Liquid Organic Hydrogen Carrier Systems Research Articles

Temperature imaging during the hydrogen release reaction from a liquid organic hydrogen carrier (LOHC) system using phosphor thermometry

Liquid organic hydrogen carrier (LOHC) systems offer a particularly interesting option for chemical hydrogen storage. In order to characterize and understand the endothermal hydrogen release from the carrier liquid and to evaluate suitable catalyst materials, knowledge of the temperature fields in the dehydrogenation reactor is important. One suitable technique for planar temperature sensing in reacting systems is phosphor thermometry. It is based on the excitation of a luminescent material by a laser pulse and detection of the subsequent phosphorescence signal. We investigated the luminescence of the thermographic phosphor (Sr,Ca)SiAIN3:Eu2+ (“SCASN:Eu2+”) dispersed in the H0-DBT / H18-DBT LOHC system in a temperature range from 400 to 600 K. A measurement cell enables repeatable and homogeneous measurement conditions of the hydrogen release reaction. A catalytic plate was put inside the heated LOHC. Temperature fields during the hydrogen release reaction were measured for the first time using the phosphorescence decay time (PDT) and the phosphorescence intensity ratio method (PIR). As expected, a strong cooling at the catalyst surface during the endothermal hydrogen release reaction could be observed, which was quantified to be in the range of 40 K.Graphical abstract

Aquatic toxicity, bioaccumulation potential, and human estrogen/androgen activity of three oxo-Liquid Organic Hydrogen Carrier (oxo-LOHC) systems

The Liquid Organic Hydrogen Carrier (LOHC) technology offers a technically attractive way for hydrogen storage. If LOHC systems were to fully replace liquid fossil fuels, they would need to be handled at the multi-million tonne scale. To date, LOHC systems on the market based on toluene or benzyltoluene still offer potential for improvements. Thus, it is of great interest to investigate potential LOHCs that promise better performance and environmental/human hazard profiles. In this context, we investigated the acute aquatic toxicity of oxygen-containing LOHC (oxo-LOHC) systems. Toxic Ratio (TR) values of oxo-LOHC compounds classify them baseline toxicants (0.1 < TR < 10). Additionally, the mixture toxicity test conducted with D. magna suggests that the overall toxicity of a benzophenone-based system can be accurately predicted using a concentration addition model. The estimation of bioconcentration factors (BCF) through the use of the membrane-water partition coefficient indicates that oxo-LOHCs are unlikely to be bioaccumulative (BCF < 2000). None of the oxo-LOHC compounds exhibited hormonal disrupting activities at the tested concentration of 2 mg/L in yeast-based reporter gene assays. Therefore, the oxo-LOHC systems seem to pose a low level of hazard and deserve more attention in ongoing studies searching for the best hydrogen storage technologies.

Demonstrating the applicability of Raman spectroscopy for the characterization of hydrogen loading of bicyclic liquid organic hydrogen carriers (LOHCs) under process-relevant conditions

During hydrogenation and dehydrogenation reactions of liquid organic hydrogen carriers (LOHC), their thermophysical properties change significantly as a function of the degree of hydrogenation (DoH). Compared with conventional techniques, Raman spectroscopy offers a contactless methodology for online process monitoring without the need of sampling. This work demonstrates the application of Raman spectroscopy for the determination of the DoH under process-relevant conditions for LOHC systems based on diphenylmethane (DPM) and benzyltoluene (BT) up to 473 K. For the DPM-based system, a previous study is extended by examining the influence of process-like disturbances. For the BT-based system, a novel approach using indirect hard modeling of the unpolarized Raman spectra is applied, which results in strongly reduced uncertainties and insensitivity towards process-like disturbances, e.g. reaction by-products. For a corresponding testing set exhibiting a pronounced fluorescent background in the Raman spectra, the DoH is determined with an average absolute deviation of 0.0057.

Does the Oxygen Functionality Really Improve the Thermodynamics of Reversible Hydrogen Storage with Liquid Organic Hydrogen Carriers?

