Catalytic transfer hydrogenation of ethyl levulinate to γ-valerolactone over supported MoS2catalysts

Article abstract of DOI:10.1039/d1cy00524c

The hydrogenation of levulinate esters to γ-valerolactone (GVL) is an important step in the transformation of biomass into biofuels. It is attractive to develop new efficient systems for the catalytic transfer hydrogenation (CTH) of levulinate esters to value-added GVL. In this work, a series of MoS2-based supported catalysts were prepared via an impregnation method for the CTH of biomass-derived ethyl levulinate (EL) to GVL. By comprehensive characterization and catalytic measurements, we found that the CTH activity of EL to GVL is closely related to the MoS2 morphology and acid distribution on the support. Among the catalysts with different supports, the AC support with abundant Lewis acid sites and large surface area facilitated the high dispersion of low stacked MoS2 slabs, and the MoS2-acid synergistic catalysis contributed to the superior activity and selectivity. The conversion of EL and the selectivity of GVL reached 97.2% and 91.2% under optimized conditions over the MoS2/AC catalyst (230 °C, 1 MPa H2, 1.5 h), respectively. We also conducted reaction kinetic experiments to reveal the relationship between the active site of the MoS2/AC catalyst and its catalytic performance, and the plausible reaction pathway and mechanism over MoS2/AC was proposed. The catalytic performance gradually declined during recycling tests due to the oxidation of MoS2 and can be easily recovered by resulfuration.

Full text of DOI:10.1039/d1cy00524c

Volume 11
Number 15
7
August 2021
Pages 5017–5362
Catalysis
Science &
Technology
rsc.li/catalysis
ISSN 2044-4761
PAPER
Na Ji et al.
Catalytic transfer hydrogenation of ethyl levulinate to
γ-valerolactone over supported MoS catalysts
2

Catalysis
Science &
Technology
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PAPER
Catalytic transfer hydrogenation of ethyl
levulinate to γ-valerolactone over supported MoS2
catalysts†
Cite this: Catal. Sci. Technol., 2021,
11, 5062
Na Ji, *ab Xinyong Diao,
Xuebin Lu,
ac
a
a
a
Zhihao Yu,
Zhenyu Liu, Sinan Jiang,
ad
a
a
a
a
Chunfeng Song, Qingling Liu,
Degang Ma and Caixia Liu
The hydrogenation of levulinate esters to γ-valerolactone (GVL) is an important step in the transformation
of biomass into biofuels. It is attractive to develop new efficient systems for the catalytic transfer
2
hydrogenation (CTH) of levulinate esters to value-added GVL. In this work, a series of MoS -based
supported catalysts were prepared via an impregnation method for the CTH of biomass-derived ethyl
levulinate (EL) to GVL. By comprehensive characterization and catalytic measurements, we found that the
CTH activity of EL to GVL is closely related to the MoS
Among the catalysts with different supports, the AC support with abundant Lewis acid sites and large
surface area facilitated the high dispersion of low stacked MoS slabs, and the MoS -acid synergistic
2
morphology and acid distribution on the support.
2
2
catalysis contributed to the superior activity and selectivity. The conversion of EL and the selectivity of GVL
Received 25th March 2021,
Accepted 3rd June 2021
reached 97.2% and 91.2% under optimized conditions over the MoS /AC catalyst (230 °C, 1 MPa H , 1.5 h),
2
2
respectively. We also conducted reaction kinetic experiments to reveal the relationship between the active
site of the MoS /AC catalyst and its catalytic performance, and the plausible reaction pathway and
mechanism over MoS /AC was proposed. The catalytic performance gradually declined during recycling
tests due to the oxidation of MoS
2
DOI: 10.1039/d1cy00524c
2
rsc.li/catalysis
2
and can be easily recovered by resulfuration.
and lignin, has been the major subject for producing all sorts
of molecules. Lignocellulose can be converted to platform
chemicals and biofuels through the sugar platform or
1
. Introduction
With the rapidly depleting reserves of fossil fuels and its
impact on global warming, more researchers are encouraged to
focus on clean and sustainable resources to produce fuels and
chemicals. Biomass is the most promising carbon feedstock
due to its abundance and low cost. Among various biomass
resources, lignocellulose, consisting of cellulose, hemicellulose
4
,5
thermochemical platform.
One of the most attractive
biomass platform chemicals is GVL, which attracted
considerable attention owing to its unique physical and
chemical properties. For instance, GVL was deemed as a green
1–3
6
organic solvent in the pretreatment of lignocellulose biomass.
Additionally, GVL can be extensively utilized as a fuel additive
suitable for petrol and diesel to enhance the fuel combustion
a
School of Environmental Science and Engineering, Tianjin Key Lab of Biomass/Wastes
3,7
Utilization, Tianjin University, 300350, Tianjin, China. E-mail: jina@tju.edu.cn
b
process.
Moreover, GVL was used as an important
Guangdong Provincial Key Laboratory of New and Renewable Energy Research
intermediate to produce high value-added chemicals like
and Development, Guangzhou 510640, China
c
8
2-methyl tetrahydrofuran and valerate esters, etc.
State Key Laboratory of Pulp and Paper Engineering, South China University of
To date, many processes have been developed for the
production of GVL from LA or its ester through
hydrogenation reaction, where they can be directly produced
Technology, Guangzhou 510640, China
d
Department of Chemistry & Environmental Science, School of Science, Tibet
University, Lhasa 850000, China
†
Electronic supplementary information (ESI) available: XRD patterns of the AC
support and MoS /AC catalysts with different Mo loadings; nitrogen adsorption–
desorption isotherms of the AC support and MoS /AC catalysts with different
Mo loadings; effects of the catalyst dosage on the conversion of EL into GVL;
effects of the Co/Ni doping on the conversion of EL into GVL over the MoS /AC
catalyst; XRD patterns of Co–MoS (Co/(Co + Mo) = 0.45) and Ni–MoS (Ni/(Ni +
Mo) = 0.45); XPS spectra of Co–MoS /AC (Co/(Co + Mo) = 0.45) and Ni–MoS
Ni/(Ni + Mo) = 0.45) catalysts; XPS spectra of the fresh and spent MoS
9–12
from cellulose with high yields in ethanol solutions.
2
Compared with LA, EL is much easier and more energy-
efficient to be separated from the alcoholic mixture than the
separation of LA from water. Therefore, it is attractive to
develop efficient systems for the catalytic hydrogenation of
EL to GVL.
