Chemichael
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Possible Dehydrogenation Alloy
Hello,
I have an idea for a dehydrogenation alloy, let me know what you think.
I was thinking of blending Lithium and aluminum in 1:1 mole ratio then powder granuals of the alloy under helium in a ballmill to make very fine
nanoparticles. Do you think if used in a reaction it would pull hydrogen to form LiAlH4, dehydrogenation the target molecule? I'm probably going to
try this in the future.
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Melgar
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No. Aluminum combining with hydrogen isn't thermodynamically favorable, and when Al-H bonds exist, they tend to transfer H atoms to other molecules
quite readily. Hence the use of LiAlH4 as a reducing agent. Most dehydrogenation reagents tend to be oxidizers, since water is very
thermodynamically stable. If the hydrogen atoms are acidic at all, then strong bases can be used to remove them.
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Chemichael
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Ok interesting, thank you!
Reaction
ΔH°(kJ/mol) ΔS°(J/(mol·K)) ΔG°(kJ/mol)
-116.3 -204.1 -44.7
Li (s) + Al (s) + 2 H2(g) → LiAlH4 (s)
Wouldn't this be a spontaneous, exothermic reaction?
The ΔH is a positive change in endothermic reactions, and negative in heat-releasing exothermic processes.
when ΔG is negative, the reaction is possibly spontaneous. When ΔG is positive, the reaction is possibly not spontaneous
[Edited on 26-1-2018 by Chemichael]
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Melgar
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The reaction you posed could actually work, maybe, at high temperatures and extremely high pressures with strict exclusion of oxygen. Lithium hydride
would form easily enough, but alane (aluminum hydride) will only form from the elements at ~10 GPa pressures:
https://en.wikipedia.org/wiki/Aluminium_hydride
I would expect lithium hydride to combine with alane to for the more-stable LiAlH4, but solid-solid reactions are always going to be impractical. And
molten, the hydrides would probably explode quite violently, unless pressures were ridiculously high for that reaction too.
That also assumes that your hydrogen is in elemental form. If your goal is dehydrogenation, there probably wouldn't be an exothermic reaction at all.
Mainly, there are practical (pressure/temperature/phase) concerns and stability concerns that you also have to take into account, in addition to ΔH
and ΔG.
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Chemichael
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I think the nanoparticles of Li-Al would be pretty reactive and would want to react in air. It would probably be used in a solventless reaction. If
kept under helium it should remain reactive. I ordered a bit of lithium and I still have to make the ball mill, but I'll definitely post my findings.
Ill have to ball mill outside away from the house just in case...should be fun. This is a project for later on for me though, since I have some other
reaction to do first. I'm interested in alloys and I'm probably going to also try making Ni5Ga3 and ball mill it until it forms a fine nanoparticle
powder and if it works as a catalyst it can be used to make methanol from CO2 and H2. I wonder if it can also reduce formic acid for example.
http://www.nature.com/nch…/journal/…/n4/full/nchem.1873....
[Edited on 3-2-2018 by Chemichael]
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