Photosynthetic Membranes Part 71. Laboratory-scale photomineralization of n-alkanols in aqueous solution by photocatalytic membranes immobilizing titianium dioxide

Kinetics of photocatalytic oxidn. of methanol, ethanol, n-propanol, n-heptanol, and n-decanol to yield intermediates, and photomineralization of intermediates to yield carbon dioxide and water, was studied in aq. soln. using a lab.-scale photoreactor and photocatalytic membranes immobilizing 30±3 wt.% of TiO2, in the presence of stoichiometric quantities of hydrogen peroxide as oxygen donor. The whole vol. of the irradiated soln. was 4.000±0.005 L, and the ratio between this vol. and the geometrical apparent surface of the irradiated side of the photocatalytic membrane 3.8±0.1 cm. A kinetic model was used, by which mineralization of substrate to CO2 was supposed to occur through one single intermediate (kinetic consts. k1), mediating the behavior of all the numerous real intermediates formed in the path from the substrate to CO2 (kinetic consts. k2). A competitive Langmuirian adsorption of both substrate and "intermediate" was also supposed to be operative, expressed by the apparent adsorption consts. K1 and K2. By Langmuir - Hinshelwood treatment of the initial rate data, the starting values of the k and K couples were obtained, from which, by a set of differential equations, the final optimized parameters, k1 and K1, k2 and K2 were calcd. While in the case of alkanoic acids, values of k1 and k2 were roughly coincident, in the case of n-alkanols investigated in this work, k2 values were higher than k1 of the same mols. This is interpreted on the basis of the closer behavior, from the photocatalytic point of view, of alkanols to hydrocarbons compared to their corresponding carboxylic acids. Furthermore, values of k2 for methanol were practically the same as those of k1 for the corresponding acid. This behavior confirms the suggestion of being methanoic acid the most representative intermediate for photodegrdn. of methanol.