Energy & Fuels 2007, 21, 1695-1698
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Hydrogen-Storage Materials Based on Imidazolium Ionic Liquids Marcelo P. Stracke, Gu¨nter Ebeling, Renato Catalun˜a, and Jairton Dupont* Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS AV. Bento Gonc¸ alVes, 9500, Porto Alegre 91501-970, RS, Brazil ReceiVed September 27, 2006. ReVised Manuscript ReceiVed NoVember 22, 2006
Simple 1-alkyl(aryl)-3-methylimidazolium N-bis(trifluoromethanesulfonyl)imidate salts that possess very low vapor pressure, high density, and thermal stability and are not inflammable can add reversibly 6-12 hydrogen atoms in the presence of classical Pd/C or Ir0 nanoparticle catalysts and can be used as alternative materials for on-board hydrogen-storage devices. These salts can hold up to 30 g L-1 of hydrogen at atmospheric pressure, which is twice that compressed hydrogen gas can attain at 350 atm.
Introduction There is no doubt that a great part of the world energy demands of this century will be fulfilled by hydrogen-based fuel cell technology.1 The storage of large quantities of hydrogen at safe pressures is one of the key factors in establishing such a hydrogen-based economy. In particular, for on-board energy storage, vehicles need compact, light, safe, and affordable hydrogen containment. Although liquid hydrogen is currently the most commonly used form in prototype automobiles, it has several disadvantages in particular for on-board storage because of its continuous boil-off. Compressed hydrogen gas is an alternative, but it only holds 15 g L-1 at 350 atm. Systems under higher pressures that could hold higher hydrogen densities are complicated by safety concerns and logistical obstacles. Other storage materials and methods,2 including molecular hydrogen adsorption on solids of large surface area,3-5 bonded atomic hydrogen in hydrocarbons or in metal hydrides6,7 and clathrate hydrates,8,9 are active research areas worldwide.10 Hydrocarbons can also be considered as a liquid storage medium for hydrogen if they can be hydrogenated and dehydrogenated. For example, cyclohexane reversibly yields six hydrogen atoms (7.1 mass %) and forms benzene, and stationary hydrogenation and dehydrogenations under steady-state conditions are managed in various chemical plants.11,12 However, the use of these compounds for * To whom correspondence should be addressed. Fax: +55-5133167304. E-mail:
[email protected]. (1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353-358. (2) Ferey, G. Nature 2005, 436, 187-188. (3) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127-1129. (4) Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91-93. (5) Kavan, L.; Dunsch, L.; Kataura, H. Carbon 2004, 42, 1011-1019. (6) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002, 420, 302-304. (7) Bluhm, M. E.; Bradley, M. G.; Mark, G.; Butterick, R., III; Kusari, U.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 7748-7749. (8) Lee, H.; Lee, J. W.; Kim, D. Y.; Park, J.; Seo, Y. T.; Zeng, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Nature 2005, 434, 743-746. (9) Mao, W. L.; Mao, H. K. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 708-710. (10) Patchkovskii, S.; Tse, J. S.; Yurchenko, S. N.; Zhechkov, L.; Heine, T.; Seifert, G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10439-10444. (11) Wang, Y. G.; Shah, N.; Huffman, G. P. Catal. Today 2005, 99, 359-364. (12) Wang, Y. G.; Shah, N.; Huffman, G. P. Energy Fuels 2004, 18, 1429-1433.
