Nanoporous [Ho2(CO2)7(H2O)2]-organic frameworks for excellent catalytic performance on the cycloaddition of CO2 into epoxides and the deacetalization-Knoevenagel condensation

Abstract

Designing nanoporous lanthanide-based metal–organic frameworks (MOFs) as robust heterogeneous catalysts has received a lot of interest in recent years. Herein, we successfully constructed a novel isomorphic nanoporous MOF {[Ho₂(TDP) (H₂O)₂]·5H₂O·4DMF}_n (named as NUC-55, NUC = North University of China) by combining [Ho₂(CO₂)₇(H₂O)₂] (abbreviated as {Ho₂}) clusters with 2,4,6-tri(2,4-dicarboxyphenyl)pyridine (H₆TDP) as structure-oriented multifunctional ligands under acidic solvothermal conditions. NUC-55 is a holmium(III)-based 3D MOF with a hierarchical porous architecture containing tetragonal microchannels (0.56 nm in diameter) and octagonal nanochannels (1.79 nm in diameter), In NUC-55, plenty of Lewis acidic and basic sites, including open Ho³⁺ sites and N_pyridine atoms, coexist. Moreover, it is worth mentioning that the void volume (∼65%) is significantly higher in NUC-55 than in most documented 3D lanthanide-based MOFs (Ln-MOFs). Catalytic experiments show that activated NUC-55 exhibits high catalytic activity in the CO₂–styrene oxide cycloaddition reactions under mild conditions, with a high turnover number of 2475 and a high turnover frequency of 619 h⁻¹. In addition, activated NUC-55 can remarkably accelerate the deacetalization-Knoevenagel condensation reactions of benzaldehyde dimethyl acetal and malononitrile. Taken together, this work can not only establish an effective self-assembly strategy for fabricating highly porous Ln-MOFs, but also provide new insights into their catalytic mechanism.