Hydrothermal Conversion of Catechol into Four-Carbon Dicarboxylic

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Hydrothermal Conversion of Catechol into Four-Carbon Dicarboxylic Acids Guodong Yin,† Fangming Jin,*,‡ Guodong Yao,‡ and Zhenzi Jing§ †

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, and §School of Material Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China ‡ School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: Conversion of lignin into value-added chemicals is attracting growing attention due to the depletion of fossil fuels and the abundant resource of lignin. In this study, hydrothermal conversion of a model compound of lignin, catechol, into valueadded four-carbon dicarboxylic acids (C4-DCAs), such as tartaric (HOOC−CH(OH)−CH(OH)−COOH), malic (HOOC− CH2−CH(OH)−COOH), and fumaric (HOOCCHCHCOOH) acids was investigated. The yield of total C4-DCAs can reach as high as 41.0%, and alkali played a key role in not only promoting the production but also avoiding the decomposition of C4-DCAs. The reaction mechanism of hydrothermal conversion catechol into C4-DCAs showed that catechol is first oxidized to o-quinone, which is then attacked by the hydroxyl radical (OH•) or the hydroperoxyl anion (HO2−) via conjugate addition to decompose into C4-DCAs. This result is helpful to facilitate studies for developing a new, green, and sustainable process to produce value-added C4-DCAs from lignin.

1. INTRODUCTION Utilization of lignin in the sustainable production of fuels and bulk chemicals is of vital importance in response to depleting fossil resources and growing environmental concerns. Lignin is the second most abundant bioresource after cellulose. Currently, pyrolysis of lignin to produce value-added fuels or chemicals has been widely studied. However, such a method results in the formation of coke, and has poor selectivity on the products. One way to add value to lignin is to depolymerize lignin into monomers or subsequently convert monomers into platform molecules. Among the processes of conversion lignin to chemicals, the hydrothermal process is one of the most promising methods,1−7 because high-temperature water (HTW) behaving as a reaction medium has unique inherent properties, including a high ion product (Kw) and a low dielectric constant, which are favorable for promoting reactions without catalysts and have a good selectivity on the products.5 Our previous research on the hydrothermal conversion of lignin and its models into formic (HCOOH) and acetic (CH3COOH) acids has shown that phenolics were first oxidized to unsaturated six-carbon dicarboxylic acids by ring-opening reactions, then to four-carbon dicarboxylic acids (C4-DCAs) by double bond cleavage, and finally to formic and acetic acids.5,8,9 C4-DCAs, such as tartaric (HOOC−CH(OH)−CH(OH)− COOH), malic (HOOC−CH2−CH(OH)−COOH), and fumaric (HOOCCHCHCOOH) acids, are not only essential industrial materials with extensive applications in food, chemical, pharmaceutical, and cosmetic industries but also potential platform chemicals.10,11 These C4-DCAs currently are produced through the catalytic oxidation of petroleum-based feedstocks, and noble metal catalysts are usually required.12−14 Although fumaric acid can also be produced from sugar or starch via bacterial fermentation,12,15 the feedstocks are costly and the reaction rate is generally low. Recently, Ma et al.16 developed a © XXXX American Chemical Society

new method for production of dicarboxylic acids (DCAs), such as malonic (HOOC−CH2−COOH), succinic (HOOC−CH2− CH2−COOH), and malic acids, from lignin oxidation with the chalcopyrite/H2O2 system at 60 °C, providing a new avenue toward lignin utilization. However, the process has a fairly long reaction time of up to 3−5 h. Our previous research found that C4-DCAs, such as maleic and fumaric acids, were detected as intermediates in the hydrothermal oxidation of lignin model compounds in minutes,9 and the presence of alkali can prevent the further oxidation of carboxylic acids effectively.17 Moreover, our previous research showed that electrodialysis is suitable for not only the separation of calcium acetate, but also the separation of the majority of small carboxylate anions including formate, succinate, propionate, and acrylate,18 and there is also great potential for this method to be applied to the separation of C4DCAs.19,20 Thus, C4-DCAs can possibly be selectively produced by hydrothermal oxidation of lignin in minutes by adding alkali and controlling reaction conditions. The aim of this work is to study the potential of production of C4-DCAs from lignin derived catechol via adding alkali catalysts and dropping the reaction temperature (Scheme 1). Lignin has varieties of complex structures and previous research has demonstrated that lignin as well as lignin models such as guaiacol, syringol, and eugenol can decompose into catechol easily under hydrothermal conditions.1,4,9,21 Thus, catechol was employed as a lignin derived phenolic monomer in this study to simplify the reaction and explore the reaction mechanism. Received: September 14, 2014 Revised: December 17, 2014 Accepted: December 18, 2014

