Highly selective oxidation of methanol to dimethoxymethane over SO42-/V2O5-ZrO2

Article abstract of DOI:10.1039/c7nj01295k

The sulfated vanadia-zirconia SO₄^2-/V₂O₅-ZrO₂ catalyst, prepared by an impregnation method and calcined at various temperatures, was found to have a high catalytic activity for methanol oxidation to dimethoxymethane in a fixed bed reactor and a batch autoclave using O₂ as the oxidant due to the crystalline V₂O₅ dispersed on the surface as redox active sites, as well as stronger acidic sites.

Full text of DOI:10.1039/c7nj01295k

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Highly selective oxidation of methanol to
dimethoxymethane over SO42À/V2O5–ZrO2†
Cite this: New J. Chem., 2017,
41, 8370
Meng Tao, Hongxia Wang, Bin Lu,* Jingxiang Zhao
and Qinghai Cai
*
The sulfated vanadia–zirconia SO42À/V2O5–ZrO2 catalyst, prepared by an impregnation method and
calcined at various temperatures, was found to have a high catalytic activity for methanol oxidation to
dimethoxymethane in a fixed bed reactor and a batch autoclave using O2 as the oxidant due to the
crystalline V2O5 dispersed on the surface as redox active sites, as well as stronger acidic sites.
Received 19th April 2017,
Accepted 7th July 2017
DOI: 10.1039/c7nj01295k
rsc.li/njc
Therefore, V2O5/TiO2 and acid-modified V2O5/TiO2 catalysts have
been paid more attention9,11 and studies on the dependence of the
Introduction
Dimethoxymethane (DMM) is considered an environmentally surface composition and structure on the catalytic performance of
friendly chemical. It is widely used as an intermediate in these catalysts have been more and more detailed, although the
organic synthesis and as an excellent solvent in the pharmaceutical contributions of different VOx species to the catalytic performance
and perfume industries because of its low toxicity.1 Recently, it was have been debated for some time.12
found that DMM can be used as a fuel additive due to its high
Zirconia is well known as the only transition metal oxide
oxygen content and cetane number,2 and thus the demand for that has both surface acidity and basicity.13 It can be directly
DMM may be considerably increased. Traditionally, DMM is used as a catalyst in reactions such as hydrogenation, alkylation,
produced by the reaction of formaldehyde (FA) with methanol condensation, etc. Also it is widely applied as a catalyst support
over acidic catalysts.3 However, the traditional approach has because zirconia possesses an excellent ion-exchange property
demerits such as being intricate and costly owing to the high and an enrichment of oxygen vacancies on the surface, as well
reaction temperature and equipment corrosion caused by the as thermal stability and chemical inertness. Furthermore, it has
acidic catalysts. This means that further development of a been recorded that methanol can react with the surface hydroxyl
potential path for DMM synthesis by the one-step selective oxidation groups of zirconia to form methoxide groups with the concurrent
of methanol, considering the economical and environmental release of water.14 Considering the superiority of zirconia, this
advantages, is required. Some catalysts used in this one step paper reports the preparation and characterization of an
procedure, including Re/g-Fe2O3,4 RuOx/SiO2,5 crystalline SO42À/ZrO2–V2O5 catalyst. The surface structure and composition
9
SbRe2O6,6 heteropoly acids,7 Cu-ZSM-5,8 V2O5/TiO2–Ti(SO4)2
of the catalyst was determined by XRD, FT-IR spectroscopy, XPS,
and VOx/TS-1 10 etc., have been reported so far, where three SEM, TEM and N2 adsorption–desorption. The catalytic performance
methanol molecules are incorporated into one DMM molecule of SO42À/ZrO2–V2O5 and the relationship between the surface nature
via oxidation of methanol to formaldehyde by O2 gas and and the catalytic activity of the catalyst for methanol oxidation to
subsequent reaction of formaldehyde with methanol. It is an DMM were investigated in a fixed bed reactor and a batch autoclave.
environmentally benign and atom-economic reaction as the The oxidation of methanol by O2 to DMM in a batch autoclave has
only byproduct is water. Irrespective of the catalytic system, all been rarely reported in the previous literature.
the studies suggest dual active sites involving redox and acidic
sites for methanol conversion. Additionally, Zhao et al. pointed out
2
¨
that the best catalyst had both redox and Bronsted acidic sites.
