Study of the Mircrostructure-Property Relationship for MFI-Type Zeolite Membranes for Xylene Separation

Effective separation of xylene isomers is possible through continuous membrane processes, specifically pervaporation, utilizing MFI-type zeolite membranes. The average pore size of MFI-type zeolite (0.6 nanometers) is large enough to allow para-xylene (molecular size ~0.56 nanometers) to pass and excludes bulkier ortho- and meta- isomers (molecular size ~0.68 nanometers). Separation through MFI-type zeolite occurs via transport differences of each isomer across the membrane and it is desired to exploit this property for use in pervaporation. Currently, MFI-type membranes have demonstrated the ability to separate xylene isomers under vapor permeation conditions where the partial pressures of xylene are extremely low resulting in impractically low flux values; at increased xylene exposure the separation capability is lost.

Observation of the differences in transport and separation capability through MFI membranes of varying microstructure and defect concentrations show these properties can significantly affect membrane performance as well as mechanical stability.

Fundamental studies demonstrate that the framework distortion experienced by the MFI zeolite at high para-xylene loading results in the loss of molecular sieving capability and is the cause of poor pervaporation separation performance rather than the presence of intercrystalline defects. These findings shift the focus from development of a defect-free membrane to one with enhanced framework rigidity. Studies into isomorphous substitution of Si in the MFI framework and synthesis of bilayer structures direct the development of silicalite/ZSM-5 bilayer membranes, which exhibit significantly, enhanced para-xylene selectivity (α = 60), reasonable flux (0.13 kg.m^-2.hr^-1), and performance stability under pervaporation conditions.