Natural amino acids/H2O as a metal- and halide-free catalyst system for the synthesis of propylene carbonate from propylene oxide and CO2 under moderate conditions

An environmentally clean, binary catalytic system of naturally occurring alpha-amino acids (AAs)/H2O has been found to deliver promising yields of propylene carbonate (PC) from propylene oxide (PO) and CO2 under moderate reaction conditions. Among the various AA systems tested, the highest yield of PC from PO-CO2 coupling was achieved with the L-His/H2O system, associated with the basic nature of L-histidine. The effect of various reaction parameters such as the catalyst loading, temperature, CO2 pressure, and time were evaluated. A plausible reaction mechanism involving synergistic interplay between the -OH groups of H2O and the COO- end of the zwitterionic L-His and the amine moiety of the imidazole ring of L-His was proposed. The process presented herein represents a low cost, ecologically safe, and efficient route with simple work-up employing readily available materials for chemical fixation of CO2 into high-value chemicals.