Kinetic processes during sorption and diffusion of aromatic molecules on medium pore zeolites studied by time resolved IR spectroscopy

Abstract

The transport of (alkyl-)aromatic molecules from the gas phase to the catalytically active sites inside the channels of ZSM-5 was studied. The sorption of the molecules on the external surface and inside the pores was followed independently by time resolved IR spectroscopy and two mechanisms for the transport, i.e., the adsorption on the external surface followed by a consecutive diffusion to sites inside the pores and the sorption of the molecules on both sites directly from the gas phase, were discussed. On the basis of the results presented the second pathway appears to be more likely, as the very low sticking probability indicates that the number of collisions of the molecules at sites in the pore entrance region is sufficiently high for a direct sorption. Nevertheless, for both pathways discussed the reorientation of the molecules from their gas phase geometry to the sorbed state is proposed to control the rate of the sorption process.