Pore crystalline

Thomas, J. M, New Microcrystalline Catalysts. Philos. Trans, R. Soc, London A, 1990, 333 173. Davis, M, E, The quest for extra-large pore, crystalline molecular sieves Chem. Eur. J. 1997, 3, 1745. Francis, R, J, O Hare, D. The kinetics and mechanisms of the crystallization of microporous materials. J. Chem. Soc., Dalton Trans, 1998, 3133. 

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

The active components of a present commercial FCC catalyst are zeolites Y (0.74 nm) and ZSM-5 (0.54 nm x 0,56 nm) whose pore sizes give limited access to the active centres for long chain and/or bulky molecules. The new extra-large pore crystalline materials MCM-41 (2.0-20 nm), VPI-5 (1.21 nm) or cloverite (1.32 nm) have significantly increased the restricted pore sizes encountered in zeolites Y and ZSM-5 and opened up interesting perspectives for the conversion of heavier feedstocks [1]. 

Richard Barter and his collaborators were the first to publish (in 1969) on the idea of extra-large pore, crystalline materials [5]. Barter and Villiger presented a series of hypothetical structures related to zeolites L, cancrinite, offretite and gmelinite that had 24 MR pores with free diameters of approximately... 

From extra-large pore crystalline materials to ordered, mesoporous materials... 

The areas of extra-large pore, crystalline solids and non-crystalline, ordered mesoporous materials both continue to flourish. By investigating further the fundamental rules involved in their syntheses, it may be possible to make new solids with the advantageous properties of both existing classes of materials. Some of the issues of concern when performing such syntheses have been outlined here and several new suggestions for research provided. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Zeolites (section C2.13) are unique because they have regular pores as part of their crystalline stmctures. The pores are so small (about 1 nm in diameter) that zeolites are molecular sieves, allowing small molecules to enter the pores, whereas larger ones are sieved out. The stmctures are built up of linked SiO and AlO tetrahedra that share O ions. The faujasites (zeolite X and zeolite Y) and ZSM-5 are important industrial catalysts. The stmcture of faujasite is represented in figure C2.7.11 and that of ZSM-5 in figure C2.7.12. The points of intersection of the lines represent Si or A1 ions oxygen is present at the centre of each line. This depiction emphasizes the zeolite framework stmcture and shows the presence of the intracrystalline pore stmcture. In the centre of the faujasite stmcture is an open space (supercage) with a diameter of about 1.2 nm. The pore stmcture is three dimensional. 

Principal Adsorbent Types. Commercially useful adsorbents can be classified by the nature of their stmcture (amorphous or crystalline), by the sizes of their pores (micropores, mesopores, and macropores), by the nature of their surfaces (polar, nonpolar, or intermediate), or by their chemical composition. AH of these characteristics are important in the selection of the best adsorbent for any particular appHcation. 

Typical pore size distributions for these adsorbents have been given (see Adsorption). Only molecular sieve carbons and crystalline molecular sieves have large pore volumes in pores smaller than 1 nm. Only the crystalline molecular sieves have monodisperse pore diameters because of the regularity of their crystalline stmctures (41). 

A surprisiagly large number of important iadustrial-scale separations can be accompHshed with the relatively small number of zeoHtes that are commercially available. The discovery, characterization, and commercial availabiHty of new zeoHtes and molecular sieves are likely to multiply the number of potential solutions to separation problems. A wider variety of pore diameters, pore geometries, and hydrophobicity ia new zeoHtes and molecular sieves as weU as more precise control of composition and crystallinity ia existing zeoHtes will help to broaden the appHcations for adsorptive separations and likely lead to improvements ia separations that are currently ia commercial practice. 

Adsorption. Although several types of microporous soHds are used as adsorbents for the separation of vapor or Hquid mixtures, the distribution of pore diameters does not enable separations based on the molecular-sieve effect. The most important molecular-sieve effects are shown by crystalline zeoHtes, which selectively adsorb or reject molecules based on differences in molecular size, shape, and other properties such as polarity. The sieve effect may be total or partial. 

These tetrahedra are arranged in a number of ways to give the different zeohtes. The stmctures are unique in that they incorporate pores as part of the regular crystalline stmctures. The pores have dimensions of the order of molecular dimensions so that some molecules fit into the pores and some do not. Hence the zeohtes are molecular sieves (qv), and they are apphed in industrial separations processes to take advantage of this property. Some zeohtes and their pore dimensions are hsted in Table 2. 

In this appHcation, ZSM-5 acts as a strong, soHd acid, and may be viewed as supported on the surfaces of the crystalline zeoHte stmcture. The older, Friedel-Crafts aluminum chloride catalyzed process for ethylbenzene produces considerably more by-products and suffers from the corrosivity of the catalyst system. Because of the intermediate pore size of ZSM-5, those reactions that produce coke from larger molecules that cannot enter the ZSM-5 pore stmcture are significantly reduced, which greatly extends catalyst lifetime. 

Characterization. The proper characterization of coUoids depends on the purposes for which the information is sought because the total description would be an enormous task (27). The foUowiag physical traits are among those to be considered size, shape, and morphology of the primary particles surface area number and size distribution of pores degree of crystallinity and polycrystaUinity defect concentration nature of internal and surface stresses and state of agglomeration (27). Chemical and phase composition are needed for complete characterization, including data on the purity of the bulk phase and the nature and quaHty of adsorbed surface films or impurities. 

Type 3A sieves. A crystalline potassium aluminosilicate with a pore size of about 3 Angstroms. This type of molecular sieves is suitable for drying liquids such as acetone, acetonitrile, methanol, ethanol and 2-propanol, and drying gases such as acetylene, carbon dioxide, ammonia, propylene and butadiene. The material is supplied as beads or pellets. 

Type 13X sieves. A crystalline sodium aluminosilicate with a pore size of about 10 Angstroms which enables many branched-chain and cyclic compounds to be adsorbed, in addition to all the substances removed by type 5A sieves. 

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