Acrylic acid hydrodeoxygenation reaction mechanism over molybdenum carbide studied by DFT calculations
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Platinum- and palladium-based catalysts are commonly used in hydrogenation reactions, but they present a great disadvantage of being quite expensive. In most cases, they can be substituted by cheaper alternative catalysts formed by transition metal carbides, such as molybdenum carbide (Mo2C). Among the reactions that can be catalyzed by Mo2C, hydrodeoxygenation (HDO) presents a great technological interest, especially in biofuel production. Nonetheless, the selectivity of carbides in HDO reactions of fatty acids is not well understood yet. In the present work, the reaction mechanism of the acrylic acid HDO over Mo2C, a fatty acid model molecule, was studied by density functional theory (DFT), with Perdew-Burke-Ernzerhof (PBE) functional and periodic boundary conditions. A global mechanism is proposed, divided in four steps, from acrylic acid to propane. In the first reaction step, decomposition by C–OH bond cleavage, with 24 kcal mol− 1 of activation energy, dominates over C=C and C=O hydrogenation. This result is in line with the absence of propanoic acid among the products and the formation of acrolein, as shown in an experimental work previously published. The proposed global mechanism is in fair agreement with the experimental findings. The main product is propane, which has the same number of carbon atoms of the reactant. This mechanism can be viewed as a model for HDO of any fatty acid catalyzed by Mo2C, since acrylic acid has the minimal structural features of fatty acids, i.e., a carboxyl group and a C=C double bond.
KeywordsDFT HDO Molybdenum carbide Fatty acid
The authors also acknowledge the Laboratório Nacional de Computação Científica (LNCC) for computational support of SDumont supercomputer under project ID 25972.
The authors acknowledge Conselho Nacional de Desenvolvimento e Pesquisa (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support.
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