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

Abstract

Metal–organic frameworks (MOFs) with small pore window sizes and narrow internal space always suffer from 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 {Ce4+2Ce3+Na(obb)6(CH3OH)(C2H5OH)}∞ (NaCe-MOF-as), featuring 1D nanochannels composed by nanocages, was obtained by the self-assembly of transition 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 Ce3+/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.