Catalytic Aspects of Liquid Organic Hydrogen Carrier Technology

Abstract

The surge in photovoltaic (PV) power generation has made it increasingly difficult to integrate the intermittent PV industry into the power grid while maintaining grid stability. The solution is to use the seasonal surplus of PV electricity to produce “green” hydrogen through water electrolysis and then use the hydrogen as an energy source or as a reactant in chemical processes in the chemical industry to produce value-added products. However, the development of advanced hydrogen storage technologies to ensure the safe handling, transportation, and distribution of H₂ is a major issue. The use of stable liquid organic hydrogen carriers (LOHCs) has emerged as a suitable technology for hydrogen storage. This review highlights prospective LOHC technologies based on reversible catalytic hydrogenation–dehydrogenation cycles of liquid organic molecules for hydrogen storage and release under mild temperature and pressure conditions. The state-of-the-art LOHC systems are critically reviewed, highlighting the most effective heterogeneous catalytic systems.

1. Introduction

Today, there is a growing demand to replace fossil-based raw materials and energy sources. The main motivation is to reduce greenhouse gas emissions. It is known that in the last 50 years (1970–2020) the carbon dioxide content of the Earth′s atmosphere has increased from 300 to 400 mg/kg, almost entirely due to the use of fossil-based raw materials.

There are several reasons for the widespread use of fossil fuels: they are cheap because they are relatively easily accessible, they have a high energy density, they can be transported efficiently, and there are well–developed, highly efficient technologies for processing them. It follows that “greener” solutions need to be developed to replace them that are competitive in the above respects.

Hydrogen, which produces only water as a by-product when converted to energy, is one of the most obvious solutions for replacing fossil fuels. However, a significant proportion of global hydrogen production (~96%) is currently based on fossil fuels. Such processes include coal gasification, which can produce what is known as brown hydrogen, and the steam reforming of natural gas, whose product is known in the literature as gray hydrogen. If the carbon dioxide extracted from the atmosphere or captured in technological processes during the steam reforming of natural gas is also utilized, greenhouse gas emissions can be significantly reduced. The hydrogen produced in this way is called blue hydrogen. In general terms, it can be said that the cheaper the technology we choose, the more significant the emissions of pollutants we can expect. If hydrogen is produced using electricity from renewable energy sources (green hydrogen), the process can be carried out without greenhouse gas emissions. Other hydrogen color codes can be found in the literature: for example, hydrogen produced using electricity from nuclear power plants is called pink hydrogen, and the product produced using solar energy is called yellow hydrogen. Hydrogen produced by the direct pyrolysis of methane is called turquoise hydrogen. In addition to hydrogen, elemental carbon is produced as a by-product. Electricity from renewable energy sources is needed to produce green hydrogen. It is well known that these energy sources are only available periodically. They are also tied to geographical location. It is also important to note that the location of use is usually far from the places with the greatest energy potential suitable for renewable hydrogen production, such as deserts and coastal wind and tidal farms. It follows that if we are to replace fossil-based raw materials and energy sources with hydrogen, the efficient implementation of a hydrogen storage and transport cycle, as shown in Figure 1, is essential.

2. Options for the Storage and Transport of Hydrogen

Hydrogen has a high energy density per unit mass (120 MJ/kg), but its volumetric energy density in the standard state is low, requiring compression or liquefaction for economic use. The use of high pressure poses serious risks during storage and transport (the risk of explosion, the ageing of tanks and pipelines) and is also expensive, as liquid hydrogen is stored at low temperatures (b.p.: −253 °C, 1 bar), and there is a loss of 0.3–3% due to evaporation [1]. Figure 2 shows the physical and chemical hydrogen storage options currently available. The density of 7–27 kg/m³ can be achieved by the compression of hydrogen, depending on the pressure applied, while a density of 70 kg/m³ can be achieved by liquefaction. A further difficulty is that although compressed hydrogen can be transported by pipeline, it cannot be transported by the methods currently used for transporting liquefied hydrocarbons (tankers, rail, trucks). Porous materials with high specific surface areas can also be used to store hydrogen. Since only weak van der Waals interactions occur between the adsorbent and hydrogen, adsorption is performed in liquid nitrogen (−196 °C). The adsorbents used include carbon-based materials [2], metal–organic frameworks (MOFs) [3], porous polymers [4], and zeolites [5]. As hydrogen adsorption is an exothermic process, the heat released results in the loss of liquid nitrogen, so the hydrogen storage efficiency cannot exceed 80%, i.e., <6.7 kWh_current/kg [6].

Metal hydrides can also be used to store hydrogen (Figure 2). The hydrogen density achieved in this way can be 40–70 kg/m³. Among the metal hydrides, magnesium hydride (MgH₂) and aluminum hydride (AlH₃) are the most promising. However, there are several barriers to their effective use. For example, the enthalpy of dehydrogenation of MgH₂ is 75 kJ/mol, but the hydrogenation–dehydrogenation of magnesium is a slow process [7]. The enthalpy of the dehydrogenation of AlH₃ is only 7 kJ/mol, and it can release 10.5 wt% hydrogen at 100 °C; however, its production is complicated because it requires high hydrogen pressures (>20,000 bar) [8]. Metal complex hydrides, such as NaAlH₄ and LiBH₄, and metal amides, such as LiNH₂, NaNH₂, and Mg (NH₂)₂, are also suitable for hydrogen storage. The main limitations to their use are the high enthalpy of the dehydrogenation and the low efficiency of the regeneration [9]. For example, a mixture of 2.0 LiNH₂-1.1 MgH₂-0.1 LiBH₄ in the presence of 3 wt% ZrCoH₃ can reversibly store 4.5–5.2 wt% hydrogen. The hydrogenation takes place at 150 °C and 70 bar pressure, whereas the dehydrogenation proceeds at 1 bar pressure and at 150 °C [10].

Covalent non-metal H-compounds (often called chemical hydrides in the scientific literature), which store hydrogen in chemical bonds are a separate group of materials used for hydrogen storage. Their main characteristics, which are also their advantages, are that they are composed of light elements (C, H, O, N), which allow a higher specific hydrogen binding per mass. In addition, most of them are liquids at room temperature, which facilitates their transport and storage, as well as heat and mass transfer during hydrogenation–dehydrogenation operations. Their main representatives are methanol, ammonia, formic acid, and the major subject of our review, the LOHCs. Methanol can store 12.5 wt% of hydrogen, which is equivalent to 99 kg/m³. From an environmental point of view, the synthesis of methanol is most advantageous when it is produced by the hydrogenation of carbon dioxide. This technology, which requires 220–280 °C and 10–80 bar pressure, is very similar to the most widely used natural gas-based process today [11]. The most efficient way of extracting hydrogen from methanol is by steam reforming, which also produces a molecule of hydrogen from water. The carbon dioxide produced in the process can be captured in an amine solution and then released and recycled back into the system. The hydrogen storage capacity of ammonia is 17.7 wt%. This corresponds to 123 kg/m³ at a pressure of 10 bar. Another important aspect is that efficient methods are available for producing, storing, and transporting ammonia. Today, a significant portion of the hydrogen required for the synthesis of ammonia is obtained by the electrolysis of water [12]. This suggests that ammonia could be an excellent hydrogen storage medium. However, recovering hydrogen from ammonia is very difficult. In the thermal decomposition of ammonia, a temperature of 520 °C is required to achieve a 25% conversion, and the complete conversion takes place at 860 °C [13]. Although it starts to decompose over Ru/C catalysts at 200 °C [14], on supported Ni catalysts, ammonia conversion of 70% or higher can be achieved at temperatures above 550 °C, and even on much more active Ru-containing catalysts, temperatures of 450 °C or higher are required for this conversion level [15]. Formic acid can store 4.4 wt% of hydrogen, which is equivalent to 53 kg/m³. In addition to the low hydrogen storage capacity, there are other problems with the use of formic acid for hydrogen storage. For example, the gas-phase reaction of carbon dioxide and hydrogen is thermodynamically inhibited, so the reaction must be carried out in an alkaline medium in the presence of a homogeneous catalyst. In addition, hydrogen recovery can only be achieved in the presence of an expensive homogeneous catalyst containing ruthenium or iridium, because in their absence, formic acid decomposes into carbon monoxide and water [16].

3. Applications of Liquid Organic Hydrogen Carriers (LOHCs) for the Storage of Hydrogen

The main advantage of LOHC materials is that they enable hydrogen to be stored and transported in a safe manner. The idea is to produce hydrogen by the electrolysis of water using “green” electricity in geographical locations where large amounts of renewable energy (wind, hydro, solar, and geothermal) are available. The resulting hydrogen is then used to hydrogenate the hydrogen-lean form of LOHC. This hydrogenation step is an exothermic process that generally takes place at a temperature above room temperature and at a pressure above atmospheric pressure at an appropriate rate. The energy density of hydrogenated LOHC is significantly higher than that of compressed or liquefied hydrogen, and it can be easily stored under normal conditions without the risk of hydrogen loss. It is also important to note that the physicochemical properties of hydrogenated LOHCs are the same as those of petrol and diesel. This means that they can be easily transported using existing infrastructure (pipelines, lorries, trains, and ships). The hydrogen stored in the LOHC can be released at its destination in a dehydrogenation reaction (200–450 °C, atmospheric pressure), and the energy stored can be converted back into electricity in a fuel cell. The hydrogen-lean form of the LOHC can then be hydrogenated again. To do this, it must be transported back to the energy-rich site, and any losses due to side reactions, transport, and storage must be compensated for.

According to Modisha et al. [17], LOHCs are organic compounds that favorably satisfy the following conditions:

• Both the hydrogenated and dehydrogenated forms should have a melting point of below −30 °C to allow transport and storage in a liquid state.
• The boiling point of both forms is above 300 °C, as the low vapor pressure of the LOHC near room temperature facilitates the purification of the released hydrogen.
• The H₂ storage capacity of the LOHC shall be greater than 56 kg/m³ or higher than 6 wt%.
• The desorption heat of hydrogen should be low (42–54 kJ/mol_H2) so that complete dehydrogenation can be achieved at low temperatures (<200 °C), even at 1 bar H₂ pressure.
• Hydrogenation–dehydrogenation can be carried out with high selectivity over as many cycles as possible.
• They should fit into today′s fuel supply infrastructure.
• They should be cheap and easy to produce.
• They must meet the applicable toxicological and ecotoxicological requirements during transport and use, i.e., they must not be classified as hazardous substances.

These criteria remained valid and make it very challenging to find the most suitable LOHC material. It has been noted [17] that none of the proposed LOHCs has been able to fully satisfy all of the above desirable features. This seems to be the case to date.

With regard to the above criteria, aromatic LOHCs containing one to three aromatic rings have emerged as possible hydrogen carriers due to their favorable properties (e.g., their hydrogenation–dehydrogenation is reversible and can be carried out in a relatively narrow temperature range with high selectivity, they have a suitable H₂ storage capacity, and both the hydrogen-rich and the hydrogen-lean forms are relatively stable compounds, especially under the severe catalytic conditions of the dehydrogenation process), and most of the recent studies are still related to these compounds. The present review focuses mainly on the catalytic aspects of these most studied aromatic LOHC materials (vide infra in Section 4).

