Extra-framework

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 previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

It must be noted that sometimes calcination is beneficial to create active species. Notable examples are the Sn-beta speciation [176] and generation of extra-framework Al-Lewis sites in beta zeolite for organic transformations... 

All the other characterization studies have been performed after the calcination step XRD experiments have shown that the material formed during the synthesis has the MFI structure Si and 27a1 MASNMR spectra indicated that this phase has a Si/Al ratio varying between 550 and 30 as a function of the sample prepared and also that no extra framework A1 is present. 

Characterization of two different framework titanium quantification of extra-framework species in TS-1 silicalites. 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

The extra-framework Ti was identified as belonging to a segregated Ti02 anatase phase. Higher level of incorporation of as much as ca. 6 and 8 % were claimed, using either silicon... 

The X-band measurements cannot identify which one of the iron sites can react with the carotenoid. Only the 95 GHz measurements (Figure 9.14) were able to demonstrate that adsorption of carotenoid results in a significant decrease of the =2.07 signal and moderate decrease of the =2.45 signal, while the intensity of the narrow line with ,= ->,=2.003, gz= 1.999 is almost unaffected. The results show that the extra-framework Fe3+ ions located on the surface of the pore are primarily responsible for carotenoid oxidation. Probably, these sites are more accessible for bulky organic molecules than the framework iron within silica walls. 

Between 3600 and 3650 cm"1 this vibration zone makes it possible to observe SiOHAl vibrations of OH groups pointing towards the supercages by means of the high frequency band at 3635 and 3645 cm"1 (for the dealuminated and non dealuminated OH, respectively). All the other peaks in this region 3600, 3620, 3630 cm"1 are due to interactions with extra framework phase. 

Between 3500 and 3600 cm"1 we can observe vibrations due to the OH groups pointing inside the sodalite cavities, i.e. the low frequency band at 3570 cm"1 and the associated band perturbed by the extra framework phase at 3525 cm"1. 

W.J. Mother, Compilation of extra-framework sites in zeolites, Butterworths, Guildford, 1982. 

Mossbauer spectra of calcined samples (Table 1). The Fe3+(3) and Fe3+(4) components are probably located in tetrahedral (framework) positions. The charge distribution around the Fe3+(3) is asymmetric (large QS), thus here the charge compensation is probably provided by Fl+, i.e. indicating the existence of Bronsted sites. The charge symmetry around Fe3+(4) is more symmetric, thus the counterion is probably Na+ or Fe(OFl)+. Fe2+ ions are probably located outside of the framework (due to their larger ionic radius). Thus, in the hydrogen a small part of Fe3+ is reduced to Fe2+, and is probable removed to extra-framework sites. 

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]. 

Dealuminated samples were obtained by hydrothermal treatment of calcined MCM-22 (Si/Al = 15) at different temperatures (673, 773, 873 K) for 2-24 h under a saturated flow of a nitrogen/steam mixture (flow rate of 200 ml min"1). These steamed samples were further treated with 6N HNO3 solution at 353 K for 4 h in order to remove the extra-framework aluminum species. 

Epoxidation of alkeneic reactants is faster on titanium-grafted silicates (such as A, B and C) than on the coprecipitated titanosilicates (such as D and E). This difference was attributed to the fact that on extra-framework titanium-grafted silicates, the catalytically active sites are virtually all exposed and accessible, whereas on the coprecipitated material some of them may be buried within the silicate walls and, thus, cannot adsorb reactant molecules. 

III.B.3. Lewis Acid Sites and Extra-Framework Aluminum. 260... 

---