Optimal Pore Chemistry in an Ultramicroporous Metal-Organic Framework for Benchmark Inverse CO2 /C2 H2 Separation

A Copper-Based Metal-Organic Framework for C2H2/CO2 Separation

A copper-based metal-organic framework, [Cu₂(PBTDA)(H₂O)₂] (UTSA-98, UTSA = the University of Texas at San Antonio; H₄PBTDA = 5',5''''-(1,4-phenylene)bis([1,1':3',1″-terphenyl]-4,4″-dicarboxylic acid)), has been solvothermally synthesized. The alternative connection of classical dicopper secondary building units and deprotonated four-branched PBTDA^4- ligands led to the formation of the three-dimensional framework of UTSA-98 with one-dimensional rhombic channels. Its guest-free phase, UTSA-98a, uptakes much more C₂H₂ (82.6 cm³/g) than CO₂ (40.3 cm³/g) at 298 K and 100 kPa, resulting in a high adsorption selectivity of 5.2. Furthermore, the efficient separation ability of UTSA-98a toward the C₂H₂/CO₂ gas mixture was further verified by laboratory-scale fixed-bed breakthrough experiments.

Tuning the Selectivity between C2H2 and CO2 in Molecular Porous Materials

A combined experimental and theoretical study of C₂H₂ and CO₂ adsorption and separation was performed in two isostructural molecular porous materials (MPMs): MPM-1-Cl ([Cu₂(adenine)₄Cl₂]Cl₂) and MPM-1-TIFSIX ([Cu₂(adenine)₄(TiF₆)₂]). It was revealed that MPM-1-Cl displayed higher low-pressure uptake, isosteric heat of adsorption (Q_st), and selectivity for C₂H₂ than CO₂, whereas the opposite was observed for MPM-1-TIFSIX. While MPM-1-Cl contains only one type of accessible channel, which has a greater preference toward C₂H₂, MPM-1-TIFSIX contains three distinct accessible channels, one of which is a confined region between two large channels that represents the primary binding site for both adsorbates. According to molecular simulations, the initial adsorption site in MPM-1-TIFSIX interacts more strongly with CO₂ than C₂H₂, thus explaining the inversion of adsorbate selectivity relative to MPM-1-Cl.

Dense Packing of Acetylene in a Stable and Low-Cost Metal-Organic Framework for Efficient C2 H2 /CO2 Separation

Porous materials for C₂ H₂ /CO₂ separation mostly suffer from high regeneration energy, poor stability, or high cost that largely dampen their industrial implementation. A desired adsorbent should have an optimal balance between excellent separation performance, high stability, and low cost. We herein report a stable, low-cost, and easily scaled-up aluminum MOF (CAU-10-H) for highly efficient C₂ H₂ /CO₂ separation. The suitable pore confinement in CAU-10-H can not only provide multipoint binding interactions with C₂ H₂ but also enable the dense packing of C₂ H₂ inside the pores. This material exhibits one of the highest C₂ H₂ storage densities of 392 g L^-1 and highly selective adsorption of C₂ H₂ over CO₂ at ambient conditions, achieved by a low C₂ H₂ adsorption enthalpy (27 kJ mol^-1 ). Breakthrough experiments confirm its exceptional separation performance for C₂ H₂ /CO₂ mixtures, affording both large C₂ H₂ uptake of 3.3 mmol g^-1 and high separation factor of 3.4. CAU-10-H achieves the benchmark balance between separation performance, stability, and cost for C₂ H₂ /CO₂ separation.

