Influence of Zeolite Protective Overlayer on the Performances of Pd Thin Film Membrane on Tubular Asymmetric Alumina Supports

The role of zeolite overlayer in the performance of Pd thin film membranes on asymmetric alumina tubular supports is analyzed. These novel multilayer membranes have a ceramic base support (asymmetric α-Al2O3 tubes) on which a crack-free Pd dense thin film was deposited by electroless plating deposition followed by creation of a further zeolite (titanium silicalite, TS-1) membrane compact layer on the top of the Pd thin film. The TS-1 layer was prepared by a two-step procedure: (i) the preseeding with TS-1 nanocrystals by dip-coating, eventually assisted by a polymer (PEI or PDADMAC), followed by (ii) a secondary growth. The composite membranes are crack-free and homogeneous, and show good mechanical properties, in terms of adhesion between metallic film and substrate, and between the zeolitic film and Pd thin film. The composite membranes were evaluated in terms of H2 permeance with respect to bare Pd membrane. The zeolite overlayer induces a change in the rate-limiting step in H2 transport, due to the partial coverage of Pd surface by zeolite, compensated by an increase in the sticking coefficient of H2 adsorption. These materials were tested to protect Pd from deactivation by propylene, for use in catalytic membrane reactors for propane dehydrogenation.