Oxidation of Ethylene over Palladium and Palladium–Gold Alloys

Partial Oxidation of Propene in the Presence of Steam

Acetaldehyde partial oxidation on the Au(111) model catalyst surface: C–C bond activation and formation of methyl acetate as an oxidative coupling product

Catalytic oxidation: I. The oxidation of ethylene over Pd and PdAu alloys

Recently we reported that Mg4V2Sb2Ox is selective for propane andn-butane Oxydehydrogenation at low hydrocarbon conversion, and that propane is oxidized in parallel reactions to propylene and COx. We report now on the kinetics of propane and propylene oxidations over this catalyst. The partial oxidations of propane and propylene and zero-order in oxygen, whereas deep oxidations of both hydrocarbons are half-order. This difference in reaction order indicates that different forms of reactive oxygen are involved in the partial and deep oxidation reactions. Presumably, nucleophilic lattice oxygen partakes in the partial oxidation, while electrophilic dissociatively adsorbed oxygen is involved in deep oxidation. A single activated surface adsorbed state of the hydrocarbons is thought to be involved in both the partial and deep oxidation reactions. An interpretation of the observed reaction kinetics in context of the Mg4V2Sb2Ox solid state chemistry, and the partial oxidation literature in general, suggests that selective oxydehydrogenation of propane occurs on isolated (Sb-O-V-O-Sb) sites, deep oxidation on multiple vicinal vanadium sites (Sb-O-V-O-V-O-Sb), and partial oxidation of propylene to acrolein on subsurface V-promoted antimony sites (Sb-O-Sb). Therefore, unproved selectivity of desired intermediates (propylene/acrolein) should be achieved by further lowering the vanadium concentration and/or through key solid state positioning of the vanadium in the catalyst lattice. Alternatively, selective doping to electronically decrease the electrophilicity of the waste forming sites and its appended oxygen should also help depress the waste forming reaction channels in favor of the desired partial oxidation channels. Finally it is anticipated that higher useful product yields would be attained with a compositionally optimized Mg-V-Sb-oxide catalyst by opting for a more stable, isolatable intermediate, e.g., acrylonitrile, by reacting propane in the presence of ammonia and oxygen (air) over this catalyst.

Photocatalytic Methane Conversion: Insight into the Mechanism of C(sp ³ )–H Bond Activation

RF plasma excitation of methane has been studied in an effort to optimize the reaction conditions for a selective partial oxidation of methane. The reaction products of RF-excited methane are C2 hydrocarbons such as ethane and acetylene when O2 is not used. The introduction of a few percent of O2, however, is found to switch the selectivity in favor of CO while CO2 formation is suppressed down to a level below a few percent. Interestingly, in the low O2 ratio regime (0–0.6), the selectivity between CO and C2 hydrocarbons is observed to vary systematically in response to the detailed reaction conditions, including flow rate, pressure and applied RF power, which are explained by the competition between coupling and partial oxidation reactions. Variation in the density and the residence time of the active species in the plasma is suggested to determine the overall reaction pathways. The present results suggest a possibility of a selective production of the partial oxidation products of methane such as CO with a high selectivity and a high conversion efficiency using controlled RF plasma from methane and O2.

Process modeling and design of the ethylene oxide production plant is mainly depended on the kinetics of the ethylene oxide production reactions. In this article, the kinetics equations for partial oxidation (epoxidation) of ethylene on silver catalyst were reviewed. There are three competitive reactions in this system: ethylene partial and total oxidation and ethylene oxide total oxidation. The reaction rate equations for these three reactions were compared and advantage and the disadvantage of kinetic models were discussed. Dichloroethane (DCE) is used in these reactions to increase the ethylene oxide selectivity. Therefore, the kinetics of the reactions considering the role of DCE is also reported. Most of the kinetics models have some weaknesses. However, one of the reviewed models was the complete model because of including all three reactions of partial and total oxidation of ethylene and total oxidation of ethylene oxide. Also, this model considered the concentration of selectivity promoter (DCE) and reverse reactions.

Catalytic properties of Sr1−xCexWO4: The role of mixed conduction in methane oxidation

Partial oxidation of CH4 by N2O on MgO below 573 K (mainly at room temperature) was investigated using TPD, IR and ESR spectroscopies. The effect of UV-irradiation was also studied. In the dark, the partial oxidation reaction did not proceed at room temperature but did when the reaction temperature was raised above 423 K. This temperature coincides well with that of decomposition of N2O, and O–2 seems to be the active oxygen species. However, most of the partial oxidation products, OCH–3, are easily oxidized above this temperature to give carbonates. Under UV-irradiation, the partial oxidation reaction of CH4 by N2O proceeded at 323 K and gave HCOO–, OCH–3 and C2–C7 hydrocarbons. Photodecomposition of N2O also easily proceeded at 323 K, and O– was formed at the initial stage. O– thus formed is an active oxygen species. Hence, it is demonstrated that decomposition of N2O is a key factor in the reaction of CH4 with N2O at low temperatures, either in the dark or under UV-irradiation. Low-coordinated surface ions play no important role in the partial oxidation with N2O as the oxidant.

