Thermodynamic Analysis of Type VI Adsorption Isotherms in MFI Zeolites

The adsorption of aromatics and alkanes in MFI zeolites is usually characterized by a characteristic S-shaped isotherm (formally type VI) caused by the presence of different potential sorption sites (i.e., channel intersections and interiors) and by an intricate panel of sorbate-sorbate interactions. Herein we show that this behavior can be conveniently modeled through a thermodynamic formulation alternative to the classical dual-site Langmuir isotherm linking the integral free energy of adsorption relative to saturation, psi/RT, expressed as a Kiselev integral, with the variable Z = 1/-ln(Pi), Pi being the relative pressure. The model is defined by a set of parameters providing direct information about the evolution of the energy heterogeneity of the solid upon adsorption, including the phase transition zone. Unlike the Dubinin-Astakhov and Dubinin-Radushkevich isotherms, lacking thermodynamic consistency at low pressures, the present formulation converges to Henry's law at low pressures. Moreover, the present formulation allows the generation of generalized isotherms at different temperatures in the low-pressure region without the need of introducing an affinity coefficient.