Aluminum T-atom fraction

For the conversion of m-xylene, the activity of H-faujasite depends strongly on its degree of dealumination. Maximum catalytic activity is obtained for aluminum T-atom fractions equal to 0.10. Surprisingly, in contrast to both theoretical predictions and to the behaviour of H-ZSM-5, for dealuminated H-faujasites the turnover frequency per protonic aluminum site exhibits a pronounced maximum when the aluminum T-atom fraction is 0.09. The present results can be rationalized if, besides the classical predictions on zeolite acidity, a new concept of "hidden acid sites" is handled. Changes of the m-xylene isomerisation and disproportionation selectivities with the degree of dealumination of faujasite are in agreement with this concept. 

Table I. Lattice constant, aluminum T-atom fraction and degree of crystallinity of NaY dealuminated with SiCl4...

Average turnover frequencies (TOF) were calculated by dividing the initial rates of formation of the products by the framework aluminium content per weight of catalyst. A maximum TOF is found when the aluminum T-atom fraction is 0.10 (Figure 5a and c). From the present results and those presented in Figure 1, it seems that with faujasite generally a maximum TOF is found for a specific aluminum fraction of the T-atoms. 

Figure 3. Ratio of the intensities of the HF and LF OH bands in function of the aluminum T-atom fraction.

Figure 4. Relative initial rates of formation (Vo) of (a), p- (o) and o-xylene ( ), and (b), toluene ( ) and trimethylbenzenes (a) in function of the aluminum T-atom fraction.

TOF with increasing aluminum T-atom fraction from 0.015 to 0.09 is in contradiction with the theoretical... 

Fig. 10. Atomic fraction of aluminum in the Ag-Al alloy, I x, as a function of potential based on the sampled-current voltammograms in Figure 9. The Ag(I) concentrations were ( ) 5.0, (A) 10.0, ( ) 15.0, (T) 20.0, and ( ) 25.0 mmol L-1. Adapted from Zhu et al. [89] by permission of The Electrochemical Society.

All transitional aluminas crystallize in disordered (and distorted) defect spinel lattices. The unit cell contains a cubic close-packed array of 32 0 ions. Electroneutrality demands that of the 24 sites in the cation sublattice only 21f are occupied, leaving 2 vacant positions. In essence, the various transitional aluminas differ in the uniformity of the anionic stacking and in the distribution of aluminum atoms over the octahedral and tetrahedral sites. In contrast to the AIO(OH) and Al(OH)3 precursors and to a-Al203, the transitional aluminas show a significant fraction of tetrahedral aluminum sites, which are readily identified and quantitated by Al MAS-NMR 58,59], The fraction of four-coordinate aluminum sites decreases in the order t] 7 8 0, in accordance... 

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