Tosylation and Characterization of Lignin in Water - ACS Publications

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Tosylation and Characterization of Lignin in Water Amadou Diop,† Houssein Awada,*,† Rachida Zerrouki,‡ Claude Daneault,† and Daniel Montplaisir† †

Lignocellulosic Materials Research Center, Université du Québec à Trois-Rivières 3351, boulevard des Forges, Trois-Rivières (Québec), Canada, G9A 5H7 ‡ Laboratoire de Chimie des Substances Naturelles, Université de Limoges, 123, Av. Albert Thomas, 87060 Limoges, France S Supporting Information *

ABSTRACT: p-Toluenesulfonyl-lignin was prepared by reacting lignin with p-toluenesulfonyl chloride (TsCl) in an aqueous medium. The reaction was performed at 25 °C. The influence of several parameters on the reaction efficiency has been studied, including the ratio of p-toluenesulfonyl chloride/hydroxyl of lignin (TsCl/OH), the amount of triethylamine (Et3N), and the reaction time. The resulting p-toluenesulfonyl-lignin samples were characterized by means of scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX), attenuated total reflection (ATR), and the X-ray photoelectron spectroscopy (XPS). Detailed structural characterization of the products, including elemental analysis, indicates that the synthetic approach leads to products without impurities. The tosylation reaction is complete after 24 h.

1. INTRODUCTION The current economic concerns require the development of new materials derived from renewable natural products that will eventually replace toxic or nonbiodegradable materials (plastics derived from oil), while providing equivalent properties.1 Lignocellulosic biomass appears to be a promising material to replace some chemical tankers. The biomass consists of 15%− 30% lignin. The structure of the lignin is based on three different cinnamyl alcohols as precursors: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol compounds (see Figure 1).2,3

matrices, several attempts have been made either by using a coupling agent or by derivation from its hydroxyl groups.10−16 To the best knowledge of the authors, there has not been a study about the tosylation of the lignin. The tosyl moiety, which is a good leaving and activating group, reacts directly with various nucleophiles. Therefore, the presence of the tosyl group increased the reactivity of the lignin in the presence of a polymer matrix containing nucleophilic groups. Commonly, the p-toluenesulfonyl polymer is prepared by adding p-toluenesulfonyl chloride to the polymer in organic solvent in the presence of a base (generally triethylamine).17 Triethylamine is used to prevent the occurrence of side reactions. With a desire to move toward greener chemistry and to prevent the use of organic solvents, the lignin was extracted with CO218 and the tosylation reaction of lignin was therefore conducted in water with p-toluenesulfonyl chloride and triethylamine at room temperature. The influence of several parameters on the reaction efficiency has been studied: the ratio of ptoluenesulfonyl chloride/hydroxyl of lignin (TsCl/OH), the ratio of triethylamine/hydroxyl of lignin (Et3N/OH), and the reaction time.

Figure 1. Chemical structure of lignin precursors.

2. MATERIALS AND METHODS

The respective aromatic constituents of these alcohols in the polymers are p-hydroxyphenyl (H), guaiacyl (4-hydroxy-3methoxyphenyl) (G), and syringyl (4-hydroxy-3,5-dimethoxyphenyl) (S) units.4 Large amounts of technical lignin are generated as byproducts of pulp and paper and traditionally have been used as an energy source. Therefore, it would be important to find new applications for it. Lignins represent potential market opportunities because of their aromatic nature and vast daily supply. Nevertheless, only about 2% (or 1 million tons per year) of the lignin extracted from the black liquor is sold.5 The use of lignin as a reinforcement in composite materials has been largely explored.6−9 The lignin-reinforced matrices often lead to poor interfacial bonding with the polymer matrix. To increase compatibility between the lignin and the polymer © 2014 American Chemical Society

