Direct Production of Vanillin from Wood Particles by Copper Oxide

Research Article pubs.acs.org/journal/ascecg

Direct Production of Vanillin from Wood Particles by Copper Oxide− Peroxide Reaction Promoted by Electric and Magnetic Fields of Microwaves Chen Qu,†,∥ Masakazu Kaneko,†,∥ Keiichiro Kashimura,‡ Kanade Tanaka,§ Suguru Ozawa,§ and Takashi Watanabe*,† †

Laboratory of Biomass Conversion, Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Kyoto 611-0011, Japan ‡ Information Technology Section, General Education Division, College of Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai, Aichi 487-8501, Japan § Fundamental Technology Development Centre, Materials Development Team, Teijin Limited, 2345 Nishihabumachi, Matsuyama, Ehime 791-8536, Japan ABSTRACT: Production of biobased functional polymers from woody biomass is becoming increasingly important for biorefineries. We studied the direct production of vanillin and vanillic acid, which are key monomer components for thermostable polymers, from a softwood Japanese cedar by an alkaline copper oxide−peroxide reaction using microwave (MW) and conventional heating (CH) reactors. MW heating at 180 °C for 10 min produced vanillin and vanillic acid in yields nearly three times higher than those produced by conventional heating (10.1% vs 3.4%). The MW and CH experiments were precisely compared using the same heating conditions and the same reaction vessel. A cavity resonator (single-mode microwave) which can separate electric (E) and magnetic (H) fields were used for wood degradation. The results revealed that the reactions were accelerated by both fields with a slightly more prominent effect of electric fields (Emax). The activation energy of experiments under CH, MW-Emax, and MW-Hmax was calculated. The yield enhancement and decrease of activation energy unequivocally indicate the MW-sensitive character of this reaction. When hardwood (Eucalyptus globulus) was used as a feedstock, syringaldehyde and syringic acid were produced together with vanillin and vanillic acid, and the maximum yield of the monomers reached 11.4% based on the original wood weight under MW heating. This method was successfully scaled up by using a 1-L scale MW reactor to give vanillin and vanillic acid at a total yield of 8.5%. KEYWORDS: Microwave effect, Lignin, Vanillin, Copper oxide

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INTRODUCTION The increasing levels of atmospheric carbon dioxide and the decline of crude oil production have stimulated the investigation of alternative carbon feedstocks such as lignocellulosic biomass. Lignin, along with cellulose and hemicellulose, are the three major components of lignocellulosic biomass. Lignin is a heterogeneous aromatic polymer composed of three phenylproane units connected by C−O−C and C−C bonds. Among the interunit bonds in lignin, β−O−4 is the most abundant linkage structure, accounting for ca. 40− 60% of the lignin linkages in spruce wood. Recently, the role of lignin in biorefineries has received considerable interest as a source of renewable aromatic chemicals with a wide range of structural diversity and added values. Among them, vanillin (4hydroxy-3-methoxybenzaldehyde) and its oxidation product vanillic acid (4-hydroxy-3-methoxybenzoic acid) have received attention in polymer chemistry. Thus, the incorporation of vanillin and vanillic acid structures to polymer chains provides © 2017 American Chemical Society

them with high thermostability and good mechanical strength characteristics comparable with those of petroleum-derived engineering plastics such as polyphenylene sulfide or polybutylene terephthalate.1 It is also reported that vanillin can be used as a biobased building-block for the synthesis of monomers bearing epoxy, cyclic carbonates, allyl, amine, alcohol, and carboxylic acid moieties for the synthesis of epoxy, polyester, polyurethane, and nonisocyanate polyurethane polymers.2 Biobased epoxy polymers with outstanding thermo-mechanical properties can be directly prepared by a lignin-to-vanillin process. The method, developed by Fache et al.,3 contains no environmentally damaging purification steps. Numerous strategies have been studied on lignin depolymerization for vanillin.4,5 Alkaline nitrobenzene oxidation is reported Received: August 19, 2017 Revised: October 13, 2017 Published: October 26, 2017 11551

DOI: 10.1021/acssuschemeng.7b02883 ACS Sustainable Chem. Eng. 2017, 5, 11551−11557

Research Article

ACS Sustainable Chemistry & Engineering

Table 1. Vanillin and Vanillic Acid Yields for the Degradation of Japanese Cedar Wood with Various Catalysts in a Milestone Reactora Entry

