Heterometallic {ZnEu}-metal-organic framework for efficient chemical fixation of CO₂

Based on a ligand-directed synthetic strategy, the acidic solvothermal reaction of ZnO, Eu2O3, and 4,4',4''-(pyridine-2,4,6-triyl)tri(1,3-benzenedicarboxylic acid) (H6PTTBA) generated a targeted robust double-walled honeycomb material {[EuIIIZnII(HPTTBA)(H2O)]·4DMF·3H2O}n (simplified as NUC-9), which featured excellent characteristics such as dual tubular nanochannels, high porosity, specific surface area, abundant exposed active metal sites, etc. Although both types of nano-channels (I and II) alternately arranged in the lattice and shaped by six rows of [EuIIIZnII(CO2)6(H2O)] SBUs possessed an equal amount of exposed active metal sites, they could be differentiated according to the discrepant inner surface functionalized by free carboxyl oxygen atoms or coordinated aqueous molecules. Moreover, an activated sample of NUC-9 exhibited better catalytic performance than documented Zn- or Eu-based MOFs for the chemical transformation of various epoxides into the related carbonates under comparatively mild conditions of 1 atm CO2 flow and 70 °C, which should be ascribed to the unsaturated Zn2+ and Eu3+ ions acting as strong Lewis acid sites and free carboxyl oxygen atoms as basic sites synergistically polarizing and activating the substrates of epoxides and CO2 and consequently promoting the reaction. Furthermore, the water-resistant framework of NUC-9 could selectively and sensitively discriminate Fe3+ in aqueous solution according to the fluorescence quenching effect. In addition, it is worth mentioning that the successful self-assembly of NUC-9 provides an effective synthetic technique by employing the designed favorable organic ligand for achieving the targeted functional model of MOFs.