Dealumination and realumination

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... 

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. Vasic, J. A. van Bokhoven, G. Pirngruber and R. Prin. Dealumination and realumination of microcrystaltine zeohte beta An XRD, FTIR and quantitative multi-nuclear (MQ) MAS NMR study. Phys. Chem. Chem. Phys. 6, 2004,447-452. 

The A1 mas and MQ MAS NMR spectroscopy methods were used to study the A1 coordination in the parent H-mordenite and dealuminated sanqile [05C1]. The parent H-mordenite displayed signals at 55 and 0 ppm corresponding to framework tetrahedral and octahedral coordinated A1 species, respectively. The sample calcinated at 923 K depicts additionally a shoitlder at <5 = 30 ppm, attributed to a combination of both distorted [4]ai (WaID) and Al. In the ammoniirm-treated sample calcinated at 923 K most of the peak at S= 0 ppm was removed, but the shoulder at <5= 30 ppm remained. There are present both and Al species. The sample calcinated at 923 K and treated in autoclave in a anunoniiun-water solution evidenced one peak at 55 ppm and a shoitlder at 30 ppm corresponding to " Al and " AID, respectively. When dealuminated zeohtes were treated at 353 K in basic solutions, a combined dissolution and realumination phenomenon could occur [89K2, 05C1]. The calcination at 673 K of the ammonium-water-treated sample renders a tine at S = 0 ppm and a shorrlder at 30 ppm In the MQ MAS spectrum, besides " A1, " AID, and A1 species, traces of A1 were detected -Fig. 35a,b,d. The ammonium-water thermal treatment transformed almost all penta- and octahedral A1 into... 

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).

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. 

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. 

---