Si-Al ratios calculation

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/Al ratios calculated using equation (23) and their comparison with bulk values [14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Brdnsted acid sites on the external surface of the zeolite crystals. The measurement of... 

Typical results are shown in Fig. 7 together with the Si/Al ratios calculated from the Si spectra [19]. As the Si/Al ratio increases, there is a corresponding increase in the relative intensities of the high-field peaks, as would be expected from the peak assignments above. Under Loewenstein s rule [20], which postulates that Al-O-Al linkages are avoided if possible, the Si/Al ratio may be calculated from the five peak areas in the Si spectrum according to... 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

The binding enetgy values are referred to Si2p( 102,6 e V) except in SiO diete the reference was Si2p (103.8 e V). Surface Co/Si ratio calculated from XPS data experimental. Sur ce Si/Al ratio for all the exchanged mordenites was... 

Distributions of structures obtained by fitting the intensity data can be compared to a most probable distribution of the sixteen structures assumming equal a priori probabilities subject to the constraint that the correct Si/Al ratio must be given. A method for calculating the most probable distribution of these structures has been previously reported (7.). 

The random A1 siting method of reference (7) was used to compute 29Si NMR intensities for comparison with experimental results reported in reference (2). The results in Table II show clearly some discrepancy between the experimental and calculated results. The variance a2 ranges from 35 to 329. The discrepancy is greatest at higher Si/Al ratios where the experimental distribution is much sharper than is expected of the maximum probability distribution of silicon and aluminum atoms. These results imply some ordering of the aluminum atoms in the lattice. 

Since an assumed distribution of structures per unit cell allows one to calculate the relative intensities of the five 29Si lines, one can choose the combination of structures which will best fit the NMR data. We have chosen to use the Box complex alogrithm to determine the distribution of structures that minimize the variance a2 between the experimental and computed relative 29Si intensities. Tables III, IV and V list results for sieves of various Si/Al ratios. The 29Si NMR data in Table III has appeared previously in the literature (2) and the data in Tables IV and V are new (Table I). At the lowest Si/Al ratio of 1.145, the best fit distribution is nearly the same as the maximum probability model with a low a2 = 28. At higher Si/Al ratios >1.9 the best fit result continues to give a low a2 between 0 and 18. 

The expansion of the crystal structure upon substitution of smaller atoms by larger ones is reflected by increasing lattice constants. For a zeolite with cubic symmetry, the lattice constant a decreases with increasing Si/Al ratio. This relation is occasionally used to calculate the Si/Al ratio of the... 

Calculate the Si/Al ratio of a Na-Y zeolite from a Si NMR spectrum with the following intensities ... 

Si NMR provides quantitative information about the framework composition, and framework Si/Al ratio is an important parameter used to tune the catalyst property. Zeolite acidity is directly related to the amount of framework Al. Framework Si/Al ratio can directly be obtained from just Si NMR alone. Si/Al ratio can be calculated from Si NMR intensities if the resonances due to different Q (nAl) species are well-resolved using Eq. (4.10), assuming there is no Al-O-Al bonds present ... 

An important feature of Si MAS-NMR spectra is that measurement of the intensity of the observed peaks allows the Si/Al ratio of the framework of the sample to be calculated. This can be extremely useful when developing new zeolites for catalysis, as much research has concentrated on making highly siliceous varieties by replacing the Al in the framework. Conventional chemical analysis only gives an overall Si/Al ratio, which includes trapped octahedral Al species and [AlCf], which has not been washed away. At high Si/Al ratios, Al MAS-NMR results are more sensitive and accurate and so are preferred. 

Leherte et al. (38-40) reported a series of MD calculations for water molecules adsorbed in the ferrierite framework. The ferrierite lattice is modeled with a Si/Al ratio of 8, with the siting for aluminum atoms taken from ab initio calculations. (The T4 site is preferentially substituted.) Four unit cells were included and the lattice and intramolecular water parameters kept rigid. Three different water concentrations were considered 23, 33, and 41 molecules in 4 unit cells. 

