Boosting reactions with microbubbles

Reacting carbon dioxide with amines is a common strategy for decarbonization of air. Mechanisms that accelerate these reactions are therefore of value. For instance, an increase in the conversion ratio (R_C) of amines has been demonstrated in microdroplets. Using electrospray ionization (ESI) to conduct and analyse microdroplet reactions, Xiaoyun Gong, Xiang Fang, and Yuanjiang Pan from the National Institute of Metrology and Zhejiang University, with colleagues from Jilin University, studied the effect of added ammonium bicarbonate (NH₄HCO₃) and found that it accelerates the reaction by acting as an internal CO₂ source in the microdroplets, boosting its reactivity with amines.

The team studied the formation of carbamic acids from CO₂ and several different diamines (1°/3°, 1°/2° and 1°/1°). They found that the R_C decreased moving from 1°/3° diamines to 1°/1° diamines. The amine structure thus plays a key role in reactivity. It is assumed that the primary amino group reacts with the CO₂ while the second acts as a general base.

Credit: Adapted with permission from Feng, L. et al. Anal. Chem. 93, 15775–15784 (2021), American Chemical Society.

Increasing the concentration of NH₄HCO₃ in the microdroplets increased the efficiency of amine conversion to carbamic acid: The R_C for N,N-dibutyl-1,3-propanediamine (DBPA) was 31.7% at 0.5 mM bicarbonate, but was found to increase and eventually plateau around 93.7% at 20 mM bicarbonate. The solvent was also shown to play a significant role in the reaction efficiency. The R_C increased from 26.1% to 93.7% when the water content (v/v in methanol) was increased from 5 to 100%, respectively. It is suggested that the level of organic solvent likely limits the level of available CO₂ in the microdroplets through two mechanisms. First, ammonium bicarbonate is less soluble in methanol so the concentration of available CO₂ is lower. A secondary effect is that this also leads to increased pH thus slowing the conversion of HCO₃^– to CO₂, which occurs in an acidic environment.

Although other bicarbonate sources such as NaHCO₃ and KHCO₃ were also found to improve the conversion of DBPA, the observed R_C values were one third of that when NH₄HCO₃ was used. In the hot gas phase produced by ESI, NH₄HCO₃ is converted into NH₃ and CO₂ through a double hydrolysis step, with almost no effect on pH. In contrast, the decomposition of both NaHCO₃ and KHCO₃ result in an increased pH, which hampers further breakdown of HCO₃⁻.

The spray voltage has only a minimal effect on the reaction but the temperature was important. The R_C of DBPA was optimal when the entrance capillary was heated to 150 °C. Lower R_C values were recorded at temperatures above 150 °C. This can likely be explained by the thermal degradation of carbamic acids at high temperatures. Lower flow rates also afforded higher R_C values by producing smaller droplets, which offer greater surface area for reactions to occur.

Finally, by using carbon-13-labelled ammonium bicarbonate, the authors were able to confirm that NH₄HCO₃ was indeed the source for CO₂, in the carbamic acids, thus supporting their proposed mechanism in which NH₄HCO₃ decomposes in the microdroplets. The presence of H⁺ accelerates the formation of CO₂ which forms microbubbles within the microdroplets, and this further increases the area of liquid–gas interface at which the reaction occurs. Overall, NH₄HCO₃ can decompose to form CO₂ via ESI and by fine-tuning various factors can be used as an additive to boost the conversion of amines with CO₂.