2.1. Materials. Samples of softwood kraft black liquor were supplied by the Kruger mill in Trois-Rivières, Quebec, Canada. Triethylamine (Et3N), p-toluenesulfonyl chloride (TsCl), ethanol, 2-chloro-4,4,5,5-tetramethyl-1, 3,2 dioxaphospholane (TMDP, 95%), cyclohexanol (99%), and pyridine (99.8%) were purchased from Sigma−Aldrich. All solvents and chemicals were commercially available, and unless otherwise stated, were used as received without further purification. Received: Revised: Accepted: Published: 16771

June 24, 2014 September 3, 2014 September 8, 2014 September 8, 2014 dx.doi.org/10.1021/ie502543p | Ind. Eng. Chem. Res. 2014, 53, 16771−16776

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2.2. Methods. 2.2.1. Kraft Lignin Precipitation. The extraction of the lignin from kraft black liquor was carried out using the method described by Nagy et al.,11 adapted to the atmospheric pressure. The black liquor with a pH of 13.4 was poured into a flask. Then, CO2 was bubbled at atmospheric pressure in order to decrease the pH (9.8) and further to precipitate lignin. 2.2.2. Preparation of p-Toluenesulfonyl-Lignin in Aqueous Medium. Tosylated lignin was prepared in aqueous medium by addition of TsCl in the presence of Et3N. The time and the amount of TsCl/Et3N were modified (see Supporting Information for details). After a given time stirring, the mixture was concentrated in a rotary evaporator and precipitated with ethanol and then filtered and washed with deionized water (300 mL) and hot ethanol (150 mL). The product was dried at 60 °C. 2.2.3. Analytical Method. The unmodified lignin is dried under vacuum for 48 h. A solvent mixture composed of pyridine and deuterated chloroform in a 1.6/1 (v/v) ratio was prepared. Twenty milligrams (20 mg) of lignin was accurately weighed into a 1-mL volumetric flask. The sample was then dissolved in 0.5 mL of the above solvent mixture. Quantitative 31 P NMR spectra of the unmodified lignin are obtained using published19 procedures with 2-chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholane as a phosphitylating reagent and cyclohexanol as an internal standard. The 31P NMR spectra of the resulting mixtures were obtained by using a Bruker 500 MHz spectrometer equipped with a broad-band inverse probe dedicated to 31P. An acquisition time of 0.2 s, a delay time of 5.00 s and a number of scan of 512 were used in each analysis. The content of hydroxyl groups was obtained by integration of the following spectral regions: aliphatic hydroxyls (149.1−144.6 ppm), syringyl (S) phenolic hydroxyls (143.3−141.9 ppm), condensed phenolic units (difference between 144.3−141.3 ppm and 143.3−141.9 ppm, as previously reported by Cateto et al.20 in 2008), guaiacyl (G) phenolic hydroxyls (140.6−138.6 ppm), p-hydroxyphenyl (H) phenolic hydroxyls (138.4−137.2 ppm), and carboxylic acids (135.3−134.4 ppm). Scanning electron microscope (SEM) coupled with energydispersive X-ray spectroscopy (EDX) (JEOL, Model JSM-5500 SEM) was used to determine the percentage of the sulfur in each sample. A voltage of 15.0 keV was used. The attenuated total reflectance (ATR) spectra were recorded, in transmittance mode, with Nicolet iS 10. The samples were scanned over the range 4000 to 400 cm−1 with a total of 64 scans at a resolution of ±4 cm−1. The chemical composition of both lignin and tosylated lignin was examined by X-ray photoelectron spectroscopy (XPS) method. Lignin and tosylated lignin were ground using an alumina mortar and pestle and pressed to form pellets. The XPS spectra of the samples were taken using an “AXIS Ultra” instrument from “Kratos Analytical”. A 225 W monochromatic aluminum source (Al Kα) was used. Survey scans were taken with steps of 1.0 eV and an analyzer pass energy of 160 eV, while the high-resolution regional spectra were recorded with steps of 0.1 eV and a pass energy of 40 eV. The pressure was typically 1 × 10−9 Torr. An area of 2 mm2 at three different spots was analyzed in order to average over the heterogeneity of the sample. The position of the detector was at an angle of 90° to the sample surface. Deconvolution analysis was performed with a SUN Sparc Station IPX computer (Vision 2). The spectrum analysis was done with casa XPS 2.3.9. All binding energies were referenced to the C 1s (C−C) peak at

