Unusual pathway of epoxide hydration over a novel CoIII(salen)-based pseudohomogeneous catalyst with excellent performance

Abstract

Due to high H2O/epoxide molar ratio for non-catalytic hydration, industrial production of bulk chemicals 1,2-diols is much energy-intensive. As a very promising route to produce 1,2-diols with low energy consumption, catalytic hydration of epoxides has aroused much interest from both academy and industry. The key is to develop a catalyst with high activity and stability at a low H2O/epoxide molar ratio. It has been demonstrated that encapsulating a homogeneous catalytic species into the cavity of porous material is a very efficient way to obtain a high-performance heterogeneous catalyst. Herein, a novel pseudohomogeneous catalyst with the distinctive Co(salen)-PF6 encapsulated in nanocages of a typical mesoporous material FDU-12 has been successfully developed, which exhibits both unexpectedly tremendously enhanced activity at the low H2O/epoxide molar ratio close to stoichiometric ratio and unique surprising recyclability without regeneration between runs in propylene oxide hydration. Especially, based on the systematic experimental design/implementation, particular structural characterizations and in-depth calculations, the pathway the newly developed catalyst going through has been investigated and determined to be an unusual bisubstrate-directly-activated cooperative mechanism (i.e. a bimolecular cooperative activation mechanism not involving Co(salen)–OH). The cooperative activation effect gives rise to the unexpectedly tremendously enhanced activity of our new catalyst, while no involving of Co(salen)–OH during the catalytic process leads to its best and unique catalytic stability in epoxide hydration among all the reported CoIII(salen)-based heterogeneous catalyst. It is believed that this study and the developed catalyst are of great significance for developing the catalytic hydration technology with low energy consumption and for developing high-performance heterogeneous catalysts.