OMS-2 Catalyst Research Articles

The remarkable effect of alkali earth metal ion on the catalytic activity of OMS-2 for benzene oxidation

Cryptomelane-type octahedral molecular sieve (OMS-2) is one of the most promising catalysts for VOCs oxidation, and introduction of metal ions in OMS-2 tunnel is widely used for tailoring its catalytic activity. Here, different types of alkali earth metal ions with the same X/Mn atomic ratio of approximately 0.012 (X represents Mg2+, Ca2+, and Sr2+) were successfully introduced into OMS-2 tunnel by a one-step redox reaction. The catalytic test showed that introducing alkali earth metal ions into tunnels had a considerable effect on the catalytic performance of OMS-2 for benzene oxidation. The Sr2+ doped OMS-2 catalyst exhibited the better catalytic activity compared with those of Mg-OMS-2 and Ca-OMS-2 samples, and was also superior to a commercial 0.5% Pt/Al2O3 catalyst, as evidenced by its low reaction temperatures of T50 = 200 °C and T90 = 223 °C (corresponding to benzene conversions at 50% and 90%, respectively). The origin of the considerable effect of alkali earth metal doping on the catalytic activity of OMS-2 catalysts was experimentally and theoretically investigated by an 18O2 isotopic labeling experiment, CO temperature-programmed reduction, O2 temperature-programmed oxidation, and density functional theory calculations. The greatest catalytic activity of Sr-OMS-2 compared with those of Mg-OMS-2 and Ca-OMS-2 samples was attributed to its highest lattice oxygen activity as well as its largest surface area. By introducing alkali earth metal ions into the OMS-2 tunnel, we developed a low-cost and highly efficient catalyst that could be used as alternative to noble metal catalysts.

Selective synthesis of pyrimidines from cinnamyl alcohols and amidines using the heterogeneous OMS-2 catalyst

Herein, an efficient aerobic oxidative synthesis of pyrimidines was carried out using cinnamyl alcohols and amidines catalyzed by manganese oxide octahedral molecular sieve (OMS-2). The heterogeneous catalytic method features base-/additive-free conditions, wide substrate scope, use of O2 as a green oxidant, and simple operation. Moreover, the OMS-2 catalyst prepared by the conventional reflux method demonstrates catalytic selectivity, activity, and excellent recyclability.

Enhancing Water Resistance of a Mn-Based Catalyst for Low Temperature Selective Catalytic Reduction Reaction by Modifying Super Hydrophobic Layers.

OMS-2 catalysts exhibit excellent selective catalytic reduction (SCR) activity at low temperature but weak H2O resistance restricts its industrial application. To remarkably improve the water resistance of Mn-based catalysts is a key technical problem. In this work, the H2O endurance and self-cleaning properties of OMS-2 catalysts are remarkably improved by the facile process, construction of hydrophobic coating. The performance of the hydrophobic layer on the bulk OMS-2 catalyst surface could be effectively controlled by adjusting the polydimethylsiloxane (PDMS) vapor deposition temperature. It is discovered that the 200 °C catalyst obtained super hydrophobic properties and formed with a contact angle of 160.3°, which not only exhibited satisfactory NH3-SCR activity at low temperatures (140-300 °C) but also dramatically improved H2O endurance and self-cleaning performance. Moreover, the mechanism of improving H2O resistance and stability of the 200 °C catalyst was investigated in detail through a series of characterizations. Although the SCR activity of the 200 °C catalyst decreased slightly because of the combination of some active species (Oα and Mn3+) with PDMS, the H2O passivation of the active species was eliminated. The advantage of self-cleaning was confirmed by the analysis of surface species and simulation experiments, which could avoid the accumulation of intermediates on the surface and promote the stability of the OMS-2 catalyst for NH3-SCR at low temperature. This method of constructing special coating might be a huge step to remarkably improve the H2O endurance properties of the catalyst and provided a new concept for future industrial application.

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.

