Framework defects

In large measure the paradigm within which work is carried out is strongly influenced by the objectives of the work, the background of the investigator, and the particular materials model under study. From a strictly fluid mechanics, hydrodynamic, or continuum framework, defect issues are not overtly at issue. From a strictly mechanical framework, the defective solid... 

SDA/framework defect sites (nonasil, SSZ-23, ZSM-12) H cpmas, hetcor [179, 180]... 

To investigate the hydration and dehydration processes of H-SAPO-34 and H-SAPO-37, H and Al MAS NMR spectroscopy was applied under CF conditions with the equipment shown in Fig. 12 (217). The chemical behavior and the change of the silicoaluminophosphate framework were monitored as nitrogen loaded with water or dry nitrogen was injected into the MAS NMR rotor filled with the silicoaluminophosphates. By this approach, the primary adsorption sites of water in silicoaluminophosphates and the variation of the aluminum coordination were observed. Furthermore, the formation of framework defects and the conditions of water desorption were characterized. 

Niobium- and tantalum-containing mesoporous molecular sieves MCM-41 have been studied by X-ray powder diffraction, 29Si MAS NMR, electron spin resonance, nitrogen adsorption and UV-Vis spectroscopy and compared with niobium- and tantalum-containing silicalite-1. The results of the physical characterization indicate that it is possible to prepare niobium- and tantalum-containing MCM-41 and silicalite-1, where isolated Nb(V) or Ta(V) species are connected to framework defect sites via formation of Nb-O-Si and Ta-O-Si bonds. The results of this study allow the preparation of microporous and mesoporous molecular sieves with remarkable redox properties (as revealed by ESR), making them potential catalysts for oxidation reactions. 

Since a hydrothermal treatment at 1100 K for 5 days leads to a healing of the zeolitic framework, most of the internal SiOH groups must be vicinal Neighbouring framework defects (vicinal SiOH groups) are transferred via dehydration into intact Si-O-Si bonds (36). 

A lower Al content definitely leads to the Si richer Beta phase, that incorporates TEA+ counterions to Al negative charges and TEAOH ionic pairs, that occupy the maximum of the intracrystalline free volume, while Na+ ions partly neutralize the Si-O" framework defect groups. 

In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-ffamework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. 

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

Silanol groups. The so-called non-acidic Si-OH band at 3742 cm"1 is more complex than expected it results from two components located at 3738 and 3743 cm". The 3738 cm-1 is predominant for well crystalline materials while the 3743 cm-1 band is only detected in the case of nearly amorphous samples (figure 9). This would mean that the 3738 and 3743 cm-1 bands correspond to Si-OH attached to the framework (defects sites) and to Si-OH attached to an extra-framework phase containing silicon, respectively. 

In general, during the synthesis of vanadosilicates, crystallization period increases with increasing framework V, while the crystallite sizes decrease [8,23]. The increase in crystallization time is probably due to V-incorporation being thermodynamically less favoured and also due to the formation of imperfect crystals as a result of framework defects brought about by V. The latter reason will also explain the decrease in crystallite size with increasing V incorporation. 

In the other constrained simulations (constraint respectively at 2.75, 3.0, 3.5, 3.75, 3.9 A) the formation of this kind of defects has been observed in the whole simulation times, and defects transform each other in a very short time ( 100 fs). However for such values of the constraint, the reacting O atom was still bound to the framework. The reactive event occurred only when the constraint was set to an NO distance of 4.0 A. After few fs, the oxygen previously trapped in the framework defects, left the four ring region and diffused in the adjacent cage colliding with the second NOj. Such collision first led to the transient species [N02---02] , that appeared in the unconstrained nitrite sodalite -I- O2 simulation. Then, such species reached rapid equilibrium with the separated NOj and O2 compounds. 

SiOH SUanol groups at the outer surface or at framework defects 211,218,221... 

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