Scientists from Bejing, China, have developed a highly selective supported Ru catalyst for the hydrogenation of benzene to cyclohexene – a compound which is very useful for nylon production. Until now, the Ru based catalysts used for this transformation deliver low selectivity and yield of the desired partial hydrogenation product, and often the use of additives is necessary. However, in this work by Buxing Han and co-workers, cyclohexene yields of 56% were achieved in water without the use of additives.
The selective hydrogenation of benzene to cyclohexene is challenging, as it is significantly more thermodynamically favorable for cyclohexane to be the primary product of the reaction. By using water as the reaction medium it is proposed that the hydrogenation of cyclohexene to cyclohexane is slowed because benezene is more soluable than cyclohexene in water under these conditions. This is also aided by the hydroxyl groups on the catalyst surface which disfavour the adsorption of cyclohexene onto the catalyst surface, thus preventing further hydrogenation. Synergistic effects between the zinc and zirconium in the catalyst support also play a crucial role in the efficiency and selectivity of the catalyst used in this work.
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Highly selective benzene hydrogenation to cyclohexene over supported Ru catalyst without additives, Huizhen Liu, Tao Jiang, Buxing Han, Shuguang Liang, Weitao Wang, Tianbin Wu and Guanying Yang, Green Chem., 2011, DOI:10.1039/c0gc00959h