Elemental mercury oxidation over manganese oxide octahedral molecular sieve catalyst at low flue gas temperature

Manganese oxide octahedral molecular sieve (OMS-2) with cryptomelane structure synthesized by a solvent-free method was employed to oxidize gaseous elemental mercury (Hg0) in coal combustion flue gas for the first time. Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) measurement were employed to characterize the catalyst. The OMS-2 with large surface area and abundant pores presented excellent Hg0 oxidation performance at a wide temperature range of 100–250 °C. At the optimal operating temperature of 150 °C, 93% Hg0 oxidation efficiency was obtained under a gas hourly space velocity as high as 1,350,000 h−1. Hg0 oxidation on OMS-2 catalyst was supposed to follow the Mars-Maessen mechanism, where the gaseous Hg0 was firstly adsorbed on the OMS-2 catalyst surface to form adsorbed Hg0, and then the adsorbed Hg0 was oxidized by the lattice oxygen over catalyst surface to form mercury oxide (HgO). Both HCl and NO in the flue gas promoted Hg0 oxidation mainly due to the formation of active species like Cl∗, NO2, which reacted with Hg0 to form volatile mercury species. SO2 inhibited the Hg0 oxidation by reducing the adsorbed HgO into gaseous Hg0 or generating sulfate on the catalyst surface. Water vapor also played an inhibitive role in Hg0 oxidation. However, the SO2 and H2O resistance of OMS-2 catalyst was much superior compared to other commercial catalysts, which makes it promising for industrial application. This knowledge is beneficial for developing economical and efficient Hg0 oxidation catalysts for coal-fired power plants.

Enhanced performance of the OMS-2 catalyst by Ag loading for the oxidation of benzene, toluene, and formaldehyde

The catalytic mechanism of formaldehyde, toluene, and benzene oxidation over xAg/OMS-2. Different concentrations of precious metal load have an important impact on the physicochemical properties of the product.

Enhanced catalytic performance by oxygen vacancy and active interface originated from facile reduction of OMS-2

Manganese oxide octahedral molecular sieves (OMS-2) is a kind of excellent redox catalysts, a key challenge still remains to further increase their catalytic performance. Herein, we report a facile reduction method to enhance the catalytic activity of OMS-2 by producing more oxygen vacancy and creating an active interface between the main phase MnO2 and the minor phase Mn3O4. Compared to the pristine OMS-2, the OMS-2 catalyst reduced in a relatively short time exhibits superior performance for dimethyl ether combustion with a starting-off temperature of 163°C and a complete combustion temperature of 230°C. Such enhanced activity can be ascribed to the higher reducibility, the more oxygen vacancy, and the active heterogeneous interface. These findings open up a new way to design highly efficient catalysts, especially the metal oxides-based catalysts, for redox reactions.

UV–Vis-infrared light-driven thermocatalytic abatement of benzene on Fe doped OMS-2 nanorods enhanced by a novel photoactivation

The catalysts of Fe doped cryptomelanetype octahedral molecular sieve (OMS-2, KMn8O16) with different Fe/Mn molar ratio of 0.079, 0.097, and 0.122 were prepared by a hydrothermal redox method. The Fe doping considerably enhances the catalytic activity of OMS-2 for the catalytic abatement of benzene as one of typical VOCs. The Fe doped OMS-2 catalyst exhibits much higher catalytic activity than pure OMS-2 and Pt/Al2O3 (a commercial precious metal catalyst). The considerable promotion in the catalytic activity of OMS-2 is ascribed to the significant enhancement in the lattice oxygen activity of OMS-2 induced by the Fe doping. Remarkably, under the full solar spectrum (UV–Vis-IR) illumination from a Xe lamp, the Fe doped OMS-2 catalyst exhibits high catalytic activity and excellent durability for benzene abatement. Its very high initial rate of CO2 production (rCO2, 602.2µmolmin−1g−1) is 4 and 179 times higher than that of OMS-2 and TiO2(P25) (a benchmark photocatalyst), respectively. Even under the λ>690nm visible-infrared (Vis-IR) illumination, the Fe doped OMS-2 catalyst still demonstrates high catalytic activity with the rCO2 of 44.3µmolmin−1g−1. The high catalytic activity of the Fe doped OMS-2 catalyst under UV–Vis-IR illumination derives from the efficient solar-light-driven (SLD) thermocatalysis. The SLD thermocatalysis is ascribed to the high thermocatalytic activity of the Fe doped OMS-2 catalyst and its efficient photothermal conversion because of its strong UV–Vis-IR absorption. It is found that the SLD thermocatalytic activity of the Fe doped OMS-2 catalyst is further enhanced by a novel photoactivation effect, which is completely unlike the well-known photocatalysis on photocatalytic semiconductors (e.g. TiO2).

