A. Kiennemann

Methanol Selective Oxidation to Formaldehyde over Iron-Molybdate Catalysts

This review deals with the important industrial reaction of formaldehyde manufacture by methanol oxidation over iron molybdate catalysts. Detailed reference is made to the used catalyst, preparation techniques (coprecipitation, sol‐gel like, mechanical mixing, etc.) including unsupported and supported catalysts, promoters and characterization methods. The controversial active phase assignment (stoichiometric versus Mo rich iron molybdate) is discussed. The proposed reaction mechanisms and kinetic laws for the main and side reactions are examined. The catalyst deactivation processes are reviewed and the role of Mo excess on these processes is underlined. Finally conclusions and perspectives are presented.

Development of catalysts suitable for hydrogen or syn-gas production from biomass gasification

Tri-metallic and ternary oxide structures were prepared, characterised and tested for two fixed-bed catalytic applications: the reforming of methane with steam and CO2; and for conditioning the gas produced by biomass steam gasification in a fluidised bed of olivine particles, with the aim of drastically reducing high and low molecular weight hydrocarbon concentrations in the product gas. Methane conversions higher than 90% were achieved for a synthetic gas feed mixture (similar of that produced in biomass gasification) using the catalyst formulation LaNi0.3Fe0.7O3 at 800°C: the space time under reaction conditions was 0.05 s, and no carbon was formed for reaction periods of up to . The catalyst contained in a secondary fixed-bed reactor placed after the gasifier was able to convert about 90% by weight of the tar present in the raw gas at 800°C: the space time under reaction conditions was , during which time no coke formation was observed on the catalyst surface.

Combined infrared spectroscopy, chemical trapping, and thermoprogrammed desorption studies of methanol adsorption and decomposition on ZnAl2O4 and Cu/ZnAl2O4 catalysts

Abstract It is shown that FT-IR spectroscopy permits discrimination to be made between methoxy (methanol) and formate species adsorbed on ZnAl 2 O 4 and CuZnAl 2 O 4 catalysts. These species were found to be less stable on copper than on ZnAl 2 O 4 . The presence of reduced copper promotes methanol transformation into formates and then into C0 2 : (i) FT-IR results show that copper formate formation from methanol adsorption occurs even at room temperature and that surface oxygen ion participates in its formation; (ii) chemical trapping experiments demonstrate that increasing copper percentage destabilizes formate species, while TPD experiments correlatively indicate an accelerated transformation of formate into CO 2 . Formyl species are detected by chemical trapping only at the end of the reaction and are therefore assumed not to participate in the decomposition reaction.

Iron molybdate catalysts for methanol to formaldehyde oxidation: effects of Mo excess on catalytic behaviour

Two iron molybdate catalysts have been prepared by coprecipitation, one stoichiometric (Mo/Fe atomic ratioD1.5) and the other with a typical industrial composition (Mo/Fe atomic ratioD3). Physicochemical characterisation shows that Mo excess brings about an increase in the surface area and deconvolution of NH3 TPD curves evidences some changes in acidity. On the other hand, surprisingly, Mo excess does not cause significant changes in the activity for methanol oxidation per unit surface area. This provides evidence that Fe2(MoO4)3 is in fact the active phase of the catalyst. The catalyst with Mo excess leads to higher selectivity for HCHO to the detriment of dimethyl ether selectivity. This result is attributable to larger oxygen availability at the catalyst surface and changes in acidity. The catalytic behaviour of the prepared catalysts was compared with that of an industrial catalyst. It was found that the prepared catalyst with an Mo/Fe composition similar to the industrial one is more active due to larger surface area.

Fe–Co based metal/spinel to produce light olefins from syngas

Abstract New metal/oxide (Co–Fe) catalysts (with no reduction or thermal pre-treatment) are efficient to produce light hydrocarbons with a low selectivity in CO 2 by the Fischer–Tropsch synthesis. The low selectivity in CO 2 is due to the occurrence of the CO 2 /H 2 reaction. These materials are stable under reaction conditions, and only few carbides are formed during the Fischer–Tropsch reaction. X-ray analyses indicate that the most degraded phase is the (Co–Fe) alloy phase in CO/H 2 reaction and the spinel phase in the CO 2 /H 2 reaction. It was demonstrated that these composites do not behave as the simple sum of a spinel phase and a (Co–Fe) alloy but have their own properties.

