Microwave-Assisted Degradation of Lignin Model Compounds in

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Microwave-Assisted Degradation of Lignin Model Compounds in Imidazolium-Based Ionic Liquids Jingying Pan,† Jie Fu,*,† Shuguang Deng,‡ and Xiuyang Lu*,† †

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027 Zhejiang, China ‡ Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico 88003, United States of America ABSTRACT: A systematic study of microwave-assisted degradation of lignin model compounds such as benzyl phenyl ether (BPE) and guaiacol, in imidazolium-based ionic liquids, was performed by evaluating the catalytic activity of 29 types of ionic liquids as both solvent and catalyst. After measuring and comparing the acidity of each ionic liquid solution for BPE and guaiacol degradation under the microwave irradiation and conventional heating conditions, it was found that the ionic liquid 1-butyl-3methylimidazolium hydrogen sulfate ([BMIM]HSO4) was the most effective for decomposing the lignin model compounds. The experimental results indicate that ionic liquid acidity is in favor of the catalytic activity for BPE and guaiacol degradation, microwave irradiation could accelerate the degradation rate by 650% for BPE and 1120% for guaiacol and significantly increase the reaction selectivity. It was also found in experiments that the ionic liquid [BMIM]HSO4 could be used for 5 times without any loss of catalytic activity. The possible mechanisms for BPE and guaiacol degradation are proposed based on the product distributions.

1. INTRODUCTION Lignocellulosic biomass consisting of cellulose,hemicellulose and lignin1,2 is the most abundant natural biomass in the world. Lignin, contributing approximately 15−30% to the total lignocellulose mass,3 is a natural amorphous polymer with a complex three-dimensional structure arising from a polymerization of phenyl propanoid monomers, including coniferyl, sinapyl, and p-coumaryl alcohol.4 The phenyl propane units are linked primarily by C−O of α- and β- bonds,5 suggesting lignin to be a possible raw material for high-value aromatic compounds. However, the existing approaches for lignin degradation are inefficient, and combustion is still the most common way of utilizing the lignin.1,6,7 Therefore, the research on converting lignin to aromatic chemicals attracts much attention. Ionic liquids, consisting of a cation and anion, are organic salts whose melting points are usually below 100 °C.8 The physical and chemical properties of ionic liquids are tunable by varying the cation−anion combinations. The cations are usually bulky organic ions, such as alkylated imidazole, pyrrole derivatives, pyridine derivatives, quarternized alkyl amines, and alkyl phosphines. Accordingly, common anions are halides, alkyl sulfates, fluorinated hydrocarbons, carboxylic acids, and amino acids.9 Ionic liquids have been developed as a green solvent for biomass conversion10−12 due to three remarkable properties: nonvolatility, tunable solubilities and designability, and thermal stability. Among many ionic liquid candidates, the imidazolium-based ionic liquids have a good solubility for lignin13−15 and thermal stability.15 The stability at high temperature of the imidazolium-based ionic liquids and their solubility for lignin make them promising reaction media for lignin degradation. Jia et al.16 studied the cleavage of β−O−4 linkage of lignin model compounds in an ionic liquid ([BMIM]Cl). Guaiacyl© 2013 American Chemical Society

glycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether were used as the β−O−4 linkage model compounds. A 70% yield of guaiacol was achieved at 150 °C after 60 min reaction in the [BMIM]Cl solution. Cox, Jia, Zhang, and Ekerdt10 also studied the degradation of β−O−4 linkage model compounds in acidic ionic liquids. Their experimental results showed that acidic ionic liquids have a high catalytic activity for the degradation of lignin model compounds, and the reactivity of model compounds in ionic liquids is not only dependent on the acidity of the reaction media. Although interesting results on β−O−4 linkage model compounds were obtained, it is necessary to extend this research to other model compounds that cannot be effectively decomposed by the existing biomass conversion technologies. Recently, the microwave irradiation method has been applied in various applications in many different fields.17 The term microwave means alternating current signals with frequencies between 0.3 and 300 GHz.18 It has been shown that microwave is able to accelerate the organic reactions,19 decrease the reaction time, or increase the selectivity in certain cases.17 Efficient heating of molecules is an advantage of microwave technology, and the microwave mainly heats the reaction system by two mechanisms: dipole rotation and ionic conduction. Dipole rotation comes from the tendency of polar molecules (dipoles or ions) trying to align themselves with the rapidly changing electric field of the microwave, which results in the friction between molecules and make them heated. Ionic conduction mechanism leads to the instantaneous superheating of an ionic substance because of the motion generated by an electric yield. By ionic conduction, ionic liquids can absorb the microwave irradiation very efficiently and Received: October 15, 2013 Revised: December 28, 2013 Published: December 30, 2013 1380

