OMS-2 Materials Research Articles

Microwave-Assisted Hydrothermal Synthesis of Cryptomelane-Type Octahedral Molecular Sieves (OMS-2) and Their Catalytic Studies

Manganese oxide octahedral molecular sieves (OMS) are important materials in environmental chemistry, electrochemistry, and heterogeneous catalysis. Here, a rapid process to prepare cryptomelane-type octahedral molecular sieve (OMS-2) nanomaterials using a microwave assisted hydrothermal technique (MW-HT) is presented. With the assistance of microwaves in the hydrothermal reaction, the preparation time of OMS-2 can be as short as 10 s; up to 4 days are required in a conventional hydrothermal reaction. Direct observation of reaction temperature and pressure in the hydrothermal reaction can be achieved in real time in the reaction process. Reaction time and temperature are two parameters chosen to examine the formation conditions of OMS-2 materials. A reaction temperature below 80 °C resulted in the formation of amorphous manganese oxide material, whereas crystalline phase OMS-2 materials were formed at increased reaction temperatures to 100 °C or above. Studies by field emission scanning electron microscop...

Catalytic thermal decomposition of ammonium perchlorate using manganese oxide octahedral molecular sieve (OMS)

Abstract The catalytic activity of mixed valent manganese oxide octahedral molecular sieve (OMS) on the thermal decomposition of ammonium perchlorate (AP) has been briefly studied. Results show that OMS plays a catalytic role on the thermal decomposition of AP, and the most effective is OMS-2. The decomposition temperature of AP has been decreased 145 °C and the heat of decomposition has been increased 0.66 kJ/g with the addition of 2 wt% OMS-2. Such effects are mainly attributed to the mixed valence of Mn 3+ and Mn 4+ existing in OMS-2 material and its high surface area.

Synthesis, characterization and catalytic oxidation activity of zirconium doped K-OMS-2 type manganese oxide materials

Zirconium doped manganese oxide OMS-2 materials have been synthesized. The synthesized zirconium doped manganese oxide OMS-2 materials have been characterized by XRD, TGA, BET and TEM techniques. All the materials showed the OMS-2 type (cryptomelane) X-ray diffraction patterns and the thermal stability and BET surface area of zirconium doped OMS-2 materials are improved compared to the parent manganese oxide (K-OMS-2) material. Transmission electron micrographs showed the rod like needle shaped morphology and zirconia dispersion on fibrous morphology of as-synthesized zirconium doped manganese oxide OMS-2 materials. As synthesized zirconium incorporated OMS-2 catalysts have been tested for the liquid phase oxidation of side chain aromatic compounds (ethylbenzene and benzyl alcohol) and cyclohexanol. Zr-K-OMS-2 (Cry) has shown high conversion for ethylbenzene and benzyl alcohol, whereas Zr-K-OMS-2 (Bir) has shown high activity for the conversion of cyclohexanol and good selectivity towards the formation of cyclohexanone.

Role of Manganese Oxide Octahedral Molecular Sieves in Styrene Epoxidation

Manganese oxide octahedral molecular sieves having 2 x 2 tunnel structure (OMS-2) and synthesized by different methods were used for studying styrene oxidation with tert-butyl hydroperoxide (TBHP) as the oxidant. The catalytic activity of the as-synthesized OMS-2 materials was investigated. The physical and chemical properties of the OMS-2 materials are related to their activity in styrene oxidation. This particular study emphasizes the acid-base properties and the porous nature of these materials, and their role in styrene oxidation. Results of styrene oxidation reveal that acidity coupled with high porosity play a crucial role in these catalytic reactions. A desired acidity coupled with pore volume found in OMS-2 synthesized by reflux methods (OMS-2(R)) and high-temperature methods (OMS-2(HT)) produces materials with higher styrene conversion and styrene oxide selectivity when compared with OMS-2 synthesized by solvent free (OMS-2(S)), microwave (OMS-2(MW)), or hydrothermal methods (OMS-2(HY)). Transition metal doped OMS-2 catalysts show better selectivity of styrene oxide when compared to their undoped catalysts.

