Charging a Liquid Organic Hydrogen Carrier with wet hydrogen from

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Charging a Liquid Organic Hydrogen Carrier with wet hydrogen from electrolysis Holger Jorschick, Alexander Bulgarin, Lukas Alletsee, Patrick Preuster, Andreas Bösmann, and Peter Wasserscheid ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05778 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Sustainable Chemistry & Engineering

Charging a Liquid Organic Hydrogen Carrier with wet hydrogen from electrolysis Holger Jorschick,a Alexander Bulgarin,a Lukas Alletsee,a Patrick Preuster,a Andreas Bösmann,b Peter Wasserscheida,b* a

Forschungszentrum Jülich, Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstr. 3, 91058 Erlangen, Germany b

Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität ErlangenNürnberg, Egerlandstr. 3, D-91058 Erlangen, Germany (*Corresponding Author’s E-mail: [email protected])

Abstract: In this contribution we explore the combination of hydrogen production via electrolysis with subsequent storage of the hydrogen via catalytic hydrogenation of the LOHC compound dibenzyltoluene (H0-DBT) without hydrogen drying. The objective is to investigate the influence of water on the performance of Pt, Pd, Rh and Ru (all alumina supported) catalysts in the hydrogenation of H0-DBT. Our study shows that H0-DBT can be readily and fully hydrogenated even in presence of large excess of water, i.e. using a water-saturated LOHCphase. The hydrogenation activity of the ruthenium catalyst is hardly affected by water, while a slight decrease in hydrogenation activity was found in presence of water for the applied Rh-, Pt- and Pd catalyst. We conclude that wet hydrogen may be utilized in charging the LOHC compound H0-DBT and thus, energy and drying equipment for producing dry hydrogen can be saved in the production of the hydrogen-rich carrier perhydro dibenzyltoluene (H18-DBT).

Keywords: Hydrogen storage, hydrogen separation, LOHC systems, hydrogenation, electrolysis, heterogeneous catalysis, water, electrolysis, renewable energy

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Introduction In the upcoming years, the transition of the energy systems toward a higher share of volatile electricity production from wind and solar energy will lead to challenges for the power grid. There will be times with electricity shortages and times with electricity overproduction. Moreover, it is very likely that there will be global exchange of renewable energy equivalents through appropriate energy logistics systems. This is due to the fact that the global distribution of renewable energy potential is uneven and does not match the regional energy demand. As a matter of fact, the potential to harvest renewable energy is limited in highly industrialized regions like central Europe, Japan, South Korea or eastern China, while it is abundant in places like Iceland, Patagonia or Western Australia.1 Thus, renewable energy storage in large quantities and efficient renewable energy transport will become more and more important in the near future. Hydrogen is a secondary energy carrier that can be generated via electrochemical water splitting and is considered to be a very capable future energy vector. However, in most cases the hydrogen generated from renewable electricity, e.g. at a wind farm, cannot be used directly on site. Therefore, flexible hydrogen storage and distribution technologies are required to couple the fluctuating energy supply by renewable sources at an energy-rich site with the energy consumption profile at an energy-lean site. Recently, the concept of chemical hydrogen storage in Liquid Organic Hydrogen Carriers (LOHC) has been introduced as one, very promising way to provide the required hydrogen logistics in the already existing infrastructure for liquid fuels.2,3 A LOHC system consists of at least one pair of liquid molecules, one hydrogen-lean compound and one hydrogen-rich compound. To store hydrogen, the hydrogen-lean compound (LOHC-) undergoes an exothermic catalytic hydrogenation reaction using hydrogen at elevated hydrogen pressure to form the hydrogen-rich compound of the LOHC system (LOHC+). For hydrogen release at times or places of hydrogen or energy need, the LOHC+ compound is converted back to LOHCin a endothermal catalytic dehydrogenation reaction at low hydrogen pressure (typically below 5 bar).3,4 With state-of-the-art water electrolyzers, a hydrogen purity of at least 99.8 % can be achieved without any gas purification. The purity is significantly reduced in part load operation due to the lower gas production. In order to meet 99.999 % purity requirements, a two-stage purification process is typically applied downstream of the electrolyzer. By means of a catalytic conversion, the oxygen content is first reduced to below 10 ppm. Subsequently, the hydrogen 2 ACS Paragon Plus Environment

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is dried by an adsorption process to remove water impurities. The hydrogen purification results in energy losses of 3 – 8 %.5,6 Note, that hydrogen drying is certainly a requirement, if the produced hydrogen is to be transported as cryogenic liquid. Cooling of raw hydrogen to -253 °C would otherwise produce solid ice that would block pipes of the liquefaction plant. In addition to the two-stage process, research activities for in-situ purification of hydrogen using modified membranes can be found in literature.7 Against this background, we were interested to clarify whether hydrogen storage in LOHC systems requires the energy-intensive drying of hydrogen from the electrolyser. This question has to be answered considering the catalytic hydrogenation reaction of LOHC- that represents the hydrogen storage step of the storage cycle. Consequently, we study in this contribution the hydrogenation of the LOHC- compound using wet hydrogen, see Figure 1.

