New ionic liquid for the dissolution of lignin

Abstract

This work aims to develop a new ionic liquid, used as an aprotic green ä solvent, to dissolve kraft lignin from black liquor. The kraft lignin was extracted through precipitation with carbon dioxide at atmospheric pressure. 1,8-Diazabicyclo[5.4.0]undec-7-ene-based ionic liquids were obtained by quaternization of the nitrogen atom with a hydrogen atom or an alkyl chain. The yields of the synthesis of the ionic liquids varied between 76 and 80%. Dissolving experiments were carried out using the lignin isolated from the black liquor of a kraft process. Up to 20% (w/w) of the lignin can be dissolved in butyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride ([DBUC4]+[Cl-]), hexyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC6]+[Cl-], and octyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC8]+[Cl-]. The time it takes to dissolve the lignin in these three liquids shows that its solubility is influenced mostly by the nature of the cations. The lignin solubility was reduced in relation to the increased length of the grafted carbon chain. The thermogravimetric analysis (TGA) showed these liquids can be used as lignin solvents from room temperature up to 300 °C (onset of degradation). Steric exclusion chromatography showed a slight decrease (6%) in the molecular weight of the lignin dissolved in these ionic liquids.

Full Article

New Ionic Liquid for the Dissolution of Lignin

Amadou Diop, Amel Hadj Bouazza, Claude Daneault, and Daniel Montplaisir *

This work aims to develop a new ionic liquid, used as an aprotic green ä solvent, to dissolve kraft lignin from black liquor. The kraft lignin was extracted through precipitation with carbon dioxide at atmospheric pressure. 1,8-Diazabicyclo[5.4.0]undec-7-ene-based ionic liquids were obtained by quaternization of the nitrogen atom with a hydrogen atom or an alkyl chain. The yields of the synthesis of the ionic liquids varied between 76 and 80%. Dissolving experiments were carried out using the lignin isolated from the black liquor of a kraft process. Up to 20% (w/w) of the lignin can be dissolved in butyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride ([DBUC₄]⁺Cl^–]), hexyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC₆]⁺[Cl^–], and octyl-1,8 diazabicyclo[5.4.0] undec-7-enium chloride [DBUC₈]⁺[Cl^–]. The time it takes to dissolve the lignin in these three liquids shows that its solubility is influenced mostly by the nature of the cations. The lignin solubility was reduced in relation to the increased length of the grafted carbon chain. The thermogravimetric analysis (TGA) showed these liquids can be used as lignin solvents from room temperature up to 300 °C (onset of degradation). Steric exclusion chromatography showed a slight decrease (6%) in the molecular weight of the lignin dissolved in these ionic liquids.

Keywords: Kraft black liquor; Kraft lignin; Ionic liquid; Biomass

Contact information: Lignocellulosics Materials Research Centre – University of Quebec at Trois Rivieres, 3351, Boul. des Forges, Trois-Rivières, QC, Canada, G9A 5H7;

* Corresponding author: Daniel.montplaisir@gmail.com

INTRODUCTION

In the context of rarefaction of fossil-fuel resources and protection of the environment, special attention has been given over the past few years to the development of processes that are more nature-friendly and to the upgrading of renewable resources. Lignocellulosic polymers represent a seemingly inexhaustible source of biomass-issued matter. Mainly exploited in the paper industry, these polymers show interesting physicochemical properties and high potential for development. To broaden the applica-tion field of these bioresources, it is highly desirable to confer them with new properties by having them chemically or structurally modified or transformed into by-products. After cellulose, lignin is the most abundant polymer in nature. Lignin is a complex tridimensional polymer made up of phenylpropane units, composed of more than 50% carbon. This tridimensional polymer forms from three phenolic precursors: guaiacyl, syringyl, and p-hydroxyphenyl (Binder et al. 2009; Gandini et al. 2002). Lignin representative formulas vary according to the source, the age, and the accuracy of the determination. One of the accepted structures is the one proposed by Adler (1977). For the past several years, lignin has been used as a combustible in the paper industry. Indeed, the residue of wood delignification, called black liquor, is evaporated and then burnt to produce energy. In recent years, research projects have attempted to upgrade part of the black liquor lignin without modifying the energy balance of the kraft mills. In fact, the lignin of this liquor has found very little use. An estimated 63 x 10⁴ million tons/year of lignin are produced in the world (Mohan et al. 2006). Only a small portion of the lignin is marketed, which is a result of the uncertainties linked to its structure and high molecular weight.

