POLYMER-SUPPORTED MO ALKENE EPOXIDATION CATALYSTS

Four chelating resins derived from crosslinked poly(chloromethylstyrene) (PCMS), four derived from crosslinked poly(glycidylmethacrylate) (PGMA), and one crosslinked poly(4-vinylpyridine) resin have been used to immobilise Mo complexes. The metal centre was introduced to the polymeric ligands by ligand exchange using MoO2(acac)2. The initially blue-coloured polymers were activated by treatment with excess tert-butylhydroperoxide to yield bright yellow resins, and it is believed that oxidation of Mo(V) to Mo(VI) occurs during this process. Each resin was then employed as a catalyst in the solution epoxidation of cyclohexene using tert-butylhydroperoxide as the oxidant at approximately 80-degrees-C. In all cases. the resin catalysts were highly active with the PGMA-based species being consistently more active, possibly due to its higher polarity and more favourable morphology. Extensive studies of the recycling of the various catalysts with monitoring of the level of Mo leaching in each reaction showed that more stable complexes were obtained with chelating ligands with two or more nitrogen donor sites. In the case of PCMS-based resin with an N-(2-hydroxypropyl)aminomethyl-2-pyridine ligand complexed with Mo(VI) with a ligand/metal ratio of approximately 2/1, the stability of the resin complex allowed recycling nine times without any loss of activity, and with the corresponding level of Mo leaching falling essentially to zero. In contrast, an analogous resin with a ligand/metal ratio of < 1/1 displayed Mo leaching which increased on recycling.