Microwave frequency effects on synthesis of cryptomelane-type manganese oxide and catalytic activity of cryptomelane precursor

Cryptomelane-type manganese oxides (OMS-2) were synthesized in the presence of microwave heating at different microwave frequencies and also using variable-frequency heating. The materials were prepared using a two-step hydrothermal procedure. Catalytic activity of the materials was tested for the oxidation of 2-thiophenemethanol. OMS-2 prepared at a high-frequency limit of 5.5 GHz showed the highest conversion (50%) to the 2-thiophenecarboxaldehyde among all of the tested OMS-2 samples. The OMS-2 precursor showed remarkable conversion (89%) in the oxidation reaction. In addition, the OMS-2 materials and the precursor showed differences in oxygen evolution based on the thermal decomposition experiments, as well as differences in porosity.

Synthesis and structural characterization of copper incorporated manganese oxide OMS-2 materials synthesized via potassium birnessite

Metal ion incorporated tunnel structured manganese oxide OMS-2 has been recently exploited as potential materials in the field of heterogeneous catalysis Cryptomelane type copper incorporated manganese oxide OMS-2 type tunnel structured material has been synthesized by hydrothermal method using birnessite-containing potassium as a parent precursor instead of using birnessite containing sodium. Crystal structure and thermal stability of as synthesized materials are characterized by using XRD and TGA. Surface area and morphology of newly synthesized copper incorporated OMS-2 type materials have been studied by BET and SEM analyses.

Synthesis and Catalytic Activity of Cryptomelane‐Type Manganese Dioxide Nanomaterials Produced by a Novel Solvent‐Free Method.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enhanced electrocatalytic activity of carbon-supported MnO x/Ru catalysts for methanol oxidation in fuel cells

This investigation reports the possible use of carbon-supported manganese octahedral molecular sieves (OMS-2) for the electrooxidation of methanol. The effect of combining these materials with a commercially available 5%Ru-carbon sample is examined. The activity of the OMS-2 materials is evaluated with respect to other forms of manganese oxides such as pyrolusite and nsutite. The OMS-2 materials show a synergistic interaction with the 5%Ru-C as well as the pyrolusite materials that enhances their electrochemical activities effectively. These OMS-2 materials are not only cost-effective but also very electrochemically active.

Synthesis and Catalytic Activity of Cryptomelane-Type Manganese Dioxide Nanomaterials Produced by a Novel Solvent-Free Method

Cryptomelane-type K-OMS-2 nanomaterials with high surface area (156 m2/g) have been synthesized via a low-temperature solvent-free method in a very short time (1 h). Field emission scanning electron microscopy and high-resolution transmission electron microscopy images reveal that these materials have nanorod morphologies with average diameters of about 10 nm and lengths of about 50 nm. These are different from the long fiberous morphologies of OMS-2 materials made by conventional reflux or hydrothermal methods. X-ray diffraction and Brunauer−Emmett−Teller studies indicate that these materials have small crystallite sizes (∼9.8 nm) and that they are mesoporous with a uniform pore size distribution centered at 12 nm. These K-OMS-2 materials show improved catalytic activity for the oxidation of alcohols compared with the conventional K-OMS-2 materials, which may be due to their higher surface areas and novel surface properties. This fast, inexpensive, and environmentally friendly solvent-free method has the potential of being used in scaled-up syntheses of K-OMS-2 and other transition-metal-ion-substituted manganese oxide nanomaterials.

Cerium incorporated ordered manganese oxide OMS-2 materials: Improved catalysts for wet oxidation of phenol compounds

Cryptomelane type manganese oxide OMS-2 material was synthesized by redox reaction between potassium permanganate and manganese sulphate in acidic medium under reflux or hydrothermal crystallization conditions. Ce was added by ion-exchange, impregnation or directly at crystallization stage. The chemical composition, structure, texture, morphology and thermal stability of the materials were measured by EDX spectroscopy, XRD, N2-adsorption, SEM and TGA. Results of the catalytic wet oxidation of phenol at 100°C indicate best performance of the well-crystallized octahedral molecular sieve with cryptomelane structure where all the accessible potassium ions were exchanged for cerium cations. OMS-2 materials containing pure CeO2 phase and excess of active oxygen species had lower stability, activity and capacity for oxidative reactive adsorption of phenol. Implementation of Ce-exchanged crystalline OMS-2 catalyst improves the wastewater treatment capacity by a factor of 1.5–2 and 3 compared with co-precipitated Mn–Ce-oxide and activated carbon, respectively.

