English

• AlH₃ possesses volumetric hydrogen density that is more than twice of liquid hydrogen. Moreover, its gravimetric hydrogen density exceeds mass fraction 10%, which is much higher than that of other metal hydrides. Such high hydrogen content makes aluminum hydride compounds an excellent hydrogen storage medium^[1-4].

  Al₂H₆ is the most common AlH₃ configuration that belongs to octahedron group and Al-H bonds stacked in different ways, thus AlH₃ possesses seven different crystal forms, including α, α′, β, δ, ξ, θ and γ. Among them, α-AlH₃ possesses excellent stability^[5]. In 1947, Finholt et al^[6] reported an organic method for the first time to prepare etherated complex of AlH₃ by reacting between AlCl₃ and LiH in ether solution. The specific reaction equation (1) is described as follows:

  ${\rm{AlC}}{{\rm{l}}_3} + 3{\rm{LiH}} \to {\rm{Al}}{{\rm{H}}_3} + 3{\rm{LiCl}} $

  (1)

  What they obtained was an ethereal solution of AlH₃, which was not stable. Based on the above method, Brower et al.^[5] prepared AlH₃ in ether solution by using LiAlH₄ and AlCl₃ as staring materials in 1976. The reaction principle is shown below (2)(3):

  $3{{\mathop{\rm LiAlH}\nolimits} _4} + {\rm{AlC}}{{\rm{l}}_3} \to 4{\rm{Al}}{{\rm{H}}_3} \cdot n{\rm{E}}{{\rm{t}}_2}{\rm{O}} + 3{\rm{LiCl}} $

  (2)

  $4{\rm{Al}}{{\rm{H}}_3} \cdot n{\rm{E}}{{\rm{t}}_2}{\rm{O}} \to 4{\rm{Al}}{{\rm{H}}_3} + n{\rm{E}}{{\rm{t}}_2}{\rm{O}} $

  (3)

  Brower′s method for AlH₃ preparation possesses the advantages of fast reaction, single crystal phase and good quality product. Some researchers tried to prepare AlH₃ by using mechanical milling method^[7-11], but AlH₃ prepared by this method is prone to extremely unstable α′-AlH₃. Saitoh et al.^[12-13] studied the direct hydrogenation of metallic aluminum under high-temperature and high-pressure condition through hydrogen flow. Stable α-AIH₃ can be obtained but the reaction condition was harsh. At present, the organic synthesis of α-AlH₃ is still a research topic, during such synthesis process, aluminum hydride etherates(AlH₃·nEt₂O) would be formed as intermediate. Therefore, deetherification and crystal transformation are key steps to obtain α-AlH₃. Nowadays, most literature reports on α-AlH₃ in the literature are formed in toluene solution^[14-17]. Obviously, the method of deetherification and crystal transformation used now is not ecofriendly, and the toxic toluene solvent is required in large quantities during the crystal phase transformation process^[18]. Moreover, it needs to be emphasized that the effect of ether content in AlH₃·nEt₂O on crystal transformation has not been explored up to date. In view of these two points, it is worth investigating the stability of AlH₃·nEt₂O and developing new methods for the α-AlH₃ preparation.

  In this work, we have successfully improved the procedure for AlH₃·nEt₂O preparation in a safe and efficient way based on Brower′s method and prepared α-AlH₃ based on AlH₃·nEt₂O in solid state form under non-toluene condition. The evolution of the AlH₃·nEt₂O during storage at room temperature was also investigated. The loss of ether in AlH₃·nEt₂O was characterized by TGA-DSC. The crystal phase of the obtained products was characterized by XRD, and the hydrogen content of the obtained AlH₃ was determined by TGA-DSC. To the best of our knowledge, this is the first report on the effect of the ether content in AlH₃·nEt₂O on crystal transformation.

  1.   Experimental

  1.1   Reagents and Instruments

  X-ray powder diffraction(XRD, PANalytical Empyrean); Thermal Gravimetric Analyzer and Differential Scanning Calorimetry(TGA-DSC, Thermo Fisher Escalab 250Xi); Scanning Electron Microscopy(SEM, Hitachi SU 8000HSD).

  Lithium aluminium hydride and aluminium chloride were purchased from Sinopharm Chemical Reagent Co., Ltd(Analytical reagent). Ethyl ether was purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD(Analytical reagent).

