Thermal resistance effect on anomalous diffusion of molecules under confinement.

Diffusion is generally faster at higher temperatures. Here, a counterintuitive behavior is observed in that the movement of long-chain molecules slows as the temperature increases under confinement. This report confirms that this anomalous diffusion is caused by the "thermal resistance effect," in which the diffusion resistance of linear-chain molecules is equivalent to that with branched-chain configurations at high temperature. It then restrains the molecular transportation in the nanoscale channels, as further confirmed by zero length column experiments. This work enriches our understanding of the anomalous diffusion family and provides fundamental insights into the mechanism inside confined systems.

Scalable Synthesis of Robust MOF for Challenging Ethylene Purification and Propylene Recovery with Record Productivity.

Ethylene (C2H4) purification and propylene (C3H6) recovery are highly relevant in polymer synthesis, yet developing physisorbents for these industrial separation faces the challenges of merging easy scalability, economic feasibility, high moisture stability with great separation efficiency. Herein, we reported a robust and scalable MOF (MAC-4) for simultaneous recovery of C3H6 and C2H4. Through creating nonpolar pores decorated by accessible N/O sites, MAC-4 displays top-tier uptakes and selectivities for C2H6 and C3H6 over C2H4 at ambient conditions. Molecular modelling combined with infrared spectroscopy revealed that C2H6 and C3H6 molecules were trapped in the framework with stronger contacts relative to C2H4. Breakthrough experiments demonstrated exceptional separation performance for binary C2H6/C2H4 and C3H6/C2H4 as well as ternary C3H6/C2H6/C2H4 mixtures, simultaneously affording record productivities of 27.4 and 36.2 L kg-1 for high-purity C2H4 (≥99.9 %) and C3H6 (≥99.5 %). MAC-4 was facilely prepared at deckgram-scale under reflux condition within 3 hours, making it as a smart MOF to address challenging gas separations.

Immobilization of the Polar Group into an Ultramicroporous Metal-Organic Framework Enabling Benchmark Inverse Selective CO2/C2H2 Separation with Record C2H2 Production.

One-step harvest of high-purity light hydrocarbons without the desorption process represents an advanced and highly efficient strategy for the purification of target substances. The separation and purification of acetylene (C2H2) from carbon dioxide (CO2) by CO2-selective adsorbents are urgently demanded yet are very challenging owing to their similar physicochemical properties. Here, we employ the pore chemistry strategy to adjust the pore environment by immobilizing polar groups into an ultramicroporous metal-organic framework (MOF), achieving one-step manufacture of high-purity C2H2 from CO2/C2H2 mixtures. Embedding methyl groups into prototype stable MOF (Zn-ox-trz) not only changes the pore environment but also improves the discrimination of guest molecules. The methyl-functionalized Zn-ox-mtz thus exhibits the benchmark reverse CO2/C2H2 uptake ratio of 12.6 (123.32/9.79 cm3 cm-3) and an exceptionally high equimolar CO2/C2H2 selectivity of 1064.9 at ambient conditions. Molecular simulations reveal that the synergetic effect of pore confinement and surfaces decorated with methyl groups provides high recognition of CO2 molecules through multiple van der Waals interactions. The column breakthrough experiments suggest that Zn-ox-mtz dramatically achieved the one-step purification capacity of C2H2 from the CO2/C2H2 mixture with a record C2H2 productivity of 2091 mmol kg-1, surpassing all of the CO2-selective adsorbents reported so far. In addition, Zn-ox-mtz exhibits excellent chemical stability under different pH values of aqueous solutions (pH = 1-12). Moreover, the highly stable framework and excellent inverse selective CO2/C2H2 separation performance showcase its promising application as a C2H2 splitter for industrial manufacture. This work paves the way to developing reverse-selective adsorbents for the challenging gas separation process.

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.

Neutral MOF Anion Receptor: Radical-Promoted Precise Anion Recognition.

Precise ion recognition plays a key role in the anionic decontamination in water. However, the established anionic recognition based on neutral or cationic anion receptor is still restricted by the inherent limitation, such as narrow application scope in organic solvent rather than water for neutral anion receptor and poor selectivity due to non-directional electrostatic interaction for cationic anion receptor. Herein, for the first time, a neutral metal-organic framework (MOF) anion receptor is shown, enabling precise anion recognition, for example, the presence of a variety of 1000-fold competitive anions does not affect the selective adsorption of the target anion at all. A radical-dominating anion-recognition mechanism is proposed for rationalizing the efficacy of the neutral MOF.

