zed (Hive Bee)
06-21-02 01:19
No 323599
      LiAlH4 Synthesis  Bookmark   

A simple, more economical synthesis of LiAlH4 has been achieved. No high temperatures, No high pressures, No wasted LiH. Other Hydrides, like NaAlH4 were also readily produced.

The author of the patent, was creating "special" and "new", complex hydrides for reversable Hydrogen storage. With that grand goal in his sights, he didn't dwell much on the synthesis of traditional hydrides. At any rate, his new method, seems to have universal application.

He basically just mixes up the two dry metal hydrides he wishes to complex, puts them in an air tight ball mill with an inert or H2 atmosphere, and lets the ball mill hammer them into the desired product. At room temperature.

LiH + AlH3........................LiAlH4

U.S. Pat. 6,251,349 June 26, 2001 Method of Fabrication of complex alkali metal hydrides.

In a particular embodiment, fabrication of the complex alkali-metal based hydrides comprises mixing powders of simple hydrides (LiH, NaH, KH) with other hydride complexes (e.g. AlH.sub.3) or other alkali metal hydrides (LiAlH.sub.4, LiBH.sub.4, NaAlH.sub.4, NaBH.sub.4, KAlH.sub.4, KBH.sub.4 etc.) in a desired proportion and applying mechanical treatment at high impact energy, for example, by grinding, agitating or ball milling, in an inert atmosphere, for instance of argon or a hydrogen atmosphere. The fabrication can be carried out at a temperature below 100.degree. C. and in particular proceeds efficiently at room temperature (20.degree. C.).

By way of example the powders may be ball milled in a high energy ball mill such as that available under the Trade-mark SPEX 8000 or Model SP2100 (from SCP. Science of St-Laurent, Quebec). These ball mills typically employ steel balls or tungsten carbide balls. Suitable operating parameters include a weight ratio of mill balls to hydrides of 30:1 to 2:1 and a milling time of 0.25 to 20 hours, more usually 2 to 5 hours, however, a wide range of grinding, agitating or ball milling conditions can be employed to effect the desired mechanical alloying.

............................................................

For those unfamiliar with ball mills. A ball mill is much like a rock tumbler used to polish stones. Ball bearings, packed inside a rolling container, tumble over, and over, grinding and pounding......"milling"...the material they are mixed with, into the finest of powders.

Note. Newbees, this kind of stuff is for the very experienced....Even then it is touchy. Making, and working with metal hydrides is dangerous, and it requires special precautions.
 
 
 
 
    Rhodium
(Chief Bee)
06-21-02 04:01
No 323609
      Help! Compared to that, I'd prefer working with ...  Bookmark   

Help! Compared to that, I'd prefer working with cyanogen bromide without a fumehood.
 
 
 
 
    Elementary
(Hive Addict)
06-21-02 04:43
No 323616
      An alternative, but I think the standard ...  Bookmark   

An alternative, but I think the standard preparation is my favourite:

Lithium Aluminium Hydride

Made by adding lithium hydride to anhydrous aluminium chloride in ethereal solution, filtering, and evapourating the solvent.

4LiH + AlCl3 = LiAlH4 + 3LiCl



John Lennon - Working Class Hero
 
 
 
 
    zed
(Hive Bee)
06-21-02 13:44
No 323749
      Not for everyone.  Bookmark   

I think few will embark upon such a project. It requires building or purchasing metal reaction vessels, and learning how to safely use them.

That being said, if you wished to produce a pound of LiAlH4 or NaAlH4, this might be the way to go. It looks scalable.

It also has economy in its favor. I am under the impression that alkali metals are somewhat expensive, and difficult to acquire now.

This proceedure produces 1 mole LiAlH4 per 1 mole LiH.

The AlCl3 method requires 4 moles LiH to produce 1 mole LiAlH4.

Personally, this process scares me less then the AlCl3 proceedure. It worries me plenty, and I'm not trying to promote it. Still, some of the bees claim to make their own LiH, and for them.....this might be do-able.
 
