Visible-Light-Driven Self-Hydrogen Transfer Hydrogenolysis of Lignin

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Visible-Light-Driven Self-Hydrogen Transfer Hydrogenolysis of Lignin Models and Extracts into Phenolic Products Nengchao Luo, Min Wang, Hongji Li, Jian Zhang, Tingting Hou, Haijun Chen, Xiaochen Zhang, Jianmin Lu, and Feng Wang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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ACS Catalysis

Visible-Light-Driven Self-Hydrogen Transfer Hydrogenolysis of Lignin Models and Extracts into Phenolic Products Nengchao Luo,†,

‡

Min Wang,† Hongji Li,†,

‡

Jian Zhang,† Tingting Hou,†,

‡

Haijun Chen,†,

Xiaochen Zhang,† Jianmin Lu,† Feng Wang†, *

†

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 ‡

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

Corresponding Author * F.W. Email: [email protected]

ACS Paragon Plus Environment

‡

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ABSTRACT

Obtaining high selectivity of aromatic monomers from renewable lignin has been extensively pursued but is still unsuccessful, hampered by the need to efficiently cleave C–O/C–C bonds and inhibit lignin proliferation reactions. Herein, we report a transfer hydrogenolysis protocol using heterogeneous ZnIn2S4 catalyst driven by visible light. In this process, alcoholic groups (CαH‒OH) of lignin act as hydrogen donors. Proliferation of phenolic products to dark substances is suppressed under visible light illumination at low temperature (below 50 °C); formation of a light and transparent reaction solution allows visible light to be absorbed by the catalyst. With this strategy, 71-91% yields of phenols in the conversion of lignin β-O-4 models, and a 10% yield of p-hydroxyl acetophenone derivatives from organosolv lignin are achieved. Mechanistic studies reveal that CαH‒OH groups of lignin β-O-4 linkage are initially dehydrogenated on ZnIn2S4 to form a “hydrogen pool”, and the adjacent Cβ–O bond is subsequently hydrogenolytically cleaved to two monomers by the “hydrogen pool”. Thus, the dehydrogenation and hydrogenolysis reaction are integrated in one-pot with lignin itself as a hydrogen donor. This study shows a promising way of supplying phenolic compounds by taking advantages of both renewable biomass feedstocks and photoenergy.

Keywords: lignin; transfer hydrogenation; heterogeneous catalysis; visible light; ZnIn2S4


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INTRODUCTION

Our society greatly relies on oxygen-containing chemicals, typical examples of which are alcohols/phenols, aldehydes/ketones, and acids/esters. Production of these chemicals is not only dependent on fossil carbon resources, resulting in anthropogenic CO2 emission, but also linked to dangerous chemical processes, involving explosive gases and requiring high-energy input. In the 21st century, increasing global awareness of excessive energy consumption has accelerated the development of sustainable chemical processes, particularly focusing on using clean photoenergy and renewable biomass feedstocks.1

Lignin, one of the most abundant renewable aromatic biopolymers, is receiving more and more attention for its potential to be converted into aromatic oxygen-containing chemicals.2-5 A chief targeted lignin structure to be broken is the β-O-4 linkage, which constitutes 43~62% of native lignin linkages.6,7 Cleavage of the C‒O bond of β-O-4 linkage by virtue of reductive method is a potential route to fragment this structure,8 and it requires external hydrogen source.6,9 Most of the currently-used hydrogen sources originate from fossil resources, among which hydrogen gas is a common one. This is manifested by examples of fragmenting diphenyl ether,10 β-O-4 type lignin models and native lignin by molecular hydrogen.11-13

However, hydrogenation of lignin with molecular hydrogen is usually conducted at elevated reaction temperature and under high reaction pressure, which easily leads to dearomatization and deoxygenation of lignin,14 as well as the proliferation reactions of oxygen-containing products.15,16 In recent years, catalytic transfer hydrogenation (CTH) has emerged as a desirable method for the transformation of biomass-derived feedstocks, and has been employed for the upgrading of


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cellulose,17,18 glycerol,19 and sugars.20,21 Fragmentation of lignin into aromatic products via CTH reactions under relatively mild conditions has been reported.22,23 In CTH reactions, alcohols are often used as both reaction solvents and hydrogen donors.24 But the dehydrogenated products of alcohols, such as aldehydes, may condense with phenolic lignin products to more recalcitrant structures, making product separation more complex.25

Scheme 1. Reductive depolymerization of lignin into oxygen-containing aromatics.

