Modelling aromatics in siliceous zeolites: a new forcefield from thermochemical studies

Abstract

A new forcefield for the modelling of interactions between aromatics and siliceous zeolites has been derived by fitting to calorimetric data on the sorption of benzene in siliceous faujasite. The calometric measurements suggest a heat of sorption of 55 kJ mol−1 at densities lower than 22 molecules per unit cell. Monte Carlo docking calculations have been carried out, using the new forcefield, to predict the most favourable binding sites for sorption of benzene in this zeolite. They show a relatively flat potential-energy surface as compared to Na-Y with a wide variety of different mainly low-symmetry binding sites. The sites are to be classified as being either near to 4-rings (58.4 kJ mol−1), 6-rings (50.0 kJ mol−1) or in the 12-ring window (43.7 kJ mol−1). These results are consistent with neutron diffraction measurements on the same system. Molecular dynamics calculations with the new forcefield suggest that two processes contribute to the motion of benzene molecules in the pore system. At low temperatures, the benzene molecules tend to be confined to a single supercage by sliding around the walls. However, at higher temperatures, the molecules have sufficient kinetic energy to move through the 12 ring window into an adjacent supercage. Minimum-energy pathways have also been calculated, based on the docking binding sites, which correlate well with this rationalisation of the transport mechanism.