Preparation and characterization of copper catalysts supported on mesoporous Al2O3 nanofibers for N2O reduction to N2

A series of mesoporous Al2O3 samples with different porous structures and phases were prepared and used as supports for Cu/Al2O3 catalysts. These catalysts were characterized by N-2 adsorption, NMR, TGA, XRD, and UV - vis spectroscopic techniques and tested for the catalytic reaction of N2O decomposition. The activity increased with the increasing calcination temperatures of supports from 450 to 900 degreesC; however, a further increase in calcination temperature up to 1200 degreesC resulted in a significant reduction in activity. Characterization revealed that the calcination temperatures of supports influenced the porous structures and phases of the supports, which in turn affected the dispersions, phases, and activities of the impregnated copper catalyst. The different roles of surface spinel, bulk CuAl2O4, and bulk CuO is clarified for N2O catalytic decomposition. Two mechanism schemes were thus proposed to account for the varying activities of different catalysts.

Nitrogen removal from sludge digester liquids by nitrification/denitrification or partial nitritation/anammox: environmental and economical considerations

In wastewater treatment plants with anaerobic sludge digestion, 15-20% of the nitrogen load is recirculated to the main stream with the return liquors from dewatering. Separate treatment of this ammonium-rich digester supernatant significantly reduces the nitrogen load of the activated sludge system. Two biological applications are considered for nitrogen elimination: (i) classical autotrophic nitrification/heterotrophic denitrification and (ii) partial nitritation/autotrophic anaerobic ammonium oxidation (anammox). With both applications 85-90% nitrogen removal can be achieved, but there are considerable differences in terms of sustainability and costs. The final gaseous products for heterotrophic denitrification are generally not measured and are assumed to be nitrogen gas (N-2). However, significant nitrous oxide (N2O) production can occur at elevated nitrite concentrations in the reactor. Denitrification via nitrite instead of nitrate has been promoted in recent years in order to reduce the oxygen and the organic carbon requirements. Obviously this achievement turns out to be rather disadvantageous from an overall environmental point of view. On the other hand no unfavorable intermediates are emitted during anaerobic ammonium oxidation. A cost estimate for both applications demonstrates that partial nitritation/anammox is also more economical than classical nitrification/denitrification. Therefore autotrophic nitrogen elimination should be used in future to treat ammonium-rich sludge liquors.

Exploring the potential of xenobiotic-metabolising enzymes as biocatalysts: Evolving designer catalysts from polyfunctional cytochrome P450 enzymes

1. Biological catalysts have the advantage of being able to catalyse chemical reactions with an often exquisite degree of regio- and stereospecificity in contrast with traditional methods of organic synthesis. 2. The cytochrome P450 enzymes involved in human drug metabolism are ideal starting materials for the development of designer biocatalysts by virtue of their catalytic versatility and extreme substrate diversity. Applications can be envisaged in fine chemical synthesis, such as in the pharmaceutical industry and bioremediation. 3. A variety of techniques of enzyme engineering are currently being applied to P450 enzymes to explore their catalytic potential. Although most studies to date have been performed with bacterial P450s, reports are now emerging of work with mammalian forms of the enzymes. 4. The present minireview will explore the rationale and general techniques for redesigning P450s, review the results obtained to date with xenobiotic-metabolising forms and discuss strategies to overcome some of the logistic problems limiting the full exploitation of these enzymes as industrial-scale biocatalysts.

Effect of the preparation technique on the catalytic properties of mesoporous V-HMS for the oxidation of toluene

Various mesoporous catalysts with vanadium loadings between 0.5 and 6 V wt.% and surface areas around 1300 m(2)/g were synthesized using the isomorphous substitution (IS) and molecular designed dispersion (MDD) techniques. Their catalytic properties were tested using toluene as a model VOC in a fixed bed reactor at temperatures between 300 and 550 degrees C. It was found that during the oxidation of toluene, over V-HMS synthesized via IS, conversion of toluene mainly results in carbon oxides, benzene, benzaldehyde and water. Total conversion is greatly improved when the vanadium content is increased from around 1.5 to 3.0 wt.%, but an increase in the textural porosity (V-TEX/V-MESO) from 0.3 to 0.6 had no discernable effect on the conversion. This can be explained by the fact that a V-TEX/V-MESO as low as 0.3 is sufficient to facilitate the access of toluene into the framework confined mesopores without any molecular transport limitations. However, when using V-HMS synthesized by MDD, conversion of toluene is greatly improved when the V-TEX/ V-MESO ratio is increased from 0.1 to 0.6. This is because the diffusion limitations are minimized by this increase. V-HMS synthesized via MDD does not exhibit selectivity to benzaldehyde, favoring total oxidation to CO and CO2. This different oxidation mechanism can be explained in terms of location, accessibility and number of active species on the surface of the HMS support. (c) 2005 Elsevier Inc. All rights reserved.