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Author: Subject: Using molecular sieves
FrankRizzo
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[*] posted on 19-8-2004 at 14:44
Using molecular sieves


Do you guys know of any online resources that detail how to use molecular sieves? I'm interested in drying both hardware store acetone and denatured alcohol, but have just found pages that either sell the beads or explain how they are made. I'm interested in how they're actually used in the laboratory.
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BromicAcid
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[*] posted on 19-8-2004 at 15:21


At my work we have a gas filter in the gas pump on the truck. It passes the gasoline above it and the water filters though it selectively and collects in a receiver. I believe it is some sort of molecular sieve but am not sure, they are readily available however.

Regardless, I am sorry to say that I have never used them in a laboratory although they seem quite useful.




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David Marx
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[*] posted on 19-8-2004 at 18:28


I've used molecular sieves several times. I recall drying THF and acetonitrile with them

How you prep them depends on where you bought them and what certification they are advertised. New out-of-bottle sieves should be good to use as-is. Open or older sieves should be dried first, as they have most likely picked up water from the atmosphere. The method I used to dry them was:

1.) Add the sieves to a 1.5 L rb flask
2.) Add an adapter and use a vacuum pump to draw a good vacuum in the flask.
3.) With the pump still running, heat the flask with a gas burner taking care to keep the flame moving so no one area of the flask warms overly and causes an implosion.
4.) Promptly remove the adapter, attach a rubber septum and backfill the flask with dry argon gas.

The beads are now ready for use. The easiest method is just to pour them directly into your solvent bottle. For a 500 mL bottle of THF I poured about 2-3 cm deep on the bottom the bottle and stored the bottle as usual.

Do you mind my asking where you are getting the sieves? I haven't seen any reasonably priced anywhere.

Hope this helps.
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svm
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[*] posted on 19-8-2004 at 19:46


david marx's method is good. you might want to repeat the vacuum-heat-backfill with dry gas cycle a couple of times and maybe shake the flask a bit to make sure all the sieves get heated.

i would do this process every time, even if the bottle is new. often times the stuff you buy isn't packaged dry.

another alternative is to just leave them in a drying oven or something at around 120 degrees centigrade for a few days. it requires that you have something to keep them hot that long, but then you don't need a vacuum and the sieves are always ready to go when you need them, so long as you keep them hot.

be careful to let the sieves cool a little before you add the solvent!
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mick
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[*] posted on 29-8-2004 at 08:12
Using molecular sieves


To check if the sieves are activated put a few on the palm of your hand and add a drop of water, if you drop them because they get to hot they are well activated. No heat, they need activating, 250-300 oC for 3 hours in an air oven or as above (100+ oC under vacuum) and put the in a heat resistant container with a good top and let them cool, test them and you will see what I mean. They can be stored above 100 oC for a while but the will gradually loose activity.
They are sodium and calcium aluminosilicates with cage like structures of varying pore size eg A3, A4, A5 etc, approx size in anstroms. To big a pore size and they might absorb the solvent as well as water. At room temp they are very strong drying agent and absorb other polar stuff that fits into the pores.
A4 seems good for most things but acetonitrile, methanol and ethanol need A3, apparently higher alcohols need powdered A3. They are not recommended for acetone because they can cause self-condensation but maybe you could dry the acetone over night, filter of the sieves and distil the acetone chucking away a fair amount of residue.
They can be reused, wash with solvent if necessary, air dry, dry in the open at approx 100 oC then re-activate in the oven. I have found that they can get a bit powdery after being reused a few times so I get rid of the powder by pouring the from one tray to another, with some paper tissue in there to stop them bouncing all over the place while blowing then with a heat gun.
To dry a solvent, batch dry it over 12 hours, 5% wt /vol and check that the previous batch of sieve was still well activated. For realy sensitive stuff the sieve needs to be filtered off and the solvent distilled before use because the sieve can leave catalytic amounts of metal ions in the solvent.
I have found them in the end of new silicone adhesive chaulking tubes, I do not know what grade. I think you might be able to get them in drying bags from photographic supplier as a replacement for silica gel. They are used in compressor filter to give very dry compressed air, I should think you can buy bags of replacement sieve from the suppliers or find a scrap compressor and see if it has got a filter with little balls or pellets in it, wash and re-activate. I not sure of the grade but maybe you could find out what make it hot besides water. Self indicating stuff might give you a higher metal ion concentration if that is going to be a problem.
Mick
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ordenblitz
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[*] posted on 29-8-2004 at 12:20


I use Grave Davison, grade 564 3a zeolite
sieves for general solvent drying.
Here is there web site where you can get more information.

http://www.gracedavison.com/eusilica/Adsorbents/product/zeol...

I ordered them from Cole-Parmer but I'd imagine they are available from many suppliers.
At one point Skylighter was selling them pretty cheap but I have no idea if he has it anymore.
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