The catalytic partial oxidation (CPO) of methane over precious metal catalyst has been shown to be an attractive way to obtain syngas (CO and H2) or H2 which can be converted to clean fuels by Fischer–Tropsch synthesis or employed in fuel cells. However, the presence of sulphur bearing compounds naturally occurring in the fuel, or added as odorants to pipe-line natural gas (approximately up to 10 ppm), can have a detrimental effect on the CPO activity. In this work the effect of sulphur addition on the catalytic partial oxidation (CPO) of methane in the low to moderate temperature regime (300-800 °C) and under self-sustained high temperature (>800 °C) condition was investigated on Rh-based catalysts supported on either La2O3 or SiO2 stabilised γ-Al2O3. Based on the results of catalytic activity measurements and in-situ FT-IR/DRIFT spectroscopic characterisation, as well as TPR/TPD studies, it has been shown that the presence of sulphur can severely suppress the formation of synthesis gas by inhibiting the steam reforming (SR) reactions during the CPO of methane. It was demonstrated that the support material plays a crucial role in the CPO of methane in the low to moderate temperature regime. In the presence of a sulphating support such as La2O3-Al2O3 the partial oxidation reaction was much less inhibited than a less sulphating support such as SiO2-Al2O3. The sulphating support acts as a sulphur storage reservoir, which minimises the poison from adsorbing on or near the active Rh sites where reactions take place. However under the typical operating conditions of methane CPO i.e. at high temperatures and short contact times over structured reactors, sulphur in the feed inhibits the SR reaction by directly poisoning the active Rh sites thus preventing the sulphur storage capacity of the support from showing any beneficial effect on the S-tolerance. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles and low temperature light-off phase. The analysis of products distribution in the effluent and heat balance demonstrated that sulphur reversibly adsorbed on Rh selectively inhibits the SR reaction path to syn-gas production. The extent of SR inhibition is greater when operating in air and diminishes at lower CH4/O2 feed ratios. The poisoning effect was also shown to be independent from the type of sulphur bearing compound and only indirectly affected by the type of catalyst support (La2O3 or SiO2 stabilised alumina) through the value of Rh dispersion. In fact by using in situ DRIFTS experiments of adsorbed CO at room temperature it was found that sulphur acts as a selective poison by preferentially adsorbing on smaller well dispersed Rh crystallites whilst larger metallic Rh sites are mostly unaffected. The adsorption of CO at room temperature before and after S poisoning is schematically represented below. Partial substitution of Rh/La-Al2O3 monolith catalysts with either Pt or Pd did not influence the way S adsorbs on highly dispersed Rh sites. Pd was found to have a detrimental effect on the overall catalytic activity and to be ineffective at improving the S-tolerance. On the other hand the partial substitution of Rh with Pt reduced the detrimental impact of S, which strongly inhibits the SR reaction on dispersed Rh sites but has a much smaller impact on Pt active sites. The improved tolerance of the bimetallic Rh-Pt catalyst against sulphur is due to its higher operating temperature related to the high oxidation activity of Pt which facilitates sulphur desorption from the catalyst and reduces its accumulation.

Flamelet/progress variable modeling of partial oxidation systems: From laboratory flames to pilot-scale reactors

