Platinum zeolites, dehydrogenation

The oxidative dehydrogenation of cyclohexane to benzene has been studied more extensively. Transition metal ion-exchanged forms of zeolite Y have been shown (34-39) to be particularly active catalysts for this reaction. Although the platinum metal ions exhibit the highest activity, CuY was found to be the most selective for benzene formation (38, 39). 

Here, first olefinic intermediates are generated over platinum. These intermediates are either cracked to form lighter olefins or cyclized and isomerized over the acidic chlorided alumina. The olefins and naphthenes thus formed are finally dehydrogenated over the platinum to form paraffins and aromatics. Zeolitic acids such as mordenite(20) and ZSM-5(21) have been substituted for the chlorided alumina as acid catalyst components. All of these acid components have been reported to boost reformed product octane. However, much of the enhanced octane observed with these materials is accompanied by enhanced cracking activity and by reduced yields. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Catalysts prepared by supporting tin and platinum in K-L zeolite exhibit high activity and selectivity for isobutane dehydrogenation and resistance to deactivation at 673-773 K. Tin Mbssbauer spectroscopy was used to show that some of the tin interacts with the platinum to form Pt/Sn alloy particles in reduced Pt/Sn/K-L catalysts while the rest of the tin is present as Sn +[45]. 

While with linear and singly-branched alkanes there is clear but not extensive evidence that on platinum catalysts the intermediates for isomerisation and for hydrogenolysis differ in their extents of dehydrogenation, with doubly-branched alkanes as exemplified by ncopentane (2,2-dimethylpropane) the situation appears not the same. In an extensive review of Arrhenius parameters for its reactions, activation energies for the two reactions were found to be of the same order, - as were orders of reaction (for Pt/KL and Pt/KY zeolites - ). On EUROPT-1 and on oriented model platinum catalysts, activation energies for total reaction increased markedly with hydrogen pressure, as indeed they should. The two reaction paths thus seem to go via the same intermediate, which might be the ap-diadsorbed species. 

The metal oxide catalysts used for hydrogenation reactions are reduced to an active form of the metal before use. Apart from metallic platinum and silver, which are used to oxidize ammonia and methanol, respectively, oxidation catalysts are usually transition metal oxides. Acidic oxides, such as alumina, sihca alumina, and zeolites are used in cracking, isomerization, and dehydrogenation reactions. These are only a few examples of the catalysts now being widely used. A more detailed list is given in Table 1.4. 

Conventional hydrocracking takes places over a bifunctional catalyst with acid sites to provide isomerization/cracking function and metal sites with hydrogenation-dehydrogenation function. Platinum, palladium, or bimetallic systems (ie, NiMo, NiW, and CoMo in the sulfided form) supported on oxidic supports (eg, silica-aluminas and zeolites) are the most commonly used catalysts, operating at high pressures, typically over 10 MPa, and temperatures above 350°C. 

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