Engineering of Pore Geometry for Ultrahigh Capacity Methane Storage in Mesoporous Metal–Organic Frameworks

Porous Aromatic Frameworks (PAFs)

Porous aromatic frameworks (PAFs) represent an important category of porous solids. PAFs possess rigid frameworks and exceptionally high surface areas, and, uniquely, they are constructed from carbon-carbon-bond-linked aromatic-based building units. Various functionalities can either originate from the intrinsic chemistry of their building units or are achieved by postmodification of the aromatic motifs using established reactions. Specially, the strong carbon-carbon bonding renders PAFs stable under harsh chemical treatments. Therefore, PAFs exhibit specificity in their chemistry and functionalities compared with conventional porous materials such as zeolites and metal organic frameworks. The unique features of PAFs render them being tolerant of severe environments and readily functionalized by harsh chemical treatments. The research field of PAFs has experienced rapid expansion over the past decade, and it is necessary to provide a comprehensive guide to the essential development of the field at this stage. Regarding research into PAFs, the synthesis, functionalization, and applications are the three most important topics. In this thematic review, the three topics are comprehensively explained and aptly exemplified to shed light on developments in the field. Current questions and a perspective outlook will be summarized.

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.

Tailoring the pore geometry and chemistry in microporous metal-organic frameworks for high methane storage working capacity

We realized that tailoring the pore size/geometry and chemistry, by virtue of alkynyl or naphthalene replacing phenyl within a series of isomorphic MOFs, can optimize methane storage working capacities, affording an exceptionally high working capacity of 203 cm3 (STP) cm-3 at 298 K and 5-80 bar.

Implementing Metal-Organic Frameworks for Natural Gas Storage

Methane can be stored by metal-organic frameworks (MOFs). However, there remain challenges in the implementation of MOFs for adsorbed natural gas (ANG) systems. These challenges include thermal management, storage capacity losses due to MOF packing and densification, and natural gas impurities. In this review, we discuss discoveries about how MOFs can be designed to address these three challenges. For example, Fe(bdp) (bdp2− = 1,4-benzenedipyrazolate) was discovered to have intrinsic thermal management and released 41% less heat than HKUST-1 (HKUST = Hong Kong University of Science and Technology) during adsorption. Monolithic HKUST-1 was discovered to have a working capacity 259 cm3 (STP) cm−3 (STP = standard temperature and pressure equivalent volume of methane per volume of the adsorbent material: T = 273.15 K, P = 101.325 kPa), which is a 50% improvement over any other previously reported experimental value and virtually matches the 2012 Department of Energy (Department of Energy = DOE) target of 263 cm3 (STP) cm−3 after successful packing and densification. In the case of natural gas impurities, higher hydrocarbons and other molecules may poison or block active sites in MOFs, resulting in up to a 50% reduction of the deliverable energy. This reduction can be mitigated by pore engineering.

Isotherms of individual pores by gas adsorption crystallography

Accurate measurements and assessments of gas adsorption isotherms are important to characterize porous materials and develop their applications. Although these isotherms provide knowledge of the overall gas uptake within a material, they do not directly give critical information concerning the adsorption behaviour of adsorbates in each individual pore, especially in porous materials in which multiple types of pore are present. Here we show how gas adsorption isotherms can be accurately decomposed into multiple sub-isotherms that correspond to each type of pore within a material. Specifically, two metal-organic frameworks, PCN-224 and ZIF-412, which contain two and three different types of pore, respectively, were used to generate isotherms of individual pores by combining gas adsorption measurements with in situ X-ray diffraction. This isotherm decomposition approach gives access to information about the gas uptake capacity, surface area and accessible pore volume of each individual pore, as well as the impact of pore geometry on the uptake and distribution of different adsorbates within the pores.

Theoretical Investigation of the Topology of Spiroborate-Linked Ionic Covalent Organic Frameworks (ICOFs)

A novel type of ionic covalent organic framework (ICOF) with a spiroborate linkage has been recently designed and synthesized by Zhang and co-workers (Ionic Covalent Organic Frameworks with Spiroborate Linkage, Angew. Chem. Int. Ed. 2016, 55, 1737-1741). The spiroborate-linked ICOFs exhibit high Brunauer-Emmett-Teller (BET) surface areas and significant amounts of H₂ and CH₄ uptakes, combined with excellent thermal and chemical stabilities. Inspired by the novel properties of ICOFs, with the aim of gaining better understanding of the structure of such spiroborate-linked ICOFs, a series of potential 3D network configurations of ICOFs connected with tetrahedral [BO₄ ]^- nodes were proposed, assuming the [BO₄ ]^- node in spiroborate segments takes a tetrahedral configuration. These ICOFs, in terms of 2D and 3D topology through torsional energy of the [BO₄ ]^- fragment, pore-size distribution, total energy of the framework, and gas adsorption properties were compared and systematically investigated by density functional theory calculations, molecular mechanics, and well-established Grand canonical Monte Carlo simulations. The results indicate that spiroborate-linked ICOFs are likely a mixture of various topologies. Among these architectures, the five-fold interpenetrating model shows the lowest energy and reasonable gas uptakes, therefore, it is considered to be the most possible structure. More importantly, the five-fold interpenetrating model, showing high CH₄ uptakes compared with several classic porous materials, represents a promising CH₄ storage material.