 
 
 
    yellium
(Hive Addict)
06-21-02 13:54
No 323751
      How do you prepare the AlH3?  Bookmark   

How do you prepare the AlH3?
 
 
 
 
    Elementary
(Hive Addict)
06-21-02 18:59
No 323849
      AlH3 Preparation  Bookmark   

According to the Merck Index, the same way you make lithium aluminium hydride :

Monograph number: 354
Title: Aluminum Hydride. 
CAS Registry number: [7784-21-6]
Molecular formula: AlH3
Molecular weight: 30.01
Composition: Al 89.92%, H 10.08%. 
Literature references: Prepd by treating lithium hydride with an ether solution of aluminum chloride:  Finholt et al., J. Am. Chem. Soc. 69, 1199-1203 (1947).
Properties: Colorless solid, nonvolatile, probably highly polymerized and containing residual ether which cannot be completely removed.

USE:  As catalyst for polymerizations; reducing agent.  Lithium aluminum hydride is a more useful reagent because of its greater solubility. 


John Lennon - Working Class Hero
 
 
 
 
    Elementary
(Hive Addict)
06-21-02 19:16
No 323858
      But  Bookmark   

I could not find any patents regarding the preparation of aluminium hydride, but I did find this one on aluminium lithium hydride :

Patent GB707851

John Lennon - Working Class Hero
 
 
 
 
    Rhodium
(Chief Bee)
06-21-02 19:28
No 323865
      Why mill AlH3 and LiH together to form LiAlH4, ...  Bookmark   

Why mill AlH3 and LiH together to form LiAlH4, when you must make the AlH3 from AlCl3/LiH anyway? Just throw in an extra equivalent of LiH in the same pot, just like in the document on my page:

../rhodium/chemistry /lah.synthesis.html
 
 
 
 
    PrimoPyro
(Hive Prodigy)
06-21-02 19:31
No 323869
      Passivation  Bookmark   

If the passive oxide coating on aluminum were removed, would not aluminum be reactive enough to form metallic hydrides in a hydrogen atmosphere?

If not, would it then form under the influence of UV radiation in said atmosphere? (the UV causes H2 to dissociate to 2H. radicals)

                                                     PrimoPyro
 
 
 
 
    zed
(Hive Bee)
06-21-02 22:14
No 323929
      An excellent point!  Bookmark   

There aren't a lot of good techniques for preparing AlH3. If you have to make it via LiH & AlCl3, you might as well go ahead and make LiAlH4 while your at it.

Still, there may be better preps of AlH3 out there. And, what if you happen to have easy access to a bunch of rocket fuel grade, or fuel cell grade....AlH3? Maybe a rocket guy (like VL) has some insight as to the availability of this material.

The technique of producing LiAlH4 via ball milling, may....or may not prove to be practical. But, it is a novel idea.

Me, I prefer hydrogenations.

Primo, those glass paintball tanks, do indeed look interesting, but from what I have been able learn so far, they are hard to find, and spendy. Do you know of a good brand? Even better, would be a tank made of 316 stainless, but I fear such a thing doesn't exist.

And, no, generally speaking, Al will not react directly with H2. AlH3 is usually produced by reacting AlCl3 with a hydride(CaH, or NaH will do) or by decomposing an alkylaluminum compound. There may however, be better methods...unknown to me. I'll look further.

Ah, back. U.S. Pat. 6,228,338 has some information on AlH3 synthesis. Seems Aluminum Alkyls may be produced via Olefin, hydrogen, and metallic aluminum, thereafter these alkyls may be used to produce AlH3. I have no idea if the proceedure is a practical one. I'll look further.

Also of note! Shulgin seems to be using AlH3 (Alane) in some of his reductions. The reaction of LiAlH4 with H2SO4, produces AlH3 in situ.

