Influence of zeolite acidity on proton conductivity of FAU embedded imidazole

Abstract

Abstract Imidazole entrapped in H-forms of FAU zeolites with various Si/Al ratios (1, 4, 40) exhibit high and stable proton conductivity (up to ∼10−3 S/cm) in a broad range of temperatures (293–393 K). The conductivity increases with temperature and imidazole loading. The number and strength of zeolite acid sites (resulting from various Si/Al) influence markedly the conductivity of composites. The samples based on zeolites with low Si/Al (1) show markedly higher conduction than these supported on the matrix with medium Si/Al (4), regardless of very similar number of the acid OH groups. The strong acid OH groups in zeolite with Si/Al = 4 result in protonation and immobilization of adsorbed imidazole, which reduce their mobility and subsequently the conduction. The low siliceous with weaker acid sites protonate imidazole and form a chain of H-bonds effective in proton propagation. Almost neutral high silica matrix provides a high mobility of dispersed imidazole, due to weak mutual interaction. The overloaded samples indicate a phase transition effect at elevated temperature due to melting of extra-matrix imidazole, which is reflected in dramatic increase in conductivity.