Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene

Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C₂ products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C₂ products for all the MnO₂-based catalysts and for lithium-doped MgO and Sm₂O₃ catalysts. The effect is greatest in MnO₂-based catalysts. When dichloromethane is added to a pure, unpromoted MnO₂ catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C₂H₄ increases from virtually zero to 56.3%. The highest C₂ selectivity observed after promotion of pure MnO₂ by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.     

The effect of alkali metal salts promoted MgO catalysts for the oxidative coupling of methane

Catalytic activities of the alkali metal salts are discussed based on experimental observations in a fixed bed flow reactor at atmospheric condition and instrumental analysis. LiCl (30 wt%) and NaCl (30 wt%) promoted MgO catalyst showed superior activity to mono alkali metal salts promoted MgO catalysts based on the C₂ yield. This suggests that the bialkali metal salts neutralize the nonselective acid sites due to synergistic effect. Moreover, it is estimated that the active sites is O^- ions.     

Oxidative coupling of methane over alkali metal oxide promoted La2O3/BaCO3 catalysts

The oxidative coupling of methane (OCM) over various alkali metal oxide promoted La₂O₃/BaCO₃ catalysts and the effects of Na₂O content on the performance of Na₂O–La₂O₃/BaCO₃ catalysts have been studied. It was found that Na₂O promoted La₂O₃/BaCO₃ catalysts had the advantages of high CH₄ conversion, C₂ selectivity and C₂H₄/C₂H₆ ratio. Na₂O might affect the properties of the catalysts through electronic and geometric effects. The highest C₂ yield (19·0–20·6%) was obtained with Na₂O–9 wt% La₂O₃/BaCO₃ catalysts of 1·0–3·0% Na₂O. The effects of reaction conditions on OCM over 3 wt% Na₂O–9 wt% La₂O₃/BaCO₃ catalysts have also been investigated. The catalysts were characterized by BET, TPD and XRD. TPD studies on 3 wt% Na₂O–9 wt% La₂O₃/BaCO₃ catalysts demonstrated that CO₂, CH₄ and O₂ could be adsorbed strongly on the catalyst. This might be related to the activation of CH₄ and the formation and regeneration of active oxygen species.     

Novel Colloidal Nanothermite Particles (MnO<Subscript>2</Subscript>/Al) for Advanced Highly Energetic Systems

Nanothermites (metal oxide/metal) are tremendously exothermic and run with self sustaining oxygen content. Manganese oxide is one of the most effective oxidizers for nanothermite applications. This paper reports on the sustainable fabrication of different nanoscopic forms of colloidal manganese oxides including: MnO₂ nanoparticles of 20 nm average particle size and Mn₂O₃ nanorods of 50 nm diameter and 1 µm length. TEM micrographs demonstrated mono-dispersed particles and rods. XRD diffractograms revealed highly crystalline materials. MnO₂ nanoparticles (oxygen content 37 wt%) can offer high oxidizing ability compared with Mn₂O₃ nanorods (oxygen content 30 wt%). The integration of colloidal particles into energetic matrix can offer enhanced dispersion characteristics; consequently stoichiometric binary mixture of MnO₂ and Al nanoparticles were re-dispersed in organic solvent. The integration of developed colloidal nanothermite particles into tri-nitro toluene offered enhanced shock wave strength by 35% using ballistic mortar test. Thanks to nanotechnology which offered sustainable manufacture and subsequent integration of one of the most effective nanothermite particles into highly energetic system.     

Structure/activity correlation for unpromoted and CeO2‐promoted MnO2/SiO2 catalysts

The goal of this study was to understand the structure–activity relationship for unpromoted and ceria‐promoted MnO_x/SiO₂ catalysts used in CO oxidation. SiO₂ and CeO₂‐promoted SiO₂ (20% CeO₂) were used as supports to prepare MnO_x/SiO₂ catalysts with various manganese (Mn) loadings. X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) data indicated a higher Mn dispersion on ceria‐promoted than on unpromoted MnO_x/SiO₂ catalysts. Analysis of the XRD patterns and Mn_2p XPS spectra indicated that Mn was present as MnO₂ on MnO_x/SiO₂ with low Mn loadings and ceria‐promoted MnO_x/SiO₂ catalysts and as mixed MnO₂/Mn₂O₃ on MnO_x/SiO₂ catalysts with high Mn loadings. Kinetic data obtained for CO oxidation on unpromoted and ceria‐promoted MnO_x/SiO₂ catalysts are presented and interpreted in correlation with the catalyst surface and bulk structure. A synergistic catalytic effect was observed in the case of the ceria‐promoted MnO_x/SiO₂ catalysts. Post‐reaction XRD and XPS analysis of catalysts indicated that the presence of ceria precludes formation of the less catalytically active Mn₃O₄ species from MnO₂ deposited initially on the SiO₂ support. This revised version was published online in July 2006 with corrections to the Cover Date.     