The pre-treatment of biomass by ionic liquids is a green and viable option for its upgrading. Ionic liquids are organic salts having a melting point lower than 100°C and often lower than room temperature. They form a very important new class of solvents because of their low vapor pressure, high ionic conductivity, and exceptional chemical stability (Welton 1999). The role of ionic liquids as “green” solvents has become increasingly important for catalytic reactions in the field of polymerization and solubil-ization of natural macromolecules (Li et al. 2007, 2008; Sievers et al. 2009, Lee et al. 2009). The systems most commonly studied are binary mixtures of anions (Cl^–, Br^–, BF₄^–, PF6^–, and CF₃SO₃^–) and cations (imidazolium, pyridinium, phosphonium, and quaternary ammonium). A wise choice of anions and cations results in the desired properties. Ionic liquids (ILs) do not degrade at high temperatures. They are non-flammable, except for “energetic” ILs, which are made of nitrate or picrate anions. Because of their almost negligible vapor pressure, they present a great thermochemical stability. However, most of the research on the treatment of biomass in an ionic liquid medium is aimed at dissolving crystalline cellulose (Kadokawa et al. 2009; Liu et al. 2011; Kuang et al. 2008; Lin et al. 2009; Hua et al. 2009 ; Zhao et al, 2009), not on the forest biomass, which is a more complex matter. In their research, Sang et al. (2009) show that an ionic liquid can dissolve wood cellulose and lignin. The difficulty of correlating the dissolution of the cellulose, lignin, and wood was revealed by Sang et al. (2009) by comparing the dissolution of lignin and wood in methylsulfatedimethylimmidazolium [mmim]⁺[MeSO₄]^–.

In the present study, we developed and characterized three new ionic liquids. We studied the role of these ionic liquids used as solvents in the pre-treatment of lignin from kraft black liquor. Particular emphasis was placed on the green aspect and the neatness of the ionic liquids. Indeed, three liquids based on 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were synthesized without solvent. DBU is part of the amidine family. An amidine is the equivalent of an amide in which the carbonyl group (C=O) was replaced by an imine group (=NH). From a chemical standpoint, amidines are much more basic than amides and are part of the stronger neutral bases. In the literature there are many protic polar ionic liquids based on DBU that are able to dissolve lignin (D’Andola et al. 2008). However, the synthesis of DBU-based solvent-free aprotic ionic liquids is less widely reported.

EXPERIMENTAL

Materials

Samples of softwood kraft black liquor were supplied by the Kruger mill in Trois-Rivières, QC, Canada. The reagents used to synthesize the ionic liquids were purchased from Sigma Aldrich. We used the following: 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU, 98%), 1-chlorobutane (99,5%), 1-chlorohexane (99%), 1-chlorooctane (99%), and ethyl acetate (99%).

Methods

Extraction of lignin from kraft black liquor

Acid precipitation was carried out, basically as described in the literature (Garcia et al. 2009; Tejado et al. 2007; Axelsson et al. 2006). The CO₂ precipitation of the lignoboost process (Nagy et al. 2010) was adapted to atmospheric pressure.

Synthesis of DBUC₄⁺Cl^–, DBUC₆⁺Cl^– and DBUC₈⁺Cl^–

1,8-Diazabicyclo[5.4.0]undec-7-ene-based ionic liquids were obtained by quarter-nization of the nitrogen atom with a hydrogen atom or an alkyl chain. During the synthesis of the aprotic ionic liquids, an equivalent of DBU (0.259 mol, i.e., 40 mL) with 1.2 equivalents of 1-chlorobutane (0.311 mol, i.e., 33.5 mL), 1-chlorohexane (0.311 mol, i.e., 43.1 mL), or 1-chlorooctane (0.311 mol, i.e., 52.8 mL) were poured into a 300-mL flask at 50 °C under agitation for about 48 h without solvent. This produced a viscous solution of DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, or DBUC₈⁺Cl^–. The excess reagents and impurities were removed with ethyl acetate. The latter were removed using a rotary evaporator at 50 °C for about 2 h. This produced a very hygroscopic orange solid. For safety reasons, we did not use DBUC₂⁺Cl^– or DBUC₃⁺Cl^– because chloroethane is in the gaseous state and the temperature of boiling chloropropane is quite low (34 to 36 °C).

Dissolving the lignin precipitated from the kraft black liquor in the ionic liquids

We added 0.5 g, 0.75 g, and 1 g of kraft lignin, respectively, in 4.5 g, 4.25 g, and 4 g of the ionic liquids, and the whole solution was placed in an oven at 105 °C. The lignin solution was then cooled to room temperature. The lignin was regenerated by adding distilled water to the solution under vigorous agitation for 2 h. The precipitated lignin was collected by centrifugation at 3400 RPM.

