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Interaction Mechanisms between Guaiacols and Lignin: The Conjugated Double Bond Makes the Difference )
V. Daniela Barrera-Garcı´ a,†,‡,^ David Chassagne,†,‡ Christian Paulin,§ Jesus Raya, Jer^ome Hirschinger, Andree Voilley,‡ Jean-Pierre Bellat,§ and Regis D. Gougeon*,†,‡ Institut Universitaire de la Vigne et du Vin “Jules Guyot”, Universit e de Bourgogne, F-21078 Dijon, France, ‡EA 581 EMMA, 1, Esplanade Erasme, AgroSup, F-21000 Dijon, France, §Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209 CNRS, Universit e de Bourgogne, F-21078 Dijon, France, and Institut de Chimie, UMR 7177 CNRS, Universit e de Strasbourg, BP 296, 67008 Strasbourg Cedex, France. ^ Present address: Facultad de Turismo y Gastronomı´a, Universidad Aut onoma Del Estado De M exico, Estado de M exico, Mexico. )
†
Received September 22, 2010. Revised Manuscript Received December 11, 2010 Lignin is considered to be responsible for a selective sorption of phenolic compounds on wood. In order to investigate the mechanisms involved, two similar guaiacol compounds;only differeing by the nature of the para side chain;were adsorbed on oak wood extracted lignin. Vapor sorption-desorption isotherms indicated that about 3.5 wt % of 4-vinylguaiacol is adsorbed near saturation whereas it is only 0.8% for 4-ethylguaiacol. For both compounds, the isotherms displayed a hysteresis though significantly greater for 4-vinylguaiacol. Analyses of the hydroxyl stretching region of FTIR spectra of the lignin/4-ethylguaiacol and lignin/4-vinylguaiacol complexes indicated that physisorption via hydrogen bonds occurs for both sorbates with lignin phenolic hydroxyl groups but which would be more condensed for the former than for the latter. According to NMR spectra, these phenolic hydroxyl groups correspond to nonetherified guaiacyl subunits. In contrast with other para substituents, the conjugated vinyl bond favors not only physisorption but also chemisorption as witnessed by the fact that upon desorption in the vapor phase, even after pumping under dynamic vacuum for several days, about 1 wt % of 4-vinylguaiacol remains adsorbed onto lignin.
Introduction Many studies in environmental science have shown that wood or wood-related materials are good adsorbents of various organic compounds, such as organochloride pesticides,1-3 monoaromatic and polyaromatic hydrocarbons,4,5 phenols,6 and lipophilic organic compounds.7 In food science, wood, and in particular oak wood, has been historically used as wine container. Far from being inert with respect to wine, it is now considered as a complex tool for the fine-tuning of wine organoleptic properties through the slow release of aromas, tannins, and oxygen through its porosity.8 However, we have shown that in the meantime oak wood can also act as a sorbent for wine compounds of organoleptic interest such as aroma compounds9-11 and stilbene derivates.12 The presence of some of these compounds, such as 4-ethylphenol and 4-vinylguaı¨ acol, is *To whom correspondence should be addressed: tel þ33 (0) 3 80 39 61 91; fax þ33 (0) 3 80 39 62 65; e-mail
[email protected].
(1) Bras, I. P.; Santos, L.; Alves, A. Environ. Sci. Technol. 1999, 33(4), 631–634. (2) Rodriguez-Cruz, S.; Andrades, M. S.; Sanchez-Camazano, M.; SanchezMartin, M. J. Environ. Sci. Technol. 2007, 41(10), 3613–3619. (3) Wang, X.; Cook, R.; Tao, S.; Xing, B. Chemosphere 2007, 66(8), 1476–1484. (4) MacKay, A. A.; Gschwend, P. M. Environ. Sci. Technol. 2000, 34(5), 839– 845. (5) Boving, T. B.; Zhang, W. Chemosphere 2004, 54(7), 831–839. (6) Mukherjee, S.; Kumar, S.; Misra, A. K.; Fan, M. Chem. Eng. J. 2007, 129 (1-3), 133–142. (7) Trapp, S.; Miglioranza, K. S. B. Environ. Sci. Technol. 2001, 35(8), 1561– 1566. (8) Garde-Cerdan, T.; Ancin-Azpilicueta, C. Trends Food Sci. Technol. 2006, 17 (8), 438–447. (9) Ramirez Ramirez, G.; Lubbers, S.; Charpentier, C.; Feuillat, M.; Voilley, A.; Chassagne, D. J. Agric. Food Chem. 2001, 49(8), 3893–3897. (10) Ramirez-Ramirez, G.; Chassagne, D.; Feuillat, M.; Voilley, A.; Charpentier, C. Am. J. Enol. Vitic. 2004, 55(1), 22–26. (11) Barrera-Garcia, V. D.; Gougeon, R. D.; Voilley, A.; Chassagne, D. J. Agric. Food Chem. 2006, 54(11), 3982–3989. (12) Barrera-Garcia, V. D.; Gougeon, R. D.; Di Majo, D.; De Aguirre, C.; Voilley, A.; Chassagne, D. J. Agric. Food Chem. 2007, 55(17), 7021–7027.