Experimental
Preparation of the catalysts
Key Lab for Photonic and Electronic Bandgap Materials, Ministry of Education,
School of Chemistry and Chemical Engineering, Harbin Normal University,
No. 1 Shida Road Limin development Zone, Harbin 150025, P. R. China.
E-mail: caiqinghai@yahoo.com, lupin630411@163.com; Fax: +86-451-88060580;
Tel: +86-451-88060580
15 g of a ZrOCl2Á8H2O aqueous solution (1.5 mol LÀ1) was
added to a three necked flask fitted with a condenser and a
stirrer. Then, an ammonia solution (25%) was added dropwise
to the solution to reach about pH 10 under violent stirring.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj01295k
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After dripping, the mixed solution was aged for 12 h, followed The catalytic test and analytical procedure
by filtering to remove the filtrate. The obtained gel was washed
The catalytic tests for methanol oxidation were carried out in a
fixed bed quartz microreactor with a 6 mm internal diameter at
atmospheric pressure. About 1 g of the catalyst sample (40 mesh)
diluted with quartz sand of the same volume and size was loaded
per run. The methanol, oxygen and nitrogen were fed into the
reactor with the molar ratio of methanol to oxygen to nitrogen in
the feed being 1 : 100 : 300 and the volume hourly space velocity
(VHSV) was 1203 mL gÀ1 hÀ1. The products, including methanol,
DMM and dimethyl ether (DME), were analyzed by online GC
(Linghua 9890A, Shanghai) with an FID detector equipped with
an HP-5 30 m  0.32 mm  0.25 mm column and a GC-MS
(Agilent GC 7890A-5975c).
with deionized water so that no chloride ions were detected.
After that, it was dried at 120 1C for 12 h, and then calcined at
800 1C for 2 h in the muffle furnace to afford ZrO2.
1 g of ZrO2 and 1 g of NH4VO3 were dissolved in 10 mL 23%
H2SO4 aqueous solution, stirring at room temperature for 12 h.
Then, the mixture was filtered to remove the filtrate. The
obtained solid was washed with deionized water to reach about
pH 7.0, followed by drying at 120 1C for 6 h and calcination at
400, 450, 500, 550 and 600 1C for 2 h to afford the acid-modified
SO42À/ZrO2–V2O5 catalysts denoted as ZrVS-x, where x represents
the calcination temperature.
The batch catalytic reaction was conducted in a stainless
steel 250 mL autoclave with a mechanical stirrer in an electric
heater. In a typical procedure, 20 mL (0.5 mol) of anhydrate
methanol and 0.2 g of the catalyst were charged into the reactor
in turn. After purging three times with O2 gas, the reactor was
pressured to an initial pressure of 1 MPa O2 gas and then
heated to 180 1C with fast stirring and the reaction was carried
out at this temperature for 6 h. At the end of the reaction, the
solution was filtered to remove the catalyst and the filtrate was
analyzed by GC (Agilent 7820) with an FID detector equipped
with an HP-5 30 m  0.32 mm  0.25 mm column and a GC-MS
(Agilent GC 7890A-5975c).
Catalyst characterization
X-ray diffraction (XRD) was performed on a Bruker AXS D8
advanced X-ray diffractometer (Cu Ka, l = 0.15406 nm, 40 kV
and 36 mA) in the step scanning mode (0.02 s per step) with the
2y range between 10 and 901. The morphologies of the catalysts
were observed using a Hitachi S-4800 scanning electron micro-
scope (SEM). The preparation procedure of the samples is as
follows: conductive adhesive was daubed on the SEM stage, and
then powder samples were uniformly sprayed by a syringe on
the stage, followed by shaking the stage to disperse the samples
and smoothing the aggregated powder with a syringe needle.
