Selective oxidation of methanol to dimethoxymethane under mild conditions over V2O5/TiO2 with enhanced surface acidity

Article abstract of DOI:10.1039/b618898b

Dimethoxymethane was synthesized from the direct oxidation of methanol with high conversion and selectivity over specially designed bifunctional V ₂O₅/TiO₂ catalysts with redox and enhanced acidic character, in which the surface acidity played an essential role for inhibiting the formation of formaldehyde through the enhanced condensation reaction of formaldehyde with methanol to produce dimethoxymethane. The Royal Society of Chemistry.

Full text of DOI:10.1039/b618898b

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Selective oxidation of methanol to dimethoxymethane under mild
conditions over V2O5/TiO2 with enhanced surface acidity{
Yuchuan Fu and Jianyi Shen*
Received (in Cambridge, UK) 2nd January 2007, Accepted 5th February 2007
First published as an Advance Article on the web 23rd February 2007
DOI: 10.1039/b618898b
Our current work demonstrated that the selectivity of DMM could
be greatly increased by simply doping Ti(SO4)2 onto the traditional
V2O5/TiO2, because of the enhanced condensation reaction
between formaldehyde and methanol on the acid sites with
enhanced acidity. The performance of the V2O5/TiO2–Ti(SO4)2
was as good as that of Re/c-Fe2O3 for the selective oxidation of
methanol to DMM.6
Dimethoxymethane was synthesized from the direct oxidation
of methanol with high conversion and selectivity over specially
designed bifunctional V2O5/TiO2 catalysts with redox and
enhanced acidic character, in which the surface acidity played
an essential role for inhibiting the formation of formaldehyde
through the enhanced condensation reaction of formaldehyde
with methanol to produce dimethoxymethane.
Ti(SO4)2 was added onto the V2O5/TiO2 catalysts by the
incipient wetness method followed by drying at 383 K and
22
Dimethoxymethane (DMM) or methylal has an extremely low
toxicity and can be used as an excellent solvent in pharmaceutical
and perfume industries, a reagent in organic synthesis1 and an
intermediate for the production of concentrated formaldehyde.2
Recently, we found that DMM could be effectively reformed to
produce H2 for fuel cells.3 DMM is usually produced by
condensation of formaldehyde with methanol over acidic cata-
lysts.4 It can also be synthesized by the direct oxidation of
methanol on crystalline SbRe2O6,5 Re/c-Fe2O3,6 heteropolyacid,7
RuOx/SiO2,8 and Cu-ZSM-5.9 The direct synthesis of DMM
would be more efficient if the process is industrialized.
calcination at 673 K. The content of SO4 was analyzed to be
about 0.5% for the 10%V2O5/TiO2–Ti(SO4)2 after calcination. (see
ESI{ for detailed procedure). The titania support used was in the
anatase form as determined by X-ray diffraction.
The catalytic reaction for the selective oxidation of methanol
was carried out in a glass reactor at atmospheric pressure. Prior to
The selective oxidation of methanol to DMM may involve two
steps: (1) oxidation of methanol to formaldehyde on redox sites
and (2) condensation of formaldehyde produced with additional
methanol to DMM on acidic sites. Thus, bifunctional catalysts
with redox and acidic characters are required for the reaction. In
addition, the relative strengths of surface acidity and redox ability
of a catalyst may be important in determining the reaction
pathways as well as the selectivity to DMM. Among the catalysts
reported to date, Re/c-Fe2O3 was the most effective with 48.4%
conversion of methanol and 91% selectivity to DMM at 513 K.6
However, there are disadvantages of using rhenium oxides since
Re is expensive and rhenium oxides are volatile.10 Liu et al. studied
the selective oxidation of methanol over H5PV2Mo10O40/SiO2
catalysts. After selective poisoning of surface protons by using an
organic base, they were able to inhibit the formation of dimethyl
ether (DME) and thereby increase the selectivity of DMM to 80%
at 453 K.7
The oxidation of methanol over V2O5/TiO2 catalysts has been
studied for the production of formaldehyde and methyl formate,
and DMM was usually taken as a by-product.11–13 In fact, the
selectivity to DMM decreased sharply with the increase of
conversion of methanol over the traditional V2O5/TiO2 catalysts.14
Laboratory of Mesoscopic Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, 22 Hankou Road, Nanjing,
210093, China. E-mail: jyshen@nju.edu.cn; Fax: +86-25-83317761;
Tel: +86-25-83594305
{ Electronic supplementary information (ESI) available: Experimental
details for the preparation of V2O5/TiO2 catalysts. See DOI: 10.1039/
b618898b
Fig. 1 Selective oxidation of methanol over V2O5/TiO2 (a) and V2O5/
TiO2–Ti(SO4)2 (b) containing 10% V2O5. FA = formaldehyde, MF =
methyl formate, DME = dimethyl ether.
2172 | Chem. Commun., 2007, 2172–2174
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Table 1 Selective oxidation of methanol to dimethoxymethane over V2O5/TiO2 and V2O5/TiO2(SO422) catalystsa
