W2C nanorods with various amounts of vacancy defects: Determination of catalytic active sites in the hydrodeoxygenation of benzofuran

Article abstract of DOI:10.1039/c6cy02702d

Transition metal carbides have been of great interest because of their noble-metal-like properties. Because of the complexity of their structures, it is crucial to design an experiment that can eliminate the influence of supports, surface carbon contamina

Full text of DOI:10.1039/c6cy02702d

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ISSN 2044-4753
PAPER
Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
rsc.li/catalysis

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W C Nanorods with Various Amounts of Vacancy Defects:
2
Determination of Catalytic Active Sites in Hydrodeoxygenation of
Benzofuran†
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
b
, a
, b
Rong Liu, Min Pang, Xiaozhen Chen, Chuang Li, Chunjian Xu,* Changhai Liang*
DOI: 10.1039/x0xx00000x
Transition metal carbides have been of great interest because of
their noble-metal-like properties. Because of the complexity of
their structures, it is crucial to design an experiment that can
eliminate the influence of supports, surface carbon
contamination and particle sizes when finding the exact catalytic
www.rsc.org/
active sites. In this work, phase-pure W
2
C nanorods (lengths of 2-
4
µm and diameters of 100-600 nm) with different amounts of
crystal defects were prepared by pyrolysis of metatungstate and
melamine hybrid nanorods with nanoscale periodic structure
synthesized in the aqueous phase. The nanoscale alternating
structure between tungsten oxide and melamine effectively promotes the reduction of tungsten oxide and the formation
of tungsten carbide, avoiding the deposition of carbon on the surface. At the same time, vacancy defects are generated
due to the deficiency of carbon. High pyrolysis temperature (900 °C), prolonged pyrolysis time (4 h), introduction of
hydrogen and proper increase of temperature ramping rate (5 °C/min) are favorable to the formation of vacancy defects.
The activity of different catalysts was evaluated by the hydrodeoxygenation of benzofuran at 320 to 350 °C. The results
2 2
show that the vacancy defect sites in W C are the key to the high reactivity of W C. The vacancy defect sites have favorable
properties on the cleavage of carbon-oxygen bonds. Caromatic-O bond is cleaved in the case of unsaturated aromatic ring,
thereby reducing the consumption of hydrogen. In addition, it is found that the apparent activation energy of each
chemical bond is linear-like positively related to its bond dissociation energy.
chemistry, experimental studies have seldom addressed this
issue and the elucidation of structure–activity relationships for
1
. Introduction
2
0
1
these materials is generally lacking. It is also important to
recognize that real catalyst surfaces can be very different from
the ideal model surfaces used in calculations. TMCs have
proven to be a class of challenging materials to work with due
to complications related to (i) phase purity, (ii) surface carbon,
(iii) surface termination, (iv) surface oxides, (v) surface defects,
Since Levy and Boudart found that tungsten carbides display
platinum-like behavior in a series of catalytic reactions, early
transition metal carbides (TMCs) have been investigated for
decades in the fields of catalysis. Extensive studies have shown
that early TMCs are catalytically active for reactions such as
2
3
4
hydrogenation,
hydrogenolysis,
dehydrogenation,
2
1
5
isomerization, methane to syngas, ammonia decomposition,
6
7
and (vi) surface roughness/area. The catalytic activity of
TMCs has been influenced by all these factors. Being able to
8
-11
12-14
deoxygenation,
hydrogen evolution reaction,
1
and
7, 17-19 decouple the influences of these six variables is of utmost
importance for understanding and optimizing the catalytic
5, 16
oxygen reduction reaction.
A number of studies
about relationship of the structure and catalytic behaviors has
been performed on experimental or theoretical investigation.
Although DFT calculations have drawn attention to the
possible role of defects and termination plane upon surface
2
1
performance of TMCs. Until now, no systematic experiment
has been carried out to find out the exact highly active
catalytic sites with serial pure TMCs which can exclude the
impacts of supports, outside carbon layers and particle sizes.
Herein it is the key issue to develop a synthetic method that
can produce single phase TMCs with high purity and various
amounts of defects, then determine the active sites in
catalysis.
a.
State Key Laboratory of Chemical Engineering, Chemical Engineering Research
Center, and School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, China. E-mail: cjxu@tju.edu.cn; Fax: +86 22 27892145
Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical
b.
Engineering, Dalian University of Technology, Dalian 116024, China. E-mail:
changhai@dlut.edu.cn; Fax: +86 411 84986353
Generally, based on the direction of carbon diffusion,
synthetic methods for TMC can be classified into two
†
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available.
See
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categories, namely, “outside-in” and “inside-out” processes. application as a high grade fuel.
ARTICLE
3
2-34
In a background that
The former one is typically the temperature-programmed research on the conversion of biomass has been motivated by
reduction (TPRe) method, in which a mixture of hydrogen and the desire to replace fossil fuels and to mitigate CO emissions
it is meaningful to
carburization of the corresponding metal oxides or metal. In study the active sites of TMCs for its effective catalysis for
2
3
5-38
4 2 6
carbon-containing gas, such as CH , C H , or CO, induced the and the associated global warming,
9
, 10, 39
this way, it is difficult to engineer TMC nanoparticles with hydrogenation and deoxygenation.
controlled properties because the high synthesis temperatures
(above 700°C) easily lead to polymeric carbon formation and
contamination at the surface of TMC particles, which severely
2
2
reduces the active sites of the catalyst. In recent years,
synthetic routes of carbides have been developed to reduce
2
3, 24
the surface carbon.
