Highly selective demethylation of anisole to phenol over H4Nb2O7modified MoS2catalyst

Article abstract of DOI:10.1039/d0cy01972k

Hydrogenolysis of lignin to obtain value-added phenolic chemicals is an important approach for its comprehensive utilization. Herein, H₄Nb₂O₇modified MoS₂catalyst with short slabs and narrow stacking degree was successfully synthesized by the one step hydrothermal method and used in the selective demethylation of anisole to phenol. The MoS₂-H₄Nb₂O₇-160 catalyst exhibited the best activity with 97.7% conversion of anisole and 98.0% selectivity of phenol under 3 MPa H₂pressure at 270 °C for 4 h, which has been rarely reported on anisole transformation over heterogeneous catalysts so far. The characterizations results demonstrated that the H₄Nb₂O₇modification reduced the slab length and stacking degree of MoS₂during the hydrothermal process and enhanced the acidity property therefore improved the cleavage ability of C_ArO-CH₃bond. This study provides a new scheme for the activity enhancement of MoS₂in lignin demethylation, laying a foundation on the improvement of lignin utilization and the development of renewable energy strategy.

Full text of DOI:10.1039/d0cy01972k

Volume 11
Number 3
7 February 2021
Pages 691–1142
Catalysis
Science &
Technology
rsc.li/catalysis
ISSN 2044-4761
PAPER
Na Ji et al.
Highly selective demethylation of anisole to phenol over
H4Nb2O7 modified MoS2 catalyst

Catalysis
Science &
Technology
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PAPER
Highly selective demethylation of anisole to
phenol over H4Nb2O7 modified MoS2 catalyst†
Cite this: Catal. Sci. Technol., 2021,
11, 800
a
Na Ji,
Zhenjiao Wang,a Xinyong Diao,a Zhichao Jia,a Tingting Li,a
*
b
a
ac
Yujun Zhao,
Qingling Liu,
Xuebin Lu,
Degang Maa and Chunfeng Songa
Hydrogenolysis of lignin to obtain value-added phenolic chemicals is an important approach for its
comprehensive utilization. Herein, H4Nb2O7 modified MoS2 catalyst with short slabs and narrow stacking
degree was successfully synthesized by the one step hydrothermal method and used in the selective
demethylation of anisole to phenol. The MoS2–H4Nb2O7-160 catalyst exhibited the best activity with 97.7%
conversion of anisole and 98.0% selectivity of phenol under 3 MPa H2 pressure at 270 °C for 4 h, which
has been rarely reported on anisole transformation over heterogeneous catalysts so far. The
characterizations results demonstrated that the H4Nb2O7 modification reduced the slab length and stacking
degree of MoS2 during the hydrothermal process and enhanced the acidity property therefore improved
the cleavage ability of CArO–CH3 bond. This study provides a new scheme for the activity enhancement of
MoS2 in lignin demethylation, laying a foundation on the improvement of lignin utilization and the
development of renewable energy strategy.
Received 9th October 2020,
Accepted 10th January 2021
DOI: 10.1039/d0cy01972k
rsc.li/catalysis
especially Mo-based sulfides, exhibited good performance in
the selective production of phenol from anisole compared
1. Introduction
Phenol, as an important chemical raw material, could be
employed for the synthesis of rubber, pesticides, and
medicines, the annual demands of which in the global
market could reach 8 million tonnes.1 Now, phenol is
prepared by treating benzene and propylene in most chemical
industries while the energy crisis has accelerated the search
for cleaner phenol production. Lignin hydrogenolysis, which
has attracted many researchers' attention in recent years
contributing to energy conservation and environmental
protection,2–5 is a relatively green process where the target
product phenol could be obtained by breaking the chemical
bond in lignin over a suitable catalyst in the presence of
hydrogen.6–9 Breaking the C–O bond of methoxyl in the
anisole (CArO–CH3) to obtain phenol efficiently is more
realistic and there are reports focused on this reaction, where
the catalyst plays the vital role in the product yield. Noble
metal,10–14 non-noble metal,15,16 sulfides,17 phosphides,18–20
and oxides21,22 have been studied and Mo-based catalysts,
with other catalysts.17,19,23–27 Great efforts have been made to
improve the selectivity of phenol from anisole conversion.
However, it can also be seen from the summary in Table S1†
that the reaction temperature needed to be above 350 °C
when the phenol yield was greater than 90%. It's meaningful
to reduce the reaction temperature for practical applications.
Wang et al.28 have reported a simple method to prepare low
crystalline MoS2 with more active sites exposed and different
ways have been studied to improve the catalytic performance
of unsupported MoS2 by changing the slab length and the
stacking degree of MoS2.29–33 However, the phenol yield was
still influenced by the weak catalyst activity under mild
conditions and the generation of benzene at certain
temperature.
It is well known that surface acidity also plays an
important role in catalytic performance,34,35 indicating that
the product selectivity could be improved availably by
changing the amount and strength of surface acidity.
Niobium-based acid has been applied to lignin conversion
and it demonstrated its ideal properties depending on its
abundant surface acidity. Wang et al.36–38 have reported a
battery of catalysts containing niobium-based acid for the
transformation of lignin and the niobyl acids group owning a
high acid amount with a better capacity of C–O bond
adsorption as well as the strong synergistic effect of active
metals and NbOx accelerated the cleavage of C–O bond,
a School of Environmental Science and Engineering, Tianjin Key Laboratory of
Biomass/Wastes Utilization, Tianjin University, Tianjin, 300350, China.
