Effect of phosphorus-modification of titania supports on the iridium-catalyzed synthesis of benzimidazoles

Abstract

Modification of titania supports for iridium catalysts by phosphorus species greatly enhanced the activity for the synthesis of benzimidazoles via hydrogen transfer. Two series of phosphorus-modified titanias were used: The iridium catalysts supported on phosphorus-doped rutile titania prepared by the hydrothermal method showed nearly 5 times higher activity for the reaction of 2-nitroaniline (1a) and benzyl alcohol (2a) to 2-phenylbenzimidazole (3aa) than the catalysts supported on unmodified rutile. XPS depth profile study substantiated that the dopant was present mainly on the surface of the rutile support. Therefore, a facile wet impregnation method was applied to modify the surface of anatase titania with phosphoric acid. The use of thus-modified titania enhanced the activity of the iridium catalysts by more than 2.7 times. The FTIR and XPS studies revealed the presence of bidentate phosphorus species on the surface of titania, and the H2-TPR study indicated that phosphorus-modification promoted the formation of iridium species reduced at higher temperature, which would be suitable for the present catalysis.