Catalyst/Oxidant

Vanillic acid (%)

Vanillin (%)

Total (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

CuO (0.15 g) CuSO4 (2.60 g) MnSO4 (3.87 g) ZnO (15 mg) CuO/ZnO (0.15 g/15 mg) CuO (0.16 g)/NaClO4 (1.5 g) Fe2O3 (0.15 g)/H2O2−NaClO (0.1 mL each) FeO (0.15 g)/H2O2/NaClO (0.1 mL each) Cu(OH)2 (0.17 g)/H2O2/NaClO (0.1 mL each) Co3O4 (0.17 g)/H2O2/NaClO (0.1 mL each) Cu2O (0.28 g)/H2O2/NaClO (0.1 mL each) CuO (0.60 g)/H2O2/NaClO (0.1 mL each) V2O3/ZnO (0.35 g/5 mg)/H2O2/NaClO (0.1 mL each) NiO/ZnO (0.18 g/5 mg)/H2O2/NaClO (0.1 mL each) MnO (5 mg)/H2O2 (0.5 mL) MnO/Pd (5 mg/5 mg)/H2O2 (3 mL) MnO/ZrO (5 mg/5 mg)/H2O2 (3 mL) MnO/Ni (5 mg/5 mg)/H2O2 (3 mL) MnO/Fe (5 mg/5 mg)/H2O2 (3 mL) MnO/Mo (5 mg/5 mg)/H2O2 (3 mL) MnO/Si (5 mg/5 mg)/H2O2 (3 mL) MnO/Pd (5 mg/5 mg)/H2O2 (3 mL) MnO/Pd−Ti (5 mg/5 mg)/H2O2 (3 mL) MnO/Pd−Ge (5 mg/5 mg)/H2O2 (3 mL) CuO (0.5 g)/H2O2 (4 mL) CuBr/Pd (0.1 g/5 mg)/H2O2 (4 mL)

0.5 0.7 0.3 0.03 0.1 0.8 0.1 0.1 1.2 0.9 0.9 1.1 0.6 0.8 0.9 1.2 1.2 1.1 1 1.1 1.1 1.0 1.1 1.0 2 0.7

1.7 2.1 0.9 0.12 0.7 3.2 0.8 0.8 2.3 1.9 3 3.5 2.1 1.2 1.8 3.3 2.2 1.6 1.6 2.1 1.9 1.7 2.4 2.8 4.2 2.1

2.4 2.8 1.2 0.2 0.8 4 0.9 0.9 3.5 2.8 3.9 4.6 2.7 2 2.7 4.5 3.4 2.7 2.6 3.2 3.1 2.7 3.5 3.8 6.2 2.8

a

Reaction conditions: Entry 1: 0.25 g of Japanese cedar wood and 5 mL of a 2N NaOH solution; Entries 2−26: 1 g of Japanese cedar wood, 20 mL of a 2N NaOH solution, 170 °C, and 80 min.

provides the first unequivocal evidence for production of the lignin monomers from wood accelerated by microwave irradiation.

as the most efficient method for lignin degradation for vanillin production; however, nitrobenzene is harmful and not suitable for industrial approach. Lignin depolymerization by homogeneous metal catalysts including Co, Cu, or V complex was studied, though the phenolic monomers yield from lignin is still generally low. The metal catalysts in the presence of oxygen (O2) or H2O2 solution as an oxidant were also used for the conversion of lignins into phenolic monomers.6−9 Ouyang et al. reported that alkaline lignin separated from wheat straw was depolymerized using H2O2 and CuO/Fe2(SO4)3/NaOH. The total yield of monophenol mixtures from the alkaline lignin in 50% aq. methanol was 17.92%.10,11 Microwave (MW) irradiation has been used in organic chemistry, and MW effects in organic synthesis have also been reported.12,13 The MW-assisted heating was reported in polar solvents in the presence of H2O2 and heterogenenous catalysts. For instance, a mesopourous catalyst La/SBA-15 gave vanillin in a yield 9.94% from organosolv beech wood lignin.14 MW reactions have also been used for producing aromatic chemicals from lignin or biomass pretreatment to increase the enzymatic saccharification yield.15,16 However, in the previous studies, microwave effects were not clearly shown because reaction vessels used were different between the microwave and conventional heating. In this study, a MW-assisted reaction system producing vanillin and vanillic acid directly from wood particles with high productivity and industrial feasibility was developed. The effects of electric (E) and magnetic (H) fields on the reaction yields were analyzed using a cavity resonator and precisely compared with conventional heating. To our best knowledge, this study