285 eV. The survey scans were used to determine the percentage of each atom present on the surface. The highresolution scans were used to evaluate the chemical states of each atom. The thermal stabilities of lignin and tosylated lignin were analyzed by thermogravimetric analysis (TGA) with a Mettler− Toledo TGA system (Model TGA/SDTA851e). Approximately 4 mg of each sample was weighed and heated in alumina crucibles. Nitrogen was purged as an inert atmosphere at a flow of 50 mL/min. Samples were heated in the temperature range from 25 °C to 700 °C at a heating rate of 10 °C/min.

3. RESULTS AND DISCUSSION 3.1. NMR 31P Results. The NMR 31P spectrum, with the signal assignment, of the unmodified lignin is presented in Figure 2.

Figure 2. Quantitative unmodified lignin.

31

P NMR spectra and signal assignment of

Table 1. Influence of the Reaction Time on the Tosylation of Lignin with 2 equiv of TsCl and 3 equiv Et3N

a

entry

TsCl/OH

Et3N/OH

reaction time (h)

% sulfura

1 2 3 4 5 6

2 2 2 2 2

3 3 3 3 3

1 2 4 24 48

1.6 3.1 3.6 5.5 5.9 6.0

Mass percentage, as determined via SEM/EDX.

The lignin contains several functional groups, of which phenolic hydroxyl and aliphatic hydroxyl groups are among the more important for the reactivity of the lignin. The concentrations of these hydroxyl groups were calculated from the 31P NMR spectra. The concentration of the condensed and the aliphatic groups were, respectively, 2.64 mmol/g and 1.27 mmol/g. The type of lignin (softwood or hardwood) is dependent on the ratio H/G/S (H = 0.07, G = 1.42, S = 0.00) calculated from the 31P NMR spectra. The absence of syringyls (S) and the low percentage of p-hydroxyphenyl group (H) indicated that the lignin extracted is softwood. Finally, the concentration of the condensed phenolic was 1.15 mmol/g (see 16772

dx.doi.org/10.1021/ie502543p | Ind. Eng. Chem. Res. 2014, 53, 16771−16776


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Table 2. Sulfur Amount Collected from EDX Analysis, as a Function of the Amount of TsCl and Et3N. The reaction time was 24 h entry 1 2 3 4 5 6 7 8 a

TsCl/OH 1 2 4 1 2 4 4

Et3N/OH

1 3 8 12

Et3N/TsCl

reaction time (h)

% sulfura

24 24 24 24 24 24 24

1.6 1.6 1.4 1.6 5.1 5.9 6.4 6.3

1 1.5 2 3

Table 3. Experimental Sulfur (%) Obtained by XPS Analysis for Unmodified Lignin and Tosylated Lignin

a

sample

TsCl/OH

Et3N/OH

reaction time (h)

sulfura

A B C D

1/1 2/1 2/1

1/1 3/1 3/1

24 4 24

0.93 4.06 4.22 5.15

Mass percentage, as determined via XPS.

