Solvent-free organocatalytic preparation of cyclic organic carbonates under scalable continuous flow conditions†
A solvent-free organocatalyzed process for the transesterification of dimethyl carbonate (DMC) with 1,2-diols under scalable continuous flow conditions is presented. Process parameters, such as temperature, residence time, DMC/glycerol molar ratio and catalyst loading are optimized for the carbonation of bio-based glycerol using 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) as a model organocatalyst. The catalytic performance of DBU is next compared with other homogeneous organic superbases including the proton sponge, Verkade's base, guanidines and phosphazenes. 2-tert-Butyl-1,1,3,3-tetramethylguanidine (Barton's base) stands as the most efficient organocatalyst, providing glycerol carbonate at 87% selectivity and 94% conversion within 2 minutes of residence time at 1 mol% loading. Representative examples of polystyrene-supported (PS) organic superbases of the amidine, guanidine and phosphazene-types are also considered as alternative heterogeneous catalysts. PS superbases typically enable up to 80 h of continuous operation with minor deactivation at elevated flow rates. The methodology is amenable to a library of other 1,2-diols, including biomass-derived substrates. Depending on the unique structural features of both substrates and products, either on-line IR or on-line NMR analytical procedures are implemented for real-time qualitative reaction monitoring. A final demonstrator showcases the transposition of the glycerol carbonation to a pilot-scale continuous flow reactor, affording the target cyclic carbonate with a 68.3 mol per day productivity (∼8 kg per day).