Zeolitic molecular sieves

The lower pressure sub-region is characterized by a considerable enhancement of the interaction potential (Chapter 1) and therefore of the enthalpy of adsorption consequently the pore becomes completely full at very low relative pressure (sometimes 0 01 or less), so that the isotherm rises steeply from the origin. This behaviour is observed with molecular sieve zeolites, the enhancement of the adsorption energy and the steepness of the isotherm being dependent on the nature of the adsorbent-adsorbate interaction and the polarizability of the adsorbate. -... 

The foregoing discussion has focused on the most important commercial molecular sieves, zeolites. New directions in the preparation of framework stmctures of different chemical composition and of large-pore molecular sieves have also appeared. 

Molecular sieve Zeolites used for ion exchange in water treatment. 

In these processes, a solid with a high surface area is used. Molecular sieves (zeolites) are widely used and are capable of adsorbing large amounts of gases. In practice, more than one adsorption bed is used for continuous operation. One bed is in use while the other is being regenerated. 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

In a sorption pump, the gas is trapped within the adsorbing material (zeolites or active charcoal) called molecular sieve. Zeolites are porous aluminium silicates which adsorb large amount of gas when cooled to low temperature (usually 77K). The pump is filled with zeolite and put in a bucket containing liquid nitrogen (see Fig. 1.11). 

F or nonconstant diffusivity, a numerical solution of the conservation equations is generally required. In molecular sieve zeolites, when equilibrium is described by the Langmuir isotherm, the concentration dependence of the intracrystalline diffusivity can often be approximated by Eq. (16-72). The relevant rate equation is ... 

Other covalently bonded catalysts are those on inorganic supports such as silica, molecular sieves (zeolites), and alumina, the complexes being held either directly via the oxygen of surface hydroxy groups or via an intermediate functional group, again especially phosphino. 

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

Molecular sieve dryers, 10 613 Molecular-sieve effects, 16 821 Molecular sieve membranes, 15 813t Molecular sieve products commercial, 16 838-839t manufacturing processes for, 16 831 Molecular sieves, 16 811-853. See also Carbon molecular sieves Zeolite entries... 

Molecular sieve technology, 14 82 Molecular sieve zeolites, 14 98. See also Zeolite entries processes for, 16 832t Molecular simulations complexity of, 16 747-748 sampling techniques for,... 

Overall Milton s concept of hydrothermal crystallization of reactive gels has been followed with various additions and modifications for most of the molecular sieve, zeolite, and zeotype materials synthesis since the late 1940s. 

Barter, R.M. (1945) Separation of mixtures using zeolites as molecular sieves. I. Three classes of molecular-sieve zeolite./. Soc. Chem. Ind., 64, 130. 

Flanigen, E.M. (1980) Molecular sieve zeolite technology-the first twenty-five years, in Proc. 5th Int. Conf. on Zeolites, Naples, Italy, June, 1980 ed. LV.C. Rees), Heyden, London, pp. 750-780. 

Flanigen, E.M. and Grose, R.W. (1971) Phosphorus substitution in zeolite frameworks. Adv. Chem. Ser., 101 (Molecular Sieve Zeolites-I), 76-101. 

Sherman, J.D. (1984) Ion exchange separations with molecular sieve zeolites, in Zeolites Science and Technology (eds G. Ohhlmann, F.R. Ribeiro, A.E. Rodrigues, LD. Rollmann, C. Naccache), NATO Scientific Affairs Division/Martinus Nijhoff, The Hague, pp.583-623... 

Inorganic materials, such as y-alumina, molecular sieves (zeolites), and glass, although being essentially metal oxides, have hydroxyl groups on the surface that can be used as the point of attachment. Capka (20) has pioneered the use of these materials by attaching groups, such as... 

A noble metal on zeolite is applied in the Hysomer process to isomerize C5/C6 alkanes in the presence of hydrogen.260,261 The once-through isomerization increases the octane numbers by 10-12 numbers. When unreacted straight-chain alkanes, however, are removed (most economically by selective absorption with molecular sieve zeolites) and recycled, then complete isomerization can result in an increase in 20 octane numbers. ... 

Ward, J. W., Int. Conf. Molecular Sieve Zeolites, 2nd, Worcester, 1970, paper 64. 

Sherry, H. S., Cation Exchange on Zeolites, Paper 28, presented at the Second International Conference on Molecular Sieve Zeolites, Worcester, Mass., 1970 Advan. Chem. Ser. (1971) 101, 350. 

Tn presenting the adsorptive properties of molecular sieve zeolites, most authors (1, 2) report isosteric heats. These are obtained from the application of the thermodynamically derived Clausius-Clapeyron type equation to experimentally measured equilibrium data. At a constant... 

Dubinin, M. M., Astarkhov, V. A., Intern. Conf. Molecular Sieves Zeolites 2nd, Worcester, 1970. 

Selectivity. Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation processes depend on equilibrium-based selectivity. Significant kinetic selectivity is. in general, restricted to molecular sieve adsorbents—carbon molecular sieves, zeolites, or zeolite analogues. 

Tlie growth in both variety and scale of gas-phase adsorption separation processes, particularly since 1970. is due in part to continuing discoveries of new porous, high surface-area adsorbent materials (particularly molecular sieve zeolites and. especially, to improvements in tlie design and modification of adsorbents. These advances have encouraged parallel inventions of new process concepts. Increasingly, the development of new... 

Zeolite Structures These are crystalline, microporous solids that contain cavities and channels of molecular dimensions (3 A to 10A) and sometimes are called molecular sieves. Zeolites are used principally in catalysis, separation, purification, and ion exchange The fundamental building block of a zeolite is a tetrahedron of four oxygen atoms surrounding a central silicon atom (i.e.. (Si04)4-). From the fundamental unit, numerous combinations of secondary building units (polygons) can be formed. The corners of these polyhedra may he Si or A1 atoms.2... 

---