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

Abstract

An environmentally clean, binary catalytic system of naturally occurring α-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.