Octahedral Molecular Sieve Research Articles

Enhanced activity of Pd–Cu(Ni) bimetallic catalysts for oxidative carbonylation of phenol

Pd–Cu(Ni) bimetallic catalysts supported on Pb-doped manganese oxide octahedral molecular sieves(Pb-OMS-2) were prepared by deposition-precipitation method, and the activities of those bimetallic catalysts were evaluated in the oxidative carbonylation of phenol to diphenyl carbonate (DPC). The physico-chemical properties of catalysts were evaluated by various characterization techniques such as X-ray diffraction (XRD), Transmission electron microscope (TEM), Scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/EDS), Temperature-programmed reduction of hydrogen (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The results showed that superior activity of bimetallic catalyst was accomplished by the addition of second metal Cu(Ni) compared to monometallic Pd catalyst. The metal particles of bimetallic catalysts were dispersed homogeneously on the surface with small particle size (3∼5 nm). The high activity and stability of bimetallic catalyst were achieved due to the formation of very active Pd–Cu(Ni) interfaces. The oxidation state of active species increased via the strong interaction between bimetallic components, and the redox capacity of Pd became stronger with the addition of the second metal. The highest DPC yield was 23.4% for Pd1Cu2 and 22.9% for Pd1Ni4 catalysts, respectively. After 20 h of reaction time, even though the DPC yield stopped growing for the monometallic catalysts, it continued to increase for the bimetallic catalysts, and the DPC yield reached about 50.0% at 24 h.

Base-free synthesis of 1,3,5-triazines via aerobic oxidation of alcohols and benzamidine over a recyclable OMS-2 catalyst

By using 4Na2SO4·2H2O2·NaCl as a reductant, manganese oxide octahedral molecular sieve (OMS-2) with enhanced surface area and mixed valence was prepared successfully. OMS-2 showed excellent catalytic ability towards aerobic oxidative synthesis of 1,3,5-triazines from benzyl alcohols and benzamidine. Methyl benzenes, DMF and DMSO could also be employed as substrates to react with benzamidine offering triazines under the heterogeneous conditions. The catalytic system features base-free conditions, broad substrate scope, high chemoselectivity, operational simplicity, catalyst recyclability and utilization of O2 as the green oxidant.

Insight into Low-Temperature Catalytic NO Reduction with NH3 on Ce-Doped Manganese Oxide Octahedral Molecular Sieves

Oxygen vacancy defect (OVD) as one of the crystal defects is critical to the reactivity of heterogeneous catalyst surface for catalytic NO reduction with NH3 (NH3-SCR). Here, we propose a simple and feasible method to enhance OVDs through Ce doping manganese oxide octahedral molecular sieves (Ce-K-OMS-2), where it was proven that Ce mainly enters the framework structure of K-OMS-2. The robust NH3-SCR catalytic test showed that the presence of Ce could lower the catalytic temperature and activity for NO conversion (∼100%) across a wide temperature span of 140–230 °C. The characterization results indicated the distortion of octahedral units after Ce substitution and the subsequent introduction of OVDs. Moreover, we demonstrated by density functional theory (DFT + U) calculations that the unique nanostructure of Ce doping K-OMS-2 with OVDs in the framework can alter the local bond lengths and the density of states, thereby obviously accelerating the low-temperature NH3-SCR. NH3 prefers to be adsorbed at the ...

Sulfate radical-mediated degradation of phenol and methylene blue by manganese oxide octahedral molecular sieve (OMS-2) activation of peroxymonosulfate.

