Understanding Microwave-Assisted Lignin Solubilization in Protic

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 4122−4129

pubs.acs.org/journal/ascecg

Understanding Microwave-Assisted Lignin Solubilization in Protic Ionic Liquids with Multiaromatic Imidazolium Cations Omar Merino,† Gabriela Fundora-Galano,‡ Rafael Luque,*,§ and Rafael Martínez-Palou*,† †

Gerencia de Transformación de Biomasa, Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, 07730 Ciudad de México, México ‡ Facultad de Química, Departamento de Física y Química Teórica, Universidad Nacional Autónoma de México, 04510 México City, México § Departamento de Química Orgánica, Universidad de Córdoba, Campus Rabanales, Edificio Marie Curie C-3, E-14014 Cordoba, Spain S Supporting Information *

ABSTRACT: The low solubility of lignin in most common solvents remains a significant challenge to be solved for its use and recovery. In this work, 18 novel protic ionic liquids (ILs) containing aromatic-substituted imidazolium cations and methanesulfonate and p-toluenesulfonate anions were synthesized and evaluated for lignin dissolution under microwave irradiation. ILs containing methanesulfonate demonstrated good lignin dissolving capacities, which can facilitate its separation and structural modifications to carry out their recovery under relatively mild and environmentally friendly conditions (few minutes, 90 °C). DFT theoretical calculations provided additional insights into the high lignin dissolution observed for the best performing ILs.

KEYWORDS: Ionic liquids, Lignin, Microwave, Solubilization, Multiaromatic cation

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INTRODUCTION

Lignin can be either processed for the production of highly valuable products such as lignosulfonates or used for different purposes that potentially include the development of dispersants, resins, and activated carbons and valuable chemicals and fuels.4−8 Nevertheless, the majority of lignin in the pulp and paper industry is still consumed as fuel in paper production processes.9 Lignin is currently separated from the pulp liquor of the lignocellulosic material using highly polluting and environmentally aggressive methods,10 whereby the development of a method for its separation and dissolution still remains of great scientific and technological interest.11 In addition, the development of a sustainable process for dissolution and regeneration of lignin could be very useful for lignin valorization in the biorefinery concept. In the present century, certain ionic liquids (ILs) have been proved to dissolve significant amounts of recalcitrant biopolymers including cellulose, chitin, and chitosan.12,13 From these results, significant efforts were devoted to the development of novel ILs capable of dissolving lignin.14−18 Several theoretical works have also attempted to explain the dissolution phenomenon.19−21 Ionic liquids have become the

Biomass comprises mainly three fractions, namely, cellulose, hemicellulose, and lignin. From a chemical point of view, lignin is an aromatic biopolymer containing phenyl-propanoids as basic structural motifs, where the benzene ring contains a variable number of hydroxyl and methoxy groups, predominantly guaiacyl-propane (methoxy-3-hydroxy-4-phenylpropane) and trimethylpropane (dimethoxy-3,5-hydroxy-4-phenylpropane) radicals. The units are linked by several C−O and C− C bonds including α-O-4, β-O-4, 4-O-5, β-5, and β−β.1 Lignin content varies approximately between 20 and 35% of the biomass weight depending on type and species of the biomass feedstock. The lignin structure also exhibits H bonds between carbonyl, hydroxyl, and ether groups and π−π interactions between the aromatic rings, which give rise to highly complex structures that are insoluble in most known organic solvents, making it difficult to carry out reactions for structural transformation or deconstruction toward high added-value compounds. Solvents capable of dissolving lignin are also very useful for its removal during biomass pretreatment, i.e., for the production of bioethanol since lignin also interferes with cellulose depolymerization.2 The solubilization of lignin remains one of the biggest challenges in lignin valorization to improve the overall biorefinery economic process.3 © 2018 American Chemical Society

Received: December 1, 2017 Revised: January 22, 2018 Published: January 27, 2018 4122

