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SYNTHETIC PHOTOSYNTHESIS FOR THE CONVERSION OF LARGE VOLUMES OF CARBON DIOXIDE REFERENCES
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526 SYNTHETIC PHOTOSYNTHESIS FOR THE CONVERSION OF LARGE VOLUMES OF CARBON DIOXIDE REFERENCES 1. EPA-US. http://www.epa.gov (accessed Jun 2013). 2. Wikipedia.org/wiki/Copenhagen Agreement. 3. (a) Kolbe, H.; Lautemann, E. Ann. 1869, 113 , 125; (b) Schmitt, R.; Burkard, E. Ber. 1877, 20 , 2699. 4. The first report was by Bassarov in 1870, see Fromm, D.; Lutzov, D. Chem. Unserer Zeit 1979, 13 , 78. 5. ICI process on stream since the 1970s. 6. Inoue, S.; Koinuma, H.; Tsuruta,T. J. Polym. Sci. B 1969, 7 , 287. 7. Inoue, S.; Koinuma, H.; Tsuruta, T. Makromol. Chem. 1969, 130 , 210. 8. Musco, A.; Perego, C.; Tartiari, V. Inorg. Claim. Acta 1978, 28 , L147. 9. Gassner, F.; Haack, V.; Janssen, A.; Elsagir, A.; Dinjus, E. EP Appl 10329720000218, 2000. 10. Behr, A.; Herdtweck, E.; Herrmann, W. A.; Keim, W.; Kipshagen, W. J. Chem. Soc. Chem. Commun. 1986, 16 , 1262–1263. 11. Aresta, M.; Quaranta, E.; Tommasi, I. New J. Chem. 1994, 18 , 133–142. 12. Behr, A.; Henze, G. Green Chem. 2011, 13 , 25–39. 13. Hoberg, H.; Peres, Y.; Kr¨uger, C.; Tsay, Y.-H. Angew. Chem. Int. Ed. 1987, 26 , 771–773. 14. Alvarez, R.; Carmona, E.; Cole-Hamilton, D. J.; Galindo, A.; Guti´errez-Puebla, E.; Monge, A.; Poveda, M. L.; Ruiz, C. J. Am. Chem.
15. Fisher, R.; Langer, J.; Malassa, A.; Walther, D.; Gorls, H.; Vaughan, G. Chem. Commun. 2006, 2510–2512. 16. Aresta, M.; Pastore, C.; Giannoccaro, P.; Kov´acs, G.; Dibenedetto, A.; P´apai, I. Chem. Eur. J. 13 , 2007, 9028–9034. 17. Lejkowski, M. L.; Lindner, R.; Kageyama, T.; B´odizs, G. ´ E.; Plessow, P. N.; M¨uller, I. B.; Sch¨afer, A.; Rominger, F.; Hofmann, P.; Futter, C.; Schunk, S. A.; Limbach, V. Chem. Eur. J. 2012, DOI: 10.1002/chem.201201757. 18. Gattrell, M.; Gupta, N.; Co, A. J. Electroanal. Chem. 2006, 594 , 1–19. 19. Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104 , 4803–4810. 20. Doherty, M. D.; Grills, D. C.; Muckerman, J. T.; Polyansky, D.E.; Fujita, E. Coord. Chem. Rev. 2010, 254 , 2472–2482. 21. Takeda, H.; Ishitani, O. Coord. Chem. Rev. 2010, 254 , 346–354. 22. Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich, L.; Gross, Z. J. Phys. Chem. A 2002, 106 , 4772–4778. 23. Aresta, M.; Quaranta, E.; Tommasi, I.; Giannoccaro, P.; Ciccarese, A. Gazz. Chim. Ital. 1995, 125 , 509. 24. Biorefinery: from Biomass to Chemicals and Fuels; Aresta, M.; Dibenedetto, A.; Dumeignil, F. Eds.; De Gruyter Publications: Berlin, Germany, 2012; Chapter 17. 25. Dibenedetto, A.; Stufano, P.; Macyk, W.; Baran, T.; Fragale, C.; Costa, M.; Aresta, M.; ChemSusChem 2012, 5 ,373–378. 26. Schneider, J.; Jia, H.; Muckerman, J. T., Fujita, E. Chem. Soc. Rev. 2012, 41 , 2036–2051. 27. Taniguchi, Y.; Yoneyama, H.; Tamura, H. Bull. Chem. Soc. Jpn. 1982, 55 , 2034–2039. 28. Taniguchi, I.; Aurian-Blajeni, B.; Bockris, J. O. M. Electrochim. Acta 1984, 29 , 923–932. 29. Bockris, J. O. M.; Wass, J. C. J. Electrochem. Soc. 1989, 136 , 2521–2528. 30. Noda, H.; Yamamoto, A.; Ikeda, S.; Maeda, M.; Ito, K. Chem. Lett. 1990, 19 , 1757–1760. 31. Ikeda, S.; Yamamoto, A.; Noda, H.; Maeda, M.; Ito, K. Bull. Chem. Soc. Jpn. 1993, 66 , 2473–2477. 32. Hirota, K.; Tryk, D. A.; Hashimoto, K.; Okawa, M.; Fujishima, A. J. Electrochem. Soc. 1998, 145 , L82–L84. 33. Kaneco, S.; Katsumata, H.; Suzuki, T.; Ohta, K. Chem. Eng. J. 2006, 116 , 227–231. 34. Ono, H.; Yokosuka, A.; Tasiro, T.; Morisaki, H.; Yugo S. New Diamond Front. Carbon Technol. 2002, 12 , 141–144. 35. Halmann, M.; Aurian-Blajeni, B. J. Electroanal. Chem.1994, 375 , 379–382 36. Hinogami, R.; Nakamura, Y.; Yae, S.; Nakato, Y. J. Phys. Chem. B 1998, 102 , 974–980. 37. Flaisher, H.; Tenne, R.; Halmann M. J. Electroanal. Chem. 1996, 402 , 97–105. 38. Ikeda, S.; Saito, Y.; Yoshida, M.; Noda, H.; Maeda, M.; Ito, K. J. Electroanal. Chem. Interfacial Electrochem. 1989, 260 , 335–345. 39. Kaneco, S.; Ueno, Y.; Katsumata, H.; Suzuki, T.; Ohta, K. Chem. Eng. J. 2009, 148 , 57–62. 40. Cottineau, T.; Morin, M.; Belanger, D. ECS Trans. 2009, 19 , 1–7. 41. Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. J. Am. Chem. Soc. 2008, 130 , 6342–6344. 42. Kumar, B.; Smieja, J. M.; Kubiak, C. P. J. Phys. Chem. 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Solar Energy 1983, 31 , 429–431. 