Supported sub-nanometer Ta oxide clusters as model catalysts for the selective epoxidation of cyclooctene (original) (raw)
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Selective oxidation is of immense importance in the synthesis of chemical intermediates and the epoxidation of alkenes by the electrophilic addition of oxygen to a carbon–carbon double bond remains one of the most significant challenges in oxidation catalysis. Although molecular oxygen is the most environmentally benign oxidant in many cases, far more reactive forms of oxygen are required to achieve reaction, and this can lead to by-products with a heavy environmental burden with respect to their disposal. We show that gold supported on graphite is a very effective catalyst for the epoxidation of cis-cyclooctene as long as catalytic amounts of a hydroperoxy species are present at the start of the reaction. Using mild solvent-free conditions the hydroperoxy initiator persists in solution for only a few minutes, being initially adsorbed on the catalyst surface. Subsequently, it decomposes to establish a reactive species that can propagate the selective oxidation process we observe. The observation of an induction period may in part be due to the adsorption of the radical initiator blocking surface sites as well as the establishment of the reactive species. We confirm that graphite is the best support and that tert-butyl hydroperoxide is the preferred initiator. We report extensive studies concerning the reusability of the gold/graphite catalyst as catalyst reusability is a key feature of green chemistry. The catalyst is found to be inhibited by the epoxide product but we demonstrate the effect of this is negligible for reused catalysts over a long reaction time.
Synthesis of TS-1 from TiO2 and Ludox Silica and Use as Catalyst for Cyclooctene Epoxidation
chemistry and materials research, 2015
The aim of this work was the synthesis of a TS-1 zeolite starting from Si and Ti precursors more adapted to scale-up than TEOS and tetra-alkyl orthotitanates. Silica and titania were used here. The resulting TS-1 material was characterized and its catalytic performance evaluated in the oxidation of cyclooctene in liquid-phase (acetonitrile) using anhydrous H 2 O 2 or tert- butyl hydroperoxide (TBHP) as oxidants. The TS-1 with Ti/Si = 0.0046 (XRF) was prepared via a hydrothermal treatment and characterized by X - ray diffraction (XRD), infrared spectroscopy (FT-IR), UV-Vis. spectroscopy. A MFI structure with high crystallinity was obtained in which the titanium atoms are tetrahedrally coordinated to four oxygen atoms. Using titanosilicalite TS-1 with MFI structure in heterogeneous catalytic oxydations of cyclooctene with aqueous H 2 O 2 and Tert-butyl hydroperoxide the epoxide of cyclooctene is detected to be highest among the obtained variety of compounds. The reaction progress is m...
TiO2-SiO2 mixed oxides: Xerogel catalyst for the Selective Epoxidation of cyclohexene
Research Journal of Pharmaceutical, Biological and Chemical Sciences
A porous TiO2-SiO2 mixed metal oxide was synthesized via sol-gel route using inorganic precursors and subsequently dried, and then calcined at different temperatures (250, 400 and 600°C). The mixed oxide was characterized by XRD, as well as by FTIR and diffuse reflectance ultraviolet-visible (DRS-UV-vis). The hydrophobicity (H) and relative abundance of Ti-O-Si linkages or Ti dispersion (D, T) were calculated. The catalysts were evaluated in the cyclohexene epoxidation with TBHP (tert-butyl hydroperoxide) as oxidant agent; the selectivity toward cyclohexene epoxide was 100 % and the conversion was 44 %. The effects of varying the catalyst calcination temperature and adding slowly the oxidant and solvent were investigated. The catalytic data obtained showed that the activity can be influenced by the coordination environment of titanium species. The insights obtained from these studies allowed a fundamental understanding of the relationships between the structural characteristics and ...