Chromium-Salophen as a Soluble or Silica-Supported Co-Catalyst for the Fixation of CO2 Onto Styrene Oxide at Low Temperatures.

Matthieu Balas,Ludivine K/Bidi,Richard Villanneau,Franck Launay

doi:10.3389/fchem.2021.765108

Abstract

Addition of a soluble or a supported CrIII-salophen complex as a co-catalyst greatly enhances the catalytic activity of Bu4NBr for the formation of styrene carbonate from styrene epoxide and CO2. Their combination with a very low co-catalyst:Bu4NBr:styrene oxide molar ratio = 1:2:112 (corresponding to 0.9 mol% of CrIII co-catalyst) led to an almost complete conversion of styrene oxide after 7 h at 80°C under an initial pressure of CO2 of 11 bar and to a selectivity in styrene carbonate of 100%. The covalent heterogenization of the complex was achieved through the formation of an amide bond with a functionalized {NH2}-SBA-15 silica support. In both conditions, the use of these CrIII catalysts allowed excellent conversion of styrene already at 50°C (69 and 47% after 24 h, respectively, in homogeneous and heterogeneous conditions). Comparison with our previous work using other metal cations from the transition metals particularly highlights the preponderant effect of the nature of the metal cation as a co-catalyst in this reaction, that may be linked to its calculated binding energy to the epoxides. Both co-catalysts were successfully reused four times without any appreciable loss of performance.

Highlights

Global warming remains one of mankind’s greatest challenges and, today, it is imperative to find environmentally friendly and cost-effective solutions to limit the increasing concentration of greenhouse gases in the atmosphere
The present work addressed the successful heterogenization of a chromium (III)-salophen complex at the surface of a mesoporous SBA-15 silica as well as its co-catalytic activity in the cycloaddition of CO2 onto styrene oxide
The synthesis of the molecular catalyst as well as its strong covalent grafting onto the support were based on simple and efficient experimental protocols

Summary

Introduction

Global warming remains one of mankind’s greatest challenges and, today, it is imperative to find environmentally friendly and cost-effective solutions to limit the increasing concentration of greenhouse gases in the atmosphere. Climate Change, 2014) For this reason, chemistry offers innovative solutions that encourage us to consider CO2 not as a waste and as a source of high valued compounds. Among the various targets considered in the literature, the formation of cyclic carbonates with a rather biodegradability through the reaction of CO2 with epoxides generates a growing interest. Cyclic carbonates have a wide range of applications such as monomers in plastics, solvents in paints, batteries, and even degreasers, or as organic intermediates for the synthesis of dimethylcarbonate (Kamphuis et al, 2019). The so-called cyclocarbonatation reaction of epoxides represents a green alternative to one of the conventional synthesis of carbonates based on the phosgenation of diols used industrially since 1833 (Fukuoka et al, 2003)

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