40 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY Martin H. G. Prechtl * and Sebastian Sahler Institute of Inorganic Chemistry, Department of Chemistry, University of Cologne, Cologne, Germany 40.1 INTRODUCTION Even today, terrestrial, nautical, and aeronautic transportation strongly depends on petrol as energy carrier. Therefore, to maintain the current modern transportation systems running in the future, novel, advanced, hence sustainable and environmental, benign energy carrier systems have to be found and further developed. One interesting and highly energetic material that may help to overcome this problem is hydrogen gas [1, 2]. It is advantageous in terms of (i) very low weight, which means high energy density per kilogram; (ii) abundance in the form of water; and (iii) environmental friendly nature as the hydrogen gas is converted again into water during combustion. However, one has to keep in mind the drawbacks of this material while storing it “physically” in the molecular dihydrogen form: (i) low density in the gaseous state, (ii) the need for portable heavy pressure tanks to carry compressed gas in a reasonable small volume, and (iii) cryogenic storage of denser liquid hydrogen requiring efficient cooling systems [1, 3–6]. An alternative pathway to store hydrogen is the “chemical storage” of reactive protons and hydrides in organic and inorganic materials, which are accessible for catalytic bond activation, hence dehydrogenation. Thus the hydrogen is stored under ambient conditions in liquid or solid form with still reasonable high density, and molecular dihydrogen gas is released on demand. To be competitive to physical storage, chemical methods are restricted to low weight materials that contain high amounts of chemically bound protic and hydridic hydrogen. One of the most interesting materials, in terms of weight efficiency (weight percent, wt%), is the Lewis pair ammonia borane (AB) ( >19 wt%). Here, the reactive protons are carried on a nitrogen moiety and the hydridic counterparts on a boron entity. Besides, also some low weight derivatives of AB are investigated for hydrogen storage. Moreover, carbon dioxide as proton and hydride acceptor in the form of formic acid (FA) is considered as it is abundant on earth. The drawback of FA is the low hydrogen content of only 4.4 wt% [7–10]. In terms of energy density (weight-to-volume ratio), AB is quite competitive and even more efficient than molecular dihydrogen gas. However, the drawback of this compound is related to the difficult regeneration of the dehydrogenated residual products after hydrogen release and some aspects in transportation for mobile application. Today’s infrastructure for the carriage and handling of energetic materials is designed for liquid fuels, therefore, solid AB for example is at a disadvantage in competition with fossil or renewable liquid fuels. Additionally, liquid fuels are superior in terms of handling during the application in engines or fuel cells. Therefore, it remains a challenge to find a liquid hydrogen storage material with accessible high hydrogen content. In this regard, ionic liquids (ILs) are quite promising because of their tunable physicochemical properties [11]. Currently, the main targets are catalytic dehydrogenation of hydrogen-enriched inorganic or organic molecules and the hydrogenation of used Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book, First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
530 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY materials for recycling purposes [12–16]. ILs revealed interesting behaviors in combination with hydrogen storage materials: (i) ILs promote hydrogen release even in the absence of catalysts, (ii) some are capable of dissolving hydrogen carriers, (iii) they help dissolve spent fuel products for regeneration purposes, and (iv) they play a role in the immobilization of catalysts for dehydrogenation and hydrogenation [16, 17]. Interestingly, all these properties can be combined, which makes the application of ILs in hydrogen storage systems as single additive especially attractive [16]. This is especially evident in combination with a transition metal catalyst, which further improves the efficiency of the hydrogen storage system in terms of fast hydrogen release and hydrogenation of the spent hydrogen carrier.