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DOI: 10.1021/ie5036447 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Previous Study and Current Concept for the Production of C4-DCAs from Lignin

The GC oven temperature was initially set to 50 °C for 2 min and was increased at a rate of 7 °C/min to 230 °C; it was held at this temperature for 10−30 min. Identification of intermediate products was made with the total and selected ion chromatograms with the aid of a computer library and data books as well as the GC retention times of products and authentic compounds. 2.3.2. HPLC Analysis. Identification of intermediate products was accomplished with the aid of retention times of products and authentic compounds. Quantitative estimation of carboxylic acids reported in this study was based on the average value of the analytical results of at least two samples with the relative errors always less than 5% for all experiments. The yield of carboxylic acids is defined as the percentage of the dicarboxylic acid to initial catechol on the carbon basis. The liquid samples were all diluted and analyzed using a high-performance liquid chromatography instrument (HPLC, Agilent 1200LC) equipped with two RSpak KC-811 (SHODEX) columns and a pair of detectors: a tunable absorbance detector (VWD) and a differential refractometer (RID); the eluent containing 2.0 mmol L−1 HClO4 was applied at a flow rate of 1.0 mL min−1.

2. EXPERIMENTAL SECTION 2.1. Materials. Catechol (analytical purity ≥98.0%), maleic acid (analytical purity ≥99.0%), fumaric acid (analytical purity ≥99.0%), oxalic acid (analytical grade, 98.0%), NaOH (analytical purity ≥96.0%), and KOH (analytical grade, 85%) were purchased from Sinopharm Chemical Reagent (China). Malic acid and tartaric acid were purchased from Wako Pure Chemicals Industries Co., Ltd. (Tokyo, Japan). H2O2 (hydrogen peroxide, 30%, Shanghai Runjie Reagent, China) was selected as an oxidant. 2.2. Experimental Procedure. All experiments were conducted in a tube reactor made of 316 stainless steel tubing (3/8 in., 1.0 mm wall thickness and 120 mm long) with two end fittings, providing an inner volume of 5.7 mL. The experimental procedure is described as follows: 1.0 mL of catechol/catechol− alkali solution containing 0.5 mmol of catechol and 1.0 mL of H2O2 solution were loaded into the tube reactor, with a water filling of approximately 35%. Then, the reactor was sealed and immersed in a salt bath, preheated to the desired temperature. During the reaction, the reactor was shaken to improve the efficiency of mixing the contents. After the desired reaction time, the reactor was removed from the salt bath and immediately cooled with cold water to room temperature. Details of this method are described in the literature.8,22−24 The reaction time was defined as the time that the reactor was kept in the salt bath. It should be noted that the typical heat-up period of the reactor to raise its temperature from 20 to 260 or 300 °C was approximately 15 s. Thus, the real reaction time is shorter than the apparent reaction time. Reaction pressures with and without H2O2 supply were about 17 and 9 MPa, respectively. All experiments were performed with a purging of the reactor with helium. 2.3. Product Analysis. 2.3.1. GC−MS Analysis. Products in the aqueous solution were esterified with methanol and analyzed using gas chromatography−mass spectrometry (GC−MS, Agilent 7890A-5975C) with an HP-INNOWAX capillary column (30 m × 0.25 mm × 0.25 μm). This esterification reaction has been described in detail elsewhere in the literature.25

3. RESULTS AND DISCUSSION 3.1. Comparison of Products in the Absence and Presence of Alkali. Experiments with catechol and H2O2 were conducted at 300 °C with and without alkali (NaOH and KOH) to test the possibility of production C4-DCAs from catechol. Figure 1 shows the GC−MS total ion chromatograms from 6 to 35 min of the esterified samples obtained with or without alkali. As shown in Figure 1, when alkali was added, peaks of pyruvic (CH3−C(O)−COOH), oxalic (HOOC−COOH), malonic (HOOC−CH2−COOH), and fumaric (HOOCCHCH COOH) acids were identified as products after esterification; no peaks of catechol existed. However, only small peaks of oxalic and fumaric acids were identified in the absence of alkali, with a large peak of catechol remaining. Further identification was performed by HPLC analysis. As shown in Figure 2a, although carboxylic acids such as oxalic (HOOC−COOH), maleic (HOOCCH B


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Industrial & Engineering Chemistry Research

Figure 1. GC−MS chromatogram of products in aqueous solution after reaction (40% H2O2 supply, 300 °C, 60 s).

