Surface engineering of a chromium metal-organic framework with bifunctional ionic liquids for selective CO₂ adsorption: Synergistic effect between multiple active sites

Targeting CO₂ capture application, a new strategy for building multiple adsorption sites in metal-organic framework MIL-101(Cr) was constructed through the incorporation of diethylenetriamine-based ionic liquid (DETA-Ac) via a post-synthetic modification approach. The DETA-Ac, with multi-amine-tethered cation and acetate anion, could not only provide additional binding sites, but also enhance the affinity of framework surfaces toward CO₂. Simultaneously, the high surface area and large cage size of MIL-101(Cr) ensured the better dispersion of IL, thus exposing more active sites for CO₂ adsorption. In addition, enough free space was still retained after functionalization, which facilitated CO₂ transport and allowed the Cr(III) sites deep within the pores to be accessed. The multiple adsorption sites originating from IL and MOF were found to synergistically affect the CO₂ capture performance of the composite. The adsorption capacity and selectivity of DETA-Ac@MIL-101(Cr) for CO₂ were significantly improved. The higher isosteric heats of adsorption (Q_st) evidenced the stronger interaction between the composite and CO₂ molecules. Moreover, a possible two-step mechanism was proposed to reveal the manner in which CO₂ bound to the IL-incorporated frameworks. Despite the relatively high initial Q_st value, the DETA-Ac@MIL-101(Cr) could be easily regenerated with almost no drop in CO₂ uptake during six cycles.