Efficient V2O5/TiO2 composite catalysts for dimethoxymethane synthesis from methanol selective oxidation

Article abstract of DOI:10.1039/c4ra16727a

A series of V₂O₅/TiO₂ composite catalysts (V₂O₅-TiO₂-Al₂O₃, V₂O₅-TiO₂-SiO₂, V₂O₅-TiO₂-Ce₂O₃ and V₂O₅-TiO₂-ZrO₂) were prepared by an improved rapid sol-gel method and the catalytic behavior for dimethoxymethane (DMM) synthesized from methanol selective oxidation was investigated. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller isotherms (BET), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H₂-TPR), NH₃ temperature programmed desorption (NH₃-TPD), infrared spectroscopy of adsorbed pyridine (Py-IR) and transmission electron microscopy (TEM) techniques. The best catalytic performance was obtained on a V₂O₅-TiO₂-SiO₂ catalyst with methanol conversion of 51% and DMM selectivity of 99% at 413 K. Furthermore, the V₂O₅-TiO₂-SiO₂ catalyst displayed an excellent catalytic stability within 240 h. Results showed that more Bronsted acidic sites were critical to increasing the DMM yield. The activity of V₂O₅/TiO₂ composite catalysts decreased with increasing Bronsted acidity, but the yield of DMM increased with an increasing amount of Bronsted acidic sites. The excellent performance of the V₂O₅-TiO₂-SiO₂ catalyst might come from its optimal acidity and redox properties, higher active surface oxygen species, together with more Bronsted acid sites.

Full text of DOI:10.1039/c4ra16727a

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Efficient V2O5/TiO2 composite catalysts for
dimethoxymethane synthesis from methanol
Cite this: RSC Adv., 2015, 5, 24795
selective oxidation†
a
a
d
Zhihong Fan,abc Heqin Guo, Kegong Fang and Yuhan Sun
*
*
A series of V2O5/TiO2 composite catalysts (V2O5–TiO2–Al2O3, V2O5–TiO2–SiO2, V2O5–TiO2–Ce2O3 and
V2O5–TiO2–ZrO2) were prepared by an improved rapid sol–gel method and the catalytic behavior for
dimethoxymethane (DMM) synthesized from methanol selective oxidation was investigated. The
physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Brunauer–
Emmett–Teller isotherms (BET), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-
programmed reduction (H2-TPR), NH3 temperature programmed desorption (NH3-TPD), infrared
spectroscopy of adsorbed pyridine (Py-IR) and transmission electron microscopy (TEM) techniques. The
best catalytic performance was obtained on a V2O5–TiO2–SiO2 catalyst with methanol conversion of
51% and DMM selectivity of 99% at 413 K. Furthermore, the V2O5–TiO2–SiO2 catalyst displayed an
excellent catalytic stability within 240 h. Results showed that more Brønsted acidic sites were critical to
increasing the DMM yield. The activity of V2O5/TiO2 composite catalysts decreased with increasing
Brønsted acidity, but the yield of DMM increased with an increasing amount of Brønsted acidic sites. The
excellent performance of the V2O5–TiO2–SiO2 catalyst might come from its optimal acidity and redox
properties, higher active surface oxygen species, together with more Brønsted acid sites.
Received 19th December 2014
Accepted 2nd March 2015
DOI: 10.1039/c4ra16727a
www.rsc.org/advances
(MF) could also be produced on redox sites and acidic sites
favored another side product of dimethyl ether (DME). There-
1. Introduction
fore, both the redox property and acidity of catalyst are needed
and should match each other in order to obtain DMM with high
selectivity.
