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Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation
international journal of hydrogen energy xxx (xxxx) xxx
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Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation €smann b, A. Bulgarin a, H. Jorschick a, P. Preuster a, A. Bo a,b,* P. Wasserscheid a
Forschungszentrum Ju¨lich, “Helmholtz-Institute Erlangen-Nu¨rnberg for Renewable Energies” (IEK 11), Egerlandstr. 3, 91058, Erlangen, Germany b €t Erlangen-Nu¨rnberg, Egerlandstr. 3, DLehrstuhl fu¨r Chemische Reaktionstechnik, Friedrich-Alexander-Universita 91058, Erlangen, Germany
highlights
graphical abstract
Impurities in the technical LOHC account for most impurities in the released H2. Water
in
perhydro
dibenzylto-
luene (H18-DBT) is a source of CO impurities. Oxygenate impurities in H18-DBT are another source of CO. The use of pre-purified H18-DBT leads to higher productivity in H2 release. Purified H18-DBT releases H2 of excellent quality, CO levels are below 0.2 ppmv.
article info
abstract
Article history:
While Liquid Organic Hydrogen Carrier (LOHC) systems offer a very promising way of
Received 6 July 2019
infrastructure-compatible storage and transport of hydrogen, the hydrogen quality
Received in revised form
released from charged LOHC compounds by catalytic dehydrogenation has been a sur-
27 September 2019
prisingly rarely discussed topic to date. This contribution deals, therefore, with a detailed
Accepted 9 October 2019
analysis of the hydrogen purity released from the hydrogen-rich Liquid Organic Hydrogen
Available online xxx
Carrier compound perhydro dibenzyltoluene (H18-DBT). We demonstrate, that high purity hydrogen (>99.999%) with carbon monoxide levels below 0.2 ppmv can be obtained from
Keywords:
the dehydrogenation of H18-DBT if the applied H18-DBT had been carefully pre-dried and
Hydrogen storage
pre-purified prior to the dehydrogenation experiment. Indeed, the largest part of relevant
LOHC systems
impurities to comply with the hydrogen quality standard for fuel cells in road vehicles (ISO
* Corresponding author. Forschungszentrum Ju¨lich, “Helmholtz-Institute Erlangen-Nu¨rnberg for Renewable Energies” (IEK 11), Egerlandstr. 3, 91058, Erlangen, Germany. E-mail address: [email protected] (P. Wasserscheid). https://doi.org/10.1016/j.ijhydene.2019.10.067 0360-3199/© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Water
14687-2) was found to originate from water and oxygenate impurities present in the
Carbon monoxide
applied, technical LOHC qualities.
Oxygenates
© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction Recently, the concept of chemical hydrogen storage in Liquid Organic Hydrogen Carriers (LOHC) has gained increasing importance [1,2]. The utilization of a high-boiling, lowmelting, neither toxic nor flammable LOHC compounds with high availability allows safe hydrogen storage and logistics on an industrial scale. Dibenzyltoluene (H0-DBT), commercially available as an isomeric mixture, e.g. under the tradename Marlotherm SH, and used on industrial scale as heat transfer fluid since the late 19600 ies, provides these features. Thus, the LOHC system dibenzyltoluene/perhydro dibenzyltoluene (H18-DBT) is a highly interesting candidate for future largescale hydrogen storage and transport applications [3]. The research dealing with this particular LOHC system has so far focused on catalyst, reactor and process developments for the hydrogenation of H0-DBT and the dehydrogenation of H18-DBT. Jorschick et al. proposed, for example, a concept for stationary energy storage that utilizes the same reactor for hydrogenation and dehydrogenation. This allows not only to use the same catalyst for charging and discharging of the LOHC system, it also shortens switching times down to seconds, as only the pressure level has to be adjusted to change from the hydrogen storage to the hydrogen release mode of operation [4]. Fikrt and colleagues investigated the dynamics of hydrogen release from H18-DBT [5]. In this work, the authors also provided some details on by-products formed during the dehydrogenation reaction. Depending on the reaction conditions C6 and C7 light boilers as well as methane were reported as by-products. In this paper, we focus on the practically very important aspect of hydrogen purity released from LOHC systems and report a much more detailed, quantitative study on the hydrogen quality obtained from H18-DBT dehydrogenation processes. In detail, we investigate the effect of water and oxygenate impurities present in the LOHC compound H18DBT on the resulting hydrogen quality obtained during the catalytic hydrogen release. Water can be introduced into the LOHC storage system if wet hydrogen (e.g. undried hydrogen from an electrolyser or water-containing, hydrogen-rich gas mixtures) is used for the catalytic LOHC charging [6]. Note that the solubility of water is higher in H0-DBT than in H18-DBT (590 ppmw vs. 60 ppmw at room temperature) [7]. Oxygenates are found in technical H0DBT to an extent of up to 1 wt%. They are presumably formed during contact of hot H0-DBT with oxygen during the production process, e.g. after the H0-DBT distillation process. Such oxygenates are irrelevant for the industrial application of H0-DBT as heat transfer oil. Their relevance for the novel application of H0-DBT as hydrogen-lean compound of a LOHC system is reported here.
Very high hydrogen quality is an important prerequisite for a number of relevant hydrogen applications. For example, specific quality requirements must be met if the released hydrogen is to be used for re-electrification in a PEM fuel cell. The relevant ISO 14687 standard distinguishes between fuel cells in road vehicles (ISO 14687-2) and for stationary applications (ISO 14687-3). Table 1 summarizes the relevant contaminants and their maximum tolerable amount to fulfil the respective specifications [8,9]. It has been described in the literature that hydrogen released from LOHC systems is of high quality, in case of hydrogen release from H18-DBT purities of 99.95%e99.97% have been reported after simple condensation of the carrier from the vapour phase [10]. From Table 1 it is obvious, however, that the tolerable contents of carbon monoxide, carbon dioxide and sulfur compounds are very low for PEM fuel cell applications. While LOHC systems are essentially free of sulfur (note, that the production of H0-DBT typically applies sulfur-free toluene as feedstock), water and oxygenates in the LOHC system may represent relevant precursors for the formation of CO and CO2. Indeed, one of the few literature reports giving more detailed information on the hydrogen quality released from H18-DBT has indicated CO contents between 5 and 15 ppmv in the produced hydrogen [11]. This earlier study provoked our interest in elucidating the source of these relevant amounts of CO in the hydrogen product from catalytic H18-DBT dehydrogenation. In principle, four different sources of oxygen may play a role in the formation of the observed, undesired traces of CO: a) Water in the H18-DBT feed; b) oxygenate impurities in the H18-DBT feed; c) water present in the pores of the catalyst support; d) oxygen from the catalyst support. To the best of our
Table 1 e Hydrogen contaminants regulated in the ISO 14687-2 and 14687-3 standards and their respective thresholds for operation of PEM fuel cells. Component Hydrogen (min.) Nitrogen Hydrocarbons (C1-basis)a Carbon dioxide Carbon monoxide Sulfur (H2Sbasis) a
Mobile Application Stationary (Type I D) Application (Type I E) 99.97 vol.-% 100 ppmv 2 ppmv
50 vol.-% max. 50 vol.-% 10 ppmv
2 ppmv 0.2 ppmv
max. 50 vol.-% 10 ppmv
0.004 ppmv
0.004 ppmv
In case that no higher hydrocarbons other than methane are present in the gas, an increased concentration of methane is tolerated. In this case, the sum of nitrogen and methane may not exceed 100 ppmv.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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knowledge, neither the effect of water nor of oxygenate impurities has been investigated to date in the context of any study dealing with the quality of hydrogen released from LOHC systems. In this study, we employ, therefore, a continuously operated laboratory-scale reactor system for systematic investigations of the impact of water and oxygenate impurities on the dehydrogenation of H18-DBT with regard to CO formation. As model substance for LOHC-derived oxygenates we apply dicyclohexylmethanol (see Fig. 7b), a compound that may form in the oxidation of dicyclohexylmethane. Dicyclohexylmethane is a suitable model compound for H18-DBT as it is chemically very similar but avoids the analytic complexity of a multi-isomeric mixture.
