Orthogonal-array dynamic molecular sieving of propylene/propane mixtures

Abstract

Rigid molecular sieving materials work well for small molecules with the complete exclusion of large ones^1-3, and molecules with matching physiochemical properties may be separated using dynamic molecular sieving materials^4-6. Metal-organic frameworks (MOFs)^7-9 are known for their precise control of structures and functions on a molecular level^10-15. However, the rational design of local flexibility in the MOF framework for dynamic molecular sieving remains difficult and challenging. Here we report a MOF material (JNU-3a) featuring one-dimension channels with embedded molecular pockets opening to propylene (C₃H₆) and propane (C₃H₈) at substantially different pressures. The dynamic nature of the pockets is revealed by single-crystal-to-single-crystal transformation upon exposure of JNU-3a to an atmosphere of C₃H₆ or C₃H₈. Breakthrough experiments demonstrate that JNU-3a can realize high-purity C₃H₆ (≥99.5%) in a single adsorption-desorption cycle from an equimolar C₃H₆/C₃H₈ mixture over a broad range of flow rates, with a maximum C₃H₆ productivity of 53.5 litres per kilogram. The underlying separation mechanism-orthogonal-array dynamic molecular sieving-enables both large separation capacity and fast adsorption-desorption kinetics. This work presents a next-generation sieving material design that has potential for applications in adsorptive separation.