Realuminated zeolites

This method has been used successfully in connection with investigations of dealuntinated (ultrastabilized) and realuminated zeolites. One should emphasize, that NMR yields the framework Si/Al ratio since only lattice Si and Al are detected, whereas elemental analysis provides the total sample composition. In addition, equation 1 is independent of the specific structure of the zeolite, but cannot be directly applied to spectra containing overlapping signals firom Si (n Al) units of crystallographically non-equivalent Si sites. 

Aluminium can be isomorphously substituted for silicon in the framework of zeolite Y by hydrothermal treatment of the dealuminated (ultrastabilised) zeolite with aqueous solutions of strong bases at elevated temperatures. The extent and efficiency of the reaction depend on the temperature, duration of treatment and especially on the kind and concentration of the basic solution. The degree of crystallinity and the thermal stability of the products are primarily controlled by the composition of the parent material. 29Si and 27A1 magic-angle-spinning NMR (MAS NMR) indicates that the extent of realumination is determined by the number of available tetrahedral Si(OAl) sites. 

Figure 1. 29Si MAS NMR spectra of the starting zeolite, the dealuminated (ultrastable) samples and the hydrothermally realuminated samples. Samples A, B, C and D (the spectra of which are identical) have been used to prepare dealuminated samples USYA, USYB, USYC and USYD, which upon treatment with KOH solution gave rise to samples Real A, Real B, Real C and Real D, respectively. Numbers above individual peaks give the n in Si(nAl).

Third, the relative intensities of the Si(nAl) signals in realuminated samples are strikingly different from those in the as-prepared zeolites with the same framework composition, which means that the distribution of Si and A1 in the treated zeolites is different. This is a consequence of the different site selectivities discussed above, but also of the fact that both the original Si(OAl) sites and the Si(OAl) sites created during ultrastabilization are available for A1 substitution. 

Since the 29Si MAS spectra of both base-treated amorphous samples (AR-7 and AR-8) contain peaks at more negative chemical shifts than in the crystalline materials (for example samples R-3 and R-4 in Figure 3) it is clear that NMR signals from the amorphous phase cannot interfere with the determination of Si/Al ratios in realuminated crystalline zeolites. Our assignment of signal 4 as due to DFT aluminium is therefore vindicated, and the reinsertion of Al into the zeolitic framework demonstrated quantitatively. 

The influence of pH value in the solution on the realumination process of BEA zeolite was investigated by mean of nitrogen adsorption, XRD, A1 MAS NMR and FT-IR. It was found that non-framework aluminum species (octahedrally coordinated aluminums) in the solution are easily reinserted into the framework of dealuminated BEA zeolite by controlling the pH value of the solution below 7. The cumene cracking activity of the realuminated BEA zeolite at pH 5.1 was comparable to that of the parent BEA zeolite. The influence of aluminum species in the solution on the realumination process was also investigated using various aluminum compounds. 

The dealumination and realumination of low sodium P-zeolite have been studied by IR spectroscopy and powder X-Ray diffraction. The calcination-dealuminated HP was easily realuminated by direct treatment with aluminate sodium solution. After transformation into protonic HP, the framework Bronsted sites (IR band at 3610 cm ) were restored and the acidity was the same as in the original Hp. The realumination of acid dealuminated Hp was difficult, but could be achieved with aluminate sodium solution by hydrothermal treatment in autoclave. This incorporation of A1 into zeolite framework increased the number of acid sites, in particular that of weak acid sites which in comparison with parent Hp. 

A. Omegna, M. Haouas, A. Kogelbauer and R. Prins. Realumination of dealuminated HZSM-5 zeolites by acid treatment A re-examination. Microporous Mesoporous Mater. 46, 2001, 177-184. 

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