Zeolites silica/alumina ratio

Because of the low rare earth content and Initially higher zeolitic silica/alumina ratio of catalyst B, its unit cell size after steaming is lower than that for catalyst A. 

The zeolites with applications to FCC are Type X, Type Y, and ZSM-5. Both X and Y zeolites have essentially the same crystalline structure. The X zeolite has a lower silica-alumina ratio than the Y zeolite. The X zeolite also has a lower thermal and hydrothermal... 

Figure 3-3. Silica-alumina ratio versus zeolite unit cell size.

Ag exchanged zeolite is used to remove iodine compounds. More recently Ag-LZ-210 , an Ag-exchanged zeoUte-Y adsorbent developed by UOP and having a high silica/alumina ratio (Si/Al > 5), has been used commercially to remove iodide from acetic acid streams [250-252]. 

It is generally accepted that aluminum deficient structures derived from type Y zeolite alter the extent of hydrogen transfer reactions which ordinarily favor the formation of paraffins and aromatics at the expense of olefins and naphthenes. This octane reducing reaction is controlled principally by the silica/alumina ratio of the zeolite and its rare earth content(1). 

The high silica/alumina ratio zeolites ZSM-5 and ZSM-11 both contain two intersecting channel systems composed of 10-membered oxygen rings. The channels in these zeolites are elliptical, with a free cross-section of 5.5 x 5.1 for the linear channels, and a cross-section of 5.6 x 5.4 for the sinusoidal channels in ZSM-5. The channel structures of these two zeolites are shown in Figure 1. 

The very high affinity of these zeolites for n-paraffins was confirmed by counterdiffusion studies, in which p-xylene was initially sorbed into the zeolite prior to n-nonane addition. The n-paraffin rapidly displaced virtually all of the sorbed p-xylene, as shown schematically in Figure 2. Similar results were obtained with both small ( 0.02y) and large (>ly) crystal forms of H-ZSM-5, as well as with a very high silica/alumina ratio (1670 1) form of H-ZSM-5. 

The high equilibrium selectivity for normal paraffins relative to aromatics observed for H-ZSM-5 and H-ZSM-11, so contrary to that reported for the lower silica/alumina ratio zeolites, may in part be due to the much higher silica/alumina ratio of these relatively hydrophobic zeolites, resulting in reduced polarity and ability to interact with polarizable molecules. However, other zeolites of comparable silica/alumina ratio, such as dealuminized H-morde-nite, exhibited no such enhanced preference for n-paraffins ] ) (see Table II). Clearly, silica/alumina ratio alone is insufficient to account for these differences. The structure of these zeolites, therefore, must play some role in the observed selectivity. 

The use of traditional and new techniques to elucidate the structure of synthetic faujasites with different silica alumina ratios, dealuminated by steaming and chemical treatment, and with and without faulting will be described. The migration and fixation of cations and the role of aluminum in the dealumination of the zeolite will be discussed. 

The REHY catalyst employed was a commercial Quantum 2000 sample with a rare earth content of 1.27 wt%. The ZSM-5 catalyst was prepared on a pilot plant spray dryer from 25% wt% zeolite, 25% wt% silica sol, and 50 wt% kaolin clay. The ZSM-5 sample used in this study analysed at 30 1 silica-alumina ratio. 

In August 1952 Breck located the powder x-ray data for mineral faujasite and realized that it was very similar to that of the X zeolite. We obtained about 50 mg. of faujasite and studied it carefully. The x-ray pattern was indeed very similar to that of X. The adsorption capacity was somewhat lower but similar. The silica/alumina ratio was 4.7 compared to 2.5 for X. The cations in faujasite were calcium, magnesium, and barium, not sodium as in X. It was clear that X and faujasite were isostructural but with different compositions. Further similarities and differences could not be studied at that time due to the limited supply of faujasite. 

Since X had been defined in our patent applications as having silica/alumina ratios between 2.0 and 3.0, and because there was a significant change in properties at a ratio of 3.0, the isostructural zeolites with ratios above 3.0 and up to 6.0 were named and patented as zeolite Y. The Y zeolite was not introduced into the market place until we had time to file appropriate patents and evaluate it as a catalyst [24],... 

XRD measurements show that calcined AFS and USY zeolites have comparable unit cell sizes. Upon steaming, the unit cell sizes for both AFS and USY reduce to identical values. Hence, framework silica-alumina ratios equilibrate to comparable levels independent of the method by which the zeolites were originally dealuminated. 

As-synthesized AFS zeolites do not contain extraframework aluminum as evidenced by Al NMR. As-synthesized USY zeolites contain appreciable amounts of extraframework material as seen by comparing framework and bulk silica-alumina ratios and by examining 27A1 spectra. Upon calcination both AFS and USY materials contain extraframework aluminum. The amount of extraframework aluminum in both AFS and USY materials increases on steaming. 

