The effects of alumina morphology and Pt electron property on reversible hydrogenation and dehydrogenation of dibenzyltoluene as a liquid organic hydrogen carrier

The morphologies and the electron property of catalysts play the very important roles in the hydrogenation and dehydrogenation of liquid organic hydrogen carriers (LOHCs) such as dibenzyltoluene (DBT). The different morphologies and pore structures of γ-Al₂O₃ and Mo_xC doped γ-Al₂O₃ were synthesized as the supports for Pt catalysts. After analyzing of various characterizations and catalytic testing, it was found that the large surface area and the mesoporous structure of catalysts are beneficial to both DBT hydrogenation and perhydro-dibenzyltoluene (H18-DBT) dehydrogenation. The doping of Mo_xC promoted the formation of the smaller Pt nanoparticles and increased Pt dispersion. The forming Pt–Mo structure is beneficial to hydrogen spillover which suppress the formation of by-product. The high Pt dispersion of 0.1 wt% Mo_xC doped Pt/Al₂O₃ catalyst plays the positive roles in increasing H18-DBT dehydrogenation activity.     

Benzyltoluene/dibenzyltoluene-based mixtures as suitable liquid organic hydrogen carrier systems for low temperature applications

In this contribution we propose mixtures of the two LOHC systems benzyltoluene (H0-BT)/perhydro benzyltoluene (H12-BT) and dibenzyltoluene (H0-DBT)/perhydro dibenzyltoluene (H18-DBT) as promising hydrogen storage media for technical applications at temperatures below ambient. The mixing of the two LOHC systems provides the advantage of a reduced viscosity of the hydrogen-rich system, for example a 20 wt% addition of H12-BT to H18-DBT reduces the viscosity at 10 °C by 80%. Interestingly, it is also found that the dehydrogenation of such mixture provides a hydrogen release productivity that is 12–16% higher compared to pure H18-DBT dehydrogenation under otherwise identical conditions. This enhanced rate is attributed to a combination of reduced hydrogen partial pressure in the reactor (due to the higher H12-BT vapor pressure), preferred H12-BT dehydrogenation (due to faster H12-BT diffusion) and effective transfer hydrogenation between the two LOHC systems.     

Burner-heated dehydrogenation of a liquid organic hydrogen carrier (LOHC) system

For a hydrogen-based economy, safe and efficient hydrogen storage is essential. Compared to other chemical hydrogen storage technologies, such as ammonia or methanol, liquid organic hydrogen carrier (LOHC) systems allow for a reversible storage of hydrogen while being easy to handle in a diesel-like manner. In our contribution, we describe for the first time the successful utilization of the exhaust gas enthalpy of a porous media burner to directly supply the dehydrogenation heat for a kW-scale dehydrogenation of the hydrogen-rich LOHC compound perhydro dibenzyltoluene (H18-DBT). Our setup demonstrates the dynamics of the dehydrogenation unit at a realized maximum hydrogen power of 3.9 kW_th, based on the lower heating value of the released hydrogen. For the intended applications with fluctuating hydrogen demand, e.g. a hydrogen refueling station (HRS) or stationary heating in buildings, a dynamic hydrogen supply from LOHC is important. Methane, e.g. from a biogas plant, is utilized in our scenario as a fuel source for the burner. Hydrogen is released within 30 min after cold start of the system. The dehydrogenation unit exhibits a power density relative to the reactor volume of about 0.5 kW_therm l⁻¹ based on the lower heating value of the hydrogen and a catalyst productivity of up to 0.65 g_H2 g_Pt⁻¹ min⁻¹ for hydrogen release from H18-DBT. An analysis of the by-products and reaction intermediates shows low by-product formation (e.g. maximum 0.6 wt.-% for high boilers and 0.9 wt.- % for low boilers) and uniform distribution of intermediates after the reaction. Thus, a relatively homogeneous temperature distribution and a uniform LOHC flow in the reaction zone can be assumed. Our findings illustrate the dynamics (heating rates of about 10 K min⁻¹) and performance of direct heating of a release unit with a burner and represent a significant step towards LOHC-based hydrogen provisioning systems at technically relevant scales.     

