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Photocatalytic Oxidation-Hydrogenolysis of Lignin #O-4 Models via Dual Light Wavelength Switching Strategy Nengchao Luo, Min Wang, Hongji Li, Jian Zhang, Huifang Liu, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02212 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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ACS Catalysis

Photocatalytic Oxidation-Hydrogenolysis of Lignin β-O-4 Models via Dual Light Wavelength Switching Strategy Nengchao Luo,a, b Min Wang,a Hongji Li,a, b Jian Zhang,a Huifang Liu,a, b and Feng Wang*a a

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

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China

b

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

ABSTRACT: One of the challenges of depolymerizing lignin to valuable aromatics lies in the selective cleavage of the abundant C–O bonds of β-O-4 linkages. Herein we report a photocatalytic oxidation-hydrogenolysis tandem method for cleaving C–O bonds of β-O-4 alcohols. The Pd/ZnIn2S4 catalyst is used in the aerobic oxidation of α-C–OH of β-O-4 alcohols to α-C=O with 455 nm light, and then a TiO2-NaOAc system is employed for cleaving C–O bonds neighbor to the α-C=O bonds through hydrogenolysis reaction by switching to 365 nm light. Interestingly, the oxidation-hydrogenolysis tandem reaction can be conducted in one pot to offer ketones and phenols (up to 90% selectivity) via dual light wavelength switching (DLWS) strategy. EPR and metal loading experiments elucidate that Ti3+ in TiO2 is in situ formed and responsible for the photocatalytic hydrogenolysis through electron transfer from Ti3+ to the β-O4 ketones.

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KEYWORDS: Lignin • Photocatalysis • Oxidation-hydrogenolysis • Dual light wavelength switching • Ti3+ INTRODUCTION Lignin is a highly-functionalized aromatic biopolymer and the most abundant renewable aromatic sources.1-4 Lignin valorization provides a sustainable way to produce low molecular weight aromatics.5-7 One of the key issues for lignin depolymerization is to break down the abundant β-O-4 linkages,8 which accounts for 43-65% of all lignin linkages.2,9 Previously oxidative,10-12 reductive,13-16 redox-neutral17-21 and solvolysis22-24 methods have been developed to cleave C–O bonds of the β-O-4 linkages. Recently an efficient two-step method, i.e. oxidation of α-C–OH of β-O-4 alcohols to β-O-4 ketones followed by C–O bonds cleavage, has attracted more interests. Theoretical studies show25,26 that the initial oxidation decreases the C–O bonds energy from 69.2 kcal mol-1 to 55.9 kcal mol-1, and making the subsequent C–O bonds cleavage easier. However, the method employs two catalysts independently working under the oxidative and reductive conditions. Therefore, it will be very interesting to conduct oxidation and hydrogenolysis reactions in one pot to simplify reaction step and maximize product yields. Photo-induced selective reactions catalyzed by heterogeneous photocatalysts have recently emerged as an efficient method for organic transformations,27 especially for oxidative,28,29 and reductive reactions.30-32 In nature, lignin is produced by photosynthesis and can be depolymerized to chemicals via light stimulation.10,25,33-35 For example, Stephenson et al. reported the reductive cleavage of the C–O bonds of β-O-4 ketones using an organometallic Ir catalyst.25 However, the pioneering work also reveals the difficulty of two-step method.


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Herein, we attempt to realize the direct cleavage of β-O-4 alcohols via tandem oxidation and hydrogenolysis reactions by switching two light wavelength (Scheme 1). Interestingly the individual oxidation and hydrogenolysis reactions can be conducted consecutively with the coexistence of the two catalysts under aerobic conditions. This work represents few examples of conducting paradoxical oxidation and hydrogenolysis reactions in one pot, while this is almost impossible for thermal reactions conducted under elevated temperature. By doing so, we realize the direct conversion of lignin β-O-4 alcohols to ketones and phenols.

