Synthesis of Tetrabutylphosphonium Carboxylate Ionic Liquids and Its Catalytic Activities for the Alcoholysis Reaction of Propylene Oxide

Four tetrabutylphosphonium carboxylate ionic liquids ([P₄₄₄₄]­[CA]) were prepared, characterized, and used as catalysts for the synthesis of propylene glycol methyl ether (PGME) from the alcoholysis reaction of propylene oxide (PO) with methanol. The effects of various parameters such as the kind of ILs, temperature, reaction time, catalyst loading, and molar ratio of the reactants on PGME yield and selectivity were also studied in detail. The results indicated that tetrabutylphosphonium butyrate ([P₄₄₄₄]­[Buty]) exhibited the best catalytic activity in comparison to three other carboxylate ILs, and PGME was produced in nearly 94% yield under mild conditions. In addition, [P₄₄₄₄]­[Buty] was found to exhibit good catalytic activities and selectivities in other alcoholysis reactions of epoxides with various alcohols, such as PO with n-propanol, PO with n-butanol, 1-(allyloxy)-2,3-epoxypropane with methanol, and 1,2-epoxybutane with methanol. [P₄₄₄₄]­[Buty] could also be recovered easily and used repetitively at least 10 times without an obvious decrease in activity and quantity. Therefore, [P₄₄₄₄]­[Buty] proved to be an effective recyclable homogeneous catalyst for the synthesis of PGME and showed potential application in industry.