Internal silanols

From the data reported in Fig. 8, it clearly emerges that the acidity of the silicalite-l/H20 and of the TS-I/H2O systems are remarkably different (compare open and full circles in Fig. 8). This difference can be explained as follows TS-1 has two main acidic sites, Ti(IV) Lewis sites and silanols, mainly located in the internal defective nests (see Sect. 3.8), while only the latter are present in silicalite-1. Addition of H2O2 to siUcaUte-l does not modify the titration curve (compare open circles with open squares in Fig. 8). This means that no additional acidic sites appear in the siUcaUte-l system upon adding H2O2, i.e., that hydrogen peroxide molecules coordinated to internal silanol do not modify their acidity. Conversely, addition of H2O2 to TS-1 moves the whole titration curve toward lower pH values, (compare full circles with full... 

Yashima et al. proposed the atom planting method as the development of the alumination through the reaction of internal silanol groups with aluminium halides. 

Relative percentage of internal silanols as measured from D20 exchange for two silica gels, Sg - BET surface area, r average pore size distribution... 

As can be observed, the high surface area (HSA) sample contains less internal silanols. The surface area of the HSA silica is larger, while the particle size is the same of that of the low surface area (LSA) silica, because of a lower average pore diameter in the HSA silica. A larger number of small pores rather than large pores can be accommodated on a particle of a given diameter. The result is a particle with more of its internal volume penetrated by smaller and more numerous pores. This leaves less internal volume for trapped or unexchangeable protons. 

Deuteration of silica gel therefore shows to be governed by the rehydration capacity of the substrate. At low pretreatment temperature the extent of H/D exchange is limited by internal silanols, which may not be reached by (heavy) water molecules. At high pretreatment temperature the exchange is restricted by the low physisorption ability of the isolated silanols. 

By contrast, ZSM-5 specimens from organic-free batches and those synthesized by n-butyl- and n-propylamine possess relatively intact lattices with only a small concentration of internal silanol groups (see Figure 9 and Table VI). 

As regards the Cu-Al-MCM-41 (Si/Al=30 and 150) and Cu-ZSM-5 (Si/Al=25) catalysts, the copper ion exchange is related to the presence of both Bronsted Si-OH-Al acid sites and internal silanols. In the case of Cu-ZSM-5 (Si/Al=25) it appears that only the Bronsted acid sites are responsible for the ion exchange (Cu/Al 0.5). The fact that the copper loading of Cu-Al-MCM-41 (Si/Al=30) is similar to that of Cu-MCM-41 and less than that of Cu-Al-MCM-41 (Si/Al=150) may be an indication of the presence of extra-framework octahedral A1 species with no ion exchange capacity (vide infra). 

MAS NMR data revealed that upon dealumination of mordenite with phosgene a part of the framework Al remains in the channels as octahedrally coordinated aluminium compounds. As a result of aluminium removal, framework vacancies and internal silanol groups are generated. The transformation of SiCl groups to SiOH groups is due to hydrolysis by ambient moisture. Framework reconstruction occurs during dealumination. 

The framework IR spectra of metallosilicates reveal a band at -960-970 cm, the band being attributed to Si-O vibrations of defect (internal) silanol groups associated with the metal locations in the framework [18]. As a consequence, the presence of the band is generally used as an evidence for the presence of the metal in lattice positions associated with defect sites [6,7]. The framework IR spectra of the calcined samples A and B are presented in Fig. 3. The spectrum of B has a band at 967 cm" and not that of A suggesting the absence of defect silanols and, presumably, the absence of vanadium in the framework positions in the latter sample. 

Surface and internal silanol groups may condense to form siloxane bridges. Strained siloxane bridges are formed on the hydroxylated silica surface by thermally induced condensation of hydroxyl groups up to about 500 °C. At higher temperatures, the strained siloxane groups are converted into stable siloxane groups (2) (Scheme II). 

A slight inflection near 3660 cm-1 can just be discerned in Figure 1 A. This band is due to perturbed or inaccessible internal silanols (self-supporting disks is increased. They are largely inaccessible to many reactant gases, and this topic is discussed in more detail later in the chapter. The 3720-and 3660-cm-1 features are not resolved in the spectrum of the precipitated silica (Fig. 2A), which instead exhibits a broad feature near 3695 cm-1. 

Surface of a Fumed Silica. Several results obtained for silica A, as received and after some contact with air, can be rationalized in the following way. The low silanol surface density (about 3.65 OH per square nanometer, internal silanols excluded), the comparatively high fraction of geminal sites (/g = 0.21), and the presence of a rather strong D2 band in the Raman spectrum indicate an only partial and selective hydrolysis of the surface after the manufacturing of silica A at high temperature. 

The following dehydroxylation stage, between 460 and 645 °C, can be interpreted by the condensation of about 0.3 OH per square nanometer absorbing around 3680 cm-1 (full width at half height (fwhh) 180 cm-1), internal silanols excluded, and of nearly the same number with t/OH 3736 cm-1 (fwhh 20 cm-1) (26). Although not firmly proved, a condensation as pairs of weakly proton donor and acceptor hydroxyls is suggested ... 

A tentative assignment of the main features of the infrared spectrum of silica A is summarized as follows 3500 cm-1, mainly SP 3620 cm-1, closest Sm in triplet sites i-j-k 3670 cm-1, internal silanols 3680 cm-1, weak proton donors Sm, Se, and Ge in pairs 3715 cm-1, terminal Gp 3736-3742 cm-1, weakly perturbed Gp, Sm, Se, and Ge 3747 cm-1, isolated Sm, Se, and single silanols ex-GP and ex-Ge. This assignment gives a possible explanation of the Raman components appearing at about 3685 and 3615 cm-1 by rehydroxylation of a silica gel pretreated at 600 °C (14). 

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