Fast Dehydrogenation Kinetics of Perhydro-N - ACS Publications

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Fast Dehydrogenation Kinetics of Perhydro-Npropylcarbazole over a Supported Pd Catalyst Yuan Dong, Ming Yang, Ting Zhu, Xuedi Chen, Guoe Cheng, Hanzhong Ke, and Hansong Cheng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00914 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Fast Dehydrogenation Kinetics of Perhydro-Npropylcarbazole over a Supported Pd Catalyst Yuan Dong, Ming Yang*, Ting Zhu, Xuedi Chen, Guoe Cheng, Hanzhong Ke, Hansong Cheng* Sustainable Energy Laboratory, Faculty of Materials science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China.

ABSTRACT: We report a kinetic study on dehydrogenation of perhydro-N-propylcarbazole over a supported Pd catalyst in the temperature range of 160 - 200 °C at 101 kPa. The dehydrogenation process was found to undergo three consecutive stages: perhydro-Npropylcarbazole → octahydro-N-propylcarbazole → tetrahydro-N-propylcarbazole → Npropylcarbazole. Detailed kinetics of all three stages was explored and the data were fitted well to a first-order reaction with the estimated apparent activation energies of the elementary steps of 90.0 kJ/mol, 90.6 kJ/mol and 96.4 kJ/mol, respectively. Full dehydrogenation of perhydro-Npropylcarbazole was achieved within 240 min at 200 °C with moderate kinetics under our experimental conditions.

KEYWORDS: Perhydro-N-propylcarbazole; Liquid organic hydrogen carrier; Catalytic dehydrogenation; Kinetics; Hydrogen storage

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1. INTRODUCTION Liquid organic hydrogen carriers (LOHCs) have been recently recognized as efficient and practical media for hydrogen storage and delivery.1-5 Hydrogen molecules are stored and released through catalytic hydrogenation and dehydrogenation without consuming the LOHC materials. Moreover, LOHCs remain in a liquid state at ambient conditions. Therefore, hydrogen storage and transportation can be carried out using existing energy infrastructure such as gasoline fueling station and pipelines, which may substantially minimize the economic impact in the transition from fossil fuel based economy hydrogen based economy.6-8 For automotive applications, catalytic hydrogenation of LOHCs can be conducted off-board in a central station under well-controlled conditions. Subsequently, the hydrogenated LOHCs in liquid form can be delivered to a fueling station and pumped safely into vehicular tanks. To save precious space in the vehicles, the tank is to be divided by a movable separator with one side containing the hydrogenated LOHC and the other for the dehydrogenated carrier. Hydrogen gas is released on demand to power fuel cell devices onboard. Carefully designed LOHC compounds and their hydrogenated counterparts, such as dibenzyl-toluene (DBT)9,

10

and several carbazole

derivatives11, 12, are usually non-toxic with low vapor pressures. Therefore, the storage technique based on LOHCs offers a substantially safer way for hydrogen containment and transport at near ambient conditions13. Conventional LOHCs were primarily based on cycloalkanes.5,

14-16

However, the

dehydrogenation temperature of these compounds is usually well above 300 °C, which may result in side reactions such as cracking or coking. It is also challenging to implement the method in vehicles given the limited space and the stringent requirement for safety.

2, 16-18

In 2004,

Cheng et al. from Air Products and Chemicals discovered that dehydrogenation thermodynamics

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and kinetics can be improved by introducing nitrogen atoms into aromatic compounds.19, 20 Since then, a series of N-hetero aromatic candidates were explored by many research groups.21-23 Nethylcarbazole (NECZ) was studied most intensively as an LOHC with a high volumetric density (55 g/L) and high boiling point (>300 °C).24-27 Full hydrogenation of NECZ was achieved over a commercial Ru/Al2O3 catalyst at 150 °C and 7 MPa in both solution and molten states.18 Dehydrogenation kinetics of perhydro-N-ethylcarbazole (12H-NECZ) was extensively investigated over various noble catalysts.

