Palladium black catalyst

The three-phase fluidized-bed reactor with countercurrent mode of operation was used by Pruden and Weber 10 to study the hydrogenation of a-methyl styrene to cumene in the presence of palladium black catalysts. They used low gas velocities so that the gas was dispersed as bubbles in the slurry. They showed that the countercurrent mode of operation was better than the slurry operation (with no liquid flow), due to improved catalyst usage and improved gas holdup characteristics. 

Because of their low selectivity platinum black and palladium black catalysts are unsuitable after absorption of the calculated amount of hydrogen the product contains substantial amounts of the starting material and of the saturated compound alongside the olefin. 

The effects of antimony, tin, and lead additions to the palladium black catalyst was analyzed [110]. Accordingly, each adatom strongly promotes formic acid oxidatirai in an electrochemical cell and reduces the amount of CO poison that develops on the catalyst surface after 1 h of oxidation. The authors attributed this effect to the third body effect (steric effect) but did not discard an electronic effect regarding that a decrease in the CO binding energy on palladium due to the presence of the adatoms, using XPS technique, was observed. 

Problem 3-8 (Levd 1) An early study of the dehydrogenation of ethylbenzene to styrene contains the following comments concerning the behavior of a palladium-black catalyst ... 

Fokin, who was also interested in precious metal catalysts, had earlier used platinum or palladium blacks to hydrogenate oleic acid at ambient temperature and much lower pressures than those used by Ipatieff. Platinum or palladium black catalysts, which were finely divided metals containing some oxygen, were then... 

Pd-C, 1,4-cyclohexadiene, 25°, 1.5 h, 95-100% yield.Palladium black, a more reactive catalyst than Pd-C, must be used to cleave the more stable aliphatic benzyl ethers. 

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

To a solution of 180 parts of -benzyl N-benzyloxycarbonyl-L-aspartvI-L-phenylalanine methyl ester in 3,000 parts by volume of 75% acetic acid is added 18 parts of palladium black metal catalyst, and the resulting mixture is shaken with hydrogen at atmospheric pressure and room temperature for about 12 hours. The catalyst is removed by filtration, and the solvent is distilled under reduced pressure to afford a solid residue, which is purified by re-crystallization from aqueous ethanol to yield L-aspartyl-L-phenylalanine methyl ester. It displays a double melting point at about 190°C and 245°-247°C. 

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

Finely divided palladium (palladium-black) used as a hydrogenation catalyst is usually pyrophoric and needs appropriate handling precautions. 

After an induction period of ca. 9 hours, the maximum productivity was reached. This was followed by a decrease in activity, which cannot solely be explained by the lack of retention. Calculations showed that at least 20% of the catalyst should still remain in the reactor after 80 h. Additional to this wash-out effect, a deactivation process took place, visible by precipitation of palladium black on the surface of the membrane. Although the catalytic system suffered from deactivation, its selectivity towards 3-phenylbut-l-ene was excellent, being 98% and 85% for the G0- and Gr catalysts respectively. 

Addition of pvp significantly extends catalyst lifetime (Scheme 2). This is a well-known protocol whereby the Pd nanoparticles are wrapped up in the polymer chains, which prevents their aggregation into catalytically inactive palladium black. 

The same catalysts which permit the addition of elementary hydrogen to a double bond are able to accelerate the opposite process—dehydrogenation, or elimination of hydrogen—when the temperature is altered. Thus cyclohexane is decomposed into benzene and hydrogen when passed over nickel or palladium black at about 300° (Sabatier, Zelinsky). The equilibrium... 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

---