Modulating the cavity micro-environments of Fe-MOFs for high-performance gas chromatographic separations

Precise modulation of cavity micro-environment in metal-organic frameworks (MOFs) is important for achieving significant separation performance. Herein, the Fe-MOF (MIL-142-BTB-BDC) with different tridentate and bidentate ligands to form cavities, was chosen as the platform to precisely modulate the cavity micro-environment and investigate the influence of cavity windows and cavity walls on gas chromatographic separations. Changing tridentate ligands on the cavity walls led to MIL-142-TATB-BDC while changing bidentate ligands on the cavity windows produced MIL-142-BTB-BDC-NH2. Mechanism investigation revealed that for MIL-142-BTB-BDC and MIL-142-TATB-BDC, altering the ligands of cavity walls had little influence on the thermodynamic interactions between MOFs and analytes while slightly reducing the kinetic diffusion rate of analytes. On the contrary, introducing amino groups on cavity windows in MIL-142-BTB-BDC-NH2 not only increased the thermodynamic interactions with analytes but also slowed down the kinetic diffusion of analytes, which resulted in poor separation performance of the MIL-142-BTB-BDC-NH2 coated column. This work provides a guide to precisely modulating the cavity micro-environment and analyzing the relationship between the cavity micro-environment and application properties of MOFs.

Rational Design of Improved Ru Containing Fe-Based Metal-Organic Framework (MOF) Photoanode for Artificial Photosynthesis

Metal-Organic Frameworks (MOFs) recently emerged as a new platform for the realization of integrated devices for artificial photosynthesis. However, there remain few demonstrations of rational tuning of such devices for improved performance. Here, a fast molecular water oxidation catalyst working via water nucleophilic attack is integrated into the MOF MIL-142, wherein Fe3O nodes absorb visible light, leading to charge separation. Materials are characterized by a range of structural and spectroscopic techniques. New, [Ru(tpy)(Qc)(H2O)]+ (tpy = 2,2':6',2″-terpyridine and Qc = 8-quinolinecarboxylate)-doped Fe MIL-142 achieved a high photocurrent (1.6 × 10-3 A·cm-2) in photo-electrocatalytic water splitting at pH = 1. Unassisted photocatalytic H2 evolution is also reported with Pt as the co-catalyst (4.8 µmol g-1 min-1). The high activity of this new system enables hydrogen gas capture from an easy-to-manufacture, scaled-up prototype utilizing MOF deposited on FTO glass as a photoanode. These findings provide insights for the development of MOF-based light-driven water-splitting assemblies utilizing a minimal amount of precious metals and Fe-based photosensitizers.

Expanding the Reticular Chemistry Building Block Library toward Highly Connected Nets: Ultraporous MOFs Based on 18-Connected Ternary, Trigonal Prismatic Superpolyhedra

The chemistry of metal-organic frameworks (MOFs) continues to expand rapidly, providing materials with diverse structures and properties. The reticular chemistry approach, where well-defined structural building blocks are combined together to form crystalline open framework solids, has greatly accelerated the discovery of new and important materials. However, its full potential toward the rational design of MOFs relies on the availability of highly connected building blocks because these greatly reduce the number of possible structures. Toward this, building blocks with connectivity greater than 12 are highly desirable but extremely rare. We report here the discovery of novel 18-connected, trigonal prismatic, ternary building blocks (tbb's) and their assembly into unique MOFs, denoted as Fe-tbb-MOF-x (x: 1, 2, 3), with hierarchical micro- and mesoporosity. The remarkable tbb is an 18-c supertrigonal prism, with three points of extension at each corner, consisting of triangular (3-c) and rectangular (4-c) carboxylate-based organic linkers and trigonal prismatic [Fe3(μ3-Ο)(-COO)6]+ clusters. The tbb's are linked together by an 18-c cluster made of 4-c ligands and a crystallographically distinct Fe3(μ3-Ο) trimer, forming overall a 3-D (3,4,4,6,6)-c five nodal net. The hierarchical, highly porous nature of Fe-tbb-MOF-x (x: 1, 2, 3) was confirmed by recording detailed sorption isotherms of Ar, CH4, and CO2 at 87, 112, and 195 K, respectively, revealing an ultrahigh BET area (4263-4847 m2 g-1) and pore volume (1.95-2.29 cm3 g-1). Because of the observed ultrahigh porosities, the H2 and CH4 storage properties of Fe-tbb-MOF-x were investigated, revealing well-balanced high gravimetric and volumetric deliverable capacities for cryoadsorptive H2 storage (11.6 wt %/41.4 g L-1, 77 K/100 bar-160 K/5 bar), as well as CH4 storage at near ambient temperatures (367 mg g-1/160 cm3 STP cm-3, 5-100 bar at 298 K), placing these materials among the top performing MOFs. The present work opens new directions to apply reticular chemistry for the construction of novel MOFs with tunable porosities based on contracted or expanded tbb analogues.

