Control of ionic interactions in silver salt-polymer complexes with ionic liquids: Implications for facilitated olefin transport

The ionic liquids 1-butyl-3-methyl imidazolium BMIM⁺NO ₃^- and BMIM⁺BF₄^-4 have been successfully utilized to increase the activity in reversible π-complexation with olefins of silver cations in silver polymer electrolytes consisting of AgNO₃ dissolved in poly(2-ethyl-2-oxazoline) (POZ). In the absence of the ionic liquids, AgNO₃ is not readily dissolved in POZ and is instead because of its high lattice energy present mostly as chemically less active ion pairs or higher order aggregates. The activation of Ag⁺ in silver polymer electrolytes in the presence of these ionic liquids is principally due to the reduction of the interactions between Ag ⁺ and NO₃^- produced by the interaction between BMIM⁺ and NO₃^-, and this effect does not occur in the presence of common salts such as NaNO₃. We investigated these systems with FT-IR, FT-Raman, and XPS and calculated the theoretical electronic structures of the relevant species. The increased activity in these membranes of Ag⁺ in reversible π-complexation with olefins was further confirmed by evaluating the sorption and desorption behavior of propylene and the performances of the membranes in the separation of olefin/paraffin mixtures.