Near infrared study on the adsorption states of NH3 and NH4+ on hydrated ZSM-5 zeolites

摘要

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 (Bronsted 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.