Silver doped Nafion-poly(pyrrole) membranes for facilitated permeation of liquid-phase olefins

Facilitated transport membranes can potentially be an alternative and/or supplementary separation technology to conventional distillation for olefin separations. In the present work, the transport of cyclohexene, 1,5-hexadiene, 1-hexene, and styrene through sodium(I) and silver(I)-form Nafion and Nafion-poly(pyrrole) composite membranes was investigated. The transport experiments were carried out in the perstraction mode at 25°C with water-free feed and sweep solutions. All experiments were performed with single-component feed solutions consisting of 0.5 M olefin in isooctane. The facilitation factors, calculated as the ratio of the total penetrant flux with the Ag+ carrier present to the penetrant diffusional flux, of cyclohexene, 1,5-hexadiene, 1-hexene, and styrene observed with Nafion membranes were 3.62, 12.4, 18.1, and 0.34, respectively. The small amount of water strongly absorbed in the membranes, observed with FTIR spectroscopy, probably contributes to the modest facilitation factors observed with cyclohexene, 1,5-hexadiene, and 1-hexene. The facilitation factors of olefins obtained from silver(I)-form Nafion-poly(pyrrole) composite membranes were >296, 127, 259, and 2.8 for cyclohexene, 1,5-hexadiene, 1-hexene, and styrene, respectively. The effectiveness of the membranes is ascribed to a molecular orbital-based hypothesis. It is hypothesized that the olefin penetrant reacts with a complex of the Ag+ and the poly(pyrrole). This hypothesis is examined with the help of ab initio molecular orbital simulations. The postulation is corroborated by a correlation between the orbital energy match required for the three-species complex formation and the olefin facilitation factors.