Highly Stable Ir/FeO_x/SiO₂Catalysts For Selective Hydrogenation Of Crotonaldehyde

Selective hydrogenation of unsaturated aldehydes to unsaturated alcohols is a critical step in the preparation of various fine chemicals, particularly in the field of fragrances chemistry and pharmaceuticals. However, the hydrogenation of the C=C bond in the unsaturated aldehyde molecule is much easier compared to that of the C=O bond, due to both thermodynamic and kinetic factors. Therefore, a large number of experimental and theoretical works have been devoted to design active and selective catalysts. In this thesis, a series of highly active and stable Ir-based catalysts were prepared and tested for selective hydrogenation of crotonaldehyde. Dominant factors that influenced the catalytic behaviors of the catalysts were investigated by various characterization techniques, by which correlationship between catalyst strcutures and performance was established. The following parts are detailed contents of this work.1. Effects of different additives on the catalytic performance of Ir/MOx/Si02catalysts.A series of Ir/MOx/SiO2catalysts with different additives were prepared using an impregnation method (M=Cr, Mn, Fe, Co, Ni and Cu), and then used for the crotonaldehyde reaction. It was found that high catalytic performance was obtained on the Ir/FeOx/SiO2catalyst. The relationship between the nature of the catalyst and catalytic properties was also discussed, which was found that the raise of catalyst activity is in favor of more Irδ+species and suitable Ir0-Irδ+interface.2. Effect of impregnation sequence of Ir and Fe on the catalytic performance of Ir/FeOx/SiO2catalystCatalysts (Ir/FeOx/SiO2and FeOx/Ir/SiO2) were prepared by an inverse impregnation sequence, and monocomponent Ir/SiO2and FeOx/SiO2catalysts were also employed to clarify the effects of impregnation sequence of Ir and Fe on the catalytic performance. When Ir was supported on the pre-formed0.1FeOx/SiO2, the Ir/0.1FeOx/SiO2catalyst kept high crotonaldehyde conversion and selectivity to crotyl alcohol. On the other hand, the FeOx/Ir/SiO2catalyst prepared by an inverse impregnation sequence showed high initial activity but deactivated quickly. The monocomponent Ir/SiO2and FeOx/SiO2catalysts had low crotonaldehyde conversions and selectivities to crotyl alcohol, which was due to the new interface (Ir-Irδ+, Ir-Fen+and Irδ+-Fen+) and the exposed FeOx species.3. The effect of Fe content on the performance of Ir/FeOx/SiO2catalyst.A series of catalysts with different iron contents (Ir/yFeOx/SiO2, the Fe/Ir molar ratio of y, y=0,0.01,0.035,0.1,1.1,3.5and11) were prepared using an impregnation method, and then employed to clarify the impact of Fe content on Ir/FeOx/SiO2catalyst for crotonaldehyde selective hydrogenation. It was found that a small amount of Fe was advantageous to the catalyst performance improvement, but when the Fe content was too high (Fe/Ir≧3.5), although the initial catalyst activity was greatly improved, but serious deactivation occurred. This may attribute to the high content of FeOx species on the catalyst surface and the new interface (Ir-Fen+and Irδ+-Fen+interface) which resulting in carbon deposit.4. The effect of reduction temperature on the performance of Ir/FeOx/SiO2catalystIr/FeOx/SiO2catalyst was prepared by impregnation method, and then reduced at different temperatures (200,300,400and500℃) to investigate the effect of reduction temperature on the performance of Ir/FeOx/SiO2catalysts. It was found that catalyst exhibited good catalytic performance when the reduction temperatures were200or300℃. While the reduction temperature was to400℃, the catalyst became less active; and the catalyst reduced at500℃showed dramatic deactivation during the reaction. And it was put down to carbon deposit on the catalyst surface after the high temperature reduction.