Dimethoxymethane (DMM), a low toxicity chemical reagent and
a valuable methanol downstream product, is widely used as an
efficient diesel fuel additive as well as an excellent solvent in the
pharmaceutical and perfume industries.1–4 DMM can be
synthesized from the selective oxidation of methanol (3CH3OH
+ 1/2O2 / CH3OCH2OCH3 + 2H2O), which may relieve the
problem of methanol surplus from coal-derived syngas.5–7
In recent years numerous efforts have been devoted to the
selective oxidation of methanol to obtain DMM.8–10 In these
reports, two types of active sites in catalysts, including both
redox sites and acidic sites, were required for DMM synthesis.11
The redox sites were considered to be involved in the initial
formation of formaldehyde (FA) from CH3OH with active lattice
oxygen atoms, while acidic sites can catalyze acetalization
reactions of FA and CH3OH to DMM. However, methyl formate
It was reported that V2O5/TiO2-based (VT-based) catalysts
exhibited excellent activity for the selective oxidation of meth-
anol to DMM in mild reaction conditions.12 However, the
physiochemical properties and catalytic performance of
VT-based catalysts were inuenced greatly by several factors,
especially the preparation method and support additives.13–16
Traditionally, the VT-based catalysts can be prepared by incip-
ient wetness impregnation,17–20 co-precipitation21 and rapid
combustion method.22 In these methods, an additional heat
treatment was typically required to obtain the desired phase
composition, which usually led to a signicant aggregation of
catalyst particles. The aggregated catalyst particles might
weaken the interaction between vanadium and supports,
leading to low reducibility,23,24 which was unfavorable for the
rst step of DMM synthesis.25,26 Moreover, this aggregation
might also decrease the number of acidic sites,25 harmful for the
second step for DMM synthesis.27 Generally, the addition of
some elements to the support may decrease the aggregation of
catalyst particles in some extent. For example, the addition of
Al, Si, Ce and Zr to V2O5/TiO2 catalyst may affect both redox and
acidic properties and dispersion of the active phase.21,28,29
However, these catalysts were prepared mainly by co-
aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Shanxi, Taiyuan 030001, P. R. China. E-mail: kgfang@sxicc.
ac.cn; Tel: +86 351 4040431
bUniversity of Chinese Academy of Sciences, Beijing, 100049, P. R. China
cShanxi Agricultural University, Shanxi, Taigu, 030801, P. R. China
dLow Carbon Conversion Center, Shanghai Advanced Research Institute, Chinese
Academy of Sciences, Shanghai, 201203, P. R. China. E-mail: yhsun@sxicc.ac.cn;
yhsun@sari.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16727a
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precipitation or incipient wetness impregnation, which may
cause the particles aggregation or the metal nonuniform
distribution. These may cause the selectivity to DMM usually
decreased sharply with the increase of methanol conversion
over traditional V2O5/TiO2 catalysts, leading to a low DMM
yield.30 Thus, it is necessary to develop new catalyst with
improved preparing method for the selective oxidation of
methanol to obtain DMM efficiently from the point of scientic
signicance and economic view.
In this article, we designed an improved rapid sol–gel
method, which overcame the demanding and not easy to
control of traditional sol–gel method, avoided the deciency of
the above methods and evenly combined the metals on
molecular level, to prepare the composite supported V2O5/TiO2
catalysts (V2O5–TiO2–Al2O3, V2O5–TiO2–SiO2, V2O5–TiO2–Ce2O3
and V2O5–TiO2–ZrO2) and get bifunctional catalysts with high
DMM selectivity and high methanol conversion simultaneously
and excellent life span in methanol selective oxidation. The
changed dispersity of vanadium, redox and acid properties of
the catalysts prepared by improved sol–gel method with stable
three dimensional structures would affect the catalytic property
apparently. The physiochemical properties of the catalysts were
characterized exhaustively together with the catalytic perfor-
mance investigation.
2.2 Catalyst characterization
X-ray diffraction (XRD) patterns were measured on a Bruker
Advanced X-ray Solutions/D8-Advance scanning from 3ꢁ to 85ꢁ
(2q) at a rate of 0.02ꢁ sꢀ1 using a Cu Ka1 radiation (l ¼
0.15418 nm) source. The applied voltage and current were 50 kV
and 35 mA, respectively.
Measurements of the BET surface area and pore volume of
catalysts were performed in a Micromeritics ASAP-2000 instru-
ment by N2 adsorption–desorption. Before analysis, the samples
were degassed at 473 K overnight.
The elemental analysis was carried out using ICP optical
emission spectroscopy (ICP-OES) with an ACTIVA spectrometer
from Horiba JOBIN YVON.
X-ray photoelectron spectroscopy spectra (XPS) were recor-
ded on a XSAM-800 spectrometer using an Al Ka (1486.7 eV)
X-ray source.