Experimental Experimental setup for the continuous dehydrogenation of H18-DBT The process flow scheme of the applied laboratory-scale reactor system applied for our continuous dehydrogenation experiments is shown in Fig. 1. H18-DBT (Hydrogenious Technologies) is stored in a heatable vessel (B01) that can be evacuated using a rotary vane pump. Vessel B01 also serves for adjusting specific water contents in the H18-DBT feed by using the temperature-dependent solubility of water in H18DBT (see Fig. 3 for details). The piping from B01 to the dehydrogenation reactor (C01) is heated to avoid separation of
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water from the organic feed. The heat for the endothermic H18-DBT dehydrogenation reaction is provided by a high temperature thermostat. Prior to the catalytic experiment, the applied commercial 0.3 wt% Pt/Al2O3 egg-shell dehydrogenation catalyst (EleMaxD 101, Clariant) is dried in the reactor for 2 h at 120 C in a stream of hydrogen (>99.999%, Linde AG) at ambient pressure. This procedure is carried out to remove remaining water from the catalyst support. Subsequently, the hydrogen pressure is increased from atmospheric to 5.5 bar(a) and the temperature is raised at a rate of 5 K min1 to 320 C under continuous flow of hydrogen. At these conditions the catalyst is held for another 6 h in order to completely reduce the metallic function of the catalyst and to convert any chemically reactive oxygen in the catalyst material to water. The long reduction time has been selected to ensure that all water formed during catalyst reduction is effectively removed from the catalyst. The catalyst bed inside the reactor is diluted with silicon carbide spheres in order to improve heat distribution for a uniform temperature in the catalyst bed [12]. The hydrogenlean LOHC and the released hydrogen leave jointly the reactor and are separated in vessel B02 that simultaneously functions as a product tank for the liquid. The hydrogen stream leaves the gas liquid separator at the top. It can pass a gas adsorber device K01 filled with activated carbon or, alternatively, K01 is by-passed. The results in this work have been obtained in experiments where K01 was by-passed to avoid any effect of the activated carbon adsorber on the gas quality. The hydrogen flow passing the valve H06 is measured using a mass flow meter (Bronkhorst, EL-Flow
Fig. 1 e Process flow diagram of the experimental set-up for continuous dehydrogenation of H18-DBT. Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Select, F-111B-2K0-RGD-33-V) and then enters the gas quality analysis. The latter applies a MultiGas™ 2031 FTIR spectrometer (MKS Instruments) equipped with a multi-pass gas cell (effective path length 5.11 m) and a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Spectra are collected at a resolution of 0.5 cm1 and the average of 128 spectra is used for quantitative analysis. In order to maximize sensitivity, the temperature of the gas cell is maintained at 35 C. Background spectra are collected by purging the gas cell with a flow of nitrogen (>99.999%, Linde AG). This technique allows for quantification of infrared active permanent gases (CO, CO2 and CH4) as well as for the detection of low-boiling hydrocarbons with detection limits in the low ppmv to subppmv range.
Preconditioning of the H18-DBT prior to dehydrogenation experiments In order to study impurity effects in the H18-DBT feedstock, it is crucial to purify the applied H18-DBT from any relevant impurities prior to purposely add impurities for the analytical studies. Low boiling hydrocarbons, water and permanent gases are removed from the LOHC in a two-step process. First, the H18-DBT sample is degassed under vacuum overnight, followed by stripping with a stream of nitrogen (5.0, Linde AG) at 60 C for another 72 h. After this procedure, IR active compounds are no longer detected in the stripping gas. Finally, tank B01 is pressurized with 1.2 bar(a) of nitrogen to prevent any impurity from the laboratory atmosphere entering the tank and changing the quality of the so-purified H18-DBT feedstock.
pumps hare hardly available and would be characterized by a significant error in the feeding rates if built for our specific purpose.
Results and discussions Hydrogen quality obtained with purified H18-DBT in the continuous H18-DBT dehydrogenation experiments Dehydrogenation experiments were carried out in a temperature range from 287 to 297 C, at atmospheric pressure and at 2.5 bar (a) hydrogen pressure. The adjusted H18-DBT flow rates corresponded to hydrodynamic residence times between 9 and 35 min. The results for the dehydrogenation experiments with dry H18-DBT are presented in Fig. 2. For a given temperature, the achievable hydrogen yield depends strongly on the applied pressure, especially at longer residence times, where the reduced thermodynamic driving force becomes obvious. It was found, that the dehydrogenation kinetics can be sufficiently well described in the range below 70% YH2 by a simple power law rate expressions assuming first order with respect to H18-DBT concentration for both ambient and elevated pressure. The apparent activation energy in this simplified model was found to be 117 kJ mol1 at ambient pressure and 149 kJ mol1 at 2.5 bar (a) hydrogen pressure. In these experiments we observed traces of carbon monoxide, depending on the LOHC flow rate. The measured amount was between 0.8 and 2.0 ppmv (see data in Fig. 4 for no
Adjustment of defined water content in the H18-DBT feedstock In order to study the effect of water on catalyst performance and by-product formation in the H18-DBT dehydrogenation, continuous dehydrogenation experiments with different H18DBT qualities are performed. For this purpose, dry and degassed H18-DBT is saturated with degassed, demineralised water at different temperatures to adjust water contents between 100 and 200 ppmw according to the temperaturedependent water-solubility of H18-DBT. The pre-conditioning is carried out by adding 500 mL of degassed and demineralised water to 1500 mL of H18-DBT. The resulting biphasic system is intensively stirred for 24 h at the selected temperature. Then, the stirrer is turned off and the mixture is allowed to settle for 24 h for complete phase separation. To double-check the water content, a sample of H18-DBT is taken by opening valve V03 with the help of a slight overpressure in B01. The sample is weighted and analysed without further pre-treatment by means of coulometric Karl Fischer Titration, using an 831 KF Coulometer (Metrohm) and HYDRANAL-Coulomat AG-H reagent (Fluka). This procedure has been found most suitable to establish well-defined, very low water contents in the H18-DBT feed. An alternative co-feeding of water to the dry H18-DBT feed has been tried but was found impractical as it would require a water pump with pumping rates in the ng min1 range. Such
Fig. 2 e Experimental results for the dehydrogenation of dry H18-DBT given in terms of hydrogen yield YH2, a YH2 of 1 corresponds to a full conversion of H18-DBT to H0-DBT. Marks represent measured data; lines are calculated from estimated kinetic parameters (k0,dehyd,1bar(a) ¼ 6.49 £ 105 ¡1 LH18-DBT g¡1 , EA,dehyd,1bar(a) ¼ 117 kJ mol¡1, cat min k0,dehyd,2.5bar(a) ¼ 2.66 £ 108 LH18-DBT gcat min¡1, EA,dehyd,2.5bar(a) ¼ 149 kJ mol¡1). Q (LOHC) ¼ 0.25e1.0 g min¡1, mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 3 e Water content in H18-DBT as a function of saturation temperature e open symbols refer to experimental data previously reported by Aslam et al. [7].
Fig. 4 e Carbon monoxide concentration in the released hydrogen in the dehydrogenation of H18-DBT as a function of the H18-DBT water content and of the H18-DBT flow rate. T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. extra water added). As dissolved atmospheric oxygen can be excluded due to the extended H18-DBT degassing process applied we investigated in a first set of experiments in more detail the influence of water in the H18-DBT on the formation of CO in the product gas.