Graphitic carbon nitride induced activity enhancement of OMS-2 catalyst for pollutants degradation with peroxymonosulfate

Low valent manganese species and surface oxygen vacancies in OMS-2 play an important role in catalytic reactions, and it is highly desirable and challenging to develop a feasible strategy of increasing the Mn(II) and Mn(III) species concentration in the oxide. Herein, the OMS-2/g-C3N4 hybrids (OMS-2/CN) were prepared by a facile refluxing approach. It was found that the MnOx precursor from the reaction of KMnO4 and MnSO4 was transformed into OMS-2 nanofibers with the formation of more Mn(II) and Mn(III) species in OMS-2 and the destruction and oxidation of g-C3N4. The hybrids exhibited higher efficiency for pollutants degradation in the presence of PMS than the pure OMS-2 or g-C3N4. There was a linear correlation between the specific initial rate and the ratio of Mn(II+III)/Mn(IV). Mechanism investigation indicated that high active manganese species or caged radicals were produced through the oxidation of Mn(II) and Mn(III) by PMS and contributed to the degradation reaction. During five consecutive cycles, the catalyst exhibited good reusability and stability. Therefore, the OMS-2/CN hybrids are promising catalysts for wastewater treatment with PMS as the oxidant.

Tuning the Synthesis of Manganese Oxides\xa0Nanoparticles for Efficient Oxidation of Benzyl Alcohol

The liquid phase oxidation of benzyl alcohol is an important reaction for generating benzaldehyde and benzoic acid that are largely required in the perfumery and pharmaceutical industries. The current production systems suffer from either low conversion or over oxidation. From the viewpoint of economy efficiency and environmental demand, we are aiming to develop new high-performance and cost-effective catalysts based on manganese oxides that can allow the green aerobic oxidation of benzyl alcohol under mild conditions. It was found that the composition of the precursors has significant influence on the structure formation and surface property of the manganese oxide nanoparticles. In addition, the crystallinity of the resulting manganese nanoparticles was gradually improved upon increasing the calcination temperature; however, the specific surface area decreased obviously due to pore structure damage at higher calcination temperature. The sample calcined at the optimal temperature of 600 °C from the precursors without porogen was a Mn3O4-rich material with a small amount of Mn2O3, which could generate a significant amount of {mathrm{O}}_2^{-} species on the surface that contributed to the high catalytic activity in the oxidation. Adding porogen with precursors during the synthesis, the obtained catalysts were mainly Mn2O3 crystalline, which showed relatively low activity in the oxidation. All prepared samples showed high selectivity for benzaldehyde and benzoic acid. The obtained catalysts are comparable to the commercial OMS-2 catalyst. The synthesis–structure–catalysis interaction has been addressed, which will help for the design of new high-performance selective oxidation catalysts.

An ultrasonic atomization assisted synthesis of self-assembled manganese oxide octahedral molecular sieve nanostructures and their application in catalysis and water treatment.

Manganese oxides of octahedral molecular sieve (OMS-2) type have important applications in oxidation catalysis, adsorption, and as battery materials. The synthesis methods employed determine their morphology and textural properties which markedly affect their catalytic activity. In this work, a room temperature ultrasonic atomization assisted synthesis of OMS-2 type materials is demonstrated. This synthesis differs from previously reported methods in that it is a simple, no-heat application that leads to a striking morphological characteristic of uniformly sized OMS-2 fibers and their self-assembly into dense as well as hollow spheres. Control of various parameters in the ultrasonic atomization assisted synthesis led to OMS-2 with high surface areas (between 136-160 m2 g-1) and mesoporosity. Catalytically these materials have higher activities in the oxidation of hydroxymethylfurfural (HMF), a bio-based chemical, (65% conversion of HMF vs. 14% with conventional OMS-2 catalyst) and a higher adsorption of lead from aqueous solutions (70% vs. 12% in conventional OMS-2 materials).

Extremely efficient full solar spectrum light driven thermocatalytic activity for the oxidation of VOCs on OMS-2 nanorod catalyst

The cryptomelane-type octahedral molecular sieve (OMS-2) nanorod catalysts with different concentration of oxygen vacancy exhibit strong absorption across the full solar spectra. The OMS-2 catalysts can efficiently transform the absorbed solar energy to thermal energy, resulting in a considerable increase of temperature up to 220°C. By combining the strong absorption across the full solar spectrum and the highly efficient thermocatalytic activity of the OMS-2 catalyst with high concentration of oxygen vacancies, full solar spectrum, visible-infrared, and infrared light driven thermocatalysis with extremely high efficiency are achieved. Under the irradiation of full solar spectrum, visible-infrared, and infrared light, the OMS-2 catalyst exhibits extremely high catalytic activity and excellent durability for the oxidation of organic pollutants such as benzene, toluene, and acetone. Under the full solar spectrum irradiation, the initial CO2 formation rate of the OMS-2 catalyst for benzene oxidation is 22.1 times larger than that of Bi2WO6/TiO2, a near-infrared photocatalyst reported recently. After the OMS-2 catalyst was recycled for 40 times, its catalytic activity remains unchanged. A novel mechanism of solar light driven thermocatalysis is proposed, and the reason why the OMS-2 catalyst with high concentration of oxygen vacancies exhibits the extremely high catalytic activity is discussed.