Infrared spectroscopy of adsorbed pyridine (Py-IR) was
applied to determine the kinds of surface acid and the
measurements were performed in a Nicolet Magna 550 spec-
trophotometer. Wafers of 15 mg cmꢀ2 were degassed overnight
under vacuum (10ꢀ3 Pa) at 673 K for 1 h and then saturated by
pyridine. Samples were evacuated to eliminate the physically
adsorbed pyridine for 30 min. The amount of Brønsted and
Lewis acid sites was calculated from the intensities of the IR
bands at ca. 1540 cmꢀ1 and 1450 cmꢀ1, respectively.
Transmission electron microscopy (TEM) images were
recorded using a JEOL-2011 microscope operated at 200 kV.
NH3 temperature programmed desorption (NH3-TPD) was
performed in a quartz micro-reactor. 200 mg of each sample was
heated in Ar at 773 K for 2 h, then NH3 was introduced to the
sample aer the sample was cooled down to 353 K under Ar
ow. Aer the sample was swept using Ar at 353 K for 1 h to
remove the weakly adsorbed NH3, the TPD experiments were
carried out with a carrier gas at a ow of 40 mL minꢀ1 Ar and the
TPD spectra were recorded using a linear heating rate of 10 K
minꢀ1 from 353 K to 900 K by a Shanghai GC-920 equipped with
a thermal conductivity detector (TCD). The desorbed NH3 was
titrated by 0.01 mol Lꢀ1 HCl. The amount of desorbed NH3 was
corresponding to the total number of acidic sites. The acidity
intensity could be determined according to the desorption
temperature of NH3, and the number of acidic sites can be
calculated based on the areas of desorption peaks.
2. Experimental section
2.1 Catalyst preparation
The composite supported V2O5/TiO2 catalysts were prepared by
improved rapid sol–gel method, including the following steps.
2 g NH4VO3 and the calculated amount of citric acid were dis-
solved in 100 mL deionized water under vigorous stirring to
form vanadium-containing solution (S1). 4 mL TiCl4 was rapidly
transferred to 100 mL of 6 mol Lꢀ1 HCl using pipet under
stirring and then the calculated amount of citric acid was added
to the solution to form titanium-containing solution (S2). The
oxide-support precursor (Al(NO3)3, or TEOS, or Ce(NO3)3, or
ZrOCl2), and the corresponding amount of citric acid were
dissolved into a limited deionized water under constantly stir-
ring to form the metal ion containing solution (S3). The S2 and
S3 were slowly dropped into S1 in turn under continuous stir-
ring to form a precipitation. The precipitate was aged for 1 h at
373 K under stirring in air to form a transparent sol. Then, the
sample was dried for 5 h at 393 K, followed by calcination at
673 K for 6 h to remove the organic materials. The obtained
catalysts were denoted as V2O5–TiO2–Al2O3 (VTiAl), V2O5–TiO2–
SiO2 (VTiSi), V2O5–TiO2–Ce2O3 (VTiCe) and V2O5–TiO2–ZrO2
(VTiZr). The theoretical amount of vanadium pentoxide of all
samples was xed at 15 wt% whatever the other metal oxide
used. The weight ratio between the TiO2 and metal oxide (Al2O3,
Hydrogen temperature-programmed reduction (H2-TPR) was
carried out with a mixture of 5% H2/N2 as the reductive gas. A
sample of about 200 mg was reduced in a ow of H2/N2 at a
heating rate of 10 K minꢀ1 from room temperature to 1000 K.
The effluent gas was detected by TCD aer the removal of
ꢀ
produced water using 5 A molecular sieves.
2.3 Catalytic test and analytic procedure
SiO2, Ce2O3 or ZrO2) supports was 1 : 1. The moral ratio between The selective oxidation of methanol was carried out in a xed
the total amount of metals (V, Ti, Al, Si, Ce or Zr) and citric acid bed micro reactor at atmospheric pressure. About 0.8 g of
was 1 : 0.6.
catalyst sample (40–60 mesh) diluted with quartz sand with the
For comparison, the V2O5/TiO2 (VTi) catalyst, containing the same size and volume. Aer the sample was activated at 673 K
same vanadium content denoted as VTi, was prepared by the for 1 h in a ow of 10% O2/Ar (70 mL minꢀ1) and cooled down to
same method.
