Zeolites as molecular sieves

Perchloryl fluoride does not attack glass at moderate temperature, but decomposes at 25°C on contact with activated Si02 or AI2O3, particularly in the presence of small amounts of H2O. With other surface-active materials, such as charcoal, ignition may take place. However, there is no reaction at room temperature with synthetic zeolites. It passes freely through a 4 A molecular sieve, but is completely absorbed... 

Figure 1. Spectra of dehydrated Co -exchanged Type A molecular sieve zeolite pure (1) and after exposure to hydrogen at 250°C (2), to ethylene oxide at 20°C (3), and to oxygen at 250°C (4)...

Zeolites, also known as molecular sieves, are inorganic substances with a nanoporous structure [13] such that molecules preferenti ly adsorb within the pores depending on their size and polarity. A well-known example is the use of 4 A molecular sieves to removes traces of water from solvents. More recently, they have been used commercially to adsorb bad odors or other unwanted volatile substances from plastic films and articles. 

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... 

The addition of activated molecular sieves (zeolites) to the asymmetric epoxidation milieu has the beneficial effect of permitting virtually all reactions to be earned out with only 5-10 mol % of the Ti-tartrate catalyst [3,4]. Without molecular sieves, only a few of the more reactive allylic alcohols are epoxidized efficiently with less than an equivalent of the catalyst. The role of the molecular sieves is thought to be protection of the catalyst from (a) adventitious water and (b) water that may be generated in small amounts by side reactions during the epoxidation process. 

Rabo, J.A. Poutsma, M. L. Molecular Sieves Zeolites-II. Advances in Chem. Series No. 102 American Chemical Society Washington DC, 1971 p 284... 

The points discussed above can be demonstrated by the case of simultaneous production of O2 and N2 enriched gases from ambient air. Air can be fractionated by selectively (thermodynamic) adsorbing N2 over O2 and Ar on a zeolite [3], or selectively (kinetic) adsorbing O2 over N2 and Ar on a molecular sieve carbon [4,5]. 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

Zeolite Membrane Reactors - The need for highly permselective membranes, which are stable to high temperatures and resist chemical attack, has led to great interest in microporous materials, especially molecular sieve zeolites. Although a great deal of work is currently underway to develop zeolite membranes, few studies of zeolite membrane reactors are available. This reflects... 

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