Small-pore zeohtes

Despite the difference ia the nature of the surface, the adsorptive behavior of the molecular sieve carbons resembles that of the small pore zeoHtes. As their name implies, molecular sieve separations are possible on these adsorbents based on the differences ia adsorption rate, which, ia the extreme limit, may iavolve complete exclusion of the larger molecules from the micropores. 

Another catalytically important zeohte is ZSM-5 (81). There is a three-dimensional network of pores in this zeohte, represented in Figure 16. A set of straight parallel pores is intersected by a set of perpendicular zigzag pores. These pores are smaller than those of the faujasites (Fig. 15). ZSM-5 is classified as a medium pore zeohte, the faujasites ate large pore zeohtes, and zeohte A (Table 2) is a small pore zeohte. 

Desiccants. A soHd desiccant is simply an adsorbent which has a high affinity and capacity for adsorption of moisture so that it can be used for selective adsorption of moisture from a gas (or Hquid) stream. The main requkements for an efficient desiccant are therefore a highly polar surface and a high specific area (small pores). The most widely used desiccants (qv) are siHca gel, activated alumina, and the aluminum rich zeoHtes (4A or 13X). The equiHbrium adsorption isotherms for moisture on these materials have characteristically different shapes (Fig. 3), making them suitable for different appHcations. 

Small-pore zeolite Nu-6(2) has a NSI-type structure and two different types of eight-membered-ring channels with limiting dimensions of 2.4 and 3.2 A [54]. Gorgojo and coworkers developed mixed-matrix membranes using Nu-6(2) as the dispersed zeolite phase and polysulfone Udel as the continuous organic polymer phase [55]. These mixed-matrix membranes showed remarkably enhanced H2/ CH4 selectivity compared to the bare polysulfone membrane. The H2/CH4 selectivity increased from 13 for the bare polysulfone membrane to 398 for the Nu-6(2)/ polysulfone mixed-matrix membranes. This superior performance of the Nu-6(2)/ polysulfone mixed-matrix membranes is attributed to the molecular sieving role played by the selected Nu-6(2) zeoHte phase in the membranes. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Zeolites are crystalUne alumino silicates with a highly ordered crystalline structure. Cavities of a definitive size are formed in three-dimensional network composed of Si04 and AIO4 tetrahedra. The lattice contains cavities of varying diameters, depending on the type of zeohtes. A distinction is made between large-, medium- and small-pore zeolites. 

The reaction is well suited to zeohte catalysts. The shape selectivity principles outlined above can be used to shift the selectivity away from the thermodynamically predicted distribution and selectively form the dimethylamine product. More specifically by choosing zeohtes with small pore apertures any trimethylamine formed in the pores... 

Furthermore, a very active research field in zeolites with great potential is the reduction of nitrogen oxides by selective catalytic reduction, especially when focused on small-pore, hydrothermaUy stable zeohtes. Table 8.1 illustrates some interesting applications of zeohtes in the production of chemicals and fine chemicals and in emerging energy and environmental sustainable applications. 

Al-Beta for their activity in the conversion of DHA to methyl lactate in methanol. Clearly, Sn is the most active Lewis acid for this conversion in comparison to Zr and Ti. The Brpnsted acidic Al-Beta mostly catalyzed the formation of the di-acetal side product in line with the high framework A1 in the USY series of Sels et al. (see Fig. 8a) [110]. Not only is the conversion with Sn-Beta complete and the selectivity near 99%, the zeohte with Sn is also able to perform this reaction at a lower reaction temperature of 80°C, whereas the other metals required 115°C. A higher reaction temperature is needed in water to compensate for the slower reactirMi, but the yield remained high (entry 5, Table 1). Tsapatsis and co-workers compared hydrothermal Sn-MFl (small pore zeolite) and Sn-Beta (large pore zeolite) for the title conversirm and noticed a higher yield for the MFl topology in their cmiditions (entry 7). 

The discovery and the commercialization of small-pore zeolite supported Cu SCR catalysts raise a fundamental question why can small-pore zeolites enhance the hydrothermal stability of the Cu/zeohte catalysts, while medium- and large-pore... 

The last two contributions show the controversy about the mechanism and involvement of acid sites in the hydride transfer reactions over zeohtes. If the hydride transfer activity depends on the hydrophobicity, the rate should correlate to the acid strength of the sites, but not to the acid site density, i.e., the concentration of adjacent active sites. If the reaction is expected to proceed via two carbenium ions adsorbed on adjacent sites or one adsorbed carbenium ion on one site and a feed molecule influenced by the second adjacent site, the acid site density would be probed, but the information about the acid strength would be less obvious. However, in both cases it does not seem that these test reactions can be applied to compare large, medium, and small pore zeolite structures, due to the large (bimolecular) transition state proposed for hydride transfer reactions (unless very small molecules are used [203]). 

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. 

Cracking, a rupturing of carbon-carbon bonds—for example, of gas oils to gasohne—is favored by sihca-alumina, zeolites, and acid types generally. Zeohtes have pores with small and narrow size distribution. They crack only molecules small enough to enter the pores. To restrain the undesirable formation of carbon and C3-C4 hydrocarbons, zeolite activity is reduced by dilution to 10 to 15 percent in silica-alumina. 

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