Newsgroups: sci.chem
Subject: Re: Removing water from ether
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Date: Tue, 27 May 1997 08:05:48 +12

In article <3389867D.777B@ctch02.chm.tu-dresden.de>
Matthias Roensberg <mattr@ctch02.chm.tu-dresden.de> writes:
...
>Bruce Hamilton wrote:
>> >Is there a proven and simple method to remove water from diethyl ether?
>> Yes. Molecular Sieves.
>> Obtain the following reference from the FAQ.
>Correct, and because, afaik, molecular seeves are pretty expensive and
>not that easy to dry without a chem lab at hand ( :-) ) I'd suggest
>Sodium(Na2SO4)- or Magnesium(MgSO4) Sulfate, respectively.

From the article "sodium and calcium sulfates are surprisingly ineffective..."
% of drying after 15 minutes with 10% w/v dessicant
Dessicant			% of drying
4A Molecular Sieves		99
Calicium Chloride (powder)		86
Calcium Chloride ( pellet)		84
Magnesium Sulfate			80
Molecular Sieve (beads)		65
Calcium Sulfate			22
Sodium Sulfate			18

Given anhydrous Calcium Chloride would be a superior choice,
and is cheaper, I'd suggest that was a preferred alternative to
anhydrous NaSO4 or Magnesium Sulfate ( note that Epsom
Salts is the heptahydrate, and can't be used ).

However molecular sieves can be purchase from any automotive
A/C or refrigeration supplies shop. The drier in those systems
is filled with MS4A already activated for use. You can put them
in a sealed container with the ether ( the copper case offers the
advantage of slowing peroxide formation as well ), and it
will accept up to 20% by weight ( the small ones typically hold
25 grams of MS4A beads ) of water. Obviously they are in the bead
form, but leaving them longer ( days ) in the ether will continue
to lower the water content.

The technical grade of MS4A is actually not that expensive, when
you consider they can retain 20+% water, compared to 5% for
calcium sulphate (drierite ). From the 1997 Aldrich catalogue
( not the cheapest place in town),
- anhydrous technical -40# calcium chloride = $23.10/kg,
Magnesium sulphate = $41/500g
Molecular sieve 4A powder ( activated ) = $28.10/500g

If you use other sources of  molecular sieves drying at 250C
in an home oven ( better under a dry gas or even a vacuum ),
but straight heat is reasonably effective ) - and quickly cooled
and sealed in a clean dry tin or bottle. Moisture uptake from the
air can be checked by container weight. Although they are more
expensive than some other driers, their efficiency, large capacity,
and relatively low price makes them the dessicants of choice
for most applications. Refer to the FAQ for references to Burfields
excellect series on drying solvents.

     Bruce Hamilton

Newsgroups: sci.chem
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Re: Activating molecular sieves
Date: Sun, 24 Aug 1997 09:31:39 +12

In article <835946434wnr@dcta.demon.co.uk>]
 cfwilliams@dcta.demon.co.uk

>Can anyone tell me the minimum temperature needed to activate 4A pellet
>form molecular sieves? I know the recommended level is about 250 deg C,
>but with the equipment currently available to me I can reach only 200.
>Is the difference critical?

Nothing that can't be cured with time. Molecular Sieves can be activated
at temperatures above 150C as long as the gas has a low water content.
Most people use dry nitrogen. Depending on how much you have to dry,
either a pyrex dreschel bottle or a two litre Duran bottle with a dreschel
top inserted and sealed with plumbers threadseal PTFE tape can be used.

Just pipe in the gas through a 1/8" metal ( copper or SS ) line and
connect to the dreschel arm that goes down to the sinter. Because
zeaolites are good insulators allow plenty of time to heat and cool.
It depends on the flow and the amount of moisture ( 4A can hold
upto 22% by mass of water, I use 24-48 hours at 250C for a full
2l Duran bottle of used MS4A at 250C and 100ml/min, but new
MS4A is only left overnight.

For 200C, you should at least should double or treble the time.
For an indication of the effect of temperature, all I have to hand
is the following, which  also shows why it's better to dry under
a dry gas, rather than in air.

% mass of Water Remaining in MS 4A after Regeneration in Air
containing the Indicated Partial Pressure of Water Vapour

Temperature           Water Partial Pressure
                       1       10       25
  ( C )                    ( mm Hg )
   150                2.5      5.5      8.0
   250                1.5      3.5      3.5
   350                 0       2.0      2.0

Note that if the MS does have residual water, it isn't
catastrophic as the water vapour adsorption only really
changes by the residual water already adsorbed on the MS.

