Synthesis of Sodium borohydride (NaBH4)

[deinemudda69]

Sodium borohydride (NaBH4) is a reducing agent that finds application in chemistry, both in the laboratory and on an industrial scale.
The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.

CAUTION:
Sodium borohydride is a source of hydrogen or diborane, which are both flammable. Spontaneous ignition can result from solution of sodium borohydride in dimethylformamide.
Zhu et al. (2020) (doi.org/10.1002/anie.201915988) report a facile method for the NaBH4 synthesis with high yield and low costs.

Using this method NaBH4 is mechanochemically synthesized by ball-milling Mg, Na2B4O7·10H2O, and Na2CO3 in a 18:1:1 molar ratio. A yield of 75.7% is reaced after 20h and a yield of 78.6% is reaced after 30h.
When Na2B4O7·10H2O and Na2B4O7·5H2O are ball-milled simultaneously with Na2CO3 and Mg in a molar ratio of 0.6:0.4:1:22, NaBH4 is also successfully synthesized.

Mg can also be replaced by Al or Ca to synthesize NaBH4.
To prevent Mg oxidation by atmospheric oxygen a inert atmosphere is necessary.

Kind regards

 

[MadHatter]

Wow, that sounds great. And have you done this yourself or is it purely theoretical?
So basically it would be possible to mix aluminum powder, borax and sodium carbonate in a ball mill (with water? sorry for the noob question. Or is it a dry reaction?) and run it for 20 + hours? At room temperature? Really? All the synths I've read are done at a really elevated temperature.
The inert atmosphere is of course a huge problem. Is that as critical when using Al dust? And do you have some tips on how to practically achieve this in a clandestine setting?

 

[deinemudda69]

To extract NaBH4 after mechanochemical synthesis a suitable solvent has to be chosen.

Sodium borohydride is soluble in most polar solvents that contain a hydroxyl or an amine group.

The stability of sodium borohydride in organic solvents is dependent on the degree of hydrolysis that can occur. Sodium borohydride instability in lower alcohols (methanol, ethanol) can be overcome by the addition of a base. The presence of water accelerates the hydrolysis reaction.

The stability of sodium borohydride in solution that contains water is dependent upon the temperature and pH of the solution. The hydrolysis reaction occurs evolving hydrogen gas decreasing the stability of sodium borohydride, when temperature is increased or pH is lowered.

Kind Regards

 

[deinemudda69]

For further reading I recommend https://doi.org/10.1021/acssuschemeng.0c04354
The chemicals used in this publication are NaH and MgH as well as the pentahydrate of Na2B4O7 making it more challenging.

Don't add water, the necessary hydrogen for the NaBH4 formation originates from the hydrated Na2B4O7. Furthermore NaBH4 decomposes in water with pH < 14.

Use a sealed container filled with inert gas for milling. Residual oxygen will be consumed by the Mg powder.

Yes, ambient temperature is fine, as the linked papers both use ambient temperature.

Kind regards

 

[MadHatter]

But I would probably opt to use aluminum powder, I guess it's the same for this?
And are there any critical safety concerns involved in this? Since I'll be using a hermetically sealed container, how about build up of vapor pressure during the reaction?

 

[deinemudda69]

Yes, that's what the paper says. Maybe you try it out and tell us if it worked for you.

I consider hydrogen build up possible in the process. Metal-powders, hydrogen and NaBH4 are flammable.

Kind regards

 

[MadHatter]

Hmm, then that leaves us with the very real and practical problem of tumbling a highly flammable bomb. If hydrogen pressure builds up in the tumbler tank (that needs to be air tight to keep an N2 amosphere), sooner or later it will explode.
So the solution, I guess, would be to install some kind of safety valve on the tumbler cylinder. A unidirectional outward flow valve.
So this does not appear to be a particularly "easy" synthesis, apart from on paper. I seriously doubt you have ever undertaken this yourself? This is a very THEORETICAL "easiness", and a discussion that maybe has a better home on Sciencemadness. Where theory can fill 20 pages followed by an experimental section that proves the whole process practically impossible in four sentences ...

 

[deinemudda69]

I wonder what exactly you consider to be complicated. The procedure only requires relatively non toxic and easy to obtain reagents and a simple apparatus.

A rotating angular shaped weck jar with rubber seal, filled with grinding balls is sufficient for small scale synthesis - this is relatively air tight keeping oxygen out but not too tight preventing pressure build up.

Hydrogen formation on decomposition is the key property of metal hydrides, there is no point complaining about that.

Please let me know if there is a more simple method for the synthesis of NaBH4 that I am not aware of.

Kind regards

 

[MadHatter]

I'm sorry, I didn't mean to sound as critical as I realize I did. I am very grateful for your sharing of information! And I'm sure there's no EASIER way, but that's not saying much since many of the traditional ways apparently involves temperatures above 600 C ...
What I mean is that the devil is in the details. And the details never become really apparent until you actually try the reaction yourself. Since you're never responding to my question I have to assume that you never have.

