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Dealkylation of lignin to phenol via oxidation-hydrogenation strategy Min Wang, Meijiang Liu, Hongji Li, Zhitong Zhao, Xiaochen Zhang, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00886 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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ACS Catalysis

Dealkylation of lignin to phenol via oxidation-hydrogenation strategy Min Wang,† Meijiang Liu,†,‡ Hongji Li,†,‡ Zhitong Zhao,† Xiaochen Zhang† and Feng Wang*,† †

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of

Chemical Physics, Chinese Academy of Sciences, Dalian 116023 (China) ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ABSTRACT Lignin is a renewable and abundant aromatic polymer found in plants. We herein propose a “cutting tail” methodology to produce phenol from lignin, which is achieved by combining Ru/CeO2 catalyst and CuCl2 oxidant via an oxidation-hydrogenation route. Phenol was obtained from separated poplar lignin with 13 wt % yield. Even raw biomass, such as poplar, birch, pine, peanut, bamboo willow and straw, could be converted into phenol in 1-33 mg per gram of biomass.

KEYWORDS: Biomass • Lignin • C-C bond cleavage • Catalysis • Phenol

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Introduction Lignin, an aromatic polymer, consists of phenylpropanoid building blocks which is majorly linked by C–O/C–C bonds. It potentially serves as a renewable source for producing aromatic chemicals.1-2 Recent interests are directed to the cleavage of β-O-4 bonds to aromatics via Cβ–O/Cα–Cβ bond cleavage, such as solvolysis,3-5 pyrolysis,6-7 reduction,8-23 oxidation,24-37 and redox-neutral methods.38-40 However, these methods generally produce phenols mixture with C1-C3 alkyl-tails (including alkyl, alkene, and oxygen-containing functionality) on the para sites. It is difficult to separate out pure products. Solving this problem may rely on innovative strategy that can afford relative narrow product distribution. Alkyl-free phenols, such as phenol, guaiacol and 2,6-dimethoxyphenol, are useful chemicals. Particularly, phenol is the key component of resin, bisphenol A and prolactam, and its world consumption was up to 7 million tons per year.41 The production of phenol from lignin, instead of cumene oxidation,41 is very interesting in the viewpoint of sustainability and economics, but also challenging due to the presence of alkyl tails. Cutting the alkyl tails via Cary–Cα bond cleavage not only leads to the fragmentation of lignin polymer, but also results in the formation of alkyl-free phenol products, which is rarely explored. Sels and coworkers reported the conversion of 4-propylphenol to phenol and propene.42 Cary–Cα bond in β-O-4 ketone models was cleaved via Baeyer–Villiger (BV) oxidation, while forming a new Cary–O bond.43 Cary–Cα bond in phenolic β-O-4 models was oxidatively broken up to generate benzoquinones.44 We herein propose a route for the preparation of alkyl-free phenols by the combination of Cα–Cβ bond oxidative cleavage to acids and subsequent decarboxylation (Scheme 1). We show that the Cary–Cα bond in phenolic lignin models was facially cleaved in CuCl2 aqueous solution. The H-based β-O-4 synthetic polymer was converted into phenol in 75% yield. By combination of C–O bond hydrogenolysis over Ru/CeO2 catalyst and Cary–Cα bond oxidative cleavage, poplar lignin was converted into phenol with up to 13 wt % yield. Some raw lignocellulosic biomass feedstocks were converted into phenol with 1-33 mg per gram of biomass.


