Adsorption and diffusion properties of zeolite membranes by transient permeation

Adsorption isotherms and diffusion coefficients for light gases and butane isomers were measured for the transport pathways involved in gas permeation through H-ZSM-5 membranes by a transient permeation technique. The permeate responses to step changes in the feed were measured, and the transport was modeled as Maxwell–Stefan diffusion with single-site Langmuir adsorption in the zeolite. Isotherms measured for N2, CO2, and CH4 at 295 K were nearly identical to those measured by calorimetry on H-ZSM-5 powders. Isotherms for butane isomers were also similar to isotherms for MFI powders and heats of adsorption and diffusion activation energies were in the ranges reported in the literature. Maxwell–Stefan diffusion coefficients for all gases studied increased slightly with feed partial pressure and were similar to those measured by other macroscopic methods for zeolite membranes and crystals. Effective membrane thicknesses were also determined non-destructively for tubular zeolite membranes by the transient permeation technique.

A method was developed to rapidly measure adsorption and diffusion parameters in transport pathways in zeolite membranes by transient permeation measurements. The effective membrane thickness is also estimated nondestructively. Adsorption isotherms and thermodynamic properties determined from transport measurements of N2, CH4, CO2, n-C4, and i-C4 on H-ZSM-5 membranes are remarkably similar to adsorption isotherms measured for ZSM-5 crystals. Maxwell–Stefan diffusion coefficients determined from transient analysis are of the same order of magnitude as those measured by other macroscopic techniques indicating that transport through these zeolite membranes occurs mainly through zeolite pores. Maxwell–Stefan diffusion coefficients increase slightly with increasing feed partial pressure, indicating that they depend on concentration and that the Darken approximation does not completely model diffusion of light gases through ZSM-5 membranes. Effective membrane thicknesses calculated by the transient method are reasonable when compared to SEMs of similarly prepared membranes.