Microwave conditions, temperature range enhanced

To elucidate the cause of the microwave-induced enhancement of the rate of this reaction in more detail, transformation of 2-t-butylphenol was performed at low temperatures (up to —176 °C). This method, which is called Simultaneous cooling whilst microwave heating, was used for the first time in heterogeneous catalytic reactions [75]. At temperatures below zero the reaction did not proceed under conventional conditions. When the reaction was performed under microwave conditions in this low-temperature region, however, product formation was always detected (conversion ranged from 0.5 to 31.4%). It was assumed that the catalyst was superheated or selectively heated by microwaves to a temperature calculated to be more than 80-115 °C above the low bulk temperature. Limited heat transfer in the solidified reaction mixture caused superheating of the catalyst particles and this was responsible for initiation of the reaction even at very low temperatures. If superheating of the catalyst was eliminated by the use of a nonpolar solvent, no reaction products were detected at temperatures below zero (Section 13.3.3). 

Loupy and co-workers [68] have studied the effectiveness of microwave irradiation in increasing the enzymatic affinity and selectivity of supported lipases in esterification and transesterification reactions under dry media conditions (see Scheme 37). The esterification and transesterifications of racemic 1-phenylethanol 64 were studied in a temperature range of 70-100 °C. The lipases considered were the Pseudomonas cepacia lipase (LP) and Candida Antarctica lipase (SP-435). The initial rates and enantiomeric ratios E were significantly enhanced under microwave irradiation. Even so, in cases where classical conditions showed poor reaction, complete conversion could be achieved with increased reactivity under microwave conditions. This was largely attributed to the exclusion of the volatile by-products from the equilibrium. More importantly, the supported enzymes showed good stability and could be reused three more times in the reactions under study without loss of activity. 

An interesting experiment was proposed by Komarneni and Katsuki [19] (Table 5.1, entry 8) by stirring of contents under microwave hydrothermal conditions. Based on the results, it was concluded that stirring under hydrothermal microwave conditions in the temperature range of 150-200 °C led to enhanced crystallization of BaTiOs, in comparison with the static conditions. Additionally, stirring led to obtain smaller and more uniform, well-faceted, and better dispersed crystals. Thus, stirring under hydrothermal microwave conditions clearly affected the yields, size of particles, and their uniformity. 

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