MnOx/CeO2 catalysts for the low-temperature selective catalytic reduction of NO with NH3

CeO₂–nanorod support was synthesized by hydrothermal method and different manganese oxides (MnO, MnO_2, and Mn₂O₃) were impregnated over support by the wet-impregnation forming MnO/CeO₂-NR, MnO₂/CeO₂-NRm and Mn₂O₃/CeO₂-NR. The physico-chemical properties of the as-prepared catalysts were analyzed using x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscope–energy-dispersive x-ray spectroscopy (SEM–EDX), hydrogen-temperature-programmed reduction (H₂-TPR), and Raman spectroscopy. These catalysts were further analyzed for NO reduction using NH₃ as a reducing gas in the temperature range of 50 to 450°C. The results confirmed that MnO₂/CeO₂-NR gave the maximum NO conversion (65%) and N₂ selectivity (89%) among all catalysts. Further, MnO₂/CeO₂-NR catalyst was studied for the effect of MnO₂ loading and more than 90% NO conversion and N₂ selectivity were obtained in the temperature range of 250 to 300°C.     

Effects of precursors on the surface Mn species and the activities for NO reduction over MnOx/TiO2 catalysts

TiO₂-supported manganese oxide catalysts were prepared from two different precursors, manganese nitrate (MN) and manganese acetate (MA), and these samples were characterized by BET, XRD, TG/DTA, XPS and FT–IR. The characterization results showed that the MN precursor resulted primarily in MnO₂, accompanied with some Mn-nitrate, while the MA precursor caused mainly Mn₂O₃ species. These two different precursors also led to different surface Mn atom concentrations indicated by XPS and NH₃ adsorption. Consequently, the higher low-temperature activity of MnO_x/TiO₂ from MA precursor was attributed to higher surface Mn concentration and the surface Mn₂O₃ species.     

Partial oxidation of ethane to syngas over nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide

The catalytic activity, thermal stability and carbon deposition of various modified NiO/γ‐Al₂O₃ and unmodified NiO/γ‐Al₂O₃ catalysts were investigated with a flow reactor, XRD, TG and UVRRS analysis. The activity and selectivity of the NiO/γ‐Al₂O₃ catalyst showed little difference from those of the modified nickel‐based catalysts. However, modification with alkali metal oxide (Li, Na, K) and rare earth metal oxide (La, Ce, Y, Sm) can improve the thermal stability of the NiO/γ‐Al₂O₃ and enhance its ability to suppress carbon deposition during the partial oxidation of ethane (POE). The carbon deposition contains graphite‐like species that were detected by UVRRS. The nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide have excellent catalytic activities (C₂H₆ conversion of ~100%, CO selectivity of ~94%, 7 × 10⁴ l/(kg h), 1123 K), good thermal stability and carbon‐deposition resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low temperature water–gas shift: Differences in oxidation states observed with partially reduced Pt/MnOX and Pt/CeOX catalysts yield differences in OH group reactivity

The Pt–ceria synergy may be described as the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO), ceria, by Pt across the interface, with H₂O participating in the transition state. However, due to the rising costs of rare earth oxides like ceria, replacement by a less expensive partially reducible oxide, like manganese oxide, is desirable. In this contribution, a comparison between Pt/ceria and Pt/manganese oxide catalysts possessing comparable Pt dispersions reveals that there are significant differences and certain similarities in the nature of the two Pt/PRO catalysts. With ceria, partial reduction involves reduction of the oxide surface shell, with Ce³⁺ at the surface and Ce⁴⁺ in the bulk. In the case of manganese oxide, partial reduction results in a mixture of Mn³⁺ and Mn²⁺, with Mn²⁺ located at the surface. With Pt/CeO_X, a high density of defect-associated bridging OH groups react with CO to yield a high density of the formate intermediate. With Pt/MnO_X, the fraction of reactive OH groups is low and much lower formate band intensities result upon CO adsorption; moreover, there is a greater fraction of OH groups that are essentially unreactive. Thus, much lower CO conversion rates are observed with Pt/MnO_X during low temperature water–gas shift. As with ceria, increasing the Pt loading facilitates partial reduction of MnO_X to lower temperature, indicating metal–oxide interactions should be taken into account.     

