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Regeneration of Aluminum Hydride Using Trimethylamine David Lacina, James Reilly, Yusuf Celebi, James Wegrzyn, John Johnson, and Jason Graetz* Department of Sustainable Energies Technology, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ABSTRACT: Aluminum hydride is an attractive reducing agent and energy storage compound possessing a low decomposition temperature and a high gravimetric and volumetric hydrogen density. However, it is thermodynamically unstable at room temperature and requires extremely high pressures to form the hydride from aluminum and hydrogen gas. Here, we describe an alternate method of synthesizing AlH3 using Ti-catalyzed Al powder, H2, and trimethylamine (TMA) to form an alane adduct. The formation of trimethylamine alane occurs at modest hydrogen pressures (∼100 bar), forming the 2:1 bis complex (2 trimethylamine/AlH3). Along with the hydrogenation product, mono (1:1) and bis (2:1) standards of TMA-AlH3 were prepared and characterized using X-ray diffraction and Raman spectroscopy. X-ray absorption spectroscopy of the reaction products showed that the Ti catalyst remains with the unreacted Al powder after hydrogenation and is not present in the alane adduct. We also demonstrate that TMA can be transaminated with triethylamine to form triethylamine alane, which can easily be separated to recover AlH3.
’ INTRODUCTION Aluminum hydride (AlH3) is a promising energy storage material due to its low decomposition temperature ( 7 103 bar,1-5 which is too high for practical applications. Aluminum hydride is normally produced using an ethereal reaction between LiAlH4 and AlCl3 that results in stable byproducts that are costly to separate. We propose and demonstrate a low-energy regeneration route to produce aluminum hydride by first stabilizing AlH3 through the low-pressure formation of alane using a tertiary amine ligand to form an adduct with Ti-catalyzed aluminum and gaseous hydrogen, and then separating that adduct to recover pure AlH3.6,7 Murib et al.8 first demonstrated a similar regeneration approach in a process using high-pressure, high-energy mechanical milling in place of the Ti catalyst. In the approach discussed here, the hydrogenation reaction requires minimal agitation at low pressures, and reactants, such as the Ti catalyst, can be recycled, which results in a lower energy cost for this regeneration route. Here, we describe the formation of trimethylamine alane (TMAA) from trimethylamine (TMA = C3H9N), Ti-catalyzed Al, and H2. The challenge with this regeneration procedure is that the temperatures required for adduct separation are typically greater than the AlH3 decomposition temperature (∼100 °C).1 One solution is to exchange the amine ligand used in the formation of AlH3 with a ligand that forms a less stable bond and is easier to separate. One example is triethylamine (TEA = C6H15N), which can be separated under a partial vacuum at 70 °C.8 It is important to note, however, that, although TEAAlH3 can be prepared from a reaction with LiAlH4 or AlH3,6 the amine alane does not form directly from catalyzed Al, TEA, and H2 at moderate pressures. r 2011 American Chemical Society
Trimethylamine alane is known to form two polymorphs, a 1:1 complex (AlH3 3 (C3H9N)) and a 2:1 bis complex (AlH3 3 (C3H9N)2),9 as shown in Figure 1. Standards of both complexes were synthesized from AlH3 and trimethylamine, with the products characterized by Raman and Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). A small amount of a Ti catalyst is essential for low-pressure hydrogenation of Al with an amine to form an amine alane.6,7 However, Ti also catalyzes the decomposition reaction, and the presence of even a small amount of Ti in the reaction product makes it extremely difficult to separate the adduct and recover intact AlH3. Therefore, a detailed characterization of the Ti catalyst before and after hydrogenation is critical for the development of a complete regeneration procedure. In this study, X-ray absorption spectroscopy was used to determine the location of the Ti atoms in the reactants, products, and the unreacted material.
’ EXPERIMENTAL SECTION Materials. The following materials were obtained from SigmaAldrich: tetrahydrofuran (THF) (99.9% anhydrous), diethyl ether (99.7% anhydrous), toluene (99.8% anhydrous), trimethylamine (99.5%), AlCl3 (99.999%), LiAlH4 (reagent grade, 95%), and TiCl3 (99.995% trace materials basis). Hydrogen (99.95%) was obtained from Praxair. R-AlH3 was prepared using the Brower method.10 The synthesis of Ti-catalyzed aluminum, identified as Al*, can be found in another publication.7,11 Received: November 6, 2010 Revised: January 4, 2011 Published: February 16, 2011 3789
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Figure 1. Depiction of molecular structures for TMA alane bis and mono complexes.
