Photocatalytic Oxidation and Subsequent Hydrogenolysis of Lignin Î²

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Photocatalytic Oxidation and Subsequent Hydrogenolysis of Lignin #-O-4 Models to Aromatics Promoted by In Situ Carbonic Acid Yu Cao, Ning Wang, Xing He, Hong-Ru Li, and Liang-Nian He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03498 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Sustainable Chemistry & Engineering

Photocatalytic Oxidation and Subsequent Hydrogenolysis of Lignin βO-4 Models to Aromatics Promoted by In Situ Carbonic Acid Yu Cao, †, ‡ Ning Wang,‡ Xing He,‡ Hong-Ru Li,*,† Liang-Nian He*,‡ †College of Pharmacy, Nankai University, Tongyan Road 38, Tianjin, 300353, P R China ‡State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Weijin Road 94, Tianjin, 300071, P R China E-mail: [email protected] (L. N. He); [email protected] (H. R. Li) KEYWORDS: Carbon dioxide, Hydrogenolysis, Lignin, Photocatalysis, Selective oxidation

ABSTRACT: The cleavage of C-O bond in lignin β-O-4 model compounds to form aromatics has been achieved via a two-step process, comprising visible-light photocatalytic oxidation and in situ carbonic acid-facilitated hydrogenolysis. In the first step, with readily available persulfate as radical initiator and cheap copper as catalyst, the secondary alcohol in the β-O-4 alkyl-aryl ether linkage is selectively oxidized to the corresponding ketone in up to 99% yield under visible light irradiation. The second step features the C-O bond cleavage of lignin β-O-4 ketones promoted by in situ acidic EtOH/H2O/CO2 system in the presence of zinc powder, producing acetophenones and phenols in high yield. This protocol provides a novel alternative to selective fragmentation of β-O-4 linkage to aromatic monomers under mild reaction conditions.

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INTRODUCTION The depletion of fossil fuel has animated people to seek for renewable carbon resources. Biomass which can be converted into fuels and value-added chemicals is the most promising substitute to fossil fuels. In this aspect, lignin is one of the most abundant biomass being considered to be a potentially valuable feedstock for the production of organic aromatic chemicals.1,2 The most common structural motif in this three-dimensionally amorphous polymer is the β-O-4 alkyl-aryl ether linkage. Therefore, strenuous efforts have been devoted to break this ether linkage, thus leading to realizing efficient valorization of lignin.3-7 Very recently, a twostep process has been proved to be efficient in the ether linkage cleavage and has therefore attracted much attention. This strategy includes selective pre-oxidation, followed by lignin depolymerization, wherein about 90 kJ/mol bond dissociation energy of the Cβ-OAr bond can be reduced in the selective oxidation of the Cα-OH to Cα=O in the common β-O-4 motif,8-12 thus facilitating the C-O bond cleavage to produce high-value aromatic products in the subsequent step (Scheme 1). Scheme 1. Two-step Conversion Strategy of Lignin β-O-4 Linkage


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In this context, a variety of catalytic systems have already been developed for oxidation of the secondary benzylic alcohol in lignin model compounds, including metal-free methods such as 4AcNH-TEMPO/HNO3/HCl/O2,13

[4-AcNH-TEMPO]BF4,14

2,3-dichloro-5,6-dicyano-1,4-

benzoquinone (DDQ)/tBuONO/O2,15 and transition-metal-based catalytic systems including VOSO4/TEMPO/O2,16 Pd/C/glucose/O2,17 Cp*Ir.18 Compared with these traditional methods being generally required elevated temperatures, mild and environmentally friendly photocatalysis approaches are considered as important alternatives to perform lignin degradation under mild condtions.19,20 In this regard, Stephenson et al. have reported the efficient oxidation of β-O-4 model using the Ir-based photosensitizer combined with Pd catalyst under visible light irradiation.21 Besides, heterogeneous semiconductors such as Pd/ZnIn2S422 and carbazolic porous organic frameworks (POFs)23, metal-free photocatalytic systems, such as DDQ/NaNO2/O2,24 DDQ/tBuONO/O2,25 or 4,4′-bis(dimethylamino)benzophenone (DPA-BP)/NHPI/O215 have also been employed for selective photocatalytic oxidation of lignin β-O-4 model. However, it is still highly desired to develop more efficient photocatalytic methods without expensive photosensitizers, noble metal catalysts, or potentially toxic reagents. Persulfate is a kind of cheap, environmentally benign oxidant and free radical initiator. It is usually activated by heat or transition metal to form sulfate free radical (SO4•-),26-29 which is oneelectron oxidant and hydrogen abstraction agent. Recently, persulfate has involved in visible light-driven photocatalysis.30,31 Diverse amines,32 amides,33,34 sulfonamides,35 ethers,36 and aldehydes37 can be oxidized or undergo hydrogen abstraction pathway in the presence of SO4•- to form radical intermediates which can thereafter employed in organic reactions to yield various chemical compounds. Based on the fact that SO4•- can oxidize several kinds of compounds, we


