Selective Hydrogenation Of Crotonaldehyde Over Ir Catalysts And CeO₂ Catalysts

α,β-unsaturated alcohols are important chemical intermediates and are widely used in the production of fragrances,medicines and other fine chemicals.In industry,unsaturated alcohols are synthesized by the reaction ofα,β-unsaturated aldehydes with strong reducing agents such as Na BH₄ and Li Al H₄,resulting the large amount of waste water and thus severe pollution.In contrast,direct hydrogenation ofα,β-unsaturated aldehydes using H₂ as the reductant to produceα,β-unsaturated alcohols is an atom-economic reaction,which meets the requirements of green chemistry and therefore has received widespread attention.However,forα,β-unsaturated aldehydes containing conjugated C=C and C=O bonds,the hydrogenation of the C=C bond is much more favored than that of the C=O in respect to both thermodynamic and kinetic facts,resulting in low selectivity to desired unsaturated alcohols.Therefore,it is still a challenge to develop catalysts with high activity and selectivity for this reaction.In this thesis,supported Ir/WO₃ and Ir/TiH₂ catalysts were prepared and tested for liquid phase selective hydrogenation of crotonaldehyde.The effects of surface acidity,Ir particle size,the unique structure of TiH₂ and the reduction treatment of the catalyst on the reaction behaviors of the catalysts were investigated.In addition,metal-free CeO₂ catalysts with different morphologies were tested for gas phase selective hydrogenation of crotonaldehyde,and the effects of CeO₂ morphologies on the adsorption behaviors of crotonaldehyde on the catalysts were investigated.Detailed contents of the thesis are as follows:1.Liquid phase selective hydrogenation of crotonaldehyde on Ir/WO₃ catalystA series of Ir/WO₃ catalysts with different Ir contents were prepared by a conventional impregnation method,and tested for liquid phase selective hydrogenation of crotonaldehyde.It was found that compared to the Si O₂ and Mg O supports,the supported Ir/WO₃catalysts were much more active and selective,with a crotonaldehyde conversion of 67%and a crotyl alhol selectivity of 87%.Characterizations revealed that the improved performance of the Ir/WO₃ catalyst was related to its high surface acidity and strong metal-support interaction.In addition,it was found that the turnover frequencies of the Ir/WO₃catalysts were similar in spite of different Ir contents,indicating that the reaction was structure insensitive in the employed Ir particle sizes of 2-3 nm.This study provides experimental evidence for understanding the influence of metal-support interaction and particle size effects on reaction behavior.2.Liquid phase selective hydrogenation of crotonaldehyde on Ir/TiH₂ catalystA series of Ir/TiH₂ catalysts with different metal contents were prepared by conventional impregnation method.Liquid phase selective hydrogenation of crotonaldehyde was carried out on a series of Ir/TiH₂ catalysts,and the effect of reduction temperature on the catalytic behaviors was investigated.It was found that the Ir/TiH₂ catalyst gave much higher activity（with a crotonaldehyde conversion of 52%at 80 ^oC）and much higher selectivity to crotyl alcohol（78%）,compared to a reference Ir/TiO₂ catalyst.The enhanced reactivity was attributed to enriched oxygen vacancies on the TiH₂ surface and the strong H activation capability of the TiH₂ support.In addition,high temperature reduction resulted in the growth of the Ir particles and consequently declined turnover frequency.It was deduced that smaller Ir particles were favorable for the hydrogenation of C=O bond in the crotonaldehyde molecule and thus improved the activity.3.Gas phase selective hydrogenation of crotonaldehyde on metal-free CeO₂ catalystr-CeO₂ and c-CeO₂ with uniform morphologies were prepared via well-established hydrothermal procedures.We have studied CeO₂ nanorods（r-CeO₂）,nanocubes（c-CeO₂）,and nanopolyhedra（p-CeO₂）as the catalysts for the gas phase hydrogenation of crotonaldehyde to crotyl alcohol and found that metal-free r-CeO₂ with a relatively high density of surface oxygen vacancies exhibits considerably catalytic efficiency and an unprecedented high selectivity of 93.2%.The combined in situ characterization results and DFT calculation results demonstrate that enhanced H₂ dissociation at the solid FLP sites of r-CeO₂ surface and preferred crotonaldehyde adsorption via the C=O bond at the Ce sites adjacent to surface oxygen vacancies are responsible for the efficient catalysis of r-CeO₂.However,a further reduction of r-CeO₂ by H₂ to obtain more oxygen vacancies was observed to be unfavorable with regard to both catalytic activity and selectivity,because of the additional surface oxygen vacancies lead to the emergence of crotonaldehyde adsorption via the C=C bond,resulting in a decrease in catalytic activity and selectivity.These findings provide a potential approach to fabricate high-performance and low-cost catalysts for selective hydrogenation ofα,β-unsaturated aldehydes to unsaturated alcohols.