Download Interaction of microwave irradiation with material

Interaction of microwave irradiation with material
(Theoretical principles)
Traditionally, organic synthesis is carried out by conductive heating with an external heat
source (Bunsen burner, electric plate heater, oil bath or heating mantle). This is a
comparatively slow and inefficient method for transferring energy into the reaction system
since it depends on convection currents and on the thermal conductivity of the various
materials that must be penetrated, and results in the temperature of the reaction vessel
being higher than that of the reaction mixture. In addition, a temperature gradient can
develop within the sample and local overheating can lead to product, substrate or reagent
decomposition.
Picture 1. Inverted temperature gradient in microwave (left) versus oil bath (right) heating (the picture from
the book C.O. Kappe and A. Stadler Microwaves in Organic and Medicinal Chemisry, Wiley-VCH, Weinheim,
2005)
In contrast, microwave irradiation produces efficient internal heating by direct coupling of
microwave energy with the molecules of solvents, reagents or catalysts presented in the
reaction
mixture.
Since
the
preceding
vessels
are
typically
made
out
of
microwave-transparent materials the radiation passes through the walls of the vessel directly
into the whole reaction mixture volume and an inverted temperature gradient as compared to
conventional thermal heating results. In addition, there are other features of the microwave
heating aseptically for chemical reactions which will be described below.
Microwave irradiation (microwave) consists in electromagnetic wave in the range 300 MHz to
300 GHz that corresponds to wavelengths of 1 cm to 1 m. The microwave region of the
electromagnetic spectrum lies between infrared and radio frequencies.
Picture 2. The electromagnetic spectrum
The term "microwave" denotes the techniques and concepts used as well as a range of
frequencies. Microwaves travel in matter in the same manner as light waves: they are
reflected by metals, absorbed by some dielectric materials and transmitted without
significant losses through other materials. For example, water, carbon, foods with a high
water contains, some organic solvents are good microwave absorbers whereas ceramics,
quartz glass and most thermoplastic materials absorb microwaves slightly. To avoid
interference with telecommunications and cellular phone frequencies heating applications
must use Industrial Scientific and Medicinal frequencies (ISM bands) which are 27.12 and 915
MHz and 2.45 GHz (wavelengths 11.05 m, 37.24 and 12.24 cm). All domestic microwave
ovens, microwave reactors and other laboratory and industrial systems usually work at 2.45
GHz. The microwave photon corresponding to 2.45 GHz has energy close 0.0016 eV (~ 1
kJ/mol). From comparison with chemical bonds and Brownian motion energies (Table 1) it is
clear that microwave photon is not sufficiently energetic to break even hydrogen bond and
cannot induce chemical reaction as opposite to γ or X-ray photons, ultraviolet and visible
radiation.
Table 1. Radiation and chemical bonds energies
Radiation type
Frequency
Quantum
(GHz)
energy (eV)
Bond type
Bond energy
(eV)
γ-rays
3.0 x 1011
1.24 x 106
C-C
3.61
X-rays
3.0 x 1011
1.24 x 105
C=C
6.35
Ultraviolet
1.0 x 106
4.1
C-O
3.74
Visible light
6.0 x 105
2.5
C=O
7.71
Infrared light
3.0 x 103
1.2 x 10-2
CH
4.28
Microwave irradiation
2.45
1.6 x 10-3
OH
4.80
Radiofrequencies
1 x 10-3
4.0 x 10-9
Hydrogen bond 0.04 – 0.44
In this connection, it is very important to understand that usually the microwave heating and
microwave-assisted reactions, it is not quantum chemical effects as photochemical
transformations or microwave spectroscopy.
Microwave-enhanced chemistry is based on the efficiency of interactions of molecules with
waves and heating of materials by "microwave dielectric heating" effect, depending on the
ability of materials to absorb microwaves and convert it into heat. In the most cases, the
created heating is believed to promote the chemical transformation. Microwaves are
electromagnetic waves which consist of an electric and magnetic field component:
Picture 3. Electric (red) and magnetic (blue) field components
For most practical purposes related to microwave heating in chemistry and industry the
electric component of the electromagnetic field is important, although in the case of magnetic
materials (for example
with
transition metal oxides, organic magnets, magnetic
nonoparticles etc.) magnetic field interactions can also be of relevance.
