Polycrystalline MFI zeolite membranes: Xylene pervaporation and its implication on membrane microstructure

This paper reports experimental results on pervaporation of pure xylene isomers and their binary mixtures through α-alumina-supported zeolite MFI membranes. The membranes were permeable for all xylene isomers. Intensive fouling of the zeolite membrane was observed during the pervaporation experiments due to strong interaction of xylene molecules with the zeolite pores via π-complexation. The pervaporation flux of pure xylene isomers at 26°C in first 10h experiment decreases in the order: m-xylene>p-xylene>o-xylene. No separation of binary p-/m-xylene and p-/o-xylene mixtures could be obtained in the investigated temperature range (26-75°C). A three-type pore microstructure for the zeolite membrane is proposed to explain the pervaporation and helium permeation data. The zeolite membrane layer contains both zeolitic pores, microporous nonzeolitic pores nonselective for xylene isomers, and a very small amount of larger defect pores. The microporous nonzeolitic pores are formed possibly in the step of template removal. Presence of the microporous nonzeolitic pores and fouling by chemisorbed xylenes are responsible for the nonseparative properties of xylene pervaporation through the polycrystalline MFI membrane observed in this work. Copyright (C) 1999 Elsevier Science B.V.