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Table 1. Structegories of Ionic Liquids

System was then started, and the reactor was heated to 180 °C and hold for a certain time with the microwave irradiation of 15 W. The products in the reactor were rinsed to a 25 mL volumetric flask with methanol, and filtered by 0.45 μm filter membrane. The product samples were analyzed by a HPLC. The controlled experiments were carried out in a micro reactor with a volume of 1.67 mL, heated by a fluidized sand bath. 0.03 g lignin model compound, 30 μL H2O and 1 g ionic liquid were added to the microreactor. The reactor was then put into the preheated fluidized sand bath (180 °C) for a given period of time. The products in the reactor were rinsed to a 25 mL volumetric flask with methanol, and filtered by 0.45 μm filter membrane. The product samples were analyzed by a HPLC. 2.4. Analysis. The product samples were analyzed by an Agilent high-pressure liquid chromatograph 1100 (HPLC 1100) equipped with a variable wavelength detector. The column was 4 mm ID × 250 mm KNAUER C18 reverse-phase column; the temperature of column was set at 35 °C; the mobile phase consisted of 0.2 wt % acetic acid aqueous solution (A) and methanol (B) with a rate of 0.5 mL/min and a gradient elution: 0 min (20% B)→10 min (35% B)→20 min (35% B)→30 min (80% B)→45 min (80% B)→55 min (20% B). Product identification was achieved by comparison of retention times with those of standard solutions of pure compounds, and identities were confirmed by GC-MS (Agilent 6890GC/5973MSD). The acidity of the ionic liquids was measured by a pH meter (PHSJ4A); 0.1 mol/L aqueous solutions of each ionic liquid were prepared for the determination of pH. 2.5. Recycling of Ionic Liquids. After the reaction, 20 g water was added to the system to dissolve ionic liquid and the products. The products in aqueous phase were extracted by acetic ether for 5 times. Then, the ionic liquid was recycled by removing water with a rotary evaporator at 70 °C and dried in a vacuum at 65 °C for 12 h.

transfer energy quickly,18 so the combination of microwave and ionic liquid may efficiently catalyze some reactions. Zhang and Zhao20 reported that microwave and ionic liquids are combined to cleave lignocellulose to produce 5-hydroxymethylfurfural and furfural, suggesting that the combination of microwave and ionic liquid is effective to the degradation reactions in the biomass conversion. In this study, benzyl phenyl ether (BPE) and guaiacol were selected as the lignin model compounds, representing the α− O−4 and guaiacyl linkage in lignins. Ionic liquids are employed as a solvent and catalyst with the assists of microwave irradiation. The catalytic activities of ionic liquids were evaluated; the relationship between catalytic activity and acidity was discussed; the product distributions of BPE and guaiacol degradation were obtained; and the reaction mechanism for the degradation of BPE and guaiacol were proposed. These results provided insights for the microwave-assisted degradation of the lignin model compounds in the imidazolium-based ionic liquids.

2. EXPERIMENTAL SECTION 2.1. Materials. All the ionic liquids (>95%) were obtained from Shanghai Cheng Jie Chemical Co. Ltd., China. The chemical structure, name, abbreviation and category of the ionic liquids are summarized in Table 1. BPE (>98%) was purchased from Tokyo Kasei Kogyo Co. LTD, Japan, Guaiacol (>99%) was purchased from Aladdin, China, and 2-benzylphenol (>98%) was purchased from ACROS Organics, China. All the chemicals were used as received without any purification or pretreatment. 2.2. Apparatus. The microwave reactor (Discover System) was purchased from CEM, U.S.A. The micro reactor was assembled by the Swagelok parts21 purchased from Swagelok, U.S.A. The precision fluidized sand baths (SBL-2) was purchased from Keison, England. The pH meter (PHSJ-4A) was purchased from INESA Instrument, Shanghai, China. 2.3. Experimental Procedure. In a typical experimental run for lignin degradation, 0.03 g lignin model compound, 30 μL H2O and 1 g ionic liquid were added to the 10 mL microwave reactor. The Discover

