Molybdate ionic liquids as halide-free catalysts for CO2 fixation into epoxides

Herein we describe the syntheses of a series of molybdate and polyoxomolybdate ionic liquids (ILs), their full spectroscopic characterisation (FT-IR, H-1-, C-13-, P-31-, and Mo-95-NMR and ICP-MS), a comparison of their properties, and their applications as bifunctional catalysts for CO2 insertion into epoxides. The synthetic procedures to obtain ILs rely on anion exchange and acid-base reactions, including an innovative route for the synthesis of molybdate ionic liquids (Mo-ILs) using a halide-free organic IL precursor. The use of Mo-ILs as catalysts for CO2 fixation was investigated using 1,2-epoxyhexane as a model substrate. In the presence of 2.5 mol% of tetrabutylammonium molybdate, hexane carbonate was obtained in up to 86% yield at T = 120 degrees C, p(0)(CO2) = 30 bar in t = 9 h, under solventless conditions and without any added halide co-catalysts. The substrate scope was broadened to other 12 terminal and internal epoxides; moreover, the reaction was scaled up to 2 g of the substrate and catalyst recyclability was demonstrated up to 5 recycles.

Automated summary

In this study, the syntheses, characterisation, properties comparison, and applications of molybdate and polyoxomolybdate ionic liquids as bifunctional catalysts for CO2 insertion into epoxides were described. The synthetic methods relied on anion exchange and acid-base reactions, including a novel route for the synthesis of molybdate ionic liquids using a halide-free organic precursor. The investigations showed that tetrabutylammonium molybdate could serve as an efficient catalyst for the production of hexane carbonate from 1,2-epoxyhexane under solventless and halide-free conditions.