Understanding Microwave-Assisted Lignin Solubilization in Protic

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Understanding microwave-assisted Lignin Solubilization in Protic Ionic Liquids with Multiaromatic Imidazolium Cations Omar Merino, Gabriela Fundora-Galano, Rafael Luque, and Rafael Martínez-Palou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04535 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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ACS Sustainable Chemistry & Engineering

Understanding microwave-assisted Lignin Solubilization in Protic Ionic Liquids with Multiaromatic Imidazolium Cations. Omar Merino,a Gabriela Fundora-Galano,b Rafael Luque,c* Rafael Martínez-Paloua*

a

Gerencia de Transformación de Biomasa, Instituto Mexicano del Petróleo, Eje Central L.

Cárdenas 152, 07730, Ciudad de México. E-mail: [email protected] b

Facultad de Química, Departamento de Física y Química Teórica, Universidad Nacional

Autónoma de México, 04510, México City, México. c

Departamento de Química Orgánica. Universidad de Córdoba, Campus Rabanales, Edificio

Marie Curie C-3, E-14014 Cordoba, Spain. E-mail: [email protected]

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, eighteen novel protic ILs containing aromatic substituted imidazolium cations and methanesulfonate and p-toluenesulfonate anions were synthesized and evaluated for lignin dissolution under microwave irradiation. ILs containing methanesulphonate demonstrated good lignin dissolving capacities which can facilitate its separation and structural modifications to carry out their recovery under relatively mild and

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environment friendly conditions (few minutes, 90°C). DFT theoretical calculations provided additional insights into the high lignin dissolution observed for best performing ILs.

Keywords: Ionic liquids, lignin, microwave, solubilization, multiaromatic cation

Introduction Biomass comprises mainly of 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 20-35% 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 difficult to carry out reactions for structural transformation or deconstruction towards 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 bio-refinery economic process.3


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Lignin can be processed for either 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 (ILs) have become the center of interest for scientists and companies worldwide in the last decade.22, 23 These have been widely studied for many purposes as solvents,24 catalysts25 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 bio-refinery concepts including biomass pretreatment, hydrolysis, dissolution and purification of final biomass-derived products.30,

31

Especially, protic ILs formed by a neutralization reaction

between Brønsted acids and bases 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 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 eighteen 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 best performing ILs.

Experimental Section Materials and characterization Kraft lignin, aromatic aldehydes and amines, aromatic diketones and ammonium acetate, Iron (III) oxide, urea, choline chloride and methanesulphonic, and p-toluenesulphonic acids were purchased from Sigma-Aldrich and emplpyed as received. Microwave irradiation experiments were performed on an Anton Paar Monowave 450 with a 24 autosampler using glass closedvessels (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 photography of the microwave equipment employed in this study is provided in the SI (Figure S1). Stereo microscope (Motic model SMZ-168) at 40X 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


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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 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 hours at 100°C. 1-phenyl-1,2-propanodione (1.57 g, 0.11 mol) was then added and the reaction was further stirred for an additional time of 6 hours 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 grey solid was washed twice with water (2 x 20 mL) and recrystallized in acetone to give a white solid. Products (4a-4e) were fully characterized by 1H and


13

C NMR spectroscopy (see SI).


Yield, Boiling Point and Spectroscopic characterization by NMR of the compounds synthesized in this study are also reported in the SI. Synthesis of Protic Multiaromatic ILs Protic Multiaromatic ILs (PMILs) were obtained via microwave-assisted neutralization reactions between imidazole 4a-4e, and methanesulphonic (5a) and p-toluenesulphonic acids (5b) at 60°C during 3 minutes with the appropriate acid/base amounts to obtain 1.0 g of PMILs. PMILs were heated under vacuum (20 min) several times until obtaining constant weight of the sample during two heating consecutive 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 were added to a microwave glass closed-vessel (30 mL) containing 1.0 g of PILs dissolved in 1.0 g of acetonitrile. The vessel was closed and irradiated at 90°C during 5 min. As lignin was dissolved, additional 10 mg were 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 40X. Having a homogeneous solution, additional 5 mg of lignin were added until a heterogeneous solution was observed after 30 min of irradiation.

Re-precipitation 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 x 20 mL). The separated lignin was dried under vacuum


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overnight, cooled and weighed. The powdery light-brown, fine lignin precipitate, was collected with a yield of ca. 88%. The filtrate was condensed under vacuum at 60°C prior to analysis.

Theoretical Calculation studies All theoretical calculations were performed with the program package Gaussian 0940 while NWChem was used for the Born Oppenheimer Molecular Dynamics (BOMD) calculations.41 In both cases, 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 non-covalent 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.

Results and discussion

Continuing our study on biomass pretreatments with ILs,43 we decided to synthesize multiaromatic imidazolium cations to increase the affinity ILs/ polyaromatic systems such as lignin. On the other hand, 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, PMILs were synthesized in two steps. In the first step, multiaromatic imidazole systems (4) were prepared via three-component 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 methanesulphonic (5a) and p-toluenesulphonic (5b) acids and multiaromatic-substituted imidazoles (4) as Brønsted bases under microwave irradiation using acetonitrile (1.0 g) as solvent, 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).

Figure 1. Synthesis of Protic Ionic liquids (6a-6r) containing aromatic substituted imidazolium cations.

Following the reaction scheme shown above, eighteen PMILs were obtained in moderate to good yields and high purities as confirmed by NMR spectroscopy (see SI). As example, HSQC spectra of the aromatic region of compound 6a is shown in Figure 2, with all correspondingly assigned signals.


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

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 which favors the dissolution process 15, 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). On the other hand,



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acetonitrile is a solvent with of the system and the generation of a 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.

(a)

(b)

(c)

(d)

Fig. 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.

Table 1 summarizes all results of lignin solubilization experiments. Aliphatic substituents in C-2 of the imidazolium ring decreased lignin solubility in the IL as compared to phenyl substituents in C-2 (entries 6a vs. 6c, Table 1) or when the imidazolium contained an aryl substituent in C-3 (entries 6a vs 6e and 6g). ILs containing methanesulfonate anions favored lignin solubilization with respect to p-toluenesulfonate anions in all studied PMILs. This last observation indicates that not only the aromatic character of the species resulted in a greater solubility but the acid


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strength also played an important role. The significant effect of the ILs anion in lignin dissolution has been previously described by other authors.16, 18, 21

Table 1. Lignin solubility in PMILs under microwave irradiation (90°C). Entry

Compound

Compound structure

ID

Cation

Anion

1 2

Acetonitrile 6a

Solubilized lignin (%) -

CH3SO3-

42

N H

N

3

6b

pCH3C6H4SO3-

Understanding Microwave-Assisted Lignin Solubilization in Protic

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