Investigation of NH3 and NH4+ adsorbed on ZSM-5 zeolites by near and middle infrared spectroscopy

The adsorption states of NH3 molecules and NH4+ ions on various sites, such as hydroxyl, H+ and Na+, of ZSM-5 zeolites with different SiO2/Al2O3 ratios were investigated by near and middle infrared spectroscopy. For Na+-type and siliceous ZSM-5 zeolites, only hydrogen-bonded NH3 clusters were observed at 5021 and 4519 cm−1, due to the lack of Bronsted acid sites. The NH3 molecules interacting with the hydroxyl groups easily desorbed at room temperature (r.t.), while the molecules interacting with the Na+ sites desorbed at 473 K. For the H+-type ZSM-5, when NH3 adsorption amount was small, the NH4+ formed on the H+ sites as well as the NH3 adsorbed on the H+ (or possibly Al3+) sites were independently observed at 4846 and 5044 cm−1, respectively. As the amount of NH3 adsorption increased, the NH4+ stabilized by the 2nd layer of NH3 molecules as well as the hydrogen-bonded NH3 clusters were observed at 4782 and 5017 cm−1, respectively. The hydrogen-bonded NH3 molecules desorbed at lower than 423 K but the NH4+ ions decomposed at 573–623 K to form Bronsted acid sites.