Silica silanol group, types

The second type of interaction, displacement interaction, is depicted in Figure 10. This type of interaction occurs when a strongly polar solute, such as an alcohol, can interact directly with the strongly polar silanol group and displaces the adsorbed solvent layer. Depending on the strength of the interaction between the solute molecules and the silica gel, it may displace the more weakly adsorbed solvent and interact directly with the silica gel but interact with the other solvent layer by sorption. Alternatively, if solute-stationary phase interactions are sufficiently strong, then the solute may displace both solvents and interact directly with the stationary phase surface. 

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. 

The silica networks of mesoporous sihcas are terminated at the surfaces of the amorphous pore walls, thus resulting in terminal silanol groups on the walls. The density of the silanol groups of all mesoporous sihcas (1 to 3 nm 2) is somewhat lower than usually found for other typical silica materials (4 to 6 nm 2) [27], The specific surface areas of the different mesoporous silicas vary depending on their pore sizes, the thicknesses of their pore walls, and the density of their sihca networks. For some MCM-41- and MCM-48-type materials, surface areas of about 1000 to 1400 m2 g 1 have been reported. The surface areas of SBA-15-type materials can be > 600 m2 g 1 [27],... 

In very narrow pores, water may be held even after drying at 100-120°. Furthermore, many types of silica contain hydroxyl groups within the bulk of the structure. This is especially true of silica which has been prepared by condensation of low-molecular silicic acids. It can easily be imagined that a few silanol groups will remain in the network not having... 

It is an important result of Imelik s work (201-206) that different types of silica may behave quite differently with regard to the packing density and the thermal stability of silanol groups as well as adsorbed water. 

Figure 5.2 Molecular models that mimic the various types of silanol groups of a silica surface. (Modified from Coperet et al. [4].)...

H MAS NMR clearly shows five signals at 0.8, 1.9, 4.4, 10.1 and 12.1 ppm. The signals at 0.8, 1.9 and 4.4 are assigned to alkyl fragments bonded to silicon atoms, residual silanol groups on the silica surface and to silicon hydride species, respectively. The two downfield signals at 10.1 and 12.1 ppm are an indication of the presence of two types of zirconium hydride surface species [111]. DQ rotor-synchronized 2D H MAS NMR was used to discriminate these zirconium hydride... 

In a subsequent paper, the authors developed another type of silica-supported dendritic chiral catalyst that was anticipated to suppress the background racemic reaction caused by the surface silanol groups, and to diminish the multiple interactions between chiral groups at the periphery of the dendrimer 91). The silica-supported chiral dendrimers were synthesized in four steps (1) grafting of an epoxide linker on a silica support, (2) immobilization of the nth generation PAMAM dendrimer, (3) introduction of a long alkyl spacer, and (4) introduction of chiral auxiliaries at the periphery of the dendrimer with (IR, 2R)-( + )-l-phenyl-propene oxide. Two families of dendritic chiral catalysts with different spacer lengths were prepared (nG-104 and nG-105). 

Silica is an amorphous product with several types of silanol groups on its surface [4,5]. In Figure 2.1, the different types are shown. From the IR data, the presence of isolated silanol groups with a sharp absorption band at 3746 cm is evident. These isolated silanol groups are the more acidic ones (pK 5) and can act as the cation exchanger. Their acidity depends on the surface impurities... 

FIGURE 2.1 Type of silanol groups on the silica surface. 

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