Facile preparation of hydroxide ion or proton conductive ionic liquids by mixingtetra-n-butylphosphonium hydroxide and benzimidazole

Abstract

Tetra-n-butylphosphonium benzimidazolate ([P₄₄₄₄][BzIm]) was prepared by mixing equimolar amounts of tetra-n-butylphosphonium hydroxide ([P₄₄₄₄][OH]) and neutral benzimidazole (BzIm). This salt had a melting point (T_m) of 55 °C and good thermal stability up to 300 °C. The equimolar mixture had ionic conductivity of 1.3 × 10⁻⁴ S cm⁻¹ at 60 °C. Unequimolar mixing of these components gave rise either to hydroxide ions or proton conductive materials, depending on the mixing ratio. Addition of excess [P₄₄₄₄][OH] to benzimidazole (molar ratio of benzimidazole:[P₄₄₄₄][OH] = 1 : 2) led to good ionic conductivity (1.3 × 10⁻³ S cm⁻¹ at 60 °C); this is attributed not only to [P₄₄₄₄] cations and [BzIm] anions but also hydroxide anions. Mixtures containing excess benzimidazole were expected to be proton conductors. As an example, the mixture with molar ratio benzimidazole : [P₄₄₄₄][OH] of 3 : 2 had ionic conductivity of 4.1 × 10⁻⁴ S cm⁻¹ at 60 °C and a decomposition temperature (T_d) above 300 °C. This conductivity was higher than the value expected from the T_g and other physicochemical properties of [P₄₄₄₄][BzIm], suggesting the contribution of proton conduction. Proton conduction was further confirmed by chronoamperometric measurement using electrochemical cells equipped with platinum electrodes.