Amine–MeOH Binary Systems as Catalysts for Cyclic Carbonate Formation from Epoxides and Carbon Dioxide: A DFT Mechanistic Investigation

The detailed mechanism of propylene carbonate (PC) formation from propylene oxide (PO) and CO₂ is investigated using density functional theory (DFT) methods, catalyzed by amine/MeOH binary systems, in propylene oxide under conditions of room temperature and 1 atm. In these systems, amines (MeNH₂, Me₂NH, Me₃N, and pyrrolidine) serve as nucleophiles, while MeOH acts as a hydrogen bond donor (HBD). The catalyzed reaction pathways for PC formation consistently proceed through two transition states, ts1 and ts2, corresponding to the oxide ring-opening and final ring-closing steps, respectively. The ring-closing step was identified as the rate-determining step in all amine/MeOH binary systems. Notably, the three aliphatic amine/MeOH binary systems significantly lower activation barriers for PC formation by approximately 20 kcal mol^-1 compared to the uncatalyzed ring-closing pathway under standard conditions. The Me₂NH/MeOH binary system demonstrates slightly higher catalytic efficiency than the MeNH₂ and Me₃N systems. Furthermore, the pyrrolidine/MeOH binary system exhibits comparable catalytic performance to the Me₂NH/MeOH system. Since pyrrolidine is liquid under standard conditions, it can act as a homogeneous catalyst when paired with MeOH, enhancing mixing with PO and improving catalytic activity relative to gaseous Me₂NH.