Selective Oxidation of Methanol to Dimethoxymethane at Low Temperatures through Size-controlled VTiOx Nanoparticles

Article abstract of DOI:10.1002/cctc.201700074

A vanadium/titanium mixed-oxide catalyst with a narrow particle-size distribution, large BET surface area, high dispersion of vanadium, and enhanced acidity and redox capabilities was prepared by a cetyltrimethylammonium bromide (CTAB) surfactant-associated vanadium oxide and titanium oxide co-precipitation route. The impact of the CTAB content on the catalyst properties, including structure, chemical states, and acidity and redox activities, was investigated by X-ray diffraction, BET surface area, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, H₂ temperature-programmed reduction, and temperature-programmed desorption of ammonia. Relative to the catalyst prepared without CTAB, the resultant catalyst exhibited significantly improved low-temperature activity for the oxidation of methanol to dimethoxymethane (DMM). A maximum methanol conversion of 53 % with 93 % selectivity to DMM were achieved over the VTiS–CTAB catalyst at 120 °C, and this optimized reaction temperature is 25 °C lower than that needed for the reference catalysts and the yield of DMM is comparable.

Full text of DOI:10.1002/cctc.201700074

Accepted Article
Title: Selective oxidation of methanol to dimethoxymethane at lower
temperature through size controlled VTiOx nanoparticles
Authors: Rui Sima, Guojuan Liu, Qiyan Wang, Ping Wu, Tingting Qin,
Gaofeng Zeng, Xinqing Chen, Ziyu Liu, and Yuhan Sun
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10.1002/cctc.201700074
ChemCatChem
ICATION
e oxidation of methanol to dimethoxymethane at lower
ature through size controlled VTiOx nanoparticles
b] Guojuan Liu,[b] Qiyan Wang,[b] Ping Wu,[b] Tingting Qin, [b] Gaofeng Zeng,*[b] Xinqing
Liu,*[b] and Yuhan Sun [b]
dium / Titanium mixed oxide catalyst with narrowed candiate for its relative high DMM yield. At the same time,
e BET surface area, higher dispersion of vanadium, the reaction temperature of VTi catalysts (130-150°C) is
ty and redox capability was prepared by
a
normally lower than that of others like heteropolyacids
mmonium bromide (CTAB) surfactant associated (180°C) and ReOx (240°C). Moreover, its activity can be
2-
anium oxides co-precipitation. The impacts of CTAB further improved by introducing SO4 (VTiS) to enhance
catalyst properties of structure, chemical states, surface acidity. In the selective oxidation of methanol, the
x activity were investigated by XRD, BET, HRTEM, deep oxidation of methanol to COx, which normally
an spectroscopy, H2-TPR and NH3-TPD in details.
denpends on the reaction temperature, should be avioded
the catalyst prepared without CTAB, the resultant as far as possible. Therefore, further declination of reaction
significantly improved low temperature activity for temperature is a critical aspect for the catalyst development.
on conversion to dimethoxymethane (DMM). The Narrowing the particle size is a promising scheme to
mance of 53% methanol conversion and 93% DMM optimize the reaction temperature because it would
achieved over the VTiS-CTAB catalyst at 120°C, increase surface area and enhance the dispersibility of both
zed reaction temperature is 25 oC lower than that of V speices and acidity cites. However, to access the VTi
alysts and the DMM yield is comparable.
catalyst with well size control and high reaction
performance is still big challanges so far.
In this work, we developed a facial and efficient method to
significantly reduce the particles size of VTiS mixed oxide
and suppress the particle aggregation through cetyltrimethyl
ammonium bromide (CTAB) associated vanadium and
titanium oxides co-precipitation (designated as VTiS-CTAB-
x, where x represents the molar ratio of CTAB to TiO2 in the
catalyst preparation). The effects of CTAB content on the
catalyst properties of structure, chemical states and
catalytic performance were investigated in details.
Compared with common VTi based nanoparticles as well as
other types of catalysts, high conversion of methanol and
selectivity to DMM was achieved over the resultant VTiS-
CTAB catalysts at much lower temperatures.
sion methanol to high-value oxygenated
becoming an important node in the chain of
nomy”, which would not only upgrade the
methanol downstream but also enhance the
efficiency of methanol. Dimethoxymethane
ontoxic and environmental-friendly methanol
product, is widely used in cosmetics,
ls, paints and rubber industry. For its high
nt and cetane number, in addition, it is
a promising diesel additive.[1] Industrially,
uced via a two-stage process involving the
ethanol to formaldehyde over silver or iron-
alysts followed by condensation reaction in a
maldehyde mixture using sulfuric acid or solid
s, which is a high temperature, corrosive,
ostly process.[2] Therefore, many efforts have
op direct synthesis of DMM from methanol
tion.
ous catalysts have been proposed for the
methanol to DMM, including Mo-based
eOx[4], heteropolyacids[5], RuOx/SiO2[6], Cu–
V-based catalysts[8-10]
.