One typical example is the
2
5
carbothermal hydrogen reduction route , which leads to less
carbon deposition by applying solid carbon during the
pyrolysis. However, pure bulk TMC nanoparticles cannot be
obtained.
Recently
,
several new routes were reported that
C microspheres
nanostructured TMCs can be generated. W
2
were synthesized by heating mixtures of ammonium
metatungstate salt (AMT) (a tungsten precursor) and
resorcinol–formaldehyde polymer (a carbon precursor) for 1
2
Scheme 1. Illustration for Synthesis of W C Nanorods
2
6
hour in an argon flow and 2 hours in hydrogen flow. Multiple
phases of molybdenum carbide were acquired by thermally
decomposing a templated precursor, which was precipitated 2. Experimental
from an aqueous solution of ammonium molybdate and 4-Cl-
2
.1. Synthesis of the tungsten-based inorganic-organic hybrid
o-phenylenediamine or p-phenylenediamine by adjusting the
1
pH below 3. Nanostructured transition metal carbides based
3
All solvents and reagents were used as received. A
on the confined carburization in metal-organic frameworks precursor to W C was prepared by simply dissolving 0.43 g of
2
matrix were produced by annealing a compound consisting of melamine and 1 g of (NH ) H W O *nH O with 150 mL of
4
6
2
12 40
2
copper-based
metal-organic
frameworks
host
and distilled water at 70 °C under magnetic stirring and then the
1
2
molybdenum-based polyoxometalates guest. These methods solution was stirred at 90 °C for 12 hours with white
represent significant advances in TMCs synthesis, which can be suspension formed slowly. Then the product was collected
considered as “inside-out” process, during which the carbon from the suspensions by centrifugation, washed with ethanol
species diffuse from the bulk to the external surface. Instead and dried in an oven at 80 °C for 24 hours.
of forming coke, most of the carbon species are removed by
2
.2. Pyrolysis of the tungsten-based inorganic-organic hybrid
the hot carrier gases (e.g., Ar, N
2
) when reaching the gas-solid
interface. Although these nanostructured TMCs show high
Pyrolysis of the organic- inorganic hybrid (1g) was
catalytic performance due to their clean surface and special conducted in a quartz boat placed in a quartz tube reactor.
structures, all of them contain amorphous carbon inside and After expelling air by argon at room temperature for 3 hours,
are also not phase-pure bulk TMCs.
In this work, an organic-inorganic hybrid strategy to obtain temperature at 2 °C/min or 5 °C/min with flowing argon (70
tungsten carbide (W C) nanorods is designed with varied mL/min) . After the obtained product had been cooled slowly
amounts of defects. In a typical synthesis (Scheme 1), a hybrid to room temperature, the gas was switched off to allow the
was precipitated from an aqueous solution of (NH
slow diffusion of air back into the tube to passivate the carbide
O and melamine at 90°C. The hybrid was surface to avoid bulk oxidation. For the convenience of
the temperature was increased linearly to the target
2
4 6
)
2 12 2
H W O40*nH
pyrolyzed at 750°C for 1h with ramping rate 2 °C/min in an presentation, the products of the heat treatment of the hybrid
argon flow. The amounts of defects were tuned by changing conducted in Ar and Ar/H at A °C/min to B °C and held at B °C
2
the treating temperature, time or the gas atmosphere.
Then the W C nanorods were studied in the catalytic respectively. After the annealing and etching processes, the
2
for C h are denoted as Ar-A-B°C–Ch and Ar/H -A-B°C–Ch,
2
hydrodeoxygenation of benzofuran, in which the process of sample was obtained as black powders.
hydrogenation, hydrogenolysis, deoxygenation can be studied
2
.3. Hydrodeoxygenation of benzofuran
at the same time. Also we chose hydrodeoxygenation of
benzofuran for another important reason that the majority of
The hydrodeoxygenation of benzofuran was carried out in a
or fixed-bed stainless steel reactor (12 × 2 × 500 mm). Toward
2
7-29
the biomass-derived molecules contain either a phenolic
3
0, 31
a furanic structure.
Catalytic deoxygenation is playing an this end, 0.2 g of catalyst diluted with 2g of quartz sand was
important role in the conversion of biomass because most of loaded into the isothermal section of the reactor and reduced
them have relatively high oxygen content, limiting their direct in 4 MPa H (flow rate of 50 mL/min) at 400 °C for 2 h to
2
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remove the surface oxides. A 3 wt % solution of benzofuran in consequence of an ordered arrangement of the organic
n-decane was introduced after the reactor had cooled to molecules between the inorganic lattices. The nanorod
340°C. The conditions were set as a total pressure of 4 MPa, morphology of the hybrid with lengths of 2-5µm and
and a H /oil volume ratio of 1000:1. Liquid samples were diameters of 200-800nm was well-observed by scanning
2
collected at 1h interval after a stabilization period of 3 h. The electron microscope (SEM, Fig. S3). In addition, the hybrid
effluent composition was analyzed by an Agilent 7890-II gas components of melamine and tungstate in the nanorods have
chromatograph equipped with an OV-101 capillary column (5 also been confirmed by Infrared spectroscopy (FT-IR) (Fig. S4)
m) and a flame ionization detector. It was confirmed that the and CHN elemental analysis in conjugation with
reaction was not controlled by the internal diffusion by Weisz– thermogravimetry conducted in air (Fig. S5 and Table S3),
Prater Criterion for Internal Diffusion (shown at the bottom of showing that the hybrid contained 23.0 wt% N, 9.6 wt% C, 1.9
supporting information).
wt% H, and 49.2 wt% W.
