The Catalyst Review April 2016 - 11

SPECIAl fEATuRE
relatively constant and the alkane:oxygen ratio decreases. This is enhanced further by the influence of the pressure drop generated
by the catalyst bed that typically increases the O2 permeation rate near the end of the reactor, due to the higher pressure gradient in
that zone (Armor 1995).
There are several ways to overcome this problem: The simplest, from a chemical engineering point of view, is co-feeding a part of the
oxygen with the alkane into the reaction zone. This is simple because it means an extra pump and some extra piping and pressure
monitoring, yet doesn't require changes in the basic structure of the reactor. Alternatively, one can use a ceramic membrane of
varying thickness, a sort of hollow cone shape, starting thin at the reactor entrance and increasing towards its exit. This increases the
oxygen diffusion times along the reactor by creating a larger physical diffusion barrier.
Farrusseng et al. proposed the chemical valve concept for regulating the oxygen flux in the reactor, by using a ceramic membrane
whose permeability could be controlled by the redox properties of the gas phase (Julbe et al. 2001). They chose vanadia for this,
utilizing the reversible redox switching between the V2O5 and V2O3 species. In the case of vanadia, the redox changes also alter the
texture of the membrane, and can change the membrane permeance behavior as shown in Figure 7, right.
The redox-tunable membrane was prepared by depositing an organic solution of vanadium and phosphorous molecular precursors
inside the pores of a commercial tubular γ-Al2O3 membrane. After calcining in air at 650 °C, the internal and intermediate layers of
the asymmetric support were entirely filled with a combined ceramic material, containing V2O5 and AlPO4 crystallites. The pores of
the internal and intermediate layers were almost completely filled with the V2O5/AlPO4 composite, while the alumina grains of the
external layer were only covered by the material. The redox switching between V2O5 and V2O3 influenced the surface area, the grain
morphology, pore size distribution, and pore volume. Changing these parameters resulted in permeance differences for the oxygen,
showing that this ceramic membrane behaves like a chemical valve: its permeance is higher when it is reduced and lower when it is
oxidized.
In conclusion, the recent developments in ceramic membrane synthesis, catalysis and reactor technology open exciting opportunities
for enhancing reactant conversion and product selectivity using catalytic membrane reactors. The increasing availability of standard
and made-to-order membrane modules makes it easier for chemical companies to choose this option in their processes.

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The Catalyst Review

April 2016

The Catalyst Review April 2016

Table of Contents for the Digital Edition of The Catalyst Review April 2016