In the past two decades, ILs, often also called molten salts, revealed their high potential for application in various fields of natural science and technology, and all fields of chemistry [18–24]. Some general examples for applications are electrolytes, lubricants, surfactants, solvents, extractants, and catalyst stabilizers/immobilizers. The variety of applications of ILs is related to some thermodynamic and other physicochemical properties: (i) noninflammability, (ii) low vapor pressure, (iii) high solvation, (iv) often weakly coordinating nature, (v) tunable polarity/miscibility/viscosity, (vi) tunable acidity/basicity/functionality, (vii) good thermal stability, and, last but not the least, (viii) low melting point, defined as salts with melting points below 100 ◦ C [11, 24–27] The latter property is based on the reduced lattice energy due to large ion pairs and low symmetry of cations (Fig. 40.1) [11, 25, 28]. Although ILs were discovered about 100 years ago, in 1913, it was only the middle of the century that saw the first applications in electrochemistry and only at the end of the millennium that the potential for widespread application in other fields began to develop rapidly [29]. Modification of the backbone and side chains of the ion structures is a common way to tune their physical and chemical properties [30–34]. In this manner, task-specific ILs can be designed with tailored hydrophilicity or hydrophobicity, variable viscosity, and incorporating functional coordinating groups. In the particular case of imidazolium-based ILs, the acidity/basicity of the reaction system might play an important role because of the acidic proton in the C2-position [22, 24, 27, 35]. Dissolved metal species form carbene complexes [36]. The complex formation can be hindered by protection of the C2-position. In homogeneous as well as in heterogeneous catalysis, ILs are suitable agents for the stabilization and immobilization of the catalyst species, preventing decomposition and agglomeration [22]. One important parameter to obtain high yields is the solubility of the substrates, reactants, intermediates, and products. In case of hydrogen generation, the driving force is the extrusion of hydrogen gas from the IL mixture, as its solubility is rather low, and therefore the equilibrium can be forced to the product side [37, 38]. Interestingly, there are some examples where ILs have been employed as hydrogen storage materials and not as simple solvents. 40.3 HYDROGEN STORAGE MATERIALS: HYDROGEN-RICH MOLECULES Since molecular dihydrogen is storable as pressurized gas or as liquid at low temperature, novel hydrogen storage materials must compromise some significant advantages. The storage of molecular hydrogen as compressed gas or as liquid requires the application of heavy pressure tanks or highly isolated cryogenic tanks, which make the application less effective and very N N
N N + R 2 R 1 N + R 2 R 1 N + R 1 N + R 1 R 4 R 3 R 2 P + R 2 R 3 R 1 R 4 R 1 R 2 R 6 R 5 R 4 R 3 (a)
(b) (c)
(d) (e)
(f) BF 4 − PF 6 − − N S S F 3 C O O CF 3 O O − NTf 2 S O O O O − S O O O − − N(CN) 2 (g)
(h) (i)
(j) (k)
(l) +
Selected examples for IL-cations (left) and anions (right): (a) imidazolium, (b) pyrrolidinium, (c) pyridinium, (d) ammonium, (e) phosphonium and (f) guanidinium. The side chains R 1–6 are alkyl groups, which can also carry functionalities such as alcohols, ethers, and nitriles. Usually weakly coordinating anions are used as counterparts (g–l): ethyl sulfate, dicyanamide, bistrifluoromethanesulfonylimide (NTf 2 −
4 − , PF 6 − , etc. HYDROGEN STORAGE MATERIALS: HYDROGEN-RICH MOLECULES 531 expensive. To visualize the drawback, we consider a short example: a car powered with hydrogen would consume 6 kg of H 2
3 at ambient pressure; therefore, the hydrogen is stored in a 260-l tank at 700 bar, resulting in a total tank weight of 125 kg [4]. As a consequence, the gravimetric efficiency of high pressure storage is rather low at 4.8 wt%. Even lower gravimetric efficiencies are reached with cryogenic methods, as this method suffers from easy evaporation of the gas; moreover, thermal isolation requires the application of further materials, which means further increase in weight. In summary, these disadvantages, resulting in gravimetric inefficiency, make the application of inorganic and organic materials, carrying protic and hydridic hydrogen, quite an attractive approach. Thus, the target is to achieve higher volumetric and gravimetric hydrogen density with a superior gravimetric efficiency in the condensed phase.
