Showing papers by "John B. Moffat published in 1994"

Oxidative methane conversion to carbon monoxide and hydrogen at low reactor wall temperatures over ruthenium supported on silica

Abstract: Oxidative methane conversion to carbon monoxide and hydrogen is catalyzed over ruthenium supported on silica at reactor wall temperatures as low as 400° C when the flow rate of reactants (methane and oxygen) is significantly high. The conversion of methane and the yields of carbon monoxide and hydrogen increase with increase in the flow rate of the reactants while oxygen is always completely consumed. Addition of carbon dioxide to the reactant flow can increase the yield of carbon monoxide in the reaction, suggesting that carbon dioxide functions as an oxidant and the actual surface temperature of the catalyst is sufficiently high that thermal conversion of methane via carbon dioxide and water can take place.

Selective oxidative coupling of methane catalysed over hydroxyapatite ion-exchanged with lead

Abstract: Oxidative coupling of methane to ethane and ethene can be effectively catalysed over hydroxyapatite ion-exchanged with lead at reaction temperatures as low at 700 °C, while hydroxyapatite itself catalyses methane oxidation mainly to carbon oxides. The rate of oxygen conversion over the former apatite is almost the same as that with the latter catalyst, suggesting that the oxidation sites on the two apatites are similar. The experimental results suggest that the lead ions on the surface of apatite play an important role in both the activation of methane and stabilization of methyl radicals on the surface.

The oxidative dehydrogenation of ethane on silica-supported metal-oxygen cluster compounds

Abstract: The oxidative dehydrogenation of ethane with nitrous oxide and oxygen on silica-supported metaloxygen cluster compounds (MOCC) has been investigated. The effects of several variables such as reaction temperature, partial pressure of reactants, nature of the oxidants (N2O and O2), residence time, loading of the catalysts, and pretreatment environment, on the conversion, product distribution and the kinetics have been studied. With nitrous oxide, on unsupported H3PMo12O40 and on the silica support no acetaldehyde was observed in the product stream while significant amounts of acetaldehyde are found with the supported MOCC. With nitrous oxide as an oxidant, acetaldehyde and ethylene were the principal products, while carbon monoxide and ethylene were the predominant products with oxygen. On H3PMo12O40the conversion of ethane and the yield of acetaldehyde have maximum values at a loading of 20 wt.-%. The results from studies of the effect of contact time suggest that acetaldehyde and ethylene are primary products. Increases in the relative amounts of the oxidants produce changes in the selectivities which are strongly dependent on the nature of the oxidant. For catalyst pretreatment temperatures greater than 500°C, the conversion of ethane and selectivity to acetaldehyde decrease. At partial pressures of ethane between 0.1 and 0.8 atm the rate of ethane conversion is of approximate order 0.8 and 0.6 in ethane and nitrous oxide, respectively and 0.7 and 0.4 in ethane and oxygen, respectively. The results show striking correspondences with those reported earlier for methane with MOCC and thus suggest that the mechanisms for the two processes may depend on the lability of the terminal oxygen atoms of the MOCC anion, the production of oxygen vacancies and the ability of the oxidant to regenerate the active oxygen sites in the anion.

Formation of chlorine species over magnesium oxide in the oxidative coupling of methane in the presence of carbon tetrachloride

Abstract: The oxidative coupling of methane on magnesium oxide (MgO) has been studied in the presence of carbon tetrachloride (TCM) as a gas-phase additive. Addition of a small amount of TCM to the reactant stream improves the selectivity to C2H4, while the conversion of methane is not influenced by the additive. X-ray photoelectron spectra of the used MgO reveal the formation of chlorine species on the catalyst surface in quantities up to 0.20 of Cl/Mg (atomic ratio), although X-ray diffraction spectra of the catalyst show MgO only and the content of the chlorine species in the bulk phase estimated by X-ray fluorescence analysis is very low. It is concluded that the enhancement of the selectivity to C2H4 primarily results from the presence of surface chlorine species. The chlorinated species on the catalyst has been identified as MgCl2.

Oxidative coupling of methane on salts of magnesium doped by alkali carbonates

Abstract: The oxidative coupling of methane has been studied at 775° C on MgO, MgSO 4 , and Mg 3 (PO 4 ) 2 doped by carbonates of Li, Na, K, Rb, and Cs. The conversion and selectivity on the sulfate and phosphate in the presence and absence of tetrachloromethane (TCM) as a gas-phase additive were mainly dependent on the nature of the magnesium compounds but not on that of the solid-phase dopants, while those on the oxide were influenced by the nature of the alkali metals. The influence of TCM on the methane conversion process appears to result primarily from the interaction of TCM with the anions associated with the alkaline earth compounds

Comparison of the Oxidative Coupling Reactions of Benzene with Those of Methane of Rare Earth Oxide Catalysts

Abstract: The oxidative coupling of benzene has been compared with that of methane on La2O3, CeO2, Pr6O11, and Sm2O3 At temperatures greater than 1048 K, the gas phase oxidative coupling of benzene appears to be predominant, while the oxidation occurs catalytically at 873 K The conversion of benzene and of methane at 873 K follows the order of Sm2O3 > La2O3 > Pr6O11 > CeO2, suggesting that the abstraction of hydrogen from the aromatic and the saturated compounds depends primarily on the nature of the catalyst but not the reactant Ancillary information has also been obtained from the results of XPS analyses of both fresh catalysts and those previously used in one of the reactions

Activation of surface lattice oxygen in the oxidation of carbon monoxide on silica

Abstract: Silica prepared by the sol–gel method from ethyl orthosilicate catalyses carbon monoxide oxidation at 850 K or above while no such activity is found with a commercial silica prepared from inorganic reagents. The activity of the silica prepared by the sol–gel method is increased by repetition of the carbon monoxide oxidation reaction. Labelled reactions with 18O2 provide evidence to suggest that most of the lattice oxygen species on the surface of the silica participate in the reaction. After evacuation at 950 K or above, radical species identified as Si—O—O–—Si are generated on the silica. Si—O–—Si radical species are apparently formed during the reaction with the radical electron originating from the Si—O–—Si species in the initial stage of the reaction. Adsorption of carbon monoxide and oxygen on the Si—O–—Si species is proposed to cause formation of carbon dioxide and O– which will react with carbon monoxide and provide the radical electron to the lattice oxygen species, that is, Si—O–—Si and Si—O–—O—Si. The present work shows that the lattice oxygen of silica can be involved in oxidation if the oxygen is activated by radical species such as O–, which have been proposed as active species in catalytic oxidation processes.