Two-dimensional Mo2C: An efficient promoter for hydrogen storage and release from a liquid organic hydrogen carrier

Two-dimensional Mo₂C (2D-Mo₂C) is reported for the first time as an effective promoter of a Pt/Al₂O₃ catalyst for both the hydrogenation and dehydrogenation of the liquid organic hydrogen carrier (LOHC) pair, dibenzyltoluene (DBT) and perhydro-dibenzyltoluene (H18-DBT), respectively. Addition of 6.2 wt% 2D-Mo₂C to a Pt/Al₂O₃ catalyst resulted in a significant increase in both the degree of hydrogenation and dehydrogenation compared to the unpromoted catalyst. An analysis of the initial (120 min) perhydro-DBT dehydrogenation kinetics in the temperature range of 270–330 °C, resulted in a reduction in apparent activation energy from 119.5 ± 3.8 kJ/mol for the Pt/Al₂O₃ catalyst to 110.4 ± 5.6 kJ/mol for the 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst. The 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst was also more stable than the unpromoted catalyst over several consecutive cycles of hydrogenation and dehydrogenation. Catalyst characterization showed that addition of 2D-Mo₂C resulted in an increase in particle size and electron density of the Pt, which enhanced both the hydrogenation and dehydrogenation reactions, despite the fact that the 2D-Mo₂C alone was inactive for both reactions.     

Intermetallide catalysts for hydrogen storage on the basis of reversible aromatics hydrogenation/dehydrogenation reactions

New efficient intermetallide catalysts for hydrogen storage in reversible processes of aromatics hydrogenation and naphthene dehydrogenation were studied. These catalysts provide an enhanced activity in the dehydrogenation of saturated organic molecules, with no side reactions like cracking, hydrogenolysis, ring opening, or coke formation occurring on these catalysts. The use of intermetallides provides some hydrogen storage capacity in the low-temperature region, while their catalytic activity in the dehydrogenation affords the hydrogen supply in the high-temperature range.     

Connected evaluation of polymer electrolyte membrane fuel cell with dehydrogenation reactor of liquid organic hydrogen carrier

Recently, hydrogen energy technologies attract attention as power systems. To develop hydrogen energy systems, hydrogen storage methods with high storage density and good safety are required. Liquid organic hydrogen carrier (LOHC) is one of the novel hydrogen storage technologies. LOHC has advantages of high storage density, good safety, and easy handling. In this study, a polymer electrolyte membrane fuel cell (PEMFC) stack is operated with hydrogen released from LOHC to evaluate the feasibility of the connected operation of the PEMFC stack and LOHC dehydrogenation reactor. Dibenzyltoluene (H0-DBT) is used as a LOHC material, and the dehydrogenation of perhydro dibenzyltoluene (H18-DBT) is conducted at 240–300 °C. Released hydrogen is purified by adsorbent of activated carbon to remove impurities. However, 100–1400 ppm of methane is observed after the purification, and the PEMFC stack power is reduced from 39.4 W to 39.0 W during the operation by hydrogen dilution and physical adsorption of methane. Then, to evaluate the irreversible damage, pure hydrogen was supplied to the PEMFC stack. The stack power is recovered to 39.4 W. It is concluded that the connected operation of the LOHC dehydrogenation reactor and PEMFC stack is feasible, and the activated carbon adsorbent can be a cost-effective purification method for LOHC.     

Pressurized hydrogen from charged liquid organic hydrogen carrier systems by electrochemical hydrogen compression

We demonstrate that the combination of hydrogen release from a Liquid Organic Hydrogen Carrier (LOHC) system with electrochemical hydrogen compression (EHC) provides three decisive advantages over the state-of-the-art hydrogen provision from such storage system: a) The EHC device produces reduced hydrogen pressure on its suction side connected to the LOHC dehydrogenation unit, thus shifting the thermodynamic equilibrium towards dehydrogenation and accelerating the hydrogen release; b) the EHC device compresses the hydrogen released from the carrier system thus producing high value compressed hydrogen; c) the EHC process is selective for proton transport and thus the process purifies hydrogen from impurities, such as traces of methane. We demonstrate this combination for the production of compressed hydrogen (absolute pressure of 6 bar) from perhydro dibenzyltoluene at dehydrogenation temperatures down to 240 °C in a quality suitable for fuel cell operation, e.g. in a fuel cell vehicle. The presented technology may be highly attractive for providing compressed hydrogen at future hydrogen filling stations that receive and store hydrogen in a LOHC-bound manner.     