Methods. Hydrogenation reactions were performed in a 300 mL stainless-steel stirred reactor obtained from Parr Instruments connected to an associated gas/vacuum manifold; the operating procedures have been outlined elsewhere.6,7 The reactor was charged with 1 g of Al* (0.037 mol) and 100 mL of diethyl ether (Et2O) and sealed in an argon-filled glovebox. The reactor was attached to the gas/vacuum manifold and quickly evacuated of argon. Because trimethylamine is a gas at room temperature (vapor pressure = 1.9 bar (absolute)), the system was cooled to -70 °C using acetone and dry ice and 10.5 mL (0.119 mol) of trimethylamine was condensed into the reactor. The system was brought back to room temperature, and 65-115 bar of hydrogen gas was added to the reactor. An internal stirrer was activated, which results in an immediate decrease in the hydrogen pressure due to the dissolution of H2 in the liquid. The internal temperatures of the reactor (gas and liquid) and manifold (kept at room temperature) were recorded along with the reactor pressure. After the reaction, the reactor was returned to the glovebox and the liquid was filtered using a 0.7 μm glass fiber filter combined with a diatomite filter aid to remove the unreacted Al*. TMAA is soluble in diethyl ether, and the solution was transferred to a Buchi rotary evaporator where the solvent was removed by vacuum distillation at room temperature to recover a solid white powder of TMAA. Samples were studied at room temperature by Raman, FTIR, and X-ray diffraction. Raman spectroscopy was carried out using a Witec Alpha 300 confocal Raman microscope utilizing a helium-neon laser with an excitation wavelength of 532 nm, a 20 objective, and a 38.6 μm spot size. The resolution of the microscope is 3 cm-1. FTIR spectra were acquired with a PerkinElmer Spectrum One spectrometer. Solid trimethylamine alane was pressed into a KBr pellet prior to analysis with FTIR. Powder X-ray diffraction spectra were obtained using a Philips X-ray diffractometer with Cu KR radiation. Ti K-edge spectra were collected at beamline X-3B at the National Synchrotron Light Source. Spectra were recorded in fluorescence yield mode using a 13-element Ge detector with an energy resolution of 3 eV for all samples. All samples were loaded in a glovebox and sealed in a sample holder under argon. Studies were performed with the sample holder in a chamber under vacuum and oriented at 45° to the incident X-ray beam. Because of the low Ti concentration in these materials, the spectra were not corrected for self-absorption.
Figure 2. Reactor pressure during hydrogenation of Al* and trimethylamine to form trimethylamine alane. The inset shows the reactor pressure for a reaction using uncatalyzed aluminum.
from 105 to 100 bar is due to the direct formation of trimethylamine alane according to the following reaction (in Et2O): Al þ 2ðC3 H9 NÞ þ 3=2H2 f AlH3 3 ðC3 H9 NÞ2
FTIR spectra were collected from the product of reaction 1 before and after solvent removal (not shown) where the presence of the Al-H stretch mode at 1710 cm-1 in both liquid and solid products is a clear indication of alane formation.12 The total yield of AlH3 is 4.03 10-2 mol (based upon the H2 uptake), which was calculated using the gas law PV/ZRT = n, where Z is the compression factor.13 This corresponds to a conversion of 72% of the initial Al to AlH3 (in the form of TMA-AlH3). Similar hydrogenation reactions using other amines have shown conversion efficiencies as low as 50% and as high as 96%.6,7 Although the total product yield is a function of a number of parameters (e.g., hydrogen pressure and time), it is likely that some of the Al is unable to react because it is associated with the Ti catalyst or a surface oxide. An identical experiment using uncatalyzed aluminum, shown in the inset of Figure 2, indicates no alane formation due to the lack of pressure drop after initial hydrogen dissolution in the solvent. The amount of aluminum recovered was equal to the starting amount, and a characterization (XRD and FTIR) of the recovered solid showed no indication of AlH3. Similar results have been obtained from the formation of other alane and alanate adducts, such as triethylenediamine alane (TEDAA),7 dimethylethylamine alane (DMEAA),6 quinuclidine alane,14 and LiAlH4-THF,11,15 and suggest that the Ti catalyst is a necessary component for lowpressure hydrogenation of aluminum. Characterization. To help characterize the composition of the TMAA prepared by direct hydrogenation, standards of the mono and bis forms of TMAA were synthesized directly from TMA and R-AlH3 as shown below:
’ RESULTS AND DISCUSSION Formation by Direct Hydrogenation. The uptake of hydrogen from a slurry of Al*, TMA, and diethyl ether is shown in Figure 2. The initial pressure drop from 116 to 105 bar is attributed to H2 dissolution in the solvent. The slower pressure decrease
ð1Þ
C3 H9 N þ AlH3 f AlH3 3 C3 H9 N
ð2Þ
2C3 H9 N þ AlH3 f AlH3 3 2ðC3 H9 NÞ
ð3Þ
The synthesis was carried out in a stirred reactor at 1 atm and 23 °C over a period of 4 h. The insoluble, unreacted material was 3790
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Figure 3. (a) XRD patterns and (b) Raman spectra from the 1:1 TMAA standard (top), the 2:1 TMAA standard (middle), and the dried hydrogenation product (bottom).
