B-oriented MFI zeolite membranes for xylene isomer separation - Effect of xylene activity on separation performance

MFI zeolite membranes, especially b-oriented ones, show superb vapor permeation/separation performance for xylene isomer separation at low vapor pressures. In industrial applications these membranes should be operated at high xylene vapor pressures in the vapor separation mode or with liquid feed in the pervaporation mode. This paper reports the influence of xylene activity on xylene vapor and liquid permeation/separation performance of b-oriented MFI zeolite membranes. High quality b-oriented MFI zeolite membranes were synthesized with an organic template on modified silica substrates. For vapor permeation with binary p/o-xylene feed, both permeance (for p-xylene and o-xylene) and p-/o-xylene separation factor increase, with separation factor reaching maximum of 515 at 150^oC, then decreases with increasing temperature due to combined effects of temperature dependence of diffusivity and adsorption. The vapor permeance for both xylenes and separation factor decrease monotonically with increasing xylene activity (or vapor pressure). Lowest separation factor is observed when operated in the pervaporation mode giving p-/o-xylene separation factor of about 2. Compared to randomly oriented MFI zeolite membranes synthesized with an organic template, the b-oriented MFI zeolite membranes show better vapor permeation/separation performance and less negative effect of xylene activity on separation factor. However, both b- and randomly oriented MFI zeolite membranes exhibit a similarly poor separation factor when operated in the pervaporation mode.