Figure 3. Product distribution with or without alkali (50% H2O2 supply, 300 °C).

Figure 2. (a) HPLC-RID and (b) HPLC-VWD chromatograms of products in aqueous solution after reaction (40% H2O2 supply, 300 °C, 60 s).

CHCOOH), tartaric (HOOC−CH(OH)−CH(OH)− COOH), malonic, malic (HOOC−CH2−CH(OH)−COOH), fumaric, acetic (CH3COOH), and muconic (HOOCCH CHCHCHCOOH) acids were identified without alkali, peaks of tartaric and fumaric acids became larger when alkali of

Figure 4. Effects of KOH concentration on the oxidation of C4-DCAs (19.43 mol L−1 tartaric acid, 10.48 mol L−1 malic acid, 25.00 mol L−1 fumaric acid, 100% H2O2 supply, 260 °C).

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Scheme 2. Possible Reaction Pathway for the Oxidation of Catechol to o-Quinone

Figure 6. Products of oxidation of (a) malic acid and (b) tartaric acid from 20 to 60 s (1.0 mol L−1 KOH, 40% H2O2 supply, 260 °C).

in the presence of alkali. The reason tartaric and malic acids were not detected by GC−MS may be attributed to their susceptibility to dehydration in the esterification process in the presence of concentrated H2SO4. In addition, maleic and fumaric acids have better respondence by HPLC-VWD than by HPLC-RID; thus maleic and fumaric acids were quantified by HPLC-VWD. Since one molecule of catechol can only produce one molecule of C4DCA, the yield of the C4-DCA (Y) is defined based on the molar ratio as follows:

Figure 5. Effects of (a) H2O2 supply, (b) KOH concentration , and (c) temperature on the yield of C4-DCAs ((a) 1.0 mol L−1 KOH, 300 °C, 60 s; (b) 50% H2O2 supply, 300 °C, 60 s; (c) 1.0 mol L−1 KOH, 50% H2O2 supply, 60 s).

NaOH or KOH was added. As shown in Figure 2b, although carboxylic acids such as oxalic, maleic, fumaric, and muconic acids were identified without alkali, the peak of maleic acid became smaller when alkali of NaOH or KOH was added. These results indicate that there is great potential to selectively produce C4-DCAs such as tartaric, malic, and fumaric acids from catechol

Y=

C4‐DCA in moles ·100% initial catechol in moles

(1)

Figure 3 shows yields of C4-DCAs, including tartaric, malic, and fumaric acids, with and without alkali. In the absence of alkali, the maximum yield of total C4-DCAs, including tartaric, malic,

Table 1. Reactant Consumption and Carbon Distribution in Organic Products in Aqueous Solutiona reactant (mmol) catechol 0.5

H2O2 3.25

carbon distribution (%) formicb 12.84

aceticb 2.32

pyruvicb 2.72

malonicb 2.61

maleicb 0.49

tartaricb 12.41

malicb 16.99

fumaricb 0.75

TOCc 61.78

a Reaction conditions: 0.5 mmol of catechol, 3.25 mmol of H2O2, 1.0 M KOH, at 260 °C for 60 s. bQuantified by HPLC. cQuantified by total organic carbon (TOC).

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Figure 7. Possible reaction pathways for further oxidation of o-quinone (R1, conjugate addition; R2, keto−enol tautomerism; R3, ring-opening reaction; R4, dehydration; R5, O−O bond cleavage; R6, hydrogenation; R7, nonconjugate addition).