The FT-IR spectrum was recorded on a Vertax-80 spectrometer
in a KBr matrix in the range of 400–4000 cmÀ1. Transmission
electron microscopy (TEM) was carried out using an FEI Tecnai
F20. X-ray photoelectron spectroscopy (XPS) was performed on
Results and discussion
a Kratos-AXIS ULTRA DLDX X-ray photoelectron spectrometer. Characterization of the catalysts
The N2 adsorption/desorption isotherms were measured using
a quantachrome NOVA2000E instrument.
XRD. The XRD patterns of the catalysts ZrVS-x with various
treatment temperatures of 400, 450, 500, 550 and 600 1C are
presented in Fig. 1. It can be seen from the figure that typical
diffraction peaks of ZrO2 can be observed for all the samples.
Particularly, the samples ZrVS-450, ZrVS-500, ZrVS-550 and
ZrVS-600 exhibited broader diffraction patterns at a 2y of about
121, indicating that an amorphous phase existed as part of their
phase compositions. Crystalline V2O5 was detected at 2y values
of 20.4 and 26.21 only for the samples ZrVS-450, ZrVS-500 and
The H2-TPR measurement was performed on TP-5076 TPD/
TPR dynamic adsorption apparatus (Tianjin Xianquan Ltd co.).
The catalyst sample (1 g) was placed in a quartz reactor and
subjected to a gas flow of N2 at 0.1 MPa and 400 1C for 50 min,
followed by cooling down to ambient temperature. After that,
the sample was heated in a H2–N2 flow containing 10 vol% H2
(30 mL minÀ1) from room temperature to 800 1C at a rate of
6.4 1C minÀ1. The amount of H2-consumption during the
heating for reduction was measured by a thermal conductivity
detector (TCD).
NH3-TPD was also conducted in the same apparatus. The
catalyst sample (1 g) was pretreated in a He flow (30 mL minÀ1) at
400 1C for 50 min and then it was cooled down to room temperature.
After that, the sample was saturated with NH3 (15 mL minÀ1) for
30 min, followed by purging with He for 1 h to remove the
physically adsorbed NH3. After that, the sample was heated in a He
flow from ambient temperature to 800 1C at a rate of 6.4 1C minÀ1
.
The amount of NH3 desorption was measured by the TCD. The
surface area was measured by nitrogen adsorption at 77 K on a
quantachrome NOVA2000E apparatus after heat pretreatment
under vacuum for 3 h at a temperature of 300 1C. The content of
vanadium and zirconium was measured using an Optima 8000
ICP-OES (Perkin Elmer company) and the sulfur content in the
catalyst was determined by gravimetric analysis.
Fig. 1 XRD patterns of the ZrVS-x catalysts with various treatment
temperatures.
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Fig. 2 FT-IR spectra of the ZrVS-x catalysts with various treatment
temperatures.
ZrVS-550, indicating that crystalline V2O5 was not formed at
a treatment temperature of 400 1C. Unexpectedly, the V2O5
diffraction peaks disappeared for the sample ZrVS-600, maybe
due to the interactions between V2O5, ZrO2 and other species at
high temperature.
FT-IR. FT-IR spectra of the samples ZrVS-400, ZrVS-450,
ZrVS-500, ZrVS-550 and ZrVS-600 are shown in Fig. 2. The two
bands at 3435 and 1626 cmÀ1, attributed to hydrogen bonded
O–H groups and H–O–H groups, respectively, were observed for
all the samples. This indicated that these prepared materials
contained a number of surface hydroxyl groups even though
ZrVS-600 was calcined at a temperature as high as 600 1C. The
sharp and stronger bands at 2022 cmÀ1 were likely ascribed to
V5+QO overtone bands.15 However, a sharp band at 1026 cmÀ1
assigned to the VQO stretching vibration mode of crystalline
V2O5 appeared in the spectra of ZrVS-450, ZrVS-500 and ZrVS-550,
and was confirmed by the XRD patterns of the samples ZrVS-450,
Fig. 3 SEM and TEM images of the catalysts calcined at various temperatures.
