Microwave chemistry
Encyclopedia
Microwave chemistry is the science of applying microwave
irradiation
to chemical reactions. Microwaves act as high frequency electric field
s and will generally heat any material containing mobile electric charge
s, 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, 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.
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:
s 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 and in-situ methods. 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 reaction
s.
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...
to chemical reactions. Microwaves act as high frequency electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s and will generally heat any material containing mobile electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
s, 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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
and energy is lost due to the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the material. Microwave heating in the laboratory began to gain wide acceptance following papers in 1986, 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
Superheating
In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...
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 rateReaction rateThe reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
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 tangentLoss tangent
The loss tangent is a parameter of a dielectric material that quantifies its inherent dissipation of electromagnetic energy. The term refers to the tangent of the angle in a complex plane between the resistive component of an electromagnetic field and its reactive component.-Electromagnetic...
s 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
Dissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
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
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change 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
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...
.
In a system where the heat transfer is slow, it would be possible to have the presence of a steady state
Steady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
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
Pelletizing
Pelletizing is the process of compressing or molding a material into the shape of a pellet. A wide range of different materials are pelletized including chemicals, iron ore, animal compound feed, and more.- Pelletizing of iron ore :...
) 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 and in-situ methods. 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
Phase transfer catalyst
In chemistry, a phase transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase transfer catalysis is a special form of heterogeneous catalysis. Ionic reactants are often soluble in an aqueous phase but...
a water phase reaches a temperature of 100°C while a chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
phase would retain a temperature of 50°C, providing the extraction
Extraction (chemistry)
Extraction in chemistry is a separation process consisting in the separation of a substance from a matrix. It may refer to Liquid-liquid extraction, and Solid phase extraction....
as well of the reactants from one phase to the other. Microwave chemistry is particularly effective in dry media reaction
Dry media reaction
A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction system in the absence of a solvent. The drive for the development of dry media reactions in chemistry is* economics...
s.
External links
- AMPERE (Association for Microwave Power in Europe for Research and Education)
- Microwave Chemistry Pages --> Comprehensive introduction to "microwave chemistry"
- MAOS (Microwave-Assisted Organic Synthesis) --> General web resource guide of "microwave chemistry"
- Microwave Synthesis @ organic-chemistry.org
- homepages uconn.edu site with microwave equipment and research
- Chemical Vapor, Oxide Matalorganic
- Alternative methods of energy input: Microwave technology
- Microwave synthesis - synthetic protocols from organic-reaction.com