A molecular T-pentomino for separating BTEX hydrocarbons

Abstract

The development of methods to separate molecules (e.g., petrochemicals) are exceedingly important industrially, particularly to purify fuels and polymer precursors.1 A common approach for separations is to crystallize an organic molecule that acts as a host by either providing an enforced covalent cavity (intrinsic cavity) or that packs inefficiently (extrinsic cavity).2 Here we report a self-assembled molecule with a shape that is highly biased to completely enclosed space and, thereby, pack efficiently yet has the property of hosting and allowing for the separation of BTEX hydrocarbons (i.e., benzene, toluene, ethylbenzene, xylenes).1 The components of the host are held together by N→B bonds and form a diboron assembly with a molecular shape that conforms to a T-shaped pentomino.3 A T-pentomino is a polyomino, which is a plane figure that tiles a plane without cavities and holes, and we show the T-shaped molecule to crystallize into one of six limited polymorphic structures for T-pentomino tiling.4 The separations of the BTEX hydrocarbons occur at mild conditions while rejecting similarly shaped aromatics such as xylene isomers, thiophene, and styrene. Our observation on the structure and tiling of the host to conform to a T-pentomino allows us to develop a theory on how novel synthetic molecules that mimic the structures and packing of polyominoes3 can be synthesized and - quite counterintuitively - developed into a system of hosts with cavities used for selective and useful separations.