Temperature-dependent gel-type ionic liquid compounds based on vanadium-substituted polyoxometalates with Keggin structure

Abstract

A series of temperature-dependent gel-type ionic liquid compounds have been synthesized from 1-(3-sulfonic group) propyl-3-methyl imidazolium (abbreviated as MIMPS) and three vanadium-substituted heteropoly acids H₅SiW₁₁VO₄₀, H₅SiMo₁₁VO₄₀ and H₇SiW₉V₃O₄₀. The designed and synthesized gel-type polyoxometalate ionic liquids (POM-ILs) have demonstrated a tendency to exhibit a layered structure. Moreover, they can undergo a phase transformation from a viscous gel-state to a liquid-state below 100 °C, and ionic conductivity up to 10⁻³ S cm⁻¹ was observed at 120 °C. Cyclic voltammetry was carried out to study their electrochemical properties in organic solutions, and it was found that the oxidizability of the three POM-ILs decreases in the order: [MIMPS]₇SiW₉V₃O₄₀ > [MIMPS]₅SiMo₁₁VO₄₀ > [MIMPS]₅SiW₁₁VO₄₀. This result indicates that the redox behavior can be tuned by changing the chemical composition of the heteropolyanions.