Degree of dealumination

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

Degree of dealumination. Moderate dealumination generally increases the catalytic activity or leaves it unchanged, while advanced dealumination leads to a decrease in activity. Such a decrease is due to a loss of active sites with advanced framework dealumination. 

DEALUMINATIQN OF ZEOLITE Y Dealumination is an important process to improve the thermal stability and resistance to acid of zeolite. This is one of the main techniques for preparing zeolite catalysts (US-Y). New pores (mesopores) have been introduced during hydrothermal treatment (Fig.4), which were directly confirmed by electron microscopy. The density of mesopores depended on the degree of dealumination and the size distribution of mesopores... 

XRD patterns of dealuminated samples D-l and D-2 agree well with previous work (15). The samples are highly crystalline and the (cubic) unit cell parameter is reduced (Table n) by the dealumination of the framework. The Si/Al ratios of samples D-l and D-2 were 3.10 and 4.91, respectively, which confirms that the degree of dealumination increases with the duration of hydrothermal treatment. 

To illustrate the dealumination and structure stabilization in treated zeolites, severely treated and postcalcined ones are chosen as examples. The degree of dealumination, nevertheless, can be controlled by varying the treatment conditions. 

Expectedly, increasing amounts of AHFS added led to increased degrees of dealumination of the samples. For moderate dealumination levels (<50%) in which 15, 30 and 50% of the original aluminium was calculated to be removed, respectively, the reaction was stoichiometric within the experimental error. Hence, the actual dealumination degrees obtained were 16, 32 and 50%, respectively. However, for higher degrees (75%) the reaction was not complete any more since only 64% dealumination was achieved. 

These curves are asymptotic and the error increases with the degree of dealumination the wave-number is precise to +/-1 cm" under typical recording conditions. 

High degrees of dealumination are difficult to achieve using AHFS compared with those obtained via hydrothermal treatment, because of loss of framework crystallinity. The characterisation techniques used here have shown that silicon enrichment occurs during the AHFS treatment, leading to higher bulk Si/Al ratios. 7a1 MAS NMR appears to show the presence of aluminium species other than those teU a- or octahedrally co-ordinated. These may be the fluorinated aluminium species mentioned in earlier works. The textural properties of AHFS treated zeolites are not changed relative to the parent material in contrast to the steam dealuminated zeolites, where the introduction of secondary mesopores occurs. 

Since the work of Lee et al. [37], zeolite mordenite continues to play an extraordinary role as a shape selective catalyst for the isopropylation of biphenyl. A high degree of dealumination [37, 38] and high pressure of propylene [39] seem to be advantageous to achieve high selectivities for 4,4 -diisopropylbiphenyl. Also, selective poisoning of the external sites with tributylphosphite [40] and the use of cerium exchanged sodium mordenites [41] are reported to suppress an undesired consecutive isomerization of 4,4 -diisopropylbiphenyl once formed in the pores. 

The starting NH/-form of Y zeolite was prepared by double ion exchange of NaY zeolite (Si/Al = 2,45) with a IN solution of NH4CI. Samples of Y zeolite with different degrees of dealumination were obtained according to the described procedure [1] by the treatment of the NH4 -form of Y zeolite with a solution of (NH4)2Sip6 with adjusted pH of the reaction medium of 6,0-6,4. The thermal treatment of the dealuminated zeolite was carried out by heating sample in a muffle furnace in air at 500-550°C for 6 h under deep-bed conditions. 

The comparison of the Si/Al ratios found by chemical analysis and IR spectroscopy indicates an excess of the overall concentration of aluminium in the dealuminated zeolite over its concentration in the framework, and these values are sufficiently close for the Na - and NPl4 -forms of Y zeolite (Table 1). When the degree of dealumination increases, the divergence between the overall Si/Al value and the Si/Al ratio in the framework increases. 

Additional reflections at 20 = 18,43 21,26 30,30 appear in the X-ray diffractions patterns of the samples with high degree of dealumination. These reflections correspond to extraframework aluminium species containing fluorine, which, as reported in [3], are not washed out from zeolite cavities even by repeated washing due to their strong interaction with the zeolite and dispersion in the crystallite bulk. 

On the other hand, for high degrees of dealumination the initial activity decreases with Nai, due to the decrease in the number of Bronsted sites per unit cell. The value of the residual activity when Al-Na+ is extrapolated to zero, is very small. This means that Lewis sites give a negligible contribution to the catalyst activity. 

Table V. Variation of relative HRMAS Si-NMR line intensities of mordenite at various degrees of dealumination by acid leaching...

The experimental data obtained for La,CeY zeolites suggest the existence of a correlation between hydrothermal stability and degree of dealumination under steam. The less stable zeolites are... 

Allv/Si+Allv. Interestingly, the acid strength continues to increase when Al VSi+Al1 < 0.1, although for such ratios NMR studies concluded that the strength of acidity remains constant (9). Our results relative to the h.f. OH groups are confirmed by those relative to the l.f. ones, since the latter also become more acidic (sensitive to ethylene) for high degrees of dealumination level. 

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. 

Catalytic Activity for m-Xvlene Conversion and Degree of Dealumination. In the reaction conditions used, the... 

The aim of the present work is to investigate the alkylation activity of chemically dealiuninated Y zeolites with different framework compositions during the gas phase alkylation of isobutane with 1-butene. Zeolite Y was treated with H4EDTA solution to achieve different degree of dealumination. These samples were characterized by X-ray diffraction, ICP-AES, surface area measurements, Si and Al MAS NMR and temperature programmed desorption of ammonia. 

Dealumination of Y zeolite by H4EDTA removes aluminium not only from the framework but also from the interstitial spaces resulting in samples completely free from extraframework aluminium. However, the degree of dealumination without structural collapse was rather low. Mildly dealuminated Y zeolites are found to be more active and selective for isobutane alkylation. 

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