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A study on hydrogen uptake and release of a eutectic mixture of biphenyl and diphenyl ether
Journal of Energy Chemistry 42 (2020) 11–16
Contents lists available at ScienceDirect
Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Communication
A study on hydrogen uptake and release of a eutectic mixture of biphenyl and diphenyl ether Munjeong Jang a,b,1, Byeong Soo Shin c,1, Young Suk Jo a, Jeong Won Kang c, Sang Kyu Kwak d, Chang Won Yoon a,b,e,∗, Hyangsoo Jeong a,∗∗ a
Center for Hydrogen Fuel Cell Research, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Divison of Energy and Environment Technology, KIST School, Korea University of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea c Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, Republic of Korea d School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, 50, UNIST-gil, Ulsan 44919, Republic of Korea e KHU-KIST Department of Converging Science and Technology, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 7 March 2019 Revised 28 May 2019 Accepted 29 May 2019 Available online 4 June 2019 Keywords: Hydrogen storage Liquid organic hydrogen carrier Diphenyl ether Dehydrogenation Thermodynamics
a b s t r a c t Hydrogen storage in Liquid Organic Hydrogen Carrier (LOHC) systems is appealing for the safe storage and distribution of excess renewable energy via existing gasoline infrastructures to end-users. We present the eutectic mixture of biphenyl and diphenyl ether of its first use as a LOHC material. The material is hydrogenated with 99% selectivity without the cleavage of C–O bond, with commercial heterogeneous catalysts, which is confirmed by nuclear magnetic spectroscopy and gas chromatography-mass spectrometry. Equilibrium concentration, dehydrogenation enthalpy, and thermo-neutral temperature are calculated using a density functional theory. The results indicate that O-atom-containing material exhibits more favorable dehydrogenation thermodynamics than that of the hydrocarbon analogue. The H2 -rich material contains 6.8 wt% of gravimetric hydrogen storage capacity. A preliminary study of catalytic dehydrogenation on a continuous reactor is presented to demonstrate a reversibility of this material. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
As environmental demand to reduce CO2 emissions grows, replacements for fossil fuels have been extensively sought over the years. One of the promising alternatives to fossil fuels is the use of hydrogen as an energy carrier. Hydrogen can be utilized thermally in an internal combustion engine or electrochemically in a fuel cell, leaving water as a pollutant-free byproduct [1]. Hydrogen contains a high gravimetric energy density of 33.3 kWh/kg-H2 [2], but due to its low volumetric density of approximately 3 Wh/LH2 in ambient conditions, it is necessary to develop a safe and efficient hydrogen storage method that enables to store a large quantity of hydrogen in a limited volume [3]. Molecular hydrogen is currently stored in a liquefied form at cryogenic temperature (20 K) or in a compressed form under high pressure (700 bar) [4]. To alleviate intensive energy loss upon cooling and compres-
∗ Corresponding author at: Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. ∗∗ Corresponding author. E-mail addresses: [email protected] (C.W. Yoon), [email protected] (H. Jeong). 1 These authors contributed equally to this work.
sion of hydrogen, hydrogen storage technologies have been diversified into molecular hydrogen physisorption or chemical hydrogen storage. Among these, chemically-bound hydrogen to a liquid carrier has drawn significant attention as a safe and economical form of storage and transport using currently existing fossil fuel infrastructures [5]. The Liquid Organic Hydrogen Carrier (LOHC) is based on a concept of reversible chemical hydrogen storage as a form of liquid organic material. Hydrogen-lean materials, typically aromatic compounds, are hydrogenated to saturated hydrogen-rich materials via catalytic hydrogenation reactions. Pure hydrogen can then be released from hydrogen-rich materials through catalytic dehydrogenation reactions. LOHC materials typically contain high gravimetric (5–8 wt%) and volumetric hydrogen storage capacities (> 60 g-H2 L−1 ) [2,5] and because LOHC materials are stable at ambient conditions, there is no hydrogen loss after a long period of storage. The most studied LOHC systems are based on toluene [6], dibenzyltoluene [3,7,8], and N-ethylcarbazole [9]. We recently reported a eutectic mixture of biphenyl (35 wt%, C12 H10 ) and diphenylmethane (65 wt%, C13 H12 ) (1) as a feasible LOHC material. Although biphenyl contains a high gravimetric
https://doi.org/10.1016/j.jechem.2019.05.024 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
Scheme 1. (Top) A eutectic mixture of biphenyl and diphenylmethane employed in the previous study [10,11]. (Bottom) A eutectic mixture of biphenyl and diphenyl ether employed in this study.