25, 28-30

It is found that full hydrogen release of 12H-

NECZ can be realized below 200 °C but higher than 178 °C.30 Notably, NECZ remains solid below 70 °C, which makes applications of the LOHC material poorly compatible with the existing infrastructure. Crawford et al. examined the dehydrogenation mechanism of tetrahydrocarbazole to carbazole over palladium via a combination of deuterium exchange experiments and DFT calculations. They reported that the first dehydrogenation step of tetrahydrocarbazole occurs at either position 1 or 4 in the molecule and the C-H bond activation is the rate limiting step.31 For quinolines hydrogenation, Yamaguchi et al. found almost 100% hydrogenation of quinline to form 1,2,3,4-tetrahydroquinoline at 80 °C and 0.3 MPa over a homogeneous iridium catalyst.32 Dehydrogenation of indoline to indole was investigated over a range of heterogeneous catalysts.33 2-methylindole (2-MID)34 and N-ethylindole (NEID)35 were reported as potential hydrogen carriers in our recent publications. The major drawback of 2-MID and NEID is their low boiling points, 228 °C and 253 °C, respectively, which gives rise to high vapor pressures under dehydrogenation temperatures and thus makes clean gas separation highly challenging. This is particularly undesirable for mobile devices where space availability to accommodate equipment units is highly limited. Dibenzyl-toluene and benzyl-toluene were proposed by Arlt and Wasserscheid and their co-workers as attractive alternatives of LOHCs due

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to their low melting points (300 °C).10,

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The catalytic

dehydrogenation of dibenzyl-toluene and benzyl-toluene was reported to occur at temperatures above 250 °C. In addition, the viability of carbazole derivatives as LOHCs was investigated via the measured thermochemical property data.11, 37 In a previous study, N-propylcarbazole (NPCZ) was proposed as an effective LOHC candidate.38 NPCZ and perhydro-NPCZ (12H-NPCZ) were both found stable with no more environment and health hazards than gasoline. In comparison with NECZ, despite the slightly lower hydrogen capacity of 5.43 wt%, NPCZ exhibits a lower melting point of 48 °C, higher boiling point (>300 °C) and more facile kinetics in hydrogenation38. However, detailed dehydrogenation process of 12H-NPCZ has not been studied. In the present work, we report a systematic study on the dehydrogenation kinetics of 12HNPCZ over a 5 wt% Pd/Al2O3 catalyst for the first time. 12H-NPCZ was obtained within 2 hours at 140 °C at 7 MPa with a hydrogen storage capacity of 5.43 wt%. Dehydrogenation of 12HNPCZ was conducted in the temperatures range of 160 °C - 200 °C at 101 kPa over a 5 wt% Pd/Al2O3 catalyst to gain insight into the elementary reactive processes. Full dehydrogenation of 12H-NPCZ can be realized with moderate kinetics under mild conditions, indicating NPCZ may be a new potential candidate for hydrogen storage and delivery. 2. MATERIALS AND METHODS 2.1 Materials Ultra high purity hydrogen (99.999%) and argon (99.999%) were provided by Sichuan Ally High-Tech Company. The commercial catalysts of 5 wt% Ru and 5 wt% Pd were purchased from Shanxi Kaida Chemical Company. 2.2 Methods

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Preparation of perhydro-N-propylcarbazole. N-propylcarbazole (NPCZ) was synthesized via solid-liquid phase alkylation of carbazole following the procedure reported in our work previously.38 Hydrogenation of NPCZ was conducted in a 600 ml Parr autoclave batch reactor (Parr 4568) at 140 °C and 7 MPa. 20g of NPCZ with 2 g of 5 wt% Ru/Al2O3 was added into the reactor, which was sealed immediately. Then the reactor was flushed with hydrogen and heated to 140 °C. During the reaction process, small liquid samples were withdrawn periodically and analyzed with gas chromatography and mass spectrometry (Agilent 7890/5975C GC-MSD). Catalytic dehydrogenation of perhydro-N-propylcarbazole. The final product of NPCZ hydrogenation was used as the reactant in dehydrogenation reactions. 3 g of 12H-NPCZ with 0.6 g of 5 wt% Pd/Al2O3 catalyst was added in a 50 ml three-necked flask connected with an integrated water condenser. The reaction was performed in the temperature range of 160 - 200 °C at 101 kPa with magnetic stirring. A constant flow of Ar gas was used as a carrier to remove hydrogen gas produced in the reaction process. Liquid samples were withdrawn and diluted with hexane for analysis by GC-MSD. The GC was equipped with an FID for quantitative detection and MS was use to identify compouds with an ion source (EI). All samples were diluted with hexane and 1µL of each sample were injected through the autosampler syringe with a split ratio of 1:100. The oven temperature program was as follows: 170 °C for 1 min, subsequently by 5 °C/min to 240 °C. The temperature of ion source was 230 °C. 3. RESULTS AND DISCUSSION The hydrogen release curves with time measured in the temperature range of 160 - 200 °C and 101 kPa are presented in Figure 1. Obviously, as the temperature increases, the dehydrogenation rate rises sharply. Full dehydrogenation of 12H-NPCZ can be achieved within 300 min at 190 °C and 240 min at 200 °C. In contrast, only 90% dehydrogenation takes as long as 360 min at 180

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°C. This suggests that the reaction rate of 12H-NPCZ dehydrogenation was sensitive to the temperatures. Therefore, dehydrogenation process can be controlled in a narrow temperature range.

