We summarize and discuss recent results on the separation of close-boiling hydrocarbon mixtures by means of zeolite membranes. We focus on the separation of xylene isomers using silicalite (MFI) membranes, as well as several other hydrocarbon mixtures using faujasite membranes. In the case of the silicalite membranes, the selectivity is found to depend on the membrane microstructure. Permeation of xylene isomers through the silicalite membranes occurs through both zeolitic and non-zeolitic (intercrystalline) nanopores. This hypothesis is supported by vapor-phase permeation results on silicalite membranes synthesized with different microstructures, and by confocal microscopy experiments. In addition, a simple method for repairing calcination-induced membrane defects is presented, and its application is found to be essential in obtaining high (20-300) p-xylene/o-xylene separation factors. The faujasite membranes are found to have high selectivities (40-150) in the separation of binary mixtures containing one aromatic component, and modest selectivities (4-9) for the separation of unsaturated from saturated low-molecular-weight hydrocarbons.
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