Conversion of n-hexane over Pt/Al2O3 was employed as a model reaction to evaluate the geometrically unconstrained catalysis of Pt, which is relevant to industrially important processes such as aromatization, hydroisomerization, hydrocracking, and dehydrogenation. Ring-closure of a six-carbon chain to produce benzene can only take place by dehydrocyclization involving the terminal 1,6-carbons. The only other type of ring-closure involves 1,5-carbons and would not lead to aromatization. Instead, 1,5-ring-closure followed by hydrogenolysis can in essence cause hydroisomerization without acid catalysis. At the same time, six-membered hydrocarbons are resistant to cracking, which makes it easier to track primary products. It was also of interest to see whether n-hexane dehydrogenation would lead to an equilibrium distribution of olefinic isomers, or whether dehydrogenation was position-specific. In this study, the dehydrogenation of n-hexane over a 1wt% platinum on alumina (Pt/Al2O3) catalyst was investigated. Vapor phase reactions were carried out in a plug flow reactor operated at 397 °C, near atmospheric pressure, and using a weight hour space velocity of 7.9 h-1 based on n-hexane. Gas chromatography was employed to analyze the reaction products. Isomeric products were identified by comparison with commercially available compounds. It was found that the preferred dehydrogenation position of n-hexane was the 2-position, with products like cis-2-hexene, trans-2-hexene and methylcyclopentane being formed in higher abundance. The observed order of abundance of different isomers at 397 °C was trans-2-hexene &gt; cis-2-hexene &gt; 1-hexene &gt; cis-3-hexene ≈ trans-3-hexene. Skeletally isomerized products, 2-methylpentane and 3-methylpentane, were formed by hydrogenolysis of methylcyclopentane at the 2,3- and 3,4-positions. In the reaction product, 3-methylpentane was more abundant than 2-methylpentane. Some cyclo-alkenes and linear hexadienes, along with benzene were obtained. It was confirmed that the benzene formed during the reaction was not formed by the ring expansion of methylcyclopentane.

Dehydrogenation of n-hexane over Pt/alumina catalyst

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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