hydrogen content (7.3 wt%), biphenyl itself may not be a strong candidate for hydrogen storage materials due to its high melting point (342 K). Therefore, mixing biphenyl and diphenylmethane to a eutectic composition allows the material to be liquid at ambient temperatures and still provides a high hydrogen storage capacity (6.9 wt%, 60 g-H2 L−1 ). The hydrogenation reaction of 1 to a hydrogen-rich mixture 2 and the dehydrogenation of 2 to 1 were demonstrated successfully with a high conversion over 94% with the aid of proper catalysts [10,11]. DFT calculations indicated that dicyclohexylmethane requires a higher temperature (603 K) than bicyclohexyl (568 K) for complete dehydrogenation [11]. The addition of diphenylmethane to biphenyl provides a clear advantage of 1 to be in a liquid phase but its hydrogen-rich analogue dicyclohexylmethane provides a drawback of 2, operating at a high reaction temperature upon H2 release. Therefore, an alternative component to diphenylmethane, with a lower required temperature for complete dehydrogenation would be preferable. One of the strategies to improve dehydrogenation thermodynamics is to introduce a heteroatom (e.g., N or B) into an aromatic structure. In this regard, heteroatom-containing LOHCs have received significant attention due to their lower reaction enthalpies of dehydrogenation compared to pure hydrocarbon LOHCs [12]. For example, dehydrogenation enthalpies of 9-ethyldodecahydrocarbazole and 1,2-dihydro-1,2-azaborine at 298 K are 53.2 and 35.9 kJ mol−1 H2 respectively, compared to those of methylcyclohexane (68.3 kJ mol−1 H2 ) and perhydrodibenzyltoluene (65.4 kJ mol−1 H2 ) at 298 K [12,13]. The existence of heteroatom (X) leads to weaker X–H bond and adjacent C–H bond strengths of H2 -rich form than C–H bonds of pure hydrocarbon LOHCs [14]. Based on this concept, a variety of N-heterocycles, including N-ethylcarbazole and N-ethylindole were studied as LOHCs despite the expense [15,16]. However, oxygen-containing LO-
HCs received less attention due to their limited aromatic structures and chemically labile nature toward the hydrogenolysis (i.e., C–O bond cleavage) [17] compared to N-heterocycles, despite the apparent thermodynamic benefit of heteroatom effect. Given these previous studies in conjunction with our recent results of biphenyl and diphenylmethane [10], use of an oxygen-containing compound with biphenyl could provide improved dehydrogenation thermodynamics compared to the mixture of biphenyl and diphenylmethane if the hydrogenolysis issue is solved. We report here on different hydrogen storage properties of a novel liquid carrier, the mixture of biphenyl and diphenyl ether (Scheme 1). DFT calculations of dicyclohexyl ether (10) were initially conducted to determine whether 10 can provide a favorable dehydrogenation enthalpy as a promising LOHC candidate. We used the mGGA-TPSS functional [18] and triple numerical plus polarization basis set [19] with Grimme dispersion correction [20] for DFT calculation and dehydrogenation equilibrium concentrations on 453– 623 K range (Fig. 1a). Dicyclohexyl ether (10) indeed provides lower temperature requirements (578 K) for complete dehydrogenation than dicyclohexylmethane (8) (603 K) but not as low as bicyclohexyl (6) (568 K). We then calculated equilibrium concentrations of stepwise dehydrogenated products of 10 on 453–623 K range. A half-dehydrogenated material, cyclohexyl phenyl ether, is an intermediate containing one cyclohexyl ring and one phenyl ring. A crossover concentration of 9 and 10 is ca. 0.2 at 525 K and cyclohexyl phenyl ether concentration is ca. 0.6 (Fig. 1b). We then calculated dehydrogenation enthalpy at 298 K and thermo-neutral temperatures, where Gibbs free energy change of dehydrogenation equals zero (TGD =0 ). The thermodynamic preference for hydrogenation and dehydrogenation at this temperature is equal. Clot et al. examined TGD =0 of various N-substituted heterocycles and showed that substitution of N on a 5-membered
M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
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Fig. 1. (a) Equilibrium concentration calculations between H2 -rich form (6, 8, 10) and H2 -lean form (5, 7, 9) on 453–623 K range and (b) equilibrium concentration calculations between 10, cyclohexyl phenyl ether, and 9 on 453–623 K range.
Table 1. Calculated dehydrogenation enthalpies at 298 K and TGD =0 values. Entry
Reactant
Product
HD (kJ/mol H2 )a
TGD =0 (K)b
1 2 3
Dicyclohexyl ether (10) Cyclohexyl phenyl ether Dicyclohexyl ether (10)
Cyclohexyl phenyl ether Diphenyl ether (9) Diphenyl ether (9)
66.52 71.92 69.22
511.65 541.11 526.63
a b
DFT calculation results using the TPSS/TNP level of theory with dispersion correction. Thermo-neutral temperature, where Gibbs free energy change of dehydrogenation equals zero.