Tuning Pore Polarization to Boost Ethane/Ethylene Separation Performance in Hydrogen-Bonded Organic Frameworks.

Hydrogen-bonded organic frameworks (HOFs) show great potential in energy-saving C2 H6 /C2 H4 separation, but there are few examples of one-step acquisition of C2 H4 from C2 H6 /C2 H4 because it is still difficult to achieve the reverse-order adsorption of C2 H6 and C2 H4 . In this work, we boost the C2 H6 /C2 H4 separation performance in two graphene-sheet-like HOFs by tuning pore polarization. Upon heating, an in situ solid phase transformation can be observed from HOF-NBDA(DMA) (DMA=dimethylamine cation) to HOF-NBDA, accompanied with transformation of the electronegative skeleton into neutral one. As a result, the pore surface of HOF-NBDA has become nonpolar, which is beneficial to selectively adsorbing C2 H6 . The difference in the capacities for C2 H6 and C2 H4 is 23.4 cm3 g-1 for HOF-NBDA, and the C2 H6 /C2 H4 uptake ratio is 136 %, which are much higher than those for HOF-NBDA(DMA) (5.0 cm3 g-1 and 108 % respectively). Practical breakthrough experiments demonstrate HOF-NBDA could produce polymer-grade C2 H4 from C2 H6 /C2 H4 (1/99, v/v) mixture with a high productivity of 29.2 L kg-1 at 298 K, which is about five times as high as HOF-NBDA(DMA) (5.4 L kg-1 ). In addition, in situ breakthrough experiments and theoretical calculations indicate the pore surface of HOF-NBDA is beneficial to preferentially capture C2 H6 and thus boosts selective separation of C2 H6 /C2 H4 .

Induced-Fit-Identification in a Rigid Metal-Organic Framework for ppm-Level CO2 Removal and Ultra-Pure CO Enrichment.

Removing CO2 from crude syngas via physical adsorption is an effective method to yield eligible syngas. However, the bottleneck in trapping ppm-level CO2 and improving CO purity at higher working temperatures are major challenges. Here we report a thermoresponsive metal-organic framework (1 a-apz), assembled by rigid Mg2 (dobdc) (1 a) and aminopyrazine (apz), which not only affords an ultra-high CO2 capacity (145.0/197.6 cm3 g-1 (0.01/0.1 bar) at 298 K) but also produces ultra-pure CO (purity ≥99.99 %) at a practical ambient temperature (TA ). Several characterization results, including variable-temperature tests, in situ high-resolution synchrotron X-ray diffraction (HR-SXRD), and simulations, explicitly unravel that the excellent property is attributed to the induced-fit-identification in 1 a-apz that comprises self-adaption of apz, multiple binding sites, and complementary electrostatic potential (ESP). Breakthrough tests suggest that 1 a-apz can remove CO2 from 1/99 CO2 /CO mixtures at practical 348 K, yielding 70.5 L kg-1 of CO with ultra-high purity of ≥99.99 %. The excellent separation performance is also revealed by separating crude syngas that contains quinary mixtures of H2 /N2 /CH4 /CO/CO2 (46/18.3/2.4/32.3/1, v/v/v/v/v).

Molecular Sieving of Propyne/Propylene by a Scalable Nanoporous Crystal with Confined Rotational Shutters.

Soft porous coordination polymers (PCPs) have the remarkable ability to recognize similar molecules as a result of their structural dynamics. However, their guest-induced gate-opening behaviors often lead to issues with selectivity and separation efficiency, as co-adsorption is nearly unavoidable. Herein, we report a strategy of a confined-rotational shutter, in which the rotation of pyridyl rings within the confined nanospace of a halogen-bonded coordination framework (NTU-88) creates a maximum aperture of 4.4 Å, which is very close to the molecular size of propyne (C3 H4 : 4.4 Å), but smaller than that of propylene (C3 H6 : 5.4 Å). This has been evidenced by crystallographic analyses and modelling calculations. The NTU-88o (open phase of activated NTU-88) demonstrates dedicated C3 H4 adsorption, and thereby leads to a sieving separation of C3 H4 /C3 H6 under ambient conditions. The integrated nature of high uptake ratio, considerable capacity, scalable synthesis, and good stability make NTU-88 a promising candidate for the feasible removal of C3 H4 from C3 H4 /C3 H6 mixtures. In principle, this strategy holds high potential for extension to soft families, making it a powerful tool for optimizing materials that can tackle challenging separations with no co-adsorption, while retaining the crucial aspect of high capacity.