On the other hand, biomass has been used as hydrogen sources as it contains alcoholic groups.26,27 Lignin consists of aromatic rings interconnected by glyceryl groups. Each glyceryl group is adjacent to a Cβ‒O bond.28 Therefore, if such glyceryl hydrogen can be fully used for hydrogenation of Cβ‒O bond, or even a small amount of biomass carbohydrates, either deliberately added or entrained from upstream lignin isolation is present to supply hydrogen,29 lignin conversion will no longer require external hydrogen, and an easy-to-handle route can be achieved. Moreover, it will be more interesting if sustainable energy is input into the CTH fragmentation of lignin. Solar energy is a free and inexhaustible resource, and harnessing it is a


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hot topic in catalytic transformations.30,31 Preliminary studies have shown that using additional hydrogen donors, C‒O bond of the oxidized lignin can be cleaved under light illumination.32

Two-step consecutive route is reported for cleaving Cβ‒O bond of lignin β-O-4 linkage (Scheme 1).33,34 The first step is to oxidize CαH‒OH groups to Cα=O over an oxidation catalyst. Thereafter, the oxidized lignin is either separated out or the oxidation catalyst is filtered off. Subsequently, a hydrogenation catalyst is added to cleave the Cβ‒O bond of oxidized lignin to offer aromatic monomers. As it can be seen, the first step converts alcoholic hydrogen into unwanted water but paradoxically the second step needs external hydrogen. To resolve this paradox, in the present study, we report a one-pot, one-catalyst and visible-light-driven CTH protocol. The combination of visible light photocatalysis and CTH reaction creates an overall-sustainable process for the fragmentation of lignin, making it different from previous studies.

Semiconductor chalcogenide ZnIn2S4 has a valance band maximum (VBM) of 1.48 eV, and a conduction band minimum (CBM) of -0.76 eV.35 The desirable band position, along with considerable photo-stability enables ZnIn2S4 to be extensively studied for photocatalytic H2 evolution,36,37 degradation of organic pollutants,38,39 and organic transformations.40,41 In this work, we report the first example of ZnIn2S4 catalyst in the photocatalytic transfer hydrogenolysis reaction.

We herein present a self-hydrogen transfer hydrogenolysis protocol and a heterogeneous ZnIn2S4 photocatalyst to achieve direct conversion of model and organosolv lignin into phenolic products (Scheme 2). Under visible light illumination, the initial dehydrogenation of interlinking


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CαH–OH groups to Cα=O forms a “hydrogen pool” on ZnIn2S4. Subsequently, the adsorbed hydrogen in the “hydrogen pool” is transferred to (O=Cα)Cβ–O bond, leading to ether bond cleavage. Lignin itself acts as a hydrogen source. Additional alcohols as hydrogen donors can promote the reaction, but are not indispensable. With this strategy in hand, we achieve 71-91% yields of phenols and acetophenones from lignin β-O-4 models, and a 10% mass yield of p-hydroxyl acetophenone derivatives from dioxanesolv poplar lignin. Particularly, monomers with weight-averaged molecular weight (Mw) of about 300 Da were also observed by gel permeation chromatography (GPC), indicating the success of achieving visible-light-driven organosolv lignin fragmentation.

Scheme 2. Visible-light-driven fragmentation of lignin β-O-4 models and dioxanesolv lignin.

EXPERIMENTAL SECTION

Materials. Zn(NO3)2·6H2O (AR) and cetyl trimethyl ammonium bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co., Ltd. InCl3·4H2O (99.9%) and thioacetamide (TAA, 99%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. All the reagents were used as received without further purification. Preparation of ZnIn2S4 Catalysts. ZnIn2S4 was prepared according to a literature procedure