Liquid organic hydrogen carriers (LOHCs) are aromatic molecules that are being considered for the safe storage and release of hydrogen. The thermodynamic properties of a range of aromatic ethers were investigated using various experimental and theoretical methods to assess their suitability as LOHC materials. The absolute vapour pressures were measured for benzyl phenyl ether, dibenzyl ether and 2-methoxynaphthalene using the transpiration method. The standard molar enthalpies and entropies of vaporisation/sublimation were derived from the temperature dependence of the vapour pressures. The combustion energies of benzyl phenyl ether and dibenzyl ether were measured using high-precision combustion calorimetry, and their standard molar enthalpies of formation were derived from these data. High-level quantum chemical calculations were used to calculate the standard molar enthalpies of formation in the gas phase for benzyl phenyl ether, dibenzyl ether and 2-methoxynaphthalene. The latter values agreed very well with the experimental results obtained in this work. The thermodynamic properties of the hydrogenation/dehydrogenation reactions in liquid phase in LOHC systems based on methoxy–benzene, diphenyl ether, benzyl phenyl ether, dibenzyl ether and 2-methoxynaphthalene were derived and compared with the data for similarly structured hydrogen carriers based on benzene, diphenylmethane, 1,2-diphenylethane, 1,3-diphenylpropane and naphthalene. The influence of the oxygen functionality on the thermodynamic properties of the hydrogenation/dehydrogenation reactions was evaluated.

Thermophysical properties of the liquid organic hydrogen carrier system based on benzyltoluene considering influences of isomerism and dissolved hydrogen

The benzyltoluene (BT)-based liquid organic hydrogen carrier (LOHC) system currently considered for large-scale applications represents a mixture of regioisomers. To characterize this complex system, a comprehensive experimental database for various thermophysical properties of synthesized BT isomers and their mixtures without and with the presence of hydrogen (H2) close to vapor-liquid equilibrium is established using optical and conventional techniques at process-relevant temperatures and pressures up to 573 K and 6 MPa. The surface tension varies by less than 4% among the dehydrogenated (H0-BT) or hydrogenated (H12-BT) isomers. The density and viscosity of mixtures of H0-BT or H12-BT isomers can be described by simple mixing rules with average absolute relative deviations of 0.022% and 0.38%. With increasing H2 pressure, the viscosity remains nearly constant, while the interfacial tension decreases by up to 5%. The thermal and mutual diffusivity of H12-ortho-BT containing dissolved H2 at 6 MPa decrease and increase with increasing temperature.

Raman spectroscopy for the determination of hydrogen concentration in liquid organic hydrogen carrier systems

This work reports on the development of a calibration approach of Raman spectroscopy for the determination of the concentration of dissolved hydrogen (H2) in representative compounds and mixtures of liquid organic hydrogen carrier (LOHC) systems based on diphenylmethane and ortho-benzyltoluene (o-BT). Hydrogen solubility data are measured by the isochoric-saturation method at temperatures and pressures up to 473 K and 7 MPa where Raman spectra of the saturated liquid phase are recorded in parallel. By selecting an appropriate frequency range attributed to the LOHC signatures in isotropic spectra, a calibration which is independent of the LOHC system, the degree of hydrogenation of the LOHC samples, and temperature can be found. The transfer of the calibration to another setup is successfully tested up to 523 K and 6 MPa, where H2 solubilities in the fully hydrogenated o-BT can be determined with an average absolute deviation of 0.009 from theoretically calculated data.

