Baphomet
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Separation of NaBH4 and Glycerine
I've got a mixture of Sodium Borohydride in Glycerine, with visible crystals.
As far as I know Glycerine is miscible with acids but does not dissolve salts very well, so to hazard a guess this mixture is a slurry rather than a
(saturated) solution. Is this correct?
Also I am interested in finding out if the mixture would be OK for use in a reduction of an aldehyde. Glycerine should not interfere since it is a
polyol, correct? Thanks
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Baphomet
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I considered filtering the slurry to obtain dry crystals of NaBH4, but if anyone can think of a better method I'm all ears..
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Ozone
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Thanks, Smuv!
Ach! brainfart. NaBH4 is soluble in MeOH/EtOH (16.5 and 4%, respectively at 20°C; it also slowly reacts). A less polar alcohol such as propanol? THF
solubility is only 0.1% (don't know about what happens with a glycerol co-solvent).
Perhaps dilution with propanol to thin it (propanol or butanol might crash it) or ethanol, followed by drop-wise addition of hexane will do the trick?
Treatment with acid can lead to the dehydration of glycerol (don't heat this or you will get acrolein). Avoid.
How did you end up with NaBH4 in glycerol? I have always received a nice white powder.
Cheers,
O3
[Edited on 3-2-2009 by Ozone]
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smuv
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NaBH4 will probably slowly react with glycerin.
I agree filtering the crystals seems to be the way to go, although sodium borohydride is likely appreciably soluble in glycerin. You could probably
wash the crystals with cold ethyl acetate, or any other fairly non-polar solvent that will not dissolve/react with NaBH4.
Another option would be to add another solvent to the glycerin that will cause any dissolved NaBH4 to crash out; unfortunately, glycerin is so polar,
I cannot fathom a suitable solvent to use.
Good Luck. Its a very odd situation.
[Edit] Ozone and I posted at about the same time...
@Ozone NaBH4 is pretty soluble in ethanol, I think it would only allow more NaBH4 into solution.
Also, adding acid is a no-no not only because of glycerol dehydration (which requires a bit of heat), but also because it will quickly destroy the
borohydride.
[Edited on 2-4-2009 by smuv]
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Baphomet
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Perhaps dilution with propanol to thin it (propanol or butanol might crash it) or ethanol, followed by drop-wise addition of hexane will do
the trick?
Good idea! I'll try it with ice cold propanol.
How did you end up with NaBH4 in glycerol? I have always received a nice white powder.
Two words: fuel cell 
NaBH4 will probably slowly react with glycerin.
Correct, I'm not sure but the mixture may have had it's pH adjusted to prevent this.
I agree filtering the crystals seems to be the way to go, although sodium borohydride is likely appreciably soluble in glycerin. You could
probably wash the crystals with cold ethyl acetate, or any other fairly non-polar solvent that will not dissolve/react with NaBH4.
I will try both your & O3 ideas, then the filtering, and compare to find which method is best.
[Edited on 4-2-2009 by Baphomet]
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smuv
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Sodium borohydride reacts very slowly with isopropanol (make sure its dry) and isn't very soluble in it (I would expect especially in the cold); this
might be a good washing solvent.
I also just thought of the possibility of salting the NaBH4 out with sodium hydroxide. Although adding NaOH to glycerol will produce some water, I
think the basic conditions will not allow much NaBH4 to react.
I think trying both methods separately is a bad Idea; you should filter and wash the crystals, then take the filtrate and try ozone's ideas or the
salting out method on the filtrate.
[Edited on 2-4-2009 by smuv]
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Klute
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I would try diluting with a healthy amount of toluene or ethyl acetate, and then filter the precipitated borohydride, as mentionned above.
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smuv
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From merck index -- Glycerol
Miscible with water, alcohol. One part dissolves in 11 parts ethyl acetate, in about 500 parts ethyl ether. Insol in benzene, chloroform, carbon
tetrachloride, carbon disulfide, petr ether, oils.
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garage chemist
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Don't add alcohols, they will rapidly form the alkoxyborohydrides with evolution of hydrogen.
Add water with a small amount of NaOH in it (to stop hydrolysis) to your NaBH4-glycerine mixture until it is water-thin, and then extract the solution
several times with isopropylamine. Dry and evaporate the isopropylamine phases.
Isopropylamine is the key solvent for the extraction of NaBH4 from water in the manufacture of NaBH4, where it is initially produced as a solution in
aqueous NaOH.
Isopropylamine can also be used to purify old, partially hydrolysed NaBH4 by extraction followed by evaporation.
If you want to do purifications of NaBH4 from whatever else, get isopropylamine.
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smuv
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| Quote: | | Don't add alcohols, they will rapidly form the alkoxyborohydrides with evolution of hydrogen. |
This is only true for methanol and to a much lesser extent ethanol; Isopropanol and t-butanol will react with NaBH4 no faster than glycerin will.
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chemrox
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So he could wash it with 99% isopropyl (2-propanol) alcohol and dry the flakes or powder on a flat dish...I wonder why it was mixed that way?
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Baphomet
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Thanks for all the pointers, guys. Isopropylamine sounded like a great idea but alas, I have only plain IPA available. Chemrox was right on the money,
cold IPA wash did the trick.
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Baphomet
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Just a postscript, for anyone who wants to try in future. I did a second run with ethanol (methylated spirits dried with CuSO4) and it was
much faster at dissolving the glycerol. Ethanol reacts with NaBH4 apparently, but it is slow enough to not matter for this
application. The salt was then quickly dried and placed in an airtight container with a bag of silica.
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