The Effect of Microstructure On the Separation of Xylene Isomers through b-Oriented MFI Silicalite-1 Membranes

Separation of p-xylene molecules from xylene isomers is very important in the petroleum industry because p-xylene is a starting material to produce monomers used in the production of PET plastics and polyester clothing. Conventional method used for the separation of xylene isomers is a distillation process that separates molecules based on the difference in their volatilities. However, close boiling points of xylene isomers make the separation by distillation more costly. Zeolite membranes have attracted a great deal of research and industrial interest due to their molecular sieving ability for gas separation. Control of microstructure such as preferential orientation of pores, size and shape of grain boundary and membrane thickness are often desirable to improve gas separation performance of zeolite membranes. For instance, Lai et al.¹ has demonstrated that b-oriented MFI zeolite membranes show dramatically increased membrane performance as compared to those oriented randomly or along the other crystallographic directions. Recently Choi et al.² has improved the performance of c-oriented MFI membranes by eliminating defects in grain boundaries using rapid thermal processing. Although control of microstructure provides promising results, there are still technological challenges for the industrial applications.   

Recently, we have reported a simple synthesis route for the preparation of b-oriented silcalite-1 membranes that provided easy ways to control microstructure of membranes such as orientation, density and structure of grain boundaries.³ In this talk, we will show the performance of b-oriented silcalite-1 membranes with controlled microstructure by measuring the permeation of xylene isomers. We will present, for the first time, the effect of the grain boundary structure on the separation of xylene isomers by comparing the permeation of xylene isomers through b-oriented MFI membranes with and without removing TPA from the grain boundaries. Also, the density of grain and grain boundary are simply controlled by changing the size of seed crystals and their effects on the separation of xylene isomers will be discussed.

(1) Lai, Z. P.; Tsapatsis, M.; Nicolich, J. R. Advanced Functional Materials 2004, 14, 716-729.

(2) Choi, J.; Jeong, H. K.; Snyder, M. A.; Stoeger, J. A.; Masel, R. I.; Tsapatsis, M. Science 2009, 325, 590-593.

(3) Lee, I.; Buday, J. L.; Jeong, H. K. Microporous and Mesoporous Materials 2009, 122, 288-293.