Safe and sustainable hydrogen production using Liquid Organic Hydrogen Carriers (LOHCs)

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INTRODUCTION

The high energy content per unit of mass of molecular hydrogen (120 MJ kg-1 or 33.3 kWh kg-1) makes it key for shifting towards a decarbonized energy system and industrial chemical production. However, most worldwide H2 production occurs via hydrocarbon steam reforming (45–65 Mt/year), and this process is associated with a high CO2 footprint, though cost-effective (1). Even if, in principle, it is possible to combine a sequestration and storage technology of the co-produced CO2 with the reforming reaction, this strategy is viable only for large-scale production. Therefore, the research for green hydrogen production is becoming ever more urgent in the context of a zero-carbon energy system.
Water electrolysis enables hydrogen production with high purity (> 99.5%) without the co-generation of gas emissions; however, this is a high energy-intensive process, which makes its utilization economically unfavourable for large-scale production.
Moreover, being the lightest molecule, H2 has a very low density and this constitutes a critical aspect of its storage and transportation (1,2). To increase the energy density per unit volume, cryogenic temperatures (-253° C) or high pressures
(35-70 MPa), are required to liquefy or compress hydrogen, respectively. Alternatively, the storage at cryogenic temperatures (-120 / -190 ° C) in pressurized containers (25-30 MPa) can be realized (cryo-compressed hydrogen). However, the extreme conditions used in these technologies require technologically advanced and expensive infrastructures, not yet available in all countries.
Furthermore, major safety concerns are associated with them, mainly because of the possibility of the accidental release of hydrogen into the atmosphere. In fact, the hydrogen flammability range is wider than that of other energy carriers (4-75% v / v) (2).
To address these issues, an alternative and promising option is the use of solid or liquid materials that can store hydrogen by physisorption and/or chemisorption and release it “on-demand” (3,4). In the case of physisorption, hydrogen is stored by exploiting weak surface interactions with the material or incorporating it at cryogenic temperatures (-196 °C).
Representative classes of solid materials suitable for hydrogen sorption are nanoporous carbon-based materials, MOF (Metal-Organic Framework), COF (Covalent Organic Framework) and PAF (Porous Aromatic Framework) (3,4). However, high storage capacities are possible only at cryogenic temperatures.
In fact, the interaction between H2 and the materials is weak, of the order of ~10 kJ/mol, and a spontaneous hydrogen leakage at a higher temperature may occur.
On the other hand, hydrogen storage by chemisorption via chemical bond or complexation with liquid or solid materials, allows for reaching the highest storage densities, at moderate pressures. In these systems, hydrogen can be released under specific reaction conditions. Hydride-based materials – i.e. metal hydrides (MgH2, LiH, AlH3), complex hydrides (NaBH4, NaAlH4, Mg2FeH6), chemical hydrides (BH3NH3) – are solid chemical carriers. However, the poor reversibility, sensitivity to oxygen and humidity, and pyrophoric characteristics of these systems represent the major issues related to their employment (3,4).
To overcome the limitation of solid hydrogen carriers, research efforts to store hydrogen chemically in organic liquids are ever-increasing. As a result, the liquid systems that can store and release hydrogen via hydrogenation/dehydrogenation cycles under ambient conditions are identified with the term “Liquid Organic Hydrogen Carriers” (LOHC). Moreover, the H2 manipulation in the form of liquid reduces the potential risks associated with hydrogen handling, transport, and storage (3-7).
Thus, several hetero- and carbocyclic compounds (i.e., perhydro-N-ethylcarbazole, perhydro-dibenzyl toluene, cyclohexane, methylcyclohexane, decalin and bicyclohexyl) have been investigated from different points of view, including energetic and economic feasibility, and the technology readiness for their implementation in clean and sustainable energy system (8).
Despite their potential as hydrogen energy systems, the literature about the implementation of LOHCs as hydrogen carriers for industrial chemical transformations is lacking. In this scenario, small alcohols (e.g., isopropanol and methanol) and formic acid play a central role both as energy and hydrogen carriers (9-13).
Moreover, these LOHCs can be produced from renewable sources, representing an economical and sustainable hydrogen source (9-11).

LOHCs AS HYDROGEN AND ENERGY CARRIERS

Isopropanol (i-PrOH), which presents a 3.3 wt%
of hydrogen capacity and 0.87 kWh L-1 of energy storage density, is mainly produced via direct or indirect propylene hydration, or acetone hydrogenation. Alternatively, it can be produced by biomass biochemical conversion, which makes i-PrOH a renewable and reversible hydrogen carrier (Figure 1).
Formic acid offers a 4.4 wt% of hydrogen capacity and 1.8 kWh L-1 of energy storage. Although recently CO2 hydrogenation has been extensively investigated, the most economical option for formic acid production is the carbonylation reaction of methanol, followed by hydrolysis of the resulting formate. Moreover, formic acid can be generated as a by-product in producing chemicals such as acetic acid, formamide or polyols and prepared via hydrolysis-dehydration of hexose sugars derived from biomass (9-11). Since formic acid may competitively dehydrate to give the stable products CO and H2O, besides dehydrogenating, the development of catalysts which are selective towards the dehydrogenation reaction plays a crucial role (Figure 1).
The combined CO2 capture and hydrogenation is an advantageous strategy also for MeOH production both from an economic and sustainability point of view. (12) Alternatively, MeOH can be produced from syngas. To avoid the formation of toxic CO during MeOH dehydrogenation, it is required the presence of H2O (Figure 1), this process is associated with a low heat demand (16.5 kJ/molH2). (8) Furthermore, MeOH has a high hydrogen capacity (12.1 wt%)
and energy density (3.3 kWh L-1). It is worthy to notice that the necessity of using solvents during the methanol dehydrogenation processes may reduce these values. Indeed, from 4 to 10 wt% of hydrogen capacity and from 1.1 to 2.7 kWh L-1 of energy density can be realized for low-temperature dehydrogenation or high-temperature methanol steam reforming, respectively (8).
The significant energy density and hydrogen capacity of the above LOHCs make them promising renewable hydrogen sources for biomass transformation reactions. Indeed, the interest in biomass as an alternative to fossil feedstocks, to afford value-added products including bulk chemicals, fine chemicals, pharmaceutical ingredients, energy carriers, and materials has recently grown rapidly.
Some interesting examples of biomass upgrading using LOHCs as hydrogen sources are the transformation of levulinic acid, or its esters, into γ-valerolactone (GVL), (14-18) pyrrolidones (16,19-23) and quinolines; (23) the hydrogenolysis of furan-based bioderived compounds; (24-29) the depolymerization of lignin to phenolic compounds, (30-31) and many others (Scheme 1).