E-mail: jina@tju.edu.cn
b School of Chemical Engineering, Tianjin University, Tianjin, 300350, China
c Department of Chemistry & Environmental Science, School of Science, Tibet
University, Lhasa, 850000, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01972k
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leading to a higher hydrodeoxygenation activity. A similar
phenomenon that the modification of strong solid acid could
further improve the cleavage of C–O bond was also observed
by Liang,39 Wang,40 and Fábio B. Noronha.14 The
introduction of niobium-based acid on the supported MoS2
was also studied, and it indicated that the addition of niobic
acid possessing Lewis acid and Brønsted acid sites could
enhance the hydrodesulfurization activity efficaciously, which
was ascribed to an increase in the acidic environment, with
the crystal length and stacking degree of the MoS2 particles
changed.41–44 Inspired by previous research, we attempted to
synthesize the MoS2 catalyst with more activity sites exposed
and simultaneously created the modification of H4Nb2O7 on
MoS2, presuming that the decoration of H4Nb2O7 could
ameliorate the surface acidity of the catalyst, simultaneously
the slab length and stacking degree of MoS2 could be
reduced, resulting in a better cleavage ability of CArO–CH3
bond. To our knowledge, it was rarely reported about the
surface acidity engineering for the enhancement of
unsupported MoS2 in lignin conversion.
were prepared through the same hydrothermal process at 160
°C for 12 h. Sn2Nb2O7 was synthesized according to the
literature.45 More details of the preparation were provided in
ESI.†
2.2 Characterization
The samples were characterized by powder X-ray diffraction
on the D8-Focus powder diffractometer operated at 40 kV
and 200 mA with Ni-filtered Cu Kα irradiation (λ = 1.5406 Å).
The 2 theta was ranged from 5° to 80° at a speed of 10°
min−1. Scanning electron microscopy (SEM) was used to test
the morphology of the samples. SEM images were obtained
on a JSM-7800F thermal field emission instrument. The
nitrogen adsorption and desorption isotherms were
measured with an ASAP2020M apparatus and the surface
area was calculated using the BET method. Catalysts were
dehydrated at 300 °C for 12
h before determination.
Transmission electron microscopy (TEM) was measured on a
JEM-2100F transmission electron microscope to observe the
¯
morphology and structure of catalysts. The average length (L)
Herein, the H4Nb2O7 modified MoS2 catalysts were
synthesized by a one-step hydrothermal method and used for
the demethylation of anisole to produce phenol. The optimal
synthesis conditions were investigated, and different
characterizations were introduced to measure the effect of
H4Nb2O7 modification in detail. The influences of resultant
conditions were explored, and the suitable reaction
conditions was found to maintain higher phenol yield. The
reaction pathway was proposed. Finally, the stability of the
catalyst was tested and characterized in detail.
¯
and stack number (N) of the slabs were calculated by the eqn
(1) and (2), where ni was the number of slabs in the unit and
Li and Ni were the length of the slab and the stacking
number of the unit. n′i was the number of Mo atoms along
one side of a MoS2 slab, fe was the fraction of Mo atoms at
the edge sites, and fc was the fraction of Mo atoms at the
corner sites.
n
n
X
X
̄
L ¼
niLi=
ni
(1)
(2)
(3)
i¼1
i¼1
n
n
X
X
̄
2. Experimental section
2.1 Catalyst preparation
N ¼
niNi=
ni
i¼1
i¼1
ꢀ
ꢁ
̄
The MoS2–H4Nb2O7 catalyst was prepared by
a facile
L
ni′ ¼ 10 ꢀ
þ 1 =2
hydrothermal method while MoS2 and H4Nb2O7 were
synthesized synchronously. Typically, the steps of MoS2–
H4Nb2O7-160 synthesis were as follows: 0.2 g Sn2Nb2O7, 0.92
g ammonium molybdate, and 1.2 g thiourea were dispersed
into 48 mL deionized water, and then the mixture was
transferred into a 100 mL Teflon-lined stainless autoclave.
The pH value was adjusted to 0.9 by adding 12 mL
hydrochloric acid into the mixed solution under stirring. The
autoclave was sealed and heated in an oven at 160 °C for 12
h. When the reaction finished, the products were separated
and washed with deionized water and dried in an oven. The
3:2
ꢂ
ꢃ
fe ¼ ð6n′ − 12Þ= 3n′2 − 3n′ þ 1
(4)
(5)
i
i
i
ꢂ
ꢃ
fc ¼ 6= 3n′2 − 3n′ þ 1
i
i
The chemical valence of each element on the catalyst
surface was measured by X-ray photoelectron spectroscopy
(XPS) with a ThermoFisher ESCALABTM 250Xi spectrometer.
The acidity of the investigated catalysts was tested by NH3
catalyst
temperature for 12 h with the Mo/Nb ratio fixed at 7 was
denoted as MoS2–H4Nb2O7-X (X is hydrothermal
temperature). The catalyst synthesized at 160 °C with the Mo/
Nb ratio fixed at 7 for different hydrothermal time was
denoted as MoS2–H4Nb2O7-Yh (Y is hydrothermal time).
Different ratio catalyst was named MoS2–H4Nb2O7-Z. For
comparison, MoS2–H4Nb2O7-mix was prepared by fixing MoS2
and H4Nb2O7 physically and the Mo/Nb ratio was 7, same as
the optimal synthesis condition. H4Nb2O7 and pure MoS2
synthesized
under
different
hydrothermal
temperature-programmed desorption (NH3-TPD) on
a
Micromeritics AutoChem II 2920 automatic analyzer. The
samples were pretreated at 150 °C for 0.5 h in the N2 flow.
After that, NH3 was introduced at 50 °C until the samples
were saturated. Subsequently, the physically adsorbed NH3
was swept by inert gas at 100 °C. Afterwards, the catalysts
were heated to 700 °C at 10 °C min−1 and the desorbed NH3
was detected by continuous effluent gas monitoring equipped
with
a thermal conductivity detector (TCD). Inductively
coupled plasma optical emission spectrometer (ICP-OES) was
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conducted on a Thermo 7000 series analyzer to test the loss
of S and Nb elements in the used catalysts. 0.05 g catalyst
lattice planes of H4Nb2O7.46 Obviously, the peak at 2θ = 14.5°
was much higher than any other peaks, which meant that
H4Nb2O7 prepared in this work was dominated by (111)
crystal plane. MoS2 (JCPDS No. 37-1492) represented with
broader and gentle peak was related to the poor
crystallinity.47 After the addition of H4Nb2O7, the position of
feature peaks of MoS2 and H4Nb2O7 were not shifted after
the hydrothermal process in comparison with pure MoS2 and
H4Nb2O7 while the intensity of peaks was similar at the same
molar ratio of Mo/Nb.
was added in 10 mL chloroazotic acid (VHCl/VHNO = 3/1), then
3
the solution was digested by microwave at 120 °C for at least
20 minutes until completely dissolved. It was transferred to a
1000 mL volumetric flask and diluted for testing.
