Engineering Metal-Organic Frameworks for Selective Separation of Hexane Isomers Using 3-Dimensional Linkers

Efficient Methane Purification Using a Robust and Recoverable Hydrogen-Bonded Organic Framework

Efficient separation and purification of methane is a key step in promoting its wide application as an environmentally friendly energy carrier and an important chemical raw material. Development of single solid adsorbents simultaneously combining robust structure, high separation capacity, and easy performance recovery is highly desired but quite a challenge for realizing methane depuration. In the present study, we report an ultramicroporous hydrogen-bonded organic framework compound constructed from a tetracyano bithiophene-functionalized ligand. Similar to the acting principle of enzymes, multisite supramolecular interactions endow the material with not only extraordinary chemical stability in wide pH range including 12 M HCl and 20 M NaOH solutions but also highly selective recognition of acetylene and carbon dioxide over methane. The packing densities and adsorption selectivities toward acetylene and carbon dioxide are the highest reported for the hydrogen-bonded organic framework materials. Dynamic breakthrough experiments demonstrate its highly efficient methane purification ability from binary and even ternary mixed gases, with not only methane productivities up to 2.34 mol kg-1 but also moderate regeneration energy. Furthermore, the title compound can be easily regenerated in almost quantitative yield via simple rato-evaporation of its dichloromethane solution once its activity is attenuated or lost.

Efficient Separation of Dibranched Hexane from its Linear and Monobranched Isomers via the Synergistic Molecular Sieving and Pore Shape-Matching Strategy

The efficient separation of dibranched hexane from its linear and monobranched isomers is crucial but challenging for the production of high-RON (Research Octane Number) gasoline. Here, a strategy is presented to realize the efficient separation of high-purity (99.8%) dibranched 2,2-dimethylbutane from a ternary mixture of n-hexane/3-methylpentane/2,2-dimethylbutane by combining the sieving aperture and shape-matching cavity within the ultramicroporous metal-organic framework, Zn4O(NTB)2 (H3NTB = 4,4',4″-Nitrilotrisbenzoic acid). The static adsorption isotherms exhibit both high capacity for linear n-hexane (2.72 mmol g-1) and monobranched 3-methylpentane (2.59 mmol g-1). Dynamic breakthrough experiment shows that Zn4O(NTB)2 is capable of completely separating high-RON dibranched 2,2-dimethylbutane with a record-high productivity (0.98 mmol g-1), which is ≈1.6 times that of the previous benchmark material. Single-crystal X-ray diffraction of guest-loaded Zn4O(NTB)2 reveals that there exist high-density elongated cavities that match well with the shape of both n-hexane and 3-methylpentane, which can account for the high adsorption capacity for both of them. This work demonstrates the effectiveness of the synergistic effect of molecular sieving and shape matching in the efficient production of dibranched hexane from complex mixture.

Metal-Organic Framework with Constrained Flexibility for Benchmark Separation of Hexane Isomers

Flexible metal-organic frameworks (MOFs) are promising candidates for adsorptive separations, but achieving a balance among flexibility, adsorption capacity, and selectivity remains challenging. Herein, we report a novel flexible MOF, Ni(bhdc)(ted)0.5 (ZUL-C6), incorporating hybrid three-dimensional alkane-bridged ligands, which realizes high-capacity molecular sieving for hexane isomer separation - a critical process in the petroleum industry. The alkyl-rich, confined pore system within the ZUL-C6 framework facilitated a strong affinity for n-hexane and 3-methylpentane. However, the narrow pore size and the constrained flexibility limited the uptake of 2,2-dimethylbutane (<4.0 mg/g), accompanied by a high gate-opening pressure. The gating behavior was elucidated by guest-loaded single-crystal (SC) X-ray diffraction and density functional theory (DFT) simulations, which revealed a unique SC to SC transformation driven by the non-centrosymmetric rotation of the 3D bhdc linker and distortion of the metal cluster and pillar units, along with a high deformation energy barrier. As a result, ZUL-C6 exhibited not only significantly higher uptake and selectivity than the industrially used 5 A molecular sieve, but also the record-high nHEX/3MP breakthrough uptake (92.8/73.9 mg/g) and unprecedented 22DMB producing time (309.2 min/g, corresponding to the productivity of 770 mmol/kg and yield of 92.8 %) among reported MOFs.

