Evidence for a strong polarization of n-hexane in zeolite H-ZSM-5 by FT-IR and solid-state NMR spectroscopy

Abstract

Abstract The adsorption and interaction of n-hexane in acidic zeolites were investigated via the adsorbate-induced shifts of the FT-IR bands (Δ ν ˜ ) and 1H MAS NMR signals (Δδ) of acidic bridging OH groups (Si(OH)Al) in H-mordenite, H-Beta, 10H,Na-Y, 86H,Na-Y, H-ZSM-5, and US-Y. The plot of the experimentally determined Δ ν ˜ values as a function of Δδ shows a linear correlation. In a subsequent step, the influences of the average electronegativity Sav of the individual zeolites under study and the deprotonation energy ΔpA of their acidic hydroxyl protons on Δ ν ˜ and Δδ were investigated. The individual Sav values were calculated via the geometric average of all element electronegativities of the zeolite framework, while the ΔpA values were obtained via an evaluation of the CO-induced shifts of the FT-IR bands of Si(OH)Al groups compared with that of SiOH groups and using an empirical relationship. In the plots of the Δ ν ˜ /Sav and Δ ν ˜ /ΔpA ratios as a function of Δδ/Sav and Δδ/ΔpA, respectively, the data sets of all zeolites under study are closely arranged, excluding that of H-ZSM-5. This finding indicates that n-hexane adsorbed on zeolite ZSM-5 is not exclusively involved in the interaction with acidic Si(OH)Al groups, but is also influenced by the zeolite framework. The 13C solid-state NMR shifts of n-hexane adsorbed on the zeolites hint at a polarization of this alkane especially in the 10-ring pores of zeolite H-ZSM-5. This polarization of n-hexane in H-ZSM-5 is important for the reactivity of this alkane in heterogeneously catalyzed reactions.