Multicomponent Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are constructed from metal ions or clusters and organic linkers. Typical MOFs are rather simple, comprising just one type of joint and linker. An additional degree of structural complexity can be introduced by using multiple different components that are assembled into the same framework In the early days of MOF chemistry, conventional wisdom held that attempting to prepare frameworks starting from such a broad set of components would lead to multiple different phases. However, this review highlights how this view was mistaken and frameworks comprising multiple different components can be deliberately designed and synthesized. When coupled to structural order and periodicity, the presence of multiple components leads to exceptional functional properties that can be understood at the atomic level.

Isoreticular Contraction of Metal-Organic Frameworks Induced by Cleavage of Covalent Bonds

Isoreticular chemistry, in which the organic or inorganic moieties of reticular materials can be replaced without destroying their underlying nets, is a key concept for synthesizing new porous molecular materials and for tuning or functionalization of their pores. Here, we report that the rational cleavage of covalent bonds in a metal-organic framework (MOF) can trigger their isoreticular contraction, without the need for any additional organic linkers. We began by synthesizing two novel MOFs based on the MIL-142 family, (In)BCN-20B and (Sc)BCN-20C, which include cleavable as well as noncleavable organic linkers. Next, we selectively and quantitatively broke their cleavable linkers, demonstrating that various dynamic chemical and structural processes occur within these structures to drive the formation of isoreticular contracted MOFs. Thus, the contraction involves breaking of a covalent bond, subsequent breaking of a coordination bond, and finally, formation of a new coordination bond supported by structural behavior. Remarkably, given that the single-crystal character of the parent MOF is retained throughout the entire transformation, we were able to monitor the contraction by single-crystal X-ray diffraction.

A Novel Aluminum-Based Metal-Organic Framework with Uniform Micropores for Trace BTEX Adsorption

Metal-organic frameworks (MOFs) are potential porous adsorbents for benzene, toluene, ethylbenzene and xylene (BTEX). A novel MOF, using low toxic aluminum (Al) as the metal, named as ZJU-620(Al), with uniform micropore size of 8.37±0.73 Å and specific surface area of 1347 m²  g^-1 , was synthesized. It is constructed by one-dimensional rod-shaped AlO₆ clusters, formate ligands and 4,4',4''-(2,4,6-trimethylbenzene-1,3,5-triyl) tribenzoic ligands. ZJU-620(Al) exhibits excellent chemical-thermal stability and adsorption for trace BTEX, e.g., benzene adsorption of 3.80 mmol g^-1 at P/P₀ =0.01 and 298 K, which is the largest one reported. Using Grand Canonical Monte Carlo simulations and Single-crystal X-ray diffraction analyses, it was observed that the excellent adsorption could be attributed to the high affinity of BTEX molecules in ZJU-620(Al) micropores because the kinetic diameters of BTEX are close up to the pore size of ZJU-620(Al).

Highly Robust Microporous Metal-Organic Frameworks for Efficient Ethylene Purification under Dry and Humid Conditions

Two C₂ H₆ -selective metal-organic framework (MOF) adsorbents with ultrahigh stability, high surface areas, and suitable pore size have been designed and synthesized for one-step separation of ethane/ethylene (C₂ H₆ /C₂ H₄ ) under humid conditions to produce polymer-grade pure C₂ H₄ . Experimental results reveal that these two MOFs not only adsorb a high amount of C₂ H₆ but also display good C₂ H₆ /C₂ H₄ selectivity verified by fixed bed column breakthrough experiments. Most importantly, the good water stability and hydrophobic pore environments make these two MOFs capable of efficiently separating C₂ H₆ /C₂ H₄ under humid conditions, exhibiting the benchmark performance among all reported adsorbents for separation of C₂ H₆ /C₂ H₄ under humid conditions. Moreover, the affinity sites and their static adsorption energies were successfully revealed by single crystal data and computation studies. Adsorbents described in this work can be used to address major chemical industrial challenges.

