Oxides basic-acidic behavior

The basic—acidic behavior of the oxides of the elements is a good indicator of the metaUic—nonmetallic character of the elements. Oxides are classified as basic or acidic depending on their reactions with acids and bases. A basic oxide is an oxide that reacts with acids. Most metal oxides are basic. An acidic oxide is an oxide that reacts with bases. Most nomnetal oxides are acidic oxides. An amphoteric oxide is an oxide that has both basic and acidic properties. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

Pyrite and arsenopyrite have similar oxidation and self-induced collectorless flotation behavior. It is generally suggested that anodic oxidation of pyrite occurs according to reactions (2-24) in acidic solutions (Lowson, 1982 Heyes and Trahar, 1984 Trahar, 1984 Stm et al., 1991 Chander et al., 1993). The oxidation of pyrite in basic solutions takes place according to reactions (2-25). Since pyrite is flotable only in strong acidic solutions, it seems reasonable to assume that reaction (2-24) is the dominant oxidation at acidic solutions. Whereas pyrite oxidizes to oxy-sulfur species with minor sulphur in basic solutions. 

According to Fraser (1975a), rare earth oxides may dissociate into the melt with both basic and acidic behavior—i.e.. 

This latter reaction is very slow as written and is of more importance in the reverse, dehydration reaction.) The characterization of these metal and nonmetal oxides as acids and bases is of help in rationalizing the workings, for example, of a basic Bessemer converter in steetmatcing. The identification of these acidic and basic species will also prove useful in develop r a general definition of acid-base behavior. 

In addition to this Lewis-acid behavior, 16- and 18-electron metal complexes can act as Lewis bases, i.e., they possess accessible electron pairs. This Lewis basic character depends strongly on the donor and acceptor strengths of the ligands and is very pronounced in complexes of strong donors such as trialkylphosphines. For example, whereas CpCo(CO)2 shows little basic character and little tendency to react with electrophiles such as CH3I, CpCo(PMe3)2 is a strong metallic base .10 Such compounds are particularly reactive towards oxidative addition reactions ... 

This behavior can be explained in terms of different oxide surface acidities. Whereas silica (pH 4) exhibits acidic properties, alumina surfaces shows a much more basic character (pH 8) [5] and, therefore, allows further hydrolysis of the alkoxysilanes. The influence of the surface pH on the characteristics of anchored molecular structures has also been reported by Deo and Wachs [6] for the preparation of supported vanadia catalysts via incipient wetness. 

Figure 8.17 The trend in acid-base behavior of eiement oxides. The trend in acid-base behavior for some common oxides of Group 5A(15) and Period 3 elements is shown as a gradation in color (red = acidic blue = basic). Note that the metals form basic oxides and the non-metals form acidic oxides. Aluminum forms an oxide (purple) that can act as an acid or as a base. Thus, as atomic size increases, ionization energy decreases, and oxide basicity increases.

In the following brief descriptions of the main-group elements, we will note the metallic—nonmetallic behavior of the elements, as well as the basic—acidic character of the oxides. Although elements in a given group are expected to be similar, the degree of... 

Table 7A lists some common oxides of main-group elements. You can see that the active metal oxides are basic and that the nonmetal oxides are acidic. Between these lies a group of oxides, the amphoteric oxides. The bonding in amphoteric oxides is intermediate between ionic and covalent bonding. As a result, oxides of this type show behavior intermediate between that of acidic oxides and basic oxides, and react as both acids and bases. 

Consistent with the position of the metal-nonmetal line (and the corresponding acid-base character of metal and nonmetal oxides), boron oxide is an acid anhydride, whereas the oxides of the heavier elements progress from amphoteric to basic in behavior. Boron oxide, then, reacts with water, as shown in Equation (14.2), to produce boric acid, B(OH)3 or H3BO3 ... 

The perspectives of the use of basic zeolites as catalysts has been reviewed recently by Davis [287], Extraframework material free, alkali exchanged zeolites are used as quite mild basic catalysts. As seen earlier, light alkali-metal zeolites, such as Na-X and Na-Y, have a mild Lewis acid behavior and do not appear to have strong basic character [245], However, heavy-alkali metal zeolites like Cs-Y act actually as basic catalysts, or better as acid-base catalysts, for example, for toluene side chain alkylation. Stronger basic character arises from impregnation of alkah-zeolites with alkali salts, later decomposed to occluded alkali oxide, as evidenced by CO2 adsorption microcalorimetry. The characterization of such materials is still quite poor. 

Usanovich attempted to explain the relationship by including oxidation as a special case of acidic behavior. Reduction was considered a special case of basic behavior. Chlorine was listed as an acid and sodium as a base. This classification has more experimental justification than would appear at first glance. Sodium, when reacting with water, increases the concentration of solvent anions as do bases when dissolved in many amphoteric solvents ... 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Adolph Baeyer is credited with the first recognition of the general nature of the reaction between phenols and aldehydes in 1872 ([2,5-7] [18], Table 5.1). He reported formation of colorless resins when acidic solutions of pyrogallic acid or resorcinol were mixed with oil of bitter almonds, which consists primarily benzaldehyde. Baeyer also saw resin formation with acidic and basic solutions of phenol and acetaldehyde or chloral. Michael and Comey furthered Baeyer s work with additional studies on the behavior of benzaldehyde and phenols [2,19]. They studied a variety of acidic and basic catalysts and noted that reaction vigor followed the acid or base strength of the catalyst. Michael et al. also reported rapid oxidation and darkening of phenolic resins when catalyzed by alkaline materials. 

Both our original prediction about the effect of ionization energy on acid-base behavior and the trend which we have observed in the first three elements lead us to expect that the hydroxide or oxide of silicon should not be basic, but perhaps should be weakly acidic. This is in fact observed. Silicon dioxide, Si02, can exist as a hydrated solid containing variable amounts of water,... 

The pattern of oxidation states correlates with the pattern of acid-base behavior of d-metal oxides. Although most d-metal oxides are basic, the oxides of a given element show a shift toward acidic character with increasing oxidation number, just as the oxoacids do (recall Section 10.10). The family of chromium oxides is a good... 

Antimony trioxide is an amphoteric oxide, exhibiting both acidic and basic behavior. It dissolves in strong acids forming antimony salts e.g., reacts with aqueous hydrofluoric acid to form antimony trifluoride, SbFs. It reacts with strong alkalies to form antimonites, such as sodium or potassium anti-monites, NasSbOs or K38b03 ... 

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