Silanol lattice defects

In addition, potential local defects including gem OH groups and silanol nests may be easily detected. For example in the case of Boralite (B-MFI), water molecules held at trigonal Boron sites, gem and OH groups were identified (23) which indicate either lattice defect or dynamic equilibria involving adsorbed water molecules and dessociated water. Similar defects were also thought to be present in the case of Cr-MFI (24). 

Figure 14 shows the high-resolution MAS Si-NMR spectra of Na-mordenite and its dealuminated counterparts. The spectrum of Na-Z consists of three resonance lines at -99, -105 and -110 ppm, corresponding to Si(2Al), Si(l Al) and Si(OAl) configurations respectively. In addition, silanol groups at defect lattice sites contribute to the intensity of the NMR line at -105 ppm. As dealumination proceeds, the lines at -99 and -105 ppm decrease, while the relative intensity of the -110 ppm line increases (Table V>. 

Similarly, the low frequency overtone at 6950 cm-1 associated with acidic OH vanishes, while the silanol overtone band develops at 7325 cm-1 (9) and the ( v + 6) combination shifts to 4540 cm-1. These observations are consistent with the creation of silicon defects in the structure of dealuminated Y zeolites (10) while the weak overtone band at 7240 cm 1 is probably related to hydroxylated aluminium species extracted from the lattice (11, 12). Thus, the near-IR spectra give evidence for the decrease of the number of Bronsted acid sites as a result of dealumination. 

The framework IR spectra of metallosilicates reveal a band at -960-970 cm, the band being attributed to Si-O vibrations of defect (internal) silanol groups associated with the metal locations in the framework [18]. As a consequence, the presence of the band is generally used as an evidence for the presence of the metal in lattice positions associated with defect sites [6,7]. The framework IR spectra of the calcined samples A and B are presented in Fig. 3. The spectrum of B has a band at 967 cm" and not that of A suggesting the absence of defect silanols and, presumably, the absence of vanadium in the framework positions in the latter sample. 

Further evidence for the presence of defect sites and V-ions in the framework of sample B comes from the Si NMR spectra of the samples (Fig. 4). The Si NMR spectrum of the calcined sample B reveals a resonance attributable to defect silanols at 5 = -100 to -110 ppm which is not observed in the spectrum of A. Besides, the bands due to Q -Si observed in the range of 5 = -110 to -120 ppm are merged without resolution leading to a broad band in the spectrum of B, while they are better resolved in the spectrum of A. The broadness and the lack of resolution of the lines in the spectrum of B may be an indication of the statistical distribution of the V in the lattice. 

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. 

The post-synthesis incorporation of aluminium into the lattice of pure siliceous zeolite-p was attempted using aluminium isopropoxide as aluminating agent in a non-aqueous environment. The XRD structural analysis of the Al-grafted materials showed an increase in the unit cell parameters which was associated with the insertion of aluminium into the framework. Quantitative multinuclear NMR investigation showed that the amount of framework aluminium incorporated into the zeolite lattice was related to the concentration of defect sites in the parent Si-p zeolite. This indicated that the alumination proceeds through a mechanism which involves the reaction between Al(OPr)3 and silanol groups at defect sites. Calcination after alumination led to the completion of the process, whereby octahedral-coordinated aluminium, (partially) attached to the framework, was transformed into tetrahedral-coordinated framework aluminium. 

Figure 1.6 Schematic illustration of a fully coordinated tetrahedrally bonded titanium atom substituted for a tin atom at one of the lattice positions of TS-1 (A) and the same titanium site located near a silicon vacancy filled with hydrogen atoms to form a silanol nest (B). The Ti/ defect mechanism for the partial silanol nest model showing the preadsorbed complex of propylene on the hydroperoxy intermediate (C) and the Ti/defect mechanism for the full silanol nest model showing the preadsorbed complex of H2O2 on the titanium site. Distances in A. Color coding small white spheres, H atoms red spheres, O atoms gray spheres, C atoms large white spheres, Ti atoms green spheres. Si atoms. Adapted from Ref (191 b), with permission from The American Chemical Society.

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