SEM: (a) ZrVS-400; (b) ZrVS-450; (c) ZrVS-500; (d) ZrVS-550; and (e) ZrVS-
600. TEM: (f) ZrVS-500.
Table 1 Average pore capacities and specific surface areas of the ZVS-x
catalysts
Specific surface
Average pore
Catalyst
area (m2 gÀ1
)
capacity (cc gÀ1
)
ZVS-400
ZVS-500
ZVS-600
10.6
15.5
10.0
0.051
0.078
0.059
ZrVS-500 and ZrVS-550. For the sample ZrVS-400, the band at materials, inevitably reducing the specific surface area and the
1026 cmÀ1 was not observed, nor was it observed for the sample average pore capacity for ZVS-600. The TEM image of ZrVS-500 is
ZrVS-600, showing that the crystalline V2O5 was not formed at the very similar to its SEM image, confirming that the bulk species is
treatment temperature of 400 1C and it was likely converted into constituted of about 100–200 nm particles.
2À
VxOy species by the interaction with ZrO2 and SO4 species at
XPS. The XPS spectra of ZrVS-500 as shown in Fig. 4 exhibit
600 1C. This finding was also in agreement with the XRD analysis. the binding energies of O1s, V2p, Zr3d and S2p3/2. As seen in
SEM and the N2 adsorption–desorption technique. The Fig. 4c, the O1s spectrum has been deconvoluted into two
morphologies of the catalysts calcined at various temperatures subbands at 530.4 eV and 532.3 eV, which were respectively
were examined by electron microscopy as shown in Fig. 3. The assigned to the lattice oxygen of ZrO2 + V2O5 and the sulfate
image of ZVS-400, presented in Fig. 3a, exhibits bulk species, oxygen of SO42À. The band at 524.9 eV was likely assigned to the
which are constituted of about 100–200 nm particles. On V2p1/2 contribution. The S2p3/2 line at a binding energy of about
increasing the calcination temperature the particles were well 169.0 eV is typical of sulfur in the S6+ oxidation state (Fig. 4b),
dispersed and porosity was found for ZVS-450 and ZVS-500 as such as in Na2SO4 or Fe2(SO4)3.16 This finding suggested that
2À
shown in Fig. 3b and c. Thus, the specific surface area of the sulfur compound existed in the SO4 form, which is in
the sample ZVS-500 (15.5 m2 gÀ1) is larger than that of ZVS-400 agreement with the existence of SQO observed in the FT-IR
(10.6 m2
g
À1), as shown in Table 1. Also, the average pore spectra. However, the Zr3d lines at binding energies of 182.5 and
capacity of the former is larger than that of the latter. At the 184.6 eV may be due to the ZrO2 contribution, which is in
higher calcination temperatures of 550 and 600 1C, for agreement with the previous report.17 The V2p3/2 line (Fig. 4a) at
the samples ZVS-550 and ZVS-600, the particles dispersed on a binding energy of 517.4 eV may be assigned to the V2O5
the surface were obviously aggregated to form bulk-shaped contribution. The elemental compositions, atomic concentrations
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Table 2 Composition of ZrVS-500 obtained from XPS, ICP-OES and
chemical analysis
Elements
O1s
V2p
Zr3d
S2p
Atomic conc.a
Mass conc.a
Mass conc.b
65.4
29.0
9.2
13.0
19.7 (20.1)
21.7
54.8
47.7 (48.1)
3.6
3.2
2.5 (2.5)
a
b
Determined by XPS. Determined by ICP-OES and chemical analysis;
the data in parentheses represent the composition of the used catalyst.
Fig. 5 NH3-TPD of the ZrVS-x catalysts with various calcination temperatures.
the surface and the profiles of the ZrVS-x catalysts are shown in
Fig. 5. The profiles exhibit various broad peaks in the tempera-
ture range from 30 to 800 1C. For the profile of ZrVS-400, the
weak and wide peaks corresponding to the low temperatures of
about 115 and 185 1C could be ascribed to NH3 chemisorbed on
the weak acid sites and the peak centered at about 550 1C was
assigned to strong acid sites. However, the strength of the peak
at about 550 1C, corresponding to strong acid sites on the
surface of the ZrVS-x catalysts, was increased as the calcination
temperature was increased from 400 to 500 1C; on the contrary
the strength was decreased when the calcination temperature
was increased further to 550 1C. Additionally, a new peak
appeared at about 627 1C in the profile of ZrVS-550, which
was assigned to stronger acidic sites although its intensity was
weaker. In particular, for the sample ZrVS-600, the peak at
550 1C almost disappeared and the new broad peak was
increased and expanded from 636 to about 750 1C, indicating
its much stronger acidic sites as compared with those of the
other samples, ZrVS-400, ZrVS-450, ZrVS-500 and ZrVS-550.