2
2
2
2
2
2
/AC
/AC
(
2
Generally, according to the diversity of hydrogen sources,
three major strategies have been explored for GVL production
2
catalysts; textural properties of activated carbon and the MoS /AC catalyst; XPS
quantitative analysis of the fresh and spent 8 wt% MoS
2
/AC catalysts. See DOI:
10.1039/d1cy00524c
from EL, including H , HCOOH and alcohol H-donor
2
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Scheme 1 Routes for the production of GVL from EL.
1
3
(
Scheme 1). However, both H
2
and HCOOH strategies have
efficient production of GVL from biomass-derived levulinate
esters (Scheme 1). To our knowledge, although disulfide
catalysts like Mo and W have been widely used for the
valorisation of biomass especially in the HDO of fats and
some limitations, such as harsh reaction conditions, use of
corrosive acids, and use of precious metals and non-
environmentally friendly solvents, which limited their further
1
4
application. Fortunately, the CTH method based on the
principle of Meerwein–Ponndorf–Verley (MPV) reduction for
the hydrogenation of EL to GVL over heterogeneous catalysts
using alcohols both as the H-donor and solvent was developed
in recent decades. It is reported that the key step for the CTH
reaction lies in the selective hydrogenation of EL to form
lignin, the application of MoS -based catalysts in the CTH of
2
LA and EL to GVL has not been reported in previous studies.
Therefore, in this work, MoS
prepared via an impregnation method for the CTH of EL to
produce GVL. The role of different supports (AC, Al , SiO
TiO , ZrO ) was explored in the light of various characterization
techniques, including XRD, N physisorption, XPS, TEM, NH -
2
-based supported catalysts were
O
2 3
2
,
2
2
1
5
4
-hydroxypentanoic acid (4-HPE), and an appropriate amount
2
3
of acid can purge the dehydration and lactonization to further
TPD and Py-FTIR. Based on the characterization and catalytic
measurements, the relationship between support properties,
MoS2 morphology, and catalytic performance was deeply
discussed. We further investigated the effect of reaction
conditions, different H-donors and substrates during the CTH
reaction, and proposed a plausible reaction pathway and
1
6
generate GVL. Therefore, many efforts have been devoted to
designing efficient metal and metal oxide catalysts combined
with acid supports for CTH of EL to GVL, including Ru, Pd, Ir,
17–19
2
Ni, Cu, Co, CuO, ZrO , etc. (Scheme 1).
Although metal and metal oxide catalysts showed good
CTH activity with a rather high GVL yield (>90 °C) at a
relatively low reaction temperature (<200 °C), the topic of
developing new material has seldom been well addressed. To
the best of our knowledge, besides metal and metal oxide
mechanism over MoS
Ni on MoS /AC and the reusability of MoS
conditions were also investigated.
2
/AC. Moreover, the doping effect of Co or
2
2
/AC under optimal
catalysts, only a few studies have investigated the CTH activity 2. Experimental
2
0,21
13,22
of metal phosphides
and metal–organic hybrids,
let
2
.1. Materials
Ammonium
Ni(NO ·6H
isopropanol, 1-butanol, 2-butanol, cyclohexane dodecane and
,4-dioxane were provided by Tianjin Kemiou Chemical
alone catalysts like metal carbide, nitride and sulfide. Thus,
to achieve a new breakthrough in the CTH of EL to GVL, the
development of improved catalytic systems and in-depth
understanding of the reaction mechanism are necessary.
As aforementioned, the rational design of the catalyst
focuses on the regulation of hydrogenation and dehydration
heptamolybdate
Co(NO ·6H O,
((NH ) Mo O ·4H O),
4 6 7 24 2
)
3 2
2
O,
3
)
2
2
methanol,
ethanol,
1
Reagent Co, Ltd. Ethyl levulinate (EL), levulinic acid (LA),
methyl levulinate (ML), butyl levulinate (BL), cyclohexanone,
γ-valerolactone (GVL), isopropyl levulinate (IPL) and isopropyl
valerate (IPA) were bought from Aladdin. The above-mentioned
chemicals were directly used without any pretreatments.
Supports including AC (activated carbon), Al O , and SiO were
purchased from NORIT, Qingdao Haiyang Chemical Co. Ltd.
and DICP, respectively. ZrO2 and TiO2 were purchased from
Tianjin Kemiou Chemical Reagent Co, Ltd. The AC support was
2
ability. It has been reported that the MoS -based catalyst
exhibited good activity in the hydrodesulfurization,
hydrodenitrogenation and hydrodeoxygenation reaction, with
the cleavage of the CC and C–O (N/S) bond usually at high
2
3,24
2
3
2
temperature (>250 °C) and high pressure (>3 Mpa H
The activity of MoS2 has been linked to the availability of
edge vacancy sites on the MoS slabs, whereas coordinatively
saturated sites on basal planes are regarded as catalytically
2
).
2
2
5,26
3 2
further treated with 50 vol% HNO /H O solution at 80 °C for 12
inert.
Moreover, the edge vacancy sites endow the catalyst
catalyst possesses the
h to remove the small amount of SiO . All the supports were
2
with weak acid. To sum up, the MoS
2
dried at 400 °C in a N flow for 4 h before use.
2
following merits: economical, potential hydrogenation ability
and weak acidity, which met the requirement of the active
center for the CTH of EL to GVL.
2.2. Catalyst preparation
Inspired by the special merits of MoS
2
materials, here we
All the supported MoS catalysts were prepared using an
2
combined MoS2 with various supports to adjust the
hydrogenation and dehydration ability, aiming to achieve
incipient wetness impregnation method. Before the
impregnation, the support was vacuum dried at 120 °C for 12
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h. In a typical procedure, the support was adequately dispersed
in an aqueous solution containing a certain amount of (NH ) -
( f /f ) = (10 × L¯ /3.2 − 3)/2
(3)
e
c M
4
6
Mo
7
O
24·4H
2
O at room temperature for 24 h. Then the sample
X-ray photoelectron spectroscopy (XPS) was conducted on a
Thermo Fisher ESCALABTM 250Xi spectrometer. Peak positions
were calibrated by using a C 1s peak at 284.4 eV as a standard.