calculated, relative to the amount of phenolic and aliphatic hydroxyl of lignin. 3.2. EDX Results. The effect of reaction time on the tosylation was evaluated by carrying out EDX analysis for different reaction times. Table 1 reported the percentage of sulfur collected from the EDX analysis for each experimental condition. From the EDX analysis, we can observe that the unmodified lignin contains a sulfur amount of 1.6% (see entry 1 in Table 1). It is worth noting that the presence of sulfur is due to the procedure used in the extraction of the lignin from the biomass.22 On the other hand, for an identical number of equivalents of TsCl and Et3N, an increase in the reaction time led to an increase in sulfur amount. Since the tosyl moiety contains a sulfur atom, this increase can be attributed to the tosylation of the lignin. However, between 24 h and 48 h, the percentage of sulfur increases slightly from 5.9% to 6% (see entries 5 and 6 in Table 1). This shows that the tosylation of lignin could be complete after 24 h. The TsCl and the Et3N amounts were modified in function of the mole number of hydroxyl (phenolic and aliphatic) lignin. Table 2 reported the percentage of sulfur collected from the EDX analysis for each experimental condition. The reaction time was 24 h. We can clearly observe that when the TsCl was used, without the employment of the Et3N, the sulfur percentage was quasiunmodified (see entries 2, 3, and 4 in Table 2). However, the presence of the Et3N increased the sulfur percentage (see entries 5, 6, 7, and 8 in Table 2). As the number of equivalents of TsCl increases, the percentage of sulfur also increases. These results suggest that, without Et3N, the tosylation reaction did not work. The increase in the Et3N/TsCl ratio from 1 to 2 (see entries 5, 6, and 7 in Table 2) improves the reaction between lignin and tosyl (increased percentage sulfur of 5.1% to 6.4%). However, when the Et3N/TsCl ratio is greater than 2, the percentage of sulfur does not increase. Therefore, it is possible that the optimal Et3N/TsCl ratio for the tosylation of 2.5 g of lignin during 24 h is equal to 2. 3.3. Attenuated Total Reflectance (ATR) Results. Attenuated total reflectance (ATR) spectra of the unmodified lignin and the tosylated lignin during different experimental condition are given in Figure 3. Two equivalents (2 equiv) of TsCl and 3 equiv of Et3N were used. The ATR analysis of the unmodified lignin revealed characteristic lignin spectra that are in agreement with the literature.23,24 The obtained spectra of the unmodified lignin shows a large band at 3400 cm−1, corresponding to phenolic and aliphatic OH groups, a band at 2933 cm−1 for the C−H vibrations, and another band at 2863 cm−1, which could be attributed to the C−H stretching in methoxy groups. At the same time, unconjugated CO stretching at 1708 cm−1, aromatic skeleton C−C stretching at ∼1599 and 1509 cm−1,

Mass percentage, as determined via SEM/EDX.

Figure 3. Attenuated total reflectance (ATR) spectrum of unmodified lignin extracted by CO2 and tosylated lignin with 2 equiv of TsCl and 3 equiv Et3N for 1, 2, 4, and 24 h.

Figure 4. XPS survey spectra of (a) unmodified lignin, (b) tosylated lignin with 1 equiv of TsCl and 1 equiv of Et3N for 24 h, (c) tosylated lignin with 2 equiv of TsCl and 3 equiv of Et3N for 4 h, and (d) tosylated lignin with 2 equiv of TsCl and 3 equiv of Et3N for 24 h.

the Supporting Information for more details). This result is clearly conforms to the values of kraft lignin found in the literature.21 The numbers of equivalents of TsCl and Et3N are 16773



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Figure 5. Deconvolution of the XPS spectra in the region of sulfur binding energies for (a) unmodified lignin and (b) tosylated lignin with 2 equiv of TsCl and 3 equiv of Et3N for 24 h.

