Mass transfer processes during sorption and diffusion of aromatic molecules on medium pore zeolites

The sorption kinetics of benzene, toluene o- and p-xylene on the outer surface of crystallites and inside the pores of H/ZSM-5 was studied by following pressure modulations with fast time resolved IR spectroscopy. Two independent transport processes from the gas phase to the SiOH and SiOHAI groups were identified, which share a common pre-adsorbed state on the external surface of the H/ZSM-5. The equilibration of the molecules in the pre-adsorbed state is fast compared to the sorption rates on hydroxyl groups in the exterior (SiOH) and interior (SiOHAI). For molecules being able to enter into the pores of H/ZSM-5 (benzene, toluene, p-xylene) the uptake rates on the SiOHAI groups are higher than those on the SiOH groups. If the access of the molecules into the pores is sterically hindered (o-xylene), the presence of a sole adsorption channel leads to a faster population of the accessible SiOH groups on the external surface.