Kumar Dipanshu, Sauer Joachim, Airi Alessia, Bordiga Silvia, Galimberti Daria Ruth
Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.
Institut für Chemie, Humboldt-Universität, Unter den Linden 6, 10117 Berlin, Germany.
Phys Chem Chem Phys. 2024 Dec 18;27(1):550-563. doi: 10.1039/d4cp03861d.
Understanding how alcohol molecules interact with the Brønsted acid sites (BAS) of zeolites is a prerequisite to the design of zeolite catalysts and catalytic processes. Here, we report IR spectra for the adsorption of ethanol on a highly crystalline sample of H-ZSM-5 zeolites exposed to ethanol gas at increasing pressure. We use density functional theory in combination with a FERMI resonance model to assign the measured spectra to a single adsorbed ethanol molecule per BAS. Specifically, we assign the bands at 2450 cm and 1670 cm to a FERMI resonance between the fundamental (Z)O-H stretching band of a single-ethanol-loaded BAS and the first overtone of the (Z)O-H out-of-plane bending. We conclude that adsorbed dimers do not contribute in a noticeable way up to a concentration of almost one ethanol molecule per BAS site. We further show that hybrid functionals (B3LYP) are required to get a close match between the predicted and experimental spectra, whereas commonly used generalized gradient type functionals such as PBE incorrectly describe the potential energy surface. They overestimate the redshift of the OH stretching band on hydrogen bond formation which results in an erroneous assignment of the IR bands.
