Alkanes, photocatalytic oxidations

The reaction between alkanes (especially methane) and water is endothermic and produces syngas (see books, reviews [51], recent papers [52] and patents [53]). The photocatalytic oxidation of methane with water to produce methanol and molecular hydrogen has been patented [54]. 

Most of the photocatalytic oxidations concern aliphatic and aromatic alkanes and alkenes derivatives, and the selective oxidation of alcohols to carbonyls. 

The selective oxidation and, more generally, the activation of the C-H bond in alkanes is a topic of continuous interest. Most methods are based on the use of strong electrophiles, but photocatalytic methods offer an interesting alternative in view of the mild conditions, which may increase selectivity. These include electron or hydrogen transfer to excited organic sensitizers, such as aryl nitriles or ketones, to metal complexes or POMs. The use of a solid photocatalyst, such as the suspension of a metal oxide, is an attractive possibility in view of the simplified work up. Oxidation of the... 

Taylor RA, Law DJ, Sunley GJ et al (2009) Towards photocatalytic alkane oxidation the insertion of dioxygen into a platinum(ll)-methyl bond. Angew Chem Int Ed Engl 48 5900-5903... 

The chief advantages to using dense CO2 as the reaction medium were claimed to be its inertness, its general ability to dissolve the reactants, and its easy separation from the reaction mixture. No mechanistic work was reported, but the same mechanism as previously proposed in alkane solvents was suggested, that is, the key step was oxidative addition to the excited state of the metal complex. The C02/Rh complex system was also reported to be an effective photocatalytic system for the dehydrogenation of cyclooctane. 

Cheng, J. Y. K. Cheung, K. K. Che, C. M. Lau, T. C. Photocatalytic and aerobic oxidation of saturated alkanes by a neutral luminescent /ran5-dioxoosmium(VI) complex 0s02(CN)(2)(dpphen). Chem. Commun. 1997, 1443-1444. 

A number of reactions involving alkane photooxidation by polyoxometalates have been run under aerobic conditions. Those by PWi2O40 and Wio032 are summarized at the end of Table I (see section 5). Unfortunately these reactions, collectively, are rather uninformative with respect to mechanism. Radical chain autoxidation processes are undoubtedly present and few if any of the reported papers have addressed the requisite experiments to differentiate autoxidation from other oxidation pathways. A similar general limitation exists in the literature on photocatalytic aerobic oxidation of organic compounds by semiconductor materials [66]. 

Photocatalytic Hydrogenation of Alkenes to Alkanes in Alcoholic Suspensions of Palladium-Loaded Titanium(IV) Oxide Without the Use of Hydrogen Gas... 

Imamura K, Okubo Y, Ito T, Tanaka A, Hashimoto K, Kominami H (2014) Photocatalytic hydrogenation of alkenes to alkanes in alcoholic suspensirms of palladium-loaded titartium (IV) oxide without the use of hydrogen gas. RSC Adv 4 19883-19886... 

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