Activation of peroxymonosulfate (PMS) has been concentrated on degrading refractory organic pollutants owing to the generation of sulfate radical ([Formula: see text]) with high standard redox potential. In this study, manganese oxide octahedral molecular sieve (OMS-2) with cryptomelane type was synthesized by a new hydrothermal method to activate PMS for the degradation of phenol and methylene blue (MB) in water. The as-prepared composites were fully characterized, and the effects of PMS dosage, OMS-2 dosage, initial pollutant concentration, pH, and chloride on the degradation of phenol were elaborately investigated. Moreover, the phenol degradation was evaluated through the variations of total organic carbon (TOC) and three-dimensional excitation emission matrix (3D-EEM), and reaction intermediates were also investigated. Both electron spin resonance (ESR) spectra and comparative experiments suggested [Formula: see text] and hydroxyl radical (HO•) took part in the phenol degradation and [Formula: see text] was more significant than HO•. The fine degradation efficiency of phenol in different water source, as well as the stability after continuous use, indicated the possible application of PMS/OMS-2 in real wastewater treatment.

First-principles study of carbon capture and storage properties of porous MnO2 octahedral molecular sieve OMS-5

Based on the experimentally determined framework structure of porous MnO2 octahedral molecular sieve (OMS)-5, we used density functional theory-based calculations to evaluate the effect of Na+ cation on pore dimensionality and structural stability, and the interaction between CO2 and OMS-5. We quantified the formation energy of one CO2/unit tunnel and two CO2/unit tunnel, and projected the electronic density of states on the OMS-5 framework, CO2 molecules, and Na+ cations to reveal their individual contributions and bonding nature. Partial charge densities were also calculated to investigate CO2 adsorption behavior in the OMS-5. Our studies predict the initial stage and driving force for the adsorption of CO2 in the OMS-5, guiding the OMS material design for carbon capture and storage applications.

Dual Promotional Effects of TiO2-Decorated Acid-Treated MnOx Octahedral Molecular Sieve Catalysts for Alkali-Resistant Reduction of NOx

Alkali metals generated during waste incineration in power stations are not conducive to the control of nitrogen oxide (NO x) emission. Hence, improved selective catalytic reduction of NO x with ammonia (NH3-SCR) in the presence of alkali metals is a major issue for practical NO x removal. In this work, we developed a novel TiO2-decorated acid-treated MnO x octahedral molecular sieve (OMS-5(H)@TiO2) catalyst with improved alkali-resistant NO x reduction at low temperature, and the dual promotional effects of OMS-5(H)@TiO2 catalysts were clarified. It was found that the special structure of the acid-treated MnO x octahedral molecular sieve (OMS-5(H)) was responsible for the trapping of alkali metals and high deNO x activity at low temperature. Subsequently, the decoration by TiO2 further improved the redox properties by accelerating the high ratio of Mn4+ and Oα on the surface of the highly active (OMS-5(H)@TiO2) catalyst. Moreover, a thorough mechanism study via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTs) demonstrated that the acid treatment led to remarkable increment of acid sites, which enabled the catalyst to resist alkali metals in the form of ion exchange. Meanwhile, the decoration of TiO2 further increased the strength of the Lewis acid sites, assisting more active intermediate species to effectively take part in the deNO x reaction. Besides, a "fast SCR" process was observed to certify that the decoration of TiO2 promoted the improvement of low-temperature activity in the presence of alkali metals. The dual effects combining OMS-5(H) with TiO2 decoration in terms of alkali metal resistance and high catalytic activity at low temperature proved that the high-performance deNO x catalyst was successfully developed in this work. The work paves a way for the development of superior low-temperature SCR catalysts with improved NO x reduction efficiency in the presence of alkali metals.

Enhanced catalytic ozonation towards oxalic acid degradation over novel copper doped manganese oxide octahedral molecular sieves nanorods.