Analytical Methods

Spectroscopy NMR ³¹P and ¹H

The NMR ³¹P spectra for the lignin extracted with acid and CO₂ were obtained using 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) as a reagent of phosphorylation. All samples were dried under vacuum for 48 h, and then analyzed by NMR of ³¹P in a blend of pyridine/CDCl₃ after phosphorylation. The quantification was done using cyclohexanol as internal standard. The acquisition conditions were as reported by Monteil-Rivera et al. (2012). The chemical shifts were referenced in relation to the signal of the water product with TMDP at 132.2 ppm.

The content of hydroxyl groups was obtained by integration of the following spectral regions: aliphatic hydroxyls (149.4–145.5 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 done by Cateto et al. (2008)), guaiacyl (G) phenolic hydroxyls (140.5–138.3 ppm), p-hydroxyphenyl (H) phenolic hydroxyls (138.3–137.4 ppm), and carboxylic acids (136.0–134.0 ppm). The structures of the ionic liquids and the acetylated lignin were determined by NMR ¹H with a Mercury 200 MHz instrument.

Spectroscopy FT-IR

FTIR spectra were obtained with a Perkin Elmer instrument, System 2000, using pellets of lignin mixed with KBr (lignin content of 1%). Spectra were recorded between 400 and 4000 cm^-1 with a resolution of 2 cm^-1.

Thermogravimetry and DSC

The thermal stabilities of kraft lignin and ionic liquids were analyzed by thermogravimetric analysis (TGA) with a Mettler Toledo (TGA / SDTA851e) and scanning calorimetry (DSC) with a Mettler Toledo DSC822e. Scans were registered between 25 and 500 °C in a dynamic mode with a heating rate of 10 °C/min, under a N₂ blanket (50 mL/min). For each experiment, about 4 mg of each sample was used.

Karl Fischer

A Karl Fischer volumetric titrator was used to determine the water content of the ionic liquids. The samples were first solubilized in anhydrous methanol (Hydranal methanol dry) buffered with benzoic acid, with the titrant being the Hydranal composite 5.

GPC analysis

The distributions of the molecular weights of the lignin dissolved in the DBUC₄⁺Cl^– and of the kraft lignin were determined by gel-permeation chromate-graphy (GPC) using a multiple detection system Viscotek (Houston) equipped with a GPCmax including an integrated pump, an automatic sampler, and a deaerator. All samples were dissolved in THF (15 mg/10 mL), filtered, and analyzed by GPC based on the conditions described by Monteil-Rivera et al. (2012). All samples were acetylated according to the method described by Zhao and Liu, (2010). The samples were acetylated and dissolved in THF (15 mg/10 mL), filtered, and analyzed by GPC according to the conditions reported by Monteil-Rivera et al. (2012).

RESULTS AND DISCUSSION

Extraction of the Lignin of the Kraft Black Liquor

The black liquor lignin was extracted using two methods. Results are presented in Fig. 1. The FT-IR analysis revealed lignin characteristic spectra that are in agreement with the literature (Zhao and Liu 2010; Ewa et al. 2009; Lisperguer et al. 2009; Boeriu et al. 2004).

The alcohol functions corresponded to an absorption band at 3450 cm^-1, and the band corresponding to the C-H vibrations was at 2940 cm^-1. An absorption band for the non-conjugated carboxyl groups was observed at 1716 cm^-1 and the aromatic backbone vibrations were observed at 1610, 1521 cm^-1 and 1415 cm^-1. The C-H related to the deformation and vibration of the aromatic rings at 1458 cm^-1 and the phenolic OH groups at 1376 cm^-1 were also present. Bands corresponding to the C-H of deformation in the syringyl and the guaiacyl units were observed at 1108 and 1040 cm^-1, respectively.

Fig. 1. FT-IR spectra for the lignin-H₂SO₄ and lignin-CO₂

The aliphatic, phenolic, and carboxylic hydroxyl concentrations of the lignin-H₂SO₄ and lignin-CO₂ were determined by a quantitative NMR analysis of ³¹P after phosphorylation with 2-chloro-4,4,5,5-tetramethyl-1, and 2,3-dioxaphospholane (Fig. 2, Table 1). As in the case of the FTIR data, both ³¹P NMR spectra were similar. The distribution of these lignin alcohols showed that the proportion of phenolic alcohols was larger than the other alcohols. These results conform to those found in the literature (Nagy et al. 2010).

