Heterogeneous systems mechanism comparison

Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed false sonochemistry although many industrialists would argue that the term false may not be correct because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus sonochemical . In fact the true test for false sonochemistry is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible. 

Another problem involves the classification of these metal-based heterogeneous systems into suspension, dispersion, and emulsion polymerizations similarly to conventional systems. This is due to not only a lack of detailed analysis of reaction mechanisms and particle sizes but also fundamental differences in several aspects such as the locus of initiation and the molecular weight of polymers in comparison with the conventional counterparts. The terms suspension and emulsion will be used in the following sections for simple classification but are not based on the strict definition for conventional free radical systems. 

A study of the effect of pore geometry on foam formation mechanisms shows that snap-off" bubble formation is dominant in highly heterogeneous pore systems. The morphology of the foams formed by the two mechanisms are quite different. A comparison of two foam injection schemes, simultaneous gas/surfactant solution injection (SI) and alternate gas/surfactant solution injection (GDS), shows that the SI scheme is more efficient at controlling gas mobility on a micro-scale during a foam flood. 

Advances made in the area of mechanism have been presented so as to give a perspective on similarities and differences between different catalysts. Both classical and novel systems have been covered with considerable attention placed on comparisons. Less detail is given for certain homogeneous and heterogeneous catalyst systems reported elsewhere in this book, such as Au/titanosilicates. 

As mentioned in Sect. 2, many efficient homogeneous and heterogeneous oxidation systems based on POMs have been developed. Among them, some POM catalysts with multimetallic active sites show remarkable activity and selectivity in comparison with the conventional monometalhc complexes. According to the proposed reaction mechanisms, the oxidation catalyses by multimetallic active sites of POMs are classified into the following four categories (1) cooperative activation of oxidants, (2) simultaneous activation of oxidants and substrates, (3) stabihzation of reaction intermediates, and (4) multielectron transfer (Fig. 1). In Sect. 3, we focus on the selective oxidations by multimetallic active sites of POM-based molecular catalysts. 

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