Hydrogenation/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 ... 

Table VI shows results obtained with a mixture of dimethyl- and ethylcyclohexanes reformed with catalysts of different acidity. The two functions of a reforming catalyst—the acidity and the hydrogenation-dehydrogenation function of the platinum—are balanced carefully for...

As explained earlier, chromia-alumina can be considered as a dual functional system, having both an acidic function and a hydrogenation-dehydrogenation function. While the present discussion is primarily concerned with the molecular configuration of the chromia dehydrogenation component, neither function can be considered entirely out of context. At the present time, however, the nature of catalyst acidity in general remains somewhat obscure, despite a considerable amount of research, and only a brief review of the acid character of chromia-alumina catalysts will be given here. 

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... 

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. 

Most hydrocracking catalysts of commercial interest are dual functional in nature, consisting of both a hydrogenation-dehydrogenation component and an acidic support. The reactions catalyzed by the individual components are quite different. In specific catalysts, the relative strengths of the two components can be varied. The reactions occurring and the products formed depend critically upon the balance between these two components. 

Photocatalysis includes a large variety of reactions such us partial or total oxidations, hydrogen transfer, functionalization, rearrangements, dehydrogenation, mineralization, and so on [1],... 

Duration of a cycle of HHP operation is defined as time required for reaction hydrogenation/dehydrogenation in pair hydride system. This time determines heat capacity of HHP. Duration of a cycle depends on kinetics of hydrogenation reactions, a heat transfer between the heated up and cooling environment, heat conductivities of hydride beds. Rates of reactions are proportional to a difference of dynamic pressure of hydrogen in sorbers of HHP and to constants of chemical reaction of hydrogenation. The relation of dynamic pressure is adjusted by characteristics of a heat emission in beds of metal hydride particles (the heat emission of a hydride bed depends on its effective specific heat conductivity) and connected to total factor of a heat transfer of system a sorber-heat exchanger. The modified constant of speed, as function of temperature in isobaric process [1], can characterize kinetics of sorption reactions. In HHP it is not sense to use hydrides with a low kinetics of reactions. The basic condition of an acceptability of hydride for HHP is a condition of forward rate of chemical reactions in relation to rate of a heat transmission. 

The catalyst is called bifunctional both the carrier and the metallic particles dispersed over the carrier exhibit different catalytic functions. The carrier contains chlorine ions and, as a consequence, it has acid properties and exhibits isomerization and cyclization activities. The metal particles consist of alloys of, for example, Pt/Re which exhibit hydrogenation/dehydrogenation activity. 

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. 

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