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ACS Catalysis

Scheme 1. The production of phenol from lignin. Experimental section General information. RuCl3 was purchased from Shenyang Nonferrous Metal Research Institute. Active carbon and SiO2 were purchased from Macklin. ZSM-5 was purchased from Nankai University. CeO2 were prepared according to our previous work.45 Preparation of supported Ru catalyst. Ru/C, Ru/ZSM-5, Ru/ZSM-5 and Ru/SiO2 and Ru/CeO2 were prepared by an incipient-wetness impregnation method. RuCl3 was used as the ruthenium source. The Ru content of Ru/C, Ru/ZSM-5, Ru/SiO2 was 5 wt %. The Ru content of Ru/CeO2 was 2 wt %. Typically for the preparation of Ru/CeO2, 0.002 g of RuCl3 and 0.5 g of CeO2 was dispersed in 20 mL of deionized water, and magnetically stirred for 24 h. And then the water was evaporated by heating at 100 °C. The resulting solid was reduced at 400 °C (5 °C min-1) by H2 (5 mL min-1 H2 and 10 mL min-1 N2) for 2 h. Synthesis of lignin models. β-O-4 alcohols and ketones were prepared according to our previous work.25 The β-O-4 polymers were synthesized according to the previous report.46 The detailed procedures were provided in the supporting information. Reaction procedure. The catalytic reactions were performed in a 10-mL autoclave reactor with an internal Teflon insert. Typically, 0.2 mmol of lignin model, 2 mL of 0.25 M metal salts were added into the stainless steel autoclave with an internal Teflon insert, and then heated to 200 °C under magnetic stirring in Ar atmosphere. For the reaction of lignin extracts and raw biomass, 100 mg of substrate and ACS Paragon Plus Environment

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0.01 mmol of Ru catalyst was added and 3 MPa of H2 was charged into the reactor. The procedure for the reaction of raw biomass was the same as lignin extracts except that 0.2 mmol of AlCl3 was added. After the reaction, the water solution was extracted with 40 mL of ethyl acetate five times (8 mL × 5), and the as-obtained solution was concentrated to about 2 mL in a rotary evaporator under reduced pressure at 313 K. A certain amount of p-xylene was added into the above solution as internal standard. And then, the products were analyzed and quantified by gas chromatography−mass spectrometry (GC−MS) using an Agilent 7890A/5975C instrument equipped with an HP-5 MS column (30 m in length, 0.25 mm in diameter). For the analysis of the acid product, the acid product was first converted into the corresponding ester and then analyzed. The procedure was as follows: the solvent of extracted solution was removed in a rotary evaporator under reduced pressure at 313 K. The as-obtained residues were dissoved in 5 mL of methanol and 50 µl H2SO4 . The solution was heated at 100 oC for 6 h under Ar atmosphere. The yield was determined using internal standard method and the selectivity was determined via peak area percentage method. Column chromatography was employed to separation of phenol. Ethyl acetate and petroleum ether with 1:2 ratio was used as an eluent. Extraction of poplar lignin. To poplar sawdust (40 g) was added 1,4-dioxane (280 mL) followed by 2 mol L–1 HCl (36 mL) and the mixture was heated to a gentle reflux (390 rpm min-1) under a N2 atmosphere at 120 °C for 2.5 hour. The mixture was then allowed to cool down and the lignin containing liquor was collected by filtration. The collected liquor was partially concentrated by evaporation to give a gummy residue which was re-dissolved in 1,4-dioxane (10 mL) and then precipitated by addition to rapidly stirring water (250 mL). The crude lignin was collected by filtration and dried under vacuum. The dried crude lignin was dissolved in 1,4-dioxane (10 mL) and precipitated by dropwise addition to rapidly stirring Et2O (250 mL). The precipitated lignin was collected by filtration and dried under vacuum to give a purified poplar lignin (6 g). This lignin was used in subsequent experiments without further processing. Results and discussion ACS Paragon Plus Environment

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ACS Catalysis

Firstly, we used p-hydroxybenzoic acid to study the decarboxylation step in aqueous solution at 200 °C for 6 h under Ar atmosphere (Figure 1). The decarboxylation reaction did not occur without catalyst. Copper catalysts are known to be good for decarboxylation reaction.47 We then screened copper catalysts, including CuCl, Cu(NO3)2, Cu2O, CuO, Cu, CuSO4 Cu(OAc)2 and CuCl2 in this reaction. Although all these copper catalysts could offer phenol at a 20 mol % adding amount, CuCl2 showed the highest phenol yield with 48%. The anion may act as a ligand to affect the electronic properties of copper. We have characterized the copper salts by linear sweep voltammetry (LSV) (Figure S1). The onset reduction potential of CuCl2 is 0.25 V (vs SCE) which is much higher than Cu(OAc)2, CuSO4 and Cu(NO3)2, which may account for the higher activity. With excess amount of CuCl2, the phenol yield was increased to 79%. CO2 was detected in the gas phase (Figure S2a). Water is a better solvent than dioxane and methanol. p-Hydroxybenzoic acid was converted to p-hydroxybenzoate in methanol solvent which prohibit the decarboxylation reaction. The carboxylic acid groups is easier to dissociate to form carboxylate anion in water which may be the reason for the better performance in water solvent.

