Visible-Light-Driven Cleavage of C?O Linkage for Lignin Valorization to Functionalized Aromatics

Article abstract of DOI:10.1002/cssc.201902355

Lignin is the most abundant source of renewable aromatics. Catalytic valorization of lignin into functionalized aromatics is attractive but challenging. Photocatalysis is a promising sustainable approach. The strategies for designing well-performing photocatalysts are desired but remain limited. Herein, a facile energy band engineering strategy for promoting the photocatalytic activity of zinc–indium–sulfide (Zn_mIn₂S_m+3) for cleavage of the lignol β-O-4 bond under mild conditions was developed. The energy band structure of Zn_mIn₂S_m+3 could be tuned by controlling the atomic ratio of Zn/In. It was found that Zn₄In₂S₇ performed best for cleavage of the β-O-4 bond under visible-light irradiation, owing to its appropriate energy band structure for offering adequate visible-light absorption and suitable redox capability. Functionalized aromatic monomers with near 18.4 wt % yield could be obtained from organosolv birch lignin. Mechanistic studies revealed that the β-O-4 bond was efficiently cleaved mainly through a one-step redox-neutral pathway via a C_α radical intermediate. The thiol groups on the surface of Zn₄In₂S₇ played a key role in cleavage of the β-O-4 bond.

Full text of DOI:10.1002/cssc.201902355

DOI: 10.1002/cssc.201902355
Full Papers
Visible-Light-Driven Cleavage of CꢀO Linkage for Lignin
Valorization to Functionalized Aromatics
Jinchi Lin, Xuejiao Wu, Shunji Xie,* Liangyi Chen, Qinghong Zhang, Weiping Deng, and
Ye Wang*[a]
Lignin is the most abundant source of renewable aromatics.
Catalytic valorization of lignin into functionalized aromatics is
attractive but challenging. Photocatalysis is a promising sus-
tainable approach. The strategies for designing well-perform-
ing photocatalysts are desired but remain limited. Herein, a
facile energy band engineering strategy for promoting the
photocatalytic activity of zinc–indium–sulfide (ZnmIn2Sm+3) for
cleavage of the lignol b-O-4 bond under mild conditions was
developed. The energy band structure of ZnmIn2Sm+3 could be
tuned by controlling the atomic ratio of Zn/In. It was found
that Zn4In2S7 performed best for cleavage of the b-O-4 bond
under visible-light irradiation, owing to its appropriate energy
band structure for offering adequate visible-light absorption
and suitable redox capability. Functionalized aromatic mono-
mers with near 18.4 wt% yield could be obtained from organo-
solv birch lignin. Mechanistic studies revealed that the b-O-4
bond was efficiently cleaved mainly through a one-step redox-
neutral pathway via a Ca radical intermediate. The thiol groups
on the surface of Zn4In2S7 played a key role in cleavage of the
b-O-4 bond.
Introduction
Lignocellulose represents more than 90% of all plant biomass,
and thus is an attractive, carbon-neutral, and nonedible start-
ing material that offers a renewable alternative to fossil carbon
resources for the production of fuels and chemicals.[1,2] Lignin
is one of the three major components of lignocellulose, in ad-
dition to cellulose and hemicellulose, with a content of 10–
35%.[3] Full valorization of all three major constituents of ligno-
cellulose is vital for enhancing the economic feasibility of a bi-
orefinery.[4,5] Owing to its structural complexity and recalci-
trance, lignin transformation is one of the major challenges in
the full valorization of lignocellulose.[6]
strategies to break the linkages in lignin, including pyrolysis,
gasification, acid catalysis, and metal (Ni or a noble metal)-cat-
alyzed hydrogenolysis, usually require harsh conditions, and
thus bond breaking is nonselective, leading to low-functional-
ized products.[3,6,9–11] The two-step strategy of preoxidation and
cleavage of the b-O-4 bond for lignin depolymerization can be
performed under milder conditions but usually requires addi-
tional reductants/oxidants or acids/bases and generates large
amounts of byproducts.[6,12,13] Thus, the development of a sus-
tainable methodology for lignin depolymerization under mild
conditions is highly attractive.
Lignin is an aromatic biopolymer and the most abundant
source of renewable aromatics in nature, and thus harbors
great potential for the production of industrial bulk or func-
tionalized aromatic compounds.[6,7] Lignin depolymerization is
an intriguing task that requires the selective cleavage of strong
CꢀO and CꢀC linkages, especially the most abundant b-O-4
ether bond, which typically makes up approximately 50% of
all linkages.[6] Cleavage of the b-O-4 linkage requires a bond
Solar-energy-driven catalytic transformations, often operat-
ing under mild conditions,[14–16] offer a green strategy for the
selective depolymerization of lignin. Interestingly, in recent
years, several promising photocatalysts have been developed
for the selective depolymerization of lignin to value-added
chemicals.[17–21] Stephenson and co-workers performed thermo-
catalytic preoxidation of the CaꢀOH bond of b-O-4 alcohols to
b-O-4 ketones, then conducted reductive cleavage of the CꢀO
dissociation energy (BDE) of 54–72 kcalmolꢀ1 [8]
.
