Toward the Understanding of Chemical Absorption Processes for Post-Combustion Capture of Carbon Dioxide: Electronic and Steric Considerations from the Kinetics of Reactions of CO_2(aq) with Sterically hindered Amines

摘要

The present study reports (a) the determination of both the kinetic rate constants and equilibrium constants for the reaction of CO_2(aq) with sterically hindered amines and (b) an attempt to elucidate a fundamental chemical understanding of the relationship between the amine structure and chemical properties of the amine that are relevant for postcombustion capture of CO_2 (PCC) applications. The reactions of CO_2(aq) with a series of linear and methyl substituted primary amines and alkanolamines have been investigated using stopped-flow spectrophotometry and ~1H NMR measurements at 25.0 ℃. The specific mechanism of absorption for each of the amines, that is CO_2 hydration and/or carbamate formation, is examined and, based on the mechanism, the kinetic and equilibrium constants for the formation of carbamic acid/carbamates, including protonation constants of the carbamate, are reported for amines that follow this pathway. A Bronsted correlation relating the kinetic rate constants and equilibrium constants for the formation of carbamic acid/carbamates with the protonation constant of the amine is reported. Such a relationship facilitates an understanding of the effects of steric and electronic properties of the amine toward its reactivity with CO_2. Further, such relationships can be used to guide the design of new amines with improved properties relevant to PCC applications.