Preparation and structural characterization of polymer-supported methylrhenium trioxide systems as efficient and selective catalysts for the epoxidation of olefins.

Abstract

Novel heterogeneous compounds of methylrhenium trioxide (MTO) were prepared with poly(4-vinylpyridine) and polystyrene as polymeric supports. The wide-angle X-ray diffraction (WAXS.) analysis, performed by the application of the difference method, showed, in a representative case of the poly(4-vinylpyridine)/MTO derivatives, a slightly distorted octahedral conformation on the metal's primary coordination sphere. The Re-O and Re-C bond distances were not influenced by the polymeric nature of the ligand, while the Re-N bond distance was abnormally shorter than those previously observed for homogeneous MTO/L(n) complexes, showing a strong coordination of the rhenium atom to the support. A set of scanning electron microscopy (SEM) photographs showing the morphology of the surface of particles of poly(4-vinylpyridine)/MTO and polystyrene/MTO systems are reported. The reticulation grade of the polymer was a crucial factor for the morphology of the particles surface. Poly(4-vinylpyridine) 2% cross-linked systems were characterized by particles with very irregular shape and surface. Poly(4-vinylpyridine) 25% cross-linked systems showed particles with regular spherical shape, which morphology was similar to microcapsules obtained with polystyrene. All novel MTO compounds were efficient and selective heterogeneous catalysts for the epoxidation of olefins using environmentally friendly H2O2 as oxygen atom donor. The catalyst activity was maintained for at least five recycling experiments.