COMMUNICATION
Importance of nanostructured vanadia for selective oxidation of propane
to acrylic acid
Christian Hess,*a Ming Hoong Looi,b Sharifa Bee Abd Hamidb and Robert Schlo¨gla
Received (in Cambridge, UK) 30th August 2005, Accepted 8th November 2005
First published as an Advance Article on the web 9th December 2005
DOI: 10.1039/b512175b
channels, which can be tuned from 5 to 30 nm, and a very
narrow pore size distribution.7 Therefore, SBA-15 allows for
rigorous control of the surface geometry at the mesoscale, which is
an important ingredient for the design of catalysts.8 SBA-15
possesses a large internal surface area (>800 m2 g21), which allows
for the dispersion of a large number of catalytically active sites. Its
large pores permit access to bulky reagents. Furthermore, the thick
framework walls (3.1–6.4 nm) provide high hydrothermal stability
that exceeds those of the thinner-walled MCM-41 materials.9 By
anchoring vanadium oxide species onto the surface of the inner
walls of SBA-15, a large density of accessible, isolated, and
uniform active sites is obtained.10 Therefore, mesoporous silica
SBA-15 acts as an inert diluent, which should stabilize the
nanostructured surface vanadium oxide in its highly dispersed state
during reaction preventing it from sintering.
Highly dispersed nanostructured vanadia supported on meso-
porous silica SBA-15, prepared by controlled grafting/ion-
exchange, has been found to exhibit high selectivities in propane
partial oxidation to acrylic acid demonstrating its unique
potential for mixed metal oxide catalyst development.
There is great interest in partial oxidation of low alkanes for
producing light oxygenated compounds due to their global
abundance and the possibility to obtain the desired products
through less expensive processes and with a lower environmental
impact.1,2 However, for the selective oxidation of alkanes there
have been only a few successful commercial examples, like maleic
anhydride formation starting from n-butane.3 Partial oxidation of
propane to acrylic acid, although very promising, is not widely
studied in academia.4 Most of the catalyst systems developed for
selective oxidation of propane to acrylic acid so far include V as an
essential element and belong to one of the following three catalyst
systems: vanadium pyrophosphate (VPO) type catalysts, hetero-
poly acids and salts, and multi-component mixed oxide catalysts.
Currently, the most effective catalysts are those of Mo- and
V-based mixed metal oxides, such as Mo–V–Te–Nb oxides.4
However, a major drawback of the mixed metal oxide catalysts
appears to be the difficulty in controlling the preparation
parameters and therefore the catalyst structure, i.e. the formation
of the active phases or sites.4 In general, very little has been
reported concerning the origin of the high catalytic performance of
these systems. The existence of a well-ordered surface resembling
the structure of the bulk is dubious regarding the surface free
energy and the harsh working conditions (400 uC, steam, etc.). On
the other hand, it is well known that the active surface requires
oxygen defects to expose metal sites to the reactants,5 as may be
found e.g. in an array of linked metal–oxo clusters supported by
the bulk.6 Therefore, it is challenging to develop catalyst systems
which mimic the properties of the active catalyst surface, but at the
same time isolate the intrinsic properties of the different catalyst
components.
The mesoporous silica SBA-15 was synthesized according to the
literature using Pluronic P123 triblock copolymer (EO20PO70EO20,
BASF). The preparation of SBA-15 supported vanadia catalysts
(VxOy/SBA-15) is described elsewhere.10 Briefly, functionalization
of SBA-15 was achieved by stirring SBA-15 in toluene at 65 uC
and adding 3-aminopropyltrimethoxysilane. The contents were
filtered and washed with toluene. This white powder was stirred in
0.3 M HCl. The contents were filtered again, washed and dried in
air overnight (functionalized SBA-15). For 3.3 wt% V/SBA-15,
73 mg of butylammonium decavanadate were added to a
suspension of 1 g functionalized SBA-15 in water. The contents
were stirred, filtered, washed and dried in air. The powder was
calcined at 550 uC for 12 hours. The V content was determined by
AAS. Reactivity experiments were carried out in a Nanoflow
catalytic reactor with twelve fixed bed quartz tubular reactor. The
feed flow rate of each reactor was fixed at a gas hourly space
velocity (GHSV) of 1200 h21 (at STP) with a standard catalytic
bed volume of 0.5 ml. The feed composition was propane–oxygen–
nitrogen–steam (1 : 2.2 : 17.9 : 14.1). The products were analyzed
by two on-line gas chromatographs.
The TEM image shown in Fig. 1 corresponds to the final
3.3 wt% V/SBA-15 catalyst and confirms that the pore structure of
the support material was conserved throughout the synthesis (see
Table 1). XPS analysis revealed the presence of O, V, Si. No C was
observed. Vanadium was present mainly as V5+ (75%), besides V3+.
Fig. 2 shows the UV-VIS diffuse reflectance and Raman (532 nm,
10 mW) spectra of dehydrated 3.3 wt% V/SBA-15 and SBA-15.
Deconvolution gives rise to bands at 250 and 292 nm, respectively.
The positions of their maxima agree well with those of the
orthovanadate reference compounds Na3VO4 and Mg3V2O8.10
The corresponding Raman spectrum is dominated by a band at
1040 cm21, which has been assigned to isolated tetrahedral VO4
In the present study, for the first time, a catalyst with uniform
active vanadium sites supported on SBA-15 was prepared by
controlled grafting/anion exchange synthesis and proved to be
highly selective in propane partial oxidation to acrylic acid. SBA-
15 is a mesoporous silica material with uniform hexagonal
aFritz Haber Institute of the Max Planck Society, Faradayweg 4-6,
14195, Berlin, Germany. E-mail: hess@fhi-berlin.mpg.de;
Fax: +49 30-8413-4401; Tel: +49 30-8413-4500
bCOMBICAT Research Center, 3rd Floor, Block A, Institute of
Postgraduate Studies, University of Malaya, 50603, Kuala Lumpur,
Malaysia
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 451–453 | 451
Hess, Christian
