Microwave chemistry

Microwave chemistry

Microwave chemistry is the science of applying microwave irradiation to chemical reactions.[1][2][3][4] Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid. Polar solvents are heated as their component molecules are forced to rotate with the field and lose energy in collisions. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material. Microwave heating in the laboratory began to gain wide acceptance following papers in 1986,[5] although the use of microwave heating in chemical modification can be traced back to the 1950s. Although occasionally known by such acronyms as 'MEC' (Microwave-Enhanced Chemistry) or MORE synthesis (Microwave-organic Reaction Enhancement), these acronyms have had little acceptance outside a small number of groups.


Heating effect

Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction. The core of the sample takes much longer to achieve the target temperature, e.g. when heating a large sample of ceramic bricks.

Microwave heating is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. It is also able to heat sufficiently thin objects throughout their volume (instead of through its outer surface), in theory producing more uniform heating. However, due to the design of most microwave ovens and to uneven absorption by the object being heated, the microwave field is usually non-uniform and localized superheating occurs.

Different compounds convert microwave radiation to heat by different amounts. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than others (particularly the reaction vessel).

Microwave heating can have certain benefits over conventional ovens:

  • reaction rate acceleration
  • milder reaction conditions
  • higher chemical yield
  • lower energy usage
  • different reaction selectivities

Selective heating

A heterogeneous system (comprising different substances or different phases) may be anisotropic if the loss tangents of the components are considered. As a result, it can be expected that the microwave field energy will be converted to heat by different amounts in different parts of the system. This inhomogeneous energy dissipation means selective heating of different parts of the material is possible, and may lead to temperature gradients between them. Nevertheless, the presence of zones with a higher temperature than others (called hot spots) must be subjected to the heat transfer processes between domains. Where the rate of heat conduction is high between system domains, hot spots would have no long-term existence as the components rapidly reach thermal equilibrium. In a system where the heat transfer is slow, it would be possible to have the presence of a steady state hot spot that may enhance the rate of the chemical reaction within that hot zone.
On this basis, many early papers in microwave chemistry postulated the possibility of exciting specific molecules, or functional groups within molecules. However, the time within which thermal energy is repartitioned from such moieties is much shorter than the period of a microwave wave, thus precluding the presence of such 'molecular hot spots' under ordinary laboratory conditions. The oscillations produced by the radiation in these target molecules would be instantaneously transferred by collisions with the adjacent molecules, reaching at the same moment the thermal equilibrium. Processes with solid phases behave somewhat differently. In this case much higher heat transfer resistances are involved, and the possibility of the stationary presence of hot-spots should be contemplated. A differentiation between two kinds of hot spots has been noted in the literature, although the distinction is considered by many to be arbitrary. Macroscopic hot spots were considered to comprise all large non-isothermal volumes that can be detected and measured by use of optical pyrometers (optical fibre or IR). By these means it is possible to visualise thermal inhomogeneities within solid phases under microwave irradiation. Microscopic hot spots are non-isothermal regions that exist at the micro- or nanoscale (e.g. supported metal nanoparticles inside a catalyst pellet) or in the molecular scale (e.g. a polar group on a catalyst structure). The distinction has no serious significance, however, as microscopic hotspots such as those proposed to explain catalyst behaviour in several gas-phase catalytic reactions have been demonstrated by post-mortem methods [6] and in-situ methods.[7] Some theoretical and experimental approaches have been published towards the clarification of the hot spot effect in heterogeneous catalysts.

A different specific application in synthetic chemistry is in the microwave heating of a binary system comprising a polar solvent and a non-polar solvent obtain different temperatures. Applied in a phase transfer reaction a water phase reaches a temperature of 100°C while a chloroform phase would retain a temperature of 50°C, providing the extraction as well of the reactants from one phase to the other. Microwave chemistry is particularly effective in dry media reactions.