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with minor modifications.42 Typically, Zn(NO3)2·6H2O (304.2 mg), InCl3·4H2O (624.2 mg) and CTAB (260.6 mg) were dissolved in 20 mL of water in a 100 mL beaker and magnetically stirred for 30 min at room temperature. TAA (604.8 mg) was then added into the above solution. After being stirred for another 30 min, the mixture was transferred to a 50-mL stainless Teflon-lined autoclave, tightly sealed and placed in a 160 °C oven for 16 h. The autoclave was then naturally cooled to room temperature. CAUTION: Toxic H2S gas is generated. After being washed with absolute ethanol (3 × 25 mL) and ethanol/deionized water (1:3, 3 × 25 mL), a yellow solid was obtained after being dried in vacuum at 60 °C for 12 h. The yield was more than 95%. Extraction of Organosolv Lignin. Extraction of dioxanesolv poplar lignin was referred to a literature.34 To poplar sawdust (20 g) was added 1,4-dioxane (144 mL) followed by 2 M HCl (16 mL). The mixture was heated in a pressure flask in an Ar atmosphere for 5 h at 93 °C. The mixture was then allowed to cool down to about 50 °C and then filtered. The filtrate was concentrated in vacuum to give a gummy residue which was taken up in acetone/water (9:1, 25 mL) and precipitated by adding to vigorously stirring water (250 mL). The crude lignin was collected by filtration and dried in vacuum. The dried crude lignin was taken up in acetone/methanol (9:1) and precipitated by dropwisely adding to vigorously stirring diethyl ether (200 mL). The precipitate was collected by filtration and dried in vacuum to give a purified poplar lignin (0.38 g). Photocatalytic Conversion of Model and Organosolv Lignin. Photocatalytic reactions were conducted in a home-made quartz photoreactor with a total power of 9.6 W (455 nm LED light). Typically, ZnIn2S4, lignin models or organosolv lignin, and solvent were added into the reactor and stirred. Then the reactor headspace was replaced by the required gas (Ar, H2, air or O2), sealed and placed under light illumination for a desired reaction time under stirring. The


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autogeneous temperature measured by thermal couple was 42 °C. CAUTION: Specific protection by wearing eye goggles to shield 455 nm light is mandatory to avoid injuring eyes. After reaction, n-dodecane/1,2-dichloroethane solution as analysis internal standard was added into the reaction mixture. The mixture was then filtered through 0.22 µm Nylon syringe filter and then analyzed by gas chromatography (GC, Agilent 7890C) or gas chromatography-mass spectrometry (GC-MS, 7890A/5975C). In some case, before analyzing the liquid phase products, gas phase (0.5 mL) was injected to an online mass spectrometer (THERMOStar gas analysis system) with Ar (30 mL min-1) as the carrier gas. Signals of m/z=2, 3, 4 and 34 were analyzed. The results were shown as intensity of the signals versus relative time. Besides, significant experiments were repeated at least twice. The methods to calculate the conversion of substrates and yields of products are attached in supporting information. General Characterizations. Powder X-ray diffraction patterns (XRD) were conducted with a PANalytical X-Pert PRO diffractometer, using Cu-Kα radiation at 40 kV and 20 mA. The morphology of the as-prepared ZnIn2S4 was observed by field emission scanning electron microscopy (FE-SEM, JSM-7800F). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100. UV-vis diffuse reflectance spectra was recorded on a JASCO V-650 UV-vis spectrophotometer. Temperature-programmed-desorption (TPD) was conducted with an online mass spectrometer (THERMOStar gas analysis system) connected at the end outlet of the U-type quartz tube with Ar (30 mL min-1) as the carrier gas. GPC was analyzed by Waters e2695 with 2489 UV-Vis detector. Mobile phase was THF with a flow rate of 0.8 mL min–1. The working temperature of detector was set to 30 °C. Samples (10 mg) for GPC analysis were acetylated by acetic anhydride/pyridine (1:1, 1.5 mL) for 20 h at room temperature. The samples


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were then concentrated in vacuum. The residue was then re-dissolved in 10 mL of ethanol and concentrated. This process was repeated for 5 times. The residue was then re-dissolved in 10 mL of dichloromethane and concentrated. This process was repeated for another 5 times. The dry residue was then dissolved in 5 mL of THF, filtered through a 0.22 µm Nylon filter, and then analyzed by GPC with an injection volume of 100 µL.34

RESULTS AND DISCUSSION

Characterization of ZnIn2S4 Photocatalyst

The prepared ZnIn2S4 presents as a sphere-like morphology with a distributive size of 0.5-6 µm as observed by FE-SEM shown in Figure 1a. These microspheres are assembled with nanowrinkles protruding out of the surface. Higher magnification FE-SEM and TEM images reveal that the vertically and horizontally aligned ZnIn2S4 nanosheets aggregate into cardhouse-like porous structures (Figure 1b and 1c). The interplanar spacing of 0.322 nm (Figure 1d) can be assigned to the (102) crystal facet of ZnIn2S4 and 0.406 nm to the (006) d-spacing (Figure 1e), which demonstrates the formation of ZnIn2S4 with a hexagonal crystal phase.43,44 EDX analysis and element mappings of a single ZnIn2S4 microsphere (Figure 1f) show the uniform dispersion of Zn, In and S with a ratio of 1:1.8:3.8 (Figure S4), which is approximate to the stoichiometry of ZnIn2S4.