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RESULTS AND DISCUSSION MW-Assisted Wood Degradation for Vanillin Production. Industrially, lignin from the sulfite pulping process has been one of the major routes for the vanillin production. The counterion (e.g., sodium, calcium, potassium, magnesium, or ammonium) can alter the behavior of the lignin product.17 Recently, direct production of vanillin from wood is becoming more important from the viewpoint of biorefineries. The wood cell wall is composed of cellulose, hemicelluloses, and lignin. The tight packing structure of cell walls hinders the access of catalysts to the components, making the production of lignin monomers from wood more difficult. A new industrially feasible process to directly obtain vanillin from wood is needed. Herein, we extensively surveyed catalysts, oxidants, and solvents and found that the MW-assisted reaction of CuO with H2O2 under alkaline media produced vanillin and vanillic acid from wood at high yields (Table 1). The wood degradation products were characterized by GC-MS (Figure 1). Vanillin was the major aromatic monomer from wood degradation (Figure 2), while guaiacol, acetovanillon, and vanillic acid were also detected. The highest total yield of vanillin and vanillic acid based on dry weight of the wood (6.5%) was obtained when a milestone multimode MW reactor was applied to the reaction at 200 °C for 20 min. The MW degradation of woody biomass (170 °C, 80 min) produced the lignin monomer at a yield ca. two times higher as compared to conventional heating (4.7% vs 2.1%) at 11552


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production of singlet oxygen did not assist increasing vanillin yield.19,20 Various kinds of transition metal catalysts have been reported for lignin degradation. They play an important role in the paper pulping industry to remove lignin.15,16 The most frequently used catalysts for lignin oxidation include copper(II) oxide (CuO), cupric sulfate (CuSO4), ferric chloride (FeCl3), and iron(III) oxide (Fe2O3). A series of catalysts including CuO, cupric hydroxide (Cu(OH)2), copper(I) oxide (Cu2O), CuSO4, zirconium oxide (ZnO), cobalt (II, III) oxide (Co3O4), manganese(II) sulfate monohydrate (MnSO4·H2O), Fe2O3, iron(II) oxide (FeO), vanadium(III) oxide (V2O3), nickel(II) oxide (NiO), copper(I) bromide (CuBr), Pd, Ni, Fe, Mo, and Si were used for wood degradation with or without oxidants (H2O2 or H2O2/NaClO) under MW irradiation at 170 °C for 80 min (Table 1). Among the various metal catalysts tested, Cu complexes gave higher yields than the other metal catalysts. Especially, CuO combined with a H2O2 solution as an oxidant showed the best yield (6.2%) among the oxidants studied herein. The reaction temperature, time, and amount of reactant were also screened in this study (Tables 3−5). Higher temperatures

Figure 1. GC-MS analysis of acetylated wood degraded by a MWassisted alkaline CuO−H2O2 reaction at 200 °C for 10 min in a milestone microwave reactor.

Table 3. Vanillin and Vanillic Acid Yields after Japanese Cedar Wood Degradation at Different Reaction Temperatures in a Milestone MW Reactora

Figure 2. Structures of target lignin monomers produced by wood degradation.

the same reaction conditions. The high yields of the CuO− H2O2 reaction were comparable to those obtained under alkaline nitrobenzene oxidation (NBO) MW conditions at 170 °C for 80 min (6.2%). NBO is a known standard laboratory method for structural analysis of lignin allowing the production of vanillin and vanillic acid from softwood, and vanillin, vanillic acid, syringaldehyde, and syringic acid from hardwood at high yields. However, the alkaline NBO degradation of wood is not industrially feasible. It has a number of disadvantages (e.g., deleterious byproducts, high cost, and explosion risk). Lignin oxidation with O2 under alkaline conditions has been intensively studied. One of the limitations of this process is the low solubility of O2 in NaOH solutions.4,5,17 Hence, we used H2O2 solutions instead as an oxidant for wood degradation. Interestingly, H2O2 alone produced higher yields of vanillin and vanillic acid as compared to the mixture of H2O2 and NaClO oxidants during wood degradation (Table 2). The oxidation effect during the degradation of wood followed the order of H2O2 > O2 > NaClO (Table 2). Despite the combination of ClO− and H2O2 results in the generation of singlet oxygen, the

