Here we will attempt a brief overview of recent synthetic efforts for micropore and lower-end mesopore membranes. We will not address the very important classes of nonporous membranes, such as dense metals and solid electrolytes with applications in H2 and O2 separations, or meso- and macroporous membranes, which find applications in food processing and water treatment. Microporous materials provide high permselectivities for molecules encountered in the chemical-processing industry but suffer from low intrinsic permeabilities. Therefore, in order to bring microporous membrane materials to commercial applications, functional composites with small effective thicknesses (in the micron or submicron range) must be developed. For example, to achieve economical membrane-reactor sizes, fluxes as high as 0.1 mol/(m2 s) are desirable. Approaches to microporous membranes include modification of mesoporous membranes by sol-gel and chemical-vapor-deposition (CVD) techniques, carbonization of polymers to form molecular-sieve carbon, and polycrystalline-film growth of zeolites and other molecular sieves.
Microporous carbon is widely used for liquid or gas purification because of its strong adsorptive properties and high surface area. It is also used for air separation by pressure swing adsorption (PSA), relying on its adsorptive and molecular-sieving properties. From the standpoint of applications, microporous carbons are classified into activated carbons with pore size 0.8–2 nm, and ultramicroporous carbons or carbon molecular sieves with pores 0.3–0.6 nm. Activated carbons are used because of their strong adsorption properties, while carbon molecular sieves are useful on account of their molecular-sieving as well as adsorption properties.