Conventional
bonds by using an organometallic iridium photocatalyst.[17]
A
two-step photocatalytic strategy was developed by Wang and
co-workers,[18] who used a Pd/ZnIn2S4 catalyst for the aerobic
oxidation of CaꢀOH to ketones under l=455 nm light irradia-
tion, and then TiO2 was employed for cleaving CꢀO bonds
under l=365 nm light irradiation. However, in each step, sacri-
ficial agents are required to consume the unused electrons or
holes. Photogenerated electrons and holes may substitute re-
ductants and oxidants simultaneously, enabling the accom-
plishment of one-step redox-neutral reactions. ZnIn2S4 can be
used for the one-pot photocatalytic anaerobic conversion of
organosolv lignin to p-hydroxyacetophenone derivatives.[19]
Moreover, we have found that CdS quantum dots are powerful
[a] J. Lin, X. Wu, Dr. S. Xie, L. Chen, Prof. Dr. Q. Zhang, Dr. W. Deng,
Prof. Dr. Y. Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces
Collaborative Innovation Center of Chemistry for Energy Materials
National Engineering Laboratory for Green Chemical Productions
of Alcohols, Ethers and Esters, College of Chemistry and
Chemical Engineering,Xiamen University
Xiamen 361005 (P.R. China)
E-mail: shunji_xie@xmu.edu.cn
wangye@xmu.edu.cn
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cssc.201902355.
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catalysts for the selective cleavage of the b-O-4 bond at room
temperature under visible-light irradiation in one step. CdS
quantum dots were further applied for the photocatalytic con-
version of native lignin in birch woodmeal to functionalized ar-
omatic monomers.[20] Despite some meaningful progress in
photocatalytic lignin depolymerization, strategies for improv-
ing the catalyst activity remain limited.
The energy band structure of a photocatalyst is one of the
most important factors in determining the catalytic performan-
ces.[22,23] The positions of the conduction band (CB) and va-
lence band (VB) edges affect the redox capability, whereas the
band-gap energy controls the spectral response. The energy
band engineering strategy has become a research hot spot for
designing effective photocatalysts for water splitting, CO2 re-
duction, and organic degradation,[24–27] and has also had some
positive effects on lignin depolymerization recently.[20,21,28]
Metal sulfide semiconductors are a class of photocatalysts with
good visible-light responses, rich variability in properties, and
high photocatalytic activities,[29] as well as high potential for se-
lectively cleaving the b-O-4 bond in lignin.[18–20]
Figure 1. a) Schematic illustration of the synthesis of ZnmIn2Sm+3 (m=1–6)
samples. b)–d) TEM and high-resolution (HR) TEM images of Zn4In2S7.
nanosheet morphology (Figure 1b and Figure S3 in the Sup-
porting Information). In the case of Zn4In2S7, for example, the
Zn4In2S7 nanosheets display a multilayered structure (Fig-
ure 1c). The HRTEM image shows lattice fringes with an inter-
planar spacing of 0.32 nm, which corresponds to the (102)
crystallographic plane (Figure 1d).[30,33]
Herein, we have synthesized a series of ternary metal sulfide
photocatalysts, ZnmIn2Sm+3 (m=1–6, integer), through a simple
low-temperature hydrothermal method, and tuned the energy
band structure of photocatalysts by controlling the atomic
ratio of Zn/In. We found that Zn4In2S7 could efficiently catalyze
the selective cleavage of b-O-4 bonds in lignin model com-
pounds and organosolv lignin to aromatic monomers under
visible-light irradiation. The effect of the energy band structure
and reaction mechanism were systematically studied to gain a
deeper understanding of the overall reaction of lignin depoly-
merization. The results and insights offer a facile, promising
tool to design effective photocatalysts.
To obtain information on the optical properties and energy
band structure of ZnmIn2Sm+3 samples, we further performed
diffuse-reflectance UV/Vis studies and Mott–Schottky measure-
ments. The UV/Vis spectra of ZnmIn2Sm+3 show that the absorp-
tion edge shifts to a shorter wavelength upon increasing the
value of m from 1 to 6 (Figure 2a); this suggests that the
band-gap energy (Eg) of ZnmIn2Sm+3 increases with increasing
Zn/In atomic ratio. From plots of the modified Kubelka–Munk
function, [F(R)hu]1/2, versus the energy of exciting light, hu, the
band-gap energy for a semiconductor can be evaluated
(Figure 2b). An increase in the value of m from 1 to 6 in
ZnmIn2Sm+3 gradually increased the band-gap energy from 2.12
to 2.60 eV (Figure 2b and Table S2 in the Supporting Informa-
tion). Mott–Schottky measurements were performed to obtain
information on the flat-band potential (Efb) of ZnmIn2Sm+3 (Fig-
ure 2c and Table S2 in the Supporting Information), which was
approximately 0.15 V more positive than that of the conduc-
tion-band minimum (CBM, Ecb).[34] The CBMs of ZnmIn2Sm+3
were shifted to more negative values, from ꢀ0.47 to ꢀ0.61 V,
upon increasing of the value of m from 1 to 6 (Figure 2d).
