Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

Oxidative Coupling of Methane Over Na–W–Mn–Zr–S–P/SiO2 Catalyst: Effect of S, P Addition on the Catalytic Performance

The multi-component catalysts we prepared exhibited high performance for the oxidative coupling of methane. The highest activity of catalysts was obtained over the Na–W–Mn–Zr–S–P/SiO₂ at the temperature of 1,023 K, on which the C₂ yield was 23.5% at the methane conversion of 43.8%. XRD and XPS results showed the S, P addition could lead to the increase of lattice oxygen concentration and the formation of phase such as Na₂SO₄, Na₂Zr(PO₄)₂ on the catalysts. These phases play great roles in activity of the catalysts system.     

Basic sites on the surface of oxide catalysts responsible for oxidative methane coupling

The basic sites of various oxide catalysts for the oxidative dimerisation of methane were studied by FTIR spectroscopy of adsorbed molecular probes (chloroform and CO₂). The methods used are compared and the advantage of CO₂ as probe for specifying the basic sites is demonstrated. The strengths of the basic sites were seen to correlate with the spectral parameters of the surface carbonates. Differences in spectral responses of carbonates are attributed to the different states of oxygen participating in their formation. The concentration of the strongest sites was estimated. A study of the catalytic activity of this system indicates that the system's activity in oxidative methane coupling depends on the presence and concentration of strong basic sites on the catalyst surface.     

Investigation of reactivity and selectivity of methane coupling catalysts using isotope exchange techniques

The relationship between the activity and selectivity of four metal oxide catalysts for the oxidative coupling of methane has been compared with their basicity and their ability to effect the scission of C-H and O-O bonds. The rates of isotope exchange reactions (CH₄-D₂ and¹⁶O₂–¹⁸O₂) were used as a measure of catalyst's ability to activate both reactants. It is demonstrated that catalysts showing high C₂ selectivity in the oxidative coupling reaction activate methane strongly and oxygen weakly. The lack of direct correlation between the rates of methane conversion and bonds' activation indicates that the formation of the methyl radical cannot be explained by a simple, one-step mechanism.Visiting scientist, on leave from the Department of Chemistry, University of Zhejiang, Hangzhou, Zhejiang, 310027, Peoples Republic of China.     

Oxidative coupling of methane over sodium-chloride-added sodium zirconium phosphates

Monosodium zirconium phosphate or disodium zirconium phosphate by itself did not catalyze the oxidative coupling of methane and also the deep oxidation of methane. However, NaCl-added sodium zirconium phosphates showed markedly increased activity and high C₂₊ selectivity in the oxidative coupling of methane, which indicates that chlorine species or NaCl plays an essential role in the catalytic action. The catalytic performance became more stable with increasing content of NaCl. The primary reason for the catalyst deactivation is the loss of chlorine, and a possible secondary reason is the transformation of catalytic substance to the sodium zirconium phosphates having higher Na/Zr ratios or decomposition of sodium zirconium phosphates to zirconium oxide and sodium phosphate. Two kinds of surface chlorine species were observed, and the lower-binding-energy species is considered to be much more active than the higher-binding-energy species in methane activation, although the latter is present in a larger amount than the former.     

Oxidative coupling of methane over promoted strontium chlorapatite

Strontium zirconium phosphate, unpromoted strontium chlorapatite and strontium hydroxyapatite showed low C₂ selectivity for the oxidative coupling of methane, but promoted strontium chlorapatite catalysts showed markedly increased activity and selectivity and also exhibited stable behavior. SrCl₂ was the primary promoter and strontium zirconium oxides were considered to be acting as other promoters, but strontium zirconium phosphate and strontium carbonate seemed to be acting adversely. A promoted strontium chlorapatite catalyst which contained a slightly larger amount of SrCl₂ than needed to form the chlorapatite showed the best performance and was stable up to 50 h at 1,023 K, and the highest C₂₊ selectivity and yield were 52% and 13.8%, respectively. Although SrCl₂ was more stable than NaCl it decomposed slowly during the reaction, leaving strontium oxide or strontium carbonate behind, which is considered to result in slow deactivation of the catalyst.     

