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

Article abstract of DOI:10.1021/acscatal.7b01043

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 a heterogeneous ZnIn₂S₄ catalyst driven by visible light. In this process, alcoholic groups (C_αH-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 C_αH-OH groups of lignin β-O-4 linkage are initially dehydrogenated on ZnIn₂S₄ to form a "hydrogen pool", and the adjacent C_β-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.

Full text of DOI:10.1021/acscatal.7b01043

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Article
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
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VisibleꢀLightꢀDriven SelfꢀHydrogen Transfer Hydrogenolysis of Lignin
Models and Extracts into Phenolic Products
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†
, ‡
†
†, ‡
†
†, ‡
†, ‡
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: wangfeng@dicp.ac.cn
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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
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heterogeneous ZnIn S catalyst driven by visible light. In this process, alcoholic groups (C H‒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, 71ꢀ91% yields of phenols in the conversion of lignin βꢀOꢀ4 models, and a 10% yield of
pꢀhydroxyl acetophenone derivatives from organosolv lignin are achieved. Mechanistic studies
reveal that C H‒OH groups of lignin βꢀOꢀ4 linkage are initially dehydrogenated on ZnIn S to
α
2 4
β
form a “hydrogen pool”, and the adjacent C –O bond is subsequently hydrogenolytically cleaved
to two monomers by the “hydrogen pool”. Thus, the dehydrogenation and hydrogenolysis reaction
are integrated in oneꢀpot 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.
2 4
Keywords: lignin; transfer hydrogenation; heterogeneous catalysis; visible light; ZnIn S
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INTRODUCTION
Our society greatly relies on oxygenꢀcontaining chemicals, typical examples of which are
alcohols/phenols, aldehydes/ketones, and acids/esters. Production of these chemicals is not only
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dependent on fossil carbon resources, resulting in anthropogenic CO
dangerous chemical processes, involving explosive gases and requiring highꢀenergy input. In the
1st century, increasing global awareness of excessive energy consumption has accelerated the
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emission, but also linked to
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development of sustainable chemical processes, particularly focusing on using clean photoenergy
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and renewable biomass feedstocks.
Lignin, one of the most abundant renewable aromatic biopolymers, is receiving more and
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ꢀ5
more attention for its potential to be converted into aromatic oxygenꢀcontaining chemicals.
A
chief targeted lignin structure to be broken is the βꢀOꢀ4 linkage, which constitutes 43~62% of
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native lignin linkages. Cleavage of the C‒O bond of βꢀOꢀ4 linkage by virtue of reductive
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method is a potential route to fragment this structure, and it requires external hydrogen source.
Most of the currentlyꢀused hydrogen sources originate from fossil resources, among which
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hydrogen gas is a common one. This is manifested by examples of fragmenting diphenyl ether,
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βꢀOꢀ4 type lignin models and native lignin by molecular hydrogen.
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
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4
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deoxygenation of lignin, as well as the proliferation reactions of oxygenꢀcontaining products.
In recent years, catalytic transfer hydrogenation (CTH) has emerged as a desirable method for the
transformation of biomassꢀderived feedstocks, and has been employed for the upgrading of
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cellulose,
glycerol, and sugars.
Fragmentation of lignin into aromatic products via CTH
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reactions under relatively mild conditions has been reported.
In CTH reactions, alcohols are
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often used as both reaction solvents and hydrogen donors. But the dehydrogenated products of
alcohols, such as aldehydes, may condense with phenolic lignin products to more recalcitrant
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structures, making product separation more complex.
Scheme 1. Reductive depolymerization of lignin into oxygenꢀcontaining aromatics.
On the other hand, biomass has been used as hydrogen sources as it contains alcoholic
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groups.
Lignin consists of aromatic rings interconnected by glyceryl groups. Each glyceryl
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group is adjacent to a C
β
‒O bond. Therefore, if such glyceryl hydrogen can be fully used for
hydrogenation of C
β
‒O bond, or even a small amount of biomass carbohydrates, either
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deliberately added or entrained from upstream lignin isolation is present to supply hydrogen,
lignin conversion will no longer require external hydrogen, and an easyꢀtoꢀhandle 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.