Exceeding flexpectations: a combined experimental and computational investigation of structural flexibility in 3-dimensional linker-based metal-organic frameworks

Designing sorbents for the separation of molecules with sub-angstrom differences in size requires precise control over pore size and environment, which can be challenging to establish in the presence of structural flexibility. However, metal-organic frameworks (MOFs) that incorporate 3-dimensional (3D) linkers-ditopic ligands with 3-dimensional, sterically bulky cores-are well-suited to address this challenge, as 3D linkers enable sub-angstrom level control over pore size by mitigating the effects of structural flexibility. In this study, we used a combined computational and experimental approach to quantify flexibility in two systems of MOFs with increasing linker bulkiness, leveraging these systems to distinguish between two classes of flexibility: global and local. Specifically, we used density functional theory (DFT) calculations to understand the electronic energy landscapes of MIL-53(Al), MIL-47(V) and their corresponding 3D linker analogues of increasing bulkiness. We further characterized the mechanical properties of these materials with DFT calculations of elastic tensors and in practical compression conditions using in situ variable pressure X-ray diffraction experiments. Finally, we illustrated the importance of establishing sub-angstrom level pore control by demonstrating the effects of each type of flexibility on the adsorption properties of MOFs using grand canonical Monte Carlo simulations.

Isoreticular Tolerance and Phase Selection in the Synthesis of Multi-Module Metal-Organic Frameworks for Gas Separation and Electrocatalytic OER

Although metal-organic frameworks are coordination-driven assemblies, the structural prediction and design using metal-ligand interactions can be unreliable due to other competing interactions. Leveraging non-coordination interactions to develop porous assemblies could enable new materials and applications. Here, we use a multi-module MOF system to explore important and pervasive impact of ligand-ligand interactions on metal-ligand as well as ligand-ligand co-assembly process. It is found that ligand-ligand interactions play critical roles on the scope or breakdown of isoreticular chemistry. With cooperative di- and tri-topic ligands, a family of Ni-MOFs has been synthesized in various structure types including partitioned MIL-88-acs (pacs), interrupted pacs (i-pacs), and UMCM-1-muo. A new type of isoreticular chemistry on the muo platform is established between two drastically different chemical systems. The gas sorption and electrocatalytic studies were performed that reveal excellent performance such as high C2H2/CO2 selectivity of 21.8 and high C2H2 uptake capacity of 114.5 cm3/g at 298 K and 1 bar.

Engineering Trifluoromethyl Groups in Porous Coordination Polymers to Enhance Stability and Regulate Pore Window for Hexane Isomers Separation

Effective separation of hexane (C6) isomers is critical for a variety of industrial applications but conventional distillation methods are energy-intensive. Adsorptive separations based on porous coordination polymers (PCPs) offer a promising alternative due to their exceptional porosity and tunable properties. However, there is still an urgent need to develop PCPs with high stability and separation performance. This study investigates how substituting a methyl (-CH3) group with a trifluoromethyl (-CF3) group can regulate pores and hydrophobicity in PCPs. This precise adjustment aims to enhance stability and improve the kinetic separation performance of hydrophobic C6 isomers by considering the size and hydrophobicity of the trifluoromethyl group. Two isostructural PCPs with pcu topology, PCP-CH3 and PCP-CF3, were synthesized to vary pore diameters and hydrophobicity based on the presence of -CH3 or -CF3 groups. PCP-CF3 showed greater stability in water compared to PCP-CH3. While PCP-CH3 had high adsorption capacities, it lacked selectivity, whereas PCP-CF3 demonstrated improved selectivity, particularly in excluding dibranched isomers. Dynamic column separation experiments revealed that PCP-CF3 could selectively adsorb linear and monobranched isomers over dibranched isomers at room temperature. These findings highlight the potential of fluorine-modified PCPs for efficient isomer separation and underscore the importance of stability improvement strategies.