1038 DOI: 10.1021/la103810q
not desirable because they impart unpleasant odors to wine, and their partial removal through wood sorption would appear as a natural decontamination. In agreement with the literature, sorption levels of these compounds appeared to vary according to their physicochemical characteristics, such as the solubility or the hydrophobicity5 or according to their chemical structures.11 In particular, we showed that for two structurally similar phenolic compounds, i.e., 4-ethylguaiacol and 4-vinylguaiacol (Figure 1), a significantly higher level of sorption onto wood11 or lignin13 is observed for the latter. We suggested that the higher electron acceptor character of the vinyl group could explain this different behavior between the two parent molecules. Nevertheless, the sorption capacity of wood is also considered to be related to its chemical nature, composition, and mechanical properties.4,5,14 Wood is indeed a complex natural material mainly composed of three major polymeric components: cellulose, hemicellulose, and lignin, which represent around 40, 25, and 25% of the hardwood dry mass, respectively.15 In particular, lignin’s phenylpropane units give rise to relatively high hydrophobic regions, and wood-water partition coefficients are therefore considered to be controlled by the wood lignin content.16 Thus, in a recent work, we consistently showed that, among wood macromolecules, lignin extracted from oak wood specifically exhibits a selective sorption capacity for phenolic compounds.13 However, investigating on a molecular scale level the interaction between lignin and phenolic compounds appears somewhat (13) Barrera-Garcia, V. D.; Gougeon, R. D.; Karbowiak, T.; Voilley, A.; Chassagne, D. J. Agric. Food Chem. 2008, 56(18), 8498–8506. (14) Chirkova, J.; Andersons, B.; Andersone, I. BioResources 2007, 4(3), 1044– 1057. (15) Fengel, D.; Wegener, G. Wood. Chemistry, Ultrastructure, Reactions; Gruyter: Berlin, 1989; p 613. (16) Severtson, S. J.; Banerjee, S. Environ. Sci. Technol. 1996, 30(6), 1961–1969.
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the unusual character of the hysterisis loop associated with the swelling of the sorbent is directly connected to the mechanism of sorption.14 In a follow-up of our previous investigations,13 the aim of this study is therefore to probe from a molecular-scale point of view and in model systems the interactions involved during the sorption process of two closely related volatile phenolic compounds, namely 4-ethylguaiacol and 4-vinylguaiacol onto lignin. The combined results obtained from vapor sorption analysis, FTIR, and solid state NMR are used to propose distinct sorption mechanisms for these two phenolics.
Experimental Section
Figure 1. Chemical structure of phenolic sorbates: 4-ethylguaiacol (a), 4-vinylguaiacol (b), and of some of the representative lignin units: guaiacyl (c); syringyl (d); 5,50 -biphenol (e). The numbering of carbon atoms in guaiacyl and syringyl units as used in the text is shown for (c).
