Separation of monochlorotoluene isomers is a vital process to obtain highly pure p-chlorotoluene, which is irreplaceable in the production of medicines and pesticides. However, traditional separation methods suffer from great energy consumption, cumbersome operation or use of organic desorbents. Herein, an energy-efficient and environmentally friendly method is developed through an absorptive separation strategy based on nonporous adaptive crystals of perethylated pillar[5]arene (EtP5) and pillar[6]arene (EtP6). EtP5 and EtP6 crystals separate p-chlorotoluene from a p-chlorotoluene/o-chlorotoluene equimolar mixture with purities of 99.1% and 96.1%, respectively and show no decrease in selectivity upon cycling. The selectivity is attributed to both the stability of the final crystal structure upon guest capture and suitable host cavity size/shape. Besides, we discovered the gate-opening behavior changes of EtP5 crystals at different temperatures after absorption of p-chlorotoluene/o-chlorotoluene mixtures with various p-chlorotoluene fractions, which is helpful to understand the thermodynamics of the absorption process.