Figure 1. Hydrogen release of 12H-NPCZ at 160 - 200 °C and 101 kPa over commercial 5wt% Pd/Al2O3. Figure 2 displays the concentration distribution of all intermediates and products during dehydrogenation. Clearly, the dehydrogenation process was promoted significantly by increasing temperatures. Two intermediates, i.e. octahydro-NPCZ (8H-NPCZ) and tetrahydro-NPCZ (4HNPCZ) along with fully dehydrogenated product, NPCZ were identified. Analysis on the concentration curves of all species exhibit 12H-NPCZ can be completely consumed in a relatively short time at the selected temperatures. Then 8H-NPCZ was converted to form 4HNPCZ as soon as produced from 12H-NPCZ. The two processes could quickly reach a dynamic balance with a maximum value of 8H-NPCZ concentration. In particular, 8H-NPCZ reached the maximum concentration within 60 min at 170 °C, 30 min at 180 °C and 20 min at 190 °C. Afterwards, 4H-NPCZ was diminished to form NPCZ. We note that 4H-NPCZ appears to be kinetically stable at all reaction temperatures. At 160 °C, the concentration of 4H-NPCZ increased monotonically throughout the reaction, indicating that the consumption rate of 4H-

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NPCZ was dramatically lower than the production rate of 4H-NPCZ. 70% dehydrogenation can be obtained at 180 °C within 360 min. Further raising the temperature to 190 °C and 200 °C, 100% conversion from 4H-NPCZ to NPCZ can be realized within 300 min and 240 min, respectively. This indicates the third stage is extremely dependent on the dehydrogenation temperatures. From the concentration distribution of the reactive species, the dehydrogenation pathway can be described as three consecutive stages: 12H-NPCZ8H-NPCZ4H-NPCZNPCZ, which is the reverse reaction of NPCZ hydrogenation.38 The three consecutive stages of 12H-NPCZ dehydrogenation are shown in Scheme 1.

Figure 2. Time-dependent product distribution for dehydrogenation of 12H-NPCZ over 5 wt% Pd/Al2O3 at various temperatures. (a) 160 °C; (b) 170 °C; (c) 180 °C; (d) 190 °C; (e) 200 °C.

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Scheme 1. The dehydrogenation route of 12H-NPCZ and intermediates: (a) 12H-NPCZ, (b) 8HNPCZ, (c) 4H-NPCZ, (d) NPCZ.

Hence, the kinetic model of 12H-NPCZ dehydrogenation can be expressed as: k

k1 k2 3 12H − NPCZ  →8H − NPCZ  →4H − NPCZ  →NPCZ

where k1, k2, and k3 represent the reaction rate constants associated with the individual steps. According to previous studies, dehydrogenation of 12H-NECZ was found undergo three stages and each of the stages has been proved to follow the first order kinetics.26, 28 Therefore, we assumed the three dehydrogenation stages of 12H-NPCZ also follow first order kinetics. In the first stage of dehydrogenation, 12H-NPCZ was converted to produce 8H-NPCZ. The kinetic equation can be described in terms of the consumption of 12H-NPCZ as:

dC12 H − NPCZ = − k1 C12 H − NPCZ dt

(1)

ln(C12 H − NPCZ / C0 ) = −k1t

(2)

where C12H-NPCZ represents the concentration of 12H-NPCZ and C0 stands for the initial concentration of 12H-NPCZ. ln(C12H-NPCZ/C0) was plotted versus t at the selected temperature in Figure 3, which presented good liner relationship correlations between the experimental data and the first order reaction kinetics. From the slope, the rate constant of the first stage dehydrogenation can be derived.

