DFT study on the side reactions of Aldol condensation on MgO in the production of 1,3-butadiene from ethanol

Highlights: • Mechanisms of Aldol condensation side reactions over MgO catalyst were studied by DFT method. • Formation mechanisms of acetone are different on different crystal planes of MgO. • Ethoxyethanol formation reactions have high activity on both MgO crystal planes. • Side reactions on the surface of MgO (1 0 0) are more advantageous. Density functional theory (DFT) was used to search the transition states of all the elementary steps of the side reactions of Aldol condensation, including acetone formation and ethoxyethanol formation on MgO(1 0 0) and MgO(1 1 0) surfaces, and the corresponding energy barriers, Eas, and reaction heat, ΔE, were calculated. On this basis, the action mechanism and effects of different MgO surfaces on the two side reactions were studied and compared. The results showed that the formation mechanism of acetone is different on different crystal planes of MgO. Acetone is formed by 3-hydroxybutyraldehyde by four steps on MgO(1 0 0) surface and CH₃CH(O)CH₂CHO species by three steps on MgO(1 1 0) surface, which is difficult to occur on both MgO(1 0 0) surface and MgO(1 1 0) surface. The ethoxyethanol formation reaction has higher activity on both MgO surfaces. Based on a comprehensive comparison of the energy barriers and reaction heat of each elementary reaction on the two surfaces, we find that no matter in terms of kinetics or thermodynamics, the side reactions are more likely to occur on the MgO(1 0 0) surface.