LOHCs materials are most commonly hydrogenated–dehydrogenated using a thermal catalytic process but can also be hydrogenated–dehydrogenated using an electrocatalytic process. This involves hydrogenating the hydrogen-lean form of the LOHC at the cathode, while oxygen evolution or other oxidizing processes occur at the anode. The electrochemical dehydrogenation reaction of the hydrogen-rich molecule occurs on the anode, while a simultaneous hydrogen evolution reaction occurs on the cathode, releasing hydrogen [18]. The electrocatalytic reaction can take place on carbon-supported noble metal catalysts or their alloys with copper and nickel, in an aqueous emulsion of the LOHC material [19]. The main advantage of the process over thermochemical processes is that hydrogen is generated in situ at the cathode surface, eliminating the kinetic barrier of the thermal splitting of the H₂ molecule. It allows the reaction to take place under mild conditions, at room temperature and atmospheric pressure. Another advantage is that there is no diffusion inhibition in the reaction mixture against insoluble hydrogen molecules. On the other hand, the main disadvantage of electrocatalytic hydrogenation is that hydrogenation and hydrogen evolution are competing processes, so instead of the hydrogenation of the LOHC, only hydrogen evolution will occur at a given cathode if the ratio of the hydrogenation to hydrogen desorption rate is low (e.g., < 0.05) [20]. According to Lebedeva et al. [21], the efficiency of the process can be improved if an ionic liquid (such as N-butylpyridinium chloride-AlCl₃) is used as the electrolyte. Lee et al. [22] found significant improvements in the efficiency and lifetime of the electrocatalytic hydrogenation of toluene by replacing the Nafion membrane with a sulfonated poly (arylene ether sulfone) membrane due to the inhibited diffusion of the reactant. Sievi et al. [23] have developed a hydrogen utilization process without an external heat input through a combination of thermocatalytic and electrocatalytic processes. In the system, 2-propanol is obtained by transfer hydrogenation between perhydro-dibenzyltoluene and acetone over a Pt/C catalyst at 150–190 °C. The product is then consumed in a polymer electrolyte membrane fuel cell at 85–90 °C over a PtRu/C catalyst, while acetone is recovered. There is no molecular hydrogen involved in the process. The thermal neutrality is due to the fact that transfer hydrogenation is slightly exothermic. The heat required for the release of protons from 2-propanol is compensated by the heat of water formation in the fuel cell. Electrocatalytic hydrogen storage can also be achieved with isopropanol, phenolic compounds, and organic acids, etc. A recent review discusses these processes in detail [24].

4. The Main LOHC Materials

4.1. The Benzene–Cyclohexane System

The hydrogen storage capacity of the benzene–cyclohexane system is 7.2 wt%. Both forms are among the most commonly used chemical raw materials and are liquid at room temperature (

Table 1. Physical–chemical properties of possible LOHC compounds.

Cyclohexane		Benzene
Melting point	5 °C	Melting point	6.5 °C
Boiling point	80 °C	Boiling point	80 °C
Flash point	−18 °C	Flash point	−11 °C
Dehydrogenation pressure	1–5 bar	Hydrogenation pressure	1–50 bar
Dehydrogenation temperature	280–350 °C	Hydrogenation temperature	120–250 °C
Methylcyclohexane		Toluene
Melting point	−126 °C	Melting point	−95 °C
Boiling point	101 °C	Boiling point	111 °C
Flash point	−3 °C	Flash point	6 °C
Dehydrogenation pressure	1–5 bar	Hydrogenation pressure	1–50 bar
Dehydrogenation temperature	300–400 °C	Hydrogenation temperature	60–200 °C
Decalin		Naphthalene
Melting point	−37 °C	Melting point	79 °C
Boiling point	189 °C	Boiling point	218 °C
Flash point	57 °C	Flash point	80 °C
Dehydrogenation pressure	1–5 bar	Hydrogenation pressure	20–70 bar
Dehydrogenation temperature	270–350 °C	Hydrogenation temperature	150–330 °C
Perhydrodibenzyltoluene		Dibenzyltoluene
Melting point	−34 °C	Melting point	−30 °C
Boiling point	354 °C	Boiling point	390 °C
Flash point	-	Flash point	190 °C
Dehydrogenation pressure	1–5 bar	Hydrogenation pressure	10–50 bar
Dehydrogenation temperature	260–320 °C	Hydrogenation temperature	140–300 °C
Perhydro-N-Ethylcarbazole		N-ethylcarbazole
Melting point	−85 °C	Melting point	70 °C
Boiling point	-	Boiling point	348 °C
Flash point	-	Flash point	186 °C
Dehydrogenation pressure	1–5 bar	Hydrogenation pressure	50–70 bar
Dehydrogenation temperature	170–270 °C	Hydrogenation temperature	140–180 °C

). However, their boiling points are low—both are around 80 °C, which makes it difficult to extract high-purity hydrogen from the system. The carcinogenic effects of benzene and the flammability of cyclohexane are other disadvantages of the system.

The hydrogenation of benzene can be carried out at 120–250 °C and 1–50 bar, whereas the dehydrogenation of cyclohexane can only be carried out under more vigorous conditions (1–5 bar, 280–350 °C) due to the highly stable sp³ C-H bonds [25].

4.2. The Toluene–Methylcyclohexane System

The hydrogen storage capacity of the toluene–methylcyclohexane system is somewhat lower (6.2 wt%) than that of the benzene–cyclohexane system, but the higher boiling point and lower toxicity of the components compensate for this disadvantage. The price of toluene is low, EUR ~0.3/kg [26]. The methyl group in toluene increases the electron density of the benzene ring, making toluene easier to hydrogenate than benzene. At the same time, the dehydrogenation of methylcyclohexane only occurs under more severe conditions than that of cyclohexane, which can lead to undesirable side reactions (dealkylation, disproportionation) and catalyst poisoning.

The hydrogenation of toluene can be carried out at a pressure of 1–50 bar and a temperature of 60–200 °C, while the dehydrogenation of methylcyclohexane can be carried out at a pressure of 1–5 bar and a temperature of 300–400 °C (Table 1) [25]. The potential of the toluene–methylcyclohexane system is demonstrated by the fact that the Japanese Chiyoda Corporation has built two pilot plants to test the hydrogen storage process using a Pt/Al₂O₃ catalyst [25].

4.3. The Naphthalene–Decahydronaphthalene (Decalin) System

The hydrogen storage capacity of the naphthalene–decalin system is 7.3 wt%. Naphthalene is solid at room temperature. This is a major drawback in use, as it requires a solvent (e.g., toluene). An additional difficulty is the high stability of the tetralin formed during the hydrogenation of decalin, which can only be hydrogenated to decalin under vigorous conditions accompanied by undesirable side reactions [26]. The hydrogenation of naphthalene can be carried out at a pressure of 20–70 bar and a temperature of 150–330 °C, while the dehydrogenation of decalin can be carried out at a pressure of 1–5 bar and a temperature of 270–350 °C (Table 1) [25]. Due to the difficulties mentioned above, no pilot plant based on the naphthalene–decalin system has yet been built. The price of decalin is EUR ~0.6/kg [26].

4.4. The Dibenzyltoluene–Perhydrodibenzyltoluene System

The hydrogen storage capacity of the dibenzyltoluene–perhydrodibenzyltoluene system is 6.2 wt%. In this system, the rate of the hydrogenation and dehydrogenation reactions is fast, and a high degree of selectivity can be achieved in both directions. Dibenzyltoluene has a low melting point (−30 °C), is nonflammable, nontoxic, and not in the group of hazardous chemicals. Its thermal stability is excellent, which is why it is used in the industry as a heat transfer oil under the brand name Marlotherm^® SH. Its price is relatively low, EUR ~ 4/kg [26]. Due to its high boiling point (390 °C), it has a low vapor pressure at room temperature, which makes it possible to extract hydrogen with a high degree of purity. Another advantage of the system is that dibenzyltoluene can also be hydrogenated using gas mixtures, such as H₂/CH₄ [27] and H₂/CO₂ [28]. The hydrogenation of dibenzyltoluene takes place at a pressure of 10–50 bar and 140–300 °C, while the dehydrogenation of perhydrodibenzyltoluene takes place at a pressure of 1–5 bar and 260–320 °C (Table 1). The benzyltoluene–perhydrobenzyltoluene system is being used on an industrial scale for hydrogen storage by Hydrogenious Technologies GmbH (Erlangen, Germany) [29].

4.5. The N-Ethylcarbazole–Dodecahydro-N-Ethylcarbazole System

The hydrogen storage capacity of the N-ethylcarbazole–dodecahydro-N-ethylcarbazole system is 5.8 wt%. The nitrogen atom in N-ethylcarbazole plays an important role in reducing the dehydrogenation enthalpy of the fully hydrogenated form (~50 kJ/mol_H2), allowing the process to take place below 200 °C [30]. The ethyl group attached to the nitrogen atom lowers the melting point of the carbazole and masks the lone pair of electrons on the nitrogen atom that would otherwise poison the active sites of the catalyst [31]. The hydrogenation of N-ethylcarbazole can be carried out at 50–70 bar pressure and 140–180 °C, and the dehydrogenation of dodecahydro-N-ethylcarbazole can be carried out at near atmospheric pressure and at temperatures between 170 and 270 °C (Table 1) [25]. However, the application of this system is hampered by several unfavorable properties, such as the price of N-ethylcarbazole (EUR ~40/kg), its high melting point (70 °C), and the instability of the N-ethyl group, which is easily de-ethylated above 120 °C. It should be noted that China′s Hynertech Co., Ltd. (Wuhan, China) is using a mixture of different nitrogen-containing LOHC materials in its prototype hydrogen-powered vehicle to achieve rapid hydrogen release at low temperatures (<200 °C) [25].

5. Catalysts for the LOHC Process

5.1. Background

The basic requirement for efficient hydrogen storage using LOHC materials is the use of a catalyst that catalyzes the hydrogenation and/or dehydrogenation step with high selectivity at relatively low pressures and temperatures at high conversion levels. It is well known that hydrogenation reactions are generally favored because of their exothermic nature. For example, the enthalpy of the hydrogenation of toluene to methylcyclohexane is −205 kJ/mol. Accordingly, dehydrogenation is relatively difficult. In the reverse reaction, 260 °C is required to achieve 50% conversion [32]. Furthermore, there are well-established industrial processes for the hydrogenation of aromatics, but none for dehydrogenation, as aromatics are not produced by dehydrogenation but are obtained from petroleum by simple separation.

Both homogeneous and heterogeneous catalytic routes can be used for hydrogenation–dehydrogenation. The advantage of homogeneous phase reactions is that hydrogenation–dehydrogenation takes place under milder conditions and with higher selectivity than in heterogeneous catalytic systems [33]. Homogeneous catalytic processes use complexes based on ruthenium and iridium. However, their use is subject to the well-known difficulties of being expensive, sensitive to air, and difficult to recover from the reaction mixture [34].