Role of potassium in the aerobic oxidation of aromatic alcohols over K+-promoted Mn/C catalysts

The aerobic oxidation of aromatic alcohol on alkali metal promoted Mn/C catalysts has been investigated. A dramatic improvement of the oxidation activity can be observed when the Mn/C catalyst is modified with potassium ions. Characterizations of Raman and X-ray absorption fine structure (XAFS) evidence that potassium ions induce a large local distortion to Mn–O octahedrons in supported manganese oxides with coexistence of Mn²⁺ and Mn³⁺, which has been suggested to be a vital factor to enhance the activation of O₂ for the oxidation of benzylic alcohol.     

Microwave-accelerated solvent-free aerobic oxidation of benzyl alcohol over efficient and reusable manganese oxides

A new facile and cost-effective process involving the solvent-free oxidation of benzyl alcohol using molecular oxygen as oxidant under controlled microwave irradiation has been developed for the production of chlorine-free benzaldehyde. Influence of different catalyst parameters (different manganese oxides and other kinds of transition metal oxides) and reaction conditions (reaction period and temperature) on the process performance has been studied. Under optimized reaction conditions, the MnO₂ catalyst showed a superior catalytic performance in the highly selective oxidation of benzyl alcohol as compared to other manganese oxide materials such as MnO, Mn₂O₃ and Mn₃O₄. Moreover, a very stable catalytic activity as a function of cycling test was observed for the MnO₂ catalyst.     

Activities of γ-Al2O3-Supported Metal Oxide Catalysts in Propane Oxidative Dehydrogenation

The activities of metal oxide catalysts in propane oxidative dehydrogenation to propene have been studied. The catalysts are M/-Al₂O₃ (where M is an oxide of Cr, Mn, Zr, Ni, Ba, Y, Dy, Tb, Yb, Ce, Tm, Ho or Pr). Both transition metal oxides (TMO) and rare-earth metal oxides (REO) are found to catalyze the reaction at 350-450 °C, 1 atm and a feed rate of 75 cm³/min of a mixture of C₃H₈, O₂ and He in a molar ratio of 4:1:10. Among the catalysts, Cr-Al-O is found to exhibit the best performance. The selectivity to propene is 41.1% at 350 °C while it is 54.1% at 450 °C. Dy-Al-O has the highest C₃H₆ selectivity among the REO. At 450 °C, the other catalysts show C₃H₆ selectivity ranging from 16.2 to 37.7%. In general TMO show higher C₃H₆ selectivity than REO, which, however, show higher C₂H₄ selectivity. An attempt is made to correlate propane conversion and selectivity to C₃H₆ with metal-oxygen bond strength in the catalysts. For the TMO a linear correlation is found between the standard aqueous reduction potential of the metal cation of the respective catalyst and its selectivity to propane at 11% conversion. No such correlation has been found in the case of REO. Analyses of the product distributions suggest that for TMO propane activation the redox mechanism seems to prevail while the REO activate it by adsorbed oxygen.     

Effect of manganese on Cu/SiO2 catalysts for the dehydrogenation of cyclohexanol to cyclohexanone

The effect of the addition of manganese to Cu/SiO₂ catalysts for cyclohexanol dehydrogenation reaction was investigated. At reaction temperature of 250 °C, the conversion and the selectivity to cyclohexanone were both increased with the addition of manganese to Cu/SiO₂ catalyst. However, as the reaction temperature was further increased, higher loading of manganese in Cu/SiO₂ catalyst led to a decrease in the conversion of cyclohexanol. Manganese in Cu/ SiO₂ catalyst decreased the reduction temperature of copper oxide, increased the dispersion of copper metal, and decreased the selectivity to cyclohexene. It was found that the dehydration of cyclohexanol to cyclohexene occurred on the intermediate acid sites of catalyst. At high Mn loading, catalyst surface was more enriched with manganese in used catalyst compared to that in freshly calcined or reduced catalyst, which may account for the sharp decrease of the conversion at high temperature of 390 °C. Upon reduction, copper manganate on silica was decomposed into fine particles of copper metal and manganese oxide (Mn₃O₄).     