Preliminary studies have shown that using additional
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hydrogen donors, C‒O bond of the oxidized lignin can be cleaved under light illumination.
β
Twoꢀstep consecutive route is reported for cleaving C ‒O bond of lignin βꢀOꢀ4 linkage
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(
Scheme 1).
α α
The first step is to oxidize C H‒OH groups to C =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 C ‒O bond of oxidized lignin to
β
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 oneꢀpot, oneꢀcatalyst and visibleꢀlightꢀdriven CTH
protocol. The combination of visible light photocatalysis and CTH reaction creates an
overallꢀsustainable process for the fragmentation of lignin, making it different from previous
studies.
Semiconductor chalcogenide ZnIn
2
S
4
has a valance band maximum (VBM) of 1.48 eV, and a
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conduction band minimum (CBM) of ꢀ0.76 eV. The desirable band position, along with
considerable photoꢀstability enables ZnIn
2
S
4
to be extensively studied for photocatalytic H
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evolution,
degradation of organic pollutants,
and organic transformations.
In this work,
we report the first example of ZnIn
2 4
S catalyst in the photocatalytic transfer hydrogenolysis
reaction.
We herein present a selfꢀhydrogen transfer hydrogenolysis protocol and a heterogeneous
2 4
ZnIn S 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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C H–OH groups to C =O forms a “hydrogen pool” on ZnIn S . Subsequently, the adsorbed
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α β
hydrogen in the “hydrogen pool” is transferred to (O=C )C –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 71ꢀ91%
yields of phenols and acetophenones from lignin βꢀOꢀ4 models, and a 10% mass yield of
pꢀhydroxyl acetophenone derivatives from dioxanesolv poplar lignin. Particularly, monomers with
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weightꢀaveraged molecular weight (M ) of about 300 Da were also observed by gel permeation
chromatography (GPC), indicating the success of achieving visibleꢀlightꢀdriven organosolv lignin
fragmentation.
Scheme 2. Visibleꢀlightꢀdriven fragmentation of lignin βꢀOꢀ4 models and dioxanesolv lignin.
EXPERIMENTAL SECTION
3 2 2
Materials. Zn(NO ) ·6H O (AR) and cetyl trimethyl ammonium bromide (CTAB) were
3 2
purchased from Sinopharm Chemical Reagent Co., Ltd. InCl ·4H O (99.9%) and thioacetamide
(TAA, 99%) were purchased from Shanghai Aladdin BioꢀChem Technology Co., Ltd. All the
reagents were used as received without further purification.
2 4 2 4
Preparation of ZnIn S Catalysts. ZnIn S was prepared according to a literature procedure
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with minor modifications. Typically, Zn(NO ) ·6H O (304.2 mg), InCl ·4H O (624.2 mg) and
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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 50ꢀmL stainless Teflonꢀlined
autoclave, tightly sealed and placed in a 160 °C oven for 16 h. The autoclave was then naturally
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cooled to room temperature. CAUTION: Toxic H S gas is generated. After being washed with
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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
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literature. To poplar sawdust (20 g) was added 1,4ꢀdioxane (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 homeꢀmade quartz photoreactor with a total power of 9.6 W (455 nm LED
light). Typically, ZnIn
2 4
S , 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, H , air or O ),
2
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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,
nꢀdodecane/1,2ꢀdichloroethane solution as analysis internal standard was added into the reaction
mixture. The mixture was then filtered through 0.22 ꢁm Nylon syringe filter and then analyzed by
gas chromatography (GC, Agilent 7890C) or gas chromatographyꢀmass spectrometry (GCꢀMS,
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7890A/5975C). In some case, before analyzing the liquid phase products, gas phase (0.5 mL) was
Star
ꢀ1
injected to an online mass spectrometer (THERMO gas analysis system) with Ar (30 mL min )
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 Xꢀray diffraction patterns (XRD) were conducted with
a PANalytical XꢀPert PRO diffractometer, using CuꢀKα radiation at 40 kV and 20 mA. The
2 4
morphology of the asꢀprepared ZnIn S was observed by field emission scanning electron
microscopy (FEꢀSEM, JSMꢀ7800F). Transmission electron microscopy (TEM) images were
obtained using a JEOL JEMꢀ2100. UVꢀvis diffuse reflectance spectra was recorded on a JASCO
Vꢀ650 UVꢀvis spectrophotometer. Temperatureꢀprogrammedꢀdesorption (TPD) was conducted
Star
with an online mass spectrometer (THERMO gas analysis system) connected at the end outlet
ꢀ1
of the Uꢀtype quartz tube with Ar (30 mL min ) as the carrier gas. GPC was analyzed by Waters
–1
e2695 with 2489 UVꢀVis detector. Mobile phase was THF with a flow rate of 0.8 mL min . 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 reꢀdissolved in 10 mL of ethanol and
concentrated. This process was repeated for 5 times. The residue was then reꢀdissolved 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 ꢁm Nylon filter, and then
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analyzed by GPC with an injection volume of 100 ꢁL.