Among them,
Ti) catalyst is considered as a promising
Figure 1 X-ray diffraction (XRD) patterns (a) and N2 sorption isotherms
of VTiS-CTAB catalysts (b).
[a]
[b]
R. Sima,
College of Environmental and Chemical Engineering,
Shanghai University
99 Shangda Road, Shanghai 200444, China
R. Sima, G. Liu, Q. Wang, P. Wu, T. Qin, Prof. G. Zeng, Prof. X.
Chen, Prof. Z. Liu, Prof. Y. Sun
CAS Key Laboratory of Low-carbon Conversion Science and
Engineering,
Shanghai Advanced Research Institute, Chinese Academy of
Sciences
The structure of VTiS-CTAB-x (x=0-1.3) catalysts were
measured by XRD. As shown in Figure 1a, the XRD patterns of
all samples display similar peaks at 25.3°, 38.6°, 48.0°, 53.9°,
55.1°, 62.7°, 70.3°, 75.0° and 82.6°, which were assigned to
(101), (112), (200), (105)，(211)，(204)，(220)，(215) and
(224) of tetragonal anatase TiO2 (JCPDS, No. 21-1272),
respectively. With different calcination temperature, TiO2 would
exist as anatase or rutile phase. It is important to avoid rutile
formation in the preparation of VTi based catalysts for its weak
acidity and declined surface area.[11] For VTiS-CTAB-0, pure
100 Haike Road, Shanghai 201210, China.
Email: zenggf@sari.ac.cn (G. Z.); liuziyu@sari.ac.cn (Z. L.)
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cctc.201700074
ChemCatChem
ICATION
can be obtained with calcination at 400°C for 5 h.
was formed for the catalysts with CTAB under
onditions because the strong exothermal
of CTAB at 350°C would elevate the real
inner particles and therefore induce the phase
O2 from anatase to rutile (Figures S1 & S2). The
be successfully avoided by remaining at 300°C
omposition of CTAB (Figure 1a). On the other
e signals of vanadia were observed from XRD
e 1a). But the elemental measurement suggests
atios of these VTiS-CATB samples are in the
25 (Table S1). Therefore, it reveals that vanadia
hly dispersed on the TiO2 surface or the size of
ne is out of the detection limit of XRD. Estimated
quation, the particle size of VTiS-CTAB-x was
uced with the increase of CTAB amount in the
m 19 nm for VTiS-CTAB-0 to 7 nm for VTiS-
Figure S3).
ws the N2 sorption isotherms of VTiS-CTAB
mples exhibit the type-IV isotherms with obvious
op in the region of P/P0>0.45 due to the capillary
the mesopores.[12] The corresponding pore-size
suggests bimodal distributions (Figure S4). The
ere generated from the intra-aggregation of
nd the large pores were contributed by the inter-
catalysts.[13] With increasing of CTAB content in
X-ray photoelectron spectroscopy (XPS). The ratio of V/Ti on the
near-surface of catalysts measured by XPS is much higher than
that of bulk determined by XRF, indicating that vandia was
mainly dispersed on the particle surface (Table S1). The sulfur
content was decreased with the increase of CTAB, suggesting
that the sulfur species were not tightly anchored to the catalysts
and therefore lost during the decomposition of CTAB in
calcination. At the same time, TG-MS results in Figure S6
proved that CTAB was completely decomposed to gaseous
species and no carbon source from CTAB was residual in
catalyst due to no signal of CO/CO2 was detected.
In the preparation of VTiS-CTAB-0, the precipitated
nanoparticles tend to further aggregation in order to reduce their
surface energy.[15] It is known that the precipitated particles
feature a negatively charged nature due to the abundant
hydroxyls on the particle surface. Therefore, the cationic
surfactant CTAB can facilely adsorb on the particle surface
through opposite charges attraction. The adsorbed CTAB can
protect particles from agglomeration through the formation of
micelles shell with the hydrophobic tail group inward towards
the center nanoparticles and the hydrophilic head group
diverging out towards the aqueous solution.[16] On the other
hand, the adsorption of CTAB results in the enhancement of
electrostatic repulsion between particles, which restrains the
aggregation of particles.[17] In the calcination, the surfactant
CTAB surrounding the particles help in keeping the particles
apart from each other and suppressing the formation of
agglomerates through neck connect and sintering.[18] Obviously,
the CTAB coverage and the surface charge density depend on
the addition amount of CTAB in the catalyst preparation.
