Efficient Electrochemical Lead Detection by a Histidine-Grafted Metal-Organic Framework MOF-808 Electrode Material.

As the excessive presence of heavy metals in the environment significantly affects human health, it becomes necessary to develop efficient, selective, and sensitive methods for their detection. In this study, a novel electrochemical sensor for the detection of Pb2+ ions is described. The proposed sensor is based on a glassy carbon electrode (GCE) modified by a thin film of histidine-grafted metal-organic framework (MOF-808-His). The MOF-808 was obtained solvothermally, and then postsynthetically modified by substituting the coordinated acetate with histidinate. By electrochemistry, the MOF-808-His-modified GCE demonstrated high charge selectivity, while electrochemical impedance spectroscopy (EIS) and kinetic studies gave a lower charge transfer resistance (4196 Ω) and a better standard heterogeneous electron transfer rate constant (1.80 × 10-5 cm s-1) on MOF-808-modified GCE. These results indicated a swift and direct electron transfer rate from [Fe(CN)6]3-/4- to the electrode surface. Using square wave anodic stripping voltammetry (SWASV), the rapid and highly sensitive determination of Pb2+ was achieved on MOF-808-His-modified GCE. By optimizing the accumulation-detection parameters including pH of the detection medium, deposition time and potential, and concentration, a remarkable limit of detection (LoD, based on a signal-to-noise ratio of 3) of (1.12 × 10-10 ± 0.10 × 10-10) mol L-1 was obtained, with a sensitivity of (9.6 ± 0.1) µA L µmol-1. After interference and stability studies, the MOF-808-His-modified GCE was applied to the detection of Pb2+ in a tap water sample with a concentration of 10 µmol L-1 Pb2+.

When Polymorphism in Metal-Organic Frameworks Enables Water Sorption Profile Tunability for Enhancing Heat Allocation and Water Harvesting Performance.

The development of thermally driven water-sorption-based technologies relies on high-performing water vapor adsorbents. Here, polymorphism in Al-metal-organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans- or cis- µ-OH-connected corner-sharing AlO4(OH)2 octahedra. Specifically, [Al(OH)(muc)] or MIP-211, is made of trans, trans-muconate linkers, and cis-µ-OH-connected corner-sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL-53-muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL-53-muc, to P/P0 ≈ 0.3 in MIP-211. Solid-state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis-positioning in MIP-211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP-211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP-211 is among the best adsorbents for adsorption-driven air conditioning and water harvesting from the air.

Enhanced sorption in an indium-acetylenedicarboxylate metal-organic framework with unexpected chains of cis-µ-OH-connected {InO6} octahedra.

Single crystals of the new metal-organic framework (MOF) In-adc (HHUD-4) were obtained through the reaction of linear acetylenedicarboxylic acid (H2adc) with In(NO3)3·xH2O as a racemic conglomerate in the chiral tetragonal space groups P4322 and P4122. Fundamentally different from other MOFs with linear linkers and trans-µ-OH-connected infinite {MO6} secondary building units as in the MIL-53-type, the linear adc2- linker leads to the formation of cis-µ-OH connected {InO6} polyhedra, which have otherwise only been found before for V-shaped ligands, as in CAU-10-H. A far-reaching implication of this finding is the possibility that trans-µ-OH/straight MIL-53-type MOFs will have polymorphs of CAU-10-H cis-µ-OH/helical topology and vice versa. HHUD-4 is a microporous MOF with a BET surface area of up to 940 m2 g-1 and a micropore volume of up to 0.39 cm3 g-1. Additionally, HHUD-4 features good adsorption uptakes of 3.77 mmol g-1 for CO2 and 1.25 mmol g-1 for CH4 at 273 K and 1 bar, respectively, and a high isosteric heat of adsorption of 11.4 kJ mol-1 for H2 with a maximum uptake of 6.36 mmol g-1 at 77 K and 1 bar. Vapor sorption experiments for water and volatile organic compounds (VOCs) such as benzene, cyclohexane and n-hexane yielded uptake values of 135, 269, 116 and 205 mg g-1, respectively, at 293 K. While HHUD-4 showed unremarkable results for water uptake and low stability for water, it exhibited good stability with steep VOC uptake steps at low relative pressures and a high selectivity of 17 for benzene/cyclohexane mixtures.

