C-H activations of methanol and ethanol and C-C couplings into diols by zinc-indium-sulfide under visible light

Article abstract of DOI:10.1039/c9cc09205f

Herein, an environmentally friendly CoP/Zn₂In₂S₅ catalyst is reported as a visible-light photocatalyst for the selective activation of the α-C-H bond of methanol to generate ethylene glycol with a selectivity of as high as 90%. The catalytic system also illustrates the first example of visible-light-driven dehydrogenative coupling of ethanol to 2,3-butanediol.

Full text of DOI:10.1039/c9cc09205f

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DOI: 10.1039/C9CC09205F
COMMUNICATION
C−H activation of methanol and ethanol and C−C coupling into
diols by zinc-indium-sulfide under visible-light
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
‡
‡,
Haikun Zhang, Shunji Xie, * Jinyuan Hu, Xuejiao Wu, Qinghong Zhang, Jun Cheng and Ye Wang*
2 2 5
In S
catalyst is reported theromocatalysis in many cases,5-7 among which the
Herein, an environmentl-friendly CoP/Zn
as a visible-light photocatalyst for selective activation of the α-C−H photocatalytic MTEG has shown some preliminary success.
4
,8
bond in methanol to generate ethylene glycol with selectivity as Yanagida and co-workers have reported that EG could be
high as 90%. The catalytic sysmtem also illustrated the first exmaple formed on ZnS (bandgap energy 3.6 eV) under ultraviolet (UV)
8
of visible-light-driven dehydrogenative coupling of ethanol to 2,3- irradiation, yet with low activity. In the solar spectrum, UV light
butanediol.
only makes up ~3% of the total solar energy, whereas visible-
light makes up ~44%. Therefore, it is very attractive to develop
5
3
Selective activation of the inert sp α-C−H bond within an
alcohol and direct formation of C−C bond with the coupling
partners is a promising way to produce multi-carbon alcohol or
a visible-light-responsive catalyst to improve the utilization of
solar energy. In our previous work, methanol was
photocatalytically converted to EG under visible-light
irradiation for the first time with high efficiency and selectivity
1
polyalcohol. Among various reactions in this field, the direct
coupling of methanol to ethylene glycol (Eq. 1, denoted as
MTEG) is known as a “dream catalytic reaction”, which is
expected to be solved in the 21st century.
4
2
using a MoS /CdS photocatalyst. However, the toxicity of CdS
has motivated us to develop a more environmentally friendly
visible-light photocatalyst.
2
CH
3
OH → HOCH
2
CH
2
OH + H
2
(1)
Metal sulfide is a class of photocatalyst with good visible-
light response and hydrogen evolution activity, especially
ternary metal sulfides, which have shown richer variability in
Ethylene glycol (EG) is the world's largest diol in demand,
underpins almost every aspect of our daily life, such as chemical
manufacturing, and transportation. In current industrial
2
9
properties and higher activity than binary metal sulfides.
process, EG is primarily produced from petroleum-derived
ethylene via epoxidation and subsequent hydrolysis of ethylene
epoxide, which suffers from high price of ethylene, low yield of
EG, and high energy consumption. In contract, the MTEG
utilizes methanol, which can be produced on a large scale from
Therefore, it is possible to develop an environmentally friendly
ternary metal sulfide catalyst for efficient photocatalytic
conversion of methanol to EG and H under visible-light
2
irradiation, which has not been realized in previous literatures.
Herein, a series of environmentally friendly ternary metal
2
a wide range of carbon resources, such as natural gas, coal,
sulfide photocatalysts, zinc-indium-sulfide (Zn
were synthesized by a simple method. We found that the few-
layer Zn In nanosheets showed higher activity and selectivity
for EG production than ZnIn and Zn In under visible-light
irradiation. The modification of Zn In with cobalt phosphide
CoP) could significantly enhance the formation of EG and H
m 2
In Sm+3, m = 1-3),
3
biomass, and CO
2
. The direct coupling of methanol to EG and
H
2
is also an economic process with high atom utilization.