A series of copper doped manganese oxide octahedral molecular sieves (Cu-OMSx-T) with different Cu/Mn ratios and hydrothermal temperatures were successfully synthesized and used for catalytic ozonation towards oxalic acid (OA) degradation. The as-prepared Cu-OMSx-T composites were comprehensively investigated by BET, FT-IR, XPS and etc. characterizations. The results indicated that the Cu doping would increase the specific surface area, change chemical bonds, and promote the transformation of multivalent metals and the generation of oxygen vacancies. It was noteworthy that the hydrothermal temperature played an important role in the morphology of Cu-OMSx-T composites and the Cu/Mn molar ratios greatly influenced the catalytic activities. Amongst, the Cu-OMS0.5-140 achieved the optimum catalytic activity with 97.3% of OA degradation efficiency and 98.8% of mineralization rate in 30 min at pH 6.0. Moreover, hydroxyl radical and superoxide radical were identified as the major reactive radicals and the catalytic mechanism for OA degradation enhancement was also elucidated. In addition, the Cu-OMS0.5-140 exhibited great stability and reusability with high OA mineralization rate (>90%) and low metal release after five times recycle. Overall, the results indicated that the synthesized Cu-OMS0.5-140 is an efficient, stable, and recyclable ozonation catalyst, and could be a promising alternative material for water purification.

Structure and electrochromism of two-dimensional octahedral molecular sieve h\u2019-WO3

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h’-WO3, built of (WO6)6 tunnel cavities. h’-WO3 is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h’-H0.07WO3. Gentle heating results in h’-WO3 with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.

Single-Step Hydrogenolysis of Furfural to 1,2-Pentanediol Using a Bifunctional Rh/OMS-2 Catalyst.

Hydrogenolysis of biomass-derived furfural (FFA) to 1,2-pentanediol (1,2-PeD) was investigated using a bifunctional catalyst with basic and metallic sites, which was synthesized by the hydrothermal method. The synthesized catalyst consisting of rhodium (Rh) supported on an octahedral molecular sieve (OMS-2) of different loadings, such as 0.5, 1, and 1.5% w/w, was studied, and 1% (w/w) loading gave the best results. This 1% w/w Rh/OMS-2 catalyst showed excellent catalytic activity and selectivity for the hydrogenolysis reaction because of better dispersion of rhodium, later revealed by characterization. Furthermore, 1% Rh/OMS-2 catalyst was well characterized in virgin and reused states using various techniques such as Fourier-transform infrared spectroscopy, NH3-temperature-programmed desorption (TPD), CO2-TPD, temperature-programmed reduction, H2 pulse chemisorption, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller surface area, X-ray photoelectron spectroscopy, Raman spectroscopy, and differential scanning calorimetry–thermogravimetry analysis. The catalyst showed a higher surface area of 72 m2/g and the average size of the highly dispersed Rh metal of ∼2 nm. The studies were performed in a batch reactor; the catalyst offered almost 100% conversion of FFA with 87% selectivity to 1,2-PeD at 160 °C and 30 atm hydrogen pressure in 8 h. The reaction mechanism and kinetic model have been developed using a dual-site Langmuir–Hinshelwood–Hougen–Watson mechanism. The activation energies were 12.3 and 27.6 kcal/mol, correspondingly. The catalyst was found to be active, selective, and reusable.

Niobium-substituted octahedral molecular sieve (OMS-2) materials in selective oxidation of methanol to dimethoxymethane.

Octahedral molecular sieve (OMS-2) refers to a one-dimensional 2 × 2 framework of octahedral manganese oxo units based on the cryptomelane-type framework. Herein, we describe a niobium (Nb) substituted mixed metal oxide of Nb and Mn where the cryptomelane-type framework is retained. These materials are hydrothermally synthesized from the reaction of potassium permanganate, manganese sulfate, and homogeneous niobium(v) precursors. Niobium incorporation up to 31 mol% can be achieved without destroying the one dimensional 2 × 2 framework. The yields of the materials vary between 70 and 90%. These materials are analyzed by powder XRD, BET isotherm, TEM, SEM, XRF, and XPS studies. The synthesized materials show promising activity in selective oxidation of methanol to dimethoxymethane (DMM) at 200 °C. Normalized activity correlations followed the trend 21% Nb-OMS-2 > 15% Nb-OMS-2 > 31% Nb-OMS-2 > 68% Nb-OMS-2 > K-OMS-2. A fluctuation in methanol conversion was observed around 125–150 °C in most samples, suggesting this to be a catalytically important temperature regime when forming active sites for DMM production.

