Drying Reagents using Molecular Sieves

Molecular sieves are a family of adsorbents that can be used to dry gases and organic liquids. They have a crystalline structure and come in different types depending on the size of the pores in the crystals. For example, type 3A is named for the size of the pores (3 ngstroms), and is used to remove moisture from unsaturated hydrocarbons and polar fluids such as methanol, ethanol, and acetone. Type 3A will adsorb a molecule less than 3 in size (e.g., water) and has an adsorption capacity of about 22% by weight.

To dry polar liquids such as methanol, add a slight excess of drying agent to the liquid. Check the Certificate of Analysis (COA)for the water content of the liquid. For example, let's assume the COA states the water content is 1.0%. This equates to 40 grams of water in a 4 liter bottle of product. 40 g of water divided by 22% molecular sieve capacity = 181.8 g. To insure an excess, use 200 g of molecular sieve. Keep in mind that an opened bottle of a hygroscopic product, such as methanol,may have a higher moisture content, depending on how many times the bottle has been opened for use. How often you would need to dry a solvent is dependent on your application, use, and humidity.

Add the determined amount of molecular sieve, mix thoroughly and allow the liquid to stand. After a few minutes, the drying agent settles to the bottom of the container. You can remove the molecular sieve from the product by decanting or filtering. The molecular sieve can be re-used once it has been regenerated ( )

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