C. Mirodatos

Deactivation of supported nickel catalysts during the reforming of methane by carbon dioxide

Abstract Several supported nickel catalysts were tested for the methane reforming reaction at 700°C. The initial activity is found to depend essentially on the state of the nickel phase (reduction and dispersion) and little on its environment (support, additive). The product distribution is controlled by the WGS equilibrium. The zero order aging process is mostly due to carbon deposition, though slight Ni sintering also occurs. Among the deposited carbon, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles. Another form, less stable and arising from methane activation, is rapidly accumulated on the catalyst, but at a low level and limited extent.

A new concept in zeolite-catalyzed reactions: Energy gradient selectivity

A study of the cracking of n-heptane, n-octane, and isooctane on a variety of zeolites (erionite, offretite, ZSM-5, mordenite, and faujasite) has shown that the formation of light products (C1, C2, C3) is favored for zeolites with small cavities or tortuous channels (erionite, highly decationated offretite, and ZSM-5). The C4 production is kept high when molecules have access only to rather large channels or cavities (cationic offretite, mordenite, and faujasite). It is proposed that the occurrence or not of secondary reactions, which determines the ratios solC4C3 and solC4C2, depends on an “energetic” selectivity which characterizes cages or channels. The smaller and the more tortuous is the space where the molecules move, the higher are the field and field gradient and the more are secondary reactions favored. The ratio solC4C3 (or solC4C2) is proposed as an “energy gradient selectivity” index. This index does not depend on factors which are related to shape selectivity.

Low temperature oxidative dehydrogenation of ethane over catalysts based on group VIII metals

Abstract Unsupported Fe, Co and Ni catalysts are active in the oxidative dehydrogenation of ethane (ODHE) at low temperature. A conversion of 1% is achieved at 585, 438 and 487 K, respectively. The selectivity towards ethylene is found to be ca. 60% at low conversion. As the reaction temperature increases, the selectivity remains nearly constant for Ni whilst it decreases for Fe and Co. This behaviour has been explained by comparing the catalytic properties of Co and Ni towards ethane and ethylene oxidation. The ODHE intrinsic activity sequence (Co>Ni>Fe) is similar to that observed for the homomolecular exchange of oxygen, confirming that these catalysts are in an oxidized state in the course of the ODHE reaction. The study of the reduction and oxidation of unsupported and silica-supported nickel catalysts using magnetic methods has shown that the catalytic activity is not related to the ease of the Ni 2+ Ni 0 transition. TAP experiments carried out over Ni have revealed that the oxygen species involved in the reaction are irreversibly held by the catalyst at 573 K (possibly O − ) from which a reaction mechanism is proposed. Furthermore, this series of experiments have shown that ethane is irreversibly adsorbed and that CO 2 originates from a parallel-consecutive scheme.

Catalytic partial oxidation of methane over ni-, co- and fe-based catalysts

Abstract Partial oxidation of methane to synthesis gas has been investigated over Fe-, Co- and Ni-based catalysts in fixed bed reactor in view of a possible application in a fluidized bed reactor. Catalytic testing of the oxidic catalysts show an increasing activity for total oxidation of methane in the following order: Fe2O3>CoO>NiO. After reduction of the metal, the Ni and Co catalysts give equilibrium conversion to synthesis gas. Co is much more easier oxidized compared to Ni at decreasing temperature under experimental conditions and equilibrium is not followed at temperatures below 550°C. Low reforming activity is obtained over the Fe catalyst. Investigations at 1000–1100°C are necessary to further explore Fe as a reforming catalyst.

Characteristics and performance in the oxidative dehydrogenation of propane of MFI and V-MFI zeolite membranes

Original V-MFI zeolite membranes in/on alumina tubes have been prepared, characterized and tested for the oxidative dehydrogenation of propane. The morphological and textural characteristics of the V-MFI membrane were found to be very similar to those of V-free MFI membranes. The insertion of vanadium species has been checked by elemental analysis, XRD, FTIR and ESR. The single gas permeance studies showed that both MFI and V-MFI membranes were free of macrodefects and provided an activated transport for both C3H8 and O2 above 400°C, with a Knudsen-like permselectivity. Both MFI and V-MFI membranes were found to produce propene with about 40% selectivity but with higher O2 and C3H8 conversions for the V-MFI. In the studied conditions, between 550 and 650°C, the oxygen distributor configuration was not found to improve the reactor performance compared to the flow-through one.

