[rolnor]

I want to use ground 4Å molecular sieves in a refluxing toluene solution. Is there a problem with this, does the sieves release part of the absorbed water at 110°C?

[TheUnassuming]

You can and I've done so before, but they aren't the most effective at elevated temp for exactly the reason you point out. Technically sieves have water in a equilibrium with the environment and this equilibrium shifts towards release as the temp increases.  You can always just add a large excess and hope for the best. 

I've had better luck with running this sort of reaction under cycling nitrogen and periodic additions of additional dry solvent to help drive the reaction forward.  It seems to work well with toluene if I remember right because it makes such a good azeotrope with water.

[pgk]

Yes, they do.
Molecular sieves absorb water in their cavity by a physical way and thus, they excrete absorbed water at elevated temperatures.
Note that the “apparent” boiling point of water in this case, is not 100oC but it is 84oC, which corresponds to the toluene/water azeotrope.
By the way, you can preliminary dry toluene by azeotropy, using a Dean-Stark trap and then run your reaction in the same flask. 

[rolnor]

OK, I have thought of a Dean Stark-trap but I need to eliminate water very fast and it takes some time for this apparatus to start, if there is water present for just a few minutes the reaction gives byproducts. I am going to try calcium hydride and also large excess ground molecular sieves. What I have tried so far is bis-trimethylsilyl acetamide and this works fine. I can not use P2O5 because my compound can react with this drying-agent. Thanx.

[rolnor]

The reaction worked poorly with the sieves, probably because the are weakly acidic, only 26% yield. With calcium hydride I got 77%, very nice.

[TheUnassuming]

That's great!

Should have asked it before, for archive/later search purposes if nothing else.  What was the reaction you were running?

[wildfyr]

I thought sieves were weakly basic?

[clarkstill]

Agreed - isnt this why you shouldn't use them to dry acetone?

[rolnor]

I am not sure, the formula given by wikipedia for 4Å is
NaO2*Al2O3*2SiO2*9/2H2O, is this basic?

[AWK]

https://www.sigmaaldrich.com/chemistry/chemical-synthesis/learning-center/technical-bulletins/al-1430/molecular-sieves.html
But note - you are not using water solution (5 % slurry - pH ~10.5) hence these values are rather useless.

[rolnor]

Thanx, they are clearly not acidic, good reading.
Also, they only work up to 90°C. I give you a "molsnack"!
I dont want to tell the forum wich reaction it is, I am going to publish this.

[pgk]

The chemical formula is not NaO2.Al2O3.2SiO2.9/2H2O but it is Na2O.Al2O3*2SiO2*9/2H2O and which indicates a strongly basic, co-crystalline compound due to the presence of Na2O.
However, molecular sieves are moderately basic and thus, the stoichiometric formula Na12Al12Si12O48·27H2O (zeolite type A) is more successful as indicating hydrate co-crystals of sodium silicate and sodium aluminate that are basic after hydrolysis.
The confusion about acidity starts from the amphoteric nature of Al2O3 (if adopting the mixed oxides formula) that may produce salts with both acids (e.g. AlCl3) and bases (e.g. Na2Al2O4). Anyway, molecular sieves are basic and not acidic.
But although molecular sieves are moderately basic in water, their basicity increases in organic solvents and thus, they are not compatible with strong acids, aliphatic halides (E2 elimination), etc.

[rolnor]

Thanx pgk, I wrote the formula wrong. I give you a "molsnack"!

[pgk]

Obviously, it was a typing error but note that NaO2 (sodium superoxide) also exists.
Thanks for the "molsnack".