Aluminum Hydride Separation Using N-Alkylmorpholine - The Journal

Jun 20, 2013 - We describe experimental and theoretical studies of several amine·alane adducts for alane separation. First, N-alkylmorpholine·alane ...
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Aluminum Hydride Separation Using N‑Alkylmorpholine Chengbao Ni,*,† Liu Yang,‡ James T. Muckerman,‡ and Jason Graetz†,§ †

Sustainable Energy Technologies Department and ‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States S Supporting Information *

ABSTRACT: We describe experimental and theoretical studies of several amine·alane adducts for alane separation. First, N-alkylmorpholine·alane adducts (NMM·AlH3 and NEM·AlH3; NMM = N-methylmorpholine, NEM = N-ethylmorpholine) were synthesized and characterized by NMR, IR, and XRD studies. Because of the bifunctionality (or dual coordination mode) of N-alkylmorpholine, NMM·AlH3 and NEM·AlH3 exhibit significantly improved thermal stability compared with the related amine·alane adducts. In the solid state, both NMM·AlH3 and NEM·AlH3 are polymers, which readily dissociate into monomers in donor solvents, as suggested by IR spectroscopy. In addition, the cost- and energy-effective transamination of (amine)2·AlH3 with NMM (or NEM) has been achieved. Because of the fast reaction kinetics, the transamination reaction could be combined with hydrogenation of Al metal to prepare NMM·AlH3 in a single step, further improving the efficiency of the process. Moreover, the thermal decomposition pathways of NMM·AlH3 and NEM·AlH3 have been elucidated. While NMM·AlH3 decomposes to Al metal directly, NEM·AlH3 can be selectively decomposed to give AlH3 under certain conditions. The dramatically different thermal properties of N-alkylmorpholine·AlH3 could be attributed to the different steric hindrance and basicity of N-alkylmorpholine compounds. Compared with the Et3N/Et3N·AlH3 process, our new approach using N-alkylmorpholine significantly improves the kinetics, selectivity, yields, and energy efficiency of AlH3 recovery. Lastly, theoretical calculations of molecular geometries, absolute free energies, Al−H vibrational frequencies, and thermodynamics of amine·alane adducts with different structures are in good agreement with experimental observations and provide further information for the interactions between amines and AlH3.

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INTRODUCTION AlH3 has been a subject of studies for decades because of its wide applications in materials science.1−5 In recent years, extensive effort has been devoted to the energy-efficient regeneration of AlH3, due mainly to its favorable energystorage properties: high volumetric (1.48 g/mL) and gravimetric (∼10%) hydrogen capacities and moderate decomposition temperature (2 equiv) gave only NMM·AlH3 after standard workup. NEM·AlH3. A procedure similar to that used for NMM·AlH3, in which AlH3 (0.060 g, 2.0 mmol) and NEM (0.253 g, 0.275 mL, 2.2 mmol) were used, gave NEM·AlH3 as a white solid (0.276 g, 95.2% based on AlH3). 1H NMR (400.14 MHz, toluene-d8): δ 0.96 (t, 3J = 7.2 Hz, 3H, CH3), 2.22−2.30 (m, 6H, NCH2), 3.60 (t, 3J = 4.8 Hz, 4H, OCH2), 3.98 (s, br, 3H, Al-H). 13C NMR (100.62 MHz, toluene-d8): δ 9.8 (CH3), 53.6 (NCH2CH2), 55.3 (CH2CH3), 64.3 (OCH2). Al−H (cm−1): 1783 (solution), 1733 and 1782 (solid). Regardless of the amount of NEM, NEM·AlH3 was the only isolated product by this method. Transamination. Slightly modified procedures17 were employed for the NMM reactions. At room temperature, a mixture of NMM (10−15 mmol) and (Me3N)2·AlH3 (or (EtMe2N)2·AlH3, 2 mmol) in Et2O (2 mL) was stirred under reduced pressures (∼ 20 mmHg). Upon evaporation of liquid materials, a white solid was obtained, which was identified to be NMM·AlH3 by NMR, IR, and XRD analyses. The NEM reaction may require multiple additions of NEM (5−10 mmol, 3−5 times) to achieve complete reaction. Transamination of NMM·AlH3 (1.5 mmol) with NEM (10− 15 mmol) was conducted in toluene (2 mL) at 65 °C under reduced pressures. The process was monitored by IR and XRD analyses. With repeated additions of NEM (three to five times), NEM·AlH3 was obtained in ca. 80% yield. Hydrogenation of Al* in the Presence of NMM (or NEM). The experimental setup is similar to the reported procedures.19 Typically, NMM or NEM (50 mmol), Al* (0.500 g, 18.5 mmol), and Et2O (90 mL) were sealed in a 300 mL stainless Parr reactor, which was purged with H2 twice before a final pressure of ca. 1000 psi was applied. Upon mechanical stirring, dissolution of H2 gas occurred as the reactor pressure dropped initially. However, no further decrease in pressure was observed, even after 2 days. The reaction mixture was filtered in the glovebox; the colorless filtrate showed no Al−H absorptions in the IR spectra. Also, no products were obtained after evaporation of solvents. Combining Hydrogenation with Transamination. The experimental setup is similar to the reported procedures.19 Typically, EtMe2N (5.0 mL, 3.3 g, 45.2 mmol), Al* (0.500 g, 18.5 mmol), NMM (5 mL, 4.61 g, 20.7 mmol), and Et2O (90

