Depolymerization of Lignin to Aromatics by Selectively Oxidizing


Jun 12, 2017 - A novel strategy for the oxidative cleavage of C–C and C–O bonds in a series of model substrates including β-O-4 lignin model comp...
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Research Article pubs.acs.org/journal/ascecg

Depolymerization of Lignin to Aromatics by Selectively Oxidizing Cleavage of C−C and C−O Bonds Using CuCl2/Polybenzoxazine Catalysts at Room Temperature Xiaorong Ren, Ping Wang, Xinyu Han, Geng Zhang, Jiangjiang Gu, Cong Ding, Xinsheng Zheng,* and Feifei Cao* Department of Chemistry, College of Science, Huazhong Agricultural University, No.1 Shizishan Street, Hongshan District, Wuhan 430070, People’s Republic of China S Supporting Information *

ABSTRACT: A novel strategy for the oxidative cleavage of C−C and C−O bonds in a series of model substrates including β-O-4 lignin model compounds using CuCl2/polybenzoxazine composites catalysts with H2O2 as oxidant at room temperature showed good conversions (up to 88%) and an over 96% total selectivity to aromatic monomers within 2 h. This approach then succeeded in application to actual lignin depolymerization monitored by gel permeation chromatography (GPC), 1H NMR, and 2D-NMR (HSQC). The results suggest that lignin can be effectively degraded into an array of functionalized dimer−trimeric aromatic acids, aldehydes, phenols, etc., obtained from the selective cleavage of aliphatic C−C and C−O bonds in the major linkages β-O-4′ aryl ethers, resinols, and p-hydroxycinnamyl alcohols while leaving the natural aromaticity intact. Furthermore, the mechanistic insights into the catalytic reactions reveal a two-electron transfer process involved with a phenoxy radical, and an oxidation-then-cleavage route proposed for lignin depolymerization. The clean process, mild reaction conditions, and high aromatics selectivity indicate that it is a promising heterogeneous catalytic system for oxidative depolymerization of lignin to valuable aromatic chemicals. KEYWORDS: Lignin valorization, Copper, Oxidative cleavage, Room temperature, Aromatic compounds



INTRODUCTION Lignin, the second most abundant biopolymer on Earth after cellulose, is one of the few renewable sources of aromatic polymers.1,2 These value-added aromatic compounds are important platform chemicals for industrial synthesis of pharmaceuticals, pesticides and fine chemicals.3,4 Currently, aromatics are mainly obtained from petroleum, coal tar, and other industrial chemicals.5,6 Considering the world’s diminishing petroleum reserves and increasingly serious environmental pollution, it is very necessary to find a sustainable alternative to the petroleum-based processes.7,8 Production of aromatic compounds from renewable biomass resources, especially lignin, exactly provides an important strategy for replacing fossil-based processes and reducing our impact on the environment.9,10 However, producing such value-added aromatic chemicals from lignin requires selective cleavage of C− C and/or C−O bonds between aromatic rings while preserving the natural aromaticity, which is still a great challenge due to its complex three-dimensional amorphous structure.11−13 To address this problem, careful control of reaction conditions and use of selective catalytic system are required. In recent years, many approaches have been proposed for reduction or oxidation of lignin to generate aromatic compounds using homogeneous or heterogeneous catalysts.14−19 For example, Jones and co-workers reported the © 2017 American Chemical Society

depolymerization and hydrodeoxygenation of switchgrass lignin to phenols and guaiacol derivatives with a formic acid hydrogen source overnight at 350 °C in sand bath.20 Besides, Yang has described catalytic cleavage of C−O bonds in biomass-derived lignin to aromatic monomers in a highly concentrated ZnCl2 solution under 4 MPa H2 at 200 °C.21 Furthermore, under oxygen atmosphere with cerium oxide-supported palladium nanoparticles (Pd/CeO2) as catalyst, organosolv lignin was converted to vanillin, guaiacol, and 4-hydroxybenzaldehyde with poor yields (no more than 5.2%).22 Despite the successful conversion of lignin in all the aforementioned reports, these processes required either elevated temperature, high pressure, or the use of noble metal-based catalysts. Recently, copperbased catalysts have received increasing attention because copper sites might help to retain the aromaticity of the products in the catalytic cleavage of lignin models.23−25 The investigation of copper complex-catalyzed oxidation of a variety of biomassderived compounds has demonstrated some intriguing selectivity, especially Cu(II) with ligands containing π bonds.26,27 However, most previous studies on selective catalytic cleavage of C−C and C−O bonds in lignin or its Received: March 8, 2017 Revised: May 17, 2017 Published: June 12, 2017 6548