2.3 Activity evaluation
The catalytic activity measurements were carried out in a 50
mL autoclave. The prepared catalyst (0.135 g) without any
pretreatment and anisole solution (20 mL, 0.125 mol L−1,
n-dodecane as the internal standard, n-hexane as the solvent)
were placed into the autoclave. Hydrogen was used to purge
the reactor and kept at an initial pressure. The reaction was
carried out at a certain temperature within a specified time.
When the reaction finished, the liquid products were
separated and analyzed by GC (SP-7890) using a flame
ionization detector (FID) with a 30 m × 0.25 mm × 0.25 μm
WondaCap WAX capillary column. Conversion and products
yield were calculated using the following formula where c
represented the mole concentration:
The crystallinity and purity of MoS2–H4Nb2O7 catalysts
prepared under different temperatures were tested by XRD.
As shown in Fig. S1,† samples prepared under different
hydrothermal temperature presented
structure.
a
similar crystal
And as the hydrothermal temperature increased, the
intensity of H4Nb2O7 first increased and then decreased,
while the sample MoS2–H4Nb2O7-140 showed the best
H4Nb2O7 purity and crystallinity. It can be seen the XRD
spectra of the MoS2–H4Nb2O7-180 and MoS2–H4Nb2O7-200
catalysts contained few peaks of H4Nb2O7 and there were
some Nb2O5 formed during the hydrothermal reaction
because of the higher temperature. The peaks of MoS2
represented the same phenomenon which can be seen from
the mountain slope around 2θ = 14.4°, the characteristic
plane (002) of MoS2, which signified the formation of layered
structure.48,49 With the increase of temperature, the peak
intensity of (002) increased firstly and then decreased. At
lower temperatures, less S2− was not sufficient to form the
layered structure so the characteristic peaks of (002) of MoS2
were not obvious and the Mo2S3 component formed
unexcepted. MoS2–H4Nb2O7-160 represented broader (002)
peak with high intensity indicated that a good structure of
MoS2 obtained.47 While the XRD spectra of the samples
prepared under different hydrothermal time, as shown in
Fig. S2,† represented that the hydrothermal time had a little
influence on the crystal formation. A weak fluctuation of the
(002) plane peak intensity could be attributed to the loss of S
at the edge of MoS2 due to the corrosion at low pH value.47
As the XRD spectra of different Mo/Nb ratio samples shown
in Fig. S3,† there were no obvious differences among the
peaks of MoS2, while the intensity of H4Nb2O7 peaks
increased with the augment of its content.
ꢀ
ꢁ
cðresidual anisoleÞ
cðinitial anisoleÞ
Conversion ¼ 1 −
× 100%
(6)
cðproductÞ
Product yield ¼
× 100%
(7)
cðinitial anisoleÞ
3. Results and discussion
3.1 Catalysts characterization results
To identity the crystal phase structure of the catalysts, XRD
measurements were carried out. As shown in Fig. 1, high
crystallinity H4Nb2O7 and MoS2 were synthesized by the
hydrothermal method. Peaks located at 14.5°, 27.9°, and
29.2° were matched well with the (111), (311), and (222)
The morphological characteristics of the samples were
investigated using scanning electron microscopy and the
results were shown in Fig. 2. As shown in Fig. 2(a and b),
pure MoS2 consisted of stacked sheets with a smooth surface,
and H4Nb2O7 presented as microspheres as shown in
Fig. 2(c and d). Microspheres provided more foothold for the
growth of MoS2 particles and the addition of H4Nb2O7
resulted in a decrease in the size of the MoS2 particles
distinctly. Moreover, the agglomeration between MoS2
particles was effectively inhibited, and the similar
phenomena have been obtained by Klimova.43 Generally, on
the one hand, reducing the size and enhancing the particle
dispersion of the catalyst could increase the number of active
Fig.
1 XRD spectra of H4Nb2O7, MoS2, MoS2–H4Nb2O7-mix, and
MoS2–H4Nb2O7-160.
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slab length, which was consistent with XRD results. The
increase in MoS2 dispersion with the embellishment of
H4Nb2O7 was also agreed with the SEM results. The average
length of the MoS2 phase in the catalysts was reduced from
3.1 nm to 2.4 nm and the stacking degree of the MoS2
particles was decreased, which were attributed the addition
of H4Nb2O7 to unsupported MoS2. More morphology
parameters of MoS2 in different catalyst were listed in Table
S2.† The structural model of MoS2 was described as stacks of
several slabs and the active sites were the sulfur vacancy at
the edge sites and the corner sites. According to the
literature, smaller slab length, which was conductive to more
accessible active phase, was beneficial to expose more corner
sites where the formation of sulfur vacancy was easier.34,51,52
As shown in Table S2,† the smaller slab length of MoS2
resulted from the H4Nb2O7 modification led to a higher
corner-to-edge ratio, which was consistent with the
conclusion of the literature. In addition, studies have shown
that the shorter slab length was in favor of better
hydrodesulfurization capability.43,44 Based on the similarity
of the two catalytic reactions and the decrease in the average
length of MoS2 slabs resulted from the modification of
Fig. 2 SEM images of (a and b) MoS2 and (c and d) MoS2–H4Nb2O7-
160.
sites exposed on the margin, which has been proved to be
the main factor for the catalytic activity of MoS2 catalysts.50
On the other hand, size reduction and dispersion promotion
were beneficial to the increase of specific surface area. As
expected, after the modification of H4Nb2O7, surface area of
the catalyst increased from 6.4 m2 g−1 to 15.9 m2 g−1. In a
word, the incorporation of an appropriate amount of
H4Nb2O7 reduced the size and agglomeration of MoS2
particles and improved the specific surface area of the
catalyst, which may provide more exposed active sites for the
reaction.
For more direct evidences, the change of the morphology
of sulfide active phases was characterized by TEM
measurements, and the images were shown in Fig. 3. The
typical layered structure was observed and 0.65 nm
interplanar distance was attributed to the (002) plane of
MoS2. The stacking number and the slab length of MoS2
slabs were statistically analyzed more than 80 stacks from
different parts of each sample, and the results were listed in
Table 1. MoS2 showed a lower stacking number and small
H4Nb2O7,
a stronger catalytic capability of the MoS2–
H4Nb2O7-160 sample was expected.
