Comparison of homogeneous and heterogeneous conditions

As discussed earlier, careful comparisons of the rates of MW irradiated reactions in homogeneous media, particularly in polar solvents, at atmospheric pressure show that these rates are the same or only slightly higher than similar reactions under 

In fact, most reports of substantial rate enhancements by MW in open vessels are in reactions performed under dry-media conditions, in which the reactants are absorbed on inert solid supports [11-16] Since one or more of the reactants is usually a liquid at the reaction temperature [44], these reactions are performed under heterogeneous conditions. Although it is difficult to perform kinetic studies on these reactions, there is little doubt that they generally occur considerably faster than similar conventionally heated reactions. 

There have also been a number of reports of accelerations of reactions in the liquid and gaseous phase in the presence of a solid catalyst which absorbs MW irradiation [81]. 

These results suggest that selective heating of the catalyst to temperatures higher than that of the liquid phase is responsible for the modest increases in rate in the presence of a heterogeneous catalyst. 

Recently, Hajek and Radoiu observed that MW not only increase the rate of heterogeneous catalytic reactions, but also affect the product selectivity [85], The results were explained in terms of MW-induced polarization, involving the absorption of MW by highly polarized reagent molecules on the active site of the catalyst. On the other hand there is little, if any, activation of homogeneous catalytic reactions in polar solvents [86], presumably due to the high absorbent power of MW irradiation by the solvent. 

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