Adsorption of benzene at the hydroxylated (111) external surface of faujasite

Using atomistic techniques we have investigated the adsorption of benzene at the external surface and sub-surface of purely siliceous faujasite. Our calculations reveal distinct optimal adsorption sites compared to those identified in the zeolite bulk crystalline environment. The magnitude of these external surface adsorption energies (30-43 kJ mol(-1)) is consistent with physisorption and therefore, at low concentrations and elevated temperatures, adsorbates will be bound transiently to the surface. Using a constrained optimisation procedure we have identified a minimum energy pathway for benzene diffusion from the external surface to the crystal interior, which shows an energy barrier of 18 kJ mol(-1) (compared to 31 kJ mol(-1) in the bulk) for the adsorption/desorption process. We therefore suggest that surface to sub-surface diffusion (and vice versa) is relatively facile and occurs on a shorter time-scale than bulk diffusion.