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Figure 1. SEM and TEM images of ZnIn2S4. (a) Low magnification SEM image with nanowrinkles. (b) High magnification SEM image. (c) Low magnification TEM image. (d) and (e) HRTEM image. (f) Element mappings of S, Zn and In.

The diffuse reflectance UV-Vis spectra of ZnIn2S4 were recorded (Figure 2). ZnIn2S4 shows strong absorption in the visible light region, typically at 455 nm (Figure 2a), which makes the catalyst suitable for photocatalytic reactions under blue LED light illumination. A steep absorption edge relevant to band gap is evident in the visible light region, indicative of an intrinsic transition of ZnIn2S4 rather than transitions from impurity levels.45,46 The band gap of ZnIn2S4 was calculated to be 2.16 eV via Kubelka-Munk plot (Figure 2b). The absorption edge of ZnIn2S4 therefore was 574 nm, showing relatively high solar light harvesting efficiency. Then, we


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calculated the CBM and VBM of ZnIn2S4 according to the following equations.47 ECB = X – Ee – 0.5Eg

(Eq. 1)

EVB = ECB + Eg

(Eq. 2)

Where ECB is the CBM, EVB is the VBM, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent elements, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the band gap of the semiconductor. ECB of ZnIn2S4 is calculated to be -0.72 eV according to Eq. 1, and EVB is determined to be 1.44 eV by Eq. 2. These values are close to the reported ones.35

Figure 2. (a) Diffuse reflectance UV-vis spectra of ZnIn2S4. (b) The plot of transformed Kubelka-Munk function.

Fragmentation of 2-Phenoxy-1-Phenylethanol

We used 2-phenoxy-1-phenylethanol (1a) as a dimeric lignin β-O-4 model to investigate the catalytic activity of ZnIn2S4. In the presence of ZnIn2S4 photocatalyst, 1a was converted into acetophenone (1b) and phenol (1c) in 83% and 90% yields, respectively (Table 1, entry 1), which were

produced

via

Cβ–O

bond

scission.48

We

also


observed

byproducts

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2-phenoxy-1-phenylethanone (1d) and 1,4-diphenyl-1,4-butanedione (1e) with yields of 6% and 2%, respectively. Byproduct 1d is a dehydrogenated product from 1a, and 1e is the Cβ-coupling product of two acetophenones. Visible light illumination is indispensable for the reaction (Table 1, entry 2). Considering the fact that ZnIn2S4 photocatalyst could absorb visible light up to 574 nm (equals to 30% of the solar energy reaching the earth), we then tried to fragment 1a with solar light as the light source. Likewise, acetophenone and phenol were obtained with yields of 39% and 30% in 3 h, respectively (Table 1, entry 3), signifying the possibility for the fragmentation of lignin with solar light. By contrast, when other commonly-used photocatalysts were employed for this reaction, such as Pd/TiO2, Bi2WO6, g-C3N4 and MoS2 (Table 1, entries 4-6), the Cβ–O bond was not cleaved. These photocatalysts are well-performed in aerobic conditions, but show nearly no activity in hydrogen acceptorless conditions.49-51 The two binary sulfides, CdS and In2S3 (Table 1, entries 8-9), only gave 16% and 8% yields of 1d, respectively. These results indicate that photo-induced e‒ is not the active species for Cβ–O bond cleavage, but other reductive species on ZnIn2S4 surface. We will discuss it in the following section.

The reaction atmosphere greatly influences 1a fragmentation. Similar yields of acetophenone and phenol were obtained when the reaction was conducted in either Ar or H2 atmosphere (Table 1, entries 1, 10), but the yield of 1d was slightly higher in H2 atmosphere, suggesting gaseous H2 is not a hydrogen source for the reaction. Switching reaction atmosphere to air decreased the yields of acetophenone and phenol to less than 30% and increased the yield of 1d to 61% (Table 1, entry 11). The reaction in pure oxygen only offered 1d (Table 1, entry 12). These results show that the oxidative atmosphere inhibits the hydrogenolysis of Cβ–O bond, but is beneficial for the oxy-dehydrogenation of 1a to 1d. The possible reason is that the adsorbed hydrogen is oxidized


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and removed from catalyst surface. The removal of such active hydrogen suppresses the subsequent Cβ–O bond cleavage.