Then, we calculated the valence-band maximum (VBM, Evb) of
ZnmIn2Sm+3, according to Evb =Eg +Ecb. The VBMs of ZnmIn2Sm+3
were shifted to more positive values, from 1.65 to 1.99 V, upon
increasing the value of m from 1 to 6 (Figure 2d). Thus, the
energy band structure of ZnmIn2Sm+3 can be tuned by control-
ling the atomic ratio of Zn/In (Figure 2d).
Results and Discussion
Synthesis and characterization of ZnmIn2Sm+3 photocatalysts
The series of ternary metal sulfide photocatalysts, ZnmIn2Sm+3
(m=1–6, integer), studied herein were synthesized through a
facile, low-temperature, hydrothermal method. As illustrated in
Figure 1a, Zn(CH3COO)2, InCl3, and thioacetamide were used as
Zn, In, and S sources, respectively, to prepare ZnmIn2Sm+3 in an
aqueous solution at 958C. Energy-dispersive X-ray spectrosco-
py (EDX) analysis (Figure S1 in the Supporting Information)
confirmed that the elemental contents of Zn, In, and S of all
ZnmIn2Sm+3 samples were close to that of the stoichiometric
composition (Table S1 in the Supporting Information). Our
powder XRD studies show that the XRD patterns of ZnmIn2Sm+3
samples present similar profiles and all diffraction peaks can
be indexed to a hexagonal wurtzite crystal lattice (Figure S2 in
the Supporting Information).[30] With an increase of the Zn/In
atomic ratio in ZnmIn2Sm+3, the position of the (006) peak shift-
ed slightly to a higher angle (Figure S2 in the Supporting Infor-
mation); this may be owing to the more compact layered
structure of ZnmIn2Sm+3, which is caused by the ionic radius of
Zn2+ (0.74 ꢁ) being smaller than that of In3+ (0.76 ꢁ).[31,32] TEM
images of the ZnmIn2Sm+3 samples all display aggregative
Photocatalytic performance of ZnmIn2Sm+3 and effect of
energy band structure
The ZnmIn2Sm+3 (m=1–6) samples were examined for the pho-
tocatalytic conversion of a model lignin compound, 2-phen-
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Table 1. Photocatalytic conversion of PP-ol with ZnmIn2Sm+3 (m=1–6)
photocatalysts under visible-light irradiation.[a]
Entry
Catalyst
Conv.
[%]
Yield [%]
AP
Phol
PP-one
1
2
3
4
5
6
ZnIn2S4
Zn2In2S5
Zn3In2S6
Zn4In2S7
Zn5In2S8
Zn6In2S9
46
61
83
99
80
69
41
54
74
86
70
59
40
53
68
82
65
56
5.3
7.4
6.1
9.6
11
10
[a] Reaction conditions: catalyst (10 mg), PP-ol (0.10 mmol), CH3CN/H2O
(5.0 mL, 1:1 v/v), N2 atmosphere, 4 h, Xe lamp (l=400–780 nm),
600 mWcmꢀ2
.
ZnIn2S4. The yields of AP and Phol increased nearly linearly in
the first 2 h, then further increased to 86 and 82%, respective-
ly, after 4 h of reaction (Figure 3a). Zn4In2S7 was stable during
the reaction; only a slight deactivation was observed in the
first three recycling experiments, and then the performance
tended to remain the same in later cycles (Figure 3b).
Figure 2. a) Diffuse reflectance UV/Vis spectra and b) the corresponding
plots of modified Kubelka–Munk function versus the energy of exciting light
of ZnmIn2Sm+3 (m=1–6). c) Mott–Schottky plots and d) band structure dia-
gram of ZnmIn2Sm+3 (m=1–6).
oxy-1-phenylethanol (PP-ol), at room temperature under the ir-
radiation of simulated visible sunlight (Xe lamp, l=400–
780 nm). As shown in Table 1, all ZnmIn2Sm+3 (m=1–6) photo-
catalysts showed unique performances in breaking the b-O-4
bond in PP-ol, producing acetophenone (AP) and phenol
(Phol) in almost stoichiometric amounts, with a small amount
of 2-phenoxy-1-phenylethanone (PP-one). The photocatalytic
activities of ZnmIn2Sm+3 for the conversion of PP-ol strongly de-
pended on the atomic ratio of Zn/In. Zn4In2S7 exhibited signifi-
cantly higher activity than that of other ZnmIn2Sm+3 photocata-
lysts; it was approximately two times higher than that of
Figure 3. a) Time course process of Zn4In2S7 for the photocatalytic conver-
sion of PP-ol under visible-light irradiation. b) Repeated use of Zn4In2S7 for
the photocatalytic conversion of PP-ol under visible-light irradiation. Reac-
tion conditions: catalyst (10 mg), PP-ol (0.10 mmol), CH3CN/H2O (5.0 mL, 1:1
v/v), N2 atmosphere, 4 h, Xe lamp (l=400–780 nm), 600 mWcmꢀ2
.