AB is a lightweight, hydrogen-rich Lewis pair, therefore an interesting material for this research area [6, 10, 17, 39–41]. It consists in total of 19.6 wt% hydrogen, with equal amounts of hydrides and protons. Simple thermal treatment (130 ◦ C) of this air- and water-stable solid material results in a hydrogen release of 14 wt% [4]. The dehydrogenation of AB might be accompanied by the formation of ammonia, diborane, and borazine. This is problematic because these volatile compounds contaminate the gas stream and decrease the efficiency owing to incomplete dehydrogenation of the substrate. Moreover, fuel cells are sensitive against ammonia. The complete dehydrogenation of AB would need elevated temperatures greater than 500 ◦ C. Under these conditions, the major by-products are solid materials such as insoluble polyborazylene as well as boron nitride (Fig. 40.2). The latter is highly stable in terms of its mechanical, thermodynamical, and chemical properties. Therefore, complete dehydrogenation of AB is not the major focus due to the high process temperature of the hydrogen generation and the stability of boronitride. The desired aim is the selective partial dehydrogenation of AB that results in a competitive hydrogen storage capacity of approximately 14 wt%. The major drawback and challenge is related to the formation of the discussed by-products, as the AB-based “spent fuel” products are difficult to regenerate, hence their reconversion into the starting material, AB. One attempt for the regeneration of the dissolved spent fuels is their reduction with hydrazine in liquid ammonia, where the complete/selective reformation of AB has been observed, accompanied by nitrogen extrusion [42]. Besides AB, some of its derivatives have also been investigated for hydrogen storage, especially to overcome the previously mentioned problems. The derivatives hydrazine borane (HB) [13], guanidinium borohydride (GBH) [12], ethylenediamine bisborane (EDB) [43], methylguanidinium borohydride (Me-GBH) [44], and different alkyl amine boranes are the most interesting (Fig. 40.3) [45, 14]. Another compound class between molecular and metal hydride storage materials is that of metal amido boranes, which is not discussed in this chapter [46]. HB provides comparable weight efficiency as AB, and most of the hydrogen is thermally accessible [13]. The capacity of this material can be improved by blending it with a hydride donor, as pure HB contains four protons and three hydrides, therefore, the excess proton in HB should be combined with an external hydride donor. By blending HB with the lowest weighing metal hydride, lithium hydride, hydrogen production is significantly improved when applying a molar ratio of 1 : 1 B − N + H H H H H H B − N + H H H H H H B − N + N + H H H H H H H H B − H H H H N + B − N + B − N + H H H
H H H H H H H H H B − H H H H − H 2 +AB − H 2 − H 2 +AB
+AB N + B − N + B − N + H H H H B − H H H H H H H B − H H H H N + B − N + B − N + H H H H H H H H N + H H H B − H H H H H H B − H H H N + H H H H H B N B N B N B N B N B N B N B N − m H 2 − m H 2 + n AB + n AB
x Figure 40.2 Decomposition pathways of ammonia borane. 532 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY N +
− H
H H H H Ammonia borane (AB) 13 wt% (19 wt%)
N N + B −
H H H H H H Hydrazine borane (HB) 13 wt% (18 wt%)
N H 2 N
2 H 2 N B-
H H H Guanidinium borohydride (GBH)
(13 wt%)
N H 2 + + H 2 N B B − Ethylene diamine bisborane (EDB) 9 wt% (11 wt%)
Me N + B − H H H
Me Dimethylamine borane (DMAB) 3.4 wt% (7 wt%)
H H H H H H −
Selected AB-related hydrogen storage materials (in gray: protons and hydrides that are initially capable for dehydrogenation. of HB and LiH, giving a blend containing 14.8 wt% of hydrogen, while neat HB contains only slightly more hydrogen (15.4 wt%). This blended HB was capable of releasing approximately 12 wt% of hydrogen at 150 ◦ C in 4.5 h, where the hydrogen produced contained ammonia as the single impurity in concentrations below 1%. The solid residual spent fuel shows reactivity in the presence of water and remains insoluble in organic solvents. The analog hydrazine bisborane adduct contains slightly more hydrogen than HB, but it is not quite suitable as hydrogen storage material, as it revealed to be quite instable at elevated temperatures ( >160 ◦
Another interesting material is GBH and its blends [12]. The protic moieties are carried by the cation, and the counterion contains the hydrides, resulting in a content of 10.8 wt% of hydrogen gas, theoretically, and 10.6 wt%, experimentally. Interestingly, the thermal decomposition is exothermic, therefore above a certain temperature, the hydrogen generation is accompanied by heat formation, and consequently it remains self-sustaining after initial heating. The dehydrogenation of GBH showed almost complete hydrogen release with ammonia as impurity of approximately 5mol%. Similar to the discussed HB, GBH contains two excess protons, therefore blending this material with external hydrides is an elegant way for optimization. EDB is a suitable material, carrying two excess hydrides, thus blending GBH in an efficient manner. The GBH–EDB blend in an equimolar ratio is suitable for release of hydrogen in high yields with a low concentration of ammonia as impurity (Fig. 40.4). A blend in wt% ratio of 40 : 60 yields 10.1 wt% of hydrogen and ammonia impurity as low as 0.026 mol%. Similar to the previously mentioned AB derivatives, the major drawback of this blend is the solid state and the related problems in terms of regeneration of the spent fuel. Neat EDB is also suitable for fast hydrogen generation with high yields and high purities at process temperatures below 200 ◦ C, showing no significant amounts of impurities [43]. The reaction rates of the hydrogen release from EDB are in the same range as the ones for AB-based systems. 