Alterations in vibrational spectra of adsorbed water in MIL-101(Cr) and functionalized MIL-101(Cr) using molecular simulations

Abstract

The vibrational dynamics of adsorbed water, as reflected in its vibrational density of states (VDOS) spectra, differ significantly from those of bulk water. This study employs molecular simulations to explore these modifications in water confined within MIL-101(Cr) and its sulfonic acid-functionalized derivative, MIL-101-SO₃H(Cr). Various water models are tested, with systematic adjustments made to identify the one that best replicates the experimental infrared (IR) spectrum. The optimized model is then used to analyze the vibrational properties of adsorbed water. Normal mode analysis (NMA) decomposes the vibrational signal into the underlying normal modes of water molecules, allowing for a detailed examination of water adsorbed at specific framework sites. At low pressures, water molecules preferentially bind to high-affinity sites, such as the unsaturated Cr centers in MIL-101(Cr) and the sulfonic acid hydroxyl (–OH) groups in MIL-101-SO₃H(Cr), exhibiting distinct asymmetric stretching modes compared to bulk water. However, as water uptake increases at higher pressures, the VDOS spectra of adsorbed and bulk water begin to converge, signaling the onset of capillary condensation.