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Ind. Eng. Chem. Res. 2010, 49, 1018–1026
Kinetics of Hydrogen Uptake and Release from Heteroaromatic Compounds for Hydrogen Storage Farnaz Sotoodeh and Kevin J. Smith* Department of Chemical & Biological Engineering, UniVersity of British Columbia, 2360 East Mall, VancouVer, BC, Canada V6T 1Z3
The kinetics of hydrogen uptake by N-ethylcarbazole and carbazole, and the kinetics of H2 release from their hydrogenated products, is reported. The hydrogenation of N-ethylcarbazole at 130-150 °C on Ru was well described by a network of first-order stepwise hydrogenation reactions. The hydrogenation of N-ethylcarbazole was significantly faster than the hydrogenation of carbazole, and in both cases, >95% selectivity to the completely hydrogenated products, dodecahydro-N-ethylcarbazole and dodecahydrocarbazole, was achieved. The dehydrogenation of dodecahydro-N-ethylcarbazole at 101 kPa and 150-170 °C, proceeded to 100% conversion of the reactant over a Pd catalyst within 1 h. However, only 69% of the stored H2 was recovered due to a low selectivity to N-ethylcarbazole. In the case of dodecahydrocarbazole dehydrogenation at 101 kPa and 170 °C over the same Pd catalyst, 28% H2 recovery was obtained due to the strong adsorption of the product carbazole on the catalyst surface that also resulted in a slower rate of dodecahydrocarbazole dehydrogenation compared to dodecahydro-N-ethylcarbazole. 1. Introduction One approach to hydrogen storage for fuel-cell vehicles is to use organic compounds that have a high capacity to bind hydrogen covalently. Storage and release of the hydrogen is achieved by hydrogenation and dehydrogenation of the organic compounds, and these reactions are done in the presence of a catalyst. For transportation applications, the H2 storage (hydrogenation) would be done off-board the vehicle. Also, the candidate compounds must have high hydrogen release rates, more than 5.5 wt % H2 storage capacity (calculated as the mass of H2 liberated divided by the mass of hydrogenated compound), melting points below -40 °C and boiling points above 200 °C.1,2 Organic heteroaromatic compounds such as carbazole have attracted attention because they meet the storage capacity targets and the boiling point criteria. Although some results have been reported for H2 storage on heteroaromatic compounds, limited data are available on the kinetics of the reactions involved in H2 uptake and release.3-7 Carbazole and N-ethylcarbazole, when hydrogenated to dodecahydrocarbazole and dodecahydro-N-ethylcarbazole, have hydrogen storage capacities of 6.7 and 5.8 wt %,5,8,9 respectively. They are, therefore, potential candidates for hydrogen storage applications. However, the hydrogenation-dehydrogenation kinetics of these compounds are not well established, especially at the low temperature (
