Dr. Masayuki Shirai, Leader of Organic Synthesis Team, Supercritical Fluid Research Center (SFRC) of the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, has developed technology for synthesizing decalin, a promising material of hydrogen storage for fuel cell through the combination of supercritical carbon dioxide with a supported rhodium catalyst. The process has merits of lower temperature, high selectivity, high efficiency, long life owing to lack of catalyst aging, easy recovery of decalin, and capability of recycling carbon dioxide solvent. It is expected to contribute to the implementation of hydrogen storage materials synthesis system to reduce the environmental burden.

The results of this study will be reported at the Meeting of the Catalysis Society of Japan to be held at Sendai September 27~30, 2004.

The SFRC-AIST has been dedicated to R&D of environment-conscious organic synthesis process based on supercritical water and carbon dioxide. The study about the hydrogenation of naphthalene using supercritical carbon dioxide and a supported rhodium catalyst indicates that naphthalene can be efficiently hydrogenated, yielding decalin at 60°C with 100 % naphthalene conversion rate and 100 % selectivity. The hydrogenation of naphthalene results in tetralin or decalin with aromatic ring hydrogenated, partly or completely, respectively. In the conventional naphthalene hydrogenation, tetralin can be obtained readily, while it is rather difficult to synthesize decalin at high concentration in a single step of reaction. Decalin is used for hydrogen storage materials for distributed fuel cell and non-aromatic solvent. The conventional naphthalene hydrogenation is carried out at 200°C or higher reaction temperatures using supported platinum catalysts. Consequently, adverse decomposition byproducts and aromatic polymers are produced to decrease the yield. Besides, it has a demerit of carbon deposition over the surface of catalyst to degrade the catalysis in the course of reaction. The newly developed process lowers the reaction temperature extensively, upgrades the decalin selectivity drastically, and cleans the catalyst surface by solvent effect of supercritical carbon dioxide, to ensure repeated and long-term use of catalyst. In this way, the process is meritorious in terms of energy saving and reduction of environmental burden.

The conventional method of naphthalene hydrogenation to synthesize tetralin and decalin has been carried out using supported platinum catalysts and at the reaction temperatures higher than 200°C. The process has some demerits such as decomposition byproducts including polymers produced at higher reaction temperatures to decrease the yield, and contamination of catalyst surface to reduce the activity and to curtail the life. Moreover, with this process, hydrogenation is halted at the level of tetralin to affect the decalin selectivity markedly. With the newly developed process, using supercritical carbon dioxide and a supported rhodium catalyst, decalin is obtained from naphthalene at the reaction temperature 60°C and 100 % yield.

The newly developed synthetic technology is superior to the conventional method in respect to the following points:

• By using supercritical carbon dioxide, naphthalene is hydrogenated at temperatures as low as 60°C, much lower than the reaction temperature in the conventional technology (higher than 200°C), which prevents the catalyst activity from being degraded.
• The use of carbon dioxide solvent improves the yield of decalin extensively.
• After the completion of synthetic reaction, the products can be readily isolated, and both catalyst and carbon dioxide can be easily recovered for recycling.