The oxidative coupling of methane by sodium promoted lanthanide catalysts

The characteristics of La₂O₃ catalysts with three different supports (BaCO₃, MgO, and ZnO), two different alkaline metal promoters (Na₂O and Na₂CO₃), and a china clay have been investigated. The relationship of the surface basicity of the support to the activity and selectivity of the catalyst were studied by TPD. The results showed that the surface basicity of these three supports decreased in the following order: BaCO₃ > MgO > ZnO. The choice of Na₂O as a promoter in La₂O₃/(BaCO₃ or MgO) had a better effect than the choice of Na₂CO₃. The Na₂O promoter could increase not only the catalytic activity and the C₂ selectivity, but also lower the optimal reaction temperature. Addition of 15% clay into the La₂O₃–alkaline metal catalysts could increase the strength of the catalyst and lower the optimum reaction temperature.     

Surface Properties of CaO (or BaO)–La2O3–MgO Catalysts and Their Performance in Oxidative Coupling of Methane

CaO–La₂O₃–MgO and BaO–La₂O₃–MgO catalysts with different compositions have been studied for their bulk and surface properties (viz. crystal phases, surface area, acidity/acid strength distribution, basicity/base strength distribution, etc.) and catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different processing conditions (reaction temperature, 700–850°C; CH₄/O₂ ratio in feed, 3·0, 4·0 and 8·0 and GHSV, 102000 and 204000 cm³ g⁻¹ h⁻¹). The surface acidity and strong basicity of La₂O₃–MgO are found to be increased due to the addition of a third component (CaO or BaO), depending upon its concentration in the catalyst. The addition of CaO or BaO to La₂O₃–MgO OCM catalyst causes a significant improvement in its performance. Both the CaO- and BaO-containing catalysts show a high activity and selectivity at 800°C, whereas, the activity and selectivity of BaO-containing catalysts at 700°C is lower than that of CaO-containing catalysts. © 1997 SCI.     

Oxidative dehydrogenation of ethane over Co–BaCO3 catalysts using CO2 as oxidant: effects of Co promoter

Co–BaCO₃ catalysts exhibited high catalytic performance for oxidative dehydrogenation of ethane (ODE) using CO₂ as oxidant. The maximal formation rate of C₂H₄ was 0.264 mmol · min⁻¹ · (g · cat.)⁻¹ (48.0% C₂H₆ conversion, 92.2% C₂H₄ selectivity, 44.3% C₂H₄ yield) on 7 wt% Co–BaCO₃ catalyst at 650 °C and 6000 ml. (g · cat.)⁻¹. h⁻¹. Co–BaCO₃ catalysts were comparatively characterized by XRF, N₂ isotherm adsorption-desorption, XRD, H₂-TPR and LRs. It was found that Co⁴⁺–O species were active sites on these catalysts in ODE with CO₂. The redox cycle of Co–O species played an important role on the catalytic performance of Co–BaCO₃ catalysts. On the other hand, the co-operation of BaCO₃ and BaCoO₃ was considered to be one of possible reasons for the high catalytic activity of these catalysts.     

Oxidative coupling of methane and oxidative dehydrogenation of ethane over strontium-promoted rare earth oxide catalysts

Sr-promoted rare earth (viz. La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxide catalysts (Sr/rare earth ratio = 0·1) are compared for their performance in the oxidative coupling of methane (OCM) to C₂ hydrocarbons and oxidative dehydrogenation of ethane (ODE) to ethylene at different temperatures (700 and 800°C) and CH₄ (or C₂H₆)/O₂ ratios (4–8), at low contact time (space velocity = 102000 cm³ g⁻¹ h⁻¹). For the OCM process, the Sr–La₂O₃ catalyst shows the best performance. The Sr-promoted Nd₂O₃, Sm₂O₃, Eu₂O₃ and Er₂O₃ catalysts also show good methane conversion and selectivity for C₂ hydrocarbons but the Sr–CeO₂ and Sr–Dy₂O₃ catalysts show very poor performance. However, for the ODE process, the best performance is shown by the Sr–Nd₂O₃ catalyst. The other catalysts also show good ethane conversion and selectivity for ethylene; their performance is comparable at higher temperatures (≥800°C), but at lower temperature (700°C) the Sr–CeO₂ and Sr–Pr₆O₁₁ catalysts show poor selectivity. © 1998 SCI.     

