What is the difference of zeolites and molecular sieves?

Zeolites are a group of alumino-silicate minerals, with a solid crystalline structure, three-dimensional, microporous, and well-defined structures that contain aluminum, silicon, and oxygen in their regular framework. Zeolites may occur either in nature or are manufactured synthetically, most zeolites used commercially are produced synthetically. Zeolite crystals are porous, the pores have the unique characteristic of being all of the same size. The diameter of these pores can be as small as a very tiny molecule such as water.

Molecular sieves
are a certain kind of zeolites, which for their pore size and chemical composition have particularly developed adsorption properties, which means to separate or remove one substance from another on a molecular scale. While the properties of natural zeolites are fixed, synthetic zeolites can be tailored to meet industrial specifications. Therefore, synthetic zeolites dominate the molecular sieve market among which the "A" and "X" structures are the most commonly used.