Structure and properties of microporous molecular sieve materials and membranes
Molecular sieving materials such as zeolites owe their unique characteristics to their crystalline frameworks and well-ordered pore structures consisting of pores of molecular dimensions. Such crystals can exert pronounced size and shape effects on molecules and ions that interact with their pore structure, leading to the possibility of energy-efficient separations of mixtures by the mechanism loosely referred to as “molecular sieving”. Much research is currently being devoted to the synthesis of new molecular sieves, as well as the fabrication of molecular sieve membranes for continuous separations processes.
Separations involving molecules of similar shapes and size require the ability to discriminate between molecules that differ in size by a few tenths of an angstrom. Some commercially important separations in this category include nitrogen/methane (natural gas purification) and oxygen/nitrogen (air separation). Recently, researchers at Engelhard Corporation discovered that heat-treated crystals of the titanosilicate ETS-4 were able to perform these separations, the results being dependent on the heat-treatment temperature. Our detailed investigation of this so-called “molecular gate” effect reveals that continuous structural contractions and structural disorder occur in ETS-4 upon heat-treatment. We have explained the molecular gate effect on the basis of the average changes in size and shape of the transport controlling apertures within the ETS-4 framework. It is found that the average size of these apertures can be finely adjusted so as to make possible various separations with the same material.
We have also investigated the commercially important separation of xylene isomers using membranes of the zeolite silicalite. We have found that silicalite membranes can display very unusual permeation characteristics in the presence of xylene isomers. In this work, we argue that the permeation characteristics of the silicalite membrane are a result of a complicated interplay between the xylene molecules, the host zeolite framework, and the intercrystalline membrane porosity.
Finally, we discuss the structures of two new and interesting materials, solved using techniques of X-ray crystallography and high-resolution electron microscopy. These studies have led to the elucidation of a novel heteroepitaxial growth of one zeolite on nanofibers of another zeolite, and the first layered silicate with three-dimensionally microporous layers.