hydroxyl of lignin. However, between 4 and 24 h, the bands corresponding to hydroxyl decreases slightly. It can be concluded that most of the hydroxyl group disappeared after 24 h. This shows that the tosylation of lignin is complete after 24 h. This results are in good agreement with those observed by EDX and supported that, for an amount of TsCl/Et3N of 2/3, a reaction time of 24 h is sufficient to make the tosylation of the lignin. 3.4. XPS Analysis. Because the surface composition can differ from the “bulk” composition, the unmodified lignin and the modified lignin were also characterized by using XPS. A wide scan (i.e., survey spectra) and high resolution scans were collected to verify the composition and the functionalization of the samples. The XPS survey spectra of both the unmodified lignin and the tosylated lignin with different experimental conditions are shown in Figure 4. A comparison between the survey XPS spectrum presented in Figure 4 reveals two items of major information. First, the C and O are the predominant species for both the untreated lignin and the tosylated lignin. Second, a small amount of sulfur (0.93%) is detected for the untreated lignin and this amount increased after tosylation and is depended on the experimental condition. By comparison with the percentage collected from EDX, we notice that the percentages measured by XPS are slightly lower. This difference can be due to the fact that the XPS is a technique limited to the very top surface layer. Table 3 summarizes the sulfur amount extracted from the survey XPS spectra. It was observed that the percentage of sulfur was dependent on both reaction time and the amount of TsCl and Et3N. The increase in reaction time, from 4 h to 24 h, for the same amount of TsCl and Et3N, increased the sulfur amount from 4.22% to 5.15%. On the other hand, for a predefined reaction time (24 h), the increase in the amount of the TsCl and Et3N led to an increase in the sulfur percentage, from 4.06% to 5.15%. These results are in good agreement with those observed by EDX and ATR. Sulfur Signal Check. Figure 5 shows S 2p spectra of unmodified lignin, and tosylated lignin using 2 equiv of TsCl and 3 equiv of Et3N. The binding energies of the S 2p core level are split into two components (S 2p1/2 and S 2p3/2) arising from spin−orbit coupling. For the unmodified lignin, S 2p spectrum is deconvoluted into two subpeaks assigned to S 2p1/2 (165.12 eV) and S 2p3/2 (164.10 eV). After tosylation, the presence of two new sulfur

Figure 6. Curves of the first derivatives of the thermogravimetric analysis (DTG) of unmodified lignin, lignin-1/1 (tosylated lignin with 1 equiv of TsCl and 1 equiv of Et3N for 24 h and lignin-2/3 (tosylated lignin with 2 equiv of TsCl and 3 equiv of Et3N for 24 h).

Figure 7. Condensation of lignin during the heating inspired from Gardner et al.25

C−O stretching of guaiacyl groups at 1271 cm−1, and aromatic C−H deformation of guaiacyl groups at 1216 cm−1 were observed. In addition, C−O(H) and C−O(C) stretching of first-order aliphatic OH and ether groups (1030 cm−1), and aromatic C−H out-of-plane bending at 817 cm−1 were also present. The ATR spectrum of tosylated lignin shows the presence of two new bands at 1171 and 1370 cm−1. These two bands correspond to the tosyl moiety attached to the lignin. The spectrum also shows a decrease in the characteristic band of the hydroxyl group (3400 cm−1). The presence of these two bands and the decrease in hydroxyl groups can be related to the presence of the tosyl groups. For severals equivalents of TsCl and Et3N, identical for all reactions, an increase in the reaction time led to a decrease in 16774


Industrial & Engineering Chemistry Research subpeaks can be clearly observed and the S 2p spectrum is deconvoluted into four subpeaks. At the low bending energy, the two peaks (S 2p1/2: 170.14 eV and S 2p3/2: 169.21 eV) are assigned to the sulfur presented in the unmodified lignin, whereas, at high bending energy, the two peaks are assigned to the sulfur presented on the tosyl moiety grafted on the lignin. 3.5. TGA Analysis. The curves of the first derivatives of the thermogravimetric analysis (DTG) defined the rate of weight loss, while DTGmax represented the maximum rate of degradation and can be used to compare the heat stability of the samples. Figure 6 shows DTG curves of unmodified lignin and tosylated lignin. The TGA analysis shows a clear difference between the unmodified and the tosylated lignin. Indeed, the presence of tosyl function decreased the thermal stability of the lignin. It is worth noting that, for the unmodified lignin, two temperatures (240 and 390 °C) of degradation were clearly observed in Figure 6. When the tosyl function increased, the presence of these two temperatures was less visible. Gardner et al.25 related the degradation at 240 °C to the condensation of hydroxyl groups of lignin (Figure 7). In our case, this condensation is visible only in unmodified lignin and tosylated lignin with 1 equiv of TsCl and 1 equiv of Et3N (lignin-1/1). This behavior is absent in lignin-2/3. This is explained by the fact that there is little hydroxyl in the lignin-2/ 3. The TGA indicated that the thermal degradation of the samples occurred at ∼300 °C. The biggest DTGmax value was 390 °C, which corresponded to unmodified lignin. This value was slightly different from the DTGmax value for lignin-1/1 (311 °C) and lignin-2/3 (305 °C). This difference is explained by the fact that the high number of hydroxyls induce crosslinking fragments (condensation) of lignin; therefore, unmodified lignin gains extra thermal stability. The lignin-1/1 and lignin-2/3 do not benefit from this high thermal stability, because the hydroxyl groups were replaced by the tosyl ones.

