Ultrasound heterogeneous catalysis

However, ultrasonic rate enhancements of heterogeneous catalysis have usually been relatively modest (less than tenfold). The effect of irradiating operating catalysts is often simply due to improved mass transport (58). In addition, increased dispersion during the formation of catalysts under ultrasound (59) will enhance reactivity, as will the fracture of friable solids (e.g., noble metals on C or silica (60),(62),(62) or malleable metals (63)). 

Research on the chemical effects of ultrasound has undergone a renaissance during the past decade and has had a significant impact in a variety of areas [1, 2], Applications of sonochemistry have been developed in virtually all areas of chemistry and related chemical technologies [3, 4]. We can conceptually divide the effects of ultrasonic irradiation on heterogeneous catalysis into those that alter the formation of heterogeneous catalysts, those that perturb the properties of previously formed catalysts, and those that affect catalyst reactivity during catalysis. In practice, these three classes of effects are often deeply intertwined in reported experimental results. 

Heterogeneous catalysis is extremely important in the chemical and petroleum industries, and the applications of ultrasound to catalysis have been reviewed recently. The effects of ultrasound on catalysis can occur in three distinct stages (i) during the formation of supported catalysts, (ii) activation of pre-formed catalysts, or (iii) enhancement of catalytic behavior during a catalytic reaction. In the cases of modest rate increases, it appears likely that the cause is increased effective surface area this is especially important in the case of catalysts supported on brittle solids. More impressive accelerations, however, have included hydrogenations and hydrosilations by Ni powder. 

Such a type of deactivation behavior was observed in the case of chemotrypsin deactivation under ultrasound. As can be noticed, the same mathematical expressions as given by eq. (8.194) can be used to describe the deactivation behavior in the case of heterogeneous and enzymatic catalysis. 

Sonochemical activation can strongly improve a wide range of chemical transformations however, the influence of ultrasound on asymmetric organocatalytic reactions is still a very weakly explored area. Based on reported ejamples it could be a very attractive and useful technique for accelerating organocatalytic reactions in heterogeneous systems (e.g., phase-transfer catalysis or reaction with sohd substrates on water). Evaluation of the usefidness of ultrasound in homogenous systems (e.g., in enamine activation) is still difficult and this topic requires more systematic studies in the future. 

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