Table 1. Screening of reaction conditions for the fragmentation of lignin model 1a.a

a

Entry

Catalyst

Solvent

1 2b 3c 4 5 6 7 8 9 10d 11e 12f 13 14 15 16 17

ZnIn2S4 ZnIn2S4 ZnIn2S4 Pd/TiO2 Bi2WO6 g-C3N4 MoS2 CdS In2S3 ZnIn2S4 ZnIn2S4 ZnIn2S4 ZnIn2S4 ZnIn2S4 ZnIn2S4 ZnIn2S4 ZnIn2S4

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN THF Acetone Ethanol H2O 1,2-DCE

Conversion (%) >99 99 >94 >39 97 >99 >99 98 59

Yield of products (%) 1b 83 0 39 0 0 0 0 0 0 86 27 0 77 92 95 73 21

1c 90 0 30 0 0 0 0 0 0 84 28 0 88 93 96 68 18

1d 6 0 6 0 0 0 0 16 8 9 61 32 0 5 0 0 10

Reaction conditions: 0.1 mmol of 1a, 5 mg of catalyst, 1.0 mL of CH3CN, 9.6 W blue LEDs (455

nm), 42 °C, 4 h. The conversion of 1a and products yields were determined by GC with n-dodecane as the internal standard. b 45 °C in the dark. c Conducted with solar light illumination, 3 h (2016.7.12, 39o N, 123o E). d In 1 atm H2 atmosphere. e In 1 atm air atmosphere. f In 1 atm O2 atmosphere. THF and 1,2-DCE are short for tetrahydrofuran and 1,2-dichloroethane, respectively.

Solvents usually have great effects on photocatalytic reactions. CH3CN, THF and acetone


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(Table 1, entries 1, 13, 14) have no hydrogen-donating ability and thus reaction in these solvents is a pure self-hydrogen transfer hydrogenolysis one. Ethanol can donate hydrogen (Table 1, entry 15), analogue to the glyceryl groups of lignin, and therefore reaction in ethanol achieves the complete fragmentation of 1a to acetophenone and phenol, while ethanol itself is converted to 1,1-diethyl acetal. 1a fragmentation can also be conducted in insoluble solvent H2O (Table 1, entry 16). Chlorinated solvent is unfavorable since under illumination, the generated active Cl· species may suppress this reaction (Table 1, entry 17).52,53

Figure 3. Time-on-course process of photocatalytic Cβ‒O bond cleavage of 1a. Reaction conditions: 0.1 mmol of 1a, 5 mg of ZnIn2S4, 1.0 mL of CH3CN, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C.

The time-on-course process was also measured (Figure 3). With light on, 1a is promptly converted with the generation of acetophenone, phenol and 1d. Full conversion of 1a is reached


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after 3 h of illumination and yields of the products also reach constant values. The consistent production of acetophenone and phenol suggests they are the two parts after Cβ–O bond cleavage. In the initial stage, the yield of 1d increases to 5-6 % after 0.5 h of illumination, and remains steady to the end. The coupling product 1e (1,4-diphenyl-1,4-butanedione) is generated in 2% yield after 4 h of illumination, which accounts for the slightly lower yield of acetophenone than that of phenol at the end of reaction.

Fragmentation of Other Lignin β-O-4 Models

After optimizing reaction conditions, we then probed the generality of this photocatalytic strategy with respect to other β-O-4 models. The methoxy-substituted β-O-4 models offered Cβ–O bond cleaved products with yields of 71 to 91%, respectively (Table 2, entries 1-5); the substitution position of methoxy, either in benzene ring A or B, on the whole, did not obviously influence catalytic results. Another β-O-4 model 6a that bears a methoxyl group instead of a phenoxyl group, was converted to the corresponding acetophenone in 62% yield (Table 2, entry 6). No other by-products, including coke, ethylbenzene derivatives or those with hydrogenated aromatic rings were observed. In a hydrogen sources closed reaction, formation of ethylbenzene derivatives means a waste of hydrogen donors, which reduces Cβ‒O bond cleavage. β-O-4 model 7a bearing a Cγ‒OH group gave 7b in 13% yield though with 97% conversion (Table 2, entry 7). Besides 7b and phenol, the Cβ‒Cγ bond cleaved product 5b was obtained in 6% yield. The use of isopropanol as a co-solvent and also a hydrogen donor afforded ketone 7c instead of 7b or 5b (Table 2, entry 8), and the yield of 7c increased to 34%.

Table 2. Substrate scope of visible-light-driven Cβ‒O bond cleavage.a


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a

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Reaction conditions: 0.1 mmol of substrate, 5 mg of ZnIn2S4, 1.0 mL of CH3CN, Ar atmosphere,

9.6 W blue LEDs (455 nm), 42 °C. The conversion of substrate and yield of products were determined by GC with n-dodecane as the internal standard.

b

5b was obtained in 6% yield.

c

Reaction in 0.2 mL CH3CN + 0.8 mL isopropanol. N. M. means not measured.

