of added water. This peak is certainly due to H2O molecules in weak interaction with the surface. However, the linewidth, which is larger that in the case of CTAB/OH, suggests that the water molecules are in a confined environment, and possibly in the silica surfactant interfacial layer. After the water addition steps, the peak due to Si-OH present at 7.2 ppm in the de-hydrated sample seems to progressively shift to lower chemical shift values, and to broaden. This indicates possible exchange mechanisms between the Si-OH surface groups and the water molecules. Indeed, the reported chemical shift value of 4.8 ppm for the second signal, indicate rather weak interactions between the water molecules and the silica surface, but certainly through extensive H-bonding.

Infrared, and more specifically near-infrared experiments, have confirmed for the two samples, distinct structures for the adsorbed water layers present at the silica-surfactant interface: water molecules exhibit weaker interactions with the surface in CTAB/NaOH sample, while they tend to strongly hydrogen-bond to surface silanols in CTAB/HCl sample, and to form long range H-bonded chains and multilayers at silica surface. The presence of these water layers in CTAB/HCl accounts for the large decrease in line width of the 1 H resonance signal at 3.3 ppm due to the protons of the polar head groups, from 150 Hz in the de-hydrated sample down to 30 Hz after the second water addition. It indicates a higher mobility of the polar head groups, due to a higher solvation degree. This effect has already been discussed by Sizun et al. 58 who used methanol to increase the CTAB mobility. On the contrary, when water is added to the CTAB/OH sample, the peak linewidth does not vary with water addition, indicating that extensive solvation does not occur, due to strong electrostatic interactions between the polar head groups and the silica surface sites, which prevents the formation of water layers in the interfacial palisade.

All this study can allow proposing the schemes shown in for the surfactant-silica interfaces in CTA + templated samples prepared under acidic and basic conditions. The main difference comes from the existence of an extended network of H-bonded water molecules in the acidic case, which is certainly favored by the presence of hydrated Cl -counter-ions.

Conclusion

This work has reinvestigated the structural study of CTAB-templated mesostructured silicas prepared under highly acidic and basic conditions. We have combined highly advanced 1D and 2D solid state NMR methods based on 29 Si-1 H dipolar coupling with Near Infrared spectroscopy in order to evidence the structural differences at the silica/surfactant interface that had been proposed through the so-called S + .I -and S 0 .X -.I + interaction mechanisms. Indeed, we went one step further since we have clearly identified the presence of water molecules in the interfacial region that behave differently depending on the synthetic conditions. Silica obtained under acidic conditions shows a silanol-rich interface.

Surfactant polar heads, whose charge is completely neutralized by its own counter-ion, settle close to the silanol palisade though leaving enough room to water molecules forming extended hydrogen bonding network. When silica is prepared under basic conditions, the surface is rich in SiO -groups which are the lonely source for the neutralization of the polar head charge. This direct SiO -/N + (CH3)3 interaction induces a close proximity between surfactant polar head group and silica surface and it consequently prevents water molecules to largely interact with the silica surface.

Figure 1 .

 1 Figure 1. Two dimensional HETCOR 29 Si{ 1 H} spectra recorded on CTAB/HCl with τCP = 500 µs (a)

Figure 2 .

 2 Figure 2. Schematic principle of 1 H-29 Si-1 H double CP experiment and related NMR sequence.

Figure 3 .

 3 Figure 3. Comparison between the 1 H MAS-NMR spectra of CTAB/HCl recorded with direct detection

Figure 4 . 1 H

 41 Figure 4. 1 H-29 Si-1 H double CP NMR spectra recorded on CTAB/HCl and CTAB/OH for a series of the

Figure 5 .

 5 Figure 5. Pulse sequence for the 2D 1 H homonuclear correlation spectra shown in Figure 7: 1 H-X-1 H

Figure 6 .

 6 Figure 6. Two-dimensional homonuclear 1 H correlation spectra of CTAB/HCl taken using the pulse

Figure 7 . 1 HFigure 8 .

 718 Figure 7. 1 H NMR-MAS spectra of CTAB/HCl (a) and CTAB/NaOH (b). (B0 : 9.4 T -Sample

Figure 9 .

