The one-step oxidation of methanol to dimethoxymethane over sulfated vanadia-titania catalysts: Influence of calcination temperature

Article abstract of DOI:10.1039/c5ra09072e

Sulfated vanadia-titania catalysts were prepared by the rapid combustion method and calcined at different temperatures. The influence of calcination temperature on the physicochemical properties of the catalysts was characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H₂-TPR-MS), thermogravimetry (TG) and temperature programmed desorption of ammonia (NH₃-TPD) techniques. The catalytic activities were evaluated by the partial oxidation of methanol to dimethoxymethane (DMM). The results showed that vanadia and sulfate were highly dispersed as the catalysts were calcined at 723 and 773 K. The reducibility of the highly dispersed vanadia was stronger than the aggregated vanadia. And the larger number of acidic sites was related to the higher dispersion of sulfate. Moreover, the higher dispersion of vanadia contributed to higher methanol conversion, and the stronger reducibility combined with the larger number of acidic sites led to high DMM selectivity. As a result, the catalysts calcined at 723 and 773 K presented higher methanol conversion and DMM selectivity than those calcined at 673 K or above 823 K.

Full text of DOI:10.1039/c5ra09072e

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The one-step oxidation of methanol to
dimethoxymethane over sulfated vanadia–titania
catalysts: influence of calcination temperature
Cite this: RSC Adv., 2015, 5, 64202
*
Heqin Guo, Debao Li, Congbiao Chen, Litao Jia and Bo Hou
Sulfated vanadia–titania catalysts were prepared by the rapid combustion method and calcined at different
temperatures. The influence of calcination temperature on the physicochemical properties of the catalysts
was characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), Fourier transform infrared
spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray
photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR-MS), thermogravimetry
(TG) and temperature programmed desorption of ammonia (NH3-TPD) techniques. The catalytic activities
were evaluated by the partial oxidation of methanol to dimethoxymethane (DMM). The results showed that
vanadia and sulfate were highly dispersed as the catalysts were calcined at 723 and 773 K. The reducibility
of the highly dispersed vanadia was stronger than the aggregated vanadia. And the larger number of acidic
sites was related to the higher dispersion of sulfate. Moreover, the higher dispersion of vanadia contributed
to higher methanol conversion, and the stronger reducibility combined with the larger number of acidic
sites led to high DMM selectivity. As a result, the catalysts calcined at 723 and 773 K presented higher
methanol conversion and DMM selectivity than those calcined at 673 K or above 823 K.
Received 15th May 2015
Accepted 17th July 2015
DOI: 10.1039/c5ra09072e
www.rsc.org/advances
lower reaction temperature.13,14 While, the physicochemical
characteristics of this catalyst, especially the acidic and redox
1. Introduction
properties, are strongly related to the existing state of vanadia,
sulfate and titania.14,17–19 Generally, the high dispersion of
vanadia and sulfate contributes to stronger reducibility and
more acidic sites. Moreover, the anatase TiO2 is usually more
active than rutile TiO2, due to the fact that the properties of the
vanadium species present on the surface of the anatase phase
have different acid–basic and redox properties, while those
present on the surface of the rutile polymorph phase resemble
the properties of those of pure V2O5.20 Therefore, to obtain the
best catalytic performance of VTS catalyst in methanol oxida-
tion reaction, it is necessary to optimize the existing state of the
vanadia, titania and sulfate. A reasonable and simple approach
towards the solution of this problem is related with the opti-
mization of calcination temperature. Although several attempts
have been made to improve the catalytic performance of vana-
dia–titania catalysts by optimizing the calcination tempera-
ture,21–23 those for the sulfated vanadia–titania catalysts have
received little attention.
In the present study, the VTS catalysts were prepared by the
rapid combustion (RC) method and calcined at different
temperature. The inuence of calcination temperature on the
physico-chemical properties of catalysts were studied by XRD,
FTIR, BET, ICP-OES, XPS, TG, H2-TPR-MS and NH3-TPD tech-
niques. Moreover, the samples were also tested in the methanol
partial oxidation reaction. The aim of this work was to obtain
matched redox and acidity to optimize DMM selectivity.