Synergistic global and local flexibilities in Zr-based metal-organic frameworks enable sequential sieving of hexane isomers

Separating hexane isomers based on the branching degree is crucial for their efficient utilization in the petrochemical industry, yet remains challenging due to their similar properties. Here we report a temperature-responsive Zr-based metal-organic framework, Zr-fum-FA, capable of sequentially sieving linear, mono-, and di-branched hexane isomers. Notably, the pore structure of Zr-fum-FA dynamically transforms from segmented triangular channels to an integrated rhombic configuration as the temperature increases, leading to distinct sieving effects. At low temperatures, the narrow triangular pores allow the exclusive adsorption of n-hexane while excluding branched isomers. In contrast, the expanded rhombic pores at high temperatures enable the sieving of mono- and di-branched isomers. Mechanistic studies reveal that this unique dual-sieving behavior originates from the synergistic effects of the global framework flexibility and the local dynamics of pendent hydroxyl groups. Furthermore, we demonstrate the decoupling of global and local flexibilities via two strategies: incorporating steric hindrance to dampen the global framework dynamics and enhancing the metal node rigidity to limit the local vibrations. These findings not only provide a promising adsorbent for the challenging separation of hexane isomers but also offer rational design principles for harnessing flexibility in MOFs.

Six-Membered Ring Directed Assembly of a Zirconium Zeolitic Metal-Organic Framework for Efficient Separation of Hexane Isomers

The synthesis of zirconium MOFs with zeolite net is quite challenging due to the high connectivity of Zr6 clusters, which is far from tetrahedral connection, a requisite for zeolite net. In this work, we demonstrate a six-membered ring (6MR) strategy through mimicking of mineral zeolites with mixed ditopic and tritopic carboxylate linkers. With this strategy, the ditopic linker cross-links Zr6 clusters to form 4-connected zeolite-like nets, while the tritopic one is used to direct the formation of 6MR and simultaneously consumes extra coordination sites on the cluster. The feasibility of this strategy is shown by one zeolitic metal-organic framework (NNM-5) and this strategy has also led to the synthesis of the other dia-type zirconium MOF (NNM-6). Interestingly, as the tritopic linker not only directs the formation of 6MR but also partitions 6MR into small segments, NNM-5 with SOD topology shows a structural feature of small aperture and big cage, which has led to efficient separation of hexane isomers. With both exceptionally high n-hexane uptake (65.9 cm3 g-1) and size-exclusion selectivity, an exceptional separation capability is verified by breakthrough experiments. Calculation results demonstrate that the large difference of diffusion energy barrier due to the small aperture accounts for the underlying separation mechanism.

Visual Recognition of Volatile Organic Compounds by Photonic Nose Integrated with Multiple Metal-Organic Frameworks

The photonic nose inspired by the olfactory system is an integrated detection platform constructed by multiple sensing units as channels. However, in the detection of volatile organic compounds (VOCs), the sensing results that cannot be directly readable and the poor ability to distinguish analytes with similar chemical properties are the main challenges faced by this sensor. Here, 8 metal-organic frameworks (MOF)-based photonic crystals are used as the basic sensing units to construct a photonic nose detection platform. The microscopic adsorption of VOCs by MOFs enables the photonic crystals (PCs) to produce macroscopic structural color output, and further makes the photonic nose have specific color fingerprints for different VOCs, the response time of all PCs to VOCs can be within 1 s. Through the color fingerprint, the visual identification of VOCs produced by 5 common solvent vapors is realized, and 9 VOCs with similar chemical properties are further distinguished. In addition, the application potential of the photonic nose in the actual environment is verified by identifying different contents of benzene in the paint. It is envisaged that the MOF-based photonic nose has great reference value for the development of intelligent and multi-component synergistic functional gas sensors.

