Surface hydroxyl groups types

Alcohols react with surface hydroxyl group to form C-O-Si bonds. Because this type of bond is not very stable, trialkoxysilanes are preferred for anchoring. Exceptions to this rule are polyhydric alcohols, such as tris(hydroxymethyl)phos-phine [86], which forms multiple bonds with the surface. 

Surface protonation at the kaolinite surfaces. The excess proton density, Th.v. at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHpzc of the edge surface is about 7.5. 

DRIFT-IR Spectroscopy—Study of Surface Hydroxyl Groups DRIFT-IR spectroscopy at 298 K of self-supported wafers in the region 3500 to 3900 cm indicated the presence of four different types of terminal and bridged hydroxyl groups... 

Scheme 12.2 Different types of surface hydroxyl groups and siloxane rings and their infrared bands.

Although it was not suggested that defects are required for selective oxidation over other catalysts, the results indicated that defects and bismuth must be present for high activity and selectivity over scheelite-type catalysts. The authors concluded that the defects which were introduced into the bulk of these phases must manifest themselves in some manner at the surface. The question of how the introduction of defects into these phases affected their catalytic properties was not resolved. However, the active site for catalysis was suggested as a cation vacancy which could abstract a proton from an olefin to form the well-established allyl intermediate and should offer considerable stabilization to a surface hydroxyl group. 

It has been known for many years that the surface hydroxyl groups can be readily converted to OD simply by repeated exposure to D20. Early experiments with this type of exchange were used in order to manipulate vibrational modes to portions of the spectrum which were more transparent. Thus, for instance, isotopic substitution of D for H in surface silanol groups moves the fundamental stretching frequency from 3747 to 2750 cm 1. Peri (15) used this technique before computer subtraction was so readily available in order to determine the bond angle of the surface Sis-OH group. By recording... 

Since Kiselev1 discovered the surface hydroxyl groups on silica in 1936, many studies on the quantification of the silanol number (a0H number of hydroxyl groups per nm2) and on the characterization of the different hydroxyl types have been published. These studies can be divided into theoretical calculations, physical methods and chemical methods. 

Mixing of an aminosilane with silica gel results in a fast adsorption, by hydrogen bonding of the amine to a surface hydroxyl group.2 After adsorption, the amine group can catalyze the condensation of the silicon side of the molecule with a surface silanol. Thus siloxane bonds with the surface may be formed in the absence of water.3,4 For other silanes the siloxane bond formation requires an initial hydrolysis of the ethoxy groups or the addition of an amine in the reaction mixture.5 This general reaction scheme has been presented in chapter 8. Here we will go into further detail on the types of interaction of the aminosilane with the silica surface and the characterization of the bonded silane species. 

Therefore, it can be concluded that the polymerization takes place at the silica surface, i.e. after adsorption of the aminosilane molecules. The surface effect can be explained by the interaction of the silane NH2 group with the substrate surface. As shown above, in water solvent the hydrolyzed aminosilane molecules are stabilized by internal hydrogen bonding of the amino group to the silane hydroxyls. When the amino group is H-bonded to a surface hydroxyl group this stabilization disappears and the silane silanols can condense to form a siloxane linkage. When the reaction is performed with hydrated silica in a dry solvent (sample 1), the hydrolysis only takes place at the silica surface and can immediately be followed by the condensation reaction. In both cases, structures of type I are formed. 

The use of supports containing hydroxyl groups such as alumina, silica, Mg(OH)Cl, etc., for chemical fixing of the transition metal compound has been widespread since the early 1960s. Heat treatment (calcination) of such supports can control the number and type of surface hydroxyl groups and indirectly the amount and distribution of transition metal atoms anchored to the surface. The most commonly used Ziegler-Natta catalyst of this type is... 

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