Graphite as sensitizer and catalyst

When the reaction was conducted in the homogeneous phase at 200 °C (Table 9.4, entries 1-4), identical reaction rates and similar yields and selectivity were obtained for both heating modes. Kinetic data for the first-order equation were similar Ea (MW) = 159 3 kj mol-, Ea (A) = 160 + 3 kj mol i In A (MW) 35 + 1, 

In A (A) 34 + 1. In contrast, in the presence of graphite (57 graphite = 4 1, w/w), improved yield and selectivity were obtained under the action of MW irradiation compared with conventional heating (Table 9.4, entries 5-8 at the same bulk temperature). Chemat and Poux ascribed this phenomenon to localized superheating ( hot spots ) on the graphite surface (Section 9.4.2). 

This difference was more evident when graphite was added to the reaction mixture - similar yields (75-95% 60a) were obtained after only 5 min MW irradiation. Despite the small amount of graphite added (approx. 1 10, w/w, relative to the reagents), the reaction mixture rapidly reached a much higher temperature (250 to 300 °C) than in the absence of graphite. 

The use of graphite-supported methodology has been reported for three types of reaction - Friedel-Crafts acylation [15, 27, 66], the acylative cleavage of ethers [15], and the ketodecarboxylation of carboxylic diacids [67, 68], either with conventional heating (GS/d) or MW irradiation (GS-MW coupling). 

however, the authors describe the analysis of two commercial graphites of different purity which are used for these experiments. 

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X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

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