Microwave-Assisted Oxidative Degradation of Lignin Model

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Microwave-Assisted Oxidative Degradation of Lignin Model Compounds with Metal Salts Jingying Pan, Jie Fu,* and Xiuyang Lu Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China S Supporting Information *

ABSTRACT: A systematic study on microwave-assisted oxidative degradation of lignin model compounds, such as 2-phenoxy-1phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol, was performed by evaluating the catalytic activity of 14 types of metal salts. The acidity of each metal salt solution for the oxidative degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol under the microwave irradiation and conventional heating conditions was measured and compared. The results showed that CrCl3 and MnCl2 were the most effective for the degradation of the lignin model compounds. The acidity of metal salt is in favor of the catalytic activity for the degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4hydroxybenzyl alcohol, and microwave irradiation is able to accelerate the degradation rate in a large scale. The possible mechanisms for the degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol are proposed on the basis of the product distributions.

1. INTRODUCTION With the decline of the fossil fuels and the growing demand for renewable, abundant, and comparably cleaner alternatives to liquid fuels and chemicals produced from petroleum, biomass appears to be a promising source for fuels and chemicals.1 The utilization of lignocelluloses emerge as a very active research area in recent years.2 Lignin contributes 15−30%3 to the total mass of lignocelluloses and it is always a residue in conversion from lignocellulose biomass to fuels.4 Lignin is a polymer of substituted phenylpropane units5 linked together by C−O α and β bonds,6 which suggests lignin to be a possible raw material for high-value aromatic chemicals.3 There are three major thermochemical pathways to convert lignin: pyrolysis, oxidation, and combustion.7 The microwave oxidation was proven to be an mild and environmental friendly pathway8 compared to pyrolysis. Araújo et al.9 investigated vanillin production from lignin oxidation and low-molecularweight lignin oxidation in a batch reactor. The former showed a lower conversion to vanillin. Zakzeski et al.10 studied the oxidative degradation of lignin in ionic liquids with metal catalysts and O2 in mild conditions. The result showed that the general order of activity for the transition-metal cations investigated was Co > Cr > Fe > Ni > Mn > Cu. The anions of the metal salts, however, were not consistent, which may affect the accuracy of the results. The high cost of ionic liquids limited the widespread use of them in lignin oxidative degradation. In some cases, a mixed solvent consisting of both organic and inorganic solvents was proper for lignin degradation. Zhao et al.11 investigated catalytic oxidative degradation of lignin using water/methanol as the mixed solvent and H5PMo10V2O40 as the catalyst. A yield of 65.2% for the oxidative degradation products was achieved from pyrolytic lignin. The microwave irradiation method has been proven to be very effective in many different fields.12 Microwave is a kind of © XXXX American Chemical Society

electromagnetic wave, with frequencies between 0.3 and 300 GHz.13 It is capable of accelerating the organic reactions,14,15 decreasing the reaction time, or increasing the selectivity in certain cases.11 The polar molecules will rotate at very high speed with the rapidly changing electric field of the microwave, resulting in the friction between molecules, which makes them heated. Ionic substances in the reaction system will be heated by the motion generated by the electric field.16 In certain cases, a microwave shows a special effect on the system. Yang et al.17 investigated the special effect of a microwave on the calcium sulfate (CaSO4) crystallization. The phase and figure of CaSO4 crystals were quite different between microwave heating and traditional heating. The microwave irradiation is not rare in oxidative degradation of lignin,18−20 but the cracking of lignin is not very efficient because of the stable structure. Ouyang et al.21 investigated the oxidative degradation of lignin under microwave irradiation with H2O2, CuO, and Fe2(SO4)3 as the oxidants, obtaining a degradation rate of 90.9% and monophenolic compound yield of 11.9%. Cu2+ was capable of promoting the cleavage of the side chain and ether bond, whereas Fe3+ was capable of enhancing the oxidation ability of H2O2, indicating that metal cations had an important effect on the oxidative degradation. The effect of other cations, however, has not yet been systematically studied. Moreover, the mechanism of lignin degradation under microwave heating is now ambiguous, limiting its large-scale application.22 Extending the research to other model compounds and exploring the regularity and mechanism for the oxidative degradation would be significant for the utilization of lignin. In this study, 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol were selected as the lignin model Received: April 7, 2015 Revised: June 18, 2015