Influence of manganese oxide on the liquid-perovskite equilibrium in the CaO–SiO2–TiO2 system at 1400 °C in air

In the Selective Crystallization and Phase Separation (SCPS) process, manganese oxide is used as an additive to promote the precipitation of perovskite. However, the influence of manganese oxide on the liquid domain of the perovskite primary phase field is still unclear. In the present work, the liquid-perovskite equilibrium with the addition of 0–15 wt% Mn₃O₄ was experimentally determined using a high-temperature isothermal equilibration-quenching technique, combined with X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS). It was confirmed that manganese was mainly existed as Mn²⁺ and Mn³⁺ in the molten phase, whereas titanium existed as Ti⁴⁺. Within the composition range of the present study, the 1400 °C liquid compositions varying from 0 wt% to 15 wt% Mn₃O₄ overlapped significantly, mainly located at w (CaO)/w (SiO₂) ratios between 0.9 and 1.1. The isotherms simulated by FactSage, as well as the data from the literature, generally agreed well with the present experimental results. The calculated 1400 °C isotherms at different Mn₃O₄ levels indicated that perovskite precipitation by manganese oxide was mainly promoted by increasing the Mn₃O₄ concentration to expand the primary phase field of perovskite toward both higher and lower TiO₂ content areas.     

Comparison of Alkali Metal Promoted MgO Catalysts for Their Surface Acidity/Basicity and Catalytic Activity/Selectivity in the Oxidative Coupling of Methane

Alkali metal (viz. Li, Na, K, Rb and Cs) promoted MgO catalysts (with an alkali metal/Mg ratio of 0·1) calcined at 750°C have been compared for their surface properties (viz. surface area, morphology, acidity and acid strength distribution, basicity and base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) to C₂-hydrocarbons at different temperatures (700–750°C), CH₄/O₂ ratios (4·0 and 8·0) in feed, and space velocities (10320 cm³ g⁻¹ h⁻¹). The surface and catalytic properties of alkali metal promoted MgO catalysts are found to be strongly influenced by the alkali metal promoter and the calcination temperature of the catalysts. A close relationship between the surface density of strong basic sites and the rate of C₂-hydrocarbons formation per unit surface area of the catalysts has been observed. Among the catalysts calcined at 750°C, the best performance in the OCM is shown by Li–MgO (at 750°C). © 1997 SCI.     

Influence of La2O3 promoter on the structure of MnOx/SiO2 catalysts

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses have been used to characterize the structure of a La₂O₃-promoted MnO_x/SiO₂ catalyst, before and after its utilization in the oxidative dehydrogenation of ethylbenzene (EB). MnO_x/SiO₂ and MnO_x/La₂O₃/SiO₂ catalysts were prepared by pore volume impregnation, using aqueous solutions of (i) La³⁺-nitrate at an atomic ratio of La/Si = 0.08, and (ii) Mn²⁺-nitrate at an atomic ratio of Mn/Si = 0.10, followed by drying and calcination at 500°C in air. XRD data show no diffraction patterns specific to MnO_x on the La₂O₃-promoted MnO_x/SiO₂ catalyst, after calcination. Thus, the presence of La₂O₃ apparently favors the dispersion of manganese oxides during calcination, presumably by forming mixed Mn-La oxides. On the fresh promoted and unpromoted catalysts, after calcination, XRD and XPS analyses indicated that Mn was present mostly as MnO₂ and Mn₂O₃. In the used catalyst, Mn from the unpromoted catalyst degenerated from Mn⁴⁺ to Mn²⁺, resulting in formation of Mn₃O₄ species, whereas in the case of La₂O₃-promoted catalyst Mn remained well dispersed as MnO₂ and Mn₂O₃. It appears that La₂O₃ precludes the formation of Mn₃O₄ during the EB dehydrogenation, conserving Mn structure and oxidation state. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic reduction of NO by hydrocarbons over a mechanical mixture of spinel Ni–Ga oxide and manganese oxide

NO reduction with propylene over Mn₂O₃, spinel Ni–Ga oxide and their mechanical mixtures has been investigated. Mn₂O₃ has no activity to NO reduction, but has a high activity for NO oxidation to NO₂. Spinel Ni–Ga oxide showed an apparent activity to NO reduction only at temperatures above 400°C. Mixing of Mn₂O₃ to the Ni–Ga oxide resulted in a significant enhancement of NO reduction in the temperature range of 250–450°C. The optimal Mn₂O₃ content in the mixture catalyst was about 10–20 wt%. It is suggested that the synergetic effect of Mn₂O₃ and Ni–Ga oxide plays an important role in the catalysis of NO reduction. The Ni–Ga oxide and Mn₂O₃ mixture catalyst is superior to Pt/Al₂O₃ and Cu-ZSM-5 by showing a higher NO reduction conversion, resistance to water and negligible harmful by-product formation. Other lower hydrocarbons C₂H₄, C₂H₆ and C₃H₈ also give a maximum NO reduction conversion as high as 50%. The difference from using C₃H₆ is that the temperature at the maximum NO reduction is higher than it is with C₃H₆. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and low temperature denitrification properties of ferric oxide red supported manganese oxide and manganese cerium co-oxide catalyst

