Alkaline earth metals, oxidation numbers

By heating the metal with appropriate oxides or carbonates of alkali or alkaline earth metals, a number of mixed oxides of Ru and Os have been made. They include NasOs Og, LifiOs Og and the ruthenites , M Ru 03, in all of which the metal is situated in octahedral sites of an oxide lattice. Ru (octahedral) has now also been established by Ru Mdssbauer spectroscopy as a common stable oxidation state in mixed oxides such as Na3Ru 04, Na4Ru2 07, and the ordered perovskite-type phases M Ln Ru Og. 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

The Guerbet reaction is an important industrial process for increasing the carbon numbers of alcohols. Thus, a primary or secondary alcohol reacts with itself or another alcohol to produce a higher alcohol (Scheme 23). Alkaline earth metal oxides have been used as catalysts for the condensation of alcohols. Ueda et al. (158,159) reported the condensation of methanol with other primary or secondary alcohols having a methyl or methylene group at the )S-position they used MgO, CaO, and ZnO as catalysts. The reactions were performed with gas-phase reactants at 635 K only MgO was found to be both active and selective (>80%). 

Besides oxidative coupling of methane and double bond isomerization reactions (242), a limited number of organic transformations have been carried out with alkali-doped alkaline earth metal oxides, including the gas-phase condensation of acetone on MgO promoted with alkali (Li, Na, K, or Cs) or alkaline earth (Ca, Sr, or Ba) (14,120). The basic properties of the samples were characterized by chemisorption of CO2 (Table VI). 

The most general methodology followed to prepare alkaline earth metal oxides as basic catalysts consists of the thermal decomposition of the corresponding hydroxides or carbonates in air or under vacuum. BaO and SrO are prepared from the corresponding carbonates as precursor salts, whereas decomposition of hydroxides is frequently used to prepare MgO and CaO. Preparation of alkaline earth metal oxides with high surface areas is especially important when the oxide will be used as a basic catalyst, because the catalytic activity will depend on the number and strength of the basic sites accessible to the reactant molecules, which is dependent on the accessible surface area. 

In alkaline earth metal oxides, the (100) surface termination plane, which exposes equal numbers of anions and cations, is prevalent and, as illustrated in Figure 21.2, it can be anticipated that an entire family of different co-ordination sites, of different basicity, can be exhibited. Furthermore, it would be expected that this would lead to a dependence upon crystallite morphology and/or particle size. 

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. 

In chapters 6 and 7 we discussed the processes which occur when two solid reactants are placed in contact with each other and allowed to react so as to lower the free energy of the system. For example, a product layer of Fe3 04 is formed between FeO and Fe2 03. The quotation marks about the wiistite FeO are to remind us that the stoichiometric compound does not actually exist. Wiistite always possesses a metal deficit [23] (see section 9.4.1), In the following discussion, the quotation marks will be omitted. The reaction between FeO and Fc2 03 can also be carried out with an ionic filter inserted between the two reactants, where the conduction in the filter is purely ionic, and the transport number of one of the components of the reactants is unity. Such a solid ionic filter, which is suitable for the above reaction, is Zr02 which has been doped with an alkaline earth metal oxide. This material conducts solely by the transport of oxygen ions [24], The experimental arrangement is shown in Fig. 9-5. 

Fig. 6. The relation between the cube root of the molecular volume of /-electron metal oxides and the radius of cations with coordination number 6 and 8 (the trend lines were evaluated for alkaline earth metal oxides and for MO2/electron metal oxides (rM calculated for fo=0.140 nm)).

The 1,2-epoxides, or vicinal epoxides, include the alkylene oxides, the simplest of which is ethylene oxide. The first reported polymerization of ethylene oxide was by Wurtz in 1863 (1), who described the reaction of ethylene oxide heated in a sealed tube with water and, later, with alkali and with zinc chloride catalysts. Seventy years later, the polymerization by a large number of catalysts, including alkali and alkaline earth metal oxides and carbonates, became one of the cornerstones in the development of the macromolecular hypothesis by Herman Staudinger (2). 

The number of NBOs are proportional to the number of moles of alkali or alkaline earth metal oxides added. 

A number of basic materials such as hydroxides, hydrides and amides of alkaline and alkaline earth metals and metal oxides such as zinc oxide and antimony oxide are useful catalysts for the reaction. Acid ester-exchange catalysts such as boric acid, p-toluene sulphonic acid and zinc chloride are less... 

In its compounds, the oxidation number of every alkali metal and alkaline earth metal is equal to its group number. 

The alkaline earth metals have an oxidation number of +2 in all their compounds. 

Metals which have been used (generally in inert or reducing atmosphere) as container materials are W (melting point 3422°C), Mo (2623°C), Pt (1769°C), Fe (1538°C), Ni (1455°C), Cu (1085°C), Au (1064°C), Ag (962°C). W and Mo do not react with many elements they must be protected however from air oxidation. Pt and Au cannot be used, owing to their reactivity, for melting metallic materials they are useful for other types of synthesis. Fe, of very high purity and with very low carbon content, could possibly be used for melting alkaline and alkaline earth metals and a number of their alloys. 

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