ring tends to be more effective in lowering TGD =0 value than on a 6-membered ring [15]. The values obtained for stepwise dehydrogenation of 10 to cyclohexyl phenyl ether (Entry 1), cyclohexyl phenyl ether to 9 (Entry 2), and 10 to 9 (Entry 3) are shown in Table 1. A TGD =0 for a first cyclohexyl ring dehydrogenation is 511.65 K and 541.11 K for a second cyclohexyl ring dehydrogenation. This result indicates that the thermodynamic barrier of a second ring dehydrogenation is larger than that of the first ring dehydrogenation. Dehydrogenation enthalpy at 298 K and TGD =0 values of stepwise dehydrogenation of 6 and 8 were also calculated and presented in Table S2. It is notable that the TGD =0 (second dehydrogenation) – TGD =0 (first dehydrogenation) value is the largest for 10 (29.46 K) compared to 6 (−25.46 K) and 8 (10.19 K) (Table S2). This could explain a high intermediate equilibrium concentration (ca 0.6) of 10 (vide supra) compared to intermediate equilibrium concentrations of 6 (ca 0.13) and 8 (ca 0.4) as reported previously [11]. Full thermochemical properties (i.e., reaction enthalpy and Gibbs free energy) of stepwise dehydrogenation of H2 -rich form (6, 10) are also presented in Tables S3–S6. The obtained thermodynamic data suggest that a biphenyl and diphenyl ether mixture can also be a plausible LOHC candidate as the case of the biphenyl and diphenylmethane mixture [10]. We calculate the solid-liquid equilibria of the binary mixture of biphenyl and diphenyl ether to estimate a eutectic point. The computational details and solid-liquid equilibria diagram result are available in Supporting Information. The calculated eutectic ratio (mol%) of biphenyl to diphenyl ether was estimated to be 0.28:0.72 at 286.15 K (Fig. S2), which agrees with literature data [21]. Note that the eutectic mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%) is a commercially available material known as Dowtherm A, an efficient heat transfer liquid. Since the relative composition of biphenyl and diphenyl ether at Dowtherm A is close to that of theoretically predicted ratio, we therefore employed Dowtherm A (H2 -lean form, 3) as a LOHC to examine its
hydrogenation reactivity and selectivity against hydrogenolysis using several heterogeneous catalysts. The catalyst screening results for the hydrogenation of 3 are shown in Table 2. Hydrogenation of the mixture was conducted either at 393 K under 50 bar of H2 or 333 K under 20 bar of H2 . Ru catalysts tend to hydrogenate the most rapidly among Ru, Pd, Rh, and Pt catalysts explored although the Ru catalysts form significant amounts of hydrogenolysis products, cyclohexanol and cyclohexyl (Entries 1 and 2). For the Pd catalysts, excellent selectivity toward hydrogenation was observed for both Pd/Al2 O3 (99%) and Pd/C (98%) but the reaction proceeded relatively slowly among the catalysts explored (Entries 3 and 4). The Rh catalysts generally showed fast reaction kinetics and good selectivity; 99% selectivity could be reached under mild hydrogenation conditions (20 bar H2 at 333 K) (Entries 5–7). Fig. 2(a) presents the 1 H NMR spectra of the H2 -rich form 4 after hydrogenation reaction using Pd/Al2 O3 at 393 K under 50 bar H2 and Rh/C at 333 K under 20 bar H2 (Entries 3 and 7) clearly indicates the exclusive formation of 4 with 99% selectivity, which corresponds to a gravimetric hydrogen storage capacity of 6.8 wt%. Additionally, the product distribution analyzed by GC-MS further demonstrated infinitesimal amounts of hydrogenolysis products (Fig. 2b). Pt catalysts showed the slowest reaction kinetics among the catalysts explored and exhibited side reactions (Entries 8 and 9). In all cases, carbon-supported catalysts tended to show higher selectivity than alumina-supported catalysts. Full product distributions were analyzed by GC-MS and are presented in Table S8. Our previous hydrogenation activities of the H2 -lean form 1 showed that the fastest reaction kinetics can be reached with Ru/Al2 O3 and the slowest kinetics are achieved with Pd-based catalysts [10]. Fig. 3 exhibits the rate of hydrogen uptake of 3 (20 g) under 50 bar H2 at 393 K for 180 min with Ru, Pd, Rh, and Pt carbon-supported catalysts (0.2 g, metal 5 wt%). Within 60 min, Ru/C and Rh/C reached the full conversion (ca. 1.5 g of H2 uptake) with the conditions used whereas Pt/C exhibited half
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M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
Table 2. Catalyst screening for the hydrogenation of 3a . Entry
Catalyst
Catalyst/reactant (mol%)b
Temperature (K)
Pressure (bar)
Time (h)
Conversion (%)
Selectivityc (%)
1 2 3 4 5 6 7 8 9
Ru/Al2 O3 Ru/C Pd/Al2 O3 Pd/C Rh/Al2 O3 Rh/C Rh/C Pt/Al2 O3 Pt/C
0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06
393 393 393 393 393 393 333 393 393
50 50 50 50 50 50 20 50 50
0.5 1 189 33 2.6 0.9 18 67.5 88
100 100 100 100 100 100 100 95 56
64 89 99 98 86 94 99 64 86
b c
Reaction conditions: 20 g of reactant (a mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%)) and 0.2 g of catalyst (metal, 5 wt%). catalyst/reactant (mol%) was calculated as metal mol% divided by weighted average of 3. Selectivity was determined by the ratio of desired products over full products via GC-MS.
Fig. 2. (a) 1 H NMR spectra (CDCl3 ) after hydrogenation with 99% selectivity. (b) Concentration profiles of the hydrogenated products via GC-MS.
1.6
Ru/C Pd/C Rh/C Pt/C Ideal uptake
1.4
Hydrogen uptake (g)
a
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
20
40
60
80
100
120
140
160
180
Reaction time (min) Fig. 3. Hydrogenation uptake profiles of the H2 -lean form 3 (20 g) with several catalysts (0.2 g, metal 5 wt%) at 393 K, 50 bar up to 180 min.