Oxidant

Vanillic acid (%)

Vanillin (%)

Total (%)

1 2 3 4 5

− NaClO O2 H2O2 H2O2/NaClO

0.2 0.3 0.4 1.0 0.7

1.4 1.6 2.7 3.3 2.9

1.6 1.9 3.1 4.3 3.6

Temperature (°C)

Vanillic acid (%)

Vanillin (%)

Total(%)

1 2 3 4 5

170 180 190 200 210

0.7 0.6 1.0 1.1 0.3

3.5 2.8 4.0 5.0 2.6

4.2 3.4 5.0 5.1 2.9

a

Reaction conditions: 1 g of Japanese cedar wood, 20 mL of a 2N NaOH solution, 20 min, 0.2 g of CuO, and 1 mL of a H2O2 30% solution.

Table 4. Effects of Reaction Time on Vanillin and Vanillic Acid Yields from Japanese Cedar Wood in a Milestone MW Reactora Entry

Time (min)

Vanillic acid (%)

Vanillin (%)

Total (%)

1 2 3

10 20 30

1.0 1.1 1.0

3.3 4.4 3.0

4.3 5.5 4.0

a

Reaction conditions: 1 g of Japanese cedar wood, 20 mL of a 2N NaOH solution, 0.6 g of CuO, 1 mL of a H2O2 30% solution, and 200 °C.

Table 2. Vanillin and Vanillic Acid Yields by Degradation of Japanese Cedar Wood Using Various Oxidants in a Milestone Microwave Reactora Entry

Entry

Table 5. Effects of Catalyst and Wood Ratio on Vanillin and Vanillic Acid Yields from Japanese Cedar Wood in a Milestone MW Reactora Entry

Wood amount (g)

CuO (g)/ H2O2 (mL)

Vanillic acid (%)

Vanillin (%)

Total (%)

1 2 3 4

2 1 0.5 0.5

1.2/2 0.6/1 0.6/1 1.2/1

0.4 1.1 1.3 1.4

1.9 4.4 4.5 5.1

2.3 5.5 5.8 6.5

a

Reaction conditions: 1 g of Japanese cedar wood, 20 mL of a 2N NaOH solution, 0.6 g of CuO catalyst, 1 mL of H2O2 or/and NaClO 30% solution as oxidant, 200 °C, and 10 min.

a

Reaction conditions: Entry 1: 40 mL of a 2N NaOH solution; Entries 2−4: 20 mL of a 2N NaOH solution, 20 min, and 200 °C.

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Table 6. Degradation of Hardwood Eucalyptus globules and Beech at Different Reaction Times in a Milestone Reactora

led to higher vanillin and vanillic acid yields at same reaction time. However, temperatures higher than 200 °C led to a decrease in vanillin and vanillic acid yields, probably due to degradation of these aromatic molecules. We also found that longer or shorter reaction time did not increase the vanillin yields (Table 4). Thus, reaction for 20 min resulted in the highest vanillin and vanillic acid yields at 200 °C. The higher weight ratio of catalyst to wood powder increased the vanillin yield (Table 5). The effects can be explained by the higher contact frequency of catalysts to lignin. Thus, the highest total yield of vanillin and vanillic acid 6.5% were obtained by using a milestone multimode microwave reactor under the following conditions: 0.5 g of Japanese cedar wood, 1.2 g of catalyst CuO, 1 mL of H2O2 solution (30%), 20 mL of 2N NaOH solution, 200 °C irradiation temperature, and 20 min reaction time. The production of vanillin and vanillic acid from lignin is caused by breaking down of aryl ether linkages in β−O−4 bonds and Cα−Cβ cleavage. When dealing with phenolic β− O−4 lignin substructures, the reaction is initiated from a phenoxide ion and proceeds by elimination of a β-ether unit by nucleophilic attack of a HOO− species to a quinonemethide (QM) intermediate and subsequent Cα−Cβ cleavage. For the final vanillin formation step through Cα−C β cleavage, Tarabenko et al.21 proposed a retroaldol cleavage of the side chain, while Girerer et al.22 proposed synchronous cleavage of C−C and C−O bonds after dioxetane formation. Several pathways for the vanillin formation from phenolic and nonphenolic β−O−4 units have been proposed.18 We also successfully up-scaled the CuO−H2O2 process from 1 to 30 g of Japanese cedar wood in a 1 L MW irradiator (Figure 3). The vanillin and vanillic acid yields reached 8.5%

