CO2 fixation reaction over pyrimidinium-based dicationic ionic liquid in MIL-101(Cr)

Abstract

Modified metal-organic frameworks (MOFs) encapsulating new ionic N-heterocyclic-based functional groups, prepared via a ship-in-a-bottle approach, introduce a novel catalytic system for CO₂ conversion with epoxides. The MIL-101(Cr) framework was functionalized with novel ionic liquids (ILs) such as 1,5-bis(2-aminopyrimidinium)pentane bromide (2AMP-DBrP) and 1,5-bis(2-aminopyrimidinium)ethyl ether bromide (2AMP-DBrEE), resulting in cationic pyrimidinium-containing Cr-MOFs with free bromide ions, namely MIL101–2AMP-DBrP and MIL101–2AMP-DBrEE. Although the surface area (2456 and 2408 m²/g) and pore volume (1.44 and 1.41 cm³/g) of these encapsulated materials were reduced compared to pristine MIL-101(Cr) (3425 m²/g and 1.78 cm³/g), the CO₂ adsorption capacity at low pressure was notably increased (3.51 and 4.05 mmol g⁻¹ of CO₂). These modified MOFs demonstrated exceptional catalytic performance and stability in CO₂ fixation with various epoxides, achieving a high turnover frequency (TOF) of 82.5 h⁻¹ under mild conditions (0.1 MPa, 90 °C) without the need for co-catalysts or solvents, which are essential criteria for commercial viability. Moreover, in situ FTIR analysis suggests that the enhanced performance is due to CO₂ activation via carbamic acid formation facilitated by primary amino groups and epoxide activation through hydrogen bonding. This dual activation mechanism is critical for achieving high catalytic efficiency under mild conditions and underscores the innovative and significant advancements presented in this work compared to existing IL-MOFs.