To examine the acidity of the catalysts, the temperature-
was dried at 120 °C for 12 h. Subsequently, the sulfidation of
the dried solid was conducted in a flow of 10% H S/H at 400
2
2
°C for 4 h. Finally, the catalysts with 8 wt% Mo loading on
different supports were obtained and denoted as: MoS /AC,
programmed desorption of ammonia (NH -TPD) was
2
3
MoS /Al O , MoS /SiO , MoS /TiO , and MoS /ZrO . The MoS /
AC catalysts with different Mo loadings were denoted as:
performed on an AutoChem 2920 chemisorption analyzer. 0.1 g
catalyst was pretreated with a flow (40 mL min ) of Ar at 600
2
2
3
2
2
2
2
2
2
2
−1
xMoS /AC, where x represents the Mo loading.
°C for 1 h. After cooling down, the samples were saturated with
2
The contrast sample MoO /AC was obtained via the direct
calcination of the impregnated precursor at 400 °C for 4 h in
NH . Subsequently, the temperature rose to 800 °C at a rate of
3
3
−1
5° min and the desorption was monitored using a thermal
conductivity detector (TCD). Fourier-transform infrared (FT-IR)
spectra of adsorbed pyridine were recorded using a Tensor 27
(Bruker, Germany) spectrophotometer in the wavenumber
air. The unsupported MoS was synthesized by the sulfidation
2
of MoO3 derived from the decomposition of (NH ) Mo -
4 6
7
O
2 2 3
24·4H O. The sulfidation of the AC and Al O support in the
absence of Mo loading was also conducted to obtain
−1
range of 1400–1700 cm , using KBr pellets, with a resolution
−1
sulfurated supports, denoted as AC–S and Al O –S.
of 4 cm at 32 scans. Typically, 6.0 mg of the catalyst was
placed in a diffuse reflectance infrared Fourier transform
2
3
2
A series of supported Co- or Ni-doped MoS /AC catalysts
−3
were prepared by varying the Co/(Co + Mo) or Ni/(Ni + Mo)
molar ratio in the range of 0, 0.25, 0.45, and 0.62, while
maintaining the Mo loading at 8 wt%. The preparation
process and treatment conditions were the same as those of
spectroscopy (DRIFTS) cell (1 × 10 Pa) and heated to 400 °C
in a N flow for 2 h to obtain a relatively clean surface. The
2
sample system was cooled down to room temperature and the
background spectrum was recorded. Then, pyridine was
introduced into the sample cell for 120 min to allow the
adsorption equilibrium of pyridine. After the physically
adsorbed pyridine was purged off under vacuum conditions,
the evacuation was performed at 150 °C to measure the IR
spectra. Quantitative analysis of the concentration of Brønsted
and Lewis acid sites can be obtained using eqn (4) and (5),
MoS
Ni(NO
2
/AC with the addition of Co(NO
·6H O in the precipitation step.
3
)
2
·6H
2
O
or
3
)
2
2
2
.3. Catalyst characterization
The X-ray powder diffraction (XRD) patterns were recorded
on a D8-Focus powder diffractometer, using Cu-Kα radiation
at 40 kV and 200 mA in the range of 10° to 80° at a scanning
B B
where C and IA represent the concentration of the Brønsted
−1
acid sites and the integrated absorbance peak at 1540 cm for
−
1
speed of 5° min .
the Brønsted acid sites. C and IA represent the concentration
L
L
The Brunauer–Emmett–Teller (BET) method was used to
calculate the specific area of the mesoporous materials with
a Micromeritics TriStar surface area and porosity analyzer.
The pore volume and pore size were obtained by the BJH
method from the isotherms. Before the measurement, the
catalysts were degassed at 300 °C for 3 h on a vacuum line.
Transmission electron microscopy (TEM) was performed
of the Lewis acid sites and the integrated absorbance peak at
−1
1
450 cm for the Lewis acid sites. R is the radius of the sample
disc, and W is the weight of the sample.
−
1
2
C
B
(μmol g ) = (1.88 × IA
B
L
× R )/W
(4)
(5)
−
1
2
C
L
(μmol g ) = (1.88 × IA
× R )/W
using
a
JEM-2100F transmission electron microscope
operated at 200 kV. Before the samples test, the catalysts were
2.4. Catalyst test
ultrasonicated with ethanol and dropped onto copper grids.
The average length ( L¯ ) and stack number ( N¯ ) of the slabs and
Transfer hydrogenation of EL to GVL was performed in a 50
mL Parr reactor (Parr 4597). Typically, 2 mmol (0.2283 g) EL,
the edge-to-corner ratio of the MoS
2 e c M
slab ( f /f ) were
75 mg catalysts and 20 mL isopropanol were placed into the
calculated using eqn (1)–(3), where n was the number of
i
autoclave. After being purged three times, H was introduced
2
slabs in the unit and L
and the stacking number of the unit. The L
obtained by statistical analysis of more than 100 stacked
MoS units from different parts of each sample, which were
analyzed with a program Digital Micrograph.
i
and N
i
were the length of the slab
into the autoclave to reach a certain pressure at ambient
temperature. Then the reactor was heated up to a certain
temperature and reacted for certain hours with stirring at
i
and N were
i
2
1000 rpm. When the reaction finished, the liquid products
were extracted until they cooled down to room temperature.
The obtained liquid products were directly identified by GC-
MS (Agilent 6890A-5975C) and quantified by GC (SP-7890)
using a flame ionization detector (FID) with a 30 m × 0.25
mm × 0.25 μm WondaCap WAX capillary column. The
experiments have been repeated at least twice to ensure the
accuracy and rationality. The conversion of EL and the yield
of GVL were calculated as follows.
n
n
X
X
̄
L ¼
niLi=
ni
(1)
(2)
i¼1
i¼1
n
n
X
X
̄
N ¼
niNi=
ni
i¼1
i¼1
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ꢀ
ꢁ
S1†). This indicates that the enhancement of Mo loading on
the AC support was favorable for the oriented growth of the
molar of residual EL
molar of initial EL
Conversion ¼ 1 −
× 100%
(6)
(
100) and (110) plane of MoS
2
slabs.
TEM characterization was carried out to further observe
molar of GVL in the products
molar of initial EL
Product yield ¼
× 100% (7)
the microstructure and metal dispersion of the catalyst, as
shown in Fig. 2. The calculated space between two parallel
dark thread-like fringes was about 6.4 Å, corresponding to
2
9,30
the (002) basal planes of the hexagonal MoS material.
