Molecular Sieves

Everyone knows Silica Gel as a desiccant (a moisture adsorbent), but there are other desiccants out there – that depending on the situation, can be even more useful! In today’s Desiccant Story, we look at other desiccants that may not be as well-known but still play a big part in our everyday lives! Today, we look into Molecular Sieve.

Molecular Sieves

Molecular Sieves are standard sieves that separate wanted elements from unwanted materials. Molecular Sieves are used to remove unwanted water or gases. Coming in different pore sizes, Molecular Sieve can be used to filter out specific substances. These pore sizes are uniform (same size) throughout the entire Molecular Sieve and are measured in angstroms (Å), with standard sizes being 3Å, 4Å, 5Å, and 13X (10Å). To give you an idea of how small Angstroms are, 1A is 10−10 m, one ten-billionth of a meter! Even the smallest molecule globally, hydrogen (H2), is only 0.74Å, with water (H20), one of the smaller particles, being 2.75Å.

If you wanted to filter out water (molecule size of 2.75Å) from ethanol (molecule size of 4.4Å), you would use size 3Å or 4Å, as those sizes of Molecule Sieve would adsorb the water molecules, being small enough to enter through the pores, while leaving the ethanol molecules untouched. You would not use 5Å as that size of Molecular Sieve would adsorb both ethanol and water molecules, resulting in no filtering. The preciseness of the pores allows Molecular Sieve to be a very effective desiccant, as the user can choose a size that removes what he wants to be removed, leaving the rest undisturbed.

At the start, we mentioned that Molecular Sieve could be better than Silica Gel as a desiccant. And it is true! Too often, people only consider the current relative humidity (RH) level and forget to consider what is their desired RH level. Molecular Sieve is the preferred choice for bringing the RH level to deficient levels, or almost to 0%. Other desiccants such as Silica Gel are mostly only able to reduce the RH level to 40%.

The final aspect of Molecular Sieve, such an amazing desiccant, is its huge surface area: Its 3-Dimensional matrix structure allows for its high internal surface area – almost 1’000 m2/g – making it highly adsorbent. Does every Molecular Sieve of different pore sizes have the same adsorption capabilities? No! Due to the different sizes of the pores of Molecular Sieves, the adsorption capabilities also differ. For example, a 3Å sized Molecular Sieve would absorb 20% of its weight, while a 4Å sized Molecular Sieve can adsorb at least 21% of its weight. The adsorption capabilities of Molecular Sieves rely on factors such as relative humidity and surrounding temperature.

Uses of Molecular Sieve

  • Gas or liquid drying, purification, or separation
  • Drying of air in confined spaces
  • Hydrocracking, catalytic cracking
  • Detergent builder and water softener

Besides, unlike many desiccants, the Molecular Sieve is effective under very high temperatures in the range of 100ºC to 300ºC, making it suitable for machinery applications.

And that’s that! Today, you’ve learned about another desiccant: one that has many interesting properties and uses. Next time you’re looking for a desiccant, consider – Molecular Sieve.