Oxidation, Reduction, and Condensation of Alcohols over (MO₃)₃ (M = Mo, W) Nanoclusters

Abstract

The reactions of deuterated methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, 2-butanol, and tert-butanol over cyclic (MO3)3 (M = Mo, W) clusters were studied experimentally with temperature-programmed desorption and theoretically with coupled cluster CCSD(T) theory and density functional theory. The reactions of two alcohols per M3O9 cluster are required to provide agreement with experiment for D2O release, dehydrogenation, and dehydration. The reaction begins with the elimination of water by proton transfers and forms an intermediate dialkoxy species that can undergo further reaction. Dehydration proceeds by a β-hydrogen transfer to a terminal MO. Dehydrogenation takes place via an α-hydrogen transfer to an adjacent MoVIO atom or a WVI metal center with redox involved for M = Mo and no redox for M = W. The two channels have comparable activation energies. H/D exchange to produce alcohols can take place after olefin is released or via the dialkoxy species, depending on the alcohol and the cluster. The Lewis acidity of the metal center with WVI being larger than MoVI results in the increased reactivity of W3O9 over Mo3O9 for dehydrogenation and dehydration. However, the product selection of aldehyde or ketone and olefin is determined by the reducibility of the metal center. Our calculations are consistent with the experiment in terms of the dehydrogenation, dehydration, and H/D exchange reactions. The condensation reaction requires a third alcohol with the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas-phase Brønsted acid. This Brønsted acid-driven reaction is different from the dehydrogenation and dehydration reactions that are governed by the Lewis acidity of the metal center.

Influence of Cluster–Support Interactions on Reactivity of Size-Selected NbxOy Clusters

“…84,85 Recently, the dehydration of 2-propanol was observed on W 3 O 9 clusters on TiO 2 (110) and attributed to strong Lewis acid sites of W 6+ with oxygen double bonds, 86 and on a graphene monolayer on Pt(111), W 3 O 9 showed a higher reactivity for dehydration compared to Mo 3 O 9 because W 6+ is a stronger Lewis acid than Mo 6+ . 87 The results of our 2-propanol TPD experiments are shown in Figure 7. It is clear from Figure 7a that Nb 3 O 5 is active for dehydration of 2-propanol while Nb 3 O 7 is not; there is a propene peak centered around 460 K for Nb 3 O 5 , but no such peak is observed for Nb 3 O 7 .…”

Investigating the Influence of Au Nanoparticles on Porous SiO2–WO3 and WO3 Methanol Transformation Catalysts

“…[1][2][3][4][5][6][7][8][9][10][11] These previous studies have established a good understanding of the performance of these catalysts as a function of WO 3 domain size and support effects. The acid site reactivity of WO 3 catalysts is commonly probed using the selective transformation of methanol (MeOH) to dimethyl ether (DME).…”

Unravelling Mechanistic Aspects of the Gas‐Phase Ethanol Conversion: An Experimental and Computational Study on the Thermal Reactions of MO2+ (M=Mo, W) with Ethanol

“…Comparing the reactivity of molybdenum and tungsten dioxides cations toward ethanol with that of neutral isolated (MoO 3 ) 3 and (WO 3 ) 3 clusters,, some dissimilarities are observed. While the ethanol conversion is governed overall by the Lewis acidity of the metal center, the preference of neutral (MoO 3 ) 3 for oxidative dehydrogenation, that is, acetaldehyde formation, and the bias for dehydration, that is, ethene formation, in the case of (WO 3 ) 3 can be explained by the different reducibility of the metal center.…”

“…Related to this, a closer inspection of the reactions of WO 3 films with propanol has revealed that alcohol dehydration proceeds efficiently only on O=W VI =O sites, whereas isolated W=O sites were shown to be inactive . In addition, Dixon and co‐workers have studied the ethanol conversion over cyclic neutral (MO 3 ) 3 (M=Mo, W) gas‐phase clusters, unravelling the chemical selectivity toward oxidative dehydrogenation to acetaldehyde and toward dehydration, respectively.…”