High-temperature adsorption of n-alkanes on ZSM-5 zeolites:: influence of the Si/Al ratio and the synthesis method on the low-coverage adsorption properties

The influence of the composition and synthesis method on the low-coverage adsorption properties Of C-5-C-9 n-alkanes on ZSM-5 zeolites was studied using the pulse chromatographic technique at temperatures between 200 and 400 degreesC. Experiments were performed with materials having Si/Al ratios between 12 and 400, synthesized with and without an organic template. For all ZSM-5 samples, the Henry adsorption constants increase exponentially with the carbon number, while zero-coverage adsorption enthalpies increase in a linear way. With decreasing Al content, the Henry constants and adsorption enthalpies decrease. An increase in adsorption enthalpy of 10.1 kJ/mol per added -CH2-group is observed for an Si/Al ratio of 400, while an increase of 12.1 kJ/mol is found for an Si/Al ratio of 15. The contribution to the adsorption entropy per carbon atom depends on the ZSM-5 composition and varies between 11.2 and 14.4 J/(mol K). A significant effect of the synthesis method on the Henry constants, adsorption enthalpies and entropies is observed. All ZSM-5 samples synthesized using organic templates show the same unique relationship between adsorption enthalpy and entropy, different from that of zeolites synthesized without organic template. (C) 2003 Elsevier Science Inc. All rights reserved.