Xylene Separation Performance of Composition-Gradient MFI Zeolite Membranes

High-quality MFI-type zeolite membranes are effective at separation of xylene isomers under ideal conditions. Due to significant interaction between p-xylene and MFI-type zeolite framework, as a result of strong adsorption, molecular sieving is only possible at low loadings of the isomers. At higher loadings, the distortion induced to the MFI pore size is large enough to allow access to the larger isomer causing p-xylene and o-xylene interactions to dominate the separation capability of the membrane through competitive adsorption. Previous studies have shown that under these conditions, o-xylene can transport faster than p-xylene causing a severe reduction in separation capability. This chapter reports on the xylene separation performance of a bilayer MFI-type zeolite membrane as well as operating conditions that favor the transport of the smaller p-xylene over o-xylene at high loadings (under pervaporation). The membrane was formed by consecutive growth of zeolite layers via traditional hydrothermal growth of ZSM-5 (Si/Al=20) and silicalite layers. A structure directing agent was not used in the synthesis of these bilayer membranes to prevent secondary deposition of zeolite crystals between the layers. Enhanced xylene separation performance in terms of membrane selectivity and stability was observed for membranes with the two-layer zeolite structure. The top layer is sacrificial and serves two functions: (1) sealing of intercrystalline defects from transport and (2) reduction of the loading on the underlying layer to prevent transport of o-xylene. Silicalite on ZSM-5 proved to be the optimum layer combination in pervaporation testing, exhibiting steady-state α_PX/OX of 20-60 with moderate PX fluxes ~0.05-0.13kgm^-2h^-1 (5%PX:95%OX feed concentration, 96h). This is a marked improvement over the conventional pervaporation performance of single layer MFI-type zeolite membranes.