The reaction of acetone and ammonia on acid zeolites

Abstract

The reaction of acetone and ammonia adsorbed on HZSM-5 was studied using FT infrared spectroscopy (FTIR) coupled with mass spectroscopy (MS). The formation of an ion-pair surface complex of protonated dimethylketimine (DMKH⁺)/skeletal oxygen anion was observed in the IR spectra at room temperature. The complex is characterized by the stretching vibration ν(C2N) of the >C2NH₂⁺ group at 1707 cm^-1. The assignment was confirmed by a new band at 1683 cm^-1, corresponding to >C2NHD⁺ group formed after the H/D exchange of one proton bonded to nitrogen at temperatures above 120°C. The formation of DMKH⁺ was also found on other acidic zeolites, in particular H-β, HY, and HM. It is suggested that the activity of zeolites in the formation of DMKH⁺ increases with their increasing acidity. Gaseous desorption products during temperature-programmed desorption, probed by MS, revealed the evolution of acetonitrile, propene, allene, and isobutene. The experimental results are compared with quantum chemical calculations based on the optimized model system of DMKIH⁺/zeolite cluster using density functional theory.