M. Yu. Sinev

Kinetic and structural studies of oxygen availability of the mixed oxides Pr{sub 1{minus}{ital x}}M{sub {ital x}}O{sub {ital y}} (M=Ce, Zr)

One composition of Pr{endash}Ce mixed oxide and a range of compositions of Pr{endash}Zr mixed oxide were prepared by coprecipitation methods and characterized by x-ray powder diffraction, thermogravimetric analysis, and x-ray photoelectron spectroscopy. Based on phases formed, the PrO{sub {ital y}}{endash}ZrO{sub 2} system in an oxygen-containing atmosphere at moderate temperatures (up to 800{endash}1000{degree}C) is analogous to that of CeO{sub 2}{endash}ZrO{sub 2}. Addition of either Ce or Zr to pure Pr oxide affects both the total amount of oxygen which can be reversibly exchanged between oxide and gas phase and the kinetics of the redox processes: Ce dramatically increases the amount (per Pr atom) and lowers the temperature of exchange, Zr slightly decreases the amount and also lowers the temperature of exchange, and both modifiers speed up the rate. These observations are rationalized in terms of bulk and surface structural features of the mixed oxides. {copyright} {ital 1996 Materials Research Society.}

Kinetics of oxidative coupling of methane: bridging the gap between comprehension and description

The development of notions about the mechanism of the oxidative coupling of methane (OCM) over oxide catalysts and corresponding progress in its kinetic description are reviewed and discussed. The latter becomes essential at the stage of scaling up and optimization of the process in pilot and industrial reactors. It is demonstrated that the main achievements in the development of kinetic models can be reached by combining the approaches conventionally used in homogeneous gas-phase kinetics and in heterogeneous catalysis. In particular, some important features of the OCM process can be described if several elementary reactions of free radical species (formation and transformation) with surface active sites are included into the detailed scheme of methane oxidation in gas. However, some important features, such as a non-additive character of the reciprocal influence of methane and ethane in the case of their simultaneous presence in the reaction mixture, cannot yet be described and comprehended in the framework of schemes developed so far. Possible ways towards an advanced kinetic model, accounting the main principles of catalyst functioning (redox nature of active sites) and pathways of product formation (via free radicals) are traced.

Kinetic modeling of heterogeneous-homogeneous radical processes of the partial oxidation of low paraffins

Abstract A new approach to kinetic modeling of heterogeneous-homogeneous radical processes of the partial oxidation of low paraffins over oxide catalysts is suggested. It is based on the elucidation of gas phase and heterogeneous reactions taking part in the overall process and on the correlation between their thermochemistry and kinetics. The model calculations explain: (1) the magnitudes of the reaction rates at different temperatures; (2) the mechanism of catalysts re-oxidation; (3) the influence of the addition of peroxides on the rate of oxidative coupling of methane.

On the role of heterogeneous and homogeneous processes in oxidative dehydrogenation of C3-C4 alkanes

Abstract The effects of reactor arrangement, catalyst particle size and void volume on the oxidative dehydrogenation (ODH) of propane and iso -butane are studied. In an empty quartz reactor, a high conversion of alkane (up to 36% with a 40% ODH selectivity) can be achieved. Packing of the reactor with different solids (quartz, γ-Al 2 O 3 , complex V-containing oxide catalysts) leads to a drastic change in reaction parameters, which is strongly dependent on the chemical nature of solids and their particle size. The analysis of observed phenomena indicates that: (a) the termination of gas-phase chain reaction takes place on the surface of any solid material; (b) certain solids behave as active generators of free radicals, thus initiating the new reaction pathway which develops both in the gas-phase and on the solid surface.

Physico-chemical properties of V-Sb-oxide systems and their catalytic behaviour in oxidative dehydrogenation of light paraffins

Catalysts prepared as bulk VSb0.1Ox and supported V2O5/Al2O3. V2O5-Sb2O3/Al2O3 and Sb2O3/Al2O3 (containing 0.5, 1 or 2 theoretical monolayers of V2O5 or Sb2O3) were tested in the oxidative dehydrogenation of iso-butane at 550 degrees C in i-C4H10:O-2 :He=20:10:70 gas mixture. Fresh and used catalysts were characterised by BET, XRD and XPS, Reactivity and thermochemistry of active oxygen taking part in the redox cycle with ethane and hydrogene were studied using in situ differential scanning calorimetry. Temperature-programmed desorption of O-2 in He Row was also investigated and in situ DRIFT was applied to investigate surface species of the catalysts in flows of i-C4H10, O-2 and i-C4H10/O-2 mixture. Supported VSbyOx catalysts are more active and selective than bulk one. V-only supported catalysts display a high efficiency due to the high reactivity of VOX-species. In bulk catalyst, the surface is enriched with antimony. In supported samples, the surfaces V/Sb are close to the calculated ones. In the presence of antimony, the amount of active oxygen species and their reactivity in redox transformation is improved. The rates of vanadium reduction and reoxidation are also higher. Compared to V-only catalysts, supported V-Sb-catalysts display a lower coking activity and higher on-stream stability

Propane partial oxidation to acrolein over combined catalysts

A novel approach for the partial oxidation of propane to acrolein, based on the use of layers of combined catalysts in a single reactor, provides good yields of acrolein with selectivity above 62%. The results depend strongly on the layer configuration, and reveal new mechanistic features for the process.