Pt/Al2O3 as efficient catalyst for the dehydrogenation of Dodecahydro-N-ethylcarbazole

The use of dodecahydro-N-ethylcarbazole in hydrogen storage for liquid organic hydrogen carrier systems holds potential for achieving large-scale application. In order to investigate the relationship between dodecahydro-N-ethylcarbazole dehydrogenation performance and Pt particles size, a series of monometallic Pt/Al2O3 catalysts with different Pt particles size from 0.70 nm to 5.12 nm were controlled synthesized by a simple impregnation method. The obtained catalysts are systematically characterized via XRD, 27Al MAS NMR, SEM-EDS, TEM, N2-adsorption, CO-IR, H2-TPR and NH3-TPD analyses, which provide essential micro-structure of these catalysts. The dehydrogenation of dodecahydro-N-ethylcarbazole over Pt/Al2O3 is a typical consecutive reaction, which follows a reaction sequence of dodecahydro-N-ethylcarbazole → octahydro-N-ethylcarbazole → tetrahydro-N-ethylcarbazole → N-ethylcarbazole. These dehydrogenation steps are quite sensitive to the Pt particle size. Large Pt particle can greatly enhance the reaction rates in octahydro-N-ethylcarbazole → tetrahydro-N-ethylcarbazole and tetrahydro-N-ethylcarbazole → N-ethylcarbazole steps. However, these two steps are difficult to take place in small Pt particles. The high performance of large Pt particle in dehydrogenation can be ascribed to its highly exposed (111) crystal face. The optimized catalyst Pt5.0/γ-Al2O3 with Pt average size of 4.63 nm demonstrated exceptional performance in dodecahydro-N-ethylcarbazole dehydrogenation at 180 °C and 101 kPa, and the dodecahydro-N-ethylcarbazole conversion of 100 %, N-ethylcarbazole selectivity of 96.44 % and dehydrogenation efficiency of 98.73 % can be achieved. These results provide valuable insights for the development of efficient metal-based catalysts for liquid organic hydrogen carriers hydrogen storage system.

Thermodynamics of reversible hydrogen storage: Does alkoxy-substitution of naphthalene yield functional advantages for LOHC systems?

The reversible hydrogenation/dehydrogenation of aromatic molecules, known as liquid organic hydrogen carriers (LOHCs), is considered an attractive option for the safe storage and release of elemental hydrogen. The LOHC systems based on the alkoxy-naphthalene/alkoxy-decalin studied in this work can become potentially attractive from the point of view of the thermodynamic conditions of the reversible hydrogenation/dehydrogenation processes. This work reports the results of a complex experimental investigation of the thermochemical properties of the reactants of the LOHC systems. The enthalpies of formation were measured using high-precision combustion calorimetry, the enthalpies of vaporization and sublimation were derived from the vapor pressure–temperature dependencies measured using the transpiration method, and the melting temperatures and enthalpies of fusion were measured using the differential scanning calorimetry method. The liquid-phase enthalpies of formation of methoxy- and ethoxy-substituted naphthalenes and methoxy- and ethoxy-substituted decalins were derived and used for the thermodynamic analysis of hydrogenation/dehydrogenation reactions and transferhydrogenation reactions.

Benzyl-methylbenzyl-benzene: Improving hydrogen storage and release performance of dibenzyltoluene based liquid organic hydrogen carrier

Liquid organic hydrogen carrier (LOHC) technology enables the safe, easy, and cost-effective storage and transportation of large quantities of hydrogen using inexpensive and mass-produced compounds. The LOHC materials play a crucial role in hydrogen storage capacity, reaction kinetics, reaction temperature, and reversibility. However, the development and optimization of LOHC materials is limited due to using of mass-produced materials for bulk hydrogen logistics. Herein, we propose a molecular engineering approach to overcome the limitations of commercial dibenzyltoluene (DBT)-based LOHC systems, considered one of the most promising LOHC materials based on the heat transfer oil, Marlotherm SH (MSH), owing to its superior physical properties (wide liquid range, and non-flammable properties), low price, and low toxicity. We devised a new chemical structure, benzyl-methylbenzyl-benzene (BMB), with highly improved dehydrogenation/hydrogenation performance achieved by rearranging the methyl group of DBT to overcome the limitations of slow reaction kinetics in terms of its chemical structure. The BMB exhibits a 150 % faster hydrogenation than MSH at 150 °C, and it releases 170 % more hydrogen compared to DBT at 270 °C. In addition, we analyzed the dehydrogenation/hydrogenation pathways according to the structures of DBT and BMB. Furthermore, we found that BMB (27 %) is inevitably generated in the MSH synthesis process alongside DBT (73 %). Based on this, we propose a streamlined and sustainable engineered method for improved MSH material manufacturing with a high BMB ratio (∼70 %). The scientific findings provide a fundamental understanding of DBT-based LOHC systems and new insights into the design of novel LOHC candidates.