2.4. Characterization
2
3.2. Characterization of W C
X-ray diffraction (XRD) patterns were taken on a Rigaku
As the inorganic-organic hybrid is periodic nanostructures in
D/MAX 2400 diffractometer with Cu Kα(40 kV, 100 mA) which the inorganic W-O layers and the organic species
radiation. The nitrogen and carbon contents were studied by (melamine iron) are stacked alternately, such a chemical
an Elementar Vario ELsystem. The morphology of the solid intimacy significantly facilitates the process of oxidation –
powder was obtained on scanning electron microscopy (SEM) reduction and formation of carbide in the preparation of W
using a NOVA NanoSEM 450 system. Transmission electron Heat treatment of the hybrid under argon resulted in well-
microscopy (TEM) was performed using Philips CM200 FEG crystallized pure phase W C at 750 °C (See XRD, Fig. 1). The
transmission electron microscope (accelerating voltage 200 diffraction peaks due to W C can be detected at 700 °C (Fig.
kV) using high-resolution imaging and energy-dispersive X-ray S6a). Although the products were dominated by W C at 800 °C
2
C.
2
2
2
spectroscopy (EDX). Fourier transform infrared spectra was or higher temperatures, a trace amount of metal tungsten was
performed at room temperature on a Nicolet Impact 410 detected by XRD and its amount was increased with
-1
spectrometer with a spectral resolution of 4 cm . Raman temperature (Fig. S6b and table S3). By further increasing to
investigation was performed on DXR Microscope 900 °C, the new diffraction peaks at 35.44 and 48.16° were
a
spectrometer with an excitation wavelength of 514 nm at observed which can be attributed to WC (Fig. S6b). When the
room temperature. The Brunauer−Emmeꢀ–Teller (BET) heat rate changed to 5 °C /min, the diffraction peaks due to
surface area was studied by static nitrogen physisorption at 77
2
W C also can be detected at 700 °C (Fig. 6Sc), but a trace of
K subsequent to outgassing at 300 °C until the pressure was metal tungsten started to appear even at 750 °C and more
lower than 5 mbar. XPS measurements were carried out in an metal tungsten (Fig. 6Sd) was produced comparing to that of
ultrahigh-vacuum setup equipped with a Gammadata-Scienta using 2°C/min at the corresponding temperature (Table S3),
SES 2002 analyzer. Thermal decomposition of the tungsten- which is attributed to the character of “inside-out” processes
based inorganic-organic hybrid was performed with a Cahn TG- that the carbon diffusion speed is not fast enough to supply
2
131 thermo-balance in pure argon or air at a heating rate of 5 carbons for the forming of W
C/min from room temperature to 900 . The outlet was temperature ramping speed is high. In such a condition, the
coupled with a HP 5973 quadrupole mass spectrometer more tungsten in the surface means the more vacancy defects
through a silica capillary tube heated to 200 to prevent due to a lack of carbon atom (See dark spots in red circles,
condensation. Multiple signals were monitored at m/ z = 17, HRTEM, Fig. 2 and S25).
8, 28, and 44, corresponding to NH , H O, CO/N , and CO
species respectively.
2
C on the surface when
°
℃
℃
1
3
2
2
2
3
. Results and discussion
3
.1. Reaction of (NH
Comparing the XRD patterns (Fig. S1) of (NH
O and the product obtained from reactions
O, a new series
of peaks occurred at the low 2θ angle for the product and the
peaks due to air-dried (NH O disappeared.
The sharp peak, centered at 7.51° in the low 2θ range is
4 6 2 12 2
) H W O40*nH O with Melamine
4 6
)
H
2 12 2
W O40*nH
between melamine and (NH
4
)
6
H
2
W
12
O40*nH
2
4
)
6
H
2
W
12
O40*nH
2
4
0
characteristic of
a
highly ordered nanostructure.
