When agricultural residues are processed to ethanol, lignin and silica are some of the main byproducts. Separation of these two products is difficult and the chemical interactions between lignin and silica are not well described. In the present study, the effect of lignin-silica complexing has been investigated by characterizing lignin and silica coprecipitates by FTIR and solid state NMR. Silica particles were coprecipitated with three different lignins, three lignin model compounds, and two silanes representing silica-in-lignin model compounds. Comparison of 29Si SP/MAS NMR spectra revealed differences in the distribution of silanol hydroxyl groups among different coprecipitates. These differences are dependent on the lignin type. The results are interpreted that the underlying mechanism of the interactions is the formation of hydrogen bonds between lignin aliphatic hydroxyl or carboxyl groups and the silanols, but not a condensation of the silica-in-lignin among the silica particles and not the formation of C-O-Si bonds.
Bioenergy production is a tool to combat climate change, however, there are technical challenges that hinder its economic feasibility. The presence of silica in biomass is one of the problems. For example, in combustion applications, silica causes fouling which decreases boiler performance (Pekarovic et al. 2006), or in the course of the process of converting lignocellulosics to ethanol, the silica is mixed with the lignin rich distillation residue, which limits the possibility to further utilize the lignin.
Silica, the polymerized form of silicic acid, is the main Si containing component in plants, and in grasses such as wheat and rice it can comprise 5–20% of the total biomass. The function of silica in higher plants has been related to crop yield, salinity, and drought stress as well as resistance to lodging (so the plant shoots are not displaced from their vertical position easily), pathogens, herbivores, and metal toxicity (Epstein 2001). Plants take up silica actively as silicic acid. The transport is genetically controlled and several genes responsible for silica uptake are known (Bauer et al. 2011). The mechanism by which silicic acid is deposited and converted to silica is still unknown as well as the mode of interaction between silica and organic compounds in the plant cell wall.
The characterization and application of lignin from second generation of bioethanol plants (Kristensen et al. 2008; Kaparaju and Felby 2010; Hansen et al. 2013; Yelle et al. 2013) have revealed that there is a strong interaction between lignin and silica, which is not yet fully understood, and which has an impact on lignin structure, properties, and applications. For understanding the association between silica and lignin, the structure-property relationships have to be elucidated, which may enable a better control concerning the lignin properties for specific applications. The structure of lignin has been extensively investigated by spectroscopy (Gosselink et al. 2004), scattering techniques (Petridis et al. 2011), and microscopy (Brunow 2005), just to name a few. Given the large amount of silica that can be found in lignin, surprisingly little is known about the spatial distribution of Si in lignin as well as the possible associations and bonding types between these components (Nour et al. 2004).
One linkage possibility is that OH groups on the surface layer of silica form hydrogen bonds with polar regions or groups of lignin (Emsley 1980) (Figure 1a). Silica surfaces contain a large number of silanol groups with zero (called Q4), one (Q3), or two OH groups (Q2) (Tuel et al. 1990; Legrand et al. 2005) (Figure 2a). If OH groups are responsible for the silica-lignin linkage, steric forces could increase the stability of silica in the polymeric matrix. For example, silicon could form stable complexes with lignin by changing coordination state, as described for some polyols in aqueous solutions (Kinrade et al. 1999) and for pyrogenic silica (Ogenko 1993), which would further increase its stability in aqueous systems.
Another possibility is the existence or formation of Si-O-C covalent bonds between silica and lignin during biomass processing (Figure 1b). Under laboratory conditions, it is possible to generate such bonds via alcoxysilane coupling agents (Rasmussen et al. 2014). It is important to note, however, that silane-based coupling agents are much more efficient in bonding with lignin than pure silica. Because the formation of this kind of bonding is not a spontaneous process under physiological conditions, it can be speculated that an enzyme catalyzed mechanism could be responsible for the formation of such structures.
A third possibility is a combination of both mechanisms, i.e. that some silica molecules are covalently bound to the lignin acting as promotors, while the others are bound by hydrogen bonding interactions.
The silica-lignin interactions are difficult to investigate under physiological conditions. The low water solubility is one obstacle, as both silica and lignin are only water soluble at pH >10 for some lignin types (Evstigneev 2010). Additionally, despite the fact that the concentration of silica in grass lignin is high, the concentration of the 29Si nuclei, i.e. the only isotope of Si that can be detected by NMR, is too low to be detected directly by 29Si NMR (Argyropoulos and Menachem 1998). Therefore, experimental precipitates with increased silica concentrations are needed to study silica and lignin bonding by 29Si NMR.