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.     

Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on ZrO<Subscript>2</Subscript> supported Ni catalysts for autothermal reforming of CH<Subscript>4</Subscript>

The effect of La₂O₃ content in Ni-La-Zr catalyst was investigated for the autothermal reforming (ATR) of CH₄. The catalysts were prepared by the coprecipitation method and had a mesoporous structure. Temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) indicated that a strong interaction developed between Ni species and the support with the addition of La₂O₃. Thermogravimetric analysis (TGA) and H₂-pulse chemisorption showed that the addition of La₂O₃ led to well dispersed NiO molecules on the support. Ni-La-Zr catalysts gave much higher CH₄ conversion than Ni-Zr catalyst. The Ni-La-Zr containing 3.2 wt% La₂O₃ showed the highest activity. The optimum conditions for maximal CH₄ conversion and H₂ yield were H₂O/CH₄=1.00, O₂/CH₄=0.75. Under these conditions, CH₄ conversion of 83% was achieved at 700 °C. In excess O₂ (O₂/CH₄>0.88), the catalytic activity was decreased due to sintering of the catalyst.     

Effect of La2O3 promoter on NiO/Al2O3 catalyst in CO methanation

A series of NiO/Al2O3 catalysts promoted by different La₂O₃ contents were prepared by impregnation method. The physicochemical properties of NiO-La₂O₃/Al₂O₃ were characterized by N₂ adsorption-desorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR) and H₂ chemisorption. The effect of La₂O₃ on the activity of NiO/Al₂O₃ for CO methanation was investigated in a fixed bed reactor. A lifetime test, as well as thermogravimetric (TG) analysis, was performed to investigate the stability performance and anti-carbon deposition of catalysts. The results showed that the addition of La₂O₃ can restrain the growth of NiO particles, increase the H₂ uptake and Ni dispersion, and therefore enhance the activity of catalysts. When the La₂O₃ content was 3 wt%, a CO conversion of 98% and a selectivity to CH₄ of 96% were obtained at 400 °C. Furthermore, the catalyst NiO-La₂O₃/Al₂O₃ with 3 wt% La₂O₃ content displayed highly stable performance in long-term tests, especially exhibiting good anti-carbon deposition property.     

Effects of promoters on catalytic activity and carbon deposition of Ni/γ‐Al2O3 catalysts in CO2 reforming of CH4

Catalytic reforming of methane with carbon dioxide was studied in a fixed‐bed reactor using unpromoted and promoted Ni/γ‐Al₂O₃ catalysts. The effects of promoters, such as alkali metal oxide (Na₂O), alkaline‐earth metal oxides (MgO, CaO) and rare‐earth metal oxides (La₂O₃, CeO₂), on the catalytic activity and stability in terms of coking resistance and coke reactivity were systematically examined. CaO‐, La₂O₃‐ and CeO₂‐promoted Ni/γ‐Al₂O₃ catalysts exhibited higher stability whereas MgO‐ and Na₂O‐promoted catalysts demonstrated lower activity and significant deactivation. Metal‐oxide promoters (Na₂O, MgO, La₂O₃, and CeO₂) suppressed the carbon deposition, primarily due to the enhanced basicities of the supports and highly reactive carbon species formed during the reaction. In contrast, CaO increased the carbon deposition; however, it promoted the carbon reactivity. © 2000 Society of Chemical Industry     

High-temperature in situ FTIR spectroscopy study of LaOF and BaF2/LaOF catalysts for methane oxidative coupling

