Nano-confined NHC-Al interfaces for efficient CO2 chemical fixation

Abstract

Nano-confined interfacial NHC-Al sites in Al2O3-supported ionic liquid phases (SILPs) act as engineered catalysts for selective CO2 fixation into epoxides, with NHC-Al adduct formation confirmed by solid-state nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). The synergistic combination of NHC-Al adducts and IL moieties generates membrane-like nano-confined environments at the solid–liquid interface. These confined environments regulate substrate access and product diffusion during CO2 absorption and fixation, which exhibit a high affinity for CO2 capture under mild reaction conditions. Size-selective transport within the nano-confined SILP architecture favors small-sized epoxides over bulky ones, leading to enhanced reaction rates and selectivity. The NHC@SILP-PMImAl2O3 catalyst, containing a lower fraction of NHC-Al adduct (15%) and a high proportion of imidazolium-chloride ion pairs, exhibited the highest activity, achieving a turnover frequency of 16.09 h−1 for epichlorohydrin at 1-bar CO2 and 70°C. In contrast, the NHC-Al SILPs with a high NHC-Al adduct (41%) showed a significantly reduced performance (59.1 TONs). This comparison highlights that catalytic efficiency is governed by a balance between NHC-Al formation and the availability of nucleophilic chloride species within the nano-confined environment. Chloride-assisted CO2 cycloaddition in SILPs proceeds via Al2O3 surface hydroxyl-mediated epoxide activation rather than imidazolium C2-H pathways typical of neat ionic liquids, as supported by density functional theory.