increase in the yield of total C4-DCAs with the addition of alkali. Generally, aqueous hydrogen peroxide (H2O2) can give hydroxyl radical (OH•) (eqs 2−5)26,27 and dissociate to hydroperoxyl anion (HO2−) (eq 6)28−30 before decomposing to molecular oxygen and both the OH• and HO2− are thought to be the active species responsible for the oxidizing property of H2O2. Since H2O2 is a weak acid, the presence of alkali can lead to the production of HO2− being increased greatly (eqs 7−9), resulting in the oxidation of catechol promoted. Thus, this is also a reason why the presence of alkali increase the yield of total C4-DCAs. In addition, it seems that KOH is more effective than NaOH at

and fumaric acids, was only approximately 9.5% for 40 s. However, the maximum yield of total C4-DCAs increased to approximately 27.9% for NaOH and approximately 37.3% for KOH. The results suggest that the yield of total C4-DCAs increased greatly with the addition of alkali. 3.2. Role of Alkali in Improving Production of C4-DCAs. As shown in Figure 2, compared to a large peak of catechol that remained without alkali, only a small peak of catechol was observed with NaOH, and no peak of catechol was observed with KOH. These results suggest that the alkali can dramatically promote the decomposition of catechol, which may explain the E


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indicates that a low H2O2 supply mainly leads to a selective oxidation of catechol to C4-DCAs, whereas too much H2O2 supply of 60% may lead to the further oxidation of C4-DCAs, resulting in a dramatic decrease of the yield of total C4-DCAs. Subsequently, effects of KOH concentration on the yield of C4-DCAs were investigated at 300 °C for 60 s with 50% H2O2 supply. As shown in Figure 5b, an increase in the KOH concentration led to an increase in the yield of total C4-DCAs, reached a maximum of 31.4% at 1.0 mol L−1 KOH, and substantially decreased with a further increase in KOH concentration. Among these C4-DCAs, the yield of tartaric acid (HOOC−CH(OH)−CH(OH)−COOH) increased to approximately 10.9% at 1.5 mol L−1 KOH and then decreased to 0, the yield of fumaric acid (HOOCCHCHCOOH) increased to approximately 14.0% at 1.0 mol L−1 KOH and then decreased to approximately 10.0%, and the yield of malic acid (HOOC−CH2−CH(OH)−COOH) decreased from approximately 14.6 to 5.0%. Yields of tartaric, fumaric, and malic acids are sensitive to KOH concentration, and raising the KOH concentration below 1.0 mol L−1 can promote the production of tartaric and fumaric acids greatly but aggravate the decomposition of malic acid considerably. These results indicate that the increase in KOH concentration below 1.0 mol L−1 can promote the production of C4-DCAs. The effect of reaction temperature on the yields of the C4DCAs is shown in Figure 5c. An increase in reaction temperature from 260 to 320 °C resulted in a decrease in the yield of total C4DCAs from approximately 41.0 to 16.0%, including the decreases of tartaric acid (HOOC−CH(OH)−CH(OH)−COOH) from 15.9 to 2.1% and malic acid (HOOC−CH2−CH(OH)− COOH) from 24.2 to 3.8%, but an increase of fumaric acid (HOOCCHCHCOOH) from 0.9 to 14.0%. The results suggest that the increase in reaction temperature is not favorable for the production of C4-DCAs, except for fumaric acid. Thus, hydrothermal conversion of catechol into C4-DCAs is conducted at 260 °Cthe lowest operating temperature of our salt bath. The consumption of catechol and H2O2 and the carbon distribution in organic products under optimal conditions are shown in Table 1. 3.4. Possible Reaction Mechanisms. Our previous study on the oxidation of lignin model compounds with H2O2 without alkali reported that catechol is first oxidized to o-quinone and then to muconic acid (HOOCCHCH−CHCH COOH), which is further oxidized to carboxylic acids with lower molecular weight.9 Devlin and Harris32 suggested a similar result. In this study, as alkali was used, catechol would first react with the alkali to form an anion, and then be oxidized to oquinone by OH• or HO2−. One proposed pathway is summarized in Scheme 2. First, catechol 1 dissolved in alkaline solution to form catecholate anion 2, which subsequently was oxidized by OH• or HO2− to form the o-quinone 3. Since transtrans-muconic acid (HOOCCHCHCHCH COOH) was also detected as a intermediate product in the presence of alkali, it is possible that the o-quinone was first oxidized to muconic acid which then was further oxidized to the C4-DCAs. To test this assumption, experiments with trans-transmuconic acid in 1.0 mol L−1 KOH at 260 °C from 20 to 60 s were conducted. As results, only oxalic (HOOC−COOH) and pyruvic (CH3−C(O)−COOH) acids were detected, but no tartaric (HOOC−CH(OH)−CH(OH)−COOH) or malic (HOOC−CH2−CH(OH)−COOH) acid was detected in the reaction products, which means that the production of tartaric and malic acids was not from the oxidation of muconic acid.