M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
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Fig. 4. (a) Flow rate of molecular hydrogen from a continuous dehydrogenation reactor at 573 K and 613 K over time with Pt/C catalyst (0.1 g, 5 wt%) with a feed rate of 0.02 g min−1 , (b) 1 H NMR spectra (CDCl3 ) after dehydrogenation of 4.
conversion and Pd/C showed the slowest reaction kinetics. The product distribution graphs determined by GC-MS and 1 H NMR spectra after the hydrogenation of 3 for 180 min were presented in Figs. S5 and S6. With the H2 -rich form 4 (99%) obtained, we explored a preliminary dehydrogenation reactivity with a Pt/C (Pt 5 wt%) catalyst on a continuous reactor at 573 K (Fig. 4a). Platinum on carbon has previously shown fast kinetics on dehydrogenation reaction of perhydro-dibenzyltoluene [8]. Then 4 was continuously added with a feed rate of 0.02 g min−1 , and hydrogen gas was produced with an average of 10 mL min−1 . When the reactor temperature was increased to 613 K, H2 gas was released, with an average of 12 mL min−1 . The amount of H2 produced was equivalent to 62% conversion at 573 K, and 73% conversion at 613 K, respectively. The 1 H NMR spectra after dehydrogenation reaction of 4 are shown in Fig. 4(b). Full product distributions were analyzed by GC-MS and are presented in Table S9. Although the reaction conversion was 99% with a Pt/C catalyst at 573 K (Table S10), significant amounts of cleavage products along with other impurities were observed. When dehydrogenation reaction of 4 was carried out with a Pd/C (Pd 5 wt%) catalyst at 573 K, 90% selectivity but with 19% conversion was observed (Table S10). Studies on the optimization of dehydrogenation reaction condition and the effect of dehydrogenation reaction temperature of 4 are currently underway. In summary, we examined the potential of a eutectic mixture of biphenyl and diphenyl ether as a new LOHC candidate. Computational studies have indicated that dicyclohexyl ether, a H2 rich form of diphenyl ether, provides a favorable dehydrogenation enthalpy of 69.22 kJ/mol at 298 K over that of diphenylmethane. The commercially-available eutectic mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%), Dowtherm A, was successfully hydrogenated to a H2 -rich mixture of bicyclohexyl and dicyclohexyl ether with Pd/Al2 O3 at 393 K under 50 bar of H2 or Rh/C at 333 K under 20 bar of H2 without scission of C–O bond (99% selectivity). The H2 -rich mixture with a gravimetric hydrogen storage ca-
pacity of 6.8 wt% was dehydrogenated with Pt/C catalyst at 573 K and 613 K under atmospheric pressure, proving reversibility of this material. Further studies are underway for (i) optimization of the dehydrogenation reaction conditions including catalysts screening, and (ii) determination of experimental dehydrogenation reaction enthalpies. Acknowledgments This work was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M1A2A2074688), KISTI-HPC (KSC-2018-CRE-0022) for computational resources, as well as the KIST institutional program funded by the Korea Institute of Science and Technology (2E29610). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.05.024. References [1] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, Phys. Today (2004) 39–45. No. December. [2] D. Teichmann, W. Arlt, P. Wasserscheid, Int. J. Hydrog. Energy 37 (2012) 18118–18132. [3] G. Do, P. Preuster, R. Aslam, A. Bösmann, K. Müller, W. Arlt, P. Wasserscheid, React. Chem. Eng. 1 (2016) 313–320. [4] D. Teichmann, W. Arlt, P. Wasserscheid, R. Freymann, Energy Environ. Sci. 4 (2011) 2767–2773. [5] P. Preuster, C. Papp, P. Wasserscheid, Acc. Chem. Res. 50 (2017) 74–85. [6] F. Alhumaidan, D. Cresswell, A. Garforth, Energy Fuels 25 (2011) 4217–4234. [7] A. Fikrt, R. Brehmer, V.-O. Milella, K. Müller, A. Bösmann, P. Preuster, N. Alt, E. Schlücker, P. Wasserscheid, W. Arlt, Appl. Energy 194 (2017) 1–8. [8] N. Brückner, K. Obesser, A. Bösmann, D. Teichmann, W. Arlt, J. Dungs, P. Wasserscheid, ChemSusChem 7 (2014) 229–235. [9] R.H. Crabtree, ACS Sustain, Chem. Eng. 5 (2017) 4491–4498.
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[10] D.J. Han, Y.S. Jo, B.S. Shin, M. Jang, J.W. Kang, J.H. Han, S.W. Nam, C.W. Yoon, Energy Technol. 7 (2019) 113–121. [11] M. Jang, Y.S. Jo, W.J. Lee, B.S. Shin, H. Sohn, H. Jeong, S.C. Jang, S.K. Kwak, J.W. Kang, C.W. Yoon, ACS Sustain, Chem. Eng. 7 (2019) 1185–1194. [12] K. Müller, J. Völkl, W. Arlt, Energy Technol. 1 (2013) 20–24. [13] K. Müller, K. Stark, V.N. Emelyanenko, M.A. Varfolomeev, D.H. Zaitsau, E. Shoifet, C. Schick, S.P. Verevkin, W. Arlt, Ind. Eng. Chem. Res. 54 (2015) 7967–7976. [14] R.H. Crabtree, Energy Environ. Sci. 1 (2008) 134–138. [15] E. Clot, O. Eisenstein, R.H. Crabtree, Chem. Commun. 22 (2007) 2231–2233.