varying amounts of Al metal.18 In contrast, the ether·alane adducts (THF·AlH321 and Et2O·AlH322) decompose cleanly to AlH3 polymorphs under heat and vacuum. Unfortunately, these ether·alane compounds cannot be obtained via hydrogenation of Al metal or ligand-exchange reactions of (amine)2·AlH3 with THF or Et2O.18,23 In search of a suitable Lewis base that facilitates the energyefficient separation of AlH3, we turned to bifunctional molecules that have both a tertiary amine group and an ether group because such a combination may promote both transamination and thermal decomposition. In this regard, Nalkylmorpholine derivatives (NMM and NEM, NMM = Nmethylmorpholine, NEM = N-ethylmorpholine, Figure 1a) are

Figure 1. Molecular structures of (a) NMM and NEM and (b) amine·alane adducts. Type I: 2:1 adduct; type II: 1:1 monomer; type III: 1:1 dimer. L is a tertiary amine.

ideal candidates because of their bifunctionality (or dual coordination mode), which was observed in the crystal structure of NMM·AlH3.24 While the synthesis of NMM·AlH3 was reported,24 its thermal properties and potential applications in alane regeneration have received little attention. We report the synthesis and characterization of N-alkylmorpholine·AlH3 compounds, the cost- and energy-effective transamination of (amine)2·AlH3 with N-alkylmorpholine, and the separation of alane from NEM·AlH3. In addition, computations of molecular geometries, absolute free energies, Al−H vibrational frequencies, and thermodynamics of amine·alane adducts with different structures were performed to better understand the interactions between amines and alane. The calculated results are in good agreement with experimental observations.

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EXPERIMENTAL SECTION General. All materials handling was carried out in an argonfilled glovebox. All chemicals were obtained from commercial vendors unless otherwise noted. Liquid chemicals were dried over 3 Å molecular sieves prior to use. Al* and γ-AlH3 were prepared according to published procedures.19,22 NMR spectra were recorded with use of a Bruker 400 spectrometer in toluene-d8. 1H (or 13C) NMR resonances are reported in ppm relative to solvent resonances, taken as 7.13 (or 137.91) for toluene. Infrared spectra were recorded on a Perkin-Elmer SpectrumOne spectrometer either in a 0.1 mm CaF2 cell (liquid) or in a KBr pellet (solid). X-ray powder diffraction (XRD) patterns were collected on a Phillips Cu Kα diffractometer. Yields indicate the amount of isolated compound; reactions are unoptimized. (Me3N)2·AlH3. This compound was obtained as a white solid by two methods: direct reaction of Me3N with γ-AlH325 and hydrogenation of Al* in a mixture of Me3N and Et2O.18 1H NMR (400.14 MHz, toluene-d8): δ 2.17 (s, 18H, CH3), 3.41 (s, br, 3H, Al-H). 13C NMR (100.62 MHz, toluene-d8): δ 47.5 (CH3). Al−H (cm−1): 1701 (solution), 1706 (solid). When 14984