Tritopic Triazatruxene Ligands for Multicomponent Metal-Organic Frameworks

Multicomponent metal-organic frameworks (MOFs) are built up from multiple ligands that are geometrically distinct. These ligands occupy specific positions in the MOF lattice. Installing different functionalities at precise locations in the framework is an important step in making MOFs for specific applications. This can be achieved by designing functionalized ligands for multicomponent MOFs. Here, we report a simple synthetic procedure for a new tritopic triazatruxene based tricarboxylic acid, H₃ tat. We show that this ligand can be symmetrically derivatized with various substituents on its nitrogen centres. We report a new isoreticular series of well-ordered quaternary MOFs based on these new triazatruxene ligands together with two linear carboxylate ligands and Zn₄ O clusters. These MOFs are isostructural to the previously reported MUF-77 series and show similar high surface areas and large pore volumes. Furthermore, H-bonding between the NH sites of the incorporated triazatruxene ligands and guest molecules is employed to modify their fluorescence behavior.

Facile fabrication of hierarchical porous ZIF-8 for enhanced adsorption of antibiotics

Aiming for improve mass transfer rate of antibiotics adsorption from water, a strategy of building larger pores (>2 nm) in microporous MOFs has been put forward. However, most of reported approaches are complicated and inefficient. Herein, a facile one-spot approach to fabricate hierarchical porous Zeolitic Imidazolate Framework-8 (HpZIF-8) was developed, where poly(diallyldimethylammonium chloride) (PDDA) was selected as structure-directing agent to modulate the growth of microporous ZIF-8 (mZIF-8). The final products with meso- and macropores exhibit hierarchical porosity. The mechanism was a two-step process: First, crystal nucleus aggregated initiated by electrostatic interaction between cationic PDDA and deprotonated 2-MI anions. Second, Ostwald ripening process and orientated growth occurred with further growth of crystals. For removing Tetracycline Hydrochloride (TH) and Chloramphenicol (CP) from water, hierarchical porous HpZIF-8-10(D) (D = 1.0, 1.5, 2.0) showed larger adsorption capacity than mZIF-8-10 despite of decreased BET surface area, which could be attributed to novel hierarchical porous structures. The adsorption kinetics and isotherms of TH and CP by HpZIF-8-10(1.5) were analyzed. The strategy present here may provide new thoughts for designing more abundant MOF structures and further expand their application range.

Water-Stable Europium 1,3,6,8-Tetrakis(4-carboxylphenyl)pyrene Framework for Efficient C2H2/CO2 Separation

Compound {(Me₂NH₂)₃[Eu₇(μ₃-O)₂(TBAPy)₅(H₂O)₆]·12.5DMF} _n (JXNU-5), constructed from the 1,3,6,8-tetrakis(4-carboxylphenyl)pyrene (TBAPy^4-) ligand and one-dimensional (1D) europium carboxylate rods, is presented. JXNU-5 has a three-dimensional framework with 1D channels. The strong coordination bonds between Eu^III ions with high charge densities and carboxylate O atoms as well as strong π···π-stacking interactions between pyrenes lead to a water-resistant JXNU-5, which was verified by powder X-ray diffraction and surface area measurements. The breakthrough simulations and experiments demonstrate that an efficient C₂H₂/CO₂ (50/50 mixture) gas separation at ambient conditions was achieved with JXNU-5. The calculation results show that the dominating interactions between the absorbed C₂H₂ molecules and host framework are hydrogen bonds associated with the carboxylate O atoms exposed on the pores. Thus, an elegant example of a water-stable metal-organic framework for effective C₂H₂/CO₂ separation is demonstrated.

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.

The chemistry of multi-component and hierarchical framework compounds

This review is expected to provide a library of multi-component hierarchically porous compounds, which shall guide the state-of-the-art design of future porous materials with unprecedented tunability, synergism and precision.

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.