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Figure 3. The kinetics of the first stage of 12H-NPCZ dehydrogenation at various temperatures: (■) 160 °C, (●) 170 °C, (▲) 180 °C, (▼)190 °C, (◀)200 °C. In the second stage dehydrogenation, 8H-NPCZ was produced from 12H-NPCZ meanwhile consumed to produce 4H-NPCZ. Therefore, the concentration equation of 8H-NPCZ can be expressed as:

dC8 H − NPCZ = k1C12 H − NPCZ − k2C 8 H − NPCZ dt

C8 H − NPCZ =

k1C0 (e − k1t − e − k2t ) k2 − k1

(3)

(4)

where C8H-NPCZ represents the concentration of 8H-NPCZ. A set of k values can be solved using iteration method with MatLab R2015a based on the reaction model described in Eq. (4) and optimize k values which would result in a minimum mismatch value. There are several initial parameters, including time, concentration of 8H-NPCZ and a set of starting values of k. The calculations would stop if the convergence criteria were satisfied. The calculation details are given in the Supporting Information.

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The third stage of 12H-NPCZ dehydrogenation was the consumption of 4H-NPCZ to form NPCZ. Hence, the reaction rate is equal to the production rate of NPCZ. Consequently, the rate constant of the third stage was estimated as Eq. (5). The plot of lnCNECZ as a function of time was displayed in Figure 4.

dC NPCZ = k3C NPCZ dt

(5)

Figure 4. The kinetics of the third stage of 12H-NPCZ dehydrogenation at various temperatures: (▼)160 °C, (▲) 170 °C, ( ◀)180 °C, (●) 190 °C, (■) 200 °C. Based on the law of conservation of mass, the concentration equation of 4H-NPCZ can be described as:

C4 H − NPCZ = C0 −

C0 (k 2e − k1t − k1e − k2t ) − C ' e − k3t (t > 0) (6) k2 − k1

where C4H-NPCZ represents the concentration of 4H-NPCZ and C’ is a constant. Table 1 lists the derived rate constants and activation energies of three dehydrogenation stages at 160 °C - 200 °C. The corresponding values of k1 (marked as k1c) coupled with k2 obtained by iteration method were presented as well. It is can be seen that the values of k1c were basically

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consistent with k1 derived from the first dehydrogenation stage. As the temperature increases from 160 °C to 200 °C, the rate constants increase significantly. According to Arrhenius equation, the apparent activation energy of each stage of 12H-NPCZ dehydrogenation were readily derived to be 90.0 kJ/mol, 90.6 kJ/mol and 96.4 kJ/mol, respectively, as shown in Figure 5. Indeed, the activation energies of each stage derived from our experiment were really close. However, the apparent activation energies can be influenced by many factors, such as the properties of catalysts, use of solvent and the design of reaction conditions, etc. Table 1. The reaction rate constants and reaction activation energy of each dehydrogenation stages over 5 wt% Pd/Al2O3 at 160 - 200 °C.

a

Temperature

k1

k2 (k1a)

k3

(°C)

min-1

min-1

mol·L-1·min-1

160

0.01915

0.00958 (0.01964)

0.00881

170

0.02596

0.01074 (0.02599)

0.00938

180

0.06004

0.02207 (0.05042)

0.01815

190

0.10016

0.04162 (0.10858)

0.02544

200

0.13687

0.06999 (0.13874)

0.05366

Ea(kJ/mol)

90.0±8.4

90.6 ±10.6 (90.9±9.1)

96.4±9.7

R2

0.98

0.95 (0.97)

0.98

the corresponding values of k1 to the values of k2 obtained by iteration method.

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Figure 5. Arrhenius plot using experimental rate constants from each stage of 12H-NPCZ dehydrogenation at 160 - 200 °C. (a) 12H-NPCZ →8H-NPCZ; (b) 8H-NPCZ →4H-NPCZ; (c) 4H-NPCZ →NPCZ.