Recent Advances in Reversible Liquid Organic Hydrogen Carrier Systems: From Hydrogen Carriers to Catalysts.

Liquid organic hydrogen carriers (LOHCs) have gained significant attention for large-scale hydrogen storage due to their remarkable gravimetric hydrogen storage capacity (HSC) and compatibility with existing oil and gas transportation networks for long-distance transport. However, the practical application of reversible LOHC systems has been constrained by the intrinsic thermodynamic properties of hydrogen carriers and the performances of associated catalysts in the (de)hydrogenation cycles. To overcome these challenges, thermodynamically favored carriers, high-performance catalysts, and catalytic procedures need to be developed. Here, significant advances in recent years have been summarized, primarily centered on regular LOHC systems catalyzed by homogeneous and heterogeneous catalysts, including dehydrogenative aromatization of cycloalkanes to arenes and N-heterocyclics to N-heteroarenes, as well as reverse hydrogenation processes. Furthermore, with the development of metal complexes for dehydrogenative coupling, a new family of reversible LOHC systems based on alcohols is described that can release H2 under relatively mild conditions. Finally, views on the next steps and challenges in the field of LOHC technology are provided, emphasizing new resources for low-cost hydrogen carriers, high-performance catalysts, catalytic technologies, and application scenarios.

Advances in liquid organic hydrogen carriers: developing efficient dehydrogenation strategies.

In pursuit of global carbon neutrality, countries are intensifying their efforts to harness clean energy sources. Hydrogen emerges as a superior alternative to traditional fossil fuels and plays a crucial role in the global energy shift. Liquid Organic Hydrogen Carrier (LOHC) systems are lauded for their high hydrogen storage capacity, ease of handling, and safe and efficient transportation, positioning them as effective solutions for extensive hydrogen storage and international distribution. Nevertheless, the dehydrogenation of hydrogen-rich LOHCs is slow, requiring high temperatures and substantial energy inputs. Addressing these challenges by reducing energy demands and improving dehydrogenation rates is essential for advancing LOHC technology. This paper comprehensively examines various LOHC systems, focusing on the selection of carriers and dehydrogenation catalysts, and their dehydrogenation efficacy. It also highlights our recent contributions in photocatalytic LOHC and outlines future research directions to enhance LOHC technology.

Revealing the Effects of Weak Surfactants on the Dynamics of Surface Fluctuations by Surface Light Scattering.

Surfactant monolayers at liquid interfaces induce a viscoelastic behavior that influences the dynamics of surface fluctuations probed by surface light scattering (SLS). Recent thermophysical property research on viscosity and interfacial tension of liquid organic hydrogen carrier (LOHC) systems based on diphenylmethane suggested that such viscoelastic effects may also be present here, although not being expected a priori. To prove the hypothesis that the LOHC intermediate cyclohexylphenylmethane (H6-DPM) can induce a surfactant-like behavior, binary mixtures of diphenylmethane (H0-DPM) or dicyclohexylmethane (H12-DPM) with small amounts of H6-DPM were studied by SLS in combination with conventional viscometry and tensiometry and molecular dynamics simulations between (303 and 473) K. Only in mixtures with H0-DPM which has a slightly larger surface tension than H6-DPM, the presence of the latter compound causes a significant effect on the dynamics of surface fluctuations, especially on their damping. In analogy to the concentration-dependent behavior observed for a monolayer of a highly amphiphilic ionic surfactant on the surface of water at ambient temperature, the orientation of H6-DPM molecules with respect to the surface appears to change from a preferentially perpendicular to a parallel alignment with increasing temperature. This demonstrates that viscoelastic effects including accompanied surface orientation effects can be resolved by SLS even for weakly asymmetric surface-active molecules such as H6-DPM in its diluted mixtures with very similar species.

