Development of reusable palladium catalysts supported on hydrogen titanate nanotubes for the Heck reaction

Abstract

Palladium catalysts supported on hydrogen titanate nanotubes were prepared by adsorption of palladium(II) acetate from dichloromethane solutions. At low Pd(OAc)2 loadings (up to 5wt.%), the catalysts contained only highly dispersed chemisorbed Pd0 and Pd2+O species (XPS). Characteristic signals of Pd(OAc)2 were not detected in these catalysts by FT-IR, FT-Raman, of XPS, indicating decomposition of the precursor salt due to the strong metal–support interaction. An increase in the palladium loading resulted in an increase in the proportion of Pd0 species and the appearance of some supported Pd(AcO)2 (XPS). In the sample with the highest palladium loading (8.84wt.% of Pd(OAc)2), formation of small (∼1.2–2.4nm) palladium-containing clusters was detected on the support surface by HRTEM. Catalytic activity was tested in the Heck reaction between 4-bromobenzaldehyde or 4-bromostyrene and styrene. Only the E-isomers of the corresponding cross-coupled products were obtained. The activity and TON numbers of the supported Pd catalysts were higher than those of the palladium(II) acetate under homogeneous catalysis conditions. Strong palladium–support interaction resulted in the generation of catalytically active Pd(0) species without additional reduction pretreatment of the catalyst. The possibility of reuse of the same catalysts in several catalytic cycles was tested. It was found that both the Pd loading in the catalysts and the amount of water in the support have a strong influence on the catalyst’s deactivation. A catalyst with stable activity, able to be recycled at least five times, was prepared with Pd loading of 5wt.% of Pd(AcO)2 using titanate nanotubes calcined at 350°C for 2h prior to the preparation of the catalyst.