Structural characterisation of copper-containing manganese oxide octahedral molecular sieve (Cu-OMS-2) materials by X-ray absorption spectroscopy and cyclic voltammetry

X-ray absorption spectroscopy (X-ray absorption near-edge structure including pre-edge features and extended X-ray absorption fine structure studies) and cyclic voltammetry analyses were used to study copper substituted manganese oxide octahedral molecular sieve (Cu-OMS) microporous materials. The results from the X-ray absorption spectroscopy study have been interpreted in terms of possible locations of Cu 2+ ions in the microporous zeotypic OMS-2 structure. According to the synthetic route, the Cu 2+ ions are expected to interact with the surface atoms, and do not replace any of the framework atoms. The results are consistent with copper interacting with lattice oxygen and manganese atoms, and are therefore, also consistent with copper being sited within the channels of OMS-2. Qualitative pre-edge studies on manganese in copper doped OMS-2 show octahedral Mn 4+. The latter confirmed by cyclic voltammetric analyses giving similar voltammograms as for the manganese dioxide and non-mobility for Mn 2+ in the OMS-2 material.

Effects of Alkali Metal and Ammonium Cation Templates on Nanofibrous Cryptomelane-type Manganese Oxide Octahedral Molecular Sieves (OMS-2)

Four alkali cations (Li+, Na+, K+, and Rb+) and ammonium (NH4+), have been successfully used as templates for the synthesis of cryptomelane-type manganese oxide octahedral molecular sieves (A−OMS-2, A = the template cation). H−OMS-2 material has been prepared by the calcination of NH4−OMS-2. These A−OMS-2 materials have been characterized by X-ray diffraction, transmission electron microscopy, high-resolution scanning electron microscopy, thermogravimetric analysis, temperature-programmed desorption, inductively coupled plasma analysis, and BET surface area pore size distribution measurements. Catalytic evaluation of these A−OMS-2 materials has been done with the aerobic oxidation of cyclohexanol to cyclohexanone. All of these A−OMS-2 materials have cryptomelane tunnel structures with different degrees of crystallinity and nanofibrous morphology. The nature of the cation templates affects the formation and microstructures of the A−OMS-2 materials. The physical and chemical properties, such as the thermal stability, chemical composition, and average oxidation state of manganese, greatly depend on the nature of cations and amount of tunnel water. Catalytic activities of these A−OMS-2 materials are related to their surface areas.

Effect of using polyvinyl alcohol and polyvinyl pyrrolidone in the synthesis of octahedral molecular sieves

Octahedral molecular sieves (OMS-2) have been reported as catalysts for oxidation reactions. Interest exists in improving the properties of these materials. Effects of using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) in the synthesis of OMS-2 have been studied. The structure of OMS-2 was kept when PVA or PVP were used as indicated by XRD and Fourier transform infrared spectra data. The use of PVA or PVP promoted an increase in surface area (measured by Brunauer–Emmett–Teller) and a decrease in particle size (measured by XRD). Besides increasing surface area and decreasing particle size of OMS-2, PVA and PVP were useful to improve the film hardness of OMS-2 samples applied on glass surfaces as measured by the pencil hardness test, and Knoop microhardness test. Film hardness is an important property for possible applications of OMS-2 materials in continuous flow reactor systems. By using PVA or PVP as non-chelating agents, an increase in surface area from 59 to 114 (m 2/g), a decrease in particle size, from 29.8 to 12.1 nm, and a hardness value of 4H using the pencil hardness test, and 17.73 H K by Knoops micro hardness tests for OMS-2 prepared with PVA were observed.

Characterization of Manganese Oxide Octahedral Molecular Sieve (M−OMS-2) Materials with Different Metal Cation Dopants

Reflux methods were used to prepare different metal cation (M: Cu2+, Zn2+, Ni2+, Co2+, Al3+, or Mg2+) doped manganese oxide octahedral molecular sieve (M−OMS-2) materials with cryptomelane structure. Thorough characterization has been carried out for these M−OMS-2 materials. SEM micrographs of M−OMS-2 materials reveal their typical fibrous morphology. The results of ICP−AES elemental analyses for M−OMS-2 materials shed light on the distribution of doped metal cations in the cryptomelane structure. TGA and XRD studies indicate that the prepared M−OMS-2 materials are more thermally stable in O2 (up to 600 °C) than in inert (e.g., N2 or He) atmospheres (up to 500 °C). Phase transformations from cryptomelane to hausmannite or bixbyite were observed when M−OMS-2 materials were heated above these stable temperatures. Comprehensive N2 sorption analyses confirmed that M−OMS-2 materials are porous materials, which contain micropores and mesopores, with total BET surface areas in the range of 70−110 m2/g. Basicity studies of synthetic M−OMS-2 materials with TPD of adsorbed CO2 revealed that all pretreated M−OMS-2 materials have several different basic sites with varied strengths and amounts on their surfaces. TPD studies of NH3 adsorbed on the pretreated M−OMS-2 materials demonstrate that all M−OMS-2 materials also have similar acidities.

