Evidence on Primary Pore Size Dependence of C-C Bond Coupling Inside Zr-Based Metal-Organic Frameworks

The effect of pore structures in porous catalysts on the catalytic behavior of guest molecules has been discussed for decades. However, there is still no clear evidence to show that the guest molecule is stably confined within the designed local environment. The local concentration of reactants caused by the confined space may affect the catalytic reaction. Herein, we show a probe study that analyzes the influence of the primary pore size on the product selectivity in the aldol condensation of acetaldehyde/ethanol mixtures (C-2), in which the reactants have been shown to be thermodynamically stable in relatively large pores (10-30 angstrom) of metal-organic framework (MOF) catalysts (such as UiO-66, NU-1000, NU-901, UiO-67, etc.) rather than the micropores (<10 angstrom) during the condensation of the C-2 molecule, which is explained by solid-state nuclear magnetic resonance ssNMR analysis. This actually suggests that the promoting effect of the primary pore on ethanol upgrading selectivity is only feasible once the guest molecule has been able to stabilize within the pore during catalysis.