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Cuesta Zapata P.M.,INIQUI | Nazzarro M.S.,CONICET | Gonzo E.E.,INIQUI | Parentis M.L.,INIQUI | Bonini N.A.,INIQUI
Catalysis Today | Year: 2015

Cr/SiO2 mesoporous materials (Cr-MCM-41) were obtained by hydrothermal synthesis. The effect of hydrothermal treatment and calcination temperature on the structure and catalytic behavior of synthesized materials were analyzed. The materials were characterized by XRD, N2 adsorption-desorption, TEM-SEM, TG-DTA, FTIR-HATR, DRUV-vis and XPS. The hydrothermal treatment promoted the incorporation of Cr3+ into the silica network, improving their resistance to oxidation. The calcination in air produces the simultaneous presence of Cr3+ and Cr6+ ions, and their relative amounts depend both on the hydrothermal pretreatment and the calcination temperatures. The Cr3+ ions constituted coordinative unsaturated acid centers responsible of the cyclohexanol to cyclohexene dehydration reaction. Moreover, the Cr6+ ions correspond to either isolated monochromate species linked to the silica structure or to polychromate aggregates, which produce crystalline α-Cr2O3 clusters after calcination in air to 450°C. The monochromate centers were active for the dehydrogenation reaction. So, although dehydration was the main reaction, the dehydrogenation reaction to cyclohexanone also was observed after calcination in air to 450°C. © 2015 Elsevier B.V. Source


Cuesta Zapata P.M.,CONICET | Parentis M.L.,INIQUI | Gonzo E.E.,INIQUI | Bonini N.A.,INIQUI
Applied Catalysis A: General | Year: 2013

Cr/SiO2 catalysts (4-8% Cr, w/w), obtained from Cr(III) salts and tetraethylortosilicate (TEOS), were prepared by the sol-gel method. After TEOS hydrolysis in ammoniacal solution (pH: 9.5), the gel was treated under hydrothermal conditions at 120-220 C. The solids were characterized by N 2 sorptometry, TG-DTA, DRUV-vis, XRD, and FTIR. In vacuum or N 2 atmosphere, Cr3+ ions were stable up to 450 C. However, calcination in air promotes the partial oxidation of Cr3+ to Cr 6+ as the temperature increases from 200 to 450 C. The amount of Cr6+ formed depends on the temperature of the hydrothermal treatment. 2,6-Lutidine adsorption allows to determine the evolution, with respect to calcination temperature, of both Lewis and Brönsted acid centers. Cr 3+ ions were related to the presence of Lewis sites, and Cr 6+ ions to the development of weak Brönsted acid sites. Accordingly, the 2-propanol dehydration reaction rate shows an increase when the catalysts are treated in N2 at 450 C, while catalysts calcined in air develop a lower acidity. The dehydration reaction is attributed to the presence of Lewis acid centers. © 2013 Elsevier B.V. Source


Cuesta Zapata P.M.,INIQUI | Nazzarro M.S.,CONICET | Gonzo E.E.,INIQUI | Parentis M.L.,INIQUI | Bonini N.A.,INIQUI
Catalysis Today | Year: 2016

Cr/SiO2 mesoporous materials (Cr-MCM-41) were obtained by hydrothermal synthesis. The effect of hydrothermal treatment and calcination temperature on the structure and catalytic behavior of synthesized materials were analyzed. The materials were characterized by XRD, N2 adsorption-desorption, TEM-SEM, TG-DTA, FTIR-HATR, DRUV-vis and XPS. The hydrothermal treatment promoted the incorporation of Cr3+ into the silica network, improving their resistance to oxidation. The calcination in air produces the simultaneous presence of Cr3+ and Cr6+ ions, and their relative amounts depend both on the hydrothermal pretreatment and the calcination temperatures. The Cr3+ ions constituted coordinative unsaturated acid centers responsible of the cyclohexanol to cyclohexene dehydration reaction. Moreover, the Cr6+ ions correspond to either isolated monochromate species linked to the silica structure or to polychromate aggregates, which produce crystalline α-Cr2O3 clusters after calcination in air to 450 °C. The monochromate centers were active for the dehydrogenation reaction. So, although dehydration was the main reaction, the dehydrogenation reaction to cyclohexanone also was observed after calcination in air to 450 °C. © 2015 Elsevier B.V. Source


TiO2 organoclays are potential adsorbents for aqueous pollutants and subsequently they could favour the photocatalysis of them. For this reason, the study of their surface and textural characteristics is important for the improvement of those processes. In this work TiO2 pillared organoclays were studied. The surfactant (hexadecyltrimethilammonium bromide) was introduced post-pillaring or during the pillaring process. The effect of the use of different acids of hydrolysis and different surfactants ratios was investigated. The clays were characterized by thermogravimetric analysis, infrared spectroscopy and nitrogen sorptometry. It was found that the highest adsorption capacity post-pillaring of the surfactant is near to 0.5 times the CEC of the original pillared clay, independently of the amount of surfactant offered. An increment in the amount of the surfactant introduced only increased the ratio adsorbed as ionic pair. In these samples the CEC measured after the adsorption was between 0.26 and 0.47 times the original CEC. It was observed a decrease in the total pore volume and the BET surface for every sample. The surface hydrophobicity of the samples with post-pillaring adsorption was higher than in those with co-adsorption. In conclusion, it was demonstrated that the incorporation of surfactant modifies the surface and textural properties of the material in different extent in accordance with the incorporation stage and the amount of surfactant used. © 2015 Universidade Federal do Rio de Janeiro. All rights reserved. Source

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