Acid Gas Capture by Nitrogen Heterocycle Ring Expansion.

Acid gases including CO, OCS, CS, and SO are emitted by industrial processes such as natural gas production or power plants, leading to the formation of acid rain and contributing to global warming as greenhouse gases. An important technological challenge is to capture acid gases and transform them into useful products. The capture of CO, CS, SO, and OCS by ring expansion of saturated and unsaturated substituted nitrogen-strained ring heterocycles was computationally investigated at the G3(MP2) level. The effects of fluorine, methyl, and phenyl substituents on N and/or C were explored. The reactions for the capture CO, CS, SO, and OCS by 3- and 4-membered N-heterocycles are exothermic, whereas ring expansion reactions with 5-membered rings are thermodynamically unfavorable. Incorporation of an OCS into the ring leads to the amide product being thermodynamically favored over the thioamide. CS and OCS capture reactions are more exothermic and exergonic than the corresponding CO and SO capture reactions due to bond dissociation enthalpy differences. Selected reaction energy barriers were calculated and correlated with the reaction thermodynamics for a given acid gas. The barriers are highest for CO and OCS and lowest for CS and SO. The ability of a ring to participate in acid gas capture via ring expansion is correlated to ring strain energy but is not wholly dependent upon it. The expanded N-heterocycles produced by acid gas capture should be polymerizable, allowing for upcycling of these materials.