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DOI: 10.1021/acs.energyfuels.5b00735 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels compounds, representing the β-O-4 structures, hydroxyl in side chains, and phenolic hydroxyl in lignin. A total of 5 kinds of organic solvent and water were employed as mixed solvents, and 14 kinds of metal salts were used as the catalysts with the assistance of microwave irradiation. Li, Na, and K were chosen to represent the alkali metals; Mg, Ca, and Ba were chosen to represent the alkaline earth metals; and Cr, Mn, Fe, Co, Ni, Cu, Zn, and Cd were chosen to represent the transition elements. The catalytic activities of metal salts were evaluated; the relationship between catalytic activity and pOH of metal salt aqueous solutions was discussed; the product distributions of 4hydroxybenzyl alcohol, vanillyl alcohol, and 2-phenoxy-1phenylethanol oxidative degradation were obtained; and the reaction mechanisms for the oxidative degradation of 4hydroxybenzyl alcohol, vanillyl alcohol, and 2-phenoxy-1phenylethanol were proposed. These results provided insights for the microwave-assisted oxidative degradation of the lignin model compounds with metal salts.

A total of 9.0 g of compound 1 (45 mmol) and phenol (5.0 g, 53 mmol) were dissolved in 200 mL of N,N-dimethylformamide (DMF), mixed with KOH (3.0 g, 53 mmol), and stirred overnight at room temperature to produce compound 4 (2-phenoxy-1-phenylethanone). The product was then extracted by ethyl ether and H2O, dried over Na2SO4, and recrystallized from ethanol. Then, 2.5 g of compound 4 (11 mmol) and 0.2 g of compound 5 (5.5 mmol) were dissolved in 100 mL of methanol and stirred for 2 h. A saturated solution of ammonium sulfate (200 mL) followed by CHCl3(200 mL) was used to purify the product. The organic layer was separated, washed by water (100 mL, 2 times), dried, and recrystallized from ethanol (compound 6 was obtained). 2.4. Experimental Procedure. In a typical experimental run for lignin degradation, 0.05 g of lignin model compound, 0.005 g of metal salts, 2.5 g of H2O, 2.5 g of organic solvent, and 1 g of H2O2 were added to a 10 mL microwave reactor. The Discover System was started, then the reactor was heated to 80 °C and held for 10 min with the microwave irradiation of 50 W. The products in the reactor were rinsed to a 25 mL volumetric flask with organic solvent and filtered by a 0.45 μm filter membrane. The samples were analyzed by highperformance liquid chromatography (HPLC). The controlled experiments were carried out in a glass jacket reactor with a volume of 25 mL, heated by a water bath. After the temperature of the water bath cabinet was stable at 80 °C, 0.05 g of lignin model compound, 0.005 g of metal salts, 2.5 g of H2O, 2.5 g of organic solvent, and 1 g of H2O2 were added to the jacket reactor and then the reactor was connected with the constant temperature water bath cabinet. After the desired reaction time, the reactor was disconnected from the water bath cabinet and the products in the reactor were rinsed to a 25 mL volumetric flask with organic solvent and filtered by a 0.45 μm filter membrane. The product samples were analyzed by HPLC. For the experiments for gas chromatography/mass spectrometry (GC/MS) analysis, 0.05 g of lignin model compound, 0.005 g of CrCl3, 2.5 g of H2O, 2.5 g of acetonitrile, and 1 g of H2O2 were added to a 10 mL microwave reactor. The Discover System was started, then the reactor was heated to 80 °C and held for 10 min with the microwave irradiation of 50 W. Excessive NaCl was added to the product, and the aqueous layer was separated. The organic layer was analyzed by GC/MS. 2.5. Analysis. The product samples were analyzed by an Agilent high-pressure liquid chromatograph 1260 (HPLC 1260 Infinity) equipped with a variable wavelength detector. The column was a 4 mm inner diameter × 250 mm KNAUER C18 reverse-phase column; the temperature of column was set at 35 °C; and 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 (30% B) → 20 min (30% B) → 45 min (80% B) → 60 min (20% B). Product identification was achieved by comparison of retention times to those of standard solutions of pure compounds, and identities were confirmed by GC/MS (Agilent 6890GC/5973MSD). The pH of the metal salt solutions was measured by a pH meter (PHSJ-4A). A total of 5 g/L aqueous solution of each metal salt was prepared for the determination of pH.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium chloride (NaCl, AR, ≥99.5%), potassium chloride (KCl, AR, ≥99.5%), barium chloride (BaCl2, AR, ≥99.5%), ferric chloride (FeCl3, ≥97.0%), cupric chloride (CuCl2, ≥99.0%), zinc chloride (ZnCl2, ≥98.0%), cadmium chloride (CdCl2, AR, ≥99.0%), and hydrogen peroxide (H2O2, ≥30%) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Lithium chloride (LiCl, ACS, ≥99.0%), magnesium chloride (MgCl2, ACS, 99.0− 102%), chromium chloride hexahydrate (CrCl3·6H2O, AR, ≥98.0%), manganese(II) chloride (MnCl2, AR, ≥99.0%), and 4-hydroxybenzyl alcohol (≥97.0%) were purchased from Aladdin Industrial Corporation, China. Calcium chloride (CaCl2, ≥96.0%) and acetonitrile (AR, ≥99.0%) were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. Nickel(II) chloride (NiCl2, ≥98.0%) and 4-hydroxy3-methoxybenzyl alcohol (vanillyl alcohol, ≥98.0%) were purchased from Alfa Aesar, a Johnson Matthey Company, China. Cobalt chloride (CoCl2, ≥98.0%) was purchased from Dongfeng Chemical Plant, Jiangsu, China. All of the chemicals above were used as received without any purification or pretreatment. 2-Phenoxy-1-phenylethanol was synthesized following the procedure of Strassberger et al.23 2.2. Apparatus. The microwave reactor (Discover System) was purchased from CEM, Matthews, NC. The pH meter (PHSJ-4A) and superconstant temperature water bath (CH1015) were purchased from INESA Instrument, Shanghai, China. CEM Discover adopted focused circular automatic coupling singlemode microwave technology, ensuring reproducible reaction conditions and results, making the energy density of the single-mode microwave field up to 900 W/L. The instrument includes five parts: main apparatus DU9046, infrared temperature control system, IntelliVent pressure control system, magnetic stirring system, and 10 mL reaction system. The frequency of microwave irradiation is 2450 MHz, and the maximum output microwave power is 300 W. The maximum temperature of the infrared temperature control system is 300 °C, and the temperature accuracy is ±0.1 °C. IntelliVent ensures operator safety by offering an automated overpressure venting capability. Reactions that exceed 300 psi (20 bar) are automatically and safely vented in a controlled manner before the operator can access the vial. The accuracy of the IntelliVent pressure control system is ±1 psi. 2.3. Synthesis of 2-Phenoxy-1-phenylethanol. 2-Phenoxy-1phenylethanol was synthesized following the procedure of Strassberger et al.23