Scheme 1. Photocatalytic cleavage of β-O-4 alcohols into ketones and phenols by oxidationhydrogenolysis reaction via DLWS strategy. RESULTS AND DISCUSSION Initially, we focused on the hydrogenolysis of the C–O bond of β-O-4 ketone (1b) using heterogeneous photocatalyst under N2 atmosphere. TiO2 (anatase, size 40 nm) was employed in this study. Ethanol was used as solvent and also the hydrogen source. The reaction was conducted at room temperature, but the autogenous temperature reached 65 ºC. We found that the reaction could not be conducted in dark and TiO2 selectively catalyzed the C–O bond cleavage of 1b (Table 1, entry 1, 2). The addition of base enhanced the activity (Figure S1a in the Supporting Information), and NaOAc showed the best performance with 93% and 90% yields


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of 1c and 1d, respectively. But NaOAc or TiO2 alone showed moderate conversion of 1b and gave low yields of phenol and acetophenone products (99

MeO

MeO 2b

1c, 85

O O

O

MeO

3

98

7

3c, 78

3b O

OMe

1d, 68

OH

O

O

MeO

4

7

97

MeO

MeO 3c, 74

4b O O

MeO

5

2d, 57

OH

OMe

MeO

O MeO

93

7 MeO

MeO 3c, 66

5b O

OMe

6b, 94%

O

MeO

6 OH

5d, 67

O

OH

O MeO

2d, 75

OH

OMe

+ MeO

7h 3c, 79%

+ MeO 6d, 30%

2d, 9%


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a

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Reaction conditions: 0.1 mmol b, 5 mg TiO2, 0.5 equiv. NaOAc, 0.75 mL ethanol, N2

atmosphere, 5 h, 5.6 W blue LED (365 nm). To find the active crystal facet for the C-O bonds cleavage of β-O-4 ketones, different anatase TiO2 was studied. Anatase TiO2 with exposed (101), (111) and (001) crystal facets were prepared according to the literature.41-43 The octahedron shape in Figure 1a indicates the anatase TiO2 mainly exposes (101) crystal facet. Figure 1b shows the TEM image of a parallelogram with an angle of 82º between (101) and (011) crystal facets, which is in accord with the (111) crystal facet. Figure 1c shows the TiO2 nanosheets mainly exposes (001) crystal facet. In the hydrogenolysis cleavage of the C–O bond of 1b, TiO2 that mainly exposes with (101) crystal facet is the most active crystal facet.


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Figure 1. HR-TEM images of TiO2. (a) TiO2 exposes 95% of (101) crystal facet. (b) TiO2 exposes 70% of (111) crystal facet. (c) TiO2 mainly exposes (001) crystal facet. (d) The results of C–O bond cleavage of 1b catalyzed over TiO2 exposed with different crystal facets. Reaction conditions: 0.1 mmol 1b, 5 mg TiO2, 0.5 equiv. NaOAc, 0.75 mL ethanol, N2 atmosphere, 2 h, 5.6 W blue LED (365 nm). We then studied the reaction mechanism of C–O bond cleavage of 1b. h+, e–, •OH, H2O2 and •O2− are identified as the key active species in photocatalytic reactions.36 Given that hydrogenolysis of 1b was conducted in N2 atmosphere and without H2O, only h+, e– and alkoxy radicals are considered to be the potential active species in our system. K2S2O7 known as an electron scavenger decreased the 1b conversion from 99% to 55% (Table 1, entry 5),37,39 suggesting that e– generated under light illumination is essential for the reaction. Na2C2O4 was used as h+ trapping agent, but nearly no inhibition of 1b hydrogenolysis was observed (Table 1, entry 6). Ethanol which is also a trapping agent might trap h+ and shields the prohibition effect of Na2C2O4.40 tBuOH is known as a typical scavenger for •OH and slowed down the hydrogenolysis of 1b significantly (Table 1, entry 7),37 however, the lack of O2 and H2O in this system excludes the presence of •OH. We suppose the generation of •OEt in our reaction system and may be trapped by tBuOH, thus, •OEt plays an important role in the hydrogenolysis of 1b. To understand the role of e– in the hydrogenolysis of 1b, we then loaded noble metal to TiO2 (Table S1). The loading of 2 wt% noble metal to TiO2 decreased the efficiency of the hydrogenolysis of 1b distinctly (Table S1, entry 2-6). This is contrary to the recognition that metal loadings on TiO2 facilitate the separation of h+ and e–,44,45 which reflects on the more efficient photocatalytic properties46. We postulate here that e– do not directly participate in the C–O bonds cleavage. Y. Shiraishi47 has encountered similar phenomenon and he proposed that


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reduction was not caused by e–, but Ti3+ on the surface. Metal particles loaded on TiO2 surface may cover the Ti3+ sites and suppress the adsorption of 1b,47 thus, giving inferior results for hydrogenolysis of 1b. We thus speculate that Ti3+ on TiO2 surface may directly participate in the C–O bond hydrogenolysis cleavage of 1b.

