Evidence for Parallel Pathways in the Facilitated Transport of Alkenes through Ag+-Exchanged Nafion Films

Facilitated transmembrane fluxes for feed solutions containing 1,5-hexadiene, 1-hexene, and n-hexane are reported for hydrated Ag+−Nafion membranes that vary in thickness between 8 and 155 μm. Fluxes for n-hexane are virtually undetectable. Analysis of the fluxes for 1,5-hexadiene and 1-hexene suggests that two transport pathways are operating in parallel. The fluxes associated with one pathway are inversely proportional to membrane thickness and lead to a limiting diene/monoene separation factor of 47. The fluxes associated with the second pathway are independent of membrane thickness and lead to a limiting separation factor of 100. The differences in the selecivity for the two pathways result in separation factors for the 8−155 μm membranes ranging from 50 to 70. Concentrations of solutes in the membranes under equilibrium and transport conditions and the rate of 1,5-hexadiene extraction from membranes as a function of time are determined and are used to infer properties associated with the two pathways.