Research Note pubs.acs.org/IECR
Liquid Organic Hydrogen Carriers: An Upcoming Alternative to Conventional Technologies. Thermochemical Studies. Sergey P. Verevkin,* Vladimir N. Emel’yanenko, and Andreas Heintz Department of Physical Chemistry, University of Rostock, Dr.-Lorenz-Weg. 1, 18059 Rostock, Germany
Katharina Stark and Wolfgang Arlt Lehrstuhl für Thermische Verfahrenstechnik der Universität Erlangen Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany S Supporting Information *
ABSTRACT: A system based on the catalytic hydrogenation/dehydrogenation reactions of N-ethylcarbazole is one of the most promising as the new class of the liquid organic hydrogen carrier (LOHC) compounds. Enthalpy of formation of the liquid dodecahydro-N-ethylcarbazole (fully hydrogenated N-ethylcarbazole) was measured using combustion calorimetry. Vaporization enthalpy for this compound was derived from vapor pressure−temperature dependence measured by transpiration. The enthalpy of formation of the gaseous dodecahydro-N-ethylcarbazole was derived and validated with the high-level quantum chemical calculation. Vapor pressures of the liquid N-ethylcarbazole (0.0008 bar) and dodecahydro-N-ethylcarbazole (0.01 bar) at a practical and relevant temperature (400 K) were assessed from the new experimental data. It has turned out that these vapor pressures were low enough to fulfill the basic requirement for an LOHC. promising LOHCs.2 Proposed hydrogenation/dehydrogenation process pathways are shown in Figure 1.
1. INTRODUCTION Developing a hydrogen economy is one possible approach for establishing a sustainable and clean energy supply. However, the efficient and safe storage of hydrogen is probably the greatest barrier for a widespread commercialization of hydrogen as a fuel. Therefore, the development of new hydrogen storage materials is a challenging task. An auspicious alternative to a conventional approach of hydrogen storage is to use unsaturated organic compounds that have a high capacity to bind hydrogen covalently. Storage and release of the hydrogen is achieved by catalytic hydrogenation and subsequent dehydrogenation of the organic compounds.1 In this reversible process, the liquid organic hydrogen carrier (LOHC) compounds are not consumed and can be reused for hydrogen storage. For transport applications, the H2 release (dehydrogenation) would be done on-board and the H2 charging and storage (hydrogenation) would be done off-board the vehicle. Organic aromatic heterocyclic organic compounds containing the elements O, N, S, and B have attracted the most interest, because of their thermal stability and high storage capacity, as well as because of the fact that the hydrogenation/ dehydrogenation can be done under reasonable conditions. They are considered to offer a practical solution for the generation, storage, transportation, and utilization of hydrogen. In contrast to the physical trapping of hydrogen gas in a substrate, the heteroaromatic systems appear as high-boiling liquids that do not require compression or cryogenic procedures. Possible candidates should provide rather high hydrogen release rates with more than 5 wt % of H2 storage (liberated H2 mass, relative to the mass of hydrogenated compound).2 Moreover, a possible LOHC should possess a low vapor pressure, high stability, and low toxicity. Nowadays, Nethylcarbazole (NEC) is considered as one of the most © 2012 American Chemical Society
Figure 1. Hydrogen storage with N-ethyl-carbazole.
