Shifting C₂H₂/CO₂ Adsorption and Separation in Pillar-Layered Metal-Organic Frameworks Finely-Regulated by Molecular Rotation

The efficient separation of C₂H₂/CO₂ mixture is crucial for industrial applications. A promising strategy is proposed herein to fine-tune the C₂H₂/CO₂ adsorption and separation by pillar-layered metal-organic framework (MOF) adsorbents via molecular rotation. Keeping the same ultramicroporous architecture, three Zn-X-TRZ (TRZ = 1,2,4-triazole) adsorbents are prepared with X-pillar rotors varying from 9,10-anthracenedicarboxylic acid (ADC), 1,4-naphthalenedicarboxylic acid (NDC) to 1,4-benzenedicarboxylic acid (BDC). Remarkably, the introduction of the largest ADC rotors enables Zn-ADC-TRZ with superior C₂H₂-selective thermodynamic-separation ability (highest heat of adsorption and IAST selectivity values) but poor dynamic-separation performance caused by steric hindrance. Conversely, Zn-BDC-TRZ with the smallest rotors exhibit moderate CO₂-selective thermodynamic-separation ability, but excellent CO₂/C₂H₂ dynamic-separation ability with high-purity C₂H₂ produced at 298 K (>99.5% obtained from 70% C₂H₂ mixture and >99.95% from 90% C₂H₂ mixture). In contrast, Zn-NDC-TRZ with medium NDC rotors shows comparable interactions and steric hindrance with C₂H₂ and CO₂, making both the thermodynamic and dynamic C₂H₂/CO₂ separation are unfavorable. Overall, controllable adsorption of C₂H₂ and CO₂ is successfully achieved and shifted from C₂H₂-selective to CO₂-selective separation regulated by the pillared molecular rotation within pillar-layered MOFs, providing a useful route for practical gas adsorbent exploration.