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

Targeting CO 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 CO. 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 CO adsorption. In addition, enough free space was still retained after functionalization, which facilitated CO 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 CO capture performance of the composite. The adsorption capacity and selectivity of DETA-Ac@MIL-101(Cr) for CO were significantly improved. The higher isosteric heats of adsorption (Q) evidenced the stronger interaction between the composite and CO molecules. Moreover, a possible two-step mechanism was proposed to reveal the manner in which CO bound to the IL-incorporated frameworks. Despite the relatively high initial Q value, the DETA-Ac@MIL-101(Cr) could be easily regenerated with almost no drop in CO uptake during six cycles.