Figure 2. EPR spectra of TiO2 suspension in ethanol after irradiation (recorded at 77 K) and the insets are photos of the TiO2 suspension. Electron paramagnetic resonance (EPR) technique was used to study the nature of Ti3+ and its role in C–O bond cleavage of 1b at 77 K in N2 atmosphere. Nearly no paramagnetic signals were detected in dark (Figure 2). Nevertheless, when the suspension was illuminated for 1h, the color of TiO2 changed from white to blue. A broad signal with g=1.935 appeared, which can be assigned to Ti3+.48-54 The in situ formed Ti3+ is very active and quickly oxidized when exposed to air, as evidenced by the change of the color from blue to white. When β-O-4 ketone


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1b was added, the signal decreased obviously and the color of the colloidal changed to jade green. We therefore conclude that Ti3+ is formed in situ by trapping electrons generated under light irradiation, and eventually, the electrons transfer from Ti3+ to 1b.

Scheme 2. Proposed reaction mechanism for photocatalytic C–O bond cleavage of β-O-4 ketones via hydrogenolysis. Based on the above results, a tentative reaction mechanism is proposed (Scheme 2). With the illumination of 365 nm LED light, holes and electrons are formed instantly. Holes oxidize ethanol to generate ·OEt while electrons reduce surface Ti4+ to Ti3+. The adsorption of 1b to the in situ formed Ti3+ leads to the weakening of the C–O bond of 1b, and finally results in the cleavage of 1b. NaOAc may play an important role in the hydrogen ion transfer from ethanol to the adsorbed –OPh species.55 Besides, the detection of 1e confirms the formation of 1f as an intermediate. TiO2-NaOAc system is suitable for the C–O bonds cleavage of β-O-4 ketones. We then attempted to realize the direct C–O bonds cleavage via tandem oxidation and hydrogenolysis


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process. We found Pd/ZnIn2S4 showed high selectivity (97%) in the oxidation of the α-C–OH of 1a (Table S2, S3) under 455 nm LED light illumination. Furthermore, Pd/ZnIn2S4 can be used in combination with TiO2-NaOAc system for the direct cleavage of β-O-4 alcohols in one pot without changing solvent and atmosphere but just by switching the light wavelength from 455 nm to 365 nm. The C–O bonds were cleaved to afford ketones and phenols even under aerobic conditions. In the first step, β-O-4 alcohols were oxidized to β-O-4 ketones over Pd/ZnIn2S4 under 455 nm light illumination. Then, the hydrogenolysis cleavage of C–O bonds was achieved over TiO2 under 365 nm light illumination even in aerobic conditions. Table 3 presents the results for the C–O bonds cleavage of β-O-4 alcohols via DLWS. Entry 2 indicates this two-step oxidation-hydrogenolysis reaction can also be conducted with the co-existence of the two wavelengths. As a result, this dual light wavelength strategy can be conducted in two modes, dual light wavelength switching (DLWS) and dual light wavelength co-existence. Entries 3-5 suggest –OMe on either side of benzene ring lead to relatively poor yields of C–O bonds cleaved products for easier oxidation of phenol parts in aerobic conditions. Besides, by comparing entries 3-5 in Table 3 and entries 2-4 in Table 2, we speculate Pd/ZnIn2S4 may influence the C-O bond cleavage of the generated β-O-4 ketones in the presence of two catalysts when switching to 365 nm LED. Further work is needed to optimize this strategy to obtain better yields of C–O bonds cleaved products.