DOI: 10.1021/acssuschemeng.7b04535 ACS Sustainable Chem. Eng. 2018, 6, 4122−4129

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Synthesis of protic ionic liquids (6a−6r) containing aromatic-substituted imidazolium cations. microscope (Motic model SMZ-168) at 40× magnification was used to confirm lignin dissolution. Proton (1H) and carbon-13 (13C) nuclear magnetic resonance (NMR) spectra were obtained on a Bruker (300 MHz) model AVANCE IIITM with a magnetic induction field of 7.05 T using tetramethylsilane (TMS) as internal standard using the deuterated solvent specified in each case. Protic Ionic Liquids (PILs) Synthesis. The synthesis of multiaromatic PILs (MPILs) was carried out in three steps as detailed below. Synthesis of 2,3,4-Triphenyl-4-methyl-1H-imidazole. In the first step, aromatic imides were synthesized by reacting aromatic amines (1.0 g, 0.10 mol) and benzaldehyde or terbutylaldehyde (1.4 g, 0.11 mol) in methanol (10.0 mL) for 3 h at 30 °C. Upon reaction completion, the solvent was evaporated under a vacuum to obtain a beige solid product. Synthesis of Multiaromatic Imidazole Compounds. The synthesis of multiaromatic imidazole compounds was carried out following a previously described modified procedure.39 In a typical reaction, imides (1.95 g, 0.10 mol) were mixed with Fe3O4/choline chloride (0.03 g), followed by the addition of ammonium acetate (0.827 g, 0.10 mol). The mixture was stirred for 3 h at 100 °C. 1-Phenyl-1,2propanodione (1.57 g, 0.11 mol) was then added, and the reaction was further stirred for an additional 6 h at 100 °C. The mixture was cooled down to room temperature, followed by the addition of water (15 mL) and dichloromethane (20 mL). The solid phase was separated by filtration, and a gray solid was washed twice with water (2 × 20 mL) and recrystallized in acetone to give a white solid. Products (4a−4e) were fully characterized by 1H and 13C NMR spectroscopy (see Supporting Information). Yield, boiling point, and spectroscopic characterization by NMR of the compounds synthesized in this study are also reported in the Supporting Information. Synthesis of Protic Multiaromatic ILs. Protic multiaromatic ILs (PMILs) were obtained via microwave-assisted neutralization reactions between imidazoles 4a−4e and methanesulfonic (5a) and ptoluenesulfonic acids (5b) at 60 °C for 3 min with the appropriate acid/base amounts to obtain 1.0 g of PMILs. PMILs were heated under a vacuum (20 min) several times until obtaining a constant weight of the sample during two consecutive heating processes, followed by cooling the samples to room temperature in a vacuum desiccator. Studies of Lignin Solubility. To determine lignin solubility, 10 mg of lignin was added to a microwave glass closed-vessel (30 mL) containing 1.0 g of PILs dissolved in 1.0 g of acetonitrile. The vessel

center of interest for scientists and companies worldwide in the past decade.22,23 These have been widely studied for many purposes as solvents,24 catalysts,25 and absorbents to remove pollutants,26,27 finding applications even in certain industrial processes.28,29 ILs also have begun to play important roles in several steps of biorefinery concepts including biomass pretreatment, hydrolysis, dissolution, and purification of final biomass-derived products.30,31 Protic ILs formed by a neutralization reaction between Brønsted acids and bases, especially, are an interesting family of ILs as they can be simply obtained with an inherent ability of transferring protons to the developed hydrogen-bond network (which gives them a high proton conductivity in the absence of water).32,33 Due to the ionic character, ILs absorb the microwave irradiation very efficiently, being a synergistic couple for green chemistry.34,35 The application of the dielectric heating of microwaves as a tool to accelerate the dissolution process of biopolymers in ILs is well-known and has been previously reported.36−38 In this work, the synthesis of 18 novel protic ILs containing aromatic-substituted imidazolium cations and methanesulfonate and p-toluenesulfonate anions is described. A dissolution study of lignin in these ILs under microwave irradiation was subsequently conducted, followed by DFT calculations to explain the dissolution phenomena observed for the best performing ILs.

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EXPERIMENTAL SECTION

Materials and Characterization. Kraft lignin, aromatic aldehydes and amines, aromatic diketones and ammonium acetate, iron(III) oxide, urea, choline chloride and methanesulfonic, and p-toluenesulfonic acids were purchased from Sigma-Aldrich and employed as received. Microwave irradiation experiments were performed on an Anton Paar Monowave 450 unit with a 24 autosampler using glass closed-vessels (30 mL). The system allows the control of temperature, pressure, and power of the radiation and counts on a camera to follow the course of the reaction and/or the dissolution process. A photograph of the microwave equipment employed in this study is provided in the Supporting Information (Figure S1). A stereo 4123


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Figure 2. HSQC spectra of the aromatic region of compound 6a.

