(371c) Tailoring Molecular Structures of N-Heterocyclic Imine Toward Energy-Efficient Electrochemical CO2 Separation

Conventional amine sorbents for thermal CO₂ capture face long-standing challenges. Strong amine-CO₂ binding (reaction enthalpy of 70–90 kJ/mol) demands significant thermal energy for regeneration. Linear amines like monoethanolamine (MEA), diethanolamine (DEA), and diisopropanolamine (DIPA) are prone to thermal degradation, forming irreversible cyclic byproducts (e.g., urea, 2-oxazolidone), which reduce capture efficiency and add operational costs and environmental concerns. Moreover, the thermochemical process often requires burning additional fossil fuels, lowering net CO₂ removal efficiency.

This work explores N-heterocyclic imine (NHI) as a novel CO₂ capture sorbent to address these issues. NHI’s structure provides a high dimensionality to tailor and fine-tune its reaction enthalpy with CO₂. The cyclic structure of NHI is expected to be more stable than the linear amine structure as the pre-existing ring precludes the chance of cyclization reactions. After CO₂ capture, the lone pair on the CO₂-binding nitrogen of NHI could be toggled electrochemically to control CO₂ release or capture on NHI, enabling integration with renewable energy sources to reduce the process's carbon footprint. Despite its promise, prior research on NHI primarily focused on crystallographic studies, critical engineering parameters like reaction enthalpy ∆H_NHI-CO2, CO₂ loading capacity, and reaction kinetics remain unexplored. This study aims to bridge these gaps, advancing NHI as a sustainable CO₂ capture solution.

In this study, a series of NHIs with various backbone, sidechain, and exocyclic functional groups were firstly synthesized and calorimetry tests were conducted to measure their ∆H_NHI-CO2. The results showed that the additional double bond π electrons on NHI backbone are necessary for NHI to capture CO₂. Further adding electron-withdrawing or donating group on the same unsaturated backbone was shown to be an effective approach to fine-tune the ∆H_NHI-CO2. The steric hindrance effect on NHI-CO₂ was also studied by installing different alkyl functional groups on the sidechain and exocyclic positions. It was observed that an increasing steric hindrance would decrease the ∆H_NHI-CO2, resulting in more stable NHI-CO₂ adducts. The achievable CO₂ loading of NHI were measured with volumetric method using 15% CO₂ and compared with MEA and DEA as the standard. From the measurements, NHI demonstrated stronger CO₂ scrubbing capability, faster reaction kinetics, and higher CO₂ loading compared to the amine counterparts. To explore NHI’s potential in electrochemical CO₂ separation, cyclic voltammetry of various NHIs were conducted under both Ar and CO₂ conditions, and the results demonstrated that the exocyclic nitrogen could be selectively oxidized and such oxidation is shown to enable CO₂ release from NHI. The reversibility of the NHI redox was also discussed. Overall, this work serves as an initial exploration of NHI’s engineering properties as both a thermochemical and electrochemical redox sorbent, providing a foundation for future application-specific research.