Hydrogenation-dehydrogenation sequence

The carbocation may rearrange, eliminate a proton to produce an olefin, or crack at a beta position to yield an olefin and a new carbocation. Under an atmosphere of hydrogen and in the presence of a catalyst with hydrogenation-dehydrogenation activity, the olefins are hydrogenated to paraffinic compounds. This reaction sequence could be represented as follows ... 

The reactions of the various xylenes and ethylbenzene have been studied by Pitts and associates (P7). It was found that isomerization reactions among the three xylenes were catalyzed by acidic catalysts, but that interconversions between the xylenes and ethylbenzene required the presence of a hydrogenation-dehydrogenation component. Furthermore, it was found that the conversion of xylenes to ethylbenzene increased with decreasing temperature. Since lower temperatures are more favorable for hydrogenation, it has been suggested that the reaction proceeds by a sequence of steps such as the following (P7, W3) ... 

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Many reactions like the skeletal isomerizations, hydrogenation-dehydrogenation can be described by sequence of first order reactions (see eg. reaction sequence 2, Table 1.). The expressions for the reversible rates with and without activation energy corrections are given by equations (14) and (16) respectively. 

A noteworthy feature of the Seager-Anderson experiment was that after hydrogenation of a diode it could be restored practically to its prehydrogenation state by heating for 10 minutes at 200°C. This process of hydrogenation and dehydrogenation could be repeated many times, producing the same sequence of C-V profiles each time. 

Maxted (43) has pointed out that in the transition metals, which are the most effective hydrogenation and dehydrogenation catalysts, the paramagnetic susceptibilities parallel their sequence of catalytic activity. 

Dehydrogenation of A2-imidazolines (416 Z = NR) gives imidazoles, but requires quite high temperatures and a catalyst such as nickel or platinum. Alternatively, hydrogen acceptors such as sulfur or selenium can be used (70AHC( 12)103). A2-Imidazoline derivatives (417) are thermally converted into imidazoles (418) by a retro-Diels-Alder sequence (93JOC3387). 

Warburg and Christian showed that the color of this old yellow enzyme came from a flavin and proposed that its cyclic reduction and reoxidation played a role in cellular oxidation. When NADP+ was isolated the proposal was extended to encompass a respiratory chain. The two hydrogen carriers NADP+ and flavin would work in sequence to link dehydrogenation of glucose to the iron-containing catalyst that interacted with oxygen. While we still do not know the physiological function of the old yellow enzyme,b the concept of respiratory chain was correct. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

Kinetic studies at short residence times first suggested the following reaction sequence ethylnaphthalene dehydrogenates to vinylnaphthalene vinylnaphthalene dehydrocyclizes to acenaphthylene and finally acenaphthylene is hydrogenated to acenaphthene. However, further work by Isagulyants and co-workers, using 14C-labeled 1-vinylnaphthylene, shows that over platinum on alumina at 470°C, acenaphthene and acenaphthylene are formed from both 1-ethylnaphthalene and 1-vinylnaphthalene. Vinylnaphthalene dehydrocyclizes about three times faster than ethylnaphthalene. The vinylnaphthalene intermediate remains adsorbed on the catalyst surface during the reaction (48). 

Complexes such as (62) can be submitted to hydrogenation and oxidative dehydrogenation processes of the type already described for related systems. The most interesting investigation has actually been carried out on complexes of a tetramethyl substituted ligand prepared from a nontemplate reaction sequence. In particular, oxidative dehydrogenation affords macrocyclic chro-mophores with six and seven double bonds (Scheme 24).118,141... 

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