An effective and fast hydrogenation of NEC to dodecahydroN-ethylcarbazole (DEC) on a supported Ru catalyst is feasible already at an acceptable temperature of ∼400 K.2 The complete recovery of H2 from DEC can also be easily achieved at similar temperatures and ambient pressure using Pd-supported catalysts.1 Feasibility and kinetics of hydrogenation/dehydrogenation processes was carefully studied in refs 1 and 2. Thermochemical properties are required for optimization of the practical applications. In this work, we have performed, for the first time, experimental (combustion calorimetry and vapor pressure measurements) as well as the high-level quantum chemical studies of dodecahydro-N-ethylcarbazole. Received: Revised: Accepted: Published: 12150
July 17, 2012 August 28, 2012 August 28, 2012 August 28, 2012 dx.doi.org/10.1021/ie301898m | Ind. Eng. Chem. Res. 2012, 51, 12150−12153
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Research Note
Table 1. Thermochemical Data at T = 298 K (p° = 0.1 MPa) for the Molecules Studied in This Work compound N-ethyl-carbazole dodecahydro-Nethylcarbazole
state
ΔcHm ° (kJ mol−1)
ΔfHm ° (kJ mol−1)
Δgl Hm ° (kJ mol−1)
° (g)exp ΔfHm (kJ mol−1)
° (g)a ΔfHm (kJ mol−1)
cr liq
−7437.6 ± 1.8 [ref 12] −8862.2 ± 3.7
70.6 ± 2.6 [ref 12]b −219.8 ± 4.2
97.1 ± 1.0 [ref 13]c 68.4 ± 0.5
167.7 ± 2.8 −151.4 ± 4.4
164.5d −150.7e
Theoretical value calculated using G3(MP2). bMolar enthalpy of formation in the liquid state ΔfHm ° (liq., 298 K) = (83.4 ± 2.6) kJ mol−1 was cr −1 ° = (12.8 ± 0.4) kJ mol at 298 K, reported in ref 13. cMolar enthalpy of sublimation ΔgcrHm °. calculated using the enthalpy of fusion Δl Hm d Calculated using the bond separation reaction C14H13N + 29CH4 = (CH3)3N + 20C2H6. eCalculated using the atomization procedure. a
2. EXPERIMENTAL PROCEDURE AND METHODS OF AB INITIO CALCULATIONS 2.1. Materials. Dodecahydro-N-ethylcarbazole (DEC) was synthesized at the Chair of Chemical Reaction Engineering (University Erlangen−Nuremberg) by catalytic hydrogenation of N-ethylcarbazole. The main impurities were structurally similar compounds with boiling points close to the main component. A commercially available microspinning band column from NORMAG GmbH was used to purify the DEC. This special type of distillation allows for an effective separation of close-boiling substances. The spinning band provides a thin film that leads to an enhanced mass transfer and, thus, to a large vapor−liquid boundary area with low pressure drop. DEC was distilled twice under vacuum (10 mbar) to reduce the impurity concentrations to a final purity of >99.9% analyzed by GCFID/MS (Agilent Model 7890A). A water content of 761 ppm in the sample for combustion experiments was measured using Karl Fischer titration. Samples were kept and handled under a nitrogen stream in a special glass device furnished with a septum for sampling using a syringe. 2.2. Transpiration Method. Vapor pressures and the enthalpy of vaporization (Δg1H°m) of the DEC were determined using the method of transpiration in a saturated stream of nitrogen.3 The saturated vapor pressure pi at different temperature Ti was calculated from the amount of product collected within a definite period of time in a cold trap. Assuming that Dalton’s law of partial pressures is valid when applied to the nitrogen stream saturated with the substance i of interest, values of pi were calculated: pi =
miRTa VMi
V = VN2 + Vi
(VN2 ≫ Vi )
energy equivalent of the calorimeter (εcalor) was determined with a standard reference sample of benzoic acid (sample SRM 39j, NIST). From eight experiments, εcalor was measured to be 14885.6 ± 0.9 J K−1. For the reduction of the data to standard conditions, conventional procedures5 were used. Auxiliary quantities for this procedure are collected in Table S2 in the Supporting Information (ESI). Corrections for nitric acid formation were based on titration with 0.1 mol dm −3 NaOH(aq). We used a value of −59.7 kJ mol−1 for the molar energy of formation of 0.1 mol dm−3 HNO3 from N2, O2, and water.6 The atomic weights have been used as recommended by the IUPAC Commission.7 2.4. Computations. Standard first-principles molecular orbital calculations were performed with the Gaussian 03 Rev. B04 series of programs.8 Energies were obtained at the G3(MP2) level of theory. G3 theory is a procedure for calculating energies of molecules containing atoms of the first and second row of the periodic table based on ab initio molecular orbital theory. A modification of G3 theory using reduced orders of Moller−Plesset perturbation theory is the G3(MP2) theory.9 For all the species included in this study, a careful conformational analysis with the full geometry optimizations were carried out at the HF/6-31G(d) level. The corresponding harmonic vibrational frequencies were evaluated at the same level of theory to confirm that the found optimized structures correspond to potential energy minima of zero-point vibrational energies. Zero-point energy (ZPE) values were scaled by an empirical factor of 0.8929. All the minima found at the HF/6-31G(d) level were again fully reoptimized at the MP2(FULL)/6-31G(d) level. G3(MP2) theory uses geometries from second-order perturbation theory and scaled ZPEs from Hartree−Fock theory, followed by a series of single-point energy calculations at the MP2(Full), QCISD(T), and MP2/GTMP2Large levels of theory (for details, see ref 9). The enthalpy values at T = 298 K of studied compounds have been determined according to standard thermodynamic procedures.10
