Use of Molecular Sieves in Process Chemistry

Using inorganic salts for drying organic solutions on scale requires adding a solid into a reactor containing organic solvent. Considerable time is needed for filtering and washing steps and this contaminates several vessels, the pumps, and connecting lines.  The use of azeotropes is sometimes an alternative but there are many cases where these do not exist. Drying with a column of molecular sieves is an alternative. It is not without its own drawbacks but in a situation where there are few options it needs to be considered.

If the reaction being optimized involves an equilibrium involving a small molecule, molecular sieves are an effective tool for shifting the equilibrium using the principle of Le Chatelier.  Commercial molecular sieves are synthetic aluminosilicate solids called zeolites. These inorganic polymers have structures that contain cavities interconnected by channels that normally contain water. Many solid materials contain interstitial water. What sets zeolites apart is that even after the water has been driven off the inorganic architecture remains the same and the unaltered cavities remain. Different zeolite structures have channels with different pore diameters and so can hold and retain molecules of different sizes removing them from a solution.  Sieves designated 3A can only host water or ammonia and nothing larger. Type 4A can also accept methanol, ethanol, hydrogen sulfide, carbon dioxide, sulfur dioxide, ethylene, ethane, or propylene. Type 5A absorbs small straight chain alkanes but excludes branched molecules and rings larger than four-membered.
The operational problem with molecular sieves is that either the solution in contact with the sieves must be very vigorously agitated or the sieves must be first ground to a powder to expose a high surface area to the solution. The high stirring grinds the sieves even more and makes recycling of a consistently sized material impossible; therefore, at-scale the most common way to operate is to rapidly pump the solution that is to be treated in a cycle through a bed of sieves.
This can achieve a high rate of absorption of any molecule,which fits the interstices of the channels. They are taken out of the solution and into the zeolite.  When a zeolite's capacity is exhausted it can be reactivated with vacuum and heat.