Alkanes atmospheric transformation

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

Because oxygen, carbon dioxide, methane, and other alkanes are completely miscible with dense supercritical water, combustion can occur in this fluid phase. Both flameless oxidation and flaming combustion can take place. This leads to an important application in the treatment of organic hazardous wastes. Nonpolar organic wastes such as polychlorinated biphenyls (PCBs) are miscible in all proportions in supercritical water and, in the presence of an oxidizer, react to produce primarily carbon dioxide, water, chloride salts, and other small molecules. The products can be selectively removed from solution by dropping the pressure or by cooling. Oxidation in supercritical water can transform more than 99.9 percent of hazardous organic materials into environmentally acceptable forms in just a few minutes. A supercritical water reactor is a closed system that has no emissions into the atmosphere, which is different from an incinerator. 

Hydrocarbons emitted into the atmosphere react with OH and other transient oxidants (NO3 radicals, O3, Cl-atoms) to form oxygenates. Hydrocarbons have a wide variety of atmospheric lifetimes. Methane is the least reactive hydrocarbon and has an atmospheric lifetime with respect to OH radicals of about 10 years. The lifetimes (days) of ethane, propane, n-butane, and n-decane are about 47, 11, 4.9, and 1, respectively. Alkenes are more reactive than alkanes of the same carbon number and are transformed into oxygenates much more quickly by reactions with OH and O3. For example, the lifetimes for reaction with OH for ethene, propene, and trans-2-butene are about 1.3 days, 10 h, 4.3 h, respectively. In addition, the alkenes react rapidly with ozone to form oxygenates. In the presence of 60 ppb of O3, the lifetimes of ethene, propene, and trans-2-butene with repect to reaction with ozone are 4.9 days, 18 h, and 1.0 h, respectively. Of course, the nature of the products formed by reactions of the hydrocarbons with... 

---