Main-group catalysis for H2 purification based on liquid organic hydrogen carriers

Abstract

Molecular hydrogen (H2) has been hailed as one of the most important energy carriers of the future, based on its high gravimetric energy density and the low environmental impact of its combustion product. In the mid-term future, a huge amount of H2 will be produced from a variety of hydrocarbon sources including biomass through the in-depth purification of crude H2, which is a gaseous mixture of H2, CO, CO2, and other components. Processes for the removal of such contaminants prior to H2 storage are of particular importance to prevent the deactivation of metal-based catalysts used in fuel cells and the chemical industry; however, bypassing these purification processes is desirable, given their energy consumption and environmental impact, which ultimately increases the cost of the produced H2. Here, we demonstrate a molecular-based strategy to separate H2 from a gaseous mixture of H2/CO/CO2/CH4 and simultaneously store it in N-heterocyclic compounds that act as liquid organic hydrogen carriers (LOHCs), which can be applied for the production of H2 by subsequent dehydrogenation. A shelf-stable triaryl borane successfully catalyzes the hydrogenation of these N-heteroaromatic compounds in the co-presence of substantial amounts of CO, CO2, CH4, and H2O, which is commensurate to a simultaneous separation and storage of H2 (catalyst turnover number of up to 1480), and the subsequent dehydrogenation (H2 recovery), both of which proceed without solvents. Our results demonstrate that LOHCs can potentially be used for H2 purification in addition to their well-established use in H2 storage.