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Yang et al.25 reported a study on dehydrogenation of perhydro-N-ethylcarbazole over alumina supported Rh, Ru, Pt, and Pd catalysts. It is found that palladium is the most active catalyst among the metals studied. Sotoodeh et al.39-41 prepared 0.5-10 wt% Pd/SiO2 catalysts by wet impregnation and calcination in He and air. They found Pd particle size and Pd loading both have significant influence on perhydro-N-ethylcarbazole dehydrogenation. Complete H2 release can be obtained over the 4 wt% Pd/SiO2 catalyst prepared by calcination in He with a Pd particle size of 9 nm within 1.6 h at 170 °C. Increasing Pd loading to 5wt%, the time for achieving final H2 recovery was increased to 3 hours with a k1 of 0.1139 min-1. On the other hand, the Pd/SiO2 catalysts calcined in air show inferior activity than that calcined in He due to the decrease in the metal dispersion. For example, it took longer than 17h to realize full dehydrogenation of perhydro-N-ethylcarbazole over 5wt% Pd/SiO2 catalyst calcined in air with a k1 of 0.0075 min-1. In our work, the commercial 5wt% Pd/Al2O3 catalyst was used and the reaction rate of the first stage was derived to be 0.02596 min-1. Clearly, the dehydrogenation rate could be further improved by reducing the Pd loading and optimizing the preparation methods of catalysts. As an ideal LOHC, the parameter of stability during dehydrogenation is important. In fact, the dealkylation of perhydro-N-alkylcarbazole in dehydrogenation under ultra-high vacuum conditions has been reported by investigating the dehydrogenation mechanism on Pt(111)42、 Pd(111)43 and Pd/Al2O3 model catalysts44. It is found the metal type and the morphology of metal deposit both strongly effect perhydro-N-alkylcarbazole degradation via C-N bond breakage.45 NPCZ exhibited a reletively slower conversion to carbazole compared to NECZ in dehydrogenation on Pt model catalysts reported by Gleichweit et al..46 In our experiments, almost no side products were detected during dehydrogenation based on the method described in Methods, which may be attributed to the extremely low concentration of the by-product. Hence,

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the splitting mode of detection method was set as splitless for identication of the side products. Consequently, traces of carbazole was detected during 12H-NPCZ dehydrogenation at 170 - 200 °C. Figure 6 displays the concentration distribution of dealkylated products - carbazole. At 160 °C, no carbazole was detected, probably because the amount of carbazole is below the detection limit of the experiment. As the temperature increases from 170 °C to 200 °C, the dealkylation rate increases significantly. Nevertheless, only minor amounts of dealkylated carbazole were produced even at 200 °C after heating 12H-NPCZ for 6h (300 °C) and a low dehydrogenation temperature, NPCZ was found to be a highly promising LOHC with fast dehydrogenation kinetics. Dehydrogenation of 12H-NPCZ was carried out over 5 wt% Pd/Al2O3 catalyst at 160 - 200 °C. Full dehydrogenation of 12HNPCZ can be achieved within 300 min at 190 °C and 240 min at 200 °C without forming any by-

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products. Dehydrogenation pathway can be divided into three stages: 12H-NPCZ→8H-NPCZ→ 4H-NPCZ→NPCZ. The apparent activation energies of 12H-NPCZ→8H-NPCZ and 8H-NPCZ →4H-NPCZ were calculated to be 90.0 kJ/mol and 90.6 kJ/mol. The conversion of 4H-NPCZ→ NPCZ was found most likely to be rate limiting step with an apparent activation energy of 96.4 kJ/mol. The reversible catalytic hydrogenation and dehydrogenation of NPCZ can be continuously repeatedly five times with hydrogen storage capacity loss less than 1%. This study demonstrates that N-propylcarbazole is a promising liquid organic hydrogen carrier (LOHC) candidate. ASSOCIATED CONTENT Supporting Information Details on the calculation results by MatLab R2015a. AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected]. (Ming Yang ) * E-mail address: [email protected]. (Hansong Cheng) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21233006, No. 21473164 and No. 21603195), the Fundamental Research Funds for Central Universities (CUGL170405 and CUG180604), Natural Science