removed by filtration, and the filtrate was vacuum-dried to recover the solid TMAA alane product. The XRD pattern from the direct hydrogenation adduct is shown in Figure 3 and indicates no trace of Al*, which was removed by filtration. The XRD patterns from standards of mono(trimethylamine) alane and bis(trimethylamine) alane, prepared by reactions 2 and 3, respectively, are also illustrated in Figure 3a with their Raman spectra shown in Figure 3b. The expected XRD peak positions of both complexes of TMAA, shown in Figure 1, were calculated from the data of Humphries (mono),9 Atwood (mono),16 and Heitsch (bis)17 using an orthorhombic space group (cmca) with a = 10.09 Å, b = 8.86 Å, and c = 12.54 Å. The data calculated for bis(trimethylamine) alane are in agreement with the synthesized standard XRD pattern. However, the peak positions calculated for mono(trimethylamine) alane, based upon the published structural data (space group R3c, a = 14.08 Å and R = 90.08°),9,16 did not result in a pattern that matched the 1:1 standard. Mono(trimethylamine) alane is known to be a severely disordered dimer in the solid phase,16 which makes it difficult to accurately calculate the XRD peak positions. Raman spectra of the standards formed by reactions 2 and 3 are consistent with vibrational spectra for other 1:1 and 2:1 complexes.12,18 These data confirm that the mono(trimethylamine) alane standard is the 1:1 complex due to the strong Al-H stretch mode at 1795 cm-1.12,18-20 The XRD pattern from the direct hydrogenation (reaction 1) product closely matches the pattern from the bis complex (formed by reaction 3). The peak positions of the strongest reflections for the mono and bis adducts (standards) are listed in Table 1. The four strongest peaks for bis-TMAA (13.9°, 15.2°, 17.8°, and 19.4°) correspond nicely with the four main peaks observed in the hydrogenation product, whereas the strongest reflection for mono-TMAA (18.3°) is not present in the reaction product pattern. This supports the Raman and FTIR results, which also suggest that the direct hydrogenation product is primarily bis(trimethylamine) alane. Location of Titanium. The reaction precursors and products were characterized using X-ray absorption spectroscopy to determine the location and concentration of the Ti (before and after hydrogenation). The reaction product, TMAA, is soluble in Et2O and is easily separated from the unreacted solids
Table 1. XRD Peak Positions and Normalized Peak Intensity for the 10 Strongest Reflections of the Mono(trimethylamine) Alane Standard and Bis(trimethylamine) Alane Adduct mono(C3H9N) 3 AlH3 2θ
intensity
bis(C3H9N) 3 AlH3 2θ
intensity
5.4
0.059
13.9
0.241
15.9 18.3
0.045 1.000
15.2 17.8
0.214 0.159
19.5
0.272
19.4
1.000
20.5
0.022
21.3
0.032
21.7
0.077
22.5
0.042
23.8
0.096
27.9
0.027
29.5
0.062
29.4
0.038
34.6
0.094
34.5
0.056
38.3
0.162
38.1
0.091
in the reactor. The Ti K-edge spectra and the Fourier transform of the k2-weighted EXAFS data (with phase shift correction) are shown in Figure 4 for the starting Al*, the TMAA product, and the unreacted solids recovered from the reactor after hydrogenation. The edge onset (4966 eV) and the general shape of the Ti K edge for both the starting Al* and the unreacted solids are similar, indicating no change in the local atomic or electronic structure around the Ti. In addition, the k2-weighted EXAFS data in Figure 4b show a similar local coordination with the dominant peak at 2.65 Å, indicating Ti-Al pairs similar to TiAl3. Previous XAS experiments on Ti-catalyzed sodium alanate (NaAlH4) showed the formation of a similar Ti-Al species.21 These results suggest that much of the Ti resides in the unreacted Al with a similar local atomic structure to the starting Al*, most likely with a higher Ti concentration. The solid TMAA product was recovered by, first, filtering off the unreacted material to recover a clear TMAA solution (in Et2O), which was then vacuum-dried. The spectra for the TMAA adduct shows no increase in intensity at the expected Ti K-edge energy (4966 eV), which indicates that no detectable titanium is present in the adduct. Experiments with other soluble tertiary amine alane compounds (not published) also show no Ti in the 3791
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Figure 4. (a) Ti K edges from the starting Al*, unreacted Al, and the TMAA solid adduct. (b) Fourier transform of the k2-weighted EXAFS data showing the local Ti coordination for all samples.
