LOHC Publications

Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy

Abstract

Conspectus

The need to drastically reduce CO2 emissions will lead to the transformation of our current, carbon-based energy system to a more sustainable, renewable-based one. In this process, hydrogen will gain increasing importance as secondary energy vector. Energy storage requirements on the TWh scale (to bridge extended times of low wind and sun harvest) and global logistics of renewable energy equivalents will create additional driving forces toward a future hydrogen economy. However, the nature of hydrogen requires dedicated infrastructures, and this has prevented so far the introduction of elemental hydrogen into the energy sector to a large extent. Recent scientific and technological progress in handling hydrogen in chemically bound form as liquid organic hydrogen carrier (LOHC) supports the technological vision that a future hydrogen economy may work without handling large amounts of elemental hydrogen. LOHC systems are composed of pairs of hydrogen-lean and hydrogen-rich organic compounds that store hydrogen by repeated catalytic hydrogenation and dehydrogenation cycles. While hydrogen handling in the form of LOHCs allows for using the existing infrastructure for fuels, it also builds on the existing public confidence in dealing with liquid energy carriers. In contrast to hydrogen storage by hydrogenation of gases, such as CO2 or N2, hydrogen release from LOHC systems produces pure hydrogen after condensation of the high-boiling carrier compounds.

This Account highlights the current state-of-the-art in hydrogen storage using LOHC systems. It first introduces fundamental aspects of a future hydrogen economy and derives therefrom requirements for suitable LOHC compounds. Molecular structures that have been successfully applied in the literature are presented, and their property profiles are discussed. Fundamental and applied aspects of the involved hydrogenation and dehydrogenation catalysis are discussed, characteristic differences for the catalytic conversion of pure hydrocarbon and nitrogen-containing LOHC compounds are derived from the literature, and attractive future research directions are highlighted.

Finally, applications of the LOHC technology are presented. This part covers stationary energy storage (on-grid and off-grid), hydrogen logistics, and on-board hydrogen production for mobile applications. Technology readiness of these fields is very different. For stationary energy storage systems, the feasibility of the LOHC technology has been recently proven in commercial demonstrators, and cost aspects will decide on their further commercial success. For other highly attractive options, such as, hydrogen delivery to hydrogen filling stations or direct-LOHC-fuel cell applications, significant efforts in fundamental and applied research are still needed and, hopefully, encouraged by this Account.

2016 Chemica Engineering & Technology Cover Image

Coupling of a Liquid Organic Hydrogen Carrier System with Industrial Heat

Abstract

Energy storage is needed for adjusting fluctuating renewable energies to the actual energy demand. A liquid organic hydrogen carrier (LOHC) system is examined for its suitability as a storage system for industrial applications. Integration of the LOHC system, e.g., into a cement plant allows for optimized utilization of waste heat. The exhaust heat from the cement plant can increase the efficiency of the storage system. The effectiveness of the LOHC system can be elevated by twelve percentage points. The working electricity costs of the cement plant can be significantly reduced by adding a LOHC storage system.

ChemSusChem cover

Carbon Dioxide-Free Hydrogen Production with Integrated Hydrogen Separation and Storage

Abstract

An integration of CO2-free hydrogen generation through methane decomposition coupled with hydrogen/methane separation and chemical hydrogen storage through liquid organic hydrogen carrier (LOHC) systems is demonstrated. A potential, very interesting application is the upgrading of stranded gas, for example, gas from a remote gas field or associated gas from off-shore oil drilling. Stranded gas can be effectively converted in a catalytic process by methane decomposition into solid carbon and a hydrogen/methane mixture that can be directly fed to a hydrogenation unit to load a LOHC with hydrogen. This allows for a straight-forward separation of hydrogen from CH4 and conversion of hydrogen to a hydrogen-rich LOHC material. Both, the hydrogen-rich LOHC material and the generated carbon on metal can easily be transported to destinations of further industrial use by established transport systems, like ships or trucks.

