Sodium fluoride enables room-temperature synthesis of dimethyl carbonate

Abstract

Carbon dioxide (CO2) in the atmosphere of the Earth represents an abundant and largely free source of carbon, which remains heavily underestimated nowadays. The disbalance between the emission and utilization of CO2 puts the sustainable development of humanity at risk because of its possible correlation with global climate change. The valorization of CO2 is an important strategy to attend to the above-mentioned challenges. CO2 can be converted to dimethyl carbonate by grafting to methanol (CH3OH) in the presence of the catalyst. The presently known catalytic solutions require high temperatures and pressures to be employed. Herein, we report sodium fluoride as a new catalyst offering an unprecedentedly low activation barrier, 23 kJ/mol, for the carboxylation of CH3OH. The fluoride anion coordinates the hydroxyl hydrogen of CH3OH thanks to forming a very strong H-bond. Upon the collision with CO2, the proton detaches, and CO2 grafts to CH3O* while becoming the carboxyl group. The reaction finishes by methylating CH3OC(O)O– via the methyl radical originating from another CH3OH molecule (esterification stage). The hydrogen fluoride molecule is an intermediate in this reaction providing the proton and fostering esterification. The subsequent in-lab experiments showed that much milder conditions can be used to synthesize DMC out of CH3OH. Under 80 °C and 40 bar, we obtained an outstanding DMC yield of 21.1 %. However, the synthesis was also possible at lower temperatures giving DMC yields of 16.3 % at 65 °C, 14.7 % at 55 °C, and 10.8 % at 25 °C. The unprecedented possibility to obtain a non-negligible production of an organic carbonate at room temperature represents a cornerstone advance in the field of CO2 valorization and promises huge energy savings. Given the current global amount of the yearly prepared DMC amounting to over one million tons and the demand for this chemical by emerging chemical technologies, the achieved result is deemed to be of paramount importance. The exemplified catalytic mechanism is also interesting in the context of other syntheses, in which a low-energetic production of the methoxy moieties is a cornerstone.