Effect of water in H18-DBT on CO formation in catalytic dehydrogenation For this purpose, we saturated the pre-purified H18-DBT feedstock with water at different temperatures. The water content of saturated H18-DBT measured by means of coulometric Karl-Fischer titration is shown in Fig. 3 as a function of
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temperature. Our values were found to be in very good agreement with solubility data previously reported by Aslam et al. [7]. For our dehydrogenation experiments we applied H18-DBT containing 120, 140, and 193 ppmw of water, respectively. The water content of the carefully dried H18-DBT reference feed contained less than 2 ppmw of water. The carbon monoxide content of the product gas obtained during the dehydrogenation of these four different H18-DBT qualities is shown in Fig. 4. For a constant flow rate of H18-DBT feed, a linear increase of the CO content in the product gas was found with increasing water content in the feed. This result indicates that water in the feed or in the applied catalyst contributes at least partly to the produced carbon monoxide. Interestingly, the carbon monoxide concentration in the produced hydrogen was also found to increase almost linearly with the flow rate of the pre-purified H18-DBT. The reason for this important observation will be discussed in detail below. Moreover, we observed that the water content of the applied H18-DBT had only a minor influence on the catalyst activity. After switching from wet LOHC containing 193 ppmw of water back to the dry feed with 2 ppmw water, dehydrogenation activity of the catalyst increased only by 1.5e3.5%. The activity loss increases with increasing flow rate of the water saturated H18-DBT. We explain this observation by the fact that less water is fed to the reactor at lower LOHC flow rate. To further elucidate the effect of water on the formation of CO during dehydrogenation of H18-DBT, we subsequently conducted experiments with 18O-labelled water. In case of water being the cause of CO formation, we expected the formation of C18O. This labelled substance should show a shift to lower wave numbers in the infrared spectrum compared to C16O. 18 O) was added to For this experiment, 1 ml of H18 2 O (97% the degassed, dried H18-DBT, resulting in a water content of 47 ppmw. This contaminated H18-DBT was dehydrogenated using the same catalyst as in the above described experiments. A section of the IR spectrum of the produced gas is shown in Fig. 5 and compared to a spectrum collected in absence of H18 2 O. In the reference spectrum a) absorption patterns of C16O and C16O2 are observed at 2150 and 2350 cm1, respectively. In contrast, the addition of H18 2 O leads in spectrum b) to an additional absorption pattern at lower wave numbers that can be attributed to C18O and C18O2 in the gas. This proves that water is indeed able to cause CO and CO2 formation during the dehydrogenation of H18-DBT, probably by reaction with some carbon deposits on the catalyst. Another important result of the experiments with H18 2 O was that not all detected CO and CO2 were 18O-labelled but still significant amounts of C16O and C16O2 were found in the product gas. The amount of C16O and C16O2 was found clearly above the level to be expected from the H16 2 O impurities in the applied H18 2 O. This finding, together with the observed dependency of CO formation on the applied LOHC flow rate, suggested an additional source of oxygen in the system that was related to the applied H18-DBT feedstock. In order to elucidate this aspect, we investigated in the next set of experiments the role of oxygenate impurities in the technical
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 5 e a) Infrared spectrum of the gas phase released from dry H18-DBT; b) after addition of H18 2 O; c) residual spectrum after subtraction of 12C16O absorption pattern from b); T ¼ 287 C, p ¼ 1 bar(a), Q (H18-DBT) ¼ 1.0 g min¡1, mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. H18-DBT feed for the observed formation of CO traces in our catalytic dehydrogenation experiments.
Effect of oxygenates during the dehydrogenation of H18DBT Our analytical efforts to identify oxygenates in the H18-DBT feed centred on oxygen-containing contaminants with low vapour pressure, as these contaminants were obviously not removed in the applied degassing process. Traces of high boiling carbonyl compounds and phenols were found in technical dibenzyltoluene by means of GC-MS analysis (e.g. m/ z 286, corresponding to the formula C21H18O) and UV/Vis after derivatization with 2,4-dinitrophenyl hydrazine (following a method reported by Farhoosh [13]). Oxygenates in the LOHC (e.g. the presence of hydroxy- or carbonyl-functionalized LOHC-derived compounds) may undergo decarbonylation and dehydration at typical dehydrogenation conditions, thereby promoting the formation of carbon monoxide and light boiling side-products. In our experiments, the analytical detection of oxygenates in the applied H18-DBT was only possible after extraction of these polar compounds with MeOH from the LOHC compound. This makes an exact quantitative determination of these traces difficult as the liquid-liquid equilibrium of the extraction falsifies the results. Nevertheless, we took the detection of oxygenated LOHC compounds in the methanol extract as clear indication that our applied technical H18-DBT contained such oxygenates in relevant amounts. Our next set of experiments was inspired by the assumption that oxygenated LOHC-compounds should undergo chemical transformations during the catalytic hydrogenation and dehydrogenation procedures applied for charging and discharging the LOHC system with hydrogen. Indeed, the cleavage of C-O bonds by heterogeneous platinum catalysts at elevated hydrogen partial pressures has been described in the
literature [14] as well as alumina-catalyzed dehydration reactions [15]. Therefore, we hypothesized that the level of oxygenate impurities in the applied H18-DBT should go down if the material has been used in several subsequent hydrogenation/dehydrogenation cycles if special care is taken that the hot LOHC mixture does not come in contact with air or oxygen during the process steps. In order to prove this hypothesis, we performed dehydrogenation experiments with H18-DBT that was freshly obtained by hydrogenation of technical H0-DBT and compared the results with experiments using a H18-DBT material that had been hydrogenated and
Fig. 6 e Carbon monoxide concentrations and hydrogen flow rates obtained during dehydrogenation of once hydrogenated (H18-DBT1) and recycled LOHC material (H18-DBT3, two complete hydrogenation/dehydrogenation cycles prior to the hydrogenation leading to the applied H18-DBT). T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 7 e a) Molar flow rates of carbon monoxide in the dehydrogenation of H18-DBT1 (filled symbols) and H18-DBT3 (open symbols) with varying DCMeOH content. T ¼ 297 C, p ¼ 1 bar(a); mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3; b) Slopes of the regression lines of a) as a function of the purposely added DCMeOH content in the H18-DBT feed.
dehydrogenated several times prior to the dehydrogenation experiment probed for hydrogen purity. The corresponding results are depicted in Fig. 6. As expected, a remarkable improvement in hydrogen product quality was observed when the recycled H18-DBT3 (two complete hydrogenation/dehydrogenation cycles prior to the hydrogenation leading to the applied H18-DBT) was dehydrogenated compared to the once hydrogenated H18DBT1. Note, that also the dehydrogenation activity increased significantly when applying H18-DBT3. We explain this very interesting finding with the lower CO content in the reaction system in case of H18-DBT3 dehydrogenation. CO coordinates strongly to Pt and leads to a reduced availability of active Pt centres for the target H18-DBT dehydrogenation reaction. These results are very encouraging, as the overall concept of hydrogen storage via LOHC systems is based on multiple recycling of the carrier fluid. Note, that the observed differences cannot be explained by different degrees of hydrogenation in the applied LOHC feedstocks as they were identical (95% as confirmed by 1H-NMR spectroscopy). Hydrogen obtained from the dehydrogenation of H18-DBT3 contained 0.22 ppmv CO in the worst case, while concentrations as low as 0.08 ppmv were achieved in the best case. In the next set of experiments, both H18-DBT qualities were purposely contaminated with dicyclohexylmethanol (98 wt% DCMeOH, Sigma Aldrich) to study the effect of defined oxygenate amounts (0.1 and 0.2 wt% DCMeOH) on the obtained hydrogen product quality. The corresponding dehydrogenation results are summarized in Table 2. For the recycled H18-DBT3 LOHC feed, addition of 0.1 wt% dicyclohexylmethanol caused a strong decrease in the observed dehydrogenation activity (corresponding to a drop 1 in Pt-based productivity). from 1.37 to 0.84 gH2 g1 Pt min Further increase of the alcohol contamination to 0.2 wt% had no additional negative effect on the catalyst activity. Interestingly, the catalyst deactivation observed during the
dehydrogenation of DCMeOH-contaminated H18-DBT3 was found to be fully reversible. Switching back to pure H18-DBT3 feed led to the high initial catalyst productivity and hydrogen product gas quality (see numbers in brackets in Table 2). In case of the “once hydrogenated” technical H18-DBT1, DCMeOH addition had no negative effect on the catalyst activity but higher amounts of DCMeOH led to a higher amount of CO in the hydrogen product gas. To further elucidate the interplay of oxygenate impurities and CO found in the hydrogen product gas, Fig. 7a shows the molar flux of CO as a function of LOHC quality, LOHC flow rate and purposely added dicyclohexylmethanol. The molar flux of carbon monoxide is found to increase linearly with increasing LOHC flow rate (Fig. 7a) confirming that in this experiment the main source of oxygen is the applied LOHC itself. The slopes of the regression lines indicate the oxygen content of the applied LOHC material. Obviously, the slope is greatly affected by the amount of purposely added DCMeOH (Fig. 7b). Without addition of DCMeOH, the amount of CO gives an indication of the level of oxygenate impurities in the applied H18-DBT feed. Thus, the oxygen content of the applied H18-DBT1 can be estimated to be 81 ppmw, while the applied H18-DBT3 contained only 14 ppmw of total oxygen.
