Crucial role of alkali metal ions and Si/Al ratio in selective adsorption of 1-octene using faujasite zeolites

Abstract

Linear α-olefins (LAOs) are conventionally purified from paraffins via energy-intensive superfractionation. Adsorptive separation with zeolite-based adsorbents is a promising alternative to distillation for olefin/paraffin purification. However, very few zeolites with different Si/Al ratios and metal ion types have been tested to separate LAOs in the liquid phase. In this study, we investigated the ability of various alkali metal ion-exchanged faujasites with different Si/Al ratios to separate 1-octene/n-octane mixtures. We prepared low-silica X (LSX), X, and Y zeolites loaded with Li+, Na+, K+, and Rb+ via ion exchange in an aqueous solution. The 1-octene adsorption capacities and selectivities were analyzed via liquid-phase batch adsorption experiments. Among LSX, X, and Y exchanged with the Na+ and Li+, LSX which had the lowest Si/Al ratios exhibited the highest selectivity. The 1-octene selectivities for LSX were in the following order: Rb+ ≈ K+ < Na+ < Li+. LiLSX demonstrated the greatest separation efficiency among the zeolites owing to the presence of the largest number of cation sites and the highest charge density of Li+. The affinity constants calculated from the Langmuir-type adsorption isotherms and enthalpies of adsorption suggest that cation–π interactions between the C = C bond in olefins and metal ions influence selective adsorption. Density functional theory calculations support this theory of intermolecular interactions. Furthermore, a series of adsorption and desorption breakthrough experiments using a column packed with LiLSX validated its applicability for separating 1-octene and n-octane. We believe these adsorbents can be modified further and widely applied in the purification of higher olefins from chemical and biochemical products.