tricky because of the great structural similarity between phenolics and the major lignin molecular units (Figure 1). To that respect, highly selective analytical methods such as Fourier transform infrared (FTIR) and solid state nuclear magnetic resonance (NMR) spectroscopies are likely to be able to probe sites that are possibly involved in the interaction process. FTIR has indeed been extensively used for the characterization of wood and its biopolymers and in particular for the semiquantitative analysis of lignin functional groups17-21 as a function of botanical origin, extraction procedure, or thermal treatment. Likewise, and despite the broader line widths inherent to the solid state, 1H and/or 13 C NMR spectroscopy has been successfully applied to study the molecular structure of wood and lignin and particularly the changes induced by thermal modifications.21-24 An advantage of NMR over FTIR for the analysis of wood is that signals from lignin and celluloses are clearly separated. Even within lignin signals, the major subunits, for instance guaiacyl (G) and syringyl (S) (Figure 1), can be distinguished, allowing both a semiquantitative analysis of them and a specific assessement of their possible involvement in sorbate-sorbent interactions. Besides spectroscopic methods, mechanistic informations can also be obtained from measurements of the vapor sorption by wood or lignin. These methods have obviously become references for the analysis of the microstructure of porous materials.25 However, wood and related biopolymers are swelling sorbents, and most of the sorption studies have involved “active” sorbates, in particular water.14,26,27 If these studies have primarily provided information on the structure of wood-related biopolymers, the use of different vapors (water, methanol, benzene) has shown that (17) Faix, O. Holzforschung 1991, 45(s1), 21–28. (18) Pandey, K. K. J. Appl. Polym. Sci. 1999, 71(12), 1969–1975. (19) Boeriu, C. G.; Bravo, D.; Gosselink, R. J. A.; van Dam, J. E. G. Ind. Crops Prod. 2004, 20(2), 205–218. (20) Martı´ nez, A. T.; Almendros, G.; Gonzalez-Vila, F. J.; Fr€und, R. Solid State Nucl. Magn. Reson. 1999, 15(1), 41–48. (21) Delmotte, L.; Ganne-Chedeville, C.; Leban, J.; Pizzi, A.; Pichelin, F. Polym. Degrad. Stab. 2008, 93(2), 406–412. (22) Maunu, S. L. Prog. Nucl. Magn. Reson. Spectrosc. 2002, 40, 151–174. (23) Wikberg, H.; Liisa Maunu, S. Carbohydr. Polym. 2004, 58(4), 461–466. (24) Lesage, A.; Bardet, M.; Emsley, L. J. Am. Chem. Soc. 1999, 121(47), 10987– 10993. (25) Gregg, S. I.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (26) Frandsen, H. L.; Svensson, S.; Damkilde, L. Holzforschung 2007, 61, 175– 181. (27) Lequin, S.; Karbowiak, T.; Brachais, L.; Chassagne, D.; Bellat, J.-P. Am. J. Enol. Vitic. 2009, 60(2), 138–144.
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Chemicals. Lignin (average particle size of 0.15 μm) was extracted from pedunculata oak wood by precipitation to dioxane as detailed elsewhere.13 Characterization of this lignin by N2 adsorption, FTIR, and 1H-13C solid-state NMR showed its nonporous character and its classification as a GS 4 type17 (Supporting Information). Ethanol, dioxane, and 4-vinylguaiacol were purchased from Aldrich-Sigma Co., and 4-ethylguaiacol was bought from TCI-EP, each with a minimum purity of 98%. Ultrapure water was obtained from a Milli-Q System (Millipore, Bedford, MA). Isothermal Adsorption Experiments. Adsorption under controlled vapor pressure was used to study the adsorption and the desorption of water, ethanol, 4-ethylguaiacol, and 4-vinylguaiacol in vapor phase. Isothermal adsorption experiments have been performed using a homemade McBain type thermobalance. The sample weight was about 20 ( 0.02 mg. Temperature was measured at (0.5 K with a thermocouple located near the sample, and saturated vapor pressure was recorded with a MKS Baratron gauge with a relative precision of 1%. The experimental accuracy is (0.02 mg for the adsorbate weight. Adsorption-desorption isotherms were obtained in the following way: the sample was first evacuated in situ at 353 K under a dynamic vacuum (10-5 hPa) for 24 h, defining the reference anhydrous state. The temperature was then decreased down to the desired adsorption temperature, the dynamic vacuum being maintained. Once the adsorption temperature was reached, the sorbate vapor was introduced into the thermobalance. The adsorption-desorption isotherm was recorded step by step using a static method. Once a plateau of mass was reached, a following equilibrium was performed by changing the sorbate vapor pressure. All sorption isotherms were obtained at 298 K. Investigated relative pressures p/ps ranged from about 10-5 to 0.98 for ethanol and 0.80 for 4-ethylgaiacol and for 4-vinylguaiacol. In the