To identify the surface chemical state of the samples, the
XPS measurements of the samples were performed, as shown
in Fig. 4. As for pure MoS2, the XPS spectra of the Mo 3d
region showed signals of Mo4+, Mo5+, Mo6+, and S 2s. The
presence of Mo5+ and Mo6+ on the surface may be related to
the formation of oxysulfide species and MoO3-like
compounds.41 For the spectra of MoS2–H4Nb2O7-160, there
were some noticeable differences. The intensity of Mo4+ and
S 2s peaks increased significantly but the intensity of peaks
corresponding to Mo5+ and Mo6+ were reduced to varying
degrees in the spectra of MoS2–H4Nb2O7-160, which meant
that the concentration of Mo oxide species was reduced. It
indicated that there may be an interaction between Mo
species and H4Nb2O7 so that the incorporation of H4Nb2O7
Fig. 3 TEM images and length distributions of (a–c) MoS2 and (d–f) MoS2–H4Nb2O7-160.
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Table 1 Structure properties of the prepared catalysts
Sample
SBET (m2 g−1
)
Stacking numbera
Lengtha (nm)
MoS2
MoS2–H4Nb2O7-160
6.4
15.9
5.6
4.6
3.1
2.4
a
As determined from TEM micrographs (average value).
Fig. 4 XPS spectra of (a) Mo 3d and (b) S 2p of MoS2 and MoS2–H4Nb2O7-160.
could accelerate the reduction process of Mo oxide species. It
was clear that the intensity of the S 2s signal on MoS2–
H4Nb2O7-160 was higher than that on MoS2, corresponding
to the S2− in MoS2, which was positively related to the
concentration of activity component on the catalyst surface.
This was consistent with the previous SEM and TEM
conclusions that niobic acid modification could effectively
increase the number of active sites on MoS2. The signals
located at 161.7 and 162.8 eV were assigned to the S2− state
revealed that the modification of H4Nb2O7 increased the
desorption peak area and shifted it to a higher temperature,
which meant that the concentration and strength of acid
sites on the catalysts both increased significantly after the
addition of H4Nb2O7, whether through physical mixing or
hydrothermal synthesis. Although both MoS2–H4Nb2O7-mix
and MoS2–H4Nb2O7-160 catalysts had a similar amount of
acid sites, the MoS2–H4Nb2O7-160 catalyst synthesized by the
hydrothermal method exhibited more stronger acid sites than
that of the MoS2–H4Nb2O7-mix catalyst. The formation of
2−
and S22−-like chemical states, respectively. The SO4 peak
located at 168.5 eV on the MoS2 catalyst was absent on the
MoS2–H4Nb2O7-160 catalyst, which was consistent with the
conclusion that the proper addition of H4Nb2O7 could
enhance the reduction process to a certain extent so that the
sulfidation was promoted. Therefore, the area of S2− peak
increased after the modifying process, representing the
improvement in the content of MoS2. As for H4Nb2O7, the
binding energy of Nb had no shift after the combination
according to the spectra of Nb 3d shown in Fig. S4.†
stronger acid sites confirmed that there was a strong
interaction between MoS2 and H4Nb2O7 coming into being
The acidity of the catalyst surface plays an important role
in its catalytic performance. Fig. 5 represented the NH3-TPD
profiles of the MoS2, MoS2–H4Nb2O7-mix, and MoS2–
H4Nb2O7-160 catalysts. It was well known that the area of the
NH3 desorption peak was related to the total acidity on the
catalyst and the strength of acid sites was in connection with
the NH3 desorption temperature.39 As shown in Fig. 5, peak
around 304 °C was identified as the weak acid sites, and
peaks centered at 396.3 °C and 434.8 °C were corresponded
to the strong acid sites. The total acidity of MoS2, MoS2–
H4Nb2O7-mix, and MoS2–H4Nb2O7-160 was 6.3 mmol g−1,
16.4 mmol g−1, and 16.7 mmol g−1, respectively. Results
Fig. 5 NH3-TPD profiles of H4Nb2O7 modified MoS2 catalysts.
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during the hydrothermal synthesis process. Generally, the
stronger acid sites, affecting the electron-deficient character
of the active metal and enhancing the adsorption of C–O
bond, were more favorable for the cleavage of the C–O bond
of the methoxy group, which was more conducive to the
demethylation of anisole.14
played. With an appropriate amount of H4Nb2O7 added into
the catalyst by the hydrothermal method, the yield of phenol
improved significantly which reached 95.7% with 97.7%
conversion of anisole obtained. The 98% selectivity of phenol
was achieved over the MoS2–H4Nb2O7-160 catalyst, which was
much higher than the result of MoS2–H4Nb2O7-mix,
indicating that the hydrothermal method was more
conducive to the niobic acid modification process and the
activity enhancement. It was worth mentioning that the
increase of acid amount of MoS2–H4Nb2O7-mix and MoS2–
H4Nb2O7-160 was about the same, while the morphology
change and more stronger acid sites formed by the
interaction between MoS2 and H4Nb2O7 during the
hydrothermal process were more effective in the
improvement of cleaving ability of CArO–CH3 bond so the
MoS2–H4Nb2O7-160 catalyst exhibited better catalytic activity.
Compared with the results reported before, the catalyst as
prepared in this work could catalyze anisole to obtain high
yield phenol under relatively mild conditions, as listed in
Table S1.† Furthermore, the formation of benzene was also
effectively inhibited at higher temperature, as shown in Table
S3.†
Based on the above results, the effects of H4Nb2O7
modification and the presence of interaction between MoS2
and H4Nb2O7 can be concluded easily. For one thing, the
addition of H4Nb2O7 promoted the dispersion of Mo species,
then the reduction and sulfidation process were accelerated
resulting in the increasing of MoS2 concentration, as
depicted in SEM and XPS results. At the same time, the
growth of MoS2 slabs was slightly influenced due to the
modification of H4Nb2O7 with shorter slab length and narrow
stacking degree obtained. For another, stronger acid sites
formed during the hydrothermal process, which were
essential for the cleavage of CArO–CH3 bond. In a word, the
effects of H4Nb2O7 decoration were manifested in the
concentration and microstructure change of MoS2 and the
strong interaction between MoS2 and H4Nb2O7 reflected by
the more strong acid sites.