Figure 1: Hydrogen production by electrochemical water splitting with integrated hydrogen storage via LOHC- hydrogenation using the wet hydrogen without drying. The LOHC system dibenzyltoluene (H0-DBT)/perhydro dibenzyltoluene (H18-DBT), that has been proposed by some of us in 2014,8 is particularly interesting for technical application because its hydrogen-lean compound (H0-DBT) is available at industrial scale as isomeric mixture (tradename Marlothern®SH) for a very attractive price (2-4 €/kg on ton scale). Moreover, the system is characterized by benign and well-investigated toxicological characteristics, favorable thermophysical properties and excellent reversibility in the hydrogen storage process with fast kinetics in hydrogenation and dehydrogenation if suitable heterogeneous catalysts are applied.9,10 The hydrogenation of H0-DBT with pure hydrogen has been described in several papers in the recent years. Brückner et al.11 studied the hydrogenation of H0-DBT in cyclohexane using ruthenium catalysts on alumina and carbon supports in the temperature range of 80 – 160 °C using hydrogen pressures in the range of 10 to 70 bar. The solvent-free hydrogenation of H0DBT using alumina-supported Ru-catalyst at 180 °C in a batch autoclave was published by Dürr et al..12 The work elucidated the influence of hydrogen pressure and stirrer speed on the 3 ACS Paragon Plus Environment


reaction rate systematically. Mechanistic studies that revealed the hydrogenation sequence of the three different aromatic rings in H0-DBT were carried out by Do et al..13 As integral part of the Hot Pressure Swing Reactor concept, the hydrogenation of H0-DBT with platinum catalysts on alumina was investigated in the temperature range of 160 – 310 °C.9 All the named studies focussed on kinetic studies for the scale-up toward industrial H0-DBT hydrogenation processes. Such processes have been realized in the meanwhile in the scale of 10 kg/h hydrogen (300 kW power based on the Lower Heating Value, LHV, of the bound hydrogen), e.g. by Hydrogenious Technologies GmbH, Erlangen.14 Note, that H0-DBT hydrogenation with relatively pure, but wet hydrogen leads to water accumulation in the H18-DBT product. However, according to data published by Aslam et al.15, the water solubility in H18-DBT is only 60 ppmw (22 °C). It is significantly lower than the water solubility in H0-DBT (590 ppmw at 22 °C). Due to this low water solubility at ambient temperature, the aqueous and the organic phase will separate in the product tank and watersaturated H18-DBT (containing 60 ppmw of water) is obtained by simple decantation. In general, the effect of water impurities in the applied hydrogen has been rarely studied for catalytic hydrogenation reactions using heterogeneous catalysts. Exceptions are found where the authors tried to increase the selectivity in partial hydrogenation reactions by a competing adsorption of water at the catalyst. An example is the partial hydrogenation of benzene to cyclohexene in multiphase liquid reaction systems including water.16 Struijk et al. described the formation of a benzene-rich water layer around the catalytic sites, that promote desorption of cyclohexene and reduces full hydrogenation to cyclohexane.17,18 These authors linked the hydrophilicity of the catalyst to the observed selectivity to cyclohexene. To achieve a high selectivity to cyclohexene up to 50 mol% of water had to be added to the reaction mixture. An example of detrimental influence of water on the catalyst activity was given by Meille et al..19 These authors investigated the catalytic hydrogenation of α-methylstyrene using Pd on alumina and observed a significant decrease in hydrogenation rate already for water concentration as low as 100 ppm. The objective of this contribution is to investigate the influence of water on the performance of Pt, Pd, Rh and Ru on alumina catalysts in the hydrogenation of H0-DBT. For this purpose, hydrogenation experiments were carried out in a batch autoclave reactor set-up comparing reaction rate and by-product formation for pure hydrogen vs. hydrogen contaminated with defined amounts of water as feed gas. 4 ACS Paragon Plus Environment

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Results and discussion Hydrogenation of H0-DBT with pure hydrogen The catalysts were investigated and evaluated in terms of their initial productivity, stability and the formation of gaseous and liquid by-products. Building on the previous work of Dürr et al.12 and Do et al.13, we first studied Ru as catalytic active metal. A commercial egg-shell catalyst with a Ru loading of 0.5 mass% was applied for this purpose. The obtained degrees of hydrogenation over reaction time are shown in Figure 2. The displayed experiments have been carried out using 30 bar of hydrogen pressure in a temperature range of 150 to 260 °C.

