Microwave-heating

Microwaves have wavelengths between 1 mm and 1 m and hence have similar frequencies to radar and telecommunication devices. So as not to cause interference with these systems the frequency of radiation that can be emitted by household and industrial appliances is strictly regulated, with most appliances operating at a fixed frequency of 2.45 GHz. To some extent this reduces the flexibility of such equipment. 

The overall mechanism of how energy is imparted to a substance under microwave irradiation is complex, consisting of several different aspects. 

From the above discussion it will be evident that whilst certain materials can be heated selectively, the energy will soon be uniformly distributed throughout a homogeneous reaction medium. Microwaves may be considered a more efficient source of heating than conventional steam- or oil-heated vessels since the energy is directly imparted to the reaction medium rather than through the walls of a reaction vessel. 

Microwave heating (MWH) is the second method for the fabrication of nanomaterials that will be discussed in this chapter. Microwaves are electromagnetic radiation, whose wavelengths lie in the range 1 mm to 1 m (frequency range 0.3 to 300 GHz). A large part of the microwave spectrum is used for communication 

However, we should mention the pioneering work of Chou and Phillips, where metallic iron and iron oxide particles were produced by injecting ferrocene into the afterglow region of a low-pressure, low-power, plasma, generated using a micro-wave power source [160]. This gas phase reaction was carried out as part of an attempt to explore the feasibility of using flow-type microwave plasmas for the production of metal nanoparticles. 

A short introduction to as to how the irradiated molecule is affected by micro-waves is first presented. It is based on the reviews of Rao and Mingos [159, 160]. 

A dielectric material is one that contains either permanent or induced dipoles 

The resulting phase displacement S, acquires a component / sin 5 in phase with the electric field, and thus resistive heating occurs in the medium. This is described as dielectric loss and causes energy to be absorbed from the electric field. 

Microwave heating is a close cousin of dielectric heating the main difference being the higher frequencies of microwaves, ranging from about 1000 to 100,000 MHz. This is about two to three orders of magnitude higher than the frequency spectrum 

The rate of heating by microwave energy is described by the same equation used for radio frequency heating, Eq. 5.87. Thus, the amount of heating depends on the field strength, frequency, and loss factor. The latter factor is a material property the first 

The heat is generated throughout the material however, the power level reduces with depth of penetration. The depth at which the power is reduced to one-half is given by  

From these expressions, one can see that the depth of penetration reduces with increasing frequency and with increasing loss factor. 

The use of domestic microwave ovens is to be greatly discomaged - although cheap, they allow for very little control and give no information about precise conditions and therefore fall short of proper scientific reqnirements. They are also dangerous, with the possibility of fires and explosions Purpose-made commercial equipment is mnch more versatile and, with feedback control from sensors for temperature and pressure, is much safer, and provides proper control and output data. 

Reaction methodology nsnally needs to be modified for microwave reactions. Solvent-free reactions are popnlar and eliminate the risk from flammable solvents. Reactions can also be carried out on pastes or with the reactants adsorbed onto solid supports, such as alumina or bentonite. 

The existence, or not, of non-heating effects of microwave radiation on reactions is somewhat controversial, as the energy of microwaves is too low to break chemical bonds directly. However, microwave irradiation certainly can prodnce different results to conventional heating, for example isomer ratios in A-alkylation of azoles. It can be argued that this is just due to the different rates of heating, but it can be a nsefnl featnre.  

Batch scale-np of microwave reactions can be carried out to a certain extent (1 kg or so ), but for larger reactions it is difficnlt, dne to the limited depth of penetration of the radiation. However, combination of 

The relative contribution of these two heating mechanisms depends on the mobility and concentration of the sample ions and on the relaxation time of the sample. If the ion mobility and concentration of the sample ions are low, then sample heating will be entirely dominated by dipole rotation. On the other hand, as the mobility and concentration of the sample ions increase, microwave heating will be dominated by ionic conduction and the heating time will be independent of the relaxation time of the solution. As the ionic concentration increases, the dissipation factor will increase and the heating time decrease. The heating time depends not only on the dielectric absorptivity of the sample but also on the microwave system design and the sample size. 

There is a clear difference between conductive and microwave heating because the vessels used in conductive heating are usually poor conductors of heat, they take time to heat and transfer such heat to the solution. Also, because the liquid vaporizes at the surface, a thermal gradient is established by convection currents so only a small portion of the fluid is at the temperature of the heat applied to the outside of the vessel. Therefore, with conductive heating, only a small portion of the fluid is above the boiling point of the solution. 

On the other hand, microwaves heat all of the sample fluid simultaneously without heating the vessel. Therefore, with microwave heating, the solution reaches its boiling point very rapidly. Because the rate of heating is so much more rapid, substantial localized superheating can occur [14]. Table 5.1 shows the effect of microwave heating on several solvents [20]. 