Acetylenedicarboxylate as a linker in the engineering of coordination polymers and metal-organic frameworks: challenges and potential.

Despite its simplicity as a short and rod-like linear linker, acetylenedicarboxylate (ADC) has for a long time been somewhat overlooked in the engineering of coordination polymers (CPs) and especially in the construction of porous metal-organic frameworks (MOFs). This situation seems to be stemming from the thermosensitivity of the free acid (H2ADC) precursor and its dicarboxylate, which makes the synthesis of their CP- and MOF-derivatives, as well as the evacuation of guest molecules from their pores, challenging. However, an increasing number of publications dealing with the synthesis, structural characterization and properties of ADC-based CPs and MOFs, disclose ways to tackle this obstacle. In this regard, using mostly room temperature solution synthesis or mechanochemical synthesis, and very rarely solvothermal synthesis, the ADC linker has successfully been used to form one-, two-, and three-dimensional CPs with metal cations from almost all groups of the periodic table of the elements, whereby its carboxylate groups adopt mainly all types of known coordination modes. ADC-based CPs feature properties, including negative thermal expansion, formation of non-centrosymmetric networks, long-range magnetic ordering, and solid-state polymerization. The first ADC-based microporous MOFs were obtained with Ce(IV), Hf(IV) and Zr(IV), in which the presence of the -CC- triple-bond within their backbone results in high hydrophilicity, high CO2 adsorption capacity and enthalpy, as well as the uptake of halogen vapors. This discloses the potential of ADC-MOFs for gas storage/separation and water adsorption-based applications. Furthermore, H2ADC/ADC was discovered to undergo facile in situ hydrohalogenation to yield halogen-functionalized fumarate-based CPs/MOFs. This review surveys investigations on ADC-based coordination polymers and metal-organic frameworks, and is intended to stimulate interest on this linker in chemists working in the fields of crystal chemistry or materials science.

Probing the limits of linker substitution in aluminum MOFs through water vapor sorption studies: mixed-MOFs instead of mixed-linker CAU-23 and MIL-160 materials.

We report a systematic study on the possibility of forming mixed-linker metal-organic frameworks (MOFs) spanning between the aluminum MOFs CAU-23 and MIL-160 with their 2,5-thiophenedicarboxylate (TDC) and 2,5-furandicarboxylate (FDC) linkers, respectively. The planned synthesis of a mixed-linker MOF, combining TDC and FDC in the framework turned out to yield a rather largely intricate mixture of CAU-23 and MIL-160. This is due to the different opening angles of 150° for TDC versus 120° for FDC and the concomitant cis-trans versus cis-only OH-bridges in the infinite secondary building unit {Al(µ-OH)(O2C-)} chains. At the same time, the CAU-23 phase is accompanied by the polymorphic MIL-53-TDC phase with trans-only OH-bridges. The measurement of water vapor sorption isotherms was the method of choice to confirm the formation of mixed MOFs instead of mixed-linker phases. Thereby, the water sorption isotherms indicate the simultaneous formation of both MOF phases, albeit they do not exclude mixed-linker MOFs which may have formed at low levels of substitution. The differentiation via powder X-ray diffractometry (PXRD), IR-spectroscopy and nitrogen sorption was either not conclusive enough or impossible, due to similarities of the neat MOF phases. The synthesized MOF mixtures within the TDC : FDC ratios of 38 : 62 up to 82 : 18 exhibit two or three uptake steps in the water sorption isotherm, with the first two corresponding to an overlay from the individual water sorption isotherm of CAU-23 and MIL-160 and a third one from the additional MIL-53-TDC.

Acetylenedicarboxylate-based cerium(iv) metal-organic framework with fcu topology: a potential material for air cleaning from toxic halogen