Pillar[5]arene Based Conjugated Microporous Polymers for Propane/Methane Separation through Host–Guest Complexation

We present an efficient strategy for the preparation of conjugated microporous polymers incorporating pillar[5]arenes (P5-CMPs) with surface areas up to 400 m² g^–1 via Pd-catalyzed Sonogashira–Hagihara cross-coupling reaction of triflate functionalized pillar[5]arene with 1,4-diethynylbenzene and 4,4'-diethynyl-1,1'-biphenyl linkers. In an effort to transfer intrinsic properties of pillar[5]arene, that is its ability to form strong host–guest complexes with linear hydrocarbons in solution, into the solid-state, we investigated the affinity of P5-CMPs toward propane gas. Unlike previously reported porous solids, which showed isosteric heats of adsorption ($Q$_st) for propane in the range of 32.9–36.9 kJ mol–1 at zero coverage and increasing $Q$_st with rising loading due to intermolecular interactions between propane molecules, we observed very high $Q$_st values up to 53 kJ mol^–1 at zero coverage, which gradually decreased to ~35 kJ mol^–1 with increasing loadings. This observation indicates strong supramolecular host–guest complexation between propane and pillar[5]arene via multiple C–H/$π$ interactions, i.e., “macrocyclic effect” arising from the ideal size match of kinetic diameter of propane to the cavity of pillar[5]arene. This approach also allowed us to introduce thermodynamic selectivity for the separation of saturated hydrocarbons with low polarizability. High affinity of P5-CMPs for propane facilitated its efficient breakthrough separation from a simulated natural gas mixture (methane:propane, 9:1) at 298 K.