Therefore, the particle size was gradually reduced with the
increase of CTAB until the excess CTAB was added.
e
amounts of the intra-aggregated pores
ased owing to the decomposition of CTAB in the
erated more intra-aggregated pores. At the same
ggregated pore size was decreased from 15 nm
0 to 5 nm for VTiS-CTAB-1.3, which indicates
gation of catalysts were suppressed and the
ticle size was significantly decreased with CTAB
y of catalysts was measured by TEM, as show in
. The particle size of VTiS-CTAB-0 estimated by
ing 100 particles is in the range of 10-28 nm with
f
17 nm (Figure 2a). As comparison, the
mensions were significantly declined with the
AB, in which the average size is 14/12/10/6/7 nm
-0.2/0.4/0.6/1.0/1.3, respectively (Figures 2c &
EM images of all the catalysts reveal that the
ructure of the interplanar spacing was 0.35 nm,
natase phase TiO2 (101). Therefore, the TEM
both particle size and phase structure of VTiS-
nsistent with the XRD results. The selected area
ion (SAED) pattern of VTiS-CTAB-0 exhibits a
nature (Figure 2b inset). But such polycrystalline
ed in the pattern of VTiS-CTAB-1.0 (Figure 2d
e decrease of particle dimensions.[14] Similar to
, no vanadia crystalline structure was observed,
vanadia was highly dispersed on the surface of
ry fine size.
se of particle size, the Brunauer-Emmett-Teller
area of catalyst was remarkably increased,
39 m2/g for VTiS-CTAB-0 to 68 m2/g for VTiS-
urther to 125 m2/g for VTiS-CTAB-1.3 (Table S1).
on of catalysts was analyzed by X-ray
RF), energy dispersive spectrometer (EDS) and
Figure 2 Transmission electron microscopy (TEM) images of (a) VTiS-
CTAB-0 and (c) VTiS-CTAB-1.0; HRTEM images and selected area
electron diffraction (SAED) patterns of (b) VTiS-CTAB-0 and (d) VTiS-
CTAB-1.0.
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10.1002/cctc.201700074
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COMMUNICATION
o determine the surface chemical states, the catalysts
ere investigated by Raman spectroscopy and XPS.
igure 3a shows the Raman spectra of VTiS-CTAB
atalysts from 200-1200 cm-1. The bands at 642, 516, and
06 cm-1 appeared in all catalysts are assigned to Ti-O
roup of anatase.[10] The bands at 285, 531, 705 and 1005
m-1 assigned to the crystalline V2O5 were detected from
TiS-CTAB-0 and then gradually weaken with CTAB
dditions. For VTiS-CTAB-0.8/1.0/1.3, none of these V2O5
rystal signals can be observed excepting one shoulder
eak at 531 cm-1. At the same time, bands at 1034 cm-1,
ssigned to the V=O (vanadyl) stretch vibration of
etrahedrally coordinated vanadium in O=V(-O)3 [19], and
47 cm-1 due to V-O-V linkage for polymeric vanadate
pecies[20] were arisen from VTiS-CTAB-0.6 and further
tensified for VTiS-CTAB-1.0. It is reasonable to deduce
hat vanadium species exist as crystalline V2O5 for VTiS-
TAB-0 and then transfer to polymeric vanadate species
or VTiS-CTAB-0.6/1.0 and finally form monomeric for VTiS-
TAB-1.3. Furthermore, it also suggests that vanadium
may exist in lower valences rather than V5+ with the addition
f CTAB.
O1s spectra can be divided into three peaks at 529.9 ± 0.1,
530.9 ± 0.1, and 531.9 ± 0.1 eV (Table S2), which assigned to
lattice oxygen of TiO2 + V2O5 oxides, -OH and sulfate oxygen,
respectively. [10] The sulfate oxygen content decreased with the
increment of CTAB, which is consistent with the sulfur
concentration in catalyst bulk as shown in Table S1. The S2p1/2
curves at the binding energy of 169.1 ± 0.1 eV is a typical signal
of S6+ oxidation state[23], suggesting that the sulfur compound is
2-
existed as SO4
.