Corresponding to its d-value, the interlayer distance was
determined to be 1.175 nm. The uniform periodic lattice
Fig. 1. XRD patterns of W
2
C originating from the heat treatment of the hybrid at
(1.175 nm) was clearly identified by a high-resolution
different conditions. The standard pattern of W
2
C (PDF# 35-0776) is shown (dashed
transmission electron microscope (TEM, Fig. S2). Accordingly, lines) at the bottom.
the constant interlayer distance has to be
a direct
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The pyrolysis products obtained, similar to the hybrid, surface roughness. The smaller surface area of Ar-2-900°C-1h
appear to be thinner nanorods with lengths of 2-4µm and is attributed to the aggregation of some W C nanorods at
diameters of 100-600 nm (See SEM, Fig. 2a, S7, S8 and S9). No higher temperature (Fig. S9). The pore-size distribution
residual carbon was detected in W C nanorods by Raman (average pore diameter =8.1nm) of Ar-2-750°C-1h (Fig. S14)
spectra (Fig. S10) and HRTEM of W C (Fig. S25). Only W, C and indicates that there are abundance of mesopores and very few
O signals were detected by the energy dispersive spectroscopy micropores in the W C nanorods. The small surface area and
attached on SEM (Fig. S11), with the small signal of O being the shortage of micropores are further evidences of high purity
due to the surface oxidation of W C. The uniform distribution of W C nanorods. For unsupported carbide, a high surface area
of W and C elements is illustrated by EDX elemental mapping typically comes from amorphous carbon, unless there are
images (Fig. S11). The atomic ratios of W to C in all of the W special well-developed pore structures. Since the products are
products determined by CHN elemental analysis and TG were phase-pure W C with high purity, the BET surface area could
2
2
2
2
2
2
2
C
2
about 2 (Table S2), equal to the theoretical value in W
2
C.
be considered as the intrinsic area value of a perfect W
surface for catalytic reaction.
2
C
Fig. 3. XPS W 4f spectra of Ar-2-750°C-1h
3
.3. Formation mechanism of W C
2
In order to understand how the conversion from the hybrid
to W C was carried out in detail, the gases evolved during the
2
heat treatment in argon were monitored using an on-line mass
spectrometer to provide insight into the reaction pathway.
TG curve and MS curves (Fig. S23) are divided into four
regions. The first region in the range of 100 – 270 °C
represents the vaporization of physically adsorbed water and
the slow dehydration of the hybrid. The second step starting
from about 320 °C is mainly attributed to the triggered
sublimation and decomposition of melamine. The sublimated
Fig. 2. SEM image of (a) Ar-2-750°C-1h and high-resolution TEM images of samples: (b) melamine can be observed in the inner wall of the quartz tube
Ar-2-750°C-1h; (c) and (d) Ar-2-900°C-1h; (e) and (f) Ar-2-750°C-1hH
2
after heat treatment. The initial decomposition of melamine
and the reduction of WO begin to occur. Some of the O and N
x
The W-4f XPS spectra of Ar-2-750°C-1h (Fig. 3), Ar-2-750°C- atoms are removed through its combination with the
h and Ar-2-900°C-1h (Fig. S12) confirmed that W were neighboring H to form H O and NH with a peak maximum at
4
2
3
2 3
present in the W C and WO state at the surface. The tungsten 390 °C. In the third region, the further decomposition of
carbide/oxide surface ratios in the three samples were all melamine and the degradation of WO by C are in progress
x
estimated to be about 64:36. It is believed that some part of with forming NH , CO / N and CO after 400 °C. With a peak at
3
2
2
surface W
certified that there was very little amount of residual N at the to be due to the pyridine N of melamine. The elimination of O
surface of W C nanorods because of the absence of sharp and N is intensified through the formation of CO/N (peak at
peaks in N 1s (Fig. S13).
The BET surface areas of the products distributed from After 600°C, it is considered as the last region in which the
2
3
C was oxidized in air after preparation. It was also 425 °C, the NH released at higher temperature is considered
2
2
500°C) and an appreciable amount of CO (peak at 440°C).
2
2
2
1
8.5m /g (Ar-2-750°C-1h) to 14.7m /g (Ar-2-900°C-1h) (Fig. residue C, N and W are combined together. The relatively
S14 and Table S2) and the differences from each other were stable WC N is formed and then transform to W C when
x
y
2
small. Also TEM images (Fig. S25) showed that they had similar temperature is higher than 700°C (XRD, Fig. S6). Based on the
surface structures, so that they had very little differences in CHN analysis data, after 650°C, the overall atom ratio W
:
C
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keeps to be at 2 and N
: W decreases with increasing aromatics yield and lower hydrogen consumption than that by
temperature. Then it can be concluded that N is removed in noble metals.
the form of N as temperature climbs up. Consequently,
2
vacancy sites are formed with the removal of N because there
is not enough carbon to replenish. Furthermore, almost no
carbon species are formed after 600°C, which is the reason for
the undetected residue carbon at the surface of W
whole process of the conversion from the hybrid to W
summarized in these transformations
2
C. The
2
C is
：
Scheme 2. The reaction pathway of benzofuran hydrodeoxygenation over W
catalysts
2
C
In view of the proposed reaction pathway, the following
2
rates (reacted molecules per cm catalyst surface per second)
Because the direction of carbon diffusion is inside to out, N
is removed in the form of N at high temperature and
melamine (C ) is component rich of N and short of C, a
variety of W C catalysts with different amounts of vacancy
2
is defined: RConv represents the number of double bonds being
hydrogenated, i.e. the conversion number of benzofuran; RCOC
represents the cleavage number of C-O-C; RDO represents the
cleavage number of the aromatic carbon and oxygen bonds
3 6 6
H N
2
defects are obtained by controlling pyrolysis process.
