Facet-engineered spinel Co3O4 for homogeneous-like efficiency in selective toluene oxidation to benzaldehyde

Heterogenization of Co-based catalyst can effectively solve the catalyst recovery problem encountered by homogenous catalyst Co(OAc) 2 , but there may generate an obvious performance decrease for the heterogenized cobalt site ascribed to the increased mass transfer resistance. Hence, how to alleviate or eliminate the performance decline for Co-based heterogeneous catalyst is an intriguing scientific question. In this study, we reported the application of crystal engineering strategy on Co 3 O 4 to bridge the performance gap between homogeneous catalyst and heterogeneous catalyst in the selective oxidation of toluene into benzaldehyde. Four distinct spinel Co 3 O 4 catalysts, predominantly exposing the {220}, {222}, {440}, and {311} crystal facets respectively, were prepared and utilized as heterogeneous catalysts. The Co 3 O 4 -P sample predominantly exposing the {220} facets exhibited the optimal catalytic activity for toluene oxidation, achieving a toluene conversion of 56.1 % and a benzaldehyde selectivity of 67.6 %, which was significantly superior to that of homogenous catalyst Co(OAc) 2 . The enhanced performance of this material is primarily attributed to the abundant reversible Co 2+ /Co 3+ redox couples on its surface, which maintains sufficient Co 2+ active sites throughout the catalytic process and facilitates toluene oxidation reactions. In addition, Co 3 O 4 -P also exhibited the maximum specific surface area, the highest Co 2+ /Co 3+ ratio, and the greatest oxygen vacancy density, which together give rise to its best catalytic performance. Density functional theory (DFT) calculations further corroborated these findings. According to quenching experiments and in-situ FTIR characterizations, the possible selective toluene oxidation mechanism over Co 3 O 4 -P catalyst was proposed. Moreover, the recycled Co 3 O 4 -P sample could be reused for more than five consecutive reactions, demonstrating its eminent re-usability.