Multivariate Zeolitic Imidazolate Framework Derived Hollow NiCo2O4 for Selective Alkene Oxidation by Oxygen Vacancy Engineering.

Selective oxidation of alkene to valuable chemical feedstocks is challenging due to the seesaw effect between conversion and selectivity. Here, multivariate zeolitic imidazolate frameworks (ZIFs) are transformed into bimetallic oxides with varied morphologies via tannic acid (TA) etching and calcination. Impressively, TA etching results in the formation of oxygen vacancies and pore expansion, and the hollow architecture is generated after pyrolysis, favoring the exposure of more active sites and rapid mass diffusion, as well as inducing prominent void-confinement effects. Besides, the incorporation of nickel ions can introduce an additional type of Lewis acid sites into the resulting bimetallic oxides. The obtained Co3Ni1-TA1.4-10-400 exhibits an effective styrene-to-benzaldehyde oxidation with nearly total conversion and a 99% yield (5 h, 90 °C), approximately 1.9 and 8.3 times higher than those of Co3Ni1-TA1.4-10 (51%) and Co3Ni1-O (12%), respectively. Mechanistic investigations for the conversion of styrene to benzaldehyde further indicate that the reaction possibly proceeds via a synergistic mechanism involving both a metal-peroxide pathway and a free-radical pathway. Furthermore, in the presence of sodium bicarbonate, Co3Ni1-TA1.4-10-400 can prove effective in catalytic epoxidation of styrene. This study provides a valid approach for the construction of ZIF derivatives with abundant oxygen vacancies and unsaturated metal sites as promising catalysts for fine and commodity chemical production.