Petroleum is rich in valuable aromatic compounds, but separating them by distillation is costly because of the compounds’ high boiling points. Porous solids are potential separation tools, but so far zeolites and metal-organic frameworks have not worked well for aromatic molecules containing more than eight carbons. Using a strategy based on shape mimicry, researchers led by Andrew I. Cooper at the University of Liverpool, in England, have now shown separation of large aromatics by purely organic frameworks (Nat. Chem., DOI: 10.1038/nchem.1550). They constructed frameworks from building blocks that are structurally similar to the molecule they aim to extract. In a proof-of-principle study, they built a framework with mesitylene-like building blocks to separate mesitylene from its structural isomer, 4-ethyltoluene. They find that mesitylene gets stuck trying to pass through the framework, while 4-ethyltoluene passes through. The trick, Cooper says, is building frameworks with enough flexibility to allow some molecules to pass through but not so flexible that everything can. He hopes the strategy can be extended to separation of other large organic molecules.