Periodic Trends in Highly Dispersed Groups IV and V Supported Metal Oxide Catalysts for Alkene Epoxidation with H2O2

摘要

Supported metal oxides are important catalysts for selective oxidation processes like alkene epoxidation with H2O2. The reactivity of these catalysts is dependent on both identity and oxide structure. The dependence of the latter on the synthesis method can confound attempts at comparative studies across the periodic table. Here, SiO2-supported metal oxide catalysts of Ti(IV), Zr(IV), Hf(IV), V(V), Nb(V), and Ta(V) (all of groups IV and V) were synthesized by grafting a series of related calixarene coordination complexes at surface densities less than similar to 0.25 nm(-2). Select catalysts were investigated by solid state NMR, UV-visible, and X-ray absorption near-edge spectroscopies. As-synthesized and calcined materials were examined for the epoxidation of cyclohexene and styrene (1.0 M) with H2O2 (0.10 M) at 45 and 65 degrees C. Nb catalysts emerge as high-performing materials, with calcined Nb SiO2 proceeding at a cyclohexene turnover frequency of 2.4 min(-1) (>2 times faster than Ti-SiO2) and with 85% selectivity toward direct (nonradical) epoxidation pathways. As-synthesized Zr, Hf, and Ta catalysts have improved direct pathway selectivities compared with their calcined versions, particularly evident for Ta-SiO2. Finally, when the materials are synthesized from these precursors but not simple metal chlorides, the direct pathway reaction rate correlates with Pauling electronegativities of the metals, demonstrating clear periodic trends in intrinsic Lewis acid catalytic behavior.