Figure 2: Hydrogenation of H0-DBT using Ru on alumina (0.05 mol% Ru applied as 0.5 mass% Ru loading, egg-shell) and 30 bar hydrogen pressure at different temperatures.

The hydrogenation of H0-DBT is characterized by a high hydrogen uptake at the beginning of the reaction. From a Degree of Hydrogenation (DoH) of 0.5 on, the reaction rate and the hydrogen uptake per time decrease. At a reaction temperature of 150 °C, the reaction seems to ―1 be slightly inhibited initially, resulting in an initial productivity P20-50 of 2.5 gH2gRu min ―1 and

a DoH of 0.72 is reached after 5 h reaction time. The hydrogenation experiments at more ―1 elevated temperatures show initial productivities P20-50 of 3.3 gH2gRu min ―1 at 180 °C, 5.9 gH2 ―1 ―1 min ―1 at 210 °C and 4.7 gH2gRu min ―1 at 240 °C, respectively. After 5 h reaction time, gRu



DoHs between 0.78 and 0.81 are reached for these temperatures. Interestingly, an even higher temperature, leads to a reduced catalytic activity. A more detailed view on the course of the Ru-catalyzed reaction is given in Figure 3, where the different intermediates of the H0-DBT hydrogenation at 180 °C are shown as function of the reaction time (Figure 7). After 90 min, almost 100 % H0-DBT conversion is achieved. At that point in time, the hydrogenation of H6-DBT to H12-DBT progressed to a H12-DBT content of 77 % in the reaction mixture. From there on, the hydrogen uptake is almost constant and limited by the comparably slow kinetics of the H12-DBT to H18-DBT reaction, the final reaction step of the hydrogen storage process. After 300 min reaction time, no H6-DBT is left in the reaction mixture.

Figure 3: Composition of the reaction mixture over time at a temperature of 180 °C using Ru on alumina (0.05 mol% Ru applied as 0.5 mass% Ru loading, egg-shell).

Note, that we applied in this study a technical H0-DBT feed. The latter is a commercial product (Marlotherm®SH, SASOL, Marl, Germany) whose original application is to serve as heat transfer fluid with an application range up to 350 °C. The technical H0-DBT is an isomeric mixture of different regioisomers. Moreover, the technical feed contains 0.7 % lower boiling (e.g. benzyltoluene, toluene, benzene) and 1.1 % higher boiling compounds from its production process. During the course of the hydrogenation reaction, we observed that the light-boiler 6 ACS Paragon Plus Environment

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fraction increases slightly to 1.1 %, while the amount of higher-boiling components remained constant. The analysis of the gas phase after the batch hydrogenation experiments with pure hydrogen revealed the presence of components, such as carbon dioxide, methane, ethane and propene. These could be the result of decomposition side-reactions. The detected CO2-concentration in the gas phase was below 50 ppm. We assume that traces of water from the catalyst support could act as source of oxygen for the observed formation of CO2. If this hypothesis is true, the reaction with wet hydrogen should enhance the formation of CO2 in the hydrogenation considerably. It can be seen from Table 1 that with the Ru-based catalyst, very high methane concentrations were found in the off-gas at temperatures above 200 °C. This is a strong indicator that the Rucatalyst promotes breakage of C-C bonds and formation of hydrogenated fragments under these harsh conditions. In contrast, the Rh-, Pt- and the Pd-catalyst showed no such drastic increase of methane at temperatures above 200 °C. Table 1: Gas phase analysis after hydrogenation of H0-DBT using alumina supported Ru-, Pt-, Rh- and Pd catalysts at different temperatures; mean values over five hydrogenation runs. T °C 150 180 210 240 260

Ru/Al2O3 CO2 CH4 ppm ppm 6 485 8 1502 17 11193 46 49994 48 154430

Pt/Al2O3 CO2 CH4 ppm ppm 15 68 66 93 42 240 54 368 74 574

Rh/Al2O3 CO2 CH4 ppm ppm 45 319 40 537 19 515 29 844 42 1376

Pd/Al2O3 CO2 CH4 ppm ppm 35 103 25 232 36 327 77 421 38 560

In a next set of experiments, we studied a commercial egg-shell catalyst with a Pt loading of 0.3 mass% in the temperature range of 150 to 260 °C in more detail. A similar type of Pt catalyst already showed very high activity in H0-DBT hydrogenation with dry hydrogen.9 In order to achieve a highly resolved monitoring of the reaction process over time, we reduced the molar Pt to LOHC ratio to 0.025 mol%, compared to 0.05 mol% with Ru. The Pt on alumina catalyst shows a significantly stronger dependence on the reaction temperature (see Figure 4). After a reaction time of 4 h the DoH was only 0.26 at 150°C but 0.90 at 180 °C. A further increase in temperature led to full hydrogenation after 4 h at 210 °C, after 2 h at 240 °C and after 1 h at 260 °C. The initial productivity P20-50 increased from 1.4 gH2 7 ACS Paragon Plus Environment


gPt―1min ―1 at 180 °C to 17.3 gH2gPt―1min ―1 at 260 °C. From the initial productivities, an apparent Arrhenius activation energy of 78 kJ mol-1 was calculated.

