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

Engineering of Stable Anionic/Neutral MOFs with Zinc-Adeninate Building Units for Efficient C2H2/CO2 Separation

Using adenine and metal ions to form secondary building units (SBUs), further connected by a highly symmetrical multicarboxylic linker to construct an amino-modified porous framework with high porosity, is an effective strategy. By regulating the deprotonation and hydrolysis capacity of the synthesized solvent, it is possible to obtain different charged frameworks. In this work, two stable anionic/neutral MOFs, (Et2NH2)[Zn3(TCPE)(adenine)2CH3COO]·DEF·3H2O (1) and [Zn3.5(adenine)(TCPE)1.5(DMA)(H2O)0.5]·2DMA·2H2O (2), have been synthesized based on zinc-adeninate building units and symmetric tetrakis(4-carboxyphenyl)ethylene (H4TCPE) in N,N-diethylformamide (DEF) and N,N-dimethylacetamide (DMA) reaction systems, respectively. 1 is an anionic framework based on 1D rod zinc-adeninate SBU, containing 1D rectangular (14.3 × 6.3 Å2) and square (14.3 × 14.3 Å2) channels. While 2 is a neutral framework built from isolated zinc-adeninate SBU, it contains hexagonal cages with a dimension of 5.5 Å in the structure. Both of them have high porosity (61.6% for 1 and 46.3% for 2) and high stability in a wide range of pH. 1 and 2 show high C2H2 adsorption capacity at 298 K (48.1 and 70.1 cm3 g-1, respectively) and selective capacity for C2H2/CO2 mixtures, which was confirmed by the breakthrough experiments. Furthermore, the interaction between the frameworks and gas molecules has also been explained by theoretical calculation. This work provides a good example of the design and regulation of porous structures for adsorption and separation functions.

Water-Etched Approach to Hierarchically Porous Metal-Organic Frameworks with High Stability

The development of hierarchically porous metal-organic frameworks (MOFs) with high stability is desirable to expand their applications but remains challenging. Herein, an anionic sodalite-type microporous MOF (Yb-TTCA; TTCA3- = triphenylene-2,6,10-tricarboxylate) was synthesized, which shows outstanding catalytic activities for the cycloaddition of CO2 into cyclic carbonates. Moreover, the microporous Yb-TTCA can be transformed into a hierarchical micro- and mesoporous Yb-TTCA by water treatment with the mesopore sizes of 2 to 12 nm. The hierarchically porous Yb-TTCA (HP-Yb-TTCA) not only exhibits a high thermal stability up to 500 °C but also shows a high chemical stability in aqueous solutions with pH values ranging from 2 to 12. In addition, the HP-Yb-TTCA displays enhanced performance for the removal of organic dyes in comparison with microporous Yb-TTCA. This work provides a facile way to construct hierarchically porous MOF materials.

Two Structurally Similar Co5 Cluster-Based Metal-Organic Frameworks Containing Open Metal Sites for Efficient C2H2/CO2 Separation

To reasonably design and synthesize metal-organic frameworks (MOFs) with high stability and excellent adsorption/separation performance, the pore configuration and functional sites are very important. Here, we report two structurally similar cluster-based MOFs using a pyridine-modified low-symmetry ligand [H₄L = 2,6-bis(2',5'-dicarboxyphenyl)pyridine], [(NH₂Me₂)₂][Co₅(L)₂(OCH₃)₂(μ₃-OH)₂·2DMF]·2DMF·2H₂O (1) and [Co₅(L)₂(μ₃-OH)₂(H₂O)₂]·2H₂O·4DMF (2). The structures of 1 and 2 are built from Co₅ clusters, which have one-dimensional open channels, but their microporous environments are different due to the different ways in which ligands bind to the metals. Both MOFs have extremely high chemical stabilities over a wide pH range (2-12). The two MOFs have similar adsorption capacities of C₂H₂ (144.0 cm³ g^-1 for 1 and 141.3 cm³ g^-1 for 2), but 1 has a higher C₂H₂/CO₂ selectivity of 3.5 under ambient conditions. The difference in gas adsorption and separation between the two MOFs has been compared by a breakthrough experiment and theoretical calculation, and the influence of the microporous environment on the gas adsorption and separation performance of MOFs has been further studied.

A microporous chromium-organic framework fabricated via solvent-assisted metal metathesis for C2H2/CO2 separation

Removal of CO₂ or C₂H₄ from C₂H₂ is still a challenging task due to their similar physical-chemical properties. Here, a bifunctional ligand decorated with amino and sulfoxide groups, 5',5''''-sulfonylbis (2'-amino-[1,1':3',1''-terphenyl]-4,4''-dicarboxylic acid) (H₄L), was employed to construct a new microporous iron-organic framework (Fe-MOF) with the formula [(Fe₃O)(L)_1.5(H₂O)₃]_n. This MOF can serve as a parent structure to obtain the isostructural Cr-MOF by solvent-assisted metal metathesis. Furthermore, the gas adsorption and separation performance of these two MOFs were systematically investigated. Compared to Fe-MOF, Cr-MOF shows a moderately higher CO₂, C₂H₂ and C₂H₄ uptake capacity. Additionally, Cr-MOF can selectively adsorb C₂H₂ over CO₂ and C₂H₄. The separation potential towards C₂H₂/C₂H₄ and C₂H₂/CO₂ was further established via IAST calculations of mixture adsorption equilibrium. IAST selectivity values of Cr-MOF are 3.4 for C₂H₂/C₂H₄ and 6.9 for C₂H₂/CO₂ at 298 K and initial pressure, indicating its potential C₂H₂ separation ability.