In this paper, two complementary analytical techniques are used to characterize silica lignin coprecipitates. Possible changes in the structure of the silica particles are examined by solid state 29Si NMR, and possible changes on the lignin structure are assessed by ATR-FTIR. Three different technical lignin preparations are observed (Sigma-organosolv, Protobind 1000, and Protobind 6000). The following compounds are additionally tested: coprecipitates of three lignin model compounds (guaiacol, creosol, and veratryl alcohol) and two silanes (trimethyl-phenoxy-silane and methoxytrimethylsilane) acting as silica-in-lignin model compounds (Figure 3).
Materials and methods
Organosolv lignin from Sigma-Aldrich (Brøndby, Denmark) and two soda lignins (Protobind 1000 lignin and Protobind 6000) from GreenValue SA (Lausanne, Switzerland) are in focus. The colloidal silica suspension LUDOX TM-30 was purchased from Sigma-Aldrich (Brøndby, Denmark). Lignin model compounds (guaiacol, creosol, veratryl alcohol), and silane model compounds [trimethyl(phenoxy)silane and methoxytrimethylsilane] were acquired from Sigma-Aldrich (Brøndby, Denmark) and used without further purification. Ludox TM-30 colloidal silica was coprecipitated with different lignin types, lignin model compounds, and two commercial silanes representing silica-in-lignin model compounds. A brief description of the compounds is summarized in Table 1.
|Protobind 1000||Protobind 6000||Organosolv|
|Plant source||Wheat straw||Wheat straw||50/35/15 (%) maple/birch/poplar|
|Solub. in aq. alkali||Very high||Very high||Medium|
|Si content (ppm)||2367±73||816±39||148±92|
StD is calculated from four measurements.
Multi-elemental analyses of lignin fractions were conducted by inductively coupled plasma-optical emission spectroscopy (ICP-OES). For the analysis, 10–50 mg of material was mixed with 500–2500 μl 70% HNO3 (SCP Science, Quebec, Canada), 250–1000 μl 15% H2O2 (Sigma-Aldrich, Brøndby, Denmark), and 40–200 μl 49% HF (Sigma-Aldrich) and digested in a pressurized microwave oven (Ultrawave, Milestone Inc., Sorisole, Italy) for 10 min with a starting pressure of 40 bar at 240°C. After digestion, samples were diluted to a final 3.5% acid concentration with Milli-Q water (Milli-Q Plus, Bedford, MA, USA) before measurement by means of an ICP-OES (Model Optima 5300 DV, PerkinElmer) equipped with a HF resistant sample introduction kit. Quantification was performed by an external 10 point calibration standard P/N 4400-132565 and P/N 4400-ICP-MSCS (CPI International, Amsterdam, The Netherlands). A certified reference material (CRM) NCS 73013 Spinach leaf was analyzed together with the samples to evaluate the accuracy and precision of the analysis.
Lignin was suspended in demineralized water, the pH adjusted to 12 with a solution of NaOH (30%) and stirred for 1 h at room temperature (r.t.) by magnetic stirring. Ludox TM-30 silica dispersion was added and stirred for another hour; the pH was then slowly adjusted to 2 with 0.1 M HCl. Lignin: silica ratios of 0.3, 0.6, and 1.4 were reached by mixing 3, 4, or 5 g of lignin with 30, 20, or 10 ml of Ludox silica, respectively (assuming 30% suspension in H2O and 1.2 g ml-1 density as reported by the supplier). The lignin-silica coprecipitate was filtered and washed with demineralized water three times to remove the acid and dried at 40°C for 10 days. The same procedure was done with lignin model compounds (1 ml of Ludox silica was mixed with 1 or 2 ml of either guaiacol, creosol, or veratryl alcohol) and silica-in-lignin model compounds (1 ml of Ludox silica was mixed with 1 or 2 ml of either trimethyl-phenoxy-silane or methoxytrimethylsilane).
ATR-FTIR spectra from the coprecipitates were obtained in triplicates on a ThermoFischer Scientific Nicolet 6700 FTIR spectrometer (Fisher Scientific, Roskilde, Denmark) equipped with a Goldengate ATR accessory. Spectra were collected with a 4 cm–1 resolution, 100 background scans, and 50 scans for each sample spectrum.
