Layered structured polyoxovanadate catalysts: Synthesis and performance in olefin epoxidation

Olefin epoxidation is a crucial transformation in organic synthesis, with significant applications in the pharmaceutical and fine chemical industries. The development of efficient catalysts that can use molecular oxygen as an environmentally benign oxidant under mild conditions remains a significant challenge. By employing diverse neutral ligands, two novel inorganic–organic hybrid polyoxovanadates were successfully synthesized: [Zn^II(cyclam)(H₂O)][V^V₈O₁₅(OH)₁₂] (1) and [Zn^II(phen)]₂[V^V₁₂O₃₂]·H₂O (2) (cyclam = 1,4,8,11-tetraazacyclotetradecane, phen = 1,10-phenanthroline). The two compounds exhibit unique two-dimensional (2D) layered structural characteristics: compound 1 forms a wave-like 2D layer, whereas compound 2 exhibits a quasiplanar 2D extended structure. These structural features make the vanadium active centers more accessible to substrates, thereby significantly improving catalytic performance. Under optimized conditions, using molecular oxygen as the oxidant, compound 1 achieved a 99.9% conversion rate with 95.4% selectivity for the terminal alkene styrene at 50 °C and a 97.7% conversion rate with over 99% selectivity for the internal alkene cyclooctene at room temperature. Similarly, compound 2 also exhibited high catalytic activity. Compared with other catalysts reported in the literature, the two synthesized compounds exhibit excellent catalytic performance and stability under mild conditions and can be recycled multiple times without significant loss of activity. Structural analysis revealed that the 2D layered structures of these compounds provide additional accessible vanadium active sites, facilitating interactions between substrates and catalysts and thus enhancing catalytic efficiency.