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

Abstract

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 Brönsted 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 Brönsted acid sites.