Structure-Performance Relationship Of Platinum-based Catalysts In Selective Hydrogenation Of Cinnamaldehyde

Cinnamaldehyde contains two functional groups of carbon-carbon double bond(C=C)and carbon-oxygen double bond(C=O).Its semi-hydrogenated cinnamyl alcohol is widely used in the fields of medicine,chemical industry and perfume.In view of thermodynamics,the C=C bond is easier to activate hydrogenation than C=O bond,and then generate saturated aldehyde product because of its lower bond energy and the ?4 adsorption mode.Therefore,how to design and synthesize the catalyst to improve the hydrogenation selectivity of the C=O functional group is very important.Pt-based catalysts are widely used in hydrogenation reactions and have excellent performance.However,the selectivity to cinnamyl alcohol is generally low in the selective hydrogenation of cinnamaldehyde.In order to obtain higher cinnamyl alcohol selectivity,it can be improved by adding a second metal into Pt-based catalysts.The second metal can not only improve the dispersibility of the Pt nanoparticles(NPs),but also affect the crystallography and electronic structure of Pt NPs,thereby improving the selectivity to the target product.In this thesis,carbon nanotubes(CNTs)and Al2O3 were used as supports,and Co or Fe was introduced into Pt NPs.The effect of the second metal on the crystal and electronic structures of Pt NPs is analyzed in detail,and the structural evolution of the catalysts during the liquid phase reaction is explored.In addition,the "Identical Location-Transmission Electron Microscopy(IL-TEM)"method was used to reveal the catalyst deactivation mechanism.The details are as follows:1.The investigation on the structure-performance relationship of PtxCoy NPs supported on oxidized CNTs(oCNTs)in the selective hydrogenation of cinnamaldehyde.A series of PtxCoy NPs with different Pt/Co atomic ratios were prepared by the oleylamine reduction method,then PtxCoy-oCNTs catalysts were synthesized by supporting NPs on oCNTs.The effects of different Co contents in Pt NPs and their microstructure and electronic structures were systematically studied.And their catalytic performance in the selective hydrogenation of cinnamaldehyde was tested.The introduction of Co in a Pt-based catalyst can not only improve the dispersion and stability of Pt NPs,but also make them more active in the cinnamaldehyde hydrogenation reaction.Meanwhile,the interaction between Pt and Co also affects the electronic structure of Pt.Electron-deficient Pt inhibits the adsorption and strengthening of C=C double bonds in the selective hydrogenation reaction of cinnamaldehyde,which promotes the adsorption and hydrogenation of C=O double bonds.It makes that PtCo3-oCNTs have excellent cinnamaldehyde hydrogenation selectivity.Furthermore,the Pt-Co interaction was revealed in detail.IL-TEM method has been used in this reaction,by which the deactivation process of the catalysts was clearly illustrated.2.The study on the structure-performance relationship of PtxFey-Al2O3 catalyst in the selective hydrogenation of cinnamaldehyde.PtxFey-Al2O3 catalyst was prepared by incipient wetness impregnation method.The effects of the support,pretreatment atmosphere,and different Fe contents on the hydrogenation of cinnamaldehyde were studied in detail.The optimal catalyst is Pt3Fe-Al2O3,which was prepared by treating the catalyst precursor on Al2O3 support under Ar gas condition and with a Pt/Fe ratio of 3:1.PtxFey NPs are uniform dispersion on Al2O3 supports.PtxFey-Al2O3 catalysts were used for the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol.It was found that the conversion and selectivity gradually improved with increasing Fe content.The Pt3Fe-Al2O3 catalyst exhibits the best catalytic performance.However,when the amount of Fe is further increased,the excess Fe covers the active components on the NP surface,resulting in a decrease in catalytic performance.In the cycle stability tests of Pt3Fe-Al2O3 catalyst,the conversion and the selectivity can be maintained at about 88%and 76%,respectively.The fresh,used and cycled catalysts were collected and characterized by XRD and XPS.The analysis indicates that the structure and electronic state of the synthesized PtxFey-Al2O3 remained unchanged after recycled.IL-TEM method has been used in this reaction,revealing that the interaction between the NPs and the supports was strong and the catalyst was relatively stable.