Delicate Softness in a Temperature-Responsive Porous Crystal for Accelerated Sieving of Propylene/Propane

Interpenetrated In(III)-MOF with Multiple Recognition Sites for Single-Step Ethylene Purification

An interpenetrated indium(III) metal-organic framework (MOF), NTUniv-73, with a rarely reported tetrameric indium cluster is developed for streamlining ethylene purification from C2 gases. At 298 K, the adsorption capacities exhibited a complete reversal sequence of C2H6 > C2H2 > C2H4. Grand canonical Monte Carlo simulation indicated that the corners in a octahedral cage facilitated the C2H2/C2H4 separation, while the pocket-like aperture situated between adjacent octahedral cages allows for full contact of C2H6. Breakthrough experiments illustrated that NTUniv-73 could yield pure C2H4 in a single step with a productivity of 0.42 mmol g-1.

Direct Ethylene Purification from a Four-Component Gas Mixture by a Microporous MOF with Aromatic Pore Surface and Carboxylates

The single-step purification of ethylene (C2H4) from a mixture of carbon dioxide (CO2), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) was achieved through MOF Compound-1, where the aromatic pore surface and carboxylates selectively recognized C2H6 and CO2, respectively, resulting in a reversal of the adsorption orders for both gases (C2H6 > C2H4 and CO2 > C2H4). Breakthrough testing verified that the C2H4 purification ability could be enhanced 2.6 times after adding impure CO2. Grand Canonical Monte Carlo (GCMC) simulations demonstrate that there are interactions between CO2 and C2H6 molecules as well as between CO2 molecules themselves. These interactions contribute to the enhancement of the C2H4 purification ability upon the addition of CO2 and the increased adsorption of CO2.

Systemic regulation of binding sites in porous coordination polymers for ethylene purification from ternary C2 hydrocarbons

The global demand for poly-grade ethylene (C2H4) is increasing annually. However, the energy-saving purification of this gas remains a major challenge due to the similarity in molecular properties among the ternary C2 hydrocarbons. To address this challenge, we report an approach of systematic tuning of the pore environment with organic sites (from -COOH to -CF3, then to -CH3) in porous coordination polymers (PCPs), of which NTU-73-CH3 shows remarkable capability for the direct production of poly-grade C2H4 from ternary C2 hydrocarbons under ambient conditions. In comparison, the precursor structure of NTU-73-COOH is unable to purify C2H4, while NTU-73-CF3 shows minimal ability to harvest C2H4. This is because the changed binding sites in the NTU-73-series not only eliminate the channel obstruction caused by the formation of gas clusters, but also enhance the interaction with acetylene (C2H2) and ethane (C2H6), as validated by in situ crystallographic and Raman analysis. Our findings, in particular the systematic tuning of the pore environment and the efficient C2H4 purification by NTU-73-CH3, provide a blueprint for the creation of advanced porous families that can handle desired tasks.

Relay Adsorption in Metal-Organic Frameworks for One-Step Helium Purification at Ambient Temperature

One-step He purification from natural gas represents a crucial solution for addressing the global He shortages. The prevailing method to produce high-grade He involves cryogenic distillation and ultralow temperature adsorption processes, which is highly cost- and energy-intensive. Separating and purifying He at ambient temperature is a great challenge because the fundamental limitation lies in the boiling point, polarizability, and kinetic diameters of CH4/N2/He gases. In this study, we seek to implement a relay adsorption strategy using Ni(ina)2 and MIL-100(Cr) metal-organic frameworks (MOFs) to produce high-purity He from ternary mixtures (CH4, N2, and He) at ambient temperature. The CH4/He selectivity in Ni(ina)2 and N2/He selectivity in MIL-100(Cr) both reach record 15.39 and 128.49, respectively, making the relay adsorption for helium purification highly efficient. The breakthrough experiments show that the two MOFs can sequentially adsorb CH4 and N2 in ternary mixtures, producing He with a purity of up to 99.99% in one step. The remarkable separation performance and stability of these MOFs underscore the industrial potential in purifying He at ambient temperature.

Solvents regulate the packing porosity of a bilayer metal-organic cage

Metal-organic cages (MOCs) are an emerging class of porous materials with promising applications. However, controlling the configuration of the cage packing, which can influence the overall porosity of the materials, remains a difficulty, as many factors can influence the cage assembly and stacking. Herein, we report a solvent strategy to fine-tune the packing configuration of a bilayer MOC, a small triangular prism cage (six Cu ions act as vertices, three nitrate ions act as pillars, and six nitrate ions act as caps) incorporated into a large triangular prism cage (another six Cu ions act as vertices, a couple of oxygen atoms act as pillars and six ligands (L1: 3,5-bis(pyridine-3-yl)-4H-1,2,4-triazole) act as a jointed cap) by the coordination between the triazole nitrogen from L1 and the inner vertex Cu ions. The involved solvents water, acetonitrile (MeCN) and N,N'-dimethylformamide (DMF) form hydrogen bonds with this bilayer MOC, resulting in three different types of packing associated with systemically tuned porosity (NTU-93: 12.2%, NTU-94: 19.3%, and NTU-95: 42.1%). Gas adsorption and breakthrough tests demonstrate that NTU-95 has potential ability for C2H2/C2H4 separation. This work not only shows a case of finely tuned packing of coordination cages, but also provides a powerful tool that may be extended to other cage families.

