Microwave chemistry
Microwave chemistry is the science of applying
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.
Acting as internal heat source, microwave absorption is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy.[7] 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. Microwave volumetric heating (MVH) overcomes the uneven absorption by applying an intense, uniform microwave field.
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
Microwave chemistry is applied to organic chemistry [8] and to inorganic chemistry.[9][10][11][12][13][14]
Selective heating
A heterogeneous system (comprising different substances or different phases) may be
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
A different specific application in synthetic chemistry is in the microwave heating of a
Microwave effect
There are two general classes of microwave effects:
- Specific microwave effects.
- Non-thermal microwave effects.
A review has proposed this definition[17] and examples of microwave effects in organic chemistry have been summarized.[18]
Specific microwave effects are those effects that cannot be (easily) emulated through conventional heating methods. Examples include: (i) selective heating of specific reaction components, (ii) rapid heating rates and temperature gradients, (iii) the elimination of wall effects, and (iv) the superheating of solvents. Microwave-specific effects tend not to be controversial and invoke "conventional" explanations (i.e. kinetic effects) for the observed effects.[19]
Non-thermal microwave effects have been proposed 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. Such effects are controversial.
Catalysis
Application of microwave heating to heterogeneous catalysis reactions has not been explored intensively due to presence of metals in supported catalysts and possibility of arcing phenomena in the presence of flammable solvents. However, this scenario becomes unlikely using nanoparticle-sized metal catalysts.[7]
References
- ^ "Microwaves in Organic Synthesis". Organic Chemistry Portal. Retrieved 23 October 2018.
- doi:10.1039/b411438h
- ^ Developments in Microwave-assisted Organic Chemistry. C. Strauss, R. Trainor. Aust. J. Chem., 48 1665 (1995).
- ^ Dry media reactions M. Kidwai Pure Appl. Chem., Vol. 73, No. 1, pp. 147–151, 2001.[1]
- ^ Microwaves in Organic and Medicinal Chemistry, 2nd, Completely Revised and Enlarged Edition, Wiley-VCH, Weinheim, 2012 http://eu.wiley.com/WileyCDA/WileyTitle/productCd-3527331859.html
- ^ a b c Pizzetti, Marianna (May 2012). "Heterogeneous catalysis under microwave heating" (PDF). La Chimica & l'Industria (4). Società Chimica Italiana: 78–80.
- ^ 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)
- ^ 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
- 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
- ^ http://www.isis.rl.ac.uk/isis2005/reports/15301.PDF[permanent dead link]
- ^ Kappe, C. O. Angew. Chem. Int. Ed. 2004, 43, 6250-6285.
- ^ De la Hoz, A.; Diaz-Ortiz, A.; Moreno, A. Chem. Soc. Rev. 2005, 164-178.
- ^ "The science behind industrial microwave". Massalfa. 23 October 2018.