Cokes into the hierarchiacal zeolites (nature/location and toxicity/reactivity)

Le craquage du méthylcyclohexane (MCH) à 450 °C et la conversion de l'éthanol (EtOH) en hydrocarbures à 350 °C sous 30 bar sont effectués sur zéolithes HZSM-5 (de taille de cristallite micrométrique et nanométrique) hiérarchisées. Ces deux réactions modèles mais complexes conduisent à la formation du coke, qui est toxique en MCH et potentiellement actif en EtOH. La toxicité (Tox) et la réactivité du coke dépendent fortement des propriétés texturales des catalyseurs. Dans ce travail, il est démontré que quelle que soit la réaction, le coke dans le cas des zéolithes taille micrométriques est « lourd », il est principalement constitué d'alkylphénanthrènes et alkylpyrènes et est localisé dans les micropores. Dans les zéolithes de taille nanométriques et hiérarchisées (méso-microporeux), le coke est plutôt « léger », formé majoritairement d'alkylbenzènes et alkylnaphtalènes ; ce coke qualifié de léger, est localisé en surface externe. Le coke situé dans les canaux et intersection de la zéolithe HZSM-5 est plus toxique (Tox ≥ 1) que celui situé en surface externe (Tox < 1). La diminution du chemin de diffusion offre également un avantage certain lors de la régénération des catalyseurs en abaissant les températures d'élimination totale de ces cokes. Les effets des propriétés texturales sur les performances catalytiques et la désactivation sont nettement plus marqués dans le cas de EtOH (réaction plus sensible) que MCH.The Methylcyclohexane (MCH) cracking at 450 °C and the ethanol (EtOH) conversion into hydrocarbons at 350 °C under 30 bar are performed over Hierarchical HZSM-5 zeolites (with micro- and nanometer crystal size). These two model but complex reactions lead to the formation of coke which is toxic with MCH and active with EtOH. The toxicity (Tox) and the reactivity of coke depend strongly on the catalysts textural properties. In this work, it's shown that whatever the reaction, coke in the case of micrometric zeolites is "heavy" and consists mainly of alkylphenanthrenes and alkylpyrenes located into the micropores. In nano-sized and hierarchical (meso-microporous) zeolites, coke is rather "light" and consisting mostly of alkyl benzenes and naphthalenes located on the external surface. The coke located into the channels and at the channels intersections of HZSM-5 zeolite is more toxic (Tox ≥ 1) than that located on the external surface (Tox <1). The decrease in the diffusion path also offers a clear advantage in the catalysts regeneration by lowering the temperature of total coke removal. The effect of textural properties on the catalytic performances and the deactivation are more pronounced in the case of EtOH (more sensitive reaction) than MCH

Dynamic Adsorption on Fixed-Bed Column of Manganese Oxoanions (MnO<SUB>4</SUB><SUP>-</SUP>) in Aqueous Media on Activated Carbon Prepared from Palm Nut Shells

International audienceThe study of the influence of operating parameters of dynamic adsorption on fixed-bed column of manganese oxoanions (MnO 4-) in aqueous media on granular activated carbon (GAC), prepared from the shells palm nuts of Gabon, was carried out. The operating parameters studied were the particle size, the concentration of the initial solution (C 0) of MnO 4-, the flow rate (D) and the pH of the media. The results obtained on the study of the influence of operating parameters show that the best adsorption capacities at saturation (Q sat) of MnO 4-ion on the CAG were obtained with particle size between 0.04 < x < 0.1 (5.90 mg.g-1); with flow rate of 3 mL.min-1 (8.36 mg.g-1) and when the pH of the initial solution was equal to 3.5 (27.01 mg.g-1). Also, these results showed that the bed of prepared GAC appeared more effective when C 0 was low (10 mg.L-1). The kinetic models of the different studies carried out show that the pseudo-first-order kinetic model best describes the adsorption of MnO 4-ions on the GAC. The results of the intraparticle diffusion model indicate that the adsorption of MnO 4-follows a multi-step process and that the intraparticle diffusion is not the limiting step. In addition, the surface adsorption plays a predominant role in the adsorption mechanism of MnO 4-ions on activated carbon studied in fixed-bed column dynamics

On the remarkable resistance to coke formation of nanometer-sized and hierarchical MFI zeolites during ethanol to hydrocarbons transformation

International audienceThe impact of the textural properties of H-ZSM-5 zeolites in the conversion of ethanol to hydrocarbons at 623 K and under 3.0 MPa pressure is investigated. We highlight that the lifetime of the catalysts is not correlated to the coking rate but to the number of pore mouths. The addition of macropores (fluoride leaching) or mesopores (alkaline leaching) to micron-sized zeolites is a simple approach to increase the number of pore mouths and reduce the diffusion path length of molecules in the micropores. However, the most efficient way is to reduce the zeolite crystal size to nanometers. The longest catalyst lifetime (>100 h) is obtained with a hierarchical nanometer-sized zeolite even though most of its acid sites are poisoned. The important impact of the nature of coke on the catalysts regeneration is also highlighted