Solid-state NMR spectra were recorded on a Bruker Avance 400 spectrometer (Bruker, Coventry, England) (9.4 T) operating at 400.13, 100.62, 79.49 MHz for 1H, 13C, and 29Si, respectively. A double tuned solid-state probe equipped with 4 mm (o.d.) spinners was applied. The single-pulse (SP) MAS and cross-polarization (CP) MAS spectra were recorded under the conditions: spin-rate of 9 kHz, 296 K, ramped CP (Metz et al. 1994) with contact times of 2 ms for 13C and 8 ms for 29Si in acquisition of CP/MAS spectra, and 1H TPPM decoupling (80 kHz rf-field strength) (Bennett et al. 1995) during acquisition (49.3 ms for 13C and 42.6 ms for 29Si). Recycle delays and number of scans were 8 s/1024 for the 13C CP/MAS spectra, whereas 2 s/8192 and 256 s/256 for the 29Si CP/MAS and SP/MAS NMR spectra, respectively. For 13C CP/MAS spectra rf-field strengths of 80 kHz were employed for 1H and 13C during CP, whereas 1H and 29Si rf-field strengths of 55.6 kHz were utilized during CP for the 29Si CP/MAS spectra. The 29Si SP/MAS spectra were recorded with a pulse combined with a flip-angle of 54.7° (rf-field strength 55.6 kHz). All 13C MAS spectra were referenced (externally) to the carbonyl resonance in α-glycine at 176.5 ppm, whereas the 29Si MAS spectra were referenced to (externally) to the resonance of 3(-methylsilyl)-1-propanesulfonic acid Na salt at 1.4 ppm. All spectra were apodized by a Lorentzian linebroadening of 10 Hz. The 29Si CP/MAS and SP/MAS spectra were deconvoluted by the software OriginPro 9.1 (OriginLab, Northampton, MA, USA).
The spectra were fitted with non-linear least squares fitting analysis based upon the Levenberg-Marquardt method (OriginPro 9.1). The results were considered accurate when the spectra exhibited clearly defined peaks and the sum of the peaks closed to 100%. Furthermore, the χ2 tolerance value should reach the 1E-6 limit in the normalized spectra, and the adjusted correlation coefficient (R2) should have a value between 0.98 and 0.99.
The normalized areas of the Gaussians functions in the deconvolution of 29Si SP/MAS NMR spectra were expressed as percentage of the total area. The amount of silanol groups relative to total silicon molecules was determined based on the following equation (Igarashi et al. 2003):
Results and discussion
Coprecipitates with lignin
The 13C NMR spectra of all coprecipitates are almost identical to the spectra of the pure lignin counterparts (Figure 4a and b). These spectra contain resonances previously assigned to lignin (Bardet et al. 2009), thus the coprecipitates contain significant amounts of lignin. The resonance at 55.8 ppm originates from the aromatic methoxy groups and the resonance at 147.5 ppm originates from C3 or C5 of non-etherified syringyl units or C1 or C4 of guaiacyl units. The resonance at 105.4 ppm is most likely due to overlapping resonances from C1 in cellulose and aromatic carbons in lignin. Comparison of these peak intensities and those in the region 84–90 ppm indicates that the cellulose content in organosolv lignin is higher than in Protobind 1000 and 6000 lignins. However, according to the specifications of the producer, Sigma organosolv lignin contains <0.5% and Protobind lignin <3%, residual carbohydrates. The signal at 14.6 ppm (methyl group in aliphatic chains) is only observed in organosolv lignin.
The coprecipitates were also analyzed by single pulse magic angle spinning (SP/MAS) 29Si NMR, which provides a quantitative spectrum, and cross-polarization magic angle spinning (CP/MAS) 29Si NMR to enhance the signal intensity of Si nuclei close to the hydrogen atom due to polarization transfer from 1H via heteronuclear dipolar couplings (Legrand et al. 2005). The 29Si MAS NMR spectra reveal resonances at approx. -90, -100 and -110 ppm. These signals are due to different chemical environments for the silicon atoms. The latter is for the bulk of the silica, which is bound to four bridging oxygen atoms (Rimola et al. 2013). This environment, known as Q4, features four neighboring [SiO4] groups attached to a given silicon atom. The resonances -100 and -90, are typical for chemical environments, in which the silicon is bound to one or two hydroxyl groups, or three or two [SiO4], and the corresponding environments are denoted as Q3 or Q2, respectively. The 29Si CP/MAS and SP/MAS spectra were deconvoluted by means of Gaussian line shapes. As an example, the 29Si MAS NMR spectra for Ludox TM-30 silica along with results of deconvolution are shown in Figure 2a, b, and c.
