Hydrogenation function of the catalyst

The isomerization of n-hexane at 250 °C, 26 bar. a H2/hydrocar-bon (HC) molar ratio of 22.4, and a WHSV of 3.3 g.g". h over fresh Pd-NiSMM led to a conversion of 56 %, almost without cracking. This isomerization activity was totally and irreversibly destroyed after injection of about 10 molecules pyridine per g Pd-NiSMM. Benzene hydrogenation over the poisoned catalyst (260 °C, 26 bar, H2/HC = 25) showed that the hydrogenation function of the catalyst was still active enough to hydrogenate benzene totally to cyclohexane, indicating that the metallic sites had only been partially poisoned, it at all. 

The hydrogenating function of the catalyst is enhanced (ie, the content of olefins in the products decreases [93] ... 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

As already shown by Wiese et al. [17] mass transport rates in biphasic catalysis can be dramatically influenced by hydrodynamics in a tube reactor with Sulzer packings. Above all, the volume rate of the catalyst phase in which the substrates are transported by diffusion plays a decisive role in accelerating the mass transport rate. This effect was also investigated for citral hydrogenation in the loop reactor. Overall reaction rates and conversions as a function of the catalyst volume rate can be seen in Fig. 15. 

In considering the function of the catalyst in reactions of hydrogen, it is useful to distinguish between three types of reactions. 

Observations that a given catalyst is frequently active in a wide variety of hydrogenation reactions suggest that the function of the catalyst is to activate the hydrogen molecule, in the sense of forming a reactive complex with it, which can then enter into reactions with a variety of substrates (Polanyi, S). This concept appears to have validity particularly in relation to the first two types of reactions considered above. Reactions of the third type frequently require also activation of another reactant, however, and... 

A more detailed interpretation of the chemistry of catalytic cracking was based on studies with pure hydrocarbons.121-123 A simplified summary put forward by Heinemann and coworkers123 (Fig. 2.1) shows how Cg open-chain and cyclic alkanes are transformed to benzene by the action of both the hydrogenating (metal) and acidic (halogenated alumina) functions of the catalyst. 

Considerable evidence exists that indicates the selective oxidation of propylene proceeds via the formation of a symmetrical allyl species. Subsequent steps may vary as a function of the catalyst. Some catalyst systems may abstract a second hydrogen atom before the insertion of oxygen. Others may add molecular oxygen, forming a hydroperoxide intermediate, which may then subsequently decompose into acrolein and water. 

A remaining question about the elimination process on a solid surface is the geometric structure of the adsorbed molecule during reaction. If the elimination is a concerted reaction, the acidic function of the catalyst surface (which binds the amine part of the reactant) and the basic function (which abstracts the p -hydrogen atom) should act simultaneously. In that case, it seems as if the reacting amine can assume only a configuration close to the syn configuration shown in Fig. 7. However, syn eliminations are less common than trans eliminations, and we have shown that the elimination of ammonia from amines occurs primarily via trans elimination [F. Rota, V. Ranade, and R. Prins, J. Catal. 200 (2001), to be published]. This problem was realized even... 

Table 3. Surface-Specific Activity of Rh for CO Hydrogenation as a Function of the Catalyst Support...

There is, nevertheless, some evidence (35, 36), based in NiNaY and NiMo/ alumina/Y model catalyst systems, that the amount of coke formed is reduced with increasing intimacy of mixing of the two functions at the submicron level. This concept is further supported by the reported relatively high performance of NiW/ASA (amorphous silica-alumina) cogel HC catalysts which, it is claimed, exhibit an excellent distribution of the NiW hydrogenation function throughout the catalyst particles (37). 

The role of the catalyst from the perspective of the majority of thiophene molecules is not to activate thiophene. Rather, its function is to line up weakly bound (and, therefore, mobile) thiophene molecules for subsequent reaction with hydrogen atoms generated at the active site. It is possible that the main function of the catalyst is not to activate thiophene molecules but rather to dissociate dihydrogen molecules, thus generating mobile H atoms, the concentration of which... 

Effect of Oxygen on the Platinum Functions. On the basis of the mechanism of sintering in a non-hydrogen atmosphere (2) and in order to protect the platinum functions of the catalysts, we treated them at 482 494°C with different oxygen concentration from 0 to 21% in N2 for 4 8 hrs. The results are shown in Table III. 

Table VI shows the effects of oxygen and hydrocarbons in H2 on the platinum functions of the catalyst B-7 after reduction at 482°C. The oxygen concentration up to 10% in H2 keeps the catalyst still in good dispersion and high activity. It was proved by another experiment that all oxygen completely formed water, when a mixture of 10%O2 and 90% H2 was passed through the catalyst at 482°C. It indicates that some oxygen contained in hydrogen dose not give visible effect on the platinum dispersion and its catalytic functions during the reduction stage.

The time (six to twelve hours) required to complete the reaction is a function of the pressure of the hydrogen, acthdty of the catalyst, and purity of the ethyl adipate. Unless a high pressure of hydrogen is used originally or the reaction vessel is of large capacity (2 1.) it will be necessary to introduce more hydrogen into the reaction vessel so that the pressure is never less than 1500 lb. per sq. in. if the reaction is to run smoothly to completion. 

Although in favorable circumstances the reaction may be effected thermally, it is usually conducted as an acid-catalyzed reaction. Both Brpnsted and Lewis acids are effective. The principal function of the catalyst is to accelerate the formation of the enehydrazine from the arylhydrazone [5,6]. It is not usually necessary to isolate the latter the reaction can be conducted as a one-pot procedure starting from ketone or aldehyde and arylhydrazine. Catalysts employed include mineral acids (hydrochloric, sulfuric, polyphosphoric), organic acids such as acetic acid, and metal-halide Lewis acids. Among the most generally reported are ZnCl2 and alcoholic hydrogen chloride. 

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