(445b) Odh of Propane over Several V2o5/Tio2-Sio2 and V2o5/Tio2 Catalysts: Understanding the Structure-Reactivity Relationship

Supported
vanadium oxides are extensively used as an effective catalyst for various
selective catalytic oxidation processes. Among various oxide supports
TiO₂ has drawn special attention since it imparts higher intrinsic
activity for the vanadium oxide phase. However, commercially available
TiO₂ (Degussa, P25, 50 m²/g)
undergoes anatase to rutile
phase transition and a loss of surface area at higher temperature. Furthermore,
anatase to rutile phase
transition occurs at much lower temperature in the presence of vanadium oxide.
The vanadium oxide content also plays a role. Therefore, synthesis of high
surface area thermally stable TiO₂ is an interesting area of
research. To increase the surface area
and thermal stability, TiO₂ can be intimately mixed by various
techniques with another high surface area thermally stable oxide support such as
SiO₂, Al₂O₃ etc.

The present
study deals with the synthesis of high surface area
TiO₂-SiO₂ (Ti-Si) mixed oxide
supported vanadium oxide catalyst and applying it for the propane oxidative
dehydrogenation (ODH) reaction. The
results obtained for mixed oxide supported catalysts were compared with a vanadia-titania catalyst prepared from commercially
available TiO₂ (Degussa, P25). For this
purpose a series of well-characterized
V₂O₅/TiO₂ (xVTi) and
V₂O₅/TiO₂-SiO₂ (xVyTi-Si) catalysts are considered. The kinetic parameters
for the ODH reaction, the pre-exponential factors and activation energies, are
estimated by optimizing an objective function using Genetic Algorithm (GA).  To understand the structure-reactivity
relationship in more detail the catalysts will be characterized under in-situ condition with simultaneous
collection of reaction data.

A series of vanadium oxide catalysts
supported on Ti-Si mixed oxide support were
synthesized by incipient wetness impregnation method to understand the effect of
vanadium oxide loading on the structure-reactivity relationship and kinetic
parameters for propane ODH reaction. A series of vanadium oxide catalysts
supported on pure TiO₂ (P25 Degussa) were
also synthesized for comparative purposes. The Ti-Si
mixed oxide support was synthesized by incipient wetness impregnation (IWP) and
sol-gel (SG) techniques. The precursor used for TiO₂ was titanium
ethoxide (TE, Aldrich) and for SiO₂ was
tetra ethyl orthosilicate (TEOS). For mixed oxide
supports, the IWP of pretreated SiO₂ (aerosil) was carried out with an incipient volume of
TE-ethanol solution. The paste formed was kept in desiccators for drying for 8h
at room temperature. In the SG route TEOS was added to the ethanol solution of
TE and surfactant (Ti: Surfactant: EtOH =1:0.1:20).
The entire mixture was refluxed for 2h at 358 K.  Stoichiometric
amounts of HCl-double distilled water solution were
then continuously added drop-wise and stirred. The gel formed was aged for 24hr
and dried in a vacuum descicator for 48hr. Finally the
supports were calcined at 723 K. The details for the
synthesis of vanadium oxide supported catalysts can be found elsewhere [1, 2].
The vanadia supported catalysts were tested for
the oxidative dehydrogenation of propane reaction, which was carried out in a
down flow tubular quartz reactor. The reaction temperature was varied from 613
to 673 K and the C₃H₈:O₂ ratio was varied from
1:1 to 3:1. For kinetic parameter estimation a Mars-van Krevelen reaction model was used. Additional experimental
details can be found elsewhere [1].

The BET surface area of various supported vanadium oxide
catalysts and Ti-Si mixed oxide support reveal no
significant changes for the VTi samples, whereas a
decrease in surface area was observed for the VTi-Si
mixed oxide support. This result suggests that the BET surface area of pure
TiO₂ (P25) is not affected after synthesis. However, the
corresponding surface area of the TiO₂ phase in the mixed oxide
support is not known. Raman spectroscopic studies reveal that for the
Ti-Si mixed oxide support prepared by IWP possesses
bulk titania above 30 wt %
TiO₂. Furthermore, Raman spectra of V50Ti-Si samples reveals the
presence of surface vanadia species up to 7.5 wt.% V₂O₅ (7.5V50Ti-Si) and bulk
V₂O₅ is observed for the 10V50Ti-Si sample.

