The green and efficient formation of the commercial cyclic carbonates from CO2 and epoxides promoted by metal-free quaternary phosphonium-boron catalysts and DMAP

Abstract

From the perspective of green chemistry and achieving net zero emissions, converting CO₂ into cyclic carbonates is one of the important strategies for CO₂ fixation in applied chemistry and industrial sectors. In this context, the novel metal-free phosphonium-boron catalysts (PBC₁–PBC₈) were synthesized and various spectroscopic tools such as ¹H, ¹³C, and ¹¹B NMR, FT-IR, UV–Vis, LC-MS/MS, elemental analysis, TGA-DTA together melting point measurement were utilized for structural characterization. The Lewis acidity of the metal-free boron-based catalysts (PBC₁–PBC₈) was determined using the Gutmann–Beckett method and resonances were obtained in the range of 51.11–50.18 ppm, respectively. Followed by, novel metal-free phosphonium-boron compounds were used for the coupling reaction of CO₂ with different epoxides to obtain target cyclic carbonates under atmospheric and high-pressure conditions (1 atm or 1.6 MPa, 100 °C, 2 h) for carbon neutrality and an alternative to the toxic and expensive phosgene gas. The binary catalytic system of metal-free phosphonium-boron catalyst (PBC₈)/DMAP presented high catalytic activity in solvent-free conditions at 100 °C, 1 atm (53.1 % yield and 97.4 % selectivity) and 1.6 MPa (95.4 % yield with 98.0 % selectivity) and 2 h with using epichlorohydrin (ECH). Furthermore, the effects of the amount of used catalyst on the catalytic conversion were examined and it was determined that the optimum (PBC₈)/ECH ratio for CO₂ coupling reactions was 1/1000.