Poisoning of Solid Acids

The poisoning of acid centers can easily be explained. Acid centers can be neutralized and thus poisoned by basic compounds such as alkali and alkaline earth compounds and especially organic bases. Alkali and alkaline earth compounds are normally used as promoters and are generally not present in process streams. 

In contrast, nitrogen-containing bases are contained in many crude-oil fractions. In a typical starting material, 25 35 % of the nitrogen compounds have basic character. The sensitivity of solid acids towards these poisons correlates directly with their basicity. For example, pyridine, quinoline, amines, and indoles are basic, while pyrrole and carbazole are nonbasic. These poisons are best removed by hydrogenation, together with sulfur and most of the heavy metal poisons. 

However, in some cases partial catalyst poisoning is desired, for example to lower the catalyst activity or to influence the selectivity. A well-known example is the addition of ppm quantities of H2S in catalytic reforming with nickel catalysts. Compared to platinum, nickel has a higher hydrogenolysis activity, which leads to formation of gases and coke. Sulfur selectively poisons the most active hydrogenolysis centers and thus drastically influences the selectivity towards the desired isomerization reactions. 

Other partially poisoned catalysts have long been used in the laboratory. Supported palladium catalysts, poisoned with lead (Lindlar catalysts), sulfur, or quinoline, are used for the hydrogenation of acetylenic compounds to cis-olefins. Another 

In the Rosenmund reaction (Eq. 5-84), acid chlorides are hydrogenated to aldehydes. The catalyst is a supported palladium catalyst (5 % Pd/BaS04) poisoned by sulfur compounds such as quinoline, tiourea, or thiophene to prevent further reduction of the aldehyde. 

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