| Abstract
| - The adsorption equilibria of a series of n-alkanes and 2-methyl- and 3-methyl-branched alkanes in the C5−C9range on ZSM-22 zeolite samples were determined using the tracer chromatographic technique at temperaturesbetween 343 and 623 K. Adsorption experiments were performed with a calcined ZSM-22 sample referredto as “open” ZSM-22 and an as-synthesized ZSM-22 zeolite sample in which the 1,6-diaminohexane templatewas left inside the pores, denoted as “closed” ZSM-22. In the “open” ZSM-22, the zeolite micropores areavailable for adsorption. In the “closed” ZSM-22, adsorption can occur in the pore mouths and on the externalsurface. The closing of the channels of ZSM-22 results in a significant decrease in the Henry constant andadsorption enthalpy and entropy of linear alkane molecules. Contrarily, the adsorption enthalpy and entropyof branched alkanes on “open” and “closed” ZSM-22 are nearly identical, indicating that the preferred adsorptionsites 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 entropywas observed. The entropy losses upon adsorption inside the narrow ZSM-22 channels are much larger thanin channel openings. These data support the pore mouth catalysis model for alkane hydro-isomerization onZSM-22 type catalysts.
|