Figure 4: Hydrogenation of H0-DBT using Pt on alumina (0.025 mol% Pt applied as 0.3 mass% Pt loading, egg-shell) and 30 bar hydrogen pressure at different temperatures.

A detailed chromatographic analysis of the composition of the liquid organic phase was carried out for the experiment at 240 °C. It shows that full conversion of H0-DBT was already achieved after 20 min. After 45 min, all formed H6-DBT species were converted to higher hydrogenated components. At this point in time, the reaction mixture consisted of H12-DBT and H18-DBT in equal proportions. Full hydrogenation was achieved after two hours. The detailed composition of the reaction mixture over time can be found in Figure 7. During the reaction, the light-boiler fraction increased slightly to 1.0 % and the amount of higher-boiling components decreased to 0.7 %. Due to the difficult chromatographic separation of the hydrogenated isomeric mixture, assignment of single products is difficult. It is not possible to clearly conclude from the spectra whether the decrease of heavies is due to cleavage of high boilers during hydrogenation or due to the low boiling points of some hydrogenated heavies. The analysis of the gas phase after 240 min reaction time showed an increase in the mean methane concentration from 68 ppm at 150 °C to 574 ppm at 260 °C, see Table 1. The mean CO2-concentration in the gas phase is independent of the reaction temperature below 75 ppm. 8 ACS Paragon Plus Environment

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The same investigations were carried out with the Rh on alumina and the Pd on alumina catalyst. To the best of our knowledge, the use of catalysts based on Pd and Rh for the hydrogenation of 0-DBT has not been described in the literature. For this reason, two commercial powder catalysts with a metal loading of 5.0 mass% were used for these first investigations. It turns out that rhodium has a similar activity compared to ruthenium, but with a much lower by-product formation, see Table 1. Palladium requires a much higher temperature for moderate hydrogenation activity, with relatively little by-product formation. The hydrogenation of H0-DBT using Rh and Pd on alumina with dry hydrogen at different temperatures is summarized in the Electronic Supporting Information. In the following, the influence of water on the activity and selectivity of these hydrogenation catalyst systems is described. Hydrogenation of H0-DBT using wet hydrogen Based on the determined activity and selectivity of the catalysts in the hydrogenation with pure hydrogen, further investigations with wet hydrogen were carried out at two different temperature levels. First, the results of low-temperature hydrogenation experiments at 180 °C using Ru and Rh on alumina will be discussed. This is followed by investigations with Pt and Pd on alumina at a temperature of 240 °C. The hydrogenation experiments using wet hydrogen as feedstock were performed with 10 and 20 mol% added water based on the amount of H0DBT. We deliberately decided to work with very large amounts of water to adjust a gas-liquidliquid-solid reaction system with a water-saturated LOHC-phase. We did so to ensure good reproducibility of our results as an insufficient amount of water could condense in cold parts of the reactor head leading to variable water contents dissolved in the LOHC phase. The applied 20 mol% water addition corresponded to a H2/H2O ratio of about 1.5 (40 mol% H2O in H2) under the applied conditions. All catalysts were employed as powders of a particle size of about 100 µm. The effect of water on the hydrogenation of H0-DBT over the alumina supported Ru and Rh catalysts at 180 °C is summarized in Figure 5.



Figure 5: Influence of water on hydrogenation of H0-DBT over Ru/Al2O3 and Rh/Al2O3 at 180 °C; 30 bar hydrogen pressure; 0.6 mol H0-DBT; 1200 rpm; 0.05 mol% Ru applied as 0.5 mass% Ru on alumina; 0.05 mol% Rh applied as 5.0 mass% Rh on alumina.