following, the amounts adsorbed on the lignin, ma, are expressed as percentages of dried sample weight (wt %). Adsorption Experiments of 4-Vinylguaiacol and 4-Ethylguaiacol in Solution. Each phenolic compound was dissolved in a water/ethanol (85/15 v/v)) solution to a concentration of 1 g L-1. This high concentration was chosen in order to achieve a sufficiently high sorbate/sorbent ratio. For each phenolic, lignin was put in contact with the hydroalcoholic solution and stirred during 7 days at room temperature. Afterward, the solid phase was recovered by filtration through 0.22 μm cellulose filters, and the filter was rinsed once with the hydroalcoholic solution. The recovered powder (referred to in the following as 4-EG and 4-VG for ethyl-4-guaiacol sorbed lignin and vinyl-4-guaiacol sorbed lignin, respectively) was subsequently dried at ambient temperature for 12 h and then kept over P2O5 desiccant for FTIR analyses or equilibrated at a 0.3 water activity for NMR analyses. For comparison, a sample of lignin was put in contact with the hydroalcoholic solution alone and recovered through the same procedure (referred to as LI in the following). FTIR Spectroscopy. IR spectra were obtained on an IFS 28 (Bruker, Germany) infrared spectrometer. Spectra of lignin samples (0.5% (w/w) lignin in KBr) were recorded between 700 and 4000 cm-1 at a resolution of 4 cm-1 in the transmittance mode using KBr as reference. Deconvolutions of spectra were realized DOI: 10.1021/la103810q
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Figure 2. Adsorption (open symbols)-desorption (full symbols) isotherms of water (O, b) and ethanol (0, 9) on lignin at 298 K. 28
using the WINFIT software. Each component was first manually adjusted for the position and the width. Then automatic optimization was run until convergence of the model was reached. NMR Experiments. 1H-13C cross-polarization magic angle spinning (CP-MAS) NMR experiments were run on a Bruker DSX 300 spectrometer operating at frequencies of 300.1 and 75.5 MHz for 1H and 13C, respectively. All the spectra were acquired with a Bruker double-channel 4 mm MAS probe at spinning speeds of 10 kHz. The 1H-13C CP-MAS spectra were obtained by crosspolarization, with spinal64 proton dipolar decoupling. HartmannHahn matching for the 1H-13C CP-MAS experiments was set on adamantane for 1H and 13C radio-frequency fields of ca. 60 kHz. Chemical shifts for 1H and 13C spectra were referenced to the signal of water (4.87 ppm) and to the methylene signal of adamantane (29.47 ppm), respectively. Other experimental details are indicated in the figure captions. As for FTIR, deconvolution of spectra were realized using the WINFIT software.28
Results Adsorption-Desorption Isotherms of Pure Gases. Because of their swelling abilities, wood and lignin do not behave as rigid sorbents, and a characteristic feature of vapors sorption is the occurrence of adsorption hystereses over the whole region of relative vapor pressures, with larger loops for lignin-rich fibers than for polysaccharide-rich fibers14,29 (Figure 2). Many studies have proposed explanations for these hystereses with a convincing one corresponding to the sorbent deformation in the presence of sorbates, proposed as a universal cause of sorptive hysteresis by Tvardovski et al.30 In this scheme, during sorption, the initial porosity formed by contacts between globular particles of lignin is increased due to the thermal motion of the sorbate (water or ethanol in Figure 2), thus creating larger pores still with narrow openings.14 However, upon desorption, these narrow openings kinetically hinder the relaxation of the sorbent matrix back to the initial state, and the major amount of the sorbate remains in the pores for high to medium relative pressures.14,29 As can be seen from Figure 2, both ethanol and water vapors lead to the same sorption-desorption behavior on lignin with a marked hysteresis.14 If near saturation the adsorbed mass of ethanol (16 wt %) is about 3 times higher than that of water (5.5 wt %), the actual molar sorption capacities are nearly the same for water and ethanol, i.e., 3.1 and 3.6 mmol g-1 of lignin, respectively. For the two vapors, desorption is complete, and subsequent sorption-desorption cycles are identically reproduced (results not shown). We can thus (28) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70– 76. (29) Hill, C. A. S.; Norton, A.; Newman, G. J. Appl. Polym. Sci. 2009, 112(3), 1524–1537. (30) Tvardovski, A. V.; Fomkin, A. A.; Tarasevich, Y. I.; Zhukova, A. I. J. Colloid Interface Sci. 1997, 191(1), 117–119.
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Figure 3. Adsorption (open symbols)-desorption (full symbols)
isotherms of 4-ethylguaiacol (O, b) and 4-vinylguaiacol (0, 9) on lignin at 298 K.
