Silica/alumina ratio

The product distrihution is influenced hy the catalyst properties as well as the various reaction parameters. The catalyst activity and selectivity are functions of acidity, crystalline size, silica/alumina ratio, and even the synthetic procedure. Since the discovery of the MTG process. 

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.

A conventional FCC unit can be an olefin machine with proper operating conditions and hardware. Catalysts with a low unit cell size and a high silica/alumina ratio favor olefins. Additionally, the addition of ZSM-5, with its lower acid site density and very high framework silica-alumina ratio, converts gasoline into olefins. A high reactor temperature and elimination of the post-riser residence time will also produce more olefins. Mechanical modification of the FCC riser for millisecond cracking has shown potential for maximizing olefin yield. 

Soil components, silica, and alumina are solubilized, in low concentration, and can react, or crystallize, to form new clays. In addition, clays from any source change over time and become simpler and simpler. Silica is more soluble than alumina and so the silica alumina ratio decreases over time. Eventually, this leads to deposits of alumina that are used as an aluminum ore for the production of aluminum metal. Although these reactions are considered to be very slow on a human timescale, they do occur. 

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). 

It is necessary, however, to maximize the intermediate olefin product at the expense of the aromatic/paraffin product which makes up the gasoline ( ). The olefin yield increases with increasing temperature and decreasing pressure and contact time. Judicious selection of process conditions result in high olefin selectivity and complete methanol conversion. The detailed effect of temperature, pressure, space velocity and catalyst silica/alumina ratio on conversion and selectivity has been reported earlier ( ). The distribution of products from a typical MTO experiment is compared to MTG in Figure 4. Propylene is the most abundant species produced at MTO conditions and greatly exceeds its equilibrium value as seen in the table below for 482 C. It is apparently the product of autocatalytic reaction (7) between ethylene and methanol (8). 

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. 

A similar experiment was performed with a large crystal (>lp) H-ZSM-5 of 75 silica/alumina ratio, and also with a sample of 1670 1 silica/alumina ratio H-ZSM-5. 

Two grams of an aqueous solution containing 3.2% each n-butyl alcohol, iso-butyl alcohol, and tert-butyl alcohol was added to lg high silica/alumina ratio H-ZSM-5 (Si02/Al20, = 1670). Sorption of tert-butyl alcohol appeared negligible, and sorption of the other two isomers was calculated relative to tert-butyl alcohol. 

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 high hydrophobicity of ZSM-5 permits the selective sorption of organic compounds dissolved in water. A high (1670 1) silica/alumina ratio ZSM-5 preferentially sorbed n-butyl alcohol (90.7% sorbed) from iso-butyl alcohol (17.3% sorbed) and tert-butyl alcohol ( 0% sorbed), all dissolved in water. 

The processes described below are the evolutionary offspring of the fluid catalytic cracking and the residuum catalytic cracking processes. Some of these newer processes use catalysts with different silica/alumina ratios as acid support of metals such as Mo, Co, Ni, and W. In general the first catalyst used to remove... 

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. 

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 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. 

In mid 1954 Breck proposed that it should be possible to synthesize the X structure with silica/alumina ratios as high as 4.7, found in faujasite, and possibly higher. He further hypothesized that the higher ratio materials, with lower aluminum and exchangeable cation content, would be more stable to acid attack... 

N. A. Acara, Breck s synthesis assistant, collected samples of X made over the past few years and sent some of them in for chemical analysis. Heretofore most lots of X had never been analyzed. Those that had, always gave silica/alumina ratios very close to 2.5 with none higher than about 2.7. This analysis of old lots of X showed some lots with ratios as high as 2.83 and 2.92. With this background, Breck and Acara soon learned how to routinely synthesize X with ratios between 3.0 and 4.0. Later analyses of additional old lots of X showed ratios of 3.05, 3.47, and 3.48. We had made high silica X before but had not recognized it. 

Beginning in 1956, E. M. Flanigen learned how to make X with silica/alumina ratios between 4.0 and 5.7. Now with a full range of ratios from 2.5 to 5.7, we were able to study systematically the variation in properties with alumina content. Breck s original hypothesis proved to be correct. The high silica forms were more stable to acid attack and to high temperatures in the presence of water vapor than the low silica forms [23]. 

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. 

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