Acid sites are associated with framework A1 or other trivalent atoms. The number of the acid sites is proportional to the concentration of framework A1 or other trivalent atom. The strength of the acid sites in most zeolites is inversely proportional to the concentration of framework A1 up to about a silica/alumina ratio of 10. The nature of the heteroatom also affects acid strength. A1 zeolites are much more acidic than Ga- or Fe-zeolites. B-zeolites have very weak acidity. ALPO4-S have no exchangeable cations and therefore no acidity. 

Most industrial shape selective catalytic processes today use medium-pore zeolites from the "pentasil" femily. (The name refers to the five-membered rings in their framework and to their high silicon content.) ZSM-5 is by far the most important member of this family. It has high acid catalytic activity and it is very stable The silica/alumina ratio in ZSM-S varies from the teens to the thousands. High silica/alumina ratios give hydrophobidty, high acid strength, and thermal, hydrothermal, and acid stability. 

This study was undertaken with the objective of closely following the relationships of silica/alumina ratio with catalytic properties of ZSM-5 zeolite in toluene transformation under hydrodealkylation process conditions. It was hoped that the data would reveal the reaction pathway of the dealkylation and subsequently shed light on the mechanism of H-ZSM-5 catalyzed transformation of toluene. 

X- and Y-type zeolites in a Na" form were used as adsorbents The silica-alumina ratios in the X- and Y-type zeolites measured by the chemical analysis, were 2.5 and 4 9, respectively NO2 was obtained commercially and used without fiirther purification. The zeolite samples were activated at 673 K for 4 h under vacuum of lO" Torr. About 500 Torr of NO2 was admitted to the zeolite sample at room temperature... 

The pH 10 data permit some generalization about the relationship between zeolite composition and stability in HH4NO3. In particular, structural stability decreases as either framework or chemical silica/alumina ratio increases, f hile mechanistic inferences (e gw intracrystalline buffering or "propping" by nonframiework alumina) cannot be drawn from these data, several points are... 

Source of Activity in Zeolites.—Most of the experimental work designed to elucidate the catalytically active sites in zeolites has used faujasitic zeolites. This has been reviewed recently in detail.Much is broadly applicable to non-faujasitic zeolites, but in this section three factors influencing catalytic activity are emphasized silica/alumina ratio crystal structure modification of the zeolite by thermal treatment, cation exchange, etc. 

Zeolites Crystalline alumosilicates, three-dimensional network of silica and alumina with adjusted silica to alumina ratio Hydrophobic or hydrophilic depending on the silica/alumina ratio, cation exchanger, Br0nsted and Lewis acidity... 

Among various microporous adsorbents such as alumina, sihca, clays, molecular sieves, etc., the HY zeolite was found to be best at promoting the acylation of 2,3-dimethyl-2-butene with acetic anhydride. The influence of numerous experimental parameters on the course of the reaction was investigated. Variations in the silica/alumina ratio of the zeolite, or in the relative proportions of reagents and catalyst, markedly affected the yield of 3,3,4-trimethyl-4-penten-2-one, whereas the reaction time and temperature were less influential. The procedure was extended to various other alkenes and it was possible to regenerate and to reuse the solid catalyst without significant loss of activity. 

Silica-Alumina Ratio Effects on Zeolite Crystallization in the Presence of Trioctylamine... 

The structural stability of a Variety of commercially available hydrothermally dealuminated Y type zeolites in high pH ammonium salt solutions was studied It was found that such zeolites can lose large amounts of crystallinity, the susceptibility to structure loss increasing with salt concentration and pH, and with increases in both the chemical and framework silica/alumina ratio of the zeolite,... 

Zeolites were selected for these studies because they possessed the types of properties (high crystallinity, low sodium content, and wide range of silica/alumina ratio) generally valued in catalyst development work All were commercial (not developmental) products of either the Linde Division of Union Carbide Corporation (Danbury, Connecticut) or Conteka, B.V. (Surte, Sweden). 

Surface acidify, as well as hydrophobicity of zeolites, are usually related to their composition, in particular the silica/alumina ratio (SAR). In the present descriphon, this crucial parameter will be reported between brackets at the end of fhe zeolife-fype code [es BEA(26)]. ... 

Two other features of zeolites are their silica alumina ratio and acidity and these are closely related. The silica alumina ratio can be varied since these are... 

J. Turkevich (Princeton University, Princeton, N. J. 08540) In connection with Figure 3, I would like to know how the silicon-aluminum ratio is determined. The synthetic zeolites and maybe natural ones are obtained from silica-rich gels which increase in silica-alumina ratio in the mother liquid as the silicon-aluminum ratio increases in the crystals. Silica may be occluded and adsorbed as a colloidal gel on the zeolite and difficult to wash off. The silica ratio as measured will be higher and the lines asymetrically lower. 

Figure 2.9 shows the various silica/alumina ratios for some common zeolites plotted against the aluminium content. They range from zeolite A with a ratio close to 1 1, through zeolite X (from 2 1 to 3 1), zeolite Y (from 3 1 to 6 1), and mordenite (10 1), to ZSM-5 with ratios from 10 1 to infinity (indicating all silica and no alumina, a structure known as a silicalite). This series also ranks these zeolites in the order of their hydrophilic nature, with the most hydrophilic (zeolite A) first. 

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