Silanols protonated

The surface silanols can still catalyze the polymerization of triethoxysilane molecules even if there is one or more adsorbed layers of water on the surface. Presumably the triethoxysilane molecule H-bonds to the surface via water molecules after which it requires only an electron shift to make a silanol proton available to the alkoxysilane molecule. This sequence is illustrated by the following structures ... 

We simulated thermal decomposition of the supported Cu(acac)2 by removing one of the ligands and the surface silanol proton, then forming a bond between the Cu cation and the surface siloxide. This structure was subjected to the PM3 energy minimization to produce the equilibrium geometry shown in Figure 8. 

The base can be a co-solvent, such as trialkylamine, or the Lewis base function can be designed into the metal complex using O-bearing ligands such as al-koxyamines, acetylacetonates, etc. The optimum amount of co-solvent is 1-2 equivalents of the metal complex present in the solvent. The nitrogen atoms in ethylenediamine are not sufficiently basic to activate the surface protons from silica, but ethylenediamine complexes of some metal cations, such as Cu(II), readily exchange for the protons in zeolites which are more acidic than silanol protons. ... 

When in situ dosing onto two different samples of Pt/silica (45, 48) and onto Cu/MgO was used (49), no evidence for spillover was found from NMR. Only one detailed study based on fully relaxed spectra led to observation of a non zero spillover (4T). In a Ru/Si02 catalyst, the silanol protons w ere exchanged for deuterons, the sam.ple was evacuated at 623 K, and a reference NMR spectrum w as taken at room, temperature. The sample was then exposed to 20 Torr of H2, an NMR spectrum was taken, and the difference with respect to the reference was calculated (line in Fig. 14). This represents the sum of reversible and irreversible hydrogen on the metal (resonating at -65 ppm) and spilled over on the support (at about 3 ppm). Then the sample was pumped out at room temperature for 10 min, and again a difference spectrum with the reference state was obtained (dashed line in Fig. 14) this represents irreversible hydrogen both on the support and on the metal. Similar in situ NMR techniques were used... 

FiC5. 14. NMR spectra of Ru/SiOi. The initial number of silanol protons has been reduced by exchange with deuterium. Both traces are difference spectra with respect to the state after initial evacuation. The continuous line represents a sample under 20 Torr of H, gas, and the dashed line represents a sample after pumping away the reversible hydrogen. There is both reversible and irreversible spillover to the support (signal at. 3 ppm), and ther e is rever sible and irreversible chemisorption on the metal (sigiral at 65 ppm). [Reproduced with permission from Uner et al. (47). ... 

The Iinewidth of the peak due to silanol protons in every zeolite did not... 

Proton NMR of a borosilicate molecular sieve was reported by Scholle and co-workers in a study of the hydroxyl groups in borosilicate, silicalite, and ZSM-5 (45). In this study, the silanol protons in both borosilicate and aluminosilicate materials were observed at a chemical shift of -2 ppm relative to Me4Si. A low field resonance attributed to hydroxyls associated with the heteroatom (B or Al) was shifted to higher field for the boron-containing sieve relative to the aluminum-containing sieve (—3.5 ppm vs -6 ppm). This was interpreted as indicating that the borosilicate hydroxyls are less acidic than those of the aluminosilicate, consistent with the IR results reported above. 

An important question to be explained by the chemical mechanism is why base catalysis often leads to condensed structures. In 1950, Aelion et al. (13) pointed out that the condensation reaction in base-catalyzed systems was faster than with acid catalysis and that the microstructure of the final product was different. Many of these facts are due to the opposite effects of, for example, substituents, on silicon on the stabilization of the transition state in base- and acid-catalyzed reactions (15). For the base-catalyzed condensation reaction to take place, a silicon atom has to be attacked by a deprotonated silanol oxygen the nucleophile. The acidity of the silanol proton increases as the basicity of the other groups bonded to the silicon decreases. This feature implies that poly silicic acid is a stronger acid than Si(OH)4 (20). Therefore, monomers react preferentially with higher polymerized species. 

The proton-active modifiers which were used for controlled protonolysis of the active Al—C bonds include long chain alcohols, carboxyhc acids, silanols, proton-active surfactants, and sugars (see Table 2.4). 

Impurities, such as Na, K, or Al, are sometimes picked up during the synthesis of aquasols in alkaline medium and may be occluded inside the colloidal particles, taking the place of the silanol protons (as with sodium or potassium) or forming isomorphic tetrahedra with an extra negative valence on the surface or inside the particles (as with aluminum) (Figure 3.12). 

Beutel et al. (2001) studied the interactions of phenol with zeolite NaX using solid-state H and Si MAS and Si CP-MAS NMR spectroscopy. The H MAS NMR spectrum of NaX degassed at 400°C included peaks at 5h= 1.24,2.05,4.05, and 5.11 ppm. The signal at 2.05 ppm is characteristic of silanol protons at the external surface of the zeolite. On NaY zeolite (Si/Al=2.4), a peak at 5h= 1.6 ppm was found after pretreatment in air and in vacuum at 400°C and assigned to traces of water adsorbed on cations. Therefore, the peak at 8h=1.24 ppm was attributed to free hydroxyl protons of water molecules strongly adsorbed on Na+ ions. The peaks at great 8h values could be attributed to water bound to different active sites. 

A very different system is used, however, in preparing a catalyst supported by silica, which contains only weakly acidic silanol groups. In preparing a Pt on silica catalyst, Pt(NH3)4 is typically used to carry out the ion-exchange reaction with silanol protons. 

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