Double-bond addition, promotion surfaces

Addition to Double Bonds and Elimination Reactions. Both Lewis and Br nsted acidity of mineral surfaces can promote addition and elimination reactions (37). Equation 14 shows an example for an addition/elimination equilibrium catalyzed by Br nsted acidity ... 

The nature of the surface site where acetylene hydrogenation occurs has been discussed extensively. It was proposed that two types of active sites are present on a Pd/Al203 catalyst. l One type promotes the hydrogenation of both triple and double bonds, whereas the other catalyzes only double-bond hydrogenation. This latter site can be poisoned by the addition of carbon monoxide to the reaction mixture as evidenced by the marked increase in reaction selectivity observed in the hydrogenation of acetylene in the presence of carbon monoxide.32.33 other modifiers can presumably act in the same way. 

The second major aspect of the surface chemistry of chromia-alumina that has to be considered is the acidic nature of its surface. The exact chemical nature of the acid sites of solid oxide catalysts such as alumina or silica-alumina has been a subject of considerable research and speculation for a number of years, yet despite these efforts a fully satisfactory chemical description of catalyst acidity has not been obtained. Nevertheless, in the case of chromia-alumina, there is good evidence for the existence of acid sites of one kind or another on the surface. Voltz and Weller (29), for example, studied the chemisorption of quinoline on chromia-alumina, with and without potassium promotion, and at the same time measured their titrable acidities in aqueous suspensions. Both methods indicated that chromia-alumina was acidic, and that the addition of potassium decreased the acidity. This observation was supported by the fact that the double bond isomerization of 1-pentene, normally an acid-catalyzed reaction, proceeded quite readily over pure chromia-alumina, but less readily over a chromia-alumina treated with potassium. 

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