Tungstate ionic liquids as catalysts for CO2 fixation into epoxides

Abstract

Herein we describe the syntheses of a series of ammonium, phosphonium, imidazolium and diazabycicloundecenium tungstate and peroxotungstate ionic liquids, their full spectroscopic characterisation (FT-IR, 1H-, 13C-and 183W-NMR) and a comparison of their properties and possible applications in catalysis. The synthetic procedures to obtain the ionic liquids rely on anion exchange and acid-base reactions – including an innovative route for the synthesis of tungstate and peroxotungstate ionic liquids using, for the first time, a halide-free organic ionic liquid as precursor. The tungstate ionic liquids were used as catalysts for CO2 fixation in styrene oxide as well as in a series of other epoxides to yield the corresponding carbonates. Under optimized conditions, styrene carbonate is obtained in up to 67 % yield at 90 °C with just butylmethylimidazolium tungstate, and in 91 % yield by coupling tetrabutylammonium tungstate and bromide. Preliminary tests indicate that the same catalysts can also promote epoxidation reactions, paving the way for their use in the direct oxidative carboxylation of olefins.