Investigation of inverse shape selectivity in alkane adsorption on SAPO-5 zeolite using the tracer chromatography technique

Adsorption of linear and branched alkanes (C4–C8 range) on a microporous silicoaluminophosphate material (SAPO-5) was studied using the chromatographic technique at 353–573 K. Under these experimental conditions of low micropore occupancy specific branched molecules adsorb preferentially over the corresponding linear isomers. Such inverse shape selectivity was encountered with isobutane, isopentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, and 2,2-dimethylhexane. Other skeletal isomers exhibit the conventional shape selectivity; i.e., their adsorption is less favorable compared to the linear alkane with the same carbon number. The occurrence of inverse shape selectivity versus conventional shape selectivity can be explained in terms of compensation between the adsorption enthalpy and entropy. The branched species that exhibit inverse shape selectivity have a higher adsorption enthalpy compared to the linear chains, counterbalancing the negative contribution of adsorption entropy to the adsorption equilibrium.