In situ FTIR spectroscopy was used to characterize the oxygen adspecies and its reactivity with CH₄ over LaOF and 15 mol% BaF₂/LaOF catalysts at OCM temperature (750-800°C). It was found that gas-phase oxygen was activated on the surface of LaOF and 15 mol% BaF₂/LaOF, which had been pretreated under vacuum at 750 or 800°C, forming O ₂ ^- species at high temperature (750-800°C). At 750°C, the adsorbed O ₂ ^- species can react with pure CH₄ accompanied by formation of gas-phase C₂H₄ and CO₂, and there is a good correlation between the rate of disappearance of surface O₂and the rate of formation of gas-phase C₂H₄. The O ₂ ^- species was also observed over the catalysts under working condition, and it reacted with CH₄ in a manner that was consistent with its role in a catalytic cycle. These results suggest that O ₂ ^- may be the active oxygen species for OCM reaction over these catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methane coupling catalysts based on zirconium combined with lanthanum

Two series of zirconium and lanthanum oxide based catalysts were synthesised, characterised (XRD, ESCA, BET) and tested for their effectiveness in the oxidative coupling of methane (OCM). Starting materials and the preparation method (ZrO₂ and wet impregnation, ZrO(NO3)₂·₄H₂O + La(NO3)₃·6H₂O and coprecipitation) determined the bulk composition of the catalysts (ZrO₂ + La₂O₃, La₂O₃ * 2ZrO₂). Doping with alkali (Li, Na, K) or alkaline earth metal (Mg, Ca, Sr) increased the selectivity to hydrocarbons. The best C₂-hydrocarbon yields were obtained with the Sr-doped ZrO₂ + La₂O₃ catalyst (16.1%) and the Li-doped La₂O₃ * 2ZrO₂ catalyst (17.3%).     

Catalytic reduction of N2O with CH4 and C3H6 over Ag–Rh/Al2O3 bimetallic catalyst in the presence of oxygen

A study of nitrous oxide (N₂O) reduction with methane (CH₄) and propene (C₃H₆) in the presence of oxygen (5%) over Ag/Al₂O₃, Rh/Al₂O₃ and Ag–Rh/Al₂O₃ catalysts, with Ag and Rh loadings of 5 wt% and 0.05 wt% respectively, has been performed. From the results, it was observed that the Ag–Rh bimetallic catalyst was the most active for both nitrous oxide removal (more than 95%) and hydrocarbon oxidation. This high activity seems to be connected with a synergistic effect between Ag and Rh. The findings from X‐ray diffraction and X‐ray photoelectron spectroscopy studies showed also, that there were no strong interactions (eg alloying) between Ag and Rh. Copyright © 2005 Society of Chemical Industry     

The effects of La2O3 on the structural properties of La2O3–Al2O3 prepared by the sol–gel method and on the catalytic performance of Pt/La2O3–Al2O3 towards steam reforming and partial oxidation of methane

The effect of La₂O₃ content on the structural properties and catalytic behavior of Pt/xLa₂O₃–Al₂O₃ catalysts in steam reforming of methane and partial oxidation of methane was investigated. There was a decrease in the density of Pt sites with the increase of La₂O₃ loadings according to Fourier transform infrared spectroscopy of adsorbed CO and to dehydrogenation of cyclohexane results. However, transmission electron microscopy data indicates an opposite trend. This apparent disagreement could be due to the partial coverage of Pt sites by LaO_x species. CH₄ turnover rates and specific rates of steam reforming of methane increased for higher La₂O₃ loadings. The Pt/Al₂O₃ catalyst was strongly deactivated during partial oxidation of methane, while La₂O₃-containing catalysts exhibited higher stability. The increase of activity observed during the reactions was ascribed to the ability of the [LaPt_xO]Pt⁰-like species to promote the gasification of coke. This cleaning mechanism led to higher accessibility of the active sites to CH₄.     

