Revealing the Mechanistic Details for the Selective Deoxygenation of Carboxylic Acids over Dynamic MoO₃ Catalysts

Abstract

The selective activation of renewable carboxylic acids could enable the formation of a variety of highly valuable renewable products, including surfactants, valuable dienes, and renewable hydrogen carriers. A kinetic study is performed to enhance understanding of the selective deoxygenation of carboxylic acid on promoted MoO 3 at mild temperatures. Although several studies indicate that deoxygenation of oxygenated biomass-derived compounds on MoO 3 occurs via a reverse Mars−van Krevelen mechanism, this study suggests that the deoxygenation of pentanoic acid (PA) on an oxygen vacancy can also be explained by a Langmuir−Hinshelwood mechanism. A detailed analysis of the experimental data indicates that the incorporation of Pt on MoO 3 shifts the reaction order with respect to hydrogen from 1 to 0.5 at a low partial pressure of PA. We reveal that the rate-determining step (RDS) shifts upon the incorporation of Pt from H 2 dissociation to H addition to adsorbed acid molecules. We further illustrate how the RDS can shift as a function of PA coverage. The inhibition effect of PA and its possible causes are discussed for both MoO 3 and 0.05 wt % Pt/MoO 3 catalysts. Here, we decouple promoter effects from the creation of highly active sites located at the Pt/MoO 3 interface. The nature of the active site involved upon Pt incorporation is also studied by separating Pt from MoO 3 at a controlled distance using carbon nanotubes as hydrogen bridges, confirming that the kinetically relevant role of Pt is to serve as a promoter of the MoO 3 .

Kinetic Analysis of the Hydrodeoxygenation of Aliphatic Volatilized Lignin Molecules on Bulk MoO3: Elucidating the Formation of Alkenes and Alkanes

“…By splitting the HDO rate into individual values for the primary propylene and propane products, nearly indistinguishable acetone dependencies of −0.22 ± 0.11 and −0.16 ± 0.13, respectively, were observed. This negative order for the deoxygenation pathway agrees with recent reports of a −0.22 reaction order for the partial deoxygenation of pentanoic acid to pentanal over bulk MoO 3 for similar partial pressure ranges. , Notably, these observed negative orders for aliphatic molecules on MoO 3 are in contrast with the kinetics previously observed for anisole and phenol with MoO 3 /ZrO 2 , as well as 2-propanol and m -cresol on oxygen-modified MoC 2 . ,, As the reaction temperature increased to 380 °C, the acetone order for HDO and aldol condensation increased to 0.02 ± 0.06 and 1.07 ± 0.04 over the same pressure range, indicating a shift in the equilibrium of the most abundant reactive intermediate(s) (MARI) on the molybdenum oxide surface. The consistently observed order difference of ∼1 for the two reactions under multiple conditions indicated that the pathways compete for the same surface sites.…”