A Controllable Gate Effect in Cobalt(II) Organic Frameworks by Reversible Structure Transformations

Metal-Organic Framework Materials for the Separation and Purification of Light Hydrocarbons

The separation and purification of light hydrocarbons (LHs) mixtures is one of the most significantly important but energy demanding processes in the petrochemical industry. As an alternative technology to energy intensive traditional separation methods, such as distillation, absorption, extraction, etc., adsorptive separation using selective solid adsorbents could potentially not only lower energy cost but also offer higher efficiency. The need to develop solid materials for the efficiently selective adsorption of LHs molecules, under mild conditions, is therefore of paramount importance and urgency. Metal-organic frameworks (MOFs), emerging as a relatively new class of porous organic-inorganic hybrid materials, have shown promise for addressing this challenging task due to their unparalleled features. Herein, recent advances of using MOFs as separating agents for the separation and purification of LHs, including the purification of CH₄ , and the separations of alkynes/alkenes, alkanes/alkenes, C₅ -C₆ -C₇ normal/isoalkanes, and C₈ alkylaromatics, are summarized. The relationships among the structural and compositional features of the newly synthesized MOF materials and their separation properties and mechanisms are highlighted. Finally, the existing challenges and possible research directions related to the further exploration of porous MOFs in this very active field are also discussed.

The crystal structure of poly[(μ3-2,4,6-tris[4-(1H-imidazol-1-yl)phenyl]-1,3,5-triazine-k3N:N′:N′′)-(nitrato-k2O,O)-(nitrato-k1O)zinc(II)] - N,N-dimethylacetamide (1/2), C38H39N13O8Zn

C38H39N13O8Zn, monoclinic, P21/c (no. 14), a = 13.0155(7) Å, b = 32.7011(14) Å, c = 9.8464(4) Å, β = 110.228(1)°, V = 3932.4(3) Å3, Z = 4, Rgt(F) = 0.0476, wRref(F2) = 0.1358, T = 150(2) K.

The crystal structure of 2,4,6-tris[4-(1H-imidazol-1-yl)phenyl]-1,3,5-triazine — dimethylformamide (1/1), C33H28N10O

C33H28N10O, triclinic, P1̄ (no. 2), a = 14.7148(9) Å, b = 15.5185(10) Å, c = 16.1128(9) Å, α = 113.219(6)°, β = 96.735(5)°, γ = 115.259(6)°, V = 2872.0(3) Å3, Z = 4, Rgt(F) = 0.0426, wRref(F2) = 0.1108, T = 150(2) K.

Enhanced Gas Uptake in a Microporous Metal-Organic Framework via a Sorbate Induced-Fit Mechanism

Physical adsorption of gas molecules in microporous materials is an exothermic process, with desorption entropy driving a decrease in uptake with temperature. Enhanced gas sorption with increasing temperature is rare in porous materials and is indicative of sorbate initiated structural change. Here, sorption of C₂H₆, C₃H₆, and C₃H₈ in a flexible microporous metal-organic framework (MOF) {Cu(FPBDC)]·DMF}_n (NKU-FlexMOF-1) (H₂FPBDC = 5-(5-fluoropyridin-3-yl)-1,3-benzenedicarboxylic acid) that increases with rising temperature over a practically useful temperature and pressure range is reported along with other small molecule and hydrocarbon sorption isotherms. Single X-ray diffraction studies, temperature-dependent gas sorption isotherms, in situ and variable temperature powder X-ray diffraction experiments, and electronic structure calculations were performed to characterize the conformation-dependent sorption behavior in NKU-FlexMOF-1. In total, the data supports that the atypical sorption behavior is a result of loading-dependent structural changes in the flexible framework of NKU-FlexMOF-1 induced by sorbate-specific guest-framework interactions. The sorbates cause subtle adaptations of the framework distinct to each sorbate providing an induced-fit separation mechanism to resolve chemically similar hydrocarbons through highly specific sorbate-sorbent interactions. The relevant intermolecular contacts are shown to be predominantly repulsion and dispersion interactions. NKU-FlexMOF-1 is also found to be stable in aqueous solutions including toleration of pH changes. These experiments demonstrate the potential of this flexible microporous MOF for cost and energy efficient industrial hydrocarbon separation and purification processes. The efficacy for the separation of C₃H₆/C₃H₈ mixtures is explicitly demonstrated using NKU-FlexMOF-1a (i.e., activated NKU-FlexMOF-1) for a particular useful temperature range.