40.3.2 Formic Acid and Carbon Dioxide as Cheap Hydrogen Storage Materials The research on the catalytic dehydrogenation of FA resulting in dihydrogen and carbon dioxide has undergone a renaissance in recent years (Fig. 40.5) [3, 7–9, 47–62]. Although FA contains only 4.4 wt% hydrogen, less than that in the previous mentioned AB derivatives, a great advantage is the relatively easy availability owing to the enormous natural feedstock of carbon dioxide; therefore, this represents a very cheap and environmentally benign hydrogen storage material. Interestingly, FA has been used as hydrogen source for hydrogen transfer reactions for several decades [63–74]. 40.4 HYDROGEN STORAGE SYSTEMS INVOLVING IONIC LIQUIDS The physicochemical properties of ILs make this solvent class highly interesting for application in hydrogen storage systems. In general, ILs are known to be highly solvating, and, owing to their tunable polarity, they are capable of solubilizing polar as well as apolar substances, hence molecular hydrogen storage materials. Moreover, owing to the negligible vapor pressure of ILs, the purity of the hydrogen gas stream can be enhanced; on the contrary, volatile solvents and impurities would contaminate the generated hydrogen gas. Besides, as later discussed herein, ILs may promote the dehydrogenation of certain HYDROGEN STORAGE SYSTEMS INVOLVING IONIC LIQUIDS 533 0 5 10 15 20 25 30 35 40 45 50 55 60 0 1 2 3 4 5 6 7 8 9 10 11 12 0 1 2 3 4 5 6 7 8 9 10 11 12 H 2 yield (wt%) H 2 yield NH 3 impurity NH 3 content (mol%) EDB (wt%) Figure 40.4 GBH–EDB self-sustaining thermal decomposition. A mixture containing 60 wt% EDB is not self-sustaining [12]. H 2
2 H O OH [M], base +
Reversible splitting of formic acid in presence of metal catalysts. hydrogen storage materials [17, 75]. ILs are also known to be suitable in stabilizing metal complexes and metal nanoparticle catalysts [9, 18, 22, 24, 76]. Furthermore, basic or acidic cocatalysts can be anchored in a side chain of the IL cation [9]. Most interestingly, lightweight, hydrogen-rich ILs can be directly used as hydrogen storage material, combining the previously mentioned properties [12, 44]. 40.4.1 ILs as Promoting Media for the Dehydrogenation of Ammonia Borane It has been shown that certain ILs themselves are suitable for promoting the dehydrogenation of AB (Figs. 40.6 and 40.7) [17, 75]. IL-promoted thermal dehydrogenation of AB containing 50 wt% of AB in ionic liquid shows that some ILs are suitable for generating at least 2 equiv. of hydrogen gas per AB from the protic and hydridic entities (Fig. 40.7, a–c). The majority of the tested ILs significantly support the dehydrogenation; however, the initial rates of the H 2 -generation are not fully related to the overall amount of released hydrogen. A tendency for the capability to promote dehydrogenation might be tentatively assigned to (i) an anion effect, (ii) coordinating or noncoordinating properties of the anion (and basicity), (iii) the volume of the anion, and (iv) the polarity of the ILs [77–79, 25, 11, 80, 81]. The tendency fits quite well with tested ILs from Entry A to J, where, with the largest anion (Entry J), most likely no promoting effect has been observed. The efficiency for dehydrogenation allows only the generation of 1 equiv of hydrogen gas, which is similar to the thermal decomposition of neat AB (Fig. 40.7). The best results for the IL-promoted H 2 -generation from AB has been obtained with [BMIM][Cl]; therefore, it has been studied in more detail [75]. The studies showed that the reaction rates and the hydrogen yield can be improved (Fig. 40.8). At a temperature as low as 85 ◦ C, the hydrogen yield and reaction rate in the presence of 50 wt% [BMIM][Cl] is significantly higher than in the absence of IL, as also the weight efficiency of both systems, in comparison with that of neat AB (Fig. 40.8A (b)). Moreover, the analysis of the gaseous phase clearly showed that only traces of borazine are detectable. This might be due to the suppression of the borazine formation or the dissolution in the liquid phase. As depicted in the reaction profile (Fig. 40.8A (a)), the influence of the IL [BMIM][Cl] during the dehydrogenation is drastic, as the reaction starts immediately, 534 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY N +
N + N N + N Cl − [BMMIM]Cl Cl − [BMIM]Cl O S O O O − [EMMIM][EtSO 4 ] N + N BF 4 − [BMIM]BF 4 N + N O S O O O − [MMIM][MeSO 4 ] N + N OTf − [BMIM]OTf N +
OTf − [EMMIM]OTf N + N I − [BMIM]I N + N PF 6 − [BMIM]PF 6 N + N Tf 3 C − [EMMIM]Tf 3 C A B C D E F G H I J Efficient promoting ILs (more polar) Inefficient promoting ILs (more apolar)
Structures of promoting ILs for AB dehydrogenation; from A to J (left–right) with decreasing efficiency for promoting the dehydrogenation of AB [75]. 2.5
2 1.5
1 0.5
0 Equivalent of H 2 0
10 15 20 25 Time (h)
a b c d e f g h i j Figure 40.7 H 2 -release measurements of 50 wt% AB (250 mg) at 85 ◦ C in 250 mg of (a) [BMMIM]Cl, (b) [BMIM]Cl, (c) [EMMIM][EtSO 4 ], (d) [BMIM]BF 4 , (e) [MMIM][MeSO 4 ], (f) [BMIM]OTf, (g) [EMMIM]OTf, (h) [BMIM]I, (i) [BMIM]PF 6 , and (j) [PMMIM]Tf 3 C. Reprinted with permission from Reference 75. Copyright (2009) American Chemical Society. 2.5 (a)
(b) 2 1.5 1 0.5
0 0 100 200 300
400 a a b b c d e Equivalent of H 2 2.5
2 1.5
1 0.5
0 Equivalent of H 2 Time (min) 0 100
200 300
400 Time (min) Figure 40.8 (A) H
2 -release measurements at 85 ◦ C of (a) IL-promoted generation with 50 wt% AB in [BMIM]Cl and (b) neat AB. (B) H 2 -release measurements of 50 wt% AB in [BMIM]Cl at (a) 110, (b) 105, (c) 95, (d) 85, and (e) 75 ◦ C. Reprinted with permission from Reference 75. Copyright (2009) American Chemical Society.
