The Energetic and Entropic Contributions Controlling the Orientation of Alkyl Substituted Aromatic Molecules in the Pores of MFI Zeolites

The energetic and entropic contributions controlling the orientation of benzene, toluene and pxylene on acidic (H/ZSM-5) and non acidic (Silicalite-1) MFI type materials were studied using calorimetry to determine the integral uptake and IR spectroscopy for monitoring the individual coverage of the SiOH and SiOHA1 groups. The Langmuir model, used to describe the sorption isotherms, indicates the presence of sterically constrained sorption structures, which primarily differ in the decrease of entropy. Energetically the sorption is controlled by the interaction between the molecules and the pore walls, while the localized interaction with the bridging hydroxy groups is only a minor energetic contribution. However, if the molecules are located close to SiOHA1 groups perturbed hydroxy groups are formed, their frequency reflects the local constraint of the molecules at the sorption sites.