Highly Selective Adsorption of Carbon Dioxide over Acetylene in an Ultramicroporous Metal-Organic Framework

Separating carbon dioxide from fuel gases like hydrocarbons by physical adsorbents is industrially important and more energy-efficient than traditional liquid extraction or cryogenic distillation methods. It is very important while very challenging to develop CO₂ -selective adsorbents, considering CO₂ is less polarizable than light hydrocarbon molecules, particularly those simultaneously with almost identical molecular dimensions and physical properties, such as acetylene. Herein, an ultramicroporous metal-organic framework constructed from copper(II) and 5-fluoropyrimidin-2-olate, termed Cu-F-pymo, is carefully studied under different activations for inverse separation of CO₂ from C₂ H₂ . The partially desolvated Cu-F-pymo can exclusively capture CO₂ over C₂ H₂ with very high selectivity exceeding 10⁵ under ambient conditions, the highest ever reported. Sorption experiments and modeling studies reveal that such molecular sieving effect is attributed to the suppression of C₂ H₂ adsorption from the blockage of the preferential sites for C₂ H₂ by residual water molecules. The inverse separation is further confirmed by column breakthrough studies given that highly pure acetylene (>99.9%) can be directly harvested from the gas mixture. Cu-F-pymo also shows remarkable stability under harsh conditions.

Efficient C2H2/CO2 Separation in Ultramicroporous Metal-Organic Frameworks with Record C2H2 Storage Density

Physical separation of C₂H₂ from CO₂ on metal-organic frameworks (MOFs) has received substantial research interest due to the advantages of simplicity, security, and energy efficiency. However, that C₂H₂ and CO₂ exhibit very close physical properties makes their separation exceptionally challenging. Previous work appeared to mostly focused on introducing open metal sites that aims to enhance the C₂H₂ affinity at desired sites, whereas the reticular manipulation of organic components has rarely been investigated. In this work, by reticulating preselected amino and hydroxy functionalities into isostructural ultramicroporous chiral MOFs-Ni₂(l-asp)₂(bpy) (MOF-NH₂) and Ni₂(l-mal)₂(bpy) (MOF-OH)-we targeted efficient C₂H₂ uptake and C₂H₂/CO₂ separation, which outperforms most benchmark materials. Explicitly, MOF-OH adsorbs substantial amount of C₂H₂ with record storage density of 0.81 g mL^-1 at ambient conditions, which even exceeds the solid density of C₂H₂ at 189 K. In addition, MOF-OH gave IAST selectivity of 25 toward equimolar mixture of C₂H₂/CO₂, which is nearly twice higher than that of MOF-NH₂. Notably, the adsorption enthalpies for C₂H₂ at zero converge in both MOFs are remarkably low (17.5 kJ mol^-1 for MOF-OH and 16.7 kJ mol^-1 for MOF-NH₂), which to our knowledge are the lowest among efficient rigid C₂H₂ sorbents. The efficiencies of both MOFs for the separation of C₂H₂/CO₂ are validated by multicycle breakthrough experiments. DFT calculations provide molecular-level insight over the adsorption/separation mechanism. Moreover, MOF-OH can survive in boiling water for at least 1 week and can be easily scaled up to kilograms eco-friendly and economically, which is very crucial for potential industrial implementation.

Interpenetration symmetry control within ultramicroporous robust boron cluster hybrid MOFs for benchmark purification of acetylene from carbon dioxide

The separation of C2H2/CO2 is an important process in industry but challenged by the trade-off of capacity and selectivity owning to their similar physical properties and identical kinetic molecular size. Herein, we report the first example of symmetrically interpenetrated dodecaborate pillared MOF, ZNU-1, for benchmark selective separation of C2H2 from CO2 with a high C2H2 capacity of 76.3 cm3 g-1 and record C2H2/CO2 selectivity of 56.6 (298 K, 1 bar) among all the robust porous materials without open metal sites. Single crystal structure analysis and modelling study indicated that the interpenetration shifting from asymmetric to symmetric mode provided optimal pore chemistry with ideal synergistic "2+2" dihydrogen bonding sites for tight C2H2 trapping. The exceptional separation performance was further evidenced by simulated and experimental breakthroughs with excellent recyclability and high productivity (2.4 mol/kg) of 99.5% purity C2H2 during stepped desorption process.