Immobilization of Amino-site into a Pore-Partitioned Metal-Organic Framework for Highly Efficient Separation of Propyne/Propylene

Adsorptive separation of propyne/propylene (C3H4/C3H6) is a crucial yet complex process, however, it remains a great difficulty in developing porous materials that can meet the requirements for practical applications, particularly with an exceptional ability to bind and store trace amounts of C3H4. Functionalization of pore-partitioned metal-organic frameworks (ppMOFs) is methodically suited for this challenge owing to the possibility of dramatically increasing binding sites on highly porous and confined domains. We here immobilized Lewis-basic (-NH2) and Lewis-acidic (-NO2) sites on this platform. Along with an integrated nature of high uptake of C3H4 at 1 kPa, high uptake difference of C3H4-C3H6, moderated binding strength, promoted kinetic selectivity, trapping effect and high stability, the NH2-decorated ppMOF (NTU-100-NH2) can efficiently produce polymer-grade C3H6 (99.95 %, 8.3 mmol ⋅ g-1) at room temperature, which is six times more than the NO2-decorated crystal (NTU-100-NO2). The in situ infrared spectroscopy, crystallographic analysis, and sequential blowing tests showed that the densely packed amino group in this highly porous system has a unique ability to recognize and stabilize C3H4 molecules. Moving forward, the strategy of organic functionalization can be extended to other porous systems, making it a powerful tool to customize advanced materials for challenging tasks.

Molecular-Sieving Separation of Methanol/Benzene Azeotrope by a Flexible Metal-Organic Framework

Separation of methanol/benzene azeotrope mixtures is very challenging not only by the conventional distillation technique but also by adsorbents. In this work, we design and synthesize a flexible Ca-based metal-organic framework MAF-58 consisting of cheap raw materials. MAF-58 shows selective methanol-induced pore-opening flexibility. Although the opened pores are large enough to accommodate benzene molecules, MAF-58 shows methanol/benzene molecular sieving with ultrahigh experimental selectivity, giving 5.1 mmol g-1 high-purity (99.99%+) methanol and 2.0 mmol g-1 high-purity (99.97%+) benzene in a single adsorption/desorption cycle. Computational simulations reveal that the preferentially adsorbed, coordinated methanol molecules act as the gating component to selectively block the diffusion of benzene, offering a new gating adsorption mechanism.

Fine-regulation of gradient gate-opening in nanoporous crystals for sieving separation of ternary C3 hydrocarbons

The adsorptive separation of ternary propyne (C3H4)/propylene (C3H6)/propane (C3H8) mixtures is of significant importance due to its energy efficiency. However, achieving this process using an adsorbent has not yet been accomplished. To tackle such a challenge, herein, we present a novel approach of fine-regulation of the gradient of gate-opening in soft nanoporous crystals. Through node substitution, an exclusive gate-opening to C3H4 (17.1 kPa) in NTU-65-FeZr has been tailored into a sequential response of C3H4 (1.6 kPa), C3H6 (19.4 kPa), and finally C3H8 (57.2 kPa) in NTU-65-CoTi, of which the gradient framework changes have been validated by in situ powder X-ray diffractions and modeling calculations. Such a significant breakthrough enables NTU-65-CoTi to sieve the ternary mixtures of C3H4/C3H6/C3H8 under ambient conditions, particularly, highly pure C3H8 (99.9%) and C3H6 (99.5%) can be obtained from the vacuum PSA scheme. In addition, the fully reversible structural change ensures no loss in performance during the cycling dynamic separations. Moving forward, regulating gradient gate-opening can be conveniently extended to other families of soft nanoporous crystals, making it a powerful tool to optimize these materials for more complex applications.

Flow Channel with Wrinkles and Calcium Sites in a Ca-MOF for Direct One-Step Ethylene Purification from C2 Gases and MTO Products Separation

The strategy of flow channel with wrinkles and calcium sites for single-step C2H4 purification from C2 gases and methanol-to-olefins (MTO) products separation was realized in FJI-Y9. The adsorption amounts showed a total reversal order of C3H6 > C2H6 > C2H2 > C2H4 at 298 K. Modeling indicated that the wrinkles and Ca2+ facilitated the full contact of C3H6 and C2H6. Breakthrough experiments illustrated that FJI-Y9 could yield pure C2H4 in a single step with a productivity of 0.78 mmol g-1. In a lone adsorption/desorption cycle for MTO product separation, the productivities of C3H6 and C2H4 were 1.96 and 1.29 mol g-1, standing as the highest recorded values.

Tuning Multiple Counter-Anions in Porous Coordination Polymers with Topology for Acetylene/Ethylene Separation

The efficient separation of acetylene (C2H2) and ethylene (C2H4) is an important and complex process in the industry. Herein, we report a new family of lcy-topologic coordination frameworks (termed NTU-90 to NTU-92) with Cu3MF6 (M = Si, Ti, and Zr) nodes. These charged frameworks are compensated by different counterbalanced ions (MF62-, BF4-, and Cl-), yielding changes in the size of the window apertures. Among these frameworks, NTU-92-a (activated NTU-92) shows good adsorption selectivity of C2H2/C2H4 and also significant ability in recovering both highly pure C2H4 (99.95%) and C2H2 (99.98%). Our work not only presents a potential alternative for energy-saving purification of C2 hydrocarbons but also provides a new approach for tuning the function of charged porous materials.

Flow Channel with Recognition Corners in a Stable La-MOF for One-Step Ethylene Production

Single-step ethylene (C2H4) production from acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) mixtures was realized via the strategy of a flow channel with recognition corners in MOF NTUniv-64. Both the uptake amounts and the enthalpy of adsorption (Qst) showed the same order of C2H2 > C2H6 > C2H4. Breakthrough testing also verified the above data and the C2H4 purification ability. Grand Canonical Monte Carlo (GCMC) simulations indicated that uneven corners could precisely detain C2H2 and C2H6, in which the C-H···π interaction distance between C2H2 (2.84 Å) and C2H6 (3.03 Å) and the framework was shorter than that of C2H4 (3.85 Å).

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.