RESULTS AND DISCUSSION
2 4
Characterization of ZnIn S Photocatalyst
The prepared ZnIn
m as observed by FEꢀSEM shown in Figure 1a. These microspheres are assembled with
nanowrinkles protruding out of the surface. Higher magnification FEꢀSEM and TEM images
reveal that the vertically and horizontally aligned ZnIn nanosheets aggregate into
cardhouseꢀlike porous structures (Figure 1b and 1c). The interplanar spacing of 0.322 nm (Figure
d) can be assigned to the (102) crystal facet of ZnIn and 0.406 nm to the (006) dꢀspacing
Figure 1e), which demonstrates the formation of ZnIn S with a hexagonal crystal phase.
2 4
S presents as a sphereꢀlike morphology with a distributive size of 0.5ꢀ6
ꢁ
2 4
S
1
2 4
S
43,44
(
2
4
EDX analysis and element mappings of a single ZnIn
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 ZnIn
2 4
S microsphere (Figure 1f) show the
2 4
S .
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Figure 1. SEM and TEM images of ZnIn S . (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.
2 4 2 4
The diffuse reflectance UVꢀVis spectra of ZnIn S were recorded (Figure 2). ZnIn S 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
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of ZnIn
2
S
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rather than transitions from impurity levels.
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The band gap of ZnIn S was
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calculated to be 2.16 eV via KubelkaꢀMunk plot (Figure 2b). The absorption edge of ZnIn S
therefore was 574 nm, showing relatively high solar light harvesting efficiency. Then, we
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calculated the CBM and VBM of ZnIn S according to the following equations.
2
4
E
CB = X – E
e
– 0.5E
g
(Eq. 1)
(Eq. 2)
E
VB = ECB + E
g
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59
60
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, E is the energy of free
electrons on the hydrogen scale (4.5 eV), and E is the band gap of the semiconductor.
e
g
E
CB of ZnIn
2
S
4
is calculated to be ꢀ0.72 eV according to Eq. 1, and EVB is determined to be
35
1
.44 eV by Eq. 2. These values are close to the reported ones.
2 4
Figure 2. (a) Diffuse reflectance UVꢀvis spectra of ZnIn S . (b) The plot of transformed
KubelkaꢀMunk function.
Fragmentation of 2ꢀPhenoxyꢀ1ꢀPhenylethanol
We used 2ꢀphenoxyꢀ1ꢀphenylethanol (1a) as a dimeric lignin βꢀOꢀ4 model to investigate the
2 4 2 4
catalytic activity of ZnIn S . In the presence of ZnIn S photocatalyst, 1a was converted into
acetophenone (1b) and phenol (1c) in 83% and 90% yields, respectively (Table 1, entry 1), which
4
8
were produced via C –O bond scission.
