Insights into the Chemical Mechanism for CO_2(aq) and H~+ in Aqueous Diamine Solutions - An Experimental Stopped-Flow Kinetic and ~1H/~(13)C NMR Study of Aqueous Solutions of N/N-Dimethylethylenediamine for Postcombustion CO_2 Capture

摘要

src="http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2018/esthag.2018.52.issue-2/acs.est.7b05226/20180111/images/medium/es-2017-05226v_0006.gif">In an effort to advance the understanding of multiamine based CO₂ capture process absorbents, we report here the determination of the kinetic and equilibrium constants for a simple linear diamine N,N-dimethylethylenediamine (DMEDA) via stopped-flow spectrophotometric kinetic measurements and 1H/13C NMR titrations at 25.0 °C. From the kinetic data, the formation of monocarbamic acid (DMEDACOOH) from the reaction of DMEDA with CO₂(aq) is the dominant reaction at high pH > 9.0 (k₇ = 6.99 × 103 M–1·s–1). Below this pH, the formation of protonated monocarbamic acid (DMEDACOOH₂) via the pathway involving DMEDAH+ and CO₂(aq) becomes active and contributes to the kinetics despite the 107-fold decrease in the rate constant between the two pathways. 1H and 13C NMR spectra as a function of decreasing pH (increasing HCl concentration) at 25.0 °C have been evaluated here to confirm the protonation events in DMEDA. Calculations of the respective DMEDA nitrogen partial charges have also been undertaken to support the NMR protonation study. A comparison of the DMEDA kinetic constants with the corresponding data for piperazine (PZ) reveals that despite the larger basicity of DMEDA, the enhanced and superior kinetic performance of PZ with CO₂(aq) above its predicted Bronsted reactivity is not observed in DMEDA.