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Author: Subject: Molecular sieves - care and feeding
ahill
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[*] posted on 11-9-2016 at 03:48
Molecular sieves - care and feeding


I dont have any experience with molecular sieves - but the other day I ordered some 3A zeolite molecular sieves from ebay with a mind to using them for drying solvents.

I just gave them a go - I added 150g of sieves to 250 ml of 95% ethanol - and very gently agitated occasionally for 6 hours. I then filtered out the sieves and checked the concentration again - 99% ! The ethanol was now quite turbid tho - the sieves were kind of dusty, and it must have been pretty fine, cause it went straight thru my 1845 25um filter paper. I kind of thought I might need to re-distill - but I was hoping not to have to. Also - now I only have 208ml of ethanol - so they have absorbed significantly more alcohol than water.

The sieves are balls - about 5mm diameter - they seem to have a lighter coating over a darker inside - I know this 'cause quite a few are broken - which brings me to my first question - should I be thowing away the broken ones ? And should I have washed them before using ? And between uses ? And what about my 30 odd ml of ethanol - should I be expecting losses like that ? Would that have something to do with the broken balls ?



maybe 1% are broken like this ..and they look really grungy in this photo - but to the eye they are a fairly consistent ivory colour.



[Edited on 11-9-2016 by ahill]
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[*] posted on 11-9-2016 at 05:53


Between uses, you'll need to reactivate them by strong heating. Broken ones should still work fine. Your turbidity was probably from dust left over from the manufacturing process, so if you repeat the experiment, you'll likely observe much less of it. The lost ethanol could also have just been adhering to the exterior of the sieves, instead of being absorbed by them.



As below, so above.
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[*] posted on 11-9-2016 at 08:18


I stick my 3A's in the oven for about 2 hours at 250 C to regenerate them, then let them cool.

You can tell if they're 'activated' by placing a small pile on a GLOVED hand and add a squirt of water - they get very hot very fast if they're activated (hence the glove !).

The dusty stuff filters out easily, especially from ethanol.




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careysub
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[*] posted on 11-9-2016 at 10:44


According to the literature, mol sieves can absorb up to 22% of their weight in water, and oven dried sieves retain 2-3% water (you need vacuum drying to remove that).

When drying 250 mL or 95% ethanol (% V/V) you have 12.5 g of water, so you might try first drying with about 65 g of sieves (theoretical capacity 14 g or so, 13 g with 2% water content) to soak up most of the water, then using 15-25 g of sieves to get maximum drying with less ethanol absorption loss. Also, waiting 24 hours might be better to get maximum absorption for both batches.

This paper describes the preparation of super-dry ethanol with sieves, but their starting "wet" ethanol has only 0.14% water, and they are drying it to 10-25 ppm (0.001-0.0025%). 5% mol sieves (W/V) is sufficient to achieve super-dryness after 5 days.

The paper also recommends KOH to dry ethanol, which is attractive due to its low cost (in the U.S. anyway), and which I have used with success to dry ethanol.

However, I then discovered that KOH is soluble in ethanol so the advice to use KOH seems questionable, unless it is insoluble in dry ethanol. Does anyone know?

Attachment: Solvent_Drying.pdf (1.8MB)
This file has been downloaded 197 times

[Edited on 11-9-2016 by careysub]
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[*] posted on 11-9-2016 at 11:50


PUBCHEM says KOH is soluble in Ethanol, but dosnt specify if the Ethanol is Anhydrous.

It does mention Anhydrous KOH, but i thought that was a bit of an oxymoron? Surely KOH is never really Anhydrous? I have presumed KOH like NaOH always contains some water?

So if it does contain water then would it make any difference if the Ethanol was dry? Wouldnt the Ethanol take water from the KOH and therefore become wet and the KOH would be soluble in it?? If you see what i mean...


Interesting question though.

[Edited on 11-9-2016 by NEMO-Chemistry]

Not sure how reliable researchgate is but the question was asked there.

https://www.researchgate.net/post/How_can_I_prepare_an_alcoh...

The gist of it being KOH is soluble in absolute Ethanol. however it also mentions that KOH is never completely dry.......

[Edited on 11-9-2016 by NEMO-Chemistry]

Found something on SIGMA

According to this Anhydrous KOH is soluble in Ethanol https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/...