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ACS Sustainable Chemistry & Engineering models employ homogeneous catalysis, which usually accompanied by high consumption of energy and relatively low aromatics selectivity.28,29 Therefore, it is of great importance to find a selective heterogeneous catalytic oxidative system for efficient conversion of lignin to aromatic compounds at relatively low temperature with a nonprecious catalyst. In the present work, a newly devised type of CuCl2/ polybenzoxazine (CuCl2/PBOZ) composite was prepared and used as a heterogeneous catalyst for oxidative cleavage of C−C and C−O bonds in lignin models, as well as extracted lignin with H2O2 as oxidant at room temperature and ambient pressure, which exhibited remarkable catalytic activity and excellent selectivity to aromatics. The as-prepared polybenzoxazine (PBOZ), based on phenol, formaldehyde, and melamine, possesses high thermal and chemical stability.30 It was chosen as an excellent ligand for Cu(II), in that the existence of nitrogen, oxygen heteroatom π bonds in its structure. The results demonstrated that our catalytic system was able to selectively cleave C−C and C−O bonds in aliphatic chains of lignin while preserving the aromatic rings, generating aromatic compounds in substantial amounts. To our best knowledge, the reaction conditions are the mildest by far among all the reported oxidative cleavage of C−C and C−O bonds in lignin in heterogeneous catalytic processes.

Scheme 1. General Synthetic Procedure for CuCl2/PBOZ Micronanospheres and Their Proposed Structures



RESULTS AND DISCUSSION To start with, chalcone 1a was selected as a model substrate to study the oxidative cleavage of C−C bonds as it contained the diphenylpropane structural aliphatic chains between two aromatic rings without C−O bonds and was still simple enough for carrying out meaningful experiments on laboratory scale. Likewise, aryl ethers 1e−g were then chosen as lignin mimics to investigate the C−O bonds cleavage since they possessed the corresponding lignin linkages. On the basis of these researches, several realistic β-O-4 lignin models (1h−j) and even actual lignin were then tested to conduct a comprehensive survey on simultaneous cleavage of C−C and C−O bonds. Aqueous hydrogen peroxide was used as oxidant for the implementation of the oxidation reaction under mild conditions.31 Preparation and Characterization of the Composite Catalysts. The CuCl2/PBOZ composites were prepared from CuCl2·2H2O and the as-prepared benzoxazine monomers in the mixed solvent of toluene and methanol by inverse suspension polymerization in methyl-silicone oil, as shown in Scheme 1. The morphology of the obtained CuCl2/PBOZ composite was examined by scanning and transmission electron microscopy (SEM and TEM). It can be seen from Figures 1a and 1b that the composite was composed of many nonuniform spheres with a diameter of about tens of nanometers to several micrometers. Moreover, these micronanospheres of different sizes were interconnected with each other and exhibited a corallike three-dimensional (3D) structure. Thus, there were abundant obvious cavities or macropores between stacked bodies, which gave a rough surface and facilitated the contact between the composite catalysts and the substrates in the heterogeneous catalytic process. Meanwhile, the surface composition of the CuCl2/PBOZ composite was investigated by energy dispersive spectrum (EDS). As shown in Figure 1c, the composite contained copper, chlorine, carbon, nitrogen and oxygen (silicon may possibly come from methyl-silicone oil), indicating that the CuCl2/PBOZ composite was successfully

Figure 1. (a, b) SEM images, (c) EDS spectrum, and (d) TEM image of CuCl2/PBOZ.

synthesized. Besides, the TEM image of CuCl2/PBOZ in Figure 1d revealed that the spheres were regular in shape and the whole structure of the composite was in good agreement with that of SEM images. According to the nitrogen adsorption/desorption isotherms (Figure S1), CuCl2/PBOZ showed a typical type II sorption behavior and possessed a Brunauer−Emmett−Teller (BET) specific surface area of 7.08 m2/g, indicating the macroporous multilayer adsorption of CuCl2/PBOZ composite. This is well consistent with the result of SEM images. Additionally, the structure of the composite was studied by X-ray diffraction (XRD), FTIR and X-ray photoelectron spectroscopy (XPS), as shown in Figures S2a−c. The integrated results from XRD in Figure S2a demonstrate that 6549