Dimethoxymethane (CH3OCH2OCH3, DMM), as an important
green downstream product of methanol, is widely used as a
diesel additive and building block in organic synthesis. There
are numerous reports that DMM can be synthesized by the one-
stage oxidation of methanol (3CH3OH + 1/2O2]CH3OCH2OCH3
+ 2H2O).1–10 In these cases, two types of active sites, namely
redox sites and acidic sites, are required for DMM synthesis.
Here, the redox sites with active lattice oxygen atoms are
considered to be involved in the initial formation of formalde-
hyde (FA) from CH3OH, whereas the acidic sites could catalyze
the desired acetalization reactions of FA and CH3OH to DMM.
Moreover, the acidic sites were also favorable for dimethyl ether
(DME) formation, and the redox sites can catalyze methanol to
FA/methyl formate (MF). Therefore, exact matching/
cooperation of the redox properties and acidity is critical in
getting high DMM selectivity.
Several catalysts, such as heteropoly acids,1,2 SbRe2O6,3 sup-
ported rhenium oxide,4 Mo12V3W1.2Cu1.2Sb0.5Ox,9 Cu–ZSM-5,11
FeMo12 and sulfated vanadia–titania,13–16 have been applied in
DMM synthesis. Among the reported catalysts, the sulfated
vanadia–titania catalysts (VTS) have been considered to be
promising for DMM synthesis with high DMM selectivity at
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, China. E-mail: houbo@sxicc.ac.cn; Fax: +86
351 4040087; Tel: +86 351 4040087
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Balzers. The NH3-TPD spectra were recorded in a xed-bed
2. Experimental
2.1 Catalyst preparation
reactor system equipped with a gas chromatograph. The cata-
lyst (200 mg) was pretreated at 773 K under Ar ow (60 ml minꢀ1
)
for 2 h and then cooled to 373 K. Then NH3 was introduced into
the ow system. The TPD spectra were recorded at a temperature
rising rate of 10 K minꢀ1 from 373 to 900 K.
The VTS catalysts were prepared by the rapid combustion (RC)
method. In the typical synthesis: 13.89 ml TiCl4 was added to
37.86 ml 67 wt% HNO3 under vigorously stirring under N2
atmospheres, and then 6.22 g NH4VO3, 3.37 g Ti (SO4)2 and
73.7 g carbamide were added to the solution, the obtained
mixture was stirred until the NH4VO3 and carbamide were
completely dissolved, and then calcined under 673, 723, 773,
823 and 873 K, respectively, for 15 min. The obtained catalysts
were marked as VTS-673, VTS-723, VTS-773, VTS-823 and
VTS-873, respectively. For comparison, the sulfated vanadia–
titania sample without calcination and the pure rutile TiO2
calcined at 973 K was prepared by the same method, which were
marked as VTS and TiO2-R, respectively.
2.3 Catalyst test
The catalytic oxidation reaction was carried out in a continuous
ow xed-bed reactor containing catalyst (1.0 g) diluted with
ground quartz. Before reaction, catalysts were treated in owing
10% O2/Ar (85 cm3 minꢀ1) for 2 h at 673 K. And then the
methanol was introduced into the reaction zone by bubbling
O2/Ar (1/9) through a glass saturator lled with methanol
(99.9%) maintained at 278 K. The feed composition was main-
tained as Ar : O2 : CH3OH ¼ 84.6 : 9.4 : 6.0 (v/v/v). The reaction
products were analyzed by on-line gas chromatography
(GC-950) using a Propack T column and a TDX-01 column
connected to TCD detector and FID detector, respectively. The
gas lines were kept at 373 K to prevent condensation of the
reactant and products. The reaction was carried out at atmo-
2.2 Catalyst characterization
The specic surface areas of the catalysts were measured by the
BET method with a Micromeritics model ASAP 2000 using
ꢁ
nitrogen at ꢀ196 C. Prior to measurements, all catalysts were
outgassed at 150 ꢁC under 1 ꢂ 10ꢀ5 Torr residual pressure. XRD
spheric pressure. The product selectivity was calculated on
P
carbon molar base: Si ¼ Yini/ Yini ꢂ 100%, where i is CH3-
OCH2OCH3, CH3OCH3, HCHO, HCOOCH3, 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.
