Supramolecular Interactions Induce Dynamics in Metal-Organic Layers to Selectively Separate Acetylene from Carbon Dioxide.

We report the synthesis and structural characterization of a 2D metal-organic framework with AB-packing layers, [Co2(pybz)2(CH3COO)2]·DMF (Co2, pybz= 4-(4-pyridyl)benzoate), containing a stable (4,4)-grid network fabricated by paddle-wheel nodes, ditopic pybz, and acetate ligands. After removal of the guest, the layer structure is retained but reorganized into an ABCD packing mode in the activated phase (Co2a). Consequently, the intralayer square windows (7.2 × 5.0 Å2) close, while the interlayer separation is decreased slightly from 3.69 to 3.45 Å, leaving a narrow gap. Importantly, the dangling methyl group of the acetate with H-bonds to the adjacent layers and also the well-distributed π-π interactions between the aromatic rings of neighboring layers facilitate the structural stability. These weak supramolecular interactions further allow for favorable dynamic exfoliation of the layers, which promotes efficient adsorption of C2H2 (41.6 cm3 g-1) over CO2 with an adsorption ratio of 6.3 (0.5 bar, 298 K). The effective separation performance of equimolar C2H2/CO2 was verified by cycling breakthrough experiments and was even tolerable to moisture (R.H = 52%). DFT calculations, in situ PXRD, and PDF characterization reveal that the favorable retention of C2H2 rather than that of CO2 is due to its H-bond formation with the paddle-wheel oxygen atoms that triggers the increase in interlayer separation during C2H2 adsorption.

Highly Efficient CO2 Capture from Wet-Hot Flue Gas by a Robust Trap-and-Flow Crystal.

Highly selective CO2 capture from flue gas based on adsorption technology is among the largest challenge on the horizon, due to its high temperature (>333 K), lower partial pressure (0.1-0.2 bar), and competition from water. Due to the designable and tunable pore system, porous coordination polymers (PCPs) have been considered as the most exciting discoveries in porous materials. However, the rational design and function-led preparation of the pore system that permits highly selective CO2 capture from flue gas (CO2/N2/O2/CO/H2O) remains a great challenge. Herein, we report a highly selective CO2 capture from wet-hot (363 K, RH = 40%) flue gas by a robust trap-and-flow crystal (NTU-67). Crystallographic analysis showed that the flow channel provides plausible CO2 traffic, while the confined trap works as an accommodation for captured gas molecules. Further, the hydrophobic pore surface endows the function of the channels that are not influenced by hot moisture, a major obstacle to overcome direct CO2 capture by PCPs. The integral nature of NTU-67, including good stability in SO2, meets the key prerequisites that are usually considered for practical applications. The molecular insight and highly efficient CO2 capture make us believe that different nanospace with their own duties may be extended into ingenious design of more advanced adsorbents for cost-effective and promising for CO2 capture from flue gas.

Confining Water Nanotubes in a Cu10O13-Based Metal-Organic Framework for Propylene/Propane Separation with Record-High Selectivity.

Energy-efficient separation of propylene (C3H6)/propane (C3H8) is in high demand for the chemical industry. However, this process is challenging due to the imperceptible difference in molecular sizes of these gases. Here, we report a continuous water nanotube dedicatedly confined in a Cu10O13-based metal-organic framework (MOF) that can exclusively adsorb C3H6 over C3H8 with a record-high selectivity of 1570 (at 1 bar and 298 K) among all the porous materials. Such a high selectivity originates from a new mechanism of initial expansion and subsequent contraction of confined water nanotubes (∼4.5 Å) caused by C3H6 adsorption rather than C3H8. Such unique response was further confirmed by breakthrough measurements, in which one adsorption/desorption cycle yields each component of the binary mixture high purity (C3H6: 98.8%; C3H8: >99.5%) and good C3H6 productivity (1.6 mL mL-1). Additionally, benefiting from the high robustness of the framework, the water nanotubes can be facilely recovered by soaking the MOF in water, ensuring long-term use. The molecular insight here demonstrates that the confining strategy opens a new route for expanding the function of MOFs, particularly for the sole recognition from challenging mixtures.

Rotation configuration control of the sp2 bond in the diimidazole-dicarboxylate linker for the isomerism of porous coordination polymers.

Porous isomers constructed from the same building blocks but different topology break the general preferred coordination rule of organic linkers and metal nodes, representing an invaluable opportunity for enriching their pore chemistry. Herein, a new group of porous isomers (termed as NTU-69 and NTU-70) was prepared from a C2v symmetric diimidazole-dicarboxylate ligand and mononuclear Cu ion. The structural differences arise from the different rotation configuration of the sp2 bond in the ligand, leading them to exhibit completely different topologies of unc (NTU-69) and sod (NTU-70) as well as framework rigidness. This rotation configuration of the sp2 bond can be controlled by the different acidity of the synthetic solution and the metal/ligand ratio. Gas adsorption and IAST selectivity show that NTU-70 features high potential for CH4 purification from C2H4, C2H6, C3H6 and CO2 mixtures at room temperature. The insight from this work establishes a new bridge between the ligand design and controlled construction of porous isomers.

Pore engineering of metal-organic frameworks for ethylene purification.

