Control of microwave-generated hot spots. Part IV. Control of hot spots on a heterogeneous microwave-absorber catalyst surface by a hybrid internal/external heating method

A problem with microwave-absorbing heterogeneous catalysts (MAHCs) in non-polar solvents is resolved with a novel approach that also uses an external heating bath in combination with microwave heating. In non-polar solvents, the microwave radiation dielectrically and selectively heats only the catalyst resulting in the frequent occurrence of hot spots under these conditions. However, such hot spots can be controlled through a combined (hybrid) internal/external heating methodology (MAHS). Moreover, high temperatures can be maintained with significant energy saving. The potential benefit of MAHS has been examined by carrying out the synthesis of 4-methylbiphenyl using the Suzuki–Miyaura coupling reaction in toluene solvent in the presence of palladium catalytic particles supported on activated carbon particulates (Pd/AC). The hybrid internal/external heating method saved 65% of microwave energy and increased the chemical yield of 4-methylbiphenyl nearly twofold in comparison with a conventional microwave heating method.