Nonlinear 3D Ligand-Based Metal-Organic Framework for Thermodynamic-Kinetic Synergistic Splitting of Mono-/Dibranched Hexane Isomers

The selective splitting of hexane isomers without the use of energy-intensive phase-change processes is essential for the low-carbon production of clean fuels and also very challenging. Here, we demonstrate a strategy to achieve a complete splitting of the high-RON dibranched isomer from the monobranched and linear isomers, by using a nonlinear 3D ligand to form pillar-layered MOFs with delicate pore architecture and chemistry. Compared with its isoreticular MOFs with the same ted pillar but different linear 3D or linear 2D in-layer ligands, the new MOF constructed in this work, Cu(bhdc)(ted)0.5 (ZUL-C5), exhibited an interesting "channel switch" effect which creates pore space with reduced window size and channel dimensionality together with unevenly distributed alkyl-rich adsorption sites, contributing to a greatly enhanced ability to discriminate between mono- and dibranched isomers. Evidenced by a series of studies including adsorption equilibrium/kinetics/breakthrough tests, guest-loaded single-crystal/powder XRD measurement, and DFT-D modeling, a thermodynamic-kinetic synergistic mechanism in the separation was proposed, resulting in a record production time for high-purity 2,2-dimethylbutane along with a high yield.

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.

Thermally activated structural phase transitions and processes in metal-organic frameworks

The structural knowledge of metal-organic frameworks is crucial to the understanding and development of new efficient materials for industrial implementation. This review classifies and discusses recent advanced literature reports on phase transitions that occur during thermal treatments on metal-organic frameworks and their characterisation. Thermally activated phase transitions and procceses are classified according to the temperaturatures at which they occur: high temperature (reversible and non-reversible) and low temperature. In addition, theoretical calculations and modelling approaches employed to better understand these structural phase transitions are also reviewed.

Efficient CO2 Capture and Separation in MOFs: Effect from Isoreticular Double Interpenetration

Severe CO2 emissions has posed an increasingly alarming threat, motivating the development of efficient CO2 capture materials, one of the key parts of carbon capture, utilization, and storage (CCUS). In this study, a series of metal-organic frameworks (MOFs) named Sc-X (X = S, M, L) were constructed inspired by recorded MOFs, Zn-BPZ-SA and MFU-4l-Li. The corresponding isoreticular double-interpenetrating MOFs (Sc-X-IDI) were subsequently constructed via the introduction of isoreticular double interpenetration. Grand canonical Monte Carlo (GCMC) simulations were adopted at 298 K and 0.1-1.0 bar to comprehensively evaluate the CO2 capture and separation performances in Sc-X and Sc-X-IDI, with gas distribution, isothermal adsorption heat (Qst), and van der Waals (vdW)/Coulomb interactions. It is showed that isoreticular double interpenetration significantly improved the interactions between adsorbed gases and frameworks by precisely modulating pore sizes, particularly observed in Sc-M and Sc-M-IDI. Specifically, the Qst and Coulomb interactions exhibited a substantial increase, rising from 28.38 and 22.19 kJ mol-1 in Sc-M to 43.52 and 38.04 kJ mol-1 in Sc-M-IDI, respectively, at 298 K and 1.0 bar. Besides, the selectivity of CO2 over CH4/N2 was enhanced from 55.36/107.28 in Sc-M to 3308.61/7021.48 in Sc-M-IDI. However, the CO2 capture capacity is significantly influenced by the pore size. Sc-M, with a favorable pore size, exhibits the highest capture capacity of 15.86 mmol g-1 at 298 K and 1.0 bar. This study elucidated the impact of isoreticular double interpenetration on the CO2 capture performance in MOFs.

