Aqueous Phase Hydroalkylation and Hydrodeoxygenation of Phenol by Dual Functional Catalysts Comprised of Pd/C and H/La-BEA

Aqueous phase catalytic phenol hydroalkylation and hydrodeoxygenation have been explored using Pd/C combined with zeolite H-BEA and La-BEA catalysts in the presence of H₂. The individual steps of phenol hydrogenation, cyclohexanol dehydration, or alkylation with phenol were individually investigated to gain insight into the relative rates in the cascade reactions of phenol hydroalkylation. The hydroalkylation rate, determined by the concentrations of phenol and cyclohexanol in phenol hydroalkylation, required the hydrogenation rate to be relatively slow. The optimized H⁺/Pd ratio was 21, which allowed achieving comparable cyclohexanol formation rates via phenol hydrogenation and consumption rates from alkylation with phenol in phenol hydroalkylation. La-BEA was shown to be more selective for hydroalkylation than H-BEA in combination with Pd/C, because cyclohexanol dehydration was retarded selectively compared to alkylation of phenol. This indicates that dehydration is solely catalyzed by Brønsted acid sites, while alkylation can be achieved in the presence of La³⁺ cations

Dynamic Phase Separation in Supported Pd–Au Catalysts

SiO₂-supported Pd–Au catalysts with Pd/Au molar ratios varying from 0.8 to 7.0 were used as catalysts for vinyl acetate synthesis under industrial conditions. Continued operation of the bimetallic catalysts at 150 °C led to the formation of the Pd₁Au₁ phase in the particles, with the remaining Pd atoms forming Pd nanoparticles by leaching of Pd as acetate. The presence of these phases was monitored by X-ray absorption spectroscopy (XAS) of the used catalysts. Temperature-resolved in situ XRD of the reduced samples in an inert atmosphere confirmed the phase separation into a Pd-rich phase and a Au-rich phase above 160 °C. CO adsorption and XRD of the catalysts used at 180 °C showed that phase separation also took place during vinyl acetate synthesis. The pronounced temperature dependence of the morphology and surface composition of the bimetallic Pd–Au catalysts determines the selectivity; the activity; and, in particular, the stability during vinyl acetate synthesis

Ni-Catalyzed Cleavage of Aryl Ethers in the Aqueous Phase

A novel Ni/SiO₂-catalyzed route for selective cleavage of ether bonds of (lignin-derived) aromatic ethers and hydrogenation of the oxygen-containing intermediates at 120 °C in presence of 6 bar H₂ in the aqueous phase is reported. The C–O bonds of α-O-4 and β-O-4 linkages are cleaved by hydrogenolysis on Ni, while the C–O bond of the 4-O-5 linkage is cleaved via parallel hydrogenolysis and hydrolysis. The difference is attributed to the fact that the C_aliphatic–OH fragments generated from hydrolysis of α-O-4 and β-O-4 linkages can undergo further hydrogenolysis, while phenol (produced by hydrolysis of the 4-O-5 linkage) is hydrogenated to produce cyclohexanol under conditions investigated. The apparent activation energies, <i>E</i>_a(α-O-4) < <i>E</i>_a(β-O-4) < <i>E</i>_a(4-O-5), vary proportionally with the bond dissociation energies. In the conversion of β-O-4 and 4-O-5 ether bonds, C–O bond cleavage is the rate-determining step, with the reactants competing with hydrogen for active sites, leading to a maximum reaction rate as a function of the H₂ pressure. For the very fast C–O bond cleavage of the α-O-4 linkage, increasing the H₂ pressure increases the rate-determining product desorption under the conditions tested

Atomistic Engineering of Catalyst Precursors: Dynamic Reordering of PdAu Nanoparticles during Vinyl Acetate Synthesis Enhanced by Potassium Acetate

The presence of potassium acetate (KOAc) on bimetallic PdAu catalysts increases the rate of reaction for vinyl acetate (VA) formation from ethene and acetic acid by a factor of 10 and the selectivity by 20%. The dynamic transitions of typical supported catalyst precursors with an atomic Pd/Au ratio of 2/1 were explored during synthesis in the presence and absence of KOAc. The dopant induces reordering of PdAu toward a Pd₁Au₁ phase, while Au-enriched Pd₄₀Au₆₀ bimetallic particles form primarily in the absence of KOAc. Pd–acetate species are generated via leaching of Pd from PdAu precursor particles during the reaction. These species are Pd₃(OAc)₆ and Pd₂(OAc)₄ in the absence of KOAc and K₂Pd₂(OAc)₆ in the presence of KOAc. Palladium in K₂Pd₂(OAc)₆ can be readily reduced by C₂H₄ to Pd⁰, while Pd₃(OAc)₆, which contains more stable, bridged acetate ligands remains stable. Reduced Pd either forms dispersed Pd⁰ or is incorporated into the metal particles. KOAc enhances rates and selectivity to VA by stabilizing, on the one hand, active Pd species at the bimetallic surface. On the other hand, KOAc enriches acetic acid close to the surface and forms Pd surface acetates, postulated to enhance the rate and the selectivity to VA by suppressing ethylene adsorption and oxidation

Oxidative Dehydrogenation of Ethane on Dynamically Rearranging Supported Chloride Catalysts

