Hydrogen production from homocyclic liquid organic hydrogen carriers (LOHCs): Benchmarking studies and energy-economic analyses

Highlights

• Affordable homocyclic LOHCs can easily store H₂ with a storage capacity of 6–8 wt% at ambient conditions.

• Benchmarking studies for homocyclic LOHC dehydrogenation were conducted.

• Model Pt/γ-Al₂O₃ catalyst and physicochemical properties of representative homocyclic LOHCs were studied.

• Energy analysis and economic assessment in maritime transport scenarios were performed.

Abstract

The construction of a cost-effective hydrogen infrastructure is needed to perpetuate the deployment of the hydrogen economy. Liquid organic hydrogen carriers (LOHCs), generally possessing a hydrogen storage capacity of 6–8 wt%, can store a large volume of hydrogen for an extended period at ambient temperature and pressure. LOHCs are also highly compatible with conventional petroleum production and transport infrastructure. The proper selection among strong LOHC candidates is of paramount importance; hence, benchmarking studies are required for a fair comparison. Herein, the dehydrogenation characteristics of different homocyclic (C_xH_y-) LOHCs are studied in a high-throughput screening system with continuous fixed-bed reactors. Four homocyclic LOHCs, including methylcyclohexane, hydrogenated biphenyl-based eutectic mixtures, perhydro-monobenzyltoluene, and perhydro-dibenzyltoluene, are dehydrogenated using a 0.5 wt% Pt/Al₂O₃ heterogeneous catalyst under the identical test protocol for comparative analysis. We further discuss post-mortem analyses on the used catalyst, physicochemical properties of LOHCs, energy analysis, and economic assessment of a maritime transport scenario for each LOHC, suggesting a future strategy to promote the practical use of homocyclic LOHCs.

Introduction

Establishing an economical hydrogen infrastructure is needed to promote the hydrogen economy. The existing hydrogen storage technologies are under a two-pronged strategy: Physical hydrogen storage (e.g., high-pressure gaseous hydrogen storage and low-temperature liquefied hydrogen storage) and material-based hydrogen storage (e.g., chemical hydrides and metal hydrides) [1], [2], [3], [4], [5]. In recent years, liquid organic hydrogen carriers (LOHCs), a type of material-based hydrogen storage materials with fully reversible hydrogenation and dehydrogenation cycles, have come into the spotlight from the hydrogen research community due to their reversibility without accompanying CO₂ emission [6], [7], [8], [9]. In general, LOHCs have hydrogen storage capacities of 6–8 wt%, and they can store hydrogen at ambient temperature and pressure in highly stable forms [9]. Unlike physical hydrogen storage based on compression or liquefaction of hydrogen, LOHCs do not require ultra-high-pressure vessels or cryogenic hydrogen tanks and, consequently, play a vital role in large-scale storage and low system maintenance costs. LOHCs also benefit from high volumetric hydrogen storage density (1–2.5 kW h/L_LOHC(l); alternatively 0.03–0.075 kg_H2/L_LOHC(l)) compared to compressed hydrogen gas with low volumetric density (0.038 kg_H2/L_LOHC(l) at 70.0 MPa) [6]. Besides, the LOHC material properties are similar to gasoline and diesel, allowing us to utilize most conventional petroleum processing and transport infrastructure. For these reasons, the LOHC is one of the most attractive options, along with liquid hydrogen and ammonia as a carbon–neutral solution for international hydrogen transportation [10]. These options can be applied to various transport modes (e.g., trucks, pipelines, or ships) depending on distance, terrain, and end-use. Compared with the liquid hydrogen, the LOHC is considered to be more favorable for long-distance transport using ships due to its low boil-off losses and ship investment (i.e., retrofitting) cost, which can lead to low specific transport costs [3]. However, the LOHC requires high energy demand for the dehydrogenation process at the end-use location and the properties of dehydrogenation vary with each substance, which can be directly related to economic performance. Therefore, it is necessary to examine the dehydrogenation properties of each LOHC material precisely for deducing suitable LOHC in terms of total hydrogen cost.

