Zeolite silanol groups

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.

Rh and Ni complexes of this ligand were anchored to the silanol groups of silica and a USY zeolite. The catalysts were used in the hydrogenation of N-acyldehydrophenylalanine derivatives (Scheme 7.6). 

Fig. 3 represents the IR spectra in the hydroxyl region and in the OH bending region for the activated 4A and 5A zeolite samples. The outgassed 5A 86 sample IR spectrum exhibits a weak absorption at 3744 cm 1 which corresponds to non acidic external silanol groups and a large contribution between 3700 cm 1 and 3500 cm 1 (Fig. 3a). This... 

Here, we have taken advantage of the low silanol content in pure silica LTA and CHA type zeolites synthesized in fluoride medium to get better resolved 29Si and H solid state NMR spectra in the silanol region. This has allowed us to investigate the nature of the silanol groups in zeolites and their interaction with water molecules. 

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]. 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

He found that these zeolites were hydrophobic and attributed this to the absence of silanol groups and the formation of Si-O-Si bonds in the vacancies generated by dealumination. 

A series of Beta zeolites have been synthesized in the presence of tetraethylammonium hydroxide (TEA). Samples with Si/Al ratio in the 7-100 range have been characterized by X-ray powder diffraction, I.R. spectroscopy, and pyridine adsorption. The fraction of TEA which is compensating the charge of the framework aluminum is removed at temperatures higher than those required to remove "occluded" TEA. Three hydroxyl bands are observed at 3740 cm l (silanol groups), 3680 cm" (extraframework Al) and 3615 cm 1 (acid hydroxyl groups interacting with pyridine). 

The presence of water and amines at high temperatures may produce a process equivalent to a shallow bed calcination, and this could explain the formation of silanol groups by partial breaking of the Si-O-Al bonds in the zeolite framework. Probably, both types of occluded TEA exist in the zeolite. 

Calcination of the samples at 500°C causes the formation of structural deffects and silanol groups. The number of silanols formed during calcination increases when increasing the Si/Al ratio of the zeolite. 

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 ... 

Figure 20 shows the H MAS NMR spectra of zeolite HZSM-5 (ksi/wai — 21.5) before (a) and after (b) adsorption of acetone-tfg at room temperature. The ll MAS NMR signals at 4.0 and 1.8 ppm are attributed to bridging OH groups (SiOHAl) and silanol groups (SiOH), respectively (Fig. 20a). Upon adsorption of 0.33 mmol...

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

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