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Structure, porosity, and redox in porous manganese oxide octahedral layer and molecular sieve materials

The control of synthesis methods allows the preparation of unique materials such as octahedral layer (OL) and octahedral molecular sieve (OMS) systems. These mixed valent porous materials are excellent semiconductors. Conductivity in porous molecular sieve materials is unusual and leads to more favorable characterization such as less charging in relation to insulating materials like clays and zeolites. Semiconductivity of OL and OMS materials allows potential applications as sensors, redox catalysis, and in fuel cells. This review will focus on the use of composition and redox to control the structure, morphology, porosity, catalytic activity, and conductivity of such materials. A major emphasis in this field has been on the generation of new materials and the mechanism of formation of such systems.

[J. Mater. Chem., 2008, 18, 1623–1631]

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Framework Doping of Indium in Manganese Oxide Materials: Synthesis, Characterization, and Electrocatalytic Reduction of Oxygen

Indium-doped manganese oxide materials have been hydrothermally synthesized. Doped indium was found to be in framework sites in synthetic cryptomelane, based on the results of XRD, average oxidation state (AOS) of manganese, Raman, and IR. TEM images show that indium-free OMS-2 and K-In-OMS-2(R=1/5) have a morphology of nanorods. Cyclic voltammetry (CV) was used to test the electrocatalytic activity in the oxygen reduction. CV results show that manganese oxide doped with indium generates electrocatalytic activity for the O 2 reduction reaction.

[Chemistry of Materials (2008), 20(6), 2069-2071]

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Adsorptive and Acidic Properties, Reversible Lattice Oxygen Evolution, and Catalytic Mechanism of Cryptomelane-Type Manganese Oxides as Oxidation Catalysts

Cryptomelane-type manganese oxides have been synthesized, characterized, and tested in the total oxidation of volatile organic compounds and CO oxidation. The structural, compositional, morphological, acid-base, physisorptive-chemisorptive, and thermal stability properties (especially the reversible evolution of lattice oxygen) have been studied in detail using ICP-AES, HRSEM, XRD, IR and adsorbate-IR, N2 and CO2 physisorption at 77 and 273 K, respectively, TPD-MS, and TGA-DSC techniques. Kinetic and mechanistic studies for the catalytic function have been conducted and related to the characterization results. Cryptomelane has shown to be highly microporous, by using CO2 physisorption, and highly hydrophobic, possessing both Bro¨nsted and Lewis acid sites. A part of the lattice oxygen atoms can be reversibly removed from the framework and recovered at elevated temperature without changing the framework structure. These lattice oxygen atoms can react with CO even at room temperature and are active sites for the oxidation of benzene. The consumed lattice oxygen atoms are replenished by gaseous oxygen to complete a catalytic cycle. The ease of reversible evolution of lattice oxygen, together with the high porosity, hydrophobicity, and acidity, leads to the excellent oxidation
properties of OMS-2.

[Journal of the American Chemical Society (2008), 130(10), 3198-3207]

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Porous Manganese Oxide Octahedral Molecular Sieves and Octahedral Layered Materials

This Account first gives a historical overview of the development of octahedral molecular sieve (OMS) and octahedral layer (OL) materials based on porous mixed-valent manganese oxides. Unique properties of such systems include excellent semiconductivity and porosity. Materials that are conducting and porous are rare and can offer novel properties not normally available with most molecular sieve materials. The good semiconductivity of OMS and OL systems not only permits potential applications of the conductivity of these materials but also allows characterization of these systems where charging effects are often a problem. Porous manganese oxide natural materials are found as manganese nodules, and these materials when dredged from the ocean floors have been used as excellent adsorbents of metals such as from electroplating wastes and have been shown to be excellent catalysts. Rational for synthesis of novel OMS and OL materials is related to the superb conductivity, microporosity, and catalytic activity of these natural materials.

[Accounts of Chemical Research (2008), 41(4), 479-487]

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Shape Evolution of Single-Crystalline Mn2O3 Using a Solvothermal Approach

A new and facile route was developed to manipulate the growth of hierarchically ordered Mn2O3 architectures via a solvothermal approach. Various solvents are employed to control the product morphologies and structures. Mn2O3 with unique cuboctahedral, truncated-octahedral, and octahedral shapes are obtained, and a possible formation mechanism is proposed.