Wood species

Time (min)

Vanillin (%)

Syringaldehyde (%)

Syringic acid (%)

Total (%)

E. globulus

10 20 30 20 30

0.8 1.2 1.0 1.1 1.7

7.7 8.2 8.8 1.2 4.9

1.0 1.2 1.6 0.1 0.5

9.5 10.6 11.4 2.2 7.1

Beech a

Reaction conditions: 20 mL of a 2N NaOH solution, 0.6 g of CuO, and 1 mL of 30% H2O2 were reacted with hardwood particles (1 g) at 200 °C. The amount of vanillic acid was under the HPLC detection range.

MW Irradiation Effects on Wood Degradation. MW irradiation accelerates temperature increase, reaction rates, and product yields depending on the reaction media, catalysts, substrates, and frequency employed. It was reported that MW dramatically reduced the reaction time and produced high reaction yields.23−26 However, the mechanism behind these MW-sensitive effects is still not well understood, and few reports were found evidencing faster lignin degradation rates using the same reaction vessel and temperature profile.14,16 Using the same sealed reaction vessel (Figure 7) and reaction temperature, we comparatively analyzed the production of vanillin and vanillic acid from woody biomass by the CuO− H2O2 reaction under MW heating and conventional heating. We found a more than a two times increase in the production yields under MW irradiation as compared to conventional heating. This new reaction offers a good model for understanding the MW-sensitive effects during biomass degradation. To analyze the effect of MW more in detail, we used a cavity perturbation heating system which separates Emax and Hmax points (Figures 6 and 7). Remarkably, the yields at the Hmax and Emax points were, respectively, 2.38 and 2.97 times higher than that obtained under conventional heating at 180 °C for 10 min. (Figure 4).

Figure 3. Picture of the 1 L-scale microwave reactor.

based on the weight of wood. The high yields obtained using low-cost commodity chemicals such as CuO and H2O2 is attractive from the viewpoint of industrial applications. The yield of phenolic aldehydes from wood depends on the plant species used as a raw feedstock. In addition to softwood Japanese cedar, hardwood Eucalyptus globulus and Japanese beech were used for the wood degradation experiments (Table 6). In contrast to softwood lignin, hardwood lignin is typically comprised of guaiacyl and syringyl unit in varying ratios, thereby resulting in, apart from vanillin and vanillic acid, syringaldehyde and syringric acid as monomeric products. The total yield of vanillin, vanillic acid, syringaldehyde, and syringric acid from Eucalyptus globules reached 11.4% based on the weight of wood. Syringaldehyde is used directly or as a precursor of fine chemicals for pharmaceutical, food, and cosmetic applications. For example, syringaldehyde was reported to exhibit peroxyl scavenging activity.17,18

Figure 4. Comparison of the vanillin and vanillic acid yields by MWH, E and conventional heating in a cavity perturbation heating system.

The Arrhenius equation was used for the analysis of the MW thermal effect by calculating the apparent activation energy from the solution temperature and the kinetic reactivity. The first-order rate constants for the reactions involved in wood degradation at 160, 170, 180, and 190 °C were determined by plotting the natural log of the values of k against 1/T to determine the Arrhenius parameters (Table 7). The apparent activation energy for vanillin production under conventional heating was calculated as Ea = 57.6 kJ/mol. The apparent activation energy decreased upon MW irradiation as compared to conventional heating. In the production of vanillin, the 11554


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covered with a thin layer of Celite to remove insoluble materials. The residue was washed twice with 20 mL of a 2N NaOH solution. All the filtrates were combined and acidified with concentrated hydroxide chloride (35%) to pH less than 2.0. The acidified products were extracted with ethyl acetate, dried over MgSO4, and filtrated. The solvent was evaporated under reduced pressure. The final product was redissolved in methanol for quantitative high-performance liquid chromatography (HPLC) analysis. (Figure 5)

Table 7. Activation Energy Forvanillin and Vanillic Acid Production at Different Heating Conditionsa Heating condition

Vanillin δG (kJ/mol)

Vanillic acid δG (kJ/mol)

Oil bath MW-Hmax MW-Emax

57.6 54.3 50.9

33.9 48.9 15.6

a

Activity perturbation heating system was used; 5 mL of a 2N NaOH solution, 0.3 g of CuO, and 0.25 mL of a 30% H2O2 solution were reacted with Japanese cedar wood particles (0.125 g) at 160, 170, 180, and 190 °C for 10 min.

