Multilayer diffusion behavior and the swelling effect of 1-butene and propylene in dry AgNO3-doped perfluorocarbon type ion-exchange membranes

The transport mechanism of 1-butene and propylene through dry AgNO3-doped perfluorosulfonate ion-exchange membranes was investigated on the basis of the results obtained from sorption and permeation experiments. A derivation of a multilayer diffusion equation was carried out in order to interpret the transport mechanism of gases or vapors through microporous materials, assuming that the BET n-layer adsorption takes place and that the transport of the adsorbed molecules is controlled by the activated diffusion mechanism. The transport mechanism of 1-butene and propylene through dry AgNO3-doped PSM membranes can be explained by multilayer diffusion together with swelling of the surrounding polymer pendant side chains by the sorbed alkene molecules. The diffusion coefficients of the sorbed alkene molecules in the first adsorbed layer, Dr, are always lower than those of the sorbed alkene molecules in the second and higher adsorbed layers, D-2. This indicates that the interaction between the silver salts and the alkene molecules is stronger than that between the alkene molecules themselves, leading to a lower mobility of the sorbed alkene molecules in the first adsorbed layer than those in the second and higher adsorbed layers. The large difference in the permeation flux between 1-butene and propylene is mainly due to the difference in the sorbed concentration between these two alkenes at the same gas pressure in the membrane.