Si-Al ratios determination

High-Resolution 29Si MAS NMR Peak Intensities in Synthetic Faujasites Determined by Computer Simulation, with Si/Al Ratios Determined from Spectref... 

Dealuminated by steaming between 973 and 1163 K followed by acid leaching with HCl. Values in parentheses are Si/Al ratios determined with NMR. 

Finally the spectra are simulated by varying R, x, and w, where w is the linewidth at half maximum. The experimental and theoretical spectra for steam dealuminated omega and offretite samples are illustrated in Figure 3. The correspondence between the experimental and theoretical spectra are excellent. One has also to emphasize, that the computation yields R = Si/Al, where the A1 atoms are only those in tetrahedral sites of the structure noted as (Si/Al)jy in Figure 3. The Si/Al ratio determined by chemical analysis corresponds, on the other hand, to global composition including both tetrehedral framework and tetra-and/or octahedral non-framework aluminium atoms. The presence of extraframework A1 can be noted in both cases because the Si/Al values are systematically lower than the (Si/Al)jy values. 

The strong decrease in catalytic activity is not in line with the moderate increase of overall Si/Al ratio determined with A1 MAS NMR (Tablel). The activity of the overgrown MTT t3ipe zeolites rather reflects the aluminum content on the surface of the crystals such as determined by XPS on the 75/25 sample. The 75/25 sample wi its surface Si/Al ratio of 244 is ca. 10 times less active than core material having a surface Si/Al ratio of 38. The synthesized MTT tj e zeolites become less active in hydroisomerization with increasing shell thickness. This is a logical evolution since the acid sites necessary for pore mouth and key lock mechanisms are to be located on the zeolites surface [14]. 

Namba et al. [171,172] subjected H-ZSM-5 to silicon tetrachloride vapor at temperatures between 450 and 650 °C and observed only a slight increase in the bulk Si/Al ratio from 19 to 24, while the surface Si/Al ratio determined by XPS increased, depending on the reaction temperature, from 18 to 39. Thus, the external crystal shell was preferentially dealuminated upon contact with SiCl4, obviously due to diffusion restrictions. In this way the contribution of the surface layer to the catalytic activity of ZSM-5 zeolite could be diminished and the shape-selectivity effect enhanced. In contrast, Thomas et al. [173] reported that aluminum could be removed from the lattice of ZSM-5 by treatment with SiCl4 at540°C. 

A partial dealumination of ferrierite increasing the bulk Si/Al ratio (determined by EDAX) from 4.6 to 7.0 was found upon treatment with SiCl4 at 550°C [152]. However, the few presented data did not evidence isomorphous substitution and do not give any structural information about the product Considering the small pore openings of the two-dimensional channel system of ferrierite, it is probable that the observed dealumination is accompanied by a gradual lattice collapse. 

MCM-41 can be synthesized with Si/Al ratio >15. After calcination the pore diameter was determined by adsorption techniques and by thermoporometry (ref. 9) to amount to 4.0 nm. 

Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase. 

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. 

Various zeolites have been studied as the dispersed phase in the mixed-matrix membranes. Zeolite performance in the zeolite/polymer mixed-matrix membrane is determined by several key characteristics including pore size, pore dimension, framework structure, chemical composition (e.g., Si/Al ratio and cations), crystal morphology and crystal (or particle) size. These characteristics of zeolites are summarized in Chapter 6. 

Acidity. Pyridine adsorption on samples of Beta zeolite with Si/Al ratio between 7 and 40 allow us to determine the acidic nature of the hydroxyl groups and their evolution as a function of its aluminum content. 

A lot of work [4,68,69] has been done using A1-NMR and Si-NMR spectra on the isostructural synthetic zeolite fl to determine Si/Al distribution in the two tetrahedral sites. These data clearly indicate that Si and Al distribution in zeolite (I is not random in nature, with an A1tz/A1ti ratio in the range 0.9-1.6, depending on the different Si/Al ratios and different synthesis conditions, and with a preferential location in the 6-memberedring, as in natural mazzite. 

A binder—free Na-Y zeolite with Si/Al ratio of 2.29 was obtained from Strem Chemical Co., La,Na—Y and Cs,Na-Y zeolites were prepared by exchanging Na-Y zeolite with LaCls and CsCl solution at room temperature. The percentage of metal ion exchanged in a zeolite has been determinated by Inductively-Coupled-Plasma Atomic Emission Spectroscopy and the number is used as prefix for the samples, e.g., Cs exchanged level of 667. is represented as 66Cs,Na-Y sample. 

The starting zeolite L, K9,5Al9,5Si26.5072, was supplied by Toso Co. It was ion-exchanged in aqueous ammonium nitrate solution, dried and further heated in order to obtain (H,K)9.5Al9,5Si26.5072 samples. The Si/Al ratio was determined by Si-MAS-NMR spectroscopy and the potassium content by activation analysis. 

The reaction of phenol benzoylation catalyzed by a H-P zeolite (Si/Al ratio 75) has been studied, with the aim of determining the reaction scheme. The only primary product was phenylbenzoate, which then reacted consecutively to yield o- and p-hydroxybenzophenone and benzoylphenylbenzoate isomers, via both intermolecular and intramolecular mechanisms. 

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. 

These distances are all considerably below the average T-0 and 0-0 distances based on the Si/Al ratio. This exemplifies the fact that interatomic distances are invariably shortened on symmetrization. All experimentally determined T-0 distances in analcime (i6, 7) are also significantly shorter than 1.656 A. This must be because of symmetrization, and this finding represents in itself evidence of pseudosymmetry. 

The Si/Al ratios of samples were determined by conventional chemical analysis. The contents of boron and sodium were analyzed by using the inductively coupled plasma (ICP) technique. 

Klinowski et al. (58) examined a great number of such models, and found that for most Si/Al ratios more than one ordering scheme is compatible with the Si(nAl) intensities determined by 29Si MAS NMR. They chose between the various ordering schemes on the basis of three criteria ... 

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