Mechanism for Acetone and Crotonaldehyde Production during Steam Reforming of Ethanol over La0.7Sr0.3MnO3-x Perovskite: Evidence for a Shared C4 Aldol Addition Intermediate

A mechanistic study was conducted on the catalytic conversion of ethanolover La0.7Sr0.3MnO3-xperovskite catalysts in the presence and absence of water. The studysought insights into the path of C-C coupling toward acetone and crotonaldehyde andalso into clarifying whether the lack of previous reports of C-C coupling overLa0.7Sr0.3MnO3-x(100) could be due to apressure gap. Several types of experiments wereperformed at 400-800 K:flow experiments with a torr range reactant gasflown overLa0.7Sr0.3MnO3-xpowders; ultra-high vacuum experiments with continuous gas exposuresto a La0.7Sr0.3MnO3-x(100) single-crystal sample; and torr range continuous gas exposuresto a La0.7Sr0.3MnO3-x(100) single-crystal sample. When ethanol and water wereflown over La0.7Sr0.3MnO3-xpowders at 400-800 K,the products detected were ethene, acetaldehyde, acetone, crotonaldehyde, CO, CO2, and H2. Acetone was catalytically producedover both the La0.7Sr0.3MnO3-xpowder and the La0.7Sr0.3MnO3-x(100) single-crystal sample at temperatures of 700-800 K whenreaction conditions were on the order of 1 Torr of reactant gas and with an excess of water relative to ethanol (1 ethanol/9 water).Isotopic labeling with deuterium was used to gain insights into the C-C coupling reaction mechanism and paths in species withthree and four carbons (C3 and C4 species). Additionally, steady-state isotopic transient kinetic analysis (SSITKA) experiments +simulations using carbon labeling of the ethanol feed were performed. Three mechanistic paths were considered for the C-Ccoupling step: thefirst two paths, A and B, involve coupling between two intermediates which are both in oxygen vacancies; and thethird path, C, involves coupling between one intermediate in an oxygen vacancy and one intermediate outside of an oxygen vacancy.The results suggest that the dominant path to the C3 product, acetone, depends on the conditions. The less active path (attributedto path A or B) occurs at 600-700 K and involves coupling between two irreversibly bound species. The more active path(attributed to path C) requires an excess of water, becomes dominant at 600-800 K, and involves coupling between one irreversiblybound species and one reversibly bound species. Based on these various observations from experiments and simulations, anelementary step is proposed for acetone formation involving a previously unreported C4 transition state that is formed after aldoladdition. Density functional theory calculations were performed based on this hypothesis, and it confirmed that this specific andpreviously unreported aldol addition path to acetone does exist and that this path consistent with the experimental data. In this path,C-C formation occurs to create a C4 intermediate that is bound to an oxygen vacancy, then a hydrogen transfer with C-C bondbreaking occurs that results in the production of the acetone molecule. The proposed mechanism is also consistent with theexperimental observation that acetone formation has a greater thanfirst-order dependence on the water vapor pressure.

Automated summary

A mechanistic study was conducted on the catalytic conversion of ethanol over La0.7Sr0.3MnO3-x perovskite catalysts in the presence and absence of water. The study aimed to gain insights into the path of C-C coupling toward acetone and crotonaldehyde and to clarify the role of water in the reaction. The results provided important information on the mechanism of ethanol catalytic conversion and the formation of acetone and crotonaldehyde.