by R. Mark Wilson
The calcium-based structure is an efficient sieve, despite the mere 0.3 Å size difference between the two filtrates.
The separation of molecules from mixtures is a critical process in the petrochemical industry. As a case in point, the global production of ethylene out of ethylene–ethane mixtures exceeded 150 million metric tons in 2016. Because the molecules’ sizes differ by a mere 0.3 Å, engineers rely on repeated distillation and compression in harsh conditions to produce that yield. The well-established technique consumes up to 1.5 GJ per metric ton—10 times the energy cost of simpler membrane-based filters. Banglin Chen (University of Texas at San Antonio), Wei Zhou (NIST in Gaithersburg, Maryland), and their colleagues have now developed a metal–organic framework (MOF) that in ambient conditions efficiently separates the molecules—no distillation or compression required.
Metal–organic frameworks are synthetic, porous materials made up of metal clusters, which form nodes, and organic ligands, which connect the nodes. Because of the versatility of the concept, MOFs can be designed with tailor-made properties. Chen, Zhou, and colleagues showed how an organic molecule, squaric acid H2C4O4, can be linked by calcium ions to make the MOF known as UTSA-280, whose structure is depicted here. The green, light coral, and gray nodes represent Ca, O, and C atoms, respectively.
As a nontoxic, widely available, and inexpensive metal, Ca is an appealing ingredient. The researchers made use of Ca ions for their coordination bonding with squaric acid, whose small size and square symmetry prevent the acid’s rotation in the lattice and facilitate the formation of a rigid, microporous structure. The restraint produces similarly rigid cylindrical channels just wide enough to admit ethylene but block ethane. What’s more, the material is stable in water and is expected to easily scale up to industrial volumes.