Cysteamine-Anchored MOF through Post-Synthetic Modification Strategy for the Effective Removal of Mercury from Water.

The introduction of cysteamine functionality, referred to as Q-ZIF-67-SH, was successfully achieved through postsynthetic modification while maintaining the structural and thermal stability of the quasi metal-organic framework Q-ZIF-67. By subjecting ZIF-67 to controlled partial deligandation at 310 °C under an air atmosphere, a substantial number of unsaturated cobalt sites were generated within the quasi ZIF-67 (Q-ZIF-67) structure. These unsaturated cobalt sites facilitated effective coordination with cysteamine, resulting in the development of the thiol-functionalized framework Q-ZIF-67-SH. The potential of these metal-organic frameworks (MOFs) for the adsorptive removal of hazardous Hg(II) was investigated. Various factors, such as the type of sorbent, pH, adsorbent dosage, initial concentration of Hg(II), and presence of coexisting ions, were thoroughly examined and comprehensively explained. Thiol-anchored MOF significantly enhanced the efficiency of Hg(II) removal, achieving an impressive removal rate of up to 99.2%. Furthermore, it demonstrated a maximum adsorption capacity of 994 mg g-1 and a distribution coefficient of 2.5 × 106 mL g-1. A good correspondence with pseudo-second-order kinetics and the Langmuir model was observed through the fitting of adsorption kinetics and the isotherm model. The thermodynamic data strongly indicate that the adsorptive removal of Hg(II) is characterized by endothermicity and spontaneity. This signifies that the process is energetically favorable and has potential for efficient Hg(II) removal. Therefore, the Q-ZIF-67-SH sorbent emerges as a promising and advantageous option for the removal of Hg(II) from water.

Defect-Engineered Metal-Organic Framework/Polyimide Mixed Matrix Membrane for CO2 Separation.

Defect-engineered metal-organic frameworks (MOFs) with outstanding structural and chemical features have become excellent candidates for specific separation applications. The introduction of structural defects in MOFs as an efficient approach to manipulate their functionality provides excellent opportunities for the preparation of MOF-based mixed matrix membranes (MMMs). However, the use of this strategy to adjust the properties and develop the separation performance of gas separation membranes is still in its early stages. Here, a novel defect-engineered MOF (quasi ZrFum or Q-ZrFum) was synthesized via a controlled thermal deligandation process and incorporated into a CO2-philic 6FDA-durene polyimide (PI) matrix to form Q-ZrFum loaded MMMs. Defect-engineered MOFs and fabricated MMMs were investigated regarding their characteristic properties and separation performance. The incorporation of defects into the MOF structure increases the pore size and provides unsaturated active metal sites that positively affect CO2 molecule transport. The interfacial compatibility between the Q-ZrFum particles and the PI matrix increases via the deligandation process, which improves the mechanical strength of Q-ZrFum loaded membranes. MMM containing 5 wt.% of defect-engineered Q-ZrFum exhibits excellent CO2 permeability of 1308 Barrer, which increased by 99 % compared to the pure PI membrane (656 Barrer) at a feed pressure of 2 bar. CO2/CH4 and CO2/N2 selectivity reached 44 and 26.6 which increased by about 70 and 16 %, respectively. This study emphasizes that defect-engineered MOFs can be promising candidates for use as fillers in the preparation of MMMs for the future development of membrane-based gas separation applications.

Quasi-HKUST Prepared via Postsynthetic Defect Engineering for Highly Improved Catalytic Conversion of 4-Nitrophenol.

