Control of microwave-generated hot spots. Part V. Mechanisms of hot-spot generation and aggregation of catalyst in a microwave-assisted reaction in toluene catalyzed by Pd-loaded AC particulates

Abstract

Hot spots are generated when carrying out the heterogeneous Suzuki–Miyaura cross coupling reaction for the synthesis of 4-methylbiphenyl in toluene solvent in the presence of Pd/AC catalyst (AC: activated carbon; see for example parts I–IV11For part I, see Ref. [8]; for part II, see Ref. [9]; for part III, see Ref. [2]; and for part IV, see Ref. [10].). Controlling these hot spots could render the microwave-assisted catalyzed reaction more effective. Accordingly, the present article examines the mechanism by which the hot spots are generated through particle aggregation observed by means of a high-speed camera; the influence of particle size was also examined. Moreover, the formation of hot spots within the spatial gap between two AC particles was simulated by an electromagnetic field analysis and subsequently evidenced experimentally. The heterogeneous Suzuki–Miyaura coupling reaction for the synthesis of 4-methylbiphenyl in toluene solvent in the presence of activated carbon (AC; no Pd) under microwave irradiation has been re-visited to ascertain what the effect of the reagents might be as to whether or not hot spots are formed. The presence of the reagents used in the synthesis of 4-methylbiphenyl did cause a firm connectivity between the activated carbon particles, which changed with the directions of the electric field and the magnetic field. The relationship between the generation of by-products and the formation of hot spots has also been considered in the synthesis of 4-methylbiphenyl in toluene solvent catalyzed by Pd/AC.