Hydrocarbon oxidation, factors affecting

Kagawa and Toyama in Tokyo followed 20 normal 11-yr-old school children once a week from June to December 1972 with a battery of pulmonary-function tests. Environmental factors studied included oxidant, ozone, hydrocarbon, nitric oxide, nitrogen dioxide, sulfur dioxide, particles, temperature, and relative humidity. Temperature was found to be the most important environmental factor affecting respiratory tests. The observers noted that pulmonary-function tests of the upper airway were more susceptible to increased temperature than those of the lower airway. Although the effect of temperature was the most marked, ozone concentration was significantly associated with airway resistance and specific airway conductance. Increased ozone concentrations usually occur at the same time as increased temperature, so their relative contributions could not be determined. 

We investigated the oxidation of only propane at the high temperature (430°C.) pic darret—first with factors affecting the type of reaction, then with analyses for some reaction products (aldehydes, hydrogen peroxide, alcohols, and hydrocarbons). All experimental data can be explained by a radical reaction mechanism which is discussed. 

Selectivity is determined by a competition between the rate of butadiene production and the rate of degradation of butene and other hydrocarbon intermediates. Various investigators have considered factors affecting selectivity (31,35-37). Generally speaking, along the selective oxidation pathway, the first abstraction of H to form rc-allyl can be viewed as a combination of an acid-base and a redox reaction ... 

Vedrine, J., Millet, J. and Volta, J. (1996). Molecular Description of Active Sites in Oxidation Reactions Acid-Base and Redox Properties, and Role of Water, Catal. Today,32,. 115-123. Oyama, S. (1996). Factors Affecting Selectivity in Catalytic Partial Oxidation and Combustion Reactions, Heterogeneous Hydrocarbon Oxidation, 638, pp. 2-19. 

This chapter does not intend to indicate that these observations are of general validity for the SCR-HC reaction and catalysts. Indeed, the results show that still further effort is necessary to understand the surface chemistry of this process. There are several factors affecting NOx removal efficiency with hydrocarbons, such as dispersion, coordination, and local electronic states of the metal cations. The oxide matrix surrounding the active sites is not inert, but plays both an indirect role (through electronic influence on the active centres) and a direct role (role of interface sites, surface acido-base centres — both Brpnsted and Lewis sites — and oxygen vacancies). The role of the oxide matrix shows several analogies with the role of the zeolite matrix in metal cation containing zeolites for SCR-HC [61]. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

In co-oxidations of undiluted hydrocarbons over a range of compositions, the change in reaction medium may affect over-all rates of oxidation by a factor of 2 or 3 (14,19, 37). Further, while many autoxidations are autoaccelerating, others are autoretarding (37). 

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

---