Table 2 e Effect of dicyclohexylmethanol contamination in H18-DBT1 and H18-DBT3 on catalyst activity and carbon monoxide concentration in the hydrogen product stream. Q (H18-DBT) ¼ 0.75 g min-1, T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. DCMeOH content/wt%
0 0.1 0.2
H18-DBT1
H18-DBT3
Q (H2)/mL min1
xCO/ ppmv
Q (H2)/mL min1
xCO/ ppmv
112 112 112
2.2 5.8 8.3
191 (194) 130 130
0.2 (0.2) 1.9 3.5
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 8 e a) Methane concentration detected in the hydrogen product gas as a function of the measured carbon monoxide content and hydrogen pressure, T ¼ 287e297 C, Q (H18-DBT1) ¼ 0.25e1.0 g min¡1, wH2O ¼ 1.4e193 ppmw; b) Inversely proportional relationship between molar flow rates of CO/CH4 and the hydrodynamic residence time, T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. These findings support our previous conclusion that repetitive hydrogenation and dehydrogenation of the technical H0-DBT/ H18-DBT LOHC system reduces oxygen-containing impurities and thus leads to higher hydrogen product quality during LOHC recycling. Our results indicate that any suitable actions to reduce the level of oxygenate impurities in the H0-DBT starting material (including a production technology that avoids formation of any oxygenate impurities) would result in a hydrogen product quality that meets the very strict CO levels for fuel cell vehicles.
methane is mainly formed through hydrogenolysis of the methyl group of the toluene moiety in the Hx-DBT compounds [5]. We consider our new findings as highly relevant for the practical use of the H0-DBT/H18-DBT LOHC system as they indicate that impurities so far attributed to partial LOHC decomposition do mainly origin from impurities in the technical LOHC system and strongly decrease over repetitive hydrogen storage cycles.
Conclusion Effect of carbon monoxide concentration on methane formation We have demonstrated above that traces of contaminants such as water and oxygenates promote the formation of carbon monoxide during the dehydrogenation of H18-DBT. Carbon monoxide, however, is not necessarily the final product of this reaction. Instead, CO may react in the hydrogen-rich reaction mixture of the dehydrogenation unit further to methane. It is therefore interesting to monitor the formation of methane as a function of feedstock quality and applied reaction conditions (in particular hydrogen pressure). The corresponding results are displayed in Fig. 8. Interestingly, our results clearly suggest a correlation between carbon monoxide and methane concentrations. Their reciprocal dependencies on the hydrodynamic residence time confirms, in addition, that methane is primarily formed in the dehydrogenation system under investigation by methanation of carbon monoxide and possibly also of carbon dioxide. This conclusion could be further confirmed by independent CO methanation experiment under the temperature and pressure conditions of the H18-DBT dehydrogenation experiment (see Supporting Information). The finding that a significant part of the methane in the hydrogen product of H18-DBT dehydrogenation experiments stems from CO hydrogenation is in disagreement with our own earlier assumption claiming that
Our study provides valuable details on the quality of hydrogen released from the industrially promising LOHC compound perhydro dibenzyltoluene (H18-DBT) by catalytic dehydrogenation under technically relevant conditions. As a key result we found that impurities from the technical production process that are present in the applied H0-DBT material account for the largest part of the relevant gaseous byproducts. We could demonstrate, that water dissolved to a very small but measurable extent in the hydrogen-charged H18-DBT is not an inert compound under the conditions of the catalytic LOHC dehydrogenation but acts as source of oxygen for the formation of CO and CO2. This has been unequivocally demonstrated by IR studies with 18O-labelled water. By purposely adding the model compound dicyclohexylmethanol to the H18-DBT feedstock we could, furthermore, disclose the important influence of LOHC oxygenate impurities on the quality of the released hydrogen gas. A strong correlation between the amount of purposely added dicyclohexylmethanol and the CO (and methane) levels found in the product gas was observed. It has also been shown that CO formed during the catalytic dehydrogenation is partly converted in the dehydrogenation reactor to methane. With these new and important insights we could strongly enhance the quality of the released hydrogen up to full
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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compliance with the strict standards for hydrogen fuel cell vehicles by two different measures: a) Applying recycled LOHC material led to a strong increase in the quality of the released hydrogen; water and oxygenate impurities in the LOHC material have been found to decrease during several hydrogenation/dehydrogenation cycles and, thus, the use of recycled LOHC material led to a much higher hydrogen quality with steeply reduced CO contents; b) Careful drying and prepurifying of the perhydro dibenzyltoluene prior to the dehydrogenation experiment (to eliminate water and oxygenates from the applied LOHC material) led equally to a steeply improved hydrogen product quality. Applying one of these two measures, high purity hydrogen (>99,999%) with carbon monoxide levels well below 0.2 ppmv can be obtained from the dehydrogenation of H18-DBT.
Acknowledgements The authors thank Stephan Du¨rr and Franziska Auer for fruitful discussions. Furthermore, infrastructural support by the Energie Campus Nu¨rnberg (funded by the Freestate of Baveria) and by the cluster B1 of the Kopernikus project “Power2X” (funded by the German ministry for education and research, BMBF) is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.067.