This direct synthesis of EG from methanol has not been
2
2 5
S
S
2 4
3
2 6
S
achieved through thermocatalysis, because of the
2
2 5
S
4
thermodynamic limitations. However, photocatalysis has been
(
2
.
found to be able to make up for the incapability of
We also demonstrated the first example of the visible-light-
driven dehydrogenative coupling of ethanol to 2,3-butanediol
over CoP/Zn
The fabrication of a series of ternary metal sulfides
In m+3 (m = 1-3) is schematically illustrated in Fig. 1a.
Initially, the multi-layer Zn In m+3 was prepared at 90 °C via a
facile low-temperature hydrothermal method using
Zn(CH COO) , InCl , and thioacetamide as the precursors.
2 2 5
In S catalyst.
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, China.
E-mail: shunji_xie@xmu.edu.cn, wangye@xmu.edu.cn
Zn
m
2
S
m
2
S
†
Electronic Supplementary Information (ESI) available: Experimental details,
catalytic performances, EDX results, XRD patterns, TEM images, Mott-Schottky
plots, UV-vis spectra, PL spectra. See DOI: 10.1039/x0xx00000x
3
2
3
‡
These authors contributed equally to this work.
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Journal Name
Table 1 Photocatalytic performances of Zn
some binary metal sulfides for MTEG
Formation rate (mmol gcat h )
m
In
2
S
m+3 (m = 1-3) in comparison with
View Article Online
DOI: 10.1039/C9CC09205F
–1
–1
b
–
Selectivity (%)
e /
+
a
Catalyst
ZnIn
EG
0
HCHO
0.46
0.42
0.43
0.36
0
HCOOH
H
2
h
EG
0
68
84
83
-
HCHO
100
32
16
17
-
HCOOH
2
S
4
0
0.46
0.83
1.6
1.3
0
1.0
0.97
1.04
1.03
-
0
0
0
0
-
Zn1.5In
2
S
4.5
0.44
1.1
0.90
0
0
0
0
0
Zn
Zn
2
In
In
2
S
S
5
3
2
6
In
2
S
3
ZnS
0
0
0
0
-
-
-
-
ZnS(UV)c
CdSd
1.3
0.46
2.2
0.38
0.067
0
3.4
0.75
0.92
0.90
54
71
43
29
1.3
0
3
Reaction conditions: solution, 76 wt% CH
3
OH + 24 wt% H
2
O, 5.0 cm ; atmosphere,
N
7
2
; light source, 300 W Xe lamp; UV-vis light, λ = 320-780 nm; visible light, λ = 400-
80 nm. a The ratio of electrons and holes consumed in product formation was
–
+
calculated by the equation of e /h = [2 × n(H
n(HCHO) + 4 × n(HCOOH) + 4 × n(CO) + 6 × n(CO
CH
were observed on ZnS, which were not listed in Table 1. b Selectivity was
2
) + 2 × n(CH
4
)]/[2 × n(EG) + 2 ×
2
)]. Only trace amount of CO, CO ,
2
4
calculated on a molar carbon basis. c Reaction was carried out under UV-vis light
irradiation. d Reaction was carried out under visible light irradiation.4
the Ecb of ZnIn
Fig. 1 (a) Schematic illustration of the low-temperature hydrothermal and −0.52, −0.61, and −0.72 V (vs. NHE, pH = 7), respectively, all
ultrasonic process for Zn In
m+3 synthesis. (b) TEM image and HRTEM image more negative than the redox potential of H O/H (−0.42 V vs.
inset) of Zn In . (c) AFM image of Zn
profiles of Zn In for lines in c. (e) Energy levels of several related redox couples
and band-edge positions of some semiconductors.