Imparting Multifunctionality in Zr-MOFs Using the One-Pot Mixed-Linker Strategy: The Effect of Linker Environment and Enhanced Pollutant Removal

The development of mixed-linker metal-organic frameworks (MOFs) is an efficient strategy to improve the performance of MOFs. Herein, we successfully integrate tetrakis(4-carboxyphenyl)porphyrin (TCPP) into different Zr-MOFs via a facile one-pot solvothermal synthesis while preserving the integrity of their frameworks. The functional groups, length of primary linkers, and the inner pore structure significantly affected the properties of the synthesized TCPP@MOFs, such as surface area, average pore size, and ¹O₂ productivity. Among them, TCPP@PCN-777 demonstrated the largest surface area (2386 cm²/g, as measured by N₂ uptake) and the highest ¹O₂ generation rate (1.15 h^-1, [¹O₂]_ss = 2.66 × 10^-12 M) under irradiation. The TCPP loading was also shown to affect the crystal phase, morphology, surface area, and photochemical properties of the synthesized MOFs. Therefore, TCPP@PCN-777s with various TCPP loadings were synthesized to investigate the optimum loading. The optimized TCPP@MOF, TCPP@PCN-777-30, was evaluated for its removal of model contaminant ranitidine (RND) through both adsorption and photodegradation. TCPP@PCN-777-30 showed a higher adsorption capacity toward RND than both the parent MOF (PCN-777) and commercially available activated carbon, and effectively degraded RND in aqueous solution (>99% photodegradation in 1 h). With irradiation, TCPP@PCN-777-30 showed a minimal loss in adsorption efficiency over four consecutive treatment cycles, confirming the reusability of the material enabled through the incorporation of TCPP into the MOF structure. This work not only developed an efficient multifunctional material for environmental remediation but also forwarded knowledge on the effect of linker environment (i.e., functional groups, framework structure, and linker ratio) on the properties of TCPP@MOFs to guide future research on mixed-linker MOFs.

Biomimetic catalysts of iron-based metal-organic frameworks with high peroxidase-mimicking activity for colorimetric biosensing

The field of metal-organic framework (MOF)-based biomimetic catalysts has achieved great progress but is still in its infancy. The systematic investigation of the tailored construction of MOF-based biomimetic catalysts is required for further development. Herein, two iron-based MOFs, namely, [(Fe₃O)₂(H₂O)₄(HCOO)(L)₂]_n (HUST-5: H₆L = hexakis(4-formylphenoxy) cyclotriphosphazene; HUST = Huazhong University of Science and Technology) and [(Fe₃O)(H₂O)₃(L)]_n (HUST-7) have been fabricated through the assembly of different iron clusters and hexa-carboxylate ligand under the control of the added acid species. The two MOFs exhibit distinct secondary building units (SBUs) and topological structures, which could be used as biomimetic catalysts for the systematic comparisons of structural characteristics and the catalytic activity. Both MOFs possess catalytic activity similar to that of natural peroxidases towards the catalysis of the oxidation of a variety of substrates. Significantly, HUST-5 and HUST-7 can effectively catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ accompanied by significant colorimetric biosensing. With same compositions, different catalytic performances were obtained due to differences in the porous structures and characteristics of SBUs in two Fe-MOFs, which was also validated by theoretical calculation results. Furthermore, the phenomenon of colorimetric biosensing could be significantly suppressed by the addition of ascorbic acid (AA) during the oxidation process of TMB. It was observed from these findings that a facile colorimetric biosensing platform for detecting H₂O₂ and ascorbic acid has been successfully explored. Therefore, this work provides another unique perspective for the tailor-made preparation of stable MOF-based peroxidase mimics with excellent catalytic performance and colorimetric biosensing.