patterns were measured on a Bruker Advanced X-ray Solutions/
ꢀ
D8-Advance using Cu Ka radiation (l ¼ 1.5404 A). The skeletal
FT-IR spectra were recorded with a Bruker Vector 22 FTIR
spectrophotometer (DTGS detector) operating in the 4000–400
cmꢀ1 range, with a resolution of 2 cmꢀ1 and 100 acquisition
scans. In each experiment, 2 mg of sample were mixed with 198
mg of KBr. A spectrum was recorded at room temperature. TG
was carried out on a SetaramTGA-92 analyzer with a heating
3. Results and discussion
3.1 Physico-chemical characterization
speed of 10 K minꢀ1 in the 303–1073 K range under air in a ow The textual properties of the catalysts are listed in Table 1. The
of 50 ml minꢀ1. The elemental analysis (Ti, V, and S) was per- surface area of the catalysts increased with increasing calcina-
formed by inductively coupled plasma-optical emission spec- tion temperature, and reached the maximum (120.19 m2 gꢀ1) on
troscopy (ICP-OES). The XPS measurements were performed in a VTS-773. While, as the calcination temperature continued to
Physical Electronics Company Quantum-2000 Scanning ESCA increase, the surface area decreased, only 12.58 m2 gꢀ1 on
Microprobe equipped with a Al X-ray source (23.3 mA and 1.5 VTS-873.
kV), a solid angle acceptance lens and a hemispherical electron
The XRD patterns of the catalysts are shown in Fig. 1. On
analyzer. Samples were compressed in small cup under the VTS-673, no diffractions of titania was observed, indicating
pressure of 5 kg cmꢀ2 for 30 s and supported on a holding that the titania might exist as amorphous. While, the typical
ceramic carousel. The positive charge, developed on the samples diffractions of anatase TiO2 (TiO2-A) was observed on VTS-723
due to the photoejection process was compensated by a charge and VTS-773. On VTS-823 and VTS-873, the TiO2-A and rutile
neutralizer (low energy electron and low energy ion beam). The TiO2 (TiO2-R) were coexisted. While, the intensities of TiO2-R
residual pressure in the spectrometer chamber was 5 ꢂ 10ꢀ7 Pa diffraction peaks were stronger on VTS-873 than VTS-823.24
during data acquisition. The analyzed area of the sample was This result indicated that the increasing temperature caused
100 mm and the energy region of the photoelectrons were scan- the transformation of TiO2-A to TiO2-R. Moreover, no
ned at a pass energy of 29.35 eV. The resolution was 0.68 eV. The diffraction peaks of crystalline V2O5 were observed as the
binding energies of O 1s, V 2p, Ti 2p and S 2p were referenced to calcination temperature was below 823 K, implying that the
the C 1s band at 284.8 eV. The data was treated on Phi Multipack vanadia was highly dispersed or the V2O5 crystalline was less
Program, Gaussian/Lorentzian ¼ 80%. Atomic concentration than
4
nm.14 The typical diffraction peaks of V2O5
ratios were calculated by correcting the measured intensity crystalline14,25,26 appeared on VTS-873, which indicated the
ratios with the manufacturer supplied sensitivity factor. The H2- aggregation of vanadia.