the reaction temperature, the gasied methanol (99.9%) was
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introduced into the reactionꢀz1one. The gas hourly space velocity VTiCe samples.31 Whereas, sample VTiZr exhibited much broader
(GHSV) was 12 000 mL gcat hꢀ1. The feed composition was diffraction peaks indicating more amorphous nature. This was
maintained at Ar : O2 : methanol ¼ 84.15 : 9.35 : 6.50 (v/v). The relevant with the catalyst calcination temperature.32 Most impor-
reaction products were analyzed by an on-line gas chromatog- tantly, no lines due to crystalline V2O5 were seen in the spectra.
raphy (GC-950) using a Propack T column and a TDX-01 column This observation clearly indicated that vanadium oxide was
connected to TCD detector and FID detector, respectively. The present in a highly dispersed.32 The crystalline V2O5 was detected
gas lines were kept at 393 K to prevent condensation of the on VTiCe and VTiAl catalyst. For the other catalysts, no crystalline
reactant and products.
V2O5 was observed, implying that the vanadium was highly
The product selectivity was calculated on carbon molar base: dispersed on TiO2 or the V2O5 crystalline was less than 4 nm
(beyond the detection capacity of the power XRD technique).33
In order to conrm the dispersion of vanadium, the surface
and bulk composition of the prepared samples were studied by
XPS and ICP techniques (see Table 1). The bulk vanadium
contents were similar for all the catalysts. The surface vanadium
on VTiSi, VTiZr and VTi catalysts was higher than that of bulk,
P
Si ¼ Yini/ Yini ꢂ 100%
wherein, i is DMM, FA, DME, MF and COx, Si is the selectivity of
product i, Yi is the number of carbon atom of product i, and ni is
the molar of product i.
suggesting the vanadium oxide was mainly dispersed on the
surface of the catalysts. However, the surface vanadium on
VTiCe and VTiAl catalysts was lower than those in the bulk,
3. Results
3.1 Physico-chemical characterization
indicating that the vanadium mainly existed on the bulk of the
catalysts. Calculation shows that V atom numbers per nm2 on
VTi catalyst was 5.6, which agrees well with the report that 7–8 V
atoms per nm2 was required to form the monolayer-disper-
sion.34 The values were only 3.6 and 6.4 on VTiSi and VTiZr
catalysts, respectively, inferring the high dispersed V species
which were half less than the theoretical value. However, the
values were 16.5 for VTiCe catalyst, which was almost two times
larger than the theoretical value, because of its low surface area.
And the values were 11.5 for VTiAl catalyst, which was larger
than the theoretical value too. Thus the crystalline V2O5
appeared as conrmed by XRD measurement.
Fig. 1 shows the XRD patterns of VTi, VTiAl, VTiSi, VTiCe and
VTiZr catalysts. As can be seen, the typical diffraction peaks
characteristic of anatase TiO2 appeared for VTi, VTiAl, VTiSi, and
The isotherms of all samples in Fig. S1† showed the type of
IV with the hysteresis loops at relative pressures of 0.4–1.0,
which was characteristic for mesoporous materials.35 The pore
size distribution for VTi, VTiCe and VTiZr catalysts, centered at
about 3.5 nm, while the peak centered at 5.3 and 9.0 nm
appeared for VTiSi and VTiAl catalyst respectively (Table 1). The
pore volume of VTiAl increased slightly to 0.25 cm3 gꢀ1
compared with that of VTi sample (0.15 cm3 gꢀ1) while the
number increased sharply to 0.55 cm3 gꢀ1 for VTiSi sample,
which may favor the subsequent reaction. The pore volume of
VTiCe, however, decreased to 0.08 cm3 gꢀ1 while that of VTiZr
kept almost unchanged.
Fig. 1 X-ray diffraction patterns of VTi, VTiAl, VTiSi, VTiCe and VTiZr
catalysts.