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Table 3. Photocatalytic cleavage of β-O-4 alcohols via DLWS.a OH

O O

OMe

OH

Pd/ZnIn2S4, TiO2, NaOAc

MeO

455/365 nm, O2, ethanol

MeO

+ MeO

a

Entry

c

Time (h)b

Substrate

Conv. (%)

d

Yield of products (%)

OH

OH

O

O 1

28/5

94

1c

1a

1d 1d, 76

1c, 94

OH

OH

O

O 2c

28

1c

97

1a

1d 1c, 97

OH

1d, 84

OH

O

O 3

99

40/7 MeO

1c

2a

1c, 87

OH

OMe

4d

2d, 82

OH

O 40/7

58

O

MeO 3c

1d 1d, 35

3c, 33

3a OH

OH

OMe O

5e

2d MeO

O

MeO 88

50/10

3c

2d MeO

MeO 4a

a

3c, 8

2d, 28

Reaction conditions: 0.1 mmol a, 10 mg Pd/ZnIn2S4, 5 mg TiO2, 0.5 equiv. NaOAc, 0.75 mL

ethanol, O2 atmosphere, 5.6 W LED (illuminated with 455 nm LED for hours and then switches to 365 nm LED). b The value before slash indicates the reaction time with 455 nm LED while the value behind slash indicates the reaction time with 365 nm LED. consists of 2.8 W of 455 nm and 365 nm LED.

d

c

With mixed light source

The other products mainly contain 3b (14%

yield). e The other products mainly contain 4b (45% yield). The time course of the DLWS strategy was recorded for the C–O bond cleavage of 1a (Figure 3). Only litter conversion of 1a was observed with TiO2 as catalyst. However, the


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selective oxidation of 1a to 1b occurred in the presence of Pd/ZnIn2S4 and TiO2 with the illumination of 455 nm light, and nearly no C–O bond cleaved products were observed. Upon switching the light wavelength from 455 nm to 365 nm, 1b decreased promptly and converted to phenol and acetophenone via C–O bond cleavage, and the consumption of 1b further promoted the conversion of 1a. Besides, the C–O bond cleaved products were observed after light wavelength switching and the yields were relatively high (>70%). This indicates that the combination of two photocatalysts, oxidative and reductive one, can realize photocatalytic C–O bonds cleavage via DLWS strategy in one pot.

Figure 3. Time course of photocatalytic C–O bond cleavage of 1a via DLWS. Cx represents the concentration of 1a, 1b, 1c or 1d at certain reaction time. C0 is the initial concentration of 1a. □


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represents C1a/C0 with TiO2-NaOAc merely, and early no 1b, 1c and 1d were detected. ■ C1a/C0, ● C1b/C0, ♦ C1c/C0, and ▲ C1d/C0 in the presence of Pd/ZnIn2S4 and TiO2-NaOAc. Finally, we applied this DLWS strategy to the oxidation-hydrogenolysis cleavage of some organosolv lignin, however, no products were observed by GC after reaction. This results may be caused by the heavy dark color of lignin, as reported by Stephenson,25 who reported the suppression effects of dark brown color of lignosulfonate in the reductive C–O bond cleavage of β-O-4 models in batch conditions. However, when the photocatalytic reduction reaction was conducted in a flow reactor, the dark brown color of lignosulfonate showed nearly no suppression effects. This study may suggest that if we expose catalyst close to light source, such as employing micro flow reactor adopted by Stephenson,25 there still lies the possibility of converting original lignin by photocatalysis. We are now investigating how to immobilize heterogeneous photocatalyst on a flow reactor, and apply this technique to depolymerize lignin. CONCLUSIONS In summary, we have demonstrated a photocatalytic oxidation-hydrogenolysis strategy for the selective C–O bonds cleavage of β-O-4 models in one pot. This method consists of two steps including selective photocatalytic oxidation of α-C–OH over Pd/ZnIn2S4 and hydrogenolysis of C–O bonds over TiO2-NaOAc in ethanol. The tandem reaction is realized by switching the wavelength of light. In depth work reveals that the in situ formed Ti3+ activates the C–O bonds. We believe this dual light wavelength switching strategy is particularly useful for converting the highly-functionalized molecules requiring multiple or cascade reactions. EXPERIMENTAL SECTION Chemicals and materials.


