Defective Silicalite

Keywords Silicalite-1 (S-l), defective silicalite MFI framework, entrapped (CuO)n nanoclusters. 

Raman experiments on sllicalite and TS-1 with excitation wavelengths of 1064 nm (non resonant) and 244 nm (resonant) show that (i) the main features associated with Ti insertion in the lattice are vibrations at 1125 and 960 cm" the former being drastically enhanced by UV-resonance, while the latter is not (ii) a mode is observed at 978 cm" on defective silicalites and TS-1, which we attribute to the Si-0 stretching in silanols. The proximity of the 960 and 978 cm" modes has prompted us to re-examine IR spectroscopy in the same region in order to distinguish the 960 cm band from defect modes. 

Bordiga, S., Ugliengo, R, Damin, A., et al. (2001). Hydroxyls Nests in Defective Silicalites and Strained Structures Derived upon Dehydroxylation Vibrational Properties and Theoretical Modelling, Top. Catal, 15, pp. 43-52. 

Defective Silicalite is an all-silica MFI zeolite, which is non-hydrophobic and weakly acidic as a consequence of the abundant polar defects (Si-OH nests) generated by the stmcture to compensate the Si atoms vacancies [7]. IR spectra in the vqh stretching frequency region give a clear evidence of the presence of Si-OH nests, which are characterized by a different geometrical arrangement according to the synthesis procedure and/or post-synthesis treatments, as illustrated in ref. [36]. 

V. Bolis, C. Busco, S. Bordiga, P. UgUengo, C. Lamberti, A. Zecchina, Calorimetric and IR spectroscopic study of the interaction of NH3 with variously prepared defective silicalites— comparison with ab initio computational data. Appl. Surf. Sci. 196(1-4), 56-70 (2002). doi 10. 1016/S0169-4332(02)00046-6... 

Fig. 4 a IR spectra, in the OH stretching region, of from top to bottom, TS-1 samples (full line spectra) with increasing Ti content, from 0 (silicalite-1, dashed spectrum) to 2.64 atoms per imit cell. All samples have been activated at 120 °C. Adapted from [24] with permission. Copyright (2001) by the ACS. b Schematic representation of the preferential location of Ti atoms and Si vacancies in the MFI framework (upper part) and their interplay (lower part). Yellow and red sticks represents Si and O of the regular MFI lattice blue balls refer to Ti, and red and white balls to O and H of defective internal OH groups... 

From the data reported in Fig. 8, it clearly emerges that the acidity of the silicalite-l/H20 and of the TS-I/H2O systems are remarkably different (compare open and full circles in Fig. 8). This difference can be explained as follows TS-1 has two main acidic sites, Ti(IV) Lewis sites and silanols, mainly located in the internal defective nests (see Sect. 3.8), while only the latter are present in silicalite-1. Addition of H2O2 to siUcaUte-l does not modify the titration curve (compare open circles with open squares in Fig. 8). This means that no additional acidic sites appear in the siUcaUte-l system upon adding H2O2, i.e., that hydrogen peroxide molecules coordinated to internal silanol do not modify their acidity. Conversely, addition of H2O2 to TS-1 moves the whole titration curve toward lower pH values, (compare full circles with full... 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Infrared and Raman Band at 960 cm-1 assigned to Ti-O-Si vibration (Caution Si-OH and defect sites in silicalites also show this feature). 

The atom-planting method for the preparation of several metallosilicates with MFI structure was studied. By the treatment of silicalite or ZSM-5 type zeolite with metal chloride vapor at elevated temperatures, metal atom could be introduced into the zeolite framework. From the results of alumination of silicalite it is estimated that the metal atoms are inserted into defect sites, such as hydroxyl nests in zeolite framework. The metallosilicate prepared had both Bronsted and Lewis acid sites with specific acid strength corresponding to the kind of metal element. 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

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. 

Figure 2a shows the 29Si MAS NMR spectra of calcined NbS-1, TaS-1 and ZSM-5 The spectral shapes of NbS-1 and TaS-1 are similar to those of aluminum-containing ZSM-5 with ns/nAi = 45, Three lines are observed for NbS-l(41) and TaS-l(74) whose chemical shifts are -103, -112 and -115 ppm. The line at -103 ppm is due to the presence of [SiO (OH)] units in defect sites within the silicalite-1 structure This assignment was confirmed by H - 29Si crosspolarization (CP) experiments in which the intensity of the signal at -103 ppm increased... 

An increase in hydrophobicity by inserting Ti into the framework is further substantiated by the crystallization curves the crystallinity of silicalite-1 decreased with increasing crystallization time after reaching the maximum. The presence of defect sites would make the material hydrophilic, resulting in an easy attack by adsorbed H20, and thus a collapse of the framework (4.1) ... 

Third, elemental analyses and thermogravimetric measurements of the products obtained in the absence of DMSO point to the possible presence of missing T sites defects (3.). This is because on every 96 Si T sites evidently more than the 4 TPA entities expected for an ideal as made silicalite sample (IS) are incorporated. Interestingly, Si-NMR spectroscopy also indicates that a large amount of defects (35%) are present in these samples (vide infra). The apparent presence of missing T sites defects may point to the D5R synthesis model being operative (cf. the introductory comments and Figure 1). 

Second, the elemental analyses of the ZSM-5 samples (20-261 prepared via standard synthesis routes do not point to missing T sites defects since these samples contain the normal (1 ) value of 3.5 - 4 TPA entities per 96 T sites (see Table IV). This observation has been confirmed by thermogravimetric measurements. Therefore, the defects in these materials are more likely to originate from hydrolysed SiOSi linkages. Interestingly, the silicalite sample prepared from clear solution at low temperature (sample 27, Table IV) most probably does contain missing T sites defects. This assertion is based on the observed high -103 ppm NMR intensity and the occlusion of more than 4 TPA entities per 96 T sites (see also Table I, samples 1-4). 

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