Microwave energy ionic liquids

Intermolecular Diels-Alder or hetero Diels-Alder reactions have been greatly improved by using microwave technology - again with higher reaction rates and improved yields [3j]. Remarkable improvements in rate acceleration and selectivity enhancement for a variety of intermolecular Diels-Alder reactions have also been accomplished in the past two decades by application of catalysts such as Lewis acids. Recently, many such examples have been reported under microwave conditions in polar solvents or ionic liquids as energy-transfer medium. These reactions have also been developed in open vessels by adsorption of the reactants on mineral solid supports or using neat reactants. 

The preparation of 1,3-dialkylimidazolium halides by conventional heating in solvent under reflux requires several hours to afford reasonable yields and also uses a large excess of alkyl halides and/or organic solvents as the reaction medium. To circumvent these problems Varma and coworkers [106] investigated the preparation of a series of ionic liquids 72 (Scheme 8.74), using microwave irradiation as the energy source, by simple exposure of neat reactants, in open containers, to microwaves by use of an unmodified household MW oven (240 W). 

Process intensification can be considered to be the use of measures to increase the volume-specific rates of reaction, heat transfer, and mass transfer and thus to enable the chemical system or catalyst to realize its full potential (2). Catalysis itself is an example of process intensification in its broadest sense. The use of special reaction media, such as ionic liquids or supercritical fluids, high-density energy sources, such as microwaves or ultrasonics, the exploitation of centrifugal fields, the use of microstructured reactors with very high specific surface areas, and the periodic reactor operation all fall under this definition of process intensification, and the list given is by no means exhaustive. 

Roberts and Strauss, 2005). As was described earlier, an added advantage to microwave chemistry is that often no solvent is required. In recent years, many commercial reactors have come on the market and some are amenable for scaling up reactions to the 10 kg scale. These new instruments allow direct control of reaction conditions, including temperature, pressure, stirring rate and microwave power, and therefore, more reproducible results can be obtained. For most successful microwave-assisted reactions, a polar solvent that is able to absorb the energy and efficiently convert it to heat is required, however, even solvents such as dioxane that are more or less microwave transparent can be used if a substrate, coreagent or catalyst absorbs microwaves well. In fact, ionic liquids have been exploited in this field as polar additives for low-absorbing reaction mixtures. 

Therefore we intended and managed to design and build a so-called microwave dielectrometric measurement device in which - based on the compensation of phase change due to the microwave energy absorption of the liquid sample - the dielectric properties of ionic liquids can be successfully measured in a continuous and automatic way. 

Stable ruthenium, rhodium, and iridium metal nanoparticles have been reproducibly obtained by facile, rapid, and energy-saving microwave irradiation under an argon atmosphere from their metal-carbonyl precursors [M(x)(CO)(y)] in the ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate (Redel et al., 2009 Vollmer et al., 2010). The metal nanoparticles synthesized have a very small (<5 nm) and uniform size and are prepared without any additional stabilizers or capping molecules as long-term stable metal nanoparticles-ionic liquid dispersions. The ruthenium, rhodium, or iridium nanoparticles dispersed in ionic liquids are highly active and easily recyclable catalysts for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane. 

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