DFT Simulational Study Of MPV Reaction And Cross Condensation,Hydrogenation Side Reactions On MgO Catalyst

Butadiene is an important organic raw-materials,as the building block widely applied in chemical industry.Ethanol is abundant in China,and the process of ethanol to butadiene has a broad application prospect.The major catalyst system of this process is MgO/SiO₂,of which MgO is regarded as the effective active component towords the reactions from ethanol to butadiene.In this work,.the simulation researches on MPV reaction and coss condensation,butenol hydrogenation side reactions in the process from ethanol to butadiene on MgO by Density Functional Theory（DFT）method were conducted systematically.It is hoped that the concerned mechanism could be further cleared to provide theoretical support for catalyst design and development.Firstly,the models of mainly exposed surfaces MgO（100）and MgO（110）of MgO were builded,and their structural and electronic properties were systematically studied by DFT method.Comparing to MgO（100）,the acidity of Mg site on Mg（110）was stronger,and the basicity of O site on Mg（110）was stronger.The adsorption configurations of ethanol and the co-adsorption configurations of ethanol/ethoxy and crotonaldehyde on MgO（100）and MgO（110）were confirmed,and the adsorption properties were investigated by DFT method.The results of density of states analysis indicated that ethanol showed a weak interaction with MgO（100）,while showed a strong interaction with MgO（110）,where the dissociative adsorption occured;and the co-adsorbed molecules showed a weak interaction with MgO（100）,whereas a strong interaction with MgO（110）,Secondly,MPV reactions on MgO（100）and MgO（110）were studied by DFT method.On MgO（100）surface,it was difficult for the conversion from ethanol to dehydrogenate directly,so it could be co-adsorbed with crotonaldehyde directly to react via MPV reaction,where the ethanol dehydrogenation,proton transfer and proton return reactions occured in turn,and finally crotyl alcohol and acetaldehyde were produced.The rate-limiting step was the proton transfer reaction,the corresponding activation energy was 0.87 eV.On MgO（110）surface,the dissociative adsorption on ethanol occured,and the produced ethoxy groups were co-adsorbed and reacted with crotonaldehyde via MPV,where proton transfer reaction occurred,and finally butenoloxy and acetaldehyde were produced.Activation energy for this step was 0.39 eV.Butenoloxy group would not conduct the proton return reaction,but was dehydrogenated to 1,3-butadiene directly.Finally,the cross condensation and hydrogenation side reactions were studied by DFT method.On MgO（100）surface,the acetaldehyde produced from MPV reaction reacted with crotonaldehyde by cross condensation,where acetaldehyde olefinization,the nucleophilic addition and proton transfer reactions occured.The rate-limiting step was the olefinization reaction,of which the activation energy was 0.74 eV.Crotyl alcohol reacted by hydrogenation side reaction,where two hydrogenation reactions occurred simultaneously,the second hydrogenation reaction was the rate-limiting step with the activation energy of 1.10 eV.On MgO（110）surface,the acetaldehyde produced from ethoxy dehydrogenation reacted with crotonaldehyde by coss condensation,where the acetaldehyde olefinization and nucleophilic addition occurred simultaneously.The rate-limiting step was the olefinization reaction,the corresponding activation energy was 0.39 eV.Butenoloxy reacted by hydrogenation side reaction,where two hydrogenation reactions occurred simultaneously,the second hydrogenation reaction was the rate-limiting step with the activation energy of 3.63eV.