Palladium dehydrocyclization

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

The nature of the metal Dehydrogenating properties are important for Cg dehydrocyclization. Platinum, palladium, iridium, and rhodium... 

Erivanskaya and co-workers also studied the dehydrocyclization of 2-n-butylnaphthalene over supported palladium, rhodium, and iridium catalysts (56-55). Palladium-alumina showed the lowest C6-dehydrocyclization activity, but was the most active for the C5-dehydrocyclization of 2-n-butyl-naphthalene. A later study showed, however, that this enhanced activity was due to the high chlorine content of the palladium-alumina catalyst and not to some mysterious inherent catalytic activity of palladium (56). 

It has been reported (115) that n-heptane and n-octane dehydrocyclize upon alloying of palladium with silver. The dehydrocyclization products are to a considerable degree dealkylated. 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

Another approach to the mechanisms of dehydrocyclization was the comparative study of the reactions of 2,2,3-trimethylpentane (1), 2,2,4-trimethylpentane (2) and 2,2,4,4-tetramethylpentane (3) (Scheme 43) on platinum and palladium films (70). On palladium, dehydrocyclization is... 

At 300°C on palladium films, the relative rates of dehydrocyclization of 1, 2, and 3 are 2.6, 0.2, and < 10 respectively. This strongly suggests that, on palladium, dehydrocyclization involves 7i-olefin formation. Indeed 3, which cannot dehydrogenate without skeletal rearrangement, does not undergo detectable dehydrocyclization below 360°C, and the low dehydrocyclization rate for 2 is probably due to steric hindrance arising from the )S-methyl substituent on the required rt-adsorbed olefin (96). The drastic decrease of the dehydrocyclization rate from 1 to 2 with increasing substitution is thus consistent with this mechanism (Scheme 44). 

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