Water Vapour Adsorption Isotherms at 10 mm Partial Pressure

Temperature          Dry          2% residual water
   ( C )            % mass            % mass
    38                22               19
    93                15               12.5
   149                 6                4
   204                 3.5              1.5


         Bruce Hamilton

From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: Solvent Drying Woes
Date: Fri, 29 Aug 1997 18:28:33 GMT

TGordy@bgsu.edu (Terrance Gordy) wrote:
..
>Can someone help me?  I was drying Pentane using the Ketyl method
>and my solution changed colors from purple to brown.
...
I'm not certain why you are using that method, as it's probably
not the easiest technique. I assume you mean the pentane fraction
(30-40C) from petroleum. We routinely use it by sparging with
nitrogen, overnight drying using molecular sieves, followed by
distilling through an adiabatic column under a gentle nitrogen
purge.

Note - first dry the sieves at 250C overnight under a stream of
nitrogen - if you use an ordinary or vacuum oven, ensure that
they aren't exposed to air when cooling or removing - they can
be stored for several weeks in an airtight container under
nitrogen - household preserving jars are good for storing larger
quantities when glassware supply is limited.

Also note that the low boiling point means that good temperature
control of the circulating water bath used to heat the
distilling flask is essential for best results. We have found
that such a system enables us to use the cheapest grades of
30-40C (pentane) and 60-80C (hexane) petroleum spirit ( we use
Shell X4 or Mobil Pegasol 1516 for the hexane fraction. They
can be pretreated by shaking with concentrated sulfuric acid
to remove non-alkanes - and we have found that it is not
usually necessary for the pentane fractions - which are
usually much lower in unsaturates - capillary GC quickly
checks what's present in technical grades - lots of peaks in
the pentane fraction indicates unsaturates are present, and
sulfuric acid treatment should first be used ). If you are
using normal pentane, or normal hexane, they will already
be free of impurities.

       Bruce Hamilton

From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem.analytical
Subject: Re: molecular sieve question
Date: Sat, 5 Sep 1998 13:11:37 +12

Matthew S. Klee wrote:
> My understanding is that the designations for inorganic molecular sieves
> is NOT based on pore size, but rather on some function of [crystal?]
> lattice structure and/or composition.  It is only by happenstance that
> molesieve 5A has 5 angstrom pores.  What would 13X be then?  Reference to
> an inorganic text or perhaps CRC reference might prove more helpful than
> my recollection on the specifics.

From memory, the nominal pore diameter of 13X is 10 angstrom. Some
classification systems for molecular  sieves use the number of oxygen atoms
( likewise silicon and aluminium atoms ) used to construct the rings of the
pores to classify the MS, with 8 oxygen = small, 10 = medium, and 12 = large.
However, for most purposes,the common MS ( 3A, 4A, and 5A ) designations can
be considered the nominal pore diameter in angstroms, with 13X being the
exception ( 10A ) .

>  Here is a teaser relating to molecular sieves as GC stationary phases
> that might break a few more assumptions:
> What is the retention mechanism responsible for separations of gases by
> molecular sieves?
> Think.  Let's see what the consensus is before I add my two cents (that
> way I can ensure that I am "correct", of course).

Wow!, you intend to detail the mechanism, *and* expect to have some
readers still awake at the end of your post? :-). Ignoring the effect of
polarity/diectric constant on retention - which is important when trying
to dehydrate solvents, then my perception is that the common mechanism
is sieving ( surprised? ) - once the water has been removed by drying.

The smaller molecules race through the small pores with larger ( detained )
molecules taking longer, thus elution of symmetrical molecules of the
appropriate size is usually by increasing molecular size. Complicating
the sieving issues are:-
1.  Even larger molecules may not be able to enter the pores, and thus pass
     through rapidly because they are unretained.
2. Some molecules - such as n-butane - enter the pores easily because
    of their small cross-sectional size, but their long structure hinders
    their  movement within, and compromises their ability to quickly pass
    through the tunnel, so theiy elute later than expected based on their size.
3. Irregular molecules - such as i-butane - may be prevented from
    completely entering the pores, and so pass through quicker than
    expected, but not as quick as completely excluded molecules.

Thus, I consider the molecules are separated by sieving, but it's a good
thing that you didn't ask about the interactions that cause the molecules
to utilise the active sites present in the molecular sieves as they pass
through....

           Bruce Hamilton