A Robust Molecular Porous Material for C2 H2 /CO2 Separation

A Lewis basic site rich metal-organic framework featuring a hydrogen-bonded acetylene nano-trap for the efficient separation of C2H2/CO2

The physical separation of C2H2 from CO2 on metal-organic frameworks (MOFs) has received a substantial amount of research interest due to its advantages of simplicity, security, and energy efficiency. However, the exploitation of ideal MOF adsorbents for C2H2/CO2 separation remains a challenging task due to their similar physical properties and molecular sizes. Herein, we report a unique C2H2 nano-trap constructed using accessible oxygen and nitrogen sites, which exhibits energetic favorability toward C2H2 molecules. This material exhibits a good acetylene capacity of 55.31 cm3 g-1 and high C2H2/CO2 selectivity of 7.0 under ambient conditions. We have combined in situ IR spectroscopy and in-depth theoretical calculations to unravel the synergistic interactions driven by the high density of accessible oxygen and nitrogen sites. Furthermore, dynamic breakthrough experiments confirmed the capability of TUTJ-201Ni for the separation of binary C2H2/CO2 mixtures. This study on Ni-based MOFs will enrich Lewis basic site rich MOFs for gas adsorption and separation applications.

Light and Guest Responsive Behavior in a Porous Coordination Network Enabled by Reversible [2+2] Photocycloaddition

Stimuli-responsive physisorbents that undergo reversible structural transformations induced by external stimuli (e.g. light, guests, or heat) offer the promise of utility in gas storage and separation. Whereas reports on guest or light-responsive sorbents have increased in recent years, we are unaware of reports on sorbents that exhibit both light and guest-induced structural transformations. Herein, we report that the square lattice, sql, topology coordination network Zn(fba)(bis) ⋅ 2DMF (sql-5,6-Zn-α, 5=trans-4,4'-bis(1-imidazolyl)stilbene=bis, 6=2,2-bis(4-carboxyphenyl)hexafluoropropane=H2fba) underwent single-crystal-to-single-crystal transformation (SCSC) upon activation, affording nonporous sql-5,6-Zn-β. Parallel alignment at 3.23 Å of olefinic moieties on adjacent bis ligands in sql-5,6-Zn-α enabled SCSC [2+2] photocycloaddition upon exposure to UV light (365 nm) or sunlight. sql-5,6-Zn-α thereby transformed to mot-5,6-Zn-α, which was subsequently activated to the narrow pore phase mot-5,6-Zn-β. sql-5,6-Zn-β and mot-5,6-Zn-β both exhibited S-shaped adsorption isotherms characteristic of guest-induced structural changes when exposed to CO2 at 195 K (type-F-IV and type F-I, respectively). Cycling experiments conducted upon sql-5,6-Zn-β reduced particle size after cycle 1 and induced transformation into a rare example of a shape memory coordination network, sql-5,6-Zn-γ. Insight into this smorgasbord of SCSC phase changes was gained from in situ PXRD, single crystal XRD and 1H NMR spectroscopy experiments.

Fine Tuning the Hydrophobicity of a New Three-Dimensional Cu2+ MOF through Single Crystal Coordinating Ligand Exchange Transformations

The synthesis, characterization, and single-crystal-to-single-crystal (SCSC) exchange reactions of a new 3D Cu2+ MOF based on 5-aminoisophthalic acid (H2AIP), [Cu6(μ3-ΟΗ)3(ΑΙΡ)4(HΑΙΡ)]n·6nDMF·nH2O - UCY-16·6nDMF·nH2O, are reported. It exhibits a 3D structure based on two [Cu4(μ3-OH)2]6+ butterfly-like secondary building units, differing in their peripheral ligation, bridged through HAIP-/AIP2- ligands. This compound displays the capability to exchange the coordinating ligand(s) and/or guest solvent molecules through SCSC reactions. Interestingly, heterogeneous reactions of single crystals of UCY-16·6nDMF·nH2O with primary alcohols resulted not only in the removal of the lattice DMF molecules but also in an unprecedented structural alteration that involved the complete or partial replacement of the monoatomic bridging μ3-OH- anion(s) of the [Cu4(μ3-OH)2]6+ butterfly structural core by various alkoxy groups. Similar crystal-to-crystal exchange reactions of UCY-16·6nDMF·nH2O with long-chain aliphatic alcohols (CxH2x+1OH, x = 8-10, 12, 14, and 16) led to analogues containing fatty alcohols. Notably, the exchanged products with the bulkier alcohols UCY-16/n-CxH2x+1OH·S' (x = 6-10, 12, 14, and 16) do not mix with H2O being quite stable in this solvent, in contrast to the pristine MOF, and exhibit a hydrophobic/superhydrophobic surface as confirmed from the investigation of their water contact angles and capability to remove hydrophobic pollutants from aqueous media.

