Recent Manganese Oxide Octahedral Molecular Sieves (OMS–2) with Isomorphically Substituted Cationic Dopants and Their Catalytic Applications

The present report describes the structural and physical–chemical variations of the potassium manganese oxide mineral, α–MnO2, which is a specific manganese octahedral molecular sieve (OMS) named cryptomelane (K–OMS–2), with different transition metal cations. We will describe some frequently used synthesis methods to obtain isomorphic substituted materials [M]–K–OMS–2 by replacing the original manganese cationic species in a controlled way. It is important to note that one of the main effects of doping is related to electronic environmental changes, as well as to an increase of oxygen species mobility, which is ultimately related to the creation of new vacancies. Given the interest and the importance of these materials, here, we collect the most recent advances in [M]–K–OMS–2 oxides (M = Ag, Ce, Mo, V, Nb, W, In, Zr and Ru) that have appeared in the literature during the last ten years, leaving aside other metal–doped [M]–K–OMS–2 oxides that have already been treated in previous reviews. Besides showing the most important structural and physic-chemical features of these oxides, we will highlight their applications in the field of degradation of pollutants, fine chemistry and electrocatalysis, and will suggest potential alternative applications.

Tunable Mn Oxidation State and Redox Potential of Birnessite Coexisting with Aqueous Mn(II) in Mildly Acidic Environments

As the dominant manganese oxide mineral phase in terrestrial and aquatic environments, birnessite plays an important role in many biogeochemical processes. The coexistence of birnessite with aqueous Mn2+ is commonly found in the subsurface environments undergoing Mn redox cycling. This study investigates the change in Mn average oxidation state (AOS) of birnessite after reaction with 0.1–0.4 mM Mn2+ at pH 4.5–6.5, under conditions in which phase transformation of birnessite by Mn2+ was not detectable. The amount of Mn2+ uptake by birnessite and the equilibrium concentration of Mn(III) proportionally increased with the initial concentration of Mn2+. The Mn AOS of birnessite particles became 3.87, 3.75, 3.64, and 3.53, respectively, after reaction with 0.1, 0.2, 0.3, and 0.4 mM Mn2+ at pH 5.5. Oxidation potentials (Eh) of birnessite with different AOS values were estimated using the equilibrium concentrations of hydroquinone oxidized by the birnessite samples, indicating that Eh was linearly proportional to AOS. The oxidation kinetics of bisphenol A (BPA), a model organic pollutant, by birnessite suggest that the logarithms of surface area-normalized pseudo-first-order initial rate constants (log kSA) for BPA degradation by birnessite were linearly correlated with the Eh or AOS values of birnessite with AOS greater than 3.64.

High-Resolution Transmission Electron Microscopy of Defects in Todorokite (Oms-1)

Todorokite is a manganese oxide mineral that occurs in widespread terrestrial and marine deposits. It is a major component of manganese nodules which occur in large quantities on ocean and some lake floors (>1012 tons). Todorokite has been linked to the presence of potentially mineable transition elements in manganese nodules. Todorokite is an octahedral molecular sieve material and has been termed by some in the chemical community as OMS-1. Many of the properties of synthetic and natural varieties have been characterized and potential applications include use for adsorption, as oxidation catalysts and in electrochemical devices.The todorokite structure consists of a framework of MnO6 octahedra that are edge- and corner-linked to form tunnel structures. Several tunnel structures have been found in naturally occurring todorokite ranging from the most basic 3×3 structure to 3×8 and larger structures. Large cations and water occur in the tunnel interiors.

A Study of Cuban Todorokite

Todorokite is a hydrated manganese oxide mineral containing small amounts of sodium, potassium, calcium, barium, magnesium, iron, and aluminum. From the reports of other investigators and observations in our laboratories, todorokite appears to be a more widely occurring mineral than generally recognized.It was found by X-ray diffraction that a sample of todorokite from Charco Redondo, Cuba, had a high degree of preferred orientation and that milling for a prolonged period of time was necessary to obtain random orientation in a mounted specimen. The position of the preferred orientation direction with respect to the X-ray beam is the principal factor determining the relative intensities on an X-ray diffraction pattern. Based on fiber data, the unit cell of todorokite is considered to be pseudo-orthorhombic or triclinic with α, β, and γ angles varying slightly from 90° and cell dimensions of a0 = 9.65A, b0 = 10.29A, and c0 = 2.84A, This deviation from orthogonality may account for the diffuse and distinct reflections typical of X-ray diffraction patterns of todorokite. Optical data and a hypothesized paragenesis of Cuban todorokite are also included.

Birnessite, a new manganese oxide mineral from Aberdeenshire, Scotland

SummaryA manganese pan near Birness contains grains of an optically uniaxial negative mineral near (Na0·7Ca0·3)Mn7O14·2·8H2O, giving an X-ray powder pattern similar to that of synthetic materials described as ‘manganous manganite’ and δ-MnO2. Material giving a similar pattern has been described from a natural occurrence in Canada, but no mineral name was assigned; the name birnessite is now proposed. The mineral is probably formed by air-oxidation of manganous oxides under alkaline conditions.