Dehydrogenation/isomerization reaction

These dehydrogenation/isomerization reactions are typically undesirable because the product acids are generally low in value, highly stable in aqueous solution and corrosive towards catalysts and equipment. 

The isomerization reaction, which is acid-site controlled, includes the conversion of alkylcyclopentanes into alkylcyclohexanes, which, in turn, are quickly converted to aromatics by dehydrogenation. In addition, isomerization also includes the conversion of feed n-paraffms into higher octane I-paraffins. 

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. 

For example, in the ring isomerization reaction, methylcyclopentane forms a methylcyclopentene intermediate in its reaction sequence to cyclohexane. The intermediate can also further dehydrogenate to form methylcyclo-pentadiene, a coke precursor. Bakulen et al. (4) states that methylcyclo-pentadiene can undergo a Diels-Alder reaction to form large polynuclear aromatic coke species. Once any olefinic intermediate is formed, it can either go to desired product or dehydrogenate further and polymerize to coke precursors. This results in a selectivity relationship between the desired products and coke formation as shown on the next page. 

The elements of range in value from 0 to 1 and are the ratio of the reformer kinetic constants at time on stream t to the values at start of cycle. At any time on stream t, the deactivation rate constant matrix K(a) is determined by modifying the start-of-cycle K with a. From the catalytic chemistry, it is known that each reaction class—dehydrogenation, isomerization, ring closure, and cracking—takes place on a different combination of metal and acid sites (see Section II). As the catalyst ages, the catalytic sites deactivate at... 

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) ... 

Such high activity in hydrogen transfer reactions smoothes the way for a series of selective transformations of synthetic interest. Thus, a versatile and efficient catalyst could potentially promote hydrogenation, dehydrogenation, and a combination of the two reactions in order to set up isomerization reactions. 

In addition to the paraffin isomerization processes, naphthene isomerization also proved useful during the war in the manufacture of toluene. In making toluene by the Shell dehydrogenation process, good yields depend upon raising the methylcyclohexane content of the feed by isomerizing the dimethylcyclopentanes. This isomerization reaction was used commercially at one refinery in the Midwest and one on the Pacific Coast. 

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