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Topic: Acidity of Molecular Sieves  (Read 7010 times)

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Offline baum0372

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Acidity of Molecular Sieves
« on: July 17, 2014, 11:48:30 PM »
Has anyone ever noticed if molecular sieves are slightly acidic?  I require some dry DMF and am concerned that the acidic residue from the sieves may affect the purity of the solvent.

I have the suspicion that sieves are slightly acidic, as I tried to make some dry ethyl vinyl ether by storing over 4Å sieves, and within a few hours the solution was noticeably more viscous, and after 2-3 days, the solution was a gel.  I suspect the vinyl ether polymerized due to acid in the sieves.

Anyone else had problems with sieves?

Offline kamiyu

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Re: Acidity of Molecular Sieves
« Reply #1 on: July 18, 2014, 04:47:20 AM »
Your post made me recall one fact about molecular seive, that soem site in the seive is super acidic (more acidic than pure H2SO4)

Not sure if my memory is right, tough.

Offline Arkcon

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Re: Acidity of Molecular Sieves
« Reply #2 on: July 18, 2014, 06:19:34 AM »
Your post made me recall one fact about molecular seive, that soem site in the seive is super acidic (more acidic than pure H2SO4)

Not sure if my memory is right, tough.

I'd like you to come up with a definitive reference for this fact as well.  As I understand it, molecular sieves are basically expanded refractory materials, generally, clays or zeolites.  http://en.wikipedia.org/wiki/Molecular_sieve  They might be slightly basic, but more acidic than sulfuric seems very far fetched.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline willug

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Re: Acidity of Molecular Sieves
« Reply #3 on: July 18, 2014, 07:42:05 AM »
Reminds me of this post at Daniel Sejer's 'Curley Arrow' blog;

http://curlyarrow.blogspot.co.uk/2010/04/anhydrous-solvents-part-3-acetone-and.html

Offline kamiyu

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Re: Acidity of Molecular Sieves
« Reply #4 on: July 18, 2014, 07:44:44 AM »
1) I am sorry I cannot come up with a reference about the acidity of molecular sieve. What I wanted to say was I once (many years ago) heard that MS can be super acidic.

2) Indeed, you can google the information with "molecular sieve superacid" or "molecular sieve acidic". I think you can get pretty much information there

Offline TheUnassuming

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Re: Acidity of Molecular Sieves
« Reply #5 on: July 19, 2014, 04:20:11 PM »
Mol sieves are generally considered to be "basic", though you could argue perhaps that its not acting as a traditional base but simply absorbing protons.  From the general absorbent idea they could perhaps be acidic if they absorbed hydroxide ions as well.  That said, I've only observed them acting functionally as a base.
When in doubt, avoid the Stille coupling.

Offline wildfyr

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Re: Acidity of Molecular Sieves
« Reply #6 on: August 04, 2014, 01:48:01 PM »
For one thing, I know you can't dry acetone over sieves because it causes aldol condensation

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