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hypothesized benzylic alcohol in β-O-4 motif may undergo the similar reaction in the presence of SO4•-, thus perform selective benzylic oxidation to obtain β-O-4 ketone. In addition, the C-O bond cleavage for the more labile β-O-4 ketones is the key step for the lignin conversion to access industrially useful aromatic monomers. The acid-promoted reduction is a well-known process, wherein the acid additive has a great influence on the conversion of βO-4 ketone and could increase product yield significantly. The frequently-used acid additives include Brønsted acid (e.g., HCOOH,11,12,14,233 H3PO438) and Lewis acid (e.g., BF3·OEt2)39,40. Recently, we have firstly applied in situ methylcarbonic acid derived from CO2 and MeOH for highly selective hydrogenolysis of β-O-4 ketones via a homogeneous PdCl2 catalysis.41 In situ alkylcarbonic acid/carbonic acid provides environmental benefits from the viewpoint of green chemistry such as self-neutralization, simple post-processing and none waste disposal. Notably, combined with various metal powders (Zn, Fe, Pd/C, Pt/Al2O3), in situ alkylcarbonic acid/carbonic acid has been reported to realize the reduction of ketones,42 aldehydes,43 imines,44,45 nitroarenes,46-49 sulfoxides.50 We envisaged such protocol could be further extended to the reduction of lignin β-O-4 ketone. Herein, a two-step strategy was used to facilitate the cleavage of β-O-4 linkage in lignin model compounds as shown in Scheme 1. The combination of one-electron oxidant and hydrogen abstraction reagent SO4•- with Cu catalysis was employed in the pre-oxidation step, and the oxidized lignin model compounds then underwent in situ EtOH/H2O/CO2 acid- promoted hydrogenolysis in the presence of zinc powder, producing acetophenone and phenol in high yield.


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RESULTS AND DISCUSSION Photocatalytic Oxidation of Lignin β-O-4 Models. We first used 2-phenoxy-1-phenylethanol (1a) as the model substrate, K2S2O8 as the oxidant and photoinitiator. No oxidation product was observed (Table 1, entry 1). Due to those metal compounds such as Cu(II), Fe(III) being known to efficiently conduct electron transfer oxidation of benzylic radicals,51-54 they were screened for this purpose. Delightedly, 2-phenoxy-1-phenylethanone (2a) was obtained in 57% yield when CuBr2 was added (entry 2). Among metal bromides, CuBr2 showed the best catalytic activity (entry 2 vs. 3-7). However, other cupric salts were almost inactive (entries 8-12). Besides, cuprous salts performed inadequate activity either (entries 13-15). The blank experiments revealed that the presence of an oxidant e.g. K2S2O8 is prerequisite for running this kind photocatalytic reaction (entry 16). Furthermore, the reaction can hardly occur in the absence of light irradiation (entries 17-18). Complete inhibition of the reaction by TEMPO demonstrated that the reaction goes through a free radical pathway (entry 19). Based on the above results, it is inferred that the metal salt play important roles in both photolysis of K2S2O8 and oxidation of benzylic radical since K2S2O8 can’t be stimulated by visible light directly to generate the radical. It is likely Cu2+ may form the charge-transfer complex with bromide, β-O-4 model compound or even solvent and persulfate molecule to promote the photolysis of S2O82- under the irradiation of visible light.55 We then optimized the oxidants and solvents as listed in Table 2. Oxygen, H2O2, and t

BuOOH were ineffective oxidants for this reaction, while K2S2O8 worked well (entries 2-4 vs.