The H2-TPR profiles of the samples ZrVS-400 and ZrVS-500
are displayed in Fig. 6. As shown in the figure, an intense
Fig. 4 XPS spectra of ZrVS-500.
and mass concentrations measured by XPS and ICP-OES were H2-consumption peak at around 527 1C is observed in the
obtained as shown in Table 2. From these data, the surface H2-TPR profile of the ZrVS-500 sample. The H2-consumption was
composition of the ZrVS-500 was confirmed to be Zr6V2.5SO18. attributed to the reduction of the vanadium species from V5+ to
However, the bulk composition of Zr, V and S determined by ICP- V3+.17 For the ZrVS-400 sample, very weak H2-consumptions at
OES and chemical analysis is also shown in the table. The higher about 450 and 533 1C were observed, which were likely attributed
content of V suggests that some of V2O5 or other VOx species exist in to VxOy species.
the catalyst bulk phase. In addition, the dispersion of V2O5 or other
Catalytic activity. The catalytic performance of the various
VOx species on the catalyst was nonuniform since the content of Zr, catalysts calcined at different temperatures (ZrVS-400, ZrVS-450,
V, S and O in various areas of the catalyst surface measured by EDS ZrVS-500, ZrVS-550 and ZrVS-600) was investigated for the
was very different from each other, as shown in Fig. SI1 (ESI†).
oxidation of methanol by O2 to dimethoxymethane (DMM)
NH3-TPD and H2-TPR. The NH3-TPD method was used to and the results are presented in Fig. 7. As seen in the figure,
characterize the amount and acid strength of the acid sites on the catalytic activities for all these catalysts were increased as
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Fig. 8 The catalytic activity of ZrVS-500 at various feed flows of methanol.
Reaction conditions: O2 5 mL, N2 15 mL, catalyst 1 g and at a temperature of
200 1C; Con – conversion, Sel – selectivity.
Fig. 6 H2-TPR profiles of (a) ZrVS-500 and (b) ZrVS-400.
it has a much stronger acidity than ZrVS-400, ZrVS-450, ZrVS-500
and ZrVS-550. This finding suggested that the redox active sites
that arose from the crystalline V2O5 on the surface of the catalyst
were vital for methanol oxidation because the first step of methanol
oxidation to formaldehyde is the rate determining one.10e
Interestingly the DMM selectivity, at all times o8 h, remained
at about 90% when using the catalyst ZrVS-500.
As the flows of O2 and N2 were constant, the effect of the
feed flow of methanol on methanol conversion and DMM
selectivity of ZrVS-500 is presented in Fig. 8. As shown in the
figure, the conversion was increased as the flow of methanol
increased from 0.02 to 0.05 mL minÀ1, up to a maximum value
of ca. 40%. At that time the selectivity almost remained at about
90% during the reaction time from 4 to 8 h. The DMM selectivity
was lower at a flow of methanol of 0.02 mL minÀ1 as compared
with other flows. This phenomenon was likely ascribed to
further oxidation of DMM at the relatively higher O2 content
of this system. When the methanol flow was continuously
increased, the conversions were adversely decreased as a whole,
which was related to the dilution of methanol with oxidant.
Fig. 9 shows the effect of the reaction temperature on the
methanol conversion and DMM selectivity. The conversion
Fig. 7 The catalytic activity of the ZrVS-x catalysts with various calcination
temperatures. Reaction conditions: methanol 0.5 mL, O2 5 mL, N2 15 mL
and at a temperature of 200 1C; Con – conversion, Sel – selectivity.
