Oxidation and dehydrogenation of alcohols by glauconite, a natural Fe(II)- and Fe(III)-containing sheet silicate, and ferri-sepiolite, its molecular sieve analogue

Abstract

Glauconite, a natural three layer sheet silicate with Fe(II) and Fe(III) ions in octahedral positions, has been investigated by X-ray diffraction and Mössbauer spectroscopy after both hydrogen and oxygen treatments at ambient temperatures. The results show that in hydrogen up to 673 K Fe(III) ions are reduced to Fe(II) ions without any change in the silicate structure. At 873 K in hydrogen the silicate structure is unstable, and small amounts of α-Fe can be detected. In oxygen at 673 K the Fe(II) ions are oxidized without any change in the silicate structure. The same occurs in an argon atmosphere at 673 K, most likely due to decomposition of the hydroxyl groups and/or the crystal water attached to the Fe(II) ions. Preliminary investigations have been performed on the reactivity of alcohols on glauconite and on ferri-sepiolite (xylotile), which is a molecular sieve composed of ribbons of three-layer sheet silicate with a composition similar to that of glauconite. Both minerals dehydrate and dehydrogenate alcohols in an inert atmosphere and oxidize them as well in the presence of oxygen. The oxidation and dehydrogenation activities can be ascribed to the Fe(II)/Fe(III) redox couple. Ferri-sepiolite additionally shows selectivity towards small size products, which can be ascribed to its molecular sieve structure.