HYDROGEN STORAGE SYSTEMS INVOLVING IONIC LIQUIDS 535 no induction period is necessary, and the total yield is much higher. Interestingly, this effect remains even with lower IL loadings (20 wt%). The total hydrogen release at 75 ◦ C is comparable to the one at 85 ◦ C of neat AB, but the IL-based system has a higher reaction rate and no induction period; resulting in a more efficient system. The reaction rate of the AB/IL-system is sensitive toward the reaction temperature, where the maximum of the total yield is almost reached at only 85 ◦
2 ) after circa 4 h. Temperatures above 85 ◦ C do not significantly improve total yield, but the reaction time is much shorter, for example at 110 ◦ C, 2.3 equiv. of hydrogen are generated in less than 30 min. 40.4.2 Ionic Liquid Stabilizers for Nanoscale Catalysts in Dehydrogenation Reactions For the evaluation of the catalytic activity of molecular and nanoscale catalysts for the dehydrogenation of AB, an ideal model system is dimethyl amino borane (DMAB). In contrast to AB, DMAB does not tend to oligomerize as the two methyl groups completely suppress further dehydrogenation as a result of the absence of additional protons, hence only 1 equiv. of hydrogen gas is generated and oligomers do not form. Instead, DMAB derivatives are selectively dehydrogenated, forming only cyclic dimers or trimers [15]. DMAB-based systems are quite interesting for mechanistic studies, but the lower content of accessible hydrogen make them unattractive for application as hydrogen storage materials. DMAB is converted into its cyclic dimer under hydrogen extrusion via diammoniate of diborane as intermediate [82]. Obviously, DMAB can also act as reducing agent, for example, an excess of DMAB reduces dimeric Rh(II) hexanoate resulting in the formation of rhodium nanoparticles and the IL dimethyl ammonium hexanoate (Fig. 40.9). The small-sized (2 nm) nanoparticles exhibit high activity for the dehydrogenation of DMAB at low temperatures with high conversions (Fig. 40.9). The IL dimethyl ammonium hexanoate formed in situ showed that it can act as a stabilizer for the nanocatalysts to prevent agglomerization or deactivation.
Metal complexes are capable of catalytic dehydrogenation of AB and its derivatives. In this manner, reaction rates and applied temperatures can be optimized towards a continuous hydrogen release in portable hydrogen storage tanks [15, 83]. Combining the promoting properties of ILs for dehydrogenation of AB derivatives with those of metal catalysts for dehydrogenation, the catalytic activity, reaction rates, and total hydrogen yield should further improve. It has been shown that the addition of 5 mol% transition metal-based precatalysts (Rh, Ru, Pd, Ni) to a 1 : 1 mixture of [BMIM][Cl] and AB resulted in significantly lower reaction temperatures (45–85 ◦ C), than in the absence of metals. Moreover, hydrogen generation is also improved (Fig. 40.10) [16]. The metal precursors depicted in Fig. 40.10 have not been studied in detail, therefore the active species remain unclear; however, these metal salts and complexes are well known to form metal nanoparticles under reductive conditions, for example, hydrogen atmosphere, and at higher temperatures, some even undergo thermal decomposition in ionic liquid media [23]. Therefore, in some cases at least, metal(0) nanoparticles might play an important role in the dehydrogenation of AB, where, in the case of Rh(0) nanoparticles and DMAB, isolated particles have been proven to be active for the dehydrogenation (see Section 40.3.2) [82]. The time-resolved reaction monitoring showed that most metal compounds improved the reaction rates and total hydrogen yields. Another approach to optimize the selectivity, reaction rates, and total yields, is the formation of AB derivative blends. For example, the blend of AB with sec-butyl amine borane (SBAB) is successfully dehydrogenated in IL with higher selectivity, which means suppression of oligomerization [14]. Instead, it has been observed that this blend in [EMIM][EtSO 4 ] results in the single formation of a cyclic trimer, a substituted monomolecular borazine derivative. On the contrary, oligomeric by-products and insoluble polyaminoboranes are not formed; however, the formation of volatile borazine (from AB dehydrogenation) or contamination by ammonia and diborane in the gas stream has not been reported in detail (Fig. 40.11). The alkyl chain in SBAB results in a loss of efficiency and the active hydrogen content drops to 5 wt%. The metal-catalyzed [Rh(O 2
5 H 11 ) 2 ] 2 + 6 Me
2 HNBH
3 2/n Rh(O) n + 4 [Me
2 H 2 N] + [C 5 H 11 CO 2 ] − + 2 B
2 H 6 + Me 2 N H 2 B NMe
2 BH 2 2 Me 2 HNBH 3 Rh-NPs
IL +toluene, r.t. Me 2 N H 2 B NMe
2 BH 2 2 H 2 +
Rhodium nanoparticle formation in presence of DMAB and subsequent catalytic dehydrogenation of DMAB. Adapted from Reference 82. 536 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY 1.8 1.6
1.4 1.2
1.0 0.8
0.6 0.4
0.2 0.0
0 2 4 6 8 Time (h) 10 12 14 16 18 20 Eq. H 2 [Rh(COD)Cl] 2 RhCl
3 RuCl
3 Ru(COD)Cl 2 Ni(COD)
2 Pd(DBA)
2 NiCl
2 Ni 2 B No catalyst Figure 40.10 Hydrogen evolution from AB-decomposition at 65 ◦ C catalyzed by different metal-catalyst precursors in [BMIM][Cl].Reproduced from Reference 16 with permission of The Royal Society of Chemistry. (See insert for color representation of the figure.) N + H H B − H H
N +
− B − H H H N + H H H
2 N + B − N + B − N + B − H
H H H H H H H −3 H 2 N + B − N + B − N + B − H H H [Ru], 80 °C SBAB
5 wt%
3 [Ru] =
Ru PMe
3 PMe
