Phase Transition and Structural Heterogeneity; Benzene Adsorption on Silicalite

Adsorption equilibria have been gravimetrically measured at 10, 20, 30, 50, and 70°C for benzene on pure crystalline silicalite. Isotherms change from type I to type IV with decreasing temperature, indicating heterogeneous behavior. The transition temperature is determined to be 38 ± 1°C by isobaric experiments. Silicalite is energetically homogeneous since there are no cations present. The observed structural heterogeneity is due to the comparable sizes of benzene molecules and the silicalite pore system. Silicalite is modeled as two types of homogeneous pores where a van der Waals model describes local equilibrium. Model parameters are calculated via nonlinear regressions with prescribed pore distribution determined from crystallographic considerations. Data and model predictions are in good agreement with highly significant parameters.