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Author: Subject: Different molecular sieves question
Mateo_swe
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[*] posted on 21-7-2021 at 06:37
Different molecular sieves question


Molecular sieves are used to dry solvents and chemicals.
I read 3A Molecular sieves are used to dry ethanol to absolute ethanol.
4A Molecular sieves are used to dry solvents for example DMSO.
The number indicates poresize in the sieves, how big molecules it can bind to if i understood this correct.

But what i dont understand is, if 3A molecular sieves can dry ethanol, that is bind the water molecules to the sieves, why is 4A molecular sieves used when drying solvents?
The water molecules are small enough to bind to 3A sieves, why is 4A sieves used to dry (bind the water molecules) in solvents like DMSO?

Does anybody know?
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Tsjerk
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[*] posted on 21-7-2021 at 11:19


4A sieves are cheaper because 3A sieves are made from 4A sieves.
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S.C. Wack
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[*] posted on 21-7-2021 at 14:16


One would think so but 3A is cheaper at Aldrich. 4A is the original so it's going to be mentioned, bought, and used often. 4A is said to have a slightly larger capacity, but this does not necessarily mean better drying.



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Oxy
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[*] posted on 21-7-2021 at 21:21


Quote: Originally posted by Mateo_swe  

But what i dont understand is, if 3A molecular sieves can dry ethanol, that is bind the water molecules to the sieves, why is 4A molecular sieves used when drying solvents?
The water molecules are small enough to bind to 3A sieves, why is 4A sieves used to dry (bind the water molecules) in solvents like DMSO?

Does anybody know?


4A sieves have larger holes which can trap larger molecules so they remove not only water but also other compounds.
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Mateo_swe
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[*] posted on 18-8-2021 at 06:17


OK.
So one could also use 4A molecular sieves to dry 96% ethanol to get absolute ethanol, is that correct?
Or would the ethanol molecule also get bound to the 4A sieves like the water molecules, making it not work?
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macckone
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[*] posted on 18-8-2021 at 13:18


No 4A sieves will not preferentially absorb water from ethanol.
Ethanol is small enough to fit in a 4A sieve.

3A sieves will absorb water but not ethanol.
To some extent they will also absorb methanol but it is right at the pore size so you can still use 3A to dry methanol.
But with methanol, it is not as efficient and your product is not as dry.

Also molecular sieves work best at removing more polar compounds from less polar compounds.
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Tsjerk
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[*] posted on 18-8-2021 at 13:40


To dry 96% ethanol you should first use a drying agent like MgSO4, to get it to 99%+

You can do it with just sieves, but you will need more and solvents tend to stick and you will have a relatively large mechanical loss.

Distilling off ethanol from a drying agent that doesn't release water at that temperature is best, after that you can use sieves to store that ethanol over, to remove the last water while standing.

CaO will give anhydrous ethanol after some reflux and distillation. Also magnesium works great, that will react with ethanol to give the etoxide. When distilling you will get dry ethanol.

[Edited on 18-8-2021 by Tsjerk]
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Syn the Sizer
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[*] posted on 18-8-2021 at 13:47


My understanding is what Oxy said, the different pore size is used to also absorb impurities from the solvent, like lets just say hypothetically your DMSO is also contaminated with ethanol, sure you could distill it off but just drop in the larger sieves and you take care of both.
Ethanol may have been a bad example just because of its low bp, but that is the idea.

As well I am sure you would end up with absolute ethanol using 4A sieves but also lose a ton of ethanol with the water in the sieves.




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