2 4 2 2 5 3 2 6
S , Zn In S , and Zn In S were calculated to be
m
2
S
2
2
(
2
2
S
S
2 5
5
2 2 5
In S . (d) The AFM-measured height
NHE, pH = 7) (Fig. 1e). Moreover, the valence-band maximum
vb) can be calculated according to Evb = E + Ecb. The Evb of
ZnIn , Zn In , and Zn In were 1.75, 1.75, and 1.83 V (vs.
NHE, pH = 7), respectively, which were all significantly lower
more positive) than the redox potential of EG/CH OH (−0.37 V
vs. NHE, pH = 7) (Fig. 1e). These results indicate that the energy
band positions of Zn In m+3 could provide sufficient redox
ability for the photocatalytic conversion of methanol to EG and
, thermodynamically speaking.
The Zn In m+3 (m = 1-3) catalysts were exploited for the
2
(
E
g
S
2 4
2
2
S
5
3
2 6
S
The few-layer Zn
treatment. The stoichiometries of each element in Zn
m = 1-3) were tuned by altering the Zn/In ratio in the feedstock.
m
In
2
S
m+3 was then generated by ultrasonic
(
3
m 2 m+3
In S
(
m
2
S
The energy-dispersive X-ray spectroscopy (EDX) analysis was
applied to confirm that the elemental compositions of Zn, In,
H
2
and S in all of the Zn
the stoichiometric compositions (Fig. S1 and Table S1, ESI†).
XRD patterns of Zn In m+3 samples present similar profiles (Fig.
S2, ESI†), which correspond to a hexagonal wurtzite crystal
phase.10 The morphology of Zn
In m+3 was identified by
transmission electron microscopy (TEM) (Fig. 1b and Fig. S3,
ESI†), from which ZnIn , Zn In , and Zn In present similar
sheet-like nanostructure. Taking Zn In for example, the
HRTEM image of Zn In displays a few-layer structure (Fig. 1b,
inset). The AFM image confirms that Zn In present a 2D
m 2
In Sm+3 samples were corresponding with
m
2
S
photocatalytic coupling of methanol to EG under visible-light
irradiation, and the reaction selectivity was found to be
significantly influenced by the chemical composition of the
catalysts. As shown in Table 1, HCHO was formed as the major
m
2
S
m
2
S
carbon-based product over ZnIn
Meanwhile, H was formed along with HCHO. In contrast,
Zn In displayed a very high selectivity towards EG production
84%). The estimated ratio of photogenerated electrons and
holes according to Eqs. 2-4 was close to 1.0, which indicates that
HCHO was the major byproduct along with EG and H . The
formation rate of EG over Zn In
could reach 1.1 mmol gcat−1
2 4
S , and no EG was detected.
2
S
4
2
2
S
5
3
2 6
S
2
2
2 5
S
2
2 5
S
2
2 5
S
(
2
2 5
S
structure with a smooth surface morphology (Fig. 1c). The
corresponding height profiles in Fig. 1d show that the typical
2
2
2 5
S
thickness of the Zn
nm. Considering that the c parameter of the unit cell of Zn
is 30.85 Å (Fig. S4, ESI†) according to JCPDS card (No. 89-1906),
the Zn In thickness is well in agreement with 1-3 times of the
thickness of a unit cell along the [001] axis, validating the few-
layer structure of the Zn In nanosheets.
The UV-vis spectra shows that the absorption edge of
In m+3 shifts slightly to a shorter wavelength with increased
Zn/In atomic ratio (Fig. S5, ESI†). The bandgap energy (E ) can
be evaluated from the modified Kubelka-Munk plots (Fig. S5,
ESI†), which were 2.27, 2.36, and 2.55 eV for ZnIn , Zn In
2
In
2
S
5
nanosheets are ~3.07, 6.17, and 9.25
−1
h , which was more than two times higher than that over CdS
In S
2 2 5
4
nanorods reported in our previous work. Moreover, the
Zn1.5In
2
S
4.5 with lower Zn/In atomic ratio than Zn
lower EG formation rate and selectivity than Zn
Zn In with higher Zn/In atomic ratio than Zn In
almost the same EG selectivity as Zn In , but with slightly
lower photocatalytic activity than Zn In Thus, the
photocatalytic activity of Zn In m+3 for MTEG strongly
2
In
2
S
5
showed
. The
showed
2
2 5
S
2
In
2
S
5
3
2
S
6
2
2 5
S
2
2 5
S
2
2 5
S
2
2 5
S .
