Isomeric Nonpolar Amino Acid–Derived Metal–Organic Frameworks for Xenon/Krypton Separation

Efficient xenon/krypton separation remains challenging due to their similar physicochemical properties. Herein, we demonstrate that ligand isomerism can be leveraged as an effective structural handle for constructing new metal-organic frameworks from readily available, low-cost amino acids. Using leucine and isoleucine-two constitutional regioisomers among proteinogenic amino acids that possess the largest nonpolar alkyl side chains-we construct a pair of zinc-based metal-organic frameworks, Zn-LEU and Zn-ILE, which share identical connectivity yet differ subtly in side-chain branching. Zn-ILE retains a more robust framework under a range of conditions, whereas Zn-LEU undergoes a pronounced phase transformation under relatively mild conditions. This structural integrity, combined with a precisely tailored nonpolar pore environment (∼4.4 Å), enables Zn-ILE to exhibit a ∼40% increase in Xe uptake and superior Xe/Kr selectivity over its isomer. Dynamic breakthrough experiments further validate the Xe/Kr separation performance under representative operating conditions, including humid streams and ultradilute xenon concentrations (400 ppm). We further formalize a cost-normalized figure of merit that quantifies dynamic xenon capture per unit synthetic input, under which Zn-ILE exhibits cost-normalized Xe productivity of 2.21 × 10⁻³ mmol USD⁻ ¹, ranking among the most cost-efficient MOF-based xenon sorbents reported to date.