Halogenated melamine formaldehyde polymers: Efficient, robust and cost-effective bifunctional catalysts for continuous production of cyclic carbonates via. CO2-epoxide cycloaddition

Abstract

Halogenated porous melamine polymers were demonstrated to be efficient catalysts for CO2-epoxide cycloaddition, selectively (>99%) producing C3-C12 cyclic carbonates in excellent yields (upto 99%) under solvent and co-catalyst free conditions. The halogenated polymers outperformed benchmark catalysts incorporating only basic (N-doped carbon, ZIF-8, N-rich melamine polymer) or nucleophilic (TBAB, KI) sites. The superior catalytic performance of these inexpensive polymers was attributed to their unique surface chemistry incorporating abundant, stable basic N sites (amine N and protonated N) and nucleophilic (Cl-, Br- or I-) that enabled simultaneous activation of both epoxide and CO2 molecule (supported by kinetic and DFT studies). Further, among halogenated polymers a Br- containing material (PMFBr) presented highest activity owing to its balanced CO2-philicity and strong nucleophilicity. Most importantly, PMFBr was robust, reusable and maintained stable performance for continuous production of C3-C4 cyclic carbonate (120 °C, 0.3–0.83 h-1 WHSVepoxide and 15 bar) in a fixed-bed reactor during 60–190 h TOS.