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

Facilitated transport membranes can potentially be an alternative and/or su pplementary separation technology to conventional distillation for olefin s eparations. In the present work, the transport of cyclohexene, 1.5-hexadien e, 1-hexene, and styrene through sodium(I) and silver(I)-form Nafion and Na fion-poly(pyrrole) composite membranes was investigated. The transport expe riments were carried out in the perstraction mode at 25 degreesC 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 facil itation factors, calculated as the ratio of the total penetrant flux with t he Ag+ carrier present to the penetrant diffusional flux, of cyclohexene, 1 .5-hexadiene, 1-hexene, and styrene observed with Nafion membranes were 3.6 2, 12.4, 18.1, and 0.34, respectively. The small amount of water strongly a bsorbed in the membranes, observed with FTIR spectroscopy, probably contrib utes to the modest facilitation factors observed with cyclohexene. 1,5-hexa diene, acid 1-hexene. The facilitation factors of olefins obtained from sil ver(I)-form Nafion-poly(pyrrole) composite membranes were >296, 127, 259, a nd 2.8 for cyclohexene, 1,5-hexadiene, 1-hexene, and styrene, respectively. The effectiveness of the membranes is ascribed to a molecular orbital-base d hypothesis. It is hypothesized that the olefin penetrant reacts with a co mplex of the Ag+ and the poly(pyrrole). This hypothesis is examined with th e help of ab initio molecular orbital simulations. The postulation is corro borated by a correlation between the orbital energy match required for the three-species complex formation and the olefin facilitation factors. (C) 20 01 Elsevier Science B.V. All rights reserved.