Oxides acidic character

Notice that the acidic character is associated with the ability of aluminium to increase its covalency from three in the oxide to six in the hydroxoaluminate ion, [Al(OH)g] the same abihty to increase covalency is found in other metals whose oxides are amphoteric, for example... 

The acidic character of siUca is shown by its reaction with a large number of basic oxides to form siUcates. The phase relations of numerous oxide systems involving siUca have been summarized (23). Reactions of siUca at elevated temperatures with alkaU and alkaline-earth carbonates result in the displacement of the more volatile acid, CO2, and the formation of the corresponding siUcates. Similar reactions occur with a number of nitrates and sulfates. Sihca at high temperature in the presence of sulfides gives thiosiUcates or siUcon disulfide, SiS2. 

It was noted that the content of functional groups on the surface of studied A1,03 was 0,92-10 mol/g of acid character for (I), FOS-IO mol/g of basic character for (II). The total content of the groups of both types was 1,70-lO mol/g for (III). The absence of appreciable point deviations from a flat area of titration curves in all cases proves simultaneously charges neutralization character on the same adsoi ption centers and non-depending on their density. The isoelectric points of oxide surfaces have been detenuined from titration curves and have been confirmed by drift method. 

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... 

The range of oxidation states of a d-block element increases toward the center of the block. Compounds in which the d-block element has a high oxidation state tend to be oxidizing those in which it has a low oxidation state tend to be reducing. The acidic character of oxides increases with the oxidation state of the element. 

Chlorine is one of the strongest oxidants whether it is in the elementary form or as oxidised anions, with oxidation states of +l (hypochlorites) to +VII (perchlorates). The chloride ion with an oxidation state of -I is very stable (octet electronic structure) only hydrochloric acid is dangerously reactive, linked to its strongly acidic character. This explains the nature of the dangerous reactions which have already been described and have caused a large number of accidents. The accidental aspect is aggravated by the fact that the derivatives mentioned in this paragraph are much used. 

Selenoprotein A is remarkably heat stable, as seen by the loss of only 20% of activity on boiling at pH 8.0 for lOmin (Thrner and Stadtman 1973). Although selenoprotein A contains one tyrosine and no tryptophan residues, it contains six phenylalanine residues and thus has an unusual absorbance spectrum (Cone et al. 1977). Upon reduction, a unique absorption peak emerges at 238 nm, presumably due to the ionized selenol of selenocysteine, which is not present in the oxidized enzyme. The activity of selenoprotein A was initially measured as its ability to complement fractions B and C for production of acetate from glycine, in the presence of reducing equivalents (e.g., dithiothreitol). Numerous purification schemes were adopted for isolation of selenoprotein A, all of which employed the use of an anion exchange column to exploit the strongly acidic character of the protein. 

The problem of the role of acidity in the oxidation reaction has been examined. To this end silicalites containing both Ti(IV) and Al(III),- or Fe(III) or Ga(III) have been synthesized [24-26] and used in the epoxidation of propylene. It is well known that trivalent elements introduced in the framework impart definite acidic character to the material. The results obtained under very similar experimental conditions are given in Table 2. 

The effect of chemisorption temperature on the ammonia uptake capacity of 6.5 wt% V20c/Ti02 is shown in Fig. 1. Ammonia chemisorption capacities increase with temperature upto 150°C and then decrease with further Increase up to 400°C. It is worth noting that there is considerable NH uptake even at 400°C. These results are in accordance with the reported literature. A number of studies have been reported on the acidic character of supported transition-metal oxides (22,34-38). Ammonia on V20g can be either adsorbed in the form of NH species on Bronsted acid sites or coordlnatively bonded to vanadium ions on Lewis acid sites (39,40). The latter species were observed up to 250°C,... 

The one-stage conversion of propene to acrylic acid is much more difficult than the selective production of acrolein. The process is essentially a two-step process in which acrolein is the intermediate product. Further oxidation leads to acrylic acid. In fact, contrasting catalyst properties are required for these reaction steps. The acrylic acid production demands an acidic catalyst surface, while a basic, or only weakly acidic character is preferred for the selective acrolein formation. Therefore, enhanced combustion and by-product formation are unavoidable. 

Deeper oxidation products like furan and, particularly, maleic acid anhydride, can be produced by catalysts that have a stronger oxidative power than the above type-(a) catalysts, but, at the same time, have retained the capacity to transfer oxygen selectively to the organic molecule (a capacity which is absent in the type-(b) catalysts). Besides, a more acidic character of the catalyst surface is probably required to produce an acidic product like maleic anhydride effectively. The most interesting catalysts of this group are V2Os-based catalysts and certain molybdates and Mo03-based catalysts. 

General.—The compounds of niobium are not so numerous as those of vanadium. The following oxides,1 Nb203, Nb02, Nb205, are known, but only the pentoxide gives rise to salts, viz. the niobates.2 The acid character of niobium pentoxide or niobic acid is very weak the niobates are decomposed, for instance, by carbon dioxide, and are readily hydrolysed to the pentoxide. Niobic acid is, in fact, very comparable in its method of preparation and behaviour to silicic acid and stannic acid. 

H4SiMo12O40 on the support for catalytic oxidation of methanol (184). At coverages larger than 0.25 monolayer, the selectivities remain constant dimethyl ether is the main product (about 80%), showing the acidic character of the catalyst. Below 0.25 monolayer, the acidic character is lowered, and dimethyl ether rapidly disappears. 

Another important point is that, when prepared from pure raw materials, titanium silicates do not have an appreciable acidic character, as demonstrated by the high yields that can be obtained even in applications with acid-sensitive products like propylene oxide. In contrast, mixed oxides of titanium and silicon have been described as being strongly acidic (Tanabe et al., 1981), The reasons for the difference are not clear and deserve further attention. 

The acid-base properties and the ionic-covalent character of an element s oxide depend on both the element s position in the periodic table and its oxidation number. As Figure 14.6 shows, both the acidic character and the covalent character of an oxide increase across the periodic table from the active metals on the left to the electronegative nonmetals on the right. In the third row, for example, Na20... 

Both the acidic character and the covalent character of different oxides of the same element increase with increasing oxidation number of the element. Thus, sulfur(VI) oxide (S03) is more acidic than sulfur(IV) oxide (S02). Reaction of S03 with water gives a strong acid (sulfuric acid, H2SC>4), whereas reaction of S02 with water yields a weak acid (sulfurous acid, H2S03). The oxides of chromium exhibit the same trend. Chromium(VI) oxide (Cr03) is acidic, chromium(III) oxide (Cr203) is amphoteric, and chromium(II) oxide (CrO) is basic. 

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