We also observed byproducts
β
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ꢀphenoxyꢀ1ꢀphenylethanone (1d) and 1,4ꢀdiphenylꢀ1,4ꢀbutanedione (1e) with yields of 6% and
β
%, respectively. Byproduct 1d is a dehydrogenated product from 1a, and 1e is the C ꢀcoupling
product of two acetophenones. Visible light illumination is indispensable for the reaction (Table 1,
entry 2). Considering the fact that ZnIn 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
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2 4
S
(
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 commonlyꢀused photocatalysts were employed for
2 2 6 3 4 2 β
this reaction, such as Pd/TiO , Bi WO , gꢀC N and MoS (Table 1, entries 4ꢀ6), the C –O bond
was not cleaved. These photocatalysts are wellꢀperformed in aerobic conditions, but show nearly
4
9ꢀ51
no activity in hydrogen acceptorless conditions.
The two binary sulfides, CdS and In S (Table
2 3
1
, entries 8ꢀ9), only gave 16% and 8% yields of 1d, respectively. These results indicate that
‒
photoꢀinduced e is not the active species for C
β
–O bond cleavage, but other reductive species on
2 4
ZnIn S 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 H atmosphere (Table 1,
2
2 2
entries 1, 10), but the yield of 1d was slightly higher in H atmosphere, suggesting gaseous H 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 C –O bond, but is beneficial for the
β
oxyꢀdehydrogenation 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
β
subsequent C –O bond cleavage.
a
Table 1. Screening of reaction conditions for the fragmentation of lignin model 1a.
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Conversion
%)
Yield of products (%)
1b 1c 1d
90
Entry Catalyst
Solvent
(
1
2
3
4
5
6
7
8
9
ZnIn2S4
CH CN
>99
<1
45
83
0
6
0
3
b
c
ZnIn
2
2
S
4
4
CH CN
0
30
0
3
ZnIn
S
CH
CH
CH
3
CN
3
CN
3
CN
39
0
6
Pd/TiO
Bi WO
gꢀC N
2
19
0
2
6
7
0
0
0
CH CN
5
0
0
0
3
4
3
MoS
2
CH
CH
CH
CH
3
CN
3
CN
3
CN
3
CN
5
0
0
0
CdS
26
0
0
16
8
In
ZnIn
ZnIn2S4
2
S
3
17
0
0
d
10
2
S
4
>99
>94
>39
97
86
27
0
84
28
0
9
e
11
CH CN
61
32
0
3
f
12
ZnIn
ZnIn
ZnIn
ZnIn
2
2
2
2
S
4
S
4
S
4
S
4
3
CH CN
13
THF
77
92
95
73
21
88
93
96
68
18
14
Acetone
Ethanol
>99
>99
98
5
15
16
17
0
ZnIn2S4
ZnIn
H O
2
0
2
S
4
1,2ꢀDCE
59
10
a
Reaction conditions: 0.1 mmol of 1a, 5 mg of catalyst, 1.0 mL of CH
nm), 42 °C, 4 h. The conversion of 1a and products yields were determined by GC with
3
CN, 9.6 W blue LEDs (455
b
c
nꢀdodecane as the internal standard. 45 °C in the dark. Conducted with solar light illumination,
o
o
d
e
f
3
h (2016.7.12, 39 N, 123 E). In 1 atm H
2 2
atmosphere. In 1 atm air atmosphere. In 1 atm O
atmosphere. THF and 1,2ꢀDCE are short for tetrahydrofuran and 1,2ꢀdichloroethane, respectively.
3
Solvents usually have great effects on photocatalytic reactions. CH CN, THF and acetone
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6
(
Table 1, entries 1, 13, 14) have no hydrogenꢀdonating ability and thus reaction in these solvents is
a pure selfꢀhydrogen 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 1,1ꢀdiethyl
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acetal. 1a fragmentation can also be conducted in insoluble solvent H
2
O (Table 1, entry 16).
Chlorinated solvent is unfavorable since under illumination, the generated active Cl· species may
52,53
suppress this reaction (Table 1, entry 17).
