Palladium, supported hydrogen

The disadvantages of the general method using supported palladium and hydrogen are a lack of selectivity and overreduction. Where selectivity is not an important requirement, Pd/C is a widely used catalyst for the hydrogenolysis of the C-O bond in benzyl ethers. 

For this reason, these catalysts are also known under the name of supported hydrogen-bonded (SHB) catalysts and, in conjunction with Pd° particles on the same support material, have contributed to generate active heterogeneous systems for the hydrogenation of benzenes in aprotic solvents. Irrespective of the substrate, the combined single-site/dispersed-metal catalyst RhI-Pd0/SiO2 shown in Figure 16.5a was from four- to six-fold more active than supported palladium... 

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) ... 

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. 

Even though the versatility of palladium in hydrogenation reactions is recognized, the explanation of its catalytic properties is still far from being satisfactory. The chapter by Z. Karpinski gives a comprehensive survey of Catalysis by Supported, Unsupported, and Electron-Deficient Palladium. ... 

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). 

A highly stereoselective synthesis of ( )-gephyrotoxin (20) has been carried out. An interesting feature is the reversal of the stereochemical course of the hydrogenation of the vinylogous amide (21) by the use of an alumina support. Hydrogenation of (21) over palladium on charcoal gives the amino-alcohol (22) as... 

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]. 

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... 

The results of this experiment are shown in Fig. 1.7, which displays the plots of the rate constants for ethylidyne removal from both clean Pd(l 11) (A) and alumina-supported palladium ( ) as a function of hydrogen pressure. The values for these two samples are in good agreement, in accordance with the observation that a Pd(lll) single crystal is a good model for the supported hydrogenation catalyst (see below). 

Thicker, self-supporting, dense metal membranes are known. These are tubular and are usually commercially successful palladium-silver hydrogen separation membranes were of this type.21 Currently, Power and Energy, Inc. also fabricates this type of membrane, although planar membranes are more common due to easier fabrication and a greater variety of fabrication methods. 

In the liquid phase at room temperature, using alcohol as a solvent and palladium supported on barium sulfate as catalyst, the only products observed from 1-butyne hydrogenation were 1-butene (98%) and n-butane (2%) (57). The gas phase reaction using 0.03% palladium on alumina catalyst gave 1-butene (99.1%), cis- and product distributions were maintained until at least 76% removal of the parent hydrocarbon but isomerization and hydrogenation of the 1-butene occurred after complete removal of the alkyne. Thus, l-butjme must displace 1-butene from the surface before its isomerization can occur, and it must prohibit the re-entry of 1-butene into the reacting surface layer. This represents the operation of a powerful thermodynamic factor. 

The hydrogenation of nitrobenzene was carried out on palladium supported on the carbon nanofibers prepared according to the preceding method without further purification and compared to a commercial palladium catalyst supported on a high surface area activated charcoal (Aldrich, 970 m /g). 

Platinum and palladium supported on carbon give a higher activity and selectivity as compared to Ru catalyst. The different products distribution observed on this latter catalyst is related to a different rate of hydrogenation of the intermediates. 

E. Joannet, C. Homy, L. Kiwi-Minsker, A. Renken, Palladium supported on filamentous active carbon as effective catalyst for liquid-phase hydrogenation of 2-butyne-1,4-diol to 2-butene-l,4-diol, Chem. Eng. Sci. 57 (2002) 2453. 

The partial hydrogenation of dienes was successfully carried out by Michalska et al. [21] using palladium supported on heterocyclic polyamides. Under the reaction conditions employed (MeOH, atmospheric pressure, 25 °C) the resin-sup-ported catalyst was able to selectively hydrogenate one of the two double bonds present. Recycling experiments proved the high stability of the used catalysts. For example, in 11 hydrogenation runs with 2-methyl-l,3-pentadiene, which is equivalent to 4300 catalytic cycles per palladium atom, neither loss of activity nor changes in selectivity were observed. 

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