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references
[1] Preuster P, Papp C, Wasserscheid P. Acc Chem Res 2017;50(1):74e85. [2] Teichmann D, Arlt W, Wasserscheid P, Freymann R. Energy Environ Sci 2011;4(8):2767e73. € smann A, Teichmann D, Arlt W, [3] Bru¨ckner N, Obesser K, Bo Dungs J, Wasserscheid P. ChemSusChem 2014;7(1):229e35. [4] Jorschick H, Preuster P, Durr S, Seidel A, Muller K, Bosmann A, Wasserscheid P. Energy Environ Sci 2017;10(7):1652e9. € smann A, [5] Fikrt A, Brehmer R, Milella V-O, Mu¨ller K, Bo Preuster P, Alt N, Schlu¨cker E, Wasserscheid P, Arlt W. Appl Energy 2017;194:1e8. € smann A, [6] Du¨rr S, Mu¨ller M, Jorschick H, Helmin M, Bo Palkovits R, Wasserscheid P. ChemSusChem 2017;10(1):42e7. [7] Aslam R, Mu¨ller K, Arlt W. J Chem Eng Data 2015;60(7):1997e2002. [8] ISO 14687-2. Hydrogen fuel - product specification - Part 2: proton exchange membrane (PEM) fuel cell applications for road vehicles. 2012. [9] ISO 14687-3. Hydrogen fuel - product specification - Part 3: proton exchange membrane (PEM) fuel cell applications for stationary appliances. 2014. € smann A, Wasserscheid P. [10] Jorschick H, Vogl M, Preuster P, Bo Int J Hydrogen Energy, in press. https://doi.org/10.1016/j. ijhydene.2019.10.018. [11] Luschtinetz T, Sklarow A, Gulden J. Appl Mech Mater 2016;839:165e9. Milorad P. AIChE J [12] Al-Dahhan Muthanna H, Dudukovic 2004;42(9):2594e606. [13] Farhoosh R, Moosavi SMR. J Food Lipids 2006;13(3):298e305. [14] Watson MJ. Johnson Matthey Technol Rev 2014;58(3):156e61. [15] Lee J, Szanyi J, Kwak JH. Molec Cataly 2017;434:39e48.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation €smann b, A. Bulgarin a, H. Jorschick a, P. Preuster a, A. Bo a,b,* P. Wasserscheid a
Forschungszentrum Ju¨lich, “Helmholtz-Institute Erlangen-Nu¨rnberg for Renewable Energies” (IEK 11), Egerlandstr. 3, 91058, Erlangen, Germany b €t Erlangen-Nu¨rnberg, Egerlandstr. 3, DLehrstuhl fu¨r Chemische Reaktionstechnik, Friedrich-Alexander-Universita 91058, Erlangen, Germany
highlights
graphical abstract
Impurities in the technical LOHC account for most impurities in the released H2. Water
in
perhydro
dibenzylto-
luene (H18-DBT) is a source of CO impurities. Oxygenate impurities in H18-DBT are another source of CO. The use of pre-purified H18-DBT leads to higher productivity in H2 release. Purified H18-DBT releases H2 of excellent quality, CO levels are below 0.2 ppmv.
article info
abstract
Article history:
While Liquid Organic Hydrogen Carrier (LOHC) systems offer a very promising way of
Received 6 July 2019
infrastructure-compatible storage and transport of hydrogen, the hydrogen quality
Received in revised form
released from charged LOHC compounds by catalytic dehydrogenation has been a sur-
27 September 2019
prisingly rarely discussed topic to date. This contribution deals, therefore, with a detailed
Accepted 9 October 2019
analysis of the hydrogen purity released from the hydrogen-rich Liquid Organic Hydrogen
Available online xxx
Carrier compound perhydro dibenzyltoluene (H18-DBT). We demonstrate, that high purity hydrogen (>99.999%) with carbon monoxide levels below 0.2 ppmv can be obtained from
Keywords:
the dehydrogenation of H18-DBT if the applied H18-DBT had been carefully pre-dried and
Hydrogen storage
pre-purified prior to the dehydrogenation experiment. Indeed, the largest part of relevant
LOHC systems
impurities to comply with the hydrogen quality standard for fuel cells in road vehicles (ISO
* Corresponding author. Forschungszentrum Ju¨lich, “Helmholtz-Institute Erlangen-Nu¨rnberg for Renewable Energies” (IEK 11), Egerlandstr. 3, 91058, Erlangen, Germany. E-mail address: [email protected] (P. Wasserscheid). https://doi.org/10.1016/j.ijhydene.2019.10.067 0360-3199/© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Water
14687-2) was found to originate from water and oxygenate impurities present in the
Carbon monoxide
applied, technical LOHC qualities.
Oxygenates
© 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Introduction Recently, the concept of chemical hydrogen storage in Liquid Organic Hydrogen Carriers (LOHC) has gained increasing importance [1,2]. The utilization of a high-boiling, lowmelting, neither toxic nor flammable LOHC compounds with high availability allows safe hydrogen storage and logistics on an industrial scale. Dibenzyltoluene (H0-DBT), commercially available as an isomeric mixture, e.g. under the tradename Marlotherm SH, and used on industrial scale as heat transfer fluid since the late 19600 ies, provides these features. Thus, the LOHC system dibenzyltoluene/perhydro dibenzyltoluene (H18-DBT) is a highly interesting candidate for future largescale hydrogen storage and transport applications [3]. The research dealing with this particular LOHC system has so far focused on catalyst, reactor and process developments for the hydrogenation of H0-DBT and the dehydrogenation of H18-DBT. Jorschick et al. proposed, for example, a concept for stationary energy storage that utilizes the same reactor for hydrogenation and dehydrogenation. This allows not only to use the same catalyst for charging and discharging of the LOHC system, it also shortens switching times down to seconds, as only the pressure level has to be adjusted to change from the hydrogen storage to the hydrogen release mode of operation [4]. Fikrt and colleagues investigated the dynamics of hydrogen release from H18-DBT [5]. In this work, the authors also provided some details on by-products formed during the dehydrogenation reaction. Depending on the reaction conditions C6 and C7 light boilers as well as methane were reported as by-products. In this paper, we focus on the practically very important aspect of hydrogen purity released from LOHC systems and report a much more detailed, quantitative study on the hydrogen quality obtained from H18-DBT dehydrogenation processes. In detail, we investigate the effect of water and oxygenate impurities present in the LOHC compound H18DBT on the resulting hydrogen quality obtained during the catalytic hydrogen release. Water can be introduced into the LOHC storage system if wet hydrogen (e.g. undried hydrogen from an electrolyser or water-containing, hydrogen-rich gas mixtures) is used for the catalytic LOHC charging [6]. Note that the solubility of water is higher in H0-DBT than in H18-DBT (590 ppmw vs. 60 ppmw at room temperature) [7]. Oxygenates are found in technical H0DBT to an extent of up to 1 wt%. They are presumably formed during contact of hot H0-DBT with oxygen during the production process, e.g. after the H0-DBT distillation process. Such oxygenates are irrelevant for the industrial application of H0-DBT as heat transfer oil. Their relevance for the novel application of H0-DBT as hydrogen-lean compound of a LOHC system is reported here.
Very high hydrogen quality is an important prerequisite for a number of relevant hydrogen applications. For example, specific quality requirements must be met if the released hydrogen is to be used for re-electrification in a PEM fuel cell. The relevant ISO 14687 standard distinguishes between fuel cells in road vehicles (ISO 14687-2) and for stationary applications (ISO 14687-3). Table 1 summarizes the relevant contaminants and their maximum tolerable amount to fulfil the respective specifications [8,9]. It has been described in the literature that hydrogen released from LOHC systems is of high quality, in case of hydrogen release from H18-DBT purities of 99.95%e99.97% have been reported after simple condensation of the carrier from the vapour phase [10]. From Table 1 it is obvious, however, that the tolerable contents of carbon monoxide, carbon dioxide and sulfur compounds are very low for PEM fuel cell applications. While LOHC systems are essentially free of sulfur (note, that the production of H0-DBT typically applies sulfur-free toluene as feedstock), water and oxygenates in the LOHC system may represent relevant precursors for the formation of CO and CO2. Indeed, one of the few literature reports giving more detailed information on the hydrogen quality released from H18-DBT has indicated CO contents between 5 and 15 ppmv in the produced hydrogen [11]. This earlier study provoked our interest in elucidating the source of these relevant amounts of CO in the hydrogen product from catalytic H18-DBT dehydrogenation. In principle, four different sources of oxygen may play a role in the formation of the observed, undesired traces of CO: a) Water in the H18-DBT feed; b) oxygenate impurities in the H18-DBT feed; c) water present in the pores of the catalyst support; d) oxygen from the catalyst support. To the best of our
Table 1 e Hydrogen contaminants regulated in the ISO 14687-2 and 14687-3 standards and their respective thresholds for operation of PEM fuel cells. Component Hydrogen (min.) Nitrogen Hydrocarbons (C1-basis)a Carbon dioxide Carbon monoxide Sulfur (H2Sbasis) a
Mobile Application Stationary (Type I D) Application (Type I E) 99.97 vol.-% 100 ppmv 2 ppmv
50 vol.-% max. 50 vol.-% 10 ppmv
2 ppmv 0.2 ppmv
max. 50 vol.-% 10 ppmv
0.004 ppmv
0.004 ppmv
In case that no higher hydrocarbons other than methane are present in the gas, an increased concentration of methane is tolerated. In this case, the sum of nitrogen and methane may not exceed 100 ppmv.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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knowledge, neither the effect of water nor of oxygenate impurities has been investigated to date in the context of any study dealing with the quality of hydrogen released from LOHC systems. In this study, we employ, therefore, a continuously operated laboratory-scale reactor system for systematic investigations of the impact of water and oxygenate impurities on the dehydrogenation of H18-DBT with regard to CO formation. As model substance for LOHC-derived oxygenates we apply dicyclohexylmethanol (see Fig. 7b), a compound that may form in the oxidation of dicyclohexylmethane. Dicyclohexylmethane is a suitable model compound for H18-DBT as it is chemically very similar but avoids the analytic complexity of a multi-isomeric mixture.