3. RESULTS AND DISCUSSION 3.1. Evaluation of Organic Solvents for Oxidative Degradation of Lignin Model Compounds. In the oxidative degradation of lignin model compounds, H2O and B


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1-phenylethanol degradation was benzoic acid and 2-hydroxy-1phenylethanone. Table 1 shows the effects of different types of metal salts on the degradation of 2-phenoxy-1-phenylethanol and the selectivity to benzoic acid and 2-hydroxy-1-phenylethanone at 80 °C. The overall yields of degradation with CrCl3, MnCl2, and CuCl2 were higher than the others (58.6, 38.8, and 52.6%). In the experiments with LiCl and CdCl2, the selectivities were very high (93.1 and 111.0%) but the conversions of 2-phenoxy-1-phenylethanol were low (18.2 and 3.4%). The conversions of 2-phenoxy-1-phenylethanol in the experiments with NaCl, KCl, MgCl2, CaCl2, BaCl2, FeCl3, and ZnCl2 were high (>40%), but the overall selectivities were relatively low (≤50.7%). In the experiments with CoCl2 and NiCl2, both the conversions of 2-phenoxy-1-phenylethanol (15.7 and 12.5%) and the overall selectivities (34.0 and 16.5%) were low. The main products of vanillyl alcohol degradation were vanillin and vanillic acid. Table 2 shows the effects of different types of metal salts on the degradation of vanillyl alcohol and the selectivity to vanillin and vanillic acid at 80 °C. The overall yields of degradation with CrCl3, MnCl2, and CoCl2 were higher than the others (71.3, 61.8, and 37.5%). In the experiments with FeCl3 and CuCl2, the conversions of vanillyl alcohol were very high (100.5 and 99.7%) but the overall selectivities were low (7.4 and 8.2%). The overall selectivities in the experiments with MgCl2, CaCl2, BaCl2, NiCl2, and ZnCl2 were high (≥58.0%), but the conversions of vanillyl alcohol were low (≤22.6%). In the experiments with LiCl, NaCl, KCl, and CdCl2, both the conversions of vanillyl alcohol (≤14.1%) and the overall selectivities (≤40.8%) were low. The main products of 4-hydroxybenzyl alcohol degradation were hydroquinone, 4-hydroxybenzaldehyde, and 4-hydroxybenzonic acid. Table 3 shows the effects of different types of metal salts on the degradation of 4-hydroxybenzyl alcohol and the selectivities to hydroquinone, 4-hydroxybenzaldehyde, and 4-hydroxybenzonic acid at 80 °C. The overall yields of degradation with CaC2, CrCl3, MnCl2, and ZnCl2 were higher than the others (41.9, 62.6, 33.8, and 30.4%). In the experiments with FeCl3 and CuCl2, the conversions of 4-