Thermochemistry and Reactivity of Lattice Oxygen in V–Sb Oxide Catalysts for the Oxidative Dehydrogenation of Light Paraffins

Differential scanning calorimetry is used to study in situthe properties of strongly bound (lattice) oxygen in vanadium-containing supported catalysts for the oxidative dehydrogenation of paraffins C2–C4. Evidence is found that the process occurs via a stepwise redox mechanism with the participation of lattice oxygen from the catalyst. When the supported component is modified by an antimony additive, the amount of reactive oxygen increases and redox processes accelerate. Simultaneously, the rate of coke formation decreases. Additional modification by Bi and Ba leads to a further increase in the amount of reactive oxygen. In all cases, oxygen bound to vanadium atoms is responsible for the redox properties of the systems. The observed effects are analyzed from the standpoint of the ratio between different forms of active oxygen.

Effect of redox treatment on methane oxidation over binary catalyst

Publisher Summary This chapter discusses the oxidative transformations of methane with a catalyst system that combines an oxide and a metal component. The presence of both components gave rise to complex oscillation phenomena. The influence of pretreatment and reaction conditions over a wide range of parameters on the oscillatory process was studied. The possible role of mass transfer and the balance of heat in the reactor were analyzed and a model for the role of the components in the binary catalyst system is suggested. A chromel–alumel thermocouple (diameter 0.3 mm, sheathed in a quartz cover or bare) was placed co-axially into the reactor filled with oxide catalyst, making it possible to detect temperature oscillations, accompanying concentration oscillations. This thermocouple in bare form also acts as the metal component in the oxide-metal binary system. The surface area of the thermocouple filament is ∼ 7.5x10 -5 m -2 . The effect of the state of the surface on the kinetic behavior was studied, using various feeder, including the methane-oxygen mixture alternating with inert (He), oxidizing (O 2 ), and reducing (H 2 ) gases. If the oxide component is removed from the reactor, no conversion of reactants is observed, indicating a very low activity of the metal filament in methane oxidation. No reaction occurs also when ethane is added to the methane–oxygen mixture. Oxidation of methane in the presence of such a binary oxide-metal catalyst proceeds in an oscillatory regime and both temperature and concentration oscillations take place. Oscillations arise at the temperature at which the rate of reaction over the oxide component becomes noticeable (∼500 °C).

Modeling of oxidative transformations of light alkanes over heterogeneous catalysts

An approach to modeling catalytic oxidative transformations of light alkanes (LAs) based on the use of thermochemical data is developed. The fact that LAs are virtually not adsorbed on the surface of oxidation catalysts seriously limits the possibility of experimentally studying the mechanisms of their transformations. In addition, LAs, the least reactive organic compounds, are oxidized at elevated temperatures even on the most active catalysts, a circumstance that makes the contribution from the homogeneous process to the overall conversion rate significant. For this reason, it is necessary to develop multilevel models capable of describing of the elementary reactions of LAs and intermediate products of their transformations (including free radicals of various types) with active sites of catalysts as well as the process as a whole. The proposed approach, based on the experimental data on the thermochemical properties of redox active sites, is applicable to describing processes occurring in the presence of an oxide catalyst. It makes it possible to estimate the kinetic parameters of the elementary interactions of molecules and radicals with active sites in reduced and oxidized states, that is, to solve the problem of the initial stage of modeling of a multistep process. A number of examples of applying this approach to studying oxidative transformations (partial oxidation and oxidative coupling) of methane, oxidative dehydrogenation of C2–C4 alkanes, and reoxidation of catalysts are considered. The prospects of modeling the heterogeneous-homogeneous oxidation of light alkanes are discussed.

Free radicals as intermediates in oxidative transformations of lower alkanes

Publisher Summary Experimental facts and theoretical concepts, existing in the literature, indicate that the formation of free radicals play an important role in a number of catalytic oxidation reactions. This chapter discusses several studies analyzing the contribution of free radicals to several oxidative transformations of lower alkanes over oxide catalysts. It has been showed that the formation of free radicals in the interaction of alkane molecules with the surface of oxides may prove to be energetically preferable, as compared to any other mechanisms of their activation. The fractions of radicals transformed into different final products depend on the reaction conditions (temperature and oxygen pressure) and on the number of carbon atoms in the alkane molecule. The fate of radicals captured by the surface sites with the formation of the alkoxy groups depends on the number of carbon atoms in the alkane molecule, as well as on the properties of the catalyst surface. The most energetically favorable process of lower alkanes activation over oxide catalysts is a homolytic C–H bond dissociation with the formation of free radicals. The difference in energy expenditures for the formation of free alkyl radicals causes the difference in reactivities between C1–C4 alkanes. The main factors determining the efficiency of different oxides, as catalysts for lower alkanes oxidation, are the H-atom affinity of strong oxidizing surface sites and the oxygen binding energy. These thermochemical factors cause the rates and directions of free-radical reactions and, as a result, the catalytic activity and selectivity to certain products.