Hydrogenation of aromatic and heteroaromatic compounds - a key process for future logistics of green hydrogen using liquid organic hydrogen carrier systems

This review deals with the chemical storage of green hydrogen in the form of Liquid Organic Hydrogen Carrier (LOHC) systems. LOHC systems store hydrogen by an exothermal catalytic hydrogenation reaction that converts the hydrogen-lean compounds of the LOHC system to their hydrogen-rich counterparts. All compounds of a technically suitable LOHC system are liquids and this offers the advantage of simple logistics of chemically bound hydrogen in the existing infrastructure for fuels. On demand, hydrogen can be released from the hydrogen-rich LOHC molecule in an endothermal catalytic dehydrogenation at low hydrogen pressure (typically below 5 bar). Our contribution deals first with available sources of green hydrogen for a future hydrogen economy and then describes established technical processes to produce clean hydrogen from technically hydrogen-rich gas mixtures. Subsequently, the review focuses on the hydrogenation of aromatic and heteroaromatic compounds as the key step of the LOHC-based hydrogen storage cycle. Special emphasis is given to the hydrogen-charging of hydrogen-lean LOHC compounds with various gas mixtures demonstrating that such a Mixed Gas Hydrogenation (MGH) process offers the technical potential to selectively extract hydrogen in a chemically bound form that enables very efficient hydrogen logistics. In this way, low cost hydrogen sources can be connected to high value hydrogen application, e.g. hydrogen filling stations for clean mobility applications, to enable a future hydrogen economy.