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J. Phys. Chem. C 2007, 111, 19148-19152
Direct and Reversible Synthesis of AlH3-Triethylenediamine from Al and H2 Jason Graetz,† Santanu Chaudhuri,‡ James Wegrzyn,† Yusuf Celebi,† John R. Johnson,† Weimin Zhou,† and James J. Reilly*,† Department of Energy Sciences and Technology, BrookhaVen National Laboratory, Upton, New York 11973, and Applied Sciences Laboratory and Institute for Shock Physics, Washington State UniVersity, Spokane, Washington 99210 ReceiVed: August 24, 2007; In Final Form: October 2, 2007
Aluminum hydride, AlH3, is the most well-known alane. Though thermodynamically unstable under ambient conditions, it is easily prepared in a metastable state that will undergo controlled thermal decomposition to produce H2 and Al at around 100 °C. AlH3 contains 10.1 wt % hydrogen and has a density of 1.48 g/mL and is therefore of interest for on-board automotive hydrogen storage. ∆Hf and ∆Gf298K for R-AlH3 are -9.9 and 48.5 kJ/mol AlH3, respectively. The latter value yields an equilibrium hydrogen fugacity of ∼5 × 105 atm at 298 K, which is equivalent to a hydrogen pressure of ∼7 × 103 atm. Thus, the direct regeneration of AlH3 from spent Al with gaseous H2 is deemed impractical. This paper describes an alternate approach to the regeneration of AlH3 via a low-temperature, low-pressure, reversible reaction using Ti-doped Al powder and triethylenediamine (TEDA). The adduct is formed in a slurry of the Al powder and a solution of TEDA in THF in contact with H2. The AlH3-TEDA product is insoluble and precipitates from solution. The reaction, forward or reverse, depends on the departure of the actual pressure of H2 gas above or below the equilibrium pressure. Pressure-composition isotherms in the range of 70-90 °C are presented from which thermodynamic data were calculated. A possible reaction mechanism is described. The relevance of this system to hydrogen storage applications is also noted.
Introduction AlH3 is a covalently bonded, binary hydride that forms a metastable solid at room temperature. Recently, substantial interest has developed in AlH3 for use as a source of hydrogen for low-temperature fuel cell applications since it contains 10.1 wt % hydrogen and has a density of 1.48 g/mL. AlH3 was originally synthesized in an ether solvated form by Finholt et al.1 and later in a nonsolvated form by Brower et al.2 The latter study noted the existence of seven crystalline phases, namely, R, R′, β, γ, δ, , and ζ. The crystal structures of the four main phases R,3 R′,4 β,5 and γ,6,7 are polymeric with octahedral AlH 6 units connected through H bridging bonds. All of the AlH3 phases are thermodynamically unstable under ambient conditions although they are usually metastable with little or no decomposition at room temperature. The decomposition of crystalline R-AlH3 occurs in a single step, as shown below:
R-AlH3 f Al + 1.5H2
(1)
Recently, Graetz and Reilly8 prepared the R, β, and γ phases of AlH3 using the procedure described by Brower et al.2 The newly prepared phases are very reactive, and all have a H content approaching 10 wt %. They must be prepared and handled in a drybox in the absence of air. All will decompose readily at ∼100 °C at a rate and pressure suitable for hydrogen storage applications. Currently R-AlH3 is of interest as a hydrogen fuel source for fuel cell vehicles as it meets the storage and H2 delivery criteria specified by DOE for the Freedom Car and Fuel Partnership.9
However, although reaction 1 occurs readily, the reverse reaction does not. The heat of formation, ∆Hf, was reported by Sinke et al.10 to be -11.4 kJ/mol AlH3 and 46.6 kJ/mol AlH3 for ∆Gf298. Graetz and Reilly11 determined that ∆Hf and ∆Gf298 for the formation of R-AlH3 are -9.9 and 48.5 kJ/mol AlH3, respectively. The latter value yields a room-temperature equilibrium hydrogen fugacity of 5 × 105 atm, which is equivalent to a hydrogen pressure of approximately 7 × 103 atm.12 In view of these data the direct regeneration of AlH3 from spent Al with gaseous H2 is deemed impractical. This poses a major problem for the use of AlH3 as a hydrogen fuel source since it cannot be easily regenerated. In view of the above, the regeneration problem was approached from a different direction. Wiberg and Amberger13 have pointed out that alane, as borane, has rich organometallic chemistry and will react with a variety of organic compounds. In 1964 Ashby14 demonstrated the direct synthesis of an AlH3 adduct of triethylenediamine (TEDA) using activated aluminum powder and TEDA in tetrahydrofuran (THF) at a hydrogen pressure of 345 atm. This paper is devoted to a description of the properties of AlH3-TEDA synthesized at room temperature in a similar, but reversible, reaction at a much lower pressure. The key feature which enables the reaction to proceed as written below is the incorporation of a titanium catalyst in Al using a special technique. The reaction may be rewritten as follows:
Al(Ti) + TEDA + 1.5H2 T Al(Ti)H3-TEDA
(2)
Experimental Section * Corresponding author. E-mail:
[email protected]. † Brookhaven National Laboratory. ‡ Washington State University.
Materials. Hydrogen was obtained from Praxair specified as 99.95% pure; the following were obtained from Sigma-
10.1021/jp076804j CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007
Direct and Reversible Synthesis of AlH3-TEDA Aldrich; THF (99.9% anhydrous), AlCl3 (99.999%), n-undecane (C11H24, 99%), LiAlH4 (reagent grade 95%), and TEDA (C6H12N2), also listed in the Aldrich catalog as 1,4-diazabicyclo[2,2,2]octane, 98%). TEDA has hexagonal symmetry15 with a ) 6.14 and c ) 9.46 Å. Preparation of Catalyzed Al. The use of Ti doping to prepare a reversible NaAlH4 was first described by Bogdanovic and Schwickardi.16 The properties of the Ti-doped alanate were so greatly improved with respect to reaction kinetics and reversibility that the material attracted wide attention as a high content, H storage compound. We have adapted a similar doping procedure to prepare titanium-catalyzed Al(Ti) in an argon-purged drybox. First, AlH3 was prepared using the synthesis route of Brower et al.2 which involves reacting LiAlH4 with AlCl3 in ether. We modified that synthesis by the addition of ethereal TiCl3 to give a final Al0.98Ti0.02 ratio. Upon the addition of the TiCl3 there was an immediate release of hydrogen indicating decomposition of AlH3 and/or LiAlH4. Upon removal of ether and by drying for 1 h at 80 °C, a black, activated Al(Ti) powder was produced. The drying step completely decomposed any residual AlH3 as indicated by X-ray diffraction (XRD) of the black powder which showed only the presence of Al. The exact state of Ti in the Al metal is not known at this time, but it is likely similar to that in the Al component of Ti-doped NaAlH4 after decomposition as discussed below. Synthesis. Hydrogenation and dehydrogenation reactions were carried out in a 300 mL stainless steel stirred reactor (Parr Instruments) rated for 200 atm maximum operating pressure. The reactor was loaded in the Ar-filled glovebox with 1.5 g of Al(Ti), 17 g of TEDA, and 90-100 mL of THF. TEDA is completely soluble in THF, whereas Al(Ti) is not. The reactor was sealed, removed from the glovebox, attached to a gasvacuum manifold, and quickly evacuated to remove most of the argon remaining in the reactor. Hydrogen gas was added to the reactor at pressures of