The coprecipitated silica-lignin particles have a particular distribution of Q4, Q3, and Q2 groups that appear to be lignin specific (Table 2, Figure 5). All the coprecipitates show a higher amount of Q2 species compared with the pure silica dispersion, while the SiOH% content is higher in almost all of the lignin coprecipitates. Organosolv lignin appears to have a small change in the distribution of Q4, Q3, and Q2 groups compared with the Ludox silica precipitate. The two Protobind soda-lignins have an increased number of SiOH% content, i.e. the amount of Q3 and Q2 species is elevated. For Protobind 1000, the effect is more evident for higher silica loadings in the coprecipitate, while in Protobind 6000 samples the tendency is opposite.
|Silanol species||Ludox silica||Lignin types with their L/S ratios|
|Organosolv||Protobind 1000||Protobind 6000|
1.4, 0.6, 0.3: Lignin/silica ratio. SiOH: amount of silanol groups relative to total silicon sites.
The resonances for Q2 and Q3 species were enhanced via 29Si CP/MAS NMR and the spectra were normalized to the intensity of the Q4 signals (Figure 6). The spectra show that the Q3 and Q2 resonances for the coprecipitates are much more intense than the resonances for the Ludox TM-30 silica dispersion. Due to the higher amounts of organic material in the lignin/silica samples, a higher proton density throughout the sample is obtained, and this makes it possible to transfer 1H polarization to 29Si from not only directly attached hydroxyl groups, but from any hydrogen within a <5 Å distance from the silicon atoms. Therefore, also protons from organic compounds close to the silica surface may contribute to the polarization transfer. In Figure 6a, b, and c, the effect of this is clearly seen as the Q3 and Q2 resonances are more intense in the lignin/silica samples compared to the pure silica sample. At 0.3 and 0.6 L/S ratios, organosolv lignin shows the highest relative intensities while Protobind 1000 and 6000 lignins show lower but equal intensities. At 1.4 S/L ratio, Protobind 6000 and organosolv show the same relative intensities while Protobind 1000 shows a lower one.
To compare the amplified signals in the 29Si CP/MAS spectra, the Q2/Q3 ratio was calculated (Table 2). This ratio indicates that the proportion of Q2 sites for organosolv lignin is higher than for both Protobind 1000 and 6000. The Q2/Q3 ratio does not vary as a function of silica loading (lignin/silica ratio) for organosolv (Q2/Q3 between 0.22 and 0.23) or Protobind 1000 (0.18–0.19), while for Protobind 6000 the values vary slightly but without an evident correlation with the L/S ratio (0.14–0.21). However, the ratios must be considered with some caution, as the CP will be more efficient for Q2-sites than Q3-sites due to more abundant hydrogens (in OH groups) close to the Si. Conversely, the rather long (8 ms) contact time may favor weaker H-Si dipolar couplings (longer distance between nuclei).
The variations of the silica component in the silanol moiety (Table 2) means that the percentage of silanol groups (SiOH) is higher than the percentage of siloxane groups (Si-O-Si) in the lignin coprecipitates than in the silica alone. Opposed to siloxane groups, silanols groups can interact with polar groups or molecules via hydrogen bonding (Rimola et al. 2013). Both the higher SiOH% and lower Q4 values for the Protobind lignins suggest a stronger interaction between these lignin types and the silica particles than in the case of organosolv lignin. However, the proportion of Si-(OH)2 to Si-(OH)1 groups, or Q2/Q3 ratio, shows a higher proportion of Q2 species in organosolv lignin than in the two Protobind lignins. In other words, organosolv lignin contains more Si-(OH)2 functionalities, suggesting that this lignin has higher functionalization in each silicon site. Accordingly, there is a different degree of reaction between silica and lignin that depends on the lignin type (lignin source), which is stronger for Protobind lignins (wheat) than for organosolv lignins (maple, birch, poplar), while the processing conditions effect the observed differences. The minor differences in SiOH% between Protobind 1000 and 6000 lignins and the smaller Q2/Q3 ratio for Protobind 6000 than for 1000 (Table 2) can also be summarized as the influence of various amounts of interacting functional groups. More acid is needed to produce Protobind 6000 than Protobind 1000 lignin. The acids increase the degree of oxidation of the resulting material, as β-O-4 ether linkages are cleaved under acidic conditions (Li et al. 2007) leading to more aromatic and aliphatic hydroxyl groups as well as carboxyl groups (Table 2). The 29Si NMR spectra do not show any other major shift, probably because the formation of covalent bonds, if any, are below the detection limit (Loy et al. 2000).