The role of lattice oxygen on the activity and selectivity of the OMS-2 catalyst for the total oxidation of toluene

The role of lattice oxygen on the activity and selectivity of the OMS-2 catalyst synthesized by the hydrothermal method was investigated for the catalytic oxidation of toluene. The OMS-2 catalyst exhibited excellent activity for the catalytic oxidation of toluene, but the by-product benzaldehyde was detected at low temperatures. A DRIFT study showed that the lattice oxygen was both the adsorption site and active catalytic site. The nature of the replenished surface lattice oxygen was found to influence the selectivity, and toluene that was adsorbed on the lattice oxygen replenished by gaseous oxygen could be oxidized easily into benzaldehyde. A possible reaction mechanism has been proposed that could elucidate the toluene oxidation activity over OMS-2 involving two reaction routes, either with or without gaseous oxygen.

Full solar spectrum light driven thermocatalysis with extremely high efficiency on nanostructured Ce ion substituted OMS-2 catalyst for VOCs purification.

The nanostructured Ce ion substituted cryptomelane-type octahedral molecular sieve (OMS-2) catalyst exhibits strong absorption in the entire solar spectrum region. The Ce ion substituted OMS-2 catalyst can efficiently transform the absorbed solar energy to thermal energy, resulting in a considerable increase of temperature. By combining the efficient photothermal conversion and thermocatalytic activity of the Ce ion substituted OMS-2 catalyst, we carried out full solar spectrum, visible-infrared, and infrared light driven catalysis with extremely high efficiency. Under the irradiation of full solar spectrum, visible-infrared, and infrared light, the Ce ion substituted OMS-2 catalyst exhibits extremely high catalytic activity and excellent durability for the oxidation of volatile organic pollutants such as benzene, toluene, and acetone. Based on the experimental evidence, we propose a novel mechanism of solar light driven thermocatalysis for the Ce ion substituted OMS-2 catalyst. The reason why the Ce ion substituted OMS-2 catalyst exhibits much higher catalytic activity than pure OMS-2 and CeO2/OMS-2 nano composite under the full solar spectrum irradiation is discussed.

The facile synthesis of a magnetic OMS-2 catalyst for decomposition of organic dyes in aqueous solution with peroxymonosulfate

A magnetically recyclable nanocomposite, manganese oxide octahedral molecular sieves (OMS-2) coated Fe3O4 nanoparticle, was prepared by a facile solvent-free process, and used for peroxymonosulfate activation and pollutants degradation.

Transition metal doped cryptomelane-type manganese oxide for low-temperature catalytic combustion of dimethyl ether

The catalysts of transition metals (Fe, Co, Ni, Cu, Cr) doped cryptomelane-type manganese oxide (M-OMS-2) were prepared and investigated for the catalytic combustion of dimethyl ether (DME). The catalysts were characterized by XRD, FT-IR, BET, TEM, ICP-AES, XPS and H2-TPR techniques. The effects of surface area, the average oxidation state, and the redox properties on catalytic activities were studied. The results showed that 100% CO2 selectivity can be realized over the doped OMS-2 catalysts. The M-OMS-2 catalysts displayed the nanorod morphology with a 13nm diameter and the length ranging from 50 to 220nm. The Cu doped OMS-2 catalyst exhibited the best activity, achieving the light-off temperature (the temperature acquired for 10% DME conversion, T10) and full-conversion temperature (the temperature acquired for 90% DME conversion, T90) of 171 and 180°C, respectively. DME catalytic combustion over M- OMS-2 catalysts was found to follow a Mars–van-Krevelen mechanism with the presence of redox between Mn4+/Mn3+ and Mm+/M(m−1)+ (M=Fe, Co, Ni, Cu, Cr).