(1)
where R is the universal gas constant, mi the mass of the collected compound, Mi its the molar mass, and Vi its volume contribution to the gaseous phase. VN2 is the volume of the carrier gas passed through the saturator while mi is collected. Ta is the temperature of the soap film bubble flow meter, which is used to measure the gas flow in the system. The volume of transporting gas (VN2) was determined from the flow rate and time measurements. Thermal decomposition of the sample was not observed at all temperatures of investigation by gas chromatography (GC) monitoring of the transported material. 2.3. Combustion Calorimetry. Measurements of the enthalpy of combustion of the liquid DEC have been determined using an isoperibolic calorimeter equipped with a static bomb and a stirred water bath. The samples were placed (under an inert atmosphere in a glovebox) in polyethylene capsules and burned in oxygen at a pressure of 3.04 MPa. The detailed procedure has been described previously.4 The combustion products were examined for carbon monoxide (Dräger tube) and traces of soot, but neither was detected. The
3. RESULTS AND DISCUSSION 3.1. Vapor Pressure and Vaporization Enthalpy of DEC. Experimental vapor pressures of DEC measured in this work are presented in Table S1 in the ESI. Equations 2 and 3 have been used to fit the data: R ln pi = a +
⎛T ⎞ b + Δlg Cp ln⎜ ⎟ T ⎝ T0 ⎠
Δlg Hm(T ) = −b + Δlg CpT
(2) (3)
where pi is the vapor pressure; a and b are adjustable parameters (see Table S1 in the ESI); T0 is an arbitrarily chosen reference temperature (T0 = 298 K), and Δgl Cp = −101.8 J mol−1 K−1 is the difference of the molar heat capacities of the 12151
dx.doi.org/10.1021/ie301898m | Ind. Eng. Chem. Res. 2012, 51, 12150−12153
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Values of the vaporization enthalpy of DEC from Table S1 in the ESI can now be used together with the results from combustion experiments (see Table 1) for further calculation of the gas standard enthalpies of formation (ΔfHm ° (g)) at 298 K. The resulting experimental values of ΔfH°m(g) are given in the last column in Table 1 , and they can be now compared with the theoretical results from quantum chemical calculations, together with other parent compounds (see Table 2). We have
gaseous and liquid phases, which was calculated from the isobaric molar heat capacity Clp = 351.0 J mol−1 K−1, using the group contribution method of Chickos and Acree.11 Experimental results, and parameters a and b, are listed in Table S1 in the ESI and Table 1. In order to assess the uncertainty of the vaporization enthalpy, the experimental data were approximated with the linear equation ln(pi) = f(T−1) using the method of least squares. The uncertainty in the enthalpy of vaporization was assumed to be identical with the average deviation of experimental ln(pi) values from this linear correlation (uncertainties in values of Δg1Cp are not taken into account). The experimental p−T dependence for DEC measured in this work has allowed us to assess the vapor pressure of only 0.01 bar (1366 Pa) for this compound at a practically relevant temperature of 400 K. For N-ethylcarbazole, the vapor pressure is at this temperature is even lower: 0.0008 bar (80 Pa).13 Hence, one of the basic requirements for an LOHC system is fulfilled successfully. 3.2. Enthalpies of Formation from Combustion Calorimetry. Results of combustion experiments on DEC are summarized in Table 1. The values of the standard specific energy of combustion (Δcu°), the standard molar enthalpy of combustion (ΔfHm ° ), and the standard molar enthalpy of formation in the liquid state (ΔfHm° (l)) were based on the reaction C14 H 25N + 20.25O2 = 14CO2 + 12.5H 2O + 0.5N2
Table 2. Results of G3(MP2) Calculation of the Standard Enthalpy of Formation (ΔfH°m(g)) for Dodecahydro-Nethylcarbazole in the Gas Phase at 298 K ΔfH°m(g) (kJ mol−1)
reaction C14H25N = 14C + 25H + 1N C14H25N + 20CH4 = NH3 + 17C2H6 C14H25N + 17CH4 = (CH3)3N + 14C2H6 a
(AT) (BS1) (BS2)
−150.7a −145.2 −144.5
Experimental value ΔfH°m(g) = −151.4 ± 4.4 (see Table 1).