The transformation of lignin-related phenolic compounds into biofuels and chemical compounds by LOHC-assisted catalytic reduction processes is another fast-growing research area (11, 32-35). In the following paragraph, a brief, dedicated overview of representative reports from our group is given.

LOHC-ASSISTED UPGRADING OF LIGNIN-DERIVED PHENOLIC COMPOUNDS

The hydrogenation of phenol to cyclohexanone is a key transformation to access several value-added compounds. Indeed, cyclohexanone represents a precursor for the synthesis of Nylon 6,6 and Nylon 6. The reaction can either proceed through the hydrogenation of phenol to cyclohexanol and its subsequent dehydrogenation to cyclohexanone, or by a single-step selective hydrogenation path. This latter is a step- and waste-minimized process, ensuring a decrease in the associated costs and the environmental impact.
However, the selectivity of the process is a crucial parameter to be controlled with an appropriate catalytic system since cyclohexanone is highly reactive and tends to form over-hydrogenated by-products.
Combining heterogeneous catalysis and continuous flow technology defines an efficient, selective, and sustainable protocol (36). The cheap, commercially available Pd/C (10 wt%) was chosen as a catalyst. The reaction conditions have been optimized by varying several parameters, including the pH values of the reaction mixture and the nature of the base controlling the pH (NaOH, KOH, Bu4NOH, NH4OH). An 80 % yield of cyclohexanone was obtained by passing the mixture of phenol and sodium formate (H2-source) in NaOH aqueous solution at pH 12 into the flow reactor packed with Pd/C, and thermostated at 90 °C (36).
In a subsequent development, the Pd/C 10%wt was replaced with a tailor-made heterogeneous POLITAG (POLymeric Ionic TAG) catalytic system (37). This catalyst features pincer-type ionic ligands covalently immobilized on polymeric resin, that are able to stabilize Pd(0) nanoparticles. The selection of the appropriate ionic tag enabled the selective hydrogenation of phenol to cyclohexanone with an isolated yield of 87% (selectivity = 100%) under continuous flow conditions. The tailor-made catalyst was stable under the reaction conditions, allowing for the conversion of over 100 mmol of different phenolic substrates showing satisfactory results (67-100% yield) (Scheme 2). (37) The catalyst stability was further confirmed by the negligible Pd leaching (0.01 ppm).

The selective production of cyclohexanone represents a key step also in domino processes. The first example of hydrogenation/reductive amination of phenols to substituted cyclohexylamines in an aqueous medium was performed using Pd/C (10 wt%) as a heterogeneous catalyst and sodium formate as a hydrogen source (38). The reaction was first studied in batch conditions under microwave irradiation and isolated yields up to 96% were obtained. The catalyst recycling experiments demonstrated efficient recovery and reuse in at least five consecutive reaction runs. The use of flow conditions enabled the efficient conversion of selected phenols to cyclohexylamines on a gram-scale and in the presence of a reduced amount of catalyst compared to the batch protocol (2.7 mol% vs 5 mol%).

 

CONCLUSIONS

LOHCs represent a promising alternative to compressed or liquefied hydrogen to boost the shift toward a carbon-neutral hydrogen economy, compatible with the existing infrastructures. Moreover, the “on-demand” H2 release during chemical processes lead to the definition of safe protocols. In particular, formic acid and its salts, and small alcohols (i-PrOH and MeOH) play a central role as energy carriers as well as H-source in the biomass valorisation reactions to afford high added-value compounds.
In the context of biomass valorization via LOHC systems, to define efficient and selective protocols, the choice of catalyst is a key parameter. In addition, the tuning of reaction conditions and the implementation of continuous flow reactors are pivotal for a reliable technology in large-scale applications. It has been demonstrated that combining heterogeneous catalysis with energy-efficient technologies (i.e., continuous flow and microwave irradiation) in LOHCs-assisted lignin-derived phenols transformations leads to a safe process with minimized environmental impact.

ACKNOWLEDGEMENTS

We gratefully acknowledge the Università degli Studi di Perugia and MIUR for financial support to the project AMIS, through the program “Dipartimenti di Eccellenza – 2018-2022”.

 

Figure 1. Schematic representation of LOHCs systems (a) isopropanol, (b) formic acid and (c) methanol.

 

Scheme 1. Examples of biomass chemical transformations exploiting LOHCs.

 

Scheme 2. Selective hydrogenation of different substituted phenolic compounds using sodium formate as in situ hydrogen source.

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