Catalytic conversions in green aqueous media: Highly efficient biphasic hydrogenation of benzene to cyclohexane catalyzed by Rh/TPPTS complexes Constantinos Vangelis a , Achilleas Bouriazos a , Sotiris Sotiriou a , Markus Samorski b , Bernhard Gutsche b , Georgios Papadogianakis a, * a University of Athens, Department of Chemistry, Industrial Chemistry Laboratory, Panepistimiopolis-Zografou, 157 71 Athens, Greece b Cognis GmbH, Process Development, Henkelstr. 67, 40589 Düsseldorf, Germany article info Article history: Received 12 April 2010 Revised 1 June 2010 Accepted 1 June 2010 Keywords: Hydrogenation Benzene Cyclohexane Rhodium TPPTS Water-soluble Two-phase system Aqueous medium abstract Exceptionally, high catalytic activities (TOF > 204,000 h 1 ) have been achieved in the hydrogenation of benzene to cyclohexane catalyzed by water-soluble Rh/TPPTS complexes [TPPTS = P(C 6 H 4 -m-SO 3 Na) 3 ] in green aqueous/organic two-phase systems. The influence of several operating parameters on the biphasic benzene hydrogenation reaction was investigated. A great advantage of this reaction is that the highest catalytic activity was achieved at a low volume ratio of aqueous/benzene phase of 0.4. Rh/ TPPTS catalytic complexes, which are stable toward sulfur-containing compounds, were easily recovered by a simple phase separation, and recycling experiments showed that the catalytic activity remained high in five consecutive runs. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction The hydrogenation of benzene to cyclohexane is one of the most important industrially practiced aromatic compounds hydrogena- tion reactions [1]. Millions of tonnes of benzene are hydrogenated to cyclohexane per year, and about 90% of world’s production of cyclohexane is used for manufacturing nylon 6 and nylon 6.6, which are about 90% of all polyamides. Nowadays, almost the whole world’s production capacity of cyclohexane is manufactured by the route of the hydrogenation of benzene. However, despite its apparent simplicity, the hydrogenation reaction of benzene has evolved through many variations and has given rise to many differ- ent processes. The successful industrial hydrogenation of benzene suitable for petrochemical cyclohexane production requires the resolution of three major critical problems: (i) the reaction is strongly exothermic with a DH 0 f (473 K) of 214.2 kJ mol 1 , (ii) the cyclohexane must be pure, and (iii) the low stability of hetero- geneous catalysts e.g., nickel catalysts which require extremely pure benzene feedstocks with less than 1 ppm sulfur in order to remain effective in the liquid phase under mild conditions. The strongly exothermic hydrogenation reaction requires careful con- trol of temperature, pressure and residence time in order to achieve quantitative conversion of benzene at very high cyclohex- ane selectivity. Thus, significant formation of methylcyclopentane, which is favored at higher temperatures, has to be suppressed to- gether with other by-products such as methylpentane, n-hexane, n-pentane and methane obtained by isomerization and hydro- cracking side reactions. These undesired by-products can be sepa- rated from the cyclohexane product only by complicated separation methods such as rectification, extraction and by molec- ular sieves [1]. A large number of industrial processes for the heterogeneous catalytic hydrogenation of benzene to cyclohexane have been developed, which are carried out in the liquid or in the gas phase. Among the well-known liquid phase benzene hydrogenation pro- cesses are the Mitsubishi, Sinclair and Institut Franais du Ptrole (IFP) process [1]. The IFP process uses a Raney nickel heteroge- neous catalyst in a bubble column reactor at temperatures be- tween 200 and 230 °C under 50 bar. The main drawbacks with this process are due to the fact that the Raney nickel catalyst pos- sesses a low stability toward sulfur-containing compounds unavoidable in benzene feedstock and that the catalyst is pyro- phoric to a greater or less extent. Benzene feedstock requires deep desulfurization to a sulfur content of <1 ppm, which considerably increases the cost of the whole process. Well-known gas phase 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.06.004 * Corresponding author. Fax: +30 210 72 21 800. E-mail address: papadogianakis@chem.uoa.gr (G. Papadogianakis). Journal of Catalysis 274 (2010) 21–28 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat