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.     

Kinetic studies of the oxidative dimerization of methane on thin-film MgO and Li/MgO

The oxidative dimerization of methane to ethane over thin-film MgO and Li/MgO catalysts prepared under well-controlled, ultrahigh vacuum (UHV) conditions has been studied using a combination UHV/reactor cell system that allows elevated pressure kinetic studies and surface science techniques to be employed. Reactions investigating temperature, pressure and pretreatment activation have been performed to determine ethane dependence on each reaction parameter. Agreement between turnover frequencies, activation energies and lithium promotional effects of the thin-film MgO and previously reported results for powdered MgO indicates that the thin-film catalyst is an excellent model catalyst for the oxidative coupling of methane.     

Liquid phases in Li:MgO as studied by thermoanalytical methods, electron microscopy, and electrical conductivity measurements

The phase relations in parts of the system Mg-Li-O-H-C are reviewed and some features investigated by TG, DTG, DSC, SEM, TEM, and electrical conductivity measurements on a commercial Li:MgO catalyst. Electrical conductivity measurements on samples with varying Li contents are used to monitor the presence of highly conducting surface melts under controlled atmospheres at high temperatures. The measurements indicate that the solubility of lithium in MgO bulk increases with increasing oxygen activity, as predicted by defect theory. The solubility of Li in MgO at 700°C in CO₂-rich, oxidizing atmosphere is estimated to be of the order of magnitude of 0.1 mol-% (0.02 wt-%).     

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.     

Acid/base properties of MgO studied by high resolution electron energy loss spectroscopy

The acid/base properties of model MgO surfaces have been studied using various probe molecules with acid strengths ranging from those of carboxylic acids and alcohols to alkenes and alkanes. High-resolution electron energy-loss spectroscopy (HREELS) data show that carboxylic acids, methanol and water dissociate heterolytically on MgO surfaces. Ethylene and ethane, however, are found to adsorb associatively. Thermally generated surface defect sites exhibit stronger basic character and are capable of dissociating ethane. The present studies demonstrate the capabilities of HREELS for the investigation of the chemical properties of insulating materials.     

Hartree–Fock periodic study of the chemisorption of small molecules on TiO2 and MgO surfaces

We present ab initio periodic Hartree–Fock calculations (CRYSTAL program) of the adsorption of small molecules on TiO₂ and MgO. These may be molecular or dissociative, depending on the acidic and basic properties of the molecules in gas phase and of the nature of the surface oxide. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages of the molecule; they also depend on the surface oxide; the oxygen atom of MgO, in spite of its large charge, is poorly reactive and dissociation on MgO is not favorable.

The surface hydrosyl of MgO are more basic than the O of the lattice and water is not dissociated under adsorption. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs basic behavior, but in terms of hard and soft acidity. MgO surface is a “soft” acidic surface that reacts preferentially with the soft base, CO₂.

Another important factor is the adsorbate–adsorbate interaction: favorable cases are the sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface, so that their mutual orientation resembles the geometry of the CO₂ dimer.     

Preparation and characterization of MgO powders having Ca or Ba as surface dopants

The tendency of large dopant cations towards surface segregation has here been utilised to prepare high surface area MgO powders carrying Ca²⁺ or Ba²⁺ as surface impurities in amounts equivalent to 0.2 to 5 monolayers of CaO or BaO. Computational approaches had pointed to possibilities for reconstruction of such impurity monolayers into “rumpled” arrangements as a means of minimizing their energies: the rumpled arrangement favoured for Ca²⁺/MgO would have alternate oxygen anions displaced outward from the original (100) plane, whereas that favoured for Ba²⁺/MgO would involve large outward displacement of alternate Ba²⁺ ions. Ions displaced in that way would necessarily feature lower coordination to counter-ions than species on non-rumpled surfaces, and hence could be expected to exhibit higher surface reactivity and/or catalytic activity. This paper presents results of experiments aimed at: (i) preparation of powdered MgO materials having monolayer amounts of CaO or BaO segregated upon their surfaces; and (ii) testing whether such monolayer-doped Ca²⁺/MgO and Ba²⁺/MgO materials exhibit physical and/or chemical properties indicative of the “rumpling” favoured by computations. Chemical comparisons made between surface reactivities of Ca²⁺/MgO and Ba²⁺/MgO versus the pure MgO support include: lability of surface oxide ions, as manifested in ease of heterophase oxygen isotope exchange at 623–873 K, and relative activities for oxidative coupling of methane at 923–958 K. Physical comparisons include: extent and spectral distribution of surface-sensitive luminescence; BET surface areas; X-ray diffraction and scanning electron microscopy.     

DFT Investigation of Formaldehyde Adsorption Characteristics on MgO Nanotube

The adsorption characteristics of formaldehyde on to MgO nanotube along inner surface, outer surface and terminating end are studied using DFT method with B3LYP/LanL2DZ basis set. The favorable adsorption site is discussed in terms of adsorbed energy which is found to be adsorption of C atom in HCHO with O atom in MgO along inner surface, outer surface and terminating end. The average energy gap variations for all the possible adsorption sites in MgO nanotube are reported. Mulliken population analysis confirms the transfers of electrons from MgO nanotube to HCHO. The conductivity of MgO base material is influenced by the energy gap variation when HCHO is adsorbed on to MgO nanotube. The result of the present study reveals that synthesizing MgO in nanotube form will enhance HCHO sensing characteristics.     

Study of the oxidative methylation of acetonitrile to acrylonitrile with CH4 over Li/MgO catalysts

The oxidative methylation of acetonitrile to acrylonitrile with methane for temperatures in the range 550–730°C over Li/MgO follows a radical mechanism. The reaction proceeds via the formation of radicals at the α-carbon of acetonitrile and methyl radicals from methane. The coupling of these radicals leads to propionitrile which is further transformed to acrylonitrile via oxidative dehydrogenation. Experimental evidences indicate that the reaction is Langmuir–Hinselwood. The Li⁺O⁻ surface sites of Li/MgO are the active centers for the activation of both methane and acetonitrile. Oxygen is absolutely necessary for the formation of the corresponding radicals from methane and acetonitrile but it must be provided at a controllable manner in order to avoid undesired oxidation reaction of nitriles. The decomposition of acetonitrile which would lead to CH₄ and HCN does not take place. However, the increase of the nitrile chain length favors the breaking of the C–C bond between the cyanide group and the α-carbon of the corresponding nitrile.     

Influence of precursors of Li2O and MgO on surface and catalytic properties of Li‐promoted MgO in oxidative coupling of methane

The influence of the catalyst precursors (for Li₂O and MgO) used in the preparation of Li‐doped MgO (Li/Mg = 0.1) on its surface properties (viz basicity, CO₂ content and surface area) and activity/selectivity in the oxidative coupling of methane (OCM) process at 650–750 °C (CH₄/O₂ feed ratio = 3.0–8.0 and space velocity = 5140–20550 cm³ g⁻¹ h⁻¹) has been investigated. The surface and catalytic properties are found to be strongly affected by the precursor for Li₂O (viz lithium nitrate, lithium ethanoate and lithium carbonate) and MgO (viz magnesium nitrate, magnesium hydroxide prepared by different methods, magnesium carbonate, magnesium oxide and magnesium ethanoate). Among the Li–MgO (Li/MgO = 0.1) catalysts, the Li–MgO catalyst prepared using lithium carbonate and magnesium hydroxide (prepared by the precipitation from magnesium sulfate by ammonia solution) and lithium ethanoate and magnesium acetate shows high surface area and basicity, respectively. The catalysts prepared using lithium ethanoate and magnesium ethanoate, and lithium nitrate and magnesium nitrate have very high and almost no CO₂ contents, respectively. The catalysts prepared using lithium ethanoate or carbonate as precursor for Li₂O, and magnesium carbonate or ethanoate, as precursor for MgO, showed a good and comparable performance in the OCM process. The performance of the other catalysts was inferior. No direct relationship between the basicity of Li‐doped MgO or surface area and its catalytic activity/selectivity in the OCM process was, however, observed. © 2000 Society of Chemical Industry     

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.     

