Selevtive Hydrogenation Of Crotonaldehyde Over Gold & Platinum Catalysts Supported On Ce-Based Oxides

α,β-Unsaturated alcohol is of very important industrial values and is produced mainly by selective hydrogenation ofα,β-unsaturated aldehyde.However,the hydrogenation of the C=C bond is thermodynamically preferable over the C=O bond, which leads to undesirable product such as saturated aldehydes.Therefore,it remains a challenge for the researchers in catalysis field to further increase the selectivity forα,β-unsaturated alcohol fromα,β-unsaturated aldehyde.Crotonaldehyde is a typitalα,β-unsaturated aldehyde.Recent studies on selective hyrogenation of crotonaldehyde to produce crotyl alcohol have been focused on the employment of supported gold and platinum catalysts.Currently,the thermodynamic stability of supported gold catalysts is still not satisfactory,and the mechanism of supported gold-and platinum-catalyzed hydrogenation requires further clarification.In this dissertation,Au and Pt catalysts supported on Ce-based oxides were prepared via deposition-precipitation method and applied in the selective hydrogenation of crotonaldehyde.Characterizations of the supported catalyst were achieved by XRD, Raman,FT-IR,TG-DTA,SEM,TEM,BET and CO chemical absorption.A series of experiments have been designed and carried out to examine the influence of electronic environment on the catalyst surface and the composition of the supports on the selective hydrogenation of the conjugated C=O and C=C bond,so as to elucidate the active center of the hydrogenation of the C=O bond and the reason for the deactivation of the catalyst. The contents of the dissertation are as follows:1.Preparation and characterization of Ce-based oxides.Ce-based solid solutions were prepared by an improved citrate sol-gel method,which mainly involved the following two steps:1)calcination(or carbonization)of the citrate precursor at high temperature under N₂ atmosphere(the step resulted in intermediate mixture of Ce-based oxides and carbon powder due to lack of oxygen);2)removal of the carbon powder at low temperature in air.With the improved citrate sol-gel method,Ce_0.8Pr_0.2O_Y solid solutions with a cubic fluorine structure and more oxygen vacancies were synthesized, with particle size smaller than 10nm and specific surface area higher than 92.1m²/g.It can be concluded that the formation of the Ce_0.8Pr(0.2)O_Y solid solution occurred in the calcination process at 800℃in N₂ and its textural structure would not be influenced by subsequent removal of carbon powder at low temperature in air.Since carbonization process is a key step for the improved citrate sol-gel method,the carbonization temperature would have a vital effect on the textural structure of Ce-based solid solution. Low carbonization temperature resulted in weak crystallization of the Ce-Zr oxide, which would aggregate into big particles with low surface area in subsequent carbon removal process.While high carbonization temperature could provide sufficient thermodynamic force to afford high degree of crystallization of Ce-Zr oxide,and finally lead to the formation of Ce_0.8Zr_0.2O₂ solid solution with higher surface area and porous structure.2.Selective hydrogenation of crotonaldehyde over Au catalysts supported on Ce-based oxides.For the selective hydrogenation of crotonaldehyde over Au/CeO₂ catalysts,the catalytic properties are not only related to the surface area of CeO₂ support, but also to the gold loadings.Using CeO₂ with lower surface area(70m²/g)as the support,the gold loading of 3%in Au/CeO₂ catalysts gave satisfactory catalytic results: 12%conversion of crotonaldehyde and 58%selectivity of crotyl alcohol at steady state. When the gold loading is lower than 3%,the catalytic properties of Au/CeO₂ catalysts decreased linearly;while with higher gold loading,the catalytic properties did not increase noticeably.Besides,a series of Au/Ce_xZr_1-xO₂ were prepared with an attempt to furher improve the thermodynamic stability of Au catalyst.Studies found that among the Au/Ce_xZr_1-xO₂ examined,Au/Ce_0.8Zr_0.2O₂ catalyst could afford higher selectivity of crotyl alcohol(62%)than Au/CeO₂ in the steady state,The result may be attributed to the presence of more oxygen vacancies in the mixed oxides,which can promote the absorption of C=O bond,thus favoring the production of more crotyl alcohol.In addition,Au/Ce_0.8Zr_0.2O₂ exhibited longer catalyst life at 180℃,thus showing its promising prospect as a catalyst suitable for industry production.3.Selective hydrogenation of crotonaldehyde over Pt catalysts supported on Ce-based oxides.In this chapter,the in situ technique was used to pretreat the Pt/CeO₂ catalysts so that the particle size and morphology of platinum could remain intact but the surface chemical environment of CeO₂ support change.The pretreatment of the catalysts allowed us to investigate the effect of Ce⁴⁺or Ce³⁺on the selective hydrogenation of crotonaldehyde.It was found that Ce³⁺ions in Pt/CeO₂ catalyst can transfer electrons to Pt particles,thus enhancing the absorption of C=O bond and improving the selectivity of crotyl alcohol.On the other hand,Ce³⁺ions could also induce the coke deposition on the surface of Pt/CeO₂ catalyst,thus leading to the deactivation of the Pt catalysts.We further explored the hydrogenation of crotonaldehyde over Pt/Ce_xSm_1-xO_2-δcatalysts,and the main results could be concluded as:1)Pt/Ce_xSm_1-xO_2-δcatalysts have better catalytic properties than Pt/CeO₂ catalysts;2) the doped quantity of Sm has an important effect on the catalytic properties of Pt catalysts.For example,Pt/Ce_0.8Sm_0.2O_2-δhas the preferable initial conversion of crotonaldehyde(53.5%)and selectivity of crotyl alcohol(76.5%);3)For Pt/Ce_0.8Sm_0.2O_2-δ,high temperature pretreatment can increase the selectivity of crotyl alcohol,which could be accounted for by the formation of big particle of platinum that favors the adsorption and hydrogenation of C=O bond.4.Selective hydrogenation of crotonaldehyde over Au and Pt catalysts supported on other oxides.As control experiments,Au/TiO₂ and Pt/Pr₆O₁₁catalysts were prepared and applied in the hydrogenation of crotonaldehyde.For the Au/TiO₂ catalysts,different crystal phases of TiO₂ can greatly affect the catalytic properties.The selectivity of crotyl alcohol on pure anatase support is higher than that on pure rutile support.Furthermore, the best catalytic properties of Au catalyst came from the use of mixed crystal phases as the support,where both the catalytic activity and the selectivity of crotyl alcohol were enhanced.For Pt/Pr₆O₁₁catalyst,the initial selectivity of crotyl alcohol can reach 75% provided that the catalyst was subjected to high temperature reduction at 700℃.High temperature reduction can produce plenty of Pr³⁺ions in Pt/Pr₆O₁₁catalysts.Such Pr³⁺ ions can donate electrons to Pt particles,thus favoring the adsorption of C=O bond and the generation of crotyl alcohol.Analogous to Pt/CeO₂ catalysts,low valent Pr³⁺ions can induce the coke deposition on the catalyst surface,and subsequently make the Pt/Pr₆O₁₁catalysts exhibit decreased catalytic properties.