The images shown on the cover were taken from papers in this issue: (top left) Tie-line plot for the equilibrium compositions (mole fraction) of the water−hexane−IPA system (see DOI: 10.1021/acs.jced.5b00542). (top right) Temperature dependence of electrical conductivity and Arrhenius plots (inset) for varied mole fractions of LiCl−KCl−CsCl molten salts (see DOI: 10.1021/acs.jced.5b00682). (bottom) Relationship between relative critical pore size and surface charge density of various monatomic metal cations (see DOI: 10.1021/acs.jced.5b00945).

Melting Points of Potential Liquid Organic Hydrogen Carrier Systems Consisting of N-Alkylcarbazoles

Abstract

Liquid organic hydrogen carriers (LOHCs) represent an attractive concept for storing hydrogen by the hydrogenation of usually aromatic compounds. One of the best investigated LOHCs is N-ethylcarbazole because of its favorable thermodynamic properties. However, its high melting point of 343.1 K could be a major drawback particularly in mobile applications. Therefore, it is desired to decrease the melting point of N-ethylcarbazole without significantly changing favorable properties such as the storage density or the reaction behavior of the carrier compound. To investigate the solid–liquid behavior during hydrogenation, the melting points of pure N-ethylcarbazole derivatives with increasing degree of hydrogenation as well as the liquidus line of the binary mixture of N-ethylcarbazole and N-ethyl-dodecahydro-carbazole were measured. Because of their structural and chemical resemblance binary mixtures consisting of different alkylcarbazole combinations were analyzed regarding their potential for a melting point depression. By the appropriate combination of N-alkylcarbazoles, it is possible to achieve a considerable melting point decrease to 297.1 K.

Abstract Image

Front cover React. Chem. Eng., 2016,1, 345-345 DOI: 10.1039/C6RE90014C

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

Abstract

The catalytic hydrogenation of the LOHC compound dibenzyltoluene (H0-DBT) was investigated by 1H 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 1H 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/Al2O3-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.

Separation and Purification Technology Cover

Development of a liquid chromatographic method for the separation of a liquid organic hydrogen carrier mixture

Abstract

Liquid organic hydrogen carriers (LOHC) are an interesting option for hydrogen storage and transportation. This concept is based on reversible hydrogenation and dehydrogenation of a carrier compound for uptake and release of hydrogen respectively. Among others, dibenzyltoluene is a potential LOHC due to its reasonable hydrogen storage capacity (6.2 ma-%) and high thermal stability. However, a huge number of stable intermediates with different degrees of hydrogenation are observed in a partially hydrogenated reaction mixture of dibenzyltoluene. For the process development and studies of the dibenzyltoluene reaction mechanism, it is crucial to determine physico-chemical properties of its various partially hydrogenated fractions, which requires their isolation from the reaction mixture. In this work, a reversed-phase high performance liquid chromatography (RP-HPLC) method for the separation and purification of partially hydrogenated mixtures of dibenzyltoluene is presented. The method was developed and validated at analytical scale and successfully scaled up to semi-preparative scale. The mixture was separated into four fractions according to their degree of hydrogenations using phenylhexyl silica stationary phase and a mobile phase consisting of acetone/water (96/4, v/v). Fractions with purity above 98% and yield higher than 90% were obtained in a semi-preparative column with an internal diameter of 50 mm.

Catalysis Letters Cover

Dicyclohexylmethane as a Liquid Organic Hydrogen Carrier: A Model Study on the Dehydrogenation Mechanism over Pd(111)

Abstract

We have studied the dehydrogenation of the liquid organic hydrogen carrier (LOHC) dicyclohexylmethane (DCHM) to diphenylmethane (DPM) and its side reactions on a Pd(111) single crystal surface. The adsorption and thermal evolution of both DPM and DCHM was measured in situ in ultrahigh vacuum (UHV) using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (HR-XPS). We found that after deposition at 170 K, the hydrogen-lean DPM undergoes C-H bond scission at the methylene bridge at 200 K and, starting at 360 K, complete dehydrogenation of the phenyl rings occurs. Above 600 K, atomic carbon incorporates into the Pd bulk. For the hydrogen-rich DCHM, the first stable dehydrogenation intermediate, a double π-allylic species, forms already at 190 K. Until 340 K, further dehydrogenation of the phenyl rings and of the methylene bridge occurs, yielding the same intermediate that is formed upon heating of DPM to this temperature, that is, DPM dehydrogenated at the methylene bridge. The onset for the complete dehydrogenation of this intermediate occurs at a much higher temperature than after adsorption of DPM. This behavior is mainly attributed to coadsorbed hydrogen from DCHM dehydrogenation. The results are discussed in comparison to our previous study of DPM and DCHM on Pt(111) revealing strong material dependencies.