1). In addition, (NH4)2S2O8 as oxidant gave better result in comparison with using K2S2O8 (entry 5 vs. 1), being ascribed to better solubility of the oxidant in the reaction mixture. On the other hand, 1,4-dioxane was shown to be the appropriate solvent with the best yield and selectivity


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(entry 7 vs. entries 5-8). Acetone and dichloromethane gave poor results (entries 6, 8). It is speculated that the 1,4-dioxane may stabilize the free radical intermediate produced in the reaction. Further reducing the amount of (NH4)2S2O8 or CuBr2 resulted in a decrease in 2a yield (entries 9-10).

Table 1. Metal Salts Promoted Photo-Oxidation of β-O-4 Model Compoundsa

Entry

Catalyst

Conversion (%)

Yield (%)b

1

--

trace

trace

2

CuBr2

59

57

3

KBr

trace

trace

4

FeBr3

28

25

5

CoBr2

31

28

6

NiBr2

43

8

7

AgBr

24

2

8

CuCl2

5

trace

9

Cu(OAc)2

trace

trace

10

Cu(CF3SO3)2

trace

trace

11

CuSO4

trace

trace

12

Cu(NO3)2

trace

trace

13

CuBr

8

6

14

CuI

5

2

15

CuCl

trace

trace


6

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16c

CuBr2

trace

trace

17d

CuBr2

trace

trace

18e

CuBr2

9

7

19f

CuBr2

trace

trace

a

Reaction conditions: 1a (0.1 mmol, 21.5 mg), catalyst (10 mol%), K2S2O8 (5 equiv., 135.0 mg), diethyl ether (1 mL), 475 nm visible light, r.t., 10 h. b Yield was determined by GC with 1,3,5-trimethoxybenzene as internal standard. c Without oxidant. d In the dark. e 60oC in the dark. f 5 equiv. TEMPO (86.0 mg) was added.

Table 2. Optimization of Model Oxidation Reactiona

Entry

Oxidant

Solvent

Conversion (%)

Yield (%)b

1

K2S2O8

diethyl ether

59

57

2

O2

diethyl ether

8

5

3

H2O2

diethyl ether

trace

trace

4

t

BuOOH

diethyl ether

11

9

5

(NH4)2S2O8

diethyl ether

64

63

6

(NH4)2S2O8

acetone

17

3

7

(NH4)2S2O8

1,4-dioxane

99

96

8

(NH4)2S2O8

dichloromethane

50

20

9c

(NH4)2S2O8

1,4-dioxane

58

56

10d

(NH4)2S2O8

1,4-dioxane

73

71

a

General conditions: 1a (0.1 mmol, 21.5 mg), CuBr2 (10 mol%, 2.2 mg), oxidant 5 equiv., solvent 1 ml, 475 nm visible light, r.t., 10 h. b Yield was determined by GC with 1,3,5-trimethoxybenzene as internal standard. c (NH4)2S2O8 (2.5 equiv., 57.0 mg). d CuBr2 (5 mol%, 1.1 mg).


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With this photo-oxidation protocol in hand, the suitability of the substrates was investigated. As listed in Table 3, the substrates with various electron donating substituents or electron withdrawing substituents on the phenoxyl ring were successfully oxidized under the optimal conditions. It is worth noting that electronegativity of the substituent at the para-position of the phenoxyl ring has remarkable influence on the reaction outcome. The electron-donating group such as methyl, methoxy reduced the reactivity significantly (2b, 2c vs. 2a) while the electronwithdrawing substituent i.e. bromo or chloro at para-position gave nice results (2d, 2e vs. 2a). Notably, the substrates with electron donating substituents attached to the ortho or meta position of the phenoxyl ring or on the phenylethanol ring also worked well to afford the corresponding oxidation products 2f-2j with good to excellent yields and selectivities. It is speculated that the substrate can coordinate to Cu2+, thus leading to altering the redox potential of the copper complex. When the substrate bearing electron-donating group at the para-position of the phenoxyl ring was used, the redox potential of the copper complex may shift toward negative value, thus the oxidation of benzylic radical occurs more difficultly (2b and 2c) than the substrates without electron-donating group at the para-position of the phenoxyl ring. To assess the dependence of reactivity upon the light, an on-off experiment was carried out as depicted in Figure 1a. After being irradiated for 4 h, the light was turned off and it was found that the yield of 2a would not increase. When the reaction system was re-irradiated from 8 h, 2a yield is increasing, being implied the reaction continued. Therefore, light not only acts as a radical-initiator, but also a continuous supply of energy. Then the kinetics of the reaction was monitored as shown in Figure 1b. The yield of 2a was measured at different reaction time, and the yield versus time followed an S-Shaped curve. The reaction was found to have an induction