Fig. 2. NMR ³¹P spectra of samples of lignin extracted with sulfuric acid and CO₂ after phosphorylation with TMDP (solvent: pyridine/CDCl₃)

Table 1. Concentration of Aliphatic OH, Phenolic OH, and COOH Groups Present in the Samples of Lignin-H₂SO₄ and Lignin-CO₂ by NMR ³¹P

Synthesis and Properties of Ionic Liquids

The yields of the synthesis of the ionic liquids varied between 76 and 80%. The structures of the resulting compounds were determined by NMR ¹H and described below in Figs. 3, 4, and 5.

Fig. 3. Structure of DBUC₄⁺Cl^–

Fig. 4. Structure of DBUC₆⁺Cl^–

Fig. 5. Structure of DBUC₈⁺Cl^–

The heat stabilities of all three ionic liquids are presented in Fig. 6 (DTG). The TG analyses indicated that the thermal degradation of the three liquid samples occurred at about 300 °C. The DTG curves indicate the rate of weight loss, while DTGmax repre-sents the maximum rate of degradation and can be used to compare the heat stability of the samples. The lowest DTGmax was 354 °C, corresponding to DBUC₈⁺Cl^–. This value was slightly different of the DTGmax of DBUC₄⁺Cl^– (368 °C) and DBUC₆⁺Cl^– (362 °C). It should be noted that in spite of this slight difference in the DTGmax, there was a tendency for a reduction of the heat stability that depended on the size of the cation. Indeed, the large organic cation of the ionic liquids generated few interactions and gave a low melting point and a low heat resistance. If the length or the volume of the alkyl chain grafted on the DBU backbone increased, this phenomenon was amplified. According to Larsen et al. (2000), the decrease of the ionic liquid heat resistance comes mostly from their incapacity to organize themselves into a compact network.

The DSC analysis of DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^– showed three endothermic absorption bands at 130 °C, 162 °C, and 158 °C, respectively, which corresponded to the evaporation of water in the liquids. The band corresponding to DBUC₄⁺Cl^– was more pronounced than the bands corresponding to the other liquids (Fig. 7). The evaporation of the water contained in an ionic liquid temperature depends on the nature of the ions and the viscosity of the liquid. According to the literature (Cammarata et al. 2001), the ionic liquid anions develop hydrogen bonds with the water. On the other hand, the size of the cation influences the evaporation of water (Bonhôte et al. (1996), with a constant anion, the viscosity increases as the side chain of the cation increases. This observation is in agreement with our results. Trapped water in liquids DBUC₆⁺Cl^– and DBUC₈⁺Cl^–] are more difficult to remove because they are more viscous than DBUC₄⁺Cl^–. This observation explains the difference in evaporation temperature of water in different ionic liquids.

Fig. 6. Curves of the first derivatives (DTG) of the thermograms of DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^–

Fig. 7. DSC Analysis of the ionic liquids [DBUC₄]⁺[Cl^–], DBUC₆Cl^–, and DBUC₈Cl^–]

The Karl Fisher analysis of the ionic liquids (Table 2) showed that the longer the chain was on the DBU, the less hygroscopic was the liquid. These results confirmed the DSC analysis.

Table 2. Water Content of the Ionic Liquids DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^–

Dissolving the Lignin-CO₂ in DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^–

As explained previously, CO₂ precipitation is more environmentally friendly than acid extraction. Consequently, the lignin-CO₂ system was used for all the dissolution tests. Lignin-CO₂samples were added to the ionic liquids without pre-treatment and heated to 105 °C. The dissolving times for the lignin in each ionic liquid are given in Table 3, and dissolution is represented in Fig. 8. The solubility was checked by ultrasound. After sonication, if the solution remained transparent without deposit, then the dissolution was regarded as complete.

Table 3. Dissolving Lignin-CO₂ in [bmim]⁺Cl^–

The results of the lignin-CO₂ dissolution showed that its solubility was a function

of the size of the ionic liquid cation. The larger the cation is, the less soluble it is. For the same percentage of lignin, the dissolution time increased with the size of the cation. For example: the time for dissolution 5% lignin in DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^– were, respectively, 2 h, 50 min; 4 h, 00 min; and 4 h, 40 min. The solubilization was essentially due to the development of ionic and hydrogen bonds between the entities of the ionic liquid and the polymeric chains (Fig. 9).

a                 b

Fig. 8. (a) 4% of Lignin-CO₂ solubilized in DBUC₄⁺Cl^–; (b): Ionic Liquid (DBUC₄⁺Cl^–) alone

Fig. 9. Interactions between ionic liquid and lignin-CO₂

The results that follow show the combined influence of ionic liquid treatment and water precipitation. It is difficult to study only the effect of the ionic liquid on lignin because one cannot have a lignin precipitated with water only. Lignin from kraft black liquor is precipitated with an acid solution or with CO₂.