Today two main effects of microwave irradiation on meter are distinguished:
1.
Thermal effects results from dipolar polarization and ionic conduction.
2. Electrostatic polar effects which lead to dipole-dipole-type interactions between the
dipolar molecules and the charges in electric field. In this way, more polar states
appear to be more stable in the electric field. This phenomenon could be the origin of
specific non-thermal microwave effects.
The electric component causes heating by two main mechanisms: dipolar polarization and
ionic conduction. The interaction of electric field component with the polar molecules is called
the dipolar polarization mechanism.
When exposed to microwave
frequencies, the molecular dipoles align in the applied electric field. As the applied field
oscillates, the dipole field attempts to follow these oscillations and the energy is lost in the
form of heat through molecular friction and dielectric loss. The amount of heat generated by
this process is directly related to the ability of the molecules to align itself with the frequency
of the applied field. If the dipole does not have enough time to realign (high-frequency
irradiation) or reorientation is too slow (low-frequency irradiation) no heating occurs. The
frequency 2.45 GHz usually used in all commercial systems lies between these two extremes
and gives the molecule dipole time to align in the field, but not to follow the alternating field
precisely. Therefore, as the dipole reorientates to align itself with the electric field, the field is
already changing and generates a phase difference between the orientation of the field and
that of the dipole. This phase difference causes energy to be lost from the dipole by molecular
friction and collisions, giving rise dielectric heating.
In this way, the interaction between microwaves and the polar matter is not a quantum
mechanical resonance phenomenon. Transitions between quantized rotational bands are not
involved and the energy transfer is not a property of a specific molecule, but the result of a
collective phenomenon involving the bulk. The heat is generated by frictional forces occurring
between the polar molecules, the rotational velocity of which has been increased by the
coupling with the microwaves. Thus, gases cannot be heated under microwave, since the
distances between the rotating molecules are too great. Similarly, ice is practically
microwave transparent, since the water dipoles are constrained in a crystal lattice and cannot
move as freely as in the liquid state.
The second major heating mechanism is the ionic conduction.
During the ionic conduction, the
dissolved charged particles in a sample (usually ions) oscillate back and forth under the
influence of the electric component of microwave irradiation. They collide with their
neighboring molecules or atoms. And these collisions cause agitation or motion, creating heat.
The conductivity principle is a much stronger effect than the dipolar rotation mechanism with
regard to the heat-generating capacity. That’s why the media containing ions are heated
more efficiently by microwaves than just polar solvents.
There are three possibilities for rationalizing of rate-enhancements observed in
microwave-assisted process: thermal effects (kinetics), specific thermal microwave effects
and specific non-thermal microwave effects.
Thermal (kinetic) effect
In the majority of cases the reason for the observed rate enhancement of chemical processes
is a purely thermal (kinetic) effect. It means that high reaction temperature achieved rapidly
under microwave irradiation of polar materials cause increase in the chemical reaction rate.
According to Arrhenius low:
k = A exp (-ΔG#/RT)
a transformation that requires 68 days to reach 90% conversion at 27 °C will show the same
degree of conversion within 1.61 seconds at 227 °C (Table 2).
Table 2. Relationship between temperature and time for a typical first-order reaction (A = 4x1010 mol-1s-1, ΔG#
= 100 kJ mol-1)
Temperature (°C)
Rate constant, k (s-1)
Time (90% conversion)
27
1.55 x 10-7
68 days
77
4.76 x 10-5
13.4 hours
127
3.49 x 10-3
11.4 minutes
177
9.86 x 10-2
23.4 seconds
227
1.43
1.61 seconds
Thus, the simple rule for the thermal microwave effect is ‘The higher temperature, the higher
reaction rate’. In this case, the same reaction rate can be principally achieved by conductive
heating reproducing the temperature profile of the microwave heated reaction mixture.
For example, hydrogenation of 1,4-butadien was performed using both the thermal (oil bath)
and microwave heating under the same temperature and time, but the yield of the
hydrogenated product appeared to be much higher under microwave irradiation conditions:
Picture 6 From the lecture of Dr. Mike Collins on the 5th International Conference on Microwaves in Organic
Synthesis (April, 2007, Imperial College, London, UK)
Such phenomena are usually rationalized by Specific microwave effects.
