Microporous silica and zeolite membranes for hydrogen purification

Microporous amorphous silica and zeolite membranes are made as thin films on a multilayer porous support. The membranes have a network of connected micropores with similar to 0.5-nm diameters. Net transport of small molecules on this network occurs under the driving force of a gradient in chemical potential. Favorable combinations of sorption selectivity and diffusion mobility in the membrane materials lead to high H-2 fluxes and good selectivity with respect to other gases. The membranes show potential for application in H-2 separation under harsh conditions. Amorphous silica membranes show very high H-2 fluxes because they can be made very thin; silicalite-type zeolite membranes are expected to have a better operational stability. To make the membranes a viable option, improvements are needed in reducing membrane defects and manufacturing costs and enhancing reproducibility and operational stability. This article summarizes the state of the art, provides relevant definitions, and outlines the base design and long-term specifications of viable supported membrane structures. This is followed by an overview of transport properties, synthesis, and operational stability of the membrane and the supporting structures. Directions for future research programs are provided by demonstrating how the selection of the actual membrane composition and supporting structure can be derived from an application-based design. The success of such a design depends critically on fundamental studies of membrane transport, strength, and operational stability.