Metal-Organic Framework Based Hydrogen-Bonding Nanotrap for Efficient Acetylene Storage and Separation.

The removal of carbon dioxide (CO2) from acetylene (C2H2) is a critical industrial process for manufacturing high-purity C2H2. However, it remains challenging to address the tradeoff between adsorption capacity and selectivity, on account of their similar physical properties and molecular sizes. To overcome this difficulty, here we report a novel strategy involving the regulation of a hydrogen-bonding nanotrap on the pore surface to promote the separation of C2H2/CO2 mixtures in three isostructural metal-organic frameworks (MOFs, named MIL-160, CAU-10H, and CAU-23, respectively). Among them, MIL-160, which has abundant hydrogen-bonding acceptors as nanotraps, can selectively capture acetylene molecules and demonstrates an ultrahigh C2H2 storage capacity (191 cm3 g-1, or 213 cm3 cm-3) but much less CO2 uptake (90 cm3 g-1) under ambient conditions. The C2H2 adsorption amount of MIL-160 is remarkably higher than those for the other two isostructural MOFs (86 and 119 cm3 g-1 for CAU-10H and CAU-23, respectively) under the same conditions. More importantly, both simulation and experimental breakthrough results show that MIL-160 sets a new benchmark for equimolar C2H2/CO2 separation in terms of the separation potential (Δqbreak = 5.02 mol/kg) and C2H2 productivity (6.8 mol/kg). In addition, in situ FT-IR experiments and computational modeling further reveal that the unique host-guest multiple hydrogen-bonding interaction between the nanotrap and C2H2 is the key factor for achieving the extraordinary acetylene storage capacity and superior C2H2/CO2 selectivity. This work provides a novel and powerful approach to address the tradeoff of this extremely challenging gas separation.

Creating High-Number Defect Sites through a Bimetal Approach in Metal-Organic Frameworks for Boosting Trace SO2 Removal.

Herein, we represent a bimetallic approach to enhance the defect number, leading to eight defect sites per node in a metal-organic framework, showing both a higher SO2 adsorption capacity and higher SO2/CO2 selectivity. The results can be further strongly supported by density functional theory calculations.

Efficient Separation of Trace SO2 from SO2/CO2/N2 Mixtures in a Th-Based MOF.

The emission of sulfur dioxide (SO2) from flue gases is harmful since trace SO2 impairs human health and the natural environment. Therefore, developing new metal organic frameworks (MOFs) to capture this toxic molecule is of great importance in flue gas desulfurization. In this work, we synthesized a new MOF, namely, ECUT-Th-60, which consists of two distinct channels (3.0 Å × 4.1 Å and 2.3 Å × 4.8 Å). It shows SO2 uptakes of around 2.5 mmol/g at 0.1 kPa and 3.35 mmol/g at 1 bar, which are higher than those of CO2 and N2 under identical conditions. Both simulated and experimental breakthrough tests proved that ECUT-Th-60 can separate trace SO2 from SO2/CO2 mixtures. Impressively, complete separation of SO2 from SO2/CO2/N2 mixtures under both dry and humid conditions was also proved in ECUT-Th-60, predicting its potential application in flue gas desulfurization.

One-Step Ethylene Purification from Ternary Mixtures in a Metal-Organic Framework with Customized Pore Chemistry and Shape.