the reaction time increased from 1 h to 8 h. The conversion was
almost unchanged as the time increased from 8 to 9 h, achieving
thermodynamic equilibrium at this time. However, the calcination
temperature during the preparation of the catalysts evidently had
an impact on their catalytic activities. With an increase in the
calcination temperature from 400 to 500 1C for a reaction time
47 h, the catalytic activities of the catalysts ZrVS-400, ZrVS-450
and ZrVS-500 decreased in the order ZrVS-500 4 ZrVS-450 4
ZrVS-400. Subsequently, the activities decreased for ZrVS-550
and ZrVS-600. The activity difference between these catalysts
calcined at various temperatures was surely ascribed to their
surface structure and composition. As shown by previous analyses
by XRD and FT-IR spectroscopy, the V2O5 crystalline phase
dispersed on the catalyst surface was found in the samples
ZrVS-450, ZrVS-500 and ZrVS-550. Additionally, stronger strengths
of the strong acidic sites were also found on the surfaces of these
three catalysts. Notably, ZrVS-500 possessed a stronger acidic
property than the other two catalysts. Also, the N2 adsorption–
desorption technique provided positive information that the
specific surface area of ZrVS-500 is high. As a result, ZrVS-500
presented excellent catalytic activity for the oxidation of methanol,
which was attributed to the crystalline V2O5 existing on the surface
achieving redox active sites, a higher acidic nature and increased
specific surface area. As for ZrVS-600, it presented the lowest
Fig. 9 The catalytic activity of ZrVS-500 at various reaction temperatures.
Reaction conditions: methanol 0.5 mL, O2 5 mL, N2 15 mL and catalyst 1 g;
activity for methanol oxidation among these catalysts although Con – conversion, Sel – selectivity.
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increased from 23.9 to 38.0% on ZrVS-500 as the reaction
temperature increased from 160 to 200 1C at a reaction time
of 8 h; the selectivity was basically constant at this condition
being 488.0%. When the temperature continuously increased
from 200 to 220 1C, a sharp drop of the conversion was
observed in the range of the reaction time from 4 to 9 h. This
behavior was probably a result of thermodynamic constraint for
DMM synthesis. It is well known that the direct synthesis of
DMM from methanol oxidation includes two steps; one is the
oxidation of methanol to generate formaldehyde HCHO, and the
other is the condensation of the HCHO produced and methanol.
The former requires redox active sites for oxidation and the latter
needs acidic sites on the catalyst surface for condensation. The
reaction formulae are shown in the following, respectively:
Fig. 10 Stability of the catalyst ZrVS-500.
CH3OH + 1/2O2 = HCHO + H2O
(1)
(2)
2CH3OH + HCHO = H3COCH2OCH3 + H2O
Among them, the oxidative dehydrogenation of methanol to
HCHO is thermodynamically favorable since DG1 = À186.4 kJ molÀ1
at 180 1C and À190.6 kJ molÀ1 at 240 1C,18 which indicates that the
temperature increase is advantageous to the oxidation step from
the point of view of the reaction thermodynamics. However, the
value of DG1 for the condensation of methanol and HCHO to
DMM positively shifted from À15.5 to À7.7 kJ molÀ1 when the
temperature increased from 180 to 240 1C. Therefore, the
temperature increase is disadvantageous for DMM formation
from the point of view of the reaction thermodynamics, which
led to the low conversion as the reaction temperature increased to
220 1C. Also, the DMM selectivity decreased when the temperature
increased up to 220 1C because the high temperature caused the
side reaction to be easily carried out.
Fig. 11 XRD patterns of the fresh and reused catalyst.
In comparison with previous reports on methanol oxidation
over SO42À/V2O5–TiO2,2,9c this system possessed a better DMM
selectivity. A high selectivity (485%) was kept over almost all
the Zr–V–S catalysts, such as ZrVS-450, ZrVS-500, ZrVS-550 and
ZrVS-600. However, the only byproduct is dimethyl ether.
