Supramolecular Entanglement in a Hydrogen-Bonded Organic Framework Enables Flexible-Robust Porosity for Highly Efficient Purification of Natural Gas.

The development of porous materials with flexible-robust characteristics shows some unique advantages to target high performance for gas separation, but remains a daunting challenge to achieve so far. Herein, we report a carboxyl-based hydrogen-bonded organic framework (ZJU-HOF-8a) with flexible-robust porosity for efficient purification of natural gas. ZJU-HOF-8a features a four-fold interpenetrated structure with dia topology, wherein abundant supramolecular entanglements are formed between the adjacent subnetworks through weak intermolecular hydrogen bonds. This structural configuration could not only stabilize the whole framework to establish the permanent porosity, but also enable the framework to show some flexibility due to its weak intermolecular interactions (so-called flexible-robust framework). The flexible-robust porosity of ZJU-HOF-8a was exclusively confirmed by gas sorption isotherms and single-crystal X-ray diffraction studies, showing that the flexible pore pockets can be opened by C3H8 and n-C4H10 molecules rather by C2H6 and CH4. This leads to notably higher C3H8 and n-C4H10 uptakes with enhanced selectivities than C2H6 over CH4 under ambient conditions, affording one of the highest n-C4H10/CH4 selectivities. The gas-loaded single-crystal structures coupled with theoretical simulations reveal that the loading of n-C4H10 can induce an obvious framework expansion along with pore pocket opening to improve n-C4H10 uptake and selectivity, while not for C2H6 adsorption. This work suggests an effective strategy of designing flexible-robust HOFs for improving gas separation properties.

Tunable Low-Pressure Water Adsorption in Stable Multivariate Metal-Organic Frameworks for Boosting Water-Based Ultralow-Temperature-Driven Refrigeration.

The green water-based adsorption refrigeration is considered as a promising strategy to realize near-zero-carbon cooling applications. Although many metal-organic frameworks (MOFs) have been developed as water adsorbents, their cooling performance are commonly limited by the insufficient water uptakes below P/P0 = 0.2. Herein, the development of multivariate MOFs (MTV-MOFs) is reported to highly modulate and boost the low-pressure water uptake for improving coefficient of performance (COP) for refrigeration. Through ligand exchange in the pristine MIL-125-NH2 , a series of MTV-MOFs with bare nitrogen sites are designed and synthesized. The resulting MIL-125-NH2 /MD-5% exhibits the significantly improved water uptake of 0.39 g g-1 at 298 K and P/P0 = 0.2, which is three times higher than MIL-125-NH2 (0.12 g g-1 ) and comparable to some benchmark materials including KMF-1 (0.4 g g-1 ) and MIP-200 (0.36 g g-1 ). Combined with its low-temperature regeneration, fast sorption kinetics and high stability, MIL-125-NH2 /MD-5% achieves one of the highest COP values (0.8) and working capacities (0.24 g g-1 ) for refrig-2 under an ultralow-driven temperature of 65 °C, which are higher than some best-performing MOFs such as MIP-200 (0.74 and 0.11 g g-1 ) and KMF-2 (0.62 and 0.16 g g-1 ), making it among the best adsorbents for efficient ultralow-temperature-driven refrigeration.

Incorporation of multiple supramolecular binding sites into a robust MOF for benchmark one-step ethylene purification.

One-step adsorption separation of C2H4 from ternary C2 hydrocarbon mixtures remains an important and challenging goal for petrochemical industry. Current physisorbents either suffer from unsatisfied separation performance, poor stability, or are difficult to scale up. Herein, we report a strategy of constructing multiple supramolecular binding sites in a robust and scalable MOF (Al-PyDC) for highly efficient one-step C2H4 purification from ternary mixtures. Owing to suitable pore confinement with multiple supramolecular binding sites, Al-PyDC exhibits one of the highest C2H2 and C2H6 uptakes and selectivities over C2H4 at ambient conditions. The gas binding sites have been visualized by single-crystal X-ray diffraction studies, unveiling that the low-polarity pore surfaces with abundant electronegative N/O sites provide stronger multiple supramolecular interactions with C2H2 and C2H6 over C2H4. Breakthrough experiments showed that polymer-grade C2H4 can be separated from ternary mixtures with a maximum productivity of 1.61 mmol g-1. This material can be prepared from two simple reagents using a green synthesis method with water as the sole solvent, and its synthesis can be easily scaled to multikilogram batches. Al-PyDC achieves an effective combination of benchmark separation performance, high stability/recyclability, green synthesis and easy scalability to address major challenges for industrial one-step C2H4 purification.

Multivariate Hydrogen-Bonded Organic Frameworks with Tunable Permanent Porosities for Capture of a Mustard Gas Simulant.

Precise synthesis of topologically predictable and discrete molecular crystals with permanent porosities remains a long-term challenge. Here, we report the first successful synthesis of a series of 11 isoreticular multivariate hydrogen-bonded organic frameworks (MTV-HOFs) from pyrene-based derivatives bearing -H, -CH3 , -NH2 and -F groups achieved by a shape-fitted, π-π stacking self-assembly strategy. These MTV-HOFs are single-crystalline materials composed of tecton, as verified by single-crystal diffraction, nuclear magnetic resonance (NMR) spectra, Raman spectra, water sorption isotherms and density functional theory (DFT) calculations. These MTV-HOFs exhibit tunable hydrophobicity with water uptake starting from 50 to 80 % relative humidity, by adjusting the combinations and ratios of functional groups. As a proof of application, the resulting MTV-HOFs were shown to be capable of capturing a mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES) from moisture. The location of different functional groups within the pores of the MTV-HOFs leads to a synergistic effect, which resulted in a superior CEES/H2 O selectivity (up to 94 %) compared to that of the HOFs with only pure component and enhanced breakthrough performance (up to 4000 min/g) when compared to benchmark MOF materials. This work is an important advance in the synthesis of MTV-HOFs, and provides a platform for the development of porous molecular materials for numerous applications.

