Bottom-up fabrication of DABCO-based bicationic ionic liquid-grafted MIL-101(Cr): Facilitating simultaneous activation of CO2 and epoxides for cyclic carbonate synthesis

Abstract

Efficient activation of substrates as an orthogonal pathway for facilitating CO2 fixation into cyclic carbonates, poses ongoing challenges for the ingenious design and development of heterogeneous cooperative catalysts. Herein, a bottom-up strategy was employed to stepwise graft chloromethyl, triethylenediamine (DABCO), and chloroethylamine onto the organic linkers, forming bicationic ionic liquid tethered to MIL-101(Cr). The introduced –NH2 and quaternary ammonium groups dangling within the micro-mesopores ensured the fast enrichment and polarization of CO2 molecules, while the inherent unsaturated Cr3+ sites bonded with the epoxides to promote the simultaneous activation. The ample Cl- counter-anions further nucleophilically attacked to proceed the ring opening process, triggering an upgradation in catalytic performance for cycloaddition. After optimized by the response surface methodology (RSM), a chloropropene carbonate (CPC) yield of 97.8 % with a selectivity of 99.3 % were attained over the developed Cl[TNH2]Cl@MIL-101(Cr) at mild conditions (104.5 °C, 1.07 MPa, 3.75 wt% of catalyst, 1.91 h) in the absence of any solvent or co-catalyst. Furthermore, Cl[TNH2]Cl@MIL-101(Cr) displayed good durability, universality, and recyclability. The synergy of multiple sites was testified by the in-situ DRIFTS spectra and DFT calculations, and thus the potential catalytic mechanism was deduced. The implement of this work could shed some lights on the rational assembling of robust IL@MOFs nanocomposites and offered novel avenues for the fabrication of high-efficiency catalysts for practical CO2 conversion.