The
TiO₂ (P-25) and prepared x%Ti-Si mixed
oxide support were characterized by XRD for samples calcined at different temperatures. From the XRD
pattern  it was observed that
TiO₂ (P-25) undergoes anatase to rutile phase transition starting from 873 K, whereas no
anatase to rutile phase
transition was observed for Ti-Si mixed oxide support
even up to 1073 K. These studies appear to suggest that the SiO₂
substrate stabilizes the anatase phase and
retards the anatase to rutile phase transition. XRD pattern of 2 and 4 VTi samples calcined at different
temperatures reveal anatase to rutile phase transition starting
at 873K and
723K respectively. No anatase to rutile phase transition was observed even up to 1073K for
2V50Ti-Si, whereas the rutile phase appeared at 973 K
for the 4V50Ti-Si sample. The relative intensity of rutile phase increases rapidly with an increase in calcination temperatures for 2VTi and 4VTi. These results
suggest that the Ti-Si support is thermally more
stable than the TiO₂ (P-25) support even in the presence of surface
vanadia species and ease of phase transition is
related to vanadium oxide concentration. Similar results were also obtained from
DTA analysis of the above samples.

To see the effect of calcination on reactivity, four catalyst samples 2VTi, 4VTi,
2V50Ti-Si and 4V50Ti-Si were pre-calcined at different
temperature varying from 723 to 1073 K and were tested for propane ODH. It was
observed that the propene yield and propane conversion decreases with an
increase in calcination temperature for the 2VTi
catalyst, whereas no significant change in propene yield and conversion was
observed for 2V50Ti-Si catalysts. A drastic decrease in propene yield and
propane conversion was observed with increase in calcination temperature for the 4VTi sample, whereas propene
yield and propane conversion decreased gradually for the 4V50Ti-Si sample. This
result reveals that deactivation of 2VTi, 4Ti and 4V50Ti-Si catalyst is
associated with the loss of surface active vanadia
sites, which is accompanied by the anatase to rutile phase transition.

Contact time studies were carried
out with 2VTi, 2VxTi-Si (x =30-70 wt %) and yV/50Ti0₂-Si0₂
(y =2-7.5 wt %) catalysts at 653 K using propane to oxygen ratio of 2:1.
From contact time
studies it was
observed that propane conversion
increases with
increase in titania content
in the mixed oxide support. The increase in activity of 2VxTi-Si
with increase in titania
content appears to
suggest an increase in vanadia concentration on the
TiO₂ phase supported on SiO₂. It was also observed that
propane conversion is a function of vanadia loading
for V50Ti-Si catalysts.  At iso-contact time the 2V50Ti-Si revealed similar activity as the 2VTi sample.
The propene
selectivity decreases
at iso-conversion with increase in titania content in the mixed oxide
support and is different from the 2VTi catalyst.
However, propene selectivity increases at iso-conversion with increase in vanadia loading for V50Ti-Si series of catalysts.
At iso-contact time the propene yield was also observed to
increase with increase in titania content of 30 to 50% and remained constant at
higher titania content mixed oxide supported
catalysts. An increase in propene yield at iso-contact
time was also observed with increase in vanadia
loading.

For kinetic parameter estimation for the
above mentioned catalysts are shown in Table 1. Analysis of kinetic parameter
reveals that the pre-exponential factor, k₁₀, for propene formation
increases with increases in vanadia loading for
samples below mono layer coverages. The
pre-exponential factor for CO_x formation,
k₂₀, also increases with increase in vanadia loading. The activation energy for the propene
formation (69-72 kJ/mol) is relatively independent of vanadia loading, whereas the activation energy for CO_x formation increases slightly with loading
from 46 to 54 kJ/mole. Kinetic parameters
obtained for the 2V50Ti-Si sample are comparable to those for the 2VTi catalyst
suggesting similar active surface vanadia sites
present on both supports. It was reported that the increase in propene yield for
a series reaction is achieved by increasing the rate constant ratio
k₁/k₂ [1,2]. The increase of
k₁/k₂ is possible by changing the catalyst or changing the
reaction temperature. In the present study, the k₁/k₂
ratio increases with an increase in vanadia loading.
This result suggests that propene yield increases with increase in vanadia loading. Similar trend in increase of propene yield
are also observed from contact time study. The actual scenario behind the
increase of k₁/k₂ ratio by modifying the catalysts is not
well understood. For this it is necessary to study the reaction under in-situ condition which is currently
underway.

Table 1:
Kinetic parameter for four catalysts for MVK model, T_m =643.15 K

References:
1.     
Routray K., Reddy K.R.S.K., Deo
G. Oxidative Dehydrogenation of propane on   
V₂O₅/TiO₂
catalysts: Understanding the effect of support by parameter estimation. Appl. Catal. A: Gen. 2004; 265:
103-113.

2. Singh
   R.P., Banares M., Deo G. Effect phosphorus modifier
   on V₂O₅/TiO₂ catalysts: ODH of Propane, J.
   Catal. 2005; 233: 388-398.