First off: what milling media and what milling method should one use? I have now gotten hold of the full text of the article, and they use a shaker type of ball mill producing 1000 cycles per minute. This is not your ordinary rock tumbler and not accessible to the clandestine cook. Since the amount of cycles was tightly linked to the result of the procedure in the article (possibly something to do with the amount of energy transferred to the material?), I'd say the results of tumbling in an ordinary rotary tumbler is perfectly unknown. That is, until you actually DO it.

Second: even though the materials are non-toxic, the reaction produces, like you wisely mentioned several times, highly flammable H2 in an unknown amount, and possibly, even likely, some heat. All this in a tightly sealed moving container with milling media that can produce sparks if not chosen very wisely, is a good description of a small bomb. So I would say there's definitely a good point complaining about that before the explosion rather than after .

So there's that. Not that it shouldn't be tried, but it's not a very easy process to undertake. An interesting one, definitely, on a scientific level! I can't however find any reference to using Al in the article, or am I reading it sloppily?
And if you or I actually try this reaction any time soon, I'm thinking absolute ethanol mixed with NaOH as solvent to purify the product. Should leave any unreacted borax and carbonate unsolved and available to filter out. What do you think?

Also kind regards!

 

[deinemudda69]

I agree with that. Depending on the milling method the milling times certainly varies.

The container is missing an oxidizing agent.

The mentioned paper says "Mg can also be replaced by Al or Ca to synthesize NaBH4 (Supporting Information, Figure S11)"


A solution of NaBH4 in ethanol releases 33% of the available hydrogen in 4 hours.

I have no reliable information about NaBH4 stabilization with NaOH in ethanol but I assume that it could work.

The high stability and temperature dependent solubility of NaBH4 in isopropyl alcohol renders it a more convenient solvent for extraction.

High purity NaBH4 (99.6 to 99.9%) may be obtained by recrystallization from dimethyl ether of diethylene glycol.

Kind regards

 

[MadHatter]

Yes, but it also contains Mg. I'm partly from the pyrotechnic world, and powdered Mg is a substance I know to treat with the utmost respect. It can form ignitable compounds with things you thought were completely inert, and ball milling of just about anything with Mg is only done with rigorous security routines in place. Shedding of the milling media, the container, the side products of these in combination with the H20 from the borax ... and then, if an explosion occurs, you have steel/ceramic balls shooting out ...

 

[deinemudda69]

What kind of safety measures would you recommend for the mechanochemical NaBH4 synthesis?

Do you think one should prefer electrochemical reduction of metaborate into borohydride over the mechanochemical synthesis?

 

[MadHatter]

Well, first off I'm definitely not convinced that this is even feasible without a high-energy ball mill. I think we're talking about weeks of milling, which in combination with the safety concerns renders the whole process useless in a clandestine setting. The article clearly states that NaBH4 starts to decompose if the milling time is longer than 10 h, and that the yield slopes down from thereon. They speculate that this is due to shedding of Fe from the milling media with subsequent build-up of side product Fe2B. This may be due to the high-energy nature of the mill, but it may also be an issue with a rotary tumbler. Say that the tumbler drum is made from PVC, or some other plastic, or from rubber ... what would be shedded from that with that kind of milling time?

If one WERE to do it, I'd say you have to be damn sure it's done under an inert atmosphere that has to be kept during the whole milling process. In the article, everything takes place in argon atmosphere in a glove box.
Next up, the presence of H2O in the mix is worrisome, and I'd never start or even be in the same room as the ball mill while its running. I'd never run it in a building where others are living, or even in a populated area (these are all basic safety procedures in pyrotechics BTW). And then there is the whole issue of actually, eventually, opening the container and let air in ... I actually don't know what realistic safety procedure to apply to that situation. A robotic arm?

 

[deinemudda69]

I really appreciate your warnings. This may prevents a accident.
One must clearly pay attention to adequate safety measures especially when the reaction is done in large scale.

I am thinking about aluminum oxide ceramic grinding balls to reduce this problem. In combination with a ceramic or glass jar I expect the wear not to significantly impede the reaction.


Elif Sanli et al. (2010) report a method for the electrochemical synthesis of NaBH4 at room temperature and atmospheric pressure. The use of Ag gauze electrode as catalyst is interesting, and still a relatively simple process when compared to the common industrial methods. doi:10.1016/j.jpowsour.2009.11.091

How do you guys get you NaBH4 are you not synthesizing it by yourself?
Please let me know if I am missing a synthesis route that is more convenient.

Kind regards

 

[T0lek511]

Oh that is good synthesis because in my country is NaBH4 so expensive