Thermogravimetric analysis is widely used to study how organic polymers decompose. TG curves reveal the weight loss of substances in relation to the temperature of thermal degradation, while the first derivative of that curve (DTG) shows the corres-ponding rate of weight loss. Results corresponding to the two lignins are shown in Fig. 10. The thermogram of kraft lignin-CO₂ treated with ionic liquid was shifted to lower temperatures. This suggests that the treated lignin degraded faster than the untreated kraft lignin-CO2.

The first derivative of the weight loss curve of kraft lignin-CO₂ treated with the ionic liquid showed two peaks, at 265 and 366 °C (Fig. 10). Kraft lignin-CO₂ gave a DTGmax of 381 °C. Temperatures of 366 °C and 381 °C were assigned to the respective degradation of lignin treated with ionic liquid and kraft lignin (Toledano et al. 2010; Toshihiro et al. 2009). This difference in degradation temperature could be explained by a loss of mass of the kraft lignin-CO₂ after dissolution in the ionic liquid. A temperature of 265 °C could be attributed to condensation or the degradation of a small fragment of lignin mass. A peak representing weight loss was observed around 90 °C, which corresponded to the absorbed water evaporation.

Fig. 10. DTG of kraft lignin-CO₂ and kraft lignin-CO₂ dissolved in DBUC₄⁺Cl^– and then regenerated

All samples were acetylated according to the method described by Zhao and Liu (2010). The samples were acetylated and dissolved in THF (15 mg/10 mL), filtered, and analyzed by GPC according to the conditions reported by Monteil-Riviera et al. (2012).

The molecular weight distributions are shown in Fig. 11, while the corresponding values of Mn, Mw, and Mw/Mn are given in Table 4.

. 

Fig. 11. Distribution of the molecular weight of the reference lignin and the reference lignin dissolved in DBUC₄⁺Cl^– and then regenerated

Table 4. Comparison of the Distribution of the Molecular Weight of the Reference Lignin and the Reference Lignin Dissolved in DBUC₄⁺Cl^– and then Regenerated

The masses found are consistent with the literature (Gosselink et al. 2004; Sun and Tomkinson 2002). It should be noted that masses M_n and M_w of the regenerated lignin were slightly lower than those of the reference lignin, after dissolving in DBUC₄⁺Cl^–. This is in line with the degradation observed with our thermogravimetric analysis.

CONCLUSIONS

1. This work showed that the kraft lignin of black liquor can be precipitated with CO₂ at atmospheric pressure. This is a viable, clean method that gives the same lignin that is obtained with the acid method.
2. Aprotic ionic liquids based on 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were successfully prepared with high yields. DBUC₄⁺Cl^–, DBUC₆⁺Cl^–, and DBUC₈⁺Cl^–] were synthesized without solvents with yields of 80, 78, and 76%, respectively. This approach consisted of reacting the DBU with an excess of chloroalkane, which yielded aprotic ionic liquids pure at almost 99%. The NMR ¹H of these ILs showed some impurity traces that were easily eliminated, as the impurities were made with an excess of reagents (chloroalkane) and water only.
3. Trials to optimize the temperature of the reaction have allowed us to demonstrate that this approach required temperatures that were 20 °C lower than for the synthesis of ionic liquids such as immidazolium, which requires a reaction temperature of about 70 °C.
4. This approach to the synthesis allowed a reduction in the use of greenhouse products (volatile solvents), and the pre-treatment gave lignin with promising properties, such as a reduced molecular weight. In our case, the heat treatment of kraft lignin at 105 °C in the [DBUC₄]⁺[Cl^–] gave 6% lower molecular weight.
5. These ionic liquids were viable solutions for the treatment of lignin at relatively high temperatures.
6. The work also demonstrated that it is possible to carry out the dissolution of the precipitated kraft lignin. Kraft lignin could be solubilized in these liquids at 105 °C at proportions of 20%.

ACKNOWLEDGMENTS

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

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Article submitted: May 14, 2013; Peer review completed: June 17, 2013; Revised version received and accepted: June 20, 2013; Published: July 3, 2013.