Figure 1. The decarboxylation of p-hydroxybenzoic acid. Reaction conditions: 0.2 mmol of phydroxybenzoic acid, 20 mol % of copper catalyst, 2 mL of water, Ar, 200 °C, 6 h. The results were determined by GC-MS. a 0.5 mmol of CuCl2. b Methanol as solvent. c Dioxane as solvent.


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Table 1. Substrate scope.a

a

Reaction conditions: 0.2 mmol of substrate, 2 mL of 0.25 M CuCl2 aqueous solution, Ar, 200 °C, 6 h.

The results were determined by GC-MS. b Isolated yields. c 50 mg of 2 wt % Ru/CeO2 was added and under 3 MPa of H2 atmosphere. d 0.1 M CuCl2 aqueous solution. The presence of phenolic group in the reactants is beneficial for the decarboxylation reaction. When 4-methoxybenzoic acid was used as substrate, the decarboxylation reaction could not proceed and only 2% yield of anisole was obtained (Table 1, entry 2). The presence of phenolic groups facilitates the formation of oxocarbonenium ion intermediates via reaction with the acidic site (Figure S3). The electron-withdrawing ability of oxocarbonenium ion can stabilize the negative intermediate and thus enhances the decarboxylation reaction. When a hydrogenation catalyst, such as Ru/CeO2, was used in H2 atmosphere, 4-methoxybenzoic acid was converted into phenol in 80% yield (Table 1, entry 3). pHydroxybenzoate, which is presence in lignin as PB unit, can also converted into phenol in 60% yield (Table 1, entry 4).48 Vanillic acid and syringic acid were also converted into guaiacol and syringol with 55 and 75% isolated yields, respectively (Table 1, entry 4 and 5). ACS Paragon Plus Environment

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The phenol building blocks are dominantly connected via β-O-4 linkages. Then, we investigated the combination of Cα–Cβ bond cleavage and decarboxylation for the β-O-4 cleavage. It is undesirable to carry out the reaction in oxygen atmosphere because phenols are unstable in oxygen atmospheres especially under high temperature.25-28 In our previous work, we found copper as a redox metal can be used as oxidant for the cleavage of C–C bond in the absence of oxygen.28 Here, we used CuCl2 as both oxidant for the Cα–Cβ oxidative cleavage and catalyst for the decarboxylation. The synthetic phenolic βO-4 polymer was broken down to phenol in CuCl2 solution with 75% yield [eq. (1)]. CuCl2 was reduced to copper as indicated by the X-ray diffraction characterization (Figure S4), and HCl was generated, resulting in the decrease of pH value from 5.4 to 2.5. Non-phenolic β-O-4 dimer with H unit underwent Cα–Cβ bond cleavage to acid and phenol [eq. (2)]. However, the Cary–Cα bond could not be broken via decarboxylation due to the absence of phenolic group. The β-C was converted into CO2 as evidenced by the detection of CO2 in the gas phase (Figure S2b). Previously, we have demonstrated the oxidative cleavage of β-O-4 is via β-O-4 ketone intermediates.25-28 Non-phenolic β-O-4 ketone with Cγ-OH were also underwent Cα–Cβ bond cleavage to acid and phenol [eq. (3)], indicating that the cleavage of β-O-4 dimer is probably via β-O-4 ketone. In the absence of Cα–OH hydroxyl group, no reaction happened [eq.


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(4)]. Our previous works showed that the Cα–Cβ bond in β-O-4 ketone linkage with G and S units is more difficult to be oxidatively cleaved due to the steric and electronic effect.25 The present method is selective for the cleavage of H-based β-O-4 monomer and show low efficiency in the cleavage of S- and G-based linkages [eq. (5)].