Figure 3. Timeꢀonꢀcourse process of photocatalytic C
β
‒O bond cleavage of 1a. Reaction
conditions: 0.1 mmol of 1a, 5 mg of ZnIn S , 1.0 mL of CH CN, Ar atmosphere, 9.6 W blue LEDs
2
4
3
(455 nm), 42 °C.
The timeꢀonꢀcourse 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
β
production of acetophenone and phenol suggests they are the two parts after C –O bond cleavage.
In the initial stage, the yield of 1d increases to 5ꢀ6 % after 0.5 h of illumination, and remains
steady to the end. The coupling product 1e (1,4ꢀdiphenylꢀ1,4ꢀbutanedione) 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.
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Fragmentation of Other Lignin βꢀOꢀ4 Models
After optimizing reaction conditions, we then probed the generality of this photocatalytic
β
strategy with respect to other βꢀOꢀ4 models. The methoxyꢀsubstituted βꢀOꢀ4 models offered C –O
bond cleaved products with yields of 71 to 91%, respectively (Table 2, entries 1ꢀ5); the
substitution position of methoxy, either in benzene ring A or B, on the whole, did not obviously
influence catalytic results. Another βꢀOꢀ4 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 byꢀproducts, 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 C ‒O bond cleavage. βꢀOꢀ4 model
β
γ
7a bearing a C ‒OH group gave 7b in 13% yield though with 97% conversion (Table 2, entry 7).
Besides 7b and phenol, the C
isopropanol as a coꢀsolvent 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%.
β γ
‒C bond cleaved product 5b was obtained in 6% yield. The use of
(
a
Table 2. Substrate scope of visibleꢀlightꢀdriven C ‒O bond cleavage.
β
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a
2 4 3
Reaction conditions: 0.1 mmol of substrate, 5 mg of ZnIn S , 1.0 mL of CH CN, Ar atmosphere,
9
.6 W blue LEDs (455 nm), 42 °C. The conversion of substrate and yield of products were
b
c
determined by GC with nꢀdodecane as the internal standard. 5b was obtained in 6% yield.
Reaction in 0.2 mL CH CN + 0.8 mL isopropanol. N. M. means not measured.
3
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 CH CN and H was used, 1d was not converted
3
2
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 Hꢀabstraction of C H‒OH groups and
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α
5
4
β
subsequent hydrogenolysis of C ‒O bond.
Scheme 3. Control experiments to illustrate the reaction route. Reaction conditions: 0.1 mmol
of 1d, 5 mg of ZnIn , 1.0 mL of solvent, 9.6 W blue LEDs (455 nm), 42 °C, 4 h.
2 4
S
Additional experiments were conducted to confirm selfꢀhydrogen transfer hydrogenolysis
48
(
Scheme 4). 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
(
C
β
β
–O bond can derive from another lignin model, and methoxyꢀsubstitution seems to facilitate the
–O bond cleavage. In Scheme 4 (Control 2), 5a and 1d were likewise converted with C –O
C
β
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 βꢀOꢀ4
α
lignin models, the abstracted hydrogen species from C H–OH groups accumulates on the surface
2 4
of ZnIn S to form a “hydrogen pool” which allocates hydrogen species for the hydrogenolysis of
β
C –O bond.
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Scheme 4. Investigation of selfꢀhydrogen transfer hydrogenolysis process. Reaction conditions:
0
2 4
.1 mmol of 1a and 5d for control 1 (0.1 mmol of 1d and 5a for control 2), 5 mg of ZnIn S , 1.0
3
mL of CH CN, 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 ZnIn revealed the existence of adsorbed hydrogen
2
S
4
species (inherent hydrogen species) on ZnIn S surface (Figure S5). Either due to the small
2 4
amount or inactivity of this inherent hydrogen species, no fragmentation of 1d occurred in the
presence of fresh ZnIn (Scheme 3, Eq. 3).
