Nano-containing Mg Composite Metal Oxide Preparation, Characterization And Catalytic Properties Of Study

Nanoparticles are regarded as new active catalytic materials because of their high reactivity and good selectivity. On the other hand, complex metal oxides have already been used widely in the catalysis. Complex metal oxides with different structures and compositions can be prepared from various kinds of metal oxides. The catalytic performance of complex metal oxides are often improved through fine adjustment in chemical composition, textural structure and the interaction among active components. Based on the former knowledge about the nanoparticles and complex metal oxides, nanosized complex metal oxides undoubtedly possess great potentials for application in catalysis,and the preparation, characterization and catalysis of these materials deserve intensive investigation. MgO is a kind of metal oxide with medium-strength basicity. In catalysis, it is common to improve the performance of catalysts through introduction of MgO. This dissertation is focused on two catalytic reactions of practical interest, i.e., selective oxidation of styrene and the ethylbenzene dehydrogenation in the presence of CO₂. In the work, the preparation, characterization and catalytic properties of MgxFe₃-xO₄ complex oxide catalysts and Mg-Fe complex metal oxide prepared from hydrotalcite-like precursors are studied. The main contents are as follows:1. Nanosized spinel MgFe₂O₄ were prepared by coprecipitation and citrate gel methods. In coprecipitation, NH₄OH and NH₄HCO₃ were used as precipitants. The influence of molar ratio of NH₄OH to NH₄HCO₃ and calcination temperature on the preparation was studied. In citrate gel method, the influence of citric acid/Mg2+ ratio and calcination temperature on the preparation was studied. XRD and TEM results reveal that both methods are successful for getting MgFe₂O₄ spinel nanoparticles. However, the particle size and morphology of the samples are different. The mean size of MgFe₂O₄ prepared by coprecipitation is about 80 nm, whereas the particles prepared by citrate gel method are smaller with a mean size of 30-40 nm and more dispersive. TG/DTG/DTA measurements show that the crystallization temperature of MgFe₂O₄ spinel prepared by citrate gel method is about 200oC lower than that by coprecipitation method. Such a temperature difference leads to the differences in the properties of the spinel products. The citrate gel method is more suitable for the preparation of samples with higher dispersity and smaller particle size.2. Selective oxidation of styrene with H₂O₂ as oxidant on MgxFe₃-xO₄ spinel complexoxide catalysts was studied. MgxFe₃-xO₄ catalysts obtained by citrate gel method are more active due to their higher dispersity and smaller particle size. The oxidation reaction is favored on nonstoichiometric MgxFe₃-xO₄ catalysts rather than on stoichiometric MgFe₂O₄. Styrene undergoes a C=C bond cleavage preferentially over the spinel-type catalysts to give benzaldehyde ( 65 - 70 mol%), phenylacetaldehyde and styrene oxide are formed as minor reaction products ( 10 mol%). The effects of solvent, reaction temperature, reaction time and styrene/H₂O₂ (molar ratio) on the performance of Mg_0.4Fe_2.6O₄ catalyst were studied. Aprotic solvent and lower reaction temperature are favorable for increasing the selectivity for benzaldehyde. Under the optimum reaction conditions, the styrene conversion is 32.0%, and the selectivity for benzaldehyde is 20.2%. The catalyst characterization and reaction results reveal that being rich in defect structure, and the existence of nanosized α-Fe₂O₃ crystallites in the non-stoichiometric Mg-Fe spinel complex oxide catalysts are responsible for the enhanced catalytic activity.3. The effect of metal promoters on the catalytic properties of Mg_0.4Fe_2.6O₄ was studied. The results show that adding an appropriate amount of Al enhances the activity of the catalyst and the selectivity for benzaldehyde. The effects of reaction temperature, reaction time and styrene/H₂O₂ (molar ratio) on the performance of Mg_0.4Fe...