  1.2   Synthesis and crystal transformation of AlH₃·nEt₂O

  A certain concentration of LiAlH₄ in ether solution and AlCl₃ in ether solution was separately prepared. Then, the solution of AlCl₃ in ether was dropwised to the solution of LiAlH₄ in ether, keeping the molar ratio n(LiAlH₄) to n(AlCl₃) 4:1 at -10 ℃. After 10~15 min, LiCl was removed by filtration under low temperature to obtain the AlH₃·nEt₂O in diethyl ether solution. The solvent was removed by evaporation under reduced pressure before the solid product was heated up to 30 ℃. The obtained etherated product was dried under vacuum at 45 ℃ for 2 hours, and then rapidly grounded into powder in argon environment. After grinding, it was quickly transferred to a sample tube filled with argon. The AlH₃·nEt₂O sample was stored in argon atmosphere at room temperature for varying time between 1 and 20 days. Then the argon was removed via a vacuum pump. The tube was put into the oil at 94 ℃ for 15 min, then in argon atmosphere for 45 min. Finally, the product obtained in the tube is α-AlH₃.

  2.   Results and Discussion

  The XRD of AlH₃·nEt₂O after different storage time is shown in Fig. 1. The AlH₃·nEt₂O did not change much during the first 12 days of storage. After 20 days, XRD patterns ascribed to α′-AlH₃ and γ-AlH₃ could be observed. This is due to the slow deetherification of the etherate during storage, and some of AlH₃·nEt₂O formed less stable γ-AlH₃ and α′-AlH₃ during the deetherification process. SEM images further reveal the morphology changes of the AlH₃·nEt₂O. As shown in Fig. 2, the newly prepared AlH₃·nEt₂O exhibits an overall amorphous state, with most of the particle size distributed in the range of 30~50 μm. However, needle-like γ-AlH₃ appeared after the ether complex was stored for 20 days. Moreover, the solid particles split into small particles with the size of 10~25 μm. This is because the AlH₃·nEt₂O undergoes deetherification process, which weakens the intermolecular interactions.

  Figure 1

  

  Figure 1.  XRD pattern of AlH₃·nEt₂O at room temperature after storing for different period of time

  a.1 day; b.4 days; c.8 days; d.12 days; e. 16 days; f.20 days

  下载: 全尺寸图片 幻灯片

  Figure 2

  

  Figure 2.  SEM images of fresh(A, B) AlH₃·nEt₂O, and AlH₃·nEt₂O stored for 20 days(C, D)

  下载: 全尺寸图片 幻灯片

  The self-deetherification from AlH₃·nEt₂O during storage is further confirmed by TGA(Fig. 3) and DSC(Fig. 4). There is a de-etherification process of the ether compound within 25~120 ℃, and a dehydrogenation process between 150 and 175 ℃. The ether removal rate is relatively slow within 12 days. However, as shown in Fig. 3, an obvious decrease in the ether content of AlH₃·nEt₂O was observed when the AlH₃·nEt₂O was stored longer than 12 days. With the increase of the preservation time of AlH₃·nEt₂O, the H% of its product α-AlH₃ generated decreases correspondingly(see Table S1 in Supporting Information).

  Figure 3

  

  Figure 3.  TGA curves after AlH₃·nEt₂O stored for different time

  下载: 全尺寸图片 幻灯片

  Figure 4

  

  Figure 4.  DSC curves of AlH₃·nEt₂O after stored for different time

  下载: 全尺寸图片 幻灯片

  In order to get pure α-AlH₃, the condition for crystal transformation with AlH₃·nEt₂O was first optimized based on the same AlH₃·nEt₂O. The XRD patterns of AlH₃ obtained according to transmutation procedures described in the experimental section. The solid-state dynamic vacuum crystallization method at 94 ℃ for 15 min leads to a single crystal form of pure α -AlH₃. The product obtained by vacuum crystallizing for less than 10 minutes and then transferred to normal argon atmosphere results in a mixture of α-AlH₃ with a small amount of γ-AlH₃, indicating that long time vacuum is good for deetherification and crystal transformation into pure α-AlH₃.