conclude that both vapors physically sorb on lignin and probe about the same number of adsorption sites on the surface. But they do not irreversibly modify its matrix. Sorption-desorption isotherms of 4-ethylguaiacol and 4-vinylguaiacol also display characteristic hystereses (Figure 3), but with clearly distinct features. For 4-ethylguaiacol, only about 0.8 wt % are adsorbed at saturation and the desorption with a very slight hysteresis is complete. This result indicates that 4-ethylguaiacol is likely physisorbed onto lignin, possibly through weak hydrogen bonds, in a reversible manner. For 4-vinylguaiacol, 3.5 wt % are adsorbed near saturation, which is 5 times more than 4-ethylguaiacol. Then the desorption is accompanied by a large hysteresis loop down to null pressure where nearly a third of the adsorbed maximum (∼1 wt %) remains onto lignin. Even after pumping under dynamic vacuum for several days, 4-vinylguaiacol remains adsorbed onto lignin. If a second adsorption-desorption cycle is performed, the adsorbed amounts are described by this second isotherm branch (Figure 3). These results clearly show that two distinct mechanisms of adsorption are involved for the two phenolics. It can be said, particularly from the sizes of hysteresis loops, that 4-vinylguaiacol is more “reactive” than 4-ethylguaiacol, and its adsorption likely occurs with stronger sorbent-sorbate interactions which are strong hydrogen bonds and/or covalent bonds. The covalent bond hypothesis is further supported by the fact that upon desorption some of the molecules remain adsorbed at null pressure. FT-IR Study of the Sorbate/Lignin System. In order to characterize these sorbent-sorbate interactions at equilibrium on a molecular scale level, we prepared concentrated (1 g L-1) sorbent/ sorbate mixtures by solid/liquid sorption of each phenolic in a hydroalcoholic solution. Very few FTIR studies of sorbent-sorbate interactions involving wood-related materials are actually reported in the literature, and a recent attempt to elucidate the sorption mechanisms of polyhydoxybenzenes onto lignin has shown that the simple analysis of IR bands before and after sorption does not necessarily provide convincing results.31 Without internal calibration, the major challenge in the quantitative analysis of FTIR spectra is their normalization. To that respect, the C-O stretching band at ∼1036 cm-1 is often chosen in the literature.21 In our case, however, IR bands of the sorbates are most likely already present in the sorbent.32 Therefore, all of the bands can possibly vary with the sorption and none of them can be reliably taken as constant. The fingerprint region of FTIR spectra of the three samples actually shows very few distinct unambiguous singularities from one to the other (Figure 4). A distinctive one concerns the band (31) Mohammed-Ziegler, I.; Holmgren, A.; Forsling, W.; Lindberg, M.; Ranheimer, M. Vib. Spectrosc. 2004, 36(1), 65–72. (32) Capanema, E. A.; Balakshin, M. Y.; Kadla, J. F. J. Agric. Food Chem. 2004, 52(7), 1850–1860.
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Figure 4. The 800-1800 cm-1 region of the FTIR spectra of LI (plain line), 4-EG (dotted line), and 4-VG (dashed line), tentatively normalized on the most intense band at 1125 cm-1 band. The inset magnifies the small 1152 cm-1 band unique to 4-VG. Figure 6. 1H-13C CP-MAS NMR spectra for LI (a), 4-EG (b), and 4-VG (c). As an example, the deconvolution (dashed line) and deconvoluted peaks for LI are shown. Table 1. Ratios between Areas of Deconvoluted Peaks from FTIR Spectra in Figure 5
liginn lignin þ 4-EG lignin þ 4-VG
Figure 5. OH stretching region of IR spectra for LI (a), 4-EG (b), and 4-VG (c). As an example, the deconvolution (dashed line) and deconvoluted peaks for LI are shown. Bands in the C-H stretching region (3000-2800 cm-1) were only considered for the deconvolution in order to have a correct baseline.
clearly observed at ∼1152 ppm in the spectrum of 4-VG, whereas only a shoulder can be observed in the two other spectra. This band, which could be assigned to phenyl C-H vibrations, probably reflects the presence of additionaly sorbed guaiacol moieties. Surprisingly, the 965 cm-1 band attributed to 4-vinylguaiacol monomers33 is not observed in the FTIR spectrum of 4-VG. Kubo and Kadla34 have recently shown that the detailed analysis of the hydroxyl stretching region of the FTIR spectrum could assess the distribution of intramolecular and intermolecular hydrogen bonds within and between lignin subunits. With the hypothesis that the two phenolics of our study may interact through hydrogen bonds, we recorded the FTIR spectrum of the two lignin-phenolic complexes (obtained at sorption equilibrium), with the spectrum of lignin which has been in contact with the hydroalcoholic solution (without phenolic) under the same conditions, for comparison. Figure 5 shows the OH stretching region of the three spectra and as an example the deconvolution obtained for LI. As can be seen, the three spectra present a very similar pattern for this OH region, and therefore, the same deconvolution could be realized for them (same number of peaks with line widths kept constant). In rather good agreement with Kubo and Kadla, seven bands with Gaussian peak shape could provide satisfactory fits to experimental spectra. Nevertheless and as mentionned before, only relative variations of these peaks between spectra can provide reliable informations on sorbent-sorbate interactions. Therefore, we calculated area ratios between deconvoluted peaks in the three spectra (Table 1). In particular, we considered the peaks centered at (33) Kodaira, K.; Onishi, Y.; Ito, K. Makromol. Chem., Rapid Commun. 1980, 1, 427–431. (34) Kubo, S.; Kadla, J. F. Biomacromolecules 2005, 6(5), 2815–2821.