The stability of the catalyst
In order to evaluate the catalytic property of the catalyst, the
stability of the catalyst was investigated. The results are
presented in Fig. 10. The activity of the catalyst increased from
1 to 8 h, 38.0% conversion was achieved at 8 h, and this
catalytic activity remained up to 9 h. At reaction times greater
than 9 h, the conversion obviously decreased to about 14.6% at
a reaction time of 12 h. Deactivation of the catalyst is likely
attributed to changes of the surface structure and composition
since the loss of V and S in the used catalyst was not found, as
shown in Table 2, the data in parentheses. The XRD pattern of
the used catalyst as compared with that of the fresh one is
shown in Fig. 11. As seen in the figure, the diffraction peaks of
Fig. 12 FT-IR spectra of the fresh and reused catalyst.
at 977.7, 951.4 and 925.1 cmÀ1 were likely assigned to the
V–O–V linkage for VxOy species.2 Thus, the crystalline V2O5 was
converted into VxOy species during the methanol oxidation,
which is the reason that the catalyst deactivated.
The batch reaction
the crystalline V2O5 at 20.4 and 26.21 disappeared completely in The catalytic activity of ZrVS-500 for methanol oxidation by O2 in a
the XRD pattern of the reused catalyst. At the same time, the batch reactor was also investigated. The results, as shown in Fig. 13,
band at 1026 cmÀ1, corresponding to the stretching vibration of exhibit the dependence of the catalytic activity of the catalyst ZrVS-
the VQO bond, was also not observed in the FT-IR spectrum of 500 on the reaction temperature. The conversion increased as the
the used catalyst, as shown in Fig. 12. The new bands observed temperature increased from 160 to 180 1C, the same as was obtained
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activity for methanol oxidation to dimethoxymethane with 38.0%
conversion and 83.0% selectivity. In particular, for the batch
reaction the DMM selectivity was over 95.0% with ca. 30.0%
conversion. This catalytic performance was ascribed to the
crystalline V2O5 and SO42À, contributing redox and acidic active
sites on the surface, respectively. The crystalline V2O5 on the
ZrO2 surface is particularly vital for the methanol oxidation.
The disappearance of the crystalline V2O5 on the used catalyst
surface was observed, which is the reason that the catalyst
deactivated in the experiments on the catalyst stability.
Fig. 13 The dependence of the catalytic activity on the reaction temperature.
Acknowledgements
We greatly acknowledge the financial support of this work by
the National Natural Science Foundation of China (No. 21671050),
the Foundation for academic leaders of Harbin (No. 2013RFXXJ009)
and the Natural Science Foundation of Heilongjiang Province
(No. B201119).
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Fig. 14 Reusability of the catalyst.
in the fixed bed reactor, which was likely attributed to the reaction
kinetics because the elevated temperature could enhance the
reaction rate. When the reaction temperature exceeded 180 1C,
the conversion was almost constant. This finding was probably
related to the reaction thermodynamics. Interestingly, the DMM
selectivity was slightly decreased as the reaction temperature
increased. However, it stayed over 95.0% all the time. Additionally,
in order to further evaluate the performance of the catalyst, the
recovery and reuse of the catalyst ZrVS-500 were explored. The
catalyst was recovered by filtration after each run, and then it was
reused in the next run under the same reaction conditions. It was
clear that the activity of the catalyst was basically unaffected even at
the third run (Fig. 14). This suggested that the prepared catalyst
ZrVS-500 was highly stable at least within three repeated runs. The
slightly lower activity for the second and third runs was likely
ascribed to the reduction of the recovery amount of the catalyst.
¨
157, 263; (e) J. M. Tatibouet, Appl. Catal., A, 1997, 148, 213.
¨
¨
11 A. Gervasini, P. Carniti, J. Keranen, L. Niinisto and A. Auroux,
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Conclusions
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various temperatures. The surface properties of the catalyst
were measured by XRD, FT-IR spectroscopy, SEM, TEM and XPS 16 (a) M. H. Kim, I. S. Nam and Y. G. Kim, J. Catal., 1998, 179, 350;
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NH3-TPD and H2-TPR. The results showed that crystalline V2O5 17 S. Besselmann, C. Freitag, O. Hinrichsen and M. Muhler,
2À
and SO4 ions were supported on the ZrO2 surface with the
Phys. Chem. Chem. Phys., 2001, 3, 4633.
composition Zr6V2.5SO18. The catalyst exhibited a higher catalytic 18 H. Liu and E. Iglesia, J. Catal., 2004, 223, 161.
8376 | New J. Chem., 2017, 41, 8370--8376
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