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 HydrogenCarrier (LOHC) systems. LOHC systems store hydrogen by an exothermal catalytic hydrogenationreaction that converts the hydrogen-lean compounds of the LOHC system to their hydrogen-richcounterparts. All compounds of a technically suitable LOHC system are liquids and this offers theadvantage of simple logistics of chemically bound hydrogen in the existing infrastructure for fuels. Ondemand, hydrogen can be released from the hydrogen-rich LOHC molecule in an endothermal catalyticdehydrogenation at low hydrogen pressure (typically below 5 bar). Our contribution deals first withavailable sources of green hydrogen for a future hydrogen economy and then describes establishedtechnical processes to produce clean hydrogen from technically hydrogen-rich gas mixtures.Subsequently, the review focuses on the hydrogenation of aromatic and heteroaromatic compounds asthe key step of the LOHC-based hydrogen storage cycle. Special emphasis is given to the hydrogenchargingof hydrogen-lean LOHC compounds with various gas mixtures demonstrating that sucha Mixed Gas Hydrogenation (MGH) process offers the technical potential to selectively extract hydrogenin a chemically bound form that enables very efficient hydrogen logistics. In this way, low cost hydrogensources can be connected to high value hydrogen application, e.g. hydrogen filling stations for cleanmobility applications, to enable a future hydrogen economy

Operational Stability of a LOHC-Based Hot Pressure Swing Reactor for Hydrogen Storage

Apart from hydrogen logistics, stationary hydrogen storage applications using Liquid Organic Hydrogen Carrier (LOHC) systems are also of significant interest. In contrast to the traditional use of separate hydrogenation and dehydrogenation reactors, our so‐called oneReactor technology offers the advantages of a simpler storage unit layout and high dynamics in switching from hydrogen charging to hydrogen release. Here we report repeated hydrogenation and dehydrogenation cycles with one batch of liquid carrier for LOHC stability tests under defined hydrogenation and dehydrogenation conditions. We demonstrate up to 13 hydrogenation/dehydrogenation cycles over a total of 405 h of operation including two long dehydrogenation sequences over weekends. In general, longer dehydrogenation runs, i. e. exposure of the LOHC to catalyst at low hydrogen pressure and elevated temperatures (>280 °C), showed negative effects on both activity of the subsequent cycles and by‐product formation. Concerning catalyst activity and hydrogen productivity, stable productivity was achieved (within 3 to 9 cycles) under all conditions tested. Longer hydrogenation runs led to significantly higher stability of the reaction system

Hydrogen storage using a hot pressure swing reactor

Our contribution demonstrates that hydrogen storage in stationary Liquid Organic Hydrogen Carrier (LOHC) systems becomes much simpler and significantly more efficient if both, the LOHC hydrogenation and the LOHC dehydrogenation reaction are carried out in the same reactor using the same catalyst. The finding that the typical dehydrogenation catalyst for hydrogen release from perhydro dibenzyltoluene (H18-DBT), Pt on alumina, turns into a highly active and very selective dibenzyltoluene hydrogenation catalyst at temperatures above 220 °C paves the way for our new hydrogen storage concept. Herein, hydrogenation of H0-DBT and dehydrogenation of H18-DBT is carried out at the same elevated temperature between 290 and 310 °C with hydrogen pressure being the only variable for shifting the equilibrium between hydrogen loading and release. We demonstrate that the heat of hydrogenation can be provided at a temperature level suitable for effective dehydrogenation catalysis. Combined with a heat storage device of appropriate capacity or a high pressure steam system, this heat could be used for dehydrogenation

Efficient hydrogen release from perhydro-N-ethylcarbazole using catalyst-coated metallic structures produced by selective electron beam melting

A particularly suitable reactor concept for the continuous dehydrogenation of perhydro-N-ethylcarbazole in the context of hydrogen and energy storage applications is described. The concept addresses the fact that dehydrogenation is a highly endothermic gas evolution reaction. Thus, for efficient dehydrogenation a significant amount of reaction heat has to be provided to a reactor that is essentially full of gas. This particular challenge is addressed in our study by the use of a catalyst-coated (Pt on alumina), structured metal reactor obtained by selective electron beam melting. The so-obtained reactor was tested both as a single tube set-up and as a Hydrogen Release Unit (HRU) with ten parallel reactors. In stationary operation, the HRU realized a hydrogen release capacity of 1.75 kWtherm (960 Wel in a subsequent fuel cell) with up to 1.12 gH2 min−1 gPt−1 and a power density of 4.32 kWel L−1 of HRU reactor

Experimental determination of the hydrogenation/dehydrogenation - Equilibrium of the LOHC system H0/H18-dibenzyltoluene

Liquid organic hydrogen carrier (LOHC) systems store hydrogen through a catalystpromotedexothermal hydrogenation reaction and release hydrogen through an endothermalcatalytic dehydrogenation reaction. At a given pressure and temperature theamount of releasable hydrogen depends on the reaction equilibrium of the hydrogenation/dehydrogenation reaction. Thus, the equilibrium composition of a given LOHC system isone of the key parameters for the reactor and process design of hydrogen storage andrelease units. Currently, LOHC equilibrium data are calculated on the basis of calorimetricdata of selected, pure hydrogen-lean and hydrogen-rich LOHC compounds. Yet, real reactionsystems comprise a variety of isomers, their respective partially hydrogenatedspecies as well as by-products formed during multiple hydrogenation/dehydrogenationcycles. Therefore, our study focuses on an empirical approach to describe the temperatureand pressure dependency of the hydrogenation equilibrium of the LOHC system H0/H18-DBT under real life experimental conditions. Because reliable measurements of the degree of hydrogenation (DoH) play a vital role in this context, we describe in thiscontribution two novel methods of DoH determination for LOHC systems based on 13C NMRand GC-FID measurements

Hydrogenation of the liquid organic hydrogen carrier compound dibenzyltoluene – reaction pathway determination by 1 H NMR spectroscopy

The catalytic hydrogenation of the LOHC compound dibenzyltoluene (H0-DBT) was investigated by $^{1}$H NMR spectroscopy in order to elucidate the reaction pathway of its charging process with hydrogen in the context of future hydrogen storage applications. Five different reaction pathways during H0-DBT hydrogenation were considered including middle-ring preference (middle-side-side, MSS), side-middle-side order of hydrogenation (SMS), side-ring preference (SSM), simultaneous hydrogenation of all three rings without intermediate formation and statistical hydrogenation without any ring preference. Detailed analysis of the $^{1}$H NMR spectra of the H0-DBT hydrogenation over time revealed that the reaction proceeds with a very high preference for the SSM order at temperatures between 120 °C and 200 °C and 50 bar in the presence of a Ru/Al$_{2}$O$_{3}$-catalyst. HPLC analysis supported this interpretation by confirming an accumulation of H12-DBT species prior to full hydrogenation to H18-DBT with middle ring hydrogenation as the final step