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.