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

This paper reports experimental results on pervaporation of pure xylene iso mers and their binary mixtures through alpha-alumina-supported zeolite MFI membranes. The membranes were permeable for all xylene isomers. Intensive f ouling of the zeolite membrane was observed during the pervaporation experi ments due to strong interaction of xylene molecules with the zeolite pores via pi-complexation, The pervaporation flux of pure xylene isomers at 26 de grees C in first 10 h experiment decreases in the order: m-xylene>p-xylene> o-xylene. No separation of binary p-lm-xylene and p-lo-xylene mixtures coul d be obtained in the investigated temperature range (26-75 degrees C). A th ree-type pore microstructure for the zeolite membrane is proposed to explai n the pervaporation and helium permeation data. The zeolite membrane layer contains both zeolitic pores, microporous nonzeolitic pores nonselective fo r xylene isomers, and a very small amount of larger defect pores. The micro porous nonzeolitic pores are formed possibly in the step of template remova l. Presence of the microporous nonzeolitic pores and fouling by chemisorbed xylenes are responsible for the nonseparative properties of xylene pervapo ration through the polycrystalline MFI membrane observed in this work. (C) 1999 Elsevier Science B.V. All rights reserved.