 9 Figure 9. Schematic view of the silica-surfactant interfaces in CTAB/HCl and CTAB/NaOH samples

Figure 1 .Figure 2 .

 12 Figure 1. Two dimensional HETCOR 29 Si{ 1 H} spectra recorded on CTAB/HCl with τCP = 500 µs (a)

Figure 3 .

 3 Figure 3. Comparison between the 1 H MAS-NMR spectra of CTAB/HCl recorded with direct detection

Figure 4 .

 4 Figure 4. 1 H-29 Si-1 H double CP NMR spectra recorded on CTAB/HCl and CTAB/OH for a series of the

Figure 5 .

 5 Figure 5. Pulse sequence for the 2D 1 H homonuclear correlation spectra shown in Figure 6 : 1 H-X-1 H

Figure 6 .

 6 Figure 6. Two-dimensional homonuclear 1 H correlation spectra of CTAB/HCl taken using the pulse

Figure 7 .

 7 Figure 7. 1 H NMR-MAS spectra of CTAB/HCl (a) and CTAB/NaOH (b). (B0 : 9.4 T -Sample

Table 1 .

 1 Structural parameters related to CTAB/HCl and CTAB/OH samples: cell parameter (a), chemical composition from elemental analysis data and distribution of Si sites from29 Si MAS-NMR experiments. Error margins in29 Si NMR analysis are estimated to ±1%.

	Sample	N/Si	Cl/N	Br/N	Q4	Q3	Q2
	a (Å)
		(mol.%)	(mol.%)	(mol.%)	δ (ppm) % δ (ppm) % δ (ppm) %
	CTAB/HCl 42.4 0.18±0.01 0.86±0.03 0.05±0.01 -110.3 43 -100.9 50 -91.2 7
	CTAB/OH 41.4 0.21±0.01		0.08±0.02 -109.2 48 -99.2 43 -90.5 9

Table 2 .

 2 Assignments of the bands present in the FT-IR and NIR spectra of CTAB/HCl and

	CTAB/NaOH.
	IR (cm -1 )	Assignements	NIR (cm -1 )	Assignements
		Cyclic species,
	580,1160		4000-4500 C-H from CTAB
		Cubic octameric silica cages
	1050	AS-TO3 Stretching
			5130	νOH + δHOH(S'1, S'2)
	1230	AS-LO3 Stretching
	1630	Adsorbed water	5245	νOH + δHOH(S'0)
	2700-3000	C-H from CTAB	5500-6000 C-H from CTAB
	3121	OH Stretching	6834	2νOH (S'1, S'2)
	3234	H-Bonded water	7057	2νOH (S'0)
	3326	H-Bonded Si-OH chains	7177	2νOH (S'0)
	3468	H-Bonded Si-OH chains
	3632	Terminal silanols

Table S1 .

 S1 1 H chemical shift values for CTAB/HCl and CTAB/OH samples. The values for a solution of CTAB in D2O (5.10 -2 M) are given as references.

	Captions
		Chemical shift (ppm)		Assignment
	CTAB/HCl sample CTAB/OH sample	CTAB in D2O
	0.9	0.9	0.85	CH3 (C16)
			1.27	CH2 (C4-C15)
	1.3	1.3	1.35	CH2 (C3)
			1.75	CH2 (C2)
			3.17	N-CH3
	3.3	3.3
			3.42	N-CH2 (C1)
	4.2-4.5	4.6		H2O
	4.4-4.8	5.3		Si-OH in interaction with H2O
	7.2			Si-OH in interaction with the surface

for Figures A updater quand on aura modifié les légendes

Supporting Information Available. 1 H chemical shift values for CTAB/HCl and CTAB/OH samples. 1 H NMR-MAS spectra of CTAB/HCl and CTAB/OH samples. Comparison between the classical 2D 29 Si-{ 1 H} CP-HETCOR experiment and the 2D 1 H-{ 29 Si} CP-HETCOR experiment recorded with a pulse sequence based on the 1 H- 29 Si-1 H double CP sequence. For CTAB/HCl sample, comparison between 1 H traces extracted from 29 Si{ 1 H} CP HETCOR and CPLG HETCOR experiments for Q3 (a) and Q4 (b) sites. This material is available free of charge via the Internet at http://pubs.acs.org.