As
2
3
3
. Results and discussion
shown in Table 1, it was estimated that the mean slab length
for MoS /AC, MoS /Al O , MoS /SiO , MoS /TiO and MoS2/
.1. Characterization of the catalysts
.1.1 Composition and structure. To better understand the
composition and structure of the supported MoS catalysts,
2
2
2
3
2
2
2
2
3
ZrO2 was 4.6 nm, 4.8 nm, 5.0 nm, 4.5 nm and 5.2 nm,
respectively. The edge-to-corner ratio of the MoS slab was
2
2
several characterization techniques were used to study their
properties in detail. XRD was performed to identify the
crystal structure of the catalysts, as shown in Fig. 1. The
calculated according to the literature and was regarded as
evidence of the relative amount of the MoS edge active
2
3
1
site. It seems that no apparent distinction of the edge-to-
corner ratio was found for the supported MoS2 catalysts.
However, a significant difference was found for the stacked
diffraction pattern of MoS exhibits several specific peaks at
2
1
4.4°, 33.5°, 39.5° and 58.9°, corresponding to (002), (100),
2
7,28
(103), and (110) (JCPDS 37-1492), respectively.
It has been
2 2
layers of MoS . In particular for MoS /AC, highly dispersed
reported that the existence of the (002) planes indicated the
low stacked MoS2 slabs (∼1.4 layers) on the support were
2
7
formation of a multilayer MoS structure. However, hardly
observed, which has been proved to be more active in the
2
3
2,33
any characteristic peaks of MoS
2
including the (002) planes
HDO reaction.
of the samples, Mo and S were well dispersed on the
supports. In summary, the MoS slabs on different supports
According to the STEM mapping images
can be found for the as-prepared supported MoS2 catalysts,
indicating the low stacked layers as well as high dispersion
2
of MoS
2
in the catalysts, which can be further confirmed by
displayed significant effects on the stacked layers, which
should greatly affect the catalytic activity.
the following TEM results. Moreover, broad peaks of the
(
100) and (110) planes were observed in MoS /AC and MoS /
3.1.2
Porosity
characteristics.
N
2
physisorption
2
2
Al
2
O
3
, while they were not found in MoS
2
/ZrO
2
, MoS
2
/SiO
2
measurement was performed to investigate the porous
and MoS /TiO . This was probably because of the coverage of
properties of the samples. Fig. S2† shows the adsorption–
2
2
the high crystallinity degree of the latter three supports. For
MoS /AC with an increased Mo loading from 5 wt% to 20
wt%, the intensity of the MoS2 diffraction peak
corresponding to the (100) and (110) plane gradually
increased, while the (002) plane still was not observed (Fig.
2
desorption isotherms of the AC support and MoS /AC
2
catalysts with different Mo loadings. The observed noticeable
hysteresis revealed the presence of the mesoporous nature in
the catalysts. The Brunauer–Emmett–Teller surface area
(SBET), pore volume and pore size data of the MoS
supported catalysts are summarized in Table 2. Among the
catalysts, MoS /AC possessed a quite larger BET surface area
2
-based
2
when compared to other supports, which is proved to be
beneficial for the mass transfer and the contact between the
3
3,34
reactants and the active sites.
MoS on AC, the specific surface area decreased from 1161 to
After the impregnation of
2
2
−1
5
07 m g and the pore volume decreased from 0.39 to 0.21
3
−1
cm
g
with the Mo loading increasing from 0 to 20 wt%
(
Table S1†). It is proposed that the impregnation of MoS
2
may result in the blockage of pores of the support, which is
3
5
ascribed to the so-called dilution effect.
.1.3 Surface species of the catalysts. Considering that
dehydration/cyclization on acid sites was presumably
3
3
6,37
involved in the hydrogenation of EL to GVL,
properties of MoS -based supported catalysts were
investigated by using temperature-programmed desorption
of NH (NH -TPD) and IR spectroscopy of pyridine
adsorption. Without regard to the bare acidity of MoS
itself, the main acidity of the samples should be provided
the acidic
2
3
3
2
by the supports. As shown in Fig. 3(a) and Table 2, MoS
AC possessed the highest acid amount of 2054 μmol NH
2
/
3
per g cat with two desorption peaks at 271.8 °C and 711.4
°C, which belonged to weak and strong acid sites,
2
Fig. 1 XRD patterns of the MoS catalysts with different supports (8
wt% Mo loading).
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2 2 2 3 2 2 2 2 2 2
Fig. 2 TEM images and STEM mapping of (A) MoS /AC, (B) MoS /Al O , (C) MoS /SiO , (D) MoS /TiO , and (E) MoS /ZrO .
2
Table 1 Morphological characteristics of the MoS active phase species calculated from HRTEM micrographs (8 wt% Mo loading)
a
Entry
Catalysts
Average length L¯ (nm)
Average stacking number N¯
e c M
( f /f )
1
2
3
4
5
MoS
MoS
MoS
MoS
MoS
2
2
2
2
2
/AC
/Al
/SiO
/TiO
/ZrO
4.6
4.8
5.0
4.5
5.2
1.4
2.6
4.1
3.1
3.6
5.7
6
6.3
5.5
6.6
2
O
2
3
2
2
a
Calculated from HRTEM data. The edge-to-corner ratio of the MoS
2
slab ( f
e
/f
c
)
M
was derived according to eqn (1)–(3).
3
8
respectively. It was reported that the acid-pretreatment of
AC would introduce abundant oxygen-containing functional
groups on the surface, which can act as the acid site. For
MoS
2
/Al
2
O
3
, strong acid sites located at 507.7 °C were
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2
Table 2 Textural properties and surface acidity of the supported MoS catalyst (8 wt% Mo loading)
a
2
−1
b
3
−1
b
c
Entry
Sample
S
BET (m g
)
Pore volume (cm g
)
Pore size (nm)
3
Acidity (μmol NH per g cat.)
1
2
3
4
5
MoS
MoS
MoS
MoS
MoS
2
2
2
2
2
/AC
/Al
/ZrO
763
101
118
371
64
0.31
0.17
0.29
0.53
0.27
3.9
4.3
6.2
4.4
16.4
2054
1498
312
78
2
O
2
3
/SiO
/TiO
2
2
180
a
b
c
BET surface area. Average pore volume and size were calculated by the BJH method from desorption branches. Calculated by NH
3
-TPD.
3
Fig. 3 (a) NH -TPD profiles and (b) Py-FTIR spectra of the samples.
observed and its total acid amount was 1498 μmol NH per
In order to investigate the chemical state of Mo and S in
the supported MoS catalysts, X-ray photoelectron
3
g cat. MoS
2
/TiO
2
and MoS
2
/ZrO
2
display apparent lower
2
acidity with the total acid amount of 180 and 312 μmol
spectroscopy (XPS) was carried out. As shown in Fig. 4(a),
four peaks are observed in the high-resolution XPS spectra of
Mo 3d-S 2s, where the one at 226.9 eV corresponds to S 2s of
NH per g cat, respectively. It's worth noting that MoS /SiO
2
3
2
also exhibited weak acidity with the total acid amount of 78
μmol NH per g cat. As both MoS /AC and MoS /Al
4
0
3
2
2
2
O
3
MoS
2
.
