A metal-organic framework with highly polar pore surfaces: selective CO2 adsorption and guest-dependent on/off emission properties

A 3D porous Zn(II) metal-organic framework {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)(H(2)O)]·4H(2)O} (1; H(2)dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H(4)dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)]} (1') provides 52.7% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated Zn(II) sites and pendant hydroxyl groups of H(2)dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO(2) over other gases, such as N(2), H(2), O(2) and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (λ(max)≈530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H(2)dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH···OCO hydrogen-bonding interaction in the H(2)dht linker in 1'/1.     

Ultrathin two-dimensional triptycence-based metal-organic framework for highly selective CO2 electroreduction to CO

Efficient CO₂ reduction reaction (CO₂RR) is one of the important topics in energy and environment field, but improving the electrochemical selectivity of specific product is a great challenge. Herein, we reported a unprecedented two-dimensional (2D) metal?organic framework with CuO₄ unit (denoted as Cu-HHTT, HHTT = 9,10-dihydro-9,10-[1,2]benzenoanthracene-2,3,6,7,14,15-hexaol) as the electrocatalyst for CO₂RR. Cu-HHTT exhibits high performance for CO₂RR to produce CO, namely Faradaic efficiency of 96.6% toward CO with a current density of 18 mA/cm² at the potential of ?0.6 V vs. RHE. Density function theory reveals that the desorption of CO species exhibits a lower energy barrier than that of hydrogenation of *CO intermediate, resulting in CO as the main product instead of alcohols or hydrocarbons.     

Three-dimensional pillar-layered copper(II) metal-organic framework with immobilized functional OH groups on pore surfaces for highly selective CO2/CH4 and C2H2/CH4 gas sorption at room temperature

A new three-dimensional microporous metal-organic framework Cu(BDC-OH)(4,4'-bipy)·G(x) (UTSA-15; H(2)BDC-OH = 2-hydroxy-benzenedicarboxylic acid, 4,4'-bipy =4,4'-bipyridine, G = guest molecules) with functional -OH groups on the pore surfaces was solvothermally synthesized and structurally characterized. UTSA-15 features a three-dimensional structure having 2D intercrossed channels of about 4.1 × 7.8 and 3.7 × 5.1 ?(2), respectively. The small pores and the functional -OH groups on the pore surfaces within the activated UTSA-15a have enabled their strong interactions with CO(2) and C(2)H(2) which have been revealed in their large adsorption enthalpies of 39.5 and 40.6 kJ/mol, respectively, highlighting UTSA-15a as the highly selective microporous metal-organic framework for the CO(2)/CH(4) and C(2)H(2)/CH(4) gas separation with separation selectivity of 24.2 and 55.6, respectively, at 296 K.     

A flexible metal-organic framework with double interpenetration for highly selective CO2 capture at room temperature

A flexible metal-organic framework of 1a Cu(FMA)(4,4'-Bpe)0.5(FMA=fumarate; 4,4'-Bpe=trans-bis-(4-pyridyl)ethylene) that exhibits guest molecule-controlled gate-opening adsorption has been reported, in which the flexible pores can be enlarged by CO_2 molecules rather than CH_4 and N_2 under a certain gate-opening pressure. The CO_2 uptake can be sharply improved from 6.85cm~3 g~(–1) at 0.60 atm to 33.7 cm~3 g~(–1) at 1 atm due to the gate-opening effect, thus resulting in the notably enhanced adsorption selectivities for CO_2/CH_4(32:1, v/v) and CO_2/N_2(48:1, v/v) separations at room temperature.     

Robust metal-organic framework enforced by triple-framework interpenetration exhibiting high H2 storage density

A microporous metal-organic framework 1 Zn 2(CNC) 2(DPT).G [CNC = 4-Carboxycinnamic; DPT = 3,6-Di-4-pyridyl-1,2,4,5-tetrazine; G = guest molecules] was synthesized and structurally characterized by a triply interpenetrated primitive cubic net with 1D pores of about 3.7 A. 1 is highly robust enforced by triple framework interpenetration through weak van der Waals interactions, thus the activated 1b takes up 1.28 wt % hydrogen gas and exhibits high hydrogen storage density of 95.2% at 1 atm and 77 K.     