The heterogeneous catalytic hydrogenation–dehydrogenation of LOHC materials is usually carried out on noble metal (Pt, Pd, Ru, Rh, and Au) catalysts supported on SiO₂, Al₂O₃, TiO₂, CeO₂, MgO, zeolites, carbon-based materials, etc. However, the use of transition metals (Ni, Mn, Co, Cu, etc.) is receiving increasing attention due to the high price of noble metals. In addition to single-metal catalysts, there are also multi-metal catalysts. These latter catalysts attempt to exploit the synergistic effect of metals. In addition to their advantages, heterogeneous catalytic processes can also have disadvantages, such as the low specific surface area of the catalyst, the low dispersion of the active phase, and inhibited heat and mass transport. To overcome these disadvantages, the development of catalyst supports is ongoing; for example, attempts are being made to increase the efficiency of catalysts by using various mixed oxides and mesoporous materials.

As mentioned above, there are established large-scale industrial processes for aromatics hydrogenation. The dehydrogenation of the hydrogen-rich form of the LOHC pair is much more challenging. Therefore, in the following chapter, we will first present the results obtained in the dehydrogenation of the main LOHC systems.

5.2. Dehydrogenation of the Main LOHC Substances

5.2.1. Dehydrogenation of Cyclohexane to Benzene

In the dehydrogenation of cyclohexane (Scheme 1), the best results are obtained with oxide-supported platinum catalysts. It has also been shown that the addition of other metals significantly increases the rate of hydrogen evolution, so that when rhodium is added to platinum deposited on activated carbon, the initial formation rate related to the metal content increased from 51 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ to 78 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 330 °C (

Table 2. Catalysts for the dehydrogenation of hydrogen-rich LOHCs.

Catalyst	Reactor	T (C°)	p (Bar)	$$\begin{gathered} {\mathbf{H}}_2 \mathbf{Formation} \mathbf{Rate} \\ (\mathbf{mmol}·{\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}·{\mathbf{min}}^{-1}) \end{gathered}$$	WHSV a (h−1)	Conv. (%)	Sel. (%)	Ref.
Cyclohexane
10%Pt/ACC b	P.S. c	330	1	51	-	35	100	[35]
Pt-Rh/ACC b	P.S. c	328	1	78	-	25–35	100	[35]
17%Ni,3%Cu/SiO2	flow through	350	1	54	1.6	94.9	99.5	[37]
20%Ni/ACC b	P.S. c	300	1	8.5	-	21.9	98.8	[38]
0.5%Pt/ACC b	P.S. c	300	1	0.22	-	0.4	100	[38]
20%Ni,0.5%Pt/ACC b	P.S. c	300	1	13.1	-	31.1	99.7	[38]
Methylcyclohexane
0.4%Pt/active carbon	flow through	300	1	1.4	2.5	95	100	[40]
1%Pt/CeO2	flow through	350	1	3.5	7.7	78	-	[41]
1%Pt/CeO2-SiO2	flow through	310	1	1.0	1.9	97	100	[42]
52%Ni,13%Sn + 17% SiO2	flow through	350	1	2.9	6.2	92	99	[43]
17%Ni/TiO2	flow through	375	1	0.84	1.9	86.5	96.5	[44]
1.2%Pt,0.4%Zn/self-pillared silicate-1	flow through	400	1	41.5	100	80	-	[45]
1%Pt/N-Ti3C2Tx	flow through	375	1	3.6	7.7	93	100	[46]
0.4%Pt,0.6%Fe/silicalite-1	flow through	350	1	12.9	90	28	100	[47]
0.4%Pt,2.8%Sn encap-sulated on silicate-1	flow through	300	1	2.5	1.56	88	100	[48]
Decalin
5%Pt/C (impreg.)	batch	270	1	0.45 (4 h) e	-	69	68	[49]
8%Ni,2%Cu/ACC b	P.S. c	350	1	9.0	-	-	-	[50]
Perhydrodibenzyltoluene
0.3%Pt/Al2O3	batch	310	1	-	-	85	100	[51]
5%Pt/Al2O3	flow through	300	1	10.8	10 d	5.0	100	[52]
5%Pt/CeO2	flow through	300	1	25.2	10 d	37	100	[52]
5%Pt/Al2O3	batch	270	1	-	-	58	100	[53]
1%La,5%Pt/Al2O3	batch	270	1	-	-	65	100	[53]
5%Pt/Al2O3	batch	300	1	12.5	-	48	-	[54]
Perhydro-N-Ethylcarbazole
2.5% Pd/graphene-oxide	batch	170	1	21.1 (12 h) e	-	100	85	[55]
5% Pd/C	batch	180	1	-	-	99.9	98	[56]
2.5%Pd + 2.5%Ni/Al2O3 f	batch	180	1	-	-	98	100	[57]
1% Pd/Al2O3	flow through	180	1	2.3	2.26 c	100	92	[58]

^a WHSV: weight hourly space velocity; ^b ACC: active carbon cloth; ^c pulse–spray mode; ^d values are given as liquid hourly space velocity (LHSV), cm³·$\mathrm{c}{\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-3}$·h⁻¹; ^e in parentheses: reaction time; ^f dodecahydro-N-propylcarbazole.

) [35]. Increasing the temperature on a given catalyst increases the rate of hydrogen formation [36], and the support also affects the efficiency of the catalyst [35]. As mentioned above, attempts are being made to replace noble metals with cheaper transition metals. However, even at higher temperatures, the hydrogen evolution rate on Ni-Cu/SiO₂, for example, is lower than on a platinum-containing catalyst with a similar metal content, i.e., 54 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 350 °C (Table 2) [37]. A synergistic effect was observed on the bimetallic Ni-Pt/activated carbon catalyst, where hydrogen was evolved at a rate of 13.1 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹, whereas the activity on the monometallic Pt/activated carbon and Ni/activated carbon catalysts was much lower (0.22 and 8.5 mol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹). The H₂ selectivity of the Pt-modified Ni catalyst was also higher (99.7%) than that of the catalyst without Pt modification (98.8%) [38]. Wu et al. [39] studied the dehydrogenation of cyclohexane to benzene on a dealuminated beta-zeolite-supported Ni catalyst. It was observed that when the Ni (NO₃)₂ precursor is decomposed at 300 °C in different media (air, nitrogen, or hydrogen), the subsequent reduction in hydrogen at 600 °C produces Ni particles of different sizes. Particles with an average diameter of ~1.0, 1.2, or ~1.1–25.7 nm were obtained for the sample containing 2.7 wt% Ni after pretreatment in nitrogen, hydrogen, or air. For the dehydrogenation of cyclohexane, conversions of 76.2% were observed for the sample decomposed in air, 90.9% for the sample decomposed in nitrogen, and 89.3% for the sample decomposed in hydrogen, with almost identical benzene selectivities (>92%) at 300 °C and 4 h⁻¹ space velocity.

Samples containing 9 wt% nickel were used to investigate the stability of the catalysts. For these samples, the average particle diameters were 1.3 (hydrogen), 4.5 (nitrogen), and 9.1 nm (air). The dehydrogenation efficiency of this series also decreased as the diameter of the Ni particles increased. On a sample prepared by decomposition in hydrogen at 300 °C at 13.3 h⁻¹ space velocity, it was found that after 150 h, the conversion decreased from an initial 60.5% to 50.8%, while the benzene selectivity remained above 95% throughout the process (

Table 3. Stability of catalysts used for dehydrogenation of hydrogen-rich LOHCs.

Catalyst	Reactor Type	T (C°)	p (bar)	W/F a (h−1)	$$\begin{gathered} \mathbf{Initial} \\ \mathbf{Reaction} \mathbf{Rate} \\ ({\mathbf{mmol}}_{{\mathbf{H}}_2}· \\ {\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}·{\mathbf{min}}^{-1}) \end{gathered}$$	Initial Conv. (%)	Initial Sel. (%)	TOS b (h)	Number of Recycling	Decrease of Reaction Rate (%)	Ref.
Cyclohexane
3%Ni/Beta (Si/Al > 300)	flow through	280	1	4	1.5	66	93	60	-	6	[39]
9% Ni/Beta (Si/Al > 300)	flow through	300	1	13.3	4.7	61	96	150	-	16	[39]
Methylcyclohexane
0.4%Pt/active carbon	flow through	300	1	2.5	1.4	98	100	52	-	0	[40]
1%Pt/CeO2	flow through	350	1	7.7	3.5	83	-	72	-	35	[42]
52%Ni, 13%Sn, 17%SiO2	flow through	350	1	18.5	6.2	64	100	100	-	19	[43]
17%Ni/TiO2	flow through	375	1	1.9	0.9	98	90	6	-	6	[44]
1.2%Pt, 0.4%Zn/self-pillared silicalite-1	flow through	400	1	100	41.5	80	100	100	-	0	[45]
9.5%Pt, 0.6%Fe/silicalite-1	flow through	350	1	2.2	314	100	99	72	-	6	[47]
0.4%Pt, 2.8%Sn encapsulated on silicalite-1	flow through	350	1	7.8	5.1	37	100	32	-	46	[48]
Decalin
8%Ni, 2%Cu/ ACC c	P.S. d	350	1	-	3.6	-	-	3	-	0	[50]
Perhydrodibenzyltoluene
5%Pt/Al2O3	flow through	300	1	6.5	1.3	45	-	24	-	16	[54]
Perhydro-N-Ethylcarbazole
2.5%Pd/graph-ene oxide	batch	180	1	-	5.1	100	84	-	5	15	[55]
5%Pd/C	batch	180	1	-	24	99	99	-	4	8	[56]
2.5%Pd, 2.5%Ni/Al2O3	batch	200	1	-	27	100	100	-	5	0	[57]
1%Pd/Al2O3	flow through	180	1	2.9 e	2.3	100	92	200	-	0	[58]

^a space velocity (h⁻¹); ^b time-on-stream (h), duration of stability test; ^c ACC: active carbon cloth; ^d pulse–spray mode; ^e LHSV: liquid hourly space velocity.

). The authors attribute the higher efficiency of smaller particles to their greater resistance to coking and their weaker adsorption of the product (benzene).

5.2.2. Dehydrogenation of Methylcyclohexane to Toluene

The methylcyclohexane–toluene system (Scheme 2) is a similar but a more suitable LOHC system than the cyclohexane–benzene system due to the higher boiling points of the components and, in particular, the unacceptable toxicity of benzene compared to toluene, and, therefore, receives more attention in the literature. Zhang et al. [40] studied the dehydrogenation of methylcyclohexane over a Pt catalyst impregnated on carbon obtained from the pyrolysis of waste tires. It was found that above a 0.4 wt% Pt content, the addition of a second metal has no positive effect. At the 0.4 wt% metal content, a conversion of about 98% could be achieved at 300 °C, while the toluene selectivity was ~100%, whereas the hydrogen evolution rate was 1.4 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ (Table 2). The stability of the catalyst was examined, and it was found that the dehydrogenation rate did not decrease, even after 52 h at 300 °C (Table 3).

