📝 SummarXiv

Abstract

Competitive adsorption is a major challenge in understanding catalytic activity, selectivity and reaction mechanisms in confined environments such as zeolites. This study investigated competitive adsorption in MFI-type zeolites (ZSM-5) using solid-state NMR, focusing on the interplay between toluene and water. Quantitative 1H NMR spectroscopy identified three distinct populations of adsorbed toluene evolving with increasing toluene loading. The adsorption behavior was consistent across a series of samples with Si/Al ratio ranging from 11.5 to 140. Combining 1D and 2D NMR techniques with sample engineering (e.g. pore-blocking) enabled the assignment of the populations to toluene within the zeolite channels, at the pore mouths, and adsorbed on the external crystal surface. Crucially, introducing water to toluene-loaded zeolites caused a partial displacement of toluene from the internal channels, but significant removal from the pore mouths. This dis-placement occurred even in the highly hydrophobic zeolite (Si/Al = 140), where water still preferentially adsorbed to Br{\o}nsted acid sites and silanol species. The results highlight the critical impact that competitive adsorption from solvents, products, or impurities can have on the efficiency and selectivity of zeolite-mediated transformations.