Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium
Section snippets
Membrane reactor concepts
According to the IUPAC definition a membrane reactor is a device that combines a membrane-based separation process with a chemical reaction step in one unit. Various possibilities exist for such a combination. The most widely used concept is the selective removal of products from the reaction zone (cf. Fig. 1a), which is applied first of all to equilibrium limited reactions to increase the yield beyond the corresponding equilibrium value, or, generally speaking, to repress undesired secondary
Membrane reactors based on palladium alloy membranes
It is well established that dense palladium and palladium alloy membranes are permeable for hydrogen only. Two main possibilities arise from this feature to employ these in membrane reactors, namely
- 1.
to promote a dehydrogenation reaction by removal of the produced hydrogen from a dehydrogenation catalyst through the membrane, i.e. preventing the establishment of the chemical equilibrium, or
- 2.
to carry out a hydrogenation reaction on the palladium surface with supply of hydrogen through the membrane.
Membrane reactors based on porous catalytic membranes with dispersed palladium
Porous membranes with built-in catalytic components are catalytic membranes in the true sense of the word. They can be employed in a reactor in various ways, which may be classified into the following three categories:
- 1.
Forced flow of (pre-mixed) reactants through the membrane.
- 2.
Co-current or counter-current flow of two reactant streams on opposite sides of the membrane with permeation of all species across the membrane (non-permselective).
- 3.
Co-current or counter-current flow of two reactant streams
Critical evaluation and outlook
It has been shown that catalytic membranes with interesting properties can be obtained by introducing palladium and tin in the top layer of asymmetric porous ceramic membranes, preferentially employing macro/mesoporous α-alumina or zirconia as top-layer materials. These membranes can be used as catalytic diffusers for the hydrogenation of nitrate and nitrite in water. The main advantage of the catalytic diffuser, besides the immobilisation of the active phase, is the possibility to supply the
Acknowledgements
The authors thank all partners of the European research project, Prof. G. Strukul, Dr. M. Marella, Dr. P. Ruiz, Dr. F. Luck, Ir. M. van Donk and Prof. G. Centi for the fruitful co-operation and Mrs. M. Schorr and Mr. M. Jusek (DECHEMA e.V.) for EPMA analysis, pulse CO-chemisorption and XRD measurements. Financial support by the Bavarian Catalysis Research Network FORKAT II and by the European Community is gratefully acknowledged.
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