Comparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applications

The stepwise hydrogenation of 9-ethylcarbazole to 9-ethyl-perhydrocarbazole (Pl 12[H]) via partially hydrogenated intermediate(s) was studied over a number of supported ruthenium and rhodium catalysts. The reaction pathways were modelled and the rate constants for individual hydrogenation steps were compared. It was found that the selectivities to the reaction intermediates and products were highly dependent on the electronic structure of the particular metal and the nature of the support used. Ruthenium was found to be the most active metal for this reaction but it suffered from a poor selectivity to the desired product due to the formation of a kinetically stable intermediate, 9-ethyl-octahydrocarbazole (Pl 8[H]) in short reaction time. On the other hand, rhodium catalysts with moderate activity gave a higher selectivity to the fully hydrogenated product under comparable conditions. It was also found that the presence of a hydrophilic support such as alumina or rutile can give kinetically favoured cis-isomers of the 9-ethyl-perhydrocarbazole. Regarding application of the reversible hydrogen storage concept, the storage material should be able to switch between fully hydrogen loaded and unloaded forms during hydrogenation (material regeneration) and dehydrogenation (delivery of hydrogen gas) in short times. Formation of any stable intermediates and stereo-non-favoured isomers with a particular type of catalyst can result in significant implications to the overall storage capacity as well as operation times for hydrogen gas delivery and regeneration. Thus, this study yields valuable information on the suitability of various metal catalysts for use in hydrogen storage systems based on the liquid organic hydride (LOH) concept.