Boosted ability of ZIF-8 for early-stage adsorption and degradation of chemical warfare agent simulants

Efficient adsorption of hazardous substances from the environment is crucial owing to the considerable risks they pose to both humans and ecosystems. Consequently, the development of porous materials with strong adsorption capabilities for hazardous substances, such as chemical warfare agents (CWAs), is pivotal for safeguarding human lives. Specifically, the early-stage adsorption proficiency of the adsorbents plays a vital role in determining their effectiveness as ideal adsorbents. Herein, we report the efficient adsorption of CWA simulants using thermally treated ZIF-8 (T-ZIF-8). The T-ZIF-8 samples were prepared by subjecting ZIF-8 to a simple thermal treatment, which resulted in a more positive surface charge with extra open metal sites. Although the pore volume of T-ZIF-8 decreased after thermal treatment, the positive surface charge of T-ZIF-8 proved advantageous for the adsorption of the CWA simulants. As a result, the adsorption capacity of T-ZIF-8 for the CWA simulants improved compared to that of pure ZIF-8. Notably, T-ZIF-8 exhibited a remarkably enhanced adsorption ability in the early stage of exposure to the CWA simulants, possibly due to the effective polar interactions between T-ZIF-8 and the simulants via the electron-rich components within the CWA simulants. Moreover, the enhanced adsorption capacity of T-ZIF-8 led to the fast degradation of simulant compared to pure ZIF-8. T-ZIF-8 also demonstrated excellent stability over three adsorption cycles. These findings highlight that T-ZIF-8 is an outstanding material for the early-stage adsorption and degradation of CWA simulants, offering high effectiveness and stability.

Robust Carborane-Based Metal-Organic Frameworks for Hexane Separation

Hexane isomers play a vital role as feedstocks and fuel additives in the petrochemical industry. However, their similar physical and chemical properties lead to significant challenges in the separation process. Traditional thermal separation techniques are energy-intensive and lead to significant carbon footprint penalties. As such, there is a growing demand for the development of less energy-intensive nonthermal separation methods. Adsorption-based separation methods, such as using solid sorbents or membranes, have emerged as promising alternatives to distillation. Here, we report the successful synthesis of two novel metal-organic frameworks (MOFs), NU-2004 and NU-2005, by incorporating a carborane-based three-dimensional (3D) linker and using aluminum and vanadium nodes, respectively. These MOFs exhibit exceptional thermal stability and structural rigidity compared to other MIL-53 analogues, which is further corroborated using synchrotron studies. Furthermore, the inclusion of the quasi-spherical 3D linker in NU-2004 demonstrates significant advancements in the separation of hexane isomers compared to other MIL MOFs containing two-dimensional (2D) and aliphatic 3D linkers.

Crystallographic and Compositional Dependence of Thermodynamic Stability of [Co(II), Cu(II), and Zn(II)] in 2-Methylimidazole-Containing Zeolitic Imidazolate Frameworks

We report the first systematic study experimentally investigating the effect of changes to the divalent metal node on the thermodynamic stability of three-dimensional (3D) and two-dimensional (2D) zeolitic imidazolate frameworks (ZIFs) based on 2-methylimidazolate linkers. In particular, the comparison of enthalpies of formation for materials based on cobalt, copper, and zinc suggests that the use of nodes with larger ionic radius metals leads to the stabilization of the porous sodalite topology with respect to the corresponding higher-density diamondoid (dia)-topology polymorphs. The stabilizing effect of metals is dependent on the framework topology and dimensionality. With previous works pointing to solvent-mediated transformation of 2D ZIF-L structures to their 3D analogues in the sodalite topology, thermodynamic measurements show that contrary to popular belief, the 2D frameworks are energetically stable, thus shedding light on the energetic landscape of these materials. Additionally, the calorimetric data confirm that a change in the dimensionality (3D → 2D) and the presence of structural water within the framework can stabilize structures by as much as 40 kJ·mol-1, making the formation of zinc-based ZIF-L material under such conditions thermodynamically preferred to the formation of both ZIF-8 and its dense, dia-topology polymorph.