The Transportation and Transformation of Energy Through Reversible Hydrogenation

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Carrier, Andrew James
Hydrogen Energy , Green Chemistry
Cycles of reversible hydrogenation reactions are important for at least two different energy-related applications: reversible chemical hydrogen storage and thermally regenerative fuel cells. Hydrogen fuel is a green alternative to conventional hydrocarbon fuels for transportation applications. This is because the combustion product of hydrogen is simply water, which is non-toxic and ubiquitous. Hydrogen is also an attractive fuel because of its high energy content; however, because it is a gas it has poor volumetric energy density. In Chapter 2, ionic liquids consisting of both cations and anions that can undergo reversible dehydrogenative aromatization were used to chemically store hydrogen. Cations investigated included pyridinium ions, which were easily hydrogenated but could not be regenerated through the dehydrogenation of piperidinium ions; and carbazole containing ammonium (whose synthesis failed) and imidazolium (which failed to hydrogenate) cations. The anions studied were heterocyclic carboxylates and sulfonates, these ions were observed to undergo both hydrogenation and dehydrogenation to various degrees when reacted in solution. However, as components of ionic liquids, they fail to react at a significant rate. The viscosity of the fluids was suspected to be hindering the diffusion of either hydrogen or the ions to or from the catalyst surface. In addition to using hydrogen as the primary source of energy in a vehicle, reversible hydrogenation can form the basis of a thermally regenerative fuel cell: a device that converts low grade vehicle waste heat, from a conventional engine, into electricity for the vehicles auxiliary power units. In Chapter 3, secondary benzylic alcohols, in particular 1-phenyl-1-propanol, were determined to be able to undergo dehydrogenation to the corresponding ketone rapidly and with extremely high selectivity over a palladium on silica catalyst. The dehydrogenation gave an initial rate of hydrogen evolution of 4.6 l of hydrogen per gram of palladium per minute and the enthalpy and entropy of the dehydrogenation is +56 kJ mol-1 and +117 J mol-1 K-1. This adsorbed energy can then be released as electricity in a fuel cell and be used to power auxiliary units in a vehicle without decreasing fuel economy.
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