Extra-framework aluminium species

The acid catalysed hydration reactions of a-pinene and camphene, respectively, using USY catalysts in aqueous acetone at 55 C, are studied. The catalyst samples were prepared from zeolite Y by hydrothermal treatments at temperatures ranging from 450 to 850 C. The so generated extra-framework aluminium species were kept in the samples. 

The extra-framework aluminium species (EFAL) generated by hydrothermal treatment of zeolite Y have a strong effect on catalytic activity and selectivity upon cracking, isomerization and alkylation reactions of hydrocarbons [14-17]. The effect of EFAL species on the liquid phase isomerization of a-pinene has also been studied [18]. 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

A1 mas NMR spectra of all the samples (not shown) show only one line corresponding to aluminum atoms situated at 60 ppm assigned to tetrahedral position in the framework. No peak for six coordinated (extra-framework) aluminium which occurs at 0 ppm was observed. This shows that dealumination by H4EDTA treatment removes aluminium atoms not only from the framework position but also removes extrafiamework aluminum species from the interstitial positions. It may be mentioned here that the other methods of dealumination like hydrothermal [17] or SiCU treatment [23] results in a material which contains some amount of extraframework aluminium, specially in the former case. 

Fig. 4 shows clearly that the steaming treatment conditions have a considerable impact on the variation of the acidic properties of ZSM-5 zeolites. Notably, ZSM-5-ST only possesses residual Bronsted acidity. ZSM-5-MT exhibits a lower number of pyridine molecules chemisorbed on Bronsted acid sites at room temperature, compared to ZSM-5-P. As steaming posttreatment extracts part of the framework aluminium species (acting as Bronsted acid sites) and converts these into extra-framework aluminium, (acting as Lewis acid sites), a deerease in the number of Bronsted acid sites is expected. A close look to the band at 1545 cm of ZSM-5-MT at 573 K and 673 K reveals a higher number of pyridine molecules attached to... 

For the materials derivatised during the synthesis, the effect of copper or silver addition is illustrated by A1 MAS NMR spectra in Fig. 2. The NMR spectra of the calcined samples with largest metal content of 20 wt % shows an intense line, especially for Gu-containing sample, at 8 = 52 ppm firom tetrahedrally coordinated Al. The appearance of a second signal at 8 = 0 ppm indicates that extra-framework aluminium attributed to octahedrally coordinated Al is only present in sample without metal, SiA120. Tetrahedrally coordinated Al was of special interest and highly desirable, because it is incorporated into the framework and therefore is responsible for the acid sites formation, and thus enhance the propene adsorption selectivity and capacity [16]. The octahedral aluminium species are occluded in the pores or exist as an amorphous by-product. 

Literature data on the deal umi nation with Si Cl 4 of X and Y type zeolites with Na, Li, H, Ca or La as charge compensating cations is reviewed. The properties of the final zeolite products, in particular the degree of dealumination, the crystallinity, mesoporosity, and the nature and content of extra-framework aluminium and silicon species are explained based on the chemistry involved in the different steps of the dealumination procedures. The reactivity towards SiCl4 of the framework aluminium atoms is explained based on the location of the associated charge compensating cations over accessible and hidden sites. 

Figure 1 shows the A1 NMR spectra of dealuminated H-mordenite samples. The framework tetrahedral aluminium at 54 ppm decreases and the extra-framework octahedral aluminium at ca. 0 ppm increases as a function of increasing temperature of dealumination. In the final sample dealuminated at 973 K (e), the rather broad NMR lines for both tetrahedral and octahedral aluminium show the presence of extra-framework aluminosilicate and alumina species [14]. 

Figure 1 shows the relative concentrations of Lewis (L) and Bronsted (B) acid sites, calculated from IR spectra of ad-sorpted pyridine [22, 23], as a function of the number of framework aluminium atoms per unit cell, Nai. When Nai decreases and therefore NEFAL increases, [L]/[B] increases, meaning that the increase of extra-framework species corresponds to an increase of Lewis acidity. 

In zeolites, tetrahedral framework aluminium can be distinguished from non-tetrahedral extra-framework species by means of Al MAS NMR. However, some AlPOs are known to contain 5- or 6-coordinated Al in the framework, which complicates a quantitative determination [4—6]. Quantitative methods for the monitoring of substituting metals Me are, therefore, required. In the case of transition metal ions, possible changes in the oxidation state must also be taken into account, since the charge n of [Me02] building units should directly affect the number of Bronsted acidic sites. 

The X-ray diffraction patterns of the AlPO and CoAPO samples indicate pure phases, except for some samples of CoAPO —34, which might contain amorphous material. However, in view of the product compositions (Table 1) it must be concluded that many samples are not pure since the (Al+Co)/P ratio is considerably greater than 1. In these cases, part of the aluminium and/or cobalt must be present in extra-framework species. In particular, samples prepared with high cobalt concentrations in the gel and samples of CoAPO -34 exhibit unfavourable element ratios. These samples are also often less homogeneous in that they can contain white particles, whereas the major phase exhibits a blue colour. 

Calcination generates acidic sites, but also promotes dealumination, generating aluminium species known as EFAL (Extra Framework Alumina). The removal of framework aluminium changes the UCS of the zeolite. The number of aluminium atoms in the unit cell may be calculated using the following correlation [21] ... 

Dealumination of zeolite Y is an essential step in the preparation of FCC catalysts. The depletion of the zeolite framework with aluminium is accompanied by the generation of mesopores and extra-framework silicon and aluminium species, which may play important catalytic roles. 

Dealumination of zeolite Y with SiCl4 was first reported by Beyer and Belenykaya (ref.l). In this paper some more light is shed on the chemistry of this dealumination reaction and on the mechanism of generation of mesopores and extra-framework silicon and aluminium species during dealumination with SiCl4. 

---