Combination of dimensional reduction and active site addition strategies for preparing unique {RE9}-cluster-based MOFs: efficient CO2 fixation and Knoevenagel condensation

Abstract

The current application of porous catalytic materials for organic synthesis is always confined to comparatively simple small substrates because of the diffusion barrier. Therefore, in this study, dimensional reduction and active site addition strategies were employed for preparing unique porous {RE₉}-cluster-based rare-earth metal–organic frameworks (MOFs) {[Me₂NH₂]₄[RE₉(pddb)₆(μ₃-O)₂(μ₃-OH)₁₂(H₂O)_1.5(HCO₂)₃]·6.5DMF·11H₂O}_n (MOF-RE, RE = Tb, Y, and Dy) with high-density multiple active sites. It was found that MOF-RE are rare {RE₉}-based two-dimensional (2D) networks including triangular-nanoporous (1.3 nm) and triangular-microporous (0.8 nm) channels decorated by abundant Lewis acid–base sites (open RE(III) sites and N_pyridine atoms) on the inner surface. As anticipated, due to the coexistence of Lewis acid–base sites, activated samples exhibited better catalytic activity (a yield of 96%, and a TON value of 768 for styrene oxide) than most previously reported 3D MOF materials for the cycloaddition of CO₂ and multifarious epoxides under moderate conditions. Moreover, as a heterogeneous catalyst, MOF-Tb, has excellent catalytic performance (with a TON value of 396 for benzaldehyde) for the Knoevenagel condensation reaction of malononitrile and aldehydes with high catalytic stability and recoverability. In addition, both reactions possessed high turnover numbers and frequencies. These dimensional reduction and active site addition tactics may permit the exploitation of new nanoporous MOF catalysts based on rare-earth clusters for useful and intricate organic conversions.