Reaction Mechanism Investigations

Next, we examined the reaction mechanism by carrying out control experiments. Hydrogen gas and isopropanol were used to discriminate the origin of hydrogen sources, respectively


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(Scheme 3). When 1d was subjected to reaction in CH3CN and H2 was used, 1d was not converted (Eq. 3). In comparison, when isopropanol was used, 1d was completely converted and offered acetophenone and phenol in 94% and 98% yields, respectively (Eq. 4). The above results suggest that 1a fragmentation follows a CTH route involving H-abstraction of CαH‒OH groups and subsequent hydrogenolysis of Cβ‒O bond.54

Scheme 3. Control experiments to illustrate the reaction route. Reaction conditions: 0.1 mmol of 1d, 5 mg of ZnIn2S4, 1.0 mL of solvent, 9.6 W blue LEDs (455 nm), 42 °C, 4 h.

Additional experiments were conducted to confirm self-hydrogen transfer hydrogenolysis (Scheme 4).48 When 1a and 5d with a molar ratio of 1:1 were subjected to reaction (Control 1), products derived from the fragmentation of 1a and 5d were both obtained, and the yield of 5b (50%) was higher than that of acetophenone (36%). This means the hydrogen species to cleave the Cβ–O bond can derive from another lignin model, and methoxy-substitution seems to facilitate the Cβ–O bond cleavage. In Scheme 4 (Control 2), 5a and 1d were likewise converted with Cβ–O bond cleaved under the same reaction conditions, which excludes the influence of different dehydrogenation activities with respect to lignin models. Thus, in the fragmentation of β-O-4 lignin models, the abstracted hydrogen species from CαH–OH groups accumulates on the surface of ZnIn2S4 to form a “hydrogen pool” which allocates hydrogen species for the hydrogenolysis of Cβ–O bond.


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Scheme 4. Investigation of self-hydrogen transfer hydrogenolysis process. Reaction conditions: 0.1 mmol of 1a and 5d for control 1 (0.1 mmol of 1d and 5a for control 2), 5 mg of ZnIn2S4, 1.0 mL of CH3CN, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 4 h.

Next, we studied the hydrogen species involved in the reaction by TPD and kinetic isotope effect (KIE) experiments. TPD of fresh ZnIn2S4 revealed the existence of adsorbed hydrogen species (inherent hydrogen species) on ZnIn2S4 surface (Figure S5). Either due to the small amount or inactivity of this inherent hydrogen species, no fragmentation of 1d occurred in the presence of fresh ZnIn2S4 (Scheme 3, Eq. 3).

We detected H2 and H2S in the gas phase (Figure 4a), which may be derived from the hydrogen species of 1a. This result accounts for the generation of a 6% yield of 1d in the end of the reaction. In addition, generation of H2S is an indication of the formation of S–H bond during the CTH reaction, which is possibly one of the forms of hydrogen species in the “hydrogen pool”. When a deuterated substrate, 1a-αCD, was subjected to the photocatalytic reaction, both H2 and HD were detected (Figure 4b), confirming the H-abstraction of the CαH–OH groups over ZnIn2S4


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catalyst. Another result needs attention is that H2 is generated in a larger amount than HD. We speculate H species may have a higher activity than D species to couple, leading to more H2 generation. Nevertheless, we cannot rule out the possibility that the inherent hydrogen species on ZnIn2S4 and the abstracted one from CαH–OH groups may couple to form H2.

Figure 4. Detection of the gas phase after fragmentation of 1a by mass spectrometer. (a) 1a as the substrate. (b) 1a-αCD as the substrate. The inset shows the enlarged peaks of HD and H2S. Reaction conditions: 0.1 mmol of 1a or 1a-αCD, 5 mg of ZnIn2S4, 1.0 mL of CH3CN, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 4 h.

For a deeper understanding of the hydrogen transfer from CαH–OH groups to Cβ‒O bond, the kinetic isotope effect (KIE) in 1a fragmentation was measured with two deuterated substrates 1a-αCD and 1a-αOD. A secondary KIE was obtained with a kH/kD = 1.10 ± 0.07 when 1a-αCD was subjected to reaction (Scheme 5, Eq. 5), disapproving the direct H-abstraction of α-CH.55 The time-on-course process for 1a-αCD fragmentation is recorded to gain more details (Figure S6). Different from 1a fragmentation, induction periods during the formation of acetophenone and phenol were observed, indicating slower rate of Cβ–O bond cleavage when 1a-αCD was the


