HIGH-TEMPERATURE STAINLESS-STEEL SUPPORTED ZEOLITE (MFI) MEMBRANES - PREPARATION, MODULE CONSTRUCTION, AND PERMEATION EXPERIMENTS

Continuous layers of MFI (silicalite-1; Si-rich ZSM-5) have been prepared on porous, sintered stainless steel supports. Similar metal supported MFI membranes of approximately 50 mum thickness have been grown within stainless steel membrane modules in order to perform (high-temperature) permeation experiments. As-synthesized layers are found to be gas-tight even for small molecules such as neon. The supported MFI layers remain thermomechanically stable upon calcination at 400-degrees-C in air to remove template ions (tetrapropylammonium). Gas permeation experiments have been performed using neon, methane, n-butane, and isobutane according to the Wicke-Kallenbach principle with helium as a purge gas. The sequence of the pure ps permeabilities at room temperature and 0.3 bar partial pressure difference is methane>n-butane>neon much greater than isobutane, demonstrating that the permeation is based on both adsorption and diffusion. The deviating permeation behaviour between the butane isomers is attributed to the bulkiness of isobutane, which is also reflected in the substantially lower adsorption capacity as compared with n-butane. In experiments using binary mixtures of strongly (butane isomers) and weakly (methane) adsorbing species, the permeation rate of the former is hardly affected, whereas for the latter a drop in permeability of some two orders of magnitude is observed. At higher temperatures (up to 350-degrees-C) with a constant feed composition, the methane permeation rate increases as a result of the decreased adsorption of n-butane. The MFI layer retains its separation potential after several heating and cooling cycles.