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International audience; Sodium manganates with nominal composition Na2/3MnO2 were prepared by solid state reaction between Na2CO3 and MnCO3 at 1000 °C. The composition and structure of NaxMnO2 were controlled by the rate of cooling from the temperature of preparation. This is a consequence of the capability of Na2/3MnO2 to accommodate over-stoichiometric Mn4+ ions up to 12.5%. Structural characterization was carried out by XRD powder diffractions, TEM analysis and Raman spectroscopy. The composition and oxidation state of manganese were determined by chemical analysis and magnetic susceptibility measurements. The manganese distribution in the layers was analysed using electron paramagnetic resonance (EPR) spectroscopy. By quenching from 1000 °C, the orthorhombic distorted modification is stabilized. A phase separation into orthorhombic and hexagonal modifications takes place when Na2/3MnO2 is slow cooled. The structure changes are concomitant with an increase in the oxidation state of Mn. The over-stoichiometric Mn4+ ions are accommodated in the hexagonal modification by creation of vacancies in the MnO2-layers.

International audience; This study reports on the seeded synthesis of zeolite Beta in fluoride media. Crystal growth kinetics of the material as a function of seed content, crystallization temperature and Si/Al ratio of the initial gel is studied. After the optimization of crystallization conditions a highly crystalline material with crystal size 100-300 nm is synthesized. A counterpart zeolite Beta sample with similar characteristics is obtained in hydroxyl media. The physicochemical properties of fluoride and hydroxyl media syntheiszed zeolites are compared as particular attention is paid to the nature, number and distribution of their active sites. Zeolite Beta sample synthesized in fluoride media exhibits larger number of Broensted acid sites, while the concentration of Lewis acid sites is higher in the zeolite synthesized in hydroxyl media. The two materials are tested in m-xylene transformation, where zeolite Beta synthesized in fluoride medium shows superior performance. The set of experimental data unambiguously proves that seeded synthesis of zeolites in fluoride medium is a promising alternative of the conventional zeolite synthesis in hydroxyl medium. (C) 2013 Elsevier Inc. All rights reserved.

International audience; In the present Study, low-temperature hydrothermal crystallization and vapor phase gel transformation have been employed to synthesis LTA-type zeolite Crystals of submicrometer size. The crystal growth curves under hydrothermal conditions at 35, 50, and 65 degrees C were obtained. From these curves, the induction periods and the times for reaching maximum crystallinity for respective temperatures were determined. A set of characterization methods, including XRD, FTIR, SEM, TEM, and DLS, was employed to obtain complementary information. The inspection of the crystalline solids obtained under hydrothermal conditions showed that the size of the crystals is a function of the crystallization temperature. The largest crystals (300 x 300 nm) were obtained at 65 degrees C, while much smaller particles were synthesized at lower temperatures. Second approach involved vapor-phase transformation of the solid part of initial gel. This approach has also yielded submicrometer-sized zeolite crystallites. The characteristics of crystalline particles synthesized by two methods were compared Both synthetic methods showed relatively high conversion (70-80%) of the initial gel into LTA type zeolite, which is substantial advantage in respect to nanozeolite synthesis from clear solutions.

Mineralium Deposita, 44 (6), ISSN:0026-4598, ISSN:1432-1866