Small-pore zeolite

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

Small Pore Zeolites Zeolite A (LTA) 4.1 A diameter pore, 11.4 A diameter cavity... 

The hydrocarbon exclusion by small-pore zeolites allows PSA to achieve a 10 to 30 K dewpoint depression in air-brake compressors,... 

Differential heats of NH adsorption were measured for the samples outgassed at different temperatures ranging from 400 to 800°C. Ammonia was chosen as a basic probe because its size is small, which may limitate diffusion effects in small pore zeolite materials. The variations of the differential heats of adsorption are plotted in fig. 3 as a function of the successive pulses of... 

Membranes made from zeolite materials provide separahon properties mainly based on molecular sieving and/or surface diffusion mechanism. Separation with large pore zeolite membranes is mainly based on surface diffusion when their pore sizes are much larger than the molecules to be separated. Separation with small pore zeolite membranes is mainly based on molecular sieving when the pore sizes are smaller or similar to one molecule but are larger than other molecules in a mixture to be separated. 

Some small-pore zeolite and molecular sieve membranes, such as zeolite T (0.41 nm pore diameter), DDR (0.36 x 0.44nm) and SAPO-34 (0.38nm), have been prepared recenhy [15-21]. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selechvihes due to a molecular sieving mechanism. For example, a DDR-type zeolite membrane shows much higher CO2 permeability and CO2/CH4 selechvity compared to polymer membranes [15-17]. SAPO-34 molecular sieve membranes show improved selechvity for separation of certain gas mixtures, including mixtures of CO2 and CH4 [18-21]. 

Mixed-matrix membranes comprising small-pore zeolite or small-pore non-zeolitic molecular sieve materials will combine the solution-diffusion separation mechanism of the polymer material with the molecular sieving mechanism of the zeolites. The small-pore zeolite or non-zeolitic molecular sieve materials in the mixed-matrix membranes are capable of separating mixtures of molecular species... 

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. 

Altwasser, S., Welker, C., Traa, Y., and Weitkamp, J. (2005) Catalytic cracking of n-octane on small-pore zeolites. Micropor. Mesopor. Mater., 83, 345-356. 

Small-pore zeolites can accommodate linear chain molecules, such as straightchain hydrocarbons and primary alcohols and amines, but not branched chain molecules. As discussed in the previous section, the port size can be enlarged to about 500 pm in diameter by replacing sodium ions with calcium ions. 

The methanol transformations discussed precedingly can be modified to produce high amounts of light alkenes.437 454 474 475 The key to achieve this change is to prevent C2-C4 olefinic intermediates to participate in further transformations. Such decoupling of alkene formation and aromatization can be done by the use of small-pore zeolites or zeolites with reduced acidity. Reduced contact time and increased operating temperature, and dilution of methanol with water to decrease methanol partial pressure, are also necessary to achieve high alkene selectivities. This approach has led to the development of the MTO (methanol-to-olefin) process, which yields C2-C5 alkenes with about 80% selectivity. 

As shown by Taylor and Roy (1) the behavior of small-pore zeolites does not necessarily conform to the classical definition of a zeolite. Rather, the properties evidenced by the P-zeolites, and perhaps other classes of small-pore zeolites as well, constitute a basis for possible future technical innovation in selective adsorption and heterogeneous catalysis. The zeolite structure, and hence the size and shape of its cell apertures and cavities and disposition of mobile cations may differ substantially at elevated temperatures from what it is under ambient conditions. 

In small pore zeolites with cage structure, e. g., faujasites, dye molecules encapsulated by in situ synthesis or crystallization inclusion are stable against extraction.1 2 However, these methods fail for MCM-41 due to the channel structure and the wider pore diameter (3 nm) of the host material. Covalent bonding of guests is necessary to obtain diffusion stability. Therefore, anchoring of organic molecules with catalytic functions into MCM-41 by covalent bonding was recently reported by Brunei et al.3... 

Despite the fact that both normal and monomethyl-substituted paraffins readily enter the pores of ZSM-5 and ZSM-11, preferential sorption of the normal isomer is observed under thermodynamic equilibrium, non-kinetically controlled conditions. Whereas small-pore zeolites, such as 5A and erionite, totally exclude branched hydrocarbons, and large-pore zeolites exhibit little preference, the intermediate pore-size zeolites ZSM-5 and ZSM-11 show a marked preference for sorption of the linear paraffin, even under equilibrium conditions. Competitive liquid phase sorption studies at room temperature indicated selectivity factors greater than ten in favor of n-hexane relative to... 

Large changes in relative energy are found upon variation of the Al/Si ratio. Medium- and small-pore zeolites are much more sensitive to an increase in aluminum content than the wide-pore material. This should be ascribed to stacking of the cations in the channels of the zeolite. The implications of these results for zeolite synthesis are discussed. 

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

Mote erionite is the only small pore zeolite treated.)... 

The earliest applications of zeolites utilized the molecular sieving properties of small pore zeolites, e.g. zeolite A, in separation and purification processes such as drying and linear/branched alkane separation [33]. In 1962 Mobil Oil introduced the use of synthetic zeolite X, an FCC (fluid catalytic cracking) catalyst in oil refining. In the late sixties the W. R. Grace company introduced the "ultra-... 

In substrate selectivity, access to the catalytically active site is restricted to one or more substrates present in a mixture, e.g. dehydration of a mixture of n-buta-nol and isobutanol over the small pore zeolite, CaA, results in dehydration of only the n-butanol [38] while the bulkier isobutanol remains unreacted. Product... 

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