TPR-MS was carried in continuous mode using a U-type quartz
To further study the existing state of vanadia, the FTIR
micro-reactor (3.5 mm in diameter). A sample of about 25 mg spectra was carried out (see Fig. 2). For VTS-673, VTS-723 and
was contacted with a H2 : Ar mixture (5% H2/Ar) at a total ow VTS-773 a threshold around 1000 cmꢀ1 were detected, which
rate of 60 cm3 minꢀ1. The inlet and outlet gas compositions were could be assigned to the bands for polyvanadate species (V–O–
measured using a quadrupole mass spectrometer QMC 311 V) bonded to the surface of TiO2.14 While, for VTS-823 and VTS-
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Table 1 The binding energies, surface and bulk composition of the catalysts and the distribution of sulfate
Surface
Bulk
compositiona
(mol%)
compositionb
(mol%)
Binding energy (eV)
No. of SO42ꢀ per
V/Tib unit area (nmꢀ2
Catalyst SBET (m2 gꢀ1
VTS-673 64.4
)
O 1s
Ti 2p3/2 V 2p3/2 S 2p
V
Ti
168.7 25.89 65.26 8.85 20.48 73.2
168.9 27.22 66.14 6.64 20.96 73.69 5.35 0.28
168.9 27.68 66.54 5.78 22.31 73.1 4.59 0.31
169.4 28.54 66.93 4.53 21.29 74.59 4.12 0.29
S
V
Ti
S
)
Sa/Sb
1.40
1.24
1.26
1.16
1.05
529.7 (81.23%) 458.2
532.1 (18.77%)
529.7 (85.11%) 458.4
532.0 (14.89%)
529.8 (87.87%) 458.3
532.0 (12.13%)
529.8 (90.36%) 458.4
531.9 (9.64%)
516.4
517.1
517.1
517.2
517.2
6.32 0.28
6.95
3.38
2.74
4.20
VTS-723 112.8
VTS-773 120.2
VTS-823
VTS-873
69.7
12.6
529.8 (95.36%) 458.4
531.9 (4.64%)
169.5 30.03 67.32 2.65 22.16 75.32 2.52 0.30 14.54
a
Surface composition calculated from XPS data. b Bulk composition calculated from ICP-OES data.
Fig. 2 The FT-IIR spectra of the catalysts.
Fig. 1 The XRD patterns of the catalysts.
169.5 eV (see Fig. 3.) was measured for S 2p3/2, which are typical
of sulfur in the S6+ oxidation state, as in Na2SO4 or Fe2(SO4)3.30
The analysis of bulk and surface sulfate content showed that
both the surface and bulk sulfate content decreased with the
873 a band at 1022 cmꢀ1 due to the microcrystalline V2O5
appeared, simultaneously with the decrease of the peak inten-
sity at 1000 cmꢀ1, suggesting the formation of crystalline
V2O5.14–17,25,26 Combined with the XRD results, it can be deduced
that the vanadia should be highly dispersed on VTS-673, VTS-
723 and VTS-773, while aggregated as crystalline V2O5 on VTS-
823 and VTS-873. In addition, a typical band at 1137 cmꢀ1
2ꢀ
corresponding to SO4 (ref. 27) was detected for all of the
samples, which indicated that the S existed as sulfate. While, its
intensity weakened with the increase of calcination tempera-
ture, implying the decrease of sulfate content.
Table 1 lists the binding energies (BE) as well as the surface
and bulk composition from XPS and ICP-OES. The binding
energies of Ti 3d5/2 for the catalysts were 458.2–458.6 eV, which
suggested the presence of Ti4+.27–29 The binding energies of V
2p3/2 for the catalysts were 517.1, 516.4, 517.2 eV, indicated the
surface vanadia species were fully oxidized (oxidation state
V5+).29 The common peaks of O 1s for both the catalysts were
529.6 and 532 eV, which belonged to the oxide oxygen and
sulfate oxygen, respectively.27 The binding energies at 168.7–
Fig. 3 XPS spectra of S 2p for the catalysts.
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increase of calcination temperature, which might be due to the
decomposition of sulfate (see TG result). Moreover, the surface
sulfate content was higher than that of bulk, indicating that the
sulfate mainly existed on the surface of the catalysts. Based on the
surface area and the sulfate content, the number of S (atoms per
nm2) was calculated. The values for VTS-723 and VTS-773 were
3.38 and 2.74, respectively. While, those were 6.95, 4.20 and 14.54
for VTS-673, VTS-823 and VTS-873, respectively. According to the
reported result,32 the necessary mole concentration of sulfate to
form a monolayer was 3.9 S per nm2. Thus, the sulfate was highly
dispersed on VTS-723 and VTS-773, while aggregated on VTS-673,
VTS-823 and VTS-873.
The H2-TPR-MS was carried out to conrm the existing state
of sulfate (in Fig. 4). Three types of product, H2O, SO2 and H2S,
were detected with their formation temperatures in parallel
with the hydrogen consumption. According to the literature, the
formation of H2O is due to the reduction of VOx, while the SO2
and H2S are due to the reduction of sulfate.32 For VTS-673, VTS-
823 and VTS-873, only SO2 was detected (see Fig. 4a, d and e).
