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 p erfluorosulfonate ion-exchange membranes was investigated on the basis of t he results obtained from sorption and permeation experiments. A derivation of a multilayer diffusion equation was carried out in order to interpret th e transport mechanism of gases or vapors through microporous materials, ass uming 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 swelli ng of the surrounding polymer pendant side chains by the sorbed alkene mole cules. The diffusion coefficients of the sorbed alkene molecules in the fir st adsorbed layer, Dr, are always lower than those of the sorbed alkene mol ecules in the second and higher adsorbed layers, D-2. This indicates that t he interaction between the silver salts and the alkene molecules is stronge r than that between the alkene molecules themselves, leading to a lower mob ility of the sorbed alkene molecules in the first adsorbed layer than those in the second and higher adsorbed layers. The large difference in the perm eation flux between 1-butene and propylene is mainly due to the difference in the sorbed concentration between these two alkenes at the same gas press ure in the membrane.