recovered solid adduct.22 These results indicate that the Ti facilitates the hydrogenation reaction but tends to stay with the unreacted Al* after formation. A similar study by Graham et al.23 looked at the morphology and microstructure of Al*, as well as the elemental distribution of Ti, after the hydrogenation of TEDA and Al*. Although the recovered solids clearly showed the presence of Ti, in this case, the solid adduct (TEDA alane) was insoluble and was not separated from the unreacted Al*. It is likely that the Ti was associated with the unreacted Al* and not directly incorporated into the TEDA alane. This is supported by the fact that Graham et al. found that the concentration of Ti in the nodular Al* particles was higher than in the starting Al*, again suggesting that the Ti was concentrated in the unreacted Al*. Transamination. Trimethylamine is effective at stabilizing AlH3 and facilitates the formation of alane directly from Al* and H2 under ambient conditions (the first step in the regeneration process). Attempts to separate trimethylamine alane to recover pure aluminum hydride AlH3 3 2ðC3 H9 NÞ f AlH3 þ 2C3 H9 N v
ð4Þ
were unsuccessful and resulted in the decomposition of the alane and loss of H2. This is not surprising because the free energy of separation (ΔGsep) for bis-TMAA is high (ΔGsep = 27.6 kcal/mol H2 at 298 K (calculated)),24 which makes it difficult to recover AlH3 at low temperatures. However, other less stable alane amine adducts, such as triethylamine alane (TEAA), have a lower separation energy (ΔGsep = 14.3 kcal/mol H2 (calculated))24 and can be separated by heating the liquid under an inert gas (N2) flow.6,8 Therefore, the AlH3 can be separated from TMAA if the TMA ligand can be exchanged with TEA to form TEAA.8 The “uphill” amine exchange reaction (transamination) is driven by preferentially pulling off trimethylamine (gas) in the presence of TEA (liquid). The progress of the transamination reaction can be followed using FTIR because the position of the Al-H stretch peaks for TMAA and TEAA are separated by 67 cm-1. A series of FTIR spectra are shown in Figure 5 with the initial direct hydrogenation product (Al-H peak at 1710 cm-1) on the bottom and a spectrum from a standard sample of TEAA (Al-H stretch at 1777 cm-1) on the top. Triethylamine was added to the direct hydrogenation TMAA product in diethyl ether and stirred for several hours. The transamination process is initiated upon
Figure 5. FTIR spectra for the transamination of TMAA and TEAA at different stages of the amine exchange process showing (top) the TEAA standard, (middle) post-transamination, and (bottom) TMAA.
heating the liquid to 55 °C under a sweep of nitrogen gas that keeps the pressure slightly above 1 bar. The formation of triethylamine alane is indicated by the growth of a new peak at 1777 cm-1 (middle spectrum), and after 3 h, the transamination reaction is nearly complete with the Al-H peak position indicating that the alane is predominately TEAA. Once the triethylamine has replaced the trimethylamine, the TEAA (liquid) is then separated to recover AlH3. The complete low-energy regeneration procedure for AlH3 using TMA is shown below: Hydrogenation : Al þ 2ðC3 H9 NÞ þ 3=2H2 f AlH3 3 2ðC3 H9 NÞ Transamination :
AlH3 3 2ðC3 H9 NÞ þ C6 H15 N
f AlH3 3 C6 H15 N þ 2ðC3 H9 NÞ v Separation :
AlH3 3 C6 H15 N f AlH3 þ C6 H15 N v
ð5Þ
ð6Þ ð7Þ
The regeneration steps above have been independently verified, but the integration of all three steps remains a challenge because 3792
