Exothermic barrier-free carboxylation of alkali methylates. The highest-yield synthesis of dialkyl carbonates out of alcohols and carbon dioxide rationalized and reformulated through quantum-chemical simulations

Abstract

While the Earth’s atmosphere represents an abundant source of CO2, its valorization by humanity remains at a nascent stage. Synthesizing added-value products out of CO2 and suitable co-reactants is an urgent and appealing chemical challenge to pursue in our busy times. Herein, we perform a rationalization, extension, and ultimate reformulation of the recently proposed highest-yield synthesis of dimethyl carbonate (DMC) and generally dialkyl carbonates out of alcohols and CO2 employing quantum-chemical reaction modeling. The carboxylation of pure methanol (CH3OH) by CO2 is thermochemically prohibited (+33 kJ/mol) and hindered by a heavy activation energy barrier (160 kJ/mol). The existing catalysts decrease the above barrier twice. The catalyzed carboxylation exhibits yields of 5–15 % of DMC depending on a specific catalyst, dehydration agents, and pressure elevation. In turn, the usage of alkali metal methylates – such as CH3OLi, CH3ONa, CH3OK, and CH3ORb – as reactants, instead of CH3OH, fully eliminates the activation barrier. Furthermore, alcoholates make carboxylation exothermic. We stepwise rationalize the origin of such a desirable behavior in terms of thermochemistry, molecular geometries, atomic nucleophilicities, potential energy landscapes, and computed vibrational spectra. All methylates perform decently and exhibit reasonable similarities as reactants. The most favorable reaction was recorded in the case of sodium alcoholate (according to Gibbs free energy) and lithium alcoholate (according to enthalpy). Since the carboxylation smoothly proceeds with the methylates, the synthesis can be carried out under normal conditions. This is contrary to the elevated temperatures and pressures, which have to be routinely supplied to implement the presently known catalytic schemes. Apart from an organic carbonate, the novel reactions produce LiI, NaI, KI, and RbI as major products. The consumed alkali metals need to be restored, for instance, by utilizing electrolysis, for their repetitive usages. The theoretical results are discussed tightly in the context of the recent experiments. The reactions can take place in liquid CH3OH. Exemplified computationally for the methyl radical, the invented scheme must generally work for larger hydrocarbon moieties. This report will interest synthetic chemists, whose work is devoted to the valorization of CO2.