HKUST-1 [Cu3(BTC)2(H2O)3]n·nH2OMeOH was submitted to thermolysis under controlled conditions at temperatures between 100 and 300 °C. This treatment resulted in partial ligand decarboxylation, generating coordinatively unsaturated Cu2+ sites with extra porosity on the way to the transformation of the initial HKUST-1 framework to CuO. The obtained materials retaining in part the HKUST-1 original crystal structure (quasi-MOFs) were used to promote 4-nitrophenol conversion to 4-aminophenol. Because of the partial linker decomposition, the quasi-MOF treated at 240 °C contains coordinatively unsaturated Cu2+ ions distributed throughout the Q-HKUST lattice together with micro- and mesopores. These defects explain the excellent catalytic performance of QH-240 with an apparent rate constant of 1.02 × 10-2 s-1 in excess of NaBH4 and an activity factor and half-life time of 51 s-1g-1 and 68 s, respectively, which is much better than that of the HKUST parent. Also, the induction period decreases from the order of minutes to seconds in the presence of the HKUST and QH-240 catalysts, respectively. Kinetic studies fit with the Langmuir-Hinshelwood theory in which both 4-nitrophenol and BH4- should be adsorbed onto the catalyst surface. The values of the true rate constant (k), the adsorption constants of 4-nitrophenol and BH4- (K4-NP and KBH4-), as well as the activation energy are in agreement with a rate-determining step involving the reduction of 4-nitrophenol by the surface-bound hydrogen species.

Improved catalytic hydrogen release of quasi HKUST-1 compared to HKUST-1.

On-demand hydrogen release could be an important process for the transportation of fuel in the near future. Herein it is reported that the controlled thermolysis of HKUST-1 under optimal conditions generates structural defects resulting in a quasi-HKUST-1 material showing a 20-fold enhancement of the H2 release turnover frequency at room temperature in the absence of a base.

Enhanced Catalytic Performance of Quasi-HKUST-1 for the Tandem Imine Formation.

Copper-based metal-organic framework (Cu3 (BTC)2 (H2 O)3 ]n ⋅nH2 OMeOH (HKUST-1) has been subjected to thermolysis under air atmosphere at different temperatures ranging from 100 to 300 °C. This treatment produces the partial removal of ligands, the generation of structural defects and additional porosity in a controlled way. The resulting defective materials denoted according to the literature as quasi-MOFs, were subsequently employed as heterogeneous catalysts in the one pot synthesis of N-benzylideneaniline from aniline and benzyl alcohol in open air as terminal oxidant at 70 °C under base- and dehydrating agent-free conditions. The Q-HKUST catalysts calcined at 240 °C (QH-240) was the most efficient in the series, promoting imine synthesis. Data from Knoevenagel condensation of malononitrile shows that in QH-240 the distances of Cu ions in HKUST-1 cavities are preserved, increasing the Knoevenagel activity, but a strong rearrangement takes place at 300 °C or above. The unsaturated copper active sites with simultaneous presence of micro- and mesopores in QH-240 are responsible for this excellent catalytic performance. The effective parameters on catalytic activity of QH-240 including deligandation temperature, the amount of catalyst, the ratio of reactants, and reaction temperature as well as the stability and recyclability of the catalyst were also investigated. The possible mechanism used by QH-240 follows alcohol aerobic oxidation and subsequent anaerobic condensation of aldehyde intermediate with aniline.

Sensitive Ratiometric Fluorescent Metal-Organic Framework Sensor for Calcium Signaling in Human Blood Ionic Concentration Media.

A two-dimensional fluorescent metal-organic framework (MOF) sensor, TMU-5S, was synthesized by introducing rhodamine B into the framework of TMU-5. This dye-sensitized MOF was explored as a ratiometric fluorescent sensor for Ca2+ signaling in the presence of interfering cations in ionic concentrations similar to that of blood plasma that exhibits exceptional sensitivity and selectivity to Ca2+ and faster response and recovery times than any other reported fluorescent sensor. This can be attributed to strong electrostatic interactions between basic azine groups in the narrow pore walls of TMU-5S and Ca2+ as hard-hard acid-base interactions. The results show that the two-dimensional signal transduction can reduce interfering responses from the environment and thus create exceptional sensitivity.

Mixed-Metal MOFs: Unique Opportunities in Metal-Organic Framework (MOF) Functionality and Design.