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P2/P4 (3515 cm-1/ 3396 cm-1)
(P3 þ P4)/(P6 þ P7) (3430 cm-1 þ 3396 cm-1)/ (3268 cm-1 þ 3116 cm-1)
1.13 1.09 1.05
0.75 0.71 0.89
3515 cm-1 (P2, covering the region of intramolecular H bonds in G and S units), at 3430 and 3396 cm-1 (P3 and P4, accounting for multiple formation of intermolecular H bonds within and between phenolic and hydroxyl groups), and at 3268 and 3116 cm-1 (P6 and P7, multiple formation of intermolecular H bonds involving phenolic or hydroxyl groups and condensed units of the lignin such as the biphenol unit in Figure 1). It must be noted that phenolic hydroxyl groups in condensed units are considered to form stronger H bonds than other hydroxyl groups of lignin.34 Therefore, a relative increase of the intensity of peaks at 3268 and 3116 cm-1 would witness to the formation of strong H bonds. Likewise, a relative decrease of the band at 3515 cm-1 would indicate a decrease of the number of intramolecular H bonds. As a result, Table 1 indicates that upon sorption of both phenolic sorbates there is a decrease of the P2/P4 ratio reflecting the decrease of intramolecular H bonds in the lignin matrix in favor of intermolecular H bonds, which supports the hypothesis of a sorbent-sorbate interaction through hydrogen bonds. However, if the (P3 þ P4/P6 þ P7) ratio decreases for 4-EG, it increases for 4-VG. Consequently, 4-ethylguaiacol would apparently tend to form stronger hydrogen bonds within regions of lignin condensed units than 4-vinylguaiacol which would rather form hydrogen bonds within regions of less-condensed units. 1 H-13C CP-MAS NMR Study of the Sorbate/Lignin System. As for FTIR, solid-state NMR spectra can only be compared on the basis of relative intensity changes because crosspolarization provides a semiquantitative information as a result of the efficiency of the polarization transfer between proton and carbon nuclei.35 This transfer is indeed function of the distance between those nuclei but also of the dynamics of the C-H vectors. Figure 6 shows the 1H-13C CP-MAS NMR spectrum for the three lignin samples and, as an example, the deconvolution for LI. Like for FTIR spectra, we calculated ratios between selected peak (35) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569–590.
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Table 2. Ratios between Areas of Deconvoluted Peaks from NMR Spectra in Figure 6a 153/148
153/144
LI 1.01 3.19 4-EG 0.92 (-9%) 2.76 (-13%) 4-VG 0.91 (-10%) 2.13 (-33%) a Percentages of variation with respect parentheses.
153/56 1.26 1.15 (-9%) 0.95 (-25%) to LI ratios are
135/133 69 52 (-25%) 31 (-55%) indicated in
areas for which the position and line width could be kept constant (Table 2). Indeed, the peaks pattern of the spectrum of 4-VG is clearly modified compared to the two other spectra, in particular in the 140-110 ppm region (Figure 6). Peaks that could be kept constant are at ∼153, ∼148, ∼144 (G1), and ∼56 ppm. Peaks that could be kept constant between LI and 4-EG but which slightly shifted for 4-VG are at ∼135 ppm (S4 and S1 in etherified units at S1 plus G4 in etherified units at G1), ∼133 ppm (S4 and S1 in units non-etherified at S1 plus G4 in units nonetherified at G1), and signals between 124 and 100 ppm mainly attributed to G carbons, although anomeric carbons from remaining cellulose moieties would also appear at about 105 ppm.23 A common trend of results in Table 2 is that as 4-ethylguaiacol or 4-vinylguaiacol is sorbed, there is a relative decrease of the 153 ppm peak (mostly S carbons etherified at position 1) compared to the 148 ppm peak (S and G carbons non-etherified) and the 144 ppm peak (G1 carbon non-etherified). Such decrease clearly indicates that 4-ethylguaiacol and 4-vinylguaiacol adsorption involves specific interactions with non-etherified units (free hydroxyl groups at position 1) which are more accessible to form hydrogen bonds than etherified units. Furthermore, regardless of the sorbate, the higher relative decrease observed for the 153 ppm/ 144 ppm ratios compared to 153 ppm/148 ppm ratios suggests that G units preferentially form hydrogen bonds compared to S units, for which the two bulky methoxy groups might hinder the interaction. Finally, and in particular for the 153 ppm/144 ppm ratios, the much higher decrease observed for 4-vinylguaiacol agrees with its higher adsorption observed in the vapor phase. Similar conclusions can be drawn from the variation of 135 ppm/ 133 ppm ratios (Table 2), which also correspond to a etherifed/ non-etherified ratio for S and G carbons and in particular for the G4 carbon. However, the carbon 4 of G units (and S units) is a priori further away from the hydroxyl group where hydrogen bonds should form, and the cross-polarization efficiency to this carbon should be reduced compared to carbon 1, for instance. Therefore, this relative increase of the 133 ppm peak is more probably due to a