Acid Gas Capture by Nitrogen Heterocycle Ring Expansion

Area of Science:

• Computational Chemistry
• Green Chemistry
• Materials Science

Background:

• Industrial processes emit acid gases (CO2, OCS, CS2, SO2), contributing to global warming and acid rain.
• Capturing and transforming these acid gases into valuable products presents a significant technological challenge.

Purpose of the Study:

• To computationally investigate the capture of CO2, CS2, SO2, and OCS using ring expansion of nitrogen-strained heterocycles.
• To explore the influence of substituents (fluorine, methyl, phenyl) on N- and/or C-atoms within the heterocycles.
• To assess the thermodynamic favorability and energy barriers of these acid gas capture reactions.

Main Methods:

• Density Functional Theory (DFT) calculations at the G3(MP2) level.
• Investigation of ring expansion reactions involving 3-, 4-, and 5-membered nitrogen-strained heterocycles.
• Analysis of substituent effects and correlation of reaction barriers with thermodynamics.

Main Results:

• Ring expansion reactions for capturing CO2, CS2, SO2, and OCS are exothermic for 3- and 4-membered rings but unfavorable for 5-membered rings.
• CS2 and OCS capture reactions are more exothermic than CO2 and SO2 capture due to bond dissociation enthalpy differences.
• Reaction energy barriers are highest for CO2 and OCS capture and lowest for CS2 and SO2 capture.

Conclusions:

• The ability of heterocycles to capture acid gases via ring expansion is linked to ring strain energy but not solely dependent on it.
• The resulting expanded N-heterocycles show potential for polymerization, enabling the upcycling of captured acid gases into useful materials.