Table 1 Textural properties and chemical analysis of the catalysts
Texture data
Chemical composition
C.A. (wt%)
Ti
7.0 50.5
7.2 25.5 21.3 6.7 26.3 23.8
7.4 25.5 20.7 9.0 25.1 22.1
7.1 24.8 35.8 6.2 20.4 28.0
7.4 25.6 31.0 7.9 26.0 25.3
XPS (wt%)
Ti
7.4 50.4
Sample SBET (m2 gꢀ1
)
Pore volume (cm3 gꢀ1
)
Pore diameter (nm) V density calculateda (nmꢀ2
)
V
Mb
V
Mb
VTi
177
87
0.15
0.25
0.55
0.08
0.14
3.5
9.0
5.3
3.3
3.5
5.6
11.5
3.6
16.5
6.4
—
—
VTiAl
VTiSi
VTiCe
VTiZr
277
60
154
a
Supposing that all the vanadium atoms locate on the surface. b M ¼ Al, Si, Ce, Zr.
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Table 2 showed the results of binding energies and peak Sample VTiSi had much more active oxygen than others. The
tting results. The Ti2p3/2 binding energy for all the samples reference V2p3/2 peak positions for V2O5 and V2O4 were around
were around 458 eV, which was in reasonable agreement with 517.4 and 516.3 eV, respectively.41,42 The full oxidation state of
those for Ti4+ in literature.36 The O1s peak showed two types of V5+ was predominant at catalyst surface. It was clearly seen that
oxygen species. The binding energy around 530.8 eV was the a greater amount of V4+ species was presented on the VTiSi
characteristic of lattice oxygen species of TiO2 and V2O5, and the surface than that of others, indicating that the degree of
binding energy at 532.3 eV could be attributed to active surface reduction of V2O5 was enhanced with the Si addition.
oxygen species, including surface oxygen of adsorbed oxygen
The morphology was characterized by the TEM techniques
species, weakly bonded oxygen and hydroxyl-like groups.37–40 (Fig. 2). It was found that the particles size of VTi was only 5–
Table 2 Peak-fitting results of O1s, V2p3/2 and Ti1p3/2 XPS spectra
V
4+/(V4+ + V5+
)
Osur/(Osur + Olat
)
V2p3/2 (eV)
V4+
O1s (eV)
Sample
V5+
Osur
Olat
Ti2p3/2 (eV)
(Area %)
(Area %)
a
b
VTi
516.3
516.2
516.5
516.3
516.4
517.2
517.3
517.4
517.3
517.3
532.1
532.5
532.3
532.7
532.4
529.9
529.6
529.8
529.5
529.4
458.3
458.0
458.4
458.2
458.2
34.4
29.6
56.0
30.4
32.8
33.6
22.2
65.8
5.6
VTiAl
VTiSi
VTiCe
VTiZr
23.2
a
Osur ¼ surface active oxygen. b Olat ¼ lattice oxygen.
Fig. 2 TEM images of catalysts: (a) VTi; (b) VTiAl; (c) VTiSi; (d) VTiCe; (e) VTiZr.
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6 nm. The particle size increased apparently with the addition on catalysts.45,46 Table 3 showed that VTiSi catalyst exhibited
of Al, Si, Ce and Zr. Such as the particle size of sample VTiAl, larger amount of Brønsted acidic sites and less Lewis acidic
VTiSi, VTiCe and VTiZr were about 190, 390, 260 and 200 nm, sites compared with those of VTiAl, VTiCe and VTiZr catalysts.
respectively. Metal additives affected the combination of vana- Combined to the NH3-TPD results, adding Si element to VTi
dium and titanium.
catalyst brought more acid sites which mainly were Brønsted
acidity. However, the addition of Al, Zr and Ce element to VTi
catalyst resulted in smaller amount of acid sites since the weak
interaction between V and supports.
3.2 Acid properties
Acidity was one of the most signicant surface properties for the
selective oxidation of vanadium-based catalysts.43 The acidity of
the samples was measured by NH3 adsorption experiments (see
Fig. 3 and Table 3). As it could be seen, two peaks of NH3
absorption could be identied in the temperature range of 380–
530 K, which could be attributed to weak acidic site and middle
strong acidic sites, respectively. The desorption peak at low
temperature of 380–450 K might be related to the physic-sorbed
NH3 or weak acidic sites, while desorption peak at high
temperature of 450–530 K might be attributed to the strong
acidic sites.20,44 Table 3 showed that the calculated acid sites
with weak and middle strength were 462 and 162 mmol gꢀ1 on
VTi catalyst, respectively. The addition of Si to VTi catalyst
increased the number of both weak acid sites and middle strong
acid sites. However, with the addition of Al, Ce and Zr, the acid
sites decreased. The increased acid sites for VTiSi might
improve its oxidation activity for methanol to DMM.