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substrate. This result, which is caused by the different activity of H and D species,56,57 suggests that H (D) species in the “hydrogen pool” directly participates in the Cβ‒O bond cleavage. When the reaction time was prolonged to 10 h, D was observed only in phenol and HD in the gas phase (Figure S8). In contrast to 1a-αCD fragmentation, with 1a-αOD subjected to reaction, the corresponding KIE was determined to be a primary one with a kH/kD = 2.11 ± 0.27 (Scheme 5, Eq. 6). Therefore, in the dehydrogenation of 1a, H-abstraction of α-OH is a rate-determining step.58,59 Prolonging the reaction time to 10 h, formation of deuterated acetophenone and phenol with 1a-αOD as the substrate is in accord with the CTH route (Figure S9). Due to the H/D exchange with the inherent hydrogen species on ZnIn2S4 surface, phenol with m/z=94 and 95 were both obtained (Figure S8, S9). Furthermore, deuterated acetophenone is formed when 1a-αOD rather than 1a-αCD is subjected to reaction, suggesting that the H of α-OH finally transfers to acetophenone while the H of phenol derives from the α-CH of 1a. This photocatalytic CTH reaction is different from thermal catalytic transfer hydrogenolysis, where Hδ+ of α-OH would transfer to phenol and retain its polarity if the CTH follows a monohydride mechanism.53,54 But in our case, the Hδ+ of α-OH finally transfers to acetophenone with Hδ‒ form. This indicates that the polarity of the Hδ+ of α-OH may be reversed to Hδ‒ during photocatalysis; afterwards, the Hδ‒ is transferred to acetophenone instead of phenol due to the smaller electronegativity of carbon than oxygen in the Cβ–O bond. Similarly, Hδ‒ of α-CH is also reversed during the photocatalytic reaction. As a result, polarity reversal of the abstracted hydrogen species may occur in this photocatalytic transfer hydrogenolysis process.24,25


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Scheme 5. Investigation of hydrogen transfer in 1a fragmentation by KIE experiments. Reaction conditions: 0.1 mmol of substrate, 5 mg of ZnIn2S4, 1.0 mL of CH3CN, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 25 min. The products and yields in Eq. 5 and Eq. 6 represent the reaction results with deuterated substrates. n. d. means not detected.

Based on the results above, a proposed mechanism for the fragmentation of 1a is outlined in Scheme 6. The CBM and VBM of ZnIn2S4 are located at -0.72 eV and 1.44 eV, respectively. Initial visible-light excitation of the ZnIn2S4 photocatalyst would generate e‒ in conduction band and h+ in valence band. h+ is a strong oxidant with a minimum oxidation potential of 1.44 eV and undergoes H-abstraction of α-OH groups, leading to the formation of radical 1f. Subsequent H-abstraction of α-CH groups leads to the formation of 1d. 1d is either adsorbed on ZnIn2S4 or diffuses into solvent as a byproduct. The abstracted H atoms are oxidized by h+, delivering protons adsorbed on ZnIn2S4. For a n-type ZnIn2S4 semiconductor,60 reduction by e‒ in the conduction band is slower than oxidation by h+, leading to the accumulation of e‒.61 In our previous work, reductive cleavage of the Cβ‒O bond of 1d by e‒ is not likely to occur,33 the generated protons in valence band are prone to react with the e‒, forming a “hydrogen pool”. Subsequently, the adsorbed 1d reacts with “hydrogen pool” to give Cβ‒O bond cleaved products. Meanwhile, a small portion of the reduced hydrogen species are combined to form H2 accompanied by the generation of coupling product 1e. At this stage, the hydrogen species on ZnIn2S4 surface formed by the


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photoinduced h+ and e‒ is critical to guarantee the photocatalytic cycle for the fragmentation of lignin β-O-4 models.

Scheme 6. Proposed mechanism for photocatalytic fragmentation of 1a via self-hydrogen transfer hydrogenolysis.

Fragmentation of a Poly-β-O-4 Model and Dioxanesolv Poplar Lignin

ZnIn2S4 is a heterogeneous photocatalyst, on which adsorption of high-molecular-weight organosolv lignin molecules is difficult under mild conditions though easy for dimeric lignin β-O-4 models. Thus, prior to organosolv lignin fragmentation, a more complex β-O-4 model 8a was subjected to reaction (Scheme 7). THF was used as the solvent for better solubility of 8a. After 10 h of illumination, p-hydroxyl acetophenone (8b) was produced with a mass yield of 67%. This indicates that free phenolic hydroxyl groups do not distinctly impact the yield of desired product derived from Cβ–O bond cleavage. However, a longer reaction time is needed for a high yield of 8b.


