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Offline dfodor

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Hydrogen absorbing solvents
« on: July 14, 2009, 06:46:51 AM »
Hi!

I have been looking for solvents that readily absorb hydrogen. I found e.g., hexane, methanol and THF but did not figure out which is better solvent for hydrogen. For hydrogen absorption I usually found metals for hydrogen storage but I would need it for hydrogenation. Could somebody please list them ordered with regards to absorbing potential?

The other thing is that I found that hydrogen absorption increases with temperature. I did not find any explanation so far and would appreciate if somebody could explain this exception.

Best regards

Daniel


Offline Dan

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Re: Hydrogen absorbing solvents
« Reply #1 on: July 14, 2009, 02:00:15 PM »
but I would need it for hydrogenation.

Here is a summary of my experiences with hydrogenation, to be honest I'm really not sure about hydrogen absorption, certainly in a quantitative sense, but here goes... hopefully this is useful.

Ok, for hydrogenation/hydrogenolysis reactions I personally usually use ethanol most of the time. Methanol is also good, but if you have amine functionality you can get reductive amination with traces of formaldehyde in the methanol (ie you methylate the N).

Another good solvent for hydrogenation in my experience is water.

In my experience, THF is also a very good hydrogenation/hydrogenolysis solvent. Ethyl acetate and dioxane usually give very slow hydrogenolysis of benzyl groups in particular, these would be solvents I'd screen if I wanted to say, reduce an azide in the presence of benzyl ethers.

Another point to make is that aliphatic amines can poison the catalyst (I would normally use Pd on C, Pd black, palladium hydroxide or platinum oxide catalysts) and to overcome this you can add an acid, aq HCl works well, or use acetic acid as the solvent.
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Offline dfodor

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Re: Hydrogen absorbing solvents
« Reply #2 on: July 15, 2009, 03:29:52 AM »
Thank you very much for your summary.
My problem is with THF that it forms peroxides and I am not sure whether (despite reductive atmosphere) it is safe to use it as solvent in a stainless steel autoclave. I usually work at 150 Celsius degree and do not distill it prior use (so do not remove stabilizers). It has the disadvantage that I have 2-3 peaks in the NMR spectra (varying from experiment to experiment) I cannot assign to any of the possible products and GC-MS does not show any by-product.

Of course I will try out other solvents you mentioned but I am curious whether my concerns because of by-products and safety are reasonable.

Thank you

Daniel


Offline mdhussam

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Re: Hydrogen absorbing solvents
« Reply #3 on: November 14, 2009, 03:35:50 AM »
i am searching for solvent for absorbing hydrogen
can any 1 help me?
also can any 1 tell me how hexane is use to absorb hydrogen?

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