Unraveling mechanistic features for the methane reforming by carbon dioxide over different metals and supports by TAP experiments

Abstract Four different catalysts (Ni/SiO2, Ni/Al2O3, Ru/SiO2, Ru/Al2O3) were extensively investigated in the TAP reactor, in order to elucidate the role of the metal and support in the dry reforming of methane. Over nickel, dissociative CH4 adsorption leads to H2 gas and to an accumulation of carbon species on the surface. Dissociative CO2 adsorption takes place giving CO gas and adsorbed oxygen. The latter reacts with carbon, originating from CH4, on the surface to give a second CO molecule in the rate determining step. Over ruthenium, the methane activation is the slowest step and CO2 reacts directly with adsorbed carbon to CO. The surface oxygen accumulation is minimal and therefore, the oxidation hydrogen is suppressed. Whereas, the silica support is rather inert, the alumina support participates in the reaction by delivering oxygen atoms to the metal via hydroxyl groups spill-over. In addition, a strong interaction between gaseous CO/CO2 and oxygenated adspecies is observed on alumina. The reverse water gas shift reaction takes place under the TAP conditions. The different results are discussed in terms of the hydrogen selectivity.

Acidic and cracking properties of offretite

The acidic and catalytic properties of offretite zeolites have been studied as a function of potassium content. NH3 thermodesorption and ir studies of pyridine adsorption have shown that the weaker protonic acid sites arise from the exchange of the first of the three potassium cations located in gmelinite windows. The progressive removal of the last two cations creates strong acid sites and allows the reaction molecules to enter the gmelinite cages. There is evidence for two types of Lewis acidity. One of them is suggested to arise from a partial dehydroxylation during pyridine evacuation. Both Lewis sites are strong. The isooctane cracking selectivity is modified and gives a high C3C4 ratio at the expense of iso C4 when the ion content is small enough to allow a diffusion of molecules through the gmelinite cavities.

Uses of transient kinetics for methane activation studies

Abstract The use of transient kinetics is reviewed for recent works performed in the wide area of methane valorization. The specific contribution of techniques such as steady-state isotopic transient kinetics, fast temperature transients, TAP reactor studies is illustrated for reactions involving different modes of methane activation and various types of catalysts.

Combinatorial approaches to heterogeneous catalysis: strategies and perspectives for academic research

The application of combinatorial chemistry to heterogeneous catalysis is analysed in terms of current strategies and perspectives on the industrial and academic levels. Potential methodologies for academic research laboratories are proposed with emphasis on both theoretical and practical considerations.

Selective and non-selective oxygen species determining the product selectivity in the oxidative conversion of propane over vanadium mixed oxide catalysts

The selectivity of catalytic oxidation reactions strongly depends on the availability of different oxygen species participating in the catalytic reaction. Lattice oxygen is generally assumed to act as the selective agent. However, although adsorbed, electrophilic oxygen species should lead to total oxidation, several indications have been reported that these species may also participate in selective reaction steps on the catalytic surface of several metal oxides. In the present work the role of different oxygen species on vanadium containing catalysts, V-Mg-O, V-P-O and V-Sb-O is elucidated using vacuum pulse experiments and stationary flow experiments at reduced pressure (P<1kPa). Isotopic labelled 18 O 2 was partly used as oxidant in order to distinguish between lattice and adsorbed oxygen. On V-Mg-O and V-P-O catalysts adsorbed oxygen play a significant role in total oxidation, whereas only catalytically active lattice oxygen is available on V-Sb-oxide. However, on V-Mg-O and V-Sb-O different types of lattice oxygen exist leading either to selective or non-selective products.