calculated using G3(MP2) total energies (E0) at T = 0 K and enthalpies at T = 298 K (H298) (see Table S4 in the ESI). In standard Gaussian theories, theoretical standard enthalpies of formation (ΔfHm° (g)) are calculated through atomization reactions and bond separation reactions.15 In this work, we have applied the atomization procedure (AT) and two bond separation reactions (BS1) and (BS2), as shown in Table 2. Results of the calculations are given in Table 2. It makes oneself conspicuous, that ΔfH°m(g) values calculated from both bond-separation reactions are somewhat less negative than the experimental value, but the agreement is still acceptable within ±5 kJ mol−1. At the same time, the result from the standard atomization procedure is in very good agreement with the experimental value. It is well-established that the enthalpies of formation obtained from the first-principle calculations are very sensitive to the choice of the bond separation or isodesmic reactions used for this purpose.15 In contrast to the bond separation reactions, atomization reactions suggest that the quality of the data on the right side (the constituted atoms) is the same for all compounds under study.15 We can now use procedure AT to predict the values of ΔfH°m(g) for other possible LOHC of similar structure directly from the G3(MP2) energies without constructing the ambiguous bond separation or isodesmic reactions. 3.4. Calculation of the Hydrogenation/Dehydrogenation Enthalpy. The enthalpy of hydrogenation/dehydrogenation enthalpy is the most relevant quantity for hydrogen storage. We use experimental values from Table 1 to calculate the enthalpy, ΔrH°m = (−303.2 ± 4.8) kJ mol−1 at 298 K, for the following reaction:
(4)
The value of the molar enthalpy of formation, ΔfH°m(l) of DEC has been obtained from the enthalpic balance for reaction 4, according to Hess’s law, using the molar enthalpies of formation of H2O(l) and CO2(g) as assigned by CODATA.7 A summary of combustion experiments is given in Table S3 in the ESI. The mean value of the standard specific energy of combustion is Δcu° = −42670.4 ± 7.6 J g−1, and the corresponding standard molar enthalpy of combustion is ΔcH°m = −8862.2 ± 3.7 kJ mol−1. The uncertainties assigned to ΔfH°m are twice the overall standard deviation and include the uncertainties from calibration, from the combustion energies of the auxiliary materials, and the uncertainties of the enthalpies of formation of the reaction products H2O and CO2. 3.3. Calculation of the Gas-Phase Enthalpies of Formation, Using Quantum Chemical Calculations. It was reported recently that there are six possible stereoisomers of dodecahydro-N-ethylcarbazole are theoretically distinguishable from DFT calculations.14 Conformational analysis of dodecahydro-N-ethylcarbazole was performed first by the molecular mechanics MM3 method. The geometric parameters of the molecule (dihedral angles) were modified randomly with preserving the cyclic system, and then geometry was optimized. The stable conformers found in this way were used as initial structures for optimization by the composite G3MP2 method. All quantum chemical calculations were made using the Gaussian 03 Rev. B 04 methodology. It has turned out that only three stable conformers (see the ESI) are energetically favorable. The stablest conformer is approximately ±5 kJ mol−1 different from the other two conformers. Many other conformers are also possible for dodecahydro-N-ethylcarbazole, but all of them are more than 10 kJ mol−1 less stable and their contribution to the energetics of the conformers equilibrium mixture can be neglected. We used the most stable conformer of dodecahydro-N-ethylcarbazole for the thermodynamic calculations (see the ESI).
N ‐ethylcarbazole(liq.) + 6H 2(gas) = dodecahydro‐N ‐ethylcarbazole(liq.)