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period of 30 min. It is assumed that free radical accumulated gradually during this period, and then the reaction rate gradually increased, and the yield reached 92% after 6 h. Table 3. Substrate Scope of Oxidation Reactiona

2a, 96%

2b, 26%

2c, 51%

2d, 71%

2e, 96%

2f, 88%

2g, 99%

2h, 98%

2i, 99%

2j, 72%

a

Reaction conditions: 1 0.1 mmol, CuBr2 (10 mol%, 2.2 mg), (NH4)2S2O8 (5 equiv., 114.0 mg), 1,4-dioxane (1 mL), 475 nm visible light, r.t., 10 h. b Isolated yield.


9


100

100

b

a

80 Yield (%)

80 Yield (%)

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 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20

60 40 20

0

0 0

2

4

6 Time (h)

8

10

12

0

2

4

6 8 Time (h)

10

12

Figure 1. a) On-off experiment. b) Temporal evolution of 2a during 12 h runs. Reaction conditions: 1a (0.1 mmol, 21.5 mg), CuBr2 (10 mol%, 2.2 mg), (NH4)2S2O8 (5 equiv., 114.0 mg), 1,4-dioxane (1 mL), 475 nm visible light, r.t., GC yield. Based on the above experiments, the possible reaction mechanism is proposed as shown in Scheme 2. S2O82- can generate SO4•- under the irradiation of light. As a reactive intermediate, SO4•- initiates the oxidation of the model compound to give the aryl radical cation, followed by loss of a benzylic proton. Thus, the benzylic radical intermediate I is formed.31a, 31b Subsequent oxidation of benzylic radical intermediate I occurs to afford the alkyl carbocation intermediate II. Simultaneously, Cu2+ is reduced to Cu+, which is then oxidized to Cu2+ by SO4•- to complete the catalytic cycle. Finally, the unstable intermediate II further releases proton to furnish the target product 2a.


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Scheme 2. Plausible Reaction Mechanism S2O82h OH SO4

O

SO42-

Cu2+

Cu+

- (e- + H+)

HSO4-

OH

OH O

I

O O

- H+

O

II

Hydrogenolysis of Lignin β-O-4 Ketones Promoted by in situ Carbonic Acid. We have already developed a visible-light-promoted copper-catalyzed selective oxidation of lignin β-O-4 models to produce β-O-4 ketones. Those results stimulated us to further investigate the cleavage of the C-O bond in β-O-4 ketone. Therefore, hydrogenolysis of β-O-4 ketone was further studied in the presence of in situ acid and metal powder. 2-phenoxy-1-phenylethanone (2a) was used as a model substrate and several metal powders were tested. Those metals including Mg, Mn, Fe, Ti were not effective at all (for details see Supporting Information). Satisfyingly, when zinc powder was employed as a reducing agent and water as the solvent, hydrogenolysis products i.e. acetophenone (3a) and phenol (4a) were obtained in high yields (Table 4, entry 1). This may be because the reducibility of zinc and the coordination ability of zinc atom with the carbonyl and Cα-oxygen atoms of 2a to form a five-membered cyclic transition state56 promote the C-O bond cleavage. Then, the reactivity of in situ carbonic acid and in situ alkylcarbonic acid were investigated (entries 1-4). As a result, less acidic system e.g. in situ alkylcarbonic acid (EtOH/CO2) caused the product yield dramatically decreased (entry 2 vs. 1). Notably, the best result was found with a mixed solvent of EtOH-H2O (2:1) under 2 MPa CO2 (entry 4), being ascribed to sufficient dissolving capacity and acidity. As expected, the reaction hardly proceeded