Figure 3. Microwave vessel showing changes of PMIL 6a in acetonitrile during lignin dissolution. (a) Solution of 6a in acetonitrile synthesized under microwave irradiation without lignin. (b) Solution of 6a in acetonitrile containing 10% of lignin. (c) Solution of 6a in acetonitrile containing 30% of lignin. (d) Solution of 6a in acetonitrile containing 45% of lignin with some precipitated particles. collected with a yield of ca. 88%. The filtrate was condensed under a vacuum at 60 °C prior to analysis. Theoretical Calculation Studies. All theoretical calculations were performed with the program package Gaussian 09,40 while NWChem was used for the Born−Oppenheimer molecular dynamics (BOMD) calculations.41 In both cases, the density functional theory (DFT) was used, in particular the M06-2X approach, combined with the 6-31G basis set. This functional approach was chosen as recommended (together with M06-2X and M06) for systems where main-group thermochemistry, kinetics, and noncovalent interactions are all important.42 Geometry optimizations were performed without imposing any constrains. Local minima were identified by the absence of imaginary frequencies. Thermodynamic corrections at 298.15 K were included in the calculation of relative energies.

was closed and irradiated at 90 °C for 5 min. As the lignin was dissolved, an additional 10 mg was progressively added and the vessel was irradiated for 5 additional minutes. Lignin solubilization was visually checked and confirmed by observing a solution film under an optic microscope at 40×. To a homogeneous solution was added an additional 5 mg of lignin until a heterogeneous solution was observed after 30 min of irradiation. Reprecipitation of Lignin. A certain amount of acetone/ethanol (1:1) was added to the stirring lignin solution in the PIL until lignin precipitation. The regenerated lignin was separated by vacuum filtration and washed with water, acetone, and ethanol (3 × 20 mL). The separated lignin was dried under a vacuum overnight, cooled, and weighed. The powdery light-brown, fine lignin precipitate was 4124


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ACS Sustainable Chemistry & Engineering Table 1. Lignin Solubility in PMILs under Microwave Irradiation (90°C)

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PMILs were synthesized in two steps. In the first step, multiaromatic imidazole systems (4) were prepared via a threecomponent reaction between aromatic imines (1), aromatic diketones (2), and ammonium acetate (3) using iron(III) oxide in an eutectic solvent obtained by mixing urea/choline chloride (2:1). In the second step, PMILs (6) were obtained via proton transfer from Brønsted methanesulfonic (5a) and p-toluenesulfonic (5b) acids and multiaromatic-substituted imidazoles

RESULTS AND DISCUSSION

With continuing our study on biomass pretreatments with ILs,43 we decided to synthesize multiaromatic imidazolium cations to increase the affinity of ILs/polyaromatic systems such as lignin. However, protic ILs can be easily synthesized by the proposed neutralization reaction.11,12 After the synthesis of aromatic imides (1) through a conventional reaction between aromatic aldehydes and amines, 4125


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Figure 4. (a) Microscope images of PMILs 6a (entry 2, Table 1) without lignin, (b) with 20% of lignin, (c) and with 40% of lignin (d). Microscope images of PMIL 6c (entry 4, Table 1) without lignin and (e) with 10% of lignin. (f) Image of lignin precipitation after antisolvent addition (40×).

compared to phenyl substituents in C2 (entry 6a vs 6c, Table 1) or when the imidazolium contained an aryl substituent in C3 (entry 6a vs entries 6e and 6g). ILs containing methanesulfonate anions favored lignin solubilization with respect to ptoluenesulfonate anions in all of the studied PMILs. This last observation indicates that not only the aromatic character of the species resulted in a greater solubility but also the acid strength played an important role. The significant effect of the IL anion in lignin dissolution has been previously described by other authors.16,18,21 t-Butyl substituents bonded to C2 in the imidazolium ring (entries 4−7) exerted a negative effect on lignin solubilization, probably associated with the interruption of the cycle of electronic delocalization present in all other cases, in particular by comparing it with its analogues (entries 2, 3, 8, and 9) with a higher extended π-system,45 where lignin solubility was considerably improved by substituting phenyl by t-butyl groups. A similar effect was observed by replacing C5 phenyl with methyl groups, although with a less significant loss in the dissolution capacity for the MeSO3− anion and markedly improved capacity for p-TsO (entries 2 and 3 vs 18 and 19). The thiomethoxy group at the para-position of the aromatic ring bonded to nitrogen N3 of imidazolium (entries 10 and 11) significantly influenced the dissolution capacity of lignin as compared to its ethoxylated analogue (entries 8 and 9) as well as to the unsubstituted ring analogue (entries 2 and 3). These results are hardly explained by the electronic effect of the substituent and show that the cation structure also exerts a considerable influence on the dissolution capacity. Interestingly, the amount of solubilized lignin under the optimized conditions is rather significant (ca. >40%) and can be completed in a few minutes and at a relatively low temperature in comparison to similar studies where the dissolution of lower lignin quantities is carried out at higher temperatures and longer dissolution times.38,46

