Alkaline earth metal amphoterism

The characteristics of this group are that the elements possess a valence of 3, and that the oxides, M2O3, have but a weakly developed basic character. Boron, in fact, shows practically no base-forming properties, but forms rather a weak acid. The oxide of aluminum displays both basic and acidic properties that is, it is amphoteric. The remaining elements are more distinctly base-forming than aluminum, without, however, approaching in any way the alkaline earth metals in this respect. 

It is, of course, important to recognize that in general base catalysts exhibit bifunctional behavior and are therefore, strictly, amphoteric. The same general point can also be made concerning acid catalysts, and therefore the distinction between acidic and basic solid catalysts is made on the basis of dominant behavior, although in some cases, for example Zr02, there may be no single dominant behavior [10]. An illustrative example of this is the alkaline earth metal oxides, classical base catalysts, where oxide ions behave as bases and the metal cations... 

Compounds M(OH) range from the strongly basic compounds of the alkali and alkaline-earth metals through the so-called amphoteric hydroxides of Be, Zn, Al, etc. and the hydroxides of transition metals to the hydroxy-acids formed by non-metals (B(OH)3) or semi-metals (Te(OH)6). The latter are few in number and are included in other chapters. 

Except for Be, all the alkaline earth metals are oxidized to oxides in air. The IIA oxides (except BeO) are basic and react with water to give hydroxides. Beryllium hydroxide, Be(OH)2, is quite insoluble in water and is amphoteric. Magnesium hydroxide, Mg(OH)2, is only slightly soluble in water. The hydroxides of Ca, Sr, and Ba are strong bases. 

Figure 15.8 shows the formulas of a number of oxides of the representative elements in their highest oxidation states. Note that all alkali metal oxides and all alkaline earth metal oxides except BeO are basic. Beryllium oxide and several metallic oxides in Groups 3A and 4A are amphoteric. Nonmetallic oxides in which the oxidation number of the representative element is high are acidic (for example, N2O5, SO3, and CI2O7), but those in which the oxidation number of the representative element is low (for example, CO and NO) show no measurable acidic properties. No nonmetallic oxides are known to have basic properties. 

We have seen that the alkali and alkaline earth metal hydroxides [except Be(OH)2l are basic in properties. The following hydroxides are amphoteric Be(OH)2, Al(OH)3, Sn(OH)2, Pb(OH)2, Cr(OH)3, Cu(OH)2, Zn(OH)2, and Cd(OH)2. For example, aluminum hydroxide reacts with both acids and bases ... 

Magnesium is a typical alkaline earth metal in that its hydroxide is a strong base. [The only alkaline earth hydroxide that is not a strong base is Be(OH)2, which is amphoteric.]... 

The alkaline earth metals are somewhat less reactive than the alkali metals. They almost always have an oxidation number of +2 in their compounds. The properties of the alkaline earth elements become increasingly metallic from top to bottom in their periodic group. Aluminum does not react with water due to the formation of a protective oxide its hydroxide is amphoteric. 

It is concluded that acidic oxides that possess a certain extent of basic property are favorable for promoting the condensation reaction a more basic property is required than in the case of the reaction with carboxylic acid. The acidic oxides such as H-zeolite, Si-Al, B, Mo-P, and W-P oxides, and heteropoly compounds are lacking in a basic property as catalyst for the reaction. On the other hand, alkali and alkaline earth metal oxides and amphoteric oxides combined with K2O are lacking in acidic property. 

The oxide of this alkaline earth metal is amphoteric. 

We have seen that the alkali and alkaline earth metal hydroxides [except Be(OH)2] are basic in properties. The following hydroxides are amphoteric Be(OH)2, Al(OH)3,... 

A number of trivial, yet useful and descriptive, designations continue to be used, e.g. alkali metals, alkaline earths (Be does not qualify, strictly speaking, as an alkaline earth because its amphoteric oxide is very weakly alkaline), the coinage metals (Cu, Ag, Au) and the noble (or inert, or rare) gases. 

Hydroxides M(OH) comprise a numerous class of compounds ranging from strongly basic hydroxides of alkaline metals and alkaline earths, to the so-called amphoteric hydroxides (of beryllium, aluminium, zinc and others) and the hydroxides of transition metals, and further to hydroxo-acids formed by non-metals or semi-metals. 

It is found that only alkali and alkaline earth hydroxides can promote the reaction that is, acidic and amphoteric oxides are inactive for the production of acrylonitrile. The best performances are obtained with silica-supported hydroxides of Cs, Rb, and K. The activity for the formation of acrylonitrile decreases in the order of Cs = Rb = K > Na > Li, and Ba > Ca Mg. This indicates that the activity is related to the electronegativity (basic property) of metal ions corresponding to the hydroxides supported on silica. 

It is concluded that the incorporation of a small amount of alkali or alkaline earth oxide, V20g, amphoteric oxide, or oxide of heavy metal into silica gel induces a marked increase in the activity. This finding suggests that the proton-abstraction from a methyl group of acetaldehyde can be promoted by active sites with a relatively weak base, arising from V2O5 and amphoteric oxides. On the other hand, the formation of acrolein is accompanied by two sides reactions (1) formation of CO2 and methanol by Equations (4) and (5) which is promoted mainly by acid-base dual functions, and (2) polymerization of acrolein to unidentified polymers, which is promoted by strongly acidic sites. 

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