Zeolite Si/Al ratio

Figure 10a, Scheme of the aluminosilicate framework of a typical faujasitic zeolite Si/Al ratio of LI8 (arbitrarily chosen to illustrate the ordering among tetrahedral sites) before (left half) and after (right half) exposure to SiCls, which dealuminates the zeolite (see Figure 10b),... 

Figure 10b, Magic anglespinning 29Si-NMR spectra of a typical faujasitic zeolite (Si/Al ratio 2.61) before and after dealumination by exposure to SiClk vapor...

Torre-Abreu, C Ribeiro, MF Henriques, C Delahay, G. Characterisation of CuMFI catalysts by temperature programmed desorption of NO and temperature programmed reduction. Effect of the zeolite Si/Al ratio and copper loading, Appl. Catal, B Environmental, 1997, Volume 12, Issues 2-3, 249-262. 

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. 

Zeolite Si/Al ratio (parent material) T—O—T angle6 29Si chemical shift (ppm from TMS) Signal assignment... 

It has been conclusively shown that catalytic activity in ion-exchanged faujasites is influenced by cation type (including size and charge) (8,9,40,43,46,47), cation location in the lattice (40,48), zeolite Si/Al ratio (40,48), and the presence of proton donors (49-52). 

The framework Si/Al ratio is an important property which controls activity and selectivity when zeolites are used as catalysts. It determines the number and strength of acid sites and the adsorption properties of the zeolite. Because of the different polarities of the reactants (saccharide and alcohol), there is an optimum zeolite Si/Al ratio that matches two key properties, the number of active sites and the adsorption characteristics. Furthermore, the rate of deactivation decreases as hydrophobicity increases. 

One point not mentioned above is the thermal stability of zeolites. Most zeolites are thermally stable at elevated temperatures (>200°C to 300°C), with the result that the crystalline stmcture is not lost. Typically the thermal stability of a zeohte depends strongly on the zeolite Si/Al ratio, with the general trend that increasing Si/Al leads to enhanced stability. There are also exceptions to this one in particular that will be discussed below is significant for fiuid catalytic cracking (FCQ catalysts. The thermal stability of zeolites facilitates catalytic applications as many of the reactions discussed above and below are at elevated temperatures (>200°C). 

Reaction conditions have to take into account that disproportionation is a bimolecular reaction, while isomerization is unimolecular, and furthermore the activation energy of the former is higher. From the catalyst point of view it has to be considered that disproportionation needs stronger acidity to stabilize the slightly less stable alkyl carbocations and moreover a higher active site density will favour disproportionation versus isomerization. This intum means lower zeolite Si/Al ratios. 

In situ TG-DSC measurements recorded in the presence of NO at 823 K showed a temporal process, which occurs in four steps. First step corresponds to a mass accumulation, second step to a mass constant, third step to another mass accumulation, and the last one to a quasi-constant mass process. The characteristics of these steps depend on the content of metal, on the zeolite Si/Al ratio, and on the content of rare-earth element. Figure 1 evidences this behavior for Cu-Z-168 zeolite. 

Zeolite Si/Al ratio Cu- content wt.% Cu- exchange % Ln- content wt.% Ln- exchange % Langmuir surface area m g ... 

Each type of Si( Al) n=0,1,2,3,4) species gives a characteristic Si MAS NMR signal in a well-defined range of chemical shifts (Fig. 8) [18,20,53]. The composition of the zeolite framework influences the relative intensities of the Si MAS NMR signals of the above-mentioned Si(nAl) species. The relative signal intensities are directly related to the relative concentrations of the various Si(nAl) units present in the zeolite structure. Consequently, from a careful analysis of the chemical shifts and line intensities, the specific types and relative population of the distinct Si(nAl) units present in a zeolite can be determined. The relative intensity of the lower frequency peaks increase with an increase of the zeolite Si/Al ratio (Fig. 9). Hence, the Si/Al ratio of the lattice can be calculated from the spectral intensities. 

In previous papers we demonstrated that the same nitridation approach can be also successfully applied for the incorporation of nitrogen into the framework of different Y zeolites, this making possible the preparation of porous basic catalysts active in the Knoevenagel condensation reaction [13-14]. The present work was undertaken in order to understand the modifications induced by nitridation and to provide a picture of the chemical rearrangements that occur upon nitrogen incorporation into ultrastable Y zeolite (Si/Al ratio of 13). Since catalysis is a surface phenomenon we choose to characterize the local environments of nitrogen, silicon and aluminum by X-ray photoelectron spectroscopy (XPS). A clear identification of these modifications is essential to allow a control of the preparation parameters for more efficient basic catalysts. 

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