Microwave heating nonthermal effects

Microwave dielectric heating was initially categorized by thermal effects and nonthermal effects. Thermal effects are those which are caused by the different temperature regime which can be created due to microwave dielectric heating. Nonthermal effects are effects, which are caused by effects specifically inherent to... 

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. 

More interesting fundamentally is the very strong specific nonthermal effect of microwaves, as evidenced by comparison with classical heating. This effect grows as ester reactivity falls. 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

Snpporting those who believe that a nonthermal effect does not exist, there are reports that maintain, for example, that claims of the microwave effect were probably dne to experimental mistakes when researchers tried to extrapolate concepts of conventional heating into microwave heating [100], It has been claimed that the possibility of rednced reaction temperatnres reported in some cases were nothing bnt mistakes in the measnred temperatnre dne to heat losses that occnrred becanse of condnction throngh thermoconple gnards [101],... 

The observed rate accelerations and sometimes altered product distributions compared to classical oil-bath experiments have led to speculation on the existence of specific or nonthermal microwave effects. " Historically, such effects were claimed when the outcome of a synthesis performed under microwave conditions was different from that of the conventionally heated counterpart. When reviewing the present literature, it appears that most scientists now agree that in the majority of cases the reason for the observed rate enhancements is a purely thermal/kinetic effect. Even though for the industrial chemist this discussion seems largely irrelevant, the debate on microwave effects is undoubtedly going to continue for many years in the academic world. Today, microwave chemistry is as reliable as the vast arsenal of synthetic methods that preceded it. Microwave heating not only reduces reaction times significantly, but is also known to reduce side reactions, increase yields, and improve reproducibility. 

There is, nevertheless, experimental evidence that some reactions occur faster than conventionally heated reactions at the same temperature. Although this suggests that nonthermal effects are involved, these rate enhancements could also be attributable to localized superheating or hot spots. In fact, most reports of substantial rate enhancements under the action of microwaves seem to be in heterogeneous reactions, for example under dry media conditions or when solid catalysts are used [75, 76]. In these circumstances localized heating of a microwaveabsorbing solid support or catalyst could lead to an increased reaction rate [136, 137]. 

The effect of microwave irradiation in chemical reactions can be attributed to a combination of the thermal and nonthermal effects, i.e. overheating, hot spots, and selective heating, and nonthermal effects of the highly polarizing field, in addition to effects on the mobility and diffusion that may increase the probabilities of effective collisions. These effects can be rationalized considering the Arrhenius law [36, 37] and result from modification of each of the terms of the equation k = Aexp -AGyRT)... 

What are microwave effects Microwave effects are usually effects which cannot be achieved by conventional heating. These microwave effects can be regarded as thermal or nonthermal. Thermal effects result from microwave heating, which may result in a different temperature regime, whereas nonthermal effects are specific effects resulting from nonthermal interaction between the substrate and the microwaves. 

The proposal of some authors on the operation of nonthermal effects is still controversial [120-122]. In the literature microwave effects are the subject of some misunderstanding. Most mistakes was recorded when authors considered that microwave effect means specific effect , i.e. a nonthermal effect. For that reason, let us discuss the matter in more detail. In heterogeneous catalytic reactions, differences between reaction rates or selectivity under microwave and conventional heating conditions have been explained by thermal effects. 

Significant rate accelerations and higher loadings are observed when the micro-wave-assisted and conventional thermal procedures are compared. Reactions times are reduced from 12-48 h with conventional heating at 80 °C to 5-15 min with microwave flash heating in NMP at temperatures up to 200 °C. Finally, kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) rather than to a specific nonthermal microwave effect [17]. 

A nonthermal microwave effect was not observed when identical temperature gradients were produced by classical heating and microwave irradiation and if the reaction temperature was strictly controlled. 

Microwave-assisted synthesis in general is likely to have a tremendous impact in the medicinal/combinatorial chemistry communities because it shortens reaction times, improves final yields and purities, and can carry out reactions that previously were thought impossible to do. It should be stressed that in general the rate enhancements seen in microwave-assisted synthesis can be attributed to the very rapid heating of the reaction mixture (flash heating) and the high temperatures that can be reached, rather than to any other specific or nonthermal microwave effect.40... 

Scientific studies have found that the differences between microwave and conventional pyrolysis go beyond the obvious difference in the source of heat. Other differences arise from the very high rates of heat transfer from the microwave-absorbent to the waste, the amount heat received by the primary pyrolytic products once they leave the absorbent bed and the highly reducing environment. These three aspects have been shown to have an important effect in the final products since they modify the extent of secondary and tertiary reactions. Moreover, the scientific studies have shown that a nonthermal microwave effect in these processes is unlikely to exist. Tests have showed the potential of the microwave-induced pyrolysis process for the treatment of real plastic-containing wastes and it is believed that a commercial process could be developed, for example, to recover clean aluminium from plastic/aluminium laminates. Other materials, in particular tyres, coal and medical wastes are very good candidates to be treated/recycled using microwave pyrolysis and there have been a considerable number patents filed with this goal in mind. 

Differences between chemistry observed with microwave and conventional heating can often be attributed to the different transfer rate or spatial distribution of heat. Once appropriate temperatures are known in various parts of the system, conventional laws of thermodynamics or kinetics commonly apply. Such cases may be called microwave specific effects. However, there are also cases where it is proposed that an additional effect operates, perhaps through the action of the electric field 23 such effects are commonly called nonthermal or athermal microwave effects. Although their existence is controversial in fluid phases,6,24-26 there is a strong body of evidence supporting such effects in solid phases.27-29... 

Microwave-assisted solvent-free reactions have been used by Jenekhe [146] to synthesize quinoline derivatives. An important specific nonthermal microwave effect has been observed compared with conventional heating (Eq. 60). This MW effect is consistent with mechanistic considerations, because the rate-determining step is the internal cyclization depicted in Eq. (60) resulting from nucleophilic attack of the enamine on the carbonyl moiety occurring via a dipolar transition state. 

The reactions have also been performed under solvent-free conditions with considerable success and almost quantitative yields of the products [35a]. For example, the last entry in Table 7.2 was improved to 100% when irradiation was performed for 3 min. at 50 °C. Under solvent-free conditions, the existence of nonthermal specific microwave effects has been proposed on the basis of comparison with control reactions performed using conventional heating. 

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