Easy and Efficient Drying of Molecular Sieve and Silica Gel Pellets.

Everyone probably has had the experience of working in a lab and being told the importance of that dry solvent and yet the bottle with the magical molecular sieve at the bottom just gets topped up and no one can seem to remember it ever being regenerated, right? Maybe it's time to regenerate those pellets!

3A zeolite ceramic molecular sieve pellets used for solvent drying in the EM lab can only absorb so much water and then need to be heated to 250C to regenerate by driving off the water. Most lab ovens are rated 1 to 2 kW and barely get to 250C. This is both marginal for drying and wastes a lot of energy to bake out a small volume of sieves. The baking is very easily accomplished using a heating mantle driven by a Variac (Variable AC Autotransformer) or some other power control device. Every lab/department seems to have both of these items lurking in back corners. For the 380 Watt Heating Mantle shown, the Variac set to "60" gives the right temperature; measure once with a thermocouple placed in the molecular sieve to find the proper setting and it should remain the same for that heating mantle..

Cautions.

• Air dry the molecular sieve from empty solvent bottles in a fume hood draft until they appear dry before heating at elevated temperatures. While the sieves bind much water, they also hold some solvent and you don't want to heat the pellets very hot with a lot of solvent present. (Fire hazard.)
• Operate the unit in a fume hood and make sure that no ignition sources are present and no solvent containers are anywhere near the heating mantle.

• The apparatus can be used for drying silica gel or molecular sieves. Indicating silica gel gives an easy indication of dryness when it turns very dark blue.
• The aluminum cups shown fit the heating mantle nicely; they came from an old camping mess kit. A single or double layer of heavy aluminum foil would do as well. Aluminum melts at 660C so there is little possibility of melting it.
  
• Heat the drying agent at 250C for about 2 hours in a shallow layer (not over a few cm deep) with a cover that lets water escape (but retains much heat).
• Use tongs to transfer the cup with hot sieves to an glass dessicator with a porcelain support to allow cooling without much atmospheric water uptake.
• When cool, pour into a bottle that will then receive the solvent. Glass bottles with "Poly-Seal" lined closures are very good and dry sieve can be stored in them and solvent added later.  If not bottled, dried sieve can be stored in tight  "paint cans" like the cans used to ship OsO4.
  
• It is good to gently agitate the solvent/sieve (a lab rocker table is ideal) for a while after combining, but then let it stand and settle overnight before using and do not further disturb the sieve layer so as not to resuspend any fine ceramic particles that may be present - they could stick to samples and cause damage to sectioning knife edges. Gently decant, or else pipette solvent from near the top with a clean pipette.
  

Dale A. Callaham