Towards metal-free supported quaternary ammonium halide catalysts for an optimized cycloaddition of CO2 onto styrene oxide

Abstract

The present work reports on a large selection of porous siliceous materials functionalised by quaternary ammonium salts (QAS) with various pore structures, possible doping with Zn²⁺, and different ammonium salt substituents as well as halide counterions. After characterization by TGA, N₂ physisorption, XRD, XPS and ¹³C NMR, the latter were tested as catalysts for the cycloaddition of CO₂ onto styrene oxide to produce styrene carbonate. Control experiments performed using naked silica supports and soluble QAS demonstrated the synergistic action of the silanol groups on the supports and the quaternary ammonium salts. Most importantly, it was also shown that covalent grafting confirmed the trend without reducing the catalytic activity in comparison with conditions where soluble QAS were used. However, the possibility of recycling the entire catalytic system is an obvious benefit of heterogenized QAS. Quite clearly, our work highlighted the higher catalytic efficiency of the most hindered quaternary ammonium salt (butyl groups) combined with bromide ions. The resulting catalysts were tested under three types of conditions: in dichloromethane at 3 mol% at 100 °C under 10 bar of CO₂, in benzonitrile at 0.8 mol% at 80 °C under 10 bar of CO₂ and in the absence of solvent at 0.2 mol% at 100 °C under 20 bar of CO₂. Much better TOF values, of the order of 40 to 50 h⁻¹, were obtained in the latter two cases. Finally, the advantages and disadvantages of these last two catalytic systems are discussed, particularly with regard to the development of a more sustainable chemistry.