Low-valence or tetravalent cation doping of manganese oxide octahedral molecular sieve (K-OMS-2) materials for nitrogen oxide emission abatement

Substitutionally doped oxide catalysts with abundant oxygen vacancy defects (OVDs) can effectively improve the catalytic activity efficiency.

A one-pot synthesis of Fe-doped cryptomelane type octahedral molecular sieve manganese oxide for degradation of methylene blue dye

The Fe-doped octahedral molecular sieve manganese oxides have been successfully synthesized by one-pot synthesis of the reaction between KMnO4 and glucose using sol-gel methods. The oxide products are then characterized by various techniques such as X-ray diffraction, scanning electron microscopy, atomic absorption spectroscopy and average oxidation state (AOS) of manganese in manganese oxides. The as-synthesized manganese oxide and Fe-doped manganese oxides are used as catalysts for the degradation of methylene blue dye using hydrogen peroxide as oxidants. The results indicated that the Fe-doped manganese oxide catalyst displayed much enhanced catalytic activities compared to undoped manganese oxide for methylene blue degradation. The differences in catalytic activities have been correlated with the difference in surface properties and crystallinity.

Binary copper and manganese oxide nanoparticle supported oms-2 for enhancing activity and stability toward co oxidation reaction at low temperature

The CuO, CuMnOx and MnOx catalysts were anchored on the manganese oxide support with the structure of octahedral molecular sieves (OMS-2), which were synthesized using MnSO4 and KMnO4 as precursors by are flux method under acidic conditions, by an impregnation method and tested for CO oxidation. These catalysts and OMS-2 support were characterized by the advanced analyzations of X-ray diffraction and FTIR patterns; and SEM performances; and H2-TPR profiles. For CO oxidation reaction, CuO and CuMnOx catalysts showed extremely higher activities than that of MnOx catalyst and OMS-2 support. For the lowest temperature for 100% conversion of CO (T100), the CuO and CuMnOx catalysts were observed at 55 oC and 65 oC, respectively. Due to the present of Cu2+– O2− – Mn4+ ↔ Cu+–□–Mn3+ + O2 redox couple in the structure of these solid catalyst. Additionally, the CuMnOx catalyst showed higher activity (~ 1.74 folds) and better stability than that of CuO catalyst in CO oxidation. Due to the advance functional of binary oxide structure of CuMnOx catalyst As known, CO oxidation may follow the Mars-van-Krevelen mechanism with Cu2+– O2− – Mn4+ ↔ Cu+–□–Mn3+ + O2 redox couple. This study shows the high applicating potential of CuMnOx/OMS-2 material in exhaust treatment for environmental safety.

Cobalt tetracarboxyl phthalocyanine-manganese octahedral molecular sieve (OMS-2) as a heterogeneous catalyst of peroxymonosulfate for degradation of diclofenac

A composite of cobalt tetracarboxyl phthalocyanine and amino-functionalized manganese octahedral molecular sieve (CNOMS-2) was synthesized in a simple way, and applied for degrading diclofenac in aqueous media by catalyzing of peroxymonosulfate heterogeneously. The experiment results revealed that the CNOMS-2/PMS system was highly efficient for DCF degradation in the pH range of 5–9, and the mechanism involved the generation of OH, SO4⁻ and 1O2 by the activation of PMS. The main reactive oxygen species was found to be 1O2 by radical scavenging experiments and electron paramagnetic resonance (EPR) spectroscopy. Further, a carboxylation product of DCF at m/z 340 was found to be a prominent intermediate of the reaction in this system specially, as determined by LC-MS/MS. The reusability and ion leaching concentrations under different pH were also examined to determine the application prospects of the catalyst.