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ACS Sustainable Chemistry & Engineering CuCl2 is monodispersed rather than mechanically mixed in the composite, and has an obvious effect on the curing behavior of benzoxazine, suggesting the existence of interaction between copper and benzoxazine. From the FTIR spectra exhibited in Figure S2b, the peaks shift of C−N and C−O stretch vibrations in CuCl2/PBOZ and the formation of two new bands in the farinfrared region suggest the real occurrence of coordination interaction between copper and N/O atoms in benzoxazine, which was further confirmed by XPS in Figure S2c. In theory, the decrease of binding energy of Cu inner shell electrons in CuCl2/PBOZ results from that copper obtains electrons from benzoxazine. Furthermore, the results from thermogravimetric analysis (TGA) curves in Figure S2d suggest that the CuCl2/ PBOZ composite exhibits much better thermal stability than both PBOZ and copper salt, which is of great significance for practical application in large scale. The detailed analysis of structure and thermal stability of the CuCl2/PBOZ composite see Supporting Information. The catalytic performance of the composites was evaluated by oxidative cleavage of chalcone 1a using hydrogen peroxide (30 wt % in H2O) as oxidant, with benzoic acid as the main reaction product. The influences of preparation conditions and the properties of these composites catalysts on their catalytic performance are shown in Tables S1 and S2. The CuCl2/PBOZ composite in Table S2, entry 2, which was prepared using 10 wt % CuCl2·2H2O in 1:4 mixed solvent at curing temperature of 200 °C for 2 h, showed good conversion and the highest selectivity to benzoic acid among all the tested catalysts in Table S2. This catalyst represents the CuCl2/PBOZ composite mentioned in this article unless otherwise stated. Furthermore, it was observed that individual CuCl2·2H2O or PBOZ as catalyst gave a fairly low selectivity or conversion while the CuCl2/PBOZ composite performed excellently both in the two aspects (Table S2, entries 2−4). These results suggest that the intrinsic selectivity of CuCl2/PBOZ to aromatic compounds stem from its whole structure, demonstrating that a composite catalyst for the cleavage reactions has been successfully fabricated. Besides, the control test in Table S2, entry 1 also confirmed the key role of the composite catalyst. Selective Oxidative Cleavage of C−C and C−O Bonds. To determine the optimized reaction conditions for the oxidative cleavage of C−C and C−O bonds, the effects of reaction conditions such as reaction time, temperature, solvents and oxidants on the catalytic cleavage of 1a were investigated in Table S3. On the basis of all the test results, the optimized reaction conditions were determined as 18 mol % CuCl2/ PBOZ catalyst with 19 equiv of H2O2 in 0.11 mol solvent MeCN (6.0 mL, on 0.5 mmol scale reaction) for 2 h at room temperature and pressure, with the highest conversion of 1a and yield of benzoic acid being 72% and 70%, respectively. Compared with previously reported results,32 the advantages of our catalytic system is obvious in terms of mild reaction conditions and high yields of aromatic compounds. To illuminate the influence of electronic effects of substitute groups on oxidative cleavage of C−C bonds in chalcone 1a, the optimized reaction conditions were then applied to the reaction of substituted chalcone possessing both electron-withdrawing and electron-donating groups. 4′-Nitrochalcone 1b and 4′methoxychalcone 1c were prepared similarly to the general procedure of chalcone 1a. To identify the main cleavage components, column chromatography was carried out to isolate the aromatics obtained from the cleavage of 1a−c (yields of products 5 and 6 were determined by HPLC). Table 1 shows

Table 1. Oxidative Cleavage of Substituted Chalcone 1a−c under the Optimized Reaction Conditionsa

yield [%] entry

R′

substrate

conv. [%]

1 2 3d

H NO2 OCH3

1a 1b 1c

72 68 88

b

2b, cc

3c

4c

5a−cb

6b

19 69

69 5 10

13 17 66

33 45 16

22 42 9

a

Reaction conditions: substrate (1 mmol), CuCl2/PBOZ (18 mol %), MeCN (0.22 mol), H2O2 (19 equiv). bConversions and yields determined by HPLC. cYields determined by column chromatography. d 4-Hydroxylbenzoic acid (less than 3% yield) found in products of 1c by HPLC.

the conversions of substrates and yields of the corresponding aromatic acids and phenol, the main products in each reaction. As for isolated products from 1a, benzoic acid 3 was obtained in high yield with 69%, while the yields of phenylacetic acid 4, phenylglyoxylic acid 5a, and phenol 6 were all lower than 33% (Table 1, entry 1). This result suggests the occurrence of simultaneous cleavage mainly at the Cβ−Cγ (pathway A) and Cα−Cβ bonds (pathway B), which is defined as pathway AB for convenient comparison. However, for the test of 1b, the yields of 4-nitrobenzoic acid 2b and 4 were very low while the main products were 4-nitrophenylglyoxylic acid 5b and 6 with yields of 45% and 42%, respectively (Table 1, entry 2). The result implies the occurrence of simultaneous cleavage of the Cα−Cβ (pathway B) and Cα−Caryl bonds (pathway C) (pathway BC). Furthermore, the oxidative cleavage of 1c proceeded smoothly with good yields of 4-methoxybenzoic acid 2c and 4 (69% and 66%, respectively, Table 1, entry 3), suggesting the priority of the Cβ−Cγ bond cleavage (pathway A) under the conditions. As different pathways (A, B, C, or their combination) are involved in the oxidative cleavage of C−C bonds in 1a−c, it is necessary to calculate the probability of each pathway. According to eqs S1 and S2, the probability values for the main pathway AB in 1a, the main pathway BC in 1b and pathway A in 1c were calculated as 49.4%, 65.6%, and 77.7%, respectively. The detailed results of the probability of each pathway are listed in Table S4. The aforementioned results suggest that a strong electrondonating group (such as methoxy) on the aromatic ring of lignin is more likely to promote the oxidative cleavage of C−C bonds on aliphatic chains between aromatic rings than a electron-withdrawing group (such as nitro), presumably because of different induced effect of their electrons. As we all know, the aromatic rings of natural lignin contain abundant methoxy and hydroxyl groups, which can exactly facilitate the oxidative cleavage of C−C bonds in lignin to platform aromatic compounds with our catalytic system. Furthermore, kinetics survey of chalcone 1a showed the C−C bonds cleavage reaction appears as a first-order reaction and its apparent activation energy is calculated as 80.2 kJ/mol, far below the 6550