Pt/CeO2 catalyst synthesized by combustion method for dehydrogenation of perhydro-dibenzyltoluene as liquid organic hydrogen carrier: Effect of pore size and metal dispersion

Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material due to its high storage density, low ignitability, and low cost. In this study, Pt/Al₂O₃ and Pt/CeO₂ catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). Pt/CeO₂ exhibits much faster dehydrogenation than Pt/Al₂O₃, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogenation rates, microstructural characterization by N₂ physisorption, CO chemisorption and transmission electron microscopy analysis are conducted, and the catalytic activities are evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due to the small pores in Pt/Al₂O₃, which is a rarely addressed issue for other LOHC materials. This is because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO₂ synthesized by the GNP is also compared with a commercial Pt/Al₂O₃ catalyst. The commercial Pt/Al₂O₃ catalyst shows a dehydrogenation of 17.8%/2.5 h, which is much slower than that of Pt/CeO₂ synthesized by the GNP, at 80.5%/2.5 h.     

A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl

Noble metal catalysts on mesoporous SiO₂ and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation and tercyclohexane (TCH) dehydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO₂ catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation.     

Hydrogenation of liquid organic hydrogen carrier systems using multicomponent gas mixtures

The cost of industrial hydrogen production and logistics, and the purity of hydrogen produced from different technologies are two critical aspects for the success of a future hydrogen economy. Here, we present a way to charge the Liquid Organic Hydrogen Carrier (LOHC) dibenzyltoluene (H0-DBT) with industrially relevant, CO₂- and CO-containing gas mixtures. As only hydrogen binds to the hydrogen-lean carrier molecule, this process step extracts hydrogen from the gas mixture and binds it selectively to the carrier. Pd on alumina has been identified as the most promising catalyst system for successfully hydrogenating H0-DBT using model gas mixtures resembling the compositions produced in methane reforming and in industrial coke production (up to 50% CO₂ and 7% CO). Up to 80% of the hydrogen present in the feedstock mixture could be extracted during the LOHC hydrogenation process. 99.5% of the reacting hydrogen was selectively bound to the H0-DBT LOHC compound. The purity of hydrogen released from the resulting perhydro dibenzyltoluene previously charged with the hydrogen-rich gas mixture proved to be up to 99.99 mol%.     

Boosting the hydrogenation activity of dibenzyltoluene catalyzed by Mg-based metal hydrides

Hydrogenation of dibenzyltoluene (DBT) is of great significance for the application in liquid organic hydrogen carriers (LOHCs). We successfully develop Mg-based metal hydrides (Mg₂NiH₄, MgH₂, and LaH₃) reactive ball-milling for the hydrogenation of DBT. Mg-based metal hydrides milled with 500 min exhibit the best catalytic activity, the hydrogen uptake of DBT can reach 4.63 wt% at the first 4 h and finally achieve 5.70 wt% through 20 h, which is the first time to use hydrogen storage material as a catalyst for the hydrogenation of DBT. The excellent catalytic hydrogenation performance of Mg-based metal hydrides mostly originates from numerous catalytic activity centers formed at the surfaces of Mg₂NiH₄ nanoparticles in the MgH₂ matrix. Inspired by this mechanism, more general metal hydrides can be explored for catalyzing the hydrogenation of LOHCs. The new application of Mg-based metal hydrides is beneficial to developing efficient LOHC based hydrogen storage systems and offers novel insights to hydride-based catalysts.     

Transition to renewable energy systems with hydrogen as an energy carrier

Unlike the present energy system based on fossil fuels, an energy system based on renewable energy sources with hydrogen and electricity as energy carriers would be sustainable. However, the renewable energy sources in general have less emergy than the fossil fuels, and their carriers have lower net emergy. Because of that they would not be able to support continuous economic growth, and would eventually result in some kind of a steady-state economy. An early transition to renewable energy sources may prove to be beneficial in the long term, i.e., it may result in a steady state at a higher level than in the case of a transition that starts later. Once the economy starts declining it will not be able to afford transition to a more expensive energy system, and transition would only accelerate the decline. Similarly, if a transition is too fast it may weaken and drain economy too much and may result in a lower steady state. If a transition is too slow, global economy may be weakened by the problems related to utilization of fossil fuels (such as global warming and its consequences) before transition is completed and the result again would be a lower steady state. Therefore, there must be an optimal transition rate; however, its determination would require very complex models and constant monitoring and adjustment of parameters.     