  To investigate the effect of storage time for AlH₃·nEt₂O on the crystal transformation into α-AlH₃, we carried out the XRD measurements(Fig. 5). Pure α-AlH₃ could be obtained based on AlH₃·nEt₂O stored within 1~10 days. However, the diffraction peak intensity gradually decreases with increasing the storage time. After the AlH₃·nEt₂O stored for more than 11 days, small amount of γ-AlH₃ could be formed after deetherification, which is probably due to the decomposition of AlH₃·nEt₂O during storage. In addition to α-AlH₃ and γ-AlH₃, α′-AlH₃ crystal phase was also found after deetherification if AlH₃·nEt₂O was stored for 19 days. In addition, if AlH₃·nEt₂O was stored for more than 20 days, α-AlH₃ was hardly found. We propose that α-AlH₃ formation is strongly related to the state of AlH₃·nEt₂O. It is know that the interaction force between AlH₃ and Et₂O is weak. With the increase of storage time, part of the ether separates from AlH₃, which is evidenced by the weight loss rate for AlH₃·nEt₂O before 100 ℃ as shown in Fig. 3. Thus we conclude that a high ratio ether in AlH₃·nEt₂O is favoring α-AlH₃ formation. With the increase of the storage time, the content of ether in AlH₃·nEt₂O gradually decreased. The lower the ether content in AlH₃·nEt₂O is, the more likely to form less stable γ-AlH₃ and α′-AlH₃. To investigate the morphology of transcrystalline product based on AlH₃·nEt₂O stored for various time, we carried out SEM measurements. As shown in Fig. 6, within 8 days, only α-AlH₃ could be obtained with a cubic shape and agglomeration phenomenon. After 12 days, crystal sizes are generally minimized to 1~2 μm.

  Figure 5

  

  Figure 5.  XRD patterns of the crystalline product based on AlH₃·nEt₂O with varying storage time at room temperature

  下载: 全尺寸图片 幻灯片

  Figure 6

  

  Figure 6.  SEM images of the crystalline product after preservation

  A.1 day; B.4 days; C.8 days; D.12 days; E.16 days; F.20 days.

  下载: 全尺寸图片 幻灯片

  To figure out whether the storage time of AlH₃·nEt₂O has influence on the hydrogen content of yield crystalline product, TGA measurement was employed. As shown in Fig. 7, the longer the storage time of AlH₃·nEt₂O is, the lower the hydrogen content of the product obtained by the transmutation is. The hydrogen content of the resulting product dropped rapidly if AlH₃·nEt₂O was stored for more than 12 days. As the storage time of AlH₃·nEt₂O extended to 20 days, the hydrogen content of the product decreased to 5.4%.

  Figure 7

  

  Figure 7.  Thermogravimetric curves of the crystalline product based on AlH₃·nEt₂O with the change of storage time

  下载: 全尺寸图片 幻灯片

  3.   Conclusion

  We prepared AlH₃·nEt₂O, which could be stored for more than 10 days, before it could be converted to high-quality α-AlH₃.We have also found that the content of ether in AlH₃·nEt₂O decreased with the increase of storage time. Solid-phase vacuum method for converting AlH₃·nEt₂O to α-AlH₃ was achieved, providing a possibility for large-scale α-AlH₃ production. It is also found that the content of ether in AlH₃·nEt₂O has a significant effect on the purity and quality of the produced α-AlH₃. The highest hydrogen content(9.2%) of α-AlH₃ was obtained by dynamic vacuum crystallization at 94 ℃ for 15 min and argon atmosphere for 45 min.

  Supporting information [H mass fraction of corresponding α-AlH₃] can be downloaded freely from our website(http://yyhx.ciac.jl.cn/).

• 1. [1]

     Graetz J, Reilly J J, Yartys V A. Aluminum Hydride as a Hydrogen and Energy Storage Material:Past, Present and Future[J]. J Alloys Compd, 2011, 509:  S517-S528. doi: 10.1016/j.jallcom.2010.11.115

  2. [2]

     Duan C, Cao Y, Hu L. Synergistic Effect of TiF₃ on the Dehydriding Property of α-AlH₃ Nano-composite[J]. Mater Lett, 2019, 238(1):  254-257.

  3. [3]

     Wang L, Rawal A, Aguey-Zinsou K. Hydrogen Storage Properties of Nanoconfined Aluminium Hydride(AlH₃)[J]. Chem Eng Sci, 2019, 194(2):  64-70.

  4. [4]

     Zheng J, Xiao X, He T. Enhanced Reversible Hydrogen Desorption Properties and Mechanism of Mg(BH₄)₂-AlH₃-LiH Composite[J]. J Alloys Compd, 2018, 762(25):  548-554.

  5. [5]

     Brower F M, Matzek N E, Reigler P F. Preparation and Properties of Aluminum Hydride[J]. J Am Chem Soc, 1976, 98(9):  2450-2453. doi: 10.1021/ja00425a011

  6. [6]

     Finholt A E, Jr A C B, Schlesinger H I. Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of Their Applications in Organic and Inorganic Chemistry[J]. J Am Chem Soc, 1947, 69(5):  1199-1203. doi: 10.1021/ja01197a061

  7. [7]

     Long V D, Knight D A, Paskevicius M. Novel Methods for Synthesizing Halide-Free Alane Without the Formation of Adducts[J]. Appl Phys A, 2012, 107(1):  173-181. doi: 10.1007/s00339-012-6791-z

  8. [8]

     Nakagawa Y, Lee C, Matsui K. Doping effect of Nb Species on Hydrogen Desorption Properties of AlH₃[J]. J Alloys Compd, 2018, 734(15):  55-59.