Scheme 2. Proposed reaction mechanism for the dealkylation of phenolic β-O-4 linkages. Based on the above results, a possible route for the dealkylation of β-O-4 linkage is proposed (Scheme 2). As demonstrated by our previous work,25-28 the Cα–Cβ bond in β-O-4 ketone is easier to be broken than that in β-O-4 alcohol. When 1-phenylethanol was used as probe molecules, acetophenone was detected as intermediates (Figure S5). The oxidative cleavage of C(OH)-C bond is probably proceeded via C(=O)-C intermediate, that is, oxidation of Cα–OH to ketone first takes place, and then oxidative cleavage of Cα–Cβ bond results in the formation of p-hydroxybenzoic acid. The Cα–Cβ bond cleavage is probably via hydroxyl ketone structure-like intermediate which is formed via the Cβ-H bond oxidation.25,49 CuCl2 as oxidant is reduced to copper, releasing HCl. Finally, CuCl2 catalyzes the decarboxylation of p-hydroxybenzoic acid to phenol. The α-C and β-C was converted into CO2 as evidenced by the detection of CO2 in the gas phase (Figure S2). Then, we tried to depolymerize organosolv lignin. The lignin was extracted from the lignocelluosic source via acid treatment in the dioxane solvent. Phenol in 2.6 wt % yield was obtained as the major product from poplar lignin, together with 0.2 wt % yield of guaiacol and syringol. As proved above,


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without phenolic groups, no dealkylation occurred. To further increase the phenol yield, hydrogenation catalysts were used to create more phenolic groups. With supported Ru catalyst and under hydrogen atmospheres, the yields of phenol were all increased (Figure 2a). Ru/CeO2 with 1 nm Ru cluster (Figure S6), prepared according to our previous work,50 showed the best performance and offered 13 wt % yield of phenol along with 2 wt % and 0.5 wt % yields of guaiacol and syringol, respectively. The selectivity of phenol was up to 60% (Figure 2b and S7a). As shown above, the β-O-4 linkage with G and S units is more difficult to be cleaved due to the steric and electronic effect, which explains the formation of a small amount of G- and S-based monomer. Instead, with Ru/CeO2 as a catalyst and in the absence of CuCl2, no phenol was obtained and a mixture of products, including substituted phenols and saturated compounds, was produced (Figure S7b).

Figure 2. (a) The dealkylation of extracted poplar lignin over supported Ru catalyst in CuCl2 aqueous solution. (b) The product distribution over Ru/CeO2, which was determined via peak area percentage method. Others are the aliphatic. Reaction conditions: 100 mg of poplar lignin, 0.01 mmol of Ru, 2 mL of 0.25 M CuCl2 aqueous solution, 3 MPa of H2, 200 °C, 6 h.


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The role of Ru/CeO2 is the hydrogenolysis cleavage of R–OAry bond to phenolic group and thus facilitates the decarboxylation reaction. With the addition of Ru/CeO2 and under hydrogen atmosphere, p-methoxybenzoic acid was converted into phenol in 80% yield (Table 1, entry 3). Me–OAry bond was cleaved over Ru/CeO2 to form phenolic group, resulting in the decarboxylation of p-hydroxybenzoic acid to phenol over CuCl2. By combination of Ru/CeO2 and CuCl2, p-methoxyl substituted lignin model was converted to phenol. The selectivity of Cary–Cα bond cleavage was 77% (Figure 3).

Figure 3. The dealkylation of non-phenolic lignin models over Ru/CeO2 in CuCl2 aqueous solution. Reaction conditions: 0.2 mmol of model, 50 mg of 2 wt % Ru/CeO2, 2 mL of 0.25 M CuCl2 aqueous solution, 3 MPa of H2, 200 °C, 6 h.

