Synergy of carbon defect and transition metal on tungsten carbides for boosting the selective cleavage of aryl ether C[sbnd]O bond

Article abstract of DOI:10.1016/j.apcata.2021.118023

Transition-metal carbides are attractive heterogeneous catalysts for the transformation of lignin-derived compounds. However, the effects of the surface carbon defect (C-defect) remain unclear due to coke deposition and exposed C terminations. Herein, we transform inert tungsten carbides (WC) into active catalysts via surficial C-etching using transition metals in the presence of H₂. An optimized 1% Pt–WC catalyst with synergy of Pt and C-defects can perform a 78.6% conversion in the selective hydrogenolysis of guaiacol to phenol with 89.0% selectivity at 300 °C, while the intact WC only gives a 7.1% guaiacol conversion with 51.0% phenol selectivity under the same conditions. As a result, the 1% Pt–WC catalyst exhibits 19-fold higher reaction rate and 12.6-fold higher turnover frequency than those of WC. Moreover, no substantial loss of catalytic performance has been observed with 1% Pt–WC for 50 h on stream. A rational reaction pathway is accordingly proposed through in-depth characterizations.

Full text of DOI:10.1016/j.apcata.2021.118023

Applied Catalysis A, General 613 (2021) 118023
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Synergy of carbon defect and transition metal on tungsten carbides for
boosting the selective cleavage of aryl ether C O bond
–
Huihuang Fang, Lijie Wu, Weikun Chen, Youzhu Yuan*
State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters and iChEM,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Transition-metal carbides are attractive heterogeneous catalysts for the transformation of lignin-derived com-
pounds. However, the effects of the surface carbon defect (C-defect) remain unclear due to coke deposition and
exposed C terminations. Herein, we transform inert tungsten carbides (WC) into active catalysts via surficial C-
Transition metal carbide
Carbon defect
Transition metal
Synergy effect
etching using transition metals in the presence of H
2
. An optimized 1% Pt–WC catalyst with synergy of Pt and C-
defects can perform a 78.6% conversion in the selective hydrogenolysis of guaiacol to phenol with 89.0%
CO bond hydrogenolysis
◦
selectivity at 300 C, while the intact WC only gives a 7.1% guaiacol conversion with 51.0% phenol selectivity
under the same conditions. As a result, the 1% Pt–WC catalyst exhibits 19-fold higher reaction rate and 12.6-fold
higher turnover frequency than those of WC. Moreover, no substantial loss of catalytic performance has been
observed with 1% Pt–WC for 50 h on stream. A rational reaction pathway is accordingly proposed through in-
depth characterizations.
1
. Introduction
namely, Caryl
OH (414 kJ mol ), which are characteristic components of
lignocellulose [13]. The HDO of GUA involves the cleavage of aryl C
bond, aliphatic C O bond, and the hydrogenation of arene (Scheme 1),
(356 kJ mol 1), Caryl
ꢀ ꢀ 1
3 3
OCH O CH (247 kJ mol ) and
– –
ꢀ 1
C
–
aryl
Phenol is an important platform chemical and commonly used as an
–
O
intermediate in chemical industry such as pesticide production [1]. The
production of phenol is essentially based on the petroleum-based feed-
stocks by cumene process [2]. Lignocellulose is the abundant renewable
feedstock which is mainly composed of aromatics. Transformation of
lignocellulose into aromatic chemicals like phenol is an attractive and
alternative process for fossil energy substitution [3–7]. It is well known
that the fast pyrolysis is the most cost-effective process to produce
lignin-derived bio-oil, but these bio-oils are far from commercial use due
to the formation of complex oxygenated mixtures such as (alkyl-)
guaiacols, resulting in high viscosity, overfunctionlization and chemical
instability [8,9]. Hydrodeoxygenation (HDO) is a viable and effective
bio-oil upgrading process to remove extensive oxygen contents.
–
posting the importance and challenging of catalyst design towards
different value-added products. Apparently, the phenol production re-
quires the direct hydrogenolysis of Carylꢀ OCH
3
without the cleavage of
aryl
C O
–
CH OH and saturation of arene. Two common types of
3
, Caryl
–
HDO systems including batch liquid-phase HDO using autoclave and
continuous HDO using fixed-bed reactor are employed. Usually, the
liquid-phase HDO requires high H
2
pressures (1–20 MPa) to ensure
better solubility into the reaction media, the high cost of
H
2
high-pressure equipment and additional solvent requirement [14];
whereas the continuous HDO with a fixed-bed reactor can proceed at a
2
wide range of H pressures although sometimes requires high reaction
HDO of lignin-derived oxygenates into phenolics with high selec-
–
tivity via direct cleavage of aryl C O bonds remains challenging due to
strong bonding linkages in lignin compounds and the competing side
temperatures and it suppresses the recondensation of reactants and
further hydrogenation of aromatic ring [15]. Table S1 summarizes the
previous reports on the GUA HDO by various catalysts with fixed-bed
reactors. From the point of view of HDO products, a total HDO of
GUA and lignin-derived oxygenates into hydrocarbons that can be used
as bio-fuels and has been intensively studied. However, a partial HDO
into O-containing chemicals is more valuable in lignin utilization; the
reactions such as hydrogenation of aromatic rings and dehydroxylation
[
10–12]. Guaiacol (GUA) is the most typical compound in lignin-derived
oils and regarded as the ideal model molecule in HDO reactions. It
contains three types of C O bonds with different bond energies,
–
*
Corresponding author.
E-mail address: yzyuan@xmu.edu.cn (Y. Yuan).
https://doi.org/10.1016/j.apcata.2021.118023
Received 31 October 2020; Received in revised form 26 January 2021; Accepted 29 January 2021
Available online 8 February 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
process needs lower H
2
consumption and has higher industry value.
diffusion. This method effectively slows down carbon diffusion through
the solid–solid interface and then avoids the excessive formation of
surface carbon. On the other hand, Yang et al. [48] used oxygen plasma
treatment to remove surface carbon and activate WC and thus improve
Therefore, it is important and attractive to develop novel catalysts to
achieve high product selectivity in GUA HDO and understand the rela-
tionship between the structure and performance.
