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Crystal engineering improves acetylene purification

Fine-tuning pore size and chemical affinity overcomes classic gas-separation trade-off

by Mitch Jacoby
August 19, 2021 | A version of this story appeared in Volume 99, Issue 30


A crystal that separates acetylene from carbon dioxide
Credit: Chem
This tailor-made crystal efficiently separates acetylene (left) from similarly sized carbon dioxide (right). N = blue; Si = yellow; F = turquoise; Ni = lilac; O = red; H = white; C = gray.

Industrial methods for making acetylene generally leave the commodity—a fuel and starting material for polymers—contaminated with carbon dioxide. Common procedures for purifying acetylene, including cryogenic distillation and solvent extraction, are energy intensive and expensive. Separating the gases with membranes or solid sorbents could be a low-cost alternative, but most porous materials used for gas purification do a poor job of separating mixtures of CO2 and acetylene because of similarities in molecular size, boiling points, and other properties. Efforts to improve separation typically lead to a trade-off: high selectivity goes along with low gas uptake and vice versa, leading to materials that don’t separate the gases thoroughly, or do so, but sluggishly. By designing a series of related porous crystalline materials, a team led by Michael J. Zaworotko of the University of Limerick may have overcome the trade-off (Chem 2021, DOI: 10.1016/j.chempr.2021.07.007). The group reacted a pyridine-based linker with three types of metal fluoride ligands and two metal cations, making a total of six materials that differ slightly in pore size, surface area, and affinity for acetylene over CO2. In separation tests, four of the new materials outperformed a benchmark material, which exhibits a separation selectivity of roughly 5 and gas capacity of 3.5 mmol/g. The top performer came in at roughly 28 and 4.0, respectively.


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