[J. Phys. Chem. C, 2007, 111, 14694-14697]

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Colloidal Manganese Oxide Precursor to Octahedral Layered, OL-3 Materials

This study reports one-pot synthesis of a hexagonal form of a layered manganese oxide material (OL-3) using mild conditions and low temperature. The oxidation of an aqueous solution of manganese acetate using tetramethylammonium hydroxide and hydrogen peroxide at 4 °C leads to the formation of a colloidal manganese dioxide solution. Colloidal MnO2 was then flocculated using K ions, forming disordered layered manganese oxide nano-flakes. The results indicate that the colloidal manganese oxide nano-flakes flocculated into ultra-thin, disorderly-stacked hexagonal lamellar sheets composed of a material with the chemical composition of K1.04MnO2.340.6H2O.

[Journal of Inorganic and Organometallic Polymers and Materials, 2007, 17(2), 459-467]

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Characterization and Electrocatalytic Behavior of Layered Li2MnO3 and Its Acid-Treated Form

The material H[H0.18Li0.15Mn0.67]O1.90, obtained by the delithiation of layered Li2MnO3 using acid, gives a 2-fold increase in oxygen reduction ability versus Li2MnO3. Using elemental analysis by the ICP technique, average oxidation state determination, acid-base titrations, and thermal analysis, we determined the chemical composition of the proton-exchanged material to be H[H0.18Li0.15Mn0.67]O1.90. Rietveld refinement shows that the structure of the delithiated material is a monoclinic distortion of the P 3 structure resulting in the C 2/ m space group with cell parameters a) 5.049(2) Å, b ) 8.707(2) Å, c ) 4.903(1) Å, â ) 109.17(2)°. The BET surface area for the materials increases from 18 to 57 m2/g upon delithiation. Application of H[H0.18Li0.15Mn0.67]O1.90, as an electrocatalyst in a lithium-air cell gives a specific discharge capacity of 1618 mAh g-1.

[Chem. Mater. 2007, 19, 229-234]

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Hydrothermal synthesis of structure- and shape-controlled manganese oxide octahedral molecular sieve nanomaterials

Highly uniform single-crystal Na-OMS-2 (OMS: octahedral molecular sieve), pyrolusite, and c-MnO 2 nanostructures with an interesting 3D urchinlike morphology have been successfully prepared using a hydrothermal method based on a mild and direct reaction between sodium dichromate and manganese sulfate. The crystal phases, shapes, and tunnel sizes of the manganese dioxide nanostructures can be tailored. Reaction temperature, concentrations of the reactants, and acidity of the solution play important roles in controlling the synthesis of these manganese dioxides. Field-emission scanning electron microscopy and transmission electron microscopy (TEM) studies show that the nanomaterials obtained are constructed of self-assembled nanorods. X-ray diffraction and TEM results indicate that the constituent manganese dioxide particles are single-crystalline materials. Energy dispersive X-ray analysis and magnetic studies imply that chromium cations may be incorporated into the framework and/or tunnels of the manganese dioxides. A mechanism for the growth of manganese dioxides with urchinlike architectures is proposed.

[Advanced Functional Materials, 2006, 16(9), 1247-1253]

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In situ Synthesis of Mixed-Valent Manganese Oxide Nanocrystals: An In situ Synchrotron X-ray Diffraction Study

In situ characterizations of phase transformations by in situ synchrotron powder X-ray diffraction was successfully performed for wet chemistry systems at different pH media. Quartz capillary tubes are brittle for high temperature/basic systems. The sapphire capillary tube is robust for pressurized, acidic, and basic conditions. In situ synchrotron XRD studies provide an opportunity for better control of material syntheses, which is difficult to achieve for conventional kinetic study methods as also suggested by others. For example, different surface areas for a pure KOMS-2 phase and varying tunnel structures of manganese oxides can be controlled in studies here. This method is not limited to manganese oxides but can be used for in situ characterization of other materials during synthesis using hydrothermal, sol-gel, or other methods. In addition, catalytic processes in liquid-solid, gas-solid, or solid-solid systems can also be studied in an in situ way. Combined with Rietveld refinement, oxygen/cation vacancies or defects in the catalyst structure can be investigated for a better understanding of the catalytic mechanisms. As a result, catalyst synthesis and catalytic processes can be optimized.

[J. Am. Chem. Soc., 2006, 128 (14), 4570 -4571]

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Synthesis of Single Crystal Manganese Oxide Octahedral Molecular Sieve (OMS) Nanostructures with Tunable Tunnels and Shapes

A new and facile route is reported to manipulate the self-assembly synthesis of hierarchically ordered Rb-OMS-2 and pyrolusite with an interesting flowerlike morphology by a direct and mild reaction between rubidium chromateand manganese sulfate without any organic templates. The crystal forms, morphologies, and tunnel sizes of the obtained OMS materials can be controlled. A mechanism for the growth of manganese dioxides with flowerlike architectures was proposed. The obtained products exhibit potential for use in catalysis and other applications.

