Surface silica acid

The silica-alumina surface is still more strongly acidic than the alumina surface. The acidity is less sensitive to poisoning by water. There has been much discussion whether the acidity of silica-alumina is caused by Bronsted or by Lewis acid sites. This matter has not been. settled definitely, although there is evidence that both types of acidity are present. This would explain the observation that the catalytic efficiency in different reactions may be selectively poisoned by different reagents. 

Specific adsorbents with positive surface charges. Acidic hydroxyl groups (hydroxylated acid oxides such as silica), aprotic acid centers, or small radius cations (zeolites) on the surface. Adsorbents of this type will interact with molecules which have locally concentrated electron densities, that is. Group B and Group D molecules. 

In Table 9.6 we see that the pure silicon carbide surface is basic, although the oxidized silicon carbide surface is acidic like that of silica. With these E and C values Okuyama et al. [21] were able to predict that the silica and oxidized silicon carbide surfaces are best wet by... 

Heavy and transition metals may also be separated via ion-pair chromatography on macroporous PS/DVB-resins or chemically bonded silica phases, respectively [154], The mobile phase contains complexing agents and a respective ion-pair reagent. If these columns are equilibrated with a surface-active acid such as octanesulfonic acid, metal ions such as Cu2+, Ni2+, Zn2+, and Co2+ elute in the same order as on surface-sulfon-... 

Due to the presence of silanol groups (Si-OH) on their surfaces, silica gels are weakly acidic supports hence amorphous silicas can be used to catalyze reactions that are easily catalyzed with acid. They are essentially used as supports due to their high surface areas and large pore volumes. 

Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy was used in conjunction with pyridine desorption studies to assess the acidity of a siliceous surface. An amorphous, porous silica substrate was investigated. The results contribute to an understanding of the acidic strength and the distribution of acidic sites on this material. A hydrogen-bonding interaction was observed between pyridine and the surface. Isothermal rate constants and an activation energy for the desorption process are reported and can be used as direct measures of surface site acidity. 

Heteropolyacids can also be bound to support materials by direct deposition. More firmly bonded materials can be prepared through chemical surface modification. Acids such as 12-tungstophosphate (PW12) will react with silica gels which have been treated with aminoalkylsilanes.129 The acidic site of PW12 reacts with the chemically modified surface forming ionic bonds. The latter... 

The modification of BEA zeolite by surface deposition of silica and impregnation with cerium oxide was studied as a tool to improve the selectivity of the reaction. The number of acid sites, particularly the strong ones, on BEA zeolite decreases with increasing amounts of silica deposited on its surface. Moreover, there is no severe pore blocking after deposition. On the contrary, cerium oxide impregnation affords a catalyst with decreased adsorption capacity because part of the cerium oxide is deposited in the channels of the zeolite crystals and blocks the porous system. In addition, cerium oxide modification creates new weak acid sites on the zeolite surface. Silica modification decreases catalytic activity but slightly increases selectivity with respect to all ortho-HAP, para-HAP and para-acetoxyacetophenone, in comparison to the unmodified BEA zeolite, and the stability of the catalyst is also improved after modification. The best reaction results are obtained over 16% cerium-oxide-modified catalyst, the selectivity with respect to the C-acetylated products being increased to about 70% while the conversion remains 60%-80%. 

Here a surface Lewis acid (denoted by j) abstracts a hydride ion from the methylene group adjacent to the double bond. This mechanism is in accord with the essential Lewis acid nature of the silica-alumina surface and is consistent with the previously demonstrated ability of this surface to abstract hydride ions from tertiary hydrocarbons. Since an alkenyl carbonium ion is stabilized by resonance to a greater extent than is a saturated carbonium ion, it may well be the most stable species which could form in the chemisorption of an aliphatic olefin or its precursor. It seems reasonable, therefore, to presume that such species may be involved in heterogeneous acid catalysis to a greater extent than has been generally recognized. This chemisorption process does not, of course, exclude the more conventional acid addition to the double bond which may occur under suitable circumstances but rather, it introduces an alternate path which may well exert a considerable influence on the overall course of catalytic reactions. Thus, for example, since a substituted ally lie carbonium ion may be converted to a conjugated diene by loss of a proton, it may be an important intermediate in the formation... 

The corrosion resistance of enamel is generally attributed to a surface layer of silica. Acid first dissolves the surface alkali oxide leaving a hydrated porous layer which is mainly silica, that acts as a barrier through which corrosive agents must difiltse for further attack to occur. As the affected layer deepens, the diffusion process slows down so that the enamel becomes protected by a skin of acid resisting silica. 

Other useful classes of basic probe molecules used to examine silica, alumina, and silica-alumina surfaces (as well as zeolite systems) include small organic phosphines and phosphine oxides, which rely on the highly convenient P nuclide (/ = 1/2, 100% natural abundance). As Lunsford and coworkers demonstrated for zeolites [89], the P NMR signal of trimethylphosphine is a useful probe for Bronsted acid sites on surfaces. The basis for this approach is the formation of R3P -H B( ) sites at surface Bronsted acid sites, H-B(. ... 

---