(320b) Chemical Looping Hydrogenation with Metal Oxide Bronzes for Selective Hydrogen Activation and Utilization

Hydrogen is widely used in the chemical industry for hydrogenation reactions, including hydrotreating, ammonia synthesis, methanol synthesis, and a wide range of related processes. However, industrial hydrogenation reactions require high-purity H₂ streams. As hydrogen is currently produced almost exclusively via steam reforming of methane, which produces a mix of hydrogen and CO (syngas), production of high-purity hydrogen requires energy-intensive purification processes like water-gas shift and pressure-swing adsorption. Reduction of the energy demand and cost associated with these steps could have significant impact on future hydrogen utilization.

We are presenting a novel approach towards using low-purity hydrogen streams for thermocatalytic hydrogenation reactions via chemical looping hydrogenation (CLH). Chemical looping has been studied extensively for fuel combustion and reforming reactions where it serves to selectively transport oxygen from air to a fuel oxidation step, allowing partial or total oxidation of fuels without the need for (external) air separation and without diluting the product stream with N₂. Prior work on chemical looping has been almost exclusively used for oxygen transport and, to a lesser degree, activating nitrogen and carbon. Our work constitutes the first extension of the “looping principle” to the transport and purification of hydrogen.

Our approach uses catalyst-decorated metal oxide hydrogen bronzes as H carriers and the partial hydrogenation of acetylene (PHA) to ethylene as a test reaction. Using density functional theory and electroanalytical measurements, we have identified Pt-loaded tungsten trioxide, Pt-WO₃, as a suitable H carrier. On this carrier, H₂ is first activated through dissociation at Pt sites, and then inserted into the WO₃ via hydrogen spillover/intercalation. The carrier is then exposed to a pure acetylene stream in which WO₃ release H to hydrogenate C₂H₂ with high selectivity towards ethylene (>85%). The hydrogenation selectivity could be further controlled by tuning the degree of H intercalation into WO₃. Most significantly, the H loading of the carrier could be conducted directly with a syngas mixture containing as little as 15 mol% H₂ without loss of activity or selectivity toward PHA, demonstrating the ability of the proposed process to use impure H₂ streams directly for catalytic hydrogenation reactions without the need for costly and energy-intensive H₂ separation and purification steps.