While, for VTS-723 and VTS-773 (see Fig. 4b and c), both H2S
and SO2 were detected. It was reported31,32 that the production
of SO2 and H2S during H2 reduction was correspond to the
reduction of bidentate sulfate species and the emission of only
SO2 was due to the reduction of pyrosulfate species. Combined
with the present study, it was clear that the sulfate dispersed as
bidentate sulfate on VTS-723 and VTS-773, while aggregated as
pyrosulfate sulfate on other samples. In addition, the intensity
of SO2 on VTS-673 was stronger than those on VTS-823 and VTS-
873, which might be due to the decomposition of sulfate on
VTS-823 and VTS-873 (see TG result).
To further study the reasons leading to the decreasing
content of sulfate, the thermal stability of sulfate was charac-
terized by TG on VTS (see Fig. 5). The VTS sample exhibited four
weight losing peaks at 379, 635, 768 and 884 K, respectively. The
peak at 379 K is attributed to the removal of water, while those at
635, 768 and 884 K are corresponding to the decomposition of
sulfate.31 Combined with the above results, it can be deduced
that the decreased content of sulfate was due to the decompo-
sition of sulfate under high temperature.
3.2 Redox and acidity properties
In H2-TPR process, the proles of hydrogen consumption could
reect the redox property of metal oxide.31 As shown in Fig. 4,
for VTS-673, a major reducible peak was observed at 741 K,
which shied to 720 K on VTS-723 and VTS-773, while shied to
805 K on VTS-873. This result indicated that the catalysts
calcined at 723 and 773 K exhibited stronger reducibility than
those calcined at 673 K or above 823 K. The different reducibility
of the catalysts could be due to the different existing state of
vanadia. For VTS-723 and VTS-773, the stronger reducibility was
due to the high dispersion of vanadia. While, for VTS-823 and
VTS-873, the appearance of crystalline V2O5 led to poor
reducibility.
Fig. 4 The H2-TPR-MS profiles of the catalysts: (a) VTS-673, (b) VTS-
723, (c) VTS-773, (d) VTS-823 (e) VTS-873.
The acidity of the catalysts was characterized by NH3-TPD temperature range of 400–700 K, indicating the acidity strength
technique (see Fig. 6 and Table 2). All of the catalysts showed of the samples was similar. However, the number of acidic sites
similar feature of NH3 desorption, which spread in the among the catalysts was signicantly different. For VTS-723 and
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Table 2 The NH3-TPD result and the calculated data
No. of acidic
No. of acidic sites
per sulfate species
Catalyst
sites (mmol gꢀ1
)
VTS-673
VTS-723
VTS-773
VTS-823
VTS-873
579.99
745.49
817.94
384.80
266.90
0.78
1.18
1.50
0.79
0.89
previous reports14,17 showed that the catalytic activity of vanadia-
based catalyst in methanol oxidation reaction is closely related
to the existing state of vanadia. And the highly dispersed vanadia
contributes to high activity, while the V2O5 crystalline exhibits
low activity.33–36 Combined with the XRD and FTIR results, it can
be deduced that the high dispersion of vanadia on VTS-723 and
VTS-773 contributed to high methanol conversion. On VTS-823
and VTS-873, the appearance of crystalline V2O5 and the
appearance of TiO2-R led to the low methanol conversion.
Table 3 shows the effect of calcination temperature on DMM
selectivity at similar methanol conversion (about 45%). The
DMM selectivity showed similar changing trend to methanol
conversion, and reached the maximum on VTS-723 and VTS-
773. That phenomenon could be explained by the difference
of acidity and redox properties among these catalysts. Accord-
ing to the mechanism of the one-step synthesis of DMM from
methanol reported by Liu,1 the methanol was primarily oxidized
to FA on redox sites, and then the methanol reacted with FA to
form DMM on acidic sites. Furthermore, our previous study33,34
showed that the higher DMM selectivity on the vanadium-based
catalysts was due to the stronger reducibility matched with
larger number of the acidic sites. Combined with the present
study, it can be deduced that the stronger reducibility of VTS-
723 and VTS-773 could enhance the oxidation of methanol to
FA, while the more number of acidic sites could enhance the
acetalization reactions of FA and CH3OH to DMM and nally
lead to high DMM selectivity. For catalysts calcined at 673 K or
above 823 K, the poor reducibility and small number of acidic
Fig. 5 The TG profiles of the VTS catalyst.
