Magnetisch abtrennbare Nanokatalysatoren: Brücken zwischen homogener und heterogener Katalyse

Nanocatalysis and prospects of green chemistry

Designing and developing ideal catalyst paves the way to green chemistry. The fields of catalysis and nanoscience have been inextricably linked to each other for a long time. Thanks to the recent advances in characterization techniques, the old technology has been revisited with a new scope. The last decade has witnessed a flood of research activity in the field of nanocatalysis, with most of the studies focusing on the effect of size on catalytic properties. This led to the development of much greener catalysts with higher activity, selectivity and greater ease of separation from the reaction medium. This Minireview describes the emerging trends in the field of nanocatalysis with implications towards green chemistry and sustainability.

A highly efficient and extensively reusable "dip catalyst" based on a silver-nanoparticle-embedded polymer thin film

Achieving a harmonious combination of the efficiency of homogeneous catalysts with the reusability of heterogeneous catalysts is a fundamental and challenging problem. Metal nanoparticles in a suitable matrix offer a potential solution. However an ideal design is yet to be realized, because the critical requirements of facile access to the catalyst, its durability, and ease of retrieval and reuse are difficult to reconcile. We report herein a multilayer free-standing thin-film catalyst based on silver nanoparticles, generated in situ inside poly(vinyl alcohol) by using a facile protocol, which shows excellent efficiency and extensive reusability in the prototypical reaction, the reduction of 4-nitrophenol by sodium borohydride. The "dip catalyst" film, which can start/stop the reaction instantaneously by mere insertion/removal, is used 30 times leading to a total turnover number (TON) of ≈3390, which is unprecedented for this reaction. The efficiency of the catalyst is reduced only marginally at the end of these runs, promising further extended usage. The unique advantage of convenient catalyst monitoring is illustrated by the periodic spectroscopic and microscopic examinations of the thin film, which revealed the basis of its durability. The demonstrated potential of metal-nanoparticle-embedded polymer thin films, coupled with their versatility and ease of fabrication, promises extensive applications in chemical catalysis.

Chemistry by Nanocatalysis: First example of a solid-supported RAPTA complex for organic reactions in aqueous medium

A ruthenium-arene-PTA (RAPTA) complex has been supported for the first time on an inorganic solid, that is, silica-coated ferrite nanoparticles. The resulting magnetic material proved to be a general, very efficient and easily reusable catalyst for three synthetically useful organic transformations; selective nitrile hydration, redox isomerization of allylic alcohols, and heteroannulation of (Z)-enynols. The use of low metal concentration, environmentally friendly water as a reaction medium, with no use at all of organic solvent during or after the reactions, and microwaves as an alternative energy source renders the synthetic processes reported herein "truly" green and sustainable.