Palladium poison

Hydrogenation of an alkyne can be stopped at the alkene stage by using a poisoned (partially deactivated) catalyst made by treating a good catalyst with a compound that makes the catalyst less effective. Lindlar s catalyst is a poisoned palladium catalyst, composed of powdered barium sulfate coated with palladium, poisoned with quinoline. Nickel boride (Ni2B) is a newer alternative to Lindlar s catalyst that is more easily made and often gives better yields. 

As mentioned above, alkynes can be reduced to czR-alkenes by hydrogen in the presence of Lindlar Pd, Palladium poisoned by CaSO4 or BaSO4. 

It has been stated that thiourea (about 20 per cent, of the weight of the palladium - barium sulphate) may also be used as a catalyst poison. 

The palladium may be recovered by heating the spent catalyst to redness in order to remove organic impurities this treatment may reduce some of the barium sulphate to barium sulphide, which acts as a catalytic poison. The palladium is then dissolved out with aqua regia and the solution evaporated the residue is dissolved in hot water and hydrochloric acid to form palladium chloride. 

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... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

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... 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

Figure 8 X-ray elemental imaging in a field-emission STEM (a) EDS data of Pd /Ce /alumina catalyst particle poisoned with SO2 and (b) 128 X 128 digital STEM images formed using X-ray counts collected at each image pixel for aluminum, palladium, cerium, and sulfur. (Courtesy of North-Holland Publishers) ...

As catalyst for the Rosenmund reaction palladium on a support, e.g. palladium on barium sulfate, is most often used. The palladium has to be made less active in order to avoid further reduction of the aldehyde to the corresponding alcohol. Such a poisoned catalyst is obtained for example by the addition of quinoline and sulfur. Recent reports state that the reactivity of the catalyst is determined by the morphology of the palladium surface." ... 

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. 

The poisoning effect of hydrogen when dissolved in palladium was for the first time properly described and interpreted by Couper and Eley (29) in 1950 in their study of the fundamental importance of the development of theories of catalysis on metals. The paper and the main results relate to the catalytic effect of an alloying of gold to palladium, on the parahydrogen conversion. This system was chosen as suitable for attempting to relate catalyst activity to the nature and occupation of the electronic energy... 

The similar phenomenon of poisoning in situ of a palladium catalyst by hydrogen which was in this case the product of a reaction was observed by Brill and Watson (37). The reaction studied was the decomposition of formic acid... 

A similar reaction was studied by Kowaka Jfi) who investigated the catalytic activity of palladium and its alloys with silver in the hydrogenation of ethylene. The author alluded to the poisoning effect of hydrogen pretreatment of the palladium catalyst. 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

As mentioned previously in the introduction to the present review the ability to form the hydride phase is not characteristic solely of palladium or nickel. It would be of interest, therefore, to verify the results on the poisoning effect of hydride formation in the case of nickel or palladium by comparing with the other transition 3d, 4d, and 5d metals and the rare earth (4f) metals. 

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