VTS-773, the values were 745 and 817 mmol gꢀ1, which were
much larger than those on VTS-673, VTS-823 and VTS-873. To
quantitatively analyze the effect of the type of sulfate species on
the number of acidic sites, the number of absorbed ammonia
per sulfate species was calculated. The values were 1.18 and 1.50
for VTS-723 and VTS-773, and only 0.78, 0.79 and 0.89 for VTS-
673, VTS-823 and VTS-873. As a result, the high dispersion of
sulfate on VTS-723 and VTS-773 led to larger number of acidic
sites, and the aggregation of sulfate on VTS-673, VTS-823 and
VTS-873 resulted in small number of acidic sites.
3.3 Catalytic performance
Fig. 7 shows the inuence of calcination temperature on meth-
anol conversion. As it can be seen, the VTS-723 and VTS-773 were
more active than VTS-673, VTS-823 and VTS-873. For example, at
403 K, the methanol conversion was only 19.51% on VTS-673,
while increased up to 43.53% and 42.72% on VTS-723 and VTS-
773, and then decreased to 0% on VTS-873. The catalytic
performances at other temperatures were similar to 403 K. The
Fig. 6 The NH3-TPD profiles of the catalysts.
64206 | RSC Adv., 2015, 5, 64202–64207
Fig. 7 Effect of calcination temperature on methanol conversion.
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Table 3 Influence of calcined temperature on DMM selectivitya
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of dimethyl carbonate and dimethoxy methane over Cu-
ZSM-5, J. Catal., 2006, 244, 219.
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S. Cristol, P. Fongarland, M. Capron, L. Duhamel,
E. Payen, J. L. Dubois and F. Dumeignil, Green Chem.,
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Selectivity (%)
Catalyst
FA
DME
MF
8.92
5.16
5.63
DMM
COx
0.73
0.34
0.07
VTS-673
VTS-723
VTS-773
VTS-823
VTS-873
20.35
3.23
1.98
7.26
19.90
3.36
0.94
0.57
1.10
1.23
66.64
90.33
91.76
83.21
64.46
8.28
14.09
0.15
19.90
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14 Q. Sun, Y. C. Fu, J. W. Liu, A. Auroux and J. Y. Shen, Appl.
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a
Reaction condition: Ar/O2/CH3OH ¼ 84.6/9.4/6.0 (v/v/v), 393 K, at
about 45% methanol conversions.
15 J. W. Liu, Y. C. Fu, Q. Sun and J. Y. Shen, Microporous
Mesoporous Mater., 2008, 116, 614.
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16 S. Chen, S. P. Wang, X. B. Ma and J. L. Gong, Chem.
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17 F. Roozeboom, T. Fransen, P. Mars and P. J. Gellings, Z.
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Conclusion
The calcination temperature has profound inuence on catalyst
structure, which in turn affected the acidity and reducibility as
well as catalytic performance in the one-step oxidation of
methanol to DMM. The highest dispersion of vanadia and
sulfate was obtained on VTS-723 and VTS-773. The stronger
reducibility and more acidic sites were closely related to highly
dispersed vanadia and sulfate, while the poor reducibility and
small acidic sites were relate to the aggregated vanadia and
sulfate. The highest activity and DMM selectivity obtained on
VTS-723 and VTS-773 was due to the highest dispersion of
vanadia and the corresponding stronger reducibility and large
number of acidic sites.
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Acknowledgements
´
F. Delgado, M. L. Granados, A. Martlnez-Arias, J. M. Blasco
The authors gratefully acknowledge the Key Project of Natural
Science Foundation of China (No. 21303241) and Key Project of
Natural Science Foundation of Shanxi Province (No.
2012021005-6).
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