Silanols types

The distribution of the silanol types and their desorption energies... 

Once the silanol number as a function of temperature is known, further distinction between isolated, vicinal and geminal hydroxyls should be made. Since all silanol types exhibit different reactivities, an exact knowledge of the ratio isolated/vicinal silanols is crucial for a thorough study of a modification reaction. 

The simplest possible way to estimate the concentration of the different silanol types on the silica surface consists of an integration of the corresponding infrared bands. 

Surface silanol types with their 29Si CP MAS NMR and FTIR peak position and names... 

Figure 5.23 Distribution of the silanol types as a function of temperature (in vacuo) according to Zhuravlev. total OH + free OH bridged OH 0 siloxanes.

These results are presented in figure 5.27, showing the absolute distribution of the three silanol types on the silica surface as a function of temperature. The absolute concentration of the free silanols increases in the low temperature region, due to dehydroxylation processes of bridged silanols (reaction 5.1). 

Concentration of the different silanol types, as a Junction of treatment temperature in vacuo... 

The book is divided into three major parts. The first part (Part I) reviews the characterization of the silica surface, discussing the preparation and properties of pure silica (Chapter 1), the physical characterization of the silica surface (Chapter 2), the chemistry of silica (Chapter 3), the quantification of the silanol number (Chapter 4), the distribution of the silanol types and their desorption energies (Chapter 5), the effect of surface morphology on the dehydroxilation behaviour (Chapter 6) and the related silicate materials (Chapter 7). Part II discusses the chemical modification of... 

Surface area Pore volume Pore diameter Pore size distribution Surface properties Number of silanols Types of silanols Acidity of surface... 

Structure (II) reacts as a Bronsted acid with the strong base, NH3, causing the disappearance of both the 3660 and 3570 cm i IR bands, but not a weak silanol-type band at 3745 cm i 70). In fact, the 3660 and 3570 cm i bands may be viewed as silanol-type bands whose frequency has been lowered from 3745 cm i by a Lewis acid-bs/se interaction between the electron pairs of the OH group with adjacent 3-coordinate A1 atom 70-72). Thus, it is this interaction and the resulting ability of the AIO4 tetrahedra to reform that contributes to the acidity of this group. Uytterhoeven d al. 70) have demonstrated from NH3 adsorption experiments (Fig. 16) that the transformations (I)f (II) above are essentially reversible. Venuto et al. 75) confirmed this conclusion by gross scale reconstitution studies on a 90% decationated Y sample ... 

Relative Rates of Reaction of Various Silanol Types. The... 

Silanol Types and Degree of Coverage with Total OH Groups, as a Function of Treatment Temperature (the Zhuravlev Model)... 

Silanol types fall into three categories isolated, geminal (two -OH hydroxyl groups on the same silicon atom), and vicinal (on adjacent silicon atoms), as illustrated in Figure 9.6. 

In the present work the influence of the acidity of silanol-type sites on the transformation of propane and n-hexene-3 is inspected by resorting to ab initio and semi-empirical methods. 

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