Sciencemadness Discussion Board

Recristallisation of NaBH4

EilOr - 17-3-2018 at 19:32

Hi!
I have a few hundred grams of a few years years old NaBH4
It's quality degraded a lot.

I know that it's possible to recristallise or even extract it from dyglime or liquid ammonia. Dyglime is very expensive and liquid ammonia not easy to handle without a lot of care and proper equipment. THF seems to be not practical as soloubility even when boiling would be too low I guess.

Is there another which could be used for purification?
Could it be recristallised from NaOH-solution etc. or is it too instable when heated?

If not, is it possible to make KBH4 from crude NaBH4? For example by adding solid NaBH4 to KOH solution (in IPA or water) and cooling or would be potassium borates from the inpurities the major product?

Ubya - 18-3-2018 at 08:29

i don't have any experience with NaBH4, but from this patent (https://patents.google.com/patent/US2856274) appears that you can recrystallize it in sodium hydroxide solution

Assured Fish - 19-3-2018 at 00:51

Um it should be noted that sodium borohydride generally reacts with water although im not sure if this rate is appreciable at room temperature.
Thus your patent mentioned this.
Quote:

Inaccordance with this method a small amount of water is added to the solvent used for treating the reaction mix ture of sodium borohydride and sodium methoxide to dissolve the sodium borohydride. The water reacts with some of the impurities to form an insoluble solid. While this method increases the purity of the product it does not produce a product having a sufliciently high purity for many uses. It has been known that the purity of sodium borohydride can be increased somewhat by recrystallization from water to obtain crystals of sodium borohydride dihydi'ate (NaBH -2H O). This method of purification does not produce the results desired and a substantial loss results since sodium borohydride is hydrolyzed rapidly in water. The dihydrate must be dehydrated by costly vacuum drying since the commercial form of sodium borohydride must be anhydrous.
Quote:

We have discovered that sodium borohydride of a purity of 98 percent by weight or greater in anhydrous form can be obtained by crystallization from a water solution if the solution also contains a predetermined amount of sodium hydroxide. The presence of sodium hydroxide in the solution also reduces the loss of sodium borohydride due to hydrolysis to practically nil.

This is odd, but i suppose it sounds plausible given that with no free protons present there wouldn't be enough protons present for the hydrogenation to take place given that protons are taken from the H2O as well to form sodium metaborate and 4 moles of H2 for every mole borohydride.
NaBH4 + 2 H2O → NaBO2 + 4 H2
Given that the solution would technically be already reduced by the addition of sodium to the H2O, perhaps there is a point where the H2O is no longer able to give up protons as you still need some protons left to stabilize the oxides and thus it cannot be reduced further.
Or perhaps simply the reduction of H2O with borohydride requires sites for neucleation and thus if the H2O is pure enough it does not get reduced under atmospheric conditions.
If there were an equimolar amount of NaOH present to H2O this may work although the patent does not use equimolar quantities of the 2.
Apologies this is a difficult concept for me to wrap my head around.
Certainly something i will play with should i ever acquire some sodium borohydride.

Sidmadra - 19-3-2018 at 07:55

I don't think recrystallizing it is worth the effort. Most people just use it in excess if they think it's degraded some, which generally is not an issue since it's usually used in excess anyways, and is very cheap industrial reagent to begin with.

Mabus - 20-3-2018 at 14:13

NaBH4 hydrolyzes very slowly in neutral water, but in the presence of acids (one source mentions boron oxide can be used) or certain metal salts like cobalt dichloride, it will rapidly decompose releasing hydrogen and heat. If it's not damp and was kept in closed bottle, even if it absorbed some moisture, there shouldn't be any significant degradation, unless it's really old.
As for recrystallization, it's soluble in amines, like pyridine, though I won't recommend using this route.

PirateDocBrown - 20-3-2018 at 16:09

Quote: Originally posted by Sidmadra  
... is very cheap industrial reagent to begin with.



Sure wish it were cheaper, and more readily available.