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Zn(NO3)3·6H2O, NaOAc, NaC2O4·2H2O, Na2CO3, K2CO3 and solvents are obtained from Kermel Reagent. Co. Ltd. While TiO2 (anatase, 40 nm, hydrophilic), tetrabutyl titanate and InCl4·4H2O (99.9%), thiacetamide (TAA) are purchased from Aladdin-Reagent without further purifications. β-O-4 models are synthesized as depicted in reference.56,57 Catalyst preparation. Preparation of ZnIn2S4. ZnIn2S4 is synthesized via hydrothermal method.58 Typically, Zn(NO3)3·6H2O (1 mmol) and InCl4·4H2O (2 mmol) were dissolved in 27 mL ethanol with stirring for 30 min. Then, excessive amount of TAA (8 mmol) was added into the above solution. After 30 min of aging while stirring, the solution was transferred to a 50 mL Teflon-lined stainless autoclave, sealed and heated at 160 ºC for 24 h. The system was then allowed to cool down to room temperature naturally. The final product was obtained by centrifugation and washed with deionized water and absolute alcohol for several times, and then dried overnight in vacuum. Preparation of Pd/ZnIn2S4. Loading of 1 wt% Pd on ZnIn2S4 was via impregnation method. Typically, 0.50 g ZnIn2S4 was added into 7.93 mL deionized water, then 2.07 mL 22.68 mmol/L H2PdCl4 was added. After vigorous stirring for 8 h, the suspension was reduced by 10 equiv. NaBH4. After another 10 h of stirring, the product was obtained by centrifugation and washed with deionized water for three times, and then dried overnight in vacuum for further characterization. Synthesis of anatase TiO2. TiO2 exposes of (101) and (111) crystal facets were synthesized via hydrothermal method43. Typically, 8 mL tetrabutyl titanate (TBT) was mixed with relative amount of NH4F, HF (40%), NH3·H2O (28%) and deionized water in a Teflon-lined


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autoclave, sealed and kept at 180 ºC for 24 h. Then the system was allowed to cool down to room temperature. The final product was obtained by washing with absolute alcohol for 4 times and then dried at 102 ºC for 12 h. For TiO2-(101) and TiO2-(111), the recipes were as follows: TiO2(101) (0.88 mL NH3·H2O and 0.76 mL deionized water); TiO2-(111) (0.24 g NH4F, 0.25 ml HF and 1.35 mL deionized water). TiO2-(001) was prepared by solvothermal method.42 Typically, 1.6 ml HF was added in 28 ml ethanol and 21 ml isopropanol under vigorous stirring. Then 0.01 mol TBT were added into the mixed solution by dropwise, and stirred for another 30 min to obtain a homogeneous solution. The mixture was transferred into a 100 mL Teflon-lined autoclave, sealed and heated at 180 ºC for 10 h. Then the system was allowed to cool down to room temperature. The final product was obtained by washing with absolute alcohol and water for several times and then dried at 60 ºC for 12 h. All the TiO2 adopted in reaction system were treated in 30 mL/min H2 at 400 ºC for 4 h. Photocatalytic activity test. Photocatalytic aerobic oxidation. Photocatalytic aerobic oxidation of 2-phenoxyl 1phenethyl alcohol (1a) and its derivatives were carried out in home-made LED photoreactors. Typically, 0.1 mmol 1a and 10 mg Pd/ZnIn2S4 were added into 0.75 mL solvent in a 4 mL quartz tube, then the system was replaced with O2 for 1 min before sealed with ground glass stopper and parafilm. The quartz tube was then allowed to be irradiated with 455 nm LED light. The conversion of 1a and yield of 2-phenoxyl acetophenone (1b) was determined by GC with ndodecane as the internal standard. Pd/ZnIn2S4 was recovered by centrifugation after reaction.


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Photocatalytic Reductive cleavage of 1b. Photocatalytic hydrogenolysis cleavage of 1b and its derivatives were carried out in home-made LED photoreactors. All the other procedures were the same as photocatalytic aerobic oxidation of 1a except that 5 mg of TiO2 and 0.5 equiv. of NaOAc were added instead and that O2 was replaced by N2. Besides, 365 nm light was irradiated instead. Photocatalytic C–O bond cleavage of 1a in one pot. The experimental process was nearly the same as the aerobic oxidation of 1a, except that 0.1 mmol 1a, 10 mg Pd/ZnIn2S4, 5 mg TiO2 and 0.5 equiv. NaOAc were added instead and that O2 was replaced by N2. Besides, the reaction system was initially illuminated by 455 nm LED and switched to 365 nm LED after certain reaction time. ASSOCIATED CONTENT Supporting Information. Catalyst characterization, including XRD TEM images, procedure for the synthesis of lignin models, NMR spectra and some catalytic results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * F.W. Email: [email protected]. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Project No. 21422308, 21273231) and by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300).


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