In the reference experiment without water, Ru on alumina showed at 30 bar hydrogen pressure ―1 and 180 °C an initial productivity P20-50 of 3.17 gH2gRu min ―1. Remarkably, the

hydrogenations in the presence of 1.08 g (10 mol% H2O) and 2.16 g water (20 mol% H2O) showed almost the same hydrogenation activity compared to the hydrogenation of H0-DBT without water. However, the methane and carbon dioxide fraction in the gas phase increased significantly. In the presence of 20 mol% water, we observed 6605 ppm methane and 95 ppm carbon dioxide in the gas phase. This is by the factor of 7 to 9 higher compared to hydrogenation run with pure hydrogen under otherwise identical conditions. Similar experiments were performed using Rh on alumina as catalyst. The reference experiment ―1 without water resulted in an initial productivity of 3.70 gH2gRh min ―1 and full hydrogenation

was achieved after 300 min reaction time. The addition of 10 mol% water led to a slight ―1 inhibition of the catalytic activity. The initial productivity P20-50 decreased to 2.74 gH2gRh

min ―1 and complete hydrogenation was achieved slightly after 360 minutes. In the experiment ―1 with 20 mol% water, a comparable initial productivity P20-50 of 2.91 gH2gRh min ―1 was

observed, but after 360 min reaction time only a DoH of 0.89 could be achieved. The gas phase after the experiment with the water content of 20 mol% showed a slight increase in methane 10 ACS Paragon Plus Environment

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(702 ppm vs. 522 ppm) and carbon dioxide (89 ppm vs. 36 ppm) in comparison to the experiment with pure hydrogen. Compared to the experiments with ruthenium, the inhibition effect of water on the rhodium catalyst was greater, but rhodium allowed in general for faster hydrogenation and significantly lower by-product formation. The effect of water on hydrogenation of H0-DBT over the alumina supported Pt and Pd catalysts at 240 °C is summarized in Figure 6.

Figure 6: Influence of water on hydrogenation of H0-DBT over Pt/Al2O3 and Pd/Al2O3 at 240 °C; 30 bar hydrogen pressure; 0.6 mol H0-DBT; 1200 rpm; 0.025 mol% Pt applied as 0.3 mass% Pt on alumina; 0.05 mol% Pd applied as 5.0 mass% Pd on alumina. In the hydrogenation of H0-DBT with platinum on alumina, the presence of water showed a significant influence on the hydrogenation rate. Note, that the hydrogenation with Pt at 240 °C is very effective up to a DoH 0.7 even in the presence of water with an initial productivity P2050

―1 of up to 11.4 gH2gnm min ―1. However, in the final hydrogenation step to perhydro-

dibenzyltoluene, a strong inhibition of the catalyst in the presence of water was observed. While ―1 ―1 the productivity P80-90 was still 1.59 gH2gnm min ―1 with no water, it drops to 0.81 gH2gnm ―1 min ―1 with 10 mol% water and down to 0.30 gH2gnm min ―1 with 20 mol% water. A detailed

chromatographic analysis of the composition of the liquid organic phase (see Figure 7) reveals the inhibition of the hydrogenation of H12-DBT to the fully hydrogenated product H18-DBT.



While H12-DBT has been almost completely converted to H18-DBT after 90 min with pure hydrogen, the yield of H18-DBT in the presence of 20 mol% water is only 46 %.

Figure 7: Composition of the reaction mixture over reaction time at a temperature of 240 °C using Pt on alumina with pure hydrogen (left) and in the presence of 20 mol% water based on the amount of H0-DBT (right) – conditions: 30 bar hydrogen pressure, 0.6 mol H0-DBT, 1200 rpm, 0.025 mol% Pt applied as 0.3 mass% Pt on alumina. The gas phase after the experiment with a water content of 20 mol% showed a slight increase in methane (697 ppm vs. 479 ppm) and carbon dioxide (259 ppm vs. 131 ppm) in comparison to the experiment with pure hydrogen under the identical conditions. The reference experiment with pure hydrogen using Pd on alumina at 30 bar hydrogen pressure ―1 and 240 °C was characterized by an initial productivity P20-50 of 2.36 gH2gRu min ―1. The

hydrogenation in the presence of 1.08 g (10 mol% H2O) showed the same hydrogenation activity compared to the hydrogenation of H0-DBT without water. Doubling the amount of water to 20 mol% H2O, however, resulted in a small decrease in activity and a reduction of the initial productivity by 24 %. While with pure hydrogen a DoH of 0.98 could be achieved after 360 min, the addition of water led to a DoH of 0.89 in the same reaction time. With regard to by-product formation in the gas phase, no negative effect of the presence of water was observed. The gas phase after the experiment with a water content of 20 mol% showed a slight increase for methane (550 ppm vs. 469 ppm) and a decrease for carbon dioxide (17 ppm vs. 83 ppm) in comparison to the experiment with pure hydrogen. Catalyst recycling and reuse experiments Recycling and repeated use of the powder catalyst turned out to be difficult. Depressurization of the reactor after the first hydrogenation run, caused foam formation in the reactor and 12 ACS Paragon Plus Environment

