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Topic: vacuum activation of molecular sieves--how to release the vacuum  (Read 4422 times)

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Offline Babcock_Hall

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vacuum activation of molecular sieves--how to release the vacuum
« on: November 13, 2014, 12:30:20 PM »
In a thread on calcium hydride the subject of activating molecular sieves came up.  Two people suggested using a flame:  "We don't have a vacuum oven either, so I usually heat them with a torch under high-vac.  It works great, just be careful heating up a flask under high-vac.  A post-doc in the hood next to mine was doing this once but kept the torch pointed at one point too long and the RBF distorted and dimpled in."  An alternative to this is to use an oil bath at 120 °C and a vacuum.  This link has a picture:  http://collum.chem.cornell.edu/documents/Molecular_sieves_activation.pdf

My question is how to release the vacuum safely.  We have a Firestone valve, but we usually use it to release partial vacuums.  I was a little worried about pulling the oil out of the needle valve, but one set of instructions for its use suggests closing slightly the fingertip control. 
http://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Aldrich/Bulletin/al_techbull_al119.pdf

Offline TheUnassuming

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Re: vacuum activation of molecular sieves--how to release the vacuum
« Reply #1 on: November 13, 2014, 03:06:36 PM »
An alternative to this is to use an oil bath at 120 °C and a vacuum.  This link has a picture:  http://collum.chem.cornell.edu/documents/Molecular_sieves_activation.pdf
Bah... the torch is faster and more exciting!  And what is this putting them in your hand and adding water to test if they are good, just pop one in your mouth.  Works like a charm and there is no denying if its active. :P

In all seriousness though, my setup is a little different but still has an oil trap/bubbler that can get pulled back into the system if you aren't careful.  Generally though I've found as long as you tighten the bubbler up and slowly release the vacuum into your gas system it doesn't cause much havoc if any.
When in doubt, avoid the Stille coupling.

Offline Babcock_Hall

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Re: vacuum activation of molecular sieves--how to release the vacuum
« Reply #2 on: November 13, 2014, 05:32:58 PM »
One other option, as Discodermolide noted in a different thread, is simple heat activation without a vacuum.  Burfield, Gan, and Smithers discussed this in an article in J. Applied Chem. Biotechnol. (1978) 28, 23-30.  They dried their sieves at 300-320 °C overnight, although they say that activation is complete in three hours at this temperature.  Elsewhere they recommend 250-300 °C for active sieves.

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