Exploring the potential of liquid organic hydrogen carrier (LOHC) system for efficient hydrogen storage and Transport: A Techno-Economic and energy analysis perspective

Despite its potential as an environmentally clean fuel and energy source, hydrogen storage and utilization has been significantly hampered by its extremely low volumetric density (0.08988 g/L at 1 atm), making it inefficient to store and transport. Therefore, liquid organic hydrogen carrier (LOHC) systems are being recently investigated as potential alternatives for hydrogen storage and transport. However, as a budding research area, the selection of a suitable LOHC, its deployment in hydrogen fuel stations, and its economic viability are not well established. Therefore, this study proposes a comprehensive investigation of four different LOHCs [Methylcyclohexane (MCH), Dibenzyltoluene (DBT), N-ethylcarbazole (NEC) and Naphthalene (NAP)] via Aspen HYSYS simulations. The LOHCs were compared and contrasted using their physiochemical properties, techno-economic analysis and heat network integration. The techno-economic analysis revealed that NAP-based hydrogen storage has the lowest cost among the LOHC options, while NEC shows the highest cost. However, when considering the breakeven point, the order changes to DBT, MCH, NAP, and NEC with 3, 3.8, 5.1 and 5.9, respectively. This result is attributed to the different hydrogen uptakes of the LOHCs, resulting in a longer breakeven period for NEC. Internal rate of return and net present value analysis also demonstrated the superior economic feasibility of the proposed systems. In terms of heat integration, the DBT process outperformed the other LOHCs as the most heat-efficient process with 80 % utility reduction, while the NEC process exhibited the lowest heat integration potential of 66.7 % utility reduction. Combining these findings with the physiochemical properties of the LOHCs, DBT emerges as the most attractive due to its favorable performance across multiple categories, such as toxicities, prices, energy consumptions, and material handling, using a spiderweb diagram.

Explosion risk assessment of a liquid organic hydrogen carrier system by using toluene–methylcyclohexane on varying hydrogen storage scenarios

The effects of phase state, delayed ignition time, mixing ratio, addition of hydrogen, and stoichiometric ratio on the explosion risk of toluene–methylcyclohexane (MCH) mixtures was investigated. Our findings indicated that the addition of 10% hydrogen to the toluene–MCH mixture exhibited a highest explosion risk, compared to other operating conditions. Gaseous toluene–MCH mixtures had a higher explosion risk than did liquid droplets of these mixtures. Of the toluene–MCH mixtures prepared in this study, that with a mass ratio of 7:3 had the highest explosion risk. Although the explosion intensities of the toluene–MCH mixtures were higher than those of their individual components, their explosion sensitivity was lower than that of MCH alone. Toluene had the lowest explosion risk. Toluene or MCH droplets can produce high-intensity explosions even in an oxygen-lean environment. These results indicate that the physical and chemical properties (and thus the combustion performance) of toluene–MCH mixtures are substantially affected by the mixing ratio and operating conditions. Overall, explosion prevention and control measures must be carefully considered when handling toluene–MCH mixtures for various hydrogen storage scenario, such as heating, mixture, or hydrogenation.