Zn
m
2
S
m
2
S
g
depended on the Zn/In atomic ratio.
+
OH + 2H+
2
CH
2
CH
3
OH + 2h → HOCH
2
CH
2
+
(2)
(3)
(4)
S
2 4
2
2 5
S ,
+
3
+
OH + 2h → HCHO + 2H
H + 2e → H
and Zn
Zn In
3
In
2
S
6
, respectively. The flat-band potential (Efb) of
-
2
m
2
S
m+3
could be obtained from Mott-Schottky
For comparison, In
S
2 3
and ZnS were also examined for this
measurement (Fig. S6, ESI†). The conduction-band minimum
(
reaction. In S showed poor photocatalytic performance for EG
2 3
1
1
E
cb) is ~0.15 V more negative than Efb, Ecb = Efb − 0.15 V. Thus,
and H
2
formation. ZnS was inactive under visible-light
2
| J. Name., 2019, 00, 1-4
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3
solution, 100 wt% C
2
H
5
OH, 5.0 cm ; atmosphere, N
2
; light source, 300 W Xe lamp;
View Article Online
visible light, λ = 400-780 nm.
DOI: 10.1039/C9CC09205F
the simulated sunlight irradiation (Xe lamp with AM 1.5 filter),
the CoP co-catalyst also showed remarkable promoting effect,
2 2 5
the EG formation rate over 0.25 wt%CoP/Zn In S could reach
−1
−1
1
1
8.9 mmol gcat h (Fig. 2c). The EG yield could reach 4.5% after
2 h reaction.
Ethanol is another simple primary alcohol, which has been
used as a sustainable fuel and versatile building block for the
production of various chemicals. 2,3-Butanediol (2,3-BD) is an
important chemical that has a number of applications. Here,
we demonstrated that ethanol could be directly transformed
into 2,3-BD driven by visible-light for the first time. As shown in
Fig. 3a, ethanol could be mainly oxidized to 2,3-BD and
Fig. 2 (a) Time course process of Zn
b) Repeated use of Zn In under visible light (12 h for each cycle). (c)
Photocatalytic performances of Zn In , 0.25 wt%CoP/Zn In for MTEG under
visible light and AM 1.5 light. Reaction conditions: solution, 76 wt% CH OH + 24
; light source, 300 W Xe lamp; visible light, λ =
2 2 5
In S for MTEG within 12 h under visible light.
1
3
(
2
2 5
S
2
S
2 5
2
2 5
S
3
3
wt% H
2 2
O, 5.0 cm ; atmosphere, N
acetaldehyde over Zn
significant promoting effect in ethanol-to-2,3-BD reaction (Fig.
b). Under visible light irradiation, the 2,3-BD selectivity and
formation rate over 0.25 wt%CoP/Zn In were 53% and 3.2
2 2 5
In S . The CoP co-catalyst also showed
4
00-780 nm; AM 1.5 filter.
3
irradiation, with EG formed only under UV light irradiation.
Hence, Zn In shows great superiority over its binary
counterparts (In and ZnS) for photocatalytic coupling of
methanol to EG. On the other hand, Zn In showed good
stability under the catalytic condition, with the steady
formation of EG, HCHO, and H over 12 h (Fig. 2a) and no
2
2 5
S
2
2 5
S
−1
−1
mmol gcat h , respectively.
2 3
S
Photocatalysis is a complex reaction process, and in order to
better evaluate the photocatalytic efficiency for MTEG, the
following processes are considered: the efficiencies of light
harvesting, charge separation, and surface reaction. The
photocurrent density under periodic irradiation of visible light
2
2 5
S
2
obvious deactivation was observed in the six recycling
experiments (Fig. 2b).
