Tuning cis-decalin selectivity in naphthalene hydrogenation over carbon-supported rhodium catalyst under supercritical carbon dioxide Norihito Hiyoshi a , Tomoya Inoue a , Chandrashekhar V. Rode b , Osamu Sato a , and Masayuki Shirai a, * a Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology, 4-2-1, Nigatake, 983–8551 Miyagino, Sendai, Japan b Homogeneous Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, 411008 Pune, India Received 8 August 2005; accepted 7 November 2005 Catalytic hydrogenation of naphthalene to decalin was studied over a carbon-supported rhodium catalyst in supercritical carbon dioxide solvent at 333 K, and the results were compared with those in an organic solvent. cis-, trans-Decalin and tetralin were formed from the beginning of the reaction in supercritical carbon dioxide. Higher concentration of hydrogen in carbon dioxide solvent and on the active site, and also the suppression of desorption of partially hydrogenated tetralin molecules from the active site would be responsible for higher selectivity to cis-decalin in supercritical carbon dioxide than that in an organic solvent. KEY WORDS: supercritical carbon dioxide; hydrogenation; naphthalene; decalin; hydrogen storage; carbon-supported rhodium catalyst. 1. Introduction Hydrogen storage is a key process for its utilization as a clean energy source being developed recently. Cyclic saturated hydrocarbons such as decahydronaphthalene (decalin), bicyclohexyl, methylcyclohexane etc. are pro- posed as new mobile hydrogen storage media for proton exchange membrane fuel cells [1–3]. Hydrogen storage system with cyclic saturated hydrocarbons is more stable and inexpensive than with inorganic hydrogen-absorb- ing alloys. Cyclic saturated hydrocarbons are flamma- ble; however, safer than using hydrogen cylinders, because pure hydrogen gas would detonate with a vio- lent explosion when exposed to a spark accidentally. Hydrogen can be obtained by catalytic dehydroge- nation of the cyclic saturated hydrocarbons to the cor- responding aromatic compounds such as naphthalene, biphenyl, toluene etc. [1–3] and stored by the hydroge- nation of the aromatic products [4–6]. For hydrogen production from decalin, cis-isomer is more preferable, because dehydrogenation rate of cis-decalin is faster than that of trans-decalin [1(d)]. Also, cis-decalin can be used to produce sebacic acid that can be used in the manufacture of Nylon 6, 10 and plasticizer [7]. Hydro- genation of aromatic compounds is also important for the production of a high performance diesel fuel [8]. Selective hydrogenation of naphthalene to cis-decalin is an important reaction to study from both fundamental understanding of its adsorption characteristics and industrial application point of view. Vapor and liquid phase hydrogenation of naphtha- lene over supported metal catalysts has been investi- gated by several researchers [9–14]. However, high temperature (>473 K) and acidic nature of catalyst supports used in the dearomatization form hydro- cracking byproducts and high molecular weight prod- ucts that cause the decrease in the yield of cyclic saturated hydrocarbons in vapor phase reaction [9,10,14]. Hence, dearomatization involving liquid phase hydrogenation under mild temperature (<373 K) is highly desirable; however, low reaction rates, low selectivity to decalin and difficulty in the separation of pure products from solvents become the critical issues. These drawbacks can be overcome by carrying out catalytic hydrogenation in supercritical carbon dioxide. Supercritical carbon dioxide (T c ¼ 304:2 K and P c ¼ 7:38 MPa) can be made miscible with light gasses and aromatics by proper choice of pressure and temperature conditions. Higher reaction rates and hence higher productivity, and easy separation of liquid products without using organic solvents can be achieved by conducting catalytic hydrogenations with solid cat- alysts in supercritical carbon dioxide medium [5,6,15– 23]. Especially, this technique is very effective for the hydrogenation of solid substrates to liquid products, which are soluble in supercritical carbon dioxide. Recently, we have reported that a carbon-supported rhodium catalyst was found to be highly active and * To whom correspondence should be addressed. E-mail: m.shirai@aist.go.jp Catalysis Letters Vol. 106, Nos. 3–4, February 2006 (Ó 2006) 133 DOI: 10.1007/s10562-005-9620-6 1011-372X/06/0200–0133/0 Ó 2006 Springer Science+Business Media, Inc.