Silicon-to-aluminum ratio

Schwartz, S., Kojima, M. and O Connor, C. T. Effect of silicon-to-aluminum ratio and synthesis time on high-pressure oligomerization over ZSM-5, Appl. Catal., 1989, 56, 263-280. 

The guidelines of linear-free energy relationships have also been used to capture not only the hydrocarbon stmcture/function but also catalyst structure/function relationships. Thus Liguras et al. (39) have fashioned a model where the rate constant is a function of the reactant, the reaction family, and the catalyst silicon to aluminum ratio. This fledgling approach considerably reduces the number of kinetic parameters and appears to be quite useful in the modelling of complex kinetics of hydrocarbon feedstocks. 

Many low silicon to aluminum ratio zeolites can be structurally modified to have a high silicon to aluminum ratio and, thus,... 

The source of the silica can be either the treating reagent (14) or the zeolite sample itself through rearrangement of silica from another part of the framework or from a silica impurity within the sample. (Note that in the chromic salt case, chromia rather than silica is claimed to be inserted in the structure.) Because of the dealumination and the silica insertion, the treated samples are usually found to have a higher framework silicon to aluminum ratio and a higher thermal stability than the untreated materials. Furthermore, hydrophobic surface properties usually result from substantial dealumination (7). 

Zeolite pore diameters increase as aluminum is iso-morphically substituted for silicon in the crystal structure. Given the relationship observed between the initial heat of adsorption and the silicon to aluminum ratio, we suggest that a sterically different adsorption site becomes available for nitrogen when the Si/Al is below the critical value of 4.70. 

The reversibility of the redox cycle involving and Oj was established for Fe by Boudart and co-workers (2) using Mossbauer spectroscopic techniques. They proposed that the oxygen was held between two Fe cations. Fu et al. (3) showed that Fe-Y acted as a redox catalyst for reactions of CO with NO, CO with Oj, and N2O with CO. The ability of Fe-Y to decompose NjO into its elements was established in the work of Hall and co-workers (4), who also showed that Fe-Mordenite was as active as Fe-Y, despite containing only 16% as much Fe as its Y-zeolite counterpart. This difference in catalytic activity was thought to result from differences in the environments of the Fe within the zeolite structures. The objective of the present study was to alter the cation environment and relate that environment to the catalytic activity this was accomplished by varying the silicon-to-aluminum ratio of Y-zeolite and by coexchanging Fe with Eu. 

Catalysts. The starting material for all the catalysts was Linde 13X in powder form, free of clay binder. The bulk sample had a silicon-to-aluminum ratio of 1.27 dz 0.02 16). In all cases, the zeolites were prepared by ion exchange with salt solution (either chloride, sulfate, or nitrate) of the required cation at room temperature. The extent of exchange was estimated from either analysis of the residual salt solution or determination of the residual sodium in the zeolite sample. After preparation, the exchanged zeolites were stored over saturated calcium nitrate solution. 

The following notes and symbols will be used in the other tables as well T, adsorption temperature Si/Al, silicon to aluminum ratio q, differential heat of adsorption n, surface coverage < inai < location of the maximum distribution of sites in the site energy distribution plot, with letters indicating the relative number of sites under the peak L, large I, intermediate S, small. 

Figure 6.192 shows pore dimensions of Zeolites in comparison with the diameters of hydrocarbons (in nm). The silicon to aluminum ratio also varies, with typical values such as ... 

The weak 0-H band at 3660 cm" - in the H-ZSM-5 spectrum is seen with varying intensity in most preparations of this synthetic zeolite (1,5,6,12,17). It seems to gain intensity with increasing silicon-to-aluminum ratio in the framework (5,12) and several explanations have been offered concerning its origin. Sayed et al. 

The much greater combined intensity of these bands versus that of the silanol stretch at 3745 cm-1 must be due in part to the low silicon-to-aluminum ratio for the H-offretite ( 4). However, when making such comparisons it is important to keep in mind that the silanol peak intensity can be as much a function of the surface area-to-volume ratio of the zeolite crystallites as it is of the silicon-to-aluminum ratio (.13). ... 

Far-infrared spectra of zeolites with well-dispersed cations, which are compared to A-, X- and Y-type zeolites rich in silicon, would be of special interest. However, they are distinctly more difficult to record, because the low cation concentration gives rather low IR intensities. To our own experience, high-quality spectra can be recorded for divalent alkaline earth cation-exchanged MFI-type zeolites up to a silicon-to-aluminum ratio of around fifteen. This corresponds only to about three cations per unit cell. Figure 17 shows the far-infrared spectrum of a Ba-exchanged zeolite ZSM-5 with nsi/n plS as an example [363]. 

Calculations of absolute intensities are generally not attempted. In almost all cases, particularly in practical analysis, the analyst is interested in relative knowledge. Thus, questions such as, What is the silicon to aluminum ratio in that zeolite or How much more potassium is there on the surface of thin film A than B are indicative of typical requests. Fortunately, in this form of analysis, one may generally simplify the complex equation relating intensity and concentration. The relative precision may also be substantially improved, and it may be possible to employ the following ... 

There are several crystal phases and each has some flexibility in the chanical composition [106,168,169]. The most frequently used formula is Na6[(A102)6Si02)io]-12-15H20 [106,109,126]. The silicon to aluminum ratio may vary between 1.0-2.5 [66,106,168,169] and 1.6-2.7 [106], respectively. 

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