ATR-FTIR spectra of the coprecipitates exhibited characteristic bands at 1200–1000 cm-1 and at 850–750 cm-1 positioned at higher frequency than in the case of the silica sample (Figure 7). These bands are attributed to asymmetric stretching vibration of Si-O-Si and symmetric stretching and bending of Si-O-Si, respectively (Parida et al. 2006). At low lignin:silica ratios of the three lignin coprecipitates, there is a band at 800 cm-1 characteristic of the pure silica sample, that shifts to 830 cm-1 at higher lignin:silica ratios. Variations of the 850–750 cm-1 band have been correlated to differences in the oxygen content of the samples, with shifts towards larger wavenumber values in case of higher oxygen contents, while its width becomes smaller (Tomozeiu 2011). Accordingly, the surface of the coprecipitate has a different oxygen substitution pattern than that of the silica alone. Additionally, all the Protobind 6000 lignin coprecipitates show a band at 960 cm-1 that it is absent in both the lignin and the silica samples. This band is attributed to the adsorption of organic molecules, which formed hydrogen bonding with silanol groups at the silica surface (Hino and Sato 1971).
The band at 1050 cm-1 disappears gradually from the coprecipitates with high Si load and a typical lignin band at 1100 cm-1 appears with low Si loading. These changes could be due to a simple dilution effect. However, a closer look reveals some differences between the different lignin types. For Protobind 1000 lignin, the band at 1050 cm-1 is almost absent already at low L/S ratios (0.3 and 0.6), while the band at 800 cm-1 is present and begins to shift only at a 2.8 S/L ratio. In the case of Protobind 6000, the shift of the band at 1050 cm-1 occurs more gradually, but the shift at 800 cm-1 occurs only when the L/S ratio is 11.1. Organosolv lignin shows an even more gradual shift in the 1050 cm-1 region and the shift at 800 cm-1 occurs at 5.6 L/S ratio. The FTIR-ATR method is surface selective and it is also suitable to detect adsorbate-adsorbent interactions (Burneau and Gallas 1998). Thus the band shift differences can be attributed to differences in the affinity of the different lignins to the silica, i.e. if the affinity of the lignin to the silica is higher, the intensity of the silica bands decreases faster as the silica/lignin ratio increases because more silica particles are covered with a lignin layer. Following this assumption, both Protobind 1000 and 6000 lignins seem to have a higher affinity to the silica particles in agreement with the solid-state NMR results.
The 13C NMR spectra of all coprecipitates are almost identical (Figure 4b). Probably, the interaction level between lignin and silica indicated by the FTIR spectra, takes place between the non-carbon atoms (H, O, and Si atoms), which are not detectable by 13C NMR spectra. The elemental analysis of the coprecipitates show that the silicon content does not vary significantly between different lignin types at the same lignin:silica ratio. This is interpreted that the differences found in the FTIR spectra correspond to differences in the affinity between these two substances (Table 3).
|Elements (mg g-1)||Lignins with their L/S data|
|Na||16 662||20 916||18 410||19 132||17 854||31 576|
|Si||211 277||154 625||115 498||212 461||179 109||105 626|
1.4, 0.6, 0.3: Lignin/silica ratio. N.D stands for not detectable.
Reactions with lignin model compounds
The spectra of the silica-lignin model compounds coprecipitates reveal that only veratryl alcohol was attached to the silica particles (Figures 8a and 9). The results of the 29Si SP/MAS NMR analysis indicated that only veratryl alcohol changed significantly the substitution patterns on the surface of the silica particles leading to more Q3 and Q2 species (Table 4). The pKa of the model compounds are: creosol 10.5, guaiacol 9.3 (Ragnar et al. 2000), veratryl alcohol 10.8 (Nour et al. 2004). Thus, when the coprecipitates were synthetized at pH 12, the model compounds were ionized, and so they could more readily react with the silica. After precipitation at pH 2, the coprecipitates were not anymore ionized so it is unlikely that they interacted with other molecules during the drying process.
|Ratios between Ludox silica and additives||Silanol species|
|Q4 (%)||Q3 (%)||Q2 (%)||SiOH (%)|
|Ludox silica alone||78||21||1||23|
|+ Veratryl alcohol (1:1)||59||36||5||46|
|+ Guaiacol (1:1)||77||20||3||26|
|+ Creosol (1:1)||74||24||2||28|
|+ Trimethyl(phenoxy)silane (1:1)||82||16||2|
|+ Methoxytrimethylsilane (1:1)||92||8||N.D|
In the FTIR spectrum of veratryl alcohol, the bands at 1517 cm-1, 1464 cm-1, and 1419 cm-1 as well as the shoulders at 1263 cm-1, 1238 cm-1, and 1028 cm-1 confirms the presence of adsorbed compounds on the surface of the silica precipitate (Figure 9). After the purification and drying process, the appearance of this material was a fine solid powder, which is a hint for an adsorbed compound instead of an in-depth impregnation into the inorganic matrix.