(4) as Brønsted bases under microwave irradiation using acetonitrile (1.0 g) as a solvent and adequate amounts of base 4 and acid 5 to obtain 1.0 g of PMILs (6), thus carrying out immediately the dissolution study of lignin in a one-pot process (Figure 1). Following the reaction scheme shown above, 18 PMILs were obtained in moderate to good yields and in high purities as confirmed by NMR spectroscopy (see Supporting Information). As an example, the HSQC spectra of the aromatic region of compound 6a is shown in Figure 2, with all of the correspondingly assigned signals. We decided to study lignin solubilization in the ILs under microwave irradiation for faster dissolution and in situ visualization of the dissolution process by means of the equipment camera (Figure 3). As expected, PMILs efficiently interacted under microwave irradiation (dielectric heating) and the dissolution process was accelerated under the investigated conditions. Given the multiaromatic nature of these systems, all obtained ILs were solid products, for which lignin dissolution studies were carried out using all ILs dissolved in acetonitrile (1:1). These possessed various advantages including a low viscosity media that favors the dissolution process15,44 and its clear visualization by diluting the very dark mixture (formed when lignin began to dissolve), with some ILs having high melting points. These experiments also allowed the dissolution studies to be performed at relatively low temperatures (90 °C). However, acetonitrile is a solvent in the system and generates slight pressure that favors the rapid lignin dissolution. Lignin is immiscible in acetonitrile, and therefore, the observed dissolution was only due to the interaction between PMILs and lignin. Table 1 summarizes all of the results of the lignin solubilization experiments. Aliphatic substituents in C2 of the imidazolium ring decreased lignin solubility in the IL as 4126


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ACS Sustainable Chemistry & Engineering For comparative purposes, conventional IL 1-butyl-3methylimidazolium chloride ([BMIM]Cl, entry 20), previously described as an efficient solvent for lignin,14 was evaluated for lignin dissolution following an identical microwave-assisted protocol. Under the investigated conditions, 32% of lignin was only solubilized in [BMIM]Cl, proving the improved efficiency of PMILs 6a, 6g, 6m, and 6o in lignin solubilization. However, a comparative experiment between conventional heating and microwave dielectric heating was also carried out to ascertain potential microwave effects using the three most efficient PMILs for dissolving lignin (6a, 6m, and 6o). We observed that the amount of lignin dissolved was virtually the same with both heating sources, but the times of dissolution were remarkably different as expected. Based on dielectric heating and the high polarity of the medium, the dissolution rate was significantly faster under microwave irradiation since reaction times of over 8 h were required to dissolve the same amount of lignin under conventional heating. These results are a clear indication that special microwave effects are not present in the system and that the improvements in reactions rates are clearly due to the fast and homogeneous heating achieved by ILs under microwave irradiation, in good agreement with previous results.47 Figure 4 depicts the lignin solubilization process for PMIL 6a. As a larger amount of lignin was employed in this experiment, small lumps began to form, which could be observed after the addition of 43% (using 6a) and 10% lignin (using 6c). Lignin could be separated from the IL−acetonitrile solution by adding an equal volume of acetone−ethanol, then filtering under a vacuum, and washing with distilled water (50 mL) and acetone−ethanol (50 mL). ILs could be recovered from the filtrate by vacuum evaporation of the solvents and could be reused in various dissolution/regeneration cycles without any appreciable changes in structure and dissolving abilities. Lignin is a very complex material with no periodicity. Therefore, a simplified model is required to better understand interactions and ways to deconstruct/solubilize lignin. Lignin presents three main units: coniferyl, sinapyl, and p-coumaryl alcohols. Coniferyl represents approximately 90% of softwood; in hardwood, approximately equal proportions of coniferyl and sinapyl are present. However, β-O-4, α-O-4, 5−5, β-5, 4-O-5, β−β, dibenzodioxocin, and β-1 are the most common linkages found in lignin, with β-O-4 being largely present in all lignin feedstocks (constituting approximately half of the linkage units in softwood lignin).47 On the basis of these premises, we chose a central β-O-4 linkage and two coniferyl units as terminal groups as model moieties for theoretical studies (Figure 5), with a fragment of a larger model recently proposed by TorresGarciá et al.48 In order to explore the experimentally observed solubility of lignin in ILs, at the molecular level, the interactions among them were investigated via computational chemistry. However, the modeling of these interactions is a challenging task because a large number of possible conformations for the interacting system may be present. In order to choose the most likely structures, a short BOMD simulation was preliminarily conducted. This allowed the selection of three different conformers that were then fully optimized. Lignin and ILs were optimized together and independently, where the geometries with the lowest energies were employed to calculate the energy difference.