According to the characteristic results, the modified
catalyst MoS2–H4Nb2O7-160, prepared under optimal
conditions, showed more stronger acid sites with the
microstructure and content change of MoS2 obtained. On the
one hand, lower agglomeration and shorter slab length were
beneficial for the exposing of activity sites and the
concentration of MoS2 in the catalyst was increased. More
importantly, the stronger acid sites resulted from the
interaction between MoS2 and H4Nb2O7 as determined by
NH3-TPD led to better adsorption and cleavage ability of CAr-
O–CH3 bond in the methoxy group. On the other hand, lower
stacking degree of MoS2 slabs preferred to weaker cleavage
capability of CAr–O bond.29 As a result, the good performance
of MoS2–H4Nb2O7-160 in anisole conversion and phenol
production under mild conditions was originated from the
synergistic effect between MoS2, H4Nb2O7 and stronger acidic
sites, confirming that the addition of H4Nb2O7 to MoS2 was
essential for the highly catalytic activity and phenol yield.
The catalytic performance of the catalysts prepared under
different conditions was evaluated at 270 °C, 3 MPa for 4 h,
and the results were listed. As shown in Table S4,† it was easy
to see that the activities of the samples were strongly related
to the synthesis temperature. Firstly, the formation of MoS2
was proved by the XPS results, as shown in Fig. S5.† Besides,
the H4Nb2O7 modification played diverse degrees of effect on
the activity of catalyst at different synthesis temperatures, by
comparing the results in Tables S4 and S5.† When the
resultant temperature was 120 °C, the anisole conversion and
phenol yield over MoS2–H4Nb2O7-120 were just 58.5% and
55.9%, while there was little activity increase. Then with the
temperature increased, the yield of phenol over H4Nb2O7
modified catalysts showed a parabolic trend and reached the
highest point 95.7% when the resultant temperature was 160
°C. However, when the temperature was 200 °C, the
3.2 Catalytic activity
To show the advantage of the samples modified with
H4Nb2O7 prepared by our method, a series of controlled
experiments were carried out. As listed in Table 2, the
products consisted of phenol, benzene, and cyclohexane,
where phenol was the main product. In the absence of a
catalyst, little conversion of anisole was obtained. And for the
pure H4Nb2O7 and MoS2 catalysts, the yield of phenol was
12.8% and 67.8%, respectively, indicating that H4Nb2O7 had
almost no activity and the activity of the prepared catalyst
mainly came from MoS2. For comparison, MoS2–H4Nb2O7-
mix was also tested the catalytic activity and the conversion
of anisole was 84.7% and the phenol yield was 74.1%. Higher
activity over the MoS2–H4Nb2O7-mix catalyst mainly resulted
from the difference of acidity amount between the MoS2 and
MoS2–H4Nb2O7-mix catalyst, which was consistent with the
previous statement that the increase in acidity amount could
indeed improve the demethylation ability of the catalyst.
Different combination method, different roles H4Nb2O7
Table
2
Catalytic performance in the transformation of anisole to
phenol
Yield (%)
Conv.
Catalyst
(%)
Phenol
Benzene
Cyclohexane
None
H4Nb2O7
Nb2O5
MoS2
MoS2–H4Nb2O7-mix
MoS2–H4Nb2O7-160
6.2
15.3
13.8
68.9
84.7
97.7
5.7
12.8
12.2
67.8
74.1
95.7
0.2
0.5
0.0
0.7
2.7
0.8
0.3
2.0
1.5
0.4
2.7
0.6
Reaction conditions: 270 °C, 3 MPa, 4 h.
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formation of Nb2O5 lowered the catalyst reactivity
simultaneously (entry 3, Table 2). Only 80.9% conversion of
anisole and 76.8% yield of phenol were obtained. Therefore,
160 °C was selected as the optional synthesizing temperature.
The samples obtained at 160 °C with different
hydrothermal time and Mo/Nb molar ratios were also tested
the breaking ability of CArO–CH3 bond and the results were
shown in Tables S6 and S7.† When the hydrothermal time
was 6 h, the conversion of anisole was 87.0% and the yield of
phenol was 67.3%. Extend the hydrothermal time to 12 h, the
conversion of anisole was 97.7% and the yield of phenol
reached 95.7%. About 5% yield reduction with further
extending hydrothermal reaction time may be related to the
loss of S at the edge of MoS2, as shown in Fig. S2.† Different
Mo/Nb molar ratio determined the content of MoS2 in the
unit catalyst. As shown in Table S7,† the influence of niobic
acid addition determined by the different Mo/Nb molar ratios
on the catalyst activity became significant with the
proportion increasing and it was the greatest when the ratio
was 7. Excessive niobic acid may cover the active site of MoS2
or formed agglomeration so the phenol yield decreased to
72.5% and 60.2% when the ratio was 5 and 4, respectively.
Through the control variable experiment, the MoS2–H4Nb2O7-
160 catalyst prepared under the optimal synthesis condition
(the hydrothermal temperature was 160 °C, the hydrothermal
time was 12 hours, and the Mo/Nb ratio was 7) exhibited the
best catalytic activity and the yield of phenol was the highest
95.7%.
increased from 88.5% to 97.7%. Then the yield of phenol
increased inconspicuously as the initial hydrogen pressure
further increased, which was related to the formation of
further hydrogenation products. From the results, it could be
concluded that the initial H2 pressure had a great effect on
the conversion of anisole when the pressure was lower, while
it had a little influence on the yield of hydrogenation
products when the pressure was higher. To retain the phenol
yield and save energy as much as possible, the pressure of 3
MPa of H2 was finally considered as suitable initial H2
pressure for further study of anisole demethylation.
To explore the reaction pathways, the yield of products at
different reaction time under the optimized reaction
conditions were carefully analyzed and shown in Fig. S8.†
When the reaction time was extended to 3 h, the conversion
rate of anisole increased from 75.6% to 84.2% and the yield
of phenol increased from 74.6% to 83.2%, respectively. A
greater increase occurred when the reaction time was
extended to 4 h, where the anisole conversion rate increased
to 97.7% at the same time the yield of phenol reached 95.7%.
However, when the reaction time was further extended, the
phenol yield changed little, mainly resulted from phenol
deoxidization to produce benzene which was reached from
0.8% to 2.4%. In the process of extending reaction time, the
yield of cyclohexane was increased by only 0.5%.
Anisole is
a
typical lignin monomer, and its
hydrodeoxygenation reaction process is studied to describe
according to the product distributions shown in Fig. S8.† In
the general catalytic reaction, the C–O bond breaking occurs
in two ways in the process of hydrodeoxygenation of anisole.
