Polyoxometalate-based protic alkylimidazolium salts as reaction-induced phase-separation catalysts for olefin epoxidation

Abstract

Two protic alkylimidazolium polyoxometalates, together with two corresponding aprotic N-methyl-alkylimidazolium polyoxometalates were synthesized and characterized by the methods of NMR, IR and TGAetc. Then, these salts were employed as catalysts for the epoxidation of cyclooctene in different media. The novel protic N-dodecylimidazolium peroxotungstate [HDIm]₂[{WO(O₂)₂}₂(μ–O)] was found to be a room temperature liquid molten salt (ionic liquid) and the most effective catalyst for the epoxidation of cyclooctene among these salts. The ionic liquid catalyst [HDIm]₂[{WO(O₂)₂}₂(μ–O)] can also be extended to the epoxidation of some other substrates. On the basis of this experimental observation, an efficient reaction-induced phase-separation catalyst system has been developed in this work. The reaction system can switch from tri-phase to emulsion and then to biphase and finally to all the catalyst self-precipitating at the end of the reaction, which made the recovery and reuse of the present catalyst very convenient.