Epoxidation Of Styrene With Hydrogen Peroxide Over MgO Catalysts

In this paper, a series of solid MgO were prepared by direct thermal decomposition of the Mg(NO₃)₂·6H₂O precursor. The epoxidation of styrene was studied over the MgO using hydrogen peroxide as an oxidant in the presence of acetonitrile as solvent. Preparation conditions of catalyst, such as calcination temperature and calcination atmosphere, and the reaction factors such as reaction temperature, reaction time and H₂O₂ dosage had defferent effects on the epoxidation of styrene.The structure and morphology of the samples were characterized by X-ray diffraction, transmission electron microscopy, nitrogen adsorption-desorption, thermogravimetry, CO2 temperature-programmed desorption. The results indicated the strength of basic sites on the MgO catalyst is the key factor for the excellent performance of the catalyst. Enhancement of the strength of basic sites can increase styrene conversion. In contrast, the number of basic sites, surface area, and pore volume of MgO have a relatively minor effect on catalytic activity. The MgO sample prepared by calcining Mg(NO₃)₂·6H₂O in air at 750 oC showed the most strength of basic sites and the highest catalytic activity among the MgO samples calcined at 450-850 oC catalysts. The optimized reaction conditions were styrene / H₂O₂ molar ratio = 1/6, acetonitrile as solvent, 60 oC, and 10 h. Under these conditions the conversion of styrene and the selectivity to epoxyethylbenzene were 97.0% and 88.4%, respectively. The MgO sample prepared in nitrogen at 650 oC showed higher catalytic activity after preteatment temperature 350 oC. Under the optimized reaction conditions, the conversion of styrene and the selectivity to epoxyethylbenzene were 97.0% and 93.2%, respectively.the MgO samples with different morphologies, such as large surface area(279 m²/g) MgO-4 sample, large diameter(294.3 nm) MgO-2 sample, large pore volume MgO-4 and MgO-5 sample (1.609 cm3/g and 1.478 cm³/g, respectively), obtained by other methods. The epoxidation of styrene was studied over the MgO samples. The results indicated the effect of surface area, pore size, pore volume factors of MgO samples on the epoxidation of styrene is very small. The numbers of the more strengh of basic sites on MgO have an effect on the conversion of styrene, the selectivity to epoxyethylbenzene is closely related to the most strength of basic sites. The MgO by direct thermal decomposition showed the highest catalytic activity among the MgO samples.Hydrotalcite that have the structure of memory effect can highly catalysed the epoxidation of styrene. Under the optimized reaction conditions, the conversion of styrene and the selectivity to epoxyethylbenzene were 85.1% and 91.1%, respectively. But its catalytic activity slightly lower than MgO-750's. Other metal oxides ,such as ZnO,Fe₂O₃,CuO, have low catalytic activity.The recovered catalysts after high temperature activation still showed good catalytic activity under the same conditions. With the increase of frequency of use, the catalyst activity gradually decreased.