A three-phase reactor assessment for the deployment of Liquid Organic Hydrogen Carriers (LOHCs): dybenzyltoluene and indoles mixture systems as case studies

As a result of the large increase in the production of renewable energies, energy storage and energy carrier systems are required to meet the most ambitious objectives. In this area, hydrogen stands out as one of the most powerful tools. However, its low volumetric energy density and its challenging storage conditions lead to the search for new forms of storage. Liquid organic hydrogen carriers (LOHCs) are positioned as a promising option to store hydrogen using a catalytic reversible reaction at moderate temperature. Notwithstanding, although reactor technology is the core section of the LOHC process, it has not been assessed yet. Therefore, in this work, a three-phase reactor analysis is performed including mass, heat, and momentum phenomena along with kinetics. In particular, the two most important reactors for three-phase reactions are evaluated, trickle bed and slurry. And two of the most attractive LOHCs systems, dybenzyltoluene and a mixture composed of 1,2-dimethylindole and 7-ethylindole, are considered. The influence of the operating and design variables was studied. The results show that, for hydrogenation slurry reactors, a first region may be formed where gas–liquid is the main resistance (60%–95%). It eventually ends up with a second one where kinetics becomes the controlling step. At this point, a better catalyst use is expected for the first region. Alternatively, trickle bed hydrogenation reactors are largely controlled by mass transfer over almost the whole reactor length. If the dehydrogenation process is assessed, kinetics resistance is predominant. Surrogate models are developed to optimize the reactor performance and to include the full reactor operation in futures process design analysis allowing for an effective deployment of LOHCs systems.

Review on the thermal neutrality of application-oriented liquid organic hydrogen carrier for hydrogen energy storage and delivery

The depletion and overuse of fossil fuels present formidable challenge to energy supply system and environment. The human society is in great need of clean, renewable and sustainable energy which can guarantee the long-term utilization without leading to escalation of greenhouse effect. Hydrogen, as an extraordinary secondary energy, is capable of realizing the target of environmental protection and transferring the intermittent primary energy to the application terminal, while its nature of low volumetric energy density and volatility need suitable storage method and proper carrier. In this context, liquid organic hydrogen carrier (LOHC), among a series of storage methods such as compressed and liquefied hydrogen, provokes a considerable amount of research interest, since it is proven to be a suitable carrier for hydrogen with safety and stability. However, the dehydrogenation of hydrogen-rich LOHC materials is an endothermic process and needs large energy consumption, which hampers the scale up of the LOHC system. The heat issue is thus essential to be addressed for fulfilling the potential of LOHC. In this work, several strategies of heat intensification and management for LOHC system, including the microwave irradiation, circulation of exhaust heat and direct LOHC fuel cell, are summarized and analyzed to provide suggestions and directions for future research.

Imidazolium-Based Anion Exchange Membranes for Electrochemically Converted Liquid Organic Hydrogen Carrier

Liquid organic hydrogen carrier (LOHC) is a technology that uses renewable energy for hydrogen transportation, storage and utilization. LOHC have advantages that can store and utilize hydrogen rapidly in the lack of transportation infrastructure. A membrane-based LOHC system is a promising technology due to the dynamic operation using electrochemistry. Anion exchange membranes are widely used as electrolyte for electrochemical devices. The performance of LOHC could be determined by the properties of anion exchange membranes. Thus, high anion conducting ionomers must be developed for the superior conversion rates on the electrochemical reduction of liquid toluene to methylcyclohexane. In this study, anion exchangeable polymers containing imidazolium salt as anion-conducting group were synthesized to achieve good conduction of hydroxide ions and their chemical stability. Anion exchange membranes were prepared by a pore-filling method using porous polyethylene substrate. The pore-filling anion exchange membranes were characterized in terms of ion conductivity, areal resistance, ion exchange capacity, water uptake, swelling ratio and mechanical strength. Acknowledgment This research was supported in part by the Korea Institute of Science and Technology (No. 2E31871) and by 2021 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes to Liquid Organic Hydrogen Carriers.