Production of nano zinc borate (4ZnO·B2O3·H2O) and its effect on PVC

In this study, nano sized zinc borate powder with a formula of 4ZnO·B₂O₃·H₂O was synthesized using 2ZnO·3B₂O₃·3.0–3.5H₂O as a starting chemical which was produced using a wet chemical method. After dissolving 2ZnO·3B₂O₃·3.0–3.5H₂O in an ammonia solution, the clear solution was boiled until a white powder formed. The resultant powder was characterized with XRD, FTIR, TGA and TEM. XRD, FTIR and TGA results proved that the powder was belonged to the 4ZnO·B₂O₃·H₂O. Nano composites of 4ZnO·B₂O₃·H₂O–polyvinylchloride (PVC) were produced by injection moulding by adding 1 and 5 wt% zinc borate powders into PVC to enhance its flame retardancy. Limiting oxygen index (LOI) of virgin PVC increased from 41% to 47% and 54% for the 1 and 5 wt% zinc borate added PVC, respectively. Nano zinc borate addition into the PVC does not have considerable negative effect on the mechanical properties of zinc borate–PVC composites even at high amounts of 5 wt%.     

Oxidative coupling of methane over Sm-promoted MgO: Influence of composition and preparation conditions

Influences of promoter concentration (or Sm/Mg ratio), precursor for MgO (viz. Mg-acetate, Mg-carbonate and Mg-hydroxide), calcination temperature of Sm-promoted MgO catalyst on the catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different temperatures (650–850°C) and CH₄/O₂ ratios in feed (2·0–8·0) at a high space velocity (51600 cm³ g⁻¹ h⁻¹) have been investigated. The catalytic activity/selectivity of Sm–MgO catalysts in the OCM are found to be strongly influenced by the Sm/Mg ratio, precursor used for MgO and catalyst calcination temperature. The catalyst with Sm/Mg ratio of 0·11, prepared using magnesium acetate and magnesium carbonate as a source of MgO and calcining at 950°C, is found to be highly active and selective in the OCM process. A drastic reduction in catalytic activity/selectivity is observed when the catalyst is supported on low surface area porous catalyst carriers, indicating strong catalyst–support interactions. ©1997 SCI     

Changes in Ba Phases in BaO/Al2O3 upon Thermal Aging and H2O Treatment

The effects of thermal aging and H₂O treatment on the physicochemical properties of BaO/Al₂O₃ (the NO_x storage component in the lean NO_x trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO₂ TPD. Thermal aging at 1000 °C for 10 h converted dispersed BaO/BaCO₃ on Al₂O₃ into low surface area crystalline BaAl₂O₄. TEM/EDX and XRD analysis showed that H₂O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly crystalline BaCO₃ phase segregated from the Al₂O₃ support. Crystalline BaCO₃ was formed from conversion of both BaAl₂O₄ and a dispersed BaO/BaCO₃ phase, initially present on the Al₂O₃ support material after calcinations at 1000 and 500 °C, respectively. Such a phase change proceeded rapidly for dispersed BaO/BaCO₃/Al₂O₃ samples calcined at relatively low temperatures with large BaCO₃ crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air, although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al₂O₃ NO_x trap catalysts since both processes involve potential exposure of the material to CO₂ and liquid and/or vapor H₂O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H₂O treatment is proposed.     

New estertin derivatives based on trivacant Keggin-type [β-SbW9O33]9− cluster

Two new polyoxometalate-based estertin derivatives, Na₆K₄[{Sn(C₃H₄O₂)}₂{Sn(C₃H₄O₂)(H₂O)}₂·(B-β-SbW₉O₃₃)₂]·14H₂O (1) and Na₄K₆[{Sn(C₄H₆O₂)(H₂O)}₂(WO₂)₂(B-β-SbW₉O₃₃)₂]·19H₂O (2), have been synthesized by the reaction of Na₉[B-α-SbW₉O₃₃]?19.5H₂O with Cl₃Sn(CH₂)₂COOCH₃ or Cl₃SnCH₂CH(CH₃)COOCH₃ in an aqueous KCl solution. Both compounds exhibit the sandwich-type structural feature, in which the carboxyethyltin fragments are sandwiched by two B-β-[SbW₉O₃₃]^9? units. Structural analyses reveal that the reaction process involves α → β isomerization of [SbW₉O₃₃]^9? unit and the hydrolysis of estertin into carboxyethyltin group. Both compounds display good electrocatalytic activities toward the reduction of hydrogen peroxide, and compound 1 also shows good anti-tumoral activities in vitro.     

