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     

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.     

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 coupling of methane to C2-hydrocarbons over lithium–cerium-promoted MgO and MgO–CaO catalysts

The oxidative coupling of methane to ethylene and ethane was studied over lithium–cerium-promoted MgO and MgO–CaO catalysts in the presence of molecular oxygen at 730°C and at atmospheric pressure in a continuous flow, fixed bed quartz reactor. The catalysts were prepared by an impregnation method and finally calcined at 900°C. The surface area, pore size distribution and pore volume of the catalysts were determined. The feed consisted of only methane and oxygen in the molar ratio of 2:1. The results obtained over the catalyst systems, viz. (i) lithium–cerium-promoted MgO and (ii) lithium–cerium-promoted MgO–CaO, have been compared. A relatively high C₂-selectivity has been obtained with Li–Ce-promoted MgO–CaO catalysts. The optimum yield and selectivity for C₂-hydrocarbons were found to be 21·5% and 76·8% respectively at a methane conversion of 28% over Li (7 wt%)–Ce (2 wt%)-doped MgO–CaO (3:1 wt ratio) catalyst. The various factors governing the activity and the selectivity of the catalyst systems have been discussed.     

The effect of Nb2O5 and ZrO2 additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane

Incorporation of Nb₂O₅ or ZrO₂ into both Li/MgO and Li/Na/MgO systems produced ternary and quaternary catalysts, respectively, capable of attaining optimal C₂ yields and selectivities at lower temperatures relative to the unpromoted materials. The degree of enhancement effected by these metal oxide additives was compared to that produced by Li/MgO and Li/Na/MgO catalysts promoted with SnO₂ or Co₃O₄. At reaction temperatures < 700°C, the Li/Co/MgO ternary system showed marked differences in behaviour compared to the other ternary catalysts tested. This was particularly evident in the variation in C₂ selectivity with time on stream during ageing studies of (i) untreated materials, (ii) materials pretreated in CO₂, and (iii) materials dosed periodically with CHCI₃.     

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10^–3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10^–2 molecules/sites, under the same reaction conditions. The initial C₂ formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.     

Influence of support on surface basicity and catalytic activity in oxidative coupling of methane of Li–MgO deposited on different commercial catalyst carriers

Deposition of Li–MgO catalyst on commonly used supports (containing SiO₂, Al₂O₃, SiC, ZrO₂, HfO₂, etc.) causes a drastic reduction in the catalytic activity/selectivity for the oxidative methane coupling reaction and also in both the total and strong surface basicity. The decrease in the catalytic activity/selectivity and basicity is attributed to strong chemical interactions between the catalyst and support which occur during the high temperature (750°C) calcination/pretreatment of the catalyst. The chemical interactions result in catalytically less active binary and ternary metal oxides containing Li and/or Mg, thus deactivating the Li–MgO catalyst by consuming its active components. © 1998 SCI     

Efficient catalysts for olefins from alkanes: sol–gel synthesis of high surface area nano scale mixed oxide clusters

High surface area nano scale Li/MgO oxide clusters with low lithium loadings are prepared by sol–gel method. Appreciable amounts of lithium present can be incorporated into the magnesia gel during preparation and retained in the oxide matrix after gel combustion. This limits presence of free lithium phases and helps prevent the associated sintering and loss of surface area during thermal treatments. The sol–gel method also allows to circumvent the high temperature treatments necessary to incorporate lithium into the magnesia oxide matrix, a prerequisite for the formation of [Li⁺O⁻] type defect sites which are the catalytically active sites for oxidative dehydrogenation of alkanes.     

Carbon dioxide chemistry during oxidative coupling of methane on a Li/MgO catalyst

Reactor tests, temperature-programmed reaction/desorption (TPR/D) carried out in a thermogravimetric balance and FT-IR were employed to investigate the course of CO₂ formed during oxidative coupling of methane (OCM) on a 7-wt.-% Li/MgO catalyst between 600–800°C. Initially, the carbonate free Li/MgO catalyst showed good OCM activity even at 600°C. However, its activity diminished considerably after about 20 min on stream. This coincided with the appearance of CO₂ in the OCM products and the disappearance of ethylene. During OCM, a substantial amount of the available lithium was converted to a stable carbonate which did not decompose easily even at 800°C. FT-IR and TPR revealed that carbonate formation began at 400°C and that the lithium carbonate existed in the form of monodentate (LiOCO₂) and perhaps mixed bridged [Mg(Li)O₂CO] carbonates. Simultaneous existence of a commonly accepted Li₂CO₃ cannot be excluded. The role of CO₂ produced during OCM in modifying the catalytic performance of Li/MgO is discussed.     