Namely, anisole could convert to phenol and benzene by
demethylation and demethoxylation, respectively. Direct
cleavage of C–O bond via demethoxylation is very difficult
due to the strong CAr–O bond energy.53 It could be seen that
the production of phenol is the first step of
hydrodeoxygenation of anisole and benzene could be gained
by further deoxygenation of phenol. And the side reaction
that methyl transferred from the methoxyl to the phenolic
rings to form cresols could also occur during the process.
Considering that almost no cresol or cresol hydrogenation
products existed in our results, we think this step hardly
occurred in this catalytic system. Cyclohexane could be
obtained by two pathways. One is that phenol undergoes
aromatic ring hydrogenation to form cyclohexanone and
cyclohexanol which are dehydrated to cyclohexene, then
cyclohexane is generated by the complete hydrogenation of
cyclohexene. The other way is the benzene hydrogenation.
According to the product distributions in this study,
3.3 Studies on different reaction conditions
The temperature has a great influence on the product
distributions in lignin hydrogenolysis. The effects of
temperature on the anisole conversion reaction were
investigated and the results were shown in Fig. S6.† When
the temperature increased from 260 °C to 270 °C, the yield of
phenol increased from 82.1% to 95.7%, which was attributed
to the relatively complete cleavage of the CArO–CH3 bond.
Then as the reaction temperature continued to rise to 300 °C,
although an increase in anisole conversion was achieved,
suggesting the cleavage of CArO–CH3 bond in anisole could
be promoted at the higher temperature, the yield of phenol
barely increased due to the formation of side-product
benzene. However, compared with the product distributions
over MoS2 (entry 1, Table S3†) at 300 °C, less benzene was
received under longer reaction time. It also confirmed that
the addition of H4Nb2O7 limited the cleavage of CAr–O bond.
As for the yield of cyclohexane, no significant changes were
observed as the reaction temperature increased. Therefore,
270 °C was selected as a critical temperature for the
demethylation of anisole in this catalytic system.
The results of anisole demethylation at different initial
hydrogen pressures over the MoS2–H4Nb2O7-160 catalyst were
shown in Fig. S7.† When the initial H2 pressure was changed
from 2 MPa to 3 MPa, the yield of phenol significantly
improved from 86.4% to 95.7% while the anisole conversion
Scheme 1 Reaction pathway of anisole over the MoS2–H4Nb2O7-160
catalyst.
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Table 3 Catalytic performance in the transformation of methylanisole to cresol
Substrate
Time (h)
4
Conversion (%)
81.0
Product
Selectivity (%)
80.5
6
8
6
6
85.7
98.1
96.9
98.2
92.6
91.3
76.8
77.9
Reaction conditions: 270 °C, 3 MPa.
cyclohexane was yielded by the hydrogenation of benzene.
The reaction of phenol over this catalyst was also tested, and
the product consisted of benzene and cyclohexane. Based on
the above analysis, the reaction pathway could be
summarized as the following steps shown in Scheme 1. The
first and foremost step was anisole demethylation to phenol.
The further cleavage of the CAr–O bond happened to produce
benzene. Then, the saturation of benzene yielded
cyclohexane. The first step was the main process in this
reaction while the next two steps rarely occurred to maintain
a high yield of phenol.
to the above results, especially when the methyl was
substituted in the para position, which may due to the
steric and electronic effects resulting in different energy
change.54,55
3.5 Recyclability of the catalyst
The recyclability of the MoS2–H4Nb2O7-160 catalyst was
investigated over three reaction runs for the transformation
of anisole under the optimal reaction conditions. The
catalyst recovered from each run was washed with n-hexane
for several times and then dried at ambient before next
use. It was worth mentioning that neither the conversion of
anisole nor the yield of phenol had decreased much in the
second run, indicating that the catalyst still retained a
relatively high activity. As shown in Table 4, it represented
a certain deactivation at the third run. The conversion of
anisole was decreased from 97.7% to 81.4%, accompanied
by the yield of phenol decreased from 95.7% to 78.2%. In
3.4 Transformation of other substituted anisole
Besides anisole, the transformation of other substituted
anisole to phenols was tested to evaluate the activity of
the MoS2–H4Nb2O7-160 catalyst in lignin conversion. As
shown in Table 3, the catalyst represented high activity
for the conversion of other ethers to obtain phenols.
When 4-methylanisole as the substrate, 92.6% selectivity
of p-cresol was received by extending the reaction time to
6 h. When the reaction time was further extended to 8 h,
4-methylanisole was almost converted but the selectivity of
p-cresol was 91.3% with little toluene generated. When the
methyl substituted at ortho or meta position, the
substrates were almost converted with 77.9% and 76.8%
selectivity of cresol, respectively. Therefore, the MoS2–
H4Nb2O7-160 catalyst showed ideal activity to cleave the
C–O bond of methoxy group to obtain phenols according
Table 4 Recyclability test of MoS2–H4Nb2O7-160 in the transformation
of anisole to phenol
Yield (%)
Conversion
Run
(%)
Phenol
Benzene
Cyclohexane
1st
2nd
3rd
97.7
96.0
81.4
95.7
92.9
78.2
0.8
2.5
1.6
0.6
0.6
0.6
Fig. 6 XRD spectra of fresh MoS2–H4Nb2O7-160 catalyst and used
Reaction conditions: 270 °C, 3 MPa, 4 h.
catalyst.
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4. Conclusion
The H4Nb2O7 modified MoS2 catalyst with short slabs and
narrow stacking degree was successfully synthesized by the
one step hydrothermal method and used in the selective
demethylation of anisole to phenol. Under 3 MPa H2 initial
pressure and 270 °C for 4 h, MoS2–H4Nb2O7-160 exhibited
the best performance with 97.7% conversion of anisole and
98.0% selectivity of phenol which was an ideal phenol yield
compared with other heterogeneous catalysts in the previous
studies. The stronger CArO–CH3 bond cleavage ability of the
MoS2–H4Nb2O7-160 catalyst was related to the decrease of
the slab length and stacking degree of MoS2 and the
enhancement of the surface acidic sites. Furthermore, the
stability of the MoS2–H4Nb2O7-160 catalyst was studied and
the recyclability of the catalyst was affected mainly by the
loss of S and the aggregation of MoS2 particles. This
research provides a novel way to improve the catalytic
performance of MoS2 catalyst in lignin demethylation to
phenols and promote the process of lignin utilization.