3. RESULTS AND DISCUSSION 3.1. Effect of Water Ratio on BPE Degradation. In the degradation of lignin, the cleavage of C−O linkage usually needs H2O as a reactant. Jia, Cox, Guo, Zhang, and Ekerdt12 stated that H2O plays a very important role in the pathway of cleaving C−O linkage. Figure 1 shows the effect of molar ratio of water to BPE on the degradation of BPE. At the beginning, 1381

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good catalytic activity for the conversion of BPE (81.6%) and yield to phenol (32.7%). The conversion and yield in the chlorides (No. 9−16) were higher than those obtained with No. 1−7 tetrafluoroborates. [CmMIM]Cl (No. 16) was the most active one, with a 22.5% conversion and 22.5% yield, having a 100% selectivity to phenol. Bromides (No. 17−21) could catalyze the degradation of BPE, the highest conversion of 34.0% was obtained with [BMIM]Br (No. 17), and the highest yield to phenol of 21.0% was obtained with [EMIM]Br (No. 19) . The degradation of BPE in acetate (No. 22, 23) had a low conversion and yield. Hydrogen sulfates (No. 24, 25) could effectively catalyze the degradation of BPE, and both high conversion and high selectivity were achieved for the degradation in [BMIM]HSO4(No. 24). Other anions (No. 26−29) had a negligible effect on BPE degradation. The main product of guaiacol C−O cleavage was catechol. Figure 2b shows the effects of different types of ionic liquids on the degradation of guaiacol for the selective formation of catechol at 180 °C. The error bar represents standard deviation obtained from three replicate experiments. The degradation efficiency of guaiacol was lower than that of BPE. Tetrafluoroborates (No. 1−8) was not in favor of the C−O cleavage of guaiacol. Chlorides (No. 9−16) was not effective except [CmMIM]Cl (No. 16). A conversion of 25.6% and yield of 19.1% were achieved in [CmMIM]Cl. Bromides (No. 17− 21) were similar to chlorides, which was different from the results obtained on BPE degradation. The catalytic activity of acetates(No. 22, 23) was higher than those of tetrafluoroborates, chlorides, and bromides. Hydrogen sulfates (No. 24, 25) could effectively catalyze the degradation of guaiacol, which was similar to the results obtained on BPE degradation. Also, other anions hardly catalyzed the degradation of guaiacol. The selectivity of the anions for guaiacol degradation reaction can be ranked as BF4− < CH3COO− < Cl− < Br− < HSO4−.

Figure 1. Conversion of BPE and yield to phenol in [BMIM]Cl at different ratios of H2O to BPE at 180 °C under microwave irradiation for 30 min.

the conversion of BPE and yield to phenol increased as the molar ratio of H2O to BPE increased, after reaching the maximum conversion and yield at a molar ratio of 10.2, both decreased as the molar ratio of H2O to BPE increased above 10.2. It indicates that 10.2 is the optimal molar ratio for producing phenol. Therefore, 10.2 was employed for the molar ratio of H2O to BPE (30 μL H2O to 0.03 g BPE) in the following experiments. 3.2. Evaluation of Ionic Liquids for BPE and Guaiacol Degradation. The main product of BPE C−O cleavage was phenol. Figure 2a shows the effects of different types of ionic liquids on the degradation of BPE and selectivity to phenol at 180 °C. The error bar represents standard deviation obtained from three replicate experiments. Tetrafluoroborates (No. 1− 8) did not significantly increase the conversion of BPE or yield to phenol except [CmMIM]BF4 (No. 8). [CmMIM]BF4 had a

Figure 2. Degradation of BPE and guaiacol in ionic liquids at 180 °C for 10 min under microwave irradiation, and the acidity of ionic liquids: (a) conversion of BPE, yield to phenol, and acidity of ionic liquids; (b) conversion of guaiacol, yield to catechol, and acidity of ionic liquids. 1382

