Isomerization dehydrogenation step

If the catalyst contains sufficient platinum to allow the hydrogenation-dehydrogenation steps to be in equilibrium, the isomerization can be taken as the rate-limiting step, and the rate becomes ... 

Paraffin isomerization is a bifunctional process comprising (i) hydrogenation and dehydrogenation steps catalyzed by platinum sites (ii) isomerization of the ole-... 

Phytoene (103) is incorporated " by tomato plastids into many of the expected intermediates to a- and )5-carotene (110) and (111). These two carotenoids are both labelled by [15,15 - H2]lycopene (107) as are the intermediates 5- and y-carotene (108) and (109). In the dehydrogenation steps to give lycopene, evidence was presented to suggest that the central cis-double bond of phytoene is isomerized... 

An answer to the puzzling problem of the kinetics of skeletal isomerization may be found in a recent proposal, by Frennet and his co-workers (62), that hydrocarbon adsorption involves not a single site but an ensemble of several contiguous sites of the surface (seven to eight). When using the model of bimolecular dehydrogenation steps, one should always replace the first equation in Scheme 15 by... 

Catalysts such as these are referred to as dual functionar because the platinum and the acid sites are discrete components. They cooperate in promoting the desired over-all reactions, but each appears to be responsible for certain steps. Details of the reaction mechanisms will be discussed later, but the concept of dual functionality can be illustrated by the simple reaction shown in Figure 3. Cyclohexane is dehydrogenated to benzene via cyclohexene. While the second dehydrogenation step is quite rapid, some of the cyclohexene is also trapped on acid sites and isomerized, appearing in the product largely as methyl-cyclopentane. The dual functional nature of the catalyst can be demon-... 

The bifunctionality of metal-doped zeolite catalysts is explained here for the important example of isomerization and hydrogenation. The metal content facilitates the hydrogenation and dehydrogenation steps, while the acid-catalyzed isomerization step takes place under the restricted conditions of the zeolite cavities (Scheme 7-1). 

The mechanism proposed for COA transfer dehydrogenation in the presence of tbe by Ir-13(Fl2) is close to that reported for Ir-2(H2) for the dehydrogenation step (Scheme 2.15) [129]. However, it differs for the isomerization step while bisphosphine complex Ir-2(H2) reacts reversibly with tbe to give a vinylic C-H addition product, bisphosphinite complex Ir-13 forms a it-coordinated complex (Scheme 2.17) [130]. 

See also Sinfelt, 1964, and Haensel, 1965]. The isomerization step is usually intrinsically very fast, and so the first part of the reaction has exactly the above sequence similar to an earlier qualitative study by Mills et al. [1953]. Weisz and co-workers performed experiments to prove this conjecture. They made small particles of acid catalyst and small particles containing platinum. These particles were then formed into an overall pellet. They found that a certain intimacy of the two catalysts was required for appreciable conversion of / heptane into isoheptane. Particles larger than about 90 pm forced the two steps to proceed consecutively, since the intermediate unsaturates resulting from the metal site dehydrogenation step could not readily move to the acid sites for isomerization. This involves diffusion steps that will be discussed in Chapter 3. Further evidence that olefinic intermediates are involved was obtained from experiments showing that essentially similar product distributions occur with dodecane or dodecene feeds. 

The principal reforming reactions are shown in Table 6.14. The isomerization of -paraffins proceeds via a dehydrogenation step on the platinum component of the catalyst to an -olefin intermediate. The -olefin then migrates to the acidic alumina site and isomerizes to the iso-olefin. Finally the iso-olefin is hydrogenated to the isoparaffin by the platinum. This mechanism requires good dispersion of platinum on the highly acidic, chloride-treated alumina support. 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

Cluster opened bislactams have been obtained in a one-pot reaction by treating diazidobutadiene 30 with Cjq for four days in ODCB at 55 °C [3, 35]. The first step in the reaction sequence can be assumed to be the formation of the bisazafulleroid 31 (Scheme 11.7). Subsequent addition of 2 leads to the endoperoxide intermediate 32, which isomerizes via a electrocyclic ring opening to 33 [36]. Subsequent oxidation with DDQ or O2 afforded the dehydrogenated bislactam 34. 

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