Dehydrogenation cycloalkanes

We shall not treat cracking processes here due to the complexity of these high-temperature (usually around 500°C) reactions. However, cycloalkane dehydrogenation to aromatics (Appendix A2.4.4), alkane isomerization and olefin alkylation (leading to branched alkanes from linear ones) occur via such carbocation rearrangements. 

Dehydrogenation of cycloalkanes such as cyclohexane, methylcyclohexane and decalin has been used for the storage and transportation of hydrogen fuel at an ambient temperature and pressure [2, 4, 14-20]. The advantages of cycloalkane dehydrogenation systems are as follows ... 

Scheme IV.21, The cycloalkane dehydrogenation to produce cyclopentadienyl and arene 7c-complcxcs. Reference numbers are in brackets.

K (277°C) and 650°K are 0.63 and 103 atm,3 respectively. Above about 350°C the equilibrium constants for this type of reaction are such that the aromatic is always highly favored thermodynamically over the corresponding cycloalkane. Moreover, olefin which is itself capable of further dehydrogenation to an aromatic (e.g., cyclohexene) is never observed in significant amounts under isomerization conditions. 

Another area of high research intensity is the catalytic dehydrogenation of alkanes to yield industrially important olefin derivatives by a formally endothermic (ca. 35 kcal mol-1) loss of H2. Recent results have concentrated on pincer iridium complexes, which catalytically dehydrogenate cycloalkanes, in the presence of a hydrogen accepting (sacrificial) olefin, with turnover numbers (TONs) of >1000 (Equation (23)) (see, e.g., Ref 33,... 

Finally, whilst rhenium hydride complexes have not been reported to hydrogenate alkenes, there are several reports of the dehydrogenation of alkanes in the presence of tBuCH=CH2 as an hydrogen acceptor (Scheme 6.14) [136-142]. For example, cycloalkanes are transformed catalytically into the corresponding cyclic alkene, which shows that, in principle, a Re-based catalyst could be designed. 

In the case of iridium, complex [IrH2(PPh3)2(acetone)2] BF4 (11) was the first to carry out catalytically the dehydrogenation of cycloalkanes [13, 14]. However, it was later realized that the halocarbons used as solvents reacted with 11 to produce the stable species [HL2lr(p-Cl)2(. i-X)IrL2H]BF4 (X = Cl (14) or H (15)) [16] (Scheme 13.8), and that elimination of the solvent by running the reactions in neat alkane not only improved yields but also permitted the activation of other previously unreactive cycloalkanes, such as methyl- and ethyl-cyclopentane. However, it was also noted that the system in some cases was not catalytic, due mainly to decomposition of the catalyst at the temperatures employed [16]. 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

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