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 (Qst) for propane in the range of 32.9–36.9 kJ mol–1 at zero coverage and increasing Qst with rising loading due to intermolecular interactions between propane molecules, we observed very high Qst 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.