Alkane sorption in molecular sieves: The contribution of ordering, intermolecular interactions, and sorption on Brønsted acid sites

Distinct molecular ordering of sorbed alkanes is observed in MFI zeolites when the chain length of the alkane is similar to the length of the zig-zag channels (i.e., with n-hexane and n-heptane). In contrast, sorbate-sorbate interactions lead to an increase of the heat of adsorption with increasing loading in large pore molecular sieves such as faujasites. The maximum heat released through these interactions is 80% of the heat of condensation of the substance adsorbed. At most three molecules are involved in such intermolecular interactions. A modified Langmuir model is presented that accounts for these sorbate-sorbate interactions. Subtle structural differences are concluded to be of minor importance as, e.g., the different pore structures of H-FAU and H-EMT hardly affected the alkane sorption. The contribution of the direct interaction between the alkane and the acid sites to the heat of adsorption is 10kJ/mol for MFI and 6kJ/mol for FAU. As for acidic zeolites, a compensation effect between entropy and enthalpy of sorption exists for the purely siliceous samples.