Jul 16, 2019 | ACS MATERIAL LLC
Zeolites, also commonly known as molecular sieves, are crystalline microporous materials primarily made up of SiO4and AlO4 corner-sharing tetrahedral building units. These are grown to form three-dimensional (3D) crystalline frameworks with well-defined channels and cavities of molecular dimensions. In application, their pore openings are utilized to selectively adsorb molecules smaller than the pore size and reject any molecules larger than the pore size. There are many types of zeolites which have been developed for the adsorption of various specific molecule sizes. These various types of zeolites will be discussed further in this blog.
All zeolites serve the purpose of selectively adsorbing smaller molecules from larger ones; however, it is imperative to choose the right zeolite for a specific application. Type A zeolites have distinct capabilities of adsorption by offering ion exchanges, whereas Type X frameworks have a different shape and offer much larger pore openings than the former. Type A zeolites are synthetic, hydrous, alkali aluminosilicates, with exceptionally advantageous structural channels and cavities1. The cubic framework structure is commonly used as a selective sorbent for drying gases and solvents. Aluminosilicate framework compounds, especially those of high symmetry, commonly display "polyhedral tilt" transitions, in which corner-linked networks of AlO4 and SiO4 tetrahedra distort at high pressure.2 Upon selecting Type A zeolites for crystallographic studies at high pressure based on the prediction that the unusually open cubic framework would undergo reversible tilt transitions, the zeolite crystals displayed a series of gradual and unique transitions not previously recorded. The high-pressure behavior of Type A zeolites was ultimately found to be dependent on the nature of the hydrostatic, pressure-transmitting fluid.
Type A zeolites with 4 to 8-mesh sieves are normally used in gas-phase applications, while Type X zeolites of 8 to 12-mesh size are more commonly used in liquid-phase applications. The powder forms of the 3A, 4A, 5A and 13X sieves are suitable for many specialized applications and they are all available for purchase on our ACS Material online store.
The 3A molecular sieve (2/3 K2O · 1/3 Na22O · AI2O3 · 2 SiO2 · 9/2 H2O) is made by substituting potassium cations for the inherent sodium ions of the 4A base structure. With a pore size of three angstroms (3Å), it is the smallest size currently available in the market. Since the pore size of the 3Å sieve is very close to the critical diameter of a water molecule (2.65Å), and because of the potassium cation incorporated into crystal, the 3A sieve has a very strong affinity for the polar water molecule. As indicated by Holland et al.3, the adsorption ability for a molecular sieve is also determined by the dipolar coupling moment between adsorbed molecules and cations inside the zeolite cavities. It is recognized that 3A cannot adsorb molecules besides water, ammonia, helium and hydrogen at normal temperatures. However, it is also reported that 3A cannot adsorb hydrogen at 77.4 K, and that this unusual result suggests the dependence of the pore size on temperature.
4A type sieves (Na2O · Al2O3 · 2 SiO2 · 9/2 H2O) have a continuous three-dimensional network of channels approximately 4A in diameter, in addition to larger "cages" approximately 7 Å in diameter. Water molecules (effective diameter < 2 Å) are easily adsorbed into these channels and cages. When water is used as the hydrostatic pressure medium the internal pressure in these open portions of the structure must rapidly approach the hydrostatic pressure in the diamond cell. The observed compression is thus representative of the aluminosilicate framework alone, independent of channel or cage compression. 4A type zeolites in water thus appear to be relatively incompressible, with no volume discontinuities.
The pore structure of the 5A type sieve (3/4 CaO · 1/4 Na2O · Al2O3 · 2 SiO2 · 9/2 H2O) is described as a three-dimensional network of intersecting channels. Entry to these is restricted by the eight oxygen atoms from which they are formed (approx. 3-5Å diameter). Where these channels intersect, larger pores or cages with diameters of 11.4Å are formed. The n-alkanes adsorbed within the 5A sieve can be released by dissolving the sieve in a mixture of diluted HF acid and organic solvents.
With an effective pore size of ten angstroms (10Å), the 13X sieve is formed by Na2O · Al2O3 · (2.8±0.2) SiO2 · (6-7) H2O. It is mainly used for the drying and purification of gases, purification of raw gas in air separation units (simultaneous removal of H2O and CO2), desulfurization of liquid hydrocarbons and natural gas etc. Compared to Type A Zeolites, a Type 13X crystal is more likely to be used to create high purity streams of oxygen. Type X crystals are shaped differently from Type A crystals and tend to offer much larger pore sizes, about 9 Angstroms in diameter. Aside from oxygen production, X type zeolites are commonly used in cryogenic distillation processes to deeply dehydrate LNG and LPG streams. It is critical to remove all water from these streams to prevent blockage and freezing in pipelines. There are many variations of Type X crystals with most common being the Type 13X.
ACS Material Products:
1. R. M. Barrer, Zeolites & Clay Minerals as Sorbents & Molecular Sieves, Academic Press, New York (1978).
2. R. M. Hazen and L. W. Finger, Phase Transitions 1. 1 (1979).
3. G. P. Holland, B. R. Cherry, and T. M. Alam. The Journal of Physical Chemistry B 108.42 (2004).
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