Sorption of n-pentane, 2-methylbutane and cyclopentane in microporous AlPO4-5

Abstract

Sorption isotherms for n-pentane, 2-methylbutane and cyclopentane in the aluminophosphate molecular sieve AlPO₄-5 are reported at 303, 313, 323 and 333 K. The sorption data were fitted into Langmuir, Sips and Dubinin–Radushkevich isotherms. The heats of sorption at zero coverage determined from the data in the Henry region are in the order 2-methylbutane>cyclopentane>n-pentane. Free energy and entropy changes and heats of sorption for these sorbates at various sorption coverages were also determined from the sorption data. The heat of sorption for n-pentane varies only marginally with sorption coverage. However, the heats of sorption for 2-methylbutane and cyclopentane increase significantly with the coverage indicating an increase in sorbate–sorbate interactions. The dependence of thermodynamic properties like the free energy of sorption (ΔG) and the entropy change (ΔS) for 2-methylbutane and cyclopentane on coverage show a sharp decrease and increase at sorption coverages 0.1–0.2 mmol g⁻¹ suggesting the rearrangement of molecular packing in the sorbed phase. These observations have been explained in terms of molecular interactions between the AlPO₄-5 channels and sorbate molecules.