Tailoring the pore size and shape of the one-dimensional channels in iron-based MOFs for enhancing the methane storage capacity

A Fe-based MOF with narrow rectangular channels exhibited a comparable volumetric CH₄ uptake with benchmark materials (e.g. MOF-5, MOF-205, MOF-905-NO₂, and MOF-210).

Incorporating Heavy Alkanes in Metal-Organic Frameworks for Optimizing Adsorbed Natural Gas Capacity

Metal-organic frameworks (MOFs) as methane adsorbents are highly promising materials for applications such as methane-powered vehicles, flare gas capture, and field natural gas separation. Pre- and post-synthetic modification of MOFs have been known to help improve both the overall methane uptake as well as the working capacity. Here, a post-synthetic modification strategy to non-covalently modify MOF adsorbents for the enhancement of the natural gas uptake for the MOF material is introduced. In this study, PCN-250 adsorbents were doped with C₁₀ alkane and C₁₄ fatty acid and their impact on the methane uptake capabilities was investigated. It was found that even trace amounts of heavy hydrocarbons could considerably enhance the raw methane uptake of the MOF while still being regenerable. The doped hydrocarbons are presumably located at the mesoporous defects of PCN-250, thus optimizing the framework-methane interactions. These findings reveal a general approach that can be used to modify the MOF absorbents, improving their ability to be sustainable and renewable natural gas adsorption platforms.

Construction of Porous Aromatic Frameworks with Exceptional Porosity via Building Unit Engineering

The construction of excellent porous organic frameworks (POFs) with high surface areas and stability is always a tremendous challenge in synthetic chemistry. The geometric configuration and reactive group of building unit are crucial factors to influence the structure and porosity of the resulting product. Herein, the design, synthesis, and characterization of two porous aromatic framework (PAF) materials, named PAF-100 and PAF-101, are reported via a strategy of building unit engineering. PAF-100 and PAF-101 present high Brunauer-Emmett-Teller surface areas exceeding 5000 m² g^-1 and uniform pore size distributions. Furthermore, PAF-100 and PAF-101 show high methane uptake with value of 742 and 622 cm³ g^-1 , respectively, at 298 K and 70 bar. The successful synthesis of PAFs with exceptional porosity from engineered building unit is powerful for constructing highly porous POFs.

Mesoporous Metal-Organic Frameworks: Synthetic Strategies and Emerging Applications

Metal-organic frameworks (MOFs) have attracted much attention over the past two decades due to their highly promising applications not only in the fields of gas storage, separation, catalysis, drug delivery, and sensors, but also in relatively new fields such as electric, magnetic, and optical materials resulting from their extremely high surface areas, open channels and large pore cavities compared with traditional porous materials like carbon and inorganic zeolites. Particularly, MOFs involving pores within the mesoscopic scale possess unique textural properties, leading to a series of research in the design and applications of mesoporous MOFs. Unlike previous Reviews, apart from focusing on recent advances in the synthetic routes, unique characteristics and applications of mesoporous MOFs, this Review also mentions the derivatives, composites, and hierarchical MOF-based systems that contain mesoporosity, and technical boundaries and challenges brought by the drawbacks of mesoporosity. Moreover, this Review subsequently reveals promising perspectives of how recently discovered approaches to different morphologies of MOFs (not necessarily entirely mesoporous) and their corresponding performances can be extended to minimize the shortcomings of mesoporosity, thus providing a wider and brighter scope of future research into mesoporous MOFs, but not just limited to the finite progress in the target substances alone.

Syntheses of copper–iodine cluster-based frameworks for photocatalytic degradation of methylene blue

Cu–I cluster-based MOFs show a broad range of absorption in the visible region and exhibit excellent photocatalytic degradation of methylene blue dye under visible light.

Functionalization of MOFs via a mixed-ligand strategy: enhanced CO2 uptake by pore surface modification

Two isostructural MOFs based on rare quaternary SBUs with a new (3,3,4,6)-connected topology have been assembled, which possess high porosity with highly polar pore systems, and also exhibit high uptake for CO₂ and significant selectivity for CO₂ over CH₄.

Synthesis, crystal structure, and anti-breast cancer activity of a novel metal-porphyrinic complex [YK(TCPP)(OH)2·(solvents)x]

A novel heterometallic metal-porphyrinic framework (MPFs) built from Y and K ions as nods and meso-tetra(4-carboxyphenyl)porphyrin as linkers has been successfully synthesized and characterized. The single crystal X-ray diffraction indicated that this complex 1 exhibited a bilayered architecture of the porphyrins, which is seldom seen in MPFs. In addition, in vitro anticancer activity of complex 1 on three human breast cancer cells (BT474, SKBr-3 and ZR-75-30) was further determined.