on-board processes under variable conditions is likely to be problematic because cyclohexane derivatives and their dehydrogenated aromatics are volatile, inflammable, possess low density (meaning less hydrogen per volume ratio), and can decompose under the catalytic hydrogenation-dehydrogenation process. Fortunately, we can access cyclohexane derivatives with very low vapor pressure, high density, and chemical and thermal stability, that are not inflammable. We show herein that these desired compounds can be easily accessed by the simple introduction of an imidazolium cation to the cyclohexane moiety. Experimental Section General Conditions. All preparative procedures were carried out under argon using standard Schlenk tube techniques. The hydrogenation experiments were performed by simply placing the salts (20 mmol) with the desired catalyst (1 mass % in relation to the salt) in a 100 mL stainless steel Parr reactor at 70 °C for 2e and 2f and 90 °C for 2g, under 35 atm for 2e and 50 atm for 2f and 2g. A deliberated stirring of 800 rpm was used, and the fall in the hydrogen pressure in the reactor was monitored with a pressure transducer interfaced through a Novus converter to a PC. The data were worked up via Microcal Origin 5.0. The dehydrogenation reactions were performed by melting the salts (20 mg) in the presence of Pd/C (5%) (salt/catalyst ) 100:1 in mass) and placing the obtained melts in a borosilicate glass U-type tube reactor (height, 25 cm; tube internal diameter, 2 mm; bottom bulb volume, 0.5 mL) connected to a thermal conductivity detector (TCD), maintaining a constant flux of argon (25 mL/min). The heating was performed from room temperature to 300 °C (1 °C/min), and the system was kept at this last temperature for 20 min. The IR spectra were recorded on a Bomen MB spectrophotometer as neat liquid films. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Varian Gemini 300 MHz spectrometer. The chemical shifts were measured in parts per million (ppm) relative to tetramethylsilane (TMS) or acetone-d6 as external standards. The electrospray ionization-high-resolution mass spectrometry (ESIHRMS) spectra were recorded in a Waters Micromass Q-Tof micro mass spectrometer YB 320. Differential scanning calorimetry (DSC) measurements were carried out on a thermal analyst 2100 (TA instruments), calibrated using an indium primary standard. Ionic liquid samples were sealed in Al pans (in the nitrogen-filled glove box), with an empty Al pan used as a reference. DSC measurements were carried out in a nitrogen atmosphere on samples of different sizes, and the samples were initially examined using different rates of heating and cooling. All of the DSC data discussed in the paper
10.1021/ef060481t CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
1696 Energy & Fuels, Vol. 21, No. 3, 2007 were measured on 7-12 mg samples. Thermogravimetric analysis (TGA) measurements were carried out on a thermogravimetric analyzer 2950 (DuPont instruments) calibrated using a nickel standard. TGA measurements were carried out in a nitrogen atmosphere on samples of different sizes, and the samples were initially examined using a rate of 20 °C/min for heating. All of the TGA data discussed in the paper were measured on 8-12 mg samples. Representative examples for the preparation and characterization of salts 1-3 are presented below, and all other experimental details are described in the Supporting Information. Preparation of 1-Methyl-3-[3-(phenyl)propyl]imidazolium Methanesulfonate (1c). The imidazolium salt 1c was prepared by the reaction of 3-(phenyl)propyl methanesulfonate (19.50 g, 91.0 mmol) with 1-methyl-imidazole (7.50 g, 91.0 mmol) at room temperature for 24 h. The viscous liquid obtained was washed with ethyl acetate (2 × 10 mL) and dried under reduced pressure (26.90 g, 99% yield). ESI-HRMS positive ion, calculated for [C13H17N2]+, m/z 201.1392; found, m/z 201.1372. ESI-HRMS negative ion, calculated for [CH3SO3]-, m/z 94.9803; found, m/z 94.9834. IR (film, cm-1): 3130, 3038, 2890, 1611, 1592, 1483, 1437, 1123, 992, 804. 