Tracer Chromatographic Study of Pore and Pore Mouth Adsorption of Linear and Monobranched Alkanes on ZSM-22 Zeolite

The adsorption equilibria of a series of n-alkanes and 2-methyl- and 3-methyl-branched alkanes in the C5−C9 range on ZSM-22 zeolite samples were determined using the tracer chromatographic technique at temperatures between 343 and 623 K. Adsorption experiments were performed with a calcined ZSM-22 sample referred to as “open” ZSM-22 and an as-synthesized ZSM-22 zeolite sample in which the 1,6-diaminohexane template was left inside the pores, denoted as “closed” ZSM-22. In the “open” ZSM-22, the zeolite micropores are available for adsorption. In the “closed” ZSM-22, adsorption can occur in the pore mouths and on the external surface. The closing of the channels of ZSM-22 results in a significant decrease in the Henry constant and adsorption enthalpy and entropy of linear alkane molecules. Contrarily, the adsorption enthalpy and entropy of branched alkanes on “open” and “closed” ZSM-22 are nearly identical, indicating that the preferred adsorption sites for monobranched alkanes are not located inside the pores, but specifically at the entrance of the pore. For the pore mouth adsorption mode, a characteristic compensation between adsorption enthalpy and entropy was observed. The entropy losses upon adsorption inside the narrow ZSM-22 channels are much larger than in channel openings. These data support the pore mouth catalysis model for alkane hydro-isomerization on ZSM-22 type catalysts.