Framework aluminium

Figure C2.12.8. Schematics of tlie dealumination of zeolites. Water adsorbed on a Br( msted site hydrolyses tire Al-O bond and fonns tire first silanol group. The remaining Al-0 bonds are successively hydrolysed leaving a silanol nest and extra-framework aluminium. Aluminium is cationic at low pH.

Cruz, ).M., Corma, A., and Eornes, V., Framework (1989) Framework and extra-framework aluminium distribution in (NHdjSiFs-dealuminated Y zeolites relevance to cracking catalysts. Appl. Catal, 50, 287. 

Cu ions are incorporated into molecular sieve as divalent (Cu2+(HzO)6)2+ or monovalent (Cu2+X (H20)s)+ complex cations [11]. Thus, after dehydration and subsequent reduction, Cu ions are placed in viscinity of one or two framework aluminium atoms [1], As two cationic sites with two close aluminium atoms are present in (A1)MCM-41 (cf. Chapters 3.1. and 3.2.), four different coordinations of the Cu+ ions in (A1)MCM-41 represent two cationic sites with two close aluminium atoms and two cationic sites balanced by a single aluminium atom. Thus, besides two cationic sites accommodating divalent cations, two other cationic sites being enable to accommodate only monovalent cations are present in the (A1)MCM-41 molecular sieve. Because only low exchange degree can be reached for divalent cations (cf. Table 1), sites with isolated aluminium atoms represent majority of cationic sites in (A1)MCM-41 with Si/Al > 20. 

Recently the effect of steaming on the presence of framework and extra-framework aluminium and its effect on catalytic activity has been the subject of a considerable research effort. 

Reactions of cyclo-olefins within the zeolites can generate aromatics by bi-molecular hydrogen transfer reactions. Such reactions are expected to be more favourable with REHY because of its greater framework aluminium content and because of restricted access to the inter-crystalline pores in ZSM-5. This is... 

Figure 8. Amount of coke oxidized above 450°C as a function of the number of framework aluminium atoms per unit cell (Ny ) of the zeolites.

When USY zeolite is reacted with a lower concentration and lower adding speed of nitric acid solution, only the non - framework aluminium species located in the pores are extracted and H+ ions could not attack the zeolite skeleton, The non - framework aluminium content of USY zeolites decreases with increasing the concentration and adding speed of acid solution. ... 

NMR-characterization. 27 Corbin et al. (35) were able to show by a systematic study that Z/A1 MAS NMR gives the true Si/Al ratio with a mean error of 10 %, if two conditions are met a) The amount of paramagnetic species is less than 0.05 % and b) the sample does not contain "NMR-invisible" aluminium. Chemical analyses of the samples under study showed that condition one is fulfilled. If samples contain "NMR-invisible" aluminium a difference between the concentration determined by chemical analysis and the framework aluminium concentration determined by NMR should be observed. From the absence of such a difference we conclude that "NMR-invisible" aluminium species do not exist in our samples. Also a line at the position of about 0 ppm due to octahedrally coordinated non-framework aluminium and a broad line at about 30 ppm due to tetrahedrally coordinated nonframework aluminium (36) could not be observed. The values for the concentration of framework aluminium atoms derived from the intensities of the line at about 60 ppm (see below) are in good agreement with those corresponding to the amount of alumina used in the synthesis mixtures. In conclusion, through the Al MAS NMR measurements it was possible to show that all aluminium atoms are incorporated in tetrahedrally oxygen coordinated framework positions. 

Table VI. Concentrations of hydroxyl species and framework aluminium atoms in the hydrogen (H) or sodium (Na) form of pentasil zeolites synthesized with different organic templates and without organic templates...

Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the composition of the framework by increasing the silicon content increases the thermal stability of the samples. The catalytically active centres in zeolites are the acidic (Bronsted) hydroxyl groups associated with tetrahedrally coordinated framework aluminium atoms. Catalytic activity is thus strongly dependent on the concentration and location of aluminium in the framework. 

Virtually all the extra-framework aluminium in dealuminated (ultrastabilized) zeolite Y can be re-substituted into the framework by treatment with an aqueous solution of KOH. 

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... 

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. 

Figure 2 Change with Nau the number of framework Aluminium atoms per unit cell in the number of potential protonic sites per unit cell nH+ (=Nai), in their acid strength, in the zeolite activity for reactions demanding strong acid sites (1) or catalyzed even by weak acid sites (2).

Final treatment of the host material with hydrochloric acid removes some of the extra-framework aluminium in order to make room for guest molecules. The overall procedure is described elsewhere in detail11.15. 

In general, the zeolite-Y crystallographic unit cell contracts with the removal of framework aluminium and this parameter is therefore a rough measure of acid-site... 

For the SCR of NO with propane, that is in the absence of diffusion limiting structural effects, Cu-Al-MCM-41 (Si/Al=30) catalyst results substantially less active and selective than Cu-ZSM-5 catalyst with similar Cu content and Si/Al atomic ratio. Moreover, the presence of extra-framework aluminium lowers the selectivity to CO2. These results indicate that Cu-Al-MCM-41 catalysts are not suitable for NO SCR reactions with hydrocarbons. 

The number of framework aluminium (FAL) atoms per unit cell, Alf, was calculated from ao using the correlation given by Fichtner-Schmittler et al. [6]. The atomic framework Si/Al ratio was derived from the calculated Nai. The number of extra-framework aluminium (EFAL) atoms per unit cell was calculated by the difference between the total aluminium, as determined by XRF analysis, and the framework aluminium Alf. 

For the parent material XRF analysis gave aluminium contents that were noticeably larger than the framework aluminium concentration indicating the presence of large amounts of extralattice or non-framework aluminium. A comparison of the results from Table 1 clearly demonstrate that at relatively low dealumination levels, i.e. 16 and 32%, the AHFS treatment preferentially removed EFAL species while both EFAL and FAL were removed when the AHFS concentration was increased [14,15]. Hence, for H-Y(d5o%) sample, almost 100% EFAL and 30% FAL was extracted from the parent material. Indeed, it was found for this sample that the framework Si/Al ratio obtained from the unit cell size was very close to the value obtained by XRF measurements. Our observations were consistent with the results... 

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