REALUMINATION

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. 

Under carefully controlled experimental conditions the crystallinity of the realuminated samples is retained. 

The realumination process strongly depends on parameters such as the residual sodium content of the parent material, duration of treatment, temperature, concentration of the realuminating solution and the kind of base. 

A1 MAS NMR of the dealuminated and realuminated samples are given in Figure 2. It shows that the signal at ca. 60 ppm corresponding to tetrahedral framework aluminium decreases upon dealumination and increases again on reinsertion of A1 into the framework. Any loss of... 

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).

Further support for the conclusion that extensive realumination has taken place comes from the increase in unit cell parameter and from IR spectra (4). The spectra of treated samples show shifts to lower frequencies in the framework vibration region with respect to the untreated samples, except for the T-O bending at ca. 450 cm 1 which is known (17) to be insensitive to framework composition. These changes are considerable and consistent with the increase in the framework aluminium content. The band at ca. 730 cm 1, related to the symmetric Si-O-Al or to "isolated" AIO4 tetrahedra, increases in intensity following the treatment. Furthermore, the band at ca. 812 cm-1 which is known to shift to lower frequencies and decrease in intensity with an increase in framework aluminium is clearly observed in the realuminated product. 

Contact Time. Table HI shows that realumination initially proceeds very fast. 67% of the non-framework aluminium goes back into the framework during the first hour of contact with aqueous solution of KOH. In general, increasing the time of contact while other parameters are kept constant has little effect on the extent of realumination as indicated by the small decrease of the Si/Al ratio of the framework (from 2.80 to 2.72) after the length of treatment was increased from 4 to 24 hours. 

Temperature. Table IV shows that realumination temperature plays an important role in the realumination process. Optimum A1 reinsertion is achieved at 80 5°C (sample USYA-3), while lower and higher temperatures... 

TABLE IV. The effect of temperature on realumination of sample USYA with 0.25M KOH for 24 hours... 

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. 

Table VI. The effect of the kind of base on the extent of realumination of sample USY. 0.25M aqueous solution was used at 80°C for 24 hours.

Realumination can only occur if there are sufficient suitable Si sites in the framework. Treatment of amorphous faujasites containing various kinds of aluminium leads to the formation of NFT aluminium with characteristic chemical shift and quadrupolar interactions. 

Realuminated samples R-3 and R-4 were prepared by hydrothermal isomorphous substitution (6.14) by stirring 1 g of sample D-2 in 50 ml of 0.5M and 2M KOH at 80°C for 24 hours. The treatment of amorphous alumino-silicates A-5 and A-6 in 0.5M KOH under the same conditions gave samples AR-7 and AR-8, respectively. The conditions of preparation of all samples are summarised in Table I. 

The intensity of the 27A1 MAS NMR spectrum of the realuminated sample R-3 (Figure 2) is greater than that of the dealuminated sample D-2. The F2 projection indicates that aluminium in the latter sample is in the tetrahedral coordination. The nutation spectra (Figure 3) clearly show that... 

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. 

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. 

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