Programmed fluorine binding engineering in anion-pillared metal-organic framework for record trace acetylene capture from ethylene.

Porous physisorbents are attractive candidates for selective capture of trace gas or volatile compounds due to their low energy footprints. However, many physisorbents suffer from insufficient sorbate-sorbent interactions, resulting in low uptake or inadequate selectivity when gases are present at trace levels. Here, we report a strategy of programmed fluorine binding engineering in anion-pillared metal-organic frameworks to maximize C2H2 binding affinity for benchmark trace C2H2 capture from C2H4. A robust material (ZJU-300a) was elaborately designed to provide multiple-site fluorine binding model, resulting in an ultrastrong C2H2 binding affinity. ZJU-300a exhibits a record-high C2H2 uptake of 3.23 millimoles per gram (at 0.01 bar and 296 kelvin) and one of the highest C2H2/C2H4 selectivity (1672). The adsorption binding of C2H2 and C2H4 was visualized by gas-loaded ZJU-300a structures. The separation capacity was confirmed by breakthrough experiments for 1/99 C2H2/C2H4 mixtures, affording the maximal dynamic selectivity (264) and C2H4 productivity of 436.7 millimoles per gram.

Reticular Synthesis of Metal-Organic Frameworks by 8-Connected Quadrangular Prism Ligands for Water Harvesting.

Utilization of rigid, highly connected organic linkers is critical for the reticular synthesis of functional metal-organic frameworks (MOFs). However, highly-stable MOFs (e.g. Al/Cr/Zr-based MOFs) based on rigid ligands with more than 6 coordinating functions have been rarely achieved thus far. Herein, we describe the construction of two bcu Zr-based MOFs (named ZrMOF-1 and ZrMOF-2) from peripherally extended pentiptycene ligands (H8 PEP-1 and H8 PEP-2) with rigid quadrangular prism shape possessing 8 carboxylic groups at the prism vertices. Particularly, ZrMOF-1 exhibits microporous structure with large Bruno-Emmett-Teller surface area and high water stability, endowing it a promising water harvesting material with a high water uptake capacity of 0.83 gH2O gMOF -1 at P/P0 =0.90 and 25 °C, a steep uptake at a low P/P0 of 0.30, and excellent durability over 500 water adsorption-desorption cycles. Moreover, self-consistent charge density functional tight-binding calculations were carried out, rationalizing the water adsorbing process and amount in ZrMOF-1.

Immobilization of Lewis Basic Nitrogen Sites into a Chemically Stable Metal-Organic Framework for Benchmark Water-Sorption-Driven Heat Allocations.

Developing efficient and stable water adsorbents for adsorption-driven heat transfer technology still remains a challenge due to the lack of efficient strategies to enhance low-pressure water uptakes. The authors herein demonstrate that the immobilization of Lewis basic nitrogen sites into metal-organic frameworks (MOFs) can improve water uptake and target benchmark coefficient of performances (COPs) for cooling and heating. They present the water sorption properties of a chemically stable MOF (termed as Zr-adip), designed by incorporating hydrophilic nitrogen sites into the adsorbent MIP-200. Zr-adip exhibits S-shaped sorption isotherms with an extremely high water uptake of 0.43 g g-1  at 303 K and P/P0  = 0.25, higher than MIP-200 (0.39 g g-1 ), KMF-1 (0.39 g g-1 ) and MOF-303 (0.38 g g-1 ). Theoretical calculations reveal that the incorporated N sites can serve as secondary adsorption sites to moderately interact with water, providing more binding sites to strengthen the water binding affinity. Zr-adip achieves exceptionally high COPs of 0.79 (cooling) and 1.75 (heating) with a low driving temperature of 70 °C, outperforming MIP-200 (0.78 and 1.53) and KMF-1 (0.75 and 1.74). Combined with its ultrahigh stability, excellent cycling performance, and easy regeneration, Zr-adip represents one of the best water adsorbents for adsorption-driven cooling and heating.

Immobilization of Lewis Basic Sites into a Stable Ethane-Selective MOF Enabling One-Step Separation of Ethylene from a Ternary Mixture.

Purification of C2H4 from a ternary C2H2/C2H6/C2H4 mixture by one-step adsorption separation is of prime importance but challenging in the petrochemical industry; however, effective strategies to design high-performance adsorbents are lacking. We herein report for the first time the incorporation of Lewis basic sites into a C2H6-selective MOF, enabling efficient one-step production of polymer-grade C2H4 from ternary mixtures. Introduction of amino groups into highly stable C2H6-selective UiO-67 can not only partition large pores into smaller cagelike pockets to provide suitable pore confinement but also offer additional binding sites to simultaneously enhance C2H2 and C2H6 adsorption capacities over C2H4. The amino-functionalized UiO-67-(NH2)2 thus exhibits exceptionally high C2H2 and C2H6 uptakes as well as benchmark C2H2/C2H4 and C2H6/C2H4 selectivities, surpassing all of the C2H2/C2H6-selective materials reported so far. Theoretical calculations combined with in situ infrared spectroscopy indicate that the synergetic effect of suitable pore confinement and functional surfaces decorated with amino groups provides overall stronger multipoint van der Waals interactions with C2H2 and C2H6 over C2H4. The exceptional performance of UiO-67-(NH2)2 was evidenced by breakthrough experiments for C2H2/C2H6/C2H4 mixtures under dry and wet conditions, providing a remarkable C2H4 productivity of 0.55 mmol g-1 at ambient conditions.