[Journal of Physical Chemistry B, 2006, 110(7), 3066-3070]

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Magnetic Route to Measure the Average Oxidation State of Mixed-Valent Manganese in Manganese Oxide Octahedral Molecular Sieves (OMS)

A magnetic route has been successfully applied to determine the AOS of both mixed-valent framework Mn and single oxidation states of Mn in manganese oxide OMS. The results from the magnetic method are in good agreement with titration results. The magnetic route confirms that framework manganese in KOMS-2 and OMS-6 is mixed-valent, consisting of 3+ and 4+ with Mn3+ in the low spin state. Magnetic methods can be used for identification of minor/impurity phases in paramagnetic or ferromagnetic samples as also pointed out by Wold. The requirements are that (1) the systems should show Curie-Weiss behavior over a temperature range and (2) there are no more than two paramagnetic ions present.

[Journal of the American Chemical Society, 2005, 127(17), 6166-6167]

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Shape-Controlled Synthesis of Manganese Oxide Octahedral Molecular Sieve Three-Dimensional Nanostructures

This study reports the fabrication of ordered porous manganese oxide paper-like free standing membranes (FSMs) by simply heating "pulp" -like homogeneous suspensions at low temperatures. These free-standing membranes are robust and flexible, and can be formed on a patterned substrate in order to make micropatterns. Tangled manganese oxide nanowires in suspension aggregate and align systematically to form such membranes. The paper-like membranes is composed of single-crystal cryptomelane-type manganese oxide (OMS-2) fibers.

The outstading features of this system include the unusual mechanical properties, periodicity, facile preparation, conductivity, easy of ion-exchange, malleability, and diffusional properties, which are due to the formation of hierarchical porous structures.

[Advanced Materials ( Weinheim , Germany ), 2004, 16(19), 1729-1732]

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Synthesis, characterization, and catalytic applications of manganese oxide octahedral molecular sieve (OMS) nanowires with a 2 x 3 tunnel structure

Romanechite is a natural manganese oxide with a 2x3 tunnel structure containing a majority of Ba 2+ and trace amounts of Na+ , K+ , and Sr2+ as tunnel cations. Many attempts have been made to synthesize the 2x3 tunnel structured manganese oxide in laboratories using Ba 2+ as the template. However, no successful work has been reported due to (1) the presence of intergrown hollandite impurities (barium-2 x 2 tunnel structure manganese oxide) in the synthesized products and (2) the absence of romanechite characteristic X-ray diffraction (XRD) peaks in the products, such as the (001) and (200) diffraction peaks which correspond to d = 9.7 and 7.0 Å, respectively. Hydrated Na + ions have been utilized as structure directors to successfully synthesize the Na + 2 x 3 tunnel structure manganese oxide (OMS-6) from hydrothermal treatment of Na-birnessite. XRD gave a pattern in very good agreement with the pattern of romanechite (JCPDS file 14-627) without impurity phases. High-resolution microscopy measurements showed a nanofibrous morphology of the materials with an average fiber diameter of 40 nm. Under N 2 environments, the 2 x 3 tunnel structure is stable below 550 °C and transforms into hausmannite (Mn 3 O 4 ) at temperatures of 550 °C or higher; however, under O2 environments, the 2 x 3 tunnel structure is preserved at 550 °C as measured by in situ XRD. The density functional theory (DFT) method indicated that the 2 x 3 tunnel structure has a major micropore size of 7.5 Å. NH 3 and CO 2 chemisorption results indicated that the amount of strong acidic and basic sites on the Na-2 _ 3 material is about 0.22 and 0.002 mmol/g sample, respectively. Catalytic oxidation of indene by the Na-2x3 manganese oxide showed a 96% conversion for indene and a 73% selectivity toward phthalic anhydride for a 40 h reaction at 80 °C.

[Chemistry of Materials, 2004, 16(25), 5327-5335]

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Higher Valency Ion Substitution into the Manganese Oxide Framework

Microporous manganese oxide materials with novel chemical composition have been hydrothermally synthesized by incorporation of increasing amounts of higher valency cations into the manganese oxide framework. Vanadium and niobium were doped into the cryptomelane structure, lowering the manganese oxide average oxidation state and increasing its electrical resistivity. The OMS-2 syntheses set forth here represent a novel strategy for obtaining higher valency substituted microporous manganese oxides. Their further potential as catalysts and sensors is currently under investigation.

