Extra-framework cations

Lewis acid sites may be formed following dehydroxylation of zeolite surface in H-form. At sufficiently high temperatures two Bronsted acid sites can drive off a water molecule and leave behind a coordinatively unsaturated Al site, as illustrated in Figure 13.16 [32]. Here not only the resulting tri-coordinated Al but also the tri-coordinated positively charged Si can act as a Lewis acid. Furthermore dehydroxylation may be followed by framework dealumination, leading to cationic extra-framework species like AlO AlfOHij that can act as Lewis acids [33-37]. 

Figure 10.17 Example of the agreement between experimental and calculated diagrams of a stabilise zeolite requiring the incorporation into the model of cationic extra framework species.

In agreement with previous studies, microcalorimetry confirms that in steamed products, most of the strong acid sites are poisoned by cationic extra-framework A1 species. These sites can be recovered by an optimized post-steaming acid leaching. Isomorphously substituted HY which is free of extra-framework cationic species possesses more acid sites than conventional dealuminated solids with similar framework Si/Al ratio. 

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.

The interaction of CO and acetonitrile with extra-framework metal-cation sites in zeolites was investigated at the periodic DFT level and using IR spectroscopy. The stability and IR spectra of adsorption complexes formed in M+-zcolitcs can be understood in detail only when both, (i) the interaction of the adsorbed molecule with the metal cation and (ii) the interaction of the opposite end of the molecule (the hydrocarbon part of acetonitrile or the oxygen atom of CO) with the zeolite are considered. These effects, which can be classified as the effect from the bottom and the effect from the top, respectively, are critically analyzed and discussed. 

Numerous adsorption complexes of CO and AN in Na-A and in Na-FER were investigated only some of these adsorption complexes (giving an example of each type) are summarized in Table 1. First we discuss the effect from the top due to the interaction with the secondary cation(s). The CO molecule adsorbs on the primary cation (via C end) and when the secondary extra-framework cation is at a suitable distance from the primary cation CO forms a bridged adsorption complex between the... 

The CO-TPD technique together with DFT calculations were previously successfully used to characterize monovalent copper positions in Cu-ZSM-5 and Cu-Na-FER catalysts[4, 5]. Recently it was observed that the CO molecule can also form adsorption complexes, where the CO molecule is bonded between two extra-framework cations [6]. It is likely that the formation of similar species between the Cu+ and K+ ions can also occur. The presence of adsorption complexes on such heterogeneous dual cation site was evidenced by the FTIR experiments [7]. The formation of CO complexes on dual cation sites was not considered in our previous TPD models where three types of Cu+ sites were taken into account. 

The refinement performed by Gualtieri et al. [5] evidenced that ECR-1 is formed by a strict alternation of mazzite (MAZ) and mordenite (MOR) sheets, with 4-, 5-, 6-, 8- and 12-membered tetrahedral ring forming a three dimensional ring. In the Na- form, sodium cations are distributed over 5 different extra-framework sites. The thirty-five water molecules are distributed over eleven sites. It is worth noting that the position of the cations found in ECR1 does not correspond with the site found for mazzite and mordenite. In the NH4 -exchanged form, the NH4 ions occupy three distinct extraframework sites, whereas the water molecules are distributed over the same eleven sites found for the Na-form [5],... 

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

The ammonia is released and the protons remain in the zeolite, which then can be used as acidic catalysts. Applying this method, all extra-framework cations can be replaced by protons. Protonated zeolites with a low Si/Al ratio are not very stable. Their framework structure decomposes even upon moderate thermal treatment [8-10], A framework stabilization of Zeolite X or Y can be achieved by introducing rare earth (RE) cations in the Sodalite cages of these zeolites. Acidic sites are obtained by exchanging the zeolites with RE cations and subsequent heat treatment. During the heating, protons are formed due to the autoprotolysis of water molecules in the presence of the RE cations as follows ... 

As mentioned above, an acidic zeolite can provide both protonic (Bronsted) and aprotonic (Lewis) sites. The Bronsted sites are typically structural or surface hydroxyl groups and the Lewis sites can be charge compensating cations or arise from extra-framework aluminum atoms. A basic (proton acceptor) molecule B will react with surface hydroxyl groups (OH ) via hydrogen bonding... 

For weak acceptors like charge balancing cations (Na, K, etc.), the interaction with the lone pair on the nitrogen is weak and results in a absorbance band around 1440cm . For stronger acceptors like extra-framework aluminum atoms (Lewis... 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

The typical unit cell content of zeolite L is (K,Na)gAi9Si27072.nH20 and its Si/AI ratio varies in the range of 2.6 - 3.5 [1-4]. Takaishi recently determined the distribution of Al atoms in the framework of zeolite L ly analyzing Si-MAS-NMR spectra. He thereby deduced five kinds of extra-framework cation sites as shown in Fig. 1., and estimated the relative strengths of their cation affinities [5]. 

The incorporation of Al atoms into the framework of zeolites occurs in a tetrahedral oxygen coordination and leads to negative framework charges. These framework charges are compensated by protons in acidic hydroxyl groups or by extra-framework cations such as Li , Na, Cs", Mg ", etc. Accordingly, these surface sites are responsible for the chemical behavior of zeolites in separation processes and in catalysis (199,200). 

The values of ev and ay are the well depth and size parameters, respectively, for the two interacting atoms i and j. In the case that one of the interacting atoms is a zeolite atom and the other is a sorbate atom, the cross terms ezeo-sorb and o-zeo-Sorb are determined from the Lorentz-Berthelot combination rules (7). When polarization interactions are accounted for, such as those between adsorbates and zeolite extra framework cations, Eq. (2) is written in the form... 

Moreover, removal of the organic component is accompanied by a contraction of the unit cell. The contraction is minimal in the referenced, Al-ffee, Si-MCM-41 and increases with the solids A1 content suggesting that HDTMA and TEA cations are charge-compensating framework Si-OH-Al(IV) units on the walls (7). As a result, the average pore diameter (APD) decreases with Si/Al values. Furthermore, as the crystals Si/Al value decreases, the A1(VI)/A1(IV) ratio increases due to the generation of extra-framework Al(VI)-species during calcination. 

In addition, some samples also gave a signal at 7.1 ppm from the residual NH4 cations the amount of the latter was determined by thermodesorption and subtracted from the intensity of line (3). Thus the sum of intensities of (2) and (3) gave the true total content of acidic hydroxyl groups. They had T2 of 60-75 ftscc, while sample 500 SB contained an additional free induction decay (FID) component due to extra-framework hydroxyls. 

Reference cites the literature from which the crystal data, atomic coordinates, and displacement factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if authors and users would inform us of any errors or omissions. A listing of the references for isotypic species can be found in the Atlas of Zeolite Framework Types (Baerlocher, McCusker and Olson (2007)). A list of references to structure analyses of zeolites with different cations, up to 1982, is given in the Compilation of Extra Framework Sites in Zeolites, Mortier (1982). 

Extra framework A1 in HY-type zeolites (generated during calcination and steaming) could directly react with VO+2 cations and form stable complexes thus increasing the crystal tolerance to V. 

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