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Table 1. Binding Free Energies (kcal·mol−1) at 298.15 K in Et2O for Alane Adducts of Me3N and EtMe2N Computed at the MP2 and B3LYP Levels of Theory with BSSE Correction

mL) were sealed in a 300 mL stainless Parr reactor, which was purged with H2 twice before a final pressure of ca. 1000 psi was applied. The mixture was stirred mechanically (∼300 rpm) for 1 to 2 days, by which time the pressure dropped significantly. Filtration of the mixture and evaporation of the filtrate in the glovebox gave a white solid, which was confirmed to be NMM·AlH3 by NMR, IR, and XRD analyses. The yield was 57.2% based on Al*. Similar experiments with NEM might require multiple additions of NEM during the evaporation step. Thermal Decomposition. In a Buchi vacuum furnace inside of a glovebox, NMM·AlH3 or NEM·AlH3 (1.5 mmol, neat or in toluene) was held under reduced pressures (ca. 20 mmHg) at different temperatures. The process was monitored by IR and XRD studies. Theoretical Calculations. Geometry optimizations of isolated amine molecules and their alane adducts were first carried out at the B3LYP29/6-31+G(d,p)30−32 level of theory in the gas phase. Three different structures (Figure 1) were examined. For species that do not have a gas-phase standard state, the geometry was reoptimized in a CPCM33−35 treatment of Et2O using UAHF radii. A vibrational frequency calculation on each species in Et2O at the same level of theory was performed to confirm the stationary point. The zero-point energy and thermal correction (ΔGoZPE&Th(S)) to the electronic energy were obtained as the difference between the free energy, o o , and the optimized electronic energy,EB3LYP,(S) . GB3LYP,(S) Finally, a more accurate electronic energy of each species in Et2O (EoMP2,(S)) was obtained by a single-point electronic energy calculation of the solution-phase B3LYP-optimized geometry carried out at the MP2/6-311+G(2d,2p)36−39 level in a CPCM treatment of Et2O using UAHF radii. The absolute free energy * = GoMP2(S) + of each adduct in Et2O is then calculated as GMP2(S) ΔGo→* = EoMP2(S) + ΔGoZPE&Th(S) + ΔGo→*. The standard-state correction (ΔGo→ = 1.894 kcal·mol−1), the work required to compress an ideal gas from a molar volume of 24.47 to 1 L, is included to account for the change in standard state from the gas phase (1 atm), the quantum chemical standard state for all species, to the experimental standard state in Et2O (1 mol/L). For intermolecular bonding, a counterpoise calculation was also conducted to determine the basis set superposition error (BSSE). Comparison with the benchmark calculations performed at the B3LYP level of theory (see later) suggests that MP2 gives a better description of the dominant long-range interactions in such systems and thus significantly improves the agreement with experimental data.

alane adduct

method

type I

type II

type III

Me3N EtMe2N Me3N EtMe2N

MP2/6-311+G(2d,2p) MP2/6-311+G(2d,2p) B3LYP/6-311+G(2d,2p) B3LYP/6-311+G(2d,2p)