A new nano‐(2Li2O/MgO) catalyst/porous alpha‐alumina composite for the oxidative coupling of methane reaction

The present work discloses a new methodology for the production of detached nanorods of 2Li₂O/MgO catalyst particles on the internal surface of α‐Al₂O₃ porous supports to be used as efficient catalysts for the oxidative coupling of methane reaction (OCM). The peculiarity of our preparatory recipe is the success in producing “detached” nanosized entities on the support surface. The performance of the new catalyst/support system for the OCM reaction has been evaluated using a special reactor assembly with cross flow of methane and oxygen gas streams. Under the optimum process conditions, the yield of C product is 25% at an average reaction temperature of 750°C. Under the optimum conditions, the yield of ethylene reaches 8%. It is shown that the enhanced catalytic properties of the new catalyst/support composite may be attributed to nanoeffects. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Aerogel VOx/MgO catalysts for oxidative dehydrogenation of propane

VO_x/MgO aerogel catalysts were synthesized using three different preparation methods: by mixing the aerogel MgO support with dry ammonium vanadate, by vanadium deposition from a precursor solution in toluene, and by hydrolysis of a mixture of vanadium and magnesium alkoxides followed by co-gelation and supercritical drying. The latter aerogel technique allowed us to synthesize mixed vanadium–magnesium hydroxides with the surface areas exceeding 1300 m²/g. The synthesized catalysts were studied by a number of physicochemical methods (XRD, Raman spectroscopy, XANES and TEM). A common feature of all synthesized samples is the lack of V₂O₅ phase. In all cases vanadium was found to be a part of a surface mixed V–Mg oxide (magnesium vanadate), its structure depending on the synthesis method. The VO_x/MgO mixed aerogel sample had the highest surface area 340 m²/g, showed higher catalytic activity and selectivity in oxidative dehydrogenation of propane compared to the catalysts prepared by impregnation and dry mixing. The addition of iodine vapor to the feed in 0.1–0.25 vol.% concentrations was found to increase to propylene yield by 40–70%.     

Oxidative coupling of methane using Li/MgO catalyst: Re-appraisal of the optimum loading of Li

The effect of the level of lithium carbonate doping on MgO, prepared by thermal decomposition of the basic carbonate, is re-examined. A low, sub monolayer, loading, ie. 0.2% Li₂CO₃-MgO is shown to significantly enhance both the specific activity for methane activation and the total C₂ hydrocarbon selectivity. The study indicates that the optimal loading of alkali promoters on MgO prepared in this way is considerably lower than indicated in previous studies.     

Synthesis of diphenyl carbonate from dimethyl carbonate and phenol using O2-promoted PbO/MgO catalysts

Various heterogeneously supported PbO catalysts were prepared for the synthesis of diphenyl carbonate (DPC) by transesterification of dimethyl carbonate (DMC) with phenol. MgO was found to be the best support, and the modification by oxygen further enhanced the catalytic activities of PbO/MgO catalysts. Several parameters affecting the transesterification were investigated. The yields of methylphenyl carbonate (MPC) and DPC reached 10% and 26.6%, respectively, over 10 wt% O₂-promoted PbO/MgO catalyst which was prepared by impregnation method. X-ray diffraction (XRD), differential thermal analysis (DTA) and Brunauer–Emmet–Teller (BET) technique were employed for the characterization of prepared catalysts. It was discovered that the structure and the oxidation states of lead species in PbO/MgO catalysts changed after the promotion by oxygen.     

Effect of MgO calcination temperature on the reaction products and kinetics of MgO–SiO2–H2O system

MgO-based binders have been widely studied for decades. Recently, the MgO–SiO₂–H₂O system was developed as a novel construction material, however, its reaction mechanism remains unclear. This paper investigated the reaction products and kinetics of MgO/silica fume (SF) pastes with MgO calcinated at different temperatures. The results indicate that MgO presented larger grain size after calcination at higher temperature. Mg(OH)₂ and magnesium silicate hydrate (M–S–H) gel were formed when using MgO calcined at 850, 950, and 1050°C. However, only M–S–H gel was formed when using MgO calcined at 1450°C. The reaction kinetics of MgO could be described using α = 1 − e^−k*t. The reaction rate of MgO increased with decreasing calcination temperature, increasing SF dosage, and the addition of sodium hexametaphosphate. Only M–S–H gel was formed when the reaction rate of MgO was below the demarcation line (about 0.250 × 10⁻⁶ s⁻¹), and the corresponding demarcation area was around 14 days.     

Catalytic methane coupling under periodic operation

Composition modulation strategies for improving selectivity to the C₂ products in the oxidative coupling of methane were investigated experimentally with Li₂O/MgO and CeO/Li₂O/MgO catalysts at about 100 kPa pressure and 750°C. Strategies used were 1) “methane cycling” between mixtures with different concentrations of CH₄, 2) “oxygen cycling” between mixtures with different concentrations of oxygen, 3) bang-bang cycling (air-CH₄), and 4) flushing the catalyst with an inert between exposures to reactants. Experimental C₂ yields and ethylene to total C₂ ratios showed that composition forcing is better with the Ce/Li/MgO catalyst. Methane cycling proved to be the best strategy explored.     

