EilOr
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Ti-(and DABCO) doped alanes as possible catalytic hydrogenation catalysts? (Aka AlH3 from Al and H2 in situ at low pressure)
Hi!
Most likely it's a bullshit idea, as I have no idea about the mechanisms going on, but I like to share it anyway 
Could Ti-doped alanes (AlH3, LiAlH4, NaAlH4) be used to reduce substrates and recycled in-situ by addition of H2 gas at moderate pressures?
I found tons of different articles about regeneratable Ti-doped alanes on Google, some of them using solvents like THF with DABCO as co-solvent (as
shown below) and some with NaAlH4 in plain THF with <0.5% TiCl3 addition and more.
http://scholar.google.com/scholar?q=Ti-doped+naalh4&hl=d...
For people who doesn't know how to get full articels, check www.sci-hub.la - there you can get any article for free without registration, all you need is the DOI or URL
Unfortunately most of these articles are focused on hydrogen storage, but my idea is to use these systems for catalytic hydrogenation
Example procedure for (reversible) synthesis of AlH3:
"Direct and Reversible Synthesis of AlH3, Triethylenediamine (DABCO) from Al and H2"
"Hydrogenation and dehydrogenation reactions were carried out in a 300 mL stainless steel stirred reactor (ParrInstruments) 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 ofthe argon remaining in the reactor. Hydrogen gas was added to the reactor at pressures of 35
atm."...
Full article: http://sci-hub.la/10.1021/jp076804j
So using a little DABCO (triethylendiamine), Ti-doped Al-powder and H2 forms AlH3 in high yields at moderate pressures in THF.
In my experience most reductions wich work at <50atm also work at 1-10atm with better stirring and higher catalyst load, so this might be even
possible to be done with usual glass-equipment.
AlH3 is a very strong reducing agent (even stronger than LAH but also leaving benzen-rings intact: http://sci-hub.la/10.1021/ja00959a027) only less used in laboratory because of worse soloublity in ethers and especially more extreme sensivity to
air. Though air-sensivity and soloubility would'nt be a big problem when it's formed in-situ.
So I wonder if such a system could be also used to reduce nitroalkenes, nitriles, epoxides, non-activated double bonds, carbonic acids, esters and
amides, where usual Pd/C, Raney-Ni, PtO2,... catalysts would need rather extreme conditions (like ~100-200bar, strong acids as solvents, high
temperature) - so it might it be useful for catalytic hydrogenation at ambient temperature and low or moderate pressures!?
Though I'm not sure at all about the mechanisms and if it could work on all substrates (Al(OH)3 formation in case of nitro-groups and so
on? Then maybe excess Al(Ti) could be used to compensate?).
Any thoughts about it? Could it work like this or would be more necessary? Anyone likes to try?
Unfortunatly I have no DABCO or NaAlH4, only thermodynamically less stable (so I guess more pressure necessary?) LiAlH4, and my equipment would need
stronger hoses to handle more than 20 bar, otherwise I could give it a try
And do you have an idea if there might be a good substitute for DABCO (maybe triethylamine or ethylenediamine?), as it seems to be quite expensive?
What do you think about doping LiAlH4 or NaAlH4 with TiCl3 and using them as catalyst in THF to avoid usage of DABCO
[Edited on 13-12-2017 by EilOr]
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zed
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Well. it is a thought provoking possibility.
Trust me, I printed up a copy of this one.
Thank you!
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zed
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Ummm. I'm going to read further. But, the easy generation of Alane, by hydrogenation of activated Aluminum, would be a great end, onto itself.
Provided you don't have to start by decomposing NaAlH4 or such, to produce said activated Aluminum.
Been a lot of work done with complex hydrides, and hydrogen storage. Unfortunately, the folks doing that research, are at cross purposes, with most
O. chemists.
Personally, I want to easily produce Alane, for one-use only. Make it by easy hydrogenation, use it for a difficult reduction, hydrolyse the reaction
complex, and laugh all the way to the bank.
Huge amounts of cash, have gone into creating complex hydrides as a component of possible energy storage systems. About the reducing properties of
those complex hydrides, little might be known.
The researchers want to store/release/and re-store hydrogen..... for use as a fuel. They are intent on developing a re-chargable storage media. We,
as chemistry guys, aren't generally involved, in their calculations. We are mostly, on a parallel track.
[Edited on 4-1-2018 by zed]
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EilOr
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Nice to hear that someone else is interested in that area
It could possibly substitute LAH, BH3*THF, Raney-Ni and Pd/Pt on C, for a fraction of their price, I wonder why not more research is done
[Edited on 7-1-2018 by EilOr]
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Reboot
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A lifetime ago, when I was a stupid kid, I tried something, well, stupid.
I set up a bog standard aluminum-mercury amalgam (in 'wet' methanol) inside a 2 liter pop bottle.
Once the amalgamation was complete, I tossed in a fairly generous amount of nitromethane. My main question was whether the reaction could be managed
simply by cooling the pop bottle in a tub of ice water as needed. Since the reaction was certain to release some hydrogen gas, I was also prepared to
uncap it to release pressure if needed.