Catalytic partial oxidation reactions and reactors

Catalytic partial oxidation reactions and reactors

The kinetics of low-temperature oxidation (LTO) of crude oils in porous media was studied. Isothermal integral reactor data were analyzed to obtain rate equations for the over-all rate of the partial oxidation reactions at temperatures below partial oxidation reactions at temperatures below 500 deg. F. The reaction order with respect to oxygen was found to be between 0.5 and 1.0. The order of the reaction was dependent upon the crude but independent of the properties of the porous medium. The activation energy of the reaction was insensitive to the type of crude or porous medium and is in the neighborhood of 31,000 Btu/lb mol. LTO reactions were found to be in the kinitics-influenced region. The measured reaction rates for a 19.9 deg. API and a 27.1 deg. API crude indicated higher oxidation rates under similar reaction conditions for the higher API gravity crude. Light crudes appear to be m ore susceptible to partial oxidation at low temperatures because of the react ed oxidation reactions rather than by carbon oxidation. Other information includes the fraction of reacted oxygen utilized in carbon atom oxidation by the LTO reaction and the molar ratio of CO2 and CO produced in the low-temperature region. Effect of partial oxidation of the crude on the in-situ combustion process was studied by experimentally simulating the zones preceding the combustion front where temperatures and injection rates of linear reservoir model were programmed with time according to a predesigned schedule. Oxidation of the crude at temperatures below 400 deg.F had significant effects on the behavior of the crude-oil/water system in the porous medium at elevated temperatures and on the fuel available for combustion. A substantial decline in the recoverable oil from the evaporation and cracking zones, an increase in fuel deposition, and drastic changes in fuel characteristics and coked sand properties were obtained when the crude was subjected to LTO during the simulation process. Introduction The application of thermal energy to petroleum reservoirs as a means of increasing crude oil recovery has been given a great deal of attention. In underground combustion, thermal energy is induced by the partial burning of the crude oil in situ. The production of heat by the exothermic oxidation reactions of the hydrocarbons constitutes a unique feature of the in-situ combustion process. The chemical reactions and the accompanying heat released create a new temperature profile and cause drastic redistribution in the reservoir fluid saturations. With oxygen available in the transient zones of variable temperature and hydrocarbon saturations, several oxidation reactions of differing nature can take place during an underground combustion process. Because of the complex composition of process. Because of the complex composition of crudes and the great number of reaction products that can be produced, it is convenient to classify the hydrocarbon oxidation reactions ascombustion reactions that take place in the high-temperature combustion zone (above 600 deg. F) with CO2, CO, and H2O as the principal reaction products andpartial oxidation or low-temperature products andpartial oxidation or low-temperature (LTO) reactions that occur in zones where the temperature is lower than 600 deg. F. Several partial oxidation reactions are known to take place, producing primarily water and oxygenated producing primarily water and oxygenated hydrocarbons such as carboxylic acid aldehydes, ketones, alcohols, and hydroperoxides. High-temperature combustion reactions are desirable because they generate most of the heat required for the in-situ combustion process. Partial oxidation reactions, on the other hand, are in most cases undesirable because of their adverse effect on the viscosity and distillation characteristics of the crude. SPEJ P. 253

SUMMARY\nThe current doctoral thesis studies the synthesis of Mo-V-Te-containing mixed metal oxides and the characterization of their physico-chemical and catalytic properties for alkanes selective oxidation reactions, such as the oxidative dehydrogenation of ethane and the partial oxidation of propane into acrylic acid.\nFirst, the study focuses on the effect of promoters (cations of Ga, Al or Nb) in Mo-V-Te mixed oxide catalysts prepared by hydrothermal synthesis, incorporated by wet impregnation or directly through the synthesis gel. In general, an improvement in the catalytic behavior of promoted Mo-V-Te catalysts for propane partial oxidation to acrylic acid has been observed for all the promoters studied using both incorporation methods. In the case of Ga, the best results have been observed for the impregnation method, which seems to favor the Ga incorporation mainly on the catalyst surface decreasing the density and strength of acid sites responsible for overoxidation reactions. On the other hand, the addition of Ga directly through the synthesis gel leads to its partial incorporation into the crystalline phase responsible for the activation and selective transformation of the alkane, the so-called M1 phase. This incorporation appears to be dependent on the Ga/V ratio in the synthesis gel, due to the competition found between both elements (Ga and V) to occupy similar sites in the M1 structure. Since the occupation preference is for vanadium, the lower the the Ga/V ratio in the synthesis gel, the lower the Ga amount incorporated into the catalysts frame.\nOn the other hand, the addition of cations Al or Nb cations as promoters in Mo-V-Te mixed oxide catalysts, by impregnation method, leads to an improvement of the catalytic behavior for propane partial oxidation to acrylic acid, compared to any of the Mo-V-Te catalysts with Ga incorporated as promoter. The best results have been reached with the Nb-containing catalyst for which either the catalytic activity or the\nselectivity to acrylic acid are enhanced, while only the selectivity is improved in the case of Al.\nThose materials have been also tested as catalysts for oxidative dehydrogenation (ODH) of ethane to ethylene. In this case, the differences in the catalytic behavior, among promoted Mo-V-Te catalysts or those and the pristine Mo-V-Te catalyst are not as marked as for propane partial oxidation. This fact is related to the effect of the promoter especially on the acid-base surface properties, and to the different nature of the reaction product in each case, i.e. olefin (for ethane) and unsaturated carboxilic acid (for propane). The formation and stability of the latter more sensitive to changes on the catalyst surface acidity.\nFinally, a new method has been optimized to develop efficient Mo-V-Te-Nb mixed oxide catalysts for partial alkane oxidation reactions, based on the precipitation by reflux synthesis of the catalyst precursor. The influence of different parameters for the reflux synthesis, such as temperature, time, pH and type of vanadium reactant, have been investigated and optimized in order to favor the formation of the so-called M1 phase, responsible for the activation and selective transformation of the alkane. The catalysts optimized in a short period with this new method show catalytic properties, for either ethane or propane partial oxidation, comparable to those obtained with catalysts after many years of optimization using traditional methods, i.e. hydrothermal and dry-up or co-precipitation methods. Due to the nature of the new method based on reflux synthesis, comprehension on some aspects of the synthesis mechanism of these materials has been advanced