To investigate the effect of the support, Zhang and co-workers [41] prepared hollow sphere, wire, and particle catalysts of platinum-impregnated CeO₂. It was found that the hollow sphere with the largest specific surface area and pore volume was the most efficient in dehydrogenating methylcyclohexane. The hydrogen evolution rate was found to be 3.5 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 350 °C at 78% methylcyclohexane conversion (Table 2). In their opinion, the higher platinum dispersion due to the large specific surface area and the faster material transport in the larger pore diameter pores resulted in the higher hydrogen yields. Jang et al. [42] deposited a CeO₂ layer on the surface of commercial silica gel and impregnated it with platinum. The activity of the catalyst thus prepared was found to be five to six times higher than that of Pt/SiO₂ or Pt/CeO₂. The positive effect was explained by the presence of defect sites on the surface of CeO₂, as they believe that the desorption of the toluene product is accelerated on this reduced surface. A formation rate of 1.0 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ was found on their most active catalyst (Table 2). It was observed that the initial 82% methylcyclohexane conversion decreased by 35% when the stability of the 1 wt% Pt/CeO₂ catalyst was investigated at 350 °C (Table 3). Koskin and coworkers [43] studied the dehydrogenation of methylcyclohexane using high-nickel-containing catalysts. The basic catalyst consists of 80 wt% nickel and 20 wt% SiO₂. Silica is used as a structure stabilizer. In the preparation of the catalyst series, part of the nickel was replaced by tin in amounts of 3, 6, 13, and 20 wt%. It was found that the heat treatment leads to the formation of NiSn solid solutions with different compositions (Ni₃Sn₂, Ni₃Sn, and Ni_1-xSn_x). It was suggested that the solid solutions thus formed reduce the adsorption strength of the product (toluene) on the catalyst surface, thus increasing the selectivity of the dehydrogenation of methylcyclohexane. The most active catalyst was identified as 52Ni13Sn17SiO₂ (the numbers in the catalyst identifier indicate the weight percent elemental composition of the oxide form catalyst), which converts methylcyclohexane to toluene at 350 °C at a space velocity of 6.2 h⁻¹ with a conversion of 64% and a selectivity of 100%; however, the activity decreased by 19% after 100 h (Table 3). Gao et al. [44] compared the dehydrogenation activity of Ni/TiO₂ and Ni/Al₂O₃ catalysts. It was observed that under the same conditions (375 °C, 1.9 h⁻¹ space velocity), the TiO₂-supported catalyst is much more efficient in the dehydrogenation of methylcyclohexane, since at a conversion level of 86.5%, toluene is formed with a selectivity of 96.5%, whereas on the Al₂O₃-supported catalyst at a similar conversion (96.0%), the selectivity of toluene is only 42.6%. The authors explain the advantage of the TiO₂ support by its weaker acidity. The dehydrogenation efficiency on the 17 wt% Ni/TiO₂ catalyst decreased by ~6% in 6 h (Table 3). The dehydrogenation of methylcyclohexane to toluene using low-coordinated Pt₁₃ clusters anchored by isolated ZnO_x nanorafts on silanol-rich, self-pillared zeolite nanosheets as catalysts was studied by Song et al. [45]. The reaction was tested at 400 °C on a catalyst containing 1.2–0.4 wt% platinum and zinc. The hydrogen evolution rate was found to be 41.5 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at ~87% conversion. No decrease in conversion or selectivity was observed when the reaction was monitored for 100 h (Table 3). The excellent activity is explained by orbital hybridization (Zn 3d-Pt 5d) and charge transfer between Zn and Pt, giving Pt electron-rich properties and a reduced d-band center, facilitating C-H cleavage and mitigating toluene poisoning. Ma et al. [46] studied the dehydrogenation of methylcyclohexane over a Pt/Ti₃C₂T_x (Pt-MXene, where T_X represents different surface terminations) catalyst. It was observed that doping with nitrogen significantly increases the rate of dehydrogenation. While the hydrogen evolution rate on commercial 1% Pt/TiO₂ was 0.78 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹, this value increased to 3.18 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ on 0.5% Pt/N-Ti₃C₂T_x, despite the lower Pt content. The significantly increased activity is explained by the 3D, wrinkled, porous structure of the catalyst. This increases the residence time of methylcyclohexane on the catalyst surface during gas–solid reactions and improves the mass transfer efficiency of MCH. Furthermore, it was found that the N-doped Pt/N-Ti₃C₂T_x, which optimizes the electronic coordination environment, produces sub-nanometer Pt clusters in which Pt is mainly present as Pt⁰ (80.78%) and avoids the presence of Pt (IV) with low catalytic activity. He et al. [47] incorporated PtFe₄.₇ clusters into the rigid structure of MFI zeolite and measured a significantly higher hydrogen evolution rate than for PtM₄.₇@MFI (M = Sn, Zn, In, Ga, Fe) or conventional supported catalysts (e.g., Pt/CeO₂, Pt-Mg-Al-O, etc.). A hydrogen evolution rate of ~12.9 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ was measured for the dehydrogenation of toluene at 350 °C using a 1:1 methylcyclohexane/N₂ mixture and a weight hourly space velocity (WHSV) of 90 h⁻¹. Catalyst stability is demonstrated by the fact that there is only a 6% loss of activity after 72 h of TOS at WHSV = 2.2 h⁻¹ (Table 3). The catalyst can be easily regenerated over eight consecutive reaction–regeneration cycles of >2000 h by calcination in air followed by reduction with H₂. The remarkable performance is explained by the unique bimetallic PtFe cluster structures and surrounding rigid zeolite framework. It was also found that, with careful tuning, PtFe@MFI catalysts can be made suitable for dehydrogenating other LOHC materials. On a similar principle, nanoparticles of platinum and tin can also be incorporated into a well-defined structure of zeolite. Shi et al. [48] achieved a high hydrogen evolution rate (7.8 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 350 °C) over a Pt₀.₄Sn₂.₈@Silicalite-1 catalyst with nearly 100% toluene selectivity, which dropped about 46% after 32 h (Table 3).

5.2.3. Dehydrogenation of Decalin to Naphthalene

As mentioned above, the decalin–naphthalene system (Scheme 3) has several disadvantages. For example, naphthalene is solid at room temperature, which requires the use of a solvent, and the high stability of the tetralin intermediate means that the hydrogenation–dehydrogenation process can otablenly be completed under vigorous conditions. Accordingly, there are fewer papers dealing with this system in the literature than with any of the other reactant pairs discussed in our review. In addition, it is difficult to compare the results because in the decalin–naphthalene system, the amount of hydrogen produced is usually not related to the mass of the catalyst but to the mass of the metal component. Kim et al. [49] studied the dehydrogenation of decalin using theoretical DFT calculations (DFT = Density Functional Theory) and in a batch tank reactor. They found that a Pt/C was a more active catalyst in decalin–tetralin dehydrogenation, while a similar Pd/C was the more active catalyst in tetralin–naphthalene dehydrogenation. The different behavior is explained by the different structural and chemical properties of the two metals. It was concluded that a catalyst formed by combining the two metals may be ideal for the dehydrogenation of decalin. Patil et al. [50] studied the dehydrogenation of decalin over an 8% Ni-2% Cu/activated carbon catalyst using a spray–pulse reactor and measured a hydrogen evolution rate of 3.6 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹. There was no change in the rate of hydrogen evolution for three hours (Table 3). Lee and co-workers [59] studied the dehydrogenation of decalin on carbon-supported palladium catalysts. About 3 wt% of palladium was deposited on the carbon surface by various methods (impregnation, precipitation, ion exchange, and the polyol method), and a correlation was found between the palladium dispersion and the rate of hydrogen evolution. For example, the palladium dispersion on the impregnated sample was 5.4%, and on the sample prepared by the polyol method, it was 14.0%. The hydrogen evolution rate increased by ~40% due to the higher palladium dispersion. As noted above, the use of the decalin–naphthalene system is complicated by the fact that naphthalene is solid at room temperature. Diaz et al. [60] studied the dehydrogenation of decalin in the presence of cycloalkane (cyclohexane and methylcyclohexane) and aromatic solvents (benzene, toluene) using 5 wt% Pt/C and 5 wt% Pt/Al₂O₃ catalysts. It has been observed that solvents are inhibitors of the hydrogenation of both decalin and tetralin. The competitive adsorption of solvent molecules explains this phenomenon. The dehydrogenation of decalin over a 2.5 wt% Pd/TiO₂-Al₂O₃ catalyst doped with MgO was investigated by Wang et al. [61]. A naphthalene selectivity of 60% with a conversion of 56% was achieved at 600 °C, 40 bar pressure, and WHSV = 514.7 h⁻¹. The MgO-doped samples were found to be more efficient than the pure TiO₂-Al₂O₃-supported catalysts. This was because the addition of MgO reduced the acidity of the catalysts and increased the number of oxygen vacancies, making the catalyst more resistant to coking. Wang et al. [62] studied the dehydrogenation of decalin over a 1 wt% Pt/MgAl₂O₄ catalyst. Catalyst optimization showed that the most efficient catalyst was the one prepared by the alcohol heating method with a Mg/Al molar ratio of 0.5. This catalyst was able to release 0.39 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 260 °C.