Demontis et al. (94) reported an early MD study of the sorption and mobility of benzene in zeolite NaY. The zeolite was modeled with a Si/Al ratio of 3.0, as in previous calculations for Xe and methane. The zeolite and benzene molecules were treated as rigid. The authors supported the assumption of a rigid zeolite lattice by quoting structural studies (95), in which the cell parameter of NaY zeolite was found to contract little upon uptake of benzene. It is, however, more than possible that the lattice undergoes substantial deformation without an overall change in volume quantum chemical calculations (96) have shown that the Si-O-Si bending potential is very soft. When these calculations were performed, the assumption of a rigid lattice was more a matter of computational necessity than it is today. 

Auerbach et al. (101) used a variant of the TST model of diffusion to characterize the motion of benzene in NaY zeolite. The computational efficiency of this method, as already discussed for the diffusion of Xe in NaY zeolite (72), means that long-time-scale motions such as intercage jumps can be investigated. Auerbach et al. used a zeolite-hydrocarbon potential energy surface that they recently developed themselves. A Si/Al ratio of 3.0 was assumed and the potential parameters were fitted to reproduce crystallographic and thermodynamic data for the benzene-NaY zeolite system. The functional form of the potential was similar to all others, including a Lennard-Jones function to describe the short-range interactions and a Coulombic repulsion term calculated by Ewald summation. 

In Mordenite. Smit and den Ouden (60, 144) reported a Monte Carlo investigation of methane adsorbed in mordenite of varying Si/Al ratios. In their calculations, both the zeolite and sorbate were held rigid, infinite dilution was assumed, and sorbate-zeolite interaction parameters were taken from Kiselev et al. (79). Electronic neutrality of the zeolite framework was preserved by compensating the trivalent aluminum exactly with sodium cations, located in experimentally determined crystallographic locations. 

The value of the heat of adsorption of methane in mordenite at an Si/Al ratio of 11 was calculated to be 21.4 2.6 kJ/mol. An experimental value of 23.0 kJ/mol was quoted by the authors, demonstrating the quality of the simulations. If the heat of adsorption is calculated as a function of Si/Al ratio, a marked decrease is observed (approximately 30%) around Si/Al = 6-7. This result is explained by considering the location of the sorbed molecules as a function of Si/Al ratio. In the case of Si/Al = 11, a significant proportion of the methane molecules were found to reside in the side pockets that branch off the main channel. These molecules enjoy a more favorable stabilization than those within the main channel. When the Si/Al ratio decreased to 5, the extra Na cations blocked off the side pockets the methane molecules were then all located in the main channel and the heat of adsorption was reduced (Fig. 8). 

In Figure 2 the dependence of the line widths of the 23Na resonance on the Si-Al ratio is shown. As mentioned, the results for low Si-Al ratios can be described by a model which uses calculations for occupancy of the S3 sites. Only the nuclei of the ions in the Si sites can be detected. 

Al-containing SBA mesoporous solid was prepared as reported 9 mL tetraethyl orthosilicate (TEOS) and the calculated amount of aluminum tri-tert-butoxide, in order to obtain a well defined Si/Al ratio equal to 10, were added to 10 mL of HC1 aqueous solution at pH=1.5 water. This solution was stirred for over 3 h and then added to a second solution containing 4 g triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EO20PO70EO20 Aldrich) in 150 mL of HC1 aqueous solution at pH=1.5 at 313 K. The mixture was stirred for another 1 h and allowed to react at 373 K for 48 h. The solid product was filtered, dried at 373 K, and finally calcined in air flow (9 L h 1) at 823 K for 4 h with a heating rate of 24 K h"1. The SBA-15 was prepared according to the literature [11]. In what follows, the samples are denoted AlSBA and SBA, respectively. 

The 7Li resonance in zeolites is also difficult to interpret, even though the quadrupole moment is much lower. Lechert et al. (227) believe that the 7Li linewidth is controlled by the dipole-dipole interaction with 27A1 nuclei in the aluminosilicate framework. According to Herden et al. (232) the increase of 7Li frequency from 9 to 21 MHz does not affect the second moment of the spectra in zeolites Li-X and Li-Y, which means that the quadrupolar interaction is small. The second moment was also independent of the Si/Al ratio. The mean Li-Al distance calculated from the van Vleck formula was 2.35 A. Small amounts of divalent cations reduce the movement of Li + considerably, with the activation energy for this process increasing from 30 to 60 kJ/mol. 

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