Adsorptive separation is an energy-efficient technology for the separation of C2 hydrocarbons. However, it remains a critical problem to directly produce high-purity C2 H4 from ternary C2 H2 /C2 H4 /C2 H6 mixtures by simultaneously trapping C2 H2 and C2 H6 . Herein, we report the one-step C2 H4 purification from the ternary mixture by a metal-organic framework Zn(ad)(int) (ad=adeninate; int=isonicotinate). The material combines dense heterocyclic rings and accessible uncoordinated O atoms as strong binding sites for C2 H6 and C2 H2 . Its spindle-like cage exhibits an interesting shape matching with the targeted molecules, affording Zn(ad)(int) not only high separation selectivity for C2 H6 /C2 H4 and C2 H2 /C2 H4 , but also excellent gas capacity. Breakthrough experiments show that polymer-grade C2 H4 can be separated from the ternary mixtures with a record productivity of 1.43 mmol g-1 . In situ powder X-ray diffraction and Fourier transform infrared spectrum analyses further provide deep insights into the separation mechanism.

Titanium-Oxo Cluster Assisted Fabrication of a Defect-Rich Ti-MOF Membrane Showing Versatile Gas-Separation Performance.

Although having shown great promise for efficient gas separation, relevant study of Ti-MOF membranes remains very scarce, owing to limited Ti source types and uncertain factors which dominate the separation properties. In this work, we pioneered the use of the Ti8 (µ2 -O)8 (OOCC6 H5 )16 cluster as the Ti source of MIL-125 membranes, which led to lower reaction temperature and higher missing-linker number within the framework and therefore, enhanced CO2 /N2 adsorption selectivity. The MIL-125 membrane prepared by combining single-mode microwave heating with tertiary growth displayed an ideal CO2 /N2 selectivity of 38.7, which ranked the highest among all pristine pure MOF membranes measured under comparable conditions. In addition, the ideal H2 /N2 and H2 /CH4 selectivity was as high as 64.9 and 40.7, thus showing great promise for versatile utility in gas separation.

Comprehensive Pore Tuning in an Ultrastable Fluorinated Anion Cross-Linked Cage-Like MOF for Simultaneous Benchmark Propyne Recovery and Propylene Purification.

Propyne/propylene (C3 H4 /C3 H6 ) separation is an important but challenging industrial process to produce polymer-grade C3 H6 and recover high-purity C3 H4 . Herein, we report an ultrastable TiF6 2- anion cross-linked metal-organic framework (ZNU-2) with precisely controlled pore size, shape and functionality for benchmark C3 H4 storage (3.9/7.7 mmol g-1 at 0.01/1.0 bar and 298 K) and record high C3 H4 /C3 H6 (10/90) separation potential (31.0 mol kg-1 ). The remarkable C3 H4 /C3 H6 (1/99, 10/90, 50/50) separation performance was fully demonstrated by simulated and experimental breakthroughs under various conditions with excellent recyclability and high productivity (42 mol kg-1 ) of polymer-grade C3 H6 from a 1/99 C3 H4 /C3 H6 mixture. A modelling study revealed that the symmetrical spatial distribution of six TiF6 2- on the icosahedral cage surface provides two distinct binding sites for C3 H4 adsorption: one serves as a tailored single C3 H4 molecule trap and the other boosts C3 H4 accommodation by cooperative host-guest and guest-guest interactions.

Boosting Ethane/Ethylene Separation by MOFs through the Amino-Functionalization of Pores.

Adsorption technology based on ethane-selective materials is a promising alternative to energy-intensive cryogenic distillation for separating ethane (C2 H6 ) and ethylene (C2 H4 ). We employed a pore engineering strategy to tune the pore environment of a metal-organic framework (MOF) through organic functional groups and boosted the C2 H6 /C2 H4 separation of the MOF. Introduction of amino (-NH2 ) groups into Tb-MOF-76 not only decreased pore sizes but also facilitated multiple guest-host interactions in confined pores. The NH2 -functionalized Tb-MOF-76(NH2 ) has increased C2 H6 and C2 H4 uptakes and C2 H6 /C2 H4 selectivity. The results of experimental and simulated transient breakthroughs reveal that Tb-MOF-76(NH2 ) has significantly improved one-step separation performance for C2 H6 /C2 H4 mixtures with a high C2 H4 (>99.95 %) productivity of 17.66 L kg-1 compared to 7.53 L kg-1 by Tb-MOF-76, resulting from the suitable pore confinement and accessible -NH2 groups on pore surfaces.

Robust 4d-5f Bimetal-Organic Framework for Efficient Removal of Trace SO2 from SO2/CO2 and SO2/CO2/N2 Mixtures.