5.2.4. Dehydrogenation of Perhydrodibenzyltoluene to Dibenzyltoluene

Perhydrodibenzyltoluene is one of the most promising LOHC materials because the components of the system have low melting points, high boiling points, high hydrogenation–dehydrogenation rates can be achieved, and dibenzyltoluene is a cheap, bulk chemical. Auer et al. [51] studied the dehydrogenation of perhydrodibenzyltoluene (Scheme 4) on a 0.3% Pt/Al₂O₃ catalyst and found that the hydrogen production capacity of the system increases with decreasing Pt particle size. With the catalyst containing platinum particles with an average diameter of 1.55–1.95 nm, a hydrogen evolution rate of 6 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ was achieved at 90% conversion, while with the catalyst containing 4.6 nm particles, this rate was reduced by half. It has also been observed that modifying the Pt particles with sulfur can further increase the efficiency of the catalyst by reducing the number of those overly active sites favoring undesirable side reactions. Lee et al. [52] investigated the dehydrogenation of perhydrodibenzyltoluene using 5% Pt/CeO₂ and 5% Pt/Al₂O₃ catalysts. Under identical conditions, the rate of hydrogen formation on the CeO₂-supported catalyst is 25.2 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at a conversion level of 37%, while it is only 10.8 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ at 5.0% conversion on the Al₂O₃-supported catalyst (Table 2). The authors interpreted the strikingly large difference as being due to a difference in the pore structure of the two catalysts. The average pore size of the CeO₂-supported catalyst was 12.6 nm, while the average pore size of the Al₂O_3-supported catalyst was only 4.2 nm. In their opinion, the difference in activity is caused by the different diffusion rates within the catalyst pores. The same group studied the effect of adding lanthanum to the 5% Pd/Al₂O₃ catalyst in a batch reactor [53]. It was observed that under the same reaction conditions, 65% of the hydrogen stored in perhydrodibenzyltoluene could be recovered over the catalyst promoted with 1% La, while this value was 58% with the unpromoted catalyst. They found that although the lanthanum added to the catalyst masks some of the platinum sites on the surface, this negative effect is more than compensated for by the electron-donating effect of the additive, which has a positive effect on the dehydrogenation ability of platinum. The addition of CrO_x changes the activity of Pt/Al₂O₃ catalyst in dehydrogenation of perhydrodibenzotoluene according to a volcano curve [63]. The activity increases with the addition of 0–1 wt% CrO_x and then starts to decrease with higher amounts (3–10 wt%). The authors explain the positive effect by the formation of polychromates, since at low loadings, CrO_x is present on the catalyst surface in the form of monochromates, and at higher loadings, it is present in the form of crystalline Cr₂O₃, and these two forms are not favorable for dehydrogenation reactions, since polychromates provide the appropriate Pt particle size and electron density. Musavuli et al. [54] impregnated Al₂O₃ pellets and Al₂O₃ foam with 5 wt% Pt. It was observed that at low Pt/perhydrodibenzotoluene ratios, the Al₂O₃-foam-supported catalysts released hydrogen with a higher efficiency than the pellet-supported ones (15.5 vs. 10.0 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹), which then almost leveled off at around 4 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ as the catalyst/substrate ratio was increased. The explanation for this phenomenon is that as the amount of catalyst increases, the number of Pt atoms available to the reactant decreases. The foam-supported catalyst was tested in a continuous flow reactor in three consecutive cycles and showed stable, but relatively low, activity (34–38% yield) due to the presence of a large amount of partially dehydrogenated intermediates in the product mixture (Table 3). Auer et al. [64] studied the dehydrogenation of perhydrodibenzotoluene on Pt catalysts impregnated on Al₂O₃ supports with a hierarchical pore structure. The support contained mesopores (an average pore diameter of between 10 and 25 nm) and macropores (an average pore diameter of between 500 and 1000 nm). It was observed that due to the large pore volume, significantly more Pt (0.6–1.2 wt%) could be impregnated into the hierarchical support compared to the commercial catalyst with the 0.3 wt% Pt content, resulting in a significant increase in the catalyst efficiency per unit volume. The explanation for the increase in efficiency is that the large pore volume allows higher Pt dispersions and facilitates mass transport and bubble nucleation. The catalysts were also tested in continuous operation. It was found that catalysts with a higher platinum content were more efficient than the commercial catalyst. The most efficient sample showed a 2.5-times-higher productivity than the reference (3.93 vs. 1.55 g_H2·L_cat ⁻¹·min⁻¹).

5.2.5. Dehydrogenation of Dodecahydro-N-Ehylcarbazole to N-Ethylcarbazole

The nitrogen heteroatom in the molecule (Scheme 5) significantly reduces the dehydrogenation enthalpy of the fully hydrogenated form, allowing the process to operate below 200 °C. Accordingly, Wang et al. [55] tested the reaction on a 2.5% Pd/graphene oxide catalyst at 170 °C in a simple round-bottomed flask reactor and achieved 85% hydrogen selectivity at 100% conversion after 12 h of reaction time (that is, some dodecahydro-N-ethylcarbazole was only partially converted to its dehydrogenated derivative). The conversion did not change after five cycles, with only a slight decrease in selectivity (~73%, Table 3). The importance of the role of the support is demonstrated by the fact that the authors found a 14.4-fold lower specific activity per palladium atom on the Al₂O₃-supported catalyst tested as a reference. The support effect (activated carbon, Al₂O₃, TiO₂, and SiO₂) on the reaction was investigated by Feng et al. [56]. The supports were impregnated with 5% palladium. The dehydrogenation reaction was studied in a simple round-bottomed flask reactor. The carbon-supported catalyst proved to be the most active. The researchers were able to extract all of the releasable hydrogen after only 6 h of reaction time. With the Al₂O₃-supported catalyst, this was achieved after 8 h of reaction time, with the TiO₂-supported catalyst after 10 h, while with the SiO₂-supported catalyst, only 59% of the hydrogen was released after 12 h. They found that the efficiency of the catalyst is significantly influenced by the surface area of the support and the dispersion and reducibility of Pd. The 5 wt% Pd/C catalyst maintained 99% selectivity at 99% conversion, even after 4 cycles at 180 °C (Table 3). The dehydrogenation of dodecahydro-N-propylcarbazole over 5Pd/Al₂O₃, 2.5Pd2.5Ni/Al₂O₃, and 5Ni/Al₂O₃ catalysts (the number before the metal represents the metal content in wt%) was investigated by Chen et al. [57]. It was observed that the catalyst containing only nickel showed no dehydrogenation activity, whereas a synergistic effect was observed for the Pd/Ni catalyst, i.e., the bimetallic catalyst proved to be more active than the sample containing only palladium. When the reaction was tested at 180 °C, 7 h were required for complete conversion on the bimetallic catalyst and 10 h on the 5Pd/Al₂O₃. The stability of the 2.5Pd2.5Ni/Al₂O₃ catalyst was tested over five consecutive cycles, and no loss of activity was observed (Table 3). Li et al. [65] produced a C₃N₄ (carbon nitride) mesoporous nanosheet material rich in N-vacancies by the thermal treatment of C₃N₄ doped with trace amounts of sulfur. The support was impregnated with platinum. The dehydrogenation of dodecahydro-N-ethylcarbazole was then studied. The Pd/S₀.₀₃-C₃N₄-Nv₅₅₀ catalyst released 5.41 wt% of hydrogen in 90 min at 180 °C with a selectivity of 81.1% towards N-ethylcarbazole. The excellent activity is explained by the fact that the presence of C−S bonds induces the formation of N_3C vacancies, which greatly facilitate the dispersion of loaded metals, offering more active sites. The catalyst is also better than the undoped sample in terms of pore size and pore diameter, which facilitates the internal diffusion of the adsorbed molecules. The existence of an N vacancy weakens the electronic localization of C₃N₄ carriers, modulates the electronic structure of the active metal Pd, and promotes the activation of reactants and intermediates. Fan et al. [58] investigated the dehydrogenation of dodecahydro-N-ethylcarbazole in a continuous flow reactor. The 1 wt% Pd/Al₂O₃ showed the highest hydrogen production, outperforming Pd/C and Pt/Al₂O₃. The main advantage is that the high hydrogenation activity of the Al₂O₃-supported catalyst leaves almost no partially dehydrogenated intermediates. Under the same temperature and pressure (140 °C, 1 bar), but with varying mass and heat transfer, the specific productivity of the catalyst in the micropacked bed reactor (6.15 mmol H₂·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹) is nearly 1.8 times greater than that in a batch reactor (3.41 mmol H₂·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹). The continuous dehydrogenation process exhibits excellent stability with no significant deactivation within 200 h (Table 3). Park et al. [66] studied the dehydrogenation of dodecahydro-N-ethylcarbazole on MoO_x-doped Pd/Al₂O₃. Varying the MoO_x content between 0.1 and 7.5 Mo/Pd molar ratios, the 0.18 ratio was found to be most suitable for hydrogen release, releasing 5.78 wt% of the theoretical 5.79 wt% at 170 °C and maintaining this efficiency for five consecutive cycles. The small amount of MoO_x is present in an isolated structure on the surface of the catalyst and acts as a Lewis acid center. Its presence reduces the strength of the Pd–Pd bonds, allowing greater Pd dispersion. In addition, there is a charge transfer from the Pd to the Mo surface, making the palladium electron deficient and thus enhancing the adsorption of dodecahydro-N-ethylcarbazole. Additionally, H-spillover regenerates the active sites by moving the H atoms generated after dehydrogenation, thereby maintaining the high catalytic conversion.

5.3. Hydrogenation of the Most Important LOHC Materials

As mentioned above, hydrogenation reactions take place under much milder conditions than dehydrogenation reactions. In addition, hydrogenation is an important chemical process, so there are well-developed technologies for the hydrogenation of the LOHC materials included in our review. In the following, we present some of the results recently published on the subject of hydrogenation.

5.3.1. Hydrogenation of Benzene to Cyclohexane

Benzene is not considered a promising LOHC material. However, its hydrogenation as a model compound can provide valuable results. Mokrane and co-workers [67] studied the hydrogenation of benzene (Scheme 6) on various nickel-containing, Al₂O₃-supported catalysts. By carrying out the impregnation in the presence of ethylene glycol, they were able to distribute the metal with high dispersion on the surface of the Al₂O₃ support. It was found that the rate of hydrogenation increases as the size of the nickel particles is reduced. For example, the consumption rate of 0.28 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ was found on the sample containing 0.5 wt% Ni (

Table 4. Catalysts for the hydrogenation of hydrogen-lean LOHCs.

Catalyst	Reactor	T (°C)	p (bar)	Rate of H2 Consumption (mmol $·{\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}·$ min−1)	WHSV a (h−1)	Conv. (%)	Sel. (%)	Ref.
Benzene
0.5%Ni/Al2O3	flow through	180	1	0.28	0.96	45	100	[67]
3%Pd/CeO2	flow through	200	1	0.79	1.23	99.5	100	[68]
1%Pd/Al2O3	flow through	200	1	0.97	1.48	97.5	100	[69]
10%Ni/ N-doped carbon	flow through	145	1	284	15.3	-	100	[72]
Toluene
60%Ni/Al2O3	flow through	150	1	-	-	100	100	[73]
1%Ir/SiO2	flow through	125	1	3.75	7.03	83	100	[74]
3.7%Ru/C	autoclave	60	45	20 (1 h) b	-	10.2	100	[75]
3.7%Ru,9.3%Ni/C	autoclave	60	45	518 (1 h) b	-	100	100	[75]
3.7%Ru,9.2%Co/C	autoclave	60	45	547 (1 h) b	-	100	100	[75]
3.7%Ru,4.6%Ni, 4.6%Co/C	autoclave	60	45	683 (1 h) b	-	100	100	[75]
Naphthalene
1%Pt/Al-SBA-15	autoclave	290	70	26 (1 h) b	-	100	100	[76]
2.9%Ni,9%Mo/HMS	autoclave	325	65	-	-	90	43.8	[77]
2.9%Ni,9%Mo/Al-HMS	autoclave	325	65	-	-	100	75.7	[77]
Dibenzyltoluene
0.3%Pt/Al2O3	autoclave	270	30	24 (15 min) b	-	100	100	[78]
3%Pt/Al2O3	autoclave	140	40	17 (10 min) b	-	100	100	[79]
3%Pt/Al2O3-H2 plasma	autoclave	140	40	19 (10 min) b	-	100	100	[79]
3%Pt/Al2O3-O2 plasma	autoclave	140	40	23 (10 min) b	-	100	100	[79]
N-ethylcarbazole
5%Ru/Al2O3	autoclave	150	70	3.0 (1 h) b	-	100	95	[80]
LiNi5.5	autoclave	180	70	-	-	97	100	[81]
2.5%Ru,2.5%Ni/Al2O3 c	autoclave	160	70	8.5 (2 h) b	-	100	100	[82]

^a WHSV: weight hourly space velocity; ^b in parentheses: reaction time; ^c N-propylcarbazole.