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[BMIM]HSO4 was used as the solvent and catalyst. At 10 min, the conversion of BPE under the microwave irradiation was 83.1%, while that under the conventional heating was only 35.5%. Moreover, the corresponding selectivities were 83.1% and 36.8%, respectively. For guaiacol, more significant differences between the microwave irradiation (conversion of 66.1%, selectivity of 59.7%, time of 10 min) and the conventional heating (conversion of 18.7%, selectivity of 9.1%, time of 10 min) were observed. Furthermore, the degradation rate constants of BPE and guaiacol under the microwave irradiation and conventional heating were obtained by fitting the reaction kinetic data plotted in Figure 3a using a first-order equation. For BPE, the rate constants of the degradation were 0.18 min−1 under the microwave irradiation and 0.024 min−1 under the conventional heating. For guaiacol, those were 0.10 min−1 under the microwave irradiation and 0.0082 min−1 under the conventional heating. The microwave irradiation accelerated the degradation rate by 650% for BPE and 1120% for guaiacol, and significantly increased the selectivity to phenol or catechol as compared to the conventional heating, which might be related to strong microwave absorbing nature of the ionic liquid-type triazolium intermediates. Via the mechanism of ionic conduction, the microwave irradiation absorbed can be quickly transferred to the energy facilitating the reactions. 3.4. Product Distribution. In the BPE reaction system, the products were identified as phenol, benzyl alcohol, 2benzylphenol, 4-benzyphenol, benzyl ethyl ether, benzyl ether, and etc. BPE, phenol, benzyl alcohol, 2-benzylphenol, and 4-benzyphenol were quantified by their corresponding standard curves. Other peaks in the chromatography were summed up and quantified by the standard curve of BPE, which allowed us to estimate their yields. The product distributions at 180 °C for 10 min reaction time are shown in Table 2. The low mass balance indicated that polymerization might occur, because the polymers might not be detected by a HPLC.

The ionic liquids evaluated in this work showed large differences in catalytic activity for the degradation of BPE and guaiacol, which may be caused by the properties of the ionic liquid including acidity. The pH of the solution of each ionic liquid was measured by a pH meter as described in Section 2.5. Figure 2 shows the relationships between the pH and the reactivity. Based on the results of acidity, the four most acidic ionic liquids are [CmMIM]BF4 (No. 8), [CmMIM]Cl (No. 16), [BMIM]HSO4 (No. 24), and [BsMIM]HSO4 (No. 25). As discussed above, [CmMIM]BF 4 , [BMIM]HSO 4 , and [BsMIM]HSO4 have the best catalytic performance for the degradation of BPE, and the catalytic activities of [CmMIM]Cl, [BMIM]HSO4, and [BsMIM]HSO4 were the highest for the degaradation of guaiacol, indicating that high acidity of ionic liquids is in favor of the degradation of BPE and guaiacol. Also, most of the other ionic liquids show an interesting trend: the more acidic the ionic liquid is, the better the catalytic performance it has. However, the most acidic ionic liquid ([BsMIM]HSO4) did not have the best catalytic effect for BPE or guaiacol degradation, probably owing to the steric hindrance of hydrogen sulfate. Butyl sulfonic acid is bigger than butyl, so the steric hindrance of [BsMIM]HSO4 is larger than that of [BMIM]HSO4. 3.3. Role of Microwave Irradiation. Figure 3 shows the degradation of BPE and guaiacol in ionic liquids under both microwave irradiation and conventional heating conditions. The reaction temperature was maintained at 180 °C and

Table 2. Product Distribution of BPE Degradation (180 °C, 10 min)a

Figure 3. Conversion of BPE and guaiacol, and selectivity to phenol and catechol in [BMIM]HSO4 at 180 °C under microwave irradiation and traditional heating.

a

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other*: including benzyl ethyl ether, benzyl ether, and so on. dx.doi.org/10.1021/ef402062w | Energy Fuels 2014, 28, 1380−1386

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Figure 4. Mechanism of BPE degradation in acidic ionic liquids.