Meanwhile, as it is thermodynamically favored to form metal
8
terminated surfaces under low carburizing potentials, the
(marked as Car-O), i.e. the amount of deoxygenation. RCCC
represents the cleavage number of C-C; RH2 represents the
number of hydrogen reacted (because hydrogen participated
in each process, calculating the rate of consumed hydrogen
can later be used to describe the average reaction rate of the
whole process, which also indicates the activity of the
catalyst).
favorable surface termination of the obtained W
2
C should be
W-terminated surface under argon environment and the
“
inside to out” processes of carbon diffusion.
We also tried to improve the removal of carbonitride by
replacing the inert gas argon with the mixture of argon and a
small amount of hydrogen. Not as expected, metal tungsten
was obtained at 650 °C, 750 °C and 800 °C (Fig. S15), which
may be the result of carbon-hydrogenating at elevated
temperatures. Then another new way to create more vacancy
defects was found, that is, to keep the pyrolysis process the
same as Ar-2-750°C-1h before it get to 750°C and only switch
the inert gas to the mixture of argon (70 mL/min) and
hydrogen (35 mL/min) when it get to 750°C and keep for 1h.
2 2
Denote the product as Ar-2-750°C-1hH . Its phase is W C with
a small amount of metal W (XRD, Fig. S16), which also improve
that the carbon atoms in the compound are already very
stable and have strong binds with tungsten atoms when the
temperature comes to 750°C.
3
.4. Hydrodeoxygenation of benzofuran over W
By analyzing the product distributions as a function of
contact time at different temperatures on a variety of W
catalysts (Fig. S17), the reaction pathway was proposed
Scheme 2, bold lines represent the major pathway). After the
2
C catalysts
2
C
(
furan double bond was saturated (denote this step as Conv),
the C−O −C bond was selecꢁvely cleaved at the posiꢁon of the
aliphatic carbon (denoted as COC). Then it is followed by the
cleavage of the aromatic carbon and oxygen atom (denoted as
DO), and the resulting deoxygenated product is progressively
hydrogenated. At the same time the whole process is
accompanied by a small amount of C-C bond cleavages
(denoted as CCC).
The reaction pathway is completely different from the one
2
Fig. 4. Arrhenius plots for RH2 and RDO on different W C catalysts.
over the noble metal catalysts in which the benzene rings are
4
firstly saturated followed by deoxygenation (Scheme S1).
1
Fig. S18 shows that with increasing contact time, the overall
Because of the difference, W C has resulted in higher hydrogen reaction rate (R ) was almost constant while the
2
H2
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deoxygenation rate
（
R
DO
）
was increased apparently, difference in catalytic activity was mainly attributed to the
competition between difference in the number of these active sites. These specific
hydrodeoxygenation reaction and other hydrogenation active sites should have special structures. Based on the
reactions, and they have some common catalytic active sites. HRTEM results of Ar-2-750°C-1h, Ar-2-750°C-4h, Ar-2-900°C-1h
Those catalysts showed almost equal apparent activation and Ar-2-750°C -1hH (Fig. 2 and S25), it was found that the
energy at each major reaction step (see Arrhenius plots in Fig. selected W C catalysts all contained crystal defects, such as
and Fig. S19), showing that they all had the same active sites. twin boundaries, stacking faults, dislocations, and vacancies.
Comparing the RH2 of different catalysts(Fig. 5) the overall However, the numbers of defects were different, Ar-2-750°C -
catalytic activity order is as follows: Ar-2-900°C-1h > Ar-2- 1h showed a few vacancies, and there were some twin
50°C-1hH > Ar-5-850°C-1h > Ar-5-800°C-1h, Ar-2-850°C-1h > boundaries, stacking faults, dislocations; Besides the line and
Ar-2-750°C-4h > Ar-2-800°C-1h > Ar-5-750°C-1h, Ar-2-750°C- planar defects, Ar-2-750-4h also showed a little more
h. Comparing the RDO (Fig. 5), the deoxygenation activity vacancies, and a large amount of vacancies appeared in Ar-2-
order is as follow Ar-2-750°C-1hH > Ar-2-900°C-1h > Ar-5- 900°C-1h and Ar-2-750°C-1hH . By counting and calculating the
50°C-1h > Ar-2-750°C-4h, Ar-5-800°C-1h, Ar-2-850°C-1h > numbers of vacancies on multiple HRTEM images, the
Ar-2-800°C-1h Ar-5-750°C-1h, Ar-2-750°C-1h. The statistical average numbers of carbon vacancies per square
difference in the orders of RH2 and RDO may be due to different nanometers of the catalysts were obtained: Ar-2-750°C-1hH
indicating
that
there
is
2
2
4
,
7
2
1
:
2
2
8
>
2
2
2
distributions of active sites.
(2.5 vacancies/nm ), Ar-2-900°C-1h (2.6 vacancies/nm ), Ar-5-
2
vacancies/nm ),
8
00°C-1h
(1.7
2
Ar-2-750°C-4h
(1.6
2
vacancies/nm ), Ar-2-750°C-1h (1.1 vacancies/nm ). It was
found that the numbers of vacancies and the levels of catalytic
activity were positively correlated (Fig. S26).