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flotation of the catalyst powder. The hydrophilic alumina has been found to agglomerate with condensed water at the top of the reactor where it is unavailable for the subsequent reaction. To avoid this phenomenon, the hydrogenation reactor was equipped with a catalyst basket and three successive hydrogenation runs were carried out using catalyst pellets. These experiments were first carried out without water as a reference. Subseqeuntly, the were repeated with addition of 5.3 g water prior to each hydrogenation run. These recycling runs with dry and wet hydrogen were only carried out with the Ru- and Pt-based catalysts as they showed the most promising results for the powder catalysts. When using the Ru on alumina catalyst in form of catalyst pellets (cylindrical shape with diameter = 3.3 mm and length = 3.4 mm), a somewhat lower reaction rate is to be expected due to higher mass and heat transfer resistances, compared to a catalyst powder with grain size in the 100 µm range. The degree of hydrogenation over time is compared in Figure 8 for three hydrogenation runs with dry (black circles) and wet hydrogen (red boxes).

Figure 8: Hydrogenation of H0-DBT using Ru on alumina (0.05 mol% Ru applied as 0.5 mass% Ru loading, egg-shell) at 30 bar hydrogen pressure and 180 °C; 1.47 mol H0-DBT; black circles: without water; red boxes: addition of 5.3 g water prior each hydrogenation run.

In three subsequent hydrogenation runs with pure hydrogen, a similar reaction rate and an initial ―1 productivity P20-40 of ca. 0.33 gH2gRu min ―1 was achieved. In the experiments with wet

hydrogen, no decrease of the hydrogenation rate was observed. However, comparing the 13 ACS Paragon Plus Environment


reaction rate of the Ru powder catalyst (Figure 5) and the pellet catalyst it is evident that the activity of the pellet catalyst is only 10 % of the powder catalyst due to mass transfer limitation. Over recycling and reuse of the Ru pellet catalyst, the final DoH after 22 hours of reaction decreased from 0.78 to 0.71 over three hydrogenation runs. The results of the experiments using platinum on alumina catalyst pellets (spherical shape with diameter of 3.3 mm) with pure hydrogen (black circles) and wet (red boxes) are shown in Figure 9. As found for the powder catalysts, the pelletized Pt-catalyst was significantly more active ―1 compared to its Ru-counterpart (initial productivity P20-40 = 1.87 gH2gPt―1min ―1 vs. 0.33 gH2gRu

min ―1. For both systems, the pelletized catalyst only reach about 10 % of the activity of the powdered catalyst. However, the relative influence of water on catalyst activity was more pronounced for the Pt-system, with significantly lower hydrogenation rates in the presence of water especially for the final hydrogenation step from H12-DBT to H18-DBT. With respect to catalyst reuse over three consecutive cycles it was found that a relatively large step in activity reduction occurs from run 1 to run 2. This is due to the conditioning of the catalyst under reaction conditions as can be seen from the much smaller reduction in activity from run 2 to run 3. The presence of water seems to accelerate deactivation during recycling.

Figure 9: Hydrogenation of H0-DBT using Pt on alumina (0.022 mol% Pt applied as 0.3 mass% Pt loading, egg-shell) at 30 bar hydrogen pressure and 240 °C; 1.47 mol H0-DBT; black circles: without water; red boxes: addition of 5.3 g water before each hydrogenation run.


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Effect of water on the formation of gaseous by-products The effect of water on the formation of gaseous by-products over 22 hours reaction time is summarized in Figure 10 for the first and second hydrogenation run with the pelletized Pt- and Ru-catalysts, respectively.

Figure 10: Fraction of by-products in the gas phase after the first and second hydrogenation of H0-DBT using Pt on alumina (0.022 mol% Pt applied as 0.3 mass% Pt loading, egg-shell, 240°C) and Ru on alumina (0.05 mol% Ru applied as 0.5 mass% Ru loading, egg-shell, 180°C) at 30 bar hydrogen pressure; 1.47 mol H0-DBT; case a: with pure hydrogen; case b: addition of 5.3 g water before each hydrogenation run. Regardless of the presence of water, the amount of by-products decreased from the first to the second hydrogenation when the Pt on alumina catalyst is used at 240 °C. The gas phase after the first hydrogenation with pure hydrogen contained 1978 ppm methane, 9 ppm ethane, 20 ppm propane and 75 ppm CO2. The by-product formation in the gas phase decreased to 762 ppm methane, 3 ppm ethane, 9 ppm propane and 17 ppm CO2 after the second hydrogenation run. When water was added, the level of by-products remained the same but significantly higher levels of CO2 were produced, while the amount of CH4 decreased. After the second hydrogenation with water, we analyzed 238 ppm methane, 3 ppm ethane, 15 ppm propane and 438 ppm CO2 in the off-gas. No CO could be detected in the gas phase. Either CO formation does not occur under the applied conditions in the system or the formed CO is converted to methane. Some CO may also be bound to active Pt centers of the catalyst. This would explain the somewhat enhanced deactivation in the presence of water. 15 ACS Paragon Plus Environment