The redox properties of catalysts were measured by H2-
TPR, as shown in Figure 3c. For each sample, two peaks,
one strong peak at < 500 °C associating with one weak
signal around 600 °C, were presented. The reduction
temperature in the H2-TPR profiles indicates the different
types of oxygen-containing vanadium species on TiO2
surface. With relatively low vanadium loadings, the
formation of strongly bound monomeric VOx groups with
three V–O bonds to the support and a terminal partly
hydrated V=O bond are found at ambient conditions.[24]
With the increase of vanadium loading, some V-O-support
bridges rearrange themselves into V–O–V[25], increasing the
degree of polymerization. These still-amorphous vanadia
species turn into crystalline ones when the V coverage was
further increased.[26] As monomeric and polymeric VOx
species are easier to be reduced than crystalline V2O5, the
reduction peak at lower temperature was assigned to the
amorphous vanadia species and the peak at high
temperature was contributed by crystal V2O5. Moreover, the
small peak area at higher temperature indicates that the
percentage of crystal V2O5 is very low in the total vanadia
species, which is well consistent with the XRD, TEM and
Raman results. On the other hand, the reduction
temperature was gradually decreased with the addition of
CTAB, from 484 °C for VTiS-CTAB-0 to 451 °C for VTiS-
CTAB-1.3. It indicates that the percentage of amorphous
vanadia was increased with the addition of CTAB, which
agrees with the surface V loading density in Table S1.
The surface acidity of catalysts was investigated by NH3-
TPD (Figure 3d). For VTiS-CTAB-0, two peaks were arisen
at 123°C and 270°C, corresponding to the weak acid and
medium strong acid, respectively. With the increase of
CTAB, as comparison, the peaks of weak acid were slightly
shifted to 129, 130, 134, 147 and 148°C for VTiS-CTAB-
0.2/0.4/0.6/1.0/1.3, respectively. It indicates that the catalysts
prepared with CTAB have higher acidity strength. At the
same time, the peak area of medium strong acid was
significantly increased with the addition of CTAB, which
suggests that the number of acid sites were improved
(Table S3). The enhanced acid properties of VTiS-CTAB
might be caused by the particle size effects. The electronic
properties of small size particles are different from those of
the bulk materials due to the surface imperfections. The
local charges caused by the imperfections could increase
the number of acidic sites.
gure 3 (a) Raman spectra, (b) XPS, (c) H2-TPR and (d) NH3-TPD results of
atalysts with different CTAB/TiO2 ratios from 0-1.3.
he XPS V2p3/2 peaks of all samples, as shown in Figure 3b,
ere severally composed of two Gaussian peaks with binding
nergies at 516.6 eV and 517.3 eV, assigning to V4+ and V5+,
[21]
espectively.
With the increasing of CTAB in the catalyst
reparation, the content of V4+ was gradually increased. As
ummarized in Table S2, the percentage of V5+ for VTiS-CTAB-x
urface was decreased from 66.6% (VTiS-CTAB-0) to 39.7%
VTiS-CTAB-1.3). High dispersed vanadium oxide could be
educed from V5+ to V4+ upon exposure to X-radiation in ultra-
gh vacuum,[22] which means that the increment of CTAB
ading to better dispersion of vanadium. On the other hand, the
The methanol oxidation to DMM over these VTiS-CTAB
catalysts was comparatively investigated in micro-fixed bed
reactor. As shown in Figure 4a, the conversion of methanol
over all catalysts were increased with the elevation of
reaction temperature, while the selectivity of DMM was
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10.1002/cctc.201700074
ChemCatChem
ICATION
ghtly at relatively low temperature and then
decline at higher temperature. Reflecting to
MM, it was increased with temperature firstly
eclined at elevated temperature. The
maximum DMM yield was slightly decreased
investigated (Table S5). Methanol conversion increased
obviously with increasing O2 content both at GHSV 945 and
549 h-1, while DMM selectivity declined slightly, indicating
the introduction of oxygen to methanol oxidation reaction
can improve methanol conversion, but too much O2 lead to
other the formation of by-products.
r VTiS-CTAB-0/0.2 to 120 °C for VTiS-CTAB-
indicates that the catalysts prepared by the
CTAB have superior low temperature activity
oxidation to DMM. At the temperature less
he VTiS-CTAB-0.2/0.4/0.6/1.0 catalysts also
ative higher activity than that of VTiS-CTAB-0.
example, the methanol conversion and DMM
er VTiS-CTAB-0 were 5.4% and 100%,
While methanol conversion was increase to
S-CTAB-0.2 and further rose to 18.2% and
S-CTAB-1.0/1.3 with the almost unchanged
ty. At 120°C, the VTiS-CTAB-1.0 exhibited
nol conversion and 92.8% DMM selectivity
est DMM yield of 49.6%, while only 27.4%
s obtained for VTiS-CTAB-0. The maximum
er the VTiS-CTAB-1.3, i.e. 49.1%, which is
than the yield of VTiS-CTAB-1.0, was also
o
20 C. It indicates that the optimized ratio of
the catalyst preparation is around 1.0.