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without CO2 (entry 4 vs 5-6). It is evident that the increasing acidity under relative high CO2 pressure could account for CO2 pressure effect. Further reducing the amount of zinc powder to 2 equiv, still gave a good result (entry 7). To be delighted, high efficiency was still retained even at room temperature (entry 8). Remarkably, quantitative results were achieved at 60 oC within 2 h (entry 9). Based on the above results, a mixture of a ratio of 2:1 of EtOH and H2O under 2 MPa CO2 is the most effective. Various substrates which mimic the predominant lignin substitution pattern of different oxidized lignin were investigated at the optimal reaction conditions as summarized in Table 5. Gratifyingly, these β-O-4 ketones with -OMe in either the ketone part or the phenol part were converted to the corresponding C-O bond cleavage products and the yields of phenol and ketone were 94-99 %. The over-hydrogenation of aromatic ring and carbonyl did not occur. Those results proved that the catalytic system has sufficient reduction ability and a wide substrate scope.


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Table 4. Optimization of Model Hydrogenolysis Reactiona

Zn

CO2

Entry

Solvent

T/oC

Conversion

Yield (%)b

(%)

3a

t/h

(equiv.) (MPa)

4a

1

3

2

H2O

60

12

90

87

86

2

3

2

EtOH

60

12

23

6

3

3

3

2

EtOH-H2O (1:2)

60

12

89

88

86

4

3

2

EtOH-H2O (2:1)

60

12

99

99

99

5

3

1

EtOH-H2O (2:1)

60

12

69

65

62

6

3

0

EtOH-H2O (2:1)

60

12

2

2

-

7

2

2

EtOH-H2O (2:1)

60

12

88

87

85

8

3

2

EtOH-H2O (2:1)

r.t.

12

90

90

90

9

3

2

EtOH-H2O (2:1)

60

2

>99

>99

98

a

Reaction conditions: 2a (0.5 mmol, 106.1 mg), solvent (3 mL), the total volume of solvent is 3 mL. b Yield was determined by 1H NMR with 1,3,5-trimethoxybenzene as internal standard.


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Table 5. In Situ EtOH/H2O/CO2 Acid-Promoted Hydrogenolysis of β-O-4 Ketonesa Entry

Substrate

1

Conversion (%）

Yield (%)b

99 2a

2

3a, 99

4a, 99

3a, 95

4b, 99

3a, 97

4c, 99

3b, 99

4a, 95c

3b, 99

4b, 94

3b, 99

4c, 99

99 2f

3

99 2h

4

99 2i

5

99 2k

6

99 2j

a

General conditions: 2 0.5 mmol, Zn (1.5 mmol, 98.7 mg), CO2 2 MPa, EtOH 2 mL, H2O 1 mL, 60 oC, 12 h. b Yield was determined by 1H NMR with 1,3,5trimethoxybenzene as internal standard. c Isolated yield.

It is speculated the in situ acid formed by CO2 with EtOH and H2O could act as a proton source in the hydrogenolysis of β-O-4 ketone being verified by deuterium labeling experiment. When zinc powder was stirred with 2a in D2O overnight in the presence of CO2, a characteristic


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peak at 2.61-2.63 ppm corresponds to the deuterated acetophenone (Figure 2). Moreover, the reaction in the absence of any acid did not take place under 2 MPa H2; while, addition of HCl greatly improved the reaction delivering the target product in 99% yield (for details see Supporting Information). Therefore, it is concluded that the in situ carbonic acid formed by CO2 and H2O promotes the reaction sufficiently.

Figure 2. 1H NMR (400 MHz, D2O): the mixture of 2a (0.5 mmol, 106.1 mg) and Zn (1.5 mmol, 98.7 mg). Based on the above experimental results, a possible reaction mechanism was proposed as depicted in Scheme 3. First, the in situ acid is formed by CO2 and H2O and concurrently the zinc atom coordinates with the substrate 2a to form the five-membered cycle species. Subsequent protonation leads to the formation of the target product. In deed, the solid residue in the resultant was confirmed to be zinc carbonate by XRD characterization (see Supporting Information).