organic solvent (1:1, v/v) was used as a mixed solvent. Figure 1 shows the evaluation of organic solvents for oxidative

Figure 1. Conversion of 2-phenoxy-1-phenylethanol in different solvents at 80 °C under microwave irradiation for 10 min.

degradation of lignin model compounds. In the evaluation, acetonitrile, dioxane, methanol, and ethanol were selected as the organic solvent and an equal amount of water was added to the reaction system instead of the organic solvent as a control experiment. 2-Phenoxy-1-phenylethanol was used as the lignin model compound. The conversion of 2-phenoxy-1-phenylethanol was 88% for acetonitrile and H2O as the mixed solvent, which is the highest in the evaluation. Acetonitrile and methanol had a positive effect on the oxidative degradation. Dioxane and ethanol had a negtive effect on the degradation compared to water. It indicates that acetonitrile was the optimal organic solvent for the degradation of 2-phenoxy-1-phenylethanol. Therefore, acetonitrile was employed for the organic part of the mixed solvent in the following experiments. 3.2. Evaluation of Metal Salts for Degradation of Lignin Model Compounds. The main product of 2-phenoxy-

Table 1. Evaluation of Metal Salts for 2-Phenoxy-1-phenylethanol Degradation under Microwave Irradiation (at 80 °C for 10 min)

catalyst

H2O2 conversion (%)

2-phenoxy-1-phenylethanol conversion (%)

LiCl NaCl KCl MgCl2 CaCl2 BaCl2 CrCl3 MnCl2 FeCl3 CoCl2 NiCl2 CuCl2 ZnCl2 CdCl2

75.5 71.2 75.2 70.9 65.0 65.0 85.0 78.5 99.9 4.6 12.1 98.7 63.1 5.4

18.2 57.3 53.0 42.9 55.6 41.6 55.5 45.0 53.5 15.7 12.5 93.4 55.3 3.4

2-hydroxy-1-phenylethanone yield (%)

benzoic acid yield (%)

6.9 4.3 2.3 6.4 9.1 4.4 25.3 11.6 12.0 2.1 1.2 39.7 9.5 1.5

9.2 9.3 6.7 11.6 14.6 7.9 13.0 23.9 2.0 2.3 0.0 12.5 14.7 1.4

2-phenoxy-1-phenylethanone yield (%)

benzoic acid selectivity (%)

overall selectivitya (%)

overall yieldb (%)

0.9 0.0 0.0 2.3 4.5 2.4 20.3 3.3 2.1 0.9 0.8 0.4 2.2 0.9

50.5 16.3 12.7 27.0 26.2 19.0 23.5 53.0 3.8 14.8 0.0 13.4 26.6 40.5

93.1 23.8 17.0 47.3 50.7 35.4 105.6 86.1 30.2 34.0 16.5 56.3 47.8 111.0

16.9 13.6 9.0 20.3 28.2 14.7 58.6 38.8 16.2 5.3 2.1 52.6 26.4 3.8

a

Overall selectivity shows the overall selectivity for 2-hydroxy-1-phenylethanone, benzoic acid, and 2-phenoxy-1-phenylethanone. bOverall yield shows the overall yield for 2-hydroxy-1-phenylethanone, benzoic acid, and 2-phenoxy-1-phenylethanone. C