For comparison, enthalpy of hydrogenation of a similarly shaped acenaphthene to hexahydroacenaphthylene liberates only 110 kJ mol−1 under the same conditions.16
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CONCLUSIONS We have obtained a consistent set of thermochemical properties of dodecahydro-N-ethylcarbazole in the liquid state and in the gaseous state, which are required for optimization and process simulation of the hydrogenation/dehydrogenation liquid organic hydrogen carrier (LOHC) system based on N12152
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(13) Verevkin, S. P.; Emel’yanenko, V. N.; Pimerzin, A. A.; Vishnevskaya, E. E. Thermodynamic Analysis of Strain in the FiveMembered Oxygen and Nitrogen Heterocyclic Compounds. J. Phys. Chem. A 2011, 115, 1992−2004. (14) Eblagon, K. M.; Tam, K.; Yu, K. M. K.; Zhao, S.-L.; Gong, X.-Q.; He, H.; Ye, L.; Wang, L.-C.; Ramirez-Cuesta, A. J.; Tsang, S. C. Study of Catalytic Sites on Ruthenium For Hydrogenation of N-ethylcarbazole: Implications of Hydrogen Storage via Reversible Catalytic Hydrogenation. J. Phys. Chem. C 2010, 114, 9720−9730. (15) Ohlinger, W. S.; Klunzinger, P. E.; Deppmeier, B. J.; Hehre, W. J. Efficient Calculation of Heats of Formation. J. Phys. Chem. A 2009, 113, 2165−2175. (16) Frye, C. G.; Weitkamp, A. W. Equilibrium hydrogenations of multi-ring aromatics. J. Chem. Eng. Data 1969, 14, 372−376.
ethylcarbazole. Vapor pressures of the liquid N-ethylcarbazole (0.0008 bar) and dodecahydro-N-ethylcarbazole (0.01 bar) at a practical and relevant temperature (400 K) were assessed from the new experimental data. These vapor pressures were low enough to fulfill the basic requirement for an LOHC.
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ASSOCIATED CONTENT
S Supporting Information *
Results from measurements of the vapor pressure p of compounds using the transpiration method (Table S1). Auxiliary quantities for combustion experiments (Table S2). Results for combustion experiments (Table S3). G3(MP2) total energies at 0 K and enthalpies at 298 K of the molecules studied in this work (the three most-stable conformers of dodecahydro-N-ethylcarbazole) (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Prof. Wasserscheid for the synthesis of chemical species. REFERENCES
(1) Sotoodeh, F.; Zhao, L.; Smith, K. J. Kinetics of H2 recovery from dodecahydro-N-ethylcarbazole over a supported Pd catalyst. Appl. Catal. A 2009, 362, 155−162. (2) 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. (3) Kulikov, D.; Verevkin, S. P.; Heintz, A. Enthalpies of vaporization of a series of aliphatic alcohols: Experimental results and values predicted by the ERAS-model. Fluid Phase Equilib. 2001, 192, 187− 207. (4) Emel’yanenko, V. N.; Verevkin, S. P.; Heintz, A. The Gaseous Enthalpy of Formation of the Ionic Liquid 1-Butyl-3-MethylImidazolium Dicyanoamide from Combustion Calorimetry, Vapor Pressure Mesurements, and ab initio Calculations. J. Am. Chem. Soc. 2007, 129, 3930−3937. (5) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental Thermochemistry; Rossini, F. D., Ed.; Interscience Publishers: New York, 1956; p 75. (6) NBS Tables of Chemical Thermodynamics Properties. J. Phys. Chem. Ref. Data 1982, 11 (Supplement No. 2). (7) Wieser, M. E.; Coplen, T. B. Atomic weights of the elements 2009 (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 359− 398. (8) Frisch, M. J. et al. Gaussian 03, Revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003. (9) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. Gaussian-3 theory using reduced Møller−Plesset order. J. Chem. Phys. 1999, 110, 4703−4710. (10) McQuarrie, D. A. Statistical Mechanics: Harper and Row: New York, 1976. (11) Chickos, J. S.; Acree, W. E., Jr. Enthalpies of Vaporization of Organic and Organometallic Compounds, 1880−2002. J. Phys. Chem. Ref. Data 2003, 32, 519−878. (12) Jimenez, P.; Roux, M. V.; Turrion, C. Thermochemical properties of N-heterocyclic compounds. III. Enthalpies of combustion vapour pressures and enthalpies of sublimation, and enthalpies of formation of 9H-carbazole, 9-methylcarbazole, and 9-ethylcarbazole. J. Chem. Thermodyn. 1990, 22, 721−726. 12153
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