Ammonium Bicarbonate Significantly Accelerates the Microdroplet Reactions of Amines with Carbon Dioxide

The reactions between amines and carbon dioxide (CO2) are among the most commonly used and important carbon fixation reactions at present. Microdroplets generated by electro-spray ionization (ESI) have been proved to increase the conversion ratio (R-C) of amines. In this work, we confirmed that the presence of ammonium bicarbonate (NH4HCO3) in ESI microdroplets significantly increased the R-C of amines. The R-C went up remarkably with the increase in the concentration of NH4HCO3 from 0.5 to 20 mM. The R-C of N,N-dibutyl-1,3-propanediamine (DBPA) reached 93.7% under 20 mM NH4HCO3, which was significantly higher than previous reports. The rise in R-C became insignificant when the concentration of NH4HCO3 was increased beyond 20 mM. Further investigations were made on the mechanism of the phenomenon. According to the results, it was suggested that NH4HCO3 decomposed into CO2 and formed microbubbles within the microdroplets of ESI. The microbubbles acted as direct internal CO2 sources. The conversion reactions occurred at the liquid-gas interface. The formation of CO2 microbubbles remarkably increased the total area of the interface, thus promoting the conversion reactions. C-13-labeled experiments confirmed that NH4HCO3 acted as an internal CO2 source. Factors that influenced the R-C of the reaction were optimized. Pure water was proved to be the optimal solvent. Lower temperature of the mass spectrometer's entrance capillary was beneficial to the stabilization of the product carbamic acids. The sample flow rate of ESI was crucial to the R-C. It determined the initial sizes of the microdroplet. Lower flow rates ensured higher R-C of amines. The present work implied that NH4HCO3 could be a superior medium for CO2 capture and utilization. It might offer an alternative choice for future CO2 conversion research studies. In addition, our study also provided evidence that NH4HCO3 decomposed and generated microbubbles in the droplets during ESI. Attention should be paid to this when using NH4HCO3 as an additive in mass spectrometry-based analysis.