1H NMR (CDCl3) δ (ppm): 9.77 (s, 1H), 7.49 (s, 1H), 7.48 (s, 1H), 7.40-7.14 (m, 5H), 4.28 (t, J ) 7.4 Hz, 2H), 3.96 (s, 3H), 2.96 (s, 3H), 2.69 (t, J ) 7.4 Hz, 2H), 2.22 (quintet, J ) 7.4 Hz, 2H). 13C NMR (CDCl3) δ (ppm): 139.6, 137.6, 128.4, 128.0, 126.1, 123.5, 121.9, 49.1, 39.5, 32.0, 31.2. Preparation of 1-Methyl-3-[3-(phenyl)propyl]imidazolium Bis(trifluoromethanesulfonyl)imidate (2f). To a solution of 1-methyl-3-[3-(phenyl)propyl]imidazolium methanesulfonate (1c, 26.90 g, 90.0 mmol) in water (80 mL) was added lithium bis(trifluoromethanesulfonyl)imidate (26.10 g, 91.0 mmol), and the mixture was stirred for 1 h at room temperature. The resulting crude solid compound 2f, insoluble in water, was filtered-off and dissolved in dichloromethane (200 mL). The dichloromethane solution was washed with water (2 × 50 mL), the solvent was evaporated, and the solid product 2f was dried under reduced pressure (42.80 g, 97% yield). ESI-HRMS positive ion, calculated for [C13H17N2]+, m/z 201.1392; found, m/z 201.1379. ESI-HRMS negative ion, calculated for [C2F6NO4S2]-, m/z 279.9173; found, m/z 279.9160. IR (film, cm-1): 3137, 3015, 2948, 2865, 1609, 1588, 1461, 1339, 1222, 1103, 857, 779, 722, 581. 1H NMR (acetone-d6) δ (ppm): 8.98 (s, 1H), 7.76 (s, 1H), 7.66 (s, 1H), 7.32-7.16 (m, 5H), 4.39 (t, J ) 7.4 Hz, 2H), 4.02 (s, 3H), 2.74 (t, J ) 7.4 Hz, 2H), 2.31 (quintet, J ) 7.4 Hz, 2H). 13C NMR (acetone-d6) δ (ppm): 141.5, 137.6, 129.5, 127.2, 124.9, 123.5, 123.3, 119.0, 50.3, 36.8, 33.0, 32.4. Preparation of 1-Methyl-3-[3-(cyclohexyl)propyl]imidazolium Bis(trifluoromethanesulfonyl)imidate (3b). Palladium catalyst (5 wt % on carbon, 0.40 g, 0.20 mmol, 1/100 molar ratio) was mixed under vigorous stirring with imidazolium salt 2f (9.60 g, 20 mmol) placed in a Schlenk tube. The resulting black mixture was transferred under argon to a stainless-steel reactor provided with magnetic stirring, and the system was pressurized with 50 atm of H2. The reactor was heated to 70 °C with constant stirring (800 rpm) for 104 h. After this time, the crude hydrogenation product was dissolved in dichloromethane (250 mL) and the solution was filtered through a short column packed with alumina/celite. Solvent evaporation under reduced pressure gave the product 3b as a viscous liquid (9.50 g, 97% yield). ESI-HRMS positive ion, calculated for [C13H23N2]+, m/z 207.1861; found, m/z 207.1857. ESI-HRMS negative ion, calculated for [C2F6NO4S2]-, m/z 279.9173; found, m/z 279.9169. IR (film, cm-1): 3136, 2877, 1536, 1460, 1406, 1221, 1154, 1098, 757. 1H NMR (CDCl3) δ (ppm): 8.72 (br s, 1H), 7.34 (t, J ) 1.8 Hz, 1H), 7.32 (t, J ) 1.8 Hz, 1H), 4.14 (t, J ) 7.4 Hz, 2H), 3.93 (s, 3H), 1.91-1.66 (m, 6H), 1.27-1.04 (m, 4H), 0.920.81 (m, 3H). 13C NMR (CDCl3) δ (ppm): 135.9, 123.7, 122.2, 50.4, 36.9, 36.2, 33.5, 33.5, 32.9, 27.4, 26.4, 26.1. Dehydrogenation of 1-Methyl-3-[3-(cyclohexyl)propyl]imidazolium Bis(trifluoromethanesulfonyl)imidate (3b). The salt (20 mg) was melted in the presence of Pd/C (5%) (salt/catalyst ) 100:1 in mass) and placed in a borosilicate glass U-type tube reactor (height, 25 cm; tube internal diameter, 2 mm; bottom bulb volume,
Stracke et al. Scheme 1. Reaction Paths Involved in the Preparation of Imidazolium Ionic Liquids Containing Cyclohexane Moieties
0.5 mL) connected to a TCD, maintaining a constant flux of argon (25 mL/min). The heating was performed from room temperature to 300 °C (1 °C/min), and the system was kept at this last temperature for 20 min. A sample of the black suspension thus obtained was dissolved in acetone-d6 and analyzed by 1H NMR, which indicated 60% conversion to 2f. No partially dehydrogenated products were detected.