It performed as you would expect, with a lot of gas evolution, bubbling, and ultimately a lot of heat production, but it turned out that it was
actually pretty easy to manage the reaction by shaking it a bit by hand and cooling it now and then in the tub when the pressure in the pop bottle
rose. (It was easy to tell by feel of the bottle.) I kept the bottle sealed through the entire reaction.
The strange thing was that when it was all done, when it had cooled back down to room temperature...there was absolutely no pressure built up in it.
There was no sign of the hydrogen that had been produced.
Which brings me to the topic at hand: Perhaps the hydrogen being produced was being adsorbed onto the aluminum. From there, it's easy to imagine
it being converted into hydride by the exposed aluminum.
To this day I can't say that I really know how the mechanics of the amalgam reduction work, but...perhaps it's a hydride formation in situ?
For what it's worth, in another spot test I added some nitromethane to a small amount (perhaps a gram) of aluminum that had been covered with a sodium
hydroxide solution. As it heated up, the reduction became absolutely fearsome, cracking the nitromethane all the way down to brown nitrogen oxides.
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EilOr
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Quote: Originally posted by Reboot  | A lifetime ago, when I was a stupid kid, I tried something, well, stupid.
I set up a bog standard aluminum-mercury amalgam (in 'wet' methanol) inside a 2 liter pop bottle.
Once the amalgamation was complete, I tossed in a fairly generous amount of nitromethane. My main question was whether the reaction could be managed
simply by cooling the pop bottle in a tub of ice water as needed. Since the reaction was certain to release some hydrogen gas, I was also prepared to
uncap it to release pressure if needed.
It performed as you would expect, with a lot of gas evolution, bubbling, and ultimately a lot of heat production, but it turned out that it was
actually pretty easy to manage the reaction by shaking it a bit by hand and cooling it now and then in the tub when the pressure in the pop bottle
rose. (It was easy to tell by feel of the bottle.) I kept the bottle sealed through the entire reaction.
The strange thing was that when it was all done, when it had cooled back down to room temperature...there was absolutely no pressure built up in it.
There was no sign of the hydrogen that had been produced.
Which brings me to the topic at hand: Perhaps the hydrogen being produced was being adsorbed onto the aluminum. From there, it's easy to imagine
it being converted into hydride by the exposed aluminum.
To this day I can't say that I really know how the mechanics of the amalgam reduction work, but...perhaps it's a hydride formation in situ?
For what it's worth, in another spot test I added some nitromethane to a small amount (perhaps a gram) of aluminum that had been covered with a sodium
hydroxide solution. As it heated up, the reduction became absolutely fearsome, cracking the nitromethane all the way down to brown nitrogen oxides.
|
I guess your nitromethane was used in excess and so all the H2 was consumed into H2O and methylamine, as I think metal hydrides (very basic) should be
very instable in an excess of (quite acidic) methanol. Even very stable alkali-borohydrides are quite unstable in methanol, though I think at least
cyanoborohydrides are quite stable, so there might be a little chance.
Or you bottle wasn't as airtight as you might think, H2 is a tiny molecule 
But if both could be excluded it would be REALLY interesting what might have been formed! Maybe the the Al/Hg reduction mechanism isn't an
dissolving-metal reduction mechanism after all, which would be a great find and explain a few things why Al/Hg+Alcohol behaves so differently than
Zn/HCOOH, Fe/HCl, Na/MeOH etc. 
A similar experiment should get more promising with THF as solvent in a diving bottle (and maybe a few milligrams of a Titanium-salt like used in the
patents) and a little methanol or water as hydrogen donor, which then might produce usable amounts of AlH3. Though of course producing the H2 in-situ
would be much less safe than simply buying the H2 gas or producing it by electrolysis. Maybe one day there will be a wet and safe one-pot synthesis
for NaAlH4, LAH and AlH3 from aluminiumpowder
[Edited on 7-1-2018 by EilOr]
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Reboot
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| Quote: | | A similar experiment should get more promising with THF as solvent in a diving bottle (and maybe a few milligrams of a Titanium-salt like used in the
patents) and a little methanol or water as hydrogen donor, which then might produce usable amounts of AlH3. Though of course producing the H2 in-situ
would be much less safe than simply buying the H2 gas or producing it by electrolysis. |
Well now you're giving me ideas. :-) I have a shiny new (75 ml) chemglass pressure vessel and a recently purchased tank of hydrogen. I've also
been seeing very reasonably priced 'digestion vessels' on eBay:
https://www.ebay.com/itm/50ml-PTFE-Lined-Hydrothermal-Synthe...
Apparently the common usage is for autoclaving samples in acid to break them down, but either way it's a fairly high pressure (400+ psi) reaction bomb
with a PTFE inner vessel for $50.
The idea of doping with titanium makes me wonder what role the mercury in a traditional amalgam might be playing beyond disruption of the aluminum
oxide layer. I've decided not to work with mercury compounds again, so alternatives are certainly of interest (there's been some neat work on tin
chloride in particular as an amalgamating agent for aluminum.)
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