Title

Studies of layered and pillared manganese oxide materials

Date of Completion

January 1999

Keywords

Chemistry, Inorganic|Engineering, Materials Science

Degree

Ph.D.

Abstract

Synthetic Birnessite, an octahedral layered manganese oxide material called OL-1 was synthesized with Na+, K+, Na +/Mg2+, K+/Mg2+, Na +/K+ ions as interlayer cations by redox reactions between permanganate and alcohols in a strong basic media. Chromia pillared OL-1s were prepared under reflux conditions using trinuclear chromium hydroxyl acetate as a pillaring agent followed by calcination in a N2 atmosphere at 200°C. Vanadium oxide pillared OL-1s were obtained by intercalating neutral vanadyl acetylacetonate (VOacac) or vanadium acetylacetonate (Vacac) into the interlayer of OL-1 and subsequently calcining in air at 300°C. The synthesis procedures were monitored using X-ray diffraction studies. The resultant materials were characterized by XRD, X-ray absorption, X-ray photoelectron spectra, FTIR, UV-VIS, inductively coupled plasma, transmission electron spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis, potentiometric titration, thermal analyses, TPD measurements, BET surface area and pore size distribution measurements. OL-1 materials prepared using this alcohol route showed enhanced thermal stabilities and increased Mg accommodation compared to OL-1s prepared with other methods. Based on the analysis methods developed here, Na-OL-1 exhibited recoverable and reversible structural and surface O2 oxygen species while K-OL-1 showed higher stability. Na-OL-1 had predominantly Brønsted acid sites resulting from OH groups bonded to Mn on Na-OL-1 surfaces, while the Na/Mg-OL-1 had mainly Lewis acid sites. Large porosity was obtained in chromia pillared OL-1 materials with a narrow pore size distribution centered around 18 Å. Although these materials remained “amorphous” as determined by XRD after calcination, TEM morphology studies suggest that the materials were still layered. EXAFS studies indicated the formation of Cr-O-Mn bonds in the resultant materials via comer-shared linkages of CrO6 and MnO6 octahedra. Good crystallinity in VOacac intercalated OL-1 was achieved, which after calcination, led to the formation of mesoporous VOx pillared OL-1 materials with the pore diameters being in the range of 36 to 41 Å. The intercalation of bulky neutral inorganic species into the interlayer of OL-1 through an exfoliation route simplified the pillaring process and preparative factors and made the process and pore sizes in the pillared materials controllable. ^