Microwave Activation of Catalytic Reactions

As an example, micro vave activation of the platinum catalyst under conditions vhen it vas highly sensitive to thermal treatment resulted in an increase of its catalytic activity and selectivity (from 40% to 80%) [37]. 

Durable changes of the catalytic properties of supported platinum induced by micro vave irradiation have been also recorded [38]. A substantial reduction of the time taken for activation (from 9 h to 10 min) vas observed in the activation of NaY zeolite catalyst by micro vave dehydration in comparison vith conventional thermal activation [39]. The very efficient activation and regeneration of zeolites by micro vave heating can be explained by the direct desorption of vater molecules from zeolite by the electromagnetic field this process does not depend on the temperature of the solid [40]. Interaction of the adsorbed molecules vith the micro-wawe field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of vater molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Alumina spheres polluted by carbon residues have been also reactivated by use of microwaves [42]. Their regeneration has been performed in a stream of air and in the presence of silicon carbide as an auxiliary microwave absorber. Microwave heat treatment led to full recovery of the catalyst in times varying from a half to a quarter of those required by conventional treatment. Regeneration of a commercial Ni catalyst (Ni-Al203) deactivated, presumably, by coke formation, by means of a flow of hydrogen or oxygen and water vapor under the action of microwave irradiation was, however, unsuccessful [43]. 

Microwaves are frequently used in the laboratory by synthetic organic chemists for regeneration and activation of solids such as molecular sieves, silica gel, or alumina when fast and complete drying is required. 

Heterogeneous catalysts have been used in a number of organic reactions in which microwave heating was used. There is, unfortunately, a limited number of control data, which makes comparison with conventional heating difficult. Nevertheless, 

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A detailed study of microwave activation of catalytic reactions in the liquid phase has recently been performed by Hajek et al. [58-60], Scheme 10.13. 

Although microwave activation of catalytic reactions has been the subject of many studies (Sects. 10.3.1 and 10.3.2), the mechanism of these reactions is not yet fully understood. In heterogeneous catalytic liquid/solid and gas/solid systems many results have revealed significant differences between the rates of conventionally and microwave heated reactions. As a rule, at the same temperature microwave heated reactions were faster than conventional and their rate enhancement was over one or-... 

A detailed study of microwave activation of catalytic reactions in the liquid phase has recently been performed by Hajek et al. (Scheme 13.13) [75-77]. It was found that in the catalytic transformation of 2- and 4-t-butylphenol in the liquid phase on heterogeneous KSF and KIO montmorillonite catalysts under MW and conventional conditions the microwaves affected both the rate and selectivity of the reaction. 

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