Hydroxyl nest

The main calorimetric studies on adsorption of water and ammonia on TS-1 and silicalite-1 have been reported by Bobs et al. [64,83,84,86], while other contributions came from the Auroux group [92] and Janchen et al. [93]. Cor-ma s group has investigated the interaction of water on zeolite [39]. The most important conclusion from the available literature is that calorimetric data require a very careful analysis, as probe molecules interact both with the silanols of the internal hydroxyl nests (see Sect. 3.8) and with Ti(lV) species. 

Cu2+ (or 2 CuOH+ species) ions are probably grafted to some of the hydroxyl nests, c per unit cell, according to the following formula ... 

It was also shown that thermal treatment of an ammonium zeolite under steam causes not only framework dealumination, but also a structural rearrangement in the zeolite framework. The defect sites left by dealumination are filled to a large extent by silica, which leads to a very stable, highly silicious framework (11,12) (Figure IB) Defect sites not filled by silica are occupied by "hydroxyl nests" (13). 

Figure 4.10 Dealumination of a zeolite structure and formation of a hydroxyl nest.

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. 

Namely, aluminium atoms can be inserted into the hydroxyl nests composed of four SiOH groups, and also into the lattice imperfections formed from the hydroxy nests by the dehydration. The above results and discussion have been described in detail elsewhere [11]. 

Present data do not justify the attribution of this V species to a real substitutional V site in the zeolite framework, because the amount of these V sites is very low and at present the degree of incorporation of these sites in the zeolite cannot be extended. It is therefore reasonable to assume that these V sites form at defect sites, possibly hydroxyl nests, the formation of which may be enhanced by the presence of V during hydrothermal synthesis, in agreement with Rigutto and van Bekkum (5). 

As mentioned above, it is reasonable to assume that this tetrahedral V species forms at defect sites (hydroxyl nests) in the zeolite framework, but is stabilized by this interaction in a well defined environment through V-O-Si bonds. As indicated by the characterization data, the local coordination of vanadium must be different from that found for well dispersed vanadium sites on silica. This stabilization probably limits the unselective metal-bonded propane or propylene adsorption, in agreement with the role of adsorbate bonding on the selection of partial and total oxidation pathways of ethane on vanadium supported on silica (76) and in agreement with IR evidence (Fig. 

More recently (8), another series of H-mordenites, acid-extracted to a greater degree, was examined. For these samples after drying at 110°C, the major results were (a) there was no evidence of hydroxyl nests stable above 100°C, and (b) NH3 chemisorption at 250°C and 11 torr roughly corresponded to a stoichiometric ratio (1 1, 25%) with the total amount of aluminum remaining in the lattice. 

In the acid extraction, greater removal of aluminum could have been achieved if smaller catalyst particles and repetitive extractions with HC1 had been employed. The progressively smaller loss on ignition in samples 1, 2, and 3 constitutes negative evidence for the hydroxyl nest hypothesis (8). 

The diffraction patterns of samples 1 and 3 were the same after drying at 500° C (air, 7 hr) as after drying at 110°C. This may be taken as additional evidence against the existence of appreciable amounts of hydroxyl nests in sample 3 after 110°C drying, since one would expect such hydroxyl nests to be unstable at 500° C. 

The effect of added Na+ on the infrared spectra of the SiClA-treated zeolite is shown in Figure 4. At least five major hydroxyl bands are apparent in the spectra. There are two silanol bands one at 3745 cm, which results from terminal hydroxal groups in the lattice (18), and one at 3739 cm"1, which is possibly due to hydroxyl nests. The two bands at 3635 and 3560 cm-1 are associated with the... 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

Zeolite Y. We also substituted boron into dealuminated zeolite Y. We dealuminated zeolite Y by EDTA treatment using standard methods (4). The presence of hydroxyl nests in the product was confirmed using 29Si CPMAS NMR spectroscopy. The dealuminated material incorporated 33 times more boron than zeolite Y when treated with KOH/B2O3. These data are summarized in Table 3. The boron substituted faujasite exhibits a single sharp resonance in the NMR spectrum, consistent with structural substitution. Since the substitution level was low and would not be expected to cause large shifts in the diffraction pattern, no corrected XRD data were obtained on substituted zeolite Y. 

Formation of a broad absorption band in the hydroxyl region of the infrared spectrum due to formation of defect sites or "hydroxyl nests" in dealuminated sites was introduced previously (1). 

During the course of hydrothermal dealumination (ultrastabilisation) of zeolite Y quadrupole nutation NMR detects four kinds of aluminium sites. In addition to signals from the framework (F), non-framework tetrahedral (NFT) and non-framework octahedral (NFO) aluminium there is a signal which we attribute to distorted framework (DFT) aluminium bonded to hydroxyl nests formed during dealumination. 

Fundamentally, all the modification methods follow the same principle, that is, to remove structural aluminum. After the removal, a structural defect site is usually created. A widely accepted hypothesis is that this structural defect site is a hydroxyl nest (1. It has been further hypothesized that under the proper conditions, a silica molecule would be inserted into the vacant site and, thus, anneal the structural defect (16). 

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