Author Topic: molecular sieves and absorbtion capabilities  (Read 7998 times)

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armageddon

  • Guest
molecular sieves and absorbtion capabilities
« on: September 02, 2004, 01:05:00 AM »
Hi bees!

Molecular sieves are a very useful tool for the organic chemist: depending on their pore size, they absorb only certain sizes of molecules, and leave behind all bigger molecules - hence the name "sieves".

Speaking technically, they are small globules made of special synthetic zeolithes (clay mineral), and they look like flower pot clay formed into little globules.  ;)

Their pore size is measured in Angstrom (or "A" with "°" on it, I'm not familiar with swedish letters on my keyboard...  :) ), with 3A sieves being the smallest pore diameter available (the "finest" sieves). 3A sieves are only capable of absorbing molecules having the size of water or smaller, thus they are the perfect drying agent for *every* purpose (they work with liquids as well as with gases BTW).

But does any bee know about the different absorption capabilities of different pore size sieves? I.e. what can 4A sieves absorb, what absorb 5A sieves etc.?

Having a table about which substances different mol. sieves can absorb would be handy, don't you think?

Any input would be greatly appreciated!

Greetz A


Captain_America

  • Guest
sieve
« Reply #1 on: September 02, 2004, 01:19:00 AM »
check the zeochem's site

http://www.zeochem.com/types/types.htm



they also have a list of papers covering the theory that are available upon request...

armageddon

  • Guest
Tutorial: Molecular Sieve Adsorbents
« Reply #2 on: September 02, 2004, 01:37:00 AM »
Molecular sieve adsorbents

Molecular sieve adsorbents are crystalline alumino-silicates. Their unique structure allows the water of crystallization to be removed, leaving a porous crystalline structure. These pores or "cages" want to readsorb water or other molecules. Aided by strong ionic forces caused by the presence of cations such as sodium, calcium and potassium, the molecular sieve will adsorb a considerable amount of water or other fluids. If the fluid to be adsorbed is a polar compound, it can be adsorbed with high loadings even at very low concentrations of the fluid.

This strong adsorptive force allows molecular sieves to remove many gas or liquid impurities to very low levels (PPM or less).

Another feature of molecular sieve adsorbents is its ability to separate gases or liquids by molecular size. The pore or "cage" openings are of the same size as many molecules. In the case of hydrocarbon paraffins, the normal, straight chained molecules can fit into the pores and be adsorbed while the branched chain molecules cannot enter the pores and pass by the molecular sieve adsorbents unadsorbed.




Molecular sieves type 3A: Will exclude most molecules except water. It is used for natural gas dehydration, cracked gas drying, olefins drying, ethanol drying, etc. It is usually made by ion exchanging potassium onto a Type 4A in place of sodium.

Molecular sieves type 4A: This sodium form of the crystal Type A is an inexpensive adsorbent usually used for drying. Further processed forms can be used for carbon dioxide removal.

Molecular sieves type 5A: This is the calcium exchanged form of the A crystal. The strong ionic forces of the divalent calcium ion makes it an excellent adsorbent for removing carbon dioxide, hydrogen sulfide and other weakly polar molecules. This product is also used for separating normal straight chain hydrocarbons from branched hydrocarbons.

Molecular sieves type 13X: The sodium form of the Type X crystal has a much larger pore opening than the Type A crystals. It also has the highest theoretical capacity of the common adsorbents and very good mass transfer rates. It can remove impurities too large to fit into a Type A crystal and is commonly used to separate nitrogen from oxygen.

Molecular sieves type Y: Similar to the X crystal except it has a higher silica to alumina ratio. It is better than 13X in some applications. In the high silica/alumina ratios it can adsorb hydrophobic molecules in the presence of water and has mild acid resistance.



...and somewhat related to the topic:

Pentasils: High silica/alumina ratio crystals that can remove organics from gas and liquid streams containing water.

Mordenite: A highly acid resistant adsorbent.



(Thanks Captain_America!)


armageddon

  • Guest
cooling advised
« Reply #3 on: September 05, 2004, 10:50:00 PM »
It is strongly advised to be careful when drying large amounts of lower alcohols (EtOH, MeOH) with mol. sieves 4A (and probably with other types too) - as the drying process is somewhat exothermic (better read pretty exothermic) and can lead to boilovers, if the vessel used isn't large enough and/or the drying process is left unattended without proper cooling ("proper" here means not lower than room temp. - because sieves don't work very well in the cold).

