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Hydrogenation and dehydrogenation kinetics and catalysts for new hydrogen storage liquids Sotoodeh, Farnaz

Abstract

Due to the very low density of H₂, practical storage and recovery of H₂ has been a challenge in utilizing H₂ as an alternative fuel. Organic heteroaromatics have attracted interest because of their thermal stability and high storage capacity. In this study, H₂ storage and recovery from these compounds were investigated. The kinetics of the hydrogenation/dehydrogenation reactions was studied and DFT calculations were used to understand the dehydrogenation product distribution. The hydrogenation of N-ethylcarbazole and carbazole at 403-423 K on a supported Ru catalyst was well described by first-order kinetics. The hydrogenation of N-ethylcarbazole was significantly faster than the hydrogenation of carbazole, and >95 % selectivity to dodecahydro-N-ethylcarbazole and dodecahydrocarbazole was achieved, respectively. The dehydrogenation kinetics of dodecahydro-N-ethylcarbazole was studied at 101 kPa and 423-443 K over a Pd catalyst prepared by wet impregnation and calcination in air. The reactions followed first-order kinetics with 100 % conversion but only 69 % recovery of H₂ was achieved at 443 K, due to minimal selectivity to Nethylcarbazole. The complete recovery of H₂ from dodecahydro-N-ethylcarbazole was achieved at 443 K and 101 kPa using Pd/SiO₂ catalysts prepared by incipient wetness impregnation with calcination in He. The dehydrogenation TOF and selectivity to N-ethylcarbazole were dependent upon the Pd particle size. The effect of the N heteroatom on the dehydrogenation of polyaromatics was studied by comparison of dodecahydro-N-ethylcarbazole, dodecahydrocarbazole and dodecahydrofluorene dehydrogenation over Pd catalysts. The dehydrogenation of dodecahydro-N-ethylcarbazole and dodecahydrocarbazole were structure sensitive. The dehydrogenation rate of dodecahydrocarbazole was slower than dodecahydro-N-ethylcarbazole. Despite catalyst poisoning through the N atom in dodecahydrocarbazole, the N heteroatom was found to favor dehydrogenation, making heteroaromatics better candidates for H₂ storage than aromatics. The structure sensitivity of the reactions and the observed product distribution are explained in view of DFT calculations that showed that the adsorption of dodecahydro-N-ethylcarbazole on Pd required multiple catalytic sites and the heat of adsorption was dependent upon the surface structure. The effect of the ethyl group and the N heteroatom on the dehydrogenation rate of dodecahydro- N-ethylcarbazole was also investigated by comparing the adsorption energies of dodecahydro-N-ethylcarbazole with dodecahydrocarbazole and dodecahydrofluorene.

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