-22.02 −19.11 −9.42 −5.87

−21.94 -21.61 -16.50 -15.91

−19.67 −18.49 −12.37 −10.88

Unlike Me3N (or EtMe2N), NMM and NEM can coordinate to AlH3 through nitrogen, oxygen, or a combination of both atoms.24,41 Consequently, seven types of structures are possible (Figure 2a), with morpholine to AlH3 ratios of 2:1, 1:1, and 1:2, respectively. To understand the interactions between morpholine and AlH3, we conducted theoretical calculations on all possible structures. The optimized structures of NMM·alane compounds are shown in Figure 2b; those of NEM are provided in Figure S5 in the Supporting Information. As shown in Figure 2b, when Al is four-coordinate (Type II or IV) or when the nitrogen atom is free of coordination (as in O L2·AlH3), the NMM molecule displays an unstrained chair configuration with the methyl group “cis” to oxygen. When Al is five-coordinate with N-coordination (as in NL2·AlH3), the methyl group is “trans” to oxygen. For NEM adducts, similarly distorted geometries are observed (Figure S5 in the Supporting Information). The different configurations of N-alkylmorpholine in these structures are probably caused by the different steric hindrance of the nitrogen and oxygen atoms.23 For these complexes, the calculated binding free energies are summarized in Table 2. For NMM, the 1:1 complex N NMM·AlH3 (with N-coordination) is favored because of the stronger basicity of nitrogen than oxygen. In contrast, the Odonating complex ONEM·AlH3 is preferred for NEM because the ethyl group in NEM may impede the coordination of nitrogen to AlH3. For both NMM·AlH3 and NEM·AlH3, the binding energy differences between the most stable structure and the next most stable structures are relatively small, suggesting that several structures may exist in equilibrium. It is also worth mentioning that the difference in binding energies between NEM·AlH3 and NMM·AlH3 is 0.25 kcal·mol−1. Such a small difference suggests that the transamination of NMM·AlH3 with NEM is feasible (vide infra). Synthesis. (Me3N)2·AlH3 and (EtMe2N)2·AlH3 are usually obtained by reactions of AlCl3 with LiAlH4 in the presence of excess Me3N or EtMe2N.25,42 However, because of side reactions, including coordination of amines to inorganic species (such as LiCl and AlCl3) and direct reactions between LiAlH4 and amines,27 the products from this method are usually mixtures and in some cases even contaminated by chloride impurities (AlH2Cl or AlHCl2).41,43 Thus, we prepared these compounds by direct synthesis44 and by hydrogenation of Al* (eqs 1 and 2).18

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RESULTS AND DISCUSSION Calculations of Reaction Free Energy and Molecular Geometries. Alane complexes of Me3N and EtMe2N usually adopt three types of structures (Figure 1b);1,3,40 thus, calculations of these structural types were conducted using both the MP2 and the B3LYP methods. As summarized in Table 1, the reaction energies suggest the existence of type-I and -II structures in Et2O. For each species, the B3LYP method gives a somewhat lower binding free energy than the MP2 method does. Also, in contrast with MP2, B3LYP predicts the 1:1 monomer (Figure 1b, type II) as the dominant species for alane adducts of Me3N and EtMe2N, a finding that is inconsistent with experimental results (type I and type II are both accessible).22−24,37 Such deviation may suggest that the B3LYP method is not adequate for describing long-range interactions in weakly bonded systems.

Et 2O

2L + γ‐AlH3 ⎯⎯⎯⎯→ L 2·AlH3

(1)

Et 2O

AL + 1.5H 2 + 2L ⎯⎯⎯⎯→ L 2· AlH3 L = Me3N or EtMe2N

(2)

Under vacuum (∼20 mmHg), both (Me3N)2·AlH3 and (EtMe2N)2·AlH3 slowly lose one amine molecule to form their respective 1:1 adducts.45 Additionally, sublimation of 14985

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Figure 2. Chemdraw (a) and optimized structures (b) of N-alkylmorpholine (L) alane compounds. NL, OL, and donating, and a combination of N,O-donating of L, respectively (polymeric structures are not included).

N,O

L denote N-donating, O-

Table 2. Binding Free Energies (kcal·mol−1) at 298.15 K in Et2O for the NMM and NEM Adducts Computed at the MP2 Level of Theory with BSSE Correction Type I

Type II

alane adduct

(NL, NL)

(NL, OL)

(OL, OL)

NMM NEM

−10.73 −6.26

−13.47 −11.01

−15.77 −15.51

N

Type III O

L

−14.08 -17.40

-17.65 −15.75

N

L

L

−14.25 −15.40

Type IV N,O

L

−16.11 −15.49

Table 3. 1H Chemical Shifts for Alane Adducts in C7D8 Al-H(v1/2) (Me3N)2·AlH3 (0.2 M) (EtMe2N)2·AlH3 (0.2 M) NMM·AlH3 (0.2 M) NEM·AlH3 (0.07 M) (0.14 M) (0.35 M) (1.38 M)

3.41 3.63 3.91 4.10 4.07 4.05 3.98

NCH3

(440) (280) (85) (340) (48) (32) (7)

