Indium-Organic Frameworks Based on Dual Secondary Building Units Featuring Halogen-Decorated Channels for Highly Effective CO2 Fixation

摘要

With the utilization of a "bifunctional ligand-directed strategy", three isostructural indium-organic frameworks based on dual secondary building units (SBUs) were successfully constructed with targeted structures. In their frameworks, two types of unsaturated monomeric indium SBUs-[In(OOC-)(2)(-N-)X(H2O)] and [In(OOC-)(2)(-N-)X-2](-) (X = Cl, Br, and I)-assemble to form 1D tubular channels with both open metal sites and weak base polarizing substituents. The trimeric indium SBUs [In3O(OOC-)(6)(DMA)(3)](+) serve as robust external linkers to extend into a 3D honeycomb double-walled framework with nanoscale channels. By changing the polarizing substituents in situ with different halogens (Cl-, Br-, and I-) three obtained isostructural MOFs show different channel characteristics, such as alkalinity of the polarizing substituents, acidity of the polarized open indium sites, extended channel sizes, and increased pore volumes (from -I to -Cl). Subsequently, we took the three MOFs collectively as a platform to investigate the impact of the different coordinated halide ions on channel functions, especially on CO2 adsorption and chemical conversion. Accordingly, the three nanochannel MOF catalysts exhibited highly effective performances in catalyzing cycloaddition of CO2 with large-sized epoxides, particularly styrene oxide, into value-added products-styrene carbonates with yields of 91-93% and high selectivity of 95-98%-under mild conditions. We speculated that the superior catalytic efficiencies of the three MOF catalysts could be ascribed to the synergistic effect of open indium sites as Lewis acid with different halide ions as weak base sites, which might enhance the catalytic selectivity through polarizing and activating CO2 molecules during the reaction process.