7 Search Results

M-WO_x/TiO₂ (M = Pt-, Rh-, Pd-, and Ru) catalysts were prepared and studied for tetrahydrofuran-dimethanol (THFDM) hydrogenolysis. All catalysts have a small metal particle size (< 2 nm) but show catalytic activities that differ by two orders of magnitude. From a combination of CO chemisorption and STEM, we conclude that this wide gap in activity is primarily due to overgrowth of WO_x/TiO₂ covering the metal particle of the less-active catalysts. This overgrowth decreases the number of exposed hydrogenation sites in these catalysts, lowering the overall reaction rate. The catalyst with the highest activity (Pt-WO_x/TiO₂) was studied at various pressures ofmore » H₂, with catalytic activity passing through a maximum with increasing pressure. This is likely due to changes in the oxidation state of W^δ+ on the catalyst surface, which changes from W⁶⁺ to W⁵⁺ and W⁴⁺ with increasing hydrogen pressure. Moreover, H₂ can strongly adsorb on the catalyst surface and inhibit the activity at high hydrogen pressure. The apparent activation energy of THFDM conversion over Pt-WO_x/TiO₂ is 47 kJ·mol^-1. Pt-WO_x/TiO₂ showed a decrease in conversion from 45% to 23% over 46 h on stream in a continuous flow reactor. Catalyst deactivation is likely due to leaching of W. THFDM can be efficiently converted to 1,2,6-hexanetriol (HTO) in the presence of Pt/TiO₂ and homogeneous W salts.« less

We report on the oxidative dehydrogenation (ODH) activity of silica-supported boron oxide prepared via incipient wetness impregnation. Characterization of pristine and spent catalysts with infrared, Raman, and solid-state NMR spectroscopy reveals the presence of both isolated and aggregated oxidized boron sites. The results of these investigations, in combination with our earlier work on bulk boron-containing ODH catalysts (e.g., h-BN, metal borides, and elemental boron), give direct evidence that oxidized boron species formed in situ on the surface of these materials are responsible for the exceptional catalytic behavior. Furthermore, we anticipate that investigation of supported boron materials can provide insight intomore » the structural characteristics required for selective boron-containing ODH catalysts.« less

Hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNTs) were recently reported as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins in the gas phase. Previous studies revealed a substantial increase in surface oxygen content after exposure to ODH conditions (heating to ca. 500 °C under a flow of alkane and oxygen); however, the complexity of these materials has thus far precluded an in-depth understanding of the oxygenated surface species. In this contribution, we combine advanced NMR spectroscopy experiments with scanning electron microscopy (SEM) and soft X-ray absorption spectroscopy (XAS) to characterize the molecular structure ofmore » the oxygen functionalized phase that arises on h-BN and BNNTs following catalytic testing for ODH of propane. The pristine BN materials are readily oxidized and hydrolyzed under ODH reaction conditions to yield a phase consisting of three coordinate boron sites with variable numbers of hydroxyl and bridging oxide groups which is denoted B(OH)_xO_3-x (where x = 0-3). Evidence for this robust oxide phase revises previous literature hypotheses of hydroxylated BN edges as the active component on h-BN.« less

In recent years, the on-purpose production of 1,3-butadiene (BD) from renewable resources such as ethanol has received increased attention. In that context, Lewis acid catalysts play an important role, especially in the two-step process, i.e. when a mixture of acetaldehyde and ethanol is used as substrate. As the reaction mechanism consists of many intermediates and occurs over different catalytic functionalities, it is notoriously difficult to gain molecular-level insights into the mechanism. Here, we present a study on Lewis acidic Ta-BEA and propose a plausible reaction mechanism. We developed an operando DRIFTS-MS setup that allows for precise control and analysis ofmore » changes in the gas-phase composition as well as surface species. Using this tool, we found a surface intermediate with a vibrational frequency at 1690 cm^–1 that is only formed in the presence of both ethanol and crotonaldehyde and that is likely involved in the production of BD. Our data further suggests that a subtle control of the ratio of ethanol to acetaldehyde is crucial to keep a high ethoxy coverage and to desorb the intermediate crotyl alcohol in order to achieve high BD productivity. To the best of our knowledge, this is the first dynamic operando spectroscopic study on this re-emerging reaction.« less

Tetrahydropyran-2-methanol undergoes selective C–O–C hydrogenolysis to produce 1,6-hexanediol using a bifunctional RhRe (reducible metal with an oxophilic promoter) catalyst supported on Vulcan XC-72 carbon (VXC) with >90% selectivity. This RhRe/VXC catalyst is stable over 40 h of reaction in a continuous flow fixed bed reactor. The hydrogenolysis activity of RhRe/VXC is two orders-of-magnitude higher than that of RhRe supported on Norit Darco 12X40 activated carbon (NDC). STEM–EDS analysis reveals that, compared to the RhRe/VXC catalyst, the Re and Rh component metals are segregated on the surface of the low activity RhRe/NDC catalyst, suggesting that Rh and Re in close proximitymore » (“bimetallic” particles) are required for an active hydrogenolysis catalyst. Differences in metal distribution on the carbon surfaces are, in turn, linked to the properties of the carbons: NDC has both a higher surface area and surface oxygen content. Thus, the low areal density of Rh and Re precursors on the high area NDC and/or interactions of the precursors with its O functional groups may interfere with the formation of the bimetallic species required for an active catalyst.« less