From Patent:

BACKGROUND

Aluminum hydride, also referred to as "alane," is usually prepared as a solution by the reaction of lithium aluminum hydride with aluminum trichloride. A. E. Finholt et al. (1947) J. Chem. Soc. 69:1199. The alane-containing solution, however, is not stable, as an alane-ether complex precipitates from solution shortly after preparation. In addition, attempts to isolate the nonsolvated form of alane from the ether solution result in the decomposition of the complex to aluminum and hydrogen. M. J. Rice Jr. et al. (1956) Contract ONR-494(04) ASTIA No. 106967, U.S. Office of Naval Research.

In a method for preparing non-solvated alane, alane-etherate may be desolvated in the presence of a small amount of lithium aluminum hydride. See, for example, A. N. Tskhai et al. (1992) Rus. J. Inorg. Chem. 37:877, and U.S. Pat. No. 3,801,657 to Scruggs. Non-solvated alane exhibits six crystalline phases, with each having different physical properties. The phase designated as .alpha.'-alane is essentially non-solvated and appears under a polarizing microscope as small multiple needles growing from single points to form fuzzy balls. The .gamma. phase appears as bundles of fused needles. The .gamma. phase is produced in conjunction with the .beta. phase, and both .gamma.- and .beta.-alane are metastable nonsolvated phases that convert to the more stable .alpha.-alane upon heating. The .alpha.-alane is the most stable, and is characterized by hexagonal or cubic shaped crystals that are typically 50-100 .mu.m in size. The other two forms, designated .delta.- and .epsilon.-alane, are apparently formed when a trace of water is present during crystallization, and the .zeta.-alane is prepared by crystallizing from di-n-propyl ether. The .alpha.', .delta., .epsilon. and .zeta. polymorphs do not convert to the .alpha.-alane and are less thermally stable than the .alpha.-form. For a discussion of the various polymorphs, reference may be had to F. M. Brower at al. (1976) J Am. Chem. Soc. 98:2450.

Alane consists of about 10% hydrogen by weight, thereby providing a higher density of hydrogen than liquid hydrogen. Because of the high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, alane can be used as a fuel for solid propellants or as an explosive.

Solvated alane can be synthesized by the reaction of LiAlH.sub.4 with aluminum chloride, resulting in the alane.etherate complex (equation 1). ##STR1##

In an alternative synthesis, LiAlH.sub.4 is reacted with sulfuric acid to give the alane.etherate complex (equation 2). ##STR2##

The AlH.sub.3 -ether complex is then treated with a mixture of LiAlH.sub.4 and LiBH.sub.4, and heated (equation 3). ##STR3##

The combination of LiBH.sub.4 /LiAlH.sub.4 enables use of a lower processing temperature, and .alpha.-alane is the final product after heating at 65.degree. C. under vacuum. In an alternative synthesis, Bulychev reports that .alpha.-alane can be prepared at pressures greater than 2.6 GPa and at temperatures in the range of 220-250.degree. C. B. M. Bulychev et al. (1998) Russ. J. Inorg. Chem. 43:829. Under those conditions, apparently only the .alpha.-alane form is observed.

In addition, alane can be directly synthesized by metathesis of aluminum alkyls followed by removal of the alkylaluminum byproduct in vacuum (equation 4). ##STR4##

Still another method of preparing nonsolvated alane is by bombarding an ultrapure aluminum target with hydrogen ions. However, alane thus produced has poor crystallinity.

One of the obstacles to large scale production of .alpha.-alane is the handling of the diethyl ether solution of the alane.cndot.ether complex. At concentrations of about 0.5 M or higher and temperatures above 0.degree. C. the alane.cndot.ether phase prematurely precipitates out of solution. In addition, .alpha.-alane can be contaminated with other phases of alane, and is not stable over time as the complex decomposes to hydrogen and aluminum.

Thus, although alane is potentially promising as a high energy density fuel, because of its high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, the lack of a suitable method for synthesizing alane in a stabilized form has severely limited its applicability.