Further study has been focused on the improvement of the
catalyst stability and the selective production of other value-
added chemicals.
Fig. 7 TEM images of (a) fresh MoS2–H4Nb2O7-160 and (b) the sample
after the 2nd run.
Table 5 ICP analysis of S and Nb content in MoS2–H4Nb2O7-160 before
and after used
Sample
S content (wt%)
Nb content (wt%)
Fresh
After the 2nd run
35.0
23.8
7.7
8.5
Conflicts of interest
The authors declare no competing financial interest.
Fig. 8 SEM images of (a) fresh MoS2–H4Nb2O7-160 and (b) the sample
Acknowledgements
after the 2nd run.
This work was supported by the National Key R&D Program of
China (Grant No. 2018YFB1501500) and National Natural
Science Foundation of China (21975181, 21690083).
order to investigate the reason for the decrease in catalyst
activity, several characterization tests were conducted to
compare the properties of fresh and the used catalysts. As
shown in Fig. 6, it was clear to see that the intensity of
diffraction peak of the sample after being used two times
had almost content, 32% sulfur loss occurred. Some defects
in black layers shown in Fig. 7 were observed, providing a
direct evidence of the S loss on active sites.56 Meanwhile,
results showed no Nb element loss took place. As reaction
proceeded, as shown in reduced in comparison with the
peaks of the fresh sample. The intensity decrease of
characteristic crystal plane (002) of MoS2 might be
attributed to the various degrees loss of S, which was
supported by the ICP results. As shown in Table 5, the S
content decreased from 35.0 wt% to 23.8 wt% after the
second run. According to the ratio of actual S content to
theoretical S Fig. 8, some particles aggregated and activity
sites might be covered so that the conversion and the
phenol yield of the third run were lower. As a result, the
loss of sulfur and the aggregation may be the main reasons
for the decrease in catalyst activity. CS2, which could be
used as the sulfiding agent, was added 0.2 mL in each
experimental reaction to keep the catalyst activity. As shown
in Table S8,† the addition of CS2 was effective to slow down
the deactivation of the catalyst.
References
1 S. S. Wong, R. Shu, J. Zhang, H. Liu and N. Yan, Chem. Soc.
Rev., 2020, 49, 5510–5560.
2 B. Zhang, Z. Qi, X. Li, J. Ji, W. Luo, C. Li, A. Wang and T.
Zhang, ACS Sustainable Chem. Eng., 2018, 7, 208–215.
3 Z. Q. B. Zhang, X. Li, J. Ji, L. Zhang, H. Wang, X. Liu and C.
Li, Green Chem., 2019, 21, 5556–5564.
4 X. Li, B. Zhang, X. Pan, J. Ji, Y. Ren, H. Wang, N. Ji, Q. Liu
and C. Li, ChemSusChem, 2020, 13, 4409–4419.
5 T. Li, N. Ji, Z. Jia, X. Diao, Z. Wang, Q. Liu, C. Song and X.
Lu, ChemCatChem, 2020, 12, 1–16.
6 G. Liu, H. Ma, I. Teixeira, Z. Sun, Q. Xia, X. Hong and S. C.
Tsang, Chemistry, 2016, 22, 2910–2914.
7 W. Song, T. Nie, W. Lai, W. Yang and X. Jiang,
CrystEngComm, 2018, 20, 4069–4074.
8 W. Song, S. Zhou, S. Hu, W. Lai, Y. Lian, J. Wang, W. Yang, M.
Wang, P. Wang and X. Jiang, ACS Catal., 2018, 9, 259–268.
9 N. J. X. Diao, M. Zheng, Q. Liu, C. Song, Y. Huang, Q. Zhang,
A. Alemayehu, L. Zhang and C. Liang, J. Energy Chem.,
2018, 27(2), 600–610.
10 D. P. Gamliel, S. Karakalos and J. A. Valla, Appl. Catal., A,
2018, 559, 20–29.
808 | Catal. Sci. Technol., 2021, 11, 800–809
This journal is © The Royal Society of Chemistry 2021

View Article Online
Catalysis Science & Technology
Paper
11 I. T. Ghampson, R. Canales and N. Escalona, Appl. Catal., A,
2018, 549, 225–236.
12 H. Ren, C. Li, D. Yin, J. Liu and C. Liang, RSC Adv., 2016, 6,
85659–85665.
13 R. Shu, B. Lin, J. Zhang, C. Wang, Z. Yang and Y. Chen, Fuel
Process. Technol., 2019, 184, 12–18.
14 C. A. Teles, P. M. de Souza, R. C. Rabelo-Neto, M. B. Griffin,
C. Mukarakate, K. A. Orton, D. E. Resasco and F. B.
Noronha, Appl. Catal., B, 2018, 238, 38–50.
15 E. Kordouli, B. Pawelec, C. Kordulis, A. Lycourghiotis and
J. L. G. Fierro, Appl. Catal., B, 2018, 238, 147–160.
16 Y. Yang, C. Ochoa-Hernández, V. A. de la Peña O'Shea, P.
Pizarro, J. M. Coronado and D. P. Serrano, Appl. Catal., B,
2014, 145, 91–100.
17 H. R. Rahimpour, M. Saidi, P. Rostami, B. C. Gates and
M. R. Rahimpour, Int. J. Chem. Kinet., 2016, 48, 702–713.
18 S. Wang, D. Xu, J. Zhao, W. Zheng, C. Hu, X. Wen, Y. Yang
and Y. Li, Catal. Sci. Technol., 2019, 9, 5712–5724.
19 H. Wang, M. Feng and B. Yang, Green Chem., 2017, 19,
1668–1673.
20 Y. Li, J. Fu and B. Chen, RSC Adv., 2017, 7, 15272–15277.
21 C. Ranga, R. Lødeng, V. I. Alexiadis, T. Rajkhowa, H.
Bjørkan, S. Chytil, I. H. Svenum, J. Walmsley, C. Detavernier,
H. Poelman, P. Van Der Voort and J. W. Thybaut, Chem. Eng.
J., 2018, 335, 120–132.
34 Y. Shi, C. Xiao, J. Mei, M. H. Alabsi, G. Wang, Y. Ni, Z.
Zhao, A. Duan and X. Wang, Energy Fuels, 2020, 34,
8759–8768.