Based on the product distribution, a possible mechanism is proposed in Figure 4. H+ attacks BPE to produce a transition complex, then H2O attacks the transition complex via the SN2 mechanism to yield phenol and benzyl alcohol. At the same time, the transition complex dissociates to phenol and a stable benzyl cation via the SN1 mechanism. Then, the benzyl cation attacks the para and ortho positions of phenol to produce 2benzylphenol and 4-benzylphenol. In the guaiacol reaction system, the products were identified as catechol, 2,3-dihydrobenzofuran, 1-ethoxy-4-methoxybenzene, 7-methoxybenzofuran, etc. Guaiacol, catechol and 2,3dihydrobenzofuran were quantified by their corresponding standard curves. Other peaks in chromatography were summed up and quantified by the standard curve of catechol, which can provide us a crude estimation of their yield. Their distributions at 180 °C for 10 min are shown in Table 3. The product distribution showed that the minor products such as 2,3dihydrobenzofuran, 1-ethoxy-4-methoxybenzene, 7-methoxybenzofuran occupied very few percentages. Similar with the results of BPE degradation, the mass balance of guaiacol was low. About 19% guaiacol was probably converted to polymers, which cannot be detected. Based on the product distribution, a possible reaction mechanism for guaiacol degradation is proposed in Figure 5. H+ attacks guaiacol to produce a transition complex; then, H2O attacks the transition complex via the SN2 mechanism to yield catechol and methanol. Since there is no methylcatechol detected, the reaction via the SN1

Table 3. Product Distribution of Guaiacol Degradation (180 °C, 10 min)a

a

other*: including 1-ethoxy-4-methoxybenzene, 7-methoxybenzofuran, and so on.

mechanism does not exist in the reaction system due to the instability of methyl cation. 3.5. Reusability of Ionic Liquids. The reusability is a very important factor to judge whether a catalyst can be applied in industry. Figure 6 shows that the conversion of BPE, the selectivity to phenol, and the recovery in fresh [BMIM]HSO4 and recycled [BMIM]HSO4. Five times fresh and recycled ionic 1384

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Figure 5. Mechanism of guaiacol degradation in acidic ionic liquids.

HSO4 can be reused without any loss of catalytic activity and change of chemical structure.

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: +86 571 87952683. E-mail: [email protected]. *Tel/Fax: +86 571 87952683. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21176218) and the National High Technology Research and Development Program of China (863 Program, No. 2012AA040211).

Figure 6. Recycling test on degradation of BPE in [BMIM]HSO4.

liquids were evaluated at 180 °C for 10 min. The conversion and selectivity were very similar. The conversions of 5 times activity evaluation were between 80.1 and 83.2%, and the selectivities were between 81.2 and 84.4%. The recoveries of [BMIM]HSO4 for the 4 times recycling process were all high, between 96.5 and 98.1%. These data showed an excellent reusability of [BMIM]HSO4 for the degradation of BPE. Fresh and recycled [BMIM]HSO4 were characterized by NMR; the results are shown as follows. Fresh [BMIM]HSO4: 1HNMR (300 MHz, D2O), δ: 8.54 (s, 1H, CH); 7.31 (s, 1H, CH); 7.26 (s, 1H, CH); 4.02 (t, 2H, CH2); 3.72 (s, 3H, CH3); 1.67 (m, 2H, CH2); 1.13 (m, 2H, CH2); 0.75 (t, 3H, CH3). Recycled [BMIM]HSO4: 1HNMR (300 MHz, D2O), δ: 8.55 (s, 1H, CH); 7.32 (s, 1H, CH); 7.28 (s, 1H, CH); 4.03 (t, 2H, CH2); 3.74 (s, 3H, CH3); 1.67 (m, 2H, CH2); 1.15 (m, 2H, CH2); 0.75 (t, 3H, CH3). All the chemical shifts remained the same, indicating the chemical structure of [BMIM]HSO4 have not changed after the repeated uses in the reaction. [BMIM]HSO4 was very stable under the degradation condition with the microwave irradiation.

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REFERENCES

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4. CONCLUSIONS The combination of ionic liquids and microwaves was effective for the degradation of BPE and guaiacol. Microwave irradiation could accelerate the degradation rate by 650% for BPE and 1120% for guaiacol, and significantly increase the selectivity. [BMIM]HSO4 was the most effective ionic liquid for the degradation of BPE and guaiacol. The acidity of ionic liquids had a great effect on the degradation of BPE and guaiacol. Possible mechanisms for the degradation of BPE and guaiacol are proposed based on the product distributions. [BMIM]1385

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