Silanol nests

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.

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

Interestingly, the activity of the catalyst was proportional to its external surface area. This, together with the fact that caprolactam does not fit easily into the pores of the zeolite, strongly suggests that the reaction takes place on the external surface, possibly at the pore openings. Ichihashi and coworkers [61] proposed that the reaction takes place in a silanol nest resulting from dealumi-nation (see earlier), as was also suggested by Hoelderich and coworkers [65]. 

Experimental heat of adsorption obtained by chromatography at zero coverage is compared with simulated heat of adsorption for 3 models in Table 2. Here simulation for the acid site model was closer to the experimental value than the pure siliceous model and the silanol nest model. 

A Silanol nest model ° Acid site model... 

The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. 

Post-synthesis modification involves isomorphous substitution of framework atoms with the desired redox metals either in aqueous media with soluble metal salts or in the gas phase with volatile chlorides. Incorporation of Ti into the framework of faujasite, zeolite-) and ZSM-5 has been accomplished by treating the zeolite with ammonium titanyl oxalate, TiCU or Ti(0/-Pr)4. Substitution of V for framework atoms has been reported with VOCI3 vapor. A more generalized method involving the reoccupation of the silanol nests created by the deboronation of bor-osilicates (ZSM-5 and zeolite- ) shows considerable promise for the incorporation of redox metals into the framework [79]. 

The dealumination of zeolite MCM-22 by calcination or hydrothermal treatment has been investigated by conventional multinuclear solid-state NMR and checked by ultrahigh-field NMR experiments (19.6 T) with a fast spin rate (19.1 kHz). The presence or variation of different species, sueh as silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminium, during or after dealumination has been detected, and their... 

Silicalite-1 is a high Si zeolite with an MEI structure and in which the weak Bronsted acid sites derive from the presence of defects [244, 245]. The active sites of the catalyst are silanol nests (very weak acidity) located close to the pore mouth of... 

Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

The comparison of the IR results with the results of chemical analysis is given in Table 1. It is seen that with increasing amount of carbon particles added to the reaction mixture (ZSM-5/10, ZSM-5/IMP) increasing differences in Si/Al ratio between chemical analysis and FTIR were observed. This could be explained by the presence of some extra-framework alumina species due to a higher distortion of the framework formed under conditions of the synthesis with the largest amounts of carbon black particles. The spectrum in the hydroxyl region shows a shoulder around 3726 cm, which can be assigned to the structural defects such as silanol nests [18]. 

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.

Extraction of metal cations Me from the sofid can also lead to creation of surface hydroxyls, with n OH groups created per Me species removed, in agreement with the rule of charge preservation. Typically, this chemistry can be formulated as an exchange reaction. However, more comphcated situations are also encoimtered. For example, extraction of framework Al ions from zeolites leads to the formation of so-called silanol nests (Figure 2.2). A perfect nest should consist of four Si—OH groups (26). 

Figure 2.2 Scheme illustrating the formation of silanol nests. 

H-bonded hydroxyls are also often detected (although not always discussed) in zeolites of various kinds. It is ascertained that the already discussed silanol nests are characterized by a broad band centered around 3500 cm . The nests constitute defects in the zeolite structure and are caused by extraction of a T -atom (a tetrahedraUy coordinated metal cation of the framework) and filling of the gap with four protons. However, bridging hydroxyls in zeolites can participate in strong H-bonds with the framework oxygen atoms, and these hydroxyl structures can contribute to the broad absorption around 3500 cm. ... 

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). 

A Silanol nest modd o Acid site modd... 

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