We summarize and discuss recent results on the separation of close-boiling
hydrocarbon mixtures by means of zeolite membranes. We focus on the separat
ion of xylene isomers using silicalite (MFI) membranes. as well as several
other hydrocarbon mixtures using faujasite membranes. In the case of the si
licalite membranes, the selectivity is found to depend on the membrane micr
ostructure. Permeation of xylene isomers through the silicalite membranes o
ccurs through both zeolitic and non-zeolitic (intercrystalline) nanopores.
This hypothesis is supported by vapor-phase permeation results on silicalit
e membranes synthesized with different microstructures, and by confocal mic
roscopy 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 th
e separation of binary mixtures containing one aromatic component, and mode
st selectivities (4-9) for the separation of unsaturated from saturated low
-molecular-weight hydrocarbons. (C) 2001 Elsevier Science B.V. All rights r