To date, supported metal catalysts are applied as common hydro-
genation catalysts in conversion of lignin biomass [16–22]. Previous
studies have shown that metal catalysts such as Pt [16,17], Pd [18], Ru
2
HER activity and methanol oxidation. Recently, W C nanorods with
abundant C-defects for HDO reactions have been reported [49]. Our
previous investigation has also demonstrated the significance of surface
[
19], Ni [20,21], and Cu [22] exhibit promising activity in trans-
formation of lignin-derived chemicals. Yet, the excellent ability for H
disassociation over these metals allows both of the cleavage of C
reconstruction on inert WC to promote C
–
O bond cleavage [50].
2
Importantly, Hunt et al. [51] synthesized metal-terminated tungsten
–
O
carbide nanoparticles on carbon supports by using SiO
as the template
4
and following
2
bonds and the saturation of aromatic rings, resulting in wide product
distribution, less value of products and excessive hydrogen consumption
and during the carburization in the presence of CH
etching treatment. This strategy allows the exposure of metal sites and
displayed 100-fold higher HER activities than commercial WC. There-
fore, the design of high-quality TMC-based materials (e.g., free from
surface carbon, controllable phase, and surface defects) is of great
importance. Another strategy for boosting TMC-based materials is by
incorporating second metals. Previous reports have shown that Au and
Cu supported on molybdenum carbides are highly active in (reverse)
[
23,24]. Therefore, constructing an active catalyst towards value-added
products like phenol in lignin utilization is important and highly
desirable.
Transition-metal carbides (TMCs) are considered as attractive ma-
terials in heterogeneous catalysis due to their outstanding catalytic
behavior [25–27]. TMCs are used in wide-ranging important reactions,
such as HDO, hydrogenation, isomerization, and hydrogen evolution
reactions (HER) [28–32]. TMCs also serve as excellent carriers for the
dispersion of metals due to the strong metal–support interaction and
modification of the electronic structure [33–35].
water–gas shift reactions due to their excellent capability for H
vation and C C/AC was adopted for the
O bond scission [52,53]. Ni–W
conversion of cellulose and lignin compounds and show remarkable
activities for selective C O bond cleavage [45,54]. Results indicate the
2
acti-
–
2
–
The TMC-based materials are typically synthesized by temperature-
strong synergistic interaction and surface modification between metals
and carbide supports. Thus, to construct active TMC-based materials by
introducing second metals is attractive and highly desired.
programmed reduction with gaseous carbon precursors, such as CH
4
,
3 8
C H
, and CO [36,37]. The insertion of carbon atoms into parent metals
induces the structural rearrangement and redistribution of the density of
state and then provides superior catalytic performance [38,39]. How-
ever, the rapid diffusion of carbons at the gas–solid interface normally
causes the deposition of carbon layer on the surface. Matus et al. [40]
found a graphite layer of approximately 2 nm thick with the formation of
Herein, we report a simple but powerful strategy to synthesize metal-
promoted tungsten carbide for the efficient hydrogenolysis of the aryl
–
ether C O bond. The key procedure is C-etching on inert WC surface by
metal-activated hydrogen species under thermal pretreatment. An as-
built Pt–WC catalyst exhibits unique catalytic activity through direct
Mo
2
C in the presence of CH
4
; Ma et al. [41] demonstrated the similar
aryl ether C O bond scission, whereas normal Pt/AC and Pt–WC
–
graphite layers on the carbide particles during the synthesis of
without C-etching facilitate ring saturation and show 19-fold higher
activity than intact WC. Other metals such as Pd, Ru, Ni, and Cu display
similar catalytic behaviors. Further designed experiments illustrate that
g
6 6 2
Co Mo C / C. These graphitic carbons block the pores and prevent the
exposure of active sites and the chemisorption of reactants, negatively
influencing the catalytic activity [39,42]. For example, Kimmel et al.
2
the incorporated metals serve as sites for H activation, whereas C-de-
[
42] synthesized WC with different monolayers of surface carbon and
fects are responsible for reactant adsorption. Finally, a rational reaction
pathway is proposed.
investigated the effect of these coated carbon layers. They found that the
catalytic performance decrease when the particles were coated by
several carbon layers. Carbothermal reduction was further proposed by
using solid carbon sources such as activated carbon, carbon nanotubes
2. Experimental
(
CNTs), and carbon fibers in the synthesis of metal carbides [43–45].
2.1. Catalyst preparation
Gong et al. [44] prepared phase-pure W C nanoparticles without coating
2
of surface carbon layer on CNTs and HER performance through carbon
diffusion from carbon nanotubes to metal tungsten. In addition, Xu and
Wu et al. [46,47] used metal–organic frameworks as precursors to
synthesize metal carbide nanoparticles by pyrolysis and carbon
The initial bulk WC was commercially available from Aladdin Co.
Ltd. Pt-promoted WC catalysts with different synthesis methods were
prepared as follows. In a typical procedure, 1.0 g of WC was dispersed in
50 mL of distilled water with sonication and continuous stirring for 30
Scheme 1. Reaction pathway for GUA HDO.
