(83b) Microporous and Dense Inorganic Membranes
Microporous inorganic membranes include amorphous and crystalline membranes. The practically useful crystalline microporous membranes are of polycrystalline structure, consisting of inorganic crystallites packed together without crystallite (grain) boundary gaps. Dense inorganic membranes are referred to those made of polycrystalline ceramics or metals allowing certain gas species to permeate through the crystal lattices. Several synthesis methods have been developed to prepare microporous inorganic membranes as thin films supported on porous inorganic supports with adequate mechanical strength and minimum mass transfer resistance. Most inorganic membrane materials have much better selectivity and permeability (at high temperatures) than polymeric membranes (at low temperature), with data points falling beyond the Robeson boundary line for most gas pairs. Based on their excellent properties, inorganic membranes offer potential applications for: (1) air separation by mixed oxygen ionic and electronic conducting ceramic membranes and molecular sieve carbon membranes; (2) hydrogen separation by metal, silica and zeolite and proton-conducting ceramic membranes; (3) hydrocarbon separation by zeolite and silica membranes, and (4) carbon dioxide separation by silica or zeolite membranes. These membranes offer sufficiently high permeance and selectivity for the targeted mixture and can be made into the membrane module for practical applications. However, large scale applications of inorganic membranes requires further improvement in chemical stability and reduction in membrane costs.