2 4
S
We detected H and H S in the gas phase (Figure 4a), which may be derived from the
2
2
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 H S 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, 1aꢀαCD, was subjected to the photocatalytic reaction, both H and
HD were detected (Figure 4b), confirming the Hꢀabstraction of the C H–OH groups over ZnIn S
2
2
α
2 4
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catalyst. Another result needs attention is that H is generated in a larger amount than HD. We
2
speculate H species may have a higher activity than D species to couple, leading to more H
2
generation. Nevertheless, we cannot rule out the possibility that the inherent hydrogen species on
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2 4 α 2
ZnIn S and the abstracted one from C H–OH groups may couple to form H .
Figure 4. Detection of the gas phase after fragmentation of 1a by mass spectrometer. (a) 1a as the
substrate. (b) 1aꢀαCD as the substrate. The inset shows the enlarged peaks of HD and H S.
2
Reaction conditions: 0.1 mmol of 1a or 1aꢀαCD, 5 mg of ZnIn S , 1.0 mL of CH CN, Ar
2
4
3
atmosphere, 9.6 W blue LEDs (455 nm), 42 °C, 4 h.
For a deeper understanding of the hydrogen transfer from C
α
H–OH groups to C
β
‒O bond, the
kinetic isotope effect (KIE) in 1a fragmentation was measured with two deuterated substrates
aꢀαCD and 1aꢀαOD. A secondary KIE was obtained with a k /k = 1.10 ± 0.07 when 1aꢀαCD
1
H
D
5
5
was subjected to reaction (Scheme 5, Eq. 5), disapproving the direct Hꢀabstraction of αꢀCH. The
timeꢀonꢀcourse process for 1aꢀαCD fragmentation is recorded to gain more details (Figure S6).
Different from 1a fragmentation, induction periods during the formation of acetophenone and
β
phenol were observed, indicating slower rate of C –O bond cleavage when 1aꢀαCD was the
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6,57
substrate. This result, which is caused by the different activity of H and D species,
suggests
that H (D) species in the “hydrogen pool” directly participates in the C
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 1aꢀαCD fragmentation, with 1aꢀαOD subjected to reaction, the
β
‒O bond cleavage. When
0
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0
(
corresponding KIE was determined to be a primary one with a k
H
/k
D
= 2.11 ± 0.27 (Scheme 5, Eq.
58,59
6
). Therefore, in the dehydrogenation of 1a, Hꢀabstraction of αꢀOH is a rateꢀdetermining step.
Prolonging the reaction time to 10 h, formation of deuterated acetophenone and phenol with
aꢀαOD as the substrate is in accord with the CTH route (Figure S9). Due to the H/D exchange
with the inherent hydrogen species on ZnIn surface, phenol with m/z=94 and 95 were both
1
2 4
S
obtained (Figure S8, S9). Furthermore, deuterated acetophenone is formed when 1aꢀαOD rather
than 1aꢀαCD is subjected to reaction, suggesting that the H of αꢀOH finally transfers to
acetophenone while the H of phenol derives from the αꢀCH of 1a. This photocatalytic CTH
δ+
reaction is different from thermal catalytic transfer hydrogenolysis, where H of αꢀOH would
53,54
transfer to phenol and retain its polarity if the CTH follows a monohydride mechanism.
But in
δ+
δ‒
our case, the H of αꢀOH finally transfers to acetophenone with H form. This indicates that the
δ+
δ‒
δ‒
polarity of the H of αꢀOH may be reversed to H during photocatalysis; afterwards, the H is
transferred to acetophenone instead of phenol due to the smaller electronegativity of carbon than
δ‒
oxygen in the C
β
–O bond. Similarly, H of αꢀCH is also reversed during the photocatalytic
reaction. As a result, polarity reversal of the abstracted hydrogen species may occur in this
24,25
photocatalytic transfer hydrogenolysis process.
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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 ZnIn S , 1.0 mL of CH CN, Ar atmosphere,
2
4
3
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 ZnIn
2
S
4
are located at ꢀ0.72 eV and 1.44 eV, respectively.