Herein, we report a highly rare robust 4d-5f bimetal-organic framework that shows high porosity and thermal/chemical stability and thus is capable of removing trace SO2 from a SO2/CO2/N2 mixture even under humid conditions. This work not only shows a novel adsorbent for SO2 removal but also extends the function of actinium-based coordination compounds.

A Robust Cage-Based Metal-Organic Framework Showing Ultrahigh SO2 Uptake for Efficient Removal of Trace SO2 from SO2/CO2 and SO2/CO2/N2 Mixtures.

Removal of trace SO2 from an SO2-containing product is now receiving increasing attention. However, designing a robust porous adsorbent with high SO2 adsorption capacity and good SO2/CO2 selectivity, as well as validity under humid conditions, is still a challenging task. Herein, we report a porous cage-based metal-organic framework, namely ECUT-111, which contains two distinct cages with apertures of 5.4 and 10.2 Å, respectively, and shows high a BET of up to 1493 m2/g and a pore volume of 0.629 cm3/g. Impressively, ECUT-111 enables an ultrahigh SO2 uptake of up to 11.56 mmol/g, exceeding most reported top-performing adsorbents for such a use. More importantly, complete separation of trace SO2 from SO2/CO2 and SO2/CO2/N2 mixtures, especially under humid conditions, and excellent recycle use were observed for ECUT-111, suggesting its superior application in desulfurization of SO2-containing products.

High Adsorption Capacity and Selectivity of SO2 over CO2 in a Metal-Organic Framework.

Herein, we report a new metal-organic framework (MOF), namely, ECUT-77, which is built on rod-shaped secondary building units, showing a high Brunauer-Emmett-Teller surface area of 760.3 cm2/g, a pore volume of 0.4 cm3/g, and an aperture of about 1 nm. This MOF enables both high SO2 adsorption capacity up to 8.0 mmol/g at 0.92 bar and room temperature and a high SO2/CO2 selectivity of 44, resulting in excellent SO2 separation upon a ECUT-77 column from a SO2/CO2 mixture containing 2000 ppm of SO2.

Interpenetration Symmetry Control Within Ultramicroporous Robust Boron Cluster Hybrid MOFs for Benchmark Purification of Acetylene from Carbon Dioxide.

The separation of C2 H2 /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. We report the first example of symmetrically interpenetrated dodecaborate pillared MOF, ZNU-1, for benchmark selective separation of C2 H2 from CO2 with a high C2 H2 capacity of 76.3 cm3 g-1 and record C2 H2 /CO2 selectivity of 56.6 (298 K, 1 bar) among all the robust porous materials without open metal sites. Single crystal structure analysis and modeling indicated that the interpenetration shifting from asymmetric to symmetric mode provided optimal pore chemistry with ideal synergistic "2+2" dihydrogen bonding sites for tight C2 H2 trapping. The exceptional separation performance was further evidenced by simulated and experimental breakthroughs with excellent recyclability and high productivity (2.4 mol kg-1 ) of 99.5 % purity C2 H2 during stepped desorption process.

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

Isolation of CO2 from acetylene (C2 H2 ) via CO2 -selective sorbents is an energy-efficient technology for C2 H2 purification, but a strategic challenge due to their similar physicochemical properties. There is still no specific methodology for constructing sorbents that preferentially trap CO2 over C2 H2 . We report an effective strategy to construct optimal pore chemistry in a CeIV -based ultramicroporous metal-organic framework CeIV -MIL-140-4F, based on charge-transfer effects, for efficient inverse CO2 /C2 H2 separation. The ligand-to-metal cluster charge transfer is facilitated by CeIV with low-lying unoccupied 4f orbitals and electron-withdrawing F atoms functionalized tetrafluoroterephthalate, affording a perfect pore environment to match CO2 . The exceptional CO2 uptake (151.7 cm3 cm-3 ) along with remarkable separation selectivities (above 40) set a new benchmark for inverse CO2 /C2 H2 separation, which is verified via simulated and experimental breakthrough experiments. The unique CO2 recognition mechanism is further unveiled by in situ powder X-ray diffraction experiments, Fourier-transform infrared spectroscopy measurements, and molecular calculations.