Integration of a well-designed biomass pair in electrochemical hydrogen pump reactor: ethylene glycol dehydrogenation and levulinic acid hydrogenation

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Chemical heat pump based on dehydrogenation and hydrogenation of i-propanol and acetone

A chemical heat pump, based on the reversible reaction couple of the i-propanol-acetone system, was investigated experimentally. The endothermic dehydrogenation of i-propanol occurred at 80°C with a Raney nickel catalyst suspended in the liquid phase. The unreacted i-propanol was separated from gaseous products in a condenser. The exothermic hydrogenation reaction of the acetone was performed at 200°C and 1atm, in the presence of the Raney nickel catalyst. The positive value (ΔG) of the change of Gibbs free energy can make the dehydrogenation reaction of i-propanol rather difficult. This problem can, though, be overcome by the continuous removal of gaseous acetone and hydrogen products from the reaction medium. The dehydrogenation rate equation of i-propanol was obtained as V = 0.1 C_p/(1 + 7.0 C_A). The gas phase hydrogenation reaction of acetone was performed in an exothermic tubular reactor. In order to estimate energy efficiency, a simulation of the separation stage was performed. Based on these experimental and simulation results, the optimal design specifications for the chemical heat pump were determined. The maximum hydrogenation of acetone was obtained when the mole ratio of acetone to hydrogen was 4.0. Energy efficiency was increased when the conversions of hydrogenation and dehydrogenation increased.     

A feasibility analysis of hydrogen delivery system using liquid organic hydrides

The paper discusses the techno-economic feasibility of a hydrogen storage and delivery system using liquid organic hydrides (LOH). Wherein, LOH (particularly cycloalkanes) are used for transporting the hydrogen in chemical bonded form at ambient temperature and pressure. The hydrogen is delivered through a catalytic dehydrogenation process. The aromatics formed in the process are used for carrying more hydrogen by a subsequent hydrogenation reaction. Cost economics were performed on a system which produces 10 kg/h of hydrogen using methylcyclohexane as a carrier. With proprietary catalysts we have demonstrated the possibility of hydrogen storage of 6.8 wt% and 60 kg/m³ of hydrogen on volume basis. The energy balance calculation reveals the ratio of energy transported to energy consumed is about 3.9. Moreover, total carbon footprint calculation for the process of hydrogen delivery including transportation of LOH is also reported. The process can facilitate a saving of 345 tons/year of carbon dioxide emissions per delivery station by replacing gasoline with hydrogen for passenger cars. There is an immense techno-economic potential for the process.     

Development of large scale unified system for hydrogen energy carrier production and utilization: Experimental analysis and systems modeling

The world's largest class hydrogen energy carrier production, storage, and utilization system has been operated in order to obtain basic data for practical use of the system using renewable energy. In this system, an alkaline water electrolyzer is combined with hydrogenation reactors to produce methylcyclohexane (MCH). Since electrolyzer behavior directly affects hydrogenation reaction, behaviors of the 150 kW class water electrolyzer against fluctuating electricity inputs were experimentally investigated. The cell stack voltage and hydrogen flow rate changed following temporal changes of the input current, whereas the temperature response was slow due to the large heat capacity of the system. Hydrogenation reactors performance using the hydrogen from the electrolyzer are reported. Then, based on the experiment data, a numerical simulation model for the electrolyzer was developed, which predicts the experimental result using fluctuating electricity very well. Furthermore, using the simulator, the heat utilization from the hydrogenation reaction for the electrolyzer warm-up process was investigated.     