Catalytic Decomposition of 2-Propanol over Different Metal-Cation-Doped OMS-2 Materials

The catalytic properties of manganese oxide octahedral molecular sieve materials (OMS-2) with different metal cation dopants (M: Cu2+, Zn2+, Ni2+, Co2+, Al2+, or Mg2+) were investigated for the reaction of 2-propanol decomposition. Compared with other M–OMS-2 catalysts, Cu–OMS-2 catalyst has much higher conversion of 2-propanol and the highest selectivity to acetone at 300°C. As for the selectivity to propene, it is generally below 6% for M–OMS-2 materials. The reaction results are discussed in consideration of characterization results of M–OMS-2 catalysts, especially studies on their acidity and basicity, and suggest that dehydrogenation of 2-propanol is not simply catalyzed by basic or acidic and basic pair sites, but probably by active sites with redox and basic properties. In situ FTIR studies suggest that Cu–OMS-2 has more active sites for 2-propanol than Al–OMS-2 and K–OMS-2. The phase transitions from cryptomelane to hausmannite and finally to manganosite were observed with XRD studies of Cu–OMS-2 catalysts during reaction. All Cu–OMS-2 materials that either have the cryptomelane, amorphous MnOy, hausmannite, or manganosite structure show activity in the decomposition of 2-propanol to acetone. The Cu–OMS-2 materials with the hausmannite structure show the highest activity.

Effects of Cu2+ Ions on the Structure and Reactivity of Todorokite- and Cryptomelane-Type Manganese Oxide Octahedral Molecular Sieves

The concentration effects on Cu uptake into the structures and reactivity of manganese oxide octahedral molecular sieves (OMS) were investigated. Two sets of 3 × 3-tunnel structure OMS designated as OMS-1 were synthesized by hydrothermal treatment at 160 °C for 48 h. The Cu−OMS-1 series of materials (tunnel substituted) were prepared by incorporating Cu2+ ions into OL-1, which has a layered structure, at 60 °C for 24 h. [Cu]−OMS-1 materials (framework substituted) were prepared by ion exchanging Cu2+ ions into the tunnels of OMS-1 under similar conditions. Cu−OMS-2 materials characterized by 2 × 2 tunnels were prepared by the reflux method. ICP analysis shows a Cu/Mn molar ratio of 0.278 for Cu−OMS-1 labeled C as the substitution limit for a pure sample based on its XRD pattern. The Cu(II) ions easily substitute for the divalent cations (Mg2+, Mn 2+) either in tunnel or framework sites when exchange was done before hydrothermal treatment. The presence of Cu2+ ions decreases the Mn2+/Mn4+ ratio in Cu−OMS-1 samples as suggested by an increasing trend in the average oxidation state of Mn and decreasing amounts of total Mn. XPS data reveal that Cu remains in the 2+ state in all of the materials as indicated by the shake-up peaks associated with Cu in such valency. EPR spectra of Cu-containing OMS-1 materials show the six peaks of Mn2+ with a hyperfine splitting constant of 97 G, indicating an octahedral environment. The TGA profiles of [Cu]−OMS-1 show that these samples have the same thermal stability as OMS-1 but the amount of lattice oxygen evolved decreases slightly with the amount of Cu2+ in the tunnel. The thermal stability of the Cu− OMS-1 and Cu−OMS-2 structures decrease as the amount of Cu incorporated increases. The copper ions that substitute for Mn2+ in the framework are considered as defects in the structure of the crystallites which make them less stable than OMS-1 containing Mg2+ in the framework where it exhibits a stabilizing effect. Resistivity measurements show a decrease and leveling off as the concentration of Cu increases in the Cu−OMS-1 series, which is a trend consistent with doping. The resistivity of the OMS-2 structure increases with the amount of copper incorporated. Cu(II) ions replacing the divalent cations in the framework during hydrothermal synthesis possibly account for the properties observed in the Cu−OMS-1 materials. An increased catalytic activity was observed for this set of materials as the Cu/Mn ratio increased for the oxidative dehydrogenation of ethylbenzene to styrene at 300 °C at 1 atm.