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ACS Sustainable Chemistry & Engineering values in previous reports,33 indicating that the composite catalyst can greatly reduce the activation energy of the conversion reaction (Figure S4). Compared to C−C bonds, C−O bonds in lignin are more common due to the existence of abundant hydroxyl and ether groups either on aromatic rings or aliphatic chains, and ether C−O bonds are more prone to break in lignin.34 So we tested the ability of our system to catalyze the oxidative cleavage of C−O bonds in aromatic alcohol and aryl ethers under the above optimized reaction conditions. 1,3-diphenyl-2-propenol 1d was prepared from reduction reaction of chalcone 1a with NaBH4 (Scheme S2). The catalytic reaction of 1d resulted in efficient cleavage to give benzoic acid 3 and phenylacetic acid 4 in good yields with 64% and 61%, respectively (Figure 2a).

respectively (Figure 2c). Among all C−C and C−O linkages between the monomers, β-O-4 ether bond is most representative in both softwood and hardwood lignins, indicating the great value of catalytic valorization of this linkage to aromatics.36 Under the basic conditions of our system, the conversion of phenethoxybenzene 1g could reach 65% within 2 h, together with 51% and 52% yields of phenylacetic acid 4 and phenol 6, respectively (Figure 2d). On the basis of the analysis of products from 1f−g, it can be concluded that both C−O and C−C bonds can be cleaved in a given substrate, but the former can be more easily broken.37 After having gained a based understanding of the respective cleavage of C−C and C−O bonds in different substrates, a comprehensive survey on simultaneous cleavage of the two types of bonds in other realistic β-O-4 lignin model compounds were tested in Table 2. The main aromatic monomers were Table 2. Catalytic Oxidative Cleavage of Lignin β-O-4 Model Compounds 1h−ja

Figure 2. Catalytic oxidative cleavage of (a) 1,3-diphenyl-2-propenol 1d, (b) diaryl ether 1e, (c) benzyl phenyl ether 1f, and (d) phenethoxybenzene 1g. Reaction performed on a 0.5 mmol scale. Yields determined by HPLC with benzyl alcohol as internal standard.

a

Reaction conditions: substrate (1 mmol), CuCl2/PBOZ (18 mol %), MeCN (0.22 mol), H2O2 (19 equiv); Conversions and yields determined after purification with column chromatography in EA/ MeOH (24:0.1). Asterisk (*) represents 1-(3,4-dimethoxyphenyl)-2(2-methoxyphenoxy)-2-propen-1-one (4% and 3% yield, respectively) found as a side product in both reactions of 1i and 1j by HPLC. The section symbol (§) represents 3,4-dimethoxyphenylglyoxylic acid (7% yield) found in products of 1i by HPLC. bYields of ketones determined by HPLC with diphenyl ether as internal standard.

However, the catalytic cleavage of 1a still proceeded more smoothly than 1d, suggesting that the neighboring alkyl C−C bonds are significantly weakened upon the oxidation of benzyl hydroxyl to carbonyl group.22 In addition, we studied the oxidative cleavage of several typical aromatic and benzylic C−O bonds that constitute more than half of all the intermonomer linkages in lignin (Figures 2b−d).35 The diaryl ether 1e is a 4O-5 linkage mimic, one of the common lignin linkages. The result in Figure 2b showed that phenol 6 was the sole main product of the oxidative cleavage of 1e with 50% yield in our catalytic system. Moreover, the test of benzyl phenyl ether 1f (a lignin a-O-4 link mimic) proceeded smoothly to afford benzoic acid 3 and phenol 6 in moderate yields of 50% and 54%,

obtained from the cleavage products after isolation by column chromatography (yields of ketones were determined by HPLC with diphenyl ether as internal standard). Under the aforementioned optimized conditions, monolignol 1h was oxidized rather more smoothly than phenethoxybenzene 1g, with 82% conversion obtained within 2 h at room temperature, which is consistent with previous findings that methoxy groups on the aromatic ring tend to promote the oxidative cleavage 6551

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Figure 3. (a) GPC of dealkaline lignin, and reaction time-course products of the lignin sample depolymerization. (b) Temporal progression of the AE, OE, and HE indices during the lignin sample depolymerization process with our catalytic system.