A window-space-directed assembly strategy for the construction of supertetrahedron-based zeolitic mesoporous metal-organic frameworks with ultramicroporous apertures for selective gas adsorption

Despite their scarcity due to synthetic challenges, supertetrahedron-based metal-organic frameworks (MOFs) possess intriguing architectures, diverse functionalities, and superb properties that make them in-demand materials. Employing a new window-space-directed assembly strategy, a family of mesoporous zeolitic MOFs have been constructed herein from corner-shared supertetrahedra based on homometallic or heterometallic trimers [M3(OH/O)(COO)6] (M3 = Co3, Ni3 or Co2Ti). These MOFs consisted of close-packed truncated octahedral cages possessing a sodalite topology and large β-cavity mesoporous cages (∼22 Å diameter) connected by ultramicroporous apertures (∼5.6 Å diameter). Notably, the supertetrahedron-based sodalite topology MOF combined with the Co2Ti trimer exhibited high thermal and chemical stability as well as the ability to efficiently separate acetylene (C2H2) from carbon dioxide (CO2).

Isoreticular chemistry of scandium analogues of the multicomponent metal–organic framework MIL-142

MIL-142(Sc) is prepared and the limits of the isoreticular substitution of each linker type are explored and characterised by single-crystal XRD.

Hierarchically porous metal–organic frameworks: synthetic strategies and applications

Despite numerous advantages, applications of conventional microporous metal–organic frameworks (MOFs) are hampered by their limited pore sizes, such as in heterogeneous catalysis and guest delivery, which usually involve large molecules. Construction of hierarchically porous MOFs (HP-MOFs) is vital to achieve the controllable augmentation of MOF pore size to mesopores or even macropores, which can enhance the diffusion kinetics of guests and improve the storage capacity. This review article focuses on recent advances in the methodology of HP-MOF synthesis, covering preparation of HP-MOFs with intrinsic hierarchical pores, and modulated, templated and template-free synthetic strategies for HP-MOFs. The key factors which affect the formation of HP-MOF architectures are summarized and discussed, followed by a brief review of their applications in heterogeneous catalysis and guest encapsulation. Overall, this review presents a roadmap that will guide the future design and development of HP-MOF materials with molecular precision and mesoscopic complexity.

Cooperative Sieving and Functionalization of Zr Metal-Organic Frameworks through Insertion and Post-Modification of Auxiliary Linkers

A major goal of metal-organic framework (MOF) research is to control the structures and functions of materials in accordance with their specific applications. However, due to the flexible coordination modes between metal ions and organic linkers in MOFs, it is still challenging to rationally assemble a framework with deliberate structures and desired functional groups. Sometimes, two or more phases coexist in a one-pot reaction, making them difficult to separate. To this end, sieving and purification of MOF mixtures become vital for the following application. Herein, we demonstrate that the formation of zirconium-based MOFs (Zr-MOFs) can be regulated in a wider two-dimensional scale by thermodynamics using auxiliary linkers. The auxiliary linkers favor the formation of the targeted Zr-MOF by selectively binding to its coordination vacancies and therefore increasing its formation enthalpy to achieve the sieving of MOF mixture. Furthermore, the resulting mixed-linkers MOFs not only maintain porosities but also contain the installed auxiliary linkers as chemical handles to further incorporate functional groups, providing the possibility of introduction of active sites through post-modification. Finally, this synthetic strategy was applied to assemble a cooperative catalytic system in a MOF platform for CO₂ cycloaddition with epoxides. To the best of our knowledge, this is the first example of sieving and functionalization of MOFs through insertion and post-modification of auxiliary linkers.

Efficient Conversion of CO2 via Grafting Urea Group into a [Cu2(COO)4]-Based Metal-Organic Framework with Hierarchical Porosity