TEMPERATURE REACHED BY VARIOUS SOLVENTS UPON HEATING FROM ROOM TEMPERATURE USING MICROWAVE ENERGY OF 2450 MHz AND 560 W FOR 1 MIN 

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Microwave Cooking Pads. A simple and effective method of reducing fat in meat products involves the use of microwavable heating pads. 

Microwaves have successfully been used for rewarming of blood for medical appHcations (157). Another successful appHcation, not commetciali2ed as of this writing, is the use of microwave heating for rapid tissue fixation (158,159). This procedure appears to reduce the time for tissue sample analysis... 

Reports of sterilisation (qv) against bacteria by nonthermal effects have appeared, but it is generally beheved that the effect is only that of heating (164). Because microwave heating often is not uniform, studies in this area can be seriously flawed by simplistic assumptions of uniform sample temperature. 

R. A. Metaxas and R. Meredith, Industrial Microwave Heating, Peter Peregrinus Ltd., London, 1983. 

D. A. Copson, Microwave Heating, Avi Publishing Co., Westport, Conn., 1975, Chapt. 11. 

Special drying methods, such as superheated steam, solvent, vacuum, infrared radiation, and high frequency dielectric and microwave heating, are occasionally employed when accelerated drying is desired and the species being dried can withstand severe conditions without damage. None of these methods is of significant commercial importance. 

Freeze drying has also been carried out at atmospheric pressure in fluid beds using circulating refrigerated gas. Vacuum-type vibrating conveyors, rotating multishelf dryers and vacuum pans can be used as can dielectric and microwave heating. 

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

Whilst it is possible to purchase standard equipment for the steam moulding process, attempts continue to be made to make sweeping modifications to the process. These include the use of dielectric and microwave heating and the development of semicontinuous and continuous processes. 

The methods of heating TLC/HPTLC plates described above depend on thermal conduction, convection or radiation. Microwave heating involves a special form... 

T-Jumps can also be produced by microwave heating and by laser pulse absorption. These methods remove the restriction to low-resistance solvents any solvent capable of absorbing energy of the applied frequency may be used. The heating time can be extremely short with laser heating. ... 

Bu3Sn)20 BzCl. The use of microwaves accelerates this reaction. Bu2Sn(OMe)2 is reported to work better than Bu2SnO in the monoprotection of diols. The monoprotection of diols at the more hindered position can be accomplished through the stannylene if the reaction is quenched with PhMe2SiCl (45-77% yield).Microwave heating has been found to be effective for this transformation in some cases. ... 

Fig. 11 Elution profile of the ws-material from microwave-heated spruce chips after SEC [218]. Detection by refractometry index (Rl) dotted line) and UV detection at 280 nm (full line). The arrows mark the elution volume of acetylated GGM fractions...

During the past decade, MALDI-TOF MS has proven to be an effective tool for the analysis of oligo- and polymeric mannoglucans (for extensive reviews see [222,223]). SEC/MALDI mass spectrometry was employed in the analysis of hemicelluloses isolated by microwave heat-fractionation from spruce and aspen wood [94]. These methods allowed the separation and characterization of the oligo- and polysaccharide fractions derived from the xylan and mannan components of both woods [224]. 

Finally, dissolution of non-activated cellulose in LiCl/DMAc, and in ionic liquids has been accelerated by microwave irradiation [72,103,104], although the effect of microwave heating on the DP of the polymer has not been investigated. This last point is relevant in view of the fact that ILs are heated with exceptional efficiency by microwaves [105], so that care must be taken to avoid excessive localized heating that can induce chain degradation of the polymer during its dissolution. 

A one-pot synthesis of thiohydantoins has been developed using microwave heating [72]. A small subset of p-substituted benzaldehydes, prepared in situ from p-bromobenzaldehyde by microwave-assisted Suzuki or Negishi reactions, was reacted in one pot by reductive amination followed by cyclization with a thioisocyanate catalyzed by polystyrene-bound dimethyl-aminopyridine (PS-DMAP) or triethylamine, all carried out under microwave irradiation, to give the thiohydantoin products in up to 68% isolated yield (Scheme 16). 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

A tetrahydrofuran cross-linked PS-based insoluble support (/anda/el) has also been developed and shown to tolerate microwave heating by Janda and coworkers [75]. /anda/el is commercially available and reported to offer better swelling characteristics, increased homogeneity and site-accessibiUty, as well as a more organic solvent-like environment compared to divinylbenzene cross-hnked polymers. 

Rapid loading of cross-linked PS Wang resin (4-(benzyloxy)benzyl alcohol PS) with a selection of /3-ketoesters was shown to reach completion within 1-10 min if microwave irradiation at 170 °C was employed. The conventional thermal method for acetoacetylation of hydroxymethyl-functionalized polystyrene resins takes several hours therefore, microwave heating allowed for... 

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