promoting the production of carboxylic acids, and a similar result was also observed in our previous research.8,31 Since the radius of K+ is larger than that of Na+, it is proposed that KOH has a greater alkalinity, as well as a greater tendency than NaOH to react with the H in the OH groups of catechol to form o-quinone. Consequently, studies with KOH were conducted to optimize the production of C4-DCAs. H 2O2 → 2OH•

(2)

H 2O2 + OH• → HO2• + H 2O

(3)

HO2• + OH• → H 2O + O2

(4)

2HO2• → H 2O2 + O2

(5)

H 2O2 ⇋ HO2− + H+ −

(6) −

H 2O2 + OH → HO2 + H 2O

(7)

HO2− + H 2O2 → O2•− + OH• + H 2O

(8)

OH• + O2•− → OH− + O2

(9)

Moreover, our previous research has shown that the presence of alkali is effective at preventing the oxidation of formic acid.17 Since tartaric (HOOC−CH(OH)−CH(OH)−COOH), malic (HOOC−CH2−CH(OH)−COOH), and fumaric (HOOC CHCHCOOH) acids are sensitive to the presence of alkali, experiments were conducted to investigate the effect of KOH concentration on the oxidation of these dicarboxylic acid. The results are given in Figure 4. As shown in Figure 4, in the absence of KOH, the conversion of tartaric acid was approximately 90.7%; however, this value decreased to 83.4% when 0.2 mol L−1 KOH was added and increased gradually to 98.9% with the increase of KOH concentration to 1.0 mol L−1. The malic and fumaric acids showed the same trend. These results indicate that a low alkali concentration can prevent the decomposition of C4DCAs, while a higher alkali concentration can promote the degradation of C4-DCAs. The fact that a low alkali concentration leads to the decrease in the decomposition of C4-DCAs may be because a small amount of alkali make acids stay in salts, avoiding decomposition. However, a too-high alkali concentration will promote H2O2 to produce HO2− greatly, and thus facilitate the oxidation of C4-DCAs, resulting in the yields of the C4-DCAs being decreased dramatically. 3.3. Optimization of Reaction Condition for Getting High Yields of C4-DCAs. As discussed before, the OH• or HO2− generated from H2O2 can oxidize catechol to C4-DCAs. The effect of H2O2 supply was investigated first. The stoichiometric demand for complete oxidation of catechol to CO2 and water was defined as 100% H2O2 supply according to eq 10. C6H6O2 + 13H 2O2 → 6CO2 + 16H 2O

(10)

Our previous research for phenol oxidation with 60% H2O2 supply resulted in optimum yields of formic and acetic acids.8 Since a lower H2O2 supply is helpful to easily get the initial oxidation products, 9 a H 2 O 2 supply of 20−60% (H 2 O 2 concentration varies from approximately 0.65 to 1.95 mol/L) was selected in this study to produce C4-DCAs with larger molecular weights. As shown in Figure 5a, the yield of total C4DCAs increased with the increase of H2O2 supply, reached a maximum of 31.4% with 50% H2O2 supply, and then decreased sharply with the H2O2 supply increased to 60%. Such observation F


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enol 16. Enol 16 may continue conjugate addition giving enol 17, which will afford ketone 18 after O−O bond cleavage and keto− enol tautomerism. Ketone 18 is extremely unstable because C−C bond becomes fairly weak and the quinonoid ring will be destroyed, leading to the formation of tartarate (C4) 19, which can be reduced by H2O2 to maleic acid (C4) 20. In addition, HO2− is a strong nucleophile which can react with o-quinone following a nonconjugate addition mechanism. When o-quinone undergoes nonconjugate addition with HO2−, the possible reaction pathway is proposed in Figure 7C. The HO2− might bond with the carbon atom of CO group to form adduct 21, and then the quinonoid ring will be destroyed, leading to the formation of muconate (C6) 22, which can be further oxidized to pyruvic (C3) 23 and oxalic (C2) 24 acids by OH• or HO2−. Gierer36 and Ma37 also reported that H2O2 can react with oquinone mainly via this pathway in the process of pulp bleaching, leading to the production of muconic acid. However, the results of the experiment show that this pathway will not produce C4DCAs under alkaline hydrothermal conditions.