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To compare the catalytic performance of Zn4In2S7 with that
of catalysts reported previously in the literature,[19,20] we further
synthesized ZnIn2S4 nanosphere (ZnIn2S4-NS), CdS nanoparticle
(CdS-NP), and CdS quantum dot (CdS-QD) photocatalysts, ac-
cording to methods provided in the literature,[19,20] for the cata-
lytic conversion of PP-ol under the same reaction conditions
(Table S3 in the Supporting Information). These catalysts all
showed good selectivity for cleavage of the b-O-4 bond in PP-
ol to generate AP and Phol. The activity of Zn4In2S7 was ap-
proximately two times that of ZnIn2S4-NSs and four times that
of CdS-NPs, but slightly lower than that of CdS-QDs (Table S3
in the Supporting Information). However, compared with CdS,
which contains the toxic element Cd, ZnmIn2Sm+3 is a more en-
vironmentally friendly photocatalyst. The apparent quantum
yields for PP-ol conversion over the Zn4In2S7 catalyst were mea-
sured under irradiation with light of different wavelengths;
these yields were above 1.0% at wavelengths below 435 nm
and decreased upon increasing the wavelength (Figure S4 in
the Supporting Information). The longest wavelength suitable
for PP-ol conversion was found to coincide with the absorption
edge of the Zn4In2S7 catalyst; this further indicates that the
conversion of PP-ol is indeed driven by light.
surface area. The conversion rate also increased with increasing
Zn/In atomic ratio in ZnmIn2Sm+3 from ZnIn2S4 to Zn6In2S9
under l=395 nm LED irradiation (Table 2), which suggested
that the larger band gap of ZnmIn2Sm+3 with higher Zn/In
atomic ratios contributed to their higher photocatalytic activi-
ties by providing a higher redox capability. Thus, Zn4In2S7
showed the highest activity for PP-ol conversion under visible-
light irradiation (Table 1); this may be owing to its energy
band structure offering adequate visible-light absorption and a
suitable redox capability.
Mechanism for b-O-4 bond cleavage
We have performed mechanistic studies to understand the
chemistry of b-O-4 bond cleavage over the Zn4In2S7 photocata-
lyst. A control experiment with a hole scavenger (Na2S+
Na2SO3) significantly decreased the performance for the con-
version of PP-ol (Table 3, entry 2), which implied that holes
might have participated in the initial step for PP-ol conversion.
With an electron scavenger (Na2S2O8), the conversion of PP-ol
decreased from 99 to 62%, and the major product was PP-one
(entry 3). The addition of radial scavenger [5,5-dimethyl-1-pyr-
roline-N-oxide (DMPO)] also suppressed the reaction signifi-
cantly (entry 4). These phenomena indicate that both photo-
generated electrons and holes participate in cleavage of the b-
O-4 bond for the formation of AP and Phol via radical inter-
mediates.
Further studies were performed to understand the effect of
the energy band structure of ZnmIn2Sm+3. A l=395 nm light-
emitting diode (LED) was employed for the photocatalytic con-
version of PP-ol to eliminate the influence of light absorption.
As shown in Table 2, the yields of AP and Phol increased with
increasing Zn/In atomic ratio in ZnmIn2Sm+3, from ZnIn2S4 to
Zn6In2S9. Differently, we observed a decrease in activity from
Zn4In2S7 to Zn5In2S8 and Zn6In2S9 under the irradiation of a Xe
lamp (l=400–780 nm; Table 1); this probably arose from the
poor light absorption of Zn5In2S8 and Zn6In2S9 in the visible-
light range owing to an increase in the band gap. The surface
area can also play a key role in photocatalysis. The surface area
of ZnmIn2Sm+3 decreased slightly from 59 to 48 m2 gꢀ1 upon in-
creasing m from 1 to 6 (Figure S5 and Table S4 in the Support-
ing Information). The conversion rate of PP-ol per surface area
per hour was further calculated to eliminate the influence of
Previous studies on the photocatalytic conversion of lignin
have proposed two mechanisms for b-O-4 bond cleavage
(Figure 4).[19,20] Pathway A is a two-step process via the PP-one
intermediate; the preoxidation of CaꢀOH to Ca=O decreases
the BDE of the b-O-4 bond from 55 to 47 kcalmolꢀ1. Pathway B
is a one-step process via a Ca radical intermediate; cleavage of
CaꢀH to the Ca radical markedly decreases the BDE of the b-O-
Table 3. Control experiments with different reactants or additives cata-
lyzed by Zn4In2S7 photocatalyst under visible-light irradiation.[a]
Table 2. Photocatalytic conversion of PP-ol with ZnmIn2Sm+3 (m=1–6)
photocatalysts under l=395 nm light irradiation.[a]
Entry
Reactant
Additive[b]
Conv.
[%]
Yield [%]
Catalyst
Conv.
