Molecular sieve zeolites, crystalline

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... 

Zeolite molecular sieves are composed of silicon and aluminum and can be natural or manmade minerals. Molecular sieves are crystalline, hydrated aluminosilicates of (most commonly) sodium, calcium, potassium, and magnesium. The alumininosilicate portion of the structure is a three-dimensional open framework consisting of a network of A104 and Si04 tetrahedra linked to each other by sharing all of the oxygens (Sherman, 1978). Zeolites may be represented by the empirical formula... 

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... 

Molecular-sieve zeolites represent an unique class of material [12-15]. This materials are crystalline aluminosilicates with open-framework structures made up of the primary building blocks [Si04]4 and [A104]5 tetrahedra that are linked by... 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

Crystalline molecular sieve zeolites, and related molecular sieve materials that are not technically zeolites, eg, silicalite, AlP04s, SAPOs, etc. 

Kuznicki S. M., Large-pored crystalline titanium molecular sieve zeolites, US Patent 4853202(1999). 

Molecular sieve zeolites constitute a class of stationary phase that combines exclusion with specific adsorption properties. These materials, which are crystalline aluminum silicates (commonly sodium or calcium aluminum silicates), have rigid, highly uniform three-dimensional porous structures containing up to 0.5ml/g of free pore volume, resulting when water of crystallization is removed by heating. Although munerous natural zeolites are known, most practical work is done with... 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

Zeolites (molecular sieves) are crystalline aluminosilicates composed of Si04 and AIO4 tetrahedra arranged in various geometric patterns. The tetrahedra are linked together at the comers by shared oxygen ions to form ordered lattices, which are often best visualized as three-dimensional combinations of chains, layers, and polyhedra. At least... 

The molecular-sieve zeolites are distiact from other three major npore size. Although other microporous solids are used as adsorbents for the separation of vapor or liquid mixtures, the distribution of pore diameters does not enable separations based on the ssolecular-sieve effect, that is. sepurations caused by difference in the molecular size of the materials to be separated. The most impurtanr molecular-sieve effects are shown by dehydrated crystalline zsoliles. Zeolites selectively adsorb or reject molecules based on differences in molecular size, shepe. and other properties such as polarity. Daring the ndsorption of various molecules, the micropores fill and empty reversibly. Adsorption in zeolites is a matter of pore filling, and the usual surface-area concepts are not applicable. 

Due to their pore diameters, less than 1 nm, the application of zeolites in catalytic processes is limited. On the other hand, mesoporous molecular sieves such as MCM-41 and MCM-48 with pore diameters up to 10 nm [1], have insufficient thermal and hydrothermal stability. To overcome these restrictions many efforts were imdertaken to combine tihe catalytic activity and stability of microporous zeolites with the better accessibility on the active sites of mesoporous molecular sieves [2]. The majority of the studies have been focused to the transformation of the amorphous pore walls of mesoporous molecular sieves into crystalline microporous zeolites by secondary crystallization [3], the mesostructuration of zeolite precursors [4] or the synthesis of a zeolite using porous carbons as cast [5]. The first step to develop... 

During the last sixteen years the crystalline aluminosilicates, named zeolites, have attracted the attention of industrialists and scientists, because of their possible uses as catalysts, adsorbents and molecular sieves. Zeolite minerals were first identified in 1756 by Baron Crostedt, a Swedish mineralogist (Occelli and Robson 1989). They are natural adsorbents, and for this use they are efficient and more cost-effective than synthetic zeolites. However, since naturally occurring zeolites very frequently contain metal impurities, which can strongly modify their catalytic behaviour, their use as catalysts is limited and synthetic zeolites are preferred for catalytic applications. 

Synthesis of mesoporous molecular sieves with crystalline zeolite phases embedded into the channel walls. 

Molecular sieve zeolites " are hydrated, crystalline aluminosilicates which give off their crystal water without changing their crystal structure so that the original water sites are free for the adsorption of other compounds. Activation of zeolites is a dehydration process accomplished by the application of heat in a high vacuum. Some zeolite crystals show behavior opposite to that of activated carbon in that they selectively adsorb water in the presence of nonpolar solvents. Zeolites can be made to have specific pore sizes that impose limits on the size and orientation of molecules that can be adsorbed. Molecules above a specific size cannot enter the pores and therefore cannot be adsorbed (steric separation effect). 

Zeolite molecular sieves are crystalline aluminosilicates with the general formula... 

Molecular-sieve zeolites are crystalline ahiminosilicales of group lA and group DA elements such as sodium, potassium, magnesium and calcium. Chemically, they are reptesemed by the empirical formula ... 

ZMlites were first recognized as a new type of mineral in 1756. Studies of the gas-adsorpdon properties of dehydrated natural zeolite crystals more than 60 years ago led to the discovery of their molecular-sieve behavior. As microporous solids with uniform pore sizes that range from 0.3 to 0.8 nm, these materials can selectively adsorb or reject molecules based on their molecular size. This effect, with obvious commercial overtones leading to novel processes for separadon of materials, inspired attempts to duplicate the natural materials by synthesis. Many new crystalline zeolites have been synthesized, and several fulfill important functions in the cherrtical and petroleum industries. Mote than 150 synthetic zeolite types and 40 zeolite minerals ate known. The most irnportam molecular sieve zeolite adsorbents ate the synthetic Type A, Type X, synthetic mordenite, and their ion-exchanged variations, and the mineral zeolites, cha-buite and mordenite. 

Molecular sieve zeolites. These zeolites are porous crystalline aluminosilicates that form an open crystal lattice containing precisely uniform pores. Hence, the uniform pore size is different from other types of adsorbents which have a range of pore sizes. Different zeolites have pore sizes from about 3 to 10 A. Zeolites are used for drying, separation of hydrocarbons, mixtures, and many other applications. 

The most widely used catalysts for acid-catalyzed aldol condensations are the molecular sieve zeolites, for example, crystalline aluminosilicates of group I and II elements, in which the latter have been replaced by protons. The surface protons confer Br0nsted acidity. Among the acidic zeolites we can mention HZSM-5 (pentasil zeolite), HY (faujasite), or HM (mordenite). Recently, polystyrene-supported sulfonic acids such as those of the macroreticular strongly acidic cation-exchange resins (59) and acid-base functionalized mesoporous materials such as amine and sulfonic acid-containing SBA-15 (60) have been shown to promote the acid-catalyzed aldol condensation of aldehydes with ketones at low temperatures. 

Molecular sieves are crystalline zeolites with the basic formula... 

---