Operando Raman study of alumina-supported Sb–V–O catalyst during
propane ammoxidation to acrylonitrile with on-line activity measurement
M. O. Guerrero-Pérez and M. A. Bañares*
Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco, E-28049- Madrid, Spain.
E-mail: mbanares@icp.csic.es
Received (in Cambridge, UK) 13th March 2002, Accepted 29th April 2002
First published as an Advance Article on the web 17th May 2002
Operando Raman spectra during propane ammoxidation
show partially reversible structural transformations of the
active phases as a function of reaction environment.
The total flow rate was 20 ml min21 (GHSV = 3000 h21).
Yields and selectivities of the various products were determined
on the basis of the moles of propane in the feed and the products,
taking into account the number of carbon atoms in each
molecule.
The ammoxidation of propane to acrylonitrile (ACN) con-
stitutes an alternative route to conventional propylene ammox-
idation since propylene is more expensive than propane. Some
of the best performances in the synthesis of acrylonitrile from
propane are achieved with V–Sb–O,1,2 Al–V–Sb–O3,4 and Al–
V–Sb–W–O5,6 systems.
The Raman spectra of the fresh catalyst prior to and during
propane ammoxidation are shown in Fig. 1. The spectrum of the
fresh dehydrated catalyst shows a Raman band near 1024 cm21
,
typical of the terminal VNO stretching vibration, and a broad
Raman band centered at 900 cm21, typical of the stretching
vibration of the bridging V–O–V bond of the surface vanadium
oxide species. The fresh catalyst does not exhibit the Raman
bands of any antimony oxide phases.4 Surface dispersed
antimony oxide species on alumina do not appear to exhibit any
strong Raman bands, but the corresponding crystalline phases
give rise to strong Raman bands.4 The Sb and V oxides are
highly dispersed on the surface of the alumina support for the
fresh dehydrated catalyst. During catalytic operation, the
intensity of the Raman band of the surface vanadium oxide
species near 1024 cm21 tends to disappear, and the shape of the
broad Raman band centered at 900 cm21 also changes. As the
reaction temperature increases, propane ammoxidation be-
comes measurable and a broad Raman band becomes evident
around 800 cm21 (deconvolution shows that it is constituted by
two Raman bands at 835 and 795 cm21). This broad Raman
band is characteristic of the SbVO4 (rutile) phase.4 At 480 °C,
the catalyst becomes more selective to ACN formation (Fig.
1(e)), and corresponds to a transformation of surface vanadia
species into microcrystalline SbVO4 and of Sb2O4. In contrast,
CO2 and propylene were the main products at lower reaction
temperatures. This dramatic change in product distribution with
reaction temperature is not observed for other alumina-
supported Sb–V–O catalysts if the SbVO4 phase does not form.4
The broad Raman bands near 1060 and 620 cm21 correspond to
V–OC and VO–C vibrations, of adsorbed alkoxy species,
respectively.13 The concomitant loss of surface vanadium oxide
species and decrease of CO2 and propylene formation is
indicative of a change in the nature of the active phase. The
remarkable increase in the selectivity towards acrylonitrile
corresponds to a structural change in the catalyst leading to
SbVO4 formation at the expense of surface vanadium and
antimony oxide species. Surface antimony oxide species show
no appreciable catalytic activity for alkane activation; however,
alumina-supported vanadia species are active for propane ODH.
The concomitant formation of SbVO4 and Sb2O4 and a
significant alteration in the product distribution towards
acrylonitrile underlines the transformation of a surface efficient
for propane ODH to an active phase efficient for propane
ammoxidation.
Several studies about the nature of the active phase on Sb–V–
Al mixed metal oxide catalysts7,8 proposed a mixed Sb–V oxide
phase, such as SbVO4, in the presence of Sb2O4 as the active
phase. However, the details about the surface species and the
changes that they undergo during propane ammoxidation are
currently not fully understood. Scanning electron microscopy
reveals that SbVO4 microcrystals appear to grow on top of
Sb2O4 after catalytic operation, and the combination of both
phases together is more selective than either pure phase.9,10
However, it is difficult to assess the exact nature of the surface
active phase in bulk mixed metal oxides since in situ molecular
spectroscopies are usually dominated by the bulk structure. The
use of supported systems may facilitate the determination of the
structure–activity relationship since it is possible to grow
microcrystalline phases with a high surface-to-volume ratio.4
Evaluation of the structures before and after catalytic operation
provides some relevant information, but knowledge of the exact
molecular structures existing during catalytic operation will
lead to a deeper understanding of the molecular structure–
activity/selectivity relationship. This work investigates the
structures of an alumina-supported Sb–V–O catalyst during
propane ammoxidation with Raman and on-line GC measure-
ments so that both the structure and activity/selectivity
information are simultaneously obtained. The methodology that
involves the use of in situ spectroscopy during catalytic
operation with on-line activity measurement has recently been
denoted operando spectroscopy.11,12
An alumina-supported Sb–V–O catalyst was prepared as
described previously.4 The catalyst loading was selected so that
the total coverage of V + Sb would correspond monolayer
surface coverage.4 The Sb/V atomic ratio was fixed at 1. Raman
spectra were obtained with a single monochromator Renishaw
System 1000 described elsewhere.4,12 The acquisition time was
200 s. The Raman–GC spectra were measured with a home-
made reaction cell that consists of a fixed-bed quartz-
microreactor contained by quartz wool plugs on both ends with
the catalyst in powder form. No appreciable differences in
conversion and selectivity can be observed between the Raman
reaction-cell and a conventional fixed-bed microreactor. The
laser power at the sample was kept below 9 mW to prevent local
heating. The reaction feed was controlled by mass-flow
controllers and the reactor outlet was connected on-line with a
gas-chromatograph equipped with flame ionization and thermal
conductivity detectors (Varian 3800). The reactor was designed
to minimize gas-phase activation of propane. Tests were made
using 0.2 g samples with particle size in the range 0.25–0.125
mm. The catalytic reaction employed the following feed
composition: 25% O2, 9.8% propane, 8.6% ammonia in helium.
It is interesting to note that the formation of SbVO4 and
Sb2O4 become more evident after catalytic operation than after
calcination of the starting catalyst precursor. This may be
indicative that the formation of SbVO4 may require an
environment that is not net oxidizing. A significant benefit of
operando Raman spectroscopy is that it provides direct
evidence on the relevance of the environment on the oxidation
states of V and Sb oxide species in the Sb–V–O phases. The
ammoxidation reaction feed composition has a moderate
1292
CHEM. COMMUN., 2002, 1292–1293
This journal is © The Royal Society of Chemistry 2002
Guerrero-Perez