Molecular screening of metal-organic frameworks for CO2 storage

We report a molecular simulation study for CO2 storage in metal-organic frameworks (MOFs). As compared to the aluminum-free and cation-exchanged ZSM-5 zeolites and carbon nanotube bundle, IRMOF1 exhibits remarkably higher capacity. Incorporation of Na(+) cations into zeolite increases the capacity only at low pressures. By variation of the metal oxide, organic linker, functional group, and framework topology, a series of isoreticular MOFs (IRMOF1, Mg-IRMOF1, Be-IRMOF1, IRMOF1-(NH2)4, IRMOF10, IRMOF13, and IRMOF14) are systematically examined, as well as UMCM-1, a fluorous MOF (F-MOF1), and a covalent-organic framework (COF102). The affinity with CO2 is enhanced by addition of a functional group, and the constricted pore is formed by interpenetration of the framework; both lead to a larger isosteric heat and Henry's constant and subsequently a stronger adsorption at low pressures. The organic linker plays a critical role in tuning the free volume and accessible surface area and largely determines CO2 adsorption at high pressures. As a combination of high capacity and low framework density, IRMOF10, IRMOF14, and UMCM-1 are identified from this study to be the best for CO2 storage, even surpass the experimentally reported highest capacity in MOF-177. COF102 is a promising candidate with high capacity at considerably low pressures. Both gravimetric and volumetric capacities at 30 bar correlate well with the framework density, free volume, porosity, and accessible surface area. These structure-function correlations are useful for a priori prediction of CO2 capacity and for the rational screening of MOFs toward high-efficacy CO2 storage.     

Construction of a trigonal bipyramidal cage-based metal-organic framework with hydrophilic pore surface via flexible tetrapodal ligands

A pcu network can be ordered through trigonal bipyramidal nanocages as secondary building blocks, which have the striking feature of hydrophilic affinity to the polar molecules and π-π interactions for aromatic molecules.     

H2 storage and CO2 capture on a nanoscale metal organic framework with high thermal stability

A nanoscale aluminium-based metal organic framework (NMOF) with high thermal stability has been synthesized, which shows high H(2) and CO(2) uptake capacities and an excellent selectivity for CO(2) over N(2) and O(2).     

Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (135 m(2) g(-1)) and was the best compromise between capacity and selectivity for CO(2) over CO, CH(4), N(2) and O(2); as such it is a promising new selective adsorbent for CO(2) capture.     

Post-modification of metal-organic framework for improved CO2 photoreduction efficiency

Utilizing metal-organic frameworks (MOFs) to design photocatalysts for CO₂ reduction catalysts is an excellent idea but currently restricted by the relatively low activity. Enhancing CO₂ affinity and tuning the oxidation state of metal clusters in MOFs might be a solution to improve the catalytic performance. Herein, the Cl-bridge atoms in the metal clusters of a cobalt MOF were easily exchanged with OH^?, which simultaneously oxidized a portion of Co(II) to Co(III) and resulted in a much enhanced photocatalytic activity for CO₂ reduction. In contrast, the original framework does not exhibit such superior activity. Comprehensive characterizations on their physicochemical properties revealed that the introduction of hydroxyl group not only greatly increases the CO₂ affinity but also alters the oxidation state of metal clusters, resulting in significantly improved photocatalytic activities for CO₂ reduction. This work provides important insight into the design of efficient photocatalysts.     

Selective CO2 adsorption in a flexible non-interpenetrated metal-organic framework

We have prepared a flexible metal-organic framework and demonstrated that when activated by supercritical CO(2) it has greater gas sorption capacities than that activated by the heat-evacuation method, and it selectively adsorbs CO(2) over N(2) at room temperature.     

CDs assembled metal-organic framework: Exogenous coreactant-free biosensing platform with pore confinement-enhanced electrochemiluminescence

Despite the various synthesis approachs to obtain luminous carbon dots (CDs), it is still quite challenging to construct the efficient electrochemiluminescence (ECL) owing to their low ECL reactivity and easy agglomeration. Herein, an efficient and concise ECL system was skillfully constructed by taking advantage of the nitrogen and sulfur co-doped CDs (N,S-CDs) with surfaces rich in hydrazide groups as luminophors to emit intense ECL, and metal-organic framework (MOF) as the matrix to confine CDs in its nanospace. Surprisingly, the proposed CDs assembled MOF (CDs/ZIF-8) enhanced anodic ECL signal up to 250% of pure CDs under the exogenous coreactant-free condition. As a proof of concept, the highly sensitive detection of uric acid (UA) was realized by the constructed ECL platform with a low detection limit of 3.52 nmol/L ranging from 10 nmol/L to 50 µmol/L. This work expanded ideas for the application of pore confinement effect, and provided references for the detection of disease biomarkers of gout and hyperuricemia.     