2 4 3 2 6
increased in the following sequence: ZnIn S < Zn In S <
Co-catalysts are known to play crucial roles in
Zn In (Fig. S8a, ESI†), with Zn In showing the highest
2
S
2 5
2
2 5
S
photocatalysis. The Zn
by some typical co-catalysts, including CoP, Ni
Pd. Our results suggested that most co-catalysts could enhance
the H formation rate, but Pt and Pd co-catalysts significantly
increased the HCHO formation rate, which may due to their
ability for O−H bond scission and sequential hydrogen
abstraction (Fig. 2c and Fig. S7, ESI†). Only CoP, Ni
co-catalysts were capable of promoting the EG formation rate.
CoP was the most effective co-catalyst on Zn In for MTEG.
Under visible-light irradiation, with a loading of 0.25 wt%CoP,
the selectivity towards EG formation was enhanced to 90%
along with a ~5 fold increase in its formation rate ( 5.5 mmol
2
In
2
S
5
nanosheets was further modified
overall efficiency of light harvesting and charge separation.
Furthermore, the electrochemical impedance spectrum of
Zn In S shows the smallest semicircle in the Nyquist plot (Fig.
2 2 5
S8b, ESI†), indicating the lowest charge-transfer resistance that
guarantees efficient charge separation and transfer. Moreover,
the photoluminescence (PL) intensity of the emission band at
2
P, MoS , Pt, and
2
2
1
2
2 2
P, and MoS
~
494 nm due to the recombination of photogenerated electrons
and holes decreased in the following sequence: ZnIn
Zn In > Zn In (Fig. S8c, ESI†), confirming that Zn In
the highest efficient for charge separation. These results
confirm that Zn In exhibited the best performance in the
2 4
S >
2
2 5
S
3
S
2 6
2
S
2 5
2
2 5
S has
2
2 5
S
separation and transfer of photogenerated excitons, which
contribute to its high activity in the MTEG reaction. However,
the effect of Zn/In atomic ratio on surface reaction and EG
selectively requires further studies in the future.
−1
−1
g
cat h ) compared to Zn
2
In
2
S
5
alone (Fig. 2c). The CoP/Zn
2 2 5
In S
also showed the same high EG selectivity to MoS
in our previous work, while the activity still not as high as the
best performance of MoS
conditions. Moreover, under
2
/CdS reported
4
2
/CdS under similar reaction
The CoP has been demonstrated to be an efficient catalyst
2
in electrochemical H evolution and also an outstanding co-
catalyst in water spliting, benefiting from its rich active sites for
1
4
2
H evolution and high ability for charge separation. From our
TEM images (Fig. S9, ESI†), the CoP displayed an ultra-thin
porous nanosheet morphology, and kept an intimate contact
with Zn In S nanosheets, which would facilitate the charge
2 2 5
transfer. Our photocurrent density results showed that the
incorporation of CoP significantly increased the charge
separation (Fig. S10, ESI†). Furthermore, the CoP co-catalyst
2
also displayed a promoting effect on photocatalytic H evolution
when using other hole scavenger (ethanol, isopropanol, or lactic
acid) (Fig. S11, ESI†). Thus, we speculated that CoP improves EG
formation in MTEG reaction by promoting charge separation
Fig. 3 Photocatalytic performances for the coupling of ethanol to 2,3-BD under
visible light irradiation. (a) ZnIn , Zn In , and Zn In . (b) Different co-catalysts
modified Zn In
2
(co-catalyst loading amount 0.25 wt%). Reaction conditions: and H production.