  9. [9]

     Duan C W, Hu L X, Dan X. Solid State Synthesis of Nano-sized AlH₃ and Its Dehydriding Behaviour[J]. Green Chem, 2015, 17(6):  3466-3474. doi: 10.1039/C5GC00426H

  10. [10]

      Sartori S, Opalka S M, Løvvik O M. Experimental Studies of α-AlD₃ and α'-AlD₃ Versus First-Principles Modelling of the Alane Isomorphs[J]. J Mater Chem, 2008, 18(20):  2361-2370. doi: 10.1039/b718896j

  11. [11]

      Duan C W, Hu L X, Ma J L. Ionic Liquids as an Efficient Medium for the Mechanochemical Synthesis of α-AlH₃ Nano-composites[J]. J Mater Chem A, 2018, 6(15):  6309-6318. doi: 10.1039/C8TA00533H

  12. [12]

      Saitoh H, Machida A, Katayama Y. Formation and Decomposition of AlH₃ in the Aluminum-Hydrogen System[J]. Appl Phys Lett, 2008, 93(15):  151918. doi: 10.1063/1.3002374

  13. [13]

      Saitoh H, Okajima Y, Yoneda Y. Formation and Crystal Growth Process of AlH₃ in Al-H System[J]. J Alloys Compd, 2010, 496(1/2):  L25-L28.

  14. [14]

      刘海镇, 王新华, 刘永安. AlH₃的制备及放氢特性[J]. 高等学校化学学报, 2013,34,(10): 2274-2278. doi: 10.7503/cjcu20130227LIU Haizhen, WANG Xinhua, LIU Yongan. Preparations and Dehydriding Properties of AlH₃[J]. Chem J Chinese Uinv, 2013, 34(10):  2274-2278. doi: 10.7503/cjcu20130227

  15. [15]

      Xu B, Liu J P. Aluminum Hydride Polymorphs:Preparation, Characterization and Thermal Properties[J]. Adv Mater Res, 2013, 712-715:  333-336. doi: 10.4028/www.scientific.net/AMR.712-715.333

  16. [16]

      Bulychev B M, Verbetskii V N, Storozhenko P A. "Direct" Synthesis of Unsolvated aluminum Hydride Involving Lewis and Bronsted Acids[J]. Russ J Inorg Chem, 2008, 53(7):  1000-1005. doi: 10.1134/S0036023608070048

  17. [17]

      Bulychev B M, Storozhenko P A, Fokin V N. "One-Step" Synthesis of Nonsolvated Aluminum Hydride[J]. Russ Chem Bull, 2009, 58(9):  1817-1823. doi: 10.1007/s11172-009-0247-4

  18. [18]

      Xu B, Liu J, Wang X. Preparation and Thermal Properties of Aluminum Hydride Polymorphs[J]. Vacuum, 2014, 99:  127-134. doi: 10.1016/j.vacuum.2013.05.009

• 

  Figure 1  XRD pattern of AlH₃·nEt₂O at room temperature after storing for different period of time

  a.1 day; b.4 days; c.8 days; d.12 days; e. 16 days; f.20 days

  下载: 全尺寸图片 幻灯片
  

  Figure 2  SEM images of fresh(A, B) AlH₃·nEt₂O, and AlH₃·nEt₂O stored for 20 days(C, D)

  下载: 全尺寸图片 幻灯片
  

  Figure 3  TGA curves after AlH₃·nEt₂O stored for different time

  下载: 全尺寸图片 幻灯片
  

  Figure 4  DSC curves of AlH₃·nEt₂O after stored for different time

  下载: 全尺寸图片 幻灯片
  

  Figure 5  XRD patterns of the crystalline product based on AlH₃·nEt₂O with varying storage time at room temperature

  下载: 全尺寸图片 幻灯片
  

  Figure 6  SEM images of the crystalline product after preservation

  A.1 day; B.4 days; C.8 days; D.12 days; E.16 days; F.20 days.

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Thermogravimetric curves of the crystalline product based on AlH₃·nEt₂O with the change of storage time

  下载: 全尺寸图片 幻灯片
•