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

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 MoO ₂^VIT and WO₂^VIT catalysts bearing different metal loadings, respectively. Homogeneous counterparts, MoO ₂^VIN and WO₂^VIN, as well as randomly ligand-grafted heterogeneous analogues, MoO₂^VIG and WO₂^VIG, were also prepared for comparison. X-ray absorption fine structure (XAFS), pair distribution function (PDF) and UV-vis data demonstrate that MoO₂^VIT and WO₂ ^VIT adopt a more solution-like bis(8-quinolinol) coordination environment than MoO₂^VIG and WO₂^VIG, respectively. Correspondingly, the templated Mo^VI and W^VI catalysts show superior performances to their randomly grafted counterparts and neat analogues in the epoxidation of cyclooctene. It is found that the representative MoO₂^VIT-10% catalyst can be recycled up to five times without significant loss of reactivity, and heterogeneity test confirms the high stability of MoO₂^VIT-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. © 2014 Elsevier B.V.