| Cyclohexene,with active C=C bond,which is an important intermediate for organic synthetic,can be widely used in synthesizing medicine,pesticide,forage additives,polyester and other fine chemicals.Selective hydrogenation of benzene to cyclohexene is one of the best ways to obtain cyclohexene because of sufficient raw materials,simple reaction route and environment friendly.However,from the thermodynamics point of view,the Gibbs free energy of benzene hydrogenation to cyclohexene standard(A,Gm(?)=-23 kJ·mol-1)is larger than that of benzene to cyclohexane(?lGm(?)=-98 kJ·mol-1).Hence,benzene is more likely to hydrogenation to cyclohexane.Therefore,development of a catalyst that can selectively convert benzene to cyclohexene is of great importance.In this thesis,TiO2 was chosen as a carrier,and a deposition-precipitation method was employed to prepare Ru/TiO2 catalysts.Their catalytic activities of selective hydrogenation of benzene to cyclohexene were evaluated.X-ray powder diffraction,X-ray photoelectron spectroscopy,N2 physisorption,CO chemical adsorption,transmission electron microscopy,scanning electron microscopy and H2-temperature programmed reduction techniques were used to investigate their structures and chemicophysical properties.The relationship between the catalyst structures and catalytic performance is discussed in detail.The results are as follows:(1)A series of Ru/TiO2 catalysts were prepared by using P25 type TiO2 as supports after calcining at different temperatures.The effect of crystal phases of TiO2 on the catalytic performance of Ru/TiO2 catalyst was investigated.It was found that the bulk TiO2 is easier to convert from anatase phase to rutile phase than the surface one;after calcination at 650 ?,majority of the bulk TiO2 is in rutile phase,while the surface is in anatase state;the surface TiO2 transforms to rutile phase after calcined at 700 ?.The sizes of Ru particles in Ru/TiO2 catalysts increase upon increasing the calcination temperature.To a certain extent,the larger the Ru particle sizes are,the higher activity of the catalyst in converting benzene to cyclohexene is.The rutile TiO2 supported Ru catalyst shows higher cyclohexene selectivity than the anatase TiO2 supported catalyst due to its hydrophilic characteristics.The catalyst fabricated using TiO2 calcined at 600 ? show the highest yield of cyclohexene.(2)A series of Ru/TiO2-R catalysts,prepared by a deposition precipitation method,were obtained by reducing at different temperatures aiming to active the catalyst.It was concluded that larger Ru particle sizes are shown in the catalyst reduced at higher temperature;After reducing at 170 ?,the interaction between Ru and Ti species results in the formation of Ti?-with rich-electron and Rua+deficient-electron.The presence of Ru?+ enhances the adsorption of benzene and cyclohexene,thus resulting in improving its catalytic activity and cyclohexene selectivity.(3)The catalytic activity of Ru/TiO2 catalyst reduce by NaBH4 or H2 for the selective hydrogenation of benzene was investigated.It was found that the Ru particle size in catalyst reduced by NaBH4 is larger than the other one,leading to low Ru dispersion and low benzene conversion.However,in the catalyst reduced by NaBH4,the interaction between Ru and Ti leads to the existence of Ti?+ with deficient-electron and Ru?-rich-electron.The as-obtained catalyst is hydrophilic due to the hydrophilic property of Ti?+ species,thus high cyclohexene selectivity is achieved.The Ru?-species,which are weak in adsorption of benzene and cyclohexene,resulting in low benzene conversion and high cyclohexene selectivity. |
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