To address the global plastic pollution issues and the challenges of hydrogen storage and transportation, we report a system, based on the hydrodeoxygenation (HDO) of oxygen-containing aromatic plastic wastes, from which organic hydrogen carriers (LOHCs) can be derived. We developed a catalytic system comprised of Ru-ReOx /SiO2 +HZSM-5 for direct HDO of polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyphenylene oxide (PPO), and their mixtures, to cycloalkanes as LOHCs, with high yields up to 99 %, under mild reaction conditions. The theoretical hydrogen storage capacity reaches ca. 5.74 wt%. The reaction pathway involves depolymerization of PC into C15 aromatics and C15 monophenols by direct hydrogenolysis of the C-O bond between the benzene ring and ester group, and subsequent parallel hydrogenation of C15 aromatics and HDO of C15 monophenols. HDO of cyclic alcohol is the rate-determining step. The active site is Ru metallic nanoparticles with partially covered ReOx species. The excellent performance is attributed to the synergetic effect of oxophilic ReOx species and Ru metallic sites for C-O hydrogenolysis and hydrogenation, and the promotion effect of HZSM-5 for dehydration of cyclic alcohol. The highly efficient and stable dehydrogenation of cycloalkanes over Pt/γ-Al2 O3 confirms that HDO products can act as LOHCs.

Hydrodeoxygenation of Oxygen‐Containing Aromatic Plastic Wastes to Liquid Organic Hydrogen Carriers

AbstractTo address the global plastic pollution issues and the challenges of hydrogen storage and transportation, we report a system, based on the hydrodeoxygenation (HDO) of oxygen‐containing aromatic plastic wastes, from which organic hydrogen carriers (LOHCs) can be derived. We developed a catalytic system comprised of Ru‐ReOx/SiO2+HZSM‐5 for direct HDO of polycarbonate (PC), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyphenylene oxide (PPO), and their mixtures, to cycloalkanes as LOHCs, with high yields up to 99 %, under mild reaction conditions. The theoretical hydrogen storage capacity reaches ca. 5.74 wt%. The reaction pathway involves depolymerization of PC into C15 aromatics and C15 monophenols by direct hydrogenolysis of the C−O bond between the benzene ring and ester group, and subsequent parallel hydrogenation of C15 aromatics and HDO of C15 monophenols. HDO of cyclic alcohol is the rate‐determining step. The active site is Ru metallic nanoparticles with partially covered ReOx species. The excellent performance is attributed to the synergetic effect of oxophilic ReOx species and Ru metallic sites for C−O hydrogenolysis and hydrogenation, and the promotion effect of HZSM‐5 for dehydration of cyclic alcohol. The highly efficient and stable dehydrogenation of cycloalkanes over Pt/γ‐Al2O3 confirms that HDO products can act as LOHCs.

A-site effects of titanate-perovskite (ATiO3)-based catalysts on dehydrogenation of N-heterocyclic molecules

Dehydrogenation reactions in liquid organic hydrogen carrier (LOHC) systems present significant challenges, particularly when aiming for low-temperature operations while ensuring that no hydrogen remains in the substrate molecules. Enhancing catalytic performance requires modifying the adsorption behavior of the reactants and products during dehydrogenation. Perovskites have emerged as promising catalyst supports because of their ability to modify the surface chemical properties by manipulating the cations present at the A- and B- sites. This study investigated the effects of A-site cations (Ca, Sr, and Ba) in titanate-type perovskite (ATiO3)—a prototypical perovskite—on the dehydrogenation activity in LOHC systems. Remarkably, Pd/SrTiO3 exhibited outstanding performance by completely converting octahydro-N-methylindole to N-methylindole and releasing 5.76 wt% hydrogen over 8 h. Additionally, it dehydrogenated dodecahydro-N-ethylcarbazole to N-ethylcarbazole with a hydrogen release of 5.70 wt%. Furthermore, the catalyst demonstrated a stable performance after recycling tests for three times without degradation or loss of activity. The chemical state of the catalyst surface was characterized through X-ray photoelectron spectroscopy, H2-temperature programmed reduction, and chemisorption using NH3, CO2, and H2. The results revealed that the exceptional dehydrogenation activity of Pd/SrTiO3 is due to the presence of suitable surface oxygen vacancies and abundant acid–base sites.