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Foundation of Hubei Province of China (No. 2015CFA129), the Open Foundation of Teaching Laboratory of China University of Geosciences (SKJ2016042, SKJ2016034) and China University of Geosciences (Wuhan) for the program of Center for Advanced Energy Research and Technologies. REFERENCES (1) Aslam, R., Müller, K., Müller, M., Koch, M., Wasserscheid, P., Arlt, W., Measurement of Hydrogen Solubility in Potential Liquid Organic Hydrogen Carriers, Journal of Chemical & Engineering Data, 2016, 61, 643-649. (2) Crabtree, R. H., Hydrogen Storage in Liquid Organic Heterocycles, Energ. Environ. Sci., 2008, 1, 134-138. (3) Teichmann, D., Stark, K., Muller, K., Zottl, G., Wasserscheid, P., Arlt, W., Energy Storage in Residential and Commercial Buildings via Liquid Organic Hydrogen Carriers (LOHC), Energ. Environ. Sci., 2012, 5, 9044-9054. (4) Teichmann, D., Arlt, W., Wasserscheid, P., Liquid Organic Hydrogen Carriers as an Efficient Vector for the Transport and Storage of Renewable Energy, Int. J. Hydrogen Energy, 2012, 37, 18118-18132. (5) Jeong, B. H., Sotowa, K. I., Kusakabe, K., Catalytic Dehydrogenation of Cyclohexane in an FAU-type Zeolite Membrane Reactor, J Membrane Sci, 2003, 224, 151-158. (6) Wang, H., Zhou, X., Ouyang, M., Efficiency Analysis of Novel Liquid Organic Hydrogen Carrier Technology and Comparison with High Pressure Storage Pathway, Int J Hydrogen Energ, 2016, 41, 18062-18071. (7) Bourane, A., Elanany, M., Pham, T. V., Katikaneni, S. P., An Overview of Organic Liquid Phase Hydrogen Carriers, Int J Hydrogen Energ, 2016, 41, 23075-23091. (8) Yanghuan Zhang, Z. J., Zeming Yuan, Tai Yang, Yan Qi, Dongliang Zhao, Development and Application of Hydrogen Storage, JOURNALOF IRON AND STEEL RESEARCH, INTERNATIONAL, 2015, 22, 757-770. (9) Müller, K., Aslam, R., Fischer, A., Stark, K., Wasserscheid, P., Arlt, W., Experimental assessment of the degree of hydrogen loading for the dibenzyl toluene based LOHC system, Int J Hydrogen Energ, 2016, 41, 22097-22103. (10) Heller, A., Rausch, M. H., Schulz, P. S., Wasserscheid, P., Fröba, A. P., Binary Diffusion Coefficients of the Liquid Organic Hydrogen Carrier System Dibenzyltoluene/Perhydrodibenzyltoluene, Journal of Chemical & Engineering Data, 2016, 61, 504-511. (11) Stark, K., Keil, P., Schug, S., Müller, K., Wasserscheid, P., Arlt, W., Melting Points of Potential Liquid Organic Hydrogen Carrier Systems Consisting of N-Alkylcarbazoles, Journal of Chemical & Engineering Data, 2016, 61, 1441-1448. (12) Choi, I. Y., Shin, B. S., Kwak, S. K., Kang, K. S., Yoon, C. W., Kang, J. W., Thermodynamic Efficiencies of Hydrogen Storage Processes Using Carbazole-Based Compounds, Int J Hydrogen Energ, 2016, 41, 9367-9373. (13) Preuster, P., Papp, C., Wasserscheid, P., Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy, Acc Chem Res, 2017, 50, 74-85.