In general, the hydrogenation of H0-DBT using the Ru on alumina catalyst pellets at 180 °C showed a higher level of by-products. The analysis of the gas phase after the first hydrogenation with pure hydrogen revealed a contamination with 7510 ppm methane, 212 ppm ethane, 42 ppm propane, 7 ppm butane and 17 ppm CO2. The second hydrogenation showed 69 % more by-products (mainly methane) compared to the first hydrogenation. The concentration of impurities in hydrogen after 22 h reaction time with water was 10651 ppm methane, 289 ppm ethane, 56 ppm propane, 10 ppm butane and 10 ppm CO2. Thus, the addition of water led to an increase in light-alkane formation by 41 %. At the same time, a slight decrease in the amount of CO2 was observed. Similar to the experiments without water, the proportion of by-products in the second hydrogenation run increased. The gas phase after the second hydrogenation run with water contained 16131 ppm methane, 499 ppm ethane, 77 ppm propane, 13 ppm butane and 7 ppm CO2, which corresponds to an increase by 51 %. The increase in by-products in the presence of water using Ru on alumina catalyst pellets is thus significantly lower than when using the ground Ru catalyst pellets in form of a powder.

Conclusion In the present study, the suitability of alumina-supported Pt group metals for H0-DBT hydrogenation was investigated in a temperature range of 150 – 260 °C using dry and wet hydrogen. It was found that Ru and Rh are very well suited for H0-DBT hydrogenation below 200 °C. With these catalysts, higher temperatures do not lead to a rise in activity, but rather to increased by-product formation, especially with ruthenium. In contrast, Pd and Pt on alumina show very good H0-DBT hydrogenation activities with very small by-product formation. The presence of water using powdered catalyst led to a slight decrease in the reaction rate for all catalysts except ruthenium. Furthermore, an increase of lighter alkanes in the gas phase after the reaction could be observed. While the increase in by-products with Rh, Pt and Pd was about 10 – 50 %, the presence of water with the ruthenium catalyst resulted in a seven-fold increase in the alkane content in the gas phase, compared to the experiments with pure hydrogen. Our studies with catalyst pellets confirmed that H0-DBT can be readily and fully hydrogenated even in presence of large excess of water, i.e. using a water-saturated LOHC-phase. The hydrogenation activity of the ruthenium catalyst is hardly affected by water, while a slight decrease in hydrogenation activity was found in presence of water for the applied Pt catalyst pellets. 16 ACS Paragon Plus Environment

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As a closing remark, we want to point out that the amount of water in relation to the catalyst mass applied in our studies is orders of magnitude higher than in any realistic real-life application of the LOHC technology. Therefore, we conclude that wet hydrogen is well suitable for charging the LOHC compound H0-DBT. This allows the direct connection of hydrogen production via electrolysis with subsequent hydrogen storage via LOHC hydrogenation without hydrogen drying. The investment in energy and drying equipment for producing dry hydrogen can thus be avoided if the electrolysis-hydrogenation sequence is applied for the purpose of charging the LOHC compound H0-DBT with renewable energy equivalents. Beyond hydrogen storage, our findings are also interesting in the context of using wet hydrogen from electrolysis for catalytic aromatics hydrogenation reaction in the chemical industry. Such production and use of hydrogen may become relevant in the near future in order to make better use of cheap renewable electricity from renewables in the chemical industry.



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Experimental setup For quantitative comparison, the degree of hydrogenation (DoH) of a LOHC system is defined as the ratio of LOHC-bound hydrogen divided by the maximum hydrogen uptake capacity of the LOHC system under consideration, see Equation (1). 𝑡

DoH (t) =

∫𝑡 nH2,reaction (τ) 𝑑𝜏 0

nH2,max

𝑡

=

(1)

∫𝑡 nH2,reaction (τ) 𝑑𝜏 0

9 ∙ nH0 ― DBT

The productivity is defined as the hydrogen uptake ∆𝑚H2,x ― y in a period of time ∆𝑡x ― y per mass of noble metal of the applied catalyst. Px ― y (t) =

∆𝑚H2,x ― y 𝑚Pt ∙

∆𝑡x ― y =

𝑛i,H2,max ∙ (DoHi,y ― DoHi,x) ∙ MH2 𝑚Pt ∙ (𝑡y ― 𝑡x)