3 Me 3 P Cl Me 3 P Cl Figure 40.11 Trimerization of SBAB to the corresponding borazine derivative during hydrogen production and the secondary products of SBAB decomposition [14]. N + N N X − (X = Cl
− , BF
4 − , OTf − , NTf
2 − , and HCO 2 − ) Figure 40.12 Structure of the amino-functionalized IL [Et 2 NC
MIM][X] basic cocatalyst [9, 84]. dehydrogenation used 1 mol% of the ruthenium complex [RuCl 2 (PMe
4 ) 3 ] at 80 ◦ C; the active complex species or any intermediates have not been reported. 40.4.4 Functionalized ILs as Cocatalyst for the Decomposition of Formic Acid In the dehydrogenation of FA, the presence of catalytic amounts of a Lewis or Brønsted base as cocatalyst facilitates the dehydrogenation process. Indeed, FA is converted into the corresponding formate, which is then converted into hydrogen gas and carbon dioxide. Instead of adding an external base, an elegant approach is the incorporation of a Lewis base into the side chain of the IL cation, for example, there are the imidazolium-based ILs [Et 2 NC 2 MIM][X] (X = Cl −
4 − , OTf − , NTf 2 − , and HCO 2 − ). This type of IL acts as (i) solvent for the catalyst; (ii) the anchored amine simultaneously acts as a basic cocatalyst to convert the FA into the formate, which is subsequently dehydrogenated by a metal complex, and (iii) the IL is miscible with FA, which guarantees efficient contact between the catalyst and substrate (Fig. 40.12) [9, 84, 85]. The [Et 2
2 MIM] cation has been tested with several anions, such as Cl − , BF
4 − , OTf − , NTf
2 − , and HCO 2 − as cocatalyst for the FA dehydrogenation in the presence of the metal precursor, the dimeric ruthenium precatalyst [(p-cymene)RuCl 2 ] 2 HYDROGEN STORAGE SYSTEMS INVOLVING IONIC LIQUIDS 537 [9, 85]. The total hydrogen yield and reaction rates strongly depend on (i) the reaction conditions in general, (ii) temperature, (iii) type of IL, and (iv) addition or absence of external base. The strongest influence is related to the external base; in the absence of the base, the obtained turnover frequency (TOF) was 36 h −1 and in presence of the external base, the TOF rose to 627 h −1 , suggesting but not proving that the active ruthenium species is monomeric [85]. Further studies revealed that the ruthenium dimer [(p-cymene)RuCl 2 ] 2 undergoes ligand exchange in the presence of FA, and is converted into the corresponding formate/chloro-bridged [(p-cymene)RuCl(HCO 2 - )] 2 and formate/hydride-bridged [(p-cymene)RuH(HCO 2 − )] 2 dimeric complexes [9]. Therefore, it is plausible that these ruthenium dimers play a role in the catalytic cycle (Fig. 40.13). Consequently, the reported TOFs of up to 1684 h −1 without additional base suggest the role of dimeric ruthenium complexes; notably, no monomeric ruthenium species have been detected. Furthermore, the concentration of ionic liquid, respectively base, plays an important role and is contrary to the expectations from systems based on organic solvents. It has been shown that lower IL (base) concentrations result in a more efficient system, than high cocatalyst loadings (basic IL). It is assumable that this is due to the much higher viscosity of the IL in comparison to that of organic solvents, because of the high conversions, resulting in a low FA concentration in the monophasic IL/FA mixture. Consequently, the mixture becomes highly viscous as well, which hinders the hydrogen extrusion and stirring becomes more difficult. Therefore, this homogeneous catalytic system is more efficient at low IL concentrations, with high FA loadings. The robustness of this catalyst/IL/hydrogen fuel triple has been shown in (i) recycling experiments (six runs) and (ii) can be performed without any precautions against moisture or air. The reaction monitoring showed that after the initial high activity in the first run, similar reaction slopes were obtained in the next runs, except in the fifth (Fig. 40.14). Monomeric ruthenium species have been detected. The same catalyst precursor [(p-cymene)RuCl 2 ]
has also been applied for the dehydrogenative conversion of biomass in IL, that is, cellulose derivatives. Certain strongly solvating polar ILs are capable to solubilize these (in common solvents) hardly soluble materials; thus, in these media, biomass-based materials become accessible for homogeneous dehydrogenation Excess HCO 2 H,
2 , −H 2 Ru Ru Cl Cl Cl Cl Ru Ru H Cl H O O Ru Ru H Cl H O O + Cl Cl [Et 2 NC 2 MIM][Cl]
Figure 40.13 Conversion of the [(p-cymene)RuCl 2 ]
with formic acid in IL [Et 2 NC 2 MIM][Cl]. 100 80
40 20 0 0 5 10 15 20 25 Conversion (%) Time (h)
First; Second;
Third; Fourth;
Fifth; Sixth cycle Figure 40.14 Reaction profiles for FA decomposition in presence of [(p-cymene)RuCl 2 ] in IL [Et 2 NC 2 MIM][Cl] at 80 ◦ C for six cycles. Reprinted with permission from Reference 9. Copyright (2010) John Wiley & Sons. 538 IONIC LIQUIDS FOR HYDROGEN STORAGE: OPPORTUNITIES FOR ORGANOMETALLIC CHEMISTRY O OH
OH OH OH −3H 2 O O HO O H +2H
2 O O O OH O OH H
2 CO 2 OH H O [Ru(p-cymene)Cl 2 ] 2 , 180
°C [Bu
4 P][Me–P(OH)O 2 ]
180 °C [Ru]/IL 180 °C + Figure 40.15 Thermal conversion of glucose followed by catalytic dehydrogenation of formic acid in IL [86]. [86]. The studies about dehydrogenation of carbohydrates showed, for example, that glucose does not undergo conversion into the corresponding dehydrogenated product, but thermal decomposition occurred resulting in FA, which is subsequently converted into CO 2 and H 2 (Fig. 40.15). The high process temperatures make it crucial to apply ILs with high thermal stability.
