Energetic and entropic contributions controlling the sorption of benzene in zeolites

The energetic and entropic contributions controlling the sorption of benzene on acidic (H/ZSM-5) and non-acidic (Silicalite-1) MFI type materials were studied using gravimetry, calorimetry and in situ IR spectroscopy to follow the qualitative and quantitative interactions of benzene with the pores and the functional SiOH and SiOHAl groups. The model derived to describe the sorption isotherms indicates the presence of sterically constrained sorption structures for benzene in MFI type materials. The interaction of benzene with the pore walls controls the sorption energetically, while the localized interaction with the bridging hydroxy groups contributes only to a minor degree. If benzene is located close to SiOHAl groups perturbed hydroxy groups are formed. Their wavenumber reflects the local sorption geometry of benzene as well as the acid strength of the hydroxy group and the base strength of benzene. Two perturbed hydroxy groups were observed for benzene adsorbed, which are assigned to two orientations of the molecules inside the pores, i.e., with the ring parallel to the pore wall and with the ring being oriented towards the bridging hydroxy groups. At higher coverage benzene adsorbs at SiOHAl groups additionally in an unconstrained environment, most probably at the pore openings.