Then, tartaric and malic acids might come from the oxidation of the o-quinone directly. In addition, predominantly tartaric and malic acids were obtained at the beginning of the reaction and the yield of fumaric acid (HOOCCHCHCOOH) increased along with the decrease of tartaric and malic acids with increasing reaction time from 20 to 120 s (Figure 3c). Thus, tartaric and malic acids might come from the oxidation of catechol directly at the beginning of the reaction. However, fumaric acid may come from the further reaction of tartaric acid and/or malic acid. To test this assumption, experiments with tartaric and malic acids as starting materials were conducted. When malic acid was employed as the starting material, fumaric acid was the major product, and as shown in Figure 6a, the yield of fumaric acid soared from 3.6 to 34.9% while the conversion of malic acid increased from 4.3 to 59.4% with increasing reaction time from 20 to 60 s, which suggests that fumaric acid was from the dehydration of malic acid under alkaline hydrothermal conditions. A similar dehydration result was also reported in other literature.33 Thus, the yield of fumaric acid increased with malic acid decrease. As shown in Figure 6b, when tartaric acid was employed as the starting material, oxalic (HOOC−COOH) and maleic (HOOCCH CHCOOH) acids were detected as the major products, which means tartaric acid disproportionated to maleic and oxalic acids. However, maleic acid is unstable under hydrothermal conditions, and may undergo further oxidation to give oxalic acid and CO2, resulting in an extremely low yield of maleic acid over time. Based on these results, proposed pathways for further hydrothermal oxidation of o-quinone in alkaline solution are summarized in Figure 7. Generally, OH• or HO2− can attack the benzene ring via addition and free radical substitution reactions. As o-quinone does not possess aromaticity (judged by Hückel’s rule) and has a conjugate structure of CCCO, it is susceptible to radical or nucleophilic addition with OH• or HO2− following a Michael-type addition (conjugate addition) mechanism.34 The OH• was considered to be a dominant oxidizing species at high temperature.35 When o-quinone reacts with the OH• following a conjugate addition, the possible reaction pathway can be explained in Figure 7A. The OH• bonds with the carbon atom of a double bond away from the CO group to form an enolate ion (a carbanion) which, in turn, is chiefly protonated at the oxygen to give enol 4. Enol 4 undergoes the conjugate addition continually to give enol 5, which is extremely unstable and transforms to ketone 6 via the keto−enol tautomerism. Ketone 6 may undergo conjugate addition to give enol 7, which subsequently gives ketone 8 via keto−enol tautomerism. Then, ketone 8 may undergo a ring-opening reaction be oxidized to malate (HOOC−CH2−CH(OH)− COOH) 9, which can dehydrate to fumarate (HOOCCH CHCOOH) 10. It was reported that H2O2 can react with o- and p-quinone structures in the process of pulp bleaching, and HO2− was thought to be the effective oxidant.36,37 When o-quinone undergoes conjugate addition with HO2−, the possible reaction pathway is proposed in Figure 7B. Similarly, the HO2− might bond with the carbon atom of a double bond away from the C O group to give an enol 11 with a O−OH group, which is unstable and will undergo O−O bond cleavage and formation of CO group giving enol 12. Enol 12 can continue conjugate addition with the HO2− giving enol 13, which will undergo O−O bond cleavage and transform to ketone 14 via keto−enol tautomerism. Ketone 14 may undergo conjugated adduction to form enol 15, which may undergo O−O bond cleavage and give

4. CONCLUSIONS Catechol, as a model compound of lignin, can be selectively converted into four-carbon dicarboxylic acids (C4-DCAs), mainly including tartaric, malic, and fumaric acids. The yield of total C4-DCAs can reach 41.0% with 50% H2O2 supply at 260 °C for 60 s. The reaction pathway is proposed such that catechol is first oxidized to o-quinone, which is then attacked by the hydroxyl radical (OH•) or the hydroperoxyl anion (HO2−) via conjugate addition to decompose into C4-DCAs. The presence of alkali is essential: it not only can promote the oxidation of catechol to C4-DCAs, but it also can prevent the further oxidation of malic and tartaric acids.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: + 86-21-54742283. Fax: + 86-21-54742283. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the General Program of National Natural Science Foundation of China (No. 21277091) and the Key Program of National Natural Science Foundation of China (No. 21436007).

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REFERENCES

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