performed using different reaction time intervals. Besides, 2DNMR (HSQC) measurements before and after the depolymerization reaction were performed to monitor the cleavage of the characteristic interconnecting bonds within lignin. The degree of depolymerization for the lignin sample was determined by calibrated GPC analysis (Figure 3a). Gratifyingly, only after 8 h reaction time, it was observed that the majority of the lignin sample was degraded to products of a reduced size. Additionally, the results from Table S5 showed a significant decrease in molecular mass from 2609 Da before treatment to approximately 500 Da after a reaction time of 8 h. It can be also observed that longer reaction times (8 to 16 h, and then to 24 h) lead to the further depolymerization of lignin and a lower weight-average molecular weight (Mw) of products. The GPC profiles for different reaction times showed little change with each other, except for higher intensity and longer retention times, suggesting the increasing formation of low molecular weight products. The observed mass of 24 h reaction products (395 Da, polydispersity 1.22) corresponds to dimer− trimeric lignin fragments.42 Although possible slight self-repolymerization of lignin may exist, these integrated results exactly indicate that a certain degree of lignin depolymerization had occurred. In addition, a control experiment was conducted to eliminate the interference of sodium hydroxide for lignin depolymerization. It can be clearly observed from Figure S6 that NaOH almost has no catalytic effect on lignin degradation in this catalytic medium at room temperature without any catalyst and oxidant when compared with the depolymerization reaction after 16 h treatment under the standard conditions, suggesting that the vital synergistic role of the CuCl2/PBOZ composite and hydrogen peroxide. Additionally, 1H NMR allowed monitoring the oxidative cleavage of C−C and C−O bonds in lignin. According to the previously published method of metrics for classifying the course of lignin depolymerization,43 A represents the fraction of aromatic protons, O the fraction of protons on carbons that bear an oxygen (including −OH), and H those that bound to aliphatic carbons. The subscript “E” designates experimentally determined values obtained from the ratios of the integrations of defined and calibrated chemical shift regions Ri (i.e., RA, RO, and RH) in the 1H NMR spectra. We define that RA, RO, and RH are the areas for the range 8.5−6.0, 6.0−3.0, and 3.0−0.5 ppm, respectively. 2-Phenoxyethanol was used as a substance for the test of this definition (its AT and OT are both calculated as 50, while the published data give its AE and OE are 47.85 and 49.76, respectively44). Thus, the experimental H indices (AE, OE, and HE) of depolymerization products of the lignin sample

reactions. The major products obtained from 1h were guaiacol 7 and veratric acid 8 in 67% and 48% yield, respectively, along with a small amount of veratraldehyde 9 and the ketone 10h (Table 2, entry 1). Besides, erythro-dilignol 1i and its threo diastereomer 1j, the most common lignin-relevant β-O-4 linkage models, were synthesized following the reported procedure (Scheme S3).38 The catalytic oxidative of 1i, which bears an extra primary hydroxyl group from 1h, resulted in efficient cleavage to give 7 and 8 in good yields of 59% and 46%, respectively. Meanwhile, ketone 10i (8% yield) and 3,4Dimethoxyphenylglyoxylic acid (7% yield) was found and identified by HPLC, and only fewer 9 could be isolated (Table 2, entry 2). Compared with 1i, its threo diastereomer 1j was more efficiently cleaved with 83% overall conversion and afforded 7 and 8 in higher yields with 64% and 57%, respectively (Table 2, entry 3), showing that the steric hindrance plays a role in these oxidative cleavage reactions. Unexpectedly, only 4% yield of ketone 10i and no 3,4Dimethoxyphenylglyoxylic acid could be observed in the products from 1j. The ketone 10i is likely an intermediate in the formation of the guaiacol and veratric acid coproducts in both reactions of 1i and 1j with this catalytic system. We assume that the ketone 10i was first formed from the oxidation of dilignol (1i or 1j) and then underwent the aliphatic C−C and C−O bonds cleavage reactions. Indeed, the additional test of a prepared pure sample of the ketone 10i ran rather smoothly with 92% conversion to afford guaiacol 7 and veratric acid 8 in obviously higher yields of 76% and 68%, respectively than that from both 1i and 1j. Besides, 10% yield of veratraldehyde 9 was isolated and identified under the mild conditions (Figure S5). Importantly, for the oxidative cleavage of β-O-4 lignin linkages with green oxidant, our developed protocol is superior to previously reported heterogeneous catalytic systems in terms of both energy saving and sustainable chemistry.39,40 Catalytic Oxidative Depolymerization of Lignin. The research of the selective oxidative cleavage of C−C and C−O bonds in lignin models is essential for the study on lignin depolymerization that followed. Lignin model compounds provide an opportunity to assess the preliminary catalytic activity and selectivity of different systems. Nevertheless, as the structural complexity and irregular composition of actual lignin frequently weaken catalyst activity,41 a direct survey of lignin depolymerization with the aforementioned model-based results is more persuasive for the practical use of renewable biomass resources. To follow the depolymerization process of the lignin source by GPC and NMR spectroscopy, a series of tests was 6552

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Figure 4. Comparison of 2D-NMR HSQC spectrum for the aliphatic ether and alcohol region of alkaline lignin sample in DMSO-d6 ; (a) before and (b) after 16 h reaction time; (c) exemplary structures for the β-O-4′ aryl ether linkages (A, A′), resinol substructures (B) and p-hydroxycinnamyl alcohol linkages (I); for detailed 2D-NMR HSQC spectra of alkaline lignin sample see Figures S35−36.

