Separation of 2-Chloropyridine/3-Chloropyridine by Nonporous Adaptive Crystals of Pillararenes with Different Substituents and Cavity Sizes

Abstract

Two monochloropyridine isomers, 2-chloropyridine (<b>2-CP</b>) and 3-chloropyridine (<b>3-CP</b>), are in need of a more effective separation method besides rectification. Herein we offer a facile and energy-saving adsorptive separation strategy using nonporous adaptive crystals of perethylated pillar[5]arene (<b>EtP5</b>), perethylated pillar[6]arene (<b>EtP6</b>), perbromoethylated pillar[5]arene (<b>BrP5</b>), and perbromoethylated pillar[6]arene (<b>BrP6</b>), which possess different cavity sizes and substituents and have never been employed in the separation of single-substituted heterocyclic aromatic compounds. <b>BrP6</b> crystals show a marked preference for <b>2-CP</b> in the equimolar mixture of <b>2-CP</b> and <b>3-CP</b>, affording it with 96.4% purity. Single crystal diffraction experiments demonstrate that <b>BrP6</b> has stronger host-guest interactions with <b>2-CP</b> than <b>3-CP</b>. The cycling experiments demonstrate that <b>BrP6</b> crystals can be used at least five times without losing their adsorption selectivity or capacity.