Dehydrogenation of cycloalkanes

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]. 

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. 

A typical catalytic reforming unit consists of a number of fixed-bed reactors, frequently four, in series. The naphtha feedstock is vaporized and heated to the desired reaction temperature, then admitted to the first reactor. As the components in the naphtha undergo reaction during passage through the catalyst bed, the temperature of the vapor stream decreases by 70-100 C due to the endothermicity of the reaction. The major reaction occurring in the first catalyst bed is the dehydrogenation of cycloalkanes to aromatics. 

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 ... 

Develop a catalyst that can affect the reversible dehydrogenation of cycloalkanes to arenes at rates that are sufficient for vehicular applications at temperatures of > 100°C. 

There is extensive work to report on dehydrogenation of cycloalkanes on bimetallic catalysts, mainly in the supported form. Its motivation is quite clear to minimise parasitic reactions such as carbon deposition and hydrogenolysis, so as to have a catalytic system capable of working for long periods of time and at high selectivity. 

Kreuder, H., Pfeifer, P. Dittmeyer, R. (2011). Catalyst preparation for dehydrogenation of cycloalkanes in a microreactor. 1st International Symposium on Chemistry of Energy Conversion and Storage [ChemEnerJ, February 27th to March 2nd 2011, Berlin -Germany, Poster No. 20. 

Undoubtedly, the most exciting recent development in this area is the [ReH7(PPh3)2]-catalyzed dehydrogenation of cycloalkanes C H2 (n = 6, 7, or 8) in the presence of a hydrogen acceptor, for example, 3,3-dimethylbut-1-ene. With methylcyclohexane the major product is 4-methylcyclohexene, consistent with a pathway involving coordination of the least sterically... 

---