Experimental Experimental setup for the continuous dehydrogenation of H18-DBT The process flow scheme of the applied laboratory-scale reactor system applied for our continuous dehydrogenation experiments is shown in Fig. 1. H18-DBT (Hydrogenious Technologies) is stored in a heatable vessel (B01) that can be evacuated using a rotary vane pump. Vessel B01 also serves for adjusting specific water contents in the H18-DBT feed by using the temperature-dependent solubility of water in H18DBT (see Fig. 3 for details). The piping from B01 to the dehydrogenation reactor (C01) is heated to avoid separation of
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water from the organic feed. The heat for the endothermic H18-DBT dehydrogenation reaction is provided by a high temperature thermostat. Prior to the catalytic experiment, the applied commercial 0.3 wt% Pt/Al2O3 egg-shell dehydrogenation catalyst (EleMaxD 101, Clariant) is dried in the reactor for 2 h at 120 C in a stream of hydrogen (>99.999%, Linde AG) at ambient pressure. This procedure is carried out to remove remaining water from the catalyst support. Subsequently, the hydrogen pressure is increased from atmospheric to 5.5 bar(a) and the temperature is raised at a rate of 5 K min1 to 320 C under continuous flow of hydrogen. At these conditions the catalyst is held for another 6 h in order to completely reduce the metallic function of the catalyst and to convert any chemically reactive oxygen in the catalyst material to water. The long reduction time has been selected to ensure that all water formed during catalyst reduction is effectively removed from the catalyst. The catalyst bed inside the reactor is diluted with silicon carbide spheres in order to improve heat distribution for a uniform temperature in the catalyst bed [12]. The hydrogenlean LOHC and the released hydrogen leave jointly the reactor and are separated in vessel B02 that simultaneously functions as a product tank for the liquid. The hydrogen stream leaves the gas liquid separator at the top. It can pass a gas adsorber device K01 filled with activated carbon or, alternatively, K01 is by-passed. The results in this work have been obtained in experiments where K01 was by-passed to avoid any effect of the activated carbon adsorber on the gas quality. The hydrogen flow passing the valve H06 is measured using a mass flow meter (Bronkhorst, EL-Flow
Fig. 1 e Process flow diagram of the experimental set-up for continuous dehydrogenation of H18-DBT. Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Select, F-111B-2K0-RGD-33-V) and then enters the gas quality analysis. The latter applies a MultiGas™ 2031 FTIR spectrometer (MKS Instruments) equipped with a multi-pass gas cell (effective path length 5.11 m) and a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Spectra are collected at a resolution of 0.5 cm1 and the average of 128 spectra is used for quantitative analysis. In order to maximize sensitivity, the temperature of the gas cell is maintained at 35 C. Background spectra are collected by purging the gas cell with a flow of nitrogen (>99.999%, Linde AG). This technique allows for quantification of infrared active permanent gases (CO, CO2 and CH4) as well as for the detection of low-boiling hydrocarbons with detection limits in the low ppmv to subppmv range.
Preconditioning of the H18-DBT prior to dehydrogenation experiments In order to study impurity effects in the H18-DBT feedstock, it is crucial to purify the applied H18-DBT from any relevant impurities prior to purposely add impurities for the analytical studies. Low boiling hydrocarbons, water and permanent gases are removed from the LOHC in a two-step process. First, the H18-DBT sample is degassed under vacuum overnight, followed by stripping with a stream of nitrogen (5.0, Linde AG) at 60 C for another 72 h. After this procedure, IR active compounds are no longer detected in the stripping gas. Finally, tank B01 is pressurized with 1.2 bar(a) of nitrogen to prevent any impurity from the laboratory atmosphere entering the tank and changing the quality of the so-purified H18-DBT feedstock.
pumps hare hardly available and would be characterized by a significant error in the feeding rates if built for our specific purpose.
Results and discussions Hydrogen quality obtained with purified H18-DBT in the continuous H18-DBT dehydrogenation experiments Dehydrogenation experiments were carried out in a temperature range from 287 to 297 C, at atmospheric pressure and at 2.5 bar (a) hydrogen pressure. The adjusted H18-DBT flow rates corresponded to hydrodynamic residence times between 9 and 35 min. The results for the dehydrogenation experiments with dry H18-DBT are presented in Fig. 2. For a given temperature, the achievable hydrogen yield depends strongly on the applied pressure, especially at longer residence times, where the reduced thermodynamic driving force becomes obvious. It was found, that the dehydrogenation kinetics can be sufficiently well described in the range below 70% YH2 by a simple power law rate expressions assuming first order with respect to H18-DBT concentration for both ambient and elevated pressure. The apparent activation energy in this simplified model was found to be 117 kJ mol1 at ambient pressure and 149 kJ mol1 at 2.5 bar (a) hydrogen pressure. In these experiments we observed traces of carbon monoxide, depending on the LOHC flow rate. The measured amount was between 0.8 and 2.0 ppmv (see data in Fig. 4 for no
Adjustment of defined water content in the H18-DBT feedstock In order to study the effect of water on catalyst performance and by-product formation in the H18-DBT dehydrogenation, continuous dehydrogenation experiments with different H18DBT qualities are performed. For this purpose, dry and degassed H18-DBT is saturated with degassed, demineralised water at different temperatures to adjust water contents between 100 and 200 ppmw according to the temperaturedependent water-solubility of H18-DBT. The pre-conditioning is carried out by adding 500 mL of degassed and demineralised water to 1500 mL of H18-DBT. The resulting biphasic system is intensively stirred for 24 h at the selected temperature. Then, the stirrer is turned off and the mixture is allowed to settle for 24 h for complete phase separation. To double-check the water content, a sample of H18-DBT is taken by opening valve V03 with the help of a slight overpressure in B01. The sample is weighted and analysed without further pre-treatment by means of coulometric Karl Fischer Titration, using an 831 KF Coulometer (Metrohm) and HYDRANAL-Coulomat AG-H reagent (Fluka). This procedure has been found most suitable to establish well-defined, very low water contents in the H18-DBT feed. An alternative co-feeding of water to the dry H18-DBT feed has been tried but was found impractical as it would require a water pump with pumping rates in the ng min1 range. Such
Fig. 2 e Experimental results for the dehydrogenation of dry H18-DBT given in terms of hydrogen yield YH2, a YH2 of 1 corresponds to a full conversion of H18-DBT to H0-DBT. Marks represent measured data; lines are calculated from estimated kinetic parameters (k0,dehyd,1bar(a) ¼ 6.49 £ 105 ¡1 LH18-DBT g¡1 , EA,dehyd,1bar(a) ¼ 117 kJ mol¡1, cat min k0,dehyd,2.5bar(a) ¼ 2.66 £ 108 LH18-DBT gcat min¡1, EA,dehyd,2.5bar(a) ¼ 149 kJ mol¡1). Q (LOHC) ¼ 0.25e1.0 g min¡1, mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 3 e Water content in H18-DBT as a function of saturation temperature e open symbols refer to experimental data previously reported by Aslam et al. [7].
