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

Abstract

In order to understand the acidity properties of hydrated ZSM-5 zeolites having different SiO 2 /Al 2 O 3 ratios, the adsorption states of the NH 3 and NH 4 + 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 H 2 O molecules directly interacting with the H + sites (Brønsted acid sites) was observed at 5266 cm −1 . When the NH 3 molecules were admitted on the hydrated H/ZSM-5, absorption bands due to isolated NH 3 and isolated NH 4 + were simultaneously observed at 5044 and 4846 cm −1 , respectively, although most of the H + sites were occupied by H 2 O molecules. As the NH 3 adsorption amount increased, almost isolated H 2 O molecules hardly changed while the NH 3 and NH 4 + 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 H 2 O molecules interacting with the silanol groups was mainly observed at 5285 cm −1 . However, since the H 2 O molecules readily desorbed at r.t. due to the hydrophobic character of the siliceous zeolite surface, the NH 3 adsorption behavior was almost the same as the dehydrated sample. In the case of Na + -type ZSM-5, absorption band due to H 2 O molecules directly interacting with the Na + sites was observed at 5291 cm −1 . When NH 3 molecules were adsorbed on the hydrated Na/ZSM-5, absorption band due to the hydrogen-bonded H 2 O shifted to 5260 cm −1 and absorption band due to the NH 3 molecules in the NH 3 /H 2 O mixed clusters was simultaneously observed at 5014 cm −1 . When the NH 3 /H 2 O mixed clusters desorbed from Na/ZSM-5, an absorption band due to NH 3 species interacting with the Al 3+ 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 NH 3 clusters.