2

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
◦
2
at a heating rate of 10 C /min in H .
min. The required amount of H
2
PtCl
6
solution was added, and the
different Pt loadings were accordingly adjusted within the range of
0
.1%–5%. After stirring for 3 h, the mixed solution removed the excess
2
.3. Catalytic testing
◦
solvent under vacuum with an oil bath at 100 C, forming a gel-like
slurry. The slurry was left at room temperature for 12 h for further
diffusion of Pt precursors. Then the Pt supported precursors were ob-
The selective hydrogenolysis of GUA was carried out on a fix-bed
flow reactor with an auto-sampling system. In a typical procedure,
00 mg of fresh catalyst was sieved with a size of about 200 m and
loaded in the center of a quartz tubular reactor and sandwiched by
quartz sand. The catalysts were pretreated under 5% H /N at required
◦
tained after solvent removal under vacuum and dried at 100 C over-
2
μ
night. For C-etching thermal treatment, the catalysts were in situ treated
◦
◦
at 450 C with a heating ramp of 2 C/min for 2 h in the presence of pure
. The obtained catalysts were denoted as (0.1%–5%) Pt‒WC. In the
2
2
H
2
temperatures to minimize the effect of surface oxide species, and then
the catalyst bed was cooled to the target reaction temperature. Then,
designed experiments, the catalyst without C-etching was reduced at
◦
#
4
00 C for 2 h under 5% H
2
/N
2
, which was denoted as 1% Pt–WC . The
◦
2
pure H was fed into the reactor at 3.0 MPa followed by the introduction
catalysts with different C-defects were treated at 450 C for 0–6 h in the
presence of pure H
of GUA to the reactor by using a Series III digital HPLC pump (Scientific
Systems, Inc., USA) with the required weight liquid hourly space ve-
locity (WLHSV). An on-line Agilent 7890A gas chromatograph (GC)
equipped with an auto-sampling value, DB-Wax capillary column, and
flame ionization detector was used for product analysis. A GC 2060 with
a TDX column and thermal-conductivity detector was used to analyze
2
. The catalyst with almost W terminations was
◦
treated at 850 C for 2 h, denoted as 1% Pt–WC-850. All catalysts were
used directly in the conversion of GUA or stored under N
2
to prevent
additional oxidation of the carbide surface.
2
.2. Catalyst characterization
4 2 2
gas products, such as CO, CH , CO , and H O. The absence of heat and
mass transfer limitations were confirmed by the estimated Thiele
modulus and Mears criterion [55–57]. GUA conversion, product selec-
tivity and carbon balance were calculated by the following Eqs. (1) and
X–ray diffraction (XRD) patterns were obtained from a Rigaku Ul-
tima IV X–ray diffractometer equipped with Cu–K radiation (40 kV and
α
◦
◦
3
0 mA) at a scanning 2θ range of 10 –90 . The JCPDS database was used
(
2).
to identify the obtained diffraction data. Transmission electron micro-
scopy (TEM) and high-angle annular dark field-scanning TEM
(m oles of GUA)in ꢀ (m oles of GUA)out
Conversion =
× 100%
× 100%
(1)
(2)
(
HAADF–STEM) with energy-dispersive spectroscopy (EDS) were per-
(m oles of GUA)
in
formed with a Philips Analytical FEI Tecnai 20 electron microscope at an
acceleration voltage of 200 kV. Fresh samples were dispersed ultrason-
ically and then dropped and dried on copper grid with lacey support
films. In situ X–ray photoelectron spectroscopy (XPS) data were ob-
tained on an Omicron Sphera II photoelectron spectrometer equipped
m oles of ring product i
Product Selectivity =
the sum m oles of GUA consum ed
The reaction rate for GUA HDO was calculated by Eq. (3).
F
k = ln
m
1
with an in situ chamber and an Al–K
α
X–ray radiation source (h
ν=
(3)
1 ꢀ X
1
2
486.6 eV). The binding energy was calibrated using the C 1s peak at
84.5 eV. High-sensitivity low-energy ion scattering (HS–LEIS) profiles
where k represents the rate constant expressed as moles of GUA
consumed per second and per gram of catalyst, F is the molar flow rate of
GUA, m represents catalyst mass, and X is the GUA conversion. All the
catalysts were evaluated twice to ensure the accuracy of measurements
were obtained on an Ion-TOF Qtac100 instrument. He was selected as
the ion source at a kinetic energy of 3 keV with an ion flux of 6000 pA
–
2
m
and a spot size of 2000
μ
m × 2000
μ
m to obtain surface information
and minimize surface damage. The C/W atomic ratio on the surface was
(
the errors are below 5%). The activation energies calculated by the
calculated by integrating the relative intensity of W and C from HS–LEIS
Arrhenius plots were based on the performance at different tempera-
tures on the condition of conversions below 40%. The turnover fre-
and XPS measurements. Before XPS experiments, the samples were
◦
pretreated with 5% H
2
/N
2
at 400 C for 4 h to ensure these samples are
quency (TOF) values were based on the H
2
and CO chemisorption
in similar states with fresh ones before the activity test. The C/W atomic
ratio on the surface was calculated by integrating the relative intensity
of W and C from HS–LEIS measurement. To eliminate the interference of
surface oxidation as much as possible, the data were collected based on
the deconvoluted spectra by excluding the influence of oxide species. To
eliminate the interference of surface oxidation as much as possible, data
were collected based on the deconvoluted spectra by excluding the in-
uptake, indicating the moles of GUA converted by per site at the catalyst
ꢀ 1 ꢀ 1 ꢀ 1
h , for short, s ). The GUA
surface per second (mol-GUA mol-site
conversion for the TOF calculation was lower than 10%. The carbon
balance was calculated by Eq. (4). Unless otherwise noted, the carbon
balance was about 95% ± 2%.