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Table 2. Evaluation of Metal Salts for Vanillyl Alcohol Degradation under Microwave Irradiation (at 80 °C for 10 min)

a

catalyst

H2O2 conversion (%)

vanillyl alcohol conversion (%)

vanillin yield (%)

vanillic acid yield (%)


overall yieldb (%)


2.0 5.0 0.0 0.3 1.9 1.8 30.1 5.2 100.2 5.7 1.2 77.0 5.6 2.0

10.2 4.7 1.7 12.7 22.6 6.7 94.2 89.6 100.5 48.1 12.6 99.7 18.5 14.1

0.0 0.6 0.2 6.4 12.4 4.9 60.6 49.4 6.1 33.9 10.3 2.8 11.8 5.5

2.2 0.5 0.4 2.6 0.7 0.4 10.6 12.4 1.3 3.6 2.2 5.4 2.0 0.3

22.0 24.2 33.3 71.2 58.0 79.2 75.6 69.0 7.4 77.9 99.5 8.2 74.3 40.8

2.2 1.1 0.6 9.0 13.1 5.3 71.3 61.8 7.4 37.5 12.5 8.2 13.8 5.8

Overall selectivity shows the overall selectivity for vanillin and vanillic acid. bOverall yield shows the overall yield for vanillin and vanillic acid.

Table 3. Evaluation of Metal Salts for 4-Hydroxybenzyl Alcohol Degradation under Microwave Irradiation (at 80 °C for 10 min) catalyst

H2O2 conversion (%)

4-hydroxybenzyl alcohol conversion (%)

hydroquinone yield (%)

4-hydroxybenzaldehyde yield (%)

4-hydroxybenzonic acid yield (%)


overall yieldb (%)


25.0 28.0 24.4 18.5 29.9 15.4 35.9 91.1 100.2 100.2 80.6 79.2 16.8 14.2

9.5 11.9 13.4 6.4 60.2 8.1 64.3 34.6 100.0 0.7 6.3 99.4 30.3 15.8

0.0 0.0 0.0 0.0 20.0 0.9 33.2 16.8 0.0 0.0 0.0 7.7 15.3 4.4

1.9 2.7 2.0 1.5 18.2 0.8 22.6 15.3 0.8 0.6 1.1 2.7 13.2 5.5

1.5 1.4 1.4 1.2 3.8 1.6 6.8 1.7 3.1 0.2 0.1 6.9 1.9 1.2

35.5 34.8 25.5 40.7 69.7 41.0 97.4 97.9 3.9 114.5 19.4 17.4 100.2 69.6

3.4 4.1 3.4 2.6 41.9 3.3 62.6 33.8 3.9 0.8 1.2 17.3 30.4 11.0

a

Overall selectivity shows the overall selectivity for hydroquinone, 4-hydroxybenzaldehyde, and 4-hydroxybenzonic acid. bOverall yield shows the overall yield for hydroquinone, 4-hydroxybenzaldehyde, and 4-hydroxybenzonic acid.

Figure 2. Relationships between the pOH and the conversion of lignin model compounds at 80 °C under microwave irradiation for 10 min.

experiments with LiCl, NaCl, KCl, MgCl2, BaCl2, and NiCl2, both the conversions of 4-hydroxybenzyl alcohol (≤13.4%) and the overall selectivities (≤41.0%) were low. The metal salts evaluated in this work showed large differences in catalytic activity for the degradation of 2-

hydroxybenzyl alcohol were very high (100.0 and 99.4%) but the overall selectivities were low (3.9 and 17.4%). The overall selectivities in the experiments with CoCl2 and CdCl2 were high (114.5 and 69.6%), but the conversions of 4hydroxybenzyl alcohol were low (0.7 and 15.8%). In the D


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Figure 3. Comparison of conversion of lignin model compounds with or without microwave irradiation.

