Dealumination process, effect

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Bronsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid-catalyzed reactions up to a maximum at ca. 32 framework A1 atoms per unit cell. A model for strong Bronsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other A1 atoms in a 4-membered ring and (ii) complex A1 cations in the cages. The essential role of extraframework aluminum is evident from recent studies in which framework A1 has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. 

Compared to the NaCaA sample the slightly higher stability of the NaY zeolite is caused by the lower content of framework aluminium. With further dealumination of this framework a further stabilization would be expected. However, this effect is not observed. Also the assumption of a strong distortion of the framework after the dealumination process as the sole reason for the reduced stability is incorrect. 

Table 1 shows that both the dealumination process by reaction at 650 C (compare MH5.9F with MH5.9D) and the acidic dealumination decrease the volume available for the physisorption of N2 and Xe, the effect being even more marked in the case of Xe. The decrease of desorbed NH3 in TPD experiments is related to the decrease of the APV sites observed by 27A1 MAS NMR. i29Xe NMR of physisorbed Xenon, and ethylene diffusion measurements help us to better understand the deactivation phenomena. 

Zhang et al. (2001) studied the HTT effects on the structure of zeolite HZSM-5 (with micro-and nanosized particles) using (Figure 2.96), Al, and Si solid-state MAS NMR. The thermal stability of nanosized HZSM-5 ( 70 nm) is lower than that of microsized particles ( 1 J.m) due to dealumination and desilicification. After HTT at 700°C, the Brpnsted acid sites (6h = 3.9 ppm) disappear (as well as nonframework AlOH at 8h = 2.7 ppm) in nanosized HZSM-5 in contrast to microsized HZSM-5 (Figure 2.96). However, the silanol peak at 1.7 ppm increases for nanosized particles. For microsized particles, the dealumination process is dominant. Upon HTT, the amorphous silica can heal dealuminated fragments. Therefore, the hydrothermal stability of nanosized HZSM-5 particles is similar to that of the microsized HZSM-5 (Zhang et al. 2001). 

The dealumination process is associated with a change in the porosity within the crystals and may sometimes cause a drastic loss of crystallinity. The microporous adsorbents of the faujasite type are so arranged that the Si/Al ratio increases as the munber of cations and the average electrostatic field within the framework decrease. To assess the effect of the Si/Al ratio on the activity and acidity of Y zeolites, it is desirable to compare samples with similar extents of exchange, since the degree of exchange has a significant influence on the catalytic and acidic properties of faujasites. 

In the description of the preparation of ultrastabilized Y zeolite reported in [61] it was not explicitly stated that water vapor is involved in the dealumination process. Also, in numerous other papers dealing with thermal dealiunination and/or thermal stabilization of zeolites, the role of water was ignored and the stabilization process was assumed to be governed by a purely thermal effect. 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

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). 

Carbonaceous deposits are the principle cause of zeolite deactivation in processes involving hydrocarbon conversions. Firstly they can poison active sites or block their access. Secondly, their removal, which is carried out through oxidative treatment at high temjjeratures, has detrimental effects i.e., dealumination and degradation of the zeolite and sintering of supported metals. 

It seems that dealiunination with chelating agents is essentially an acid-leach-ing process where the effectiveness is enhanced by complexing of the aluminum species formed as reaction products. The process proceeds stoichiometrically (see Fig. 2) so that it can be controlled by the amount of H4EDTA calculated for dealumination to the desired level [24,29]. 

It is highly probable that lattice vacancies created by removal of silicon may be filled up in the same way and under similar conditions as those remaining after release of aluminum. Thus, recrystallization of the desilicated products to particles with well-ordered crystal structure but traversed by nanopores is obviously effected by water steam present as reaction product of the dehydroxy-lation of hydroxyl nests . After the desihcation process, the zeolite is in the sodium (or potassium) form and this is known to be highly resistant towards hydrothermal effects. Therefore, it is to be expected that steaming represents the most effective method for the stabilization of desilicated zeolites avoiding the risk of concurrent dealumination. 

Post-synthesis modification of zeolites via alteration of the alvunimun content of the framework became a most important topic of zeolite chemistry when, in the mid 1960s, the effect of stabilization through dealumination was discovered. In Chapter 3, H. K. Beyer contributes a systematic review on techniques for the dealumination of zeolites by hydrothermal treatment or isomorphous substitution amended by a section on the reverse process, i.e., introduction of aluminum into and removal of silicon from the framework. 

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