sum of m oles of C in all com pounds in the effluent
Carbon balance =
(4)
fluence of oxide species. CO and H
2
temperature-programmed desorp-
m oles of C in the GUA feed
tion (TPD) measurements were carried out with a Micromeritics
AutoChem II 2920 chemisorption analyzer. Before tests, the sample was
in situ pretreated under the condition similar to that of the activity
evaluation of the catalysts. Temperature-programmed reduction-mass
spectrometry (TPR–MS) was conducted on a Micromeritics AutoChem II
3. Results and discussion
3.1. Construction of Pt–WC with C-defects
2
920 chemisorption analyzer equipped with a mass spectrometry. The
Pt–WC was synthesized by combining methods including impreg-
nation for Pt loading and careful H thermo-treatment for C-etching
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of
GUA adsorption was conducted on a Nicolet 6700 spectrometer equip-
ped with a mercury cadmium telluride detector cooled by liquid nitro-
gen. Prior to adsorption, the catalyst samples were in situ reduced under
2
(Scheme 2). The inert bulk WC, commercially available from Aladdin
Co. Ltd., was selected as support, and Pt species was incorporated by wet
impregnation. This step was followed by C-etching through the reaction
the same conditions as that of fresh catalyst. Spectra recorded under N
2
of H
2
and carbons under thermo-treatment (detailed in Experimental
flow were used as reference. Then, GUA was introduced by dropping
trace liquid onto the powder sample. Next, the cell was evacuated to
section). XRD patterns (Fig. S1) showed that the initial WC displays
◦
◦
◦
◦
typical diffractions located at 2θ = 31.70 , 35.89 , 48.65 , and 66.23
◦
remove excess GUA at 80 C until the spectra became constant. Ther-
(PDF#65-4539). No obvious diffractions ascribed to Pt species was
found in representative 1% Pt–WC catalysts, indicating the good
dispersion of Pt species on WC support. For confirming the role of Pt
species in the C-etching treatment, TPR–MS measurement was carried
mogravimetric analysis (TGA) was carried out on a NETZSCH-TG209 F1
analyzer to evaluate the weight loss of the precursor upon calcination.
◦
◦
During the measurements, the samples were heated from 50 C to 800 C
3

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
Scheme 2. Illustration of the synthesis of active Pt–WC catalyst.
out as shown in Fig. S2. 1% Pt–WC shows a significant CH
4
signal which
and carbon
was found and no obvious
was observed. In contrast, the formation of CH could be
peak is located at about 520
distances of 0.252 nm, corresponding to the WC(100) facet (Fig. 1a). By
contrast, the surface of 1% Pt–WC roughened and acquired rich stacking
faults and black spots (Fig. 1b). The lattice fringe was 0.236 nm, which
◦
is located at about 400 C, which is due to reaction of H
2
atoms of WC (Fig. S2a). In this case, only CH
signal of C
4
2
H
6
4
was primarily ascribed to the W
2
C(002) facet. This finding was due to
to form C-defects, which
also observed in WC catalyst, while the CH
4
the gasification of carbon atoms with H
2
◦
C, which is much higher than that of 1% Pt–WC (Fig. S2b). That is to
and then hydroge-
benefited the adsorption and activation of reactants. STEM–EDS
elemental-mapping analysis demonstrated that almost all Pt species
were distributed in the W domain with high dispersion (Fig. 1c and d).
say, the Pt species is beneficial for activation of H
2
nation of C atoms of WC, achieving C-defects on their surface of WC
surface. To further investigate the structures, TEM and HAADF–STEM
images were collected, as shown in Fig. 1. WC showed a smooth surface,
clear atom formulation, and typical lattice fringes with interplanar
For WC and Pt-WC catalysts, N
2
sorption was further studied as shown in
Fig. S3 and Table S2. The BET surface area (SBET) of commercial WC
demonstrates a small surface area of 2.9 m2
g
ꢀ 1
with an average pore
Fig. 1. TEM images of (a) WC and (b) 1% Pt–WC; HAADF-STEM image (c) and EDS elemental mappings of (d) 1% Pt–WC.
4

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
diameter of 12.7 nm. With further C-etching treatment over the Pt–WC
subjected to integrated H
2
treatment activated the WC surface with rich
catalysts, the surface area has a slight increase from 2.9 to 4.0 m2
g
ꢀ 1
C-defects and W-terminations. These C-defects benefited the exposure of
active hollow sites and adsorption of reactants. Therefore, supported
Pt–WC with well-dispersed Pt species and rich C-defects was successfully
synthesized.
with average pore diameter in the range of 13.1–15.0 nm (Table S2).
TGA Results in Fig. S4 revealed easier weight loss in 1% Pt–WC than in
WC, indicating the promotion of C-etching through the incorporation of
Pt into 1% Pt–WC.
To gain insight into surficial properties, XPS and HS–LEIS analyses
were carried out. As shown in Fig. 2a, the W 4f profile in WC had two
typical peaks centered at 31.8 and 33.7 eV, corresponding to carbide
species (Fig. 2a). The carbide peaks shifted to lower binding energy with
3.2. Synergy of Pt and C-defects for guaiacol transformation
The hydrogenolysis of GUA was selected as the typical reaction for
–
aryl ether C O bond scission because it is a structure sensitive reaction
3
1.6 eV in the 1% Pt–WC catalyst. This finding implied that W had
for carbide-based catalysis. As shown in Fig. 3 and Table S4, WC dis-
played relatively low GUA conversion (7.1%) with poor phenol selec-
tivity (51.0%), whereas GUA conversion and phenol selectivity
significantly increased over 1% Pt–WC [Unless otherwise noted, 1%
Pt–WC represents the catalyst with C-etching under 450 C under H for
4 h (Table S5, Entry 5)], reaching 78.6% and 89.0%, respectively. WC*
higher electron density due to the removal of surrounding C atoms and
decrease in coordination number. As evident in Fig. 2b, the deconvo-
luted C 1s profile demonstrated a significant decrease in carbide species
with unchanged graphitic carbon species. This result indicated the
successful C-etching and the formation of low-C interstitial structure on
the outer surface of WC, confirming that photoelectrons escaping depth
can be penetrated up to a few nanometers. Accordingly, HS–LEIS was
conducted to confirm the information on the topmost layer. An obvious
decline in C intensity was found (Fig. S5). As summarized in Table S3 for
quantitative analysis, the C/W atomic ratios in WC determined by XPS
and HS–LEIS were approximately 1.12 : 1 and 16.3 : 1, respectively. The
C/W atomic ratio by XPS was similar to the phase composition of bulk
WC; however, the ratio found by HS–LEIS was relatively high, indicating
abundant C-terminations on the outer surface. By contrast, 1% Pt–WC
showed decreased C/W ratios of 0.66 : 1 and 5.3 : 1, as determined by
XPS and HS–LEIS, respectively. These results revealed that 1% Pt–WC
◦
2
◦
with C-defects was constructed at 600 C under H2 for 4 h (Table S5).