Iron oxide red (IOR) is a kind of byproduct from spray pyrolysis treatment process of pickling waste liquid. In this project, manganese oxide and manganese cerium co-oxide were loaded onto the surface of IORto prepare IOR supported manganese oxide (Mn_x/IOR) and IOR supported manganese-cerium co-oxide (Mnx-Cey/IOR) catalysts by a simple combustion method. Low temperature NH3-selective catalytic reduction (NH3-SCR) denitrification properties of the synthetic catalysts were studied. XRD, SEM, EDS, TEM, BET, XPS, H₂-TPR and NH₃-TPD were used to characterize the physiochemical properties of the catalysts. The results show that during the combustion process for preparing Mnx/IOR catalyst, MnO_x, almost all in amorphous phase, can be well distributed on the surface of the IOR carrier. The NO_x conversions of Mn_x/IOR catalyst gradually increase with the increase of MnO_x loading amount within a specific working temperature range (<200 °C). When MnO_x loading amount x is 0.43, the NO_x conversion is up to 85% at most. The low temperature NH₃-SCR activity of Mn_x/IOR catalyst can be enhanced remarkably by introducing cerium. The NO_x conversions of Mn_0.43-Ce_y/IOR catalyst at the operating temperature range (<200 °C) increase steadily with the cerium adding amount increasing. All the Mn_0.43-Ce_y/IOR catalysts show excellent denitrification efficiency (more than 90%) at about 160 °C. Compared with Mn_0.43/IOR catalyst, Mn_0.43-Ce_0.1/IOR shows excellent denitrification performance and application prospect because of its higher manganese valence state, enhanced redox capacity, larger specific surface area, and significantly improved surface acidity and anti-SO₂ poisoning ability.     

Synthesis of Biodiesel from Canola Oil Using Heterogeneous Base Catalyst

A series of alkali metal (Li, Na, K) promoted alkali earth oxides (CaO, BaO, MgO), as well as K₂CO₃ supported on alumina (Al₂O₃), were prepared and used as catalysts for transesterification of canola oil with methanol. Four catalysts such as K₂CO₃/Al₂O₃ and alkali metal (Li, Na, K) promoted BaO were effective for transesterification with >85 wt% of methyl esters. ICP-MS analysis revealed that leaching of barium in ester phase was too high (~1,000 ppm) when BaO based catalysts were used. As barium is highly toxic, these catalysts were not used further for transesterification of canola oil. Optimization of reaction conditions such as molar ratio of alcohol to oil (6:1–12:1), reaction temperature (40–60 °C) and catalyst loading (1–3 wt%) was performed for most efficient and environmentally friendly K₂CO₃/Al₂O₃ catalyst to maximize ester yield using response surface methodology (RSM). The RSM suggested that a molar ratio of alcohol to oil 11.48:1, a reaction temperature of 60 °C, and catalyst loading 3.16 wt% were optimum for the production of ester from canola oil. The predicted value of ester yield was 96.3 wt% in 2 h, which was in agreement with the experimental results within 1.28%.     

Gas phase hydrogenation of ortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) over unpromoted and alkali metal promoted-alumina supported palladium catalysts

A series of alkali metals (Li, Na, K and Cs) promoted alumina-supported palladium catalysts were prepared by a wet impregnation method and characterized by X-ray diffraction (XRD) and CO chemisorption measurements. The samples were tested for the gas phase hydrogenation of ortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) in a fixed-bed micro reactor at 250 °C under normal atmospheric pressure. The promoted-Pd/Al₂O₃ catalysts show higher conversion for O-CNB and the hydrogenation activity of O-CNB per site decreases with the increasing ionic radius of the alkali metal promoter ions. However, the selectivity for O-CAN remains more or less the same in both unpromoted and promoted catalysts and also irrespective of the nature of the alkali metal promoter ions used for promotion of alumina support. Despite, similar activity and selectivity observed between Li- and Na-promoted Pd/Al₂O₃ catalysts, the Na-promoted showed higher resistance for coke formation than a Li-promoted catalyst. The increase in the intrinsic activity of palladium site on alkali promotion has been attributed to the increase in hydrogenation activity over promoted catalysts.