[%]
Yield [%]
Conversion rate[b]
PP-one
AP
Phol
PP-one
[mmolmꢀ2 hꢀ1
]
AP
Phol
1
2
3
4
5
6
PP-ol
PP-ol
PP-ol
PP-ol
PP-one
PP-one
–
99
2.7
62
12
1.1
18
86
82
9.6
0.21
48
2.6
–
ZnIn2S4
Zn2In2S5
Zn3In2S6
Zn4In2S7
Zn5In2S8
Zn6In2S9
40
47
61
74
79
82
35
42
52
63
66
69
31
38
49
57
61
62
5.8
5.3
5.5
8.4
12
17
21
27
36
40
43
Na2S+Na2SO3
Na2S2O8
DMPO
1.3
18
9.3
0.6
16
2.1
12
8.1
0.7
17
–
CH3OH[c]
–
13
[a] Reaction conditions: Zn4In2S7 (10 mg), (0.10 mmol), CH3CN/H2O
(5.0 mL, 1:1 v/v), N2 atmosphere, 4 h, Xe lamp (l=400–780 nm),
600 mWcmꢀ2. [b] Additive concentrations: 10 mm Na2S+5 mm Na2SO3,
10 mm Na2S2O8, 5 mm DMPO. [c] CH3OH is the solvent.
[a] Reaction conditions: catalyst (10 mg), PP-ol (0.10 mmol), CH3CN/H2O
(5.0 mL, 1:1 v/v), N2 atmosphere, 1 h, LED lamp (l=395 nm),
350 mWcmꢀ2. [b] Conversion rate of PP-ol per surface area of catalyst per
hour.
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Figure 4. Proposed mechanism for b-O-4 bond cleavage in the photocatalyt-
ic conversion of lignin over the Zn4In2S7 catalyst.
4 bond from 55 to 7.8 kcalmolꢀ1. The conversion of PP-one
with or without CH3OH (as a hydrogen donor and a hole scav-
enger) was slow over the Zn4In2S7 catalyst (Table 3, entries 5
and 6), which suggested that the process via the PP-one inter-
mediate for cleavage of the b-O-4 bond of PP-ol was not a
major process for AP and Phol formation. Two similar lignin
model compounds with methoxy groups, 2-(2-methoxyphe-
noxy)-1-(4-methoxyphenyl)ethanol (methoxy-PP-ol) and 2-(2-
methoxyphenoxy)-1-(4-methoxyphenyl)ethanone (methoxy-PP-
one) were also used as reactants. Similar to the catalytic results
of PP-ol and PP-one (Scheme 1, entries 1 and 2), the transfor-
mation of methoxy-PP-one is much slower than that of me-
thoxy-PP-ol (Scheme 1, entries 3 and 4). Additional control ex-
periments were conducted to confirm the reaction pathway by
tracing the reductive cleavage of Ca=O intermediate in the
presence of CaꢀOH reactant. If methoxy-PP-one was added
with PP-ol as a reactant, the conversion of methoxy-PP-one
was only 24% (Scheme 1, entry 5). If PP-one was added with
methoxy-PP-ol as a reactant, the conversion of PP-one was
only 17% (Scheme 1, entry 6). Both results further confirm that
the major process for cleavage of the b-O-4 bond over Zn4In2S7
is the one-step Ca radical intermediate pathway; the two-step
Ca=O intermediate pathway is a minor process for cleavage of
the b-O-4 bond (Figure 4). According to this one-step Ca radi-
cal intermediate mechanism, photogenerated holes in the VB
can oxidize the CaꢀH bond of PP-ol to form the Ca radical in-
termediate, which can undergo subsequent reductive cleavage
of the CbꢀO bond by accepting photogenerated electrons
from the CB (Figure 4). A lower (more positive) VB edge will fa-
cilitate oxidation of the CaꢀH bond of PP-ol to form the Ca rad-
ical intermediate, whereas a higher (more negative) CB edge
will facilitate the reductive cleavage of the CbꢀO bond. Thus,
ZnmIn2Sm+3 with a higher Zn/In atomic ratio has a higher redox
capability for cleavage of the b-O-4 bond and shows higher ac-
tivity for PP-ol conversion under irradiation from a l=395 nm
LED (Table 2).
Scheme 1. Control experiments with different reactants catalyzed by
Zn4In2S7 photocatalyst under visible-light irradiation. Reaction conditions:
Zn4In2S7 catalyst (10 mg), reactant (0.10 mmol for entries 1–4; 0.10 mmol PP-
ol+0.10 mmol methoxy-PP-one for entry 5; 0.10 mmol PP-one+0.10 mmol
methoxy-PP-ol for entry 6), CH3CN/H2O (5.0 mL, 1:1 v/v), N2 atmosphere,
12 h, Xe lamp (l=400–780 nm), 600 mWcmꢀ2
.