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Scheme 7. Photocatalytic fragmentation of poly-β-O-4 model via transfer hydrogenation. Reaction conditions: 15 mg of 8a, 5 mg of ZnIn2S4, 1.0 mL THF, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 10 h.

Next, ZnIn2S4 photocatalyst was used to catalyze the fragmentation of organosolv lignin under visible light illumination. In light of the fact that dark brown color of organosolv lignin may suppress lignin fragmentation in photocatalysis as evidenced by a control experiment (Scheme S1) and previous work,32 we extracted dioxanesolv poplar lignin with a light yellow-brown color.34 Mixed solvent consisted of acetone and isopropanol was chosen for better solubility of dioxanesolv poplar lignin, in addition, isopropanol could provide sufficient hydrogen donors to improve yields of products. GPC was used to analysis dioxanesolv poplar lignin before and after illumination (Figure 5). Before reaction, the peak at 29.5 min is attributed to a higher molecular weight dioxanesolv poplar lignin while the peak at 34.8 min represents products with Mw of about 300 Da. After reaction, the peak area of 35.7 min increases obviously, indicative of lignin monomers formation. Meanwhile, the peak area of lignin molecules with a high Mw decreases. No darkening of lignin samples was observed (Figure 5).


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Figure 5. GPC analysis of dioxanesolv poplar lignin. Samples (10 mg) for GPC analysis were acetylated by acetic anhydride/pyridine (1:1, 1.5 mL) for 20 h at room temperature.

As GPC analysis revealed the formation of lignin monomers, we then analyzed the reaction system after illumination by GC-MS, and a variety of monomers were observed. For comparison, unreacted dioxanesolv poplar lignin were analyzed. The reaction results are presented in Figure 6a with all the peak areas normalized according to the internal standard (n-dodecane). Before reaction, aldehydes and cinnamyl alcohols derivatives were detected. These substances may be formed in extracting lignin. Quantitative analysis showed a total products mass yield of 7% (Figure 6b). After illumination for 24 h, fragmentation led to products with a total mass yield of 17% (Figure 6b). Therefore, the new generated substances with a total mass yield of 10% are derived from photocatalysis over ZnIn2S4. The 10% yield of products could also be roughly obtained from GPC results by comparing the peak areas of 35.7 min and 34.8 min. Among these products, p-hydroxyl acetophenone derivatives (marked in red), which are formed by Cβ–O bond cleavage, are in accord with the products of β-O-4 lignin model 8a, proving that the fragmentation rules of lignin models are maintained in organosolv lignin fragmentation. Other products, such as 9j, 9k and 9l, may be


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generated with photocatalysis as their mass yields increased after illumination; however, the reasons for the formation of these products could not be well explained. Overall, the formation of p-hydroxyl acetophenone derivatives suggests the success of achieving dioxanesolv poplar lignin fragmentation with this photoinduced CTH strategy.

Figure 6. (a) GC-MS spectra and (b) Quantitative analysis for photo-induced CTH fragmentation of dioxanesolv poplar lignin. The values inside and outside parentheses are yields of products from extracted dioxanesolv poplar lignin before and after reaction, respectively. Reaction conditions: 30 mg of dioxanesolv poplar lignin, 10 mg of ZnIn2S4, 0.8 mL of acetone and 0.2 mL of isopropanol, Ar atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 24 h.

CONCLUSION

In summary, visible-light-driven fragmentation of lignin β-O-4 models and dioxanesolv


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poplar lignin is achieved over ZnIn2S4 photocatalyst. During photocatalysis, the CαH‒OH hydrogen of β-O-4 models is transferred to ZnIn2S4 surface to form a “hydrogen pool”, then the hydrogen species in the “hydrogen pool” is hydrogenated to Cβ–O bond for cleavage. The photocatalytic transfer hydrogenation results in a polarity reversal, which is contrary to thermal ones when following a monohydride mechanism. Particularly, dioxanesolv poplar lignin is photocatalytically converted to p-hydroxyl acetophenone derivatives. The influence of Cγ‒OH was only partly identified, and further work is needed on this issue and on improving the products yields from native lignin. This work provides an alternative method for lignin β-O-4 model and dioxanesolv lignin fragmentation with lignin itself as a hydrogen source.

AUTHOR INFORMATION

Corresponding Author * F.W. Email: [email protected]

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.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (21422308, 21690082, 21690084), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300) and DICP (DICP ZZBS201613).


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Visible-Light-Driven Self-Hydrogen Transfer Hydrogenolysis of Lignin

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