The catalytic activity for GUA hydrogenolysis of WC* shows an
enhancement than that of intact WC, reaching 15.2% of GUA conversion
and 72.2% of phenol selectivity, respectively. However, the activity is
still much lower than that of 1% Pt–WC. That is to say, the incorporation
of Pt species enhances the hydrogenolysis of GUA compared with the
WC* with C-defects along. This is presumably due to the improved
activation of H on Pt sites and adsorption of GUA on C-defects, which
2
shows a synergistic effect over the 1% Pt–WC catalyst. In contrast, 1%
#
Pt–WC without C-etching treatment was prepared and found to have
low GUA conversion (39.3%) with diverse products. The selectivity of
phenol through C O bond cleavage was 56.7%, and ring-saturated
–
products including cyclohexanol, cyclohexenol, and cyclohexane were
also formed. On the other hand, 1% Pt/AC with the same Pt loading
demonstrated satisfactory GUA conversion but relatively different
product distribution compared with 1% Pt–WC, primarily producing
ring-saturated products. Fig. S6 shows the effect of WLHSV on the per-
formance of 1% Pt–WC and 1% Pt/AC. The GUA conversion over the 1%
Pt–WC decreases from 98.7% to 30.9% with a slight fluctuation of
phenol selectivity in the range of 87% and 93% when the WLHSV was
ꢀ
1
increased from 0.5–10.0 h
(Fig. S6a). Notably, no ring-saturated
products nor obvious change on product distribution was were found
Fig. 3. Performance for GUA conversion over different catalysts. WC* was
◦
synthesized by construction of C-defects on intact WC at 600 C under H
2
for 4
#
h. 1% Pt–WC was synthesized by wet impregnation without C-etching treat-
–
1
ment. Reaction conditions: WLHSV=3.0 h , P (H
2
) = 3.0 MPa, H
2
/GUA molar
Fig. 2. XPS profiles of WC and 1% Pt–WC: (a) W 4f and (b) C 1s.
ratio = 50, and T =300 ◦C.
5

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
with change of WLHSV. On the other hand, the 1% Pt/AC displays a
decreasing GUA conversion and the product distribution varied with the
increase of WLHSV (Fig. S6b). The cyclohexenol selectivity show a
volcano-type curve with decreasing cyclohexanol and increasing phenol
ꢀ
1
selectivity when the WLHSV was increase from 1.0 to 10.0 h , which is
presumably due to the shorter residence time of reactants on the surface
of 1% Pt/AC. To further investigate the roles of these catalysts on the
product selectivity, we evaluated the catalytic performance with similar
GUA conversion by varying the WLHSV, as shown in Fig. S7. The 1%
Pt–WC demonstrates a high selectivity of phenol while other catalysts
display wide product distributions although they have similar GUA
conversion. Accordingly, the as-built active 1% Pt–WC facilitated the
selective cleavage of aryl ether C O bonds toward phenol production.
–
Fig. 4 shows the phenol space–time yield (STY) over the 0–5.0% Pt–WC
catalysts with similar C-etching treatment. WC alone displayed rela-
–1
–1
tively low phenol STY (0.25 umol gcat
added to WC, the phenol STY, as well as GUA conversion and phenol
selectivity, drastically increased and reached the maximum of 4.77 mol
when the Pt loading was 1 wt.% (Fig. 4 and Table S4). The in-
s
). With increased amount of Pt
μ
– –1
1
Fig. 5. Arrhenius plots of reaction rate (Lnk) versus 1/T for GUA hydro-
genolysis over WC and 1.0 % Pt–WC.
g
cat
s
crease in activity was approximately 19.1-fold compared with that of
intact WC. The TOF values were further calculated to quantify the effi-
cyclohexane formed in all Pt/AC catalysts, even in the case of 0.1%
–
Pt/AC. Thus, C O bond scission and arene reduction occurred, and no
specific selectivity of products was observed among Pt/AC catalysts.
ciency of these active sites based on the H
2
and CO chemisorption up-
) and
takes, as summarized in Table S6. The intact WC shows low TOF(H
2
ꢀ 1
TOF(CO) values of 0.9 and 0.7 s , respectively. In contrast, the WC*
with C-defects displays a slight increasing TOF values since the con-
struction of C-defects improve the activation of reactants including GUA
Previous studies have shown that the Ea over Pt/C is about 93–126 kJ
ꢀ
1
mol
[17,63,64]. We performed the Arrhenius plot of as-prepared
ꢀ 1
Pt/AC and the Ea is about 93.2 kJ mol (Fig. S8). Even though the
Pt/AC displays similar Ea compared with that of Pt–WC, the phenol
selectivity is quite poor, which indicates the uncontrollable product
selectivity in GUA HDO over the Pt/AC. These results are corresponding
with previous reports [58,59]. It is worth noting that Pt–WC catalysts
and hydrogen. Significantly, there is a great increase on TOF(H
2
) and
ꢀ 1
TOF(CO) values over the 1% Pt–WC, giving values of 11.4 and 9.0 s
,
respectively, which are nearly 12.6-fold higher than those of intact WC.
