Dual active-sites of Co and oxygen vacancies in Co-doped CeO2-catalyzed toluene oxidation for the subsequent Knoevenagel condensation process

Abstract

Efficient activation and oxidative transformation of C(sp³)–H into value-added compounds by O₂ represents a sustainable synthetic pathway with high atom economy and environmentally friendly features. However, both C(sp³)–H and O₂ must be activated effectively, and this may be difficult to achieve with catalysts that contain only one type of active site. Herein, dual active-sites comprising oxygen vacancies and cobalt species were constructed in cobalt-doped nanorods of ceria for the respective activation of O₂ and C(sp³)–H, enabling efficient toluene oxidation for subsequent Knoevenagel condensation with malononitrile to yield benzylidenemalononitrile under mild conditions. Extensive experiments and theoretical simulations revealed that the oxidation of C(sp³)–H in toluene to aldehyde intermediates was realized through the spillover of active oxygen species from the oxygen vacancies to cobalt sites owing to the high capacity for oxygen mobility in the defective CeO₂. Subsequently, the facile condensation with malononitrile on CeO₂ was also promoted by the presence of cobalt sites. This dual-active-sites process provides an alternative approach for the effective oxidation of C(sp³)–H by O₂/air for subsequent transformations.