Figure 5. (a) Recycling reaction and (b) leaching test of CuCl2/PBOZ in the cleavage reaction of chalcone 1a under the optimized reaction conditions. SEM images of CuCl2/PBOZ (c) before and (d) after 3 cycles of use. Recycling reactions performed on a 1.0 mmol scale. The main products and their quantification methods were the same as the reaction described in Table 1, entry 1. The total selectivity to aromatics is calculated from reaction conversion and the sum of yields of these monomers (S% = ∑Yi/(2*C)*100%, S means the total selectivity, Y is yield of the aromatic monomer i, and C refers to the reaction conversion).

the first 16 h, it can be seen that the value of OE increases obviously from 46.41 for lignin sample to 62.92 in depolymerization products, while HE drops from 27.95 to 16.25, although they were almost stable for the next 8 h. These

before treatment and after 8 h, 16 and 24 h were analyzed and exhibited in Figure 3b. The detailed 1H NMR spectra and their AE, OE, and HE of depolymerization product mixtures for different reaction times are given in Figures S31−34. During 6553

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copper dissolved in the mother liquid will continuously catalyze the conversion during the later 3.5 h. Figures 5c and 5d present the SEM images of the CuCl2/PBOZ catalyst before and after 3 cycles of use. No significant change in morphology between recovered catalyst and the fresh one was observed except that a few spheres were slightly out of shape. Collectively, the composite catalyst exhibits good recyclability and stability. The integrated results from Table S2, entries 2−4 suggest that the catalytic cleavage reactions proceeded via different mechanisms when individually using CuCl2·2H2O and CuCl2/ PBOZ as catalyst. Additionally, the main products in the test using CuCl2·2H2O were short-chain acids, such as acetic acid, malonic acid, etc., which means aromatic rings in chalcone 1a were destroyed in this catalytic process. Inspired by this observation and experimental results, we proposed a reaction path for the catalytic cleavage of the dilignol model 1i using the CuCl2/PBOZ composite (Figure S8a). In the three-step cycle, the initial activation of the composite adds an electron to form a phenoxy radical (step i), the active phenoxy radical then catalyzes the oxidative cleavage of 1i to generate aromatics (guaiacol 7, veratric acid 8, etc.) in step ii while a one-electron reduction process takes place in the copper center at the same time. Finally, CuI(LH) is oxidatively converted into CuII(L) (the original state) by H2O2 in step iii. This phenomenon is pervasive in metal-containing redox proteins, such as galactose oxidase, which undergoes a two-electron oxidation process with the help from a tyrosine ligand.27 Thereinto, however, the oxidative cleavage of 1i (step ii) is a two-step procedure rather than a direct cleavage one. It consists of a benzyl hydroxyl oxidation to the intermediate ketone 10i by H2O2 and then a selective aliphatic C−C and C−O bonds cleavage with CuCl2/ PBOZ. To confirm this two-step strategy, fundamental research of benzyl alcohol with our catalytic system has been conducted to give benzoic acid with yield of 76% (Table S6), further suggesting that the initial oxidation of benzylic alcohol in lignin is favorable for its ultimate depolymerization.46 We propose that this mechanism also applies to the oxidative cleavage of lignin β-O-4 models 1h and 1j, as well as actual lignin. Using CuCl2·2H2O as catalyst, however, chalcone 1a only afforded short-chain acids after aromatic rings were broken in this catalytic process via a simple Fenton-like reaction,47,48 as shown in Figure S8b.

results suggest that quite a few C−C and C−O linkages in aliphatic chains of the lignin substrate have been oxidatively cleaved to generate phenols and carbonyl compounds such as aromatic acids and aldehydes et al. in substantial amounts. However, only a slight decrease of AE is observed all along the reaction in Figure 3b, suggesting that most of valuable aromatic rings are retained in the course of lignin depolymerization. The number of protons at aliphatic sites will decrease and that of hydroxyl or carbonyl protons increase at the same time once the above process occurs. After all, the aliphatic C−C and C−O linkages in lignin are the only chemical bonds to be easily oxidized when its aromatic rings stay intact. Moreover, the sum of the three experimental H indices (AE, OE, and HE) is a constant. Therefore, a great deal of functionalized aromatic compounds can be obtained in actual lignin depolymerization with our catalytic system, in agreement with the prediction from the oxidative reactions of lignin model compounds. The 2D-NMR HSQC spectra of alkaline lignin sample before (a, left side) and after 16 h degradation reaction time (b, right side) with 18 wt % of CuCl2/PBOZ at room temperature are depicted in Figure 4. The interpretation of the spectra followed the method previously outlineed by Sun.45 It can be clearly seen that the β-O-4 linkages A identified in alkaline lignin sample were completely degraded as their characteristic signals in the HSQC disappeared except that the γ-signal was still slightly present but with a significantly reduced intensity. In addition, the corresponding signals for the acetylated β-O-4 linkages A′ and resinol structures B entirely vanished likewise during the reaction. For the p-hydroxycinnamyl alcohol substructures I, the γ-signal also completely disappeared after a reaction time of 16 h. These results were well consistent with GPC measurements. Besides, the detailed HSQC spectra of aromatic regions (δC/δH 100−135/5.5−8.5 ppm) in Figures S35−36 showed that most of aromatic rings are retained in the depolymerization course, which was in accordance to the results previously obtained from 1H NMR. Consequently, it can be concluded that this mild but efficient catalytic system is active for the selective oxidative cleavage of aliphatic C−C and C−O bonds in not only β-O-4 but also other characteristic linkages in alkaline lignin, resulting in the formation of valuable aromatic compounds that are the same types of main products (aromatic acids, aldehydes, phenols and ketones et al.) formed in the cleavage reactions of lignin model compounds 1h−j since the same linkages were oxidatively cleaved with the same catalytic system in both oxidative reactions of the lignin source and these models. Recyclability and Proposed Mechanism. The recyclability of the composite catalyst was investigated by carrying out the oxidative cleavage of 1a, and the result indicated that the CuCl2/PBOZ catalyst could maintain its catalytic activity and the total selectivity to aromatics (calculated from reaction conversion and the sum of yields of these monomers) remained above 96% during all the three consecutive cycles despite somewhat loss of conversion (Figure 5a). A measurement of copper percentage in catalyst before and after use was proformed with AAS analysis (Figure S7), showing that only a slight amount of copper (0.41 wt %, reduce from 2.95 wt % to 2.54 wt %) were dissolved in the solution during all the three cycles. Moreover, a leaching test in Figure 5b showed that the conversion remained constant after filtering the catalyst out, indicating that the dissolved copper has no significant influence on the catalytic reaction, and that the catalyst action is of heterogeneous rather than homogeneous nature. Otherwise the