Ethane is oxidatively dehydrogenated with a selectivity up to 95% on catalysts comprising a mixed molten alkali chloride supported on a mildly redox-active Dy₂O₃-doped MgO. The reactive oxyanionic OCl^– species acting as active sites are catalytically formed by oxidation of Cl^– at the MgO surface. Under reaction conditions this site is regenerated by O₂, dissolving first in the alkali chloride melt, and in the second step dissociating and replenishing the oxygen vacancies on MgO. The oxyanion reactively dehydrogenates ethane at the melt–gas phase interface with nearly ideal selectivity. Thus, the reaction is concluded to proceed via two coupled steps following a Mars-van-Krevelen-mechanism at the solid–liquid and gas–liquid interface. The dissociation of O₂ and/or the oxidation of Cl^– at the melt–solid interface is concluded to have the lowest forward rate constants. The compositions of the oxide core and the molten chloride shell control the catalytic activity via the redox potential of the metal oxide and of the OCl^–. Traces of water may be present in the molten chloride under reaction conditions, but the specific impact of this water is not obvious at present. The spatial separation of oxygen and ethane activation sites and the dynamic rearrangement of the surface anions and cations, preventing the exposure of coordinatively unsaturated cations, are concluded to be the origin of the surprisingly high olefin selectivity

Stabilizing Catalytic Pathways via Redundancy: Selective Reduction of Microalgae Oil to Alkanes

A new route to convert crude microalgae oils using ZrO₂-promoted Ni catalysts into diesel-range alkanes in a cascade reaction is presented. Ni nanoparticles catalyze the selective cleavage of the C–O of fatty acid esters, leading to the hydrogenolysis of triglycerides. Hydrogenation of the resulting fatty acids to aldehydes (rate-determining step) is uniquely catalyzed via two parallel pathways, one via aldehyde formation on metallic Ni and the second via a synergistic action by Ni and ZrO₂ through adsorbing the carboxylic groups at the oxygen vacancies of ZrO₂ to form carboxylates and subsequently abstracting the α-hydrogen atom to produce ketene, which is in turn hydrogenated to aldehydes and decarbonylated on Ni nanoparticles

Formation of CO₂ and Ethane from Propionyl over Platinum: A Density Functional Theory Study

The conversion of propionyl, the most stable dehydrogenation product of 1-propanol on flat and stepped Pt(111) surfaces is explored computationally as a model for the transformation of alcohols on Pt to CO₂ and hydrocarbons. We studied a reaction network in which dehydrogenation steps or hydroxyl insertion may precede the desired C–C cleavage, via either decarboxylation or decarbonylation. In the latter case, CO₂ will be obtained via a subsequent water-gas-shift reaction. On a flat surface, the decarbonylation pathway with the highest barrier of 91 kJ mol^–1 was calculated to be preferred, as the decarboxylation reactions are inaccessible because of high barriers of the preceding dehydrogenations, 129 kJ mol^–1, or the C–C bond scission itself, 204 kJ mol^–1. At step defects the highest barrier for decarboxylation was determined to be notably lower, at 86 kJ mol^–1, making this pathway competitive for C–C scission

Mechanism and Kinetics of CO₂ Adsorption on Surface Bonded Amines

The impact of H₂O on the mechanism and kinetics of CO₂ adsorption by amine-impregnated SBA-15 has been investigated. The H₂O-free adsorption mechanism proceeds predominantly via formation of carbamates stabilized between two amine groups. Bicarbonates and surface stabilized carbamates were formed on hydrated sorbents. Film diffusion limitations were not observed during adsorption of CO₂ at high amine loadings. Gaseous water decreased the adsorption rate but increased the maximum equilibrium uptake as well as the uptake before breakthrough of the reactor bed. A water film is formed on the adsorbent particles in gas streams containing more than 5 vol %, gaseous H₂O limiting the interaction of CO₂ with the active amine sites and constraining the rates of adsorption. The higher uptake of CO₂ in the presence of H₂O vapor is a result of a change in the adsorption mechanism that increases the amine efficiency, thus leading to a higher adsorption capacity. The adsorption was fully reversible for all adsorbents at a maximum desorption temperature of 100 °C

Characterization of Fe-Exchanged BEA Zeolite Under NH₃ Selective Catalytic Reduction Conditions

Structural changes to Fe³⁺ cationic species in Fe-exchanged zeolite BEA during the selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃ were probed by UV–vis spectroscopy. The distribution between Fe²⁺ and Fe³⁺ species was characterized by IR spectroscopy of adsorbed CO. Upon heating to 723 K, some of the Fe cations formed Fe–O–Fe bonds that underwent reversible structural transformation under NH₃–SCR conditions. The in situ formed Fe oxide clusters could be dissociated to isolated Fe cations at 423 K, while at higher temperatures O-bridged Fe clusters were again formed. The structure of the Fe cluster is related to the Al distribution in the zeolite probed by Co²⁺ ion exchange. We propose here that two Fe cations bound within one six-membered ring containing an Al pair form hydroxylated dimeric Fe–O–Fe in the zeolite. This was supported by a structure simulation of a binuclear [HO-Fe­(III)–O–Fe­(III)–OH] model

Pathways for H₂ Activation on (Ni)-MoS₂ Catalysts

The activation of H₂ and H₂S on (Ni)­MoS₂/Al₂O₃ leads to the formation of SH groups with acid character able to protonate 2,6-dimethylpyridine. The variation in concentrations of SH groups induced by H₂ and H₂S adsorption shows that both molecules dissociate on coordinatively unsaturated cations and neighboring S^2–. In the studied materials, one sulfur vacancy and four SH groups per 10 metal atoms exist at the active edges of MoS₂ under the conditions studied. H₂–D₂ exchange studies show that Ni increases the concentration of active surface hydrogen by up to 30% at the optimum Ni loading, by increasing the concentration of H₂ and H₂S chemisorption sites