Sidmadra
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In what ways does crushing Molecular Seives effect their absorption capacity?
I conducted a reaction (amine/ketone condensation which I haven't been able to confirm the product yet) which releases water and should be pushed
forward as the water is absorbed. I added 3A molecular sieves to this during the reaction period, and noticed that the color of the reaction didn't
really change until the molecular sieves started to become broken up from the mechanical force of the stir bar.
I had two theories.
1) The sieves being crushed greatly raised their absorption capacity, increasing reaction rate.
2) The sieves being crushed broke up their porosity preventing any water from getting absorbed, and the color change I absorbed was some sort of base
catalyzed condensation.
I'd like to believe it is #1 but if I understand correctly, molecular sieves trap water in their pores and not through a chemical reaction, so if they
were broken up, this wouldn't be the case?
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aga
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Theoretically, crushing them would increase their surface area and adsorb more.
Practically, you'd have a pile of clay dust to throw away, and would need to buy more sieves.
It's always a balance between Best versus Practically useful.
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LearnedAmateur
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Your first point and Aga’s response pretty much sum it up. Increased surface area of pretty much anything - solid reactants, catalysts, dessicants,
etc. - increases the rate at which something occurs.
Just a quick semantic tip, molecular sieves aDsorb substances - sticking molecules to the surface via adhesion. ABsorption is when a substance is
drawn into a material like putting a paper towel into water, via capillary action. They are pretty closely related and distinctions are usually only
made when discussing the relevant topics, but different phenomena are at play and this is important on the theoretical level. Basically, adsorption is
a property of surface whereas absorption is a property of volume.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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unionised
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Unless you crush them down to a powder that's "fine" compared to the pore size (typically a few Angstrom) the molecular sieve will still behave in
much the same way.
However, since it will be in smaller bits, the diffusion of materials on, and off the material will be quicker.
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Sidmadra
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The crushing I am referring to is what I would consider "dust form". When reading I've read that molecular sieves are usually 20% binder, which could
explain why they come apart under shear and still maintain their form.
Now I just wonder how significant the adsorption capacity would be increased? Are we talking double, or something drastic in the range of 5-10x? The
beads adsorb some 20% of their weight in water... is that only at surface sites, or is the water actually diffusing effectively and thoroughly through
the beads?
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zed
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Umm. I'm reminded of a slick water removal set-up via Nurdrage. Uses sieves, but in a different manner.
https://www.youtube.com/watch?v=Ah5ds_3s5BI
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Tsjerk
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Quote: Originally posted by Sidmadra  | The crushing I am referring to is what I would consider "dust form". When reading I've read that molecular sieves are usually 20% binder, which could
explain why they come apart under shear and still maintain their form.
Now I just wonder how significant the adsorption capacity would be increased? Are we talking double, or something drastic in the range of 5-10x? The
beads adsorb some 20% of their weight in water... is that only at surface sites, or is the water actually diffusing effectively and thoroughly through
the beads? |
I don't think you gained any significant amount of absorption power. The beads don't only absorb at the surface. The are very poreus. The only thing
you did is making filtration harder for yourself.
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S.C. Wack
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Physical damage to the zeolite structure must occur to some extent, but what else can I say you're nuts if you think powdering 3A wouldn't make a
significant difference in activity. Same for silica gel. Sigma-Aldrich sells powdered sieves BTW. With some care, there's no reason why powder can't
be re-activated.
[Edited on 10-4-2018 by S.C. Wack]
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Sidmadra
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I am now having an issue but unrelated to the water absorption one. Is it possible for the powdered sieves to be adsorbing a significant amount of my
end product (25%), not allowing it to be extracted during the workup?
I repeated my amine+ketone condensation reaction a few more times because I kept ending up with less product weight than I was starting out with, and
my starting compound has a BP over 300c, so it's not at all volatile. There didn't seem to be many places for the product to be disappearing.
Procedure:
I stir 10g of Amine and 2g of Acetone with a 2x excess of molar sieves (on a 20% water absorption basis). After 24 hours, I filtered the mixture,
rinsed the flask with solvent, and then washed the powdered/crushed sieves 3 times with solvent. The combined solvents were then evaporated.
Consistently I end up with 9g of product, when there should be around 11.5-12g. That's around a 25% loss of product.
The reaction did occur as TLC indicates mostly a single product, and the product has a different color and smell to it than the original. Is it
possible 25% of the product is being lost to the surface of the sieves, and not able to be washed off?
[Edited on 11-4-2018 by Sidmadra]
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HeYBrO
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Hard to say when you're just using "amine"- what is your limiting reagent? How dry is your acetone? Acetone is notoriously difficult to dry (esp. w/
sieves) because you end up shifting the equilibrium to the enol- Perhaps that explains why you get some side product and your yield suffers. try
another compatible drying agent
[Edited on 11-4-2018 by HeYBrO]
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Sidmadra
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I think you misunderstood my message. My yield didn't suffer. Some of it just completely vanished, presumably stuck to the sieves. As far as I can
tell, yield was near quantitative based on purity of TLC. It also was definitely an imine because I reduced it with borohydride afterwards, which I
monitored by TLC as well. Even if all the acetone had evaporated, there would still have been 10g of amine left, not 9g.
Regarding your points: The Acetone was technical grade from the hardware store - I used excess sieve to combat atmospheric water absorption. I used
"amine" as short hand because the amine I'm using is a polycyclic secondary aniline with full name far too tedious to type repeatedly. The reaction
was stirred in chloroform at room temperature in a capped flask, with a near molar equivalent of acetone (around 1.1eq, about 2g).
About drying acetone with sieves, this was more 2g of Acetone dissolved in 100ml of Chloroform, rather than pure acetone. Most of the warnings about
acetone I've seen relate to aldol formation if the acetone is stored over sieves for many weeks/months. I didn't notice much of any aldol formation.
[Edited on 11-4-2018 by Sidmadra]
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HeYBrO
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I think you should rather say your conversion is quantitative, rather than your yield ( 25% of product lost = yield suffering). If your amine is too
tedious to write, write it once, then put an appropriate acronym. a tip for the future: if you want us to help you, give the full experimental
procedure from the get go (how am I supposed to know you used chloroform as a solvent, or capped it?) I am not trying to bust your chops; it just
saves time and posts if you give the full picture.
are you following a published procedure? It seems a mystery to me that it just disappears and you washed your crushed sieves 3 times... Perhaps you
could repeat the procedure but weigh the sieves, recover them at the end, dry them in an oven and weigh it? this would only really work if your
compound is non-volatile and won't be destroyed at those temperatures. But if those conditions are met, you would probably know if your compound is
sticking. Or you could soxhlet extract the sieve residue to see if anything comes off.
edit (missed the volatile comment)
[Edited on 11-4-2018 by HeYBrO]
[Edited on 11-4-2018 by HeYBrO]
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