Hydrogen is fast emerging as an industrially viable clean energy vector. It is prominently positioned as a replacement for fossil fuels due to its abundance, high gravimetric energy density, and non-polluting combustion products. A major challenge obstructing widespread use of hydrogen as an energy vector is the significant hazards and cost associated with the storage and transport of such a flammable gas. Liquid organic hydrogen storage (LOHS) is an approach to mitigating these concerns by (de)hydrogenating organic molecules, allowing hydrogen to be transported in a liquid phase. This is not only inherently safer, as expensive high pressure systems are not required, but can also leverage pre-existing transport infrastructure from the petrochemical industry. The reversible (de)hydrogenation of carrier molecules currently uses precious metal catalysts, bringing significant cost and sustainability implications. If LOHS is to be commercially viable at a range of industrial scales, there is a pressing need to develop catalysts which either leverage more sustainable materials, or make better use of the scarce elements on which current catalysts depend. Firstly, Pd/TiO2 catalysts were synthesised using a sol immobilisation technique and tested using a model reaction: the acceptor less dehydrogenation of 1-phenylethanol to form acetophenone. The Pd/TiO2 catalyst synthesised showed a 3.5 increase in TOF compared to commercial Pd/C for the acceptorless dehydrogenation of 1-phenylethanol. The catalyst was also successful in catalysing a full LOHS cycle, maintaining high selectivity and constant rates for the hydrogenation and dehydrogenation steps. Modulation of polyvinyl alcohol(PVA) concentration enabled tight control of particle size distributions and mitigated sintering effects, yielding catalysts with significantly higher active site concentrations per unit mass of Pd and enhanced stability under harsh reaction conditions. Secondly, a kinetic isotope effect (KIE) study of the cleavage of the benzylic C H bond in 1-phenylethanol was performed to provide broader mechanistic insights in acceptorless dehydrogenation reactions. A KIE of 7.4 confirmed C H bond cleavage to be the rate determining step in this system. 2D NMR analysis of the post-reaction mixture showed 2D present in the methyl groups of the p-xylene solvent. This reaction was repeated in an EPR study, which demonstrated the presence of spin-trapped H radicals, indicating a radical exchange mechanism. D/H exchange was further demonstrated across a range of catalysts and common solvents using NMR spectroscopy, indicating this phenomenon occurs readily. Given the abundance of palladium driven reactions in organic solvents in the literature, it is plausible that H radicals are present and unaccounted for in a wide range of published organic syntheses. Finally, in an attempt to decouple the use of hydrogen as a green energy vector from scarce resources for LOHS catalysts, a plasmonic Cu/TiO2 catalyst was synthesised. The catalyst was active towards the acceptorless dehydrogenation of 1-phenylethanol, and its activity doubled when exposed to visible light. The direct delivery of energy to the catalyst surface via visible light, and the use of hot-electron driven reaction mechanisms under mild reaction conditions, provides a sustainable and inherently safe approach to driving LOHS reactions using earth-abundant catalysts.
Investigations into Heterogeneous Catalysts for Hydrogenation and Dehydrogenation Reactions
Rogers, H. (Author). 24 Apr 2024
Student thesis: Doctoral Thesis › PhD