Infrared spectroscopic studies of amination of ZSM-5 zeolites

Abstract

The amination of NaZSM-5 (only 3743 cm^–1 Si—OH groups) and of NaHZSM-5 zeolites (both 3743 cm^–1 Si—OH and ‘bridged’ Si—OH⋯Al 3609 cm^–1 groups) has been studied at temperatures between 673 and 1073 K. The reaction with very dry ammonia results in the formation of Si—NH₂ and Si—NH₂⋯Al groups, in which case the amination of ‘bridged’ Si—OH⋯Al groups occurs at a lower temperature than that of Si—OH groups. At temperatures higher than 773 K NH₂ groups undergo further reaction (imination) resulting in the formation of Si—NH—Si and Si—NH—Al groups. A mechanism of amination and imination has been proposed. NH₂ groups were generated by substitution of OH groups and in a dissociative reaction of ammonia with siloxane bridges or Si—O—Al bridges. NH groups (silazane bridges) were formed by elimination of NH₃ from vicinal NH₂ groups.