The results indicate that the active sites contributed by construction of
C-defects and synergy of Pt incorporation achieve superior catalytic
behaviors for GUA conversion. Considering the great enhancement in
phenol production, Arrhenius plots were used to derive the apparent
activation energy (Ea). The intact commercial WC displays an Ea of
with the same Pt loading attained only the products with C O bond
–
cleavage, especially for phenol production. Moreover, Pt–WC catalysts
showed higher conversion than Pt/AC catalysts. This finding was pre-
sumably due to the good dispersion of Pt on the WC surface and the
C-defects for reactant adsorption, thereby altering the reaction mecha-
nism via synergistic effect. Further increasing the amount of Pt in 5%
Pt–WC resulted in obviously declined phenol selectivity, and
ring-saturated products formed (Table S7). As evidenced in Fig. S9, the
typical TEM image of 5% Pt–WC showed that extensive Pt species
overlapped with the WC surface, which can cover available C-defect
sites and greatly enhance the activation of dihydrogen, resulting in
similar catalytic property to that of 5% Pt/AC. Thus, the rational
incorporation of Pt species onto the WC surface with C-defects benefited
the activation of dihydrogen and reactants, thereby inducing the pro-
–
1
1
71.6 kJ mol (Fig. 5), which is higher than those of tungsten carbide
–1
catalysts in HDO reactions in a range of 120ꢀ 152 kJ mol [50,58,59].
This is due to the lack of active sites on the surface of commercial WC.
However, the Ea over 1% Pt–WC was 91.8 kJ mol– , which was almost
half of that on initial WC, indicating the successful construction of active
1
sites and thus resulting in a high efficiency of C O bond cleavage over
–
the C-etched 1% Pt–WC catalyst.
Supported Pt catalysts are commonly used for hydrogenation and
also for HDO reactions. Both saturation of aromatic ring through hy-
–
drogenation and the C O bond cleavage occur during the trans-
formation of lignin-derived compounds, resulting in wide product
duction of phenol via C O bond cleavage.
–
distributions [60–62]. In our case, Pt supported on activation carbon
To further investigate the effect of C-defects, 1% Pt–WC catalysts
with different C-etching treatments were fabricated and evaluated for
GUA conversion. The details of C-etching are summarized in Table S5.
Fig. 6 demonstrates the phenol STY as a function of surface C/W atomic
ratio. The changes in phenol STY showed a volcano-type trend with
increased C/W atomic ratio. At high C/W atomic ratio, the surface had
exposed abundant C terminations and was covered by numerous carbon
atoms during the carbide preparation; these carbons prevented the
(
Pt/AC) with different Pt loadings was also prepared and evaluated for
GUA conversion for comparison, as shown in Table S7. As predicted,
ring-saturated products such as cyclohexanol, cyclohexenol, and
#
chemisorption of reactants. The 1% Pt–WC catalyst without C-etching
had an atomic ratio of 14.3 C/W with 1.51
–
1
–1
μ
mol gcat s . C-etching to
decrease the C/W atomic ratio, thereby facilitating the formation of C-
defects and thus the activation and chemisorption of reactants. Conse-
quently, the phenol STY significantly increased and reached the
maximum value at the C/W atomic ratio of 7.5. Further decreasing the C
atoms resulted in a decline in the phenol STY, which was due to the
exposure of abundant W atoms. The 1% Pt–WC-850 catalyst (Table S5,
◦
Entry 7) with extensive C-etching at 850 C obtained a low phenol STY
Fig. 4. Phenol space–time yield (STY) over 0–5.0% Pt–WC with similar C-
etching treatment. Note: Low phenol STY over 5%Pt–WC was due to the for-
mation ring-saturated products (Table S3). Reaction conditions: WLHSV=3.0
with catechol as by-product (Table S8). This finding was due to the
formation of a metallic W phase, as confirmed by the XRD profile
2
–
1
/GUA molar ratio = 50, and T =300 ◦C.
(Fig. S10), in agreement with a previous report. These results revealed
h
, P (H
2
) = 3.0 MPa, H
2
6

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
Fig. 6. Phenol STY for GUA conversion as a function of surface C/W atomic
ratio in 1.0% Pt–WC catalysts with different C-etching treatments. Reaction
–
1
conditions: WLHSV=3.0 h , P (H
2
) = 3.0 MPa, H /GUA molar ratio = 50, and
2
T =300 ◦C.
the significance of rational surface-carbon removal to construct C-de-
fects. Therefore, the rational surface configuration between W and C
atoms is highly important to afford numerous hollow C-defects and
strong Pt incorporation for the promotion of phenol production.
3
.3. Understanding the origin of catalytic performance
Chemisorption behavior on catalysts is worthy of study to gain in-
sights into the proper reaction mechanism. The HDO of GUA involves
the activation and adsorption of H and GUA on the active sites of
catalyst surface. In our case, H and CO were used as typical probe
molecules, and the TPD profiles are shown in Fig. 7. The intact WC
showed nearly no H adsorption on the surface due to surface C termi-
2
2
2
Fig. 7. H
2
–TPD (a) and CO–TPD (b) of WC-based catalysts.