At least when redistilling a mixture of these alcohols over anhydrous Na2SO4, followed by thorough drying with 4A sieves, the solvent mixture started to boil - after some 5 minutes of vigorous bubbling! Luckily the flask used had enough headspace and cooling was applied soon enough to prevent boilover. I don't like boiling methanol spillages... :(

(just thought it might bee of interest)


gsus

  • Guest
4A
« Reply #4 on: September 06, 2004, 02:54:00 AM »
of course it got hot, 4A adsorbs EtOH.


armageddon

  • Guest
exactly what I imagined
« Reply #5 on: September 06, 2004, 11:41:00 AM »
Gsus: Well you could've said that earlier - as I now have some 300ml's of MeOH/EtOH-soaked sieves... :)  (I guess I can assume that MeOH is absorbed, too?)

Longimanus: That table from your catalogue is exactly what I was imagining when I started this thread - thanks a lot!

Greetz


java

  • Guest
Mineral Adsorbents , filter agents, drying agents.
« Reply #6 on: September 06, 2004, 01:47:00 PM »

http://www.sigmaaldrich.com/Brands/Aldrich/Tech_Bulletins/AL_143.html



   I.    Molecular Sieves

   II.   Activated Alumina

   III.   Activated Carbon

   IV.   Diatomaceous Earth

    V.   Ascarite II® Adsorbents

   VI.   Montmorillonites and other Mineral Adsorbents

  VII.   Drying Agents

  VIII.   Toxicity and Handling

   IX.    References


jsorex

  • Guest
You should always use different purifying...
« Reply #7 on: September 06, 2004, 04:39:00 PM »
You should always use different purifying methods, the stuff that goes in to the sieve should be already purified by other means.

But wont these thing become costly if you need to activate them and after some time not even that will do.


armageddon

  • Guest
cheap stuff
« Reply #8 on: September 07, 2004, 01:41:00 AM »
I can get 2 liters of 4A sieves for 20 bucks - so I would say no, it won't become expensive, on the contrary: they are the cheapest recyclable drying agent IMO (at least they are sold by volume! Which is usually only the case with cheap bulk chemicals..  ;) )

And reactivation is just placing them into your microwave (after having dried them in the oven) and heat on full blast for 1-2 minutes, let cool - et voilà!

But make sure to use some other drying agent (Na2SO4, MgSO4, or other cheap salts like CaCl2 where possible) prior to using sieves: they are better suited for removal of water traces, as the drying process slows down towards the end if too much water was present...


armageddon

  • Guest
AW300 is pretty cool!
« Reply #9 on: September 11, 2004, 08:48:00 AM »
Those acid resistant types have a *very* nice use in clandestine chemistry:

Need 5N HCl/IPA? Or 1N HCl/Et2O maybe? Don't like to gas everytime you need some?

Simply dissolve the needed amount of aequ.HCl in solvent of choice, dump in just enough sieves to absorb the water content of the added acid, wait - and there you go...

:)


psytech

  • Guest
great idea
« Reply #10 on: September 13, 2004, 09:09:00 AM »
now couldnt you add methylamine.hcl in your solvent, add naoh, add molecular sieves and just leave salt.In theory to obtain say dry methylamine in meoh

armageddon

  • Guest
possible
« Reply #11 on: September 13, 2004, 01:26:00 PM »
It is possible, but the problem would be that if too much NaOH is used, it would react with the solvent (alcohol). But if you use a very slight excess MeAm*HCl - yes, it will work.

Just make sure you use 3A sieves. They even soak up ammonia impurities from the MeAm... ;)

greetz


Rhodium

  • Guest
Alcohols and alkalis
« Reply #12 on: September 13, 2004, 01:55:00 PM »
It is possible, but the problem would be that if too much NaOH is used, it would react with the solvent (alcohol).

There is no reason to add more than one equivalent of base, but the above statement is wrong, or at best redundant.

The only thing which happens between an alcohol and NaOH is the equilibrium NaOH + ROH <-> RO-Na+ + H2O which is very displaced to the left anyway - and won't influence the reaction at all.


armageddon

  • Guest
Dearest Rhodium! The only thing which happens...
« Reply #13 on: September 13, 2004, 02:35:00 PM »
Dearest Rhodium!

The only thing which happens between an alcohol and NaOH is the equilibrium NaOH + ROH <-> RO-Na+ + H2O which is very displaced to the left anyway - and won't influence the reaction at all.

It will of course not influence the HCl neutralization - but as you said the reaction between alcohols and bases is an equilibrium. Because the water will be removed by the sieves, the equilibrium isn't completely shifted to the left but rather almost completely to the right, I would think??

And using a slight excess (lets say 0.001 mol) MeAm*HCl doesn't hurt anything. It just makes sure that said reaction between alcohol and base can't occur. If excess base is present, you won't get a MeAm solution comparable to a gassed one (traces of sodium alkoxide present) - if you don't use excess base, this doesn't happen.

And even if the amount of sodium methoxide (in this case MeOH was used I think) is neglectible small; still it is true that when using 1.001 equivalents methylamine hydrochloride, it will be even smaller.

Although Rhodium considers it redundant.