), which was an order of magnitude higher than that of observed on the catalyst containing 1 wt% Ni, but significantly larger metal particles. Zhou et al. [68] studied the hydrogenation of benzene over a 3% Pd/CeO₂ catalyst. The CeO₂ supports were prepared using different structure-directing compounds, called templates. It was observed that the catalyst with the largest surface area and palladium dispersion had the highest hydrogenation activity. A benzene conversion of 99.5% with 100% selectivity was measured when the reaction was run at 200 °C and 1 bar pressure (Table 4). The hydrogenation of benzene over a 1% Pd/Al₂O₃ catalyst was also investigated by the same research group [69]. A catalyst with a mesoporous structure and high specific surface area was obtained by impregnating palladium in nitric acid solution, with which it was possible to convert benzene into cyclohexane at 200 °C and atmospheric pressure with a yield of nearly 100% (Table 4). They interpreted the high yield as a result of the interaction between the partially positively charged palladium nanoparticles formed in the metal–support interactions and the delocalized electrons of benzene. The hydrogenation of benzene, biphenyl, and various alkylated benzene derivatives over an Al₂O₃-supported catalyst containing 0.12 wt% Pd, 3.8 wt% Ni, and 4.3 wt% Cr at 60 bar was investigated by Kalenchuk et al. [70]. It was observed that alkyl groups inhibit the uptake of hydrogen, and an order of activity was established as follows: benzene > biphenyl > toluene > ethylbenzene > m-xylene > p-xylene > o-xylene. The hydrogenation of benzene showed conversions of 42% after 5 h, 70% after 9 h, and 84% after 14 h at 60 bar pressure. Benzene hydrogenation can also be performed in a homogeneous catalytic reaction [71]. Among the complexes containing M = Ti, Zr, Hf, Nb, and Ta, [Hf(CH₂tBu)₄] (TOF = 1155 mol_C6H6∙molM⁻¹∙h⁻¹) and [Nb₂(μ₂-CSiMe₃)₂(CH₂SiMe₃)₄] (TOF = 1055 mol_C6H6∙molM⁻¹∙h⁻¹) were found to be the most active, surpassing the 5% Pd/C and Raney nickel catalysts, for which TOF = 393 and 72 mol_C6H6∙molM⁻¹∙h⁻¹ were measured, respectively, at 27 bar pressure and reaction times of 2.4 h (Hf), 2.1 h (Nb), and 4 h (Pd, Raney Ni). The authors studied the hydrogenation of C₆D₆ with H₂ on the Nb-containing catalyst under the same conditions and obtained d_0–12 isotopologues, whereas when the isotope exchange on C₆H₁₂ was studied with D₂, cyclohexane d_1–4 isotopologues were found. These experiments indicate that the C–H activation of hydrocarbon substrates and hydrogenation reactivity are feasible with the Nb pre-catalyst. Catalyst stability was investigated for the Nb-containing catalyst. After 4 h of reaction at 120 °C and 27 bar, the catalyst activity decreased from an initial TOF of 1055 h⁻¹ to 296 h⁻¹. In the second subsequent cycle, the initial TOF was 212 h⁻¹, which corresponds to a reduction in the rate of 28%. The hydrogenation of benzene over a 10 wt% Ni/C catalyst doped with nitrogen and boron was investigated by Ishii et al. [72]. In a study of the hydrogenation of benzene in a flow reactor, it was found that while boron doping at 145 °C barely increases the hydrogenation rate from 79.2 to 100.8 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹, nitrogen doping increases this value to 284.4 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹. This improvement in activity is attributed to the increase in the energy of the reactant adsorption on the Ni catalyst surface due to the support effect of the heteroatom-doped carbon associated with the variation of chemical states of Ni particles.

5.3.2. Hydrogenation of Toluene to Methylcyclohexane

Hydrogenating toluene (Scheme 7) is relatively simple. Lindfors and co-workers carried out kinetic measurements on an industrial Al₂O₃ catalyst (Engelhard) containing 60% nickel and obtained a 100% methylcyclohexane yield at atmospheric pressure and 150 °C [73]. They also found that higher temperatures no longer favor the reaction, as above 170 °C, activated hydrogen desorbs from the nickel surface without a reaction (more precisely, the hydrogen coverage becomes small compared to the toluene coverage, which is detrimental, assuming the Langmuir–Hinshelwood hydrogenation reaction mechanism). Janiszewska and co-workers [74] investigated the hydrogenation of toluene over a 1% Ir/SiO₂ catalyst. The SiO₂ support was subjected to acid (NH₄Cl) and alkaline (NH₃) treatments prior to iridium impregnation. It was found that the catalyst made from the acid-treated support contained more strong acid surface sites than the catalyst made from the alkali-treated or untreated support. It was also observed that acid pretreatment favored the formation of larger iridium particles (d = 2.9 vs. 1.3 nm), and the highest hydrogenation rate was found on this catalyst, with a value of 3.75 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ (Table 4). Zhu and co-workers [75] studied the hydrogenation of toluene over a 3.7% Ru/C catalyst. In order to understand the synergistic effect of nickel and cobalt, additional catalysts were prepared that also contained 3.7 wt% ruthenium, but with the addition of two other metals, the total metal content increased to ~12 wt%. The hydrogenation reaction was studied in an autoclave at a pressure of 45.5 bar and a temperature of 60 °C. No conversion could be detected on the sample without ruthenium, while a yield of 10.2% methylcyclohexane was found on the Ru/C catalyst at a hydrogenation rate of 20 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹. The methylcyclohexane yield on the bimetallic and trimetallic catalysts was 100% in all cases, with hydrogen consumption rates of 518 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ on the RuNi/C catalyst, 547 on the RuCo/C catalyst, and 683 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ on the RuNiCo/C catalyst (Table 4). The activity of the trimetallic catalyst was tested over five cycles and found to be reduced by only 4% (

Table 5. Stability of catalysts used for hydrogenation of hydrogen-lean LOHCs.

Catalyst	Reactor Type	T (°C)	p (bar)	W/F a (h−1)	$$\begin{gathered} \mathbf{Initial} \\ \mathbf{Reaction} \mathbf{Rate} \\ ({\mathbf{mmol}}_{{\mathbf{H}}_2}· \\ {\mathbf{g}}_{\mathbf{c}\mathbf{a}\mathbf{t}}^{-1}·{\mathbf{min}}^{-1}) \end{gathered}$$	Initial Conv. (%)	Initial Sel. (%)	TOS b (h)	Number of Recycling	Decrease of Reaction Rate (%)	Ref.
Benzene
Nb2(μ2-CSiMe3)2 (CH2SiMe3)4	autoclave	120	27.6	-	149	80	100	-	2	28	[71]
Toluene
3.7%Ru 4.6%Ni, 4.6%Co/C	autoclave	60	45	-	94 (1 h)	100	100	-	5	4	[75]
1.3wt%Pd/ graphite nanopla-telets	flow through	200	1	-	287	-	-	24	-	16.2	[83]
1.5wt%Pd/ Timrex (graphite)	flow through	200	1	-	127	-	-	24	-	14.6	[83]
0.039wt%Pd, 6wt%Co/ ZrO2	flow through	120	1	36.1	6	-	-	24	-	0	[85]
Naphtalene
0.8wt%Pt-Al2O3-NH2/SiO2	flow through	260	40	10	-	100	96.4	60	-	0	[86]
1wt%Pt/ H-Beta-75	flow through	220	40	10	-	96.7	79.3	40	-		[87]
Dibenzyltoluene
0.3%Pt/Al2O3	autoclave	300	30	-	37.5 (1 h)	96	100	-	4	0	[78]
3%Pt/Al2O3	batch	140	40	-	-	100	100	-	4	3.7	[79]
3%Pt/Al2O3-O2 plasma treated	batch	140	40	-	-	100	100	-	4	3.7	[79]
3%Pt/Al2O3-H2 plasma treated	batch	140	40	-	-	100	100	-	4	5.3	[79]
5%Pt/Al2O3	batch	240	50	-		100	100	-	3	61.3	[88]
N-Ethylcarbazole
LaNi5.5	batch	180	70	-	-	100	96.8	-	9	0	[81]
Ru2.5Ni2.5/Al2O3	batch	150	60	-	-	93.6	100	-	5		[82]
30%Ni, 10%La2O3, 60%Al2O3	batch	140	50	-	-	100	100	-	10	0	[89]

^a space velocity (h⁻¹); ^b time-on-stream (h), duration of stability test.

). Ojo et al. [83] studied the dehydrogenation of toluene over different carbon-supported Pd catalysts and found a correlation between the degree of crystallinity of the carbon and the hydrogenation activity. Compared to activated carbon- or SiO₂-supported catalysts, graphite nanoplatelets and the high-surface-area graphite samples showed significantly higher hydrogenation activity. The authors rule out the role of the hydrogen that is intercalated between the layers of graphite in the hydrogenation process. The positive effect is explained by the strong affinity of graphitic carbons for aromatics, such as toluene. This promotes a high concentration of toluene at the metal/graphene interface, thereby facilitating the reaction of toluene with adsorbed hydrogen, or even allowing the reaction of toluene and spilled hydrogen over the graphite support surface. The catalyst based on graphite nanoplatelets lost 17% of its activity after 24 h of reaction. The initial activity was ~0.43 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ (Table 5). Na et al. [84] investigated the hydrogenation of toluene on Pt/Silicalite-1 and Ni/Silicalite-1 catalysts. Prior to the impregnation of the metals, Silicalite-1 was treated in an autoclave for 3 h at 90 °C in an aqueous solution of a mixture of 11 M NH₃ and 0.2 M NH₄NO₃. As a result of the hydrothermal treatment, silanol nests were formed in the zeolite structure, resulting in a six-fold increase in metal dispersion compared to untreated supports with the same metal content. A higher degree of dispersion resulted in a higher hydrogenation efficiency. The reaction was carried out in a flow-through reactor at atmospheric pressure and 180 °C, using a ratio of n_(H2)/n_(toluene) = 4; the hydrogen consumption rate was increased by a factor of 2.5 for the Pt/Silicalite-1 and by a factor of 810 for the 5 Ni/Silicalite-1 catalyst. Oda et al. [85] prepared Pt-Co/m-ZrO₂-supported catalysts by alloying a trace amount of Pt on supported Co nanoparticles. Three types of catalysts were tested in the hydrogenation of toluene: (i) PtCo single-atom alloys, (ii) PtCo alloy nanoislands on Co nanoparticles, and (iii) PtCo alloy nanoparticles on a support. The efficiency of the catalysts varied in the following order: (i) > (ii) > (iii). The most important finding is that the optimal atomic to nanoscale design of the PtCo alloy catalyst achieves ultra-fast toluene hydrogenation, even with extremely low Pt usage (0.014–0.077 wt%), providing an innovative approach to saving precious metal in hydrogen storage and transport. The authors showed, by comparison with previously reported data, that their catalysts are significantly more active than those found in the literature in terms of “mol·mol_{noble metal}⁻¹·min⁻¹”. For the two catalysts with the lowest metal content, values of around 1000–1500 were measured, with only one article in the literature reporting a value around 700, while in most cases, values of below 250 are typical. Stability tests coupled with microscopy confirmed that PtCo single-atom alloy catalysis remained stable for at least 24 h, indicating that the supported Co nanoparticles also play a crucial role in preventing Pt aggregation (Table 5).