Results and Discussion It is now well-known that 1,3-disubsituted imidazolium cations associated with weak coordinating anions, such as tetrafluoroborate and N-bis(trifluoromethanesulfonyl)imidate, denominated ionic liquids,13,14 display such features, i.e., noninflammable (at least some of them),15 high density, chemical and thermal stability,16 and very low vapor pressure.17-19 Our synthetic strategy is based on the classical alkylation of 1-methyl-imidazole with alkyl methanesulfonates20 containing alky(aryl) groups to afford compounds 1 that were submitted to exchange their methanesulfonate anions for BF4, PF6, and N(SO2CF3)2 [N(Tf)2] by a simple reaction with their alkaline salts to produce compounds 2 (Scheme 1). The prepared compounds are presented in Table 1 together with some of their physicochemical properties. Compounds 2 are crystalline solids with high thermal stability (they start to decompose at temperatures above 284 °C). Some of the imidazolium salts 2 have been previously prepared and characterized as halide derivatives.21-23 Compounds 2 were then submitted to hydrogenation reactions in the presence of classical and commercially available noble metal catalysts, such as Rh/C (5%) and Pd/C (5%), to generate the cyclohexane-containing imidazolium moieties 3. Compounds containing a benzyl unit (2a, 2b, and 2d) are not stable under the hydrogenation reaction conditions (Table 1) and decompose to a mixture of products. The same instability was also observed with those containing the PF6 and BF4 anions (2a-2c). (13) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 37733789. (14) Dupont, J.; Spencer, J. Angew. Chem., Int. Ed. 2004, 43, 52965297. (15) Smiglak, M.; Reichert, W. M.; Holbrey, J. D.; Wilkes, J. S.; Sun, L.; Thrasher, J. S.; Kirichenko, K.; Singh, S.; Katritzkyc, A. R.; Rogers, R. D. Chem. Commun. 2006, 2554-2556. (16) Dupont, J.; Suarez, P. A. Z. Phys. Chem. Chem. Phys. 2006, 8, 2441-2452. (17) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831-834. (18) Wasserscheid, P. Nature 2006, 439, 797-797. (19) Neto, B. A. D.; Santos, L. S.; Nachtigall, F. M.; Eberlin, M. N.; Dupont, J. Angew. Chem., Int. Ed. 2006, 45, 7251-7254. (20) Cassol, C. C.; Ebeling, G.; Ferrera, B.; Dupont, J. AdV. Synth. Catal. 2006, 348, 243-248. (21) Dzyuba, S. V.; Bartsch, R. A. ChemPhysChem 2002, 3, 161-166. (22) Lee, J. K.; Kim, M. J. J. Org. Chem. 2002, 67, 6845-6847. (23) Moret, M. E.; Chaplin, A. B.; Lawrence, A. K.; Scopelliti, R.; Dyson, P. J. Organometallics 2005, 24, 4039-4048.
Imidazolium Ionic Liquids for Hydrogen Storage
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Table 1. Physicochemical Properties of Salts 1-3 salt
X
n
R
mp (°C)
Td (°C)
d (g/cm3)
Tg (°C)
1a 1b 1c 1d 2a 2b 2c 2d 2e 2f 2g 3a 3b 3c
MeSO3 MeSO3 MeSO3 MeSO3 BF4 PF6 PF6 N(Tf)2 N(Tf)2 N(Tf)2 N(Tf)2 N(Tf)2 N(Tf)2 N(Tf)2
0 1 2 1 0 0 2 0 1 2 1 1 2 1
H H H Ph H H H H H H Ph H H Cy
86.0 46.1 53.8 67.8
284.9 288.5 292.5 296.3 324.7 318.8 301.0 320.8 329.5 350.6 336.4 331.8 344.2 339.2
1.49 1.50 1.44 1.52 1.33a 1.44a 1.44a 1.49 1.47b 1.46b 1.48b 1.42b 1.39 1.43b
-25.3 -20.7 -27.8 -40.5
135.3 52.8 40.6 50.2 62.1 52.8 66.3
Tc (°C)
-20.2 -56.2 -59.1 -60.3 -50.7 -11.5 -22.7 -49.7
-5.5 -6.7
a
Solid compound. The density was obtained by heating the salt at the melting point. b Solidified after being kept at -25 °C.
Figure 1. Conversion of 2f (O) and 2g (9) (20 mmol) to afford 3b and 3c, respectively, at 70 °C for 2f and 90 °C for 2g under 50 atm (initial pressure) of hydrogen with a Pd/C (5%) catalyst.
However, those containing two or three CH2 spacers (n ) 1 or 2) associated with the N-bis(trifluoromethanesulfonyl)imidate anion (2e-2g) are stable and generate quantitatively the hydrogenated products 3a-3c after ca. 100 h, employing the classical Rh or Pd catalysts at 70-90 °C under 35-50 atm of hydrogen (see Figure 1). Among the various catalysts tested [Rh/C (5%), Pt/alumina, Pd-Ru/C (1 and 5%), and Pd/C (5%)] for the dehydrogenation reaction of compounds 3b and 3c, Pd/C (5%) was the most effective for hydrogen production. The dehydrogenation experiments were performed by placing the salts with the catalysts in a U-type reactor connected to a TCD. A heating rate of 1 °C/ min was used, and the results obtained with Pd/C (5%) are presented in Figure 2. The hydrogen generation starts for both compounds at ca. 230 °C, increases steadily until ca. 300 °C, and attains 60% conversion after 50 min. It is important to note that these reactions should be performed under an inert atmosphere because otherwise a significant amount of the salts decomposes. After this time, a decrease in hydrogen production was observed probably because of segregation of the ionic liquids from the catalyst as visually observed. However, at temperatures above ca. 310 °C, the mixture starts to decompose. The compounds 2f and 2g recovered from the dehydrogenation process are easily hydrogenated back to 3b and 3c, and the process can be repeated without any detectable decomposition as checked by 1H and 13C NMR. Therefore, the combination of imidazolium salts 2f and 2g with classical Pd/C (5%) can add reversibly 6-12 hydrogen atoms per ionic pair24 and
Figure 2. Dehydrogenation reaction of compounds 3b (O) and 3c (9) in the presence of Pd/C (5%) (salt/catalyst ) 100:1 in mass) as a function of the temperature using a U-type reactor with a heating rate of 1 °C/min and 25 mL/min of argon.
