Proton Conductivity of Imidazole Entrapped in H-forms of MFI Zeolites

Abstract Imidazole (Im) introduced into the channels of H-MFI zeolites (with diverse framework Si/Al) shows remarkably enhanced proton conductivity in a broad temperature range (303-393K), which increased with Im loading. Some conductivity raise is already noticed after its introduction into slilicalite-1 (i.e. SiO2) due to escalated Im dispersion and subsequently enhanced mobility inside the channels. The increase, however, becomes much more spectacular for the Im composites based on H-forms of aluminosilicate MFI zeolite matrices. It results from additional beneficial interaction of the guest Im with the zeolite acid sites, particularly OH groups, acting as additional proton carriers. Nevertheless, there is no straight correlation between the number of acid sites and the proton conductivity. It seems that the strong Lewis acid sites, contrary to Broensted centers, hinder the conduction, because of firm bonding the Im molecules and reducing their motion. Thus, both kind of sites compete with each other. The highest conduction noticed for the composites based on the H-zeolite with Si/Al=50 results not so much from pronounced contribution of Broensted centers, but rather from the small number of Lewis sites which are relatively weak. Both Grotthuss and vehicle conductivity modes could be considered.