A Six-Membered Concerted Mechanism for CO2 Capture by Amines Studied under Charged Microdroplet Reaction Conditions.

Carbon dioxide (CO2) capture and storage represents an important technological challenge. A mechanistic understanding of interactions involved in the capture process is necessary not only for technological development but also for efficient conversion of captured CO2 into value-added materials. Herein, we present a novel contained secondary electrospray ionization platform for studying the interactions of gaseous amines and CO2 gas under microdroplet reaction conditions, which enables mass spectrometry (MS) characterization of CO2 capture products and intermediates in real time. We detected [2 M + CO2 + H]+ species, which corresponds to a six-membered intermediate. DFT calculations confirmed the high stability of the protonated six-membered ring intermediate. This finding provides a plausible concerted mechanism in the microdroplet environment that excludes the involvement of thermodynamically disfavored ionic species. The carbamic acid counterpart of the final product/salt was readily characterized by tandem MS. The carbamic acid/amine salt was also isolated and characterized by Fourier transform infrared spectroscopy. By virtue of the fact that headspace vapors of amines are sampled, we were able to establish a high-throughput platform that enabled the CO2 capture capacity of five different amines to be studied in under 2 min. The same device also enabled the absolute quantification of capture capacity.