Molybdenum active sites implanted defective UiO-66(Zr) for cyclohexene epoxidation: Activity and kinetics investigation

Abstract

Defect engineering is a versatile method to modulate structure and performance of crystalline materials yet remain sophisticated. Herein, Zr-based metal organic frameworks (i.e., UiO-66) featuring controlled structural defects and single Mo active sites, namely Mo@UiO-66-xy (x represented molar equivalents and y denoted the modulator, acetic acid or formic acid, with respect to the linker), were synthesized to systematically conduct activity and kinetics investigation on microscopic level. It was determined that the predominant defects, missing-cluster defects, could be tuned to a large extent via altering molar equivalents of formic acid not acetic acid. Further, it was established that intrinsic Lewis acid acidity was positively correlated with formic acid concentration. All Mo@UiO-66-xy exhibited catalytic activity in cyclohexene epoxidation and Mo@UiO-66-100for showed the highest catalytic performance due to its relative strong Lewis acid acidity and suitable specific surface area / porosity-derived attractive high Mo sites-dispersion and mass-transfer property. Additionally, for the first time, a kinetic measurement that considered two reaction pathways and all products was performed as a function of reaction temperature to evaluate activation barrier for cyclohexene epoxidation. That was, cyclohexene epoxidation to epoxide activation energies determined for direct and radical routes were 46.12 and 102.46 kJ mol−1, respectively, and Mo@UiO-66-100for was inclined to act along direct pathway to stimulate cyclohexene epoxidation.