The GBH derivative Me-GBH is mono-methylated at one nitrogen of the guanidinium cation. This simple modification has a massive effect on the physical properties of this salt; in particular, the melting point is depressed resulting in a room temperature ionic liquid with high hydrogen content that is suitable as hydrogen storage material (Fig. 40.16) [44]. Me-GBH contains less hydrogen accessible for dehydrogenation in comparison to GBH (11 wt%). The methyl group results in distortion of the cation symmetry, which is responsible for the lower melting point of this salt [87]. Me-GBH represents the first room temperature IL with a melting point of −5 ◦ C which is suitable as hydrogen storage material. It is easily dehydrogenated and has a hydrogen content of 9.0 wt%. Unfortunately, probably owing to high viscosity of the material, kinetic studies revealed that the simple thermal dehydrogenation at 75 ◦ C is rather slow and still not satisfactory for application (2.3 equiv H 2 ). The addition of Wilkinson’s catalyst [Rh(PPh 3 ) 3 Cl] improved the yield to 2.9 equiv. of hydrogen. Interestingly, the studies showed that the decomposition of the guanidinium salts also results in mass loss that exceed the theoretical capacity. Thus, this cannot be assigned to hydrogen generation. At temperatures higher than 120 ◦ C, the gas stream is contaminated with ammonia, which indicates decomposition of the material. The quantification of the total hydrogen yield was assigned to 9.0 wt% on the basis of thermogravimetric analysis (TGA) and volumetric measurements. The dehydrogenation product of Me-GBH is solid and insoluble in the IL. A different approach focused on anchoring a cyclohexyl ring into a side chain of an imidazolium cation [76]. The imidazolium salts were mixed with palladium on carbon and heated to greater than 230 ◦ C, then with dehydrogenation, an aromatic ring started to form. Hydrogenation was then performed between 70 and 90 ◦ C, demonstrating the reversibility (Fig. 40.17). Although the hydrogen content is too low for application and the process temperature is quite high (230–300 ◦ C), this work represents the first in which an ionic liquid itself functioned as hydrogen storage material. The thermal stability of some of the employed ILs is remarkable. C +
2 NH H 2 N B − H H H H H 3 C
Methyl guanidinium borohydride (Me-GBH). N N Me n N N Me
NTf 2
2 T = 230–300 °C inert atmosphere p = 30–50 atm T = 70–90 °C Cat: Pt, Pd, Ru t = 100 h NTf
2 <2 wt% Figure 40.17 Cyclohexyl-ILs for hydrogen storage [76]. REFERENCES 539 40.5 CONCLUSION AND OUTLOOK In summary, the number of publications in the research field of molecular hydrogen storage materials is still quite limited. Some results are promising, but still provide some challenges to be overcome. ILs have some physicochemical properties that might be helpful in resolving some aspects. ILs support hydrogen generation in several ways: (i) they are highly thermally-stable solvents with low vapor pressure, which allows elevated temperatures without contamination of the gas stream, (ii) ILs promote the dehydrogenation of AB derivatives, (iii) a cocatalyst can be anchored into side chains (i.e., Lewis bases), and (iv) highly solvating ILs are promising in the search for liquid hydrogen storage materials that remain in liquid state after hydrogen release. The latter aspect is also helpful for the regeneration of the material by hydrogenation with molecular or nanoscale catalysts. The single drawback of ionic liquids as additives to neat or blended hydrogen-rich materials is the additional weight, decreasing the gravimetric efficiency. Nevertheless, in certain cases, this drawback can be fully compensated, as IL-based systems give much higher total hydrogen yields and higher reaction rates than neat materials. In addition, for sure, the development of novel hydrogen-rich ILs for the application as hydrogen storage material is another interesting challenge to be resolved.
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