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REFERENCES

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CONCLUSION Tosylated lignin was synthesized in an aqueous medium, without the use of any organic solvent, via a very simple method, in accordance with the principles of greener sustainable chemistry. The substitution of the tosyl moieties on lignin can be effectively controlled by adjusting the molar ratio of p-toluenesulfonyl chloride/hydroxyl lignin in the presence of triethylamine. The variation of the reaction time greatly influences the reaction. To 2.5 g of tosylating lignin at room temperature, it takes an Et3N/TsCl ratio equal to 2 for 24 h. The tosylated lignin offers various recoveries of lignin, because it is more reactive than the unmodified lignin. The tosylated lignin can have several applications. For example, the tosylated lignin can be used to prepare novel co-polymers. ASSOCIATED CONTENT

S Supporting Information *

Complementary data on experimental condition is available free of charge via the Internet at http://pubs.acs.org

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ACKNOWLEDGMENTS

The authors are grateful for the support of Kruger, Inc. and NSERC.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 819-376-5011, ext 4538. E-mails: houssein.awada@uqtr. ca, [email protected]. Notes

The authors declare no competing financial interest. 16775



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(19) Granata, A.; Argyropoulos, D. S. 2-Chloro-4,4,5,5-tetramethyl1,3,2-dioxaphospholane, a Reagent for the Accurate Determination of the Uncondensed and Condensed Phenolic Moieties in Lignins. J. Agric. Food Chem. 1995, 43, 1538−1544. (20) Cateto, C. A.; Barreiro, M. F.; Rodrigues, A. E.; Brochier-Salon, M. C.; Thielemans, W.; Belgacem, M. N. J. Lignins as macromonomers for polyurethane synthesis: A comparative study on hydroxyl group determination. J. Appl. Polym. Sci. 2008, 109, 3008−3017. (21) Tyrone, W., Jr.; Kosa, M.; Ragauskas, A. J. Polymerization of Kraft lignin via ultrasonification for high-molecular-weight applications. Ultrason. Sonochem. 2013, 20 (6), 1463−1469. (22) Gullichsen, J.; Paulapuro, H.; Stenius, P.; Attwood, B.; Joyce, T.; Heitner, C. Forest Products Chemistry; Stenius, P., Ed.; Papermaking Science and Technology, Book 3; Finnish Paper Engineers’ Association and TAPPI: Helsinki, Finland, 2000; pp 62−69. (23) Ye, Y.; Zhang, Y.; Fan, J.; Chang, J. Novel Method for Production of Phenolics by Combining Lignin Extraction with Lignin Depolymerization in Aqueous Ethanol. Ind. Eng. Chem. Res. 2012, 51, 103−110. (24) Tejado, A.; Pena, C.; Labidi, J.; Echeverria, J. M.; Mondragon, I. Physico-chemical characterization from different source for use in phenol−formaldehyde resin synthesis. Bioresour. Technol. 2007, 98, 1655−1663. (25) Gardner, D. J.; Schulz, T. P.; McGinnis, G. D. The pyrolytic behavior of selected lignin preparations. J. Wood Chem. Technol. 1985, 5 (1), 85−110.

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Tosylation and Characterization of Lignin in Water - ACS Publications

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