Fig. 6. Relationship between catalytic activity and tungsten content in XRD.
Although XRD results showed diffraction peaks of metal
tungsten, the lattice of metallic tungsten had not been found
in HRTEM images. In conjunction with the formation
2
mechanism of the W C, it is believed that these diffraction
peaks come into being by superimposing the diffraction peaks
of small tungsten centers dispersed in various localized regions
as a result of carbon vacancies. Some of these tungsten atoms
can still be considered as defective part of W
2
C, and they can
C. It is
also provide more W-termination for the surface of W
2
2
Fig. 5. RH2 and RDO at different temperatures on different W C catalysts.
believed that the increase of vacancy sites and the increase of
W-terminated surface is accompanied by each other. The
surface termination of carbide also has some impact on the
The catalytic activity (RH2) of Ar-2-750°C-1h was taken as the
reference 1. The relative catalytic activities of each catalyst are
8
catalysis for hydrodeoxygenation.
shown in Table S3. Ar-2-900°C-1h and Ar-2-750°C-1hH
more than twice as active as Ar-2-750°C-1h. Each catalyst is a
pure phase W C and has a surface with the same amount of
C per unit area. But they show significant differences in
activity, so not every W C has hydrogenation activities, but
2
are
2
The numbers of W C defects should be positively correlated
with the tungsten contents in XRD which is also positively
correlated with the catalytic activity (see Fig. 6). These samples
2
W
2
(Ar-5-750°C-1h, Ar-5-800°C-1h and Ar-5-850°C-1h), comparing
2
with each others, also are also correlated, but in comparison
there are special active sites which make them active. The
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with the samples from temperature ramping speed 2°C/min, important reference for the activity of chemical bond cleavage,
they exhibit much higher metal tungsten contents in XRD (Fig. and it is important for us to judge the effects of temperature
S6 and Table S3). It is believed that the rapid heating rate led on reaction rate and selectivity. The bond dissociation energy
to a large number of carbon loss from some locals, resulting in of Car-O is high, so the increase of temperature has a strong
the formation of some large metal tungsten centers. Only a influence on the DO process.
-
1
fraction of the tungsten in these large tungsten centers has
The bond dissociation energy of Car-O (464 kJ∙mol ) is larger
-1
6
high activity due to carbon vacancies. When raising the heating than that of cyclo-C H11–OH (399kJ∙mol ) after the saturation
rate to 20°C/min, tungsten carbide with a layer of sintered of the aromatic ring, and therefore the direct deoxygenation
metal tungsten on the outside is an extreme example of the requires a higher temperature than the deoxygenation after
effect of heating rate. The pure metal tungsten (Ar/H
50 C-1h) obtained by pyrolyzing the hybrid in a mixture of dissociation energy of Car-O is larger than that of C-C, the
argon and hydrogen was also evaluated by number of Car-O cleavage is significantly higher than that of C-
hydrodeoxygenation of benzofuran and it was found that the C cleavage (Fig. S22) under the catalysis of W C, indicating that
sample showed very low activity. It is concluded that pure C contains active sites with good scission effect on Car-O
2
-2- the saturation of aromatic ring. In addition, although the
7
°
2
W
2
tungsten, which is not associated with W
active site.
2
C, is not a highly and shows excellent oxophilicity that makes it suitable for
deoxygenation (Fig. S21).
2
The RH2 and RDO of Ar-2-750°C -1hH , which were placed in
3
.5. Mechanism of benzofuran hydrodeoxygenation over W
catalysts
On the basis of the above results and discussion, the defects
% respectively (Table S4). Although their XRD did not show on the surface of W C, especially the vacancy sites, exhibit high
C catalyst, their activity was catalytic activity during the hydrodeoxygenation of
significantly reduced over time. This may be because the benzofuran. The furan double bonds are saturated by the
defects are so active that even in the air at room temperature dissociated H atoms on the surface of the W C. And then the
they are gradually oxidized to form tungsten oxide and lose aliphatic−O–Caromatic linkages are adsorbed on the defects of
activity. It is also shown that WO formed by surface oxidation C, and the cleavage preference is on the weaker Caliphatic−O
is also not a highly active site for the hydrodeoxygenation of bonds rather than on the stronger Car−O bonds. The
2
C
air at 28°C for 45 days, were reduced to 41% and 9% of the
ones of the fresh catalyst respectively (Table S4). And for Ar-2-
9
00°C-1h placed in air at 28°C for 90 days, they were 18% and
6
2
any difference from the fresh W
2
·
2
C
3
W
2
benzofuran.
2
dissociated H· atoms on the W C surface are in turn added to
form ethyl phenol. After that, the Car-O bonds in ethyl phenol,
prior to the aromatic ring and C-C bond, are adsorbed on
tungsten atoms of W
consequently cleaved and saturated under the synergistic
effect of dissociated H atoms, resulting in the formation of
ethylbenzene and water after desorption. C-C is adsorbed on
the defects of W C in small amount, and then is cleaved and
saturated under the action of dissociated H atoms. At the
same time, small amount of aromatic rings after
2
C with carbon vacancies, and are
·
2
·
a
deoxygenation are saturated while non-deoxygenated
molecules need to give priority to the adsorption of oxygen
atoms.
