Electrochemical Looping Hydrogenation Mediated by Tungsten Oxide Bronze

Global decarbonization demands carbon-free energy carriers. Hydrogen fits this role, yet most US supply remains fossil-derived, so near-zero emission routes such as water electrolysis powered by renewable electricity are needed. Industrial hydrogenation to make platform chemicals such as ammonia and biofuels is a major end use of hydrogen, but storing and transporting gaseous hydrogen adds cost and risk. We present here an integration strategy called electrochemical looping hydrogenation that uses mediated hydrogen transfer to decouple hydrogen activation from substrate hydrogenation. This strategy eliminates the storage and handling of hydrogen gas and relaxes the requirement for a single catalyst that must simultaneously activate H-atoms and H-acceptor species and while suppressing hydrogen gas evolution. As a proof of concept, we used nanocrystalline tungsten trioxide to mediate hydrogen activation and transfer via the formation of tungsten oxide hydrogen bronze. We further demonstrated operation in a continuous flow reactor system that accomplishes net water electrolysis and hydrogenation of the organic n-oxide radical TEMPOL as a model substrate. Kinetic studies showed first order dependence on TEMPOL but reaction rates saturated at TEMPOL concentrations above 7 mM. We also observed an apparent first-order dependence on the bulk hydrogen concentration in the bronze. Replacing water with toluene produced no significant change in the TEMPOL hydrogen kinetics. This result, alongside insights from quantum chemical calculations, support a hydrogenation mechanism involving electron-transfer associated with TEMPOL surface adsorption alongside transfer of a lattice-bound proton, analogous to a Mars van Krevelen type mechanism.