5.3.3. Hydrogenation of Naphthalene to Decalin

The hydrogenation of naphthalene (Scheme 8) can be carried out in the most efficient way on a supported noble metal catalyst. Medina-Mendoza et al. [76] deposited 1 wt% of platinum on an aluminum-modified SBA-15 support prepared by means of solid phase ion exchange or wet impregnation methods. The hydrogenation was carried out in an autoclave at a hydrogen pressure of 70 bar and a temperature of 290 °C using n-decane as a solvent. The results of the catalytic studies showed that the activity of the catalyst prepared by solid-phase ion exchange was 26 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹, which was almost twice that observed on the sample prepared by wet impregnation. It was found that the two methods resulted in an almost identical platinum dispersion. They suggested that the solid-phase ion exchange results in more active platinum nanoparticles, i.e., with more edges and corners, which further convert the stable hydrogenation intermediate, tetralin, with greater efficiency. Naphthalene can also be hydrogenated using transition metal catalysts. Montesinos-Castellanos and co-workers [77] studied the reaction on NiMo/Al(Ti, Zr)-HMS (HMS—hexagonal mesoporous silica) catalysts in an autoclave at 70 bar hydrogen pressure and 325 °C. Their results showed that the hydrogenation of naphthalene on the NiMo/HMS catalyst was stopped at the intermediate product, tetralin. When the HMS support was modified with aluminum, titanium, or zirconium at a ratio of Si/transition metal = 40, the reaction continued under the same conditions, and a mixture of cis/trans decalin became the main reaction product. It was also found that pre-sulfidation of the catalyst further increases the efficiency of hydrogenation. It was also found that the rate of tetralin–decalin hydrogenation increases with the increasing nickel oxidation state, that the higher dispersion of sulfided Ni and Mo particles increases the rate constant of naphthalene formation, and that the presence of terminal Mo = O forms promotes the second hydrogenation step, i.e., the hydrogenation of tetralin to decalin. The results show that the decalin selectivity of the catalyst at 90% conversion is only 43.8% on the NiMo/HMS catalyst, whereas the decalin selectivity on the NiMo/Al-HMS catalyst is 75.7% at full conversion (Table 4). Zhang et al. [90] investigated naphthalene dehydrogenation on a Pd/HY zeolite with different SiO₂/Al₂O₃ ratios in an autoclave at 200 °C, 40 bar pressure, and 60 min reaction time. The ideal ratio of SiO₂ to Al₂O₃ was found to be 9.5, at which a decalin selectivity of 73.5% was measured. The catalyst appeared to be stable, and no leaching or agglomeration of the Pd nanoparticles was observed over a long period of reaction time. Based on their experimental results, they defined a mesopore–acid–metal factor that comprehensively and quantitatively describes the states of mesopores, acid sites, and Pd nanoparticles in the Pd/HY catalyst. Wang et al. [86] investigated the hydrogenation of naphthalene on core–shell hierarchical porous silica nanospheres with a radially open pore structure and a controllable pore size. The catalyst was functionalized with amino groups, then grafted with aluminum and impregnated with platinum. This catalyst showed a naphthalene conversion of 100%, a decalin yield of 96.4%, a trans/cis ratio of 7.8, a kinetic constant of 1.55 × 10⁻⁵ mol⋅g⁻¹⋅s⁻¹, and a turnover frequency (TOF) value of 11.9 min⁻¹. The outstanding activity was interpreted as a synergistic effect of its hierarchical core–shell pore structure, suitable acidity, and ultra-small Pt metal phase. No activity lost was found after 60 h TOS at 260 °C and 40 bar (Table 5). Liu et al. [87] impregnated 1 wt% platinum into a H-beta zeolite with different SiO₂/Al₂O₃ molar ratios (25, 50, 75, 100, or 150). The hydrogenation of naphthalene was investigated in a flow-through reactor using a 5% naphthalene/cyclohexane solution, between 180 and 260 °C, at a pressure of 40 bar, and a volume ratio of H₂/organic solution of 400. On the Pt/H-Beta-75 catalyst, a decalin yield of 76.7% was obtained with a naphthalene conversion of 96.7% at 220 °C. The outstanding performance was explained by the synergistic effect of the optimal mesoporous structure, the appropriate acidity, and the ideal platinum particle size. The authors tested the catalytic stability of fresh Pt/H-Beta-75 and found no significant decrease in conversion and selectivity until the catalyst was continuously used for 40 h (Table 5).

5.3.4. Hydrogenation of Dibenzyltoluene to Perhydrodibenzyltoluene

The complete hydrogenation of dibenzyltoluene (Scheme 9) was first achieved over a Ru/Al₂O₃ catalyst [26]. Brückner et al. [91] performed the experiment on an Al₂O₃ catalyst containing 5 wt% ruthenium at 50 bar pressure and 150 °C. After the first hour of reaction, they achieved about 45% of the theoretically bound hydrogen, and they needed 4 h of reaction time to reach 100% hydrogenation. Their ¹H-NMR and HPLC measurements showed that during the hydrogenation of dibenzyltoluene, the hydrogenation of the two side rings occurs first, while the hydrogenation of the middle ring requires more vigorous conditions [92]. Jorschick and co-workers [78] studied a 0.3% Pt/Al₂O₃ catalyst (a product of Clariant) in the hydrogenation of dibenzyltoluene. At 270 °C, the platinum activity was similar to that of ruthenium (24 vs. 13 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹) [18,52]. With increasing temperature, the activity of Pt/Al₂O₃ increased and the amount of hydrogen absorbed per minute was 7 g at 230 °C and 25 g at 300 °C per mol of platinum. The activity of the catalyst, which was tested at 301 °C and 30 bar, remained unchanged after four cycles (Table 5). The hydrogenation of dibenzyltoluene over a 3% Pt/Al₂O₃ catalyst was investigated by Shi and co-workers [79]. To investigate the effect of the support, two Al₂O₃ samples were prepared, one treated with oxygen plasma and the other with hydrogen plasma, prior to impregnation. They found that the hydrogen plasma treatment increased the number of oxygen defects, while the oxygen plasma treatment increased the number of surface hydroxyl groups. Their most interesting observation was that both treatments promote hydrogen transfer (spillover) from the metal to the LOHC material. The treatments increase the number of surface Pt⁰ particles, thereby improving the hydrogenation and dehydrogenation activity of the catalyst, while suppressing side reactions and increasing the long-term stability of the catalyst. Among the three catalysts, the one treated with oxygen plasma proved to be the most active, followed by the one treated with hydrogen plasma and then the untreated sample, with H₂ consumption rate values of 23, 19, and 17 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹ (Table 4). The authors found a slight loss of activity of 3.7–5.3% when the stability of the catalysts was investigated over four hydrogenation/dehydrogenation cycles (Table 5). A Raney–Ni type catalyst can also be used for the hydrogenation of dibenzyltoluene. Ali et al. [88] achieved full hydrogen saturation in 30 h at 170 °C and 70 bar using a catalyst product from Sigma-Aldrich. During the three-cycle stability test, the hydrogen capacity was 100% in the first cycle, dropping to 58.1% and 38.7% in the second and third cycles, respectively (Table 5). In a flow-through system with a Clariant′s NISAT 310 catalyst containing 50 wt% nickel, almost complete hydrogenation (98%) also occurred under milder conditions (3–15 bar, 150–170 °C, 1.3 mL/min) [93]. Liu et al. [94] studied an Al₂O₃-supported catalyst containing 5 wt% Pd, Pt, Ru, or Rh in the hydrogenation reaction of dibenzyltoluene. The most active hydrogenation catalyst was 5 wt% Rh/Al₂O₃, with a TOF of 26.5 h⁻¹ and a degree of hydrogenation of 92.7% in 2 h. It is interesting to note that of the four noble metals, the Rh-containing metal had the weakest dehydrogenating effect in the conversion of perhydrodibenzotoluene, whereas the Pt-containing metal was the most active in this reaction and had the weakest dehydrogenating activity. For both hydrogenation and dehydrogenation, Pd and Ru gave values between those of the other two metals. Zhao et al. [95] studied the kinetics of the hydrogenation of dibenzyltoluene over a 5 wt% Ru/Al₂O₃ catalyst. By systematically studying the effects of the reaction temperature, pressure, and catalyst amount, the following conclusions were reached: (i) the main barrier to the reaction is diffusion within the catalyst particles, while diffusion inhibition within the gas/liquid phase and the bulk liquid phase is negligible, (ii) the apparent reaction rate is zero order for the concentration of dibenzyltoluene, while it is first order for the hydrogen dissolved in the liquid phase, and (iii) the overall process of DBT hydrogenation over Ru/Al₂O₃ catalyst is mainly kinetically controlled by the surface reaction on the catalyst particles, with an apparent activation energy of 10.4 kJ/mol.