Ethylene production is an important and direct indicator related to the development of the petrochemical industry in a country. However, the separation and purification of ethylene is an extremely energy-consuming process. In this review, the latest progress in the purification of ethylene using metal organic frameworks (MOFs), a new type of physical adsorbent, is summarized according to four classifications of pore engineering, including pore surface functionalization, molecular sieving, controlled framework softness and dynamic pore-dominated molecular diffusion. Finally, the current challenges and future prospects in this field are also discussed.

Tuning Gate-Opening of a Flexible Metal-Organic Framework for Ternary Gas Sieving Separation.

In comparison with the fast development of binary mixture separations, ternary mixture separations are significantly more difficult and have rarely been realized by a single material. Herein, a new strategy of tuning the gate-opening pressure of flexible MOFs is developed to tackle such a challenge. As demonstrated by a flexible framework NTU-65, the gate-opening pressure of ethylene (C2 H4 ), acetylene (C2 H2 ), and carbon dioxide (CO2 ) can be regulated by temperature. Therefore, efficient sieving separation of this ternary mixture was realized. Under optimized temperature, NTU-65 adsorbed a large amount of C2 H2 and CO2 through gate-opening and only negligible amount of C2 H4 . Breakthrough experiments demonstrated that this material can simultaneously capture C2 H2 and CO2 , yielding polymer-grade (>99.99 %) C2 H4 from single breakthrough separation.

Finely Tuned Framework Isomers for Highly Efficient C2H2 and CO2 Separation.

Obtaining the optimal physiadsorbents based on the same starting materials is one of the crucial technologies that can address the increasing problem of energy-consuming separation. Herein, a group of porous coordination isomers (NTU-51 to NTU-54) with topologies of sql, dia, nbo, and kgm has been newly designed and prepared from a 4-c square node (paddlewheel cluster) and a 2-c linker (isophthalic acid derivative). Pure gas measurements revealed that they have a varied ability for selective C2H2 capture from C2H2/CO2 mixtures, originating from the fine arrangement of functional sites within these isomers as well as size-exclusive effects. Further dynamic breakthrough experiments exhibited good C2H2/CO2 (1/1, v/v) separation performance of the two isomers (NTU-53 and NTU-54) in both dry and humid gas phases (R.H. = 45%). More interestingly, stability tests and long-term measurements demonstrated a high potential of them to be used under realistic conditions.

Accelerated C2H2/CO2 Separation by a Se-Functionalized Porous Coordination Polymer with Low Binding Energy.

High-quality pure acetylene (C2H2) is a kind of crucial starting material for various value-added products. However, selective capture of C2H2 from the main impurity of CO2 via porous absorbents is a great challenge, as they possess extremely similar kinetic diameters and boiling points, as well as the explosive and reactive properties of C2H2. Herein, we report a porous coordination polymer (PCP), (NTU-55), which assembled from the coordination between a Cu dimer and a newly designed ligand with a nonmetal selenium (Se) site. Static single-component adsorption and dynamic breakthrough experiments reveal that desolvated NTU-55 can completely adsorb C2H2 from the C2H2/CO2 mixture (1/4, v/v) at 298 K, along with higher C2H2 capacity and much lower binding energy. The origin of this separation, as comprehensively revealed by density functional theory (DFT) calculations, is derived from the interaction discriminatory of C2H2 and CO2 toward accessible Se and Cu adsorption sites. To the best of our knowledge, this is the first time to find the positive effect of nonmetal Se sites for selective C2H2 capture.

Interplay of Tri- and Bidentate Linkers to Evolve Micropore Environment in a Family of Quasi-3D and 3D Porous Coordination Polymers for Highly Selective CO2 Capture.

In the design and construction of porous materials, these with exceptional structure and composition are often highly expected, as they may offer unique nanopore space for desired applications. Here, a new family of quasi-3D and a 3D porous coordination polymers (PCPs) (termed NTU-43 to NTU-50) were constructed via an evolution strategy from a layered structure (termed NTU-42). Single gas adsorption isotherms of CO2, N2, and CH4 display the dependency of gas capacity on optimized effects of pore size, functionality, and charged framework of these quasi-3D PCPs, where NTU-45 and NTU-46, the two with NH2-BDC and OH-BDC bidentate linkers (NH2-BDC = 2-aminoterephthalic acid and OH-BDC = 2-hydroxyterephthalic acid) have demonstrated outstanding ability for selective CO2 uptake. To the best of our knowledge, this is the first time to well explore the synergistic effects toward gas adsorption on a platform of quasi-3D frameworks. More importantly, the efficient CO2 capture from CO2/CH4 and CO2/N2 mixtures has been also validated by breakthrough experiments under continuous and dynamic conditions at 298 K.

Finely Tuned Porous Coordination Polymers To Boost Methane Separation Efficiency.

Absorbents with high breakthrough efficiency and weak host-guest interaction are considered to be promising candidates for an energy-saving process in feasible pressure/volume swing adsorption (PSA/VSA). Herein, two groups of finely designed Fe- and Co-based porous coordination polymers (PCPs) are proposed and validated; these possess hourglass-shaped nanochannels, through the cooperation of T-shaped ligands with shifted methyl groups. Featuring optimal nanochannels, high static adsorption, and relatively lower binding energy, one of these polymers, named NTU-30, enables significant C2 H6 /CH4 and C2 H4 /CH4 breakthrough efficiency, with approximately 1.0 or 0.6 g CH4 (100 %) harvested from the corresponding mixtures using 1 g of sample at ambient temperature. Furthermore, the positive effect of aromatic sites within NTU-30 is detected and investigated through an in situ IR study.