reduced mobility of the lignin subunits near the sites of interaction, consistently with the stiffening of the lignin local network identified by the hysterisis in vapor sorption isotherms. It must be noted, that for 4-VG, the higher stiffening is shown not only by the higher relative increase of the 133 ppm peak but also by the fact that both the ∼135 and ∼133 ppm peaks have shifted in the spectrum, indicating changes in the local environments of corresponding carbons. This reduced mobility is also suggested by the decrease of the 153/56 ppm ratios (relative increase of the 56 ppm peak) for 4-EG and 4-VG. Indeed, such an increase of the intensity of the mobile methyl carbon is likely due to a reduced dynamics of the methyl C-H bonds imposed by the stiffening of the lignin network.
Discussion The most noticeable feature of the sorption-desorption of guaiacol vapors onto lignin is the hysterisis loop that is significantly more important for 4-vinylguaiacol than for 4-ethylguaiacol, indicating that the sorption of the former leads to both a higher 1042 DOI: 10.1021/la103810q
deformation/stiffening of the lignin network and most probably chemisorption on specific sites. For 4-VG, the first adsorption branch would therefore correspond to cumulated physisorption and chemisorption processes, whereas after desorption, any subsequent adsorption would correspond to a physisorption process along the desorption branch. This is in agreement with results obtained from NMR spectra which further identify non-etherified guaiacyl subunits as specific adsorption sites. Accordingly, FTIR spectra indicate that the major mechanism of adsorption involves the formation of hydrogen bonds between 4-ethylguaiacol or 4-vinylguaiacol and lignin phenolic hydroxyl groups but which would be more condensed for the former than for the latter. This is somewhat surprising since hydrogen bonds with more condensed units are considered to be stronger,34 whereas the hysteresis loop for the adsorptiondesorption cycle of 4-ethylguaiacol is weaker than that for 4-vinylguaiacol (Figure 3). It must be noted that non-etherified G (and S) units, which are considered to be specific interaction sites according to NMR results, are found in both condensed and noncondensed units of the lignin network, although in higher quantity in the latter.36,37 Therefore, a possible explanation of our observations is that as far as hydrogen bonds are involved, 4-ethylguaiacol specifically interacts with more condensed nonetherified G units which are scarce and which correspond to regions which already exhibit local reduced plasticities. As a consequence, a local “pore deformation” due to accumulation of sorbate is hindered. In contrast, 4-vinylguaiacol does not display such a higher specificity for condensed regions, and its adsorption occurs through the interaction with less condensed non-etherified G units, though not excluding the onset of hydrogen bonds also with more condensed ones. Less condensed regions likely exhibit higher local plasticities thus enabling “pore deformation” due to, for instance, the formation of clusters at these sorption sites. As a whole, such explanation relies on a higher reactivity of 4-vinylguaiacol compared to 4-ethylguaiacol molecules. For 4-vinylguaiacol adsorption, chemisorption has indeed also been hypothetized in view of the vapor sorption isotherms and is further supported by kinetic results previously obtained which clearly showed that the adsorption equilibrium of 4-vinylguaiacol is reached more slowly than that of 4-ethylguaiacol.13 Such a chemisorption through the formation of covalent bonds would necessarily lead to the appearance of new peaks in the FTIR and the NMR spectrum. However, as already mentioned, the intrinsic nature of lignin already exhibits most of the possible covalent bonds involving C, O, and H atoms, with various local environments, therefore making unlikely such observation. A theoretically possible chemical reaction between lignin subunits and 4-vinylguaiacol would be an electrophilic addition of the phenolic acidic OH groups onto the vinyl double bond, leading to a phenyl-O-C-C-phenyl ether bond. 13C chemical shifts for these two aliphatic carbons would be about 70 and 40 ppm, respectively. Although not important, a small signal is observed in the NMR spectrum of 4-VG around 40 ppm (Figure 6) which could possibly witness to the existence of such chemical reaction. Another possible chemical reaction involving 4-vinylguaiacol molecules at the lignin interface would be its oligomerization as shown by Kodaira et al.33 These authors have indeed shown that self-polymerization of 4-vinylguaiacol can be formed independently of the use of initiators, which suggests that such reaction (36) Liiti€a, T.; Maunu, S. L.; Sipil€a, J.; Hortling, B. Solid State Nucl. Magn. Reson. 2002, 21(3-4), 171–186. (37) Chen, H. T.; Funaoka, M.; Lai, Y. Z. Wood Sci. Technol. 1997, 31, 433–440.