Mixed-metal metal-organic frameworks (MM-MOFs) can be considered to be those MOFs having two different metals anywhere in the structure. Herein we summarize the various strategies for the preparation of MM-MOFs and some of their applications in adsorption, gas separation, and catalysis. It is shown that compared to homometallic MOFs, MM-MOFs bring about the opportunity to take advantage of the complexity and the synergism derived from the presence of different metal ions in the structure of MOFs. This is reflected in a superior performance and even stability of MM-MOFs respect to related single-metal MOFs. Emphasis is made on the use of MM-MOFs as catalysts for tandem reactions.

Host-Guest Interaction Optimization through Cavity Functionalization for Ultra-Fast and Efficient Water Purification by a Metal-Organic Framework.

To achieve optimized host-guest interactions, a "cavity functionalization" approach has been applied using two isostructure MOFs, [Zn(OBA)(BPDB)0.5] n·2DMF (TMU-4) and [Zn(OBA)(H2DPT)0.5] n·DMF (TMU-34), where H2OBA = 4,4'-oxybis(benzoic acid), BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, and H2DPT = 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine. TMU-4 and TMU-34 are functionalized with azine and dihydro-tetrazine groups, respectively. Both of these functions can act as Lewis basic sites, but only the dihydro-tetrazine function inside the cavities of TMU-34 can act as a hydrogen bond donor site. These frameworks were applied in removal of pollutant dyes. The results indicate that TMU-34 can remove rose-bengal B (RB-B) from aqueous solution much more than TMU-4, through size selective and optimized host-guest interaction mechanisms. We believe that the preferred hydrogen bond interaction between RB-B free phenolate/carboxylate and dihydro-tetrazine hydrogens of TMU-34 is responsible for the drastic enhancement in the adsorption capacity and removal kinetics of TMU-34 rather than TMU-4. Our strategy in this work clearly shows that (i) high surface area and porosity as well as rational decoration of MOF cavities with organic functions are the key methods for highly efficient and fast dye removal and (ii) both adsorption and desorption times can be optimized very fast. Also, optimized interaction between TMU-34 and RB-B is strong and effective, yet not so much that it leads to long desorption time. To the best of our knowledge, this is the first paper on removal of highly toxic rose-bengal B dye from aqueous solutions using MOFs.

Ultrasound-assisted synthesis and characterization of a new metal-organic framework based on azobenzene-4,4-dicarboxylic acid: Precursor for the fabrication of Co3O4 nano-particles.

Azobenzene groups are widely known as photochromic ligands thus are particularly interesting building blocks for designing receptors for neutral or charged guests. A new metal-organic framework, [Co3(adc)3(DMF)4].2DMF (compound 1) (adc = azobenzene-4,4-dicarboxylic acid, DMF = N,N-dimethylformamide), was synthesized by solvothermal methods and structurally characterized using X-ray crystallography and a range of spectroscopic techniques. Also, nanorods of compound 1 have been synthesized by a sonochemical process and characterized by field emission scanning electron microscopy (FE-SEM) and powder X-ray diffraction (PXRD). The effect of sonication time and concentration of the initial reagents on the size and morphology of the MOF have been optimized. Results indicate that decreasing of initial concentration and increasing ultrasound radiation time lead to small size nanorods of compound 1. Thus, ultrasound radiation affects the size of nanorods. After heat treatment, the cobalt ion-based metal organic framework nanorods can be converted into porous Co3O4 nanoparticles.

Sonochemical synthesis and structural characterization of a new Zn(II) nanoplate metal-organic framework with removal efficiency of Sudan red and Congo red.