Understanding hydrogen sorption in a polar metal-organic framework with constricted channels

A high fidelity molecular model is developed for a metal-organic framework (MOF) with narrow (approximately 7.3 A?) nearly square channels. MOF potential models, both with and neglecting explicit polarization, are constructed. Atomic partial point charges for simulation are derived from both fragment-based and fully periodic electronic structure calculations. The molecular models are designed to accurately predict and retrodict material gas sorption properties while assessing the role of induction for molecular packing in highly restricted spaces. Thus, the MOF is assayed via grand canonical Monte Carlo (GCMC) for its potential in hydrogen storage. The confining channels are found to typically accommodate between two to three hydrogen molecules in close proximity to the MOF framework at or near saturation pressures. Further, the net attractive potential energy interactions are dominated by van der Waals interactions in the highly polar MOF - induction changes the structure of the sorbed hydrogen but not the MOF storage capacity. Thus, narrow channels, while providing reasonably promising isosteric heat values, are not the best choice of topology for gas sorption applications from both a molecular and gravimetric perspective.     

A microporous, moisture-stable, and amine-functionalized metal-organic framework for highly selective separation of CO2 from CH4

A new microporous amino-functionalized metal-organic framework has been synthesized by direct self-assembly, which exhibits high moisture-stability, acceptable capacity, and unprecedented high selectivity for CO(2) over CH(4), suggesting its potential application in gas separation processes like natural gas and biogas upgrading.     

A highly thermally stable ferroelectric metal-organic framework and its thin film with substrate surface nature dependent morphology

A highly thermally stable, polar MOF built from the rigid ligand benzene-1,4-dicarboxylate and the main-group metal ion Sr(2+) shows ferroelectricity. With the ultimate goal of making components for use in devices, the fabrication of MOF thin films on Al(2)O(3), SrAl(2)O(4), and Al foil substrates using the in situ solvothermal method was explored. The mechanism of macroscopic polarization reversals in the ferroelectric MOF under an ac electric field and the dependence of the morphology of the MOF thin film on the nature of the substrate surface are discussed.     

CH(4) storage and CO(2) capture in highly porous zirconium oxide based metal-organic frameworks

A series of porous Zr oxoclusters-based MOFs was computationally explored for their gas storage/capture performances. The highly porous UiO-67(Zr) and UiO-68(Zr) solids show exceptionally high CH(4) and CO(2) adsorption capacities under operating conditions that make these thermal, water and mechanical resistant materials very promising for physisorption-based processes.     

Ultrathin two-dimensional metal-organic framework nanosheets for efficient electrochemical CO2 reduction

Electrochemical CO2 reduction into CO or high-value products is regarded as a feasible pathway for energy conversion, which has attracted universal attention in...     

Designing polyoxometalate-based metal-organic framework for oxidation of styrene and cycloaddition of CO2 with epoxides

A photoactive polyoxometalate-based metal−organic framework (POMOF), NiW-DPNDI, was synthesized by combination of Ni(II) ions, [ZnW₁₂O₄₀]^6‒ anions, and N,N'-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) molecules into one single framework. NiW-DPNDI displays a three-dimensional structure by the strong anion⋅⋅⋅π interactions between the trapped [ZnW₁₂O₄₀]^6‒ anions and the electron-deficient naphthalenic ring centroids and the π−π stacking interactions between DPNDI moieties. NiW-DPNDI displayed a highly efficient hole-electron separation and ordered electron transfer under irradiation, thus ensuing its excellent photocatalysis in oxidation of styrene to produce benzaldehyde. In addition, it gave a high efficiency for styrene oxide under thermocatalytic conditions. Because the carbonic anhydrase (CA)-mimicking Ni sites and the negative electron-enriched [ZnW₁₂O₄₀]^6‒ anions are well aligned in the pores, it can promote the cycloaddition of CO₂ with epoxides under mild conditions.