Fig. 4 e Carbon monoxide concentration in the released hydrogen in the dehydrogenation of H18-DBT as a function of the H18-DBT water content and of the H18-DBT flow rate. T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. extra water added). As dissolved atmospheric oxygen can be excluded due to the extended H18-DBT degassing process applied we investigated in a first set of experiments in more detail the influence of water in the H18-DBT on the formation of CO in the product gas.
Effect of water in H18-DBT on CO formation in catalytic dehydrogenation For this purpose, we saturated the pre-purified H18-DBT feedstock with water at different temperatures. The water content of saturated H18-DBT measured by means of coulometric Karl-Fischer titration is shown in Fig. 3 as a function of
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temperature. Our values were found to be in very good agreement with solubility data previously reported by Aslam et al. [7]. For our dehydrogenation experiments we applied H18-DBT containing 120, 140, and 193 ppmw of water, respectively. The water content of the carefully dried H18-DBT reference feed contained less than 2 ppmw of water. The carbon monoxide content of the product gas obtained during the dehydrogenation of these four different H18-DBT qualities is shown in Fig. 4. For a constant flow rate of H18-DBT feed, a linear increase of the CO content in the product gas was found with increasing water content in the feed. This result indicates that water in the feed or in the applied catalyst contributes at least partly to the produced carbon monoxide. Interestingly, the carbon monoxide concentration in the produced hydrogen was also found to increase almost linearly with the flow rate of the pre-purified H18-DBT. The reason for this important observation will be discussed in detail below. Moreover, we observed that the water content of the applied H18-DBT had only a minor influence on the catalyst activity. After switching from wet LOHC containing 193 ppmw of water back to the dry feed with 2 ppmw water, dehydrogenation activity of the catalyst increased only by 1.5e3.5%. The activity loss increases with increasing flow rate of the water saturated H18-DBT. We explain this observation by the fact that less water is fed to the reactor at lower LOHC flow rate. To further elucidate the effect of water on the formation of CO during dehydrogenation of H18-DBT, we subsequently conducted experiments with 18O-labelled water. In case of water being the cause of CO formation, we expected the formation of C18O. This labelled substance should show a shift to lower wave numbers in the infrared spectrum compared to C16O. 18 O) was added to For this experiment, 1 ml of H18 2 O (97% the degassed, dried H18-DBT, resulting in a water content of 47 ppmw. This contaminated H18-DBT was dehydrogenated using the same catalyst as in the above described experiments. A section of the IR spectrum of the produced gas is shown in Fig. 5 and compared to a spectrum collected in absence of H18 2 O. In the reference spectrum a) absorption patterns of C16O and C16O2 are observed at 2150 and 2350 cm1, respectively. In contrast, the addition of H18 2 O leads in spectrum b) to an additional absorption pattern at lower wave numbers that can be attributed to C18O and C18O2 in the gas. This proves that water is indeed able to cause CO and CO2 formation during the dehydrogenation of H18-DBT, probably by reaction with some carbon deposits on the catalyst. Another important result of the experiments with H18 2 O was that not all detected CO and CO2 were 18O-labelled but still significant amounts of C16O and C16O2 were found in the product gas. The amount of C16O and C16O2 was found clearly above the level to be expected from the H16 2 O impurities in the applied H18 2 O. This finding, together with the observed dependency of CO formation on the applied LOHC flow rate, suggested an additional source of oxygen in the system that was related to the applied H18-DBT feedstock. In order to elucidate this aspect, we investigated in the next set of experiments the role of oxygenate impurities in the technical
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 5 e a) Infrared spectrum of the gas phase released from dry H18-DBT; b) after addition of H18 2 O; c) residual spectrum after subtraction of 12C16O absorption pattern from b); T ¼ 287 C, p ¼ 1 bar(a), Q (H18-DBT) ¼ 1.0 g min¡1, mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. H18-DBT feed for the observed formation of CO traces in our catalytic dehydrogenation experiments.
Effect of oxygenates during the dehydrogenation of H18DBT Our analytical efforts to identify oxygenates in the H18-DBT feed centred on oxygen-containing contaminants with low vapour pressure, as these contaminants were obviously not removed in the applied degassing process. Traces of high boiling carbonyl compounds and phenols were found in technical dibenzyltoluene by means of GC-MS analysis (e.g. m/ z 286, corresponding to the formula C21H18O) and UV/Vis after derivatization with 2,4-dinitrophenyl hydrazine (following a method reported by Farhoosh [13]). Oxygenates in the LOHC (e.g. the presence of hydroxy- or carbonyl-functionalized LOHC-derived compounds) may undergo decarbonylation and dehydration at typical dehydrogenation conditions, thereby promoting the formation of carbon monoxide and light boiling side-products. In our experiments, the analytical detection of oxygenates in the applied H18-DBT was only possible after extraction of these polar compounds with MeOH from the LOHC compound. This makes an exact quantitative determination of these traces difficult as the liquid-liquid equilibrium of the extraction falsifies the results. Nevertheless, we took the detection of oxygenated LOHC compounds in the methanol extract as clear indication that our applied technical H18-DBT contained such oxygenates in relevant amounts. Our next set of experiments was inspired by the assumption that oxygenated LOHC-compounds should undergo chemical transformations during the catalytic hydrogenation and dehydrogenation procedures applied for charging and discharging the LOHC system with hydrogen. Indeed, the cleavage of C-O bonds by heterogeneous platinum catalysts at elevated hydrogen partial pressures has been described in the
literature [14] as well as alumina-catalyzed dehydration reactions [15]. Therefore, we hypothesized that the level of oxygenate impurities in the applied H18-DBT should go down if the material has been used in several subsequent hydrogenation/dehydrogenation cycles if special care is taken that the hot LOHC mixture does not come in contact with air or oxygen during the process steps. In order to prove this hypothesis, we performed dehydrogenation experiments with H18-DBT that was freshly obtained by hydrogenation of technical H0-DBT and compared the results with experiments using a H18-DBT material that had been hydrogenated and
Fig. 6 e Carbon monoxide concentrations and hydrogen flow rates obtained during dehydrogenation of once hydrogenated (H18-DBT1) and recycled LOHC material (H18-DBT3, two complete hydrogenation/dehydrogenation cycles prior to the hydrogenation leading to the applied H18-DBT). T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3.
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 7 e a) Molar flow rates of carbon monoxide in the dehydrogenation of H18-DBT1 (filled symbols) and H18-DBT3 (open symbols) with varying DCMeOH content. T ¼ 297 C, p ¼ 1 bar(a); mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3; b) Slopes of the regression lines of a) as a function of the purposely added DCMeOH content in the H18-DBT feed.