A 3-D Zn(II) based metal-organic framework (MOF) of [Zn4(oba)3(DMF)2] was synthesized using the nonlinear dicarboxylate ligand, 4,4'-oxybis(benzoic acid) (H2oba) via sonochemical and solvothermal routes. IR spectroscopy, single-crystal X-ray crystallography, scanning electron microscopy, and X-ray powder diffraction were used to characterize these MOF samples. The effect of different times of irradiation and various concentrations of primary reagents were experimented for obtaining monotonous morphology. The results show that uniform nanoplates can be achieved by increasing the time of irradiation and decreasing the concentration. N2 adsorption was applied to examine the effect of synthesis method on porosity of the framework. Also Congo red and Sudan red dyes were employed to explore the efficiency of this MOF in removal of the dye pollutants.

Morphology-dependent sensing performance of dihydro-tetrazine functionalized MOF toward Al(III).

A pillared MOF, [Zn(OBA)(H2DPT)0.5].DMF (TMU-34), based on dihydro tetrazine functionalized pillar spacer (H2DPT=3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine) and V-shape dicarboxylate linker (H2OBA=4,4'-oxybis(benzoic acid)) was synthesized by reflux and ultrasonic methods. The effects of sonication time, initial concentration of reagents and sonication power on size and morphology have been optimized. This MOF has been characterized by scanning electron microscopy, FT-IR spectra, X-ray powder diffraction and N2 adsorption at 77K. Bulk and nano samples of TMU-34 have been applied in cation sensing for detection of Al(III) in presence of other cations (Na(I), Mg(II), Sr(II), Al(III), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II), Hg(II),Cr(III), Li(I), Fe(III), K(I)). The results show that nano powder of TMU-34 with uniform separated plate-like morphology (TMU-34-F) has higher detection limit and short response time compared to bulk material. So, in this work we show the application of luminescent metal-organic frameworks synthesized by sonochemistry approach in effective cation detection.

Double Solvent Sensing Method for Improving Sensitivity and Accuracy of Hg(II) Detection Based on Different Signal Transduction of a Tetrazine-Functionalized Pillared Metal-Organic Framework.

To design a robust, π-conjugated, low-cost, and easy to synthesize metal-organic framework (MOF) for cation sensing by the photoluminescence (PL) method, 4,4'-oxybis(benzoic acid) (H2OBA) has been used in combination with 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (DPT) as a tetrazine-functionalized spacer to construct [Zn(OBA)(DPT)0.5]·DMF (TMU-34(-2H)). The tetrazine motif is a π-conjugated, water-soluble/stable fluorophore with relatively weak σ-donating Lewis basic sites. These characteristics of tetrazine make TMU-34(-2H) a good candidate for cation sensing. Because of hydrogen bonding between tetrazine moieties and water molecules, TMU-34(-2H) shows different PL emissions in water and acetonitrile. Cation sensing in these two solvents revealed that TMU-34(-2H) can selectively detect Hg2+ in water (by 243% enhancement) and in acetonitrile (by 90% quenching). The contribution of electron-donating/accepting characteristics along with solvation effects on secondary interactions of the tetrazine motifs inside the TMU-34(-2H) framework results in different signal transductions. Improved sensitivity and accuracy of detection were obtained using the double solvent sensing method (DSSM), in which different signal transductions of TMU-34(-2H) in water and acetonitrile were combined simultaneously to construct a double solvent sensing curve and formulate a sensitivity factor. Calculation of sensitivity factors for all of the tested cations demonstrated that it is possible to detect Hg2+ by DSSM with ultrahigh sensitivity. Such a tremendous distinction in the Hg2+ sensitivity factor is visualizable in the double solvent sensing curve. Thus, by application of DSSM instead of one-dimensional sensing, the interfering effects of other cations are completely eliminated and the sensitivity toward Hg(II) is highly improved. Strong interactions between Hg2+ and the nitrogen atoms of the tetrazine groups along with easy accessibility of Hg2+ to the tetrazine groups lead to a shorter response time (15 s) in comparison with other MOF-based Hg2+ sensors.

Stimuli-Responsive Metal-Organic Framework (MOF) with Chemo-Switchable Properties for Colorimetric Detection of CHCl3.