DMM selectivity and yields of VTiS-CTAB-
eclined rapidly due to the increased formation
when the temperature higher than 130°C. In
oxidation to DMM over VTiS-CTAB catalysts,
by-products include methyl formate (MF),
(FA), dimethyl ether (DME) and COx, which
nd on the reaction temperature (Figure S7).
e selectivity of MF, FA and DME were
the elevated temperature, which is varied in
-66.8%, 0-15.2% and 0-0.5% for MF, FA and
Figure 4 Catalyst performance of VTiSS-CTAB catalysts at different reaction
temperature with GHSV 945h-1 (a), differreent GHSV at 90°C (b).
In conclusion, VTiS mixed oxide catalysts with significantly
narrowed size were successfully synthesized via a CTAB
associated co-precipitation method. In this approach, the
resultant catalysts have smaller grain size, more pores, larger
BET surface area and higher dispersion of vanadium. At the
same time, both of the oxidation property and acidity of catalyst
were enhanced. The resultant catalyst exhibited higher activity
for methanol selective oxidation to DMM at lower temperature, in
which 53.4% methanol conversion and 93% DMM selectivity
were achieved at 120°C. The low reaction temperature of
oxidation reaction is considered as the crucial factor to avoid the
deep oxidation of methanol.
o
o
to 150 C. At 150 C, a little amount of CO2
selectivity for these catalysts was detected.
e
declination of reaction temperature is
ot only suppress the formation of methanol
y-products but also avoid the deep oxidation
to COx. According to the mechanism of
27]
ective oxidation[8,
, methanol oxidation
ate-determining C-H bond activation steps in
ps to form FA via redox cycles, and then FA
h methanol to form hemiacetal intermediates
. The hemiacetal was further condensed with
rm DMM on acidic sites or oxidized to form
[28]
sites.
The active sites of this reaction are
Experimental Section
ersed vanadia species VOx on TiO2. With the
articles size, the redox and acidity properties
Catalyst synthesis:
B
catalysts were significantly improved.
h literature, the methanol conversion and
The catalyst was prepared by a modified co-precipitation. In detail, 4 g of
VOSO4 and 17.4 g of Ti(SO4)2 were dissolved in 400 mL water with
adding 8.6 g H2SO4. Then it was slowly dropped into 500 mL CTAB
solution. Further 440 mL NH3•H2O (2.8 wt.%) solution was also dropped
into the CTAB solution until the pH = 9.3. After 2 h aging, the particles
were filtered, dried and calcined at 400°C for 5 h. The catalyst was
denoted as VTiS-CTAB-x, where x is the ratio of CTAB to Ti2O.
o
ty at low temperature like 120 C is higher
er type catalysts (Table S4). [29]
also investigated the influence of GHSV on
tivity, as shown in Figure 4b. VTiS-CTAB-1.0
er methanol conversion and DMM yield than
but a slightly lower DMM selectivity with
50 to 1000 h-1. To further study the DMM
e effects of feedstock composition was also
Methanol oxidation:
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ICATION
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knowledge financial support from Natural Science
China (21506243), Shanghai Committee of
Technology (14DZ1203700) and the Youth
motion Association of CAS (Y624211401).
hanol oxidation •dimethoxy methane• size
ant • low temperature oxidation
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10.1002/cctc.201700074
ChemCatChem
ICATION
he Table of Contents (Please choose one layout)
NICATION
e catalyst with narrowed
e surface area, higher
hanced acidity and
was prepared by a
Rui Sima, Guojuan Liu, Qiyan Wang,
Ping Wu, Tingting Qin, Gaofeng Zeng,*
Xinqing Chen, Ziyu Liu,* and Yuhan Sun
Page No. – Page No.
ated co-precipitation,
ellent methanol
Selective oxidation of methanol to
dimethoxymethane at lower
temperature through size controlled
VTiOx nanoparticles
M performance at lower
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