Nonaqueous Sol−Gel Syntheses of Microporous Manganese Oxides

Abstract

Microporous manganese oxides have been prepared by nonaqueous sol−gel reactions involving tetrabutylammonium (TBA) or tetraethylammonium (TEA) permanganate and methanol in the presence of alkali cation dopants. Layered birnessite-type materials were obtained for Na+ and K+ dopants in a 0.5:1 reactant ratio with manganese. Na−birnessite was isolated in hydrated and dehydrated forms (7 vs 5.6 Å interlayer spacing) whereas only the hydrated K−birnessite was observed. Cryptomelane was generated with a K:Mn ratio of 0.25:1. Spinel manganese oxides were formed with Li+ dopants using 0.5:1 and 0.75:1 Li:Mn ratios. The materials were characterized by powder X-ray diffraction (XRD), elemental analyses, Mn oxidation state determination, thermogravimetric analysis, and scanning electron microscopy. Thin films of manganese oxides were prepared by spin coating TEAMnO4-derived sols onto glass slides. Unlike the bulk gel syntheses, only layered birnessite phases were obtained for thin films with Li+, Na+, and K+ dopants. Aerogels of K−birnessite and cryptomelane were prepared by supercritical fluid extraction of bulk gels with carbon dioxide. Surface areas were increased over the conventional sol−gel workup, but overall the values were not exceptional in these preliminary experiments.