dehydrogenated several times prior to the dehydrogenation experiment probed for hydrogen purity. The corresponding results are depicted in Fig. 6. As expected, a remarkable improvement in hydrogen product quality was observed when the recycled H18-DBT3 (two complete hydrogenation/dehydrogenation cycles prior to the hydrogenation leading to the applied H18-DBT) was dehydrogenated compared to the once hydrogenated H18DBT1. Note, that also the dehydrogenation activity increased significantly when applying H18-DBT3. We explain this very interesting finding with the lower CO content in the reaction system in case of H18-DBT3 dehydrogenation. CO coordinates strongly to Pt and leads to a reduced availability of active Pt centres for the target H18-DBT dehydrogenation reaction. These results are very encouraging, as the overall concept of hydrogen storage via LOHC systems is based on multiple recycling of the carrier fluid. Note, that the observed differences cannot be explained by different degrees of hydrogenation in the applied LOHC feedstocks as they were identical (95% as confirmed by 1H-NMR spectroscopy). Hydrogen obtained from the dehydrogenation of H18-DBT3 contained 0.22 ppmv CO in the worst case, while concentrations as low as 0.08 ppmv were achieved in the best case. In the next set of experiments, both H18-DBT qualities were purposely contaminated with dicyclohexylmethanol (98 wt% DCMeOH, Sigma Aldrich) to study the effect of defined oxygenate amounts (0.1 and 0.2 wt% DCMeOH) on the obtained hydrogen product quality. The corresponding dehydrogenation results are summarized in Table 2. For the recycled H18-DBT3 LOHC feed, addition of 0.1 wt% dicyclohexylmethanol caused a strong decrease in the observed dehydrogenation activity (corresponding to a drop 1 in Pt-based productivity). from 1.37 to 0.84 gH2 g1 Pt min Further increase of the alcohol contamination to 0.2 wt% had no additional negative effect on the catalyst activity. Interestingly, the catalyst deactivation observed during the
dehydrogenation of DCMeOH-contaminated H18-DBT3 was found to be fully reversible. Switching back to pure H18-DBT3 feed led to the high initial catalyst productivity and hydrogen product gas quality (see numbers in brackets in Table 2). In case of the “once hydrogenated” technical H18-DBT1, DCMeOH addition had no negative effect on the catalyst activity but higher amounts of DCMeOH led to a higher amount of CO in the hydrogen product gas. To further elucidate the interplay of oxygenate impurities and CO found in the hydrogen product gas, Fig. 7a shows the molar flux of CO as a function of LOHC quality, LOHC flow rate and purposely added dicyclohexylmethanol. The molar flux of carbon monoxide is found to increase linearly with increasing LOHC flow rate (Fig. 7a) confirming that in this experiment the main source of oxygen is the applied LOHC itself. The slopes of the regression lines indicate the oxygen content of the applied LOHC material. Obviously, the slope is greatly affected by the amount of purposely added DCMeOH (Fig. 7b). Without addition of DCMeOH, the amount of CO gives an indication of the level of oxygenate impurities in the applied H18-DBT feed. Thus, the oxygen content of the applied H18-DBT1 can be estimated to be 81 ppmw, while the applied H18-DBT3 contained only 14 ppmw of total oxygen.
Table 2 e Effect of dicyclohexylmethanol contamination in H18-DBT1 and H18-DBT3 on catalyst activity and carbon monoxide concentration in the hydrogen product stream. Q (H18-DBT) ¼ 0.75 g min-1, T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. DCMeOH content/wt%
0 0.1 0.2
H18-DBT1
H18-DBT3
Q (H2)/mL min1
xCO/ ppmv
Q (H2)/mL min1
xCO/ ppmv
112 112 112
2.2 5.8 8.3
191 (194) 130 130
0.2 (0.2) 1.9 3.5
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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Fig. 8 e a) Methane concentration detected in the hydrogen product gas as a function of the measured carbon monoxide content and hydrogen pressure, T ¼ 287e297 C, Q (H18-DBT1) ¼ 0.25e1.0 g min¡1, wH2O ¼ 1.4e193 ppmw; b) Inversely proportional relationship between molar flow rates of CO/CH4 and the hydrodynamic residence time, T ¼ 297 C, p ¼ 1 bar(a), mcat ¼ 4.9 g applied as 0.3 wt% Pt/Al2O3. These findings support our previous conclusion that repetitive hydrogenation and dehydrogenation of the technical H0-DBT/ H18-DBT LOHC system reduces oxygen-containing impurities and thus leads to higher hydrogen product quality during LOHC recycling. Our results indicate that any suitable actions to reduce the level of oxygenate impurities in the H0-DBT starting material (including a production technology that avoids formation of any oxygenate impurities) would result in a hydrogen product quality that meets the very strict CO levels for fuel cell vehicles.
methane is mainly formed through hydrogenolysis of the methyl group of the toluene moiety in the Hx-DBT compounds [5]. We consider our new findings as highly relevant for the practical use of the H0-DBT/H18-DBT LOHC system as they indicate that impurities so far attributed to partial LOHC decomposition do mainly origin from impurities in the technical LOHC system and strongly decrease over repetitive hydrogen storage cycles.
Conclusion Effect of carbon monoxide concentration on methane formation We have demonstrated above that traces of contaminants such as water and oxygenates promote the formation of carbon monoxide during the dehydrogenation of H18-DBT. Carbon monoxide, however, is not necessarily the final product of this reaction. Instead, CO may react in the hydrogen-rich reaction mixture of the dehydrogenation unit further to methane. It is therefore interesting to monitor the formation of methane as a function of feedstock quality and applied reaction conditions (in particular hydrogen pressure). The corresponding results are displayed in Fig. 8. Interestingly, our results clearly suggest a correlation between carbon monoxide and methane concentrations. Their reciprocal dependencies on the hydrodynamic residence time confirms, in addition, that methane is primarily formed in the dehydrogenation system under investigation by methanation of carbon monoxide and possibly also of carbon dioxide. This conclusion could be further confirmed by independent CO methanation experiment under the temperature and pressure conditions of the H18-DBT dehydrogenation experiment (see Supporting Information). The finding that a significant part of the methane in the hydrogen product of H18-DBT dehydrogenation experiments stems from CO hydrogenation is in disagreement with our own earlier assumption claiming that
Our study provides valuable details on the quality of hydrogen released from the industrially promising LOHC compound perhydro dibenzyltoluene (H18-DBT) by catalytic dehydrogenation under technically relevant conditions. As a key result we found that impurities from the technical production process that are present in the applied H0-DBT material account for the largest part of the relevant gaseous byproducts. We could demonstrate, that water dissolved to a very small but measurable extent in the hydrogen-charged H18-DBT is not an inert compound under the conditions of the catalytic LOHC dehydrogenation but acts as source of oxygen for the formation of CO and CO2. This has been unequivocally demonstrated by IR studies with 18O-labelled water. By purposely adding the model compound dicyclohexylmethanol to the H18-DBT feedstock we could, furthermore, disclose the important influence of LOHC oxygenate impurities on the quality of the released hydrogen gas. A strong correlation between the amount of purposely added dicyclohexylmethanol and the CO (and methane) levels found in the product gas was observed. It has also been shown that CO formed during the catalytic dehydrogenation is partly converted in the dehydrogenation reactor to methane. With these new and important insights we could strongly enhance the quality of the released hydrogen up to full
Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067
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compliance with the strict standards for hydrogen fuel cell vehicles by two different measures: a) Applying recycled LOHC material led to a strong increase in the quality of the released hydrogen; water and oxygenate impurities in the LOHC material have been found to decrease during several hydrogenation/dehydrogenation cycles and, thus, the use of recycled LOHC material led to a much higher hydrogen quality with steeply reduced CO contents; b) Careful drying and prepurifying of the perhydro dibenzyltoluene prior to the dehydrogenation experiment (to eliminate water and oxygenates from the applied LOHC material) led equally to a steeply improved hydrogen product quality. Applying one of these two measures, high purity hydrogen (>99,999%) with carbon monoxide levels well below 0.2 ppmv can be obtained from the dehydrogenation of H18-DBT.
Acknowledgements The authors thank Stephan Du¨rr and Franziska Auer for fruitful discussions. Furthermore, infrastructural support by the Energie Campus Nu¨rnberg (funded by the Freestate of Baveria) and by the cluster B1 of the Kopernikus project “Power2X” (funded by the German ministry for education and research, BMBF) is gratefully acknowledged.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.10.067.
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Please cite this article as: Bulgarin A et al., Purity of hydrogen released from the Liquid Organic Hydrogen Carrier compound perhydro dibenzyltoluene by catalytic dehydrogenation, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.067