nation and the lack of hollow C-defects. However, 1% Pt–WC displayed a
◦
significant desorption peak located at 78 C with a shoulder peak at 130
◦
C, indicating better activation of H
2
and hydrogen bonding on the
adsorption behavior on these catalysts. Fig. 8 shows the typical spectra
–
1
surface. In addition to similar peaks at low temperatures, 5% Pt–WC and
of GUA between 1050 and 1650 cm . The initial WC displayed several
–
–
typical peaks that contributed to the aromatic C C bond, OH group,
5
3
% Pt/AC catalysts showed an extra-large H
2
desorption peak at about
–
◦
aryl–OCH
–1
vibrations at 1500, 1362, 1259, and 1225 cm ,
20 C; this peak was ascribed to the stronger adsorption of hydrogen
and C
3
species on the two catalysts, thereby promoting the activation and
reduction of aromatic rings. These results were consistent with the
catalytic performance of Pt/AC and 5% Pt–WC. It is well accepted that
CO can be used as the typical probe molecule to investigate the
adsorption behavior of oxygenates on catalysts, especially on the surface
of metal carbides [27,38,65,66]. Previous studies show that CO binding
with C end down was favored on the metallic sites [27]. Lee et al. [65,
respectively [68]. No shift on wavenumbers was observed in this case,
indicating the weak interaction and only physical adsorption of GUA on
#
–1
the WC surface. Over 1% Pt–WC , a new band at 1494 cm was
–
1
detected with the disappearance of 1500 cm ; on the other hand, the
OH group vibration was unchanged. This finding indicated the pres-
ence of a direct electrodonation from the catalyst surface to the aromatic
–
ring. The C
aryl–OCH3 position slightly shifted to higher wavenumbers
(1271 cm 1) due to the interaction between methoxyl groups and the
–
6
6] studied the CO chemisorption behavior and used CO chemical
titration to investigate the active sites of Mo C in HDO of anisole
containing the Caryl OCH functional group), which is quite similar
with GUA HDO. They found that the hydrogenolysis rate of Caryl OCH
catalyst surface. Results suggested the strong adsorption of aromatic
2
#
–
ring and the interaction of C–O bond on 1% Pt–WC . Interestingly, 1%
(
3
–
Pt–WC with rational C-etching demonstrated different GUA adsorption
3
–
1
behaviors. For the Caryl
–
OCH vibration, the peak at 1225 cm dis-
3
bond significantly decreases with introduction of CO and they can be
restored after removing CO, indicating the CO reversible poisoning of
the active sites. As indicated by the CO–TPD measurements (Fig. 7b), CO
–
1
appeared and a new band at 1253 cm was found, which was assigned
to the vibration of asymmetric C–O bond. Moreover, the 1259 cm
–
1
◦
–
desorbed as an obvious peak at 85 C on the WC catalyst, whereas 1%
signal shifted to higher wavenumbers with unchanged OH group vi-
O bond
bration. These results illustrated the strong adsorption of C
–
Pt–WC displayed a broader and larger peak. A previous report has shown
that CO binding with a C end down is favored on metallic sites [67].
Thus, 1% Pt–WC exposed numerous metallic sites such as tungsten on
the surface. Conversely, 5% Pt–WC and 5% Pt/AC showed large CO
desorption peaks at similar temperatures, as well as a tiny peak at high
temperatures. These results indicated the strong interaction between the
substrates and catalysts, favoring the further conversion of
intermediates.
–
1
over 1% Pt–WC. The position at 1500 cm also remained. The slight
–
1
shift to higher wavenumber (1500 cm ) can be recognized as the
electron-withdrawing effect on the aromatic by the C–O bond adsorp-
tion. Thus, 1% Pt–WC showed strong adsorption of Caryl–OCH3 but
weak interaction with aromatic ring, resulting in boosted C–O bond
scission.
In situ DRIFTS analysis was carried out to further investigate the
7

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
the production of phenol.
Based on our results and above discussion, appropriate reaction
pathways were proposed, as shown in Scheme 3. In the case of 1%
Pt–WC, the incorporation of Pt species followed by the thermal treat-
ment allows the successful carbon etching on the outer surface of intact
WC which can be confirmed by the XPS and HSꢀ LEIS analysis. These C-
defects benefited the adsorption and activation of C O bond, which can
–
be observed clearly from DRIFTS profiles. Besides, the incorporated Pt
shows a better adsorption and activation of H , as shown in Fig. 7a. As
2
depicted in Scheme 3a, over 1% Pt–WC, the methoxyl groups in GUA
were trapped in hollow C-defects and bound to the exposed W sites due
to the superficial C-defect sites and then the aryl ether C
weakened. The Pt species were responsible for the greatly enhanced
activation and dissociation of H . Thus, hydrogen facilitated the attack
of the O atom to efficiently form methanol and phenol. The high
selectivity of the C O bond cleavage was due to the weak interaction
–
O bond
2
–
between the Pt surface and aromatic ring. It is worth noting that 5%
Pt–WC displays low phenol selectivity with ring-saturated products as
by-products, which is due to the extensive activation of H , resulting in
2
further hydrogenation of aromatic ring. On the other hand, extensive C-
etching over the 1% Pt–WC-850 leads to the formation of metal W phase
with almost W atoms on the outer surface of catalyst and demonstrates a
low phenol selectivity. That is to say, the balance between the C-defects
and the incorporated Pt species with rational compositions achieve
highly-efficient cleavage of Caryl
phenol. Meanwhile, Pt NPs/WC and Pt/AC were preferred for the
adsorption of the aromatic ring with interaction, as well as the weak
adsorption of the C O bond, inconsistent with previous reports. As
shown in Scheme 3b, two reaction pathways involving ring saturation
and C O bond cleavage occurred in parallel. The 2-methoxyl-cyclohex-
anol that originated from the ring saturation can be further hydroge-
nated by C O bond cleavage. Phenol produced from another pathway
–
OCH bond and thus high yield of
3
π–π
–
–
–
can be strongly absorbed on the Pt surface and hydrogenated further,
resulting in low efficiency for catalytic transformation and diverse
product distribution. Therefore, 1% Pt–WC with suitable C-etching
demonstrated a catalytic activity different from that of normal Pt cata-
#
Fig. 8. (a) DRIFTS measurement of GUA absorbed on WC, 1% Pt–WC , and 1%
Pt–WC. (b) Adsorption behavior schematics.
lysts. Pt activated the WC surface to construct C-defects for strong C
bond adsorption, replacing the activated aromatic rings on the Pt sur-
face. The exposed Pt surface enhanced the H activation, achieving high
production of phenol through C O bond cleavage.
