Size-selective catalysis enhancement in a new nanocaged MOF through the synergistic Lewis acidic–basic sites

Abstract

Metal–organic frameworks (MOFs) with small pore window sizes and narrow internal space always suffer from a low diffusion rate of reactants/products, and limited accessibility to their internal catalytic active sites, resulting in low catalytic activity. Constructing MOFs with pores and/or infinite channels in nanoscale size via self-assembly of suitable ligands and metal ions/clusters could improve the mass diffusion rate and accessibility of interior catalytic active sites, thereby boosting the catalytic activity of MOFs. Moreover, engineering Lewis acidic–basic active sites can efficiently enhance the activity of MOFs due to their synergetic catalytic effects. However, MOFs with nanocages or one-dimensional (1D) nanochannels are scarce, let alone the ones with synergistic catalytic Lewis acidic–basic active sites, applied in the catalysis field. Here, a new mixed-metal-based MOF {Ce₁³⁺Ce₂⁴⁺Na(obb)₆(CH₃OH)(C₂H₅OH)}_∞ (NaCe-MOF-as), featuring 1D nanochannels composed of nanocages, was obtained by the self-assembly of lanthanide and alkali metal ions with the long-chain-like ligand 4,4′-oxydibenzoate (obb²⁻). As expected, benefitting from the constructed 1D nanochannels composed of nanocages, as well as the engineered Lewis basic sites (the bridging oxygen atom of obb²⁻) and Lewis acidic sites (the coordinatively unsaturated Ce^3+/4+ and Na⁺ centers), NaCe-MOF-240 (NaCe-MOF activated at 240 °C under vacuum) exhibits higher reactivity and better recyclability towards Knoevenagel condensation (KC) reactions of large-sized substrates in relation to the previously reported IAM19-1 {[Ce₄(obb)₆(H₂O)₉]·(H₂O)}_∞ with smaller pore size. This work provides a blueprint for developing MOF catalysts with high catalytic activity and excellent recyclability toward the reactions of large-sized substrates.