Ligand-engineered cobalt precursors toward enhanced catalytic activity of Co@S-1 zeolite in toluene oxidation

Cobalt-doped zeolite catalysts have been identified as a potentially viable non-precious metal alternative for toluene oxidation, exhibiting characteristics reminiscent of noble metals, such as the ability to activate C-H bonds, abundance, and minimal adverse environmental impact. However, the catalytic activity is greatly reliant on the diversity of cobalt species embedded within the zeolite framework. To address this, a ligand-engineering strategy was utilized to precisely control the coordination environment of the cobalt precursor, thereby regulating the resulting cobalt species and accordingly the catalytic activity of cobalt-doped zeolite catalysts. Four distinct Co@S-1 zeolites were synthesized with disparate ligands including ethylenediamine, water, ammonia water and acetylacetone, and meanwhile applied as heterogeneous catalysts for the selective oxidation of toluene with molecular oxygen as oxidant. Among them, the Co@S-1(EDA) zeolite catalyst synthesized with ethylenediamine demonstrated superior catalytic reaction performance, achieving a toluene conversion of 63.8 % with a benzaldehyde selectivity of 68.5 % within 6 h of reaction. This eminent catalytic oxidation ability for Co@S-1(EDA) zeolite catalyst is mainly ascribed to its largest Co 2+ /Co 3+ ratio, its most oxygen vacancy amount and its largest external surface area. Furthermore, the Co@S-1(EDA) zeolite catalyst exhibited excellent re-usability, maintaining its activity over five consecutive catalytic cycles. This study emphasizes ligand engineering as an effective approach to create highly efficient and stable cobalt-based zeolite catalysts for selective hydrocarbon oxidation reactions.