Pd encapsulated and nanopore hollow fiber membranes: Synthesis and permeation studies

New types of supported Pd membranes were developed for high temperature H-2 separation. Sequential combinations of boehmite sol slip casting and film coating, and electroless plating (ELP) steps were designed to synthesize "Pd encapsulated" and "Pd nanopore" membranes supported on alpha-Al2O3 hollow fibers. The permeation characteristics (flux; permselectivity) of a series of unaged and aged encapsulated and nanopore membranes with different Pd loadings were compared to those of a conventional 1 mu m Pd/4 mu m gamma-Al2O3/alpha-Al2O3 hollow fiber membrane. The unaged encapsulated membrane exhibited good performance with ideal H-2/N-2 separation factors of 3000-8000 and H-2 flux similar to 0.4 mol/m(2) s at 370 degrees C and a transmembrane pressure gradient of 4 x 10(5) Pa. The unaged Pd nanopore membranes had a lower initial flux and permselectivity, but exhibited superior performance with extended use (200 h). At the same conditions the unaged 2.6 mu m Pd nanopore membrane had a H-2 flux of similar to 0.16 mol/m(2) s and separation factor of 500 and the unaged 0.6 mu m Pd nanopore membrane had a H-2 flux of similar to 0.25 mol/m(2) s and separation factor of 50. Both nanopore membranes stabilized after 40 h of operation, in contrast to a continued deterioration of the permselectivity for the other membranes. An analysis of the permeation data reveals a combination of Knudsen and convective transport through membrane defects. A phenomenological, qualitative model of the synthesis and resulting structure of the encapsulated and nanopore membranes is presented to explain the permeation results. (c) 2007 Elsevier B.V. All rights reserved.