Oxidation of methane to formaldehyde over Fe/SiO2 and Sn---W mixed oxides

In an attempt to develop new catalysts for the formation of formaldehyde from methane, the promotion effect of Fe on SiO₂ and that of Sn on WO₃ have been studied. The formation of formaldehyde on silica can be appreciably enhanced by the impregnation of Fe, as far as iron loadings are kept below 0.1 atom.% (Fe/Si × 100). In the case of Sn---W---Ox catalysts, both the addition of Sn to WO₃, and that of W on SnO₂ were effective to the selective formaldehyde formation. Absorption spectra (UV-Vis) and ESR measurements revealed that tetrahedrally coordinated Fe³⁺ in the silica network plays an important role in the formation of formaldehyde. A thin surface layer consisting of W and Sn oxides can account for the selective formaldehyde formation on the Sn---W---Ox catalysts.     

Can isotopic transient and formal steady-state kinetics lead to a converging surface chemistry analysis? Case study on the selective reduction of NO by CH4 over Co-ZSM-5 catalysts

The surface coverage analysis derived from two formal steady-state kinetic models is compared to values directly obtained from steady-state isotopic transient analysis (SSITKA) for the selective catalytic reduction (SCR) of NO by CH₄ over Co-ZSM-5 catalysts. It is shown that the most abundant reacting intermediates are NO _x adspecies, though no clear differentiation between the various adspecies identified by DRIFT spectroscopy was achieved. Less numerous carbon containing adspecies were identified and quantified in the reacting system, essentially as methoxy species. Nitromethane-like intermediates remained undetectable due to a very rapid transformation into N₂ and CO₂. On the basis of these converging kinetic analyses related to each elementary step of the SCR process, a microkinetic model can be derived, which allows describing transient operation, in view of a non steady-state application.     

A Novel Technology for Natural Gas Conversion by Means of Integrated Oxidative Coupling and Dry Reforming of Methane

A novel process concept for the oxidative coupling of methane followed by the oligomerization to liquids has been developed within the frame of the EU integrated project OCMOL. This technology is based on process intensification principles via cutting‐edge structured microreactor technology. It is also a fully integrated industrial process through the re‐use and the recycling of by‐products, in particular CO₂, at every process stage. The focus of this contribution is on the reaction engineering aspects of the core steps, i.e., catalysts, kinetics and reactor design for the methane coupling and reforming.     

Analysis of volume‐to‐surface ratio effects on methane oxidative coupling using microkinetic modeling

The effect of the volume‐to‐surface (V/S) ratio on the catalytic performance of a La–Sr/CaO catalyst in a fixed bed reactor under oxidative coupling of methane (OCM) conditions is investigated by adjusting the amount of diluent in the catalyst bed. It was observed experimentally that the catalyst activity, C₂ selectivity, and C₂H₄/C₂H₆ ratio are all favored at high V/S ratios. The total void volume, available in the intraparticle and the interstitial phase, was considered. A comprehensive OCM microkinetic model, explicitly distinguishing between these two phases, allowed accounting for the observed dependence of catalytic performance on V/S ratio. The major experimentally implemented variation in interstitial volume available for reaction, provoked also changes in radical concentration profiles in intraparticle phase. Given the high reaction rates occurring at this location, the experimentally observed effects with varying the V/S ratio, are attributed to concentration and, hence, reaction rate changes occurring mainly in the intraparticle phase. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2603–2611, 2018     

High-throughput approach to the catalytic combustion of diesel soot II: Screening of oxide-based catalysts

Following the development of a high-throughput (HT) methodology for the evaluation of diesel soot oxidation catalysts in a 16 parallel channels reactor, a library of over 60 catalysts was tested under optimized conditions. The catalyst compositions were chosen to include solids which specific properties like oxygen storage capacity, oxygen mobility and ionic conductivity. The key parameters for high activity appear related to the presence of active and mobile surface oxygen species, and to an appropriate catalyst particle size in order to favour the number of contacts with the soot. In contrast, high oxygen storage capacity and bulk oxygen ion mobility do not appear as relevant properties for high catalytic activity. Nine new formulations were found to perform better than the reference catalyst “high surface area (HSA) ceria” (Rhodia).     

Methane activation in exchange and oxidative dimerization reactions : a study using isotope steady-state and transient techniques

A study of methane activation (reversible for exchange reaction or irreversible for oxidative dimerization) is carried out on alkali promoted and unpromoted basic oxides (magnesia and lanthana) using isotopic steady-state and transient techniques. Rates of CH₄/CD₄ scrambling, isotopic effect between CH₄ and CD₄ coupling, surface concentration of reversibly adsorbed methane, of C₂ and CO_x precursors are determined. Mechanistic propositions focused on the role played by the surface basicity and the poisoning effect of alkali carbonate are formulated.     

Acidity Characterization of Catalyst Libraries by High-Throughput Testing

The development of acid and bi-functional catalysts with controlled properties is a matter of high interest for refinery applications. The issues of characterization and quantification of catalyst acidity in the frame of high throughput (HT) screening are outlined. The present study deals with the development of a HT method for the characterization of the Brønsted acidity of 80 bifunctional catalysts library by means of isomerization of 3,3-dimethylbutene testing. Catalytic results are validated with literature data. The choice of probe molecules, the operating conditions and deactivation issues upon testing are addressed. This method allows quantifying very accurately the acid strength. Catalyst ranking from large library can be achieved an acidity scale in conditions similar to the practical applications. The pros and cons of the use of model reactions for HT quantitative characterization are discussed.     

Catalytic membrane reactor for oxidative coupling of methane. Part II —Catalytic properties of LaOCl membranes

This work presents the potential of a LaOCI porous membrane reactor for the reaction of oxidative coupling of methane. The sol-gel method provided LaOCl membranes supported on alumina tubes which presented mesoporous texture. Actually, the severe operating conditions of this reaction caused textural instability which restricted any transport effects to the macroporous domain. However, despite moderate separation effects between methane and oxygen, beneficial effects of the membrane reactor have been observed, subject to surface composition, structural and feeding configuration requirements.     

Catalytic membrane reactor for oxidative coupling of methane. Part 1: preparation and characterisation of LaOC1 membranes

The sol-gel process was investigated in order to prepare LaOC1 inorganic membranes on commercial alumina tubular supports which were catalytically active for the oxidative coupling of methane. The preparation of LaOCI meso and microporous membranes is described on the basis of the fundamental phenomena occurring during the sol preparation and thermal treatment. Membranes were synthesised from LaC1₃ precursor in aqueous media. Important parameters, mastering the sol formation and consequently the final material textural characteristics, were the molar ratio [aceta]/La],[NH₃/La] and the total concentration of lanthanum in the sol. Without any acetate addition, turbid sols were obtained leading at 800°C to mesoporous membranes. The effect of acetate ions is shown to be of prime importance in order to limit the polycondensation reactions in the sol and to prepare microporous materials. Under these conditions, quite dense membranes were obtained at 200°C due to lanthanum acetate polymerisation. The formation of carbonates and their decomposition at 600°C explain the maximum micro-porosity observed at this temperature. When these membranes were treated at 800°C, the microporous volume decreased.