from the weak and hydridic benzylic CꢀH bond.[35,36] Moreover,
thiol groups (ꢀSH) on the surface of sulfide semiconductors
are also proposed to be the active centers for CꢀH bond acti-
vation under light irradiation.[37,38] Thus, further studies were
performed to understand the effect of ꢀSH groups on the
Zn4In2S7 surface. Bromide derivatives can react with ꢀSH
groups efficiently through a nucleophilic displacement reac-
tion,[38,39] and thus can be used as inhibitors of ꢀSH groups on
the Zn4In2S7 surface. Different amounts of (3-bromopropyl)tri-
methoxysilane (BPTMOS, as an inhibitor for ꢀSH groups) were
used to treat Zn4In2S7 in cyclohexane, and the catalytic per-
formances of treated Zn4In2S7 are shown in Figure 5. The con-
version of PP-ol decreases significantly with an increase in the
amount of BPTMOS used to treat Zn4In2S7. If the surface ꢀSH
groups of Zn4In2S7 were mostly alkylated by BPTMOS, the reac-
tion was completely inhibited. The alkylated Zn4In2S7 catalyst
was further treated with an aqueous solution of NaSH to
remove BPTMOS species and regenerate ꢀSH groups. The cata-
lytic activity of the regenerated Zn4In2S7 catalyst was restored
(Figure 5). These results indicate that the ꢀSH groups on the
In the one-step Ca radical intermediate pathway, selective
cleavage of the benzylic CaꢀH bond is the key step for Ca radi-
C
cal intermediate formation. The electrophilic thiyl radical (RꢀS ),
which is formed through the photo-oxidation of the thiol cata-
lyst (RꢀSH), has been used to selectively abstract hydrogen
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Photocatalytic conversion of various b-O-4 model com-
pounds and organosolv birch lignin
Apart from b-O-4 linkages, functional groups such as phenol,
methoxy, and g-hydroxy are also essential constituents of real
lignin. Thus, other b-O-4 model compounds with these groups
were also studied to examine the feasibility of the Zn4In2S7 cat-
alyst for lignin valorization. As shown in Table 4, Zn4In2S7 could
efficiently catalyze cleavage of the b-O-4 bond in these lignin
model compounds with different functional groups. In the re-
action products, functional groups such as phenol and me-
thoxy groups can be well preserved. Although the g-hydroxy
group could easily undergo degradation, a large fraction of g-
hydroxy groups was retained in the products, contributing to
the mild reaction conditions applied.
Figure 5. Photocatalytic performance of Zn4In2S7 after modification with dif-
ferent amounts of BPTMOS and regenerated Zn4In2S7. Regenerated Zn4In2S7
refers to BPTMOS-modified (100 mL) Zn4In2S7, which is further treated with
an aqueous solution of NaSH. Reaction conditions: catalyst (10 mg), PP-ol
(0.10 mmol), CH3CN/H2O (5.0 mL, 1:1 v/v), N2 atmosphere, 4 h, Xe lamp
The Zn4In2S7 catalyst was further applied to the conversion
of real lignin under visible-light irradiation. Compared with
lignin model compounds, the conversion of real lignin is much
more difficult, owing to its high molecular weight and cross-
linked structure.[19,20] The real lignin sample was isolated from
birch chips, a typical hardwood, by a standard organosolv pro-
cess with dioxane as a solvent (denoted as dioxasolv birch
lignin).[40] The photocatalytic depolymerization process is illus-
trated in Figure 6a. The yellow–brown dioxasolv birch lignin
solution in methanol was decolored after the reaction, which
indicated a reduced molecular weight of the substrate. The 2D
heteronuclear single quantum coherence (HSQC) NMR spectra
of the dioxasolv birch lignin and reaction products demon-
(l=400–780 nm), 600 mWcmꢀ2
.
Zn4In2S7 surface play a key role in cleavage of the b-O-4 bond.
The surface ꢀSH groups may trap holes to generate thiyl radi-
cals for the selective activation of the benzylic CaꢀH bond to
form the Ca radical intermediate for subsequent reductive
cleavage of the b-O-4 bond.
Table 4. Photocatalytic conversion of different b-O-4 lignin model compounds with the Zn4In2S7 photocatalyst under visible-light irradiation.[a]
Reactant
Time [h]
4
Conv. [%]
99
Yield [%]
86
82
4
12
12
98
81
85
91
70
88
68
76
55
48
41
74
32
16
28
16
16
20
95
86
92
92
77
12
82
[a] Reaction conditions: Zn4In2S7 (10 mg), reactant (0.10 mmol, CH3CN/H2O (5.0 mL, 1:1 v/v), N2 atmosphere, Xe lamp (l=400–780 nm), 600 mWcmꢀ2
.
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groups in these products remained almost intact
under the mild conditions.
Conclusions
We found that Zn4In2S7 was an effective photocata-
lyst for selectively cleaving the lignol b-O-4 bond in
various lignin model compounds and dioxasolv birch
lignin under visible-light irradiation. The functional
groups in the substrate could be retained in the
products during cleavage of the b-O-4 bond. A high
yield of functionalized aromatic monomers of ap-
proximately 18.4 wt% was obtained for the conver-
sion of dioxasolv birch lignin. Further studies showed
that the energy band structure of ZnmIn2Sm+3 could
be tuned by controlling the atomic ratio of Zn/In;
specifically, a higher Zn/In ratio provided a higher
redox capability but poorer visible-light absorption.
Zn4In2S7 performed best for cleavage of the b-O-4
bond under visible-light irradiation, owing to its ap-
propriate energy band structure for offering ade-
quate visible-light absorption and suitable redox ca-
pability. Mechanistic studies revealed that the major
process for cleavage of the b-O-4 bond over Zn4In2S7
was a one-step redox-neutral pathway via a Ca radi-
cal intermediate. The ꢀSH groups on the surface of
Zn4In2S7 might trap photogenerated holes to gener-
ate thiyl radicals for the selective activation of the
CaꢀH bond to form the Ca radical intermediate,
which could undergo subsequent reductive cleavage
of the CbꢀO bond by accepting photogenerated
electrons. All of these results and insights offer a
facile, promising tool to design effective catalysts for
lignin valorization to functionalized aromatics driven
by solar energy.