CONCLUSIONS A newly devised type of CuCl2/PBOZ composites was prepared and used to catalyze the selective oxidative cleavage of C−C and C−O bonds in dealkaline lignin and a series of model substrates, including the 4-O-5, α-O-4, and β-O-4 lignin models with H2O2 as oxidant at room temperature and pressure, yielding valuable aromatic compounds in substantial amounts with high activity and excellent selectivity. On the basis of the results of HSQC and GPC, the main products formed in the lignin degradation reaction are dimer−trimeric aromatic acids, aldehydes, and phenols. Besides, the CuCl2/ PBOZ composite can be reused for at least three consecutive runs while the total selectivity to aromatics remains above 96%. Furthermore, the mechanistic insights into the catalytic reactions reveal a two-electron transfer process involved with a phenoxy radical, and an oxidation-then-cleavage route proposed for lignin depolymerization. Therefore, our study has paved a practical avenue for the heterogeneous catalytic conversion of lignin into aromatics under mild conditions with a widely available metal-based catalyst and an environment6554

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ACS Sustainable Chemistry & Engineering



friendly, nontoxic oxidant, which is vital for efficient use of renewable resources for fuels and fine chemicals.



EXPERIMENTAL SECTION



ASSOCIATED CONTENT

REFERENCES

(1) Nanayakkara, S.; Patti, A. F.; Saito, K. Chemical depolymerization of lignin involving the redistribution mechanism with phenols and repolymerization of depolymerized products. Green Chem. 2014, 16, 1897−1903. (2) Delidovich, I.; Hausoul, P. J. C.; Deng, L.; Pfutzenreuter, R.; Rose, M.; Palkovits, R. Alternative Monomers Based on Lignocellulose and Their Use for Polymer Production. Chem. Rev. 2016, 116, 1540− 1599. (3) Springer, S. D.; He, J.; Chui, M.; Little, R. D.; Foston, M.; Butler, A. Peroxidative Oxidation of Lignin and a Lignin Model Compound by a Manganese SALEN Derivative. ACS Sustainable Chem. Eng. 2016, 4 (6), 3212−3219. (4) He, M.-Y.; Sun, Y.-H.; Han, B.-X. Green Carbon Science: Scientific Basis for Integrating Carbon Resource Processing, Utilization, and Recycling. Angew. Chem., Int. Ed. 2013, 52 (37), 9620−9633. (5) Ma, Y.-Y.; Du, Z.-T.; Liu, J.-X.; Xia, F.; Xu, J. Selective oxidative C-C bond cleavage of a lignin model compound in the presence of acetic acid with a vanadium catalyst. Green Chem. 2015, 17, 4968− 4973. (6) Gao, F.; Webb, J. D.; Hartwig, J. F. Chemo- and Regioselective Hydrogenolysis of Diaryl Ether C-O Bonds by a Robust Heterogeneous Ni/C Catalyst: Applications to the Cleavage of Complex LigninRelated Fragments. Angew. Chem., Int. Ed. 2016, 55, 1474−1478. (7) Zhu, R.; Wang, B.; Cui, M.-S.; Deng, J.; Li, X.-L.; Ma, Y.-B.; Fu, Y. Chemoselective oxidant-free dehydrogenation of alcohols in lignin using Cp*Ir catalysts. Green Chem. 2016, 18, 2029−2036. (8) Dabral, S.; Mottweiler, J.; Rinesch, T.; Bolm, C. Base-catalysed cleavage of lignin β-O-4 model compounds in dimethyl carbonate. Green Chem. 2015, 17, 4908−4912. (9) Huang, X.-M.; Atay, C.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. M. Role of Cu-Mg-Al Mixed Oxide Catalysts in Lignin Depolymerization in Supercritical Ethanol. ACS Catal. 2015, 5, 7359−7370. (10) Besson, M.; Gallezot, P.; Pinel, C. Conversion of Biomass into Chemicals over Metal Catalysts. Chem. Rev. 2014, 114, 1827−1870. (11) Xu, H.-J.; Yu, B.; Zhang, H.-Y.; Zhao, Y.-F.; Yang, Z.-Z.; Xu, J.L.; Han, B.-X.; Liu, Z.-M. Reductive cleavage of inert aryl C-O bonds to produce arenes. Chem. Commun. 2015, 51, 12212−12215. (12) Díaz-Urrutia, C.; Sedai, B.; Leckett, K. C.; Baker, R. T.; Hanson, S. K. Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts. ACS Sustainable Chem. Eng. 2016, 4 (11), 6244−6251. (13) Jiang, Y.-Y.; Yan, L.; Yu, H.-Z.; Zhang, Q.; Fu, Y. Mechanism of Vanadium-Catalyzed Selective C−O and C−C Cleavage of Lignin Model Compound. ACS Catal. 