Effect of non-aqueous solvents on kinetics of carbon dioxide absorption by tBu3P/B(C6F5)3 frustrated Lewis pairs

Abstract

Frustrated Lewis pairs (FLPs), combinations of sterically hindered Lewis acids and bases, are known for their ability to capture CO2. Although there have been several theoretical studies on the mechanisms of the reactions between CO2 and some FLP systems, experimental studies on the reaction kinetics have been inconclusive. In this study, the mechanism and kinetics of CO2 absorption by an FLP system consisting of tri-tert-butylphosphine (tBu3P) and tris(pentafluorophenyl)borane (B(C6F5)3) in bromobenzene, cyclopentyl methyl ether (CPME), and tert-butyl methyl ether (MTBE) were investigated using the stopped-flow method. The pseudo-first-order reaction rate constants, ko (s−1), were measured for a concentration range of 0.02–0.035 M and over a temperature range of 298–323 K. The experimental data were fitted according to modified termolecular mechanisms with average absolute relative deviations of 4.34%, 4.63%, and 3.51% for the CO2–FLP:bromobenzene, CO2–FLP:CPME, and CO2–FLP:MTBE systems, respectively. The forward reaction rate constants, k (m3 kmol−1 s−1), were calculated based on the proposed reaction mechanism. The forward reaction rate constants were higher than those for various aqueous tertiary amine systems but lower than those for aqueous monoethanolamine and piperazine systems. Moreover, the activation energies were estimated from Arrhenius plots. They were calculated to be 22.0, 19.7, and 21.8 kJ mol−1 for the CO2–FLP:bromobenzene, CO2–FLP:CPME, and CO2–FLP:MTBE systems, respectively. This study promotes the development of novel efficient solvent formulations for CO2 capture.