Selectivity (%)
Methanol
Ratee/mmol g21 h21
Catalyst
SBET/m2 g21
T/K
conv. (%)
DMM
FAb
MFc
DMEd
Methanolf
DMMg
10%V2O5/TiO2
10%V2O5/TiO2–Ti(SO4)2
92
82
433
433
433
433
433
513
40.7
60.0
24.0
6.6
48.2
48.4
10.9
88.6
41.1
33.1
91.6
91.0
61.0
1.2
43.6
61.5
0.1
2.4
b
27.5
9.8
15.0
5.1
6.0
4.6
0.7
0.4
0.3
0.3
2.4
1.0
195
287
115
31.4
462
319
7.1
85
16
3.5
141
97
10%V2O5/TiO2–Ti(SO4)2–0.8%K2CO3
10%V2O5/TiO2–Ti(SO4)2–3%K2CO3
5%V2O5/TiO2–Ti(SO4)2
N/Ah
N/Ah
95
i
10%Re/c–Fe2O3
a
16
Feed conditions: methanol : O2 : N2 = 2 : 6 : 30 ml min21, catalyst loading = 0.2 g. FA = formaldehyde. MF = methyl formate.
c
d
h
e
DME = dimethyl ether. Based on the unit mass of V or Re. Rate of conversion of methanol. Rate of formation of DMM.
f
g
i
Not available. Data from ref. 6.
The importance of surface acidity for the synthesis of DMM
from the direct oxidation of methanol was further confirmed by
poisoning the surface acidity using K2CO3. Data in Table 1 clearly
show that the addition of K2CO3 greatly decreased the conversion
of methanol and selectivity to DMM.
Table 1 also compares the activity of supported vanadia and
rhenium oxide for the selective oxidation of methanol to DMM.
The rates of conversion of methanol and formation of DMM were
found to be about 319 and 97 mmol g21 h21 on the 10%Re/
c-Fe2O3,6 and 462 and 141 mmol g21 h21 on our 5%V2O5/TiO2–
Ti(SO4)2 catalyst, respectively, with similar conversion of methanol
(48%) and selectivity to DMM (92%). Apparently, vanadia was
much more active than rhenium oxide considering the fact that the
rate of conversion of methanol was about 50% higher on the
5%V2O5/TiO2–Ti(SO4)2 than on the 10%Re/c-Fe2O3 although
the reaction temperature was 80 K lower for the reaction on the
5%V2O5/TiO2–Ti(SO4)2 than on the 10%Re/c-Fe2O3.6
Fig. 2 DMM selectivity vs. methanol conversion over 10%V2O5/TiO2
and 10%V2O5/TiO2–Ti(SO4)2.
each test, the catalysts were activated at 673 K for 1 h in a flowing
gas with O2 : N2 = 6 : 30 (ml min21). The reactants and products
were analyzed on line by GC.
Different sulfates were added into the V2O5/TiO2 for the
selective oxidation of methanol to DMM. Similar positive effect
was found for the addition of (NH4)2SO4 and H2SO4, but the
addition of K2SO4 greatly decreased the activity and selectivity to
DMM. The relevant data are provided in the ESI.{
Fig. 1 shows the results for the selective oxidation of methanol
over 10%V2O5/TiO2 and 10%V2O5/TiO2–Ti(SO4)2. Fig. 1(a) shows
that the selectivity to DMM was high (.90%) on the V2O5/TiO2 at
low temperatures (393 and 403 K) with low conversion of
methanol (,15%). With the increase of temperature, the selectivity
to DMM decreased rapidly with the increased conversion of
methanol, due to the increased selectivity to formaldehyde and
methyl formate. At 433 K, the products were mainly formaldehyde
(61%) and methyl formate (27.5%) with only 10.9% DMM. This
result indicated that the surface acidity of the V2O5/TiO2 was not
strong enough to effectively catalyze the condensation reaction,
leading to the production of a large amount of formaldehyde as
well as its oxidation product methyl formate.15
In conclusion, we have demonstrated in this work that the
bifunctional V2O5/TiO2 with redox and enhanced acidic character
was pertinent for the direct oxidation of methanol to DMM. The
reaction involves the oxidation of methanol to formaldehyde
which is then condensed with two methanol molecules to produce
DMM. Addition of an acidic sulfate, e.g., Ti(SO4)2 enhanced the
surface acidity of V2O5/TiO2 and greatly improved the selectivity
to DMM as well as the conversion of methanol, due to the
enhanced condensation reaction of formaldehyde with methanol.
The selectivity to DMM reached 89–92% with 48–60% conversions
of methanol at 433 K over the V2O5/TiO2 modified with Ti(SO4)2,
showing the potential application of the catalysts for the reaction
in industry.
Fig. 1(b) shows that the selectivity to DMM was greatly
improved with the addition of Ti(SO4)2 onto the V2O5/TiO2, even
at high conversions of methanol. The conversion of methanol
increased from 10 to 60% with the increase of temperature from
393 to 433 K, while the selectivity to DMM was maintained at
high level (.88%). The production of formaldehyde was greatly
inhibited and no formaldehyde was detected for the reaction at the
temperatures below 423 K. DMM remained the predominant
product in the temperature range used (393–433 K). Fig. 2 presents
the curves of selectivity to DMM vs. the conversion of methanol. It
is clearly seen that the selectivity to DMM decreased sharply over
the V2O5/TiO2 while it remained high over the V2O5/TiO2–
Ti(SO4)2 with the increase of conversion of methanol.
This work was supported by the National Science Foundation
of China (20373023) and the Ministry of Science and Technology
of China (2004DFB02900 and 2005CB2214003)
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