Figure 6. a) Reaction process images, quantification, and distribution of products ob-
tained from the photocatalytic depolymerization of birch lignin. b) 2D HSQC NMR spectra
of dioxasolv birch lignin before and after the reaction. Aa, Ab, and Ag denote the CꢀH
bonds for the a, b, and g carbon atoms of b-O-4 linkages. Bg denotes the CꢀH bond for
the g carbon of phenylcoumaran units. Ca, Cb, and Cg denote the CꢀH bonds for the a, b,
and g carbon atoms of resinol units. Prefixes S- and G- indicate syringyl- and guaiacyl-de-
rived compounds, respectively. Reaction conditions: Zn4In2S7 (10 mg), dioxasolv birch
lignin (20 mg), CH3OH (5.0 mL), N2 atmosphere, 24 h, Xe lamp (l=400–780 nm),
Experimental Section
Chemicals
600 mWcmꢀ2
.
Zn(CH3COO)2·2H2O, thioacetamide, 1,4-dioxane, CH2Cl2,
and acetonitrile were purchased from Sinopharm Chemi-
cal Reagent Co., Ltd. InCl3·4H2O was purchased from
Shanghai Macklin Biochemical Co., Ltd. BPTMOS was purchased
from Tokyo Chemical Industry Co., Ltd. All reagents were used as
received without further purification.
strated a significant decrease in the signals ascribed to b-O-4
linkages (Figure 6b). Based on the Aa integrals of 2D HSQC
NMR spectra before and after the photocatalytic reaction, ap-
proximately 85% of b-O-4 was cleaved (Figure 6b).[41] The
liquid products were extracted with CH2Cl2 and analyzed by
GC–MS and GC. Quantitative analysis shows that the major
soluble products are ketones (including S- and G-ketones) and
alcohols (including sinapyl and coniferyl alcohols, belonging to
S and G units, respectively; Figure 6a and Figure S6 in the Sup-
porting Information) with an S/G ratio of 3.2, which is close to
that in the original lignin (2.8), as characterized by 2D HSQC
NMR spectroscopy (Figure S7 and Tables S5 and S6 in the Sup-
porting Information). The total mass yield of functionalized ar-
Synthesis of ZnmIn2Sm+3 photocatalysts
All ZnmIn2Sm+3 (m=1–6) photocatalysts were synthesized through
a
low-temperature hydrothermal method. More specifically,
InCl3·4H2O (1.0 mmol), Zn(CH3COO)2·2H2O (m/2 mmol), and thioa-
cetamide [(m+3)/2 mmol] were dissolved in deionized (DI) water
(100 mL); the mixture was heated to 958C and maintained at the
same temperature for 5 h under vigorous stirring. The solution was
then cooled to room temperature naturally, and the precipitate
was collected by centrifugation, rinsed with DI water and absolute
omatic monomers reached 18.4 wt%, and the functional ethanol, and dried under vacuum at 608C overnight.
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lysts. TEM measurements were performed on a Philips Analytical
FEI Tecnai 20 electron microscope operated at an acceleration volt-
age of 200 kV. Samples for TEM measurements were suspended in
ethanol and dispersed ultrasonically. Drops of suspensions were
applied on a copper grid coated with carbon. The 2D HSQC NMR
spectra were acquired on a Bruker Avance III 850 MHz spectrome-
ter equipped with a 5 mm TXI 1H/13C/15N cryoprobe. The electro-
chemical Mott–Schottky measurements were performed at open-
circuit potential. An electrochemical analyzer (CHI760E instruments,
CHI, PR China) of a standard three-electrode system was used for
measurements. The sample (fixed amount of 3 mg) was coated
onto a fluorine-doped tin oxide (FTO) glass electrode. Sample-
coated FTO, Pt wire, and Ag/AgCl were used as the working, coun-
ter, and reference electrodes, respectively. An aqueous solution of
Na2SO4 (0.5 moldmꢀ3) was used as the electrolyte. All applied po-
tentials were recorded versus the Ag/AgCl (saturated KCl) reference
electrode and then converted into values for a normal hydrogen
electrode (NHE) according to Equation (2).
Evaluation of photocatalytic performances
Photocatalytic reactions were conducted in a 10 mL quartz tube,
and a 300 W Xe lamp with a UV cutoff filter (l=400–780 nm) or a
l=395 nm LED lamp were used as light sources. ZnmIn2Sm+3 pho-
tocatalyst (10 mg), solvent (5.0 mL), and lignin models (0.10 mmol)
were added to the reactor, and, with the aid of ultrasonication, the
mixture formed a well-dispersed suspension. Then, the reactor was
evacuated, purged with N2 gas, and placed under illumination; the
reaction mixture was stirred at 1000 rpm during the reaction. After
the reaction, the mixture was filtered through a 0.22 mm Nylon sy-
ringe filter and analyzed by means of HPLC.
We measured the apparent quantum yields of PP-ol conversion
over the Zn4In2S7 catalyst by using light of different wavelengths.