2.17 2.09 1.99

(Me3N)2·AlH3 and evaporation of (EtMe2N)2·AlH3 also occur under similar conditions.26,46 Thus, proper conditions need to be applied to isolate these compounds in high yields and purity. Attempts to synthesize alane adducts of NMM (or NEM) by hydrogenation of Al* were unsuccessful, due mainly to the relatively weak basicity of NMM and NEM. Thus, the direct reaction of γ-AlH3 with NMM (or NEM) was used (eq 3), giving the 1:1 adducts in good yields. We note that complexes of the types NEM·(AlH3)n (n = 1.5 or 2) were synthesized by reactions of LiAlH4 with NEM/H2SO4 or NEM·HCl/H2SO4 in different ratios;41 however, these or other related adducts, such as L2·AlH3 (L = NMM or NEM), were not isolated in our studies.

OCH2

2.52 2.16−2.24 2.08−2.21 2.10−2.24 2.12−2.26 2.22−2.30

3.57 3.58 3.59 3.61 3.65

CH2CH3 0.92 0.91 0.92 0.93 0.96

temperature for several weeks without discernible decomposition. In solution, however, they decompose slowly to Al metal in a few days, a phenomenon that is similar to other amine·alane adducts.28 The enhanced thermal stability of these adducts in the solid state is perhaps due to the bifunctionality of N-alkylmorpholine and the polymeric structures of NMM·AlH3 (or NEM·AlH3).24,41 Characterization. The alane compounds were characterized by NMR and IR spectroscopy and, in some cases, XRD studies. The 1H NMR chemical shifts (Table 3) show distinct features. First, the hydride signals of the 2:1 adducts are usually upfield by 0.4 to 0.6 ppm to those of the 1:1 compounds,25,46 indicating more shielded hydrides in (amine)2·AlH3. Thus, the Al−H chemical shift is indicative of the AlH3 coordination environment. Additionally, the proton chemical shifts exhibit concentration dependence. As shown in Figure 3, dilution of the NEM·AlH3 solution in C7D8 caused a downfield shift of the hydride signal with concurrent peak broadening, whereas the C−H signals underwent slight upfield shift.28,43 While the different macroscopic physical properties of solutions with different concentrations may be responsible for the changes in proton chemical shifts and half peak widths, the association of NEM·AlH3 molecules through the NEM oxygen atom may also contribute to such changes.

Et 2O

L + γ‐AlH3 ⎯⎯⎯⎯→ L ·AlH3 L = NMM or NEM

NCH2

(3)

Depending on the nature of amines, their corresponding alane adducts exhibit different thermal stability.47 For (Me3N)2·AlH3 and (EtMe2N)2·AlH3, decomposition to Al metal is observed at room temperature, regardless of their states (pure form or in solution).28 In contrast, NMM·AlH3 and NEM·AlH3 show different thermal behaviors. In the solid state, both compounds are quite stable and can be stored at room 14986

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however, NMM·AlH324 and the related NEM complexes NEM·(AlH3)n (n = 1.5 or 2)41 exhibited polymeric structures. Thus, the different Al−H absorptions of NMM·AlH3 (or NEM·AlH3) in Et2O and in KBr are probably due to dissociation of polymers in Et2O. To understand the different Al−H stretching frequencies in different states, we performed calculations on the thermodynamics of polymer dissociation. Two kinds of polymers with structural types II and IV (Figure 2b) as the repeating units were studied because of their relative stability as monomers. As shown in Figure 5, the polymers are constructed by NFigure 3. 1H NMR spectra of NEM·AlH3 in C7D8 at different concentrations.

The IR spectra of these adducts are shown in Figure 4. In general, the Al−H stretches for the 2:1 adducts are ca. 70 cm−1

Figure 5. Two types of NMM·alane polymers: (a) {NMM·AlH3}n and (b) {NMM·(AlH3)2}n.

alkylmorpholine molecules connecting either AlH3 (type a) or {AlH2(μ-H)}2 units (type b). The polymerization free energy is calculated as the formation free energy of a truncated polymeric structure from two repeating units (Figure S6 in the Supporting Information). For NMM·AlH3 and NEM·AlH3, the polymerization free energies are 4.86 and 5.50 kcal·mol−1, respectively. The positive, relatively small values suggest that both polymers prefer to dissociate into monomers in Et2O, a finding that supports our IR observations. Table 5. Polymerization Free Energy (kcal/mol) of Amine·Alane Adducts in Et2O

Figure 4. IR spectra of (Me3N)2·AlH3, (EtMe2N)2·AlH3, NMM·AlH3, and NEM·AlH3: (a) in a CaF2 cell (solution) and (b) in a KBr pellet (solid). It should be noted that NMM is slightly soluble in Et2O; the absorption at 1963 cm−1 is originated from Et2O.