The ligands 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine (H2 DPT), as a pillar spacer, and 4,4'-oxybis(benzoic acid) (H2 OBA), as a dicarboxylate linker, have been used to develop a dihydrotetrazine-functionalized pillared metal-organic framework (MOF), [Zn(OBA)(H2 DPT)0.5 ]⋅DMF (TMU-34), as a functionally convertible stimuli-responsive MOF-based sensor. Sonochemically synthesized TMU-34 powder has been employed in the sensing of volatile organic compounds (VOCs). The results show that TMU-34 can act as a solid-state, naked-eye visual chemosensor for the detection of chloroform in the liquid and gas phases. TMU-34, as a chemo-switchable MOF with rapid response, undergoes a significant color change from yellow to pink through reversible dynamic conversion of dihydrotetrazine into tetrazine upon exposure to chloroform in the presence of other VOCs. Whereas other colorimetric MOF-based sensors show a wide range of color changes in the presence of different analytes, TMU-34 undergoes a color change only in the presence of chloroform. A mechanism for the observed color change is proposed on the basis of UV/Vis and 1 H NMR spectroscopic studies. These observations reveal that TMU-34 represents one of the most promising candidates for the highly selective and sensitive detection of chloroform.

Porosity and dye adsorption enhancement by ultrasonic synthesized Cd(II) based metal-organic framework.

A Cd(II) based metal-organic framework, TMU-7, was synthesized by incorporation of V-shaped flexible dicarboxylate ligand and the N-donor pillar ligand using sonochemical method while morphology and size of particles were investigated and characterized by scanning electron microscopy and X-ray powder diffraction. Moreover, N2 adsorption was applied to examine the effect of synthesis method on porosity. Also TMU-7 was evaluated for its Congo red adsorption.

Ultrasonic assisted synthesis of a tetrazine functionalized MOF and its application in colorimetric detection of phenylhydrazine.

TMU-34(-2H), [Zn(OBA)(DPT)0.5].DMF, has been sonochemically synthesized by applying H2OBA, (4,4'-oxybis(benzoic acid)), as the dicarboxylate linker, and DPT, (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine), as pillar spacer. Sonication time, concentration of initial reagents, sonication power and molar ratio of pyridine as modulator has been optimized to synthesize nano powder of TMU-34(-2H) including uniform plate morphology. Nano TMU-34(-2H), can detect phenylhydrazine (PH) by color changing from pink to deep purple. As a comparison, nano and crystal samples of TMU-34(-2H) were treated to detect PH. Experiments show that nano TMU-34(-2H) has better detection limit and response time.

High organic sulfur removal performance of a cobalt based metal-organic framework.

Synthesis of a new porous cobalt based metal-organic framework, [Co6(oba)6(CH3O)4(O)2]n·3DMF (TMU-11) has been carried out to introduce a new and highly efficient adsorbent of dibenzothiophene (DBT). This compound has been synthesized by solvothermal method using a nonlinear dicarboxylate ligand and characterized by single-crystal X-ray crystallography. To study the adsorption properties of the synthesized compound, TMU-11, for DBT removal, various factors, such as amount of adsorbent, contact time and temperature were examined. On the basis of the results, maximum efficiency and reusability in DBT removal occur under the mild reaction conditions. Furthermore, the DBT removal follows the pseudo-second order reaction kinetic. The maximum adsorption value is 825mg/g. The selectivity test of DBT over naphthalene (NA) clearly shows that π-π interactions between organic linkers of TMU-11 and the aromatic ring of DBT are not responsible for the adsorption desulfurization (ADS) process and the main part of adsorption takes place on unsaturated site around Co centres. Our findings may provide some insight into the preparation of the adsorbent with superior performance in practical applications.

Improvement of Methane-Framework Interaction by Controlling Pore Size and Functionality of Pillared MOFs.

