Layered-structured polyoxovanadate catalysts: Synthesis and performance in olefin epoxidation

Olefin epoxidation represents a crucial transformation in organic synthesis, with significant applications in pharmaceutical and fine chemical industries. Developing efficient catalysts that can utilize 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: [ZnⅡ(cyclam)(H₂O)][VⅤ₈O₁₅(OH)₁₂] (1) and [ZnⅡ(phen)]₂[VⅤ₁₂O₃₂]·H₂O (2) (cyclam=1,4,8,11-tetraazacyclotetradecane, phen=1,10-phenanthroline). These two compounds exhibit unique two-dimensional layered structural characteristics: compound 1 forms a wave-like 2D layer, while compound 2 presents a quasi-planar 2D extended structure. These structural features make the vanadium active centers more accessible to substrates, thereby significantly enhancing catalytic performance. Under optimized conditions, compound 1, using molecular oxygen as the oxidant, 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. Compound 2 also demonstrated similar high catalytic activity. Compared to other catalysts reported in the literature, these two compounds exhibited excellent catalytic performance and stability under mild conditions and could be recycled multiple times without significant loss of activity. Structural analysis revealed that the 2D layered structures of these compounds provide more accessible vanadium active sites, facilitating interactions between substrates and catalysts, thus improving catalytic efficiency.