Transition metal-doped TiO2 nanowire catalysts for the oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM) on transition metal-doped TiO₂ nanowire catalysts was performed and the effects of metal dopants were studied. With transition metal doping, the electric and optical properties of nanowires were adjusted, which seemed to improve the catalytic activity and selectivity of the OCM reaction. A Mn-doped TiO₂ nanowire catalyst exhibited the highest C₂ yield with the highest (ethylene)/(ethane) ratio because of its moderate oxidation activity, while a highly active Rh-doped TiO₂ nanowire catalyst converted methane into fully oxidized CO and CO₂. The electric conductivity assessed by UV–vis absorption represented the oxidation activity of the nanowire catalysts.     

Preparation of CeO2/ZnO nanostructured microspheres and their catalytic properties

CeO₂/ZnO nanostructured microspheres with an average diameter of about 3.8 μm were synthesized by a solid-stabilized emulsion route. The CeO₂/ZnO nanostructured microspheres were characterized with SEM, XRD, CO₂-TPD, BET measurement and size analysis. Based on the oxidative coupling reaction of methane with carbon dioxide as an oxidant, the catalytic performance of the CeO₂/ZnO nanostructured microspheres was evaluated and compared with that of the CeO₂/ZnO nanoparticles. The results showed that the surfaces of the CeO₂/ZnO nanostructured microspheres consisted of petal-like structures with a petal thickness of about 90 nm and a petal depth of 0.4 μm to 0.9 μm. Using CeO₂/ZnO nanostructured microspheres as catalysts for the oxidative coupling of methane with carbon dioxide, the conversion of methane corresponded with that using the CeO₂/ZnO nanoparticles, while the CeO₂/ZnO nanostructured microspheres had much longer operating life.     

Autothermal Methane Oxidative Coupling Process over La2O3/MgO Catalysts

Some essential conditions necessary to reach an autothermal regime in methane oxidative coupling on La₂O₃/MgO catalysts were investigated. The following three ways can be suggested to transfer the process into the autothermal regime: (1) higher initial concentrations of reagents; (2) larger reactor diameter; (3) optimization of the flow rate and the preheating temperature. It was found that the optimal temperature of the autothermal regime of methane oxidative coupling is governed by the nature of the catalyst.     

Oxidative coupling of methane over strontium-doped neodymium oxide: Parametric evaluations

Since its discovery in 1982, oxidative coupling of methane (OCM) has been considered one of the most promising approaches for the on-purpose synthesis of ethylene. The development of more selective catalysts is essential to improve process economics. In this work, undoped neodymium oxide as well as neodymium oxide doped with high (20%) and low (2.5%) levels of strontium were tested in a high-throughput fashion covering a wide range of operating conditions. The catalysts were shown to be able to achieve greater than 18% C₂+ yield. Space velocity was shown to play a significant role in C₂+ selectivity. For a methane to oxygen feed ratio of 3.5, selectivity increased with increasing space velocity, reaching a maximum of 62% at a methane conversion of 30% at an optimal space velocity of ~250,000 ml/h/g. The difference in activity between the three samples was linked to the contribution of different oxygen centers.     

The catalytic low temperature oxydehydrogenation of methane. Temperature dependence,carbon balance and effects of catalyst composition

In an earlier publication [1] it has been claimed that oxidative coupling of methane to higher hydrocarbons had been obtained with close to 100% selectivity at 600 ° C and atmospheric pressure in the presence of steam over a CaNiK oxide catalyst. These results have been confirmed in longer runs. Artifacts, such as carbonate formation on the catalyst, have been excluded. The reaction is slightly exothermic. An Arrhenius plot shows that methane oxidation to CO₂ predominates at temperatures above 600 ° C and oxidative methane coupling at lower temperatures. The importance of exact catalyst composition is demonstrated.     

An experimental study of oxidative coupling of methane in a solid oxide fuel cell with 1 wt%Sr/La2O3-Bi2O3-Ag-YSZ membrane

A solid oxide fuel cell with 1 wt%Sr/La₂O₃-Bi₂O₃-Ag-YSZ membrane was applied to oxidative coupling of methane. Membrane composition had a great effect on the reaction and current generated. An increase in the current generated was accompanied by a decrease in C₂ selectivity and an increase in CH₄ conversion. There is an optimal temperature for C₂-selectivity. CH₄ conversion decreased, C₂-selectivity increased and current generated decreased slightly with a rise in total flow rates. CH₄ conversion and the current generated increased with a rise in oxygen concentration. If only C₂-selectivity and current were concerned, the higher the methane concentration, the more favourable for the cogeneration of electrical energy and ethane and ethylene. Stability of the membrane was also tested.     

