Palladium, supported support

The catalyst commonly used in this method is 5 wt % palladium supported on barium sulfate inhibited with quinoline—sulfur, thiourea, or thiophene to prevent reduction of the product aldehyde. A procedure is found in the Hterature (57). Suitable solvents are toluene, benzene, and xylene used under reflux conditions. Interestingly, it is now thought that Rosenmund s method (59) originally was successful because of the presence of sulfur compounds in the xylene used, since the need for an inhibitor to reduce catalyst activity was not described until three years later (60). 

New Synthesis of Optically Active (L)-Alaninol over Palladium Supported Catalysts... 

Once the reaction kinetics were determined and the appropriate reaction conditions were set, the analyses of different catalysts was launched. A variety of different heterogeneous catalysts were evaluated for the deprotection of 20 mmol Cbz-glycine (1, R=-H). Catalysts composed of different platinum group metals supported on activated carbon were evaluated as well as palladium supported on alumina. Figure 5 confirms that Pd supported on activated carbon is indeed the catalyst of choice for this type of hydrogenolysis reaction. 

Rossi, L.M., Nangoi, I.M. and Costa, N.J.S. (2009) Ligand-assisted preparation of palladium supported nanopartides a step toward size control. Inorganic Chemistry, 48 (11), 4640-4642. 

S. Sekimoto, H. Nakagawa, S. Okazaki, K. Fukuda, S. Asakura, T Shigemori, and S. Takahashi, A fiber-optic evanescent-wave hydrogen gas sensor using palladium-supported tungsten oxide. Sensors Actuators B 66(1-3), 142-145 (2000). 

Both uncalcined and calcined LDHs have also been shown to be effective supports for noble metal catalysts [18-25]. For example, palladium supported on Cu/Mg/Al LDHs has been used in the liquid phase oxidation of limonene [24], and on calcined Mg/Al LDHs for the one-pot synthesis of 4-methyl-2-pentanone (methyl isobutyl ketone) from acetone and hydrogen at atmospheric pressure [25]. In the latter case, the performance depends on the interplay between the acid-base and hydrogenation properties. More recently. 

The actual selectivity depends on the nature of the catalyst. For example, the following data were reported for n-hexane transformed over platinum and palladium supported on the same alumina 44) (pulse system, hydrogen carrier gas, T = 520°C) ... 

With platinum and palladium supported on acidic alumina, cyclopentanes are important intermediates of aromatization (44, 123-124). For example, n-heptane gave about 2-3 times more aromatic product than 2,4-dimethyl-pentane, whereas the formation of C5 cyclic products was about the same from both alkanes. Alkylcyclopentanes aromatized at a reasonable rate (123a). 

Nicolau et al. (187) have reported EPR absorption of platinum and palladium supported on charcoal. These workers attribute a sharp resonance, centered at p = 2.0034, to a species resulting from the interaction of the metal and the charcoal. The intensity of the EPR signal increases... 

The variations of CH4 combustion activity associated with the PdO-Pd reversible transformation are responsible for the unique thermostating ability of palladium-supported catalysts. Indeed, in the adiabatic combustion of CH4, the... 

A strong palladium-support interaction was found to be crucial to the overall activity of the catalyst. 

The exchange of a number of compounds in this category with deuterium has been examined by Burwell and his colleagues. n-Heptane has been exchanged over nickel-keiselguhr (43), reduced nickel oxide (29), a series of nickel catalysts of varying crystallite size (37), and over palladium supported on 7-alumina (43). Less extensive studies were also made with 2,3-dimethylbutane (29, 43) and n-hexane (42). 

Adsorption and Catalytic Properties of Palladium Supported by Silica, Alumina, Magnesia, and Amorphous and Crystalline Silica-Aluminas... 

Measurement of the Dispersion of the Metallic Phase. The dispersion of palladium was measured by H2-02 titration (12-14) using gravimetry. For palladium supported by amorphous supports, good agreement... 

Colloidal palladium or platinum supported on chelate resin beads were employed for the stereoselective hydrogenation of olefins 86). Colloidal palladium supported on iminodiacetic acid type chelate resin beads was prepared by refluxing the palladium chloride and the chelate resin beads in methanol-water. Using the resin-supported colloidal palladium as a catalyst, cyclopentadiene is hydrogenated to cyclopentene with 97.1% selectivity at 100 % conversion of cyclopentadiene under 1 atm of hydrogen in methanol at 30 °C. Finely dispersed metal particles ranging from 1 to 6 nm in diameter are the active species in the catalyst. 

Another indication that electronic properties of Pd may be important in hydrogenation reactions originates from the work of Carturan et al. (167), who investigated palladium supported on vitreous materials in hydrogenation of phenylacetylene. A relatively better catalytic activity of the catalyst with smaller alkaline content (Na20) suggests that an electron transfer from Pd to the support is smaller in the case of less acidic (containing more alkaline) supports. Similar metal particle sizes (2.8-3.4 nm) exhibited by all the catalysts rule out an explanation that takes into account a surface sensitivity of this reaction. 

The hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraqumone (THEAQ) at the oxygen in the presence of a palladium supported catalyst is a key step in the industrial production of hydrogen peroxide. In industrial plants, the performance of the catalyst slowly decreases because of deactivation. Two types of catalyst poisoning are operative, a reversible one, related to the presence of water, and a permanent one, probably due to the condensation of two or more anthraquinone molecules on the palladium surface. The kinetic data obtained from laboratory runs are used to simulate the performance in industrial plants. 

G. Larsen, E. Lotero, R. D. Parra, L. M. Petkovic, H. S. Silva, and S. Raghavan, Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n - butane isomerzation reaction in the presence of hydrogen, Appl. Catal A 130, 213-226 (1995). 

Calcined LDHs may also serve as effective catalyst supports. Davis and Derouane prepared, for example, a supported-platinum catalyst for aromatiza-tion of n-hexane, via the impregnation of a calcined MgAl-LDH with an aqueous solution of Pt(NH3)4Cl2 [203]. More recently, Narayanan and Krishna have studied a system of palladium supported on calcined MgAl-LDHs for phenol hydrogenation [204-207]. 

Aramendia, M. A., Borau, V., Garcia, I. M., Jimenez, C., Marinas, J. M. and Urbano, F. J. (1999) Influence of the reaction conditions and catalytic properties on the liquid-phase hydrobromi-nation of bromobenzene over palladium supported catalysts activity and deactivation. Appl. Catal. B Environ. 20, 101-110. 

Several heterogeneous catalysts have been developed for the hydroxylation of alkanes under mild conditions.68,69 One of them is the bi-catalytic system, which combines the ability of palladium to convert hydrogen and oxygen to hydrogen peroxide, with the capability of the iron ions to activate the hydrogen peroxide to hydroxylate hydrocarbons.70 Iron oxide and palladium supported on silica have been used as efficient catalysts for the oxidation of cyclohexane to the alcohol and ketone, via the in situ generation of hydrogen peroxide in an acetone solvent.71... 

Operating under supercritical conditions allows the homogeneous mixing of various reactants at high concentrations. For example, palladium supported on polysiloxanes catalyzes the hydrogenation of cyclohexene in supercritical C02 significantly faster than in a common liquid phase.180... 

Palladium supported on barium sulfate is an efficient catalyst in the hydrogeno-lysis of benzyl hydroxamates to give the corresponding hydroxamic acids [Scheme 4.134].249 With palladium on charcoal, reduction of the N-O bond was observed. 

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