Transformation of 2-t-butylphenol

To elucidate the cause of the microwave-induced enhancement of the rate of this reaction in more detail the transformation of 2-t-butylphenol was performed at low temperatures (up to -176 °C). At temperatures below zero the reaction did not proceed under conventional conditions. When the reaction was performed under micro-wave 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 105-115 °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 (see also Sect. 10.3.3). 

Let us now return to the question How to interpret or explain the fact that some reactions are affected by microwaves and some reactions are not . A detailed study of this subject has been performed by Hajek et al. [75-77] for heterogeneous catalytic liquid-phase reactions. Transformation of 2-t-butylphenol into phenol, 4-t-butylphenol, 2,4-di-t-butylphenol, and isobutene on montmorillonites as catalysts (KSF, KIO) was chosen as model reaction. Scheme 13.13. Both reactant and catalysts coupled very well with microwaves. KSF and KIO catalysts in the form of a fine powder (10-15 pm) were used to avoid creation of macroscopic hot spots (as in the presence of voluminous catalyst pellets, e.g. 5 mm [46, 47]). The results are summarized below. 

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