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’ CONCLUSIONS We demonstrate the formation of bis(trimethylamine) alane by direct hydrogenation at low pressures. The trimethylamine alane products were investigated using X-ray diffraction, Raman and Fourier transform infrared spectroscopy, and X-ray absorption spectroscopy. XAS spectra confirmed that the Ti catalyst promotes the hydrogenation reaction and is present in the Al* both before and after the reaction, but not in the solid TMAA adduct. A comparison of XRD patterns from synthesized standards of monoand bis-TMAA and the respective reaction products confirms the formation of bis(trimethylamine) alane after hydrogenation. By incorporating a transamination step, the TMA ligand can be exchanged with TEA to form TEAA, which is easily separated to recover AlH3. This simple three-step process may be a useful lowenergy pathway for the regeneration of AlH3. ’ AUTHOR INFORMATION
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(12) Greenwood, N. N.; Thomas, B. S. J. Chem. Soc. A 1971, 814–817. (13) Report LA-2271, Los Alamos Scientific Laboratory of the University of California, 1958. (14) Lacina, D.; Wegrzyn, J.; Reilly, J. J.; Johnson, J.; Celebi, Y.; Graetz, J. J. Alloys Compd. 2010. DOI: 10.1016/j.jallcom.2010.10.010. (15) Liu, X. F.; McGrady, G. S.; Langmi, H. W.; Jensen, C. M. J. Am. Chem. Soc. 2009, 131, 14 5032. (16) Atwood, L.; Bennett, F. R.; Elms, F. M.; Jones, C.; Raston, C. L.; Robinson, K. D. J. Am. Chem. Soc. 1991, 113, 8183. (17) Heitsch, C. W.; Nordman, C. E.; Parry, R. W. Inorg. Chem. 1963, 2, 508. (18) Dautel, V. R.; Zeil, W. Z. Elektrochem. 1960, 64, 1234. (19) Ehrlich, R.; Lichstein, B. M.; Perry, D. D.; Young, A. R. Inorg. Chem. 1963, 2, 650–2. (20) Dilts, J. A.; Ashby, E. C. Inorg. Chem. 1970, 9, 855–62. (21) Chaudhuri, S.; Graetz, J.; Ignatov, A.; Reilly, J. J.; Muckerman, J. T. J. Am. Chem. Soc. 2006, 128, 11404. (22) Lacina, D.; Reilly, J. J.; Graetz, J. Unpublished results, 2010. (23) Graham, D. D.; Graetz, J.; Reilly, J.; Wegrzyn, J.; Robertson, I. M. J. Phys. Chem. C 2010, 114, 15207. (24) Wong, B. M.; Lacina, D.; Nielsen, I. M. B.; Graetz, J.; Allendorf, M. D., submitted for publication, 2010.
Corresponding Author
*Phone: 1-631-344-3242. Fax: 1-631-344-2359. E-mail: graetz@ bnl.gov.
’ ACKNOWLEDGMENT D.L. and J.G. acknowledge support from the Office of Basic Energy Sciences and J.R., Y.C., J.J., and J.W. from the Metal Hydride Center of Excellence, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, under Contract No. DE-AC02-98CH1-886. Synchrotron studies were supported by the Center for Synchrotron Biosciences Grant, P30-EB009998, from the National Institute of Biomedical Imaging and Bioengineering (NIBIB). The authors thank Weimin Zhou for her assistance. ’ REFERENCES (1) Graetz, J.; Reilly, J. J.; Kulleck, J. G.; Bowman, R. C. J. Alloys Compd. 2007, 446-447, 271. (2) Saitoh, H.; Machida, A.; Katayama, Y.; Aoki, K. Appl. Phys. Lett. 2008, 93, 151918. (3) Baranowski, B.; Tkacz, M. Z. Physik. Chem. (N.F.) 1983, 135, S27–S38. (4) Konovalov, S. K.; Bulychev, B. M. Inorg. Chem. 1995, 34, 172. (5) Graetz, J.; Reilly, J. J.; Yartys, V. A.; Maehlen, J. P.; Bulychev, B. M.; Antonov, V. E.; Tarasov, B. P.; Gabis, I. E. J. Alloys Compd. 2010. DOI:10.1016/j.jallcom.2010.11.115. (6) Lacina, D.; Wegrzyn, J.; Reilly, J. J.; Celebi, Y.; Graetz, J. Energy Environ. Sci. 2010, 3, 1099. (7) Graetz, J.; Chaudhuri, S.; Wegrzyn, J.; Celebi., Y.; Johnson, J. R.; Zhou, W.; Reilly, J. J. J. Phys. Chem. C. 2007, 111, 19148–52. (8) Murib, J. H.; Horvitz, D. Synthesis of Aluminum Hydride and Tertiary Amine Adducts Therof. U.S. Patent 3,642,853, February 15, 1972. (9) Humphries, T. D.; Sirsch, P.; Decken, A.; McGrady, G. S. J. Mol. Struct. 2009, 923, 13. (10) Brower, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L.; Snover, J. A.; Terada, K. J. Am. Chem. Soc. 1976, 98, 2450. (11) Graetz, J.; Wegrzyn, J.; Reilly, J. J. J. Am. Chem. Soc. 2008, 130, 17790. 3793
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