Tuning Guest Cations to Liberate Pore Space for Efficient C4 Hydrocarbon Separation

Abstract

Although C4 hydrocarbons are important raw materials for synthetic rubber and fine chemicals, their similar physicochemical properties limit the effectiveness of traditional high-energy separation processes. Herein, we report a post-synthetic modification strategy that tunes the pore space of an anionic Cu-MOF ([Me2NH2][Cu3(μ3-OH)(pyc)3], 1) via simple cation exchange. Replacing the bulky dimethylammonium (Me2NH2+) cations with smaller potassium (K+) ions ([K][Cu3(μ3-OH)(pyc)3], 1·K) liberates additional pore volume, which significantly enhances the adsorption capacity for C4 olefins. At 298 K and 95 kPa, 1·K showed higher iso-C4H8 (74.9 cm3 g-1 vs 42.7 cm3 g-1) and C4H6 (77.9 cm3 g-1 vs 49.0 cm3 g-1) adsorption capacities than 1. The expanded pore space, rather than strong specific interactions with K+, is identified as the key factor driving the superior separation selectivity for both the isobutene/isobutane binary and the n-butene/isobutene/1,3-butadiene ternary mixtures, as confirmed by adsorption and breakthrough experiments and theoretical simulations.