Organic synthesis

  1. ^ Microwaves in organic synthesis, Andre Loupy (ed), Wiley-VCH, Weinheim, 2006, http://www.organic-chemistry.org/books/reviews/3527305149.shtm
  2. ^ Microwaves in organic synthesis. Thermal and non-thermal microwave effects, Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno, Chem. Soc. Rev., 2005, 164-178 doi:10.1039/b411438h
  3. ^ Developments in Microwave-assisted Organic Chemistry. C. Strauss, R. Trainor. Aust. J. Chem., 48 1665 (1995).
  4. ^ Dry media reactions M. Kidwai Pure Appl. Chem., Vol. 73, No. 1, pp. 147–151, 2001.[1]
  5. ^ The use of microwave ovens for rapid organic synthesis Richard Gedye, Frank Smith, Kenneth Westaway, Humera Ali, Lorraine Baldisera, Lena Laberge and John Rousell Tetrahedron Letters Volume 27, Issue 3, 1986, Pages 279-282 doi:10.1016/S0040-4039(00)83996-9
  6. ^ Zhang, X., Hayward, D.O., Lee, C. and Mingos, D.M.P. (2001) Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts. Applied Catalysis B: Environmental, 33, (2), 137-148
  7. ^ http://www.isis.rl.ac.uk/isis2005/reports/15301.PDF

R.Cecilia, U.Kunz, T.Turek. "Possibilities of process intensification using microwaves applied to catalytic microreactors" Chem. Eng. Proc. Volume 46, Issue 9, Pages 870-881 (September 2007)

Inorganic synthesis

  • Martín-Gil J, Martín-Gil FJ, José-Yacamán M, Carapia-Morales L and Falcón-Bárcenas T. "Microwave-assisted synthesis of hydrated sodium uranyl oxonium silicate". Polish J. Chem, 2005, 1399-1403.
  • J. Prado-Gonjal, M.E. Villafuerte-Castrejón,L. Fuentes and E. Morán. "Microwave-hydrothermal synthesis of BiFeO3". "Mat.Res.Bull" 44 (2009) 1734-1737
  • K.J.Rao, B.Vaidhyanathan, M.Ganduli, P.A.Ramakrishnan, Chem.Mater. 11, 1999, 882
  • J.Zhao, W.Yan, Modern Inorganic Synthetic Chemistry, Chapter 8 (2011) 173
  • R.K.Sahu, M.L.Rao, S.S.Manoharan, Journal of Materials Science 36 (2001) 4099
  • D.M.P.Mingos, D.Baghurst, Chem.Soc.Rev 20 (1991) 1

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Microwave chemistry sensor — or Surface acoustic wave (SAW) sensors consist of an input transducer, a chemically adsorbent polymer film, and an output transducer on a piezoelectric substrate, which is typically quartz. The input transducer launches an acoustic wave that… …   Wikipedia

  • Microwave Active Composition — is a brand of microwave sensitive liquid chemicals sold by Atlanta Chemical Engineering LLC. The chemical undergoes color change if it interacts with intensive microwaves. There are two major types of microwave active compositions reversible and… …   Wikipedia

  • Microwave oven — A modern microwave oven …   Wikipedia

  • Microwave effect — The phrase microwave effect is a term that is applied to a range of observations in microwave chemistry. There are two general classes of microwave effects: Specific microwave effects. Non thermal microwave effects. A recent review has proposed… …   Wikipedia

  • Microwave — This article is about the electromagnetic wave. For the cooking appliance, see Microwave oven. For other uses, see Microwaves (disambiguation). A microwave telecommunications tower on Wrights Hill in Wellington, New Zealand Microwaves, a subset… …   Wikipedia

  • Microwave plasma — is a type of plasma, that has high frequency electromagnetic radiation in the GHz range. It is capable of exciting electrodeless gas discharges. Properties of microwave excited plasma If applied in surface wave sustained mode, they are especially …   Wikipedia

  • Microwave digestion — is a common technique used by elemental scientists to dissolve heavy metals in the presence of organic molecules prior to analysis by inductively coupled plasma, atomic absorption, or atomic emission measurement. This technique is usually… …   Wikipedia

  • Microwave enhanced electrochemistry — Microwave radiation was applied in electrochemical methods in 1998 when Frank Marken and Richard G. Compton in Oxford placed a piece of platinum wire inside microwave cavity in small electrochemical cell. Categories: Microwave technologyChemistry …   Wikipedia

  • Chemistry — For other uses, see Chemistry (disambiguation). Chemistry is the science of atomic matter (that made of chemical elements), its properties, structure, comp …   Wikipedia

  • Non-thermal microwave effect — Non thermal microwave effects have been posited in order to explain unusual observations in microwave chemistry. As the name suggests, the effects are supposed not to require the transfer of microwave energy into thermal energy. Instead, the… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.