Clay mineral catalysed oxidation of phenols

Abstract

The primary aim of the project was to grow a series of nanoparticulate iron(III) oxides within aluminosilicate clay minerals (Fulacolor™ and bentonite) and employ them as oxidation catalysts for phenolic compounds using hydrogen peroxide as oxidant in Fenton or photo-Fenton processes. Thermal degradation of phenol proceeded very slowly; photocatalytic degradation, utilising daylight simulating bulbs, proved to be far more rapid by comparison. Hence, we decided to concentrate on optimising the mineralisation potential of our catalysts.

Several products, including diphenols, quinones and carboxylic acids, have been identified within the degradation pathway to carbon dioxide and water. Different product isomer ratios are produced depending upon whether the process is thermal or photochemical.

Of the catalysts synthesised, a mixed Fe(II)/Fe(III) oxide (magnetite) nanocomposite, gave the greatest rate of mineralisation: due to the presence of Fe(II) cations in the photocatalytically active iron oxide structure. Well aged (3-4 years) iron(III) nitrate activated Fulacolor™ consistently gave the slowest rates, due to the presence of haematite, the most stable iron(III) oxide. Therefore, the iron oxide species grown within the interlayer spacing has a marked effect upon the rate of reaction. Newly synthesised iron(III) oxides derived from Fe(III) nitrate exchanged clay minerals would be expected to contain goethite (-FeOOH) nanoparticles, while Fe(III) chloride activated clay minerals should contain akaganéite (-FeOOH). Both newly prepared nanocomposites showed much greater rate enhancements than the aged (haematite) nanocomposite due to the lower stability and, probably, their higher degree of defects as full Ostwald ripening has not yet occurred.

Substituted phenols appear to be degraded very rapidly. However, further work needs to be done in this area to ascertain the effect that substituents have on the rate of reaction.