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

New types of supported Pd membranes were developed for high temperature H2 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 α-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 μm Pd/4 μm γ-Al2O3/α-Al2O3 hollow fiber membrane. The unaged encapsulated membrane exhibited good performance with ideal H2/N2 separation factors of 3000–8000 and H2 flux ∼0.4 mol/m2 s at 370 °C and a transmembrane pressure gradient of 4 × 105 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 μm Pd nanopore membrane had a H2 flux of ∼0.16 mol/m2 s and separation factor of 500 and the unaged 0.6 μm Pd nanopore membrane had a H2 flux of ∼0.25 mol/m2 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.