Organosolv lignin is considered to be pure and unaltered (Mousavioun and Doherty 2010). In contrast, Protobind 1000 lignin is digested with caustic soda and the extract, which contains lignin, is treated with an acid to form a lignin slurry, which is filtered and then washed and dried. Protobind 6000 is a derivate of Protobind 1000 in which the caustic soda process is repeated. According to the literature, Protobind 1000 contains more aliphatic hydroxyl and carboxylic groups than organosolv lignin (Table 5). Nevertheless, the total phenolic hydroxyl groups are slightly lower in Protobind 1000 lignin, hence the aliphatic hydroxyl and carboxylic groups are mainly responsible for the interaction with silica.
|Functional group (mmol g-1)||Lignin type|
Only veratryl alcohol has a primary alcohol group, which may be responsible for the interaction with the silica particles, probably through hydrogen bonding. The phenol groups in guaiacol and creosol, as well as the methoxy groups on the aromatic rings in all three compounds seem to have a lesser effect on the silica surface. The formation of silica-veratryl alcohol complexes was, however, not evident in previous works with laser Raman spectroscopy as analytical instrument in focus (Nour et al. 2004). However, in the quoted study aqueous silica was applied, and veratryl alcohol was compared with molecules containing a carboxylic acid functionality. Probably, the sensitivity of the Raman spectroscopy was too low for detection of the weak interaction between the hydroxyl groups and the solid silica surface. The samples were synthesized under specific laboratory conditions both in the study of Nour et al. (2004) and in the present work, and so cross-correlating these data would be highly speculative.
Literature data support the importance of the aliphatic alcohol groups in the course of silica adsorption. Zhao et al. (1994) compared the standard free adsorption energy of linear fatty alcohols at silica gel/carbon tetrachloride and silica gel/cyclohexane interfaces with that of aromatic substances. For the former, the adsorption energy range was -20.8 to -15.5 kJ mol-1 from ethanol to octanol, while for benzoic alcohol, anisole, and toluene, the range was -17.2 kJ mol-1 to -6.48 kJ mol-1, indicating a lower adsorption with aromatic substances. Likewise, when 1- and 2-propanol were adsorbed on silica surfaces via hydrogen bonding, the contact of adsorption layers of 1-propanol brought about a longer-range attraction than that of 2-propanol due to cyclic aggregation of the latter (Mizukami and Kurihara 2003).
Reactions with silanes
The 13C CP/MAS NMR analysis indicates that a condensation took place during the selected conditions (Figure 8b). The 29Si SP/MAS NMR spectra show that – contrary to the lignin and the lignin model compounds – silanes increase the population of Q4 species (Table 4). A possible explanation is that the surface of the silica particles becomes more hydrophobic with the silane functionalization inhibiting the formation of silanol species. This suggests that the interaction between the silica-in-lignin species with the silica dispersion is not responsible for the behavior of the lignin-silica coprecipitates. Thus, evidence could not be found for the hypothesis that some silica molecules are covalently bound to the lignin acting as promotors, while the others silica molecules were bound by hydrogen bonding interactions.
The primary interactions between lignin and silica occur via hydrogen bonds. Variations in both the shifts in the Si-O-Si stretching bands and in the silanol content among different coprecipitates indicate that lignin is capable of inducing substitution pattern changes on the surface of silica particles, and interaction degree changes with lignin type and degree of lignin oxidation. Variations in the Q2/Q3 ratio of the lignin-silica coprecipitates, which reflect the presence of additional hydroxyl groups in the silica-lignin coprecipitates compared to the silica alone, further reinforce the hypothesis that lignin induce substitution pattern changes in the silica surface. Differences in lignin-silica interactions are likely due to hydrogen bonding interactions between the silica hydroxyl groups and the primary alcohol groups of lignin and do not involve formation of carbon-silica bonds.
This work was supported by the Danish National Advanced Technology Foundation via the Technology Platform “Biomass for the 21st century”. The authors gratefully acknowledge Thomas H. Hansen and Jan K. Schjørring for performing the elemental analysis of the samples.
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