Studies of oxidative dehydrogenation of ethanol over manganese oxide octahedral molecular sieve catalysts

A series of manganese oxide octahedral molecular sieve (OMS) materials were prepared and characterized by X-ray diffraction and surface area measurements (Brunauer-Emmett-Teller). Some first row transition metals (Ni 2+, Cu 2+, Fe 3+, Co 2+, Mg 2+) were doped into the framework of these octahedral molecular sieve materials to give [M]-OMS materials. The catalytic activities of these materials were studied in the oxidative dehydrogenation of ethanol in a flow reactor at different temperatures and at various ratios of ethanol to O 2. The influence of space velocity on the reaction was also tested. The experimental results showed that both the conversions and selectivities of this reaction over each individual OMS material are highly dependent on the nature of OMS materials and the transition metal dopants in their framework structures. The [Co]-OMS-1 materials show the highest conversion and selectivity to acetaldehyde of all materials. Basic sites on both OMS-1 and OMS-2 materials may be responsible for catalytically active sites in this reaction.

Electrical Resistivity Measurements on Manganese Oxides with Layer and Tunnel Structures: Birnessites, Todorokites, and Cryptomelanes

Direct-current measurements were used to determine the electrical resistivities of several manganese oxide materials having either layered or tunnel structures. Birnessite (Na-OL-1, OL=octahedral layer) consists of layers of edge- and corner-sharing Mn{sub 6} octahedral units with Na{sup +} in the interlayer regions. Todorokite and cryptomelane (Mg-OMS-1 and K-OMS-2, respectively, OMS = octahedral molecular sieve) are similarly built from MnO{sub 6} units, but in these systems the octahedral join to form 6.9 and 4.6 {Angstrom} tunnels occupied by Mg{sup 2+} and K{sup +}, respectively. Resistivities were also measured for OL-1, OMS-1, and OMS-2 materials in which (a) cation exchange was carried out at layer and tunnel sites or (b) isomorphous subsititution for Mn was performed by doping small amounts of foreign cations into the manganese oxide framework. Four probe measurements on pressed pellets reveal that OL-1 and OMS-1 materials have resistivities on the order of 10{sup 5}-10{sup 6} {Omega}cm at 298 K. OMS-2 materials have resistivities on the order of 10{sup 5}-10{sup 6} {Omega} cm at 298 K. Variable-temperature measurements establish a general pattern of increasing resistivity with decreasing temperature. However, between 153 and 293 K, OL-1 and OMS-1 materials do not obey a simple exponential variation of resistivity and temperature.more » By contrast, OMS-2 samples follow the Arrhenius relationship over a comparable temperature range. Activation energies for the conductivity OMS-2 materials were calculated to be in the range 0.5-0.6 eV. Solid-state voltammetry was used to determine the electrical resistance of OMS-1 and OMS-2 samples at higher temperatures from 298 to 673 K. A general exponential decrease in resistance with increasing temperature was observed for both classes of materials. Ac resistivity measurements show similar trends to dc resistivity data. 36 refs., 8 figs., 3 tabs.« less

Synthesis and Characterization of Octahedral Molecular Sieves (OMS-2) Having the Hollandite Structure

Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular siev (OMS) having the 2x2 hollandite structure with a one-dimensional pore diameter of 4.6 {Angstrom} Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnO{sub 4} and Mn{sup 2+}. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600 {degrees}C. OMS-2 formed by oxidation of Mn{sup 2+} under basic conditions and calcined at higher temperature (200-800 {degrees}C) is thermally stable up to 800 {degrees}C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu{sup 2+} and Fe{sup 3+}. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu{sup 2+} and Fe{sup 3+} contain nonexchangeable Mn{sup 2+}. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and A value of 85 G indicative of octahedral Mn{sup 2+} coordination. The Mn{sup 2+} EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra,more » ion-exchange data, X-ray diffraction patterns, and the theoretical simulations of diffraction data suggest that Cu{sup 2+} and Fe{sup 3+} are located in the tunnels of OMS-2. 29 refs., 8 figs., 3 tabs.« less