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would be all the more possible at the lignin surface. It must be noted that the possible presence of few quinonic groups at the lignin interface as suggested by NMR signals around 190 ppm (not shown) could provide free radicals to initiate either the oligomerization of 4-vinylguaiacol or even its grafting onto lignin similarly to the copolymerization of lignin and 1-phenylethylene reported by Meister and Chen.38 Although neither FTIR nor NMR spectra could provide unambiguous evidence of chemisorption (grafting) or oligomerization of 4-vinylguaiacol, only such mechanisms can explain the large hysteresis observed for the corresponding isotherm and the fact that not all the sorbed amount can be desorbed. The conjugation of the vinyl bond to the phenyl ring must be the driving force of these mechanisms as also illustrated by the distinct adsorption behaviors of eugenol and isoeugenol, which both display a double-bond-containing para side chain.11,13 Isoeugenol, for which the double bond is conjugated and which displays a similar wood adsorption isotherm to 4-vinylguaiacol, indeed displays an adsorption coefficient that is ∼15 times higher than that of eugenol.11,13
Conclusion Through this study, we have shown that a molecular scale discrimination of the selectivity of interaction between phenylcontaining adsorbates and a complex phenyl-based adsorbant can be obtained through a synoptic approach involving macroscopic isothermal measurements and selective FTIR and solid-state NMR spectroscopies. By comparing two closely related phenolic compounds;4-ethylguaiacol and 4-vinylguaiacol;we have shown that two different sorption mechanisms can occur, i.e., physisorption and chemisorption, the latter being correlated to the presence of a conjugated para double bond in the adsorbate. (38) Meister, J. J.; Chen, M.-J. Macromolecules 1991, 24, 6843–6848.
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Indeed, although the two sorbates display hysteresis of vapor sorption-desorption isotherms, the magnitude of the hysteresis for 4-vinylguaiacol is markedly higher than the one of 4-ethylguaiacol, indicating a higher level of sorption and a stronger interaction for the former. Physisorption through the formation of hydrogen bonds appears to be the major mechanism of sorbent/ sorbate interaction for both phenolic compounds as illustrated by the deconvolution of the OH vibration region of FTIR spectra. On the basis of 1H-13C CP-MAS spectra analysis, these hydrogen bonds appear to be formed in particular with free OH groups of lignin guaiacyl subunits. However, 4-ethylguaiacol seems to interact preferentially with more condensed regions of the lignin matrix, whereas 4-vinylguaiacol likely interacts with several kinds of OH groups. Furthermore, chemisorption most likely occurs for some of the adsorbed 4-vinylguaiacol molecules as witnessed by the fact that upon desorption in the vapor phase, even after pumping under dynamic vacuum for several days, about 1 wt % of 4-vinylguaiacol remains adsorbed onto lignin. Such chemisorption would be explained by the higher reactivity of the conjugated vinyl bond. More generally, these fundamental aspects of adsorption may be of interest for various fields ranging from nanotechnology to environmental sciences. In winemaking practices, for instance, it shows that nondesired odor-active compounds in wine can be withdrawn from the wine and irreversibly bound to wood vessels. Acknowledgment. V. Daniela Barrera Garcı´ a is grateful to CONACyT-Mexico for financial support. Marie-Laure Leonard and Jean-Marc Dachicourt are warmly thanked for FTIR support. This work was financially supported by the Region Bourgogne (Project 05/516/CP/O12/S372) and the AROBOIS Company (France). Supporting Information Available: Characterization of the extracted lignin. This material is available free of charge via the Internet at http://pubs.acs.org.
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