Synergistic Effects of Lewis Acid–Base Pair Sites─Hf-MOFs with Functional Groups as Distinguished Catalysts for the Cycloaddition of Epoxides with CO₂

Abstract

The incorporation of Lewis acid-base sites in catalysts has been considered as a significant approach to fabricating bifunctional catalysts with efficient catalytic activity for CO₂ fixation. In this paper, a series of Hafnium-based metal-organic frameworks (Hf-MOFs), NU-912(Hf) and NU-912-X(Hf)-X (X = -NH₂, -Br, -CN, and -I) derivatives assembled by Lewis acidic Hf₆(μ₃-O)₄(μ₃-OH)₄(H₂O)₄(OH)₄ (Hf₆) clusters and Lewis base-attached organic linkers, are successfully synthesized by a facile ligand functionalization method. These isostructural Hf-MOFs, which exhibit diamond channels of 1.3 nm diameter, great chemical stability, and CO₂ adsorption capacity, have been evaluated as catalysts for the CO₂ cycloaddition reaction with epoxides. Catalytic experiments reveal that the micropore environments of these MOFs have an outstanding impact on catalytic activity. Remarkably, NU-912(Hf)-I serves as an efficient heterogeneous catalyst for this catalytic reaction under mild conditions due to the high density of Lewis acid Hf₆ cluster centers and strong Lewis base functional groups, surpassing most of the reported MOF-based catalysts.