The apparent quantum yield (h) of PP-ol conversion was calculated
by using Equation (1):
h ¼ ½yDnðPP-olÞNAꢁ=ðISt=ElÞ ꢂ 100 %
ð1Þ
ENHE ¼ EAg=AgCl þ 0:197
ð2Þ
in which y represents the number of holes or electrons used for
the conversion of one PP-ol molecule, which is one, according to
the one-step process through Ca radical intermediate mechanism;
Dn(PP-ol), NA, I [Wcmꢀ2], S [cm2], and t [s] represent the molar
amount of PP-ol conversion, Avogadro’s constant, light intensity, ir-
radiation area, and reaction time, respectively. El [J] can be calcu-
lated from hc/l (l=365, 395, 420, 435, 450, 515, or 560 nm).
The Efb of a semiconductor can be obtained from Mott–Schottky
measurements through Equation (3):
1=Csc ¼ B½2=ðee0eA2NDÞꢁ½EꢀEfbꢀðkT=eÞꢁ
2
ð3Þ
in which Csc is the space-charge capacitance of the semiconductor,
e is the dielectric constant of the semiconductor (relative to
vacuum), e0 is the permittivity of the vacuum, e is the electron
charge, A is the surface area of the interface, ND is the donor carrier
density of the semiconductor, Efb is the flat band potential of the
semiconductor, E is the applied potential, k is the Boltzmann con-
stant, and T is the absolute temperature [K]. For an n-type semi-
conductor, B is equal to ꢀ1, whereas for a p-type semiconductor B
is equal to 1. Therefore, Efb could be obtained from the extrapola-
tion of 1/Csc2 =0.[34,42] The value of Ecb could be evaluated from Efb,
according to Equation (4),[34] and that of Evb was calculated accord-
ing to Equation (5).
For the photocatalytic conversion of dioxasolv birch lignin, lignin
was first extracted from birch sawdust, according to a procedure
reported in the literature.[40] Briefly, dried birch sawdust (5.0 g) was
added to 1,4-dioxane/water (100 mL, 96:4) containing 0.12m HCl in
a 250 mL vessel. The mixture was heated at 808C for 4 h, and then
it was allowed to cool to room temperature. After filtration, the fil-
trate was neutralized by the addition of a solution of bicarbonate
(1.0 g in 20 mL DI water), concentrated in vacuum to give a solid
residue, which was washed with DI water to remove salts and car-
bohydrates, and dried under vacuum at 808C for 12 h to obtain di-
oxasolv birch lignin power. Subsequently, catalyst (10 mg) and di-
oxasolv birch lignin (20 mg) were added to the reactor, and the re-
action conditions were basically the same as those for the conver-
sion of lignin model compounds, except for using CH3OH as the
solvent. After the reaction, an internal standard solution (pentade-
cane in CH2Cl2 solution) was added to the mixture, which was fil-
tered and evaporated, and the residue was extracted with CH2Cl2.
Products in the CH2Cl2-soluble fraction were identified by means of
GC–MS and quantified by GC.
Ecb ¼ Efbꢀ0:15 V
Evb ¼ Eg þ Ecb
ð4Þ
ð5Þ
Acknowledgements
For the inhibition experiment of ꢀSH groups on the Zn4In2S7 sur-
face, Zn4In2S7 catalyst (100 mg) was treated with a certain amount
of BPTMOS in cyclohexane (10 mL) at 808C for 6 h;[38] then, the
powder was collected by centrifugation, rinsed with cyclohexane,
and dried in vacuum at 608C overnight. The catalytic performance
of the treated catalysts was evaluated for the transformation of PP-
ol.
This work was supported by the National Natural Science Foun-
dation of China (nos. 21690082, 91545203, and 21503176).
Conflict of interest
The authors declare no conflict of interest.
Characterization
Keywords: biomass
· cleavage reactions · energy band
Powder XRD patterns were recorded on a Panalytical X’pert Pro dif-
fractometer by using CuKa radiation (40 kV, 30 mA). Diffuse reflec-
tance UV/Vis spectroscopic measurements were performed on a
Varian-Cary 5000 spectrophotometer equipped with a diffuse re-
flectance accessory; spectra were collected with BaSO4 as a refer-
ence. N2 physisorption was performed on a Micromeritics Tris-
tar 3020 instrument to measure the surface area of the photocata-
engineering · photochemistry · renewable resources
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Manuscript received: August 28, 2019
Revised manuscript received: September 30, 2019
Accepted manuscript online: October 4, 2019
Version of record online: && &&, 0000
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J. Lin, X. Wu, S. Xie,* L. Chen, Q. Zhang,
W. Deng, Y. Wang*
Energy band engineering: Efficient
photocatalytic depolymerization of
lignin to functionalized aromatic mono-
mers over zinc–indium–sulfide catalysts
through an energy band engineering
strategy is reported. The energy band
structure of ZnmIn2Sm+3 can be tuned by
controlling the atomic ratio of Zn/In to
find the best catalyst for cleavage of the
b-O-4 bond under visible-light irradia-
tion.
&& – &&
Visible-Light-Driven Cleavage of CꢀO
Linkage for Lignin Valorization to
Functionalized Aromatics
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