apparent activation energy followed the order δG conventional heating > δG MW-Hmax > δG MW-Emax. This result explains the higher enhancement effect for vanillin production at Emax points. The apparent activation energy for vanillic acid production also decreased at Emax points. The pre-exponential factor (A) provides a measure of the collision efficiency (i.e., probability of molecular impact). Thus, larger A values can also represent an increase in the reaction rate. However, since the production of vanillin and vanillic acid from wood involve multiple reaction steps (i.e., swelling, impregnation of the oxidative catalysts and solvents in the wood cell walls and cleavage of the ether and Cα−Cβ bonds in lignin), the calculated factor A cannot be used as a probability of molecular impact. The presence of multiple steps also complicates the interpretation of the differential apparent activation effects. However, the higher acceleration effects under E and H fields as compared to conventional heating provide unequivocal evidence of the MW-sensitive character of this reaction. Selective dielectric heating near the reaction site is one possible mechanism to explain the higher yields of vanillin and vanillic acid under MW heating. Additionally, possible changes in the reaction route under conventional heating and MW irradiation conditions can also explain these results. Higher cell wall loosening rates upon dielectric heating of the solvent is also a possible mechanism explaining the above results. The analysis of the reactions of lignin β−O−4 dimer model compounds and the determination of the dielectric parameters of each reactant and reaction media will lead to a further understanding of this unique MW-assisted reaction.

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Figure 5. Flowchart of the alkaline CuO−H2O2 wood degradation process. The wood particles (30 g) were degraded by an alkaline (600 mL) CuO−H2O2 reaction in a large-scale (1 L) MW irradiator (Japan Chemical Engineering and Machinery Co. Ltd., Osaka, Japan and Kyoto Universities, Kyoto, Japan) at 200 °C for 10 min (N = 2). Alkaline Nitrobenzene Oxidation under MW Irradiation. To a 2N NaOH solution (18 g) containing wood particles (1.0 g), 1.2 g (9.75 mmol) of nitrobenzene were added in a screw-cap reaction container and reacted under MW or conventional heating. The products were separated and purified as in the CuO−H2O2 alkaline experiments. Cavity Perturbation Experiments and Conventional Heating Experiments Conditions. A cavity perturbation heating system (Figures 6 and 7) consisted of a waveguide, semiconductor amplifier, three-stub tuner, plunger, and an isolator. The MW was focused with an iris forming waves (TE-103-mode) in the cavity. The iris had a 28 mm slit parallel to E. The plunger was placed at the end of the waveguide. The system enabled spatial separation of E and H fields forming the MW. The sample was placed at electric-field (denoted by Emax, H = 0) or magnetic-field (denoted by Hmax, E = 0) nodes. The temperature of the reactants was monitored by a fiber-optic thermometer. The wood particles (0.125g) were degraded by an alkaline (5 mL) CuO−H2O2 reaction in a cavity perturbation system (Figures 6 and 7) in a 12 mL glass container provided with a silicon cover (Milestone General, Kawasaki, Japan).The reaction was performed under E and H maximum fields at 160, 170, 180, and 190 °C for 10 min (N = 4) with constant magnetic stirring for analyzing MW sensitizer effects. The reaction temperature was controlled to increase from room temperature to 160, 170, 180, and 190 °C in 10 min and was subsequently kept at that temperature for 10 min. The reaction products were then cooled to room temperature. Conventional heating experiments were conducted under the same reaction conditions and the same vessel as the comparison experiment in an oil bath (TBX 203HA, Advantec Toyo Kaisha Ltd., Tokyo, Japan). HPLC Analysis. Vanillin, vanillic acid, syringaldehyde, and syringic acid were quantified on a Shimadzu HPLC system equipped with a SPD-M20A UV−vis diode array detector, a LC-20AD pump, and a