Photo-deposition preparation of supported Pd catalysts for non-phosgene one-step synthesis of diphenyl carbonate

Pd catalysts anchored on manganese oxide octahedral molecular sieves (OMS-2) were synthesized by photo-deposition method, and then used for one-step oxidative carbonylation of phenol to synthesize diphenyl carbonate (DPC). The results showed that the catalytic activity of Pd/OMS-2 catalyst prepared by photo-deposition was apparently better than those prepared by the traditional precipitation and impregnation method, the distribution of Pd particles was more uniform with the average size of 1.3 nm. This may result from the strong interaction between the carriers and the active Pd species. Irradiated for 5 h at pH 5.0, with the Pd loading of 2.5 wt%, the highest DPC yield of 18.1% was achieved.

Mechanochemical formation of highly active manganese species from OMS-2 and peroxymonosulfate for degradation of dyes in aqueous solution

The performance and mechanism of mechanochemical activation of peroxymonosulfate (PMS) by cryptomelane-type manganese octahedral molecular sieve (OMS-2) prepared by a refluxing method for degradation of Acid Orange 7 (AO7) have been demonstrated. Grinding of OMS-2 and PMS in presence of a small amount of H2O for a short time induced a much higher AO7 degradation efficiency compared with the OMS-2 + PMS system without pretreatment. Such degradation can also be efficiently performed by this system over a wide range of solution pH and for different organic dyes. For other manganese oxides such as OMS-2 prepared by a solvent-free method, amorphous manganese oxides, and γ-MnO2, with coexistence of Mn(III) and Mn(IV) species, great enhancement of catalytic activity was also observed. Characterization of the ground OMS-2 by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) suggested that, during grinding, low-valent manganese species are oxidized to Mn(IV) species, representing the main active species for AO7 degradation. The produced SO 4 −· and HO· radicals from Mn(III) and the rest of the PMS also take part in the degradation reaction, although their contribution is limited. In addition, the influence of some key factors including the water content and time on the grinding and AO7 degradation was explored. The results of this study provide deep insight into mechanochemical activation of PMS with manganese oxides for enhanced degradation of pollutants in aqueous solution.

Tuning manganese (III) species in manganese oxide octahedral molecular sieve by interaction with carbon nanofibers for enhanced pollutant degradation in the presence of peroxymonosulfate

The development of environmental-benign, efficient, and durable Mn based catalysts is of great importance for remediation of organic pollutants. In this study, a series of cryptomelane-type octahedral molecular sieve (OMS-2) and carbon nanofibers (CNFs) nanocomposites (OC) were synthesized by a facile refluxing approach under different conditions, and the catalytic activity was evaluated for Acid Orange 7 degradation under neutral conditions via peroxymonosulfate activation. It was revealed that the composites were more efficient than the pure OMS-2 and CNFs, due to the higher amounts of Mn(II) and Mn(III) species and the synergistic effects between OMS-2 and CNFs. The OC catalysts presented long-term stability without the leaching of Mn ions during seven consecutive cycles. Radical scavenger and EPR experiments indicated that the low valent Mn species in the composites were oxidized by PMS to produce sulfate radicals to degrade dyes. However, the structure and performance of OC were influenced significantly by CNFs dosage and refluxing time. Under a high CNFs ratio or a long refluxing time, OMS-2 was damaged and lost its catalytic activity completely. This study provides important implications for turning of the valence of Mn species in manganese oxides, and widens the practical applications of the manganese oxides/ carbon materials in wastewater treatment.

