(583g) Propane Oxidative Dehydrogenation Over Vanadium Oxide Catalysts in a Riser Reactor Simulator

Propane Oxidative Dehydrogenation
over Vanadium Oxide Catalysts in a Fluidized-Bed Reactor

S. Al-Ghamdi ^(1,2) and H. de
Lasa ⁽¹⁾

1.    
Chemical
Reactor Engineering Centre, Department of Chemical and Biochemical Engineering,
University of Western Ontario, London,
Ontario, N6G 5B9 (Canada)

2.Process Development Group, Research & Development Center, Saudi Aramco Oil Company,
Dhahran31311, (Saudi Arabia)

Abstract

            The present study reports propane
oxidative dehydrogenation (ODH) under oxygen free atmosphere over a series of
alumina-supported vanadia catalysts (VO_x/Al₂O₃)
with vanadium loading between 5-10 wt. %. A number of  physicochemical
techniques such as Brunauer-Emmett-Teller (BET) surface area,
temperature-programmed reduction (TPR), temperature programmed oxidation (TPO) ,
ammonia temperature programmed desorption (NH₃-TPD) ,
O2-Chemisorption, Laser Raman Spectroscopy (LRS) and X-ray diffraction (XRD),
are used to characterize the prepared catalyst samples. H₂-TPR
results show that the prepared γ-alumina- supported catalysts are stable
over repeated reduction and oxidation cycles with reduction temperature around between
450-500 ^oC. The XRD profile for the prepared catalysts indicates the
absence of V₂O₅ bulk surface species andhigh
dispersion of VO_x on the support surface. The VOx phase is primarily
present as vanadate or poly vanadate, which is known to be X-ray amorphous.
Moreover, XRD also indicates that no other species is formed due to the
interaction between V₂O₅ and Al₂O₃
support. It was also shown that the prepared VO_x/γ-Al₂O₃
catalysts display moderated acidity compared to that of the bare alumina
support t. The addition of Vmetals on γ-Al₂O₃
reduces the acidity of bare γ-Al₂O₃, from 16.26 cm³
STP NH₃ /g to 10.94, 11.50 and 13.05 STP NH₃ /g for
samples with vanadium loading of 5, 7 and 10 wt.%, respectively. Moreover, it
was found that the catalyst acidity increases with increasing the vanadium
loading.  Laser Raman spectroscopy indicated that the higher the vanadium
loading, the higher the degree of polymerisation of the vanadium species on the
γ-Al₂O₃ support surface and the lower the BET
surface area.

            The prepared catalysts were tested for
their catalytic performance in the oxidative dehydrogenation of propane using a
CREC Riser Simulator unit over successive reaction-oxidation cycles at 550?600
°C and atmospheric pressure. Two sets of experiments were considered: single-injection
(pulse) and multi-injections (pulses) experiments. Single-injection experiments
are used to study the interaction of propane with the fully oxidized catalyst. 
In contrast, multi-injections experiments are used to change the catalyst state
from a completely oxidized to a partially reduced one and study the influence
of the reduction degree of the catalyst on its performance in propane ODH. Fully
oxidized catalysts in single-injection experiments showed high propane conversion
between 58.89-81.98 % with low propylene selectivity between 0.61-2.19 %. And their
activity was found to increase with increasing vanadium loading. On the other
hand, given the results obtained from multi-injections (pulses) experiments
with partially reduced catalyst samples in each run in oxygen-free environment,
it appears that lattice oxygen is involved in the catalytic ODH reaction. It is
demonstrated with reactivity tests that the prepared catalysts display
promising propane conversions (11.73% - 15.11%), propylene selectivity (67.65-85.89%)
and catalyst stability during multiple reduction cycles at 4750-550 ⁰C
with increasing vanadium loading. Under such oxygen-free conditions, the oxygen
from the catalyst lattice is consumed by ODH reactions. Therefore, the oxygen
availability is expressed as the extent of catalyst oxidation during the
experiment. Changes in the extent of oxidation are described using an
exponential decay function based on propane conversion.

            On the basis of the data obtained, a
kinetic model is developed including both reactant adsorption and reaction on
the catalyst surface and catalyst's oxygen availability. Rate equations are
developed based on Mars and Van Krevelen (MvK) mechanism in which reactant
adsorption on the catalyst surface was formulated via a Langmuir?Hinshelwood
formulation. The kinetic model parameters are estimated using regression
analysis. Activation energies and Arrhenius pre-exponential constants are
calculated with their respective confidence intervals. The proposed
series-parallel kinetic model satisfactorily predicts the ODH reaction of propane
under the selected reaction conditions over the VOx/Al₂O₃
catalysts and the reaction network accounts for all product gases.

Keywords: Propane
oxidative dehydrogenation, Propylene, vanadium oxide, vanadium surface species,
lattice oxygen, riser reactor simulator.

*Corresponding author: Tel: +1-519-661-2144; Fax:
+1-519-850-2931

E-mail: hdelasa@fes.engga.uwo.ca