Barium sulfate, catalyst support

Alkali moderation of supported precious metal catalysts reduces secondary amine formation and generation of ammonia (18). Ammonia in the reaction medium inhibits Rh, but not Ru precious metal catalyst. More secondary amine results from use of more polar protic solvents, CH OH > C2H5OH > Lithium hydroxide is the most effective alkah promoter (19), reducing secondary amine formation and hydrogenolysis. The general order of catalyst procUvity toward secondary amine formation is Pt > Pd Ru > Rh (20). Rhodium s catalyst support contribution to secondary amine formation decreases ia the order carbon > alumina > barium carbonate > barium sulfate > calcium carbonate. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

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). 

Catalysts reduced with formaldehyde carry no adsorbed hydrogen and are less pyrophoric. Barium carbonate as a support may sometimes be advantageous in that the neutrality of the h3 drogenation mixture may be maintained. Barium sulfate or barium carbonate may be a better support than carbon, which may, in some instances, so strongly adsorb the derived product that recovery is difficult or incomplete. Palladium may be more completely and easily recovered from a spent catalyst where carbon rather than barium sulfate is the support. In general, the submitter prefers a catalyst prepared according to procedure C. 

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." ... 

A great many materials have been used as catalyst supports in hydrogena-tion, but most of these catalyst have been in a quest for an improved system. The majority of catalyst supports are some form of carbon, alumina, or silica-alumina. Supports such as calcium carbonate or barium sulfate may give better yields of B in reactions of the type A- B- C, exemplified by acetylenes- cjs-olefins, apparently owing to a weaker adsorption of the intermediate B. Large-pore supports that allow ready escape of B may give better selectivities than smaller-pore supports, but other factors may influence selectivity as well. 

Many workers (5,6,7,87) have compared various metals for the selective hydrogenation of lower acetylenes to olefins, and it was always found that palladium was by far the most selective. This conclusion concurs with the usual synthetic experience, but under special circumstances other metals, such as platinum, may prove more useful (35,63). The catalyst support may also have an influence (21,65). Carbon, calcium carbonate, and barium sulfate are frequently used supports. Examples of some differences are noted later,... 

Support has been shown to influence selectivity and some workers have obtained higher yields of cis isomer over palladium-on-calcium carbonate or palladium-on-barium sulfate 21), whereas others find carbon satisfactory. In general, carbon support makes the more active catalyst and it is, therefore, more prone to become hydrogen poor. 

This solution Is heated to 65°C and barium hydroxide added in quantity sufficient to make the concentration of the barium hydroxide 0.2 mol/liter. The solution is agitated and maintained at 65°C for 6 hours after the addition of the barium hydroxide. It is then cooled and neutralized to a pH of 6.8 with sulfuric acid. The precipitated barium sulfate is filtered out. A quantity of activated supported nickel catalyst containing 5 g of nickel is added. 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

Palladium catalysts resemble closely the platinum catalysts. Palladium oxide (PdO) is prepared from palladium chloride and sodium nitrate by fusion at 575-600° [29,30]. Elemental palladium is obtained by reduction of palladium chloride with sodium borohydride [27, 31], Supported palladium catalysts are prepared with the contents of 5% or 10% of palladium on charcoal, calcium carbonate and barium sulfate [32], Sometimes a special support can increase the selectivity of palladium. Palladium on strontium carbonate (2%) was successfully used for reduction of just y, (5-double bond in a system of oc, / , y, (5-unsaturated ketone [ii]. 

The y, -double bond of a linear dienone is selectively reduced over palladium in the presence of a trace of base.15,62,85,86,137 Presaturation of the catalyst with hydrogen enhances the selectivity in the reaction.137 In the absence of base, the use of strontium carbonate or barium sulfate as a support for the palladium is preferred.56,57 A benzene solvent is also beneficial.78... 

These reactions are unlike any we have encountered so far. They are heterogeneous reactions, which means that the reacting system consists of two or more phases. Usually, the metal catalyst is present as a finely divided solid suspension in the liquid or solution to be reduced. Alternatively, the metal is deposited on an inert solid support such as carbon, barium sulfate, alumina (A1203), or calcium carbonate. Then the mixture of the liquid substrate and solid catalyst is shaken or stirred in a hydrogen atmosphere. However, the actual reaction takes place at the surface of the metal catalyst and is an example of heterogeneous or surface catalysis. 

Catalytic hydrogenations take place only on the surface of the particles of a metal catalyst. The metal must therefore be very finely divided and is often mixed with a support—this is what Pd/C or Pd/BaS04 means—palladium particles deposited on a support of powdered charcoal or barium sulfate. Palladium on charcoal is probably the most commonly used catalyst, but three others deserve special mention. 

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. 

The catalytic hydrogenation of a double bond involves the adsorption of the alkene on a metal surface and the transfer of hydrogen from the surface to the double bond. Typical catalysts are finely divided forms of nickel, platinum or palladium, the latter often supported on an inert carrier such as charcoal or barium sulfate. Hydrogenations are carried out in solution, with the hydrogen at atmospheric or higher pressure. The addition of hydrogen is typically cis and from the less-hindered face of the molecule (e.g. the hydrogenation of a-pinene, 3.1). 

The most common catalyst for low- and medium-pressure hydrogenation is platinum. Platinum oxide is available from a number of suppliers and is converted to colloidal platinum in situ by hydrogenation. Palladium is another commonly used catalyst and is usually prepared on some inert support such as charcoal, barium sulfate, or calcium carbonate. The procedure for the preparation of these catalysts is given in Organic Syntheses. - A rhodium catalyst appears to be particularly effective in reducing aromatic compounds at low pressure and is available on an alumina support. ... 

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. 

As for heterogeneous catalysts, the addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals. In the latter case, the best results are obtained if the metal is finely divided over the surface of an inert support. Many materials can be used as catalyst supports, however, carbon or alumina are suitable for the majority of reactions. Calcium and barium carbonate or sulfate are also frequently used if less reactive catalysts are desired. The influence of the support is generally small compared to the effect of the metal3. The choice of metal is especially important when stereoselectivity is desired because different metals can catalyze the formation of different diastereomers upon hydrogenation. 

Similar deactivation is observed with bismuth and copper salts. A palladium catalyst poisoned with quinoline-sulfur and support on barium sulfate is used in the Rosenmund-Saytzeff reaction (p. 67). 

The deactivation of these catalysts is probably due to the formation of highly stable species, such as barium sulfate. In addition to the possible encapsulation of Pd particles, the migration of barium from the bulk to the surface of the support is probably responsible for the lower activity in methane oxidation. The possible formation of barium or lanthanum sulfate covering Pd particles will be further investigated. 

Many different catalysts have been used for catalytic hydrogenations they are mainly finely divided metals, metallic oxides or sulfides. The most commonly used in the laboratory are the platinum metals (platinum, palladium and, increasingly, rhodium and ruthenium) and nickel. The catalysts are not specific and may be used for a variety of different reductions. The most widely used are palladium and platinum catalysts. They are used either as the finely divided metal or, more commonly, supported on a suitable carrier such as activated carbon, alumina or barium sulfate. 

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