Manuel Farinha Portela

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.

Catalytic oxidative dehydrogenation of n-butane

This review deals with the catalytic conversion of n-butane by oxidative dehydrogenation as an alternative process to direct dehydrogenation, making special reference to the catalytic systems used, kinetic studies performed and reaction mechanisms proposed. Particular attention is also focused on the hydrocarbon activation process and on the main factors governing catalytic efficiency. Finally, after some general conclusions, future trends are analyzed.

Synergy effects between β and γ phases of bismuth molybdates in the selective catalytic oxidation of 1-butene

Abstract β+γ mixed Bi–Mo–O phases with a large range of composition were prepared by coprecipitation for investigating the existence of synergetic effects between β and γ phases for the oxidative dehydrogenation of 1-butene. Results from surface characterization show that the surface of the catalysts always has a composition similar to the β phase but the γ phase grows around β phase. Mixed phases present higher activity and less CO 2 selectivity than pure β and γ phases. This result leads to propose that the whole catalyst (bulk and surface) participates in the catalytic process. The synergetic effect between these two phases can be ascribed to the high oxygen mobility in the lattice of the γ phase that allow the migration of oxygen species from the γ to the β phase.

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.

Effect of alkali metal promoters on nickel molybdate catalysts and its relevance to the selective oxidation of butane

Abstract The role of alkali metal promoters (Li, Na, K, Cs) in the α and β phases of NiMoO4 for butane oxidative dehydrogenation (ODH) was studied. The catalysts were characterized by BET, ICP, X-ray fluorescence, AA, XRD, TG, DSC, IR, and XPS techniques. It was evidenced that the added alkali metals are deposited only on the catalyst surface and practically do not affect the bulk. An influence of the concentration and basicity of the promoters on the decrease of the catalyst surface area was observed. Reaction studies show that addition of promoters generally reduces the catalytic activity but the promoters induce a high selectivity to butenes and butadiene. The greater the basicity and the amount of promoter ion, the higher is the selectivity and this improvement arises largely by suppression of total oxidation. The combination of activity and selectivity effects leads to improvement of yields of C4 hydrocarbons by addition of small amounts of alkali promoters up to an optimum loading. Overdoping deteriorates yields, specially for β phases. The observed changes are understood by considering the physical and chemical effects related with the interaction of the alkali promoters with the catalyst. Basicity is shown to be fundamental to promote the selective ODH of butane in the NiO MoO3 system.

Nickel molybdate catalysts and their use in the selective oxidation of hydrocarbons

Abstract This paper reviews the preparation techniques, characterization, and use of nickel molybdate catalysts in the selective oxidation of hydrocarbons, particularly of light alkanes. Catalysts with different Ni:Mo ratios, unsupported and supported, undoped and doped, were considered. Particular attention is given to the thermal activation process for the transition of the low temperature α‐phase into the metastable β‐phase, which was shown to be more selective in some cases. Special reference is also made to the results of kinetic studies performed, to the mechanisms proposed for some important reactions, and to the nature of the active sites. Finally, after some general conclusions, future trends are analyzed.

Homo- and copolymerisation of norbornene and styrene with nickel bis(acetyl acetonate)/methylaluminoxane system

Abstract The homo and copolymerisations of norbornene and styrene with nickel bis(acetyl acetonate)/methylaluminoxane system were systematically investigated. This catalytic system shows a high activity towards homopolymerisation of both norbornene and styrene. For random copolymerisation, an increase in the initial styrene feed content leads to a gradual loss of activity relative to norbornene homopolymerisation. On the other hand, a drastic loss of activity, relative to styrene homopolymerisation, was found for very low norbornene feed contents. These results are qualitatively interpreted using the trigger coordination mechanism proposed by Ystenes. The structural characterisation of the polymers showed that norbornene polymerisation occurs via a 2,3 addition mechanism and that true copolymers are formed by random copolymerisation. At low styrene feed contents, only isolated styrene units or very short styrene sequences are present in the resulting copolymer. At higher styrene feed contents, short polystyrene sequences with more than eight styrene units may be formed. Determination of the reactivity ratios shows a much higher reactivity for norbornene (rnorbornene=17.8 and rstyrene=0.16). Size exclusion chromatography measurements have shown that polynorbornenes possess high molecular weights. Relatively low molecular weights were observed for homopolystyrene and copolymers.

Reducibility of undoped and Cs-doped α-NiMoO4 catalysts : Kinetic effects in the oxidative dehydrogenation of n-butane

Abstract Temperature-programmed reduction (TPR) was used to study the reduction of undoped and doped α-NiMoO 4 with several Cs loads. The results contributed to clarify the mechanism of the nickel molybdate reduction by hydrogen. It was found that the reducibility of the catalysts, as inferred from the temperature of reduction onset, decreases with Cs load. The catalysts were tested for the oxidative dehydrogenation of n -butane and the activity followed the same trend recorded in the TPR experiments. It was possible to conclude that the catalytic activity is related to the reducibility of the sample while the selectivity is determined by the basicity of the surface. Kinetic studies over unpromoted and 3% Cs-promoted α-NiMoO 4 evidenced that Cs doping only affects the partial order with respect to butane, which increases for dehydrogenation products and decreases for CO and CO 2 . The partial order with respect to oxygen is almost unaffected. The kinetic results were discussed based on the basicity and reducibility of the catalysts.

Oxidative dehydrogenation of ethane on the α and β phases of NiMoO4

Abstract The oxidative dehydrogenation of ethane on the α and β phases of NiMoO4 was studied, and the results compared with those previously obtained on propane in the 500–600°C range. A reaction mechanism is proposed in accordance with the surface properties, preliminary kinetic studies and catalytic tests with ethanol as a probe molecule.

Improved adhesion of polyethylene by copolymerisation of ethylene with polar monomers

Abstract The inertness of polyethylene restricts its compatibility with other materials. Thus, the adhesion between polyethylene and various substrates usually requires surface pretreatments on the polyolefin in order to obtain satisfactory results. One different approach is the production of ethylene copolymers with small amounts of polar comonomers that can act as adhesion promoters. Using metallocene based catalysts, ethylene copolymers with 10-undecenoic acid (UA) and with 5,7-dimethylocta-1,6-diene (5,7DMO) were synthesised. The adhesion strengths of the copolymers in direct tension were measured. The obtained results show that UA/ethylene and 5,7DMO/ethylene copolymers exhibit higher adhesion strengths with increasing comonomer contents. UA/ethylene copolymers are more effective in improving adhesion properties than 5,7DMO/ethylene copolymers. This difference is easily understandable on the basis of the polarities of both comonomers.