[16] D. Forberg, T. Schwob, M. Zaheer, M. Friedrich, N. Miyajima, R. Kempe, Nat. Commun. 7 (2016) 1–6. [17] K.M. Eblagon, S.C.E. Tsang, Appl. Catal. B Environ. 163 (2015) 599–610. [18] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, J. Phys. Chem. 98 (1994) 11623–11627. [19] B. Delley, J. Phys. Chem. A 110 (2006) 13632–13639. [20] S. Grimme, J. Comput. Chem. 27 (2006) 1789–1799. [21] D. Cabaleiro, C. Gracia-Fernández, L. Lugo, J. Chem. Thermodyn. 74 (2014) 43–50.
Contents lists available at ScienceDirect
Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Communication
A study on hydrogen uptake and release of a eutectic mixture of biphenyl and diphenyl ether Munjeong Jang a,b,1, Byeong Soo Shin c,1, Young Suk Jo a, Jeong Won Kang c, Sang Kyu Kwak d, Chang Won Yoon a,b,e,∗, Hyangsoo Jeong a,∗∗ a
Center for Hydrogen Fuel Cell Research, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Divison of Energy and Environment Technology, KIST School, Korea University of Science and Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea c Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul 02841, Republic of Korea d School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, 50, UNIST-gil, Ulsan 44919, Republic of Korea e KHU-KIST Department of Converging Science and Technology, Kyung Hee University, 26 Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 7 March 2019 Revised 28 May 2019 Accepted 29 May 2019 Available online 4 June 2019 Keywords: Hydrogen storage Liquid organic hydrogen carrier Diphenyl ether Dehydrogenation Thermodynamics
a b s t r a c t Hydrogen storage in Liquid Organic Hydrogen Carrier (LOHC) systems is appealing for the safe storage and distribution of excess renewable energy via existing gasoline infrastructures to end-users. We present the eutectic mixture of biphenyl and diphenyl ether of its first use as a LOHC material. The material is hydrogenated with 99% selectivity without the cleavage of C–O bond, with commercial heterogeneous catalysts, which is confirmed by nuclear magnetic spectroscopy and gas chromatography-mass spectrometry. Equilibrium concentration, dehydrogenation enthalpy, and thermo-neutral temperature are calculated using a density functional theory. The results indicate that O-atom-containing material exhibits more favorable dehydrogenation thermodynamics than that of the hydrocarbon analogue. The H2 -rich material contains 6.8 wt% of gravimetric hydrogen storage capacity. A preliminary study of catalytic dehydrogenation on a continuous reactor is presented to demonstrate a reversibility of this material. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
As environmental demand to reduce CO2 emissions grows, replacements for fossil fuels have been extensively sought over the years. One of the promising alternatives to fossil fuels is the use of hydrogen as an energy carrier. Hydrogen can be utilized thermally in an internal combustion engine or electrochemically in a fuel cell, leaving water as a pollutant-free byproduct [1]. Hydrogen contains a high gravimetric energy density of 33.3 kWh/kg-H2 [2], but due to its low volumetric density of approximately 3 Wh/LH2 in ambient conditions, it is necessary to develop a safe and efficient hydrogen storage method that enables to store a large quantity of hydrogen in a limited volume [3]. Molecular hydrogen is currently stored in a liquefied form at cryogenic temperature (20 K) or in a compressed form under high pressure (700 bar) [4]. To alleviate intensive energy loss upon cooling and compres-
∗ Corresponding author at: Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea. ∗∗ Corresponding author. E-mail addresses: [email protected] (C.W. Yoon), [email protected] (H. Jeong). 1 These authors contributed equally to this work.
sion of hydrogen, hydrogen storage technologies have been diversified into molecular hydrogen physisorption or chemical hydrogen storage. Among these, chemically-bound hydrogen to a liquid carrier has drawn significant attention as a safe and economical form of storage and transport using currently existing fossil fuel infrastructures [5]. The Liquid Organic Hydrogen Carrier (LOHC) is based on a concept of reversible chemical hydrogen storage as a form of liquid organic material. Hydrogen-lean materials, typically aromatic compounds, are hydrogenated to saturated hydrogen-rich materials via catalytic hydrogenation reactions. Pure hydrogen can then be released from hydrogen-rich materials through catalytic dehydrogenation reactions. LOHC materials typically contain high gravimetric (5–8 wt%) and volumetric hydrogen storage capacities (> 60 g-H2 L−1 ) [2,5] and because LOHC materials are stable at ambient conditions, there is no hydrogen loss after a long period of storage. The most studied LOHC systems are based on toluene [6], dibenzyltoluene [3,7,8], and N-ethylcarbazole [9]. We recently reported a eutectic mixture of biphenyl (35 wt%, C12 H10 ) and diphenylmethane (65 wt%, C13 H12 ) (1) as a feasible LOHC material. Although biphenyl contains a high gravimetric
https://doi.org/10.1016/j.jechem.2019.05.024 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
Scheme 1. (Top) A eutectic mixture of biphenyl and diphenylmethane employed in the previous study [10,11]. (Bottom) A eutectic mixture of biphenyl and diphenyl ether employed in this study.