The two main peaks at 229.6 and 232.8 eV can be
4
+
possessed strong acidity, the Py-FTIR was performed to
further analyze the acidic character of the catalyst, as shown
assigned to the Mo ions in MoS2 while the high binding
6+
energy at 233.1 eV and 236.2 eV can correspond to Mo in
MoO . For S 2p spectra shown in Fig. 4(b), two peaks at 162.5
eV and 163.9 eV were assigned to the S 2p and S 2p1/2 of
divalent sulfide ions (S ). The higher binding energy S 2p3/2
peak at 163.5 eV and S 2p1/2 peak at 163.5 eV and the S 2p3/2
in Fig. 3(b). For MoS
2
/AC, the peaks at 1490, 1540, and 1640
3
−
1
cm were pyridine-adsorbed Brønsted acid sites including
S–OH and surface –OH, while the pyridine-adsorbed Lewis
acid sites consisting of Mo species and surface O-contained
3
/2
2
−
−
1 39
2−
group showed peaks at 1450, 1490, and 1610 cm . For
MoS /Al , the Al support provides Brønsted acid sites
peak at 164.7 eV were due to monovalent sulfide ions (S2 ).
2
2
O
3
2
O
3
In addition, a broad peak at 169.0 eV representing the SO
x
from the Al–OH–Al group and Lewis acid sites from the Al
group was also observed. It can be clearly seen that most of
center. Quantitative results (Table 3) showed that both the
the surface Mo species existed as MoS , indicating the
2
AC and Al
2
O
3
support mainly provided Lewis acid sites, and
formation of MoS
2
nanoparticles on the supports. Besides, it
the calculated ratios between the B-acid and L-acid were not
more than 0.15.
is observed that the catalysts with strong acidity displayed a
6
+
4+
high Mo /Mo ratio and SO proportion. This was probably
x
2 2 2 3
Table 3 Pyridine-FTIR surface acid sites of the MoS /AC and MoS /Al O samples
−1
−1
−1
Catalysts
Brønsted acid sites (μmol g
)
Lewis acid sites (μmol g
)
Total acid sites (μmol g
)
B/L
MoS
MoS
2
/AC
/Al
73.3
34.9
6.7
4.7
80.0
39.6
0.091
0.135
2
2
O
3
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Fig. 4 XP spectra of (a) Mo 3d and (b) S 2p of the catalysts.
due to the adsorption of H
2
S species on the acid sites
MoS
2
-based catalysts with different supports demonstrated
suppressing the proceeding of the vulcanization reaction
during the sulfidation process.
significant differences in activity. A negligible reaction
occurred in the blank experiment, indicating that carbonyl
groups were thermally stable at 230 °C (entry 1). When the
2
bulk unsupported MoS was added, the EL conversion and
GVL yield rose to 51.4% and 28.5%, respectively (entry 2).
This indicates that the intrinsic hydrogenation ability derived
3
.2. Catalytic performance and discussion
The catalytic activities of the catalysts were tested by CTH of
EL at 230 °C under 1 MPa H for 1.5 h. As shown in Table 4,
2
2
from the edge site of MoS can availably catalyze the CTH of
Table 4 CTH of EL to GVL with various catalystsa
Yield (%)
Entry
Catalyst
Blank
Conv. (%)
GVL
IPL
4.5
18.8
10.0
18.2
4.8
10.8
18.9
9.1
0.3
17.6
4.0
8.7
2.3
IPA
GVL Sel. (%)
1
2
3
4
5
6
7
8
9
4.8
51.4
44.8
52.8
44.5
15.6
51.7
42.8
97.2
51.3
95.8
61.8
17.0
58.3
0.2
28.5
31.9
30.2
32.6
8.2
27.8
32.2
88.7
31.4
78.3
48.9
12.7
44.7
—
1.4
—
1.4
0.8
—
1.3
—
2.0
1.6
5.2
1.3
1.6
1.2
4.2
55.4
71.2
59.4
73.2
52.6
53.6
75.2
91.2
61.2
81.7
78.9
74.7
76.7
MoS
AC
2
Al O
2 3
ZrO
2
SiO
TiO
2
2
AC–S
MoS
Al
2
/AC
10
11
12
13
14
2
O
3
–S
MoS
MoS
MoS
MoS
2
2
2
2
2 3
/Al O
/ZrO
/SiO
/TiO
2
2
2
0.2
a
2
Reaction conditions: 2.0 mmol EL, 75 mg catalyst, 20 mL isopropanol, reaction temperature 230 °C, initial H pressure 1 MPa, reaction time 1.5 h,
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EL. However, for the bulk MoS , its edge site amount was
catalyzed the CTH reaction but also significantly affected the
MoS2 morphology. Moreover, the overwhelmingly larger
surface area of the AC support may also contribute to the high
2
relatively low and limited its activity. Thus, it is necessary to
introduce an appropriate support to realize the enhancement
of exposed MoS2 edge sites. Then, the function of various
supports with different structures and surface acidity were
dispersion of MoS slabs and was beneficial for mass transfer
2
in the catalytic process. In conclusion, the AC support with
abundant Lewis acid sites and large surface area facilitated the
high dispersion of low stacked MoS slabs, and the MoS -acid
discussed. For single supports without MoS
2
loading (entries
3–7), as we expected, a relatively low EL conversion and GVL
2
2
yield were observed due to the absence of robust
hydrogenation sites. As mentioned above, the dehydration/
cyclization on acid sites was presumably involved in the
hydrogenation of EL to GVL, and the displayed activity for
the single supports mainly comes from their surface acid
center. Therefore, it can be seen that the AC, Al O , ZrO , and
synergistic catalysis contributes to the superior activity and
selectivity in CTH of EL to GVL.
37,43,44
3.3. Study of the reaction mechanism
We conducted reaction kinetic experiments to reveal the
2
3
2
TiO2 supports with relatively higher acidity showed higher
activity with a GVL yield around 30%, whereas the SiO
support with a very small number of acid sites only achieved
an 8.2% yield of GVL. Moreover, the XPS results have
relationship between the active site of the MoS /AC catalyst
2
2
and its catalytic performance. The details are as follows.