3.3 Redox properties
The redox properties of the prepared samples were measured by
the H2-TPR techniques (Fig. 4). It was usually reported that the
reducibility of vanadium-based catalysts could be greatly
inuenced by the existing state of vanadium.47 As shown in
Fig. 4, two H2 consumption peaks at about 705 K and 793 K were
observed on VTi catalyst, which were attributed to the reduction
of the highly dispersed surface vanadium species.48 The H2
consumption peaks around 863 K and 977 K were attributed to
the reduction of bulk V2O5.49,50 This was attributed to
the following reduction sequence: V2O5 / V6O13 (863 K) and
V6O13 / V2O4 / V2O3 (977 K). VTiSi catalyst had a similar but
enhanced H2 consumption peak around 703 K which indicated
the increasing reducibility of the catalyst. The fact that VTiSi
can be reduced in lower temperature but with larger amount
shows that this catalyst could give out crystal O2 easier, as a
Chemisorption of pyridine followed by FTIR spectroscopy
was useful to probe the presence and nature of surface acid sites
Fig. 3 NH3-TPD profiles of VTi, VTiAl, VTiSi, VTiCe and VTiZr catalysts. Fig. 4 H2-TPR profiles of VTi, VTiAl, VTiSi, VTiCe and VTiZr catalysts.
Table 3 Acid distribution of V2O5/TiO2 composite catalysts
Weak acidity
Temp. (K)
Middle stronger acidity
Sample
Number (mmol gꢀ1
)
Temp. (K)
Number (mmol gꢀ1
)
Brønsted sites/Lewis sites
VTi
414
403
387
412
384
462
323
689
147
198
503
499
477
504
480
162
198
256
96
0.56
0.37
0.74
0.38
0.45
VTiAl
VTiSi
VTiCe
VTiZr
116
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result, methanol could be oxidized easier. Compared to VTi stability was carried for VTiSi catalyst at 413 K (Fig. 5). The result
catalyst, the low reduction peak at about 776 K and high showed that DMM selectivity (99%) and methanol conversion
reduction peak at about 977 K shied to higher temperature on (51%) did not change obviously within 240 h, indicating that the
VTiAl, VTiCe and VTiZr catalysts, shown the weak reducibility. VTiSi catalyst exhibited an excellent stability, which probably
related to its bigger particle size. All the catalytic data were
better than those data shown in literature.10,11,23,30,53
3.4 Catalytic activities
However, sample VTiAl showed low activity for DMM
It was reported that the selective oxidation of methanol to DMM
synthesis from methanol selective oxidation, 23% of methanol
involves two steps: (1) oxidation of methanol to FA on redox
conversion and 56% of DMM selectivity at 413 K. Aer the ZrO2
sites, and (2) condensation of the produced FA with additional
doping, the VTiZr sample showed increased DMM selectivity
methanol to form DMM on acidic sites.15 So the selective
but much lower methanol conversion compared with those of
oxidation of methanol was very sensitive to the catalysts of the
VTiSi sample. In fact, the methanol conversion kept about 28%
surface acidity and redox properties.4 The catalytic test showed
on VTiZr catalyst when reaction temperature increased from
that the reducibility and oxidizability of catalyst determines its
catalytic activity.51
Table 4 presented the catalytic performances of composite
supported V2O5/TiO2 catalysts. The sample VTi exhibited a
highest methanol conversion of 43% of and a DMM selectivity
of 89% at 413 K. The selectivity for MF was 10% and 1% for
DME. The distribution of products on VTi catalyst indicated the
surface acidity was not strong enough to effectively catalyze the
reaction of FA condensation with methanol to produce DMM,
leading to the production of MF and a small amount of DME.
The methanol conversion increased sharply with increasing
reaction temperature, meanwhile, the selectivity to DMM
decreased while the selectivity to MF and DME increased. The
sudden drop in DMM selectivity possibly came from the ther-
modynamic constrains for DMM synthesis.52
Compared to VTi catalyst, sample VTiSi showed the highest
activity of 51% methanol conversion and DMM selectivity of
99% among all samples at 413 K. Moreover, when the temper-
ature increased to 423 K, the DMM selectivity was still kept at
Fig. 5 Changes of DMM selectivity and methanol conversion with
98% with the methanol conversion of 52%. In addition, the time on stream on VTiSi catalyst at 413 K.
