Synthetic erionite

A second and more subtle area of difference is in the crystallography of the erionite phase itself (10). Table I compares x-ray diffraction intensities of low angle lines for a natural erionite (Jersey Valley, Nev.) and a synthetic erionite prepared at Esso Research Laboratories. The agreement is quite good except for those lines which have been marked by an astrisk indicating an intensity of less than half of that for natural erionite. Without exception, the designated lines (101, 201, 103, 211, 213, 311, and 401) have odd values for the 1 index. Further, their intensities are substantially less than the reference. 

Such an effect is understandable in view of the distinction between erionite and offretite structures published by Bennett and Card (2, 9). The designated lines are forbidden for the offretite structure. Card has examined our synthetic erionite product by electron diffraction and found disordered intergrowth with widely varying proportions of erionite and offretite structures (8). 

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). 

The difference is more notable in n-octane adsorption which is shown in the last 2 columns of Table II. Zeolite A shows substantially the same capacity and adsorption rate for n-octane as for n-hexane. But for erionite, both natural and synthetic, n-octane capacities, and particularly the adsorption rates are substantially reduced. Here the difference between synthetic and natural erionite adsorption rate is quite large. It is possible that this is an effect of residual cations. However, simple exchange of Na" and for H" showed little change. We believe the more probable explanation is the intergrowth of offretite in the erionite crystal. The large offretite channels could give more rapid distribution of the sorbate molecule within the synthetic erionite crystal. 

Catalyst Base Faujasite Zeolite A Natural Erionite Synthetic Erionite... 

H. Robson We know that the synthetic erionite product is highly crystalline from the intensity of x-ray diffraction peaks and the absence of the amorphous halo. Unfortunately, this does not prove the sample is fully crystalline. If amorphous material is present, it should be at a very low level. 

There is no systematic nomenclature developed for molecular sieve materials. The discoverer of a synthehc species based on a characteristic X-ray powder diffraction pattern and chemical composihon typicaUy assigns trivial symbols. The early syn-thehc materials discovered by Milton, Breck and coworkers at Uruon Carbide used the modem Lahn alphabet, for example, zeoHtes A, B, X, Y, L. The use of the Greek alphabet was inihated by Mobil and Union Carbide with the zeoHtes alpha, beta, omega. Many of the synthetic zeoHtes which have the structural topology of mineral zeoHte species were assigned the name of the mineral, for example, syn-thehc mordenite, chabazite, erionite and offretite.The molecular sieve Hterature is replete with acronyms ZSM-5, -11, ZK-4 (Mobil), EU-1, FU-1, NU-1 (ICI), LZ-210, AlPO, SAPO, MeAPO, etc. (Union Carbide, UOP) and ECR-1 (Exxon). The one pubHcaHon on nomenclature by lUPAC in 1979 is Hmited to the then-known zeoHte-type materials [3]. 

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. 

Molecular sieve effects and their influence on catalytic selectivity offer important possibilities. Chen (48) showed that for a given reaction synthetic offretite, with its 12-membered rings of oxygen ions, exhibited no selectivity where the presence of small amounts of erionite (3%) resulted in an effective blocking of the large openings and the creation of selectivity. This emphasizes the possible influence of impurities on the practical uses of zeolite catalysts. 

Zeolon 200H (Z200H), Zeolon 900 Na (Z900Na) (both synthetic large port mordenltes), and Zeolon 500 (Z500) (a natural chabazite-erionite). The approach to static equilibrium for these zeolites is shown in Figure 2. 

Synthetic zeolites of various types differ in the number of cations in their voids which are accessible for direct interaction with the molecules adsorbed. Table I lists, for typical examples of zeolites, the numbers of accessible cations Na per zeolite void and their number Z in mmole/ gram for dehydrated zeolites. When passing from zeolite NaA to zeolite L, the number of accessible cations Z—i.e., the number of adsorption centers in the void—decreases almost by a factor of 10. Therefore, in the case of zeolite L, the relative role of interactions among cations and molecules adsorbed, conventionally called electrostatic, will be approximately one order lower than for zeolite NaA. In adsorption on this zeolite of substances with slightly pronounced nonuniformity of distribution of electron density in molecules— for instance, saturated hydrocarbons— one may expect that electrostatic interactions will not play the decisive role. As a result, we obtain the limiting case of adsorption on zeolites like zeolite L and erionite with a weak electrostatic interaction. 

D. L. Peterson (California State College, Hayward, Calif. 94542) Did you examine the temperature dependence of selectivity and conversion of the Zn or H forms of either the synthetic or the natural erionites ... 

Erionite is not known to be currently mined or marketed for commercial purposes. Natural erionite has been replaced by synthetic nonfibrous zeolites. However, erionite was used as a noble metal impregnated catalyst in a hydrocarbon-cracking process, and erionite-rich blocks was also used for house building materials. Its use to increase soil fertility and to control odors in livestock production has been studied. 

Natural erionite, synthetic nonfibrous zeolite with the composition of erionite, and crocidolite type asbestos were tested at a concentration of 10mg m inhalation in rats. Pleural mesotheliomas were found in 27 of 28 rats exposed to erionite one pulmonary and one pleural tumor were found in 28 rats exposed to synthetic zeolite, and one lung carcinoma was reported in rats exposed to crocidolite. 

Ames [76] also used acid washed erionite (Linde AW-300) to construct the series Cs>K>Na. Finally Sherry [20] examined Linde T, an early synthetic product subsequently shown to be a mixed crionile/offrctite phase. This exhibited the following preferences ... 

The most common of the natural sedimentary zeolites found in the United States in mineable quantities are chabazite, clinoptilolite, erionite, and mordenite (2). Many crystalline zeolites decompose in acids, although mordenite and, to a lesser extent, erionite have been reported to be stable in acid solutions (2). The goal of this research was to evaluate the stability and cation-exchange capabilities of these common, natural, sedimentary zeolites in acidic solutions. The basic concepts of zeolite ion-exchange, usually emphasizing synthetic zeolites, may be found elsewhere... 

A band near 3700 cm appears only in the spectra of synthetic (ref. 12) but not in that of natural erionites (refs. 8,13). This seems to indicate that the 3700 cm band represents OH groups of other phases formed during the (hydrothermal) synthesis. During thermal treatment these groups are irreversibly removed via dehydroxylalion. 

Synthetic zeolites which have been resolved include the zeolite type L (14) and the zeolite Q (13), both of which have open frameworks and should have important catalysis application. Ion exchange studies (71) have contributed to an understanding of the synthetic zeolite T (19), which appears to be structurally related to the minerals offretite and erionite. 

Zeolite Catalysts. During the past 3 years, interest in zeolite catalysts has intensified greatly. Zeolites X, Y, and mordenite have been the center of most of the attention, with some recent interest in synthetic zeolites related to erionite and offiretite. Controversy still exists concerning the source of activity and the nature of the active sites in zeolites, such as the rare earth exchanged forms of zeolite Y for hydrocarbon conversion reactions. This is reviewed by Rabo (Vol. II, p. 284). At present, opinion seems to converge on the compromise that both the so-called Bronsted... 

The other synthetic samples comprised laths. In electron diffraction patterns, spots with 1 odd were always diffuse and often streaked along c (e.g., Figure 5). Assessment of the proportion of erionite in such intergrowths is technically important, as it affects diffusion rates and catalytic properties [Robson et al. (12)]. The 10.1, 20.1, and 21.1 x-ray powder lines are quite strong for ordered erionite, but they were either undetectable or very weak and diffuse for the other 2 Esso samples. 

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