Catalytic conversion of ethanol to 1,3-butadiene on MgO: A comprehensive mechanism elucidation using DFT calculations

• Density Functional Theory calculations of ethanol to form 1,3-butadiene were performed.
• Undoped MgO was used as a model catalyst with Mg3c active site.
• Aldol condensation, Prins condensation and 1-ethoxyethanol pathways explored.
• Aldol condensation was found a viable route from free energy calculations at 723 K.

In this work, we performed periodic Density Functional Theory calculations and explored reactive pathways of ethanol catalysis to catalytically form 1,3-butadiene on undoped MgO surface. We have identified critical reactive intermediates, as well as thermodynamic and kinetic barriers involved in the overall reactive landscape. The overall free energy surface was explored for the highly debated reaction mechanisms, including Toussaint’s aldol condensation mechanism, Fripiat’s Prins mechanism and mechanism based on Ostromislensky’s hemiacetal rearrangement. Thermodynamics and kinetics data calculated showed four rate limiting steps in the overall process. In particular, ethanol dehydration to form ethylene possessed lower energy barrier than dehydrogenation to yield acetaldehyde suggesting competing reactive pathways. CC bond coupling to form acetaldol (3-hydroxybutanal) is preceded with 16 kcal/mol forward reaction barrier. Direct reaction of ethylene and acetaldehyde proceeds with a free energy barrier of 29 kcal/mol suggesting that Prins condensation is an alternative route. Finally, thermodynamic stability of 1-ethoxyethanol prevents further reaction via hemiacetal rearrangement. The results here provide a first glimpse into the overall 1,3-butadiene formation mechanism on undoped MgO reactive sites in light of the vast literature discussing variety of the proposed mechanistic pathways mostly based on conventional homogenous organic chemistry reactions.

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