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The Ullmann coupling of aryl halides is a simple and efficient way to obtain symmetric biaryls. In this study, we have developed a novel blended catalyst system, consisting of a heterogeneous TiO2 photocatalyst and a supported Pd metal catalyst, to carry out the photocatalytic Ullmann coupling of various aryl halides efficiently under mild conditions. A thorough study revealed that the reaction proceeds via a radical addition–elimination mechanism, involving TiO2 photocatalysis and Pd metal catalysis. The two parts, i.e., TiO2 photocatalyst particles and supported Pd nanoparticles, can cooperatively work with comparable yield and higher selectivity than the conventional Pd-loaded TiO2 photocatalyst. The separation of the Pd nanoparticles from the TiO2 surface has some merits; it decreases the undesirable side reactions to improve biaryl selectivity and allows the modification of the metal catalyst, such as the fabrication of bimetallic catalysts, without affecting the properties of the TiO2 photocatalyst. Hence, the design of these blended catalysts including the improvement of both the photocatalyst and the metal catalyst parts independently is an attractive and promising approach to improve the entire catalytic performance.

catalysis science and technology

Photocatalytic Ullmann coupling of aryl halides by a novel blended catalyst consisting of a TiO2 photocatalyst and an Al2O3 supported Pd–Au bimetallic catalyst