4
. Conclusions
Metatungstate and melamine hybrid nanorods (lengths of 2-
Fig. 7. Relationship between apparent activation energy and bond dissociation
energy for COC, CCC and DO.
5 µm and diameters of 200-800 nm) with nanoscale periodic
structure were synthesized in aqueous phase at 90 °C. During
From the Arrhenius plots (Fig. 4, S19 and S20), the apparent pyrolyzing the hybrid, the nanoscale alternating contact
activation energy of each step is obtained in the order: DO between tungsten oxide and melamine effectively promotes
(
134.5kJ/mol) > CCC (96.4kJ/mol)> COC (86.6kJ/mol)> RH2 the reduction of tungsten oxide and the carbonization of
84.8kJ/mol)> Conv (68.5kJ/mol). The apparent activation tungsten and inverts the diffusion direction of carbon to be
(
energy of RH2 is considered as the average apparent activation from inside to outside, thereby avoiding the deposition of
energy of the whole process. In combination with the bond carbon on the surface. Meanwhile, carbon and nitrogen are
dissociation energies of Car-O, C-C, and C-O (Table S5), it is gradually lost. Carbon and tungsten are stoichiometrically
found that the apparent activation energies of these steps are combined to form W C. Since melamine has less carbon and
2
approximately linearly related to the bond dissociation more nitrogen contents, when nitrogen and carbon are locally
energies of the chemical bonds (Fig. 7). Therefore, in the lost significantly and there is not enough carbon to
future study, the bond dissociation energy can be used as an complement or timely replenished, vacancy defects will be
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generated due to the lack of carbon. Therefore, W
lengths of 2-4 µm and diameters of 100-600 nm) with
2
C nanorods 15 C. H. Liang, L. Ding, C. A. Li, M. Pang, D. S. Su, W. Z. Li and Y.
M. Wang, Energ. & Environ. Sci., 2010, , 1121-1127.
6 X. M. Ma, H. Meng, M. Cai and P. K. Shen, J. Am. Chem. Soc.,
012, 134, 1954-1957.
3
(
1
1
different number of vacancy defect sites were obtained by
controlling the pyrolysis temperature, pyrolysis time,
temperature ramping rate or the introduction of hydrogen.
2
7 L. H. Bennett, J. R. Cuthill, A. J. McAlister, N. E. Erickson and
R. E. Watson, Science, 1974, 184, 563-565.
The trace amount of metal tungsten detected by XRD can be 18 D. R. Stellwagen and J. H. Bitter, Green Chem., 2015, 17, 582-
93.
9 P. Liu and J. A. Rodriguez, J. Chem. Phys., 2004, 120, 5414-
423.
5
2
still considered as defective part of W C due to carbon atom
1
2
vacancies. High pyrolysis temperature, lengthening pyrolysis
time and introduction of proper amount of hydrogen, properly
increasing temperature ramping rate are favorable to form
5
0 A. M. Alexander and J. S. J. Hargreaves, Chem. Soc. Rev.,
2010, 39, 4388-4401.
more vacancy sites which lead to higher activity in 21 D. V. Esposito and J. G. G. Chen, Energ. & Environ. Sci., 2011,
4
2 S. T. Hunt, M. Milina, A. C. Alba-Rubio, C. H. Hendon, J. A.
, 3900-3912.
hydrodeoxygenation of benzofuran. Rich vacancy sites in the
tungsten carbides exhibit excellent oxophilicity and good
scission effect on Car-O. Car-O bond is cleaved in the case of
2
Dumesic and Y. Roman-Leshkov, Science, 2016, 352, 974-
9
78.
unsaturated aromatic ring, thereby reducing the consumption 23 S. R. Vallance, S. Kingman and D. H. Gregory, Adv. Mater.,
of hydrogen. It is found that the apparent activation energy of
each chemical bond cleavage is linearly related to its bond
dissociation energy. Therefore, bond dissociation energy can
be used as an important reference for chemical bond cleavage
process in future studies.
2007, 19, 138-+.
4 A. Celzard, J. F. Mareche, G. Furdin, V. Fierro, C. Sayag and J.
2
2
Pielaszek, Green Chem., 2005,
5 C. H. Liang, P. L. Ying and C. Li, Chem. Mater., 2002, 14, 3148-
151.
26 R. Ganesan and J. S. Lee, Angew. Chem. Int. Ed., 2005, 44
557-6560.
7, 784-792.
3
,
6
2
7 C. Zhao, Y. Kou, A. A. Lemonidou, X. B. Li and J. A. Lercher,
Angew. Chem. Int. Ed., 2009, 48, 3987-3990.
Acknowledgements
2
8 A. Berenguer, T. M. Sankaranarayanan, G. Gomez, I. Moreno,
J. M. Coronado, P. Pizarro and D. P. Serrano, Green Chem.,
We gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (No.
2
016, 18, 1938-1951.
2
3
3
3
3
9 D. A. Ruddy, J. A. Schaidle, J. R. Ferrell, J. Wang, L. Moens and
J. E. Hensley, Green Chem., 2014, 16, 454-490.
0 G. Y. Li, N. Li, J. F. Yang, L. Li, A. Q. Wang, X. D. Wang, Y. Cong
and T. Zhang, Green Chem., 2014, 16, 594-599.