The assembly of mixed [1,1';3',1'']terphenyl-4,5',4''-tricarboxylic acid (H₃TPTC) and [1,1'-biphenyl]-4,4'-dicarboxylic acid (H₂BPDC), 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H₂BPDC-NH₂), or 6-oxo-6,7-dihydro-5H-dibenzo[ d, f][1,3]diazepine-3,9-dicarboxylic acid (H₂BPDC-Urea) with Cu²⁺ ion generated the corresponding copper-paddlewheel-based metal-organic framework (MOF) [Cu₅(TPTC)₃(BPDC)_0.5(H₂O)₅] (1), [Cu₅(TPTC)₃(BPDC-NH₂)_0.5(H₂O)₅] (1-NH₂), or [Cu₅(TPTC)₃(BPDC-Urea)_0.5(H₂O)₅] (1-Urea). They are isostructural with hierarchical porosity, consisting of zero-dimensional cage (19.2 Å × 18.9 Å) and one-dimensional pillar channel (29.7 Å × 15.1 Å) in a manner of face sharing. Platon analyses revealed the porous volume ratios are 80.2%, 80.0%, and 77.8% for 1, 1-NH₂, and for 1-Urea, respectively. Thermogravimetric measurements suggested 53, 51, and 48 wt % guest molecules in 1, 1-NH₂, and 1-Urea, respectively. 1-NH₂ and 1-Urea were precisely functionalized via the introduction of amino and urea functional groups into the pillar channels. The constructed MOF 1-Urea, incorporating both exposed copper active sites and accessible urea functional groups to substrates, presents high efficiency on catalytic CO₂ cycloaddition with propene oxide to produce cyclic carbonate in the yield of 98% with a TOF value of 136 h^-1 at 1 atm and room temperature. This synergic effect provides a new strategy for designing high-efficient catalysts for CO₂ chemical conversion under ambient conditions.

Development of a new Lindqvist-like Fe6 cluster secondary building unit for MOFs

In this work, a novel Lindqvist-like Fe6 cluster was introduced as an SBU to construct a new family of Fe-cluster organic frameworks.

A Zn(ii) metal–organic framework constructed by a mixed-ligand strategy for CO2 capture and gas separation

A microporous Zn(ii) metal–organic framework has been assembled using a mixed-ligand strategy, and it exhibits high capture ability for CO₂ and good selectivity for CO₂/CH₄, C₂H₆/CH₄ and C₃H₈/CH₄.

Exploring the dicationic gemini surfactant for the generation of mesopores: a step towards the construction of a hierarchical metal–organic framework

The dicationic gemini surfactant cooperatively self-assembles with the Cu-BTC precursors to form a hierarchical microporous–mesoporous metal–organic framework.

Metal-Organic Frameworks for Heterogeneous Basic Catalysis

Great attention has been given to metal-organic frameworks (MOFs)-derived solid bases because of their attractive structure and catalytic performance in various organic reactions. The extraordinary skeleton structure of MOFs provides many possibilities for incorporation of diverse basic functionalities, which is unachievable for conventional solid bases. The past decade has witnessed remarkable advances in this vibrant research area; however, MOFs for heterogeneous basic catalysis have never been reviewed until now. Therefore, a review summarizing MOFs-derived base catalysts is highly expected. In this review, we present an overview of the recent progress in MOFs-derived solid bases covering preparation, characterization, and catalytic applications. In the preparation section, the solid bases are divided into two categories, namely, MOFs with intrinsic basicity and MOFs with modified basicity. The basicity can originate from either metal sites or organic ligands. Different approaches used for generation of basic sites are included, and each approach is described with representative examples. The fundamental principles for the design and fabrication of MOFs with basic functionalities are featured. In the characterization section, experimental techniques and theoretical calculations employed for characterization of basic MOFs are summarized. Some representive experimental techniques, such as temperature-programmed desorption of CO₂ (CO₂-TPD) and infrared (IR) spectra of different probing molecules, are covered. Following preparation and characterization, the catalytic applications of MOFs-derived solid bases are dealt with. These solid bases have potential to catalyze some well-known "base-catalyzed reactions" like Knoevenagel condensation, aldol condensation, and Michael addition. Meanwhile, in contrast to conventional solid bases, MOFs show some different catalytic properties due to their special structural and surface properties. Remarkably, characteristic features of MOFs-derived solid bases are described by comparing with conventional inorganic counterparts, keeping in mind the current opportunities and challenges in this field.

The electrochemical properties, nitrogen adsorption, and photocatalytic activities of three 3D metal–organic frameworks bearing the rigid terphenyl tetracarboxylates ligands

Three new 3D complexes derived from the rigid terphenyl tetracarboxylates ligands with different transition metal ions have been successfully prepared. In addition, the title complexes exhibit different properties.