(4)

Initial productivity considered the hydrogenation progress from a DoH of 0.20 to 0.50 only. In this range, the reaction is characterized by a nearly constant hydrogen consumption. Due to the lower reaction rate with catalyst pellets, the initial productivity was determined from a DoH of 0.20 to 0.40 in these experiments. The experiments were performed in a stainless-steel Parr batch autoclave (Type 4566) equipped with a four-blade gas entrainment stirrer (n = 1200 rpm). The reaction temperature was measured with a thermocouple Type J and controlled by a Parr Controller (Type 4875). The reactor was heated with an electric heating jacket. The cooling water (CW) in the cooling coil was provided by a cryostat (Huber Unichiller 600) at a temperature of 16 °C. The pressure in the reactor was monitored by a pressure transmitter (Ashcroft Type G2). The gas supply and discharge was carried out manually via needle valves. For our hydrogenation experiments, the catalyst and the hydrogen-lean H0-DBT were placed in the reactor. The experiments using Ru, Rh and Pd were performed with a molar catalyst to LOHC ratio of 1:2000 (0.05 mol%). For the experiments with Pt catalyst powder, the molar catalyst to LOHC ratio was reduced to 1:4000 (0.025 mol%). An overview of the applied catalysts is given in Table 2. The catalyst pellets (Ru and Pt on alumina) were ground in a planetary mill. All catalyst powders were sieved to a fraction smaller than 125 µm and vacuum dried at 110 °C for 12 hours. We performed experiments with milled catalyst pellets in a 300 mL reactor and with the catalyst pellets in a 600 mL reactor equipped with a catalyst basket.


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Table 2: List of catalyst materials applied in this study. Entry

Catalyst

Supplier

LOT No.

Pt-1

Pt/Al2O3 (0.3 wt%, pellet)

Clariant

C101

Ru-1

Ru/Al2O3 (0.5 wt%, pellet)

Hydrogenious Technologies CN23012

Pd-1

Pd/Al2O3 (5.0 wt%, powder) SigmaAldrich

10221PE

Rh-1

Rh/Al2O3 (5.0 wt%, powder) SigmaAldrich

MKBW4671V, MKBP3639V

After purging the autoclave with low-pressure argon to ensure inert atmosphere, the reactor was heated to reaction temperature (150 – 260 °C). At reaction temperature, the reactor was pressurized with pure hydrogen (grade 5.0 corresponding to a purity of >99.999 %, Linde AG). The absolute reaction pressure was controlled by a pressure regulator and kept constant by feeding hydrogen into the system. The reaction was started by increasing the stirrer speed from 300 to 1200 rpm. Liquid samples were taken during the reaction and the DoH of the organic phase was determined by 1H-NMR spectroscopy according to Do et al.13 using a JNM-ECX 400 NMR-spectrometer (Jeol Ltd.) with 64 scans at room temperature. The hydrogenation run was stopped after 20 h reaction time. After the reactor has cooled down, a gas sample was taken and the hydrogenated product was drained through the sampling tube. For gas phase analysis after 20 hour reaction time, a Thermo Fisher Scientific Trace 1310 gas chromatograph equipped with a ShinCarbon ST 100/120 (2 m length, 1 mm inner diameter) column and a flame ionization detector was applied. In order to study the stability of the catalyst, some experiments were performed with a recycle of the catalyst. In preparation of the following hydrogenation run, fresh H0-DBT was fed to the reactor and the procedure was repeated. In the experiments considering the influence of water on the hydrogenation, 10 to 20 mol% of deionized water (based on the amount of fresh H0-DBT) were added at the beginning of each hydrogenation run. Acknowledgements The authors like to thank Dr. Normen Szesni for fruitful discussions and Clariant Produkte GmbH Deutschland for providing some of the applied catalysts. The authors acknowledge financial support by the German Science Foundation through its Erlangen Excellence Cluster “Engineering of Advanced Materials”. In addition, the authors gratefully acknowledge infrastructural support by the Free State of Bavaria through its Energie Campus Nürnberg.



Supporting Information The ESI shows additional information about: -

the hydrogenation of H0-DBT over Rh on alumina with pure hydrogen (page S2)

-

the hydrogenation of H0-DBT over Pd on alumina with pure hydrogen (page S3)


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Electrolysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Electricity

Wet hydrogen

+ 9 H2 Hydrogenation

6.2 wt% H2 capacity

Pt-, Pd-, Ru-, Rh-catalyst H0-DBT (hydrogen-lean LOHC)

H18-DBT (hydrogen-rich LOHC)

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Charging a Liquid Organic Hydrogen Carrier with wet hydrogen from

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