MATERIALS AND METHODS

Materials. Japanese cedar (Cryptomeria japonica), Eucalyptus globulus, and beech (Fagus crenata Blume) wood particles (14−30 mesh) were used throughout this study. The water content of the Japanese cedar, E. globulus, and Japanese beech wood were 8.32%, 8.90%, and 9.09%, respectively. The Klason lignin content of Japanese cedar, E. globulus, and Japanese beech wood were 31.0%, 19.9%, and 23.0%, respectively.27 Hydrogen peroxide (30 wt %), copper(II) oxide (99.9%), cupric hydroxide (90%), copper(I) oxide (90%), cupric sulfate (97.5%), iron(III) oxide (95.0%) and all the other chemicals were purchased from Wako (Osaka, Japan) and used as received. Alkaline Oxidation by CuO and H2O2 by MW Irradiation and Conventional Heating Conditions. Wood particles (0.5−2.0 g) were presoaked in a 2N NaOH solution (10−20 mL) in a screw-cap test tube overnight. To a Teflon container for MW reaction, the wood slurry was transferred, and 0.1−1.2 g of CuO was added. The reaction mixture was agitated at 600 rpm with a magnetic stirrer at room temperature for a few seconds before starting MW irradiation with a MW reactor STARTSYNTH (Milestone Inc., Sorisole, Italy). The reaction temperature was programmed to increase from room temperature to 180−200 °C in 2 min and subsequently kept at this temperature for 10−80 min (N = 1). The reaction products were cooled to room temperature and filtrated through a 1-G3 glass filter 11555



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CONCLUSIONS A MW-assisted wood degradation method using CuO and H2O2 was developed for producing vanillin and vanillic acid components of lignin-based functional polymers. The total yield of vanillin and vanillic acid from Japanese cedar wood by single-mode MW irradiation reached values up to 10.1% (based on the weight of wood), which was 2.94 times higher than that obtained under conventional heating. The MW effects of the reaction system were studied using the same reaction time, heating profile, and reaction vessel. Experiments using a cavity resonator revealed that E fields were particularly effective in increasing the vanillin and vanillic acid production rates. These effects were corroborated by the lower apparent activation energy values at E and H fields as compared to conventional heating. The method developed herein showed a promising industrial feasibility for the mass production of vanillin directly from biomass considering the low-cost commodity chemicals employed and the high yields obtained.

Figure 6. (a) Schematic illustration of the MW heating system. (b) Contour distribution of E between the iris and the plunger and (c) contour of H. The heating system consists of a signal generator, a semiconductor amplifier, an isolator, a power monitor, a waveguide (WRJ-2), 3-stubs, an iris, and a plunger. The semiconductor amplifier increases the MW power generated by the signal generator (the frequency of the MW (2.45 GHz) is controlled by the signal generator). The plunger and the iris make up the cavity structure, which separates the E and H fields (b and c). The lengths (La and Lb) are controlled to prepare an adequate cavity structure for each frequency. The sample slurry is placed at the separated electronic field (Emax) or magnetic field (Hmax). The temperature is monitored by a fiber-optic thermometer.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 774 38 3640. Fax: +81 774 38 3681. E-mail: [email protected]. ORCID

Takashi Watanabe: 0000-0003-0220-4157 Author Contributions ∥

C. Qu and M. Kaneko contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST CREST Grant Number JPMJCR11B4, Analysis and Development System for Advanced Materials (ADAM), and Flagship Research Program of RISH, Kyoto University.

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REFERENCES

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Figure 7. Pictures of single mode MW heating (left) and CH heating (right) apparatus. CTO-20Acolumn oven. A reversed-phase YMC-Pack ODS-A column (4.6 mm × 250 mm) with a guard column was used. The flow rate, injection volume, and column temperature were fixed to 0.8 mL/min, 10 μL, and 50 °C, respectively. The mobile phase A was water containing 0.1% of formic acid, and the phase B was acetonitrile. The elution started by increasing A from 10% to 50% in 35 min, and the composition was kept for 5 min before immediately returning to the initial conditions. The absorbance at 254 nm was used for detection and quantification. The degradation products were dissolved in methanol, and benzaldehyde was added as an internal standard. Gas Chromatography Mass Spectrometry (GC-MS) Analyses. The wood degradation products were identified by GC-MS. The GCMS analysis was conducted on a Shimadzu GCMS-QP5050A device (Shimadzu, Co., Ltd., Kyoto, Japan) at an ionization voltage of 70 eV on a DB-5MS column (30 m × 0.25 mm i.d.; 0.25 μm film thickness; J&W Scientific). The temperature program was as follows: 50 °C for 3 min, then increased by 8 °C/min to 300 °C and held for 20 min. The other parameters were as follows: injection temperature, 250 °C; detector, 300 °C; carrier gas, He at 1.6 mL/min; injection volume, 1 μL; split ratio, 1:10; mass range, m/z 40−600. The products were identified by comparing with the mass peaks of authentic samples. 11556


