Covalent heterogenization of discrete bis(8-quinolinolato)dioxomolybdenum(VI) and dioxotungsten(VI) complexes by a metal-template/metal-exchange method: Cyclooctene epoxidation catalysts with enhanced performances

• Discrete homogeneous bis(8-quinolinol) metal sphere is created by a metal templating method.
• The composition and local structure difference is proved for templated and grafted samples.
• Templated Mo and W catalysts are superior to their corresponding grafted and neat analogues.
• Templating improves the catalyst stability.

A metal-template/metal-exchange method was used to imprint covalently attached bis(8-quinolinolato)dioxomolybdenum(VI) and dioxotungsten(VI) complexes onto large surface-area, mesoporous SBA-15 silica to obtain discrete MoO2VIT and WO2VIT catalysts bearing different metal loadings, respectively. Homogeneous counterparts, MoO2VIN and WO2VIN, as well as randomly ligand-grafted heterogeneous analogues, MoO2VIG and WO2VIG, were also prepared for comparison. X-ray absorption fine structure (XAFS), pair distribution function (PDF) and UV–vis data demonstrate that MoO2VIT and WO2VIT adopt a more solution-like bis(8-quinolinol) coordination environment than MoO2VIG and WO2VIG, respectively. Correspondingly, the templated MoVI and WVI catalysts show superior performances to their randomly grafted counterparts and neat analogues in the epoxidation of cyclooctene. It is found that the representative MoO2VIT-10% catalyst can be recycled up to five times without significant loss of reactivity, and heterogeneity test confirms the high stability of MoO2VIT-10% catalyst against leaching of active species into solution. The homogeneity of the discrete bis(8-quinolinol) metal spheres templated on SBA-15 should be responsible for the superior performances.

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