Molecular sieves are materials or substances that are highly porous at a molecular level. These substances can be used to separate one chemical or substance from another. This is a process known as desiccation. Most sieves that operate on the molecular level are desiccants. When one chemical is composed of smaller molecules than the other, the smaller molecules become trapped in the pores of the sieve. When the sieve is later removed from the mixture, the substance composed of the smaller molecules is removed along with it.
These sieves often come in the form of clays or porous glasses which are used to filter chemicals out of other mixtures. The chemical that is most typically filtered out is water, surprisingly enough. Many substances contain water naturally, and many mixtures that occur in nature are composed at least in part of water. Because water prevents some chemical reactions from occurring, it's very important that a desiccant of some kind is applied to the mixture before the reaction can take place.
The pores in the sieve must be uniform in size and shape in order for the process to work. If they aren't uniform, the molecules won't be absorbed fully or accurately. In order for this system to work, you need to be able to separate the two chemicals you're trying to separate fully, without error. If the sieve you’re using is of lower quality, you won't be able to do this effectively and the separation will be incomplete. At best, you may remove a portion of the chemical you’re trying to remove. You won't get all of it. This will make the ensuing reaction less effective. This is why sieves are so incredibly important if you're planning to implement any chemical engineering process.
One of the most common areas where these sieves are used is in the oil and gasoline industry. Petroleum is purified through desiccation using sieves on a regular basis. Most any product made from crude oil or petroleum relies upon the use of one or more sieves during development. These sieves are commonly used when catalysts are involved in a chemical reaction. This makes them highly valuable to other similar industries as well.
There's no question that these substances are important. Because they are often made from organic materials, such as the clay mentioned above, the FDA has not approved the use of desiccant sieves with consumable items such as food products. There may come a time in the future when such usage is accepted. Molecular sieves allow researchers and engineers to work with materials in environments and in ways that they wouldn't otherwise be able to. This makes sieves and important aspect of significant scientific and consumer work.