1 A. D. Sutton, F. D. Waldie, R. L. Wu, M. Schlaf, L. A. Silks and
21573031 and 21373038), the Fundamental Research Funds
for the Central Universities (DUT15ZD106) and Program for
Excellent Talents in Dalian City (2016RD09).
J. C. Gordon, Nature Chem., 2013,
2 C. Zhao, T. Bruck and J. A. Lercher, Green Chem., 2013, 15
720-1739.
5, 428-432.
,
Notes and references
1
1
2
R. B. Levy and M. Boudart, Science, 1973, 181, 547-549.
M. Pang, C. Liu, W. Xia, M. Muhler and C. Liang, Green
Chem., 2012, 14, 1272.
3 H. L. Wang, H. Ruan, H. S. Pei, H. M. Wang, X. W. Chen, M. P.
Tucker, J. R. Cort and B. Yang, Green Chem., 2015, 17, 5131-
5
4 M. Saidi, F. Samimi, D. Karimipourfard, T. Nimmanwudipong,
135.
3
4
5
6
M. K. Neylon, S. Choi, H. Kwon, K. E. Curry and L. T.
Thompson, Appl. Catal. A Gen., 1999, 183, 253-263.
L. Leclercq, M. Provost, H. Pastor and G. Leclercq, J. Catal.,
3
B. C. Gates and M. R. Rahimpour, Energ. & Environ. Sci.,
2
014, 7, 103-129.
5 R. Ma, W. Hao, X. Ma, Y. Tian and Y. Li, Angew. Chem. Int.
Ed., 2014, 53, 7310-7315.
1
989, 117, 384-395.
3
3
F. H. Ribeiro, R. A. D. Betta, M. Boudart, J. Baumgartner and
E. Iglesia, J. Catal., 1991, 130, 86-105.
J. B. Claridge, A. P. E. York, A. J. Brungs, C. Marquez-Alvarez,
6 N. Ji, T. Zhang, M. Y. Zheng, A. Q. Wang, H. Wang, X. D. Wang
and J. G. G. Chen, Angew. Chem. Int. Ed., 2008, 47, 8510-
J. Sloan, S. C. Tsang and M. L. H. Green, J. Catal., 1998, 180
5-100.
,
8
513.
8
3
3
7 R. Ooms, M. Dusselier, J. A. Geboers, B. Op de Beeck, R.
Verhaeven, E. Gobechiya, J. A. Martens, A. Redl and B. F.
Sels, Green Chem., 2014, 16, 695-707.
8 C. J. Zhu, T. Shen, D. Liu, J. L. Wu, Y. Chen, L. F. Wang, K. Guo,
H. J. Ying and P. K. Ouyang, Green Chem., 2016, 18, 2165-
7
W. Q. Zheng, T. P. Cotter, P. Kaghazchi, T. Jacob, B. Frank, K.
Schlichte, W. Zhang, D. S. Su, F. Schuth and R. Schlogl, J. Am.
Chem. Soc., 2013, 135, 3458-3464.
8
9
1
M. S. Mark, C. Cha-Jung and B. Aditya, Catal. Sci. Technol.
2016. 6, 602-616.
R. W. Gosselink, D. R. Stellwagen and J. H. Bitter, Angew.
Chem. Int. Ed., 2013, 52, 5089-5092.
0 Baddour. G. Frederick, Nash. P. Connor, Schaidle. A. Joshua,
Ruddy and A. Daniel, Angew. Chem. Int. Ed., 2016, 55, 9026-
2
174.
9 S. A. W. Hollak, R. W. Gosselink, D. S. van Es and J. H. Bitter,
Acs Catal., 2013, , 2837-2844.
3
4
4
3
0 J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti and
M. Niederberger, Angew. Chem. Int. Ed., 2006, 45, 261-265.
1 C. Y. Liu, Z. F. Shao, Z. H. Xiao, C. T. Williams and C. H. Liang,
Energ. & Fuels, 2012, 26, 4205-4211.
9
029.
1 D. R. Stellwagen and J. H. Bitter, Green Chem., 2015, 17, 582-
93.
2 H. B. Wu, B. Y. Xia, L. Yu, X. Y. Yu and X. W. Lou, Nature
Commun., 2015,
3 C. Wan, Y. N. Regmi and B. M. Leonard, Angew. Chem. Int.
Ed., 2014, 53, 6407-6410.
4 D. V. Esposito, S. T. Hunt, A. L. Stottlemyer, K. D. Dobson, B.
E. McCandless, R. W. Birkmire and J. G. G. Chen, Angew.
Chem. Int. Ed., 2010, 49, 9859-9862.
1
1
1
1
5
6
.
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W C Nanorods with Various Amounts of Vacancy Defects: Determination
2
of Catalytic Active Sites in Hydrodeoxygenation of Benzofuran
Rong Liu, Min Pang, Xiaozhen Chen, Chuang Li, Chunjian Xu, Changhai Liang
Phase-pure W C nanorods were prepared by pyrolysis of metatungstate and melamine hybrid
2
and showed excellent performnce for hydrodeoxygenation of benzofuran.