Mixed-class claims

Flavonoids as antioxidants have been reviewed several times 45s including an outline of many claims to their beneficial health effects . Due to their complex structures and different classes (eight thousand different compounds are known ), researchers often resorted to qualitative screening methods to evaluate their antioxidant potentials in mixed aqueous/lipid phases. For example, the so-called Trolox equivalent antioxidant capacity (TEAC), the concentration of Trolox with equivalent antioxidant activity of a 1 mM concentration of the substrate, is frequently used in heterogeneous systems. Unfortunately, this can be an unreliable measure of the activity of the substance, especially if initiation is also carried out in the aqueous phase. Nevertheless, there have been some efforts made to evaluate antioxidant activities of specific flavonoids using more quantitative methods in heterogeneous systems in order to mimic natural environments. A few examples are cited below to illustrate some approaches to determine flavonoid activities in micelles or lipid membranes. 

Three major classes of reaction mechanisms can be identified (1) Radical processes for cobalt and manganese carbonyl complexes that give the expected products with little stereoselectivity [11, 12, 49, 50]. Although some claims have appeared of the intervention of radicals in HDS-related hydrogenation, most of the evidence points to other types of surface mechanisms which can be better related to coordinative mechanisms therefore no further mention will be made of radical reactions in this Chapter. (2) Reactions involving Ziegler-type catalysts (a transition metal complex mixed with an alkyl aluminum co-catalyst) these poorly defined systems have proved to be difficult to study in detail [20, 22-25], and they appear rather unrelated to HDS-active... 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

Different classes of catalysts have been claimed for the oxidation of ethane to acetic acid [3], but the catalyst that gives the best performance is made of a mixed oxide of Mo/V/Nb (plus other components in minor amounts). This compound was first described in a paper by Thorsteinson et cd. 3a] - a paper that is considered nowadays a milestone in the field of the selective oxidation of alkanes, in view of the number of active phases that have been developed starting from catalysts described therein. Several patents were also issued by Union Carbide [3a-f], now Dow Chemical, regarding this system and the ETHOXENE process. The activity in ethane oxidation was attributed to the development of a crystalline phase characterized by a broad X-ray diffraction reflection at d = 4.0 A. The best composition was claimed to be Moo.73Vo.i8Nbo.o90 c, which reached 10% conversion of ethane at 286 °C with almost total selectivity to ethylene the selectivity decreased with increasing temperature, due to the formation of carbon oxides. The main peculiarity of this catalyst is its capability to activate the paraffin at low temperatures (<250 °C). 

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