Absorption/desorption of NOx process on perovskites: performances to remove NOx from a lean exhaust gas

Abstract NOx adsorption/desorption capacities of perovskites (ABO3) were measured under representative exhaust gas mixture conditions at temperatures below 550°C, with A=Ca, Sr, Ba and B=Sn, Zr, Ti. The solids exhibited good NO2 sorption capacities with a reversible adsorption to desorption process according to the sequence Ba>Sr>Ca for A, while for element B the sequence Sn>Zr>Ti is observed. In the case of alkaline earth metals, the absorption behaviour proved to be directly related to their electropositivity (Ba>Sr>Ca). The key factors which control the absorption of NO2 are the bonding energy between the element B and the oxygen atom on one hand and the electropositivity of the element A on the other. The best result was obtained with the perovskite BaSnO3. The absorption of NO2 is favoured at low temperature and in the presence of water. The addition of platinum has no significant influence upon any NO2 absorption.

Removal of NOx: Part I. Sorption/desorption processes on barium aluminate

Abstract NO x adsorption/desorption capacities of barium aluminates were measured under representative exhaust gas mixture at temperatures below 550°C. The solid doped with Pt or not, exhibits good NO 2 sorption capacities with a reversible adsorption to desorption process. With bulk BaO, desorption was observed at high temperature. The different behaviour between the two catalysts is explained by the fact that strongly bonded carbonates are formed on bulk BaO while they do not exist on barium aluminate, therefore allowing the formation of nitrates which can be decomposed by a thermal process. SO 2 poisoning was also studied.

Mechanism of deactivation of iron-molybdate catalysts prepared by coprecipitation and sol–gel techniques in methanol to formaldehyde oxidation

Stoichiometric iron-molybdate and an industrial like catalyst, with the usual Mo excess, were prepared by the normal coprecipitation technique. Additionally, a Mo rich catalyst was prepared by sol–gel like technique. The catalytic stability was tested in the presence and in the absence of water in the reactor feed. Only the stoichiometric catalyst deactivates appreciably in the tested conditions. Water in the reactor feedaccelerates this process andexhibits a markedreaction inhibition e7ect. Water seems to hamper the catalyst reoxid ation during reaction. The results of the applied characterisation techniques evidenced that the deactivation mechanism involves an increase of surface reduction (higher Fe 2+ =Fe 3+ atomic ratio) andsurface Mo loss by MoO 3 sublimation andformation of Mo–methanol and Mo–water volatile compounds. Mo species migrate from the bulk to the catalyst surface to compensate the Mo loss favoured by water. Mo excess is in fact required to have a stable, active and selective iron-molybdate catalyst for the methanol to formaldehyde oxidation. ? 2003 Elsevier Science Ltd. All rights reserved.

Methanol synthesis on Cu/ZnAl2O4 and Cu/ZnOAl2O3 Catalysts: Influence of carbon monoxide pretreatment on the formation and concentration of formate species

Abstract Chemical trapping, thermoprogrammed desorption (TPD) and FT-IR spectroscopy have been used to study the mechanism of methanol synthesis on Cu/ZnAl 2 O 4 and Cu/ZnO Al 2 O 3 catalysts. Pretreatment of the catalyst by carbon monoxide creates anionic vacancies and increases the amount of surface formate species. Three kinds of formate are formed as characterized by FT-IR spectroscopy and TPD. One is the formate located on copper; another, which desorbs at a temperature between copper formates and formates located on ZnAl 2 O 4 , is assumed to be formed in the vicinity of an oxygen vacancy. The third is located on ZnAl 2 O 4 . Implications of these results on the reaction mechanism are discussed.

Application of chemical trapping to the determination of surface species and to the study of their evolution under reaction conditions in heterogeneous catalysis

Abstract The detection of surface species by chemical trapping is described and discussed. The results obtained by scavenging intermediates in CO-H 2 and CO 2 -H 2 reactions show the relevance of this method to the study of reaction mechanisms in heterogeneous catalysis. Quantitative detection of some species (carboxylate, formyi, methoxy) is achieved and a correlation has been found between activity and surface species on a Pd/MgO-SiO 2 catalyst. A reaction scheme is proposed for the CO-H 2 and CO 2 -H 2 reactions.