Ligand-Rearrangement-Induced Transformation from a 3D Supramolecular Network to a Discrete Octanuclear Cluster: A Good Detector for Pb2+ and Cr2O7 2

A three-dimensional supramolecular framework containing both left- and right-handed helixes, {[Zn(TTPA)Cl₂]·1.63H₂O} _n (1), has been converted to a novel octanuclear cluster containing a metal-organic framework, [Zn₈(ptptp)₆Cl₂](ClO₄)₂·4CH₃OH·2H₂O (1b), induced by ligand rearrangement during a solvothermal process. The luminescent properties of 1b indicate that the material can act as a selective probe toward Pb²⁺ and Cr₂O₇ ^2-.

Defective hierarchical porous copper-based metal-organic frameworks synthesised via facile acid etching strategy

Introducing hierarchical pore structure to microporous materials such as metal-organic frameworks (MOFs) can be beneficial for reactions where the rate of reaction is limited by low rates of diffusion or high pressure drop. This advantageous pore structure can be obtained by defect formation, mostly via post-synthetic acid etching, which has been studied extensively on water-stable MOFs. Here we show that a water-unstable HKUST-1 MOF can also be modified in a corresponding manner by using phosphoric acid as a size-selective etching agent and a mixture of dimethyl sulfoxide and methanol as a dilute solvent. Interestingly, we demonstrate that the etching process which is time- and acidity- dependent, can result in formation of defective HKUST-1 with extra interconnected hexagonal macropores without compromising on the bulk crystallinity. These findings suggest an intelligent scalable synthetic method for formation of hierarchical porosity in MOFs that are prone to hydrolysis, for improved molecular accessibility and diffusion for catalysis.

Structural Engineering of Low-Dimensional Metal-Organic Frameworks: Synthesis, Properties, and Applications

Low-dimensional metal-organic frameworks (LD MOFs) have attracted increasing attention in recent years, which successfully combine the unique properties of MOFs, e.g., large surface area, tailorable structure, and uniform cavity, with the distinctive physical and chemical properties of LD nanomaterials, e.g., high aspect ratio, abundant accessible active sites, and flexibility. Significant progress has been made in the morphological and structural regulation of LD MOFs in recent years. It is still of great significance to further explore the synthetic principles and dimensional-dependent properties of LD MOFs. In this review, recent progress in the synthesis of LD MOF-based materials and their applications are summarized, with an emphasis on the distinctive advantages of LD MOFs over their bulk counterparties. First, the unique physical and chemical properties of LD MOF-based materials are briefly introduced. Synthetic strategies of various LD MOFs, including 1D MOFs, 2D MOFs, and LD MOF-based composites, as well as their derivatives, are then summarized. Furthermore, the potential applications of LD MOF-based materials in catalysis, energy storage, gas adsorption and separation, and sensing are introduced. Finally, challenges and opportunities of this fascinating research field are proposed.

Flexibility in Metal–Organic Frameworks: A Basic Understanding

Much has been written about the fundamental aspects of the metal–organic frameworks (MOFs). Still, details concerning the MOFs with structural flexibility are not comprehensively understood. However, a dramatic increase in research activities concerning rigid MOFs over the years has brought deeper levels of understanding for their properties and applications. Nonetheless, robustness and flexibility of such smart frameworks are intriguing for different research areas such as catalysis, adsorption, etc. This manuscript overviews the different aspects of framework flexibility. The review has touched lightly on several ideas and proposals, which have been demonstrated within the selected examples to provide a logical basis to obtain a fundamental understanding of their synthesis and behavior to external stimuli.

Specific K+ Binding Sites as CO2 Traps in a Porous MOF for Enhanced CO2 Selective Sorption

Metal-organic frameworks (MOFs) can be fine-tuned to boost sorbent-sorbate interactions in order to improve gas sorption and separation performance, but the design of MOFs with ideal structural features for gas separation applications remains a challenge. Herein it is reported that unsaturated alkali metal sites can be immobilized in MOFs through a tetrazole based motif and that gas affinity can thereby be boosted. In the prototypal MOF of this type-NKU-521 (NKU denotes Nankai University), K⁺ cations are effectively embedded in a trinuclear Co²⁺ -tetrazole coordination motif. The embedded K⁺ sites are exposed to the pores of NKU-521 through water removal, and the isosteric heat (Q_st ) for CO₂ is boosted to 41 kJ mol^-1 . The nature of the binding site is validated by molecular simulations and structural characterization. The K⁺ cations in effect serve as gas traps and boost the CO₂ -framework affinity, as measured by the Q_st , by 24%. In addition, the impact of unsaturated alkali metal sites upon the separation of hydrocarbons is evaluated for the first time in MOFs using ideal adsorbed solution theory (IAST) calculations and column breakthrough experiments. The results reveal that the presence of exposed K⁺ sites benefits gas sorption and hydrocarbon separation performances of this MOF.

