Immobilized iron oxide nanoparticles as stable and reusable catalysts for hydrazine-mediated nitro reductions in continuous flow.

An experimentally easy to perform method for the generation of alumina-supported Fe3O4 nanoparticles [(6±1) nm size, 0.67 wt %]and the use of this material in hydrazine-mediated heterogeneously catalyzed reductions of nitroarenes to anilines under batch and continuous-flow conditions is presented. The bench-stable, reusable nano-Fe3O4@Al2O3 catalyst can selectively reduce functionalized nitroarenes at 1 mol % catalyst loading by using a 20 mol % excess of hydrazine hydrate in an elevated temperature regime (150 °C, reaction time 2-6 min in batch). For continuous-flow processing, the catalyst material is packed into dedicated cartridges and used in a commercially available high-temperature/-pressure flow device. In continuous mode, reaction times can be reduced to less than 1 min at 150 °C (30 bar back pressure) in a highly intensified process. The nano-Fe3O4@Al2O3 catalyst demonstrated stable reduction of nitrobenzene (0.5 M in MeOH) for more than 10 h on stream at a productivity of 30 mmol h(-1) (0.72 mol per day). Importantly, virtually no leaching of the catalytically active material could be observed by inductively coupled plasma MS monitoring.

In situ preparation of silver nanocomposites on cellulosic fibers--microwave vs. conventional heating.

A green approach for the preparation of silver nanocomposites on viscose fibers using microwave and conventional heating is presented. Reduction of silver nitrate is induced by addition of 6-O chitosan sulfate (S-Chi) in aqueous media which provides steric protection and electrostatic stabilization to prevent agglomeration of the nanoparticles. The particles are formed in close spatial proximity to the fibers and adsorption of the particles via structural similarity takes place to create silver nanocomposites. All nanocomposites have been subjected to antimicrobial tests and high antimicrobial activity toward Escherichia coli bacteria has been determined. Further, the nanocomposites are characterized using different analytical techniques which reveal very similar results for both heating techniques. The only significant difference is observed concerning the shape of the nanoparticles on the viscose fibers which are slightly elongated for the microwave method in comparison to spheres observed by conventional heating. Therefore, detailed investigations on the formation of colloidal silver nanoparticles have been performed, comparing microwave dielectric and conventional heating at the exact same temperature and reaction times. These experiments resulted in nearly identical nanoparticle shape and size for both heating methods as demonstrated by dynamic light scattering, UV-vis spectroscopy and transmission electron microscopy. A wide range of parameters has been varied (temperature, AgNO3 to S-Chi ratio, reaction time, and stirring speed) to study the nanoparticle formation under microwave and conventional conditions. No evidence for the existence of so-called specific microwave effects was obtained.

Hydrazine-mediated reduction of nitro and azide functionalities catalyzed by highly active and reusable magnetic iron oxide nanocrystals.

Iron oxide (Fe3O4) nanocrystals generated in situ from an inexpensive and readily available iron source catalyze the reduction of nitroarenes to anilines with unparalleled efficiency. The procedure is chemoselective, avoids the use of precious metals, and can be applied under mild reflux conditions (65 or 80 °C) or using sealed vessel microwave heating in an elevated temperature regime (150 °C). Utilizing microwave conditions, a variety of functionalized anilines have been prepared in nearly quantitative yields within 2-8 min at 150 °C, in a procedure also successfully applied to the reduction of aliphatic nitro compounds and azides. The iron oxide nanoparticles are generated in a colloidal form, resulting in homogeneous solutions suitable for continuous flow processing. Selected examples of anilines of industrial importance have been prepared in a continuous regime using this protocol.

Microwave-assisted synthesis of CdSe quantum dots: can the electromagnetic field influence the formation and quality of the resulting nanocrystals?

Microwave-assisted syntheses of colloidal nanocrystals (NCs), in particular CdSe quantum dots (QDs), have gained considerable attention due to unique opportunities provided by microwave dielectric heating. The extensive use of microwave heating and the frequently suggested specific microwave effects, however, pose questions about the role of the electromagnetic field in both the formation and quality of the produced QDs. In this work a one-pot protocol for the tunable synthesis of monodisperse colloidal CdSe NCs using microwave dielectric heating under carefully controlled conditions is introduced. CdSe QDs are fabricated using selenium dioxide as a selenium precursor, 1-octadecene as a solvent and reducing agent, cadmium alkyl carboxylates or alkyl phosphonates as cadmium sources, 1,2-hexadecanediol to stabilize the cadmium complex and oleic acid to stabilize the resulting CdSe QDs. Utilizing the possibilities of microwave heating technology in combination with accurate online temperature control the influence of different reaction parameters such as reaction temperature, ramp and hold times, and the timing and duration of oleic acid addition have been carefully investigated. Optimum results were obtained by performing the reaction at 240 °C applying a 5 min ramp time, 2 min hold time before oleic acid addition, 90 s for oleic acid addition, and a 5 min hold time after oleic acid addition (8.5 min overall holding at 240 °C). By using different cadmium complexes in the microwave protocol CdSe QDs with a narrow size distribution can be obtained in different sizes ranging from 0.5-4 nm by simply changing the cadmium source. The QDs were characterized by TEM, HRTEM, UV-Vis, and photoluminescence methods and the size distribution was monitored by SAXS. Control experiments involving conventional conductive heating under otherwise identical conditions ensuring the same heating and cooling profiles, stirring rates, and reactor geometries demonstrate that the electromagnetic field has no influence on the generated CdSe QDs. The resulting CdSe NCs prepared using either conductive or microwave dielectric heating exhibited the same primary crystallite size, shape, quantum yield and size distribution regardless of the heating mode.