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Water Interactions in Zeolite Catalysts and Their Hydrophobically Modified Analogues

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journal contribution
posted on 04.12.2015 by Kuizhi Chen, Jarred Kelsey, Jeffery L. White, Lu Zhang, Daniel Resasco
Renewed interest in zeolite catalyst performance in the presence of variable amounts of water has prompted solid-state NMR experiments designed to identify the nature of water interaction with and within conventional and chemically modified H-ZSM-5 zeolites. Recent work has demonstrated that water can positively influence reaction rates in zeolite-catalyzed chemistries, and new interest in catalytic processing of molecules derived from biomass requires understanding the fate of water in and on zeolite catalysts, as a function of water loading. The contribution of acid site density to water adsorption within zeolites is assessed by comparing bulk uptake and molecular experiments at varying Si:Al ratios, and interpreting those results in the context of solid-state NMR results that reveal strongly adsorbed water molecules and water clusters. In situ magic-angle spinning (MAS) NMR experiments for water loadings ranging from ca. 4 to 500 water molecules per zeolite unit cell indicate the following: (1) the dominant interaction is from water adsorbed from the vapor phase at an interior acid site, and unique signals for both the water and acid site are resolved at low loadings; (2) the exchanged-averaged water/acid site chemical shift at higher loadings can be used to measure acid site titration by water; and (3) silane-treated hydrophobically modified H-ZSM-5 does not allow liquid-phase water to access interior acid sites. The in situ 1H MAS NMR method indicates that as-synthesized acidic zeolites can be rendered hydrophobic in the presence of liquid-phase water, with only a minimal reduction in the total number of acid sites.