Structural dynamics of a metal-organic framework induced by CO2 migration in its non-uniform porous structure

Stimuli-responsive behaviors of flexible metal–organic frameworks (MOFs) make these materials promising in a wide variety of applications such as gas separation, drug delivery, and molecular sensing. Considerable efforts have been made over the last decade to understand the structural changes of flexible MOFs in response to external stimuli. Uniform pore deformation has been used as the general description. However, recent advances in synthesizing MOFs with non-uniform porous structures, i.e. with multiple types of pores which vary in size, shape, and environment, challenge the adequacy of this description. Here, we demonstrate that the CO₂-adsorption-stimulated structural change of a flexible MOF, ZIF-7, is induced by CO₂ migration in its non-uniform porous structure rather than by the proactive opening of one type of its guest-hosting pores. Structural dynamics induced by guest migration in non-uniform porous structures is rare among the enormous number of MOFs discovered and detailed characterization is very limited in the literature. The concept presented in this work provides new insights into MOF flexibility.

Metal–organic frameworks that undergo structural transitions in response to external stimuli are promising for gas storage, but the mechanisms of such dynamics are poorly understood. Here the authors show that the structural transformation of ZIF-7 is induced by CO₂ migration through its non-uniform porous structure.

Two metal–organic zeolites for highly sensitive and selective sensing of Tb3+

Two metal–organic zeolites can sensitize the luminescence of the Tb³⁺ ion by energy transfer between the host frameworks and the guest Tb³⁺ ion, thus serving as Tb³⁺ ion sensors.

Single-side and double-side swing behaviours of a flexible porous coordination polymer with a rhombic-lattice structure

A rhombic porous coordination polymer can show special reversible–irreversible structural transformations with single-side and double-side swing, as well as sophisticated bond reconstitution behaviours.

New linker installation in metal–organic frameworks

Installation of new organic components (including multitopic anionic and neutral linkers) in MOFs and MOPs for multicomponent materials.

CoII immobilized on an aminated magnetic metal–organic framework catalyzed C–N and C–S bond forming reactions: a journey for the mild and efficient synthesis of arylamines and arylsulfides

The catalytic activity of Fe₃O₄@AMCA-MIL53(Al)-NH₂-Co^II NPs as a novel and inexpensive catalyst was investigated in the C–N and C–S cross coupling reactions.

Guest-Induced Switchable Breathing Behavior in a Flexible Metal-Organic Framework with Pronounced Negative Gas Pressure

Flexible metal-organic frameworks (MOFs) have attracted great interest for their dynamically structural transformability in response to external stimuli. Herein, we report a switchable "breathing" or "gate-opening" behavior associated with the phase transformation between a narrow pore (np) and a large pore (lp) in a flexible pillared-layered MOF, denoted as MOF-1 as, which is also confirmed by SCXRD and PXRD. The desolvated phase (MOF-1 des) features a unique stepwise adsorption isotherm for N₂ coupled with a pronounced negative gas adsorption pressure. For comparison, however, no appreciable CO₂ adsorption and gate-opening phenomenon with stepwise sorption can be observed. Furthermore, the polar micropore walls decorated with thiophene groups in MOF-1 des reveals the selective sorption of toluene over benzene and p-xylene associated with self-structural adjustment in spite of the markedly similar physicochemical properties of these vapor molecules.

Hyperexpandable, self-healing macromolecular crystals with integrated polymer networks

The formation of condensed matter typically involves a trade-off between structural order and flexibility. As the extent and directionality of interactions between atomic or molecular components increase, materials generally become more ordered but less compliant, and vice versa. Nevertheless, high levels of structural order and flexibility are not necessarily mutually exclusive; there are many biological (such as microtubules1,2, flagella 3 , viruses4,5) and synthetic assemblies (for example, dynamic molecular crystals6-9 and frameworks10-13) that can undergo considerable structural transformations without losing their crystalline order and that have remarkable mechanical properties8,14,15 that are useful in diverse applications, such as selective sorption 16 , separation 17 , sensing 18 and mechanoactuation 19 . However, the extent of structural changes and the elasticity of such flexible crystals are constrained by the necessity to maintain a continuous network of bonding interactions between the constituents of the lattice. Consequently, even the most dynamic porous materials tend to be brittle and isolated as microcrystalline powders 14 , whereas flexible organic or inorganic molecular crystals cannot expand without fracturing. Owing to their rigidity, crystalline materials rarely display self-healing behaviour 20 . Here we report that macromolecular ferritin crystals with integrated hydrogel polymers can isotropically expand to 180 per cent of their original dimensions and more than 500 per cent of their original volume while retaining periodic order and faceted Wulff morphologies. Even after the separation of neighbouring ferritin molecules by 50 ångströms upon lattice expansion, specific molecular contacts between them can be reformed upon lattice contraction, resulting in the recovery of atomic-level periodicity and the highest-resolution ferritin structure reported so far. Dynamic bonding interactions between the hydrogel network and the ferritin molecules endow the crystals with the ability to resist fragmentation and self-heal efficiently, whereas the chemical tailorability of the ferritin molecules enables the creation of chemically and mechanically differentiated domains within single crystals.

