Construction Of C=O Selective Hydrogenation Catalysts Based On LDHs Two-dimensional Materials With Enhanced Interfacial Electron Effect And Its Reaction Mechanism

α,β-unsaturated aldehydes,as a kind of low value-added chemical reagents,are often used as raw materials in the field of fine chemicals to prepare high value-added products because of the presence of a pair of conjugated C=C and C= in the molecule.The hydrogenation products of O bond and C=O bond are widely used in the production of medicines and perfumes and dyes.Therefore,the selective hydrogenation of α,β-unsaturated aldehydes has been extensively studied by many scholars.The initial theoretical study of the reaction found that there is a competitive hydrogenation reaction between C=C and C=O bonds in the hydrogenation process,and the thermodynamics supports C=C bond hydrogenation,and therefore,the enhancement of the selective hydrogenation performance of the C=O bond has become a common scientific problem in the field of catalysis.The interface structure of the supported catalyst has a special interface effect,which can change the adsorption form of the catalyst to the reactant and improve the activation ability of the catalytic site,constructing the interface structure and effectively adjusting the interface effect is a possible way to improve the selectivity of the C=O bond in the selective hydrogenation of unsaturated aldehydes.In this thesis,cinnamaldehyde is used as a probe molecule to study the hydrogenation reaction of α,β-unsaturated aldehydes,aiming at the problem of insufficient C=O bond selectivity,a highly dispersed active metal supported catalyst prepared with the classic two-dimensional layered material LDHs and its derivative composite metal oxide(MMO)as the catalyst carrier was proposed as the research object.Pt/CoAl-LDH based on the adjustable properties of LDHs laminate elements and controllable electronic interaction enhancement and Pt/CoAlTi-MMO based on the topological effect of LDHs structure to construct defect sites enriched,realized the innovation of catalytic materials in the reaction.The enhancement mechanism of the intrinsic activity of the C=O bond by the interface electrons was revealed,and the innovation of the performance enhancement method was realized.(1)Based on the adjustable characteristics of metal cations of LDHs laminates,using it as a catalyst carrier,the active metal Pt was loaded on the surface of the laminates.By adjusting the composition of the carrier,the selection of different divalent metal elements to cinnamaldehyde was studied.The effect of hydrogenation reaction performance,among which Co element shows the best performance.When the reaction conditions of Pt/CoAl-LDH are 70℃ and H2 pressure is 3 MPa,the conversion rate of reaction for 2 h is 95.4%,and the selectivity is 92.0%.More importantly,in order to further study the role of Co in the reaction,single metal catalysts(Pt/Co(OH)2 and Pt/Al(OH)3)were prepared as comparative samples,focusing on the electron transfer mechanism at the interface.And its mechanism of action in the catalytic process,the results show that:Pt/CoAl-LDH with a Co2+-O2--Al3+structure has an appropriate acid-base sites,which induces the generation of interface electrons that transfer electrons from the carrier to the active metal.This increases the electron cloud density around Pt0,promotes the activation and dissociation of H2,and can provide a large number of active hydrogen species for the reaction.In addition,the coordination degree of unsaturation of Pt0 at the interface of the sample is the highest,which leads to the predominant adsorption of the C=O bond,and the electron-rich Pt0 forms a strong repulsion with the C=C bond π electron cloud,and finally achieves high selectivity to the C=O bond.(2)Based on the topological effect of the LDHs structure,the Ti element is introduced into the binary CoAl-LDHs laminate to replace part of the Al element to obtain the ternary CoAlTi-LDHs,and a series of composite metal oxides CoAlTi-MMO-T(350℃,450℃,650℃)are obtained by precisely controlling the firing temperature.Using it as a carrier,further supporting active metal precursors,and obtaining a series of Pt/CoAlTi-MMO-T catalysts through a liquid phase reduction step.It was found that with the increase of the calcination temperature,the conversion rate gradually increased,and the selectivity to the C=O bond under the same conversion rate did not show a difference.Pt/CoAlTi-MMO-650℃ at 70℃,3 MPa conditions,the highest conversion rate of 98.0%and a higher C=O bond selectivity 87.3%when reacted for 2 h,focusing on the mechanism of interface electrons in the catalysis process.By adjusting the carrier composition and element content,comparing the performance of Pt/CoAl-650℃ and Pt/CoTi-MMO-650℃,it is found that the Co2+-O2--Al3+component promotes the electron transfer at the interface of the metal carrier,and the Ti3+-Ov component can promote the adsorption of Pt to the C=O bond,and the hydrogen overflow effect shown by the catalyst is consistent with the law of interface electron action,that is,the stronger the metal support,the stronger the hydrogen overflow effect and the higher the catalytic activity.Pt/CoAlTi-650℃ exhibits stronger interface electron effect and hydrogen overflow effect than Pt/CoAlTi-350℃,so the activity is improved,and the increase of the calcination temperature does not cause the increase or decrease of Ti3+-Ov content,so the selectivity to C=O bond is maintained at a high level,which also shows that the introduction of Ti element promotes the adsorption of Pt on the C=O bond and this adsorption has good stability.