Correlation of electrical properties and performance of OCM MOx/Na2WO4/SiO2 catalysts

The electrical conductivity and catalytic performance of MO_x/Na₂WO₄/SiO₂ catalysts in oxidative coupling of methane (OCM) are measured and correlated. M is V, Cr, Mn, Fe, Co or Zn. In this study, for the first time, the conductivity of the catalyst powder was measured under the OCM conditions as well as in oxygen. A definite correlation between the catalytic performance, the electrical conductivity under the OCM conditions, and the band gap of metal oxide constituent of the catalysts is observed. Manganese oxide (MnO_x with the lowest band gap) on Na₂WO₄SiO₂ with the highest electrical conductivity shows the best catalytic performance.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Oxidative coupling of methane by carbon dioxide: a highly C2 selective La2O3/ZnO catalyst

In the oxidative coupling of methane by carbon dioxide, La₂O₃/ZnO catalysts were found to have high C₂ selectivity and good stability. The coupling selectivity on La₂O₃/ZnO is about 90%, which is much higher than that on pure La₂O₃ or ZnO. The consumption ratio of carbon dioxide to methane is approximately one. X-ray diffraction analysis reveals that the structural forms of the oxides are unchanged during the reaction. The reaction mechanism for C₂ formation is discussed.     

Low temperature catalysts for oxidative coupling of methane

A simple review is given to the recent work of the oxidative coupling of methane at low temperature. Emphasis is laid on the different systems of low-temperature catalysts under conventional CH₄/O₂ co-feed conditions, and on the investigations of low-temperature oxidative coupling of methane in the presence of steam in the feed. Other approaches, e.g. oxidative coupling of methane at elevated pressure and moderate temperature, preparing ethylene by oxidative coupling reaction of methane on laser-activated solid surface, are also included.     

Oxidative coupling of methane to C2-hydrocarbons over La-promoted CaO catalysts

La₂O₃ promoted CaO [La/Ca (mol/mol) = 0.05] catalyst shows very high activity and selectivity (methane conversion: 25%, C₂-selectivity: 66% and C₂-space-time-yield: 864 mmol ·g^–1 (cat.)·h^–1) with no catalyst deactivation in oxidative coupling of methane to C₂-hydrocarbons at 800 ° C.     

Transient and isotopic studies of the oxygen transport and exchange during oxidative coupling of methane on Sr promoted La2O3

Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La₂O₃ catalysts. Results of the¹⁸O₂/¹⁶O₂ isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La₂O₃. Steps of¹⁸O₂ in the presence of a steady flow of methane over Sr/La₂O₃ catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase¹⁸O². Steps of CO₂ over catalysts in which lattice oxygen has been exchanged with¹⁸O₂, show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen.     

Catalytic properties of bismuth vanadates based catalysts in oxidative coupling of methane and oxidative dehydrogenation of propane

The oxidative coupling of methane (OCM) and the oxidative dehydrogenation of propane (ODHP) have been performed on new mixed Bi-V oxides having a --Bi₂MoO₆-like structure. The results yield a correlation between the methane conversion and the oxygen diffusivity. The introduction of metals (Fe, Cu, Sr with 10 mol%) improves the C₂ selectivity. The use of an oxide with a scheelite structure (BiVO₄) shows that, under catalytic conditions, this oxide has the same behaviour as the above solids.     

Synergistic effect of Pd in methane combustion PdMnOx/Al2O3 catalysts

The catalytic methane combustion was investigated over alumina-supported monometallic and bimetallic palladium and manganese oxide catalysts. The catalytic activity of these systems showed that palladium incorporation on MnO_x/Al₂O₃ catalyst leads to an enhancement in methane combustion. The higher catalytic activity of the PdMn/Al₂O₃ catalysts is related to a greater mobility of lattice oxygen in manganese oxide in the presence of palladium. These bimetallic catalysts also showed a significant improvement in catalysts stability with respect the monometallic ones. Surface analysis of the used catalysts revealed less amount of coke and Mn/Al and Pd/Al atomic ratios almost unchanged, which is indication of absence of active phase sintering.