Water Adsorption Dynamics within Zeolite Micropores Tracked by NIR Spectroscopy

The adsorption behaviors of H2O molecules on MS-13X and Mg2+/H+/Y zeolites were investigated using thermogravimetry-differential thermal analysis (TG-DTA) and near-infrared (NIR) spectroscopy. Total H2O uptake was evaluated to be 15.04 mmol/g for the MS-13X and 17.02 mmol/g for the Mg2+/H+/Y, corresponding to 2.5 and 2.1 multilayers, respectively. NIR spectroscopy revealed that well-dried MS-13X zeolite showed an absorption band at 5320 cm–1, attributed to isolated H2O molecules interacting with the Na+ sites. Upon exposure to atmospheric moisture, a broad absorption band near 5190 cm–1 emerged, indicating the formation of hydrogen-bonded H2O clusters. For the Mg2+/H+/Y, the absorption band attributed to the hydrogen-bonded H2O cluster was observed at 5254 cm–1. This absorption band decreased and shifted to 5284 cm–1 during desorption above 150 °C. Following complete desorption of H2O at 500 °C, the readsorption of H2O molecules was tracked during cooling. The absorption band at 5284 cm–1 reappeared at initial adsorption step, and upon exceeding the monolayer threshold of 7.1 mmol/g, the absorption band at 5254 cm–1 emerged, signaling the formation of hydrogen bonding between H2O molecules (multilayer adsorption).