–
O
2
3
.4. Appropriate reaction mechanism over 1% Pt–WC
–
The GUA contains three types of C
energies, including Caryl OCH
design of catalysts achieves regioselective cleavage of C
–
O bonds with different bind
CH and Caryl OH. Rational
O bonds to-
3
.5. Catalytic stability
–
, CarylO
–
–
3
3
–
It is well known that the stability of a catalyst is one of the important
wards different products. Previous reports demonstrate that molybde-
num carbides and tungsten carbides are efficient for direct cleavage of
issues. The susceptibility of TMC-based catalysts to undergo surface
oxidation, phase change, particle sintering, and coke deposition nor-
mally results in limitation for further industrial applications. We have
investigated the stability of 1% Pt–WC during the conversion of GUA.
Fig. 9 displays the GUA conversion and phenol selectivity over the 1%
C
–
aryl
OCH
the distinct catalytic behaviour [59,69–75]. Moreira and co-workers
73] showed that a high selectivity of phenol can be obtained in GUA
HDO without benzene production over the Mo C/CNF, which is in
3
bond in GUA conversion to produce phenol, which is due to
[
2
◦
Pt–WC as a function of reaction time on stream at 300 C under a high
WLHSV of 9 h . It can be seen that the initial GUA conversion is 38.7 %
and there is only a slight decrease after the reaction on stream for 50 h.
agreement with the previous studies in tungsten and molybdenum car-
bide catalysts [71]. Ochoa et al. [74,75] investigated the influence of
carburizing time of carbides for GUA HDO and posted the importance of
carburization process. Our previous results also showed that different
carbide compositions can catalyse the HDO of GUA with different
product selectivity [29,50]. Metal W displays high catechol by cleavage
ꢀ
1
of Caryl
by cleavage of Caryl
and benzene which indicating the non-selectivity on the cleavage of aryl
O bonds. Similarly, Tran et al. [59] investigated the bimetallic
MoWC carbide for GUA HDO and demonstrated that the introduction of
W into Mo C increased the oxophilicity to oxygenated reactants and
-activating sites, thus inducing the further transformation of phenol to
O
–
CH
3
, where the W
x
C (1 < x < 2) achieves high yield of phenol
–
OCH . In contrary, W
2
C produces phenol, anisole
3
C
–
2
H
2
benzene. These results reveal the phase control and surface construction
of metal carbides are of great importance in product selectivity in GUA
HDO. Suitable C/W ratio on the outer surface of catalyst is beneficial for
Scheme 3. Suggested pathway of GUA HDO over (a) 1% Pt–WC and (b) 1%
#
Pt–WC or 1% Pt/AC.
8

H. Fang et al.
Applied Catalysis A, General 613 (2021) 118023
3
for the selective cleavage of aryl OCH bond to produce phenol. The
–
intact WC displayed 7.1% GUA HDO conversion with 51.0 % phenol
selectivity, whereas the optimized 1% Pt–WC catalyst with C-defects and
Pt on WC afforded 78.6% GUA conversion and 89.0% phenol selectivity
◦
–1
under the reaction conditions of 3.0 M P H
2
, 300 C and 3.0 h GUA
O bond cleavage of
WLHSV. The phenol formation rate through the C
–
GUA over 1% Pt–WC was 19 times higher than that over intact WC. The
GUA conversion and phenol selectivity over 1% Pt–WC were maintained
for at least 50 h without considerable changes. Other transition metals
like Ru, Pd, Ni, and Cu could also be employed to activate the inert WC
by creating the C-defects and thus generating the synergy effects for
hydrogenolysis catalysis. We envision that our findings may provide a
deeper understanding on surface catalysis and new approaches to the
activation and application of TMCs.
Contribution
H.H. developed the sample synthesis approach and performed most
of the experiments. L.J. helped to carry out the experiments in early
stage. W.C provided help in the TEM measurements. Y.Y. designed and
guided the work. H.H. and Y.Y. wrote and revised the manuscript. All
the authors read and commented on the manuscript.
Fig. 9. GUA conversion and phenol selectivity as a function of reaction time on
–1
stream over the 1% Pt–WC. Reaction conditions: WLHSV=9.0 h , P (H ) = 3.0
2
MPa, H
2
/GUA molar ratio = 50, and T =300 ◦C.
The catalytic performance remains at a GUA conversion of 35.3 %,
which is comparable with the initial activity. The selectivity towards
phenol slightly fluctuates in the range of 86.3% and 89.6% during the
reaction. Thus, our as-built 1% Pt–WC exhibits a satisfied stability in the
hydrogenolysis of GUA. XRD patterns of fresh and spent catalysts are
shown in Fig. S11. There is no obvious changes in diffractions of tung-
sten carbides and Pt species, indicating no obvious phase deconstruction
or particle sintering in our case. TEM analysis was conduct to further
study the spent catalyst and the image is shown in Fig. S12. No obvious
particle sintering was found in the used 1% Pt–WC, which is presumably
due to the strong metal-support interaction between Pt and WC. Fig. S13
shows the XPS profiles of fresh and spent catalysts. The carbidic species
are clearly observed without change in both catalysts, although a slight
increase in intensity ascribable to oxide species is detected for the spent
catalyst, most probably due to surface oxidation. These results reveal
that the 1% Pt–WC possess a satisfying tolerance on structure changes
during the HDO of O-containing GUA. Briefly, the results demonstrate
the 1% Pt–WC catalyst shows a good performance and stability for the
selective HDO of GUA towards phenol production.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Key Research and Devel-
opment Program of China (2017YFA0206801 and 2018YFB0604701),
the National Natural Science Foundation of China (21972113), and the
Program for Innovative Research Team in Chinese Universities
(
IRT_14R31).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2021.118023.
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.6. Catalytic performance over the supported metal–WC with C-etching
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