Trans dehydrogenation, during aromatization

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. 

A curious mechanistic phenomenon found in hydrogenation is also found in dehydrogenation. Once again, we see an example of a reaction occurring on both sides of a six-membered ring. A clear example of trans dehydrogenation occurs during the Pd-catalyzed aromatization of the compound in Scheme 5.6.48 See Section 2.1.3.2. for a discussion of trans addition. 

Aromatization, however, may also be envisaged as taking place via stepwise dehydrogenation of an unbranched hydrocarbon molecule followed by ring closure of the polyunsaturated intermediates. In fact, the formation of dienes was proved during the aromatization of C6 and C7 alkanes to the corresponding aromatics over monofunctional metal oxides and metal black, and bifunctional catalysts.307 308 Radiotracer studies even allowed the detection in very low concentration of hexatriene during the aromatization of n-hexane over Pt black.309 It was also proposed that aromatics are formed from the cis isomers, whereas trans isomers may be coke precursors.213 Direct experimental evidence has recently been... 

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