Next, we tried the direct conversion of raw biomass, which majorly consists of lignin, hemicellulose and cellulose. The lignin twines with hemicellulose and cellulose, forming complex and robust structure, hampering it to be accessed by solid catalyst, and therefore is a more challenging task. The oxidationhydrogenation strategy via combination of Ru/CeO2 and CuCl2 is applicable to the raw biomass resource (Table 2). AlCl3 was added to hydrolyze cellulose to release lignin. The lignin was converted to phenol, and cellulose was majorly converted to levulinic acid (LA) and γ-valerolactone (GVL) (Figure S7c). Various raw biomass sources, including poplar, pine, birch, peanut, bamboo willow and ACS Paragon Plus Environment

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ACS Catalysis

straw, were tested, and 1-33 mg of phenol, 70-224 mg of LA and 2-48 mg of GVL per gram of biomass were obtained, respectively. The poplar showed the highest yield of phenol with 33 mg g-1.

Table 2. The conversion of raw biomass. a

a

Entry

Lignin source

Phenol / mg g-1

LA / mg g-1

GVL / mg g-1

1

Poplar

33

222

14

2

Pine

6

224

2

3

Birch

1

58

48

4

Peanut

7

196

6

5

Bamboo willow

5

70

42

6

Straw

3

97

6

Reaction conditions: 100 mg of substrate, 50 mg of 2 wt % CeO2, 0.2 mmol of AlCl3, 2 mL of 0.5 M

CuCl2 aqueous solution, 3 MPa of H2, 200 °C, 6 h. The results were expressed as mg product per gram of biomass (mg g-1).


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It should be noted that besides the three primary building blocks of H, G and S units, there are some other units, such as p-hydroxybenzoate (PB), which is similar to the H unit and can be the resource of phenol. As shown in Figure 4, the p-hydroxybenzoate (PB) unit, such as p-hydroxybenzoic acid, p-methoxylbenzoic acid and p-hydroxybenzoate, were effectively converted into phenol with around 80% yields (Table 1, entry 1, 3 and 4). It is reported that poplar lignin is rich in PB unit.48 We also characterized the poplar lignin and birch lignin by 2D HSQC NMR (Figure 4). The content of PB unit is up to 17.1% for poplar lignin, while birch lignin contains only 2.1% PB unit. This accounts for the higher yield of phenol from poplar lignin. Because poplar lignin contains 17.1% PB unit and H unit was not detected, the formation of phenol majorly comes from PB unit via ester bond cleavage.

Figure 4. 2D HSQC NMR characterization of poplar and birch lignin. Conclusion In summary, we reported an oxidation-hydrogenation strategy for conversion of lignin to phenol by combination of Ru/CeO2 and CuCl2. Ru/CeO2 hydrogenolyzes C–O bond to release phenolic group and ACS Paragon Plus Environment

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CuCl2 accounts for the oxidative cleavage of Cary–Cα bond. This strategy can be used in converting lignin extracts and native lignin. The lignin-based process is a promising and renewable route for the production of phenol ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Supporting Information. Detailed experimental procedures, the synthesis of lignin models, some experimental data, NMR spectra and GC-MS spectra. “This material is available free of charge via the Internet at http://pubs.acs.org.” ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21603219, 21690082), DICP (DICP ZZBS201613), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300), Department of Science and Technology of Dalian under the contract of 2017RQ114 and Department of Science and Technology of Liaoning province under the contract of 2015020086-101. REFERENCES (1) Rinaldi, R.; Jastrzebski, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C. A.; Weckhuysen, B. M. Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis. Angew. Chem. Int. Ed. 2016, 55, 8164-8215. (2) Wang, M.; Ma, J. P.; Liu, H. F.; Luo, N. C.; Zhao, Z. T.; Wang, F. Sustainable Productions of Organic Acids and Their Derivatives from Biomass via Selective Oxidative Cleavage of C-C Bond. ACS Catal. 2018, 8, 2129-2165. (3) Ma, R.; Hao, W.; Ma, X.; Tian, Y.; Li, Y. Catalytic ethanolysis of Kraft lignin into high-value smallmolecular chemicals over a nanostructured alpha-molybdenum carbide catalyst. Angew. Chem. Int. Ed. 2014, 53, 7310-7315. ACS Paragon Plus Environment

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Dealkylation of Lignin to Phenol via Oxidation ... - ACS Publications

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