3.3.1 Optimization of reaction conditions. The reaction
temperature can affect the temperature dramatically.
Therefore, a variety of experiments were carried out at
different temperatures viz. 210, 220, 230, 240 and 250 °C. As
shown in Fig. 5(a), a low-temperature limits the catalyst
activity and increasing the temperature effectively enhanced
the EL conversion and GVL yield. When the temperature
increased to 230 °C, EL was almost completely converted and
the yield of GVL could reach up to 88.7%. However, when the
temperature increased above 230 °C, the GVL yield decreased
remarkably. It was not surprising that a higher temperature
could improve the catalytic activity and the reaction rate.
However, the intrinsic activity of MoS2 for further C–O
cleavage and oxygen removal will be promoted under
indicated the formation of SO
x
species on the supported
MoS samples, which are easily converted to strongly acidic
2
“
sulfuric acid” in the presence of water vapor. To eliminate
the influence of the surface SO species, the catalytic activity
of the sulfurated AC and Al O support was also investigated.
x
2
3
As shown in entry 8 and entry 10, there is nearly no
difference between the un- and sulfurated support. When
MoS2 was impregnated on the supports, no obvious
improvement was observed for MoS
MoS /TiO (entries 12–14). It can be seen that the EL
conversion only increased slightly while the GVL selectivity
increased more remarkably. However, for MoS /AC and MoS
Al (entry 9 and entry 11), significant enhancement was
found with the impregnation of MoS on the support. MoS /
2 2 2 2
/SiO , MoS /ZrO and
2
2
2
2
/
4
5
2
O
3
excessive temperature conditions, which led to the decline
of GVL production.
2
2
AC presented a 97.2% EL conversion and 91.2% GVL
selectivity, while MoS /Al exhibited a 95.8% EL conversion
and 81.7% GVL selectivity.
The effect of hydrogen pressure was also studied, and the
catalyst activity is shown in Fig. 5(b). Without external H
2
added into the system, the EL conversion and GVL yield was
relatively low. Although isopropanol acted as the H-donor as
well as solvent in the CTH of EL, the sulfur vacancies on the
2
2
O
3
Many factors can affect the catalytic activity such as
particle size, dispersion, acid properties, chemical properties,
4
1
etc. In our case, it seems that the acidity of the support
mainly influences the catalytic activity of the supported MoS
edge site of MoS can hardly be created in the single function
2
2
of the H-donor. Therefore, the addition of external H could
2
catalysts. The catalyst with high acidity achieved relatively high
help to the formation of sulfur vacancies of the catalyst
termed as coordinatively unsaturated sites, which served as
active sites for the CTH reaction. It was found that the
optimized hydrogen pressure is 1 MPa; a lower or higher
hydrogen pressure would cause incomplete reaction or a
decline of GVL production.
activity, according to the catalytic performance and the NH -
3
42
TPD results. Garcia et al. demonstrated that strong metal–
support interactions occurred with the Mo precursor
interacting mainly with Lewis acid sites, and this stage has a
key effect on the genesis of the sulfide phase, and can strongly
change the morphology, activity and selectivity of the resulting
MoS2 slabs. Thus, although the AC, Al O , ZrO , and TiO
2
The influence of reaction time over MoS /AC was
investigated to understand the reaction process (Fig. 5(c)). In
the first 0.5 h, the EL conversion reached 82.8% and the GVL
yield achieved 78.1%. Trace intermediates were detected as
IPL, IPA, and other over-hydrogenated by-products derived
from GVL (not quantitative). With the extension of reaction
time, the yield of GVL increased first and reached the highest
in 1.5 h, and then decreased gradually. Meanwhile, IPL
gradually disappeared via the CTH reaction like EL. A small
amount of IPA was maintained throughout the entire reaction
and other over-hydrogenated by-products derived from GVL
were markedly increased after 1.5 h. This indicates that GVL
2
3
2
2
supports showed similar activity, their distinct difference in the
acid content significantly affects the MoS morphology, which
directly determines the hydrogenation ability of the catalyst.
According to the TEM analysis, the MoS slabs on different
2
2
supports displayed significant effects on the stacked layers.
As a consequence, the AC and Al O support with abundant
2
3
Lewis-acid sites facilitated the formation of low stacked MoS
2
slabs, and significant activity enhancement was found with
the impregnation of MoS on the AC and Al O support. This
2
2 3
indicates that the surface acid site on the support not only
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Fig. 5 Effects of the reaction conditions on the conversion of EL into GVL over the MoS
2
/AC catalyst. Reaction conditions: EL 2 mmol, catalyst 75
mg, isopropanol 20 mL. (a) Effect of reaction temperature, 1.5 h, 1 MPa H
2 2
; 8 wt% Mo loading. (b) Effect of initial H pressure, 230 °C, 1.5 h, 8 wt%
Mo loading. (c) Effect of reaction time, 230 °C, 1 MPa H , 8 wt% Mo loading. (d) Effect of Mo loading, 230 °C, 1.5 h, 1 MPa H .
2
2
was not the terminal product in this catalytic system and it will
further transform into a down-stream product.
We further explore the effect of Mo loadings on the CTH
reaction. As shown in Fig. 5(d), when adding 5 wt% of Mo on
the AC support, both the conversion and yield increased
remarkably. With the Mo loading increasing to 8 wt%, the
highest EL conversion and product selectivity were achieved.
However, a further increase in the weight percentage of the Mo
element led to a slight reduction in the selectivity towards GVL.
For the 20% MoS /AC catalyst, 84.4% yield of GVL was obtained
2
at 97.2% conversion of EL, indicating that more by-products
were produced. Similar to the conclusion on the effect of the
2
The effect of the MoS /AC catalyst dosage on the
conversion of EL into GVL in isopropanol at 230 °C with 1.5
h was investigated and the results are shown in Fig. S3.† With
the increase of the catalyst dosage, the trend of the
conversion results was similar to that of the reaction
temperature and hydrogen pressure. The products detected
by GC-MS indicated that more catalyst dosage resulted in
excessive hydrogenation of GVL. It is obvious that the
amount of the MoS
2
edge active site, as well as the acidity,
2
catalyst dosage, excessive amounts of MoS edge sites as well as
will increase with the enhancement of the catalyst dosage,
which will lead to an improvement in the excessive
hydrogenation of GVL.
acidity sites will promote excessive hydrogenation of GVL.
