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Kappe C.O.,Christian Doppler Laboratory
Chemical Society Reviews | Year: 2013

High-speed microwave chemistry has attracted considerable attention in the past two decades with new and innovative applications in organic and peptide synthesis, polymer chemistry, material sciences, nanotechnology and biochemical processes continuously being reported in the literature. In particular the introduction of benchtop single-mode microwave reactors just over ten years ago has revolutionized the way many scientists today perform reactions in the laboratory. Unfortunately, the accurate measurement of reaction temperature in these devices is far from being trivial and requires both a basic understanding of microwave dielectric heating effects and use of appropriate temperature monitoring devices. In this tutorial review frequently occurring problems in the determination of accurate reaction temperatures in single-mode microwave reactors are discussed. © 2013 The Royal Society of Chemistry.

Wagner W.,Christian Doppler Laboratory
ISPRS Journal of Photogrammetry and Remote Sensing | Year: 2010

Small-footprint (0.2-2 m) airborne laser scanners are lidar instruments originally developed for topographic mapping. While the first airborne laser scanners only allowed determining the range from the sensor to the target, the latest sensor generation records the complete echo waveform. The waveform provides important information about the backscattering properties of the observed targets and may be useful for geophysical parameter retrieval and advanced geometric modelling. However, to fully utilise the potential of the waveform measurements in applications, it is necessary to perform a radiometric calibration. As there are not yet calibration standards, this paper reviews some basic physical concepts commonly used by the remote sensing community for modelling scattering and reflection processes. Based purely on theoretical arguments it is recommended to use the backscattering coefficient γ, which is the backscatter cross-section normalised relative to the laser footprint area, for the radiometric calibration of small-footprint full-waveform airborne laser scanners. The presented concepts are, with some limitations, also applicable to conventional airborne laser scanners that measure the range and intensity of multiple echoes. © International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS).

Kappe C.O.,Christian Doppler Laboratory | Van Der Eycken E.,Catholic University of Leuven
Chemical Society Reviews | Year: 2010

First described almost a decade ago, "click" reactions such as the Cu(i)-catalyzed azide-alkyne cycloaddition (CuAAC) are widely used today in organic and medicinal chemistry, in the polymer and material science field, and in chemical biology. While most click reactions can be performed at room temperature there are instances where some form of process intensification is required. In this tutorial review, aimed at the synthetic chemistry community, examples of click chemistry carried out under non-classical reaction conditions, such as for example applying microwave heating or continuous flow processing will be highlighted. © 2010 The Royal Society of Chemistry.

Moseley J.D.,Astrazeneca | Kappe C.O.,Christian Doppler Laboratory
Green Chemistry | Year: 2011

The question "why should microwave chemistry be green?" is evaluated in the context of the twelve principles of green chemistry, with a focus on the 6th principle: design for energy efficiency. A significant number of publications on microwave-assisted organic transformations during the past 25 years describe this non-classical heating technology as being "green", assuming that microwave dielectric heating is more energy efficient than classical conductive heat transfer methods. In this Perspective article, we critically assess the energy efficiency of microwave-assisted transformations in the context of scaling-up this technology to production quantities. © The Royal Society of Chemistry.

Kappe C.O.,Christian Doppler Laboratory
Accounts of Chemical Research | Year: 2013

In the past few years, the use of microwave energy toheat chemical reactions has become an increasingly popular theme in the scientific community. This nonclassical heating technique has slowly progressed from a laboratory curiosity to an established method commonly used both in academia and in industry. Because of its efficiency, microwave heating dramatically reduces reaction times (from days and hours to minutes and seconds) and improves product purities or material properties among other advantages. Since the early days of microwave chemistry, researchers have observed rate-accelerations and, in some cases, altered product distributions as compared with reactions carried out using classical oil-bath heating. As a result, researchers have speculated that so-called specific or nonthermal microwave effects could be responsible for these differences. Much of the debate has centered on the question of whether the electromagnetic field can exert a direct influence on a chemical transformation outside of the simple macroscopic change in bulk reaction temperature.In 2009, our group developed a relatively simple "trick" that allows us to rapidly evaluate whether an observed effect seen in a microwave-assisted reaction results from a purely thermal phenomenon, or involves specific or nonthermal microwave effects. We use a microwave reaction vessel made from silicon carbide (SiC) ceramic. Because of its high microwave absorptivity, the vessel shields its contents from the electromagnetic field. As a result, we can easily mimic a conventionally heated autoclave experiment inside a microwave reactor under carefully controlled reaction conditions. The switch from an almost microwave transparent glass (Pyrex) to a strongly microwave absorbing SiC reaction vial under otherwise identical reaction conditions (temperature profiles, pressure, stirring speed) then allows us to carefully evaluate the influence of the electromagnetic field on the particular chemical transformation.Over the past five years we have subjected a wide variety of chemical transformations, including organic reactions, preparations of inorganic nanoparticles, and the hydrolysis of proteins, to the "SiC test." In nearly all of the studied examples, we obtained identical results from reactions carried out in Pyrex vials and those carried out in SiC vials. The data obtained from these investigations confirm that in the overwhelming majority of cases a bulk temperature phenomenon drives the enhancements in microwave chemistry and that the electromagnetic field has no direct influence on the reaction pathway. © 2013 American Chemical Society.

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