Erionite pore structure

Commercially significant zeolites include the synthetic zeolites type A (LTA), X (FAU), Y (FAU), L (LTL), mordenite (MOR), ZSM-5 (MFI), beta ( BEA/BEC), MCM-22 (MTW), zeolites E (EDI) andW (MER) and the natural zeolites mordenite (MOR), chabazite (CHA), erionite (ERl) and clinoptiloUte (HEU). Details of the structures of some of these are given in this section. Tables in each section lists the type material (the common name for the material for which the three letter code was established), the chemical formula representative of the unit cell contents for the type material, the space group and lattice parameters, the pore structure and known mineral and synthetic forms. 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

Since offretite is a large-pore structure, intergrowth of offretite in the erionite phase would be expected to affect the adsorption properties. Table II compares adsorption capacities for natural and synthetic erionite with Zeolite A (Ca) and synthetic faujasite (Na) (4.8 Si02/Al203). As expected, the more dense erionite structure shows lower capacity (5). There is substantial agreement between natural and synthetic erionite capacity the difference shows in adsorption rates (D/r ). The low apparent diffusivity of n-parafBns in erionite is somewhat a mystery since there does not appear to be that much difference in pore dimensions between erionite and zeolite A as predicted from their structures (6). The difference cannot be attributed to crystallite size since the natural erionite sample (laths, 0.5 /x diameter or less) has finer crystallite size than any of the synthetic materials (1-5 /x). 

These observations give rise to the question wether there exists a correlation between the formation of C3 and C4 fragments inside the pore structure of erionite and the observed formation of cyclic compounds. 

Silicoaluminophosphates (SAPOs) are a new generation of crystalline microporous molecular sieves. They have been discovered by incorporating Si into the fr unework of the aluminophosphates (AIPO4) molecular sieves. Several small-pore SAPO crystals have been synthesized. SAPO-17, SAPO-34 and SAPO-44 have pore openings of about 0.43 nm. SAPO-17 has an erionite-like structure, while SAPO-34 and SAPO-44 have a chabazite-like structure. 

Many zeolites have more than one pore structure, and Table 2.1 lists the dimensions of the largest pores in nanometres. These pores can be circular, as in zeolite A (LTA) with a diameter of 0.41 nm, whereas others are elliptical, as in erionite (ERl B) with dimensions 0.36 x 0.51 nm. As well as having different size circular or elliptical pores, zeolites may also have pores that run in just one dimension, or in two or three dimensions. We can see that with this wide range of pore sizes and orientations, the separation of various substances with different molecular sizes can be achieved. 

The major crystal structures of AlPO-n and AlPO-n-based molecular sieves are summarized in Table 1 At present, there are 31 AlPO-n materials whose crystal structure has been determined. They include 18 novel structures as well as 13 structures with framework topologies related to those found in the zeolites such as chabazite (n=34, 44, 47), erionite (n=17), gismondine (n=43), levynite (n=35), Linde type A (n=42), faujasite (n=37), and sodalite (n=20). Among these, the crystal structure of AlPO-5 has been the most widely studied. Figure 1 shows the pore structure of AlPO-5. The structure of... 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

In a cooperative effort, Linde Research and Union Carbide Nuclear Co. prospected for and located deposits of natural zeolites in Western United States. No deposits of A, X, Y, or faujasite were found. Numerous and extensive deposits of other useful zeolites were located (chabazite, erionite, mordenite, clinop-tilolite), claimed and at a later date some were mined and sold for special uses. We learned how to dealuminate zeolites while maintaining crystal structure, opening the pore and increasing the silica/alumina from 10 to about 20 in mordenite. Procedures for synthesizing A, X, and Y from clays were discovered. 

Increasing attention has been given to the structures of zeolitic crystals, not only on account of their practical value as selective sorbents, but also because of the remarkable pore systems which have been revealed. As a result considerable new information exists about the anionic frameworks, although the disposition of the relatively mobile intracrystalline water and cations is intrinsically more difficult to determine. Four structures which have certain related features, and which are of interest as molecular sieves, are those of chabazite, gmelinite, levynite and erionite, for which hexagonal unit cells may be given as follows ... 

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

The diameter of the pore is given in Angstroms (divide by 10 for nanometres). Rotate the model and find the diameters of several other pores to explore the range of pore sizes in erionite. Note that WebLab ViewerLite measures interatomic distances. These will be larger than the pore diameters quoted in the text, which have taken the van der Waals radii into account. If you have time, try out some of the other zeolite structures from the CD-ROM. 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

For amorphous silica-alumina and for faujasite structures, a Cl of 0.6 was observed, while for zeolites with ten-membered-ring (10-MR) channels (ZSM-5) values of 8 and for zeolites with 8 MR (erionite) a Cl of 38 were reported. This shows that the Cl can be used to differentiate between zeolites with medium (1 < CI< 12) to small pore sizes (CI> 12), while it is not sensitive enough to classify zeolites with pores consisting of 12 or more oxygen atoms (CI< 1). 

An 8-membered ring zeolite, erionite, gives a constraint index of 38, 10-membered ring zeolites, ZSM-5 and ZSM-11, give values of 8.3 and 8.7, respectively, while 12-membered ring zeolites, mordenite and REY, give values of 0.5 and 0.4. In this way, the constraint index is a very useful tool for estimating the pore size of zeolites of unknown structures. 

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