tional heating. The microwave irradiation accelerated the oxidative degradation dramatically. 3.4. Product Distribution and Mechanism. The samples prepared for the GC/MS analysis used CrCl3 as the catalyst, H2O/acetonitrile (1:1, v/v) as the mixed solvent, and 80 °C for 10 min as the reaction condition. In the 2-phenoxy-1-phenylethanol reaction system, the products were identified as benzoic acid, 2-hydroxy-1-phenylethanone, 2-phenoxy-1-phenylethanone, phenol, etc. 2-Phenoxy-1-phenylethanol, benzoic acid, 2-hydroxy-1-phenylethanone, 2-phenoxy-1-phenylethanone, and phenol were quantified by their corresponding standard curves. Other peaks in the chromatography were summed up and quantified by the standard curve of 2-phenoxy-1-phenylethanol, which allowed us to estimate their yields. The product distributions at 80 °C for 10 min reaction time are shown in Table 4. On the basis of the

phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol, which may be caused by the properties of the metal salts, including acidity and ionization constant. The pH of the solution of each metal salts were measured by a pH meter, as described in section 2.5. Figure 2 shows the relationships between the pOH and the conversion of lignin model compounds. Most of the metal salts show an interesting trend: as the acidity of the metal salt solution increased, the conversion of the lignin model compounds increased. The four most acidic solutions of metal salts are those of CrCl3, MnCl2, FeCl3, and CuCl2. Correspondingly, NaCl, CrCl3, and CuCl2 have the best catalytic performance for the conversion of 2phenoxy-1-phenylethanol; the catalytic activities of CrCl3, MnCl2, FeCl3, and CuCl2 were the highest for the conversion of vanillyl alcohol; and the catalytic activities of CrCl3, FeCl3, and CuCl2 were the highest for the conversion of 4hydroxybenzyl alcohol, indicating that high acidity of metal salt solutions is in favor of the degradation of the lignin model compounds. 3.3. Role of Microwave Irradiation. The microwave reactor starts timing when the reaction system gets the temperature settled, while we started timing when the water circulation was open in the conventional heating. From the heating curve for the conventional heating reaction, the temperature of the reaction system reached 80 °C within 120 s. Therefore, to ensure that the reaction time was the same for the microwave and conventional heating reactions, the time for the conventional heating reactions was 12 min, in comparison to 10 min for microwave heating reactions. Figure 3 shows the comparison of conversions of the three lignin model compounds with or without microwave irradiation. The reaction temperature was maintained at 80 °C, and H2O/acetonitrile (1:1, v/v) was used as the solvent. When using CrCl3, MnCl2, and CuCl2 as the catalyst, the conversions of 2-phenoxy-1-phenylethanol under the microwave irradiation were 55.5, 45.0, and 93.4%, while those under the conventional heating were only 29.7, 9.7, and 68.2%. For vanillyl alcohol, the conversions under microwave irradiation with these three kinds of catalysts were 94.2, 89.6, and 99.7%, while those under the conventional heating were only 49.5, 55.4, and 65.9%. For 4-hydroxybenzyl alcohol, the conversions under microwave irradiation with these three kinds of catalysts were 64.3, 34.6, and 99.6%, while those under the conventional heating were only 24.3, 14.5, and 89.2%. For the other catalysts, the conversions of the lignin model compounds were much higher under microwave irradiation than those under conven-

Table 4. Product Distribution of 2-Phenoxy-1-phenylethanol Degradation (at 80 °C for 10 min)

product distribution, a possible mechanism is proposed in Figure 4. 2-Phenoxy-1-phenylethanol is oxidized to produce 2phenoxy-1-phenylethanone, and then, the β-O-4 bond in 2phenoxy-1-phenylethanone cleavages in two pathways: between the β-C and O atoms and between the α-C and β-C atoms, with 2-hydroxy-1-phenylethanone and phenol as major products for the former pathway and benzoic acid as the major product for the latter pathway. The yield of 2-hydroxy-1phenylethanone was more than twice the yield of benzoic acid, indicating that the cleavage of the bond between β-C and O E


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Table 6. Product Distribution of 4-Hydroxybenzyl Alcohol Degradation (at 80 °C for 10 min)

Figure 4. Mechanism of 2-phenoxy-1-phenylethanol oxidative degradation.

atoms is easier than that between the α-C and β-C atoms. The reaction pathways are similar to the results from Crestini et al.24 In their research, β-O-4 dimeric compound showed cleavage at both α and β positions. In the vanillyl alcohol reaction system, the products were identified as vanillin, vanillic acid, 5-hydroxyl-4-methoxyl-7ketone-4-heptylic acid, etc. Vanillyl alcohol, vanillin, and vanillic acid were quantified by their corresponding standard curves. 5Hydroxyl-4-methoxyl-7-ketone-4-heptylic acid in the chromatography was quantified by the standard curve of vanillyl alcohol, which allowed us to estimate its yield. The product distributions at 80 °C for 10 min reaction time are shown in Table 5. On the basis of the product distribution, a possible

also detected by GC/MS. On the basis of the product distribution, a possible reaction mechanism for the oxidative degradation of 4-hydroxybenzyl alcohol is proposed in Figure 6.