Research Article

ACS Sustainable Chemistry & Engineering (10) Ouyang, X.; Tan, Y.; Qiu, X. Oxidative degradation of lignin for producing monophenolic compounds. J. Fuel Chem. Technol. 2014, 42 (6), 677−682. (11) Ouyang, X.; Ruan, T.; Qiu, X. Effect of solvent on hydrothermal oxidation depolymerization of lignin for the production of monophenolic compounds. Fuel Process. Technol. 2016, 144, 181−185. (12) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. The use of microwave ovens for rapid organic synthesis. Tetrahedron Lett. 1986, 27, 279−282. (13) Kappe, C. O.; Pieber, B.; Dallinger, D. Microwave effects in organic synthesis: myth or reality? Angew. Chem., Int. Ed. 2013, 52, 1088−1094. (14) Gu, X.; Kanghua, C.; He, M.; Shi, Y.; Li, Z. La-modified SBA15/H2O2 systems for the microwave assisted oxidation of organosolv beech wood lignin. Maderas Cienc. Tecnol. 2012, 14, 31−41. (15) Kaiho, A.; Kogo, M.; Sakai, R.; Saito, K.; Watanabe, T. In situ trapping of enol intermediates with alcohol during acid-catalysed depolymerisation of lignin in a nonpolar solvent. Green Chem. 2015, 17, 2780−2783. (16) Verma, P.; Watanabe, T.; Honda, Y.; Watanabe, T. Microwaveassisted pretreatment of woody biomass with ammonium molybdate activated by H2O2. Bioresour. Technol. 2011, 102, 3941−3945. (17) Pinto, P. C. R.; Silva, E. A. B.; Rodrigues, A. E. Lignin as Source of Fine Chemicals: Vanillin and Syringaldehyde. In Biomass Conversion; Baskar, C.; Baskar, S.; Dhillon, R. D., Eds.; Springer: London, 2012; pp 381−420. (18) Lai, Y.-Z. Chemical Degradation. In Wood and Cellulosic Chemistry; Hon, D. N. S., Shiraishi, N., Eds.; Marcel Dekker, Inc.: New York, 2001; pp 443−512. (19) Khan, A. U.; Kasha, M. Singlet molecular oxygen evolution upon simple acidification of aqueous hypochlorite: Application to studies on the deleterious health effects of chlorinated drinking water. Proc. Natl. Acad. Sci. U. S. A. 1994, 91, 12362−12364. (20) Watanabe, T.; Shirai, N.; Okada, H.; Honda, Y.; Kuwahara, M. Production and chemiluminescent free radical reactions of glyoxal in lipid peroxidation of linoleic acid by the ligninolytic enzyme, manganese peroxidas. Eur. J. Biochem. 2001, 268, 6114−6122. (21) Tarabanko, V. E.; Petukhov, D. V.; Selyutin, G. E. New Mechanism for the catalytic oxidation of lignin to vanillin. Kinet. Catal. 2004, 45, 569−577. (22) Gierer, J.; Imsgard, F.; Noren, I.; et al. Studies on the degradation of phenolic lignin units of the β-aryl ether type with oxygen in alkaline media. Acta Chem. Scand. 1977, 31, 561−572. (23) Kappe, C. O. Microwave dielectric heating in synthetic organic chemistry. Chem. Soc. Rev. 2008, 37, 1127−1139. (24) Perreux, L.; Loupy, A. A tentative rationalization of microwave effects in organic synthesis according to the reaction medium, and mechanistic considerations. Tetrahedron 2001, 57, 9199−9223. (25) Microwaves in organic synthesis; Loupy, A., Ed.; Wiley-VCH Verlag & Co: Weinheim, Germany, 2012. (26) Gedye, R.; Smith, F.; Westaway, K.; Ali, H.; Baldisera, L.; Laberge, L.; Rousell, J. The use of microwave ovens for rapid organic synthesis. Tetrahedron Lett. 1986, 27, 279−282. (27) Dence, C. W. The Determination of Lignin. In Methods in Lignin Chemistry; Lin, S. Y.; Dence, C. W., Eds.; Springer: London, 1992; pp 34−35.

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Direct Production of Vanillin from Wood Particles by Copper Oxide

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