Metal dehydrogenation function

Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

Besides Ga, other metals such as Zn (11, 12) and Pt (13) have also been used in combination with ZSM-5 zeolite for C2-C4 aromatization. However, besides aromatization, Pt also catalyzes other undesired reactions, such as hydrogenolysis, hydrogenation and dealkylation that leads to excessive formation of methane and ethane, and limits the selectivity to aromatics. Therefore, Ga- and Zn-ZSM-5 catalysts are preferred over Pt-ZSM-5 except, perhaps, in the case of the more refractory ethane, in where a higher dehydrogenating function is needed to activate the reactant. The catalytic performance of Ga and Zn/ZSM-5 for propane aromatization is compared in Table 2.2. The results obtained on the purely acidic H-ZSM-5 are also included in the table. As observed, a higher conversion and yield of aromatics is obtained for the Ga/ZSM-5 catalyst. 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

As expected from the above discussion, the amounts of cracked products and methane increase by a factor of 2 to 2.5. However, the DHC products (naphthenes and toluene) increase by a factor of 12. Under the experimental conditions, DHC can be both mono- or bi-([metal + alumina]-) functional. The rate of dehydrogenation of MCH to toluene (not shown) actually decreases for the bimetallic catalyst, from 45 to 12 mol/h/kg. 

The next example shows how catalyst bifunctionaUty can arise from the support material. Platinum metal dehydrogenates napthenes to give aromatic compounds, but it is not able to isomerize or cyclize n-alkanes. This function is adopted by the AI2O3 support with its acidic properties. The cooperation of the two catalyst components is shown schematically for the reforming of n-hexane in Scheme 5-5 [T20]. 

Bifunctional Catalysts. The complete reforming reaction sequence requires both metal-catalyzed dehydrogenation and acid-catalyzed isomerization and cyclization. It is clear that both acid and metal functionalities must exist on a satisfactory reforming catalyst. Reforming catalysts are often referred to as bifunctional, meaning that both metal and acid functions exist on the catalyst. Modern catalysts are platinum on alumina, typically modified by a number of additional elements. The platinum supplies the dehydrogenation function, and the alumina supplies the acidic function. The acidity of the alumina is enhanced through the adsorption of chloride. 

Reforming requires a catalyst with dual functions an acidic function to catalyze isomerization and cycUzation and a dehydrogenation function that requires an active metal site. Figure 6.9.5 illustrates a simplified reaction network for the example of Cg-hydrocarbons that also identifies the catalytic sites involved. 

Straight-run gasoline is composed primarily of alkanes and cycloalkanes with only a small fraction of aromatics, and has a low ON of about 50. The ON is improved by catalytic reforming of n-paraffins and cycloalkanes into branched alkanes and aromatics. The main reactions are isomerization (w- to iso-), cycli-zation, dehydrogenation, and dehydrocyclization. The bifunctional catalyst has an acidic function to catalyze isomerization and cyclization and a dehydrogenation function that requires an active metal site. Typically, platinum is used as the metal and AI2O3 for the acidity. 

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. 

HDC catalysts are characterized by their dual functionality. The cracking function is promoted by the highly acidic support, whereas the active metal phase is responsible for the hydrogenation/dehydrogenation function. The typical support is made of amorphous silica-alumina or crystalline zeolites (X and Y zeolites), the latter being the most acidic (Robinson and Dolbear, 2006). The hydrogenation function can be catalyzed by noble metals such as Pd and Pt or by metal sulfides of NiMo and NiW. Noble metals exhibit the best hydrogenation activity however, they are very... 

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... 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

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