Multivariate analysis coupled to infrared spectroscopy unravels the diversity of adsorption sites and strengths of a zeolite surface †

Abstract

Quantifying adsorption thermodynamics on heterogeneous catalysts or adsorbents remains challenging because macroscopic techniques yield averaged parameters, while vibrational spectra often contain strongly overlapping contributions from multiple adsorbed species. Here we introduce an infrared (IR)-chemometric framework that extracts site-specific adsorption thermodynamics directly from experimental IR isotherms and we benchmark it against independent microcalorimetry and density functional theory (DFT) calculations. Difference IR spectra recorded for isobutanol adsorption on H-ZSM-5 (MFI) were analysed by principal component analysis and multivariate curve resolution (MCR-ALS) under soft constraints (monotonic concentration profiles, spectral normalisation), and further refined using a hard-soft strategy in which concentration profiles are constrained by a an adsorption model. The analysis reveals three adsorption modes associated with bridging Brønsted OH groups, extra-framework Al-OH species, and silanols, providing representative pure-component spectra and site-resolved adsorption isotherms. The thermodynamic trends and site hierarchy obtained from IR-MCR-ALS are consistent with independent microcalorimetry measurements and density functional theory calculations, validating the approach. More broadly, IR-MCR-ALS offers a transferable route to a quantitative, site-resolved adsorption thermodynamics in complex porous materials.