Signs of alkylcarbonate formation in water-lean solvents: VLE-based understanding of pKa and pKs effects

Abstract

Water-lean solvents are solvents for CO2 capture which contain at least one amine and one organic diluent. Carbamate-forming amines in nonaqueous water-lean solvents typically absorb CO2 through the formation of carbamate species. Conversely, tertiary amines in nonaqueous amino-organic mixtures cannot form carbamate nor undergo bicarbonate formation. However, protic organic diluents might take part in an alkylcarbonate formation pathway: in this route, the diluent is deprotonated by the amine, and its conjugate base is then able to react directly with CO2 to form an alkylcarbonate. This is an interesting reaction mechanism, as it offers a possibility for formulating solvents that deviate from the two most common reaction pathways for CO2 absorption (i.e., carbamate and bicarbonate formation). The present study introduces a simple way for evaluating alkylcarbonate-forming water-lean solvents based on the properties of its single constituents, namely the basicity of the amine and the autoprotolysis constant of the organic diluent. Our theoretical framework shows that higher CO2 absorption capacities can be obtained for solvents containing amines with high pKa and diluents with low pKs. We assess both the challenges and possibilities for alkylcarbonate-forming solvents and propose a framework for their development and utilization in the industry.