resumo
In face of the industrial scale production of epoxides and their importance as versatile intermediates, much research effort has been put into investigating the epoxidation of olefins. Example of an industrial catalytic epoxidation process is Halcon-ARCO, involving the liquid phase reaction of propylene with tBHP in the presence of homogeneous molybdenum catalysts. The present work explores new molybdenum-based catalysts (or precursors) for the liquid phase epoxidation of olefins. Special attention was given to the identification of active species and to assess the catalyst stability, through their isolation after catalytic tests, characterization and reutilization. The epoxidation of cis-cycloctene epoxidation with tBHP (in decane, tBHPdec), at 55 ºC, was chosen as the model reaction. The studies on the catalytic performances were extended to different substrates, oxidants, solvents and heating methods. The highest catalytic activity was observed for complexes [MoO2Cl2L2] (L= dialkylamide ligand), which are more stable and easier to handle than [MoO2Cl2] and analogous complexes with L {THF, MeCN} (Chap. 2). From these complexes the formation in situ of intermediate active species of the type [(MoO2ClL2)2(μ-O)] is possible. The complex [MoO2(Lzol)], Lzol=chiral oxazoline ligand (Chap. 3), is a stable and versatile catalyst, active for the epoxidation of several olefins (high epoxide selectivities, but low enantiosselectivities), the oxidative dehydrogenation of alcohols and for sulfides sulfoxidation. The catalyst could be efficiently recycled, when dissolved in an ionic liquid (IL). The ionic complex [MoO2Cl{HC(3,5-Me2pz)3}]BF4 (Chap.4) converted into the active complexes [{MoO2(HC(3,5-Me2pz)3)}2(μ-O)](BF4)2, [Mo2O3(O2)2(μ-O){HC(3,5-Me2pz)3}] and [MoO3{HC(3,5-Me2pz)3}]; the catalyst could be efficiently recycled using an IL. The presence of water and the oxidation conditions influenced the formation of these species. Complexes [CpMo(CO)3Me] (Chap.5) and [CpMo(CO)2(η3- C3H5)] (Chap.6) gave similar active species (based on catalytic tests and FT-IR ATR spectra of the recovered solids). For [Cp'Mo(CO)2(η3-C3H5)], the influence of the type of Cp’ on the catalytic activity suggested the formation of active species with this ligand. Complexes [Mo(CO)4L] gave stable catalysts, formed in situ during the epoxidation, which may perform as heterogeneous catalysts: for L=2-[3(5)-pyrazolyl]pyridine it was formed [Mo4O12L4]; for L=ethyl[3-(2- pyridyil)-1-pyrazolyl]acetate it was formed [Mo8O24L4] (Chap.7). The use of microwave heating (MO) instead of the oil bath method (BO) resulted in an increase of the rate of the catalytic reaction, due to the faster heating of the reaction mixture (Chaps. 5 and 7). The utilization of aqueous tBHP instead of tBHPdec was preferable, since it excluded the decane from the reaction system and epoxide yields remained high (Chaps. 2 and 6); the catalytic performance was optimized by removing water from the reaction mixtures (Chaps. 4 e 7). The best result for limonene epoxidation was observed for [CpMo(CO)3Me]: 88% epoxide yield (2 h, 55 ºC, MO heating).
categoria
Engenharia química Catálise homogénea Catalisadores Epoxidação Soluções iónicas Molibdénio
autores
Patrícia dos Santos Neves
nossos autores
Patrícia dos Santos Neves
Investigador Auxiliarorientadores
Carlos Manuel Santos Silva; Anabela Tavares Aguiar Valente