The rational design of functionalized porous metal-organic frameworks (MOFs) for gas adsorption applications has been applied using three spacer ligands H2DPT (3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine), DPT (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine), and BPDH (2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) to synthesize TMU-34, [Zn(OBA)(H2DPT)0.5]n·DMF, TMU-34(-2H), [Zn(OBA)(DPT)0.5]n·DMF, and TMU-5, [Zn(OBA)(BPDH)0.5]n·1.5DMF, respectively. By controlling the pore size and chemical functionality of these three MOFs, we can improve the interactions between CO2 and especially CH4 with the frameworks. Calculated Qst(CH4) for TMU-5, TMU-34, and TMU-34(-2H) are 27, 23, and 22 kJ mol-1, respectively. These Qst values are among the highest for CH4-framework interactions. For systematic comparison, two reported frameworks, TMU-4 and TMU-5, have been compared with TMU-34 and TMU-34(-2H) in CO2 adsorption.

A nanocomposite prepared from a zinc-based metal-organic framework and polyethersulfone as a novel coating for the headspace solid-phase microextraction of organophosphorous pesticides.

The authors describe a zinc-based metal-organic framework/polyethersulfone nanocomposite (TMU-4/PES) coating deposited on a stainless steel wire via a single-phase inversion method. The nanocomposite represents a novel fiber coating for headspace solid-phase microextraction of organophosphorous pesticides (OPPs) from environmental water and soil samples. The synergistic effects of the high surface area and unique porous structure of TMU-4 as well as the rich π electron stacking and mechanical attributes of the PES polymer result in a high affinity of the composite for OPPs. Following thermal desorption, the OPPS were quantified by gas chromatography with a nitrogen-phosphorus detector. The preparation of the coating is simple, and the coated fiber is highly stable and reusable in that it can be used in about 100 consecutive extractions/desorption cycles. A central composite design was used for assessing the effect of the experimental parameters on the extraction process. Under optimized conditions, the limits of detection are in the 5-8 ng mL-1 range for the OPPs diazinon, fenitrothion, malathion and chlorpyrifos. The average repeatability and fiber-to-fiber reproducibility are 6.5% and 8.7%, respectively. The method was applied to the trace determination of OPPs in (spiked) water and soil samples where it gave good recovery (88-108%) and satisfactory reproducibility (5.9-10.1%). Graphical abstract A zinc-based metal-organic framework/polyethersulfone nanocomposite coating was prepared on a stainless steel wire via phase inversion. It was used as a novel fiber coating for headspace solid phase microextraction of organophosphorous pesticides from water and soil samples.

Ultrasonic assisted synthesis of two new coordination polymers and their applications as precursors for preparation of nano-materials.

Two new isostructural coordination polymers, [Zn3(Oba)4][Me2NH2)2]·6DMF·3H2O (1) and [Cd3(Oba)4][Me2NH2)2]·2DMF·2H2O (2), were synthesized using the nonlinear dicarboxylate ligand, 4,4'-oxybis(benzoic acid) (H2oba) and characterized by IR spectroscopy and single-crystal X-ray crystallography. Single-crystal X-ray data show that 1 and 2 are two-dimensional coordination polymers that can be extended to three-dimensional supramolecular networks by CH…O interactions. These two new coordination polymers were also sonochemically synthesized while sonication time and power of irradiation influencing size and morphology of nano-structured compounds were also studied. Moreover, calcination of these coordination polymers creates ZnO and CdO nanostructures.

Two Dimensional Host-Guest Metal-Organic Framework Sensor with High Selectivity and Sensitivity to Picric Acid.

A dye-sensitized metal-organic framework, TMU-5S, was synthesized based on introducing the laser dye Rhodamine B into the porous framework TMU-5. TMU-5S was investigated as a ratiometric fluorescent sensor for the detection of explosive nitro aromatic compounds and showed four times greater selectivity to picric acid than any state-of-the-art luminescent-based sensor. Moreover, it can selectively discriminate picric acid concentrations in the presence of other nitro aromatics and volatile organic compounds. Our findings indicate that using this sensor in two dimensions leads to a greatly reduced environmental interference response and thus creates exceptional sensitivity toward explosive molecules with a fast response.