Silica-alumina hydroxyl groups

Surface hydroxyl groups on TiOi and Si02 are typically less acidic than those on alumina. On silica snrfaces, hydroxyl groups are associated with tetrahedrally coordinated silicon atoms. Functionahzation of the snrface sUanols is readily achieved by reaction with chloro- or alkoxysilane reagents. Reactions snch as those in eqnations (2) and (3) may be nsed to add phosphine, cyclopentadienyl, and other fimctionalities to the snrface. ... 

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

Hydroxyl groups on oxide and carbon surfaces are often modeled as a one-site, two-pK model as shown in Figure 6.1. Defend this choice of model with the pH shift data for alumina (Figure 6.15). Might a different type of site be envoked for silica (Figure 6.20) and unoxidized carbon (Figure 6.26a) See [21] for more other types of acid-base group models. 

The trans elimination can take place if the basic sites of the alumina attack the hydrogen from one side of the plane and the hydroxyl group is removed from the opposite side of the plane by the acidic sites of the alumina. This may be possible if the reaction occurs within the pores of molecular dimensions (46) or within the crevices of the aluminas. Crevice sites on silica-alumina catalyst have been proposed by Burwell and co-workers (57) on the basis of racemization and exchange reactions of hydrocarbons. 

Basila (365) studied the izifrared spectrum of silica-alumina dehydrated at 500°. The technique developed by Peri and Hannan (333) was used. Only one OH stretching frequency at 3745 cm- was observed. This coincides with the absorption of isolated hydroxyl groups on pure silica. The absorption peak is not influenced by the chemisorption of water vapor at 150°. The chemisorbed water retains its molecularity, does not form hydrogen bonds with the isolated silanol groups, and is adsorbed on sites which can be poisoned by treatment with potassium acetate. 

Summarizing, it can be said that the existence of surface hydroxyl groups on silica-alumina is beyond doubt. However, in chemical reactions all of the hydroxyl groups behave just like silanol groups on silica. No conclusive evidence for the existence of hydroxyl groups bonded to aluminum ions was ever obtained. The most that can be said is that surface silanol groups are much more stable than A1—OH groups. 

The reaction between Os3(CO),2 and silica and alumina has been the object of different studies. Appropriate thermal treatment under argon of the OssjCOjn initially physisorbed leads to [Os3(CO)io( t-H)( t-OSi)] or [Os3(CO)io(n-H)(p-OAl)] surface species that are similar to those obtained from RusjCOjn [129, 130]. An increase of temperature can produce osmium oxidation by the hydroxyl groups and the breaking of the Os-Os bonds, resulting in a surface carbonyl species of Os(II), Os (CO) ( = 2 or 3), surface anchored fragments that in the case of alumina have been characterized by EXAFS [131, 132]. 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

Silica-alumina is the most popular mixed oxide support, combining characteristic features of silica and alumina, including (i) high surface area, (ii) persistent OH population at high temperature and (iii) strong Lewis acidic sites. The predominant surface hydroxyl groups on silica-alumina are [=Si-OH], while [Als-OH] surface species have not been observed [79, 89, 90]. Note that the silica-alumina bulk is mainly composed of [=Si-0-Si=] along with [=Si-0-Als] moieties (Lewis... 

By covalently bonding organic functions via the hydroxyl groups at the surface of silica or alumina, the chromatographic behavior of the solid stationary phase can be drastically modified and the problems mentioned above seem to be less important with such bonded phases. 

Substrates often consist of oxides whose surface contains hydroxyl groups with a density that strongly depends upon the nature of the oxide itself and upon the experimental conditions by which deposition is carried out. Silica, alumina, silicoaluminates and glasses are among the classical ones, and different kinds of - OH groups can be encountered on a surface, i.e. ter-... 

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