Effective　removal　of　acetaldehyde　and　propionaldehyde　from　propylene　oxide　is　important　for　polyether　industry　but　remains　a　significant　challenge　due　to　their　similar　molecular　sizes　and　relatively　close　volatilities.　Herein,　we　studied　flexible　molecular　materials,　perethylated　pillar[n]arene　crystals　(n　=　5,　6),　which　can　be　used　to　separate　acetaldehyde　and　propionaldehyde　in　propylene　oxide.　Through　a　space　size　matching　effect,　perethylated　pillar[5]arene　demonstrates　excellent　adsorption　selectivity　of　95　%　and　100　%　for　aldehydes　in　ternary　mixture　and　binary　mixture　of　propylene　oxide,　acetaldehyde　and　propionaldehyde,　respectively.　These　impressive　selective　adsorption　is　achieved　based　on　their　differential　host–guest　interactions.　Density　functional　theory　calculations　provide　a　comprehensive　explanation　of　the　adsorption　process,　revealing　that　the　adsorption　mechanisms　of　perethylated　pillar[5]arene　for　acetaldehyde　and　propionaldehyde　primarily　involve　C-H···O　hydrogen　bonding　and　C-H···π　interactions.　This　study　also　provides　a　novel　method　for　achieving　one-step　purification　of　propylene　oxide　in　industrial　applications.　©　2024　Elsevier　B.V.