NMM·AlH3 NEM·AlH3

lower than those for 1:1 compounds, a trend that is consistent with other amine·alane compounds.25,46,47 For (Me3N)2·AlH3, the Al−H absorptions in Et2O and in KBr are similar, suggesting similar structures of (Me3N)2·AlH3 in different states. In contrast, the Al−H absorption of NMM·AlH3 (or NEM·AlH3) in Et2O is quite different from that in KBr, a phenomenon that is indicative of different structures in solution and in the solid state. To get more structural information, we calculated the Al−H stretching frequencies of NMM·AlH3 and NEM·AlH3 in Et2O at the B3LYP/6-31+G(d,p) level of theory. The calculated Al− H frequencies (Table 4) suggest monomeric structures (Figure 1b, type II) for both compounds in Et2O. In the solid state,

type (a)

type (b)

-15.22 −13.21

−14.94 -14.12

For solid compounds, XRD patterns were also recorded. (See Figure S1 in the Supporting Information.) Because of the low crystallinity, the diffraction patterns are usually weak. Nevertheless, they are in good agreement with those derived from single-crystal data,24,25 and provide useful information for the transamination and thermal decomposition studies (discussed below). Transamination. Previous research has shown that the 2:1 adducts (amine)2·AlH3, formed by hydrogenation of Al metal, cannot be decomposed to AlH3 in a single step. In contrast, Et3N·AlH3, which decomposes to alane under heat and vacuum, cannot be obtained by hydrogenation.7,18 Because of this

Table 4. Calculated Al−H Stretching Frequencies (cm−1) at B3LYP/6-31+G(d,p) Level of Theory in Et2Oa type I

a

N

N

N

O

type II O

O

N

alane adduct

( L, L)

( L, L)

( L, L)

( L)

(OL)

type III NL

NMM NEM

1741 (1716) 1761 (1735)

1754 (1729) 1771 (1745)

1756 (1731) 1787 (1761)

1804 (1778) 1806 (1780)

1808 (1782) 1807 (1781)

1817, 1715 (1791, 1690) 1867, 1654 (1840, 1630)

type IV

N,O

L

1821 (1794) 1815 (1789)

Values inside the parentheses are corrected frequencies by a factor of 0.9857.48 14987

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“paradox”, the 2:1 adducts are usually transformed into Et3N·AlH3 for the separation of AlH3.7 However, this process is associated with several undesired processes, including evaporation of Et3N·AlH3 and formation of Al metal, and suffers from slow reaction rates and low energy efficiency.18 To improve such a process, NMM and NEM were utilized as the transaminating agents. Unlike the Et3N transamination that requires heating to 65 °C,17,18 the NMM reaction with (amine)2·AlH3 proceeds smoothly at room temperature. As the volatile materials evaporated, the mixture became turbid with the formation of a solid. The IR spectra (Figure 6a) monitoring the reaction

