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

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 degrees 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 degrees 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.