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(14) Pande, J. V., Shukla, A., Biniwale, R. B., Catalytic Dehydrogenation of Cyclohexane over Ag-M/ACC Catalysts for Hydrogen Supply, Int J Hydrogen Energ, 2012, 37, 6756-6763. (15) Desai, P. H., Richardson, J. T., Crystallite Size Effects in Nickel Catalysts: Cyclohexane Dehydrogenation and Hydrogenolysis, J Catal, 1986, 98, 392-400. (16) He, T., Pei, Q., Chen, P., Liquid Organic Hydrogen Carriers, Journal of Energy Chemistry, 2015, 24, 587-594. (17) Moores, A., Poyatos, M., Luo, Y., Crabtree, R. H., Catalysed Low Temperature H2 Release from Nitrogen Heterocycles, New Journal of Chemistry, 2006, 30, 1675-1678. (18) Eblagon, K. M., Rentsch, D., Friedrichs, O., Remhof, A., Zuettel, A., Ramirez-Cuesta, A. J., Tsang, S. C., Hydrogenation of 9-ethylcarbazole as a Prototype of a Liquid Hydrogen Carrier, Int. J. Hydrogen Energy, 2010, 35, 11609-11621. (19) Pez, G. P., Scott, Aaron R., Cooper, Alan C. , Cheng, Hansong Hydrogen Storage Reversible Hydrogenated of pi-conjugated Substrates, in: Air Products and Chemicals, Inc., U.S. , 2004. (20) Crabtree, R. H., Nitrogen-Containing Liquid Organic Hydrogen Carriers: Progress and Prospects, ACS Sustainable Chemistry & Engineering, 2017, 5, 4491-4498. (21) Eblagon, K. M., Tsang, S. C. E., Structure-Reactivity Relationship in Catalytic Hydrogenation of Heterocyclic Compounds over Ruthenium Black; Part B: Effect of Carbon Substitution by Heteroatom, Applied Catalysis B: Environmental, 2015, 163, 599-610. (22) Bernas, H., Kumar, N., Aho, A., Leino, R., Murzin, D. Y., Hydrogenation of 2-methylindole Using Supported Metal Catalysts, Catalysis Communications, 2014, 56, 41-44. (23) Wang, B., Yan, T., Chang, T., Wei, J., Zhou, Q., Yang, S., Fang, T., Palladium Supported on Reduced Graphene Oxide as a High-Performance Catalyst for the Dehydrogenation of Dodecahydro-N-ethylcarbazole, Carbon, 2017, 122, 9-18. (24) Wan, C., An, Y., Xu, G. H., Kong, W. J., Study of Catalytic Hydrogenation of Nethylcarbazole over Ruthenium Catalyst, Int J Hydrogen Energ, 2012, 37, 13092-13096. (25) Yang, M., Dong, Y., Fei, S., Ke, H., Cheng, H., A Comparative Study of Catalytic Dehydrogenation of Perhydro-N-ethylcarbazole over Noble Metal Catalysts, Int J Hydrogen Energ, 2014, 39, 18976-18983. (26) Dong, Y., Yang, M., Mei, P., Li, C., Li, L., Dehydrogenation Kinetics Study of Perhydro-Nethylcarbazole over a Supported Pd Catalyst for Hydrogen Storage Application, Int J Hydrogen Energ, 2016, 41, 8498-8505. (27) Stark, K., Emel’yanenko, V. N., Zhabina, A. A., Varfolomeev, M. A., Verevkin, S. P., Müller, K., Arlt, W., Liquid Organic Hydrogen Carriers: Thermophysical and Thermochemical Studies of Carbazole Partly and Fully Hydrogenated Derivatives, Industrial & Engineering Chemistry Research, 2015, 54, 7953-7966. (28) Sotoodeh, F., Smith, K. J., Kinetics of Hydrogen Uptake and Release from Heteroaromatic Compounds for Hydrogen Storage, Ind. Eng. Chem. Res., 2010, 49, 1018-1026. (29) Sotoodeh, F., Huber, B. J. M., Smith, K. J., Dehydrogenation Kinetics and Catalysis of Organic Heteroaromatics for Hydrogen Storage, Int. J. Hydrogen Energy, 2012, 37, 2715-2722. (30) Yang, M., Han, C., Ni, G., Wu, J., Cheng, H., Temperature Controlled Three-Stage Catalytic Dehydrogenation and Cycle Performance of Perhydro-9-ethylcarbazole, Int J Hydrogen Energ, 2012, 37, 12839-12845. (31) Crawford, P., Burch, R., Hardacre, C., Hindle, K. T., Hu, P., Kalirai, B., Rooney, D. W., Understanding the Dehydrogenation Mechanism of Tetrahydrocarbazole over Palladium Using a