‒
Initial visibleꢀlight excitation of the ZnIn S photocatalyst would generate e in conduction band
2
4
+
+
and h in valence band. h is a strong oxidant with a minimum oxidation potential of 1.44 eV and
undergoes Hꢀabstraction of αꢀOH groups, leading to the formation of radical 1f. Subsequent
Hꢀabstraction of αꢀCH groups leads to the formation of 1d. 1d is either adsorbed on ZnIn
2
S
4
or
+
diffuses into solvent as a byproduct. The abstracted H atoms are oxidized by h , delivering protons
60
‒
adsorbed on ZnIn S . For a nꢀtype ZnIn S semiconductor, reduction by e in the conduction
2
4
2 4
+
‒ 61
band is slower than oxidation by h , leading to the accumulation of e . In our previous work,
‒
33
reductive cleavage of the C
β
‒O bond of 1d by e is not likely to occur, 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 C
β
‒O bond cleaved products. Meanwhile, a small
portion of the reduced hydrogen species are combined to form H accompanied by the generation
2
2 4
of coupling product 1e. At this stage, the hydrogen species on ZnIn S surface formed by the
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photoinduced h and e is critical to guarantee the photocatalytic cycle for the fragmentation of
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Scheme 6. Proposed mechanism for photocatalytic fragmentation of 1a via selfꢀhydrogen
transfer hydrogenolysis.
Fragmentation of a PolyꢀβꢀOꢀ4 Model and Dioxanesolv Poplar Lignin
2 4
ZnIn S is a heterogeneous photocatalyst, on which adsorption of highꢀmolecularꢀweight
organosolv lignin molecules is difficult under mild conditions though easy for dimeric lignin
βꢀOꢀ4 models. Thus, prior to organosolv lignin fragmentation, a more complex βꢀOꢀ4 model 8a
was subjected to reaction (Scheme 7). THF was used as the solvent for better solubility of 8a.
After 10 h of illumination, pꢀhydroxyl 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 C
β
–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 polyꢀβꢀOꢀ4 model via transfer hydrogenation.
2 4
Reaction conditions: 15 mg of 8a, 5 mg of ZnIn S , 1.0 mL THF, Ar atmosphere, 9.6 W blue
LEDs (455 nm), 42 °C, 10 h.
2 4
Next, ZnIn S 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)
3
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and previous work, we extracted dioxanesolv poplar lignin with a light yellowꢀbrown color.
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 M
w
of about
300 Da. After reaction, the peak area of 35.7 min increases obviously, indicative of lignin
w
monomers formation. Meanwhile, the peak area of lignin molecules with a high M 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 GCꢀMS, 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 (nꢀdodecane). 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
6
b). Therefore, the new generated substances with a total mass yield of 10% are derived from
photocatalysis over ZnIn . 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, pꢀhydroxyl
acetophenone derivatives (marked in red), which are formed by C –O bond cleavage, are in accord
2 4
S
β
with the products of βꢀOꢀ4 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
pꢀhydroxyl acetophenone derivatives suggests the success of achieving dioxanesolv poplar lignin
fragmentation with this photoinduced CTH strategy.
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Figure 6. (a) GCꢀMS spectra and (b) Quantitative analysis for photoꢀinduced 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
2 4
conditions: 30 mg of dioxanesolv poplar lignin, 10 mg of ZnIn S , 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, visibleꢀlightꢀdriven fragmentation of lignin βꢀOꢀ4 models and dioxanesolv
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poplar lignin is achieved over ZnIn2S4 photocatalyst. During photocatalysis, the C H‒OH
α
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hydrogen of βꢀOꢀ4 models is transferred to ZnIn S surface to form a “hydrogen pool”, then the
β
hydrogen species in the “hydrogen pool” is hydrogenated to C –O bond for cleavage. The
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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 pꢀhydroxyl acetophenone derivatives. The influence of C ‒OH was
γ
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 βꢀOꢀ4 model and
dioxanesolv lignin fragmentation with lignin itself as a hydrogen source.
AUTHOR INFORMATION
Corresponding Author
*
F.W. Email: wangfeng@dicp.ac.cn
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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