Green oxidative synthesis of primary amides from primary alcohols or aldehydes catalyzed by a cryptomelane-type manganese oxide-based octahedral molecular sieve, OMS-2

In this study, a new green synthetic route to primary amides, that is, aerobic oxidative amidation of primary alcohols or aldehydes with ammonia, has been developed. In the presence of a cryptomelane-type manganese oxide-based octahedral molecular sieve (OMS-2), various kinds of structurally diverse primary alcohols or aldehydes including aromatic, olefinic, heteroaromatic, and aliphatic ones can be converted into the corresponding primary amides in moderate to high yields (20 examples from primary alcohols and 11 examples from aldehydes). Furthermore, gram-scale amidation is also effective, and the analytically pure primary amides can easily be isolated. The present catalysis by OMS-2 is truly heterogeneous in nature, and the retrieved OMS-2 catalyst can be reused several times (at least 12 times for the amidation of 2-pyridinemethanol). Though the formation rates of the corresponding primary amide are gradually decreased by repeating reuse experiments, OMS-2 can be regenerated by calcination. The present OMS-2-catalyzed amidation of primary alcohols is composed of four relay steps: (i) oxidative dehydrogenation of primary alcohols, (ii) dehydrative condensation of aldehydes with ammonia, (iii) oxidative dehydrogenation of aldimines, and (iv) hydration of nitriles to form the corresponding primary amides. All steps (i)–(iv) can be promoted by the presence of OMS-2.

Evaluation of metal-doped manganese oxide octahedral molecular sieves in catalytic reduction of NO x from cigarette mainstream smoke

A series of Ag, Cu and Co-doped manganese oxide octahedral molecular sieves (OMS-2) were synthesized and evaluated to remove nitrogen oxides (NOx) from cigarette mainstream smoke. The three kinds of catalysts were added to cigarettes for studying the capabilities of reducing NOx from cigarette mainstream smoke. The catalysis and reduction of NO in laboratory were studied. A mechanism for NOx catalytic reduction from burning cigarettes with the catalysts adding to cigarettes was described. The catalysts show excellent catalytic activity for NOx removal, especially the Ag-doped OMS-2 catalyst. 0.5% (mass fraction) Ag-doped OMS-2 catalyst has the best ability to remove NOx from cigarette mainstream smoke. The use of Ag-doped OMS-2 as catalyst for removing carcinogenic compounds from cigarette smoke will be an effective strategy to protect the environment and public health.

H 2 production through the water-gas shift reaction: An in situ time-resolved X-ray diffraction investigation of manganese OMS-2 catalyst

Manganese oxide octahedral molecular sieve (OMS-2) catalyst prepared by the reflux method was investigated for hydrogen generation via the water-gas shift reaction. Catalysts were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), The Brunauer–Emmett–Teller (BET) surface area and determination of average oxidation state (AOS). The OMS-2 catalyst showed very good catalytic activity for the water-gas shift reaction to generate hydrogen under laboratory conditions. An in situ study was conducted to monitor the structural changes in the catalyst during the water-gas shift reaction using synchrotron radiation-based time-resolved X-ray diffraction (TR-XRD). During the water-gas shift reaction, the mixed valent OMS-2 catalyst undergoes a structural transformation to form Mn 2O 3 and finally to form MnO. The study showed that OMS-2 catalysts can be used as inexpensive catalysts for hydrogen generation.

Role of Lattice Oxygen and Lewis Acid on Ethanol Oxidation over OMS-2 Catalyst

Ethanol-gasoline fuel vehicles result in a statistically significant increase in emissions of ethanol, although they are environmentally friendly due to their lower levels of carbon oxide and hydrocarbon emissions. This work focused on catalytic oxidation of ethanol on octahedral molecular sieve (OMS-2) catalyst, which was synthesized using refluxing method. The catalyst demonstrated great reactivity with acetaldehyde as the major intermediate. Small amounts of formaldehyde were detected while little acetic acid was observed in the presence of oxygen. Results suggested that ethanol adsorption occurred through the breakage of O−H bond forming ethoxide species on Lewis acid sites of the catalyst and OMS-2 could partially oxidize adsorbed ethanol to acetaldehyde and formaldehyde from the consumption of catalyst lattice oxygen in the absence of oxygen flow. Acetaldehyde and formaldehyde formed were directly oxidized to carbon dioxide in the presence of an oxygen flow. The major path of ethanol complete oxidation on OMS-2 catalyst appeared to be the direct oxidation of acetaldehyde to carbon dioxide.