Innovations in the synthesis of Fe-(exchanged)-zeolites

Several aspects on the preparation of Fe-zeolites are discussed. In contrast to the many studies highlighting the characterisation of the active sites, new approaches for incorporation of Fe are presented.Full utilization of exchange capacity of zeolites has been achieved by a controlled alkaline treatment of the parent sample. With this method, iron can be fully exchanged by liquid phase ion-exchange on ZSM5 without the formation of inactive Fe-oxides.The second topic is the-use of a mild oxidant (H2O2, and peroxides in general) to break down strong complexating equilibria during ion-exchange by controlled redox titration of the ligands. Hydrogen peroxide oxidizes effectively chelating groups releasing Fe species at a controlled rate. The method is demonstrated for the preparation of Fe-FER through Ferric-citrate.The final concept discussed is the detemplating of the zeolite with the simultaneous incorporation of the iron (combined detemplation and ionexchange). This one-pot preparation minimizes the number of steps considerably. To realize this, a strong oxidant is necessary to remove the organic template, and Fe-cations for exchange. Both requirements are met with the Fenton's-chemistry (Fe3+/2+/H2O2 mixtures) involving radical chemistry. (c) 2005 Elsevier B.V. All rights reserved.</p

Structure/metathesis-activity relations of silica supported molybdenum and tungsten oxide

In this study the influence of metal content and impregnation technique (wet or dry) on struture and metathesis activity for silica supported molybdenum and tungsten oxide is described. Structural aspects are obtained from several techniques, in particular temperature programmed reduction and Raman spectroscopy. Catalytic activity has been measured in a six-channel microflow reactor. A combination of structural and metathesis activity data leads to the conclusion that in these systems the precursors for the catalytic sites in metathesis are surface compounds and not the bulk oxides. The results suggest that a prerequisite for catalytic activity is a combination of a high degree of dispersion and an easy reducibility

Development of TiO2/Ti wire-mesh honeycomb for catalytic combustion of ethyl acetate in air

Wire-mesh sheets coated with Ti particles were developed by using an electrophoretic deposition (EPD) method. They were thermally treated at various working conditions for enhancing the coating strength. The Ti layer was suitably porous and showed a good adherence to the wire surface, due to partial sintering of Ti particles after thermal treatment at 900 degrees C. Then additional calcination at 600 degrees C resulted in the formation of a thin TiO2 layer (similar to 3 mu m) on the outer surface of the Ti layer. After making Pt/TiO2/Ti-coated wire-mesh honeycombs (WMH) successfully, they were applied in catalytic combustion of ethyl acetate in air. The activity was much higher than that of a ceramic honeycomb containing a similar catalyst composition, especially in the high temperature ranges. This is explained by the difference in external mass transfer rate between the two modules. TiO2/Ti-WMH had several additional advantages: (i) the active catalyst platinum could be directly impregnated, since TiO2 could be used as a catalyst support. This reduces the manufacturing process with one-step, viz., support washcoating. (ii) Since there is no additional washcoat, it can be expected that the possibility of catalyst loss during the operation becomes very low. (iii) A rapid thermal response due to the small heat capacity. (c) 2006 Elsevier B.V. All rights reserved.X1117sciescopu

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Characterization of silica-supported molybdenum oxide and tungsten oxide. Reducibility of the oxidic state versus hydrodesulfurization activity of the sulfided state

Thiophene hydrodesulfurization and butene hydrogenation have been studied for presulfided MoO3/SiO2 and WO3/SiO2 catalysts using a micro-flow reactor operating at atmospheric pressure. The catalysts have been prepared by dry as well as wet impregnation. The oxidic precursor catalysts were characterized by X-ray diffraction, surface area and pore volume measurements, and temperature-programmed reduction. Catalysts prepared by dry or wet impregnation are essentially the same. At low metal oxide contents the catalysts are of the monolayer type. At higher metal oxide contents also bulk compounds are present, which is demonstrated by means of X-ray diffraction as well as temperature-programmed reduction. The maximum concentration of monolayer-type compounds corresponds to approximately one transition metal atom per square nanometer of the carrier. A correlation could be established between reducibility of the oxidic monolayer-type catalysts and the activity for thiophene hydrodesulfurization. This correlation appears to be in good agreement with the one reported earlier for the analogous ¿-alumina-based catalysts. Butene hydrogenation activity of the sulfided MoO3/SiO2 and WO3/SiO2 monolayer-type catalysts also correlates with reducibility of the oxidic systems. Due to the presence of bulk compounds the turnover frequency in hydrodesulfurization as well as hydrogenation decreases at higher metal oxide contents