Lewis ionic oxides

A third, more extreme, conceptual model, based on a completely ionic picture of hypervalent bonding, can also be invoked to remove perceived conflicts with Lewis-structural principles. In PF5, for example, the completely ionic ( oxidation number ) P5+(F )5 representation... 

The surface of an ionic oxide in its fully dehydroxylated form can be considered as a 2D and rigid array of Lewis acidic and basic sites (Scheme 1 M, cationic sites with vacant orbitals O, O2 ions with available electron pairs). In flat faces, M and O sites normally occupy equivalent positions. Of course, this is not the case for defect sites (e.g., on edges and corners). Furthermore,... 

The ionic oxidant (KrF+) gives an ionic mechanism, whereas the Lewis acid in association with F2 or PtF6, which are radical oxidants, results in a radical mechanism. In all the systems the one-electron (Lewis acid-F2 or PtF6) or two-electron (KrF+) oxidizer reacts with the substrate (NF3). This leads to an electron transfer to the oxidant. Either simultaneously (for KrF+) or subsequently (for Lewis acid-F2 or PtF6), the intermediate radical cation (NF3+) is fluorinated to give NF4+. 

The surface of a fully dehydroxylated ionic oxide can be considered as an extended array of coordinately unsaturated oxygen ions and metal cations. From the acid-base point of view, they act as Lewis base and Lewis acid centers, respectively (structure I in Scheme 1). The net charge on each surface center depends on the stoichiometry, ionicity, and local surface structure, which in turn vary from one crystal plane to another. A few positions that represent geometrical defects on the surface (corners, edges, dislocations) may be characterized by enhanced acid-base character, as their energy is further increased due to additional chemical unsaturation,... 

Lewis GV, Catlow CRA (1986) Potential models for ionic oxides. J Phys C Solid State Phys 18 1149-1161... 

The second method considers the surface acidity to result from the electron acceptor characta- of the oxide surface (29). This is related to the Lewis acid-base concept where, for an ionic oxide, the acid entity is the cation with the base being the oxygen anions. For an oxide M ,Oy, the surface acidity has been shown to be related to the ionization potential (IP) of the metal M according to... 

On the contrary, dehydroxylation of ionic oxides is easier, and results in the generation of Lewis acid sites, which are sufficiently stable to stay as such in relatively mild conditions. The strongest Lewis acidic oxides in normal conditions are alumina and gallia (and silica after very strong pretreatments) that is, oxides of elements at the limit of the metallic character. The same elements also give rise to halides characterized by an even stronger Lewis acidity. 

Lewis acid sites can adsorb water as any other molecules having some amount of basicity. As discussed in the case of alcohol adsorption [127], the coordination on Lewis sites of basic but dissociable molecules activates them toward dissociation if a sufficiently basic site is located nearby. This condition can be fulfilled for ionic oxides, thus molecular coordination of water can be precursor for its dissociation, and the overall process is, in principle, reversible (Scheme 9.2). 

The reactions (both dissociative and nondissociative adsorption of water) are highly exothermic and their equilibrium is shifted toward left by heating and lowering water vapor partial pressure. On the other hand, at least for ionic oxides, it is clear that bases stronger than water can also displace water from Lewis sites. This has been found, for example, for ammonia [128]. Consequently, the number of Lewis acid sites actually available depends on the degree of dehydroxylation of the surface that depends primarily on the temperature, on the composition of the environment and on the ionicity-covalency of the oxide (Scheme 9.2). 

Also, titania is a polymorphic material the most usual phases are anatase (SG = I4i/amd, Z = 4) and rutile (S.G. P42/mnm, Z = 2), the latter being always thermodynamically stable. In addition, titanias are highly ionic oxides with medium-high Lewis acidity, significant basicity and weak Brpnsted acidity if at all. Characterization data show that on anatase stronger Lewis acid sites are usually detectable than on rutile [261,262]. 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

The four structures with three double bonds (third row) and the one with four double bonds are the most plausible Lewis structures, (b) The structure with four double bonds fits these observations best, (c) +7 the structure with all single bonds fits this criterion best, (d) Approaches (a) and (b) are consistent but approach (c) is not. This result is reasonable because oxidation numbers are assigned by assuming ionic bonding. 2.109 The alkyne group has the stiffer C—H bond because a large force constant, k, results in a higher-frequency absorption. 

A soft Lewis acid has a relatively high polarizability. Large atoms and low oxidation states often convey softness. Contrast with Hg , a typical soft acid. The ionic radius of Hg is 116 pm, almost twice the size of... 

Early attempts to fathom organic reactions were based on their classification into ionic (heterolytic) or free-radical (homolytic) types.1 These were later subclassified in terms of either electrophilic or nucleophilic reactivity of both ionic and paramagnetic intermediates - but none of these classifications carries with it any quantitative mechanistic information. Alternatively, organic reactions have been described in terms of acids and bases in the restricted Bronsted sense, or more generally in terms of Lewis acids and bases to generate cations and anions. However, organic cations are subject to one-electron reduction (and anions to oxidation) to produce radicals, i.e.,... 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

---