Selective oxidation lattice oxygen, role

The existence of the molecular radical ion 02 , of atomic O-, and of the regular ions in the lattice O2- has been firmly established. A review by Lunsford (33) presents a summary of the experimental evidence which led to the discovery of 02 and O-. The participation of these various forms of oxygen in hydrocarbon oxidation is discussed in a review by Sachtler (11). It seems clear that both adsorbed and lattice oxygen species play an important role in the selective oxidation of hydrocarbons. 

Evidence discussed in Section 6 on the role of die lattice oxygen in the oxidative coupling of methane indicated that different oxygen anions can be responsible for the selective activation of methane and that the fcamation of surface carbonates can affect CH conversion rates. Thmnodynamic analysis of the Na2C03/Na20 systems conducted by Lamoreaux et al. showed that under oxidative coupling conditions sodium carbonates are in the stable bulk phase and... 

The reason for the difference in the effectiveness between each of the crystal face to catalyze the ammoxidation of alkyl aromatics selectively is a result of the specific electronic character of the oxygen atoms associated with the vanadium atoms of the V2 O5 structure. As was learned about the role of lattice oxygen (0 ) in the selective ammoxidation of propylene to acrylonitrile (see above), hydrogen abstraction and oxygen insertion require oxygen atoms with nucleophilic character (79). On the other hand, nonselective oxidation is affected by electrophilic oxygen species, O2 and O . These are the intermediate species in the dissociative chemisorption and reduction of O2 to lattice (80). 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

In the literature there has been much debate regarding the role of the lattice or extralattice Ti in Ti silicalite for a variety of oxidation reactions. In order to have a more precise idea of the role of the lattice or surface Ti and more specifically of the role of the coordination sphere of Ti, a series of monopodal and tripodal titanium surface complexes (i. e., =SiOTi(OR)3 and ( SiOIsTiOR) were derived by the reaction of the Ti alkyl (Structure 1) and hydride species with water, oxygen, methanol, and tert-butanol. The resulting complexes were then used in the epoxidation of 1-octene by tert-butyl hydroperoxide. Tripodal complexes, especially (=SiO)3Ti( Bu), were found to be significantly more active and more selective for the epoxidation of 1-octene than their monopodal counterparts [22]. 

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