Cover of the Journal of Chemical & Engineering data vol 61 issue 1

Measurement of Hydrogen Solubility in Potential Liquid Organic Hydrogen Carriers

Abstract

Liquid organic hydrogen carriers (LOHC) are potential compounds that can facilitate chemical energy storage and hydrogen logistics using reversible hydrogenation. For the process development, the physical solubility of hydrogen in potential LOHCs is required. In this work, solubility of hydrogen in the potential LOHC systems toluene/methylcyclohexane, N-ethylcarbazole/perhydro-N-ethylcarbazole, and dibenzyltoluene/perhydrodibenzyltoluene was measured using the static isochoric saturation method. The data were measured at low pressures up to 10 bar within the temperature range of (293 to 373) K. Hydrogen solubility in hydrogenated forms of the LOHCs was found to be higher compared to the dehydrogenated forms. Solubility in all substances increased with increasing temperature within the whole temperature range under consideration. The temperature dependency of the Henry coefficient of hydrogen in the solvents was correlated using the Benson and Krause correlation.

a diagram as shown in JCED_Measurement of Hydrogen Solubility in Potential Liquid Organic Hydrogen Carriers

Thermodynamic analysis of reversible hydrogenation for heat storage in concentrated solar power plants

Abstract

Heat storage in concentrated solar power plants is required to compensate for variable availability of solar radiation. The energy density achievable with thermochemical heat storage is higher than for molten salt which represents the state of the art technology. The efficiency of different reversible hydrogenation reactions as thermochemical heat storage systems have been examined, since they can be operated at appropriate temperatures. Thermal efficiency of reversible hydrogenation based thermal energy storage can reach values up to 65.9% and an overall efficiency of up to 23.1% compared to 25.7% without heat storage. The LOHC dibenzyltoluene and the metal hydride magnesium hydride turn out to be most suitable for this application.

Cover of the Journal of Chemical & Engineering data vol 61 issue 1

Binary Diffusion Coefficients of the Liquid Organic Hydrogen Carrier System Dibenzyltoluene/Perhydrodibenzyltoluene

Abstract

Liquid organic hydrogen carrier (LOHC) systems constitute a very promising concept for future hydrogen storage and logistics. The concept builds on the conversion of excess renewable energy to hydrogen via electrolysis followed by reversible catalytic hydrogenation/dehydrogenation of a diesel-like organic carrier molecule. For an ideal design of the catalytic process, insight into reaction mechanisms and kinetics but also precise knowledge on mass transport properties are necessary. In the present study, binary diffusion coefficients in selected binary LOHC mixtures with five different compositions of perhydrodibenzyltoluene (H18-LOHC) and dibenzyltoluene (LOHC) were measured by dynamic light scattering (DLS). The compositions were defined by mixing appropriate amounts of LOHC and H18-LOHC to realize different hydrogenation degrees of the LOHC. Binary diffusion coefficients were investigated over a temperature range from (264 to 571) K with an absolute uncertainty of (3 to 25) %. Moreover, an empirical equation describing the binary diffusion coefficients of all five mixture compositions over the complete temperature range with a root-mean-square deviation of less than 3 % was established. It was observed that the binary diffusion coefficient is independent of the hydrogenation degree of LOHC at temperatures above 430 K. For lower temperatures, the binary diffusion coefficient increases with decreasing degree of hydrogenation.

Binary Diffusion Coefficients of the Liquid Organic Hydrogen Carrier System Dibenzyltoluene/Perhydrodibenzyltoluene

Binary Diffusion Coefficients of the Liquid Organic Hydrogen Carrier System Dibenzyltoluene/Perhydrodibenzyltoluene