Sciencemadness Discussion Board

Solvent drying - molecular sieves

amateur23 - 28-11-2016 at 05:00

Hello

I have read quite a lot about the use of molecular sieves for drying solvents. I noticed that most commonly people say the solvent should be left over molecular sieves for 12/24 hours. However, I would need to leave it for a week. Does anybody know if that would (somehow) affect my solvent? I would imagine it shouldn't be a problem since many solvents are normally sold over molecular sieves. However, I might be wrong - perhaps the procedure is different when you need to do it yourself. Any suggestions?

Thanks

Meltonium - 28-11-2016 at 05:03

Totally fine to leave it for a week, indefinitely even. Clean molecular sieves won't (or at least should not) affect you solvent in any way other than taking the water out of it.

Metacelsus - 28-11-2016 at 06:28

Remember, don't dry enolizable ketones (like acetone) with molecular sieves. You'll get aldol condensation reactions.

Other than that, molecular sieves are fine to leave in for extended periods of time.

Chlorine - 28-11-2016 at 06:47

If you're using molecular sieves, make sure your solvent is relatively dry, upwards of 95%.

Bert - 28-11-2016 at 07:38

Quote: Originally posted by Metacelsus  
Remember, don't dry enolizable ketones (like acetone) with molecular sieves. You'll get aldol condensation reactions.

Other than that, molecular sieves are fine to leave in for extended periods of time.


I have seen published instructions for drying acetone with molecular sieves, to be used in processing fireworks compositions containing Magnalium and bound with Parlon (chlorinated isoprene). Specifically, "go getter" self propelled colored stars-

Could you elaborate on these condensation reactions, and the time frame required

Scalebar - 28-11-2016 at 08:03

If it's an issue for you, mol sieves can be dusty as hell - I have to filter my anhydrous ethanol for optical use. We store ours over an excess ( about 25% volume ) of sieve until the bottle runs out, which can be months.

amateur23 - 28-11-2016 at 09:30

Hi

Many thanks for all the replies. I need to dry acetonitrile for a chiral derivatising reagent that is moisture sensitive. Last time I used the sieves, I had to filter the solvent as well as it was very cloudy.

Thank you for all your help.

Cryolite. - 28-11-2016 at 09:44

@Bert: As far as I know, those reactions are pretty fast: keto-enol tautomerism is relatively fast at the surface of the relatively basic molecular sieves, and condensation is again quick. To make matters worse, the water produced in the condensation gets absorbed into the sieves, preventing the reverse reaction from happening. For more information, see http://pubs.acs.org/doi/pdf/10.1021/jo00414a038

Chlorine - 28-11-2016 at 09:56

Quote: Originally posted by Cryolite.  
@Bert: As far as I know, those reactions are pretty fast: keto-enol tautomerism is relatively fast at the surface of the relatively basic molecular sieves, and condensation is again quick. To make matters worse, the water produced in the condensation gets absorbed into the sieves, preventing the reverse reaction from happening. For more information, see http://pubs.acs.org/doi/pdf/10.1021/jo00414a038


It may of been cloudy from clay dust suspended in solution.

DDTea - 28-11-2016 at 13:46

Quote: Originally posted by Bert  


Could you elaborate on these condensation reactions, and the time frame required


I've tested this myself with ~10 mL acetone in a 20 mL screw cap vial, with the bottom covered with mol. sieves. Within 4 hours, you will see a layer of yellowy condensation product surrounding the mol. sieves.

aga - 28-11-2016 at 13:58

Quote: Originally posted by DDTea  
will see a layer of yellowy condensation product surrounding the mol. sieves.

... or that could be the dust from the molecular sieves, which anyone ever having used any would recognise immediately.

Personally i know roughly Jack Shit about Chemistry, yet have actually tried to use these things, sometimes with success, sometimes with failure.

One of the most noteable things is how fragile those balls of clay are, and how much clay dust is Always present when you use them.

[Edited on 28-11-2016 by aga]

DDTea - 28-11-2016 at 14:21

Quote: Originally posted by aga  

... or that could be the dust from the molecular sieves, which anyone ever having used any would recognise immediately.


Maybe. But gas chromatography said otherwise: higher boiling products formed that were not present in the original acetone. Taken together with the color change and that these particular mol. sieves did not discolor other solvents, and I'm comfortable saying that the acetone reacted in a manner that it is known to. I'm relying on memory: this was a quick and dirty test, maybe 2 years ago, with data I no longer have access to.

S.C. Wack - 28-11-2016 at 15:48

Quote: Originally posted by amateur23  
I have read quite a lot about the use of molecular sieves for drying solvents. I noticed that most commonly people say the solvent should be left over molecular sieves for 12/24 hours. However, I would need to leave it for a week. Does anybody know if that would (somehow) affect my solvent?


Well at least it's not another one of those narrow super specific threads.
Quote: Originally posted by Bert  
I have seen published instructions for drying acetone with molecular sieves


Armarego and Perrin say 4A sieves; see Burfield and Smithers...who say don't use sieves (for acetone. 3A sieves for acetonitrile)...acetone reacts nearly instantly with strong enough bases.

Dr.Bob - 29-11-2016 at 16:57

I think acetone could be dried for minutes to an hour OK, I would not go much longer, then you could filter it is you wish to keep it clean of particulates. For most other solvents, you can dry them for days to months, we buy THF over sieves. It helps some, but not that much. But you do need to dry them well first under vacuum while heating in order to have them work well.

moonbeamstudio - 26-1-2017 at 23:22

Can someone tell me please - can silica gel (blue granules) can be used for drying solvents??
Thanks in advance!

Melgar - 27-1-2017 at 00:24

The blue is anhydrous cobalt chloride that's used as an indicator. It turns orange when it hydrates, indicating that it may need to be regenerated. Silica gel is slightly acidic, so as long as your solvent is able to handle those conditions, as well as the cobalt chloride, it should work decently well for drying.

Heavy Walter - 27-1-2017 at 06:26

I would not recommend the one with cobalt indicator.
It dissolves easily.
I have seen many contaminations.

alking - 27-1-2017 at 10:51

I always rinse my sieves off to remove any dust and then dry them before use. They come largely inactive so you have to dry them anyway, might as well clean off that dust in the process.

Dr.Bob - 27-1-2017 at 18:45

Quote: Originally posted by moonbeamstudio  
Can someone tell me please - can silica gel (blue granules) can be used for drying solvents??
Thanks in advance!


Silica gel can dry solvents, but blue stuff might leach. As well, it will not work well for polar solvents like MeOH which tend to dissolve/react with silica gel to create soluble silicates. Drying agents are best with already mostly dry solvents, there are tables of which ones work best with each solvent, just google "solvent drying agents" and you will find several, keep in mind than many require distillation after drying, such as calcium hydride, which cannot simply be filtered off of many solvents.