[Journal of the American Chemical Society, 2004, 126(25), 7774-7775]

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Higher Valency Ion Substitution into the Manganese Oxide Framework

A new route for higher valency ion substitution into the Mn oxide (OMS-2) framework is reported. Isomorphously substituted V and Nb OMS-2 were hydrothermally synthesized at 200 °C for a period of 2 days. Characterization by XRD, elemental anal., Raman spectroscopy, and resistivity studies proved that V was incorporated into the Mn oxide structure. The presence of V in the framework changes the electrical properties, making the material very attractive for H2O sensing applications.

[Journal of the American Chemical Society, 2004, 126, 7774-7775]

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Incorporation of Manganese Species into the Pores of MCM-48

The present work describes a new way to synthesize manganese species within the channels of mesoporous MCM-48 silica using permanganate as manganese precursor agent. For the first time permanganate ions were incorporated within the as-synthesized mesoporous host by ion-exchange process. The resulting materials were characterized by XRD, N2 sorption, TEM, EPR, XPS, Raman, and elemental analysis methods. The structure of the MCM-48 host material is maintained as indicated by XRD and HRTEM, and the surface areas of the MnMCM-48 materials are in the 670-930 m2/g range compared to 1090 m2/g for the host. The characterization results also indicate the presence of manganese species inside the silica pores, as well as a good dispersion of the manganese particles within the pores of the host material. Controlled manganese loading was possible to a max. of 13 wt%. No bulk manganese was detected outside the MCM-48 mesoporous silica particles.

[Chemistry of Materials, 2004, 16, 2411-2417]

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Effect of using PVA and polyvinyl pyrrolidone in the synthesis of OMS

Octahedral mol. sieves (OMS-2) have been reported as catalysts for oxidation reactions. Interest exists in improving the properties of these materials. Effects of using polyvinyl alc. (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 IR 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 (m2/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.

[Microporous and Mesoporous Materials , 2003, 63, 11-20 ]

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Effects of Alkali Metal and Ammonium Cation Templates on 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 NH 4 -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.

[Journal of Physical Chemistry B, 2003, 107, 9185-9194]

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New LDHs Containing Intercalated Manganese Oxide Species

Mn oxide species (MnOx) were intercalated within the gallery spaces of Mg-Al layered double hydroxides (LDHs). Synthesis of these materials was achieved by ion-exchange of the LDH-nitrate precursor with permanganate anion followed by reduction with organic reagents, such as glucose, EtOH, and ascorbic acid. Elemental analysis, x-ray diffraction, FTIR spectroscopy, Raman spectroscopy, HR-TEM, and N2 sorption analyses were used to characterize these materials. TEM micrographs of LDH-MnOx materials revealed platelike morphology, characteristic of hydrotalcite-like compounds. Chemical analysis results showed that permanganate anions exchanged with nitrate anions. FTIR and Raman spectroscopy confirmed the reduction of the permanganate anions after treatment with the organic reagents. The XRD diffraction patterns of LDH-MnOx revealed that the layer structure is maintained after all synthetic steps. The observed basal spacings of intercalates varied depending on the reducing agent; the largest expansion was 9.93 .ANG., corresponding to the use of ascorbic acid. The specific surface areas were also affected according to the organic reagent used, indicating that the structural modifications in the interlayer domain obsserved by x-ray diffraction also influence the microtextural properties.

[Inorganic Chemistry, 2003, 42, 5621-5631]

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Characterization of OMS with Different Metal Cation Dopants

Reflux methods were used to prepare different metal cation (M: Cu(2+), Zn(2+), Ni(2+), Co(2+), Al(3+), or Mg(2+) 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., nitrogen or helium) 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 N 2 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.

[Chemistry of Materials, 2002, 14, 940-948]

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Synthesis of Layer-Structure Birnessite by Air Oxidation

An air oxidation method was used to synthesize a layer-structure birnessite (OL-1) by the oxidn. of Mn2+. The synthesis was accomplished in a much shorter time than other methods such as the redn. of MnO4- and redox reactions between Mn2+ and MnO4-. Thorough studies of preparative parameters such as the rate and extent of oxidation, the concentration of OH-, and the aging process were made to understand the synthesis of birnessite using air oxidation. Five metal (Fe, Co, Ni, Ca, and Cd) cations were added to the initial reactant solutions as framework dopants to study their effects on the synthesis and to modify the properties of the resulting birnessite. The crystal phases, average Mn oxidation states, surface areas, and thermal properties of birnessites prepared under different conditions were studied to understand the influence of these conditions on birnessite formation. Birnessite prepared by this air oxidation method was stable up to 500 °C.

[Chemistry of Materials, 2002, 14, 2071-2077]

[Inorganic Chemistry Communications, 2001, 4, 493-495]