Figure 5. Lignin fragment used as a model for the theoretical study.

The energy of the three located supramolecular complexes, together with the most relevant features of the interactions between fragments, are reported in Table S1 (Supporting Information). Complexes were labeled as configuration I, II, and III in increasing order of energy. They were formed through H bonding (HB)-like interactions. Configuration I (Figure 6) featured the lowest energy ca. 3 kcal/mol with

Figure 6. Optimized geometry of complex I, showing the distances for H bond-like interactions with (a) anions and (b) cations of IL. Blue: lignin fragment. Red: anion. Gray: cation.

respect to that measured in configuration II. According to the Maxwell−Boltzmann distribution, this represents 99.3% of the total population of the studied supramolecular complexes. This configuration presents two interactions at distances shorter than 1.7 Å, while configurations II and III (Figures S2 and S3) have exclusively one interaction similarly short. Therefore, this structural feature seems to be responsible for the relative energies of the different modeled configurations. In addition, the strongest interactions (shortest interaction distances) were found to systematically involve H atoms in the hydroxyl groups of lignin and O atoms in the anionic fragment of the IL. This suggests that the anion plays a more important role in the IL−lignin interaction, which is in good agreement with previously proposed studies by Zhang et al.18 and Holmes.21 4127



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The Gibbs free energies of the reaction associated with the investigated configurations/complexes were all found to be negative. This is a significant finding since it may imply a spontaneous formation of such supramolecular complexes between lignin and the synthesized ILs upon close interaction. The formation of such complexes seems to explain the high lignin solubility in the particularly investigated ILs. Cartesian coordinates of atoms for the lignin fragment and 6a in the three conformers obtained by the DFT study are presented in the Supporting Information (Table S2).

CONCLUSIONS Novel protic ILs containing multiaromatic cations (PMILs) and methanesulfonate and p-toluenesulfonate anions were synthesized and evaluated in lignin dissolution. Some of these ILs, especially those containing methanesulfonate anions, demonstrated excellent lignin dissolving capacities, which could facilitate its separation and structural modifications for recovery/isolation under relatively mild and environmental friendly conditions. The PMIL exhibited the best performance for lignin dissolution (42%) achieved after a few minutes of microwave irradiation (typically 5 min) at 90 °C. Theoretical studies based on DFT calculations indicated that the Gibbs free energy associated with the formation of the IL−lignin complex is negative, suggesting that these supramolecular complexes may be spontaneously formed, which may account for the experimental observation of high lignin solubilities in these ILs at low temperatures. The strongest interactions were found to consistently involve H atoms in the hydroxyl groups of lignin and oxygen atoms in the anionic fragment of the IL. According to these results, PMILs could pave the way to their widespread utilization in biomass pretreatment under mild conditions and high biomass loadings. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04535. Yield, boiling point, and spectroscopic characterization by NMR of synthesized multiaromatic imidazole and MPILs and additional details of the DFT calculations (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rafael Luque: 0000-0003-4190-1916 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support of the research project from IMP and SENER-CONACyT Project. O.M.P. thanks CONACyT for the concession of a fellowship (no. 202187). 4128


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Understanding Microwave-Assisted Lignin Solubilization in Protic

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