Extra-framework groups

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.

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. 

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... 

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). 

Numbers of OH Groups per Cavity (4 of Unit Celt), Framework Aluminum Atoms (Al, ), Extra-Framework Aluminum (A/A — -4/gS J, and Aluminum in the Form of Mobile Hydrated Complexes... 

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. 

It is therefore clear that dehydroxylation is always accompanied by the release of Al from the framework. It seems that when the Al atom in the vicinity of a structural OH group is lost from the framework, the group is simultaneously dehydroxylated. Deep-bed treatment produces four times as much extra-framework Al as shallow-bed treatment at the same temperature. 

The maximum content of titanium in Ti-beta zeolite appears to be higher than in the other materials. A value of x = 0.038 has been reported without formation of extra-framework Ti02. From the characterization of Ti-beta zeolite by XANES and EXAFS, it has been concluded that Tilv in the calcined material is tetrahedrally coordinated, isolated from other TiIV ions, and surrounded by OSi groups. In the presence of H20, Tilv increases its coordination and very likely undergoes hydrolysis of the Ti—O—Si bonds forming TiOH and SiOH groups (Blasco et al., 1993). 

Meradotos and Barthomeuf (31) found enhanced activity in hydrothermally treated mordenites, which they accredited to bridging hydroxyl groups interacting with extra-framework Al. Lago et al. (32) found a large increase in the activity of HZSM-5 after mild steaming at 540°C, which they attributed to paired Al atoms. 

The total concentration of OH groups in dealuminated zeolite Y samples was determined using 1H-NMR techniques described (40) and is in reasonable agreement with the concentration of tetrahedrally coordinated extra-framework Al. The concentration of SiOH groups does not change in hydrothermal dealumination, but increases on acid leaching due to removal of framework Al. 

Because of the chemical implausibility of some speculations about zeolite A, high-precision X-ray analyses were made of dehydrated crystals whose chemical composition was checked by electron microprobe analysis. Data were collected for the 24 A superstructure. All diffractions are consistent with space group Fm3c except for a few weak ones. Unpublished measurements by J.J. Pluth and G.D. Price of a hydrated Na-A (i.e. as-synthesized) showed strict obeyance of Fm3c except for a very weak (111) diffraction, and the inconsistent diffractions of the dehydrated crystals are attributed tentatively to minor positional disorder of extra-framework cations (1). 

Unlike aluminum, titanium is tetravalent and can exhibit different oxidation states. Thus TS-1 is non-acidic if isomorphously substituted. TS-1 is relatively difficult to synthesize, which is probably one of the reasons the site structure was under debate for quite some time. TS-1 can only be synthesized with a maximum of 3 wt% Ti if more Ti is added extra-framework titanium is formed. Inihally, it was suggested from XAS that octahedral sites are formed in the silicalite framework [60]. However, later more and more groups suggested tetrahedral isomorphous substitution of the titanium sites in the MFl framework [61-63], It is now more or less generally accepted that the Ti is four-coordinate and has a Ti—O distance of 1.79-1.81 A. This is a significantly increased distance compared to the Si—O distance 1.605 A, which is constant for a vast number of oxides [4]. The increase in... 

Figure 13.8 Chemical shifts observed in proton NMR for a zeolite (a) sitanol. (b) and (c) acidic OH groups, (d) NH/, (e) OH linked to extra-framework Al.

HM-MOR was dealuminated with phosgene between 673 and 973 K. Chemical analysis of samples combined with A1 NMR and Si NMR data allowed us to quantify the Si/Al ratio, the concentration of extra-framework aluminium and framework vacancies. The Si NMR spectra reveal progressive dealumination with increasing temperature, accompanied by formation of SiCl and Si(OH)2 defect groups. The former yields quantitatively SiOH groups. The result showed unambiguously that framework reconstruction occurred during dealumination. 

Ferrisilicates are considered for catalytic applications, primarily in their H-form, due to the acidic function on the Si-OH-Fe groups. In addition, the extra-framework ions may also play a catalytic role (e.g. exhibiting Lewis acidity). Further, in the case of comparable size of diameter of channels ancJ reaction components, shape selectivity is imposed by the zeolite structure. Thus, the overall catalytic performance is influenced by various sources. It should also be considered that in general case the structure, the distribution of iron components among the possible oxidation and coordination states may also change under catalytic conditions in a ferrisilicate. 

Extra-framework atoms 051-056 were refined with oxygen scattering factors. In editions of the International Tables For Crystallography published after 1995, space group symbol Cmme replaces Cmma. 

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