Volume 1, Issue 3

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Sustainable chemistry is the attempt to design chemical products and processes that reduce or eliminate the use and generation of hazardous substances, minimize waste and energy consumption, favor renewable resources and integrate aspects of recycling. Since 2004, the Deutsche Bundesstiftung Umwelt (German Federal Environmental Foundation) supported more than 70 projects in the area of “Sustainable Chemistry”. In this paper, current results and approaches of these sponsored projects are reviewed, e.g., substitution of fossil raw materials and biorefinery concepts, catalytic processes, novel solvent systems including solvent-free synthesis. The application of microstructured equipment and chemical reaction under novel process regimes, i.e., so-called “novel process windows” are summarized, and a brief look at concepts for inherently safe chemicals, new business models, e.g., chemical leasing and early stage eco-balancing, is given. It is revealed that industrial sustainable chemistry is no longer an emerging trend but already a reality.

The alkylation of N -methylimidazole with dimethyl carbonate yields the respective solid zwitterion N,N ′-dimethylimidazolium-2-carboxylate, a versatile precursor for manufacturing non-halide-containing ionic liquids (ILs). This reaction requires high p,T-conditions, i.e., up to 200°C and 70 bar pressure, and even then requires reaction times up to several days under autoclave batch conditions. To perform these reactions continuously in stainless steel tubes with inner diameters in the millimeter range, precipitation of the salts must be suppressed to avoid any blockage. The achievable yield of the zwitterion is remarkably increased when a packed Al 2 O 3 catalyst is used instead of an empty tube. For example, with a catalyst-packed column, a nearly 100% yield of N,N ′-dimethylimidazolium-2-carboxylate can be achieved within 20 min at 200°C, whereas only a 67% yield in 50 min can be obtained without a catalyst at the same temperature. The transfer of the autoclave process into continuous flow is advantageous and allows a more economical scale-up.

In this work, we describe the application of a new series of solid cross-linked cyclodextrin (α-, ß-, and γ-CD) based catalysts that are obtained by reticulation with hexamethylene diisocyanate (HDI) in solutions containing Pd (II) or Cu (I) cations. Diisocyanates are efficient cross-linking agents for CDs because of their high reactivity towards hydroxyl groups. Metal content in the polyurethane-bridged CD polymers has been analyzed using ICP-MS. Pd (II) based catalysts have been successfully used in C-C couplings (Heck and Suzuki reactions), and a Cu (I) based system has been used in alkyne/azide [3+2] cycloadditions. Because metal leaching is negligible, which allows it to be recycled, and preparation is facile, this protocol is truly sustainable. As confirmed by ICP-MS analysis, the crude products do not require further purification for metal contamination. This versatile catalyst, due to its polar structure, is particularly suitable for microwave (MW)-assisted reactions that, in this study, gave the best yields in shorter reaction times. Due to their polar structure, both native CDs and cross-linked derivatives are very sensitive to dielectric heating; this effect is enhanced by the embedded cations. The design of sustainable synthetic protocols can be efficiently accomplished using a multi-faceted strategy that combines innovative solid catalysts and suitable enabling technologies.

A palladium bis- N -heterocyclic carbene complex was immobilized on polystyrene modified, magnetic carbon coated iron nanoparticles and evaluated in Suzuki-Miyaura cross-coupling reactions under conventional and microwave heating. Under the latter conditions, both aryl bromides and aryl chlorides could be employed as substrates at low loading of catalyst (0.2 mol%), which could be readily recovered by an external magnet and reused in at least four cycles. As a possible deactivation pathway of the catalyst, the formation of palladium nanoparticles in the course of the reaction that became encapsulated in the polystyrene matrix of the support is suggested.

A continuous flow microwave system based on a transmission line short-circuited waveguide reactor concept is described. The continuous flow reactor is capable of operating in a genuine high-temperature/high-pressure process window (310°C/60 bar), applying a microwave transparent and chemical resistant cylindrical γ-Al 2 O 3 tube as reaction zone. The system can be operated in an extremely energy efficient manner, utilizing 0.6–6 kW microwave power (2.45 GHz). The application of the reactor for processing four chemical transformations in a high-temperature/high-pressure regime with a throughout of 3.5–6.0 l/h is demonstrated.

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