Pd catalysts supported on hydrogen titanate nanotubes for Suzuki-Miyaura cross-coupling reactions

Abstract

Palladium catalysts supported on hydrogen titanate nanotubes (HTN) were prepared by impregnation of palladium(II) acetate, characterized by N2 physisorption, powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and tested in the Suzuki-Miyaura cross-coupling reactions between 4-bromobenzaldehyde and phenylboronic acid (reaction 1) or 4-hydroxyphenylboronic acid (reaction 2). The aim of this work was to determine the activity of these catalysts in the Suzuki-Miyaura reactions and the possibility of their reusing in several catalytic cycles. The results showed that 3.6wt.% of Pd was the optimum metal loading in the Pd(x)/HTN catalysts. The Pd(3.6)/HTN catalyst resulted in 97 and 88% yields of biphenyl products in the cross-coupling reactions 1 and 2, respectively, and its activity decreased by about 10% after five catalytic cycles. This deactivation was attributed to different reasons: an amount of the catalyst was lost during its separation by filtration after each catalytic cycle, Pd leaching, the presence of organic and inorganic species on the spent catalyst’s surface acting as poisons, as well as the agglomeration of palladium species.