constitute an alternative material for chemical hydrogen-storage devices. The hydrogenation reaction time (100 h) and the dehydrogenation temperature (230-300 °C) are relatively high, and also, the dehydrogenation yields are only moderate (60%) for practical purposes. However, as already pointed out by Jensen and Goldman, the high endothermicity of the alkane dehydrogenation process is obviously a key factor involved in the challenge of developing alkane dehydrogenation catalysts.25 Because the dehydrogenation enthalpy of typical alkanes is on the order of 28-30 kcal mol-1, reasonable rates could be obtained (in principle) even if the favorable reaction entropy does not make any contribution in the transition state.26 Evidently, a significant concentration of the dehydrogenated product(s) can only be generated by allowing hydrogen to escape, thereby permitting entropy to predominate over enthalpy, and that is the case of ionic liquids in which the solubility of hydrogen is very low.27-29 Note that the process was tested only with classical and commercially available heterogeneous catalysts. Therefore, it is probable that more efficient catalysts will be developed in the near future,30-32 especially now with the availability of compounds such as 2f and 2g that possess very low vapor pressure, high density and thermal stability, almost immiscible with hydrogen, and non-inflammable and can add reversibly up to 12 atoms of hydrogen (2.2 mass % for 3c and 1.2 mass % for 3b). Notwithstanding, this system (imidazolium salt 3c with Pd/C) can hold ca. 30 g L-1 of hydrogen at atmospheric pressure, which is twice that compressed hydrogen gas can attain (15 g L-1 at 350 atm). Moreover, we have also tested Ir0 nanoparticles of 2-3 nm prepared in imidazolium ionic liquids33 in the hydrogenation(24) Consorti, C. S.; Suarez, P. A. Z.; de Souza, R. F.; Burrow, R. A.; Farrar, D. H.; Lough, A. J.; Loh, W.; da Silva, L. H. M.; Dupont, J. J. Phys. Chem. B 2005, 109, 4341-4349. (25) Xu, W. W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska, W. C.; KroghJespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273-2274. (26) Zhu, K. M.; Achord, P. D.; Zhang, X. W.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2004, 126, 13044-13053. (27) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2002, 106, 7315-7320. (28) Dyson, P. J.; Laurenczy, G.; Ohlin, C. A.; Vallance, J.; Welton, T. Chem. Commun. 2003, 2418-2419. (29) Berger, A.; de Souza, R. F.; Delgado, M. R.; Dupont, J. Tetrahedron: Asymmetry 2001, 12, 1825-1828. (30) Liu, F. C.; Goldman, A. S. Chem. Commun. 1999, 655-656. (31) Jensen, C. M. Chem. Commun. 1999, 2443-2449. (32) Morales-Morales, D.; Redon, R.; Yung, C.; Jensen, C. M. Inorg. Chim. Acta 2004, 357, 2953-2956.
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dehydrogenation process. Gratifyingly, the hydrogenation reaction of 2f could be performed at only 5 atm of hydrogen (constant pressure) at 75 °C to afford compound 3b in 75% yield. The dehydrogenation of 3b in the presence of colloidal Ir0 could also be performed to regenerate 2f in 60% yield after 50 min at 230-300 °C.
can add reversibly up to 6-12 hydrogens atoms per ionic pair and constitute alternative materials for on-board hydrogenstorage devices. Finally, it is now possible to design more efficient chemical hydrogen-storage materials based on ionic liquids, for example, by increasing the number of cyclohexanearene units attached to the N-alkylimidazolium cation.
Conclusion
Acknowledgment. Thanks are due to CT-ENERG CNPq for finantial support.
In summary, we have demonstrated that 1-alkyl(aryl)-3methylimidazolium N-bis(trifluoromethanesulfonyl)imidate salts (33) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228-4229.
Supporting Information Available: Experimental procedures and characterization of the imidazolium salts. This material is available free of charge via the Internet at http://pubs.acs.org. EF060481T