3.3.2 Effect of hydrogen donors. In the CTH of LA and its
esters, the conversion of the reactant and the yield of GVL
4
6
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were significantly related to the readiness of deriving the
active H atom from the hydrogen donor. Thus, a variety of
protic solvents (such as methanol, ethanol, isopropanol,
was used as a solvent. When further increasing the alkyl chains
of secondary alcohol, the production of GVL significantly
decreased (2-butanol). We speculated that it is the enhancive
space steric effect of 2-butanol inhibits the hydrogen transfer
reaction. In conclusion, EL can be condensed in different
alcohol solvents to produce GVL, and we believe that isopropanol
as a hydrogen donor is worthy of further research in our system
in terms of its reactivity as well as the practical point of view.
These results further highlight that the effective transformation
1
-butanol, and 2-butanol) and aprotic solvents (such as
49
cyclohexane and 1,4-dioxane) were used to study their
effectiveness in the CTH of EL. As shown in Table 5, the
solvent significantly affects the EL conversion and GVL
selectivity under the optimized conditions. When cyclohexane
and 1,4-dioxane were used, EL was basically not converted
and little GVL was obtained. Although the addition of H2
of EL to GVL over MoS /AC catalyst follows the MPV route since
2
could activate the formation of sulfur vacancies of MoS , the
hydrogenation of EL to 4-HPE can hardly occur in the
absence of a H-donor, which further confirmed that the
the hydrogen supply capacity of the solvent plays a vital role.
3.3.3 Effect of Co/Ni doping. Previous experimental and
theoretical studies pointed out that Co or Ni doping will
increase the availability of vacancy sites by lowering the
Mo–S bond energy, and the formation of the CoMoS or
NiMoS phase presented enhanced activity in HDO, HDS and
2
MoS -catalyzed conversion of EL to GVL completely follows
2
the MPV route. Deservedly, when various alcohols served as
the solvent, higher EL conversion and GVL yield were
achieved. Notably, different alcohols with discriminative H
donor capability demonstrated significant differences in
activity. It's difficult for primary and tertiary alcohols to
5
0–52
HDN reactions.
Therefore, we further investigated the
effects of Co/Ni doping on the conversion of EL into GVL over
the MoS /AC catalyst, and the results are shown in Fig. S3.†
2
1
5,47
provide H-movement.
As a result, the CTH reaction with
2 2
However, the Co–MoS /AC and Ni–MoS /AC catalysts with
methanol or 1-butanol as a solvent achieved a high EL
conversion but a relatively low GVL selectivity. It is obvious
that when the secondary alcohol was utilized, the GVL
selectivity was much higher because of their active β-H
different doping contents present unexpectedly lower EL
conversion and GVL yield when compared to the MoS /AC
2
catalyst. The XRD and TEM analysis demonstrated the
formation of MoS
2
slabs and their high dispersion on the AC
4
8
movement.
For example, EL was almost converted
support (Fig. S5 and S6†), and the XPS results showed that
2
+
completely and the yield of GVL was 88.7% when isopropanol
Ni(Co) mainly existed as Ni(Co) in the Ni(Co)–S bond (Fig.
Table 5 The effect of different solvents on the hydrogenation of EL to GVL
Entry
1
Solvent
Conv. (%)
81.1
Yield (%)
22.2
Sel. (%)
27.3
Methanol
2
3
Ethanol
72.8
97.2
50.9
88.7
69.9
91.2
Isopropanol
4
5
6
1-Butanol
2-Butanol
Cyclohexane
78.4
87.7
18.2
46.0
67.1
9.4
58.7
76.5
51.6
7
1,4-Dioxane
1.6
0.6
37.5
2 2
Reaction conditions: EL 2 mmol, 8 wt% MoS /AC 75 mg, solvent 20 mL, 230 °C, 1.5 h, 1 MPa H .
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23
S7†). Moreover, it is obvious that Ni(Co) doping leads to a
distribution over time, we proposed a possible pathway for the
lower sulfuration degree of the Mo species. This probably
EL to GVL reaction (Scheme 2) using the MoS /AC catalyst.
2
explained why these bimetallic Co(Ni)–MoS
2
/AC catalysts are
Generally, EL was firstly converted to 4-HPE via the transfer
hydrogenation. Subsequently, 4-HPE further underwent
lactonization to form GVL. Besides, a couple of side reactions
also happened in this system. EL will be converted to IPL via
transesterification and will further transform into GVL via
transfer hydrogenation like EL, and GVL will undergo excessive
hydrogenation to form IPA, pentanoic acid, etc.
less effective than the monometallic MoS /AC catalyst.
2
3
.3.4 Effect of substrates. In order to verify the versatility
of the catalyst and better understand the reaction mechanism
during the MoS /AC catalyzed CTH reaction, several biomass-
2
derived carbonyl compounds were evaluated as substrates
under optimum reaction conditions As shown in Table 6, it
.
can be observed that the CO group in aldehydes and
ketones can be selectively hydrogenated to –C–OH with high
conversion (>90.0%) and high selectivity (>88.0%) (entries
Based on previous research, Lewis acid sites has an
important effect on the transfer hydrogenation reaction
whereas Brønsted acid sites play an important role in the
1
4,16
1
–3). This clearly indicates that EL was firstly transformed
into 4-HPE in the CTH reaction of EL to GVL, although no
-HPE was detected during the reaction because it can
lactone reaction of 4-HPE.
However, in this MoS /AC
2
catalyzed system, the MoS2 edge sites acted as the robust
hydrogenation sites and the acid sites mainly promoted the
4
1
3,53,54
quickly convert to GVL via internal esterification. In addition,
the catalyst presented good performance in transforming
different levulinate derivatives to GVL, including LA, methyl
levulinate (ML) and butyl levulinate (BL). Notably, LA and ML
were converted to GVL with the yield of 79.6% and 81.9%,
respectively (entries 4 and 5). However, BL exhibited a
relatively low activity probably due to the steric hindrance of
lactone reaction of 4-HPE. Regarding previous studies
and our previous results, we proposed plausible
a
explanation for the MPV reaction of EL to GVL over the
MoS /AC catalyst, as shown in Scheme 3. In the presence of
2
molecular H
generated on the edge of MoS
for the adsorption of EL and H-donor isopropanol.
Mo and S center interacted with both isopropanol and EL
to form a six membered ring transition state. At this stage,
the corresponding alkoxide generated from the dissociation
of isopropanol readily transferred hydride ions to attack the
carbonyl group of EL to yield 4-HPE. At the same time,
isopropanol was converted into acetone with the loss of two
2
at high temperature, the S vacancies were
2
, regarded as the active sites
5
5,56
The
4
+
2−
butyl (entry 6). Above all, the MoS
2
/AC catalyst revealed
superior activity in the selective hydrogenation of the CO
group to –C–OH, and presented great potential for the CTH
reactions of carbonyl compounds.