hydrogen content (7.3 wt%), biphenyl itself may not be a strong candidate for hydrogen storage materials due to its high melting point (342 K). Therefore, mixing biphenyl and diphenylmethane to a eutectic composition allows the material to be liquid at ambient temperatures and still provides a high hydrogen storage capacity (6.9 wt%, 60 g-H2 L−1 ). The hydrogenation reaction of 1 to a hydrogen-rich mixture 2 and the dehydrogenation of 2 to 1 were demonstrated successfully with a high conversion over 94% with the aid of proper catalysts [10,11]. DFT calculations indicated that dicyclohexylmethane requires a higher temperature (603 K) than bicyclohexyl (568 K) for complete dehydrogenation [11]. The addition of diphenylmethane to biphenyl provides a clear advantage of 1 to be in a liquid phase but its hydrogen-rich analogue dicyclohexylmethane provides a drawback of 2, operating at a high reaction temperature upon H2 release. Therefore, an alternative component to diphenylmethane, with a lower required temperature for complete dehydrogenation would be preferable. One of the strategies to improve dehydrogenation thermodynamics is to introduce a heteroatom (e.g., N or B) into an aromatic structure. In this regard, heteroatom-containing LOHCs have received significant attention due to their lower reaction enthalpies of dehydrogenation compared to pure hydrocarbon LOHCs [12]. For example, dehydrogenation enthalpies of 9-ethyldodecahydrocarbazole and 1,2-dihydro-1,2-azaborine at 298 K are 53.2 and 35.9 kJ mol−1 H2 respectively, compared to those of methylcyclohexane (68.3 kJ mol−1 H2 ) and perhydrodibenzyltoluene (65.4 kJ mol−1 H2 ) at 298 K [12,13]. The existence of heteroatom (X) leads to weaker X–H bond and adjacent C–H bond strengths of H2 -rich form than C–H bonds of pure hydrocarbon LOHCs [14]. Based on this concept, a variety of N-heterocycles, including N-ethylcarbazole and N-ethylindole were studied as LOHCs despite the expense [15,16]. However, oxygen-containing LO-
HCs received less attention due to their limited aromatic structures and chemically labile nature toward the hydrogenolysis (i.e., C–O bond cleavage) [17] compared to N-heterocycles, despite the apparent thermodynamic benefit of heteroatom effect. Given these previous studies in conjunction with our recent results of biphenyl and diphenylmethane [10], use of an oxygen-containing compound with biphenyl could provide improved dehydrogenation thermodynamics compared to the mixture of biphenyl and diphenylmethane if the hydrogenolysis issue is solved. We report here on different hydrogen storage properties of a novel liquid carrier, the mixture of biphenyl and diphenyl ether (Scheme 1). DFT calculations of dicyclohexyl ether (10) were initially conducted to determine whether 10 can provide a favorable dehydrogenation enthalpy as a promising LOHC candidate. We used the mGGA-TPSS functional [18] and triple numerical plus polarization basis set [19] with Grimme dispersion correction [20] for DFT calculation and dehydrogenation equilibrium concentrations on 453– 623 K range (Fig. 1a). Dicyclohexyl ether (10) indeed provides lower temperature requirements (578 K) for complete dehydrogenation than dicyclohexylmethane (8) (603 K) but not as low as bicyclohexyl (6) (568 K). We then calculated equilibrium concentrations of stepwise dehydrogenated products of 10 on 453–623 K range. A half-dehydrogenated material, cyclohexyl phenyl ether, is an intermediate containing one cyclohexyl ring and one phenyl ring. A crossover concentration of 9 and 10 is ca. 0.2 at 525 K and cyclohexyl phenyl ether concentration is ca. 0.6 (Fig. 1b). We then calculated dehydrogenation enthalpy at 298 K and thermo-neutral temperatures, where Gibbs free energy change of dehydrogenation equals zero (TGD =0 ). The thermodynamic preference for hydrogenation and dehydrogenation at this temperature is equal. Clot et al. examined TGD =0 of various N-substituted heterocycles and showed that substitution of N on a 5-membered
M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
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Fig. 1. (a) Equilibrium concentration calculations between H2 -rich form (6, 8, 10) and H2 -lean form (5, 7, 9) on 453–623 K range and (b) equilibrium concentration calculations between 10, cyclohexyl phenyl ether, and 9 on 453–623 K range.
Table 1. Calculated dehydrogenation enthalpies at 298 K and TGD =0 values. Entry
Reactant
Product
HD (kJ/mol H2 )a
TGD =0 (K)b
1 2 3
Dicyclohexyl ether (10) Cyclohexyl phenyl ether Dicyclohexyl ether (10)
Cyclohexyl phenyl ether Diphenyl ether (9) Diphenyl ether (9)
66.52 71.92 69.22
511.65 541.11 526.63
a b
DFT calculation results using the TPSS/TNP level of theory with dispersion correction. Thermo-neutral temperature, where Gibbs free energy change of dehydrogenation equals zero.