35 I. Vázquez-Garrido, A. López-Benítez, G. Berhault and A.
Guevara-Lara, Fuel, 2019, 236, 55–64.
36 Q. Xia, Z. Chen, Y. Shao, X. Gong, H. Wang, X. Liu, S. F.
Parker, X. Han, S. Yang and Y. Wang, Nat. Commun., 2016, 7,
11162.
37 Q. Xia, X. Zhuang, M. M.-J. Li, Y.-K. Peng, G. Liu, T.-S. Wu,
Y.-L. Soo, X.-Q. Gong, Y. Wang and S. C. E. Tsang, Chem.
Commun., 2016, 52, 5160–5163.
38 J. Xi, Q. Xia, Y. Shao, D. Ding, P. Yang, X. Liu, G. Lu and Y.
Wang, Appl. Catal., B, 2016, 181, 699–706.
39 W. Guan, X. Chen, C. Li, J. Zhang, C.-W. Tsang, H. Hu, S. Li
and C. Liang, Mol. Catal., 2019, 467, 61–69.
40 K. Peng, X. Li, X. Liu and Y. Wang, Mol. Catal., 2017, 441,
72–80.
41 E. Puello-Polo, E. Marquez and J. L. Brito, J. Sol-Gel Sci.
Technol., 2018, 88, 90–99.
42 S. Ding, A. Li, S. Jiang, Y. Zhou, Q. Wei, W. Zhou, Y. Huang,
Q. Yang and T. Fan, Fuel, 2019, 237, 429–441.
43 F. J. Méndez, O. E. Franco-López, X. Bokhimi, D. A. Solís-
Casados, L. Escobar-Alarcón and T. E. Klimova, Appl. Catal.,
B, 2017, 219, 479–491.
44 A. S. Rocha, A. C. Faro, L. Oliviero, M. A. Lélias, A. Travert, J.
van Gestel and F. Maugé, Catal. Lett., 2006, 111, 27–34.
45 C. Zhou, R. Shi, L. Shang, Y. Zhao, G. I. N. Waterhouse, L.-Z.
Wu, C.-H. Tung and T. Zhang, ChemPlusChem, 2017, 82,
181–185.
22 M. Shetty, E. M. Anderson, W. H. Green and Y. Román-
Leshkov, J. Catal., 2019, 376, 248–257.
23 T.-R. Viljava, R. S. Komulainen and A. O. I. Krause, Catal.
Today, 2000, 60, 83–92.
24 Q. Lai, C. Zhang and J. H. Holles, Catal. Sci. Technol.,
2017, 7, 3220–3233.
46 J. Wu, J. Li, X. Lü, L. Zhang, J. Yao, F. Zhang, F. Huang and
F. Xu, J. Mater. Chem., 2010, 20, 1942–1946.
25 B. Rahzani, M. Saidi, H. R. Rahimpour, B. C. Gates and
M. R. Rahimpour, RSC Adv., 2017, 7, 10545–10556.
26 N. Ji, X. Diao, X. Li, Z. Jia, Y. Zhao, X. Lu, C. Song, Q. Liu
and C. Li, Ind. Eng. Chem. Res., 2020, 59, 17287–17299.
27 J. Song, N. Ji, X. Diao, C. Song, Q. Liu and M. Zheng, Prog.
Chem., 2017, 29, 563–578.
47 K. Wu, W. Wang, S. Tan, G. Zhu, L. Tan and Y. Yang, RSC
Adv., 2016, 6, 80641–80648.
48 W. Wang, K. Wu, L. Li, S. Tan, G. Zhu, W. Li, Z. He and Y.
Yang, Catal. Commun., 2016, 74, 60–64.
49 H. Zhang, H. Lin, Y. Zheng, Y. Hu and A. MacLennan, Appl.
Catal., B, 2015, 165, 537–546.
28 W. Wang, L. Li, K. Wu, G. Zhu, S. Tan, W. Li and Y. Yang,
RSC Adv., 2015, 5, 61799–61807.
50 B. Yoosuk, D. Tumnantong and P. Prasassarakich, Fuel,
2012, 91, 246–252.
29 W. Wang, K. Zhang, Z. Qiao, L. Li, P. Liu and Y. Yang, Ind.
Eng. Chem. Res., 2014, 53, 10301–10309.
30 B. Yoosuk, D. Tumnantong and P. Prasassarakich, Chem.
Eng. Sci., 2012, 79, 1–7.
51 D. Hu, Y. Li, J. Mei, G. Wang, C. Xiao, C. Liu, Q. Meng, H. Li,
S. Fu and A. Duan, Fuel, 2020, 263, 116701–116710.
52 P. Zheng, T. Li, K. Chi, C. Xiao, J. Fan, X. Wang and A. Duan,
Appl. Catal., B, 2019, 257, 117937–117947.
31 X. Wang, C. Xiao, J. Mei, M. H. Alabsi, Y. Shi, Z. Zhao, A.
Duan, K. W. Huang and C. Xu, ACS Appl. Mater. Interfaces,
2020, 12, 40404–40414.
32 F. Cesano, S. Bertarione, A. Piovano, G. Agostini, M. M.
Rahman, E. Groppo, F. Bonino, D. Scarano, C. Lamberti, S.
Bordiga, L. Montanari, L. Bonoldi, R. Millini and A.
Zecchina, Catal. Sci. Technol., 2011, 1, 123–136.
33 J. L. Pinilla, H. Purón, D. Torres, I. Suelves and M. Millan,
Carbon, 2015, 81, 574–586.
53 X. Zhu, L. L. Lobban, R. G. Mallinson and D. E. Resasco,
J. Catal., 2011, 281, 21–29.
54 G. Li, L. Li, L. Jin, Z. Tang, H. Fan and H. Hu, Fuel Process.
Technol., 2018, 178, 371–378.
55 R. Lin, S. Mitchell, T. Netscher, J. Medlock, R. T. Stemmler,
W. Bonrath, U. Létinois and J. Pérez-Ramírez, Catal. Sci.
Technol., 2020, 10, 4282–4292.
56 W. Wang, S. Tan, K. Wu, G. Zhu, Y. Liu, L. Tan, Y. Huang
and Y. Yang, Fuel, 2018, 214, 480–488.
This journal is © The Royal Society of Chemistry 2021
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