Figure 6. Mechanism of 4-hydroxybenzyl alcohol oxidative degradation.

Table 5. Product Distribution of Vanillyl Alcohol Degradation (at 80 °C for 10 min)

4-Hydroxybenzyl alcohol was oxidized to produce 4-hydroxybenzaldehyde and 4-hydroxybenzonic acid. Phenol, formed by the decarboxylation of 4-hydroxybenzonic acid, reacted with hydroxy radicals, which were formed by microwave irradiation of H2O2, to produce hydroquinone. The yields of hydroquinone and 4-hydroxybenzaldehyde were higher than that of 4-hydroxybenzonic acid, suggesting that the first step of oxidation and the decarboxylation are more likely to occur than the oxidation to produce acid. There have been reports about free-radical reactions with metal compound catalyst and H2O2 as the oxidant.25,26 The metal salts that showed better catalytic activity (CrCl3 and MnCl2) were both multivalent elements. It is possible that these elements would be oxidized to a higher chemical valence. The ions would react with H2O2 to produce hydroxyl radicals, which can react with the lignin model compounds, as follows: chain initiation

mechanism is proposed in Figure 5. Vanillyl alcohol was oxidized to produce vanillin and vanillic acid, and then vanillic acid experiences a ring-opening reaction to produce 5-hydroxyl4-methoxyl-7-ketone-4-heptylic acid. The yield of vanillin was much higher than the others, which indicates that the first step of the oxidation is more likely to occur than the following steps. In the 4-hydroxybenzyl alcohol reaction system, the products were identified as hydroquinone, 4-hydroxybenzaldehyde, 4hydroxybenzonic acid, etc. Hydroquinone, 4-hydroxybenzaldehyde, and 4-hydroxybenzonic acid were quantified by their corresponding standard curves. Their distributions at 80 °C for 10 min are shown in Table 6. A small amount of phenol was

Men + + HOOH → Me(n + 1) + + OH− + HO• Me(n + 1) + + RCH 2OH → Men + + RC•HOH + H+

propagation RCH 2OH + HO• → RC•HOH + H 2O

Figure 5. Mechanism of vanillyl alcohol oxidative degradation. F


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termination •

•

RC HOH + HO → RCHO + H 2O

The ions with lower valence would react with H2O2 to produce hydroxyl radicals. The ions with higher valence would capture an electron from alcohols to produce aromatic radicals. Hydroxyl radicals would react with alcohols to produce aromatic radicals. Finally, the aromatic radicals would react with hydroxyl radicals to produce aldehydes. For the oxidation of 2-phenoxy-1-phenylethanol, the model compound was oxidized to ketone. For the oxidation of vanillyl alcohol and 4-hydroxybenzyl alcohol, the model compounds were first oxidized to aldehydes and then acids. The mechanism with OH• is shown as follows: RCH 2OH + HO• → RC•HOH + H 2O RC•HOH + HO• → RCHO + H 2O

RCHO + HO• → RC•O + H 2O

RC•O + HO• → RCOOH

4. CONCLUSION The combination of metal salts and microwaves was effective for the oxidative degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol. Microwave irradiation could accelerate the oxidative degradation rate of the three kinds of lignin model compounds. CrCl3 and MnCl2 were the most effective metal salts for the degradation of 2phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol. The acidity of metal salt solutions promoted the oxidative degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol. Possible mechanisms for the oxidative degradation of 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol are proposed on the basis of the product distributions.

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ASSOCIATED CONTENT

S Supporting Information *

GC/MS chromatography of 2-phenoxy-1-phenylethanol (Figure S1), vanillyl alcohol (Figure S2), and 4-hydroxybenzyl alcohol (Figure S3) (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b00735.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/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 (21476204 and 21306165) and the Zhejiang Provincial Natural Science Foundation of China (LZ14B060002 and LQ13B060001).

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REFERENCES

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Microwave-Assisted Oxidative Degradation of Lignin Model

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