Microporous Silica and Zeolite Membranes for Hydrogen Purification
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Microporous Silica and Zeolite Membranes for Hydrogen Purification Henk Verweij, Y.S. Lin, and Junhang Dong Abstract 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 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 H2 fluxes and good selectivity with respect to other gases. The membranes show potential for application in H2 separation under harsh conditions. Amorphous silica membranes show very high H2 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.
limited hydrocarbon dehydrogenation, methane reforming, and water–gas shift reactions by the effective removal of the H2 produced in the reaction zone. To make microporous membranes a viable option for these applications, improvements are needed in such interrelated properties as flux and selectivity, the occurrence of structural defects, operational stability, reproducibility, and manufacturing costs. A comprehensive review of the literature up to 2002 is provided in Reference 3. The purpose of this article is to highlight recent progress and to provide directions for future focused and comprehensive research programs. Symbols used throughout the article are summarized for the reader’s convenience in the accompanying sidebar.
Base Design, Definitions, Specifications, and Scope Supported microporous membranes have a structure as shown in Figure 3. The predesigned structure of the layers in this structure is described as a quasihomogeneous arrangement of connected particles.4 Most practical porous layers have one typical pore diameter Øp. Pore diameters are classified by IUPAC into the following categories: 䊏 microporous: Øp 2 nm; 䊏 mesoporous: 2 nm Øp 50 nm; and 䊏 macroporous: Øp 50 nm.5 Likewise, deviations from the quasihomogeneous structure are indicated as meso- and macrodefects (see Figures 4 and 5).
Keywords: ceramic, hydrogen, membrane, silica, sol-gel, transport,
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