5.3.5. Hydrogenation of N-ethylcarbazole to Dodecahydro-N-ethylcarbazole

The hydrogenation of N-ethylcarbazole (Scheme 10) using a Raney–Ni catalyst can be performed under relatively mild conditions. Ye et al. [96] carried out the reaction in an autoclave at 200 °C and 30 bar pressure, achieving complete conversion in 1 h. Kinetic measurements have shown that in the liquid phase, between 120 and 200 °C, the apparent reaction order for N-ethylcarbazole is zero, while for hydrogen it is one (over the entire temperature range, almost the entire surface of the catalyst is covered with N-ethylcarbazole; the reaction rate is proportional to the H₂ concentration, i.e., to the H₂ partial pressure). Sotoodeh and co-workers [80] prepared dodecahydro-N-ethylcarbazole with 100% conversion and 95% selectivity over a 5% Ru/Al₂O₃ catalyst at 150 °C and 70 bar after 1 h of reaction time (Table 4). The reaction mixture was 100 mL and contained 6 wt% N-ethylcarbazole dissolved in decalin and 1 g of catalyst. Liu et al. [97] hydrogenated N-ethylcarbazole in a hydrogen/methane gas mixture over a similar 5% Ru/Al₂O₃ catalyst. The reaction was carried out without a solvent at 170 °C under a hydrogen pressure of 70 bar, and complete hydrogenation was achieved in 5 h. Yu and co-workers [81] prepared a catalyst with a unique core–shell structure by mixing a mixture of La(OH)₃ and Ni(OH)₂ with CaH₂ under inert conditions, in which the LaNi₅ core was surrounded by a nickel-enriched shell. During the hydrogenation–dehydrogenation process, a transition state product with the composition LiH_X was formed in the shell, which greatly facilitated the hydrogen transfer processes in both directions. By carrying out the reaction in an autoclave, starting from 1 g of N-ethylcarbazole and 100 mg of LaNi₅.₅, performing hydrogenation at 180 °C and 70 bar pressure for 8 h and performing dehydrogenation at atmospheric pressure, 200 °C, for 6 h, over nine cycles, they managed to carry out the back-and-forth reaction with ~95% efficiency (Table 5). There have also been attempts to accelerate the hydrogenation of N-ethyl carbazole with solid metal hydrides. A mechanical mixture of Ni/Al₂O₃ and YH₃ was used as a catalyst by Wu et al. [98]. It was observed that while Ni/Al₂O₃ and YH₃ individually showed no hydrogenation activity, the mechanical mixture of the two components hydrogenated N-ethylcarbazole at almost the same rate as the 5% Ru/Al₂O₃ catalyst. By investigating the mechanism of the process using isotopic labeling, the authors propose the following mechanism: YH₃ is able to reversibly adsorb/desorb hydrogen and transfer atomic hydrogen to N-ethylcarbazole activated on Ni/Al₂O₃, while a hydrogen defect is formed in the YH₃ phase, which is then regenerated by the gas phase hydrogen. The regeneration of the catalyst is demonstrated by the fact that after a complete hydrogenation cycle, the hydrogenation rate hardly decreases after washing. XRD, TEM, and XPS measurements also confirmed that there was no change in the catalyst structure during the reaction. Chen et al. [82] investigated the hydrogenation of N-propylcarbazole over Ru-Ni alloy catalysts in an autoclave at 130–160 °C and a pressure of 40–70 bar. A synergistic effect was found when the two metals were present, as hydrogen uptake was faster and more efficient on the 2.5 wt% Ru-2.5 wt% Ni/Al₂O₃ catalyst than on the 2.5 wt% Ru/Al₂O₃ catalyst, while the 2.5 wt% Ni/Al₂O₃ catalyst showed no hydrogenation activity. Furthermore, complete hydrogenation was achieved in the presence of nickel, whereas under the same conditions a significant amount of 8H-propylcarbazole was still present in the product in the experiment carried out with a catalyst containing only ruthenium. The 2.5 wt% Ru-2.5 wt% Ni/Al₂O₃ retained its original activity after five consecutive cycles at 150 °C and 60 bar (Table 5). It was also observed that the Ru₂.₅Ni₂.₅/Al₂O₃ catalyst had a high hydrogenation performance with a saturated hydrogen uptake of 5.43 wt% and a rapid hydrogenation rate of 8.5 mmol_H2·${\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}$·min⁻¹. Zhu et al. [89] investigated the hydrogenation of N-propylcarbazole over a La₂O₃-doped Ni/Al₂O₃ catalyst in an autoclave at 140 °C and 50 bar pressure. The basic composition of the catalyst was 30 wt% Ni, 10 wt% La₂O₃, and 60 wt% Al₂O₃, which was reduced at different temperatures (450, 650, or 850 °C). Among the three catalysts, the sample prepared by reduction at 650 °C showed the fastest hydrogenation, with the rate of hydrogenation not decreasing, even after 10 consecutive cycles (Table 5). According to the authors, the introduction of La species effectively inhibited the formation of the unfavorable NiAl₂O₄ spinel structure, which in turn favored the improvement of the catalyst′s specific surface area and the balanced distribution of surface acidic sites. Importantly, these advantages also resulted in the formation of Ni-NiO interfaces in the La-Ni/Al₂O₃-650 catalyst, stable hydrogen adsorption sites, and enhanced interactions between metallic Ni particles and the support. Furthermore, the synergistic effects of the Ni⁰-Ni^δ+ structures modulated the d-band center with Ni closer to the Fermi level, thus favoring the adsorption and transformation of various reaction intermediates.

6. Future Research Directions for LOHC Hydrogen Storage

6.1. Background

Liquid organic hydrogen carriers (LOHCs) offer several advantages over other systems developed for hydrogen storage: they have a high specific energy density for stored hydrogen; are liquid at ambient temperature and pressure, making them easy to store and transport; and fit into the existing liquid fuel infrastructure. Their disadvantages include the following: relatively high temperatures are required for complete and rapid dehydrogenation, resulting in significant energy consumption; undesirable side reactions require the continuous replacement of the lost LOHC fraction; and poisoning and structural damage to the catalysts slow down the reaction. To maintain conversion, it may be necessary to periodically modify the reaction conditions (temperature, space velocity) and, if necessary, regenerate or replace the catalyst. Against this background, research and development will focus on three areas: (i) the development of new LOHC materials, (ii) the development of cheaper and more efficient catalysts than existing ones, and (iii) the development and integration of hydrogen storage systems using LOHCs into existing energy supply infrastructures.

6.2. Directions for the Development of New LOHC Materials

The main requirements to be met by LOHC materials are described in Section 3. The development of new LOHCs is taking place in line with these basic principles. LOHCs as hydrogen storage devices can be found in the literature with several applications at the basic research level, but today we can only read about three implementations at the industrial level [34]: Chyoda uses toluene–methylcyclohexane [99] and Hydrogenious Technologies GmbH uses benzyltoluene–perhydrobenzyltoluene [29], while Hynertech Corporation does not specify the LOHC material it uses [100]. One of the main objectives of the research and development efforts is to develop an LOHC material with the lowest possible dehydrogenation enthalpy, as the high energy requirement of this step is one of the weakest points of the hydrogen storage process. LOHC materials are currently produced from crude oil, which is a controversial solution from the point of view of carbon neutrality. It would be obvious to use aromatic compounds for the synthesis of LOHCs, which are present in large quantities in biomass (lignin fraction) but are hardly used today. In addition, the use of aromatic molecules extracted from polymer waste containing aromatic components, such as polyethylene terephthalate, which is currently recycled at less than 10%, as raw materials for LOHCs could be a solution.

6.3. Development of Hydrogenation and Dehydrogenation Catalysts

As shown above, the most efficient catalysts for storing hydrogen using LOHCs contain noble metals (Ru, Pt, Pd, and Ir), which makes them relatively expensive. One of the main research directions is to replace noble metals by using much cheaper transition metals (Ni, Fe, Mn, Mo, etc.). Extensive research is also being carried out to improve the efficiency of precious metal catalysts. The main objectives of catalyst development are as follows [25,34,101,102]:

• Achieve the highest possible metal dispersion, which is most easily achieved on mesoporous supports with a high specific surface area. The fine tuning of the chemical properties of the support (e.g., acidity, basicity) is also an important issue. The method of metal deposition, the development of ideal pre-treatment conditions, and the doping of the support and/or the supported metal can also improve the efficiency of the catalyst.
• An important aspect is the acceleration of hydrogen spillover between the metal component and the LOHC material in the activated state on the support. This can be facilitated by increasing the number of acidic surface hydroxyl groups (Brønsted acidity) or by introducing a hydrogen transfer additive, which, as we have seen, facilitates the hydrogenation of the LOHC in the case of the LaNi₅/LiH₃ catalyst [98], since LiH₃ is able to transfer atomic hydrogen to N-ethylcarbazole activated on Ni/Al₂O₃.
• By doping a support (e.g., with boron, nitrogen, phosphorus, or sulfur), the degree of interaction between the electron-rich benzene rings of LOHC materials and the Lewis acid sites of the catalyst can be increased, which can increase the rate of hydrogenation.
• By using two or more metals together, the synergistic effect between the metals can be exploited by creating electron-deficient sites on the catalyst surface, which then promote the adsorption of the electron-rich aromatic molecules.

In view of the above considerations, significant research efforts are being undertaken to develop advanced LOHC materials and to make the hydrogenation–dehydrogenation process more efficient. Below, we briefly present the results of some recent publications:

Gong et al. [103] developed N-ethylcarbazole by the replacement of the ethyl group with a phenyl group. Perhydro-N-phenylcarbazole has a hydrogen storage capacity of 6.9 wt% and a dehydrogenation enthalpy of 51.7 kJ/mol_H2. They managed to release 4.74 wt% hydrogen from their new LOHC material in 15 min at 270 °C. The authors tested the stability of the system for 160 h and found no significant efficiency lost. It is known that among the benzyltoluene isomers, the hydrogenation–dehydrogenation of the para form is significantly easier than that of the ortho form due to steric effects. Kim et al. [104] developed a para-selective synthesis using a beta zeolite (Si/Al = 25) catalyst. Starting from benzyl chloride and toluene, it was possible to obtain para-benzyltoluene with a selectivity of 72% (the equilibrium isomer composition is 46% ortho, 5% meta, and 49% para isomer). The authors determined the activation energy of hydrogenation over a Ru/Al₂O₃ catalyst for isomer mixtures of different compositions and found a value of 15.8 kJ/mol when the amount of the para-isomer was 49%, 15.1 kJ/mol at 55%, and 13.9 kJ/mol at a 72% para-isomer content. Schörner and co-workers [105] studied the dehydrogenation of perhydrodibenzotoluene on catalysts heated by induction heating. Three types of catalysts were prepared: (i) Pt/alumina on steel beads, (ii) Pt/alumina on a flat FeCrAl-plate, and (iii) α-alumina core with a γ-alumina shell that contains spray-dried iron oxide nanoparticle agglomerates and is impregnated with Pt. Induction heating allows rapid heating and heats up only the catalyst particle, not the reactant and reactor, significantly reducing the energy requirements of the process. The authors did not find any products from the decomposition of the LOHC material. It was found that to achieve the same hydrogen evolution rate with inductive heating, as opposed to heating the entire reactor, a 35 °C lower temperature was sufficient for the Pt/Al₂O₃ on steel beads. The efficiency of hydrogen storage using LOHCs can also be increased by microwave treatments [106]. Wang et al. [107] investigated the cyclohexane dehydrogenation over a Pt/MgAl(Sn)O catalyst. It was observed that the Pt dispersion of the microwave treated sample significantly exceeded the Pt dispersion of the oven-calcined sample, while the interaction between the tin modifier and the support was also proved to be stronger, and the pores of the microwave-treated catalyst were less clogged than the pores of the oven-calcined catalyst. The result was an increase in the rate of cyclohexane dehydrogenation. Ichikawa et al. [108] investigated the dehydrogenation of methylcyclohexane and found that the rate of hydrogen formation was significantly higher using microwave heating on a 5% Pt/carbon catalyst than in a similar experiment using conventional heating.

7. Summary

In the present literature review we described the methods suitable for hydrogen transport and storage, including their advantages and disadvantages. We then presented in detail the possibilities of using LOHC materials and described the main physicochemical properties of the most commonly used systems (benzene–cyclohexane, toluene–methylcyclohexane, naphthalene–decahydronaphthalene, dibenzyltoluene–perhydrodibenzyltoluene, and N-ethylcarbazole–perhydro-N-ethylcarbazole). In the following section, we introduced the catalysts used in the dehydrogenation of the hydrogen-rich forms of LOHC pairs and their efficiency and then described the results obtained in the hydrogenation of the hydrogen-lean forms. At the end of our review, we outlined future directions for research into hydrogen storage using LOHCs and presented some recently developed innovative solutions to increase the efficiency of hydrogen storage.