Construction of Hollow Cobalt Tetroxide Nanocages through the Metal Salt Bifunctional Etching Strategy for Catalytic Oxidation of Propane at Ultrahigh Space Velocity

In order to greatly improve the catalytic oxidation activity of propane under ultrahigh space velocity conditions, we developed a bifunctional etching strategy to prepare ZIF-67-derived hollow cage catalysts that do not use organic acid as an etchant but directly use the acidity of metal nitrates in ethanol to etch ZIF-67 assisted by ultrasound. The application of acid/base reagents in the traditional etching strategy was avoided in our proposed etching strategy, making the preparation process more concise, efficient, and environmentally friendly. To verify the superiority of the catalysts prepared by the bifunctional etching strategy, especially in the heterogeneous reaction at ultrahigh space velocity, the propane catalytic oxidation activity tests were conducted on the samples etched by Co(NO3)2·6H2O with different concentrations. The complete conversion of propane was still achieved at 240 °C on Co3O4–30, which fully proves the superiority of the hollow cage structure. Furthermore, we also verified the feasibility of the bifunctional strategy with other metal nitrates [Ni(NO3)2·6H2O, Ce(NO3)3·6H2O, and Fe(NO3)3·9H2O] and found that pH was not the only factor that affected the etching results; the difference of metal cations also had a great influence on the final morphology.