Switching Adsorbent Layered Material that Enables Stepwise Capture of C8 Aromatics via Single-Crystal-to-Single-Crystal Transformations

Separation of the C8 aromatic isomers, xylenes (PX, MX, and OX) and ethylbenzene (EB), is important to the petrochemical industry. Whereas physisorptive separation is an energy-efficient alternative to current processes, such as distillation, physisorbents do not generally exhibit strong C8 selectivity. Herein, we report the mixed-linker square lattice (sql) coordination network [Zn2(sba)2(bis)]n·mDMF (sql-4,5-Zn, H2sba or 4 = 4,4'-sulfonyldibenzoic acid, bis or 5 = trans-4,4'-bis(1-imidazolyl)stilbene) and its C8 sorption properties. sql-4,5-Zn was found to exhibit high uptake capacity for liquid C8 aromatics (∼20.2 wt %), and to the best of our knowledge, it is the first sorbent to exhibit selectivity for PX, EB, and MX over OX for binary, ternary, and quaternary mixtures from gas chromatography. Single-crystal structures of narrow-pore, intermediate-pore, and large-pore phases provided insight into the phase transformations, which were enabled by flexibility of the linker ligands and changes in the square grid geometry and interlayer distances. This work adds to the library of two-dimensional coordination networks that exhibit high uptake, thanks to clay-like expansion, and strong selectivity, thanks to shape-selective binding sites, for C8 isomers.

Highly Efficient CO2/C2H2 Separation by Porous Graphene via Quadrupole Gating Mechanism

Acetylene (C₂H₂) is an important and widely used raw material in various industries (such as petrochemical). Generally, a product yield is proportional to the purity of C₂H₂; however, C₂H₂ from a typical industrial gas-production process is commonly contaminated by CO₂. So far, the achievement of high-purity C₂H₂ separated from a CO₂/C₂H₂ mixture is still challenging due to their very close molecular dimensions and boiling temperatures. Taking advantage of their quadrupoles with opposite signs, here, we show that the graphene membrane embedded with crown ether nanopores can achieve an unprecedented separation efficiency of CO₂/C₂H₂. Combining the molecular dynamics simulation and the density functional theory (DFT) approaches, we discovered that the electrostatic gas-pore interaction favorably allows the fast transport of CO₂ through crown ether nanopores while completely prohibiting C₂H₂ transport, which yields a remarkable permeation selectivity. In particular, the utilized crown ether pore is capable of allowing the individual transport of CO₂ while completely rejecting the passage of C₂H₂, independent of the applied pressures, fed gases ratios, and exerted temperatures, featuring the superiority and robustness of the crown pore in CO₂/C₂H₂ separation. Further, DFT and PMF calculations demonstrate that the transport of CO₂ through the crown pore is energetically more favorable than the transport of C₂H₂. Our findings reveal the potential application of graphene crown pore for CO₂ separation with outstanding performance.

Highly Productive C3H4/C3H6 Trace Separation by a Packing Polymorph of a Layered Hybrid Ultramicroporous Material

Ultramicroporous materials can be highly effective at trace gas separations when they offer a high density of selective binding sites. Herein, we report that sql-NbOFFIVE-bpe-Cu, a new variant of a previously reported ultramicroporous square lattice, sql, topology material, sql-SIFSIX-bpe-Zn, can exist in two polymorphs. These polymorphs, sql-NbOFFIVE-bpe-Cu-AA (AA) and sql-NbOFFIVE-bpe-Cu-AB (AB), exhibit AAAA and ABAB packing of the sql layers, respectively. Whereas NbOFFIVE-bpe-Cu-AA (AA) is isostructural with sql-SIFSIX-bpe-Zn, each exhibiting intrinsic 1D channels, sql-NbOFFIVE-bpe-Cu-AB (AB) has two types of channels, the intrinsic channels and extrinsic channels between the sql networks. Gas and temperature induced transformations of the two polymorphs of sql-NbOFFIVE-bpe-Cu were investigated by pure gas sorption, single-crystal X-ray diffraction (SCXRD), variable temperature powder X-ray diffraction (VT-PXRD), and synchrotron PXRD. We observed that the extrinsic pore structure of AB resulted in properties with potential for selective C3H4/C3H6 separation. Subsequent dynamic gas breakthrough measurements revealed exceptional experimental C3H4/C3H6 selectivity (270) and a new benchmark for productivity (118 mmol g-1) of polymer grade C3H6 (purity >99.99%) from a 1:99 C3H4/C3H6 mixture. Structural analysis, gas sorption studies, and gas adsorption kinetics enabled us to determine that a binding "sweet spot" for C3H4 in the extrinsic pores is behind the benchmark separation performance. Density-functional theory (DFT) calculations and Canonical Monte Carlo (CMC) simulations provided further insight into the binding sites of C3H4 and C3H6 molecules within these two hybrid ultramicroporous materials, HUMs. These results highlight, to our knowledge for the first time, how pore engineering through the study of packing polymorphism in layered materials can dramatically change the separation performance of a physisorbent.