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Scheme 3. Possible Reaction Mechanism

CONCLUSION A two-step process for the cleavage of the C-O bond in lignin β-O-4 model compounds has been successfully developed. The first step features a simple and efficient photocatalytic oxidation of lignin β-O-4 models. With readily available persulfate as initiator and oxidant, cheap copper as catalyst, this method allows selective oxidation of a series of lignin model substrates to afford the corresponding β-O-4 ketones in good to excellent yields under visible light irradiation. In the second step, the reductive system composed by zinc powder and in situ EtOH/H2O/CO2 acid efficiently promotes the cleavage of C-O bond in lignin β-O-4 ketones, producing acetophenones and phenols in high yield and selectivity. The in situ acid serves as the proton source for the reduction of various substrates. Such a process with in situ acid can be inherently neutralized by depressurizing CO2, offering feature advantages for simple postprocessing and none waste disposal. The two-step protocol in this work represents a sustainable, cost-effective, environmentally benign protocol to achieve efficient and highly selective fragmentation of lignin β-O-4 model compounds to aromatic monomers. As the conversion of


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native lignin is more valuable and still remains great challenges,57,58 this protocol may pave an alternative way to promote the native lignin conversion. The intensive study will be continued in our future research.

EXPERIMENTAL SECTION General Procedure for Photocatalytic Oxidation of Lignin Model Compounds A mixture of 2-phenoxy-1-phenylethanol (1a) (0.1 mmol, 21.7 mg), (NH4)2S2O8 (5 equiv., 114.0 mg), CuBr2 (10 mol%, 2.2 mg), 1,4-dioxane (1 mL) were placed in a 10 mL threaded glass tube at room temperature. Then the reaction system was stirred for appropriate time under the irradiation of 300 W Xe lamp (475 nm). After the reaction, products were then diluted by dichloromethane and analyzed by GC with 1,3,5-trimethoxybenzene as internal standard. The residue was purified by column chromatography on silica gel (200-300 mesh, eluting with nhexane/ethyl acetate from 10:1 to 3:1) to afford the desired product. The isolated products were further identified with NMR and GC-MS analysis (see Supporting Information). General Procedure for Reduction of Lignin Model Compounds A mixture of 2-phenoxy-1-phenylethanone (2a) (0.5 mmol, 106.1 mg), Zn (3 equiv., 98.7 mg), EtOH (2 mL), H2O (1 mL) were placed in a 25 mL stainless steel autoclave equipped with an inner glass tube at room temperature. CO2 (2 MPa) was subsequently introduced into the autoclave and the system was heated under the predetermined reaction temperature for 25 min to reach the equilibration. The mixture was stirred continuously for the desired reaction time. After cooling, products were then diluted by dichloromethane and analyzed by GC and 1H NMR with 1,3,5-trimethoxybenzene as internal standard. The residue was purified by column chromatography on silica gel (200-300 mesh, eluting with n-hexane/ethyl acetate from 50:1 to


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10:1) to afford the desired product. The isolated products were further identified with NMR and GC-MS analysis (see Supporting Information).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxx. The following files are available free of charge. Spectra data and copies of 1H NMR and 13C NMR spectra of all compounds. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +86-22-23503878; Tel: +86-22-23503878 *E-mail: [email protected]; Fax: +86-22-23507760; Tel: +86-22-23507760 Author Contributions Yu Cao and Ning Wang contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was financially supported by National Natural Science Foundation of China (21472103, 21672119), the National Program on Key Research Project (2016YFA0602900), Natural Science Foundation of Tianjin (16JCZDJC39900), and the "12th Five-Year" National Science and Technology Support Plan (2015BAD15B07). REFERENCES (1) Tuck, C. O.; Pérez, E.; Horváth, I. T.; Sheldon, R. A.; Poliakoff, M. Valorization of biomass:

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SYNOPSIS Visible-light photocatalytic oxidation followed by CO2-promoted hydrogenolysis has been developed for the two-step conversion of lignin β-O-4 models, producing acetophenone and phenol in high yield.


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Photocatalytic Oxidation and Subsequent Hydrogenolysis of Lignin Î²

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