line is weaker in basicity than small acyclic amines (Me3N or EtMe2N), it may form an equilibrium with (amine)2·AlH3 (amine = Me3N or EtMe2N) under the transamination reaction conditions, giving some N-alkylmorpholine·alane species. Under reduced pressures (∼20 mmHg), the highly volatile amine (Me3N or EtMe2N) that is transaminated from (amine)2·AlH3 is removed from the reaction mixture, moving the equilibrium toward the N-alkylmorpholine·alane adduct. Also, the bifunctionality of N-alkylmorpholine and the reduced solubility of N-alkylmorpholine·alane compounds in Et2O are other possible driving forces for the reaction. Lastly, transamination of NMM·AlH3 with NEM has also been achieved. Because of structural similarities and small differences in binding free energies between NMM·AlH3 and NEM·AlH3 (Table 2), this ligand-exchange reaction is usually slow. Thus, the reaction was conducted in toluene at 65 °C. With multiple additions of NEM, the transamination proceeded to completion in several hours, giving NEM·AlH3 in ca. 80% yield. It should be noted that although heating was applied in the process, the formation of Al metal was not observed because of the improved stability of NMM·AlH3 and NEM·AlH3. Thermal Decomposition. While most amine·alane adducts decompose directly to Al metal, some other ones, such as Et3N·AlH3, have been shown to form AlH3 under certain conditions.18 Unfortunately, the separation of AlH3 from Et3N·AlH3 is associated with several undesired processes, including evaporation of Et3N·AlH3 and formation of Al metal. As a result, we studied the thermal properties (or decomposition pathways) of NMM·AlH3 and NEM·AlH3 in order to utilize them for AlH3 separation. NMM·AlH3 (solid) is stable up to 80 °C, as no reaction was observed below 80 °C. At 85 °C, NMM·AlH3 gradually decomposed to a gray solid, which was confirmed to be Al metal by XRD analysis (Figure 7c). Thus, NMM·AlH3 is not suitable for the separation of donor-free AlH3 (eq 4).

Figure 6. IR spectra of the transamination reaction of (Me3N)2·AlH3 with NMM: (a) in a CaF2 cell (solution) and (b) in a KBr pellet (solid).

vacuum

NMM· AlH3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Al + NMM ↑ + 1.5H 2↑

showed gradual growth of a new peak around 1780 cm−1 with concurrent decrease in peak intensity at 1701 cm−1, suggesting consumption of (Me 3 N) 2 ·AlH 3 and formation of 1:1 amine·AlH3 species. Upon complete reaction, a white solid was obtained in good yield, which was identified to be NMM·AlH3 by NMR, IR, and XRD analyses (Figures 6 and Figure S2 of the Supporting Information). The NEM transamination was similarly conducted. Because of the greater steric hindrance of NEM, the reaction is usually slower than that with NMM. Also, multiple additions of NEM (three to five times) might be necessary to achieve complete reaction. Upon complete transamination, NEM·AlH3 was also obtained in good yield. Compared with Et3N, NMM and NEM show several advantages as transaminating agents. First, the reaction with N-alkylmorpholine is more energy-efficient because it does not require heating and yet is much faster than that with Et3N.16−18 Additionally, both NMM·AlH3 and NEM·AlH3 are less prone than Et3N·AlH3 to form Al metal; thus, the desired transamination products are usually obtained in higher yields and purity. In contrast, the Et3N reaction often gives Et3N·AlH3 in low yields with contamination of Al metal.18 Furthermore, because of the high efficiency of the transamination reaction, it can be combined with the hydrogenation step by hydrogenating Al* in a mixture of Me3N (or EtMe2N), NMM, and Et2O, giving NMM·AlH3 in one step. Although N-alkylmorpho-

60 ∼ 70 ° C

(4)

Decomposition of NEM·AlH3 was also studied in a similar manner. In the temperature range of 20 to 55 °C, no reaction occurred. Between 60 and 70 °C and under reduced pressures (∼20 mmHg), NEM·AlH3 readily changed to a gray solid with significant loss of mass. XRD analyses showed the formation of α-AlH3 with a small amount of Al metal (Figure 7d). Compared with that of NMM·AlH3, the decomposition of NEM·AlH3 to give alane could be attributed to the greater steric hindrance and the decreased Lewis basicity of NEM. Further increasing the reaction temperature (>75 °C) of NEM·AlH3 gave mostly Al metal with a trace amount of (or no) α-AlH3. Thus, the ideal temperature range for the separation of AlH3 from NEM·AlH3 is between 60 and 70 °C (eq 5). vacuum

NEM· AlH3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ AlH3 + NEM↑ 60 ∼ 70 ° C

(5)

In general, the rate of decomposition increases with increasing temperatures; however, the yield of AlH3 decreases as the Al formation pathway becomes more favorable at higher temperatures. In addition, the thermal decomposition reaction occurs both in the solid state and in solution, with a somewhat higher rate in solution. Unlike Et3N·AlH3, which requires a metal hydride additive (such as LiH or LiAlH4) as catalyst or seed for the decomposition,7,18 NEM·AlH3 does not require 14988

