Heterogeneous oxidation effects

Effective catalysts for heterogeneous oxidations using 02 are mainly Pt and Pd with some activity by Ir70 and Ru.71 Much work has gone into alcohol oxidations that are dehydrogenations to ketones or aldehydes. Also, oxygen may be inserted at allylic positions of alkenes and these may be dehydrogenated to ketones or aldehydes.72 In the case of aldehydes, additional oxidation may be accomplished to produce acids.72,73... 

The extent of this impurity effect is surprising and is worthy of examination. Consideration of Eq. (6.111) readily shows [23] for heterogeneous oxidation that there is no apparent gas-phase reaction. This equation is now written in the form... 

Takeuchi, K., Yazawa, T., andibusuki, T. Heterogeneous photocatal34ic effect of zinc oxide on photochemical smog formation reaction of C4H8-N02-Air, Atmos. Environ., 17(ll) 2253-2258,1983. 

Anhydrous RuO has a tetragonal rutile structure with a slightly distorted octahedral structure, there being two sets of Ru-0 distances at 1.917(8) and 1.999(8) A [645] and has an extensive chemistry as a heterogeneous oxidation catalyst, a topic beyond the scope of this book. It is rarely used as a precursor for Ru oxidations, the hydrated form RuO. nH O (for brevity written below simply as RuO ) being much more effective in this respect. A procedure for converting inactive RuO (presumably the anhydrous form) to the hydrated RuO used in catalytic oxidations has been described [243]. 

From what is now known, we can treat the sulfur photochemistry independently of the bulk of the tropospheric photochemistry. The number densities of H2S and SC>2 are too small to have any significant effect on the radical photochemistry, though the reactions of S02 and H2S with HO2 and OH, respectively, are important to the sulfur cycle. Although the heterogeneous oxidation reactions of S02 and H2S may be important in polluted environments, the gas-phase oxidation reactions R6, R7,... 

Heterogeneous oxidations. KOi can effect oxidation of either 1,2- or 1,3-dihy-droxynaphthalene to 2-hydroxy-1,4-naphthoquin(jne in an aprotic medium in 60% yield. Addition of a crown ether is not essential, but results in a somewhat higher yield (80%). The yield is somewhat lower (50%) in the absence of oxygen. ... 

Heterogeneous oxidations. K02 can effect oxidation of either 1,2- or 1,3-dihy-... 

Theoretical research into the problem of combustion propagation on the surface of condensed fuels involving heterogeneous oxidation has been reported It has been found that the combustion velocity depends on thermal effects, kinetic parameters of the exothermic oxidation reaction on the surface, and on the rate of... 

The available data concerning the effect of metal compounds on the pyrolysis and combustion of polymeric materials show that there is a great potential in the control of these materials, This field is still in its embryonic stage. Much is yet unclear. In many cases the analysis is rendered difficult because of the dual function of metal compounds (especially of the d- and f-types) in redox reactions. Their ability not only to inhibit but also to promote polymer decomposition manifests itself most clearly in concentration effects The specific effects of certain metal compounds on the ignition and combustion rate of polymers indicate that heterogeneous oxidation... 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

Reaction 20, the pro-oxidant effect (Type 3), can become significant during high local concentrations of a-Toc in heterogeneous systems of lipids and wifi be discussed in Section in.C.2. Similarly, synergism (Type 4) is of particular interest in the inhibition of lipid peroxidation and will be reviewed in that section. 

The relationship between other flammability characteristics and the polymer tendency for carbonization has also been studied 112-114). Flame retardant effectiveness of coke is, primarily, related to the reduced release of fuels into the gas-phase, and to the ability of the coke to undergo heterogeneous oxidation. Therefore, not only the composition is important but also the morphological structure, porosity, specific surface area and thermophysical properties of the carbonaceous residue115, U6). 

---