Nanoporous Lanthanide-Based MOFs for Catalyzing the Cycloaddition of CO₂ and the Knoevenagel Condensation

Due to the merits of a higher charge, an empty 4f electron layer, and a larger radius, Ln3+ ions in nanoporous LnOFs usually display a strong Lewis acidity, which endows the host frameworks with excellent catalytic performance on most of the CO2-involved reactions. Herein, a combination of [Ln2(COO)7(H2O)2] clusters (abbreviated as {Ln2}) and a multifunctional structure-oriented ligand of 4,4′,4′′-(pyridine-2,4,6-triyl)triisophthalic acid (H6PTTA) under acidic solvothermal conditions offered a series of isomorphic nanoporous frameworks of {(Me2NH2)[Ln2(PTTA)(H2O)2]}n (NUC-110, Ln = Pr, Nd, Eu, Gd, Ho, Er, Tm, Yb, and Lu). Activated NUC-110 series not only own ca. 64–65% void volume composed of rare hierarchical cylindrical (17 Å) and quasi-quadrilateral (8 Å) channels and rich Lewis acid–base sites such as LnIII sites and Npyridine atoms, but also have an abnormal stability resistant to boiling water, weak acid and base solutions, and most of the organic solvents. For theoretically screening the catalytic ability of NUC-110Ln, the adsorption energy between the propylene oxide (PO) molecule and the activated host frameworks was calculated by the Dmol3 module of Material Studio software, which exhibits a binding energy between PO and NUC-110Gd, which is the smallest (−100.301 kJ/mol), suggesting that NUC-110Gd should have the most ideal catalytic performance on the cycloaddition of CO2 and epoxides. Experiments proved that the cycloaddition of CO2 and epoxides could be highly catalyzed by 0.10 mol % of NUC-110Gd catalyst under relatively mild conditions of 65 °C and 1 atm with a turnover frequency of 162 h–1. Furthermore, the Knoevenagel condensation of aldehyde and malononitrile could be accelerated by NUC-110Gd, which should be due to the fact that pyridine and defectively coordinated Gd3+ can separately activate malononitrile and aldehyde molecules. Therefore, the work not only provides a type of robust high-porosity rare-earth cluster-based MOFs but also offers a comprehensive calculation of binding energies on Ln3+ ions for the first time