Room-temperature synthesis of OMS-2 hybrids as highly efficient catalysts for pollutant degradation via peroxymonosulfate activation

Due to low cost and low toxicity, manganese oxides have been extensively explored to reduce organic pollutants in wastewater via peroxymonosulfate (PMS) activation; but the development of manganese-based catalysts with facile synthesis process at low temperatures and high efficiency is still of significant practical interest. In this paper, a simple room temperature method has been successfully developed to synthesize cryptomelane-type manganese octahedral molecular sieves (OMS-2) from KMnO4 and MnSO4 in the presence of carbon nanotubes (CNTs). The redox reaction between amorphous manganese oxide and CNTs results the decrease of manganese valences and the formation of OMS-2 phase at a low temperature of 25 °C. The changes of synthesis time, temperature and CNTs dosage altered the characteristics of the prepared materials. Compared with CNTs, OMS-2 and other manganese oxides hybrids, the synthesized catalysts demonstrated a remarkable efficiency for PMS activation to degrade organic dyes and a superior reusability during ten successive cycles. Sulfate radicals were formed as the active species in the system from the oxidation of low valent Mn species by PMS. This study not only provides a simple method to synthesize OMS-2 at room temperature, but also improves the understanding of PMS activation on manganese-based catalysts for pollutants degradation.

Effective reduction of metronidazole over the cryptomelane-type manganese oxide octahedral molecular sieve (K-OMS-2) catalyst: facile synthesis, experimental design and modeling, statistical analysis, and identification of by-products.

High concentrations of antibiotic compounds within pharmaceutical wastewater have hazardous impacts toward environment and human health. Therefore, there is an immediate requirement of efficient treatment method for removal of antibiotics from aquatic environment. In the present study, the cryptomelane catalyst-type manganese oxide octahedral molecular sieve (K-OMS-2) was synthesized in the presence of benzyl alcohol as a reducing agent and cetyltrimethylammonium bromide as a structure-directing agent and then utilized to reduce the metronidazole. The central composite design method was the experimental design adopted. The FESEM analysis revealed that the K-OMS-2 surface contained many uniformly cylindrical aggregates less than about 40nm in diameter and about 80-100nm in length. Besides, a high specific surface area of 129m2/g and average pore size of 45.47nm were recorded. According to the TGA/DTA analysis, the prepared catalyst revealed high thermal stability. The maximum metronidazole degradation (95.36%) was evident at conditions of pH = 3, catalyst mass = 0.97g/L, contact time = 200min, and metronidazole concentration = 20mg/L. Metronidazole did not form a complex with nitrate, fluoride, sulfate, or hardness. These ions exerted a negligible effect on metronidazole reduction using the K-OMS-2 catalyst, except for hardness, which reduced the removal efficiency of metronidazole by 17%. The FTIR and LC-MS revealed a complex mechanism involved in the metronidazole degradation by the K-OMS-2 involving the formation of an amino group, a hydroxyelated compound via N-denitration, and hydrogenation process on the K-OMS-2 catalyst surface.

Promoting effect of unreducible metal doping on OMS-2 catalysts for gas-phase selective oxidation of ethanol

Various unreducible-metal cations can be successfully doped into the tunnel of manganese octahedral molecular sieve (OMS-2) by using “one-pot” refluxing method to tune the acid-base and redox properties. The resulting M-OMS-2 materials were used as efficient catalysts for gas-phase aerobic oxidation of ethanol to acetaldehyde. The activity of the M-OMS-2 catalysts was demonstrated to be proportional to the surface basicity. Na-OMS-2 exhibited the highest catalytic activity at 200 °C, which is higher than and comparable to the previously reported preferred Fe-OMS-2 and Ag-OMS-2 catalysts, respectively. The beneficial effect of Na+-doping is related to the enhancement of surface oxygen vacancies, basicity and reducibility, which also result in better catalytic stability. Kinetic studies indicate that reaction rates depend weakly on the concentration of O2 and ethanol. Ethanol OH and α-CH bond cleavages are both kinetically-relevant, with the latter being the more difficult step. O2 is the most efficient oxidant and is thought to be activated at oxygen vacancies of Mn2+/Mn3+ species to form superoxide-type O2− and lattice-oxygen-type O2− species, which act as dual active sites for the rate-controlling ethanol activation steps.