The term molecular sieves describes microporous media capable of separating molecules on the basis of size. It can relate to certain carbons and silicas, as well as porous gels/resins for polymer separations, but molecular sieving is best illustrated by the aluminosilicate zeolites. The first separations were on chabazite, a natural mineral, but now synthetic zeolites (e.g., zeolites A, X Y) are widely used.
Zeolites have unique 3-D molecular architectures based on linking [AlO4]5− and [SiO4]4− tetrahedra. Their structures contain cations and water molecules, located in regular channels and cavities. Two examples are in Figure 1 (a) and (b).
In these schematic diagrams, a line represents an oxygen atom, and an intersection Si/Al. Entry to internal cavities/channels is via pore openings whose size is defined by the number of oxygen atoms forming the constricting "window". Removal of water enables zeolites to selectively adsorb (or "sorb") gaseous molecules as a function of window size. Synthetic zeolite 4A takes in, e.g. CO, H2O, NH3, N2, CH4, CH6, CO2 and excludes larger molecules (4A represents 4 Å the restricting dimensions).
Synthetic zeolite X has larger pores (7.8 Å) and can accept e.g. SF6, C6H5, naphthalene. The mechanism of molecular sieving is a complex interaction to which zeolite structure, cation content and Si/Al composition all contribute. Permeation of gases through zeolites is a diffusion process so temperature controls gas separations. Additionally, molecular polarizability/dipolar nature will contribute to some separations.
Zeolites are widely used in industry as desiccants (air supplies, refrigerators, double glazing units, vehicle braking, large scale drying of H2,O2, liquid propane gas, ethylene, propylene, natural gas). They also perform many useful separations [e.g., CO2 from natural gas and air, "sweetening" of gases (removal of S,N compounds), i-n paraffins, benzene/toluene/xylene, oxygen/nitrogen enrichment from air]. They also separate liquids (alcohol from water, p-xylene and ethylbenzene from their isomers, sugars). Zeolites are widely used as catalysts [cracking, hydrocracking, selectoforming, methanol to gasoline (MTG)] and in some cases, products selectivity can be controlled by molecular sieving (shape selective catalysis).
Breck, D. W. (1984) Zeolite Molecular Sieves, Robert. E. Krieger, Florida.
Dyer, A. (1988) An Introduction to Zeolite Molecular Sieves, John Wiley, Chichester.
Ruthven, D. M. (1984) Principles of Adsorption and Adsorption Processes, John Wiley, New York.