Influence of V2O5 content on ammoxidation of 3-picoline over V2O5/AlF3 catalysts

V₂O₅/AlF₃ catalysts with V₂O₅ loadings ranging from 2 to 15 wt% were prepared by the conventional wet impregnation method, using nonporous AlF₃·3H₂O sample as the support for impregnating NH₄VO₃. It was found that the catalysts evolve porous structures upon calcination at 723 K. The influence of V₂O₅ content was studied on ammoxidation of 3-picoline on the reduced catalysts. The catalyst with 15 wt% V₂O₅ exhibited the highest selective ammoxidation acitivity towards nicotinonitrile. The XRD and oxygen chemisorption studies revealed that vanadia is in a highly dispersed state in the catalysts.     

Higher Hydrocarbon Production through Oxidative Coupling of Methane Combined with Hydrogenation of Carbon Oxides

In the production of higher hydrocarbons, combining oxidative coupling of methane (OCM) with hydrogenation of the formed carbon oxides in a separate reactor provides an alternative to the currently applied methane conversion to syngas followed by Fischer‐Tropsch synthesis. The effects of CH₄:O₂ feed ratio in the OCM reactor and partial pressures of H₂ or/and H₂O in the hydrogenation reactor were analyzed to maximize production of C₂₊ hydrocarbons and reduce CO_x formation. The highest C₂₊ yield was achieved with low CH₄:O₂ feed ratio for OCM and removal of the formed water before entering the hydrogenation reactor.     

Ca-containing additives in PTC-BaTiO3 ceramics: effects on the microstructural evolution

Two series of Ca²⁺-modified BaTiO₃ ceramics have been prepared of the gross composition La_0.002Ba_0.998–xCa_xTi_1.01O_3.02 (0<x<0.24). In the first series, CaCO₃, BaCO₃, TiO₂ and La₂(C₂O₄)₃·9H₂O were used as starting materials. The calcination of mixtures with x⩽0.08 resulted in the formation of the corresponding titanate solid solution (Ba_1–xCa_x)TiO₃. With values of x higher than 0.08, CaTiO₃ was observed as an additional phase. In the second series, a La_0.002Ba_0.998TiO₃ starting powder was hydrothermally recrystallized in Ca(NO₃)₂ solution. High resolution imaging and analytical methods revealed that the BaTiO₃ grains are surrounded by small CaTiO₃ crystallites, which influence the evolution of the microstructure of the ceramic in the sintering process strongly by acting as seeds during the recrystallization of the matrix material. Thus, it was possible to optimize the microstructural and electrical characteristics of a ceramic of the second series by adding only 4 mol% Ca, while in case of the first series 16 mol% Ca are necessary.     

Three new polyoxometalate-based hybrids prepared from choline chloride/urea deep eutectic mixture at room temperature

Three new polyoxometalate-based hybrids {[(CH₃)₃N(CH₂)₂OH]₂(H₃O)}[Na₂(H₂O)₆][IMo₆O₂₄]·H₂O 1 , {Na₂[(CH₃)₃N(CH₂)₂OH]₄}[Al(OH)₆Mo₆O₁₈]₂·8NH₂CONH₂·4H₂O 2 and {Na₆(H₂O)₁₈[(CH₃)₃N(CH₂)₂OH]₂(CON₂H₅)₂}[NaMo₇O₂₄]₂·4NH₂CONH₂·H₂O 3 were successfully synthesized in the choline chloride/urea deep eutectic mixture at room temperature. Reactant quantities of water presents in the nonaqueous eutectic mixture solvent may influence the structure of the products. The three compounds are fully characterized by elemental analyses, IR, UV–vis, TG analyses, power X-ray diffraction and single-crystal X-ray diffraction. The photocatalytic properties of 1 and 2 are investigated.