The effect of some impurities on the basicity of MgO tested by the transformation of 2-butanol and on its catalytic performance in oxidative coupling of methane

The properties and the role of MgO in oxidative coupling of methane to C₂₊ hydrocarbons (OCM) have been studied. The transformation of 2-butanol over several unpromoted MgO samples was applied as a test reaction for the determination of the surface basicity/acidity. The specific rates of alcohol transformation and of product formation were determined in a flow silica reactor at 608±1 K. All the investigated MgO samples exhibited significant dehydrogenation activity and extremely weak dehydration activity, producing 2-butanone as the main product and only minor or even negligible amounts of butenes. The samples of MgO containing Ca compounds as impurities showed enhanced basicity, as concluded from the higher rates of ketone formation and from the presence of solely 1-butene among butenes. These samples also showed enhanced catalytic performance in the OCM process. The surface basicity of MgO, affected by the content or the absence of some impurities (Ca, Na, etc. oxides), was recognized as the decisive factor responsible for observed diversity in the catalytic performance of MgO in OCM, more significant than other surface properties or precursors.     

Influence of various promoters on the basicity and catalytic activity of MgO catalysts in oxidative coupling of methane

Addition of promoters, such as Li₂O, Na₂O, PbO, La₂O₃, MgCl₂ and CaCl₂, to MgO causes a large increase in its surface basicity (particularly strong basic sites) and catalytic activity/selectivity in oxidative coupling of methane, but the correlation between the basicity and C₂-yield is poor, indicating that factors other than basicity are also important in deciding catalytic performance.     

Corrosive and mechanical resistance of MgO ceramics under metallizing and mild chlorination of spent nuclear fuel in molten salts

The present work is aimed at the study of the unirradiated and irradiated MgO ceramics corrosion and mechanical properties in the molten LiCl at 650–750 °C with addition of UCl₃ and Li₂O(LiCl + nLi₂O and LiCl + mUCl₃ molten salts with n = 1.0 and 2.0 mol. % and m = 0.25, 0.5 and 1.0 mol. %). MgO ceramics is suggested to be used as one of materials for pyrochemical technology for recycling of spent nuclear fuel.The gravimetric method with the exposure time during 100 h was the primary method of investigation. The investigation of surface and bulk corrosion of MgO samples by scanning electron microscopy (SEM) and X-ray spectroscopy (MRSA) was performed using scanning electron microscope equipped with a x-Act 6 energy-dispersive analytic system for X-rays characteristic (XRС). Determination of corrosion losses and average corrosion rates of MgO samples was based on the assumption that the ejection of the radionuclides ⁹⁵Zr, ¹⁷⁵Hf and ¹⁸¹Hf from the MgO samples.Incorporation of Li₂O and UCl₃ in molten LiCl result in increase in the rate of MgO ceramic corrosion both at 650 °C and 750 °C and acts on MgO compressive strength (σ_cs) and on the elemental composition of MgO surface layers. Besides the increasing of UCl₃ concentration led to the bulk corrosion of MgO sample grains.Short-term mechanical tests demonstrated the transition of MgO sample destruction pattern depending on the concentration of Li₂O and UCl₃ additions in LiCl melt.Doping of molten LiCl by 0.5 mol. % of UCl₃ at 650 °C and by 0.25 mol. % of UCl₃ at 750 °C had no influence on the ultimate compression strength of irradiated and unirradiated MgO samples. Increased UCl₃ concentration totaling 0.5 mol. % in the LiCl melts at 750 °C reduced the ultimate compression strength of irradiated MgO ceramic samples by ~15%.     

Synthesis and characterization of new Mg–O–F system and its application as catalytic support

The Mg–O–F system (MgF₂–MgO) with different contents of MgF₂ (100–0%) and MgO is tested as support of iridium catalysts in the hydrogenation of toluene as a function of the MgF₂/MgO ratio. Mg–O–F samples have been prepared by the reaction of magnesium carbonate with hydrofluoric acid. The MgF₂–MgO supports, after calcination at 500 °C, are classified as mesoporous of surface area (34–135 m²·g^− 1) depending on the amount of MgO introduced. The Ir/Mg–O–F catalysts have been tested in the hydrogenation of toluene. The highest activity, expressed as TOF, min^− 1, was obtained for the catalyst supported on Mg–O–F containing 75 mol%MgF₂.     

Fabrication of Hierarchical Co/MgO Catalyst for Enhanced CO2 Reforming of CH4 in a Fluidized-Bed Reactor

This work reports facile fabrication of fluidizable hierarchical Co–MgO particles with high dispersion of Co nanoparticles (NPs) and high surface oxygen vacancies for matching with the fluidized-bed reactor. The hierarchical Co–MgO catalyst exhibits higher activity and thermal stability for CH₄–CO₂ reforming than the conventional impregnation Co/MgO catalyst. The enhanced prevention of Co NPs sintering of the hierarchical Co–MgO catalyst is attributed to the multilayer structure and the exposed MgO (111) face, which creates high surface oxygen vacancies, induces high Co dispersion to restrain Co NPs sintering, and supplies high resistance to graphite carbon deposition. A formation mechanism for the hierarchical MgO structure is also proposed. This formation of incomplete rhombohedron MgO particles with an exposed octahedral MgO (111) face is attributed to the electrostatic interaction of OH⁻. The proposed approach to improving catalytic activity by creation of hierarchical MgO particles can be expanded to other metal oxide catalysts. © 2018 American Institute of Chemical Engineers AIChE J, 65: 120–131, 2019