[Edited on 11-9-2016 by NEMO-Chemistry]
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[*] posted on 11-9-2016 at 12:05


No idea about absolutes, but my KOH will dissolve in absolute ethanol.

My NaOH too, but not so much.




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[*] posted on 11-9-2016 at 13:10


I guess one must distill ethanol dried with KOH, to remove it. Not so convenient.

And would the water stay with the KOH or follow the ethanol, defeating the whole purpose?

[Edited on 11-9-2016 by careysub]
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[*] posted on 11-9-2016 at 14:37


Quote: Originally posted by careysub  
I guess one must distill ethanol dried with KOH, to remove it. Not so convenient.

And would the water stay with the KOH or follow the ethanol, defeating the whole purpose?

[Edited on 11-9-2016 by careysub]


Turns out it might not be so simple as i thought.

Alcoholic KOH is used for water sensitive reactions, apparently the KOH becomes a stronger base.

So it would seem reasonable that Anhydrous KOH and Ethanol do actually exist.

But that is kind of getting away from drying Ethanol with KOH as water is involved.

I love the way nothing is as it first seems in chemistry. Just a guess but the water should stay with the KOH, i base this on the fact you dry Ethanol with KOH.

Therefore KOH must have a greater affinity for water than Ethanol??
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[*] posted on 12-9-2016 at 04:39


Quote:
The paper also recommends KOH to dry ethanol, which is attractive due to its low cost (in the U.S. anyway), and which I have used with success to dry ethanol.

However, I then discovered that KOH is soluble in ethanol so the advice to use KOH seems questionable, unless it is insoluble in dry ethanol. Does anyone know?


Have a look at the paper you've attached, in case of KOH a distillation was performed, see footnote 'b' in table 5 or 6.
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[*] posted on 12-9-2016 at 11:57


Quote: Originally posted by careysub  
According to the literature, mol sieves can absorb up to 22% of their weight in water, and oven dried sieves retain 2-3% water (you need vacuum drying to remove that).

When drying 250 mL or 95% ethanol (% V/V) you have 12.5 g of water, so you might try first drying with about 65 g of sieves (theoretical capacity 14 g or so, 13 g with 2% water content) to soak up most of the water, then using 15-25 g of sieves to get maximum drying with less ethanol absorption loss. Also, waiting 24 hours might be better to get maximum absorption for both batches.

This paper describes the preparation of super-dry ethanol with sieves, but their starting "wet" ethanol has only 0.14% water, and they are drying it to 10-25 ppm (0.001-0.0025%). 5% mol sieves (W/V) is sufficient to achieve super-dryness after 5 days.

The paper also recommends KOH to dry ethanol, which is attractive due to its low cost (in the U.S. anyway), and which I have used with success to dry ethanol.

However, I then discovered that KOH is soluble in ethanol so the advice to use KOH seems questionable, unless it is insoluble in dry ethanol. Does anyone know?



[Edited on 11-9-2016 by careysub]

K2CO3 works reasonably well, as does MgSO4, although I just purchased five pounds of silica gel desiccant, so I've been using that for drying everything lately.
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[*] posted on 12-9-2016 at 12:58


Quote: Originally posted by Alice  

Have a look at the paper you've attached, in case of KOH a distillation was performed, see footnote 'b' in table 5 or 6.


Thanks, I overlooked that bit (obviously). So you can distill over KOH and the water stays with the alkali. I had read elsewhere on this site that this does not work for magnesium sulfate, that at the temperature of distillation it loses water.
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[*] posted on 13-9-2016 at 05:10


KOH is much more hygroscopic than magnesium sulfate, thus the following equilibrium is shifted more on left side for KOH. Less free water means less water being removed by distillation:

DA*H2O <------> DA + H2O
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[*] posted on 13-9-2016 at 06:18


For the shorter alcohols, anything with more than 15% water should be distilled first.

K2CO3 works wonders to remove up to about 15% water. Saturated solutions of K2CO3 will actually "salt out" both MeOH and EtOH, which is impossible with most other salts. A careful finishing pass over fresh sieves leaves them very dry.

This is my preferred method to dry anything that isn't affected by the rather strongly basic K2CO3. It works great with MeOH, EtOH, THF, DCM, TCE, chloroform, and Et2O, among many others. It is not so suitable for common ketones like acetone and MEK since the basic K2CO3 catalyzes aldol condensations.




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