Near infrared study on the adsorption states of NH 3 and NH 4 + on hydrated ZSM-5 zeolites

In order to understand the acidity properties of hydrated ZSM-5 zeolites having different SiO₂/Al₂O₃ ratios, the adsorption states of the NH₃ and NH₄⁺ species on hydrated ZSM-5 zeolites were concurrently investigated by near infrared spectroscopy. In the case of hydrated H⁺-type ZSM-5, absorption band due to the H₂O molecules directly interacting with the H⁺ sites (Brønsted acid sites) was observed at 5266 cm⁻¹. When the NH₃ molecules were admitted on the hydrated H/ZSM-5, absorption bands due to isolated NH₃ and isolated NH₄⁺ were simultaneously observed at 5044 and 4846 cm⁻¹, respectively, although most of the H⁺ sites were occupied by H₂O molecules. As the NH₃ adsorption amount increased, almost isolated H₂O molecules hardly changed while the NH₃ and NH₄⁺ stabilized by the hydrogen bonds shifted to 5000 and 4771 cm⁻¹, respectively. As for the siliceous-type ZSM-5, near infrared absorption due to H₂O molecules interacting with the silanol groups was mainly observed at 5285 cm⁻¹. However, since the H₂O molecules readily desorbed at r.t. due to the hydrophobic character of the siliceous zeolite surface, the NH₃ adsorption behavior was almost the same as the dehydrated sample. In the case of Na⁺-type ZSM-5, absorption band due to H₂O molecules directly interacting with the Na⁺ sites was observed at 5291 cm⁻¹. When NH₃ molecules were adsorbed on the hydrated Na/ZSM-5, absorption band due to the hydrogen-bonded H₂O shifted to 5260 cm⁻¹ and absorption band due to the NH₃ molecules in the NH₃/H₂O mixed clusters was simultaneously observed at 5014 cm⁻¹. When the NH₃/H₂O mixed clusters desorbed from Na/ZSM-5, an absorption band due to NH₃ species interacting with the Al³⁺ sites (Lewis acid sites) was observed at 4957 cm⁻¹ since the neutral Na/ZSM-5 surface might be dealuminated by the basicity of the wet NH₃ clusters.