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     

Carbon Dioxide Reforming of Methane over Nickel Catalyst Supported on MgO(111) Nanosheets

MgO nanosheets possessing the (111) facet as the main surface were synthesized and the Ni catalyst supported on MgO(111) nanosheets was investigated for the carbon dioxide reforming of methane. The catalytic activity and carbon deposition were compared between Ni/MgO(111) and Ni/MgO(commercial) catalysts. The results showed that Ni/MgO(111) performed at a higher activity as well as a longer stability. From the characterization results, the improved catalytic performances of Ni/MgO(111) were suggested to be closely associated with both the high dispersion of active Ni particles owing to the strong metal-support interaction and the large amount of basic sites of MgO(111) due to its unusual surface properties.     

A Critical Assessment of Li/MgO-Based Catalysts for the Oxidative Coupling of Methane

Li/MgO is one of the most frequently investigated catalysts for the oxidative coupling of methane. Besides catalytic testing, it is also a suitable system to perform surface science experiments and quantum chemical calculations, which is not possible for many other active catalysts. However, the real structure of Li/MgO, the nature of the active center and the structure - activity relationship remain unclear, despite all the research that has been done. The aim of this review is to summarize the available knowledge on Li/MgO to structure and accelerate and improve the ongoing work on this catalytic system.     

Oxidative coupling of methane over doped Li/MgO catalysts

A series of zirconia doped Li/MgO catalysts with a fixed amount of zirconia and varying concentrations of lithium was used for the oxidative coupling of methane. It was found that an increase in lithium concentration resulted in a decrease in initial activity, while the selectivity was not affected. The life-time of Zr doped Li/MgO catalysts with a fixed concentration of ZrO₂ is a function of the lithium concentration. Previous results have shown that Li₂Mg₃ZrO₆ is active and selective but it is now shown to be instable under reaction conditions.     

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.     

活性氧化镁对高矿物掺和料水泥基材料固化氯离子能力的影响

研究了活性氧化镁对高掺量矿物掺和料水泥基材料固化氯离子能力的影响,采取活性氧化镁与氧化钙混掺的方法进行试验研究。实验采用自动电位滴定仪测试水泥浆中总氯离子和游离氯离子量,并借助XRD、DTG等分析其物相组成。研究结果表明：在活性氧化镁和氧化钙混掺量为10%的情况下,随着活性氧化镁掺量的增加,浆体的早期强度降低,而后期强度增进率则提高。在活性氧化镁掺量为5%时,浆体的固化氯离子量达到最大。 XRD、DTG等分析表明浆体中产生了C-S-H凝胶、镁铝水滑石和F盐。     

Segregation of Isovalent Impurity Cations at the Surfaces of MgO and CaO

Enthalpies of segregation for isovalent impurities in magnesium and calcium oxide as a function of surface concentration were calculated by using an atomistic computer simulation method. We have considered Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, and Ni²⁺, segregating to both (001) and (110) faces. The results obtained can be extrapolated to predict the behavior of other impurities including Mn²⁺, Fe²⁺, and Co²⁺, We find, for example, that Fe²⁺, Mn²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ will concentrate at the (001) surface of MgO, while Ni²⁺ will be depleted. The enthalpy of segregation is found to vary substantially with coverage particularly for the larger impurities. The enthalpy becomes less negative with increasing impurity concentration due to the increasing lattice strain until the surface is nearly saturated. Then additional stabilization is obtained by restructuring of the surface layer. We predict reconstructed surfaces for both the (001) and (110) faces, which contain a high concentration of a larger impurity ion. The enthalpy of segregation shows a maximum at around 50% surface coverage implying a bimodal surface distribution of segregant. The influence of segregation on surface energy suggests two unusual effects. The (001) surface energy of the impure crystal becomes negative for surface concentrations of impurity greater than 10% Ba²⁺ or 75% Sr²⁺ in MgO. This implies a thermodynamic barrier to sintering. At high coverages of Ba²⁺ in MgO the (110) surface becomes more stable than the (001) face suggesting that facetting may occur.     

Exafs study of the Nd ion lattice position in Nd:MgO: LiNbO3 and Nd:ZnO:LiNbO3

The local order around Nd³⁺ ions in Nd:MgO:LiNbO₃ and Nd:ZnO:LiNbO₃ codoped samples has been investigated by Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Independently from the codoping atom, the main position of the Nd ions has been determined to be near the Li lattice site.     

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.