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CONCLUSIONS We have presented the synthesis and characterization of Nalkylmorpholine·alane adducts (NMM·AlH3 and NEM·AlH3) by NMR, IR, and XRD studies. Because of the bifunctionality (or dual coordination mode) of N-alkylmorpholine, both NMM·AlH3 and NEM·AlH3 exhibit polymeric structures in the solid state and are more stable than the related amine·alane adducts.1,3 Studies of Al−H absorptions by IR spectroscopy suggest the dissociation of polymers into monomers in donor solvents. These considerations are further supported by theoretical calculations of the molecular geometries, absolute free energies, Al−H vibrational frequencies, and the thermodynamics of amine·alane adducts with different structures. In addition, the cost- and energy-effective transamination of (amine)2·AlH3 with NMM (or NEM) has been achieved, giving NMM·AlH3 (or NEM·AlH3) in high yields and purity. Because of increased stability of NMM·AlH3 and NEM·AlH3, formation of Al metal was not observed during the process. The absence of Al metal not only increases the yield of alane product but also reduces the complexity of the reaction. More importantly, because of the fast reaction kinetics (even at room temperature), the transamination can be combined with hydrogenation of Al metal to prepare NMM·AlH3 in a single step from a one-pot reaction. Lastly, the thermal decomposition pathways of NMM·AlH3 and NEM·AlH3 have been elucidated. While NMM·AlH3 decomposes to Al metal directly, NEM·AlH3 can be selectively transformed to donor-free alane. Compared to the separation of AlH3 from Et3N·AlH3, the NEM·AlH3 process greatly improves the yield of AlH3.17,18 Combining the transamination of (amine)2·AlH3 with N-alkylmorpholine with the separation of alane from NEM·AlH3, we have established a new separation pathway for donor-free AlH3 with improved yields, selectivity, and energy-efficiency.

Figure 7. XRD patterns of the decomposition products of NMM·AlH3 and NEM·AlH3: (a) Al metal (standard); (b) α-AlH3 (standard); (c) the NMM·AlH3 decomposition product; and (d) the NEM·AlH3 decomposition products.

any catalyst in the process, although a small amount of α-AlH3 (as seed/catalyst) can hasten the reaction to some extent. Thus, no external hydride impurities are introduced into the product for the NEM·AlH3 reaction. We note that the alane product from NEM·AlH3 may contain a small amount of Al metal (Figure 7d); however, the yield and selectivity of this reaction are significantly improved compared with those with Et3N·AlH3. Thus, NEM·AlH3 is a better intermediate than Et3N·AlH3 for the separation of alane.17,18 On the basis of these discussions, we propose an AlH3 regeneration pathway using N-alkylmorpholine compounds. As shown in Scheme 2, hydrogenation of Al metal (with a catalyst)

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 2. Proposed AlH3 Regeneration Pathway Using NAlkylmorpholine Compounds

XRD patterns of alane adducts and the transamination products with NMM or NEM, the 1H and 13C NMR spectra of alane compounds, and the optimized geometries for AlH3, Me3N, EtMe2N, NMM, NEM, and the related alane complexes. 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]. Tel: 631-344-3511. Present Address §

Jason Graetz: HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, CA 90265. Notes

The authors declare no competing financial interest.

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in a mixture of a strong amine (such as Me3N or EtMe2N) and Et2O readily gives (amine)2·alane,17,18 which then reacts with NMM to form NMM·AlH3. Because of the fast kinetics of transamination, it could be combined with hydrogenation of Al metal to prepare NMM·AlH3 in a single step from a one-pot reaction. Subsequently, NMM·AlH3 is transformed into NEM·AlH3 in good yields. Alternatively, transamination of (amine)2·alane with NEM can be conducted to give NEM·AlH3 directly. With NEM·AlH3 available, donor-free alane can be obtained under proper conditions.

ACKNOWLEDGMENTS We thank Yusuf Celebi, Weimin Zhou, John Johnson, James E. Wegrzyn, and James Reilly for discussions and the Chemistry Department at Brookhaven National Laboratory (BNL) for access to its NMR spectrometer. C.N. acknowledges financial support from the Goldhaber Distinguished Fellowship at BNL. This work was carried out at BNL under Contract No. DEAC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences. Calculations 14989

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