2016, 6 (7), 4399−4410. (14) Xu, C.-P.; Arancon, R. A. D.; Labidi, J.; Luque, R. Lignin depolymerisation strategies: towards valuable chemicals and fuels. Chem. Soc. Rev. 2014, 43, 7485−7500. (15) Molinari, V.; Clavel, G.; Graglia, M.; Antonietti, M.; Esposito, D. Mild Continuous Hydrogenolysis of Kraft Lignin over Titanium Nitride-Nickel Catalyst. ACS Catal. 2016, 6, 1663−1670. (16) Song, Q.; Wang, F.; Cai, J.-Y.; Wang, Y.-H.; Zhang, J.-J.; Yu, W.Q.; Xu, J. Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation-hydrogenolysis process. Energy Environ. Sci. 2013, 6, 994−1007. (17) Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J. Isolation of Functionalized Phenolic Monomers through Selective Oxidation and C-O Bond Cleavage of the β-O-4 Linkages in Lignin. Angew. Chem., Int. Ed. 2015, 54, 258−262. (18) Barta, K.; Warner, G. R.; Beach, E. S.; Anastas, P. T. Depolymerization of organosolv lignin to aromatic compounds over Cu-doped porous metal oxides. Green Chem. 2014, 16 (1), 191−196. (19) Mottweiler, J.; Rinesch, T.; Besson, C.; Buendia, J.; Bolm, C. Iron-catalysed oxidative cleavage of lignin and β-O-4 lignin model compounds with peroxides in DMSO. Green Chem. 2015, 17, 5001− 5008.

General Procedure for Oxidative Cleavage of C−C and C−O Bonds. Typically, in a 50 mL round-bottom flask, the respective catalyst was added to a solution of substrate (0.5 mmol) in acetonitrile (6.0 mL, 0.11 mol). Then aqueous hydrogen peroxide (30 wt % in H2O) was added dropwise into the above solution or the flask was equipped with the oxygen balloon, stirred for a certain time at the desired temperature. Then the mixture was cooled down, dried over MgSO4, filtered, and the solvent was removed under reduced pressure. The samples were analyzed by HPLC using benzyl alcohol as an external standard or column chromatography for NMR spectroscopy. General Procedure for Oxidative Depolymerization of Lignin. Two hundred fifty milligrams of the lignin sample (dealkaline lignin) and CuCl2/PBOZ (45 mg, 18 wt %) were added in a 50 mL round-bottom flask. Next, acetonitrile (5.0 mL, 93.5 mmol) and NaOH (5.0 mL, 10 wt %) were added followed by the addition of H2O2 (30 wt % in H2O, 0.5 mL), and the mixture was stirred at room temperature for the desired time. Then the reaction mixture was cooled down, filtered, washed with MeCN (10 mL). The solvent and water were removed under reduced pressure. The product was dried under high vacuum overnight and analyzed by GPC, 1H NMR, and HSQC. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00732. Materials and instruments, extended experimental section, nitrogen adsorption/desorption isotherms, structural characterizations of the composites, XPS spectra, screening of preparation conditions of CuCl2/ PBOZ, catalyst screening, screening of reaction conditions, equations S1 and S2, probability of each pathway for the reactions of 1a−c, reaction kinetics and activation energies, oxidation of ketone intermediate 10i, GPC for oxidative depolymerization of the lignin sample, AAS analysis, proposed mechanism, fundamental research of benzyl alcohol, NMR spectra of chalcone 1a and its derivatives 1b−d, NMR spectra of the isolated products of lignin models, 1H NMR spectra of the product mixtures of lignin, and 2D-NMR HSQC measurements (PDF)



Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

Xinsheng Zheng: 0000-0003-2255-7507 Feifei Cao: 0000-0002-4290-2032 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This research was supported by the Fundamental Research Funds for the Central Universities, China (2662015PY163), the National Natural Science Foundation of China (51173059, 21303064), and Wuhan Chenguang Science and Technology Project for Young Experts (2015070404010192). 6555

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Research Article

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