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High Propene/Propane Selectivity in Isostructural Metal–Organic Frameworks with High Densities of Open Metal Sites
Summary:
The authors from Northwestern University (USA) developed a series of isostructural M-MOF-74 materials (M = Co, Mn, Mg) featuring ultra-high densities of open Co²⁺ sites, achieving record thermodynamic propene/propane selectivity (≈46) under ambient conditions for petrochemical separations.
Background:
1. To address the energy-intensive cryogenic distillation of propene/propane, previous studies exploited MOFs with open metal sites (e.g., HKUST-1, MIL-100(Fe)), yet selectivity dropped rapidly above 0.0025 bar and scale-up issues remained.
2. The authors proposed tuning the metal identity within the identical MOF-74 topology to systematically modulate π-complexation strength, obtaining unprecedented selectivity at 1 bar/298 K.
Research Content:
1. Synthesis:
Solvothermal reaction of M(NO₃)₂·xH₂O with 2,5-dihydroxy-1,4-benzenedicarboxylic acid (H₄DOBDC) in DMF followed by 250 °C evacuation yielded crystalline M-MOF-74.
2. Characterizations:
1) BET surface areas: Co- 1247 m² g⁻¹, Mn- 1000 m² g⁻¹, Mg- 1206 m² g⁻¹; 1D hexagonal channels ~11–12 Å.
2) PXRD confirms phase purity and framework retention after propene adsorption.
3) Volumetric gas adsorption shows Co-MOF-74 uptakes 7.29 mol kg⁻¹ C₃H₆ at 1 bar/298 K (≈1.14 C₃H₆ per Co²⁺).
3. Application:
Breakthrough experiments with equimolar C₃H₆/C₃H₈ feed gave sharp separation (6.8 mol kg⁻¹ C₃H₆ capacity) over three cycles using mild propane purge regeneration.
4. Mechanism:
MP2 calculations reveal strongest π-complexation for Co²⁺ (E<sub>b</sub> = 49.0 kJ mol⁻¹; M–C = 2.60 Å). Pore volume exactly accommodates one C₃H₆ per open site, suppressing C₃H₈ uptake at high pressure—an emergent “size-matching” effect absent for C₂H₄/C₂H₆.
Outlook:
The work demonstrates that maximizing open-metal-site density within optimally sized pores furnishes a general strategy for olefin/paraffin adsorptive separations, positioning Co-MOF-74 as a promising next-generation sorbent for energy-efficient petrochemical refining.
High Propene/Propane Selectivity in Isostructural Metal–Organic Frameworks with High Densities of Open Metal Sites
Authors: Youn-Sang Bae, Chang Yeon Lee, Ki Chul Kim, Omar K. Farha, Peter Nickias, Joseph T. Hupp, SonBinh T. Nguyen, Randall Q. Snurr
DOI: 10.1002/anie.201107534
Link: https://onlinelibrary.wiley.com/doi/10.1002/anie.201107534
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