Influence of Hydronium Ions in Zeolites on Sorption

In the presence of sufficient concentrations of water, stable, hydrated hydronium ions are formed in the pores and at the surface of solid acids such as zeolites. For a medium‐pore zeolite, such as zeolite MFI, hydrated hydronium ions consist of eight water molecules and have an effective volume of 0.24 nm³. In their presence, larger organic molecules can only adsorb in the portions of the pore that are not occupied by hydronium ions. As a consequence, the available pore volume decreases proportionally to the concentration of the hydronium ions. The higher charge density (the increasing ionic strength) that accompanies an increasing concentration of hydronium ions leads to an increase in the activity coefficients of the adsorbed substrates, thus, weakening the interactions between the organic part of the molecules and the zeolite and favoring the interactions with polar groups. The quantitative understanding of these interactions makes it possible to link a collective property such as hydrophilicity and hydrophobicity of zeolites to specific interactions on molecular level.