Four 3D coordination polymers based on layers with single syn-anti carboxylate bridges: synthesis, structures, and magnetic properties

Four novel coordination polymers (CPs) based on a new 4-(3,5-dicarboxylphenyl) picolinic acid ligands (H3L), [M3(L)2(H2O)6]·4H2O (M3 = Mn3, 1; Co3, 2; Ni3, 3, Co1.01Ni1.99, 4), have been hydrothermally synthesized, and structurally and magnetically characterized. In these isomorphous CPs, octahedrally coordinated metal ions are linked by the single syn-anti carboxylate bridge (μ-COO) to give linear trinuclear motifs. The motifs are connected through the other single syn-anti carboxylate bridge (μ-COO) to give a 2D (4,4) layer, and the layers are interlinked by the L ligands into 3D frameworks. Magnetic measurement indicates that antiferromagnetic interactions between metal ions are mediated through the single syn-anti carboxylate bridges in 1 and 2, while the same carboxylate bridges in 3 transmit ferromagnetic couplings. The bimetallic CP 4 shows interesting complicated magnetic behaviors due to the competition effect of Co(ii) and Ni(ii) ions.

Ship in a breakable bottle: fluoride-induced release of an organic molecule from a Pr(iii)-linked molecular cage

A 3-dimensional networked molecular cage, , has been synthesized. This macrocycle-based framework was prepared from a solvothermal reaction involving a flexible organic building block, calix[4]pyrrole dibenzoic acid (H2), and Pr(NO3)3·6H2O. A unique feature of is that it retains free calix[4]pyrrole molecules in the framework pores. Treatment with a fluoride anion source serves to destroy the network and allows release of the organic guest. The net result is a 'molecular ship' in a 'breakable bottle'.

Controllable Fluorescence Switching of a Coordination Chain Based on the Photoinduced Single-Crystal-to-Single-Crystal Reversible Transformation of a syn-[2.2]Metacyclophane

The observation of a reversible chemical transformation corresponding to an external stimulus in the solid state is intriguing in the exploration of smart materials, which can potentially be applied in molecular machines, molecular switches, sensors, and data storage devices. The solid-state photodimerization reaction of 1,3-bis[2-(4-pyridyl)ethenyl]benzene (1,3-bpeb) in a one-dimensional coordination polymer {[Cd₂(1,3-bpeb)₂(4-FBA)₄]·H₂O}_n (4-FBA = 4-fluorobenzoate) with 365 nm UV light afforded syn-tetrakis(4-pyridyl)-1,2,9,10-diethano[2.2]metacyclophane (syn-tpmcp) in quantitative yield via a single-crystal-to-single-crystal (SCSC) transformation. Upon irradiation with 254 nm UV light, an SCSC conversion from syn-tpmcp to 1,3-bpeb was also achieved in quantitative yield within the syn-tpmcp-supported coordination polymer {[Cd₂(syn-tpmcp)(4-FBA)₄]·H₂O}_n. In particular, accompanied by the reversible transformation between 1,3-bpeb and syn-tpmcp, the coordination chain exhibits photocontrollable fluorescence-switching behavior, which makes this intelligent material an appealing candidate for practical applications.

Guest-induced reversible crystal-to-amorphous-to-crystal transformation in a Co(ii)-based metal–organic framework

A crystal-to-amorphous-to-crystal (CAC) transformation was obtained during the dehydration/rehydration process. Moreover, an amorphous sample DPPB-2 could separate methanol and ethanol from other VOCs.

Synthesis and proton conductivity of two novel molybdate polymers

Two molybdate polymers H₄[Co(phen)₃]₂[NaO(H₂O)(l-Mo₈O₂₆)]₂·2H₂O (1) and H₂[TEDA][Mo₄O₁₃]·3H₂O (2) were synthesized, proton conductivity of 1 was 3.06 × 10⁻³ S cm⁻¹.

Selective fluorescent sensing and photocatalytic properties of three MOFs based on naphthalene-1,4-dicarboxylic acid and 2,4,5-tri(4-pyridyl)-imidazole

Three MOFs display high sensitivity in the detection of NB and Ca²⁺ or Co²⁺ ions as luminescent probes. Moreover, 3 exhibits a relatively good photocatalytic degradation activity of rhodamine B.