Catalytic dual-function catalyst

Powerforming is basically a conversion process in which catalytically promoted chemical reactions convert low octane feed components into high octane products. The key to a good reforming process is a highly selective dual-function catalyst. The dual nature of this catalyst relates to the two separate catalyst functions atomically dispersed platinum to provide... 

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

Acid-catalyzed cracking and platinum-catalyzed hydrogenolysis proceed simultaneously over dual-function catalysts. The distribution of the scission products is determined by the relative strengths of the acidic and metal-type catalytic components. 

Mechanistically, this catalytic reaction proceeds via enantioselective Michael addition and the subsequent protonation of the transient enol intermediate in a stereoselective manner (Scheme 9.27). Thus, the authors proposed that the catalysts serve as a dual-function catalyst for this tandem reaction namely, the stereochemical outcome of this tandem reaction resulted from a network of hydrogen-bonding interactions between the catalyst with the reacting donor and acceptor in the addition step and, subsequently, with the putative enol intermediate (78) in the protonation step (Scheme 9.28). 

There are several techniques for preparation of high-area catalysts. Some involve formation of a support or carrier, especially alumina, silica or carbon, onto whose surface is deposited an active catalyst ingredient. However, seldom is the support inactive in the sense that it functions only to spread out the active component. The support usually influences the added ingredient through epitaxial or chemical interaction which alters the behavior of the active component. In Si02— Al203 catalysts, it is the combination of both oxides that provides for the essential acidity. In dual-function catalysts, the support can serve catalytically as the essential acid function. 

Catalysts that contain metallic constituents deposited on a support are of major scientific and industrial interest ". The support permits the metal to be present on the surface as small crystallites that have a high surface area. If these crystallites are about 1 nm in diameter, most of the atoms are exposed on the metal surface and available for catalytic action. Of great significance is the interaction of the support with the metal. This interaction can modify the catalytic properties of the metal and also slow crystallite growth processes, which result in loss of area and activity. Sometimes, as in dual-function catalysts, the support is required to provide one of the catalytic functionalities, usually the acid function. 

A major factor in the rapid commercial utilization of catalytic reforming processing for upgrading low octane naphthas, and the production of aromatics from petroleum sources, has been the development of more active and selective dual-functional catalysts. These catalysts contain a very active hydrogenation-dehydrogenation agent such as platinum, in combination with an acidic oxide support such as alumina or silica-alumina. 

Dual Function Catalytic Processes. Dual-function catalytic processes use an acidic oxide support, such as alumina, loaded with a metal such as Pt to isomerize the xylenes as weH as convert EB to xylenes. These catalysts promote carbonium ion-type reactions as weH as hydrogenation—dehydrogenation. In the mechanism for the conversion of EB to xylenes shown, EB is converted to xylenes... 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

Bis-hydroxylation. Molecular oxygen or air was used as the terminal oxidant in the osmium-catalyzed oxidation of alkenes to form cis-diols with high conversions at low catalyst amount.1298 In a triple catalytic system using H2O2 as the terminal oxidant, a cinchona alkaloid ligand has a dual function—it provides stereocontrol and acts as reoxidant via its IV-oxide. The formation of the latter is catalyzed by a biomimetic flavin component.1299... 

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