Hydrogen storage in liquid organic hydride: Selectivity of MCH dehydrogenation over monometallic and bimetallic Pt catalysts

Hydrogen storage for mobile and stationary applications is an expanding research topic. One of the more promising storage techniques relies on the reversibility, high selectivity, and high hydrogen density of liquid organic hydrides, in particular methylcyclohexane (MCH). Catalyst evaluation for MCH dehydrogenation to toluene is based on three catalytic parameters: activity, selectivity, and stability. Current catalysts, optimized for catalytic reforming, do not meet the targeted aromatic selectivity (+99%) for MCH dehydrogenation. Therefore, a range of Pt catalysts was prepared and compared with commercially available catalysts in a fixed-bed reactor under operating conditions suitable for mobile and stationary applications. The best overall performance was realized by a particular monometallic Pt catalyst. This catalyst showed superior activity, selectivity, and stability compared with other prepared and commercial catalysts. As an effort to further enhance the aromatic selectivity, this study identified the main side-reactions associated with MCH dehydrogenation, the effect of operating parameters on by-product yields, and the effect of catalyst deactivation on long-term selectivity.     

Utilization of green ammonia as a hydrogen energy carrier for decarbonization in spark ignition engines

Rising concerns about the dependence of modern energy systems on fossil fuels have raised the requirement for green alternate fuels to pave the roadmap for a sustainable energy future with a carbon-free economy. Massive expectations of hydrogen as an enabler for decarbonization of the energy sector are limited by the lack of required infrastructure, whose implementation is affected by the issues related to the storage and distribution of hydrogen energy. Ammonia is an effective hydrogen energy carrier with a well-established and mature infrastructure for long-distance transportation and distribution. The possibility for green ammonia production from renewable energy sources has made it a suitable green alternate fuel for the decarbonization of the automotive and power generation sectors. In this work, engine characteristics for ammonia combustion in spark ignition engines have been reported with a detailed note on engines fuelled with pure ammonia as well as blends of ammonia with gasoline, hydrogen, and methane. Higher auto-ignition temperature, low flammability, and lower flame speed of ammonia have a detrimental effect on engine characteristics, and it could be addressed either by incorporating engine modifications or by enhancing the fuel quality. Literature shows that the increase in compression ratio from 9.4:1 to 11.5:1 improved the maximum power by 59% and the addition of 10% hydrogen in supercharged conditions improved the indicated efficiency by 37%. Challenges and strategies for the utilization of ammonia as combustible fuel in engines are discussed by considering the need for technical advancements as well as social acceptance. Energy efficiency for green ammonia production is also discussed with a due note on techniques for direct synthesis of ammonia from air and water.     

Feasibility of renewable energy storage using hydrogen in remote communities in Bhutan

This paper considers the technical and economic feasibility of using renewable energy with hydrogen as the energy storage medium for two remote communities in Bhutan, selected to illustrate two common scenarios presenting different challenges. The Royal Government of Bhutan has published plans to provide electricity to all households in the next 20 years, but the practical problems of extending the grid over long distances and mountainous terrain will make that target difficult and expensive to achieve. Consequently, the possibility of using natural energy and diversified generation is attractive. This paper examines the use of hydro power in one community and photovoltaics with wind power in another. Hydrogen is the proposed energy storage medium in both cases. Analysis suggests that it is technically possible to use renewable energy and hydrogen for diversified power supplies and that where, as here, the costs of grid extension are high, it may also be financially viable. Thus we argue that there is a good case for establishing a test and demonstration system near the capital Thimphu for further investigation prior to use in remote locations.     

Dehydrogenation of a liquid organic hydrogen carrier compound 1-(3-cyclohexylpropyl)-3-ethylcyclohexane: A density functional theory study

As a compound for liquid organic hydrogen carrier (LOHC) applications, 1-(3-cyclohexylpropyl)-3-ethylcyclohexane was designed and its dehydrogenation reaction was investigated using density functional theory calculations. To check how this compound could be stable, vibrational frequency analysis and formation energy calculations were conducted. Our findings revealed that this LOHC compound was dynamically and chemically stable. Using Mulliken population analysis, the dehydrogenation process was clearly explained. To reduce the dehydrogenation energy, different substituents, such as N, Cl, and Br were used. Our results suggested that N-substitution could be potentially suitable to lower the dehydrogenation energy. Reaction barriers of pristine and N-substituted systems for dehydrogenation reactions were investigated through nudged elastic band methods. In addition, the gap between HOMO and LUMO was calculated to check chemical reactivity.