Specific thermal microwave effects
In addition to thermal effects, there are microwave effects that are caused by the unique
nature of the microwave dielectric heating. These effects should be defined as accelerations
of chemical transformations in a microwave field that cannot be achieved or duplicate by
conventional heating, but essentially are still thermal effects. The superheating effect of
solvent: the question of the boiling point of a liquid undergoing microwave irradiation is one
of the basic problems facing microwave heating. It was established that microwave-heated
liquids boil at temperatures above the equilibrium boiling point at atmospheric pressure
(Table 3).
Table 3. Boiling points of some polar solvents under microwave irradiation
Solvent
Water
Boiling point
(°C)
100
Boiling points under microwave exposure (°C)
Multimode
105
Monomode (100
Monomode +
W)
boiling chips
100
100
1-Heptanol
176
208
180
173
Ethyl acetate
77
102
92
77
Chloroform
61
89
85
62
Cyclohexanone
155
186
186
155
This superheating effect is because the microwave power is dissipated over the whole volume
of the solvent. The most significant way to lose excess thermal energy is by boiling. However,
this will only occur at the existing liquid-gas interfaces, in contrast to a thermally heated
solvent where boiling typically occurs at nucleation points (cavities, pits, scratches etc) on
the glass reactor surface. The superheating effect can be removed by adding boiling stones or
stirring.
Wall effect: with microwave heating, the surface of the wall of the reactor is generally not
heated since the energy is dissipated inside the whole volume of the reaction mixture.
Therefore, the temperature at the inner surface of the reactor wall is lower than that of the
bulk liquid (Picture 1).
Volumetric heating: another phenomenon characteristic of microwave dielectric heating is
rapid and even heating of the whole reaction mixture by microwaves, almost without any
temperature gradient.
Selective heterogeneous heating: one of the most important specific thermal microwave
effects results from the selective heating of strongly microwave-absorbing heterogeneous
catalysts or reagents in a less polar reaction medium. It means that in a sample containing
more than one component, only that one which couples with microwaves is selectively heated.
This effect is probably explains the difference between reaction yields demonstrated on the
Figure #, taking into account the application of heterogeneous high absorbing palladium on
charcoal catalyst.
Molecular radiators: for homogeneous mixtures an existing of so-called "molecular radiators"
(microscopic hot spots) is suggested. They are believed to be formed when a polar substance
is irradiated in non polar media. It is considered that "molecular radiators" can directly couple
microwave energy but this hypothesis is difficult to prove experimentally.
Specific non-thermal microwave effects
In contrast to specific thermal microwave effects an existing of specific non-thermal
microwave effects is suggested by some authors. These should be classified as acceleration
of chemical processes in a microwave field that cannot be rationalized in terms of either
purely thermal/kinetic or specific thermal microwave effects. Essentially, most non-thermal
effects result from a proposed direct interaction of the electric field with specific molecules in
the reaction medium and electrostatic polar effect.
Increase in the pre-exponential factor A of Arrhenius equation: pre-exponential factor A in
Arrhenius low [k = A exp (-ΔG#/RT)] is representative of the probability of molecular impact.
The collisions efficiency can be effectively influenced by mutual orientation of polar molecular
involved in the reaction. Because this factor depends on the vibration frequency of the atoms
at the reaction interface, it could be postulated that the microwave field might affect this one.
Decrease in the activation energy (-ΔG#): is the main non-thermal effect. When considering
the contribution of enthalpy and entropy to the value of -ΔG# (ΔG# = ΔH# - TΔS#), it may be
predicted that the magnitude of the –TΔS# term would decrease in a microwave-induced
reaction, this being more organized when compared with classical heating as a consequence
of dipolar polarization. To justify the reduction in the energy of activation one can assume
stabilization by the microwaves of the transition state. This is presumably true when the
polarity increases in the transition state, i.e. if the transition state is more polar than the
ground state.
Thus the controlled microwave heating is a new and efficient tool in Chemistry, which widens
the scope of thermal heating limitations and gives chemist novel and exiting possibilities.