Understanding the photocatalytic degradation by P25 TiO2 of acetic acid and propionic acid in the pursuit of alkane production

Towards the application of harvesting valuable hydrocarbon products from wastewater and associated fermentation processes, the traditional P25 Degussa TiO2-assisted decarboxylation of simple carboxylic acid substrates - in this case acetic and propionic acids - was re-examined for its alkane production capacity. Avoiding doping or the introduction of other elements into the commercially available TiO2 lattice, we modified other parameters of the reaction not yet explored. Under nitrogen, highly concentrated (50%) and pure (100%) solutions of both acetic acid and propionic acid demonstrated good selectivity for the decarboxylation over any other side reaction or complete mineralization, nearing or exacting a 1:1 hydrocarbon/CO2 ratio in each case. This behaviour proves that the direct reaction of h(+) with the acid in either case is the key parameter to favour this selectivity. In the case of acetic acid, methane's yield was increased significantly by the periodic aeration of the catalyst when exhausted under nitrogen. The modification of solution pH to more physiological and fermentation-compatible levels (pH of 6.0) lead to improve the selectivity and the ethane' yield in 50% propionic acid degradation and also resulted in virtually no unidentified carbon products. The Photo-Kolbe coupling and dehydration products, meanwhile, were favoured in the less concentrated solutions, suggesting that more dilute solutions may be more promising for deriving longer length hydrocarbons from substrates of smaller size. Mechanistically, these results also implicate that OH degrees radicals are at the origin of product generation by way of Photo-Kolbe coupling and dehydration. Finally, acetic acid and propionic acid were degraded simultaneously, in which the latter was degraded preferentially or at least initially before the former, along with the appearance of propane as a result of cross-coupling.