S
2 4
2
S
2 5
3
2 6
S
2
S
2 5
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2019, 00, 1-4 | 3
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Journal Name
The in situ electron spin resonance (ESR) spectroscopic study light-driven dehydrogenative coupling of ethanol to 2,3-BD over
View Article Online
DOI: 10.1039/C9CC09205F
2 2 5
on MTEG reaction using 5,5-dimethyl-1-pyrroline-N-oxide CoP/Zn In S catalyst. Mechanism studies show that the C−H
2
(DMPO) as a spin-trapping agent revealed the generation of the bond within methanol was activated to form •CH OH radical
without affecting the O−H group for subsequent coupling to EG,
and the α-C−H bond within ethanol was selectively activated to
form •CH(OH)CH radical for subsequent coupling to 2,3-BD.
3
The present work would offer an environmentally friendly
visible-light-driven strategy for preferential activation of α-C−H
bond in the present of O−H group under mild conditions.
This work was supported by the National Key Research and
Development Program of Ministry of Science and Technology
(2017YFB0602201), the National Natural Science Foundation of
China (Nos. 21972115, 91545203 and 21690082).
Conflicts of interest
There are no conflicts to declare.
Notes and references
1
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2 2 5 2 2 5
Fig. 4 (a) In situ ESR spectra for systems containing Zn In S and CoP/Zn In S
catalysts in methanol aqueous solution in the presence of DMPO (a spin-trapping
agent) with or without light irradiation. (b) In situ ESR spectra for systems
2
containing Zn
of DMPO. (c) Illustration of photocatalytic conversion of methanol and ethanol to
diols over CoP/Zn In
2 2 5 2 2 5
In S and CoP/Zn In S catalysts in ethanol solution in the presence
2
2 5
S .
3
4
hydroxymethyl radical (•CH
2
OH) on Zn
2 2 5 2 2 5
In S and CoP/Zn In S
5
6
7
catalysts (Fig. 4a).4 The •CH
2
OH radical should be the
intermediate by selective activation of the C−H bond within
methanol for the formation of EG. The modification of CoP on
Zn In S could significantly increase the generation of •CH OH
2 2 5 2
radicals, and thus promoted EG formation. On the other hand,
8
9
S. Yanagida, T. Azuma, H. Kawakami, H. Kizumoto and H. Sakurai, J.
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generation of α-hydroxyethyl radicals (•CH(OH)CH
Zn In S and CoP/Zn In S
2 2 5 2 2 5
3
) on
catalysts (Fig. 4b). The selectively
activated α-C−H bond of ethanol would result in the formation
of •CH(OH)CH radical, and enabled the subsequent C−C
coupling to 2,3-BD (Fig. 4c). As shown in Fig. 4c, the H
10154.
1
5
11 W. D. K. Clark and N. Sutin, J. Am. Chem. Soc., 1977, 99, 4676-4682.
1
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evolution 13 H. Lu, J. Zhao, L. Li, L. Gong, J. Zheng, L. Zhang, Z. Wang, J. Zhang and
7
3
2
may occur on the CoP surface, while the dehydrogenative
coupling of alcohols to diols may occur on the surface of
Zn In S . Thus, Zn In S is quite unique in the preferential
2 2 5 2 2 5
Z. Zhu, Energy Environ. Sci., 2011, 4, 3384-3388.
1
4 (a) C. Zhang, Y. Huang, Y. Yu, J. Zhang, S. Zhuo and B. Zhang, Chem.
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activation of the α-C−H bond in alcohols without affecting the
O−H group. The thiol group (−SH) on the surface of sulfide
semiconductor has been proposed to be the active sites for
1
1
6
selective activation of the C−H bond. In our case, the thiol
groups on Zn In surface may trap holes to generate thiyl
2
2 5
S
radicals for abstracting hydrogen from α-C−H bond of alcohols
to form the radical intermediates for subsequent C−C coupling
to diols.
2
002, 1, 696-703.
2 2 5
In conclusion, we found that the few-layer Zn In S
nanosheets was an environmentally friendly visible-light
photocatalyst for coupling methanol to EG. The modification of
Zn
2 2 5
In S nanosheets with CoP nanosheets could significantly
enhance EG formation. We also demonstrated the first visible-
4
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