Molecular Simulation Study On Acetaldehyde Aldol Condensation Reaction And Side Reactions On MgO Catalyst

1,3-butadiene is an extremely important chemical raw material with a wide range of uses.The traditional butadiene production route is the petroleum route,and the raw material is the C₄ fraction produced as a by-product of ethylene,so the output is restricted by the ethylene production.In recent years,the process of producing butadiene from ethanol has gradually received widespread attention.MgO/SiO₂catalysts have been considered as one of the most promising catalysts for the production of butadiene from ethanol.The aldol condensation reaction is the key step in the process of producing butadiene from ethanol,but the mechanism has not yet been clarified,and there are few studies on related side reactions.In this paper,the Density Functional Theory method is used to systematically study the acetaldehyde aldol condensation reaction mechanism and related by-product formation mechanisms,in order to have a deeper understanding of the mechanism of MgO/SiO₂ catalysts for the process of ethanol to butadiene.First,based on reported XRD and FESEM test of MgO/SiO₂ catalyst,we found that MgO（100）and MgO（110）surfaces were the mainly exposed surfaces.So we constructed periodic models of MgO（100）and MgO（110）surfaces.The adsorption and co-adsorption properties of the main species on the two surfaces of MgO were systematically studied.It was found that the adsorption of all the adsorbates was chemisorption,and there was an electron transfer between the adsorbate and the catalyst surface.In the case of co-adsorption,electron transfer not only exists between the adsorbate and the catalyst surface,but also occurs between the two co-adsorbed adsorbates.Secondly,in order to explore the aldol condensation reaction mechanism,the activation barrier and reaction energy of each elementary reaction on the MgO（100）and MgO（110）surfaces were calculated by DFT method.It was found that on the MgO（100）surface,acetaldehyde firstly undergoes enolization reaction,C-C bond formation reaction,and proton equilibrium reaction to generate CH₃CH（OH）CH₂CHO.Then crotonaldehyde is formed after enolization and dehydroxylation.On the MgO（110）surface,acetaldehyde undergoes enolization reaction,C-C bond formation reaction,proton equilibrium reaction,second enolization reaction and dehydroxylation reaction to form crotonaldehyde.And the lower overall barrier shows that aldol condensation reaction is more prone to occur on MgO（110）surface.Finally,by calculating the activation barriers and reaction energies of the acetone formation reactions and the ethoxyethanol formation reactions on the MgO（100）and MgO（110）surfaces,we constructed the reaction network.The analysis of the reaction network shows that on the MgO（100）surface,the ethoxyethanol formation reaction is more likely to occur than the aldol condensation reaction,which affects the main reaction selectivity.On the MgO（110）surface,the overall energy barrier of the aldol reaction is lower,which means that aldol condensation is more likely to occur,and the selectivity of the main reaction is higher.However,the ethoxyethanol produced by the side reaction is difficult to desorb,occupying the surface active sites of the catalyst,and affecting the activity of the catalyst.The barriers of acetone formation reaction on different MgO surfaces are relatively high,making acetone more difficult to generate.