ring tends to be more effective in lowering TGD =0 value than on a 6-membered ring [15]. The values obtained for stepwise dehydrogenation of 10 to cyclohexyl phenyl ether (Entry 1), cyclohexyl phenyl ether to 9 (Entry 2), and 10 to 9 (Entry 3) are shown in Table 1. A TGD =0 for a first cyclohexyl ring dehydrogenation is 511.65 K and 541.11 K for a second cyclohexyl ring dehydrogenation. This result indicates that the thermodynamic barrier of a second ring dehydrogenation is larger than that of the first ring dehydrogenation. Dehydrogenation enthalpy at 298 K and TGD =0 values of stepwise dehydrogenation of 6 and 8 were also calculated and presented in Table S2. It is notable that the TGD =0 (second dehydrogenation) – TGD =0 (first dehydrogenation) value is the largest for 10 (29.46 K) compared to 6 (−25.46 K) and 8 (10.19 K) (Table S2). This could explain a high intermediate equilibrium concentration (ca 0.6) of 10 (vide supra) compared to intermediate equilibrium concentrations of 6 (ca 0.13) and 8 (ca 0.4) as reported previously [11]. Full thermochemical properties (i.e., reaction enthalpy and Gibbs free energy) of stepwise dehydrogenation of H2 -rich form (6, 10) are also presented in Tables S3–S6. The obtained thermodynamic data suggest that a biphenyl and diphenyl ether mixture can also be a plausible LOHC candidate as the case of the biphenyl and diphenylmethane mixture [10]. We calculate the solid-liquid equilibria of the binary mixture of biphenyl and diphenyl ether to estimate a eutectic point. The computational details and solid-liquid equilibria diagram result are available in Supporting Information. The calculated eutectic ratio (mol%) of biphenyl to diphenyl ether was estimated to be 0.28:0.72 at 286.15 K (Fig. S2), which agrees with literature data [21]. Note that the eutectic mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%) is a commercially available material known as Dowtherm A, an efficient heat transfer liquid. Since the relative composition of biphenyl and diphenyl ether at Dowtherm A is close to that of theoretically predicted ratio, we therefore employed Dowtherm A (H2 -lean form, 3) as a LOHC to examine its
hydrogenation reactivity and selectivity against hydrogenolysis using several heterogeneous catalysts. The catalyst screening results for the hydrogenation of 3 are shown in Table 2. Hydrogenation of the mixture was conducted either at 393 K under 50 bar of H2 or 333 K under 20 bar of H2 . Ru catalysts tend to hydrogenate the most rapidly among Ru, Pd, Rh, and Pt catalysts explored although the Ru catalysts form significant amounts of hydrogenolysis products, cyclohexanol and cyclohexyl (Entries 1 and 2). For the Pd catalysts, excellent selectivity toward hydrogenation was observed for both Pd/Al2 O3 (99%) and Pd/C (98%) but the reaction proceeded relatively slowly among the catalysts explored (Entries 3 and 4). The Rh catalysts generally showed fast reaction kinetics and good selectivity; 99% selectivity could be reached under mild hydrogenation conditions (20 bar H2 at 333 K) (Entries 5–7). Fig. 2(a) presents the 1 H NMR spectra of the H2 -rich form 4 after hydrogenation reaction using Pd/Al2 O3 at 393 K under 50 bar H2 and Rh/C at 333 K under 20 bar H2 (Entries 3 and 7) clearly indicates the exclusive formation of 4 with 99% selectivity, which corresponds to a gravimetric hydrogen storage capacity of 6.8 wt%. Additionally, the product distribution analyzed by GC-MS further demonstrated infinitesimal amounts of hydrogenolysis products (Fig. 2b). Pt catalysts showed the slowest reaction kinetics among the catalysts explored and exhibited side reactions (Entries 8 and 9). In all cases, carbon-supported catalysts tended to show higher selectivity than alumina-supported catalysts. Full product distributions were analyzed by GC-MS and are presented in Table S8. Our previous hydrogenation activities of the H2 -lean form 1 showed that the fastest reaction kinetics can be reached with Ru/Al2 O3 and the slowest kinetics are achieved with Pd-based catalysts [10]. Fig. 3 exhibits the rate of hydrogen uptake of 3 (20 g) under 50 bar H2 at 393 K for 180 min with Ru, Pd, Rh, and Pt carbon-supported catalysts (0.2 g, metal 5 wt%). Within 60 min, Ru/C and Rh/C reached the full conversion (ca. 1.5 g of H2 uptake) with the conditions used whereas Pt/C exhibited half
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Table 2. Catalyst screening for the hydrogenation of 3a . Entry
Catalyst
Catalyst/reactant (mol%)b
Temperature (K)
Pressure (bar)
Time (h)
Conversion (%)
Selectivityc (%)
1 2 3 4 5 6 7 8 9
Ru/Al2 O3 Ru/C Pd/Al2 O3 Pd/C Rh/Al2 O3 Rh/C Rh/C Pt/Al2 O3 Pt/C
0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06
393 393 393 393 393 393 333 393 393
50 50 50 50 50 50 20 50 50
0.5 1 189 33 2.6 0.9 18 67.5 88
100 100 100 100 100 100 100 95 56
64 89 99 98 86 94 99 64 86
b c
Reaction conditions: 20 g of reactant (a mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%)) and 0.2 g of catalyst (metal, 5 wt%). catalyst/reactant (mol%) was calculated as metal mol% divided by weighted average of 3. Selectivity was determined by the ratio of desired products over full products via GC-MS.
Fig. 2. (a) 1 H NMR spectra (CDCl3 ) after hydrogenation with 99% selectivity. (b) Concentration profiles of the hydrogenated products via GC-MS.
1.6
Ru/C Pd/C Rh/C Pt/C Ideal uptake
1.4
Hydrogen uptake (g)
a
1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
20
40
60
80
100
120
140
160
180
Reaction time (min) Fig. 3. Hydrogenation uptake profiles of the H2 -lean form 3 (20 g) with several catalysts (0.2 g, metal 5 wt%) at 393 K, 50 bar up to 180 min.
M. Jang, B.S. Shin and Y.S. Jo et al. / Journal of Energy Chemistry 42 (2020) 11–16
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Fig. 4. (a) Flow rate of molecular hydrogen from a continuous dehydrogenation reactor at 573 K and 613 K over time with Pt/C catalyst (0.1 g, 5 wt%) with a feed rate of 0.02 g min−1 , (b) 1 H NMR spectra (CDCl3 ) after dehydrogenation of 4.
conversion and Pd/C showed the slowest reaction kinetics. The product distribution graphs determined by GC-MS and 1 H NMR spectra after the hydrogenation of 3 for 180 min were presented in Figs. S5 and S6. With the H2 -rich form 4 (99%) obtained, we explored a preliminary dehydrogenation reactivity with a Pt/C (Pt 5 wt%) catalyst on a continuous reactor at 573 K (Fig. 4a). Platinum on carbon has previously shown fast kinetics on dehydrogenation reaction of perhydro-dibenzyltoluene [8]. Then 4 was continuously added with a feed rate of 0.02 g min−1 , and hydrogen gas was produced with an average of 10 mL min−1 . When the reactor temperature was increased to 613 K, H2 gas was released, with an average of 12 mL min−1 . The amount of H2 produced was equivalent to 62% conversion at 573 K, and 73% conversion at 613 K, respectively. The 1 H NMR spectra after dehydrogenation reaction of 4 are shown in Fig. 4(b). Full product distributions were analyzed by GC-MS and are presented in Table S9. Although the reaction conversion was 99% with a Pt/C catalyst at 573 K (Table S10), significant amounts of cleavage products along with other impurities were observed. When dehydrogenation reaction of 4 was carried out with a Pd/C (Pd 5 wt%) catalyst at 573 K, 90% selectivity but with 19% conversion was observed (Table S10). Studies on the optimization of dehydrogenation reaction condition and the effect of dehydrogenation reaction temperature of 4 are currently underway. In summary, we examined the potential of a eutectic mixture of biphenyl and diphenyl ether as a new LOHC candidate. Computational studies have indicated that dicyclohexyl ether, a H2 rich form of diphenyl ether, provides a favorable dehydrogenation enthalpy of 69.22 kJ/mol at 298 K over that of diphenylmethane. The commercially-available eutectic mixture of biphenyl (26.5 wt%) and diphenyl ether (73.5 wt%), Dowtherm A, was successfully hydrogenated to a H2 -rich mixture of bicyclohexyl and dicyclohexyl ether with Pd/Al2 O3 at 393 K under 50 bar of H2 or Rh/C at 333 K under 20 bar of H2 without scission of C–O bond (99% selectivity). The H2 -rich mixture with a gravimetric hydrogen storage ca-
pacity of 6.8 wt% was dehydrogenated with Pt/C catalyst at 573 K and 613 K under atmospheric pressure, proving reversibility of this material. Further studies are underway for (i) optimization of the dehydrogenation reaction conditions including catalysts screening, and (ii) determination of experimental dehydrogenation reaction enthalpies. Acknowledgments This work was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M1A2A2074688), KISTI-HPC (KSC-2018-CRE-0022) for computational resources, as well as the KIST institutional program funded by the Korea Institute of Science and Technology (2E29610). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.05.024. References [1] G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, Phys. Today (2004) 39–45. No. December. [2] D. Teichmann, W. Arlt, P. Wasserscheid, Int. J. Hydrog. Energy 37 (2012) 18118–18132. [3] G. Do, P. Preuster, R. Aslam, A. Bösmann, K. Müller, W. Arlt, P. Wasserscheid, React. Chem. Eng. 1 (2016) 313–320. [4] D. Teichmann, W. Arlt, P. Wasserscheid, R. Freymann, Energy Environ. Sci. 4 (2011) 2767–2773. [5] P. Preuster, C. Papp, P. Wasserscheid, Acc. Chem. Res. 50 (2017) 74–85. [6] F. Alhumaidan, D. Cresswell, A. Garforth, Energy Fuels 25 (2011) 4217–4234. [7] A. Fikrt, R. Brehmer, V.-O. Milella, K. Müller, A. Bösmann, P. Preuster, N. Alt, E. Schlücker, P. Wasserscheid, W. Arlt, Appl. Energy 194 (2017) 1–8. [8] N. Brückner, K. Obesser, A. Bösmann, D. Teichmann, W. Arlt, J. Dungs, P. Wasserscheid, ChemSusChem 7 (2014) 229–235. [9] R.H. Crabtree, ACS Sustain, Chem. Eng. 5 (2017) 4491–4498.
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