Palladium carbon poison

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

Figure 3.1 The composition of reaction mixture as a function of reaction time in the hydrogenation of 1,5-cyclooctadiene over unpoisoned (a), phenylacetaldehyde-poisoned (b), and carbon monoxide-poisoned (c) palladium catalysts. The points are experimental values, and the curves show the simulations using the values given in Table 3.4. For the reaction conditions, see footnote b in Table 3.4. (Key 1,5-COD 1,4-COD O COE 9 COA. (For abbreviations, see Scheme 3.9.) (FromHigashijima, M. Ho, S.-M. Nishimura, S. Bull. Chem. Soc. Jpn. 1992, 65, 2960. Reproduced with permission of Chemical Society of Japan.)...

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

Hydrogenolysis of the acetylene-cumulenes 19 can be achieved by means of a partially poisoned palladium catalyst to yield the regular [22]porphyrin(2.2,2.2) 20 with cisjrans.cis,-trans configuration of the two-carbon bridges. 

We have studied the hydrogenolysis of 2-(perfluorohexyl)ethane thiocyanate to 2-(perfluorohexyl)ethane thiol. It was discovered that perfluoroalkyl thiocyanates can be reduced to thiols and co-product hydrogen cyanide with molecular hydrogen in the presence of a carbon-supported palladium-tin catalyst. This result is surprising since it is known that palladium and other gronps 8 to 10 metal catalysts are poisoned by the product thiol, traces of hydrogen snlfide byprodnct, and the hydrogen cyanide co-product. For that reason, we characterized the catalyst to understand why it was so robust under conditions that would normally poison snch a catalyst. 

It is apparent that a new synthetic methodology, preferably catalytic, is needed for the synthesis of this important class of 2-(perfinoroalkyl)ethane thiols. In this context, a variety of catalysts was examined to determine if they wonld catalyze the hydrogenolysis of 2-(perfinorohexyl)ethane thiocyanate. In the conrse of this study, much to our surprise, it was discovered that a carbon supported Pd-Sn would catalyze the reaction. It is known that palladium and other group Vtll metal catalysts are poisoned by the product thiol, traces of hydrogen sulfide byproduct, and the hydrogen cyanide co-prodnct (6), but our observations are that this catalyst is surprisingly robust in the reaction medium. 

It is well known that palladium on carbon catalysts are poisoned by hydrogen cyanide and thiol products or hydrogen sulfide (6). Therefore, it was of interest to investigate the reduction of perfluoroalkyl thiocyanates as a function of tin concentration, keeping the concentration of palladium and reaction conditions constant. Figure 15.1 delineates the % conversion vs. Sn/Pd ratio, under the same reaction conditions of 175°C, 700 psig H2 for 2 hours with 5% Pd on carbon catalysts in ethyl acetate solvent at a 1000 1 substrate catalyst molar ratio. The increase in... 

Lindlar s catalyst metallic palladium deposited on calcium carbonate and is poisoned with lead acetate and quinoline. 

Z)-Methyl styrene was easily obtained by hydrogenation of 1-phenyl-1-propyne using Lindlar s catalyst (5% palladium on calcium carbonate, poisoned with lead) in //-hexane under an atmospheric pressure of hydrogen. The mixture, containing 90% of (Z)-methyl styrene and 10% of the overreduced alkane, was used without further purification. 

As stated above, aliphatic amines are potent ligands for electrophilic transition metals and are efficient catalyst poisons in attempted alkene animation reactions. However, tosylation of the basic amino group greatly reduces its complexing ability, yet does not compromise its ability to nucleophilically attack complexed alkenes. Thus, a variety of alkenic tosamides efficiently cyclized under palladium(II) catalysis producing N-tosylenamines in excellent yield (equations 17 and 18).32 Again, this alkene amination proceeded through an unstable a-alkylpalladium(II) species, which could be intercepted by carbon monoxide, to result in an overall aminocarbonylation of alkenes. With ureas of 3-hydroxy-4-pentenyl-amines (Scheme 7), this palladium-catalyzed process was quite efficient but it was somewhat less so with... 

Tritiated GAs of very high specific radioactivity have been prepared by catalytic reduction. The 1,2 double bond of GAq can be selectively reduced, using a partially poisoned palladium catalyst, to give [1,2-3h2]GA] (7 -,(75,76), although some reduction of the 16,17 double bond and the lactone also occurs (76). Introduction of 3h at sites other than carbon atoms 1 and 2 has also been found (76). [3h]GAI has been prepared from GAy by a similar method JT7). [3h]GA was converted... 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

---