Permeation characteristics of a metal-supported silicalite-1 zeolite membrane

Permeation data are presented which give an overview of the permeation and separation characteristics of a metal supported silicalite-1 zeolite membrane over a broad temperature and pressure range. Methane, ethane, ethene, propane, propene, n-butane, i-butane, carbon dioxide, hydrogen, and i-octane are used as probe molecules and helium is used as sweep gas. One-component anti binary systems are studied in a temperature range of 193-673 K and a pressure range of 0.05-500 kPa. Large differences have been found between the different one-component permeation fluxes, which amounts up to a factor of about 500 between methane and the bulky iso-butane. The permeation fluxes at 295 K, generally decrease with increasing molecular size. The alkenes permeate faster than their corresponding alkanes. Diffusion coefficients calculated with the Maxwell-Stefan equations are in accordance with the literature. A remarkable temperature dependency has been observed. For the bulky i-butane the permeance increases steadily with temperature. For methane, ethane, and n-butane a maximum in permeation is observed and for methane and ethane also a minimum. This maximum can be explained by the combined temperature dependency of diffusion and adsorption in configurational mass transport. The minimum is explained by the occurrence of a Knudsen-like mass transport at low occupancy and high temperature. In many cases the separation selectivity of a mixture does not reflect the one-component permeation ratio. Besides molecular sieving and difference in diffusivity, difference in adsorption appears to be a key factor in separation selectivity. The permeation of weakly adsorbing molecules (e.g. hydrogen at: 295 K) can drop over two orders of magnitude in the presence of strongly adsorbing molecules(e.g. n-butane at 295 K). This results in high separation selectivities favouring the strongest adsorbing component. Typical separation selectivities for hydrogen/n-butane (at 295 K, 95 kPa/5 kPa), n-butane/i-butane (at 295 K, 50 kPa/50 kPa), and methane/i-octane (at 423 K, 25 kPa/5 kPa) mixtures, are 125, 27 and > 300, respectively. An inversion in separation selectivity is observed during a temperature programmed permeation which is explained from the temperature dependence of adsorption. The membrane appears to be very stable upon thermal cycling (193-673 K) and the permeation characteristics have changed less than 10% over the testing period of 1.5 year.