Cellulose nanocrystals-supported organocatalyst for the synthesis of cyclic carbonates via cycloaddition of carbon dioxide to epoxide.

The synthesis of cyclic carbonates can be realized via cycloaddition of CO2 to epoxides. Organocatalysts are of interest regarding specific applications. The reaction can be co-activated by a hydrogen bond donor such as cellulose. A fully recyclable catalytic system combining an organocatalyst and a polysaccharide hydrogen bond donor has never been reported. In this study, we have supported an organocatalyst onto cellulose nanocrystals (CNC). The grafting of 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) onto microcrystalline cellulose (MCC) and onto the surface of cellulose nanocrystals (CNC) was confirmed by 13C solid state NMR, FT-IR, XRD and elemental analysis. The prepared heterogeneous TBD-CNC nanocatalyst was successfully assessed on a scope of five epoxides including mono- and di-substituted. The activity was found to be higher than that of the TBD-MCC supported catalyst, and higher than the homogeneous counterpart. That latter result highlights the coactivation role of the polysaccharide. Recyclability studies showed that the catalyst is reusable for at least five cycles. Density functional theory (DFT) studies indicate that the mechanism most probably involves the formation of anionic cellulosic species. Initially, the grafted TBD deprotonates a surface hydroxy to form a cellulose alkoxide, which then performs a nucleophilic attack on CO2 or on the activated epoxide.