From microporous to mesoporous manganese oxide crystalline phases: Syntheses, characterization, and applications

Date of Completion

January 1998

Keywords

Chemistry, Inorganic

Degree

Ph.D.

Abstract

This dissertation concerns studies of microporous and mesoporous manganese oxides that are metastable, mixed valent, semiconducting, and catalytically active. Syntheses, characterization, and applications of these materials are the major parts of this research.^ (A) Microporous manganese oxides. Using inexpensive KMnO$\sb4$ to substitute for expensive Mg(MnO$\sb4)\sb2$, good quality octahedral molecular sieves (Mg-OMS-1) were made only if the (Mg$\sp{2+}$) concentration in the starting materials fell in window-1 of 0.20-0.50 M or a molas ratio of framework Mg(F) to Mn in window-2 of 3%-5%. Materials of windows 1 and 2 have colors (from grayish brown to brown, respectively), average oxidation states (from 343 to 3.36, respectively), band gaps, resistivity, and thermal stability that are all very sensitive to the variable ratios (from 0.03 to 0.05) of framework Mg to Mn.^ (B) Mesoporous manganese oxides. A new approach for making manganese oxide mesoporous structures (MOMS),via a precipitation-wrapping-oxidation (PWO) mechanism, has been introduced. The syntheses of hexagonal and cubic phases of MOMS need concentrations of surfactant CTAB (cetyltnmethylammonium bromide) of 28 and 10 weight %, respectively. TEM photographs, electron diffraction patterns, powder XRD patterns, BET surface areas, and pore size distribution all suggest the existence of a hexagonal array of mesopores for hexagonal MOMS-1. EPR and XRD results also suggest that the MOMS mesopores could be built by microcrystallites of dense phases like $\rm Mn\sb2O\sb3$ and $\rm Mn\sb3O\sb4$ as the secondary structure that is composed of(MnO$\sb6$) octahedra which form the primary structure.^ The thick-walled MOMS-1 shows a thermal stability up to $\sim$1000$\sp\circ$C a band gap of 2.46 eV, a resistivity of $\sim$1.23 $\times$ 10$\sp5$ ($\Omega\cdot$cm), an average oxidation state of 3.55 (+), and dual surface acidic functionality (many more Lewis acid sites than Bronsted acid sites). The liquid-phase catalytic oxidations of stable cyclohexane and n-hexane give conversions of $\sim$10% and $\sim$8%, respectively, suggesting a novel catalytic potential of these MOMS materials. ^

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