3
.3.5 Reaction pathway and mechanism. On the basis of
the qualitative analysis of GC-MS and the trend of product
2
Table 6 CTH of various biomass-derived carbonyl compounds over the 8 wt% MoS /AC catalyst
Entry
1
Substrates
Products
Conv. (%)
94.8
Yield (%)
87.2
Sel. (%)
92.0
2
3
97.6
98.2
93.5
92.3
95.8
94.0
4
5
6
93.2
95.2
84.7
79.6
81.9
73.2
85.4
86.0
86.4
2 2
Reaction conditions: substrate 2 mmol, 8 wt% MoS /AC 75 mg, isopropanol 20 mL, 230 °C, 1.5 h, 1 MPa H .
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hydrogenation center was extremely important to achieve
high activity in the CTH of EL to GVL.
The performance of the MoS /AC catalyst for the CTH of
2
EL was also compared with those previously reported in the
literature (Table 7). Although the MoS /AC catalyst presented
2
comparable GVL yield with those reported in the literature, a
relatively higher reaction temperature is non-negligible. It's
worth noting that external H is essential for the activation of
2
2
MoS to form sulfur vacancies as the active center, whereas it
is needless for the metal catalyst or Zr-based solid catalyst.
Hence, this MoS /AC catalyst perhaps provides a new idea for
2
the CTH of EL to GVL to a certain extent, but there are
indeed some obvious weaknesses to limit its applicability.
6
1
Strategies such as the design of Co–MoS2 metal-vacancies
or MoS
−x
6
2
2
monolayer sheets decorated with isolated Co atoms
might be able to solve these problems.
3.4. Stability and reusability of the catalyst
The economic benefit is significant in the industrial
production process, so the recyclability of the 8 wt% MoS /AC
2
Scheme 2 Plausible pathway for EL to GVL series reaction.
catalyst was tested under optimized reaction conditions. The
catalyst was separated by filtration, washed with ethanol
three times and subsequently dried for 2 h at 120 °C before
next use. The results are shown in Fig. 6. The conversion of
EL and yield of GVL gradually decreased after 4 runs. When
hydrogen atoms. No detection of 4-HPE by GC analysis
supported the fact that intramolecular transesterification of
4
-HPE into GVL was very fast via the promotion of the surface
comparing the spent catalyst with the MoO /AC catalyst,
3
acid site on the AC support, and the control step was located
in the transfer hydrogenation of EL to 4-HPE. Therefore, the
rational design of MoS2 on the acid support to engineer
similar catalytic activity was observed. According to the
3
2
literature, it may be the replacement of S with O species in
the catalyst, which leads to the sulfur loss and the formation
of Mo oxidized species.
highly dispersed low stacked MoS
2
slabs as the robust
2
Scheme 3 Plausible mechanism of the MPV route for CTH of EL to GVL over the MoS /AC catalyst.
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2
Table 7 Comparison of the catalytic performance of MoS /AC with other catalysts in the CTH of EL to GVL
a
Entry
Catalyst
H-Donor
T (°C)
P (Mpa)
t (h)
Conv. (%)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
MOF-808
Zr-HBA
Isopropanol
Isopropanol
2-BuOH
Isopropanol
2-BuOH
Isopropanol
Isopropanol
Isopropanol
130
200
150
230
150
120
r.t.
—
—
—
—
—
—
0.1 Ar
1
3
1
12
0.5
8
3
9
1.5
100
100
100
94.2
99.0
96.0
—
85.0
96.8
97.0
86.7
96.8
86.4
99.0
88.7
13
14
15
57
58
59
60
Cu–Ni/Al
2
O
3
ZrFeO
CuNi/ZrAlO
Cu/ZnO/Al
RANEY®
2
O
3
2
MoS /AC
230
97.2
This work
a
2
External H .
in the CTH of EL to GVL. The 8 wt% MoS /AC catalyst was
2
found to exhibit the highest catalytic performance in the
CTH reaction under 1 MPa H2 at 230 °C for 1.5 h in
isopropanol solvent, with the EL conversion of 97.2% and
GVL yield of 88.7%. By comprehensive characterization and
catalytic measurements, we found that the CTH activity of
2
EL to GVL is closely related to the MoS morphology and
acid distribution on the support. Among the catalysts with
different supports, the AC support with abundant Lewis
acid sites and large surface area facilitated the high
2 2
dispersion of lower stacked MoS , and the MoS -acid
synergistic catalysis contributed to the superior activity and
selectivity. We further investigated the effect of reaction
conditions, different H-donors, and substrates during the
CTH reaction, and proposed a plausible reaction pathway
2 2
and mechanism over MoS /AC. Although this MoS /AC
catalyst presented comparable GVL yield with those reported
in the literature, the harsh reaction conditions and the
obvious deactivation are non-negligible which will limit its
further application. More studies are needed to focus on
the enhancement of low-temperature activity and stability in
the future.
Fig. 6 The recycling test of 8 wt% MoS
2
/AC. Reaction conditions: EL 2
2
mmol, catalyst 75 mg, isopropanol 20 mL, 230 °C, 1.5 h, 1 MPa H .
Our XPS characterization (Fig. S8†) showed that the Conflicts of interest
4+
2−
percentages of Mo and S decreased gradually with each
cycle, and more MoO was generated, which is in accordance
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
3
with the literature and our recycling results. It's worth noting
that this may not be solely responsible for the decline in catalyst
activity after consecutive cycles, because metal sintering and
leaching also had an important impact. The intensity of the Mo
XPS signal also decreased after different uses, as shown in Table
S2,† based on the XPS quantitative analysis. Both Mo and S
contents presented apparent loss after the reaction because of
metal leaching during the reaction. Fortunately, the sulfide state
of the catalyst can be easily regenerated by sulfidation before
using for the next cycle. Therefore, we re-sulfurized the 4th
Acknowledgements
This work was supported by the National Key R&D Program of
China (Grant No. 2018YFB1501500), the National Natural
Science Foundation of China (21975181, 21690083), the
Guangdong Provincial Key Laboratory of New and Renewable
Energy Research and Development (No. E139kf0701), and the
State Key Laboratory of Pulp and Paper Engineering (South
China University of Technology), No. 202113.
2
MoS /AC and the conversion of EL and selectivity of GVL could
reach 86.4% and 88.3%, respectively.
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