CO2 conversion to cyclic carbonates catalyzed by ionic liquids with aprotic heterocyclic anions: DFT calculations and operando FTIR analysis

Quantum chemical calculations and in situ infrared spectroscopy were applied to analyze the role of CO2 activation by aprotic heterocyclic anion ionic liquids (AHA-ILs) in its reaction with propylene oxide to form propylene carbonate. Two AHA-ILs with remarkably different behavior as CO2 chemical absorbent were considered: triethyl(octyl)-phosphonium indazole, [P2228][Inda], and triethyl(octyl)-phosphonium 2-cyanopyrrol-1-ide [P2228][2-CNPyr]. The structure and energy of reaction intermediates were predicted by Density Functional Theory (DFT) method, observing that CO2 and AHA-IL reaction form an anionic carbamate that promotes a nucleophilic attack on the propylene oxide, causing the ring opening with negligible energy barrier. Later intramolecular cyclization occurs, followed by the AHA-IL regeneration and propylene carbonate production, both requiring appreciable activation energy. The proposed reaction mechanism were experimentally validated by ATR-FTIR measurements, identifying the characteristic signals of reactants, products and intermediate species. Finally, the reaction using the AHA-ILs [P2228][Inda] and [P2228][2-CNPyr] was followed over time using operando ATR-FTIR technique at different operating conditions (temperature and catalyst concentration). A close relationship between the performance of AHA-IL as CO2 chemical absorbent and CO2 conversion catalyst is revealed, opening opportunities for the efficient application of AHA-ILs in intensified process of CO2 capture and utilization.