Table 4 Catalytic activities of V2O5/TiO2 composite catalysts in the methanol selective oxidation
Selectivity (%)
Sample
VTi
Temp. (K)
Con. of methanol (%)
FA
DME
MF
DMM
COx
DMM yield
Consumption of methanol/SBET
413
423
433
443
413
423
433
443
413
423
433
444
413
423
433
443
413
423
433
443
43
50
72
95
23
24
45
48
51
52
77
91
15
20
25
30
28
29
37
40
0
0
0
1
7
19
16
1
3
2
5
1
2
7
6
2
3
7
7
1
10
26
47
66
0
0
0
0
0
89
57
23
1
0
10
11
17
1
2
1
1
0
0
0
0
0
0
0
0
0
38
29
17
1
24
28
41
54
26
39
52
55
18
19
28
33
25
33
42
50
18
20
24
26
0
VTiAl
VTiSi
VTiCe
VTiZr
42
37
34
31
0
0
0
0
0
0
0
0
0
0
2
2
56
58
62
63
99
98
56
55
98
97
93
93
99
99
93
92
13
14
28
30
50
51
43
50
15
19
23
28
28
29
34
37
0
37
39
0
0
0
0
0
0
0
1
5
6
0
0
0
0
24800 | RSC Adv., 2015, 5, 24795–24802
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413 K to 423 K, the DMM selectivity exceeded 99% at 413–423 K few acids. The weak acidity and Brønsted acidity were identied
and 92% at 433–443 K. The catalytic performance of VTiCe as the reason for high DMM yield. The best catalytic perfor-
showed a high DMM selectivity (98%) but lowest methanol mance was obtained for sample VTiSi catalyst with high activity
conversion (15%) was obtained.
for synthesizing of DMM from methanol selective oxidation.
Particularly, the VTiSi catalyst exhibited an optimal 51%
methanol conversion with a DMM selectivity of 99% at 413 K.
Furthermore, the VTiSi catalyst displayed excellent catalytic
stability. The effect of acidity on activity and selectivity reaction
explained the high catalytic performance for methanol oxida-
tion to DMM. The activity of V–Ti–M samples decreased with
increasing of Brønsted acidity, but the yield of DMM increased
with rising of amount of Brønsted acidic sites. More active
surface oxygen species and greater amount of V4+ species on
catalyst facilitated the catalytic performance. The efficient and
stable V2O5/TiO2 composite catalyst was obtained through the
improved sol–gel method.
4. Discussion
4.1 Effect of weak Brønsted acidic sites
From the NH3-TPD results shown in Fig. 3 and Table 3, the
composite TiO2–SiO2 inuenced the catalytic performances by
providing the necessary acidic sites and promoting create a
balance for the appreciate acidic and redox sites. It could be
seen that the addition of Si species to VTi catalyst could produce
more weak acidic sites. Moreover, the partial reduced V–Ti
mixed oxides resulted in an increased number of Brønsted
acidic sites on VTiSi catalyst as shown in the FTIR spectra of
pyridine absorption analysis (Table 3). The reactivity and
distribution of acidic sites revealed that the DMM yield was
directly proportional to the surface density of the Brønsted
acidic sites on VTiSi catalyst, which accounted for its highest
DMM yield among these samples. It was further conrmed by
the consumption of methanol data in Table 4, the activity of
V–Ti–M samples decreased with increasing of Brønsted acidity,
but the yield of DMM increased with rising of amount of
Brønsted acidic sites and, agreement with earlier report.8,18,30
Acknowledgements
The authors gratefully acknowledge the nancial supports from
the International Science & Technology Cooperation Project of
Ministry of Science and Technology of China (no. 2012AA051002),
Key Project of Natural Science Foundation of China (no.
21303241) and Key Project of Natural Science Foundation of
Shanxi Province (no. 2012021005-6).
4.2 The interact effect of vanadium and composite supports
References
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The VTiSi catalyst exhibited better methanol conversion and
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24802 | RSC Adv., 2015, 5, 24795–24802
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