LOHCs are divided into two groups: Homocyclic compounds with identical C–C bonds in the backbone structure (e.g., methylcyclohexane (MCH) [11], [12], [13], [14], [15], [16], [17], [18], hydrogenated biphenyl (Bicyclohexyl, BC) [19], [20], [21], hydrogenated biphenyl and diphenylmethane (H12-BPDM) [22], [23], [24], perhydro-monobenzyltoluene (H12-MBT) [25], [26], [27], [28], and perhydro-dibenzyltoluene (H18-DBT) [28], [29], [30], [31], [32], [33], [34], [35], [36]) and heterocyclic compounds containing C–N bonds (e.g., perhydro-N-ethylcarbazoles (H12-NEC) [37], [38], and octahydroindoles [39]) or C–O bonds (e.g., dicyclohexyl ether [40]). Although heterocyclic compounds with heteroatoms (N or O) capable of destabilizing neighboring C–H bonds have the advantage of lowering the dehydrogenation temperature with less endothermicity, they are still too costly to deploy on a commercial scale. It is also reported that heterocyclic LOHCs can suffer from C–N bond scission and dealkylation during dehydrogenation [41].

Not only do homocyclic LOHC compounds possess high chemical stability, but they are also off-the-shelf at reasonably low prices. Their low mobility in soil and high biodegradability have also gained significant attention [42]. Commonly, MCH (methylcyclohexane, C₇H₁₄) and H18-DBT (perhydro-dibenzyltoluene, C₂₁H₃₈) are widely known and accepted as promising homocyclic hydrogen carriers. Recently, H12-BPDM has been reported in the LOHC research community [22], [23], [24]. H12-BPDM, the hydrogenated form of a eutectic mixture of biphenyl (BP) and diphenylmethane (DM), has been contrived to utilize BC with 7.2 wt% H₂ capacity while resolving the solidification problem of biphenyl by lowering the melting point from 69.2 to 13.1 °C. Although many studies regarding the dehydrogenation of homocyclic LOHCs have been carried out as Table 1, only a few carried out comparative studies on homocyclic LOHC performance, and there has been no clear scientific consensus in the hydrogen community for testing LOHCs and catalysts and for reporting their productivity in a highly consolidated manner. LOHCs are currently competing with other promising hydrogen carriers such as compressed and liquefied hydrogen [1], [3], ammonia [43], [44], [45], and methanol [46], [47], [48]. There should be a benchmarking protocol for the adoption of promising LOHCs among various viable candidates. Otherwise, the rosy blueprint on LOHCs could end up being just a blueprint. Therefore, this study attempted to evaluate their dehydrogenation characteristics in a well-defined condition and report the standardized dehydrogenation performance quantitatively in order to contribute to drawing out optimal solutions based on commercially available homocyclic LOHCs.

Such benchmarking studies have been emphasized in relevant energy research fields such as electrolyzers [49], [50] and fuel cells [51]. Key dehydrogenation performance indicators should be quantitatively elucidated via benchmarking studies [52], while the obtained data should be qualitatively comparable with other literature. The benchmarking studies are also deemed very important for conducting legitimate techno-economic analyses on LOHCs, as many techno-economic analyses of LOHCs set assumptions regarding dehydrogenation catalyst behavior and construct costs of a dehydrogenation reactor [3], [7]. These parameters usually show a high sensitivity to the levelized cost of hydrogen [3], [7], [53], which can significantly affect policy and business decisions. Groundless approaches with a lack of realistic and practical inputs possibly misled governments and companies to pursue unpromising choices, significantly undermining R&D and business fields in the long run.

This study presents a generalizable procedure for selecting LOHC and dehydrogenation catalysts. Evaluating the H₂-release characteristics is coupled with the understanding of each LOHC material’s properties. Energy and economic assessment are performed in consideration of the current technology level. This step-by-step process is important for the successful demonstration of hydrogen production on a large scale. Specifically, the benchmarking study is based on MCH, H12-BPDM, H12-MBT, and H18-DBT. MCH and H18-DBT are chosen due to their technological readiness pioneered by Chiyoda Corporation and Hydrogenious GmbH [54], [55]. H12-MBT and H18-DBT are selected because of their high commercial readiness as a heat transfer medium under the brand Marlotherm [29]. H12-BPDM is selected as it has the highest H₂ capacity of 6.85 wt% among the four LOHCs [22], [24]. Herein, the dehydrogenation characteristics of MCH, H12-BPDM, H12-MBT, and H18-DBT are compared at identical testing protocols in a continuous lab-scale fixed bed reactor with controlled reaction conditions such as catalytic bed temperature, pressure, space velocity, and co-fed H₂ to LOHC+ ratio. In addition to the experimental investigation of dehydrogenation characteristics, comparative analyses on the four homocyclic LOHCs are conducted in terms of dehydrogenation thermodynamics, physicochemical material properties, the overall energy consumption required for hydrogen release, and the economic assessment, all of which provide preliminary considerations in the benchmarking study.