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Combined Experimental and Density Functional Theory Approach, J Phys Chem C, 2007, 111, 6434-6439. (32) Ryohei Yamaguchi, C. I., Yoshinori Takahashi, and Ken-Ichi Fujita, Homogeneous Catalytic System for Reversible Dehydrogenation-Hydrogenation Reactions of Nitrogen Heterocycles with Reversible Interconversion of Catalytic Species, J. AM. CHEM. SOC., 2009, 131, 3. (33) Dean, D., Davis, B., Jessop, P. G., The Effect of Temperature, Catalyst and Sterics on the Rate of N-heterocycle dehydrogenation for Hydrogen storage, New J. Chem., 2011, 35, 417-422. (34) Li, L., Yang, M., Dong, Y., Mei, P., Cheng, H., Hydrogen Storage and Release from a New Promising Liquid Organic Hydrogen Storage Carrier (LOHC): 2-methylindole, Int J Hydrogen Energ, 2016, 41, 16129-16134. (35) Dong, Y., Yang, M., Yang, Z., Ke, H., Cheng, H., Catalytic Hydrogenation and Dehydrogenation of N-ethylindole as a New Heteroaromatic Liquid Organic Hydrogen Carrier, Int J Hydrogen Energ, 2015, 40, 10918-10922. (36) Müller, K., Stark, K., Emel’yanenko, V. N., Varfolomeev, M. A., Zaitsau, D. H., Shoifet, E., Schick, C., Verevkin, S. P., Arlt, W., Liquid Organic Hydrogen Carriers: Thermophysical and Thermochemical Studies of Benzyl- and Dibenzyl-toluene Derivatives, Industrial & Engineering Chemistry Research, 2015, 54, 7967-7976. (37) Emel’yanenko, V. N., Varfolomeev, M. A., Verevkin, S. P., Stark, K., Müller, K., Müller, M., Bösmann, A., Wasserscheid, P., Arlt, W., Hydrogen Storage: Thermochemical Studies of NAlkylcarbazoles and Their Derivatives as a Potential Liquid Organic Hydrogen Carriers, The Journal of Physical Chemistry C, 2015, 119, 26381-26389. (38) Yang, M., Dong, Y., Fei, S., Pan, Q., Ni, G., Han, C., Ke, H., Fang, Q., Cheng, H., Hydrogenation of N-propylcarbazole over Supported Ruthenium as a New Prototype of Liquid Organic Hydrogen Carriers (LOHC), RSC Advances, 2013, 3, 24877-24881. (39) Sotoodeh, F., Zhao, L., Smith, K. J., Kinetics of H2 Recovery from Dodecahydro-Nethylcarbazole over a Supported Pd Catalyst, Appl. Catal. a-Gen., 2009, 362, 155-162. (40) Sotoodeh, F., Smith, K. J., Structure Sensitivity of Dodecahydro-N-ethylcarbazole Dehydrogenation over Pd Catalysts, J Catal, 2011, 279, 36-47. (41) Sotoodeh, F., Hydrogenation and Dehydrogenation Kinetics and Catalysts for New Hydrogen Storage Liquids, in: Chemical and Biological Engineering, The University of British Columbia, Vancouver, 2011. (42) Gleichweit, C., Amende, M., Schernich, S., Zhao, W., Lorenz, M. P. A., Höfert, O., Brückner, N., Wasserscheid, P., Libuda, J., Steinrück, H.-P., Papp, C., Dehydrogenation of Dodecahydro-N-ethylcarbazole on Pt(111), ChemSusChem, 2013, 6, 974-977. (43) Amende, M., Schernich, S., Sobota, M., Nikiforidis, I., Hieringer, W., Assenbaum, D., Gleichweit, C., Drescher, H. J., Papp, C., Steinruck, H. P., Gorling, A., Wasserscheid, P., Laurin, M., Libuda, J., Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro-N-ethylcarbazole on Pd(111), Chem.-Eur. J., 2013, 19, 10854-10865. (44) Sobota, M., Nikiforidis, I., Amende, M., Sanmartin Zanon, B., Staudt, T., Hofert, O., Lykhach, Y., Papp, C., Hieringer, W., Laurin, M., Assenbaum, D., Wasserscheid, P., Steinruck, H. P., Gorling, A., Libuda, J., Dehydrogenation of Dodecahydro-N-ethylcarbazole on Pd/Al2O3 Model Catalysts, Chemistry, 2011, 17, 11542-11552. (45) Amende, M., Gleichweit, C., Schernich, S., Hofert, O., Lorenz, M. P., Zhao, W., Koch, M., Obesser, K., Papp, C., Wasserscheid, P., Steinruck, H. P., Libuda, J., Size and Structure Effects Controlling the Stability of the Liquid Organic Hydrogen Carrier Dodecahydro-N-ethylcarbazole

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during Dehydrogenation over Pt Model Catalysts, The journal of physical chemistry letters, 2014, 5, 1498-1504. (46) Gleichweit, C., Amende, M., Bauer, U., Schernich, S., Hofert, O., Lorenz, M. P., Zhao, W., Muller, M., Koch, M., Bachmann, P., Wasserscheid, P., Libuda, J., Steinruck, H. P., Papp, C., Alkyl Chain Length-Dependent Surface Reaction of Dodecahydro-N-alkylcarbazoles on Pt Model Catalysts, J Chem Phys, 2014, 140, 204711.

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TOC GRAPHICS

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1 220x165mm (300 x 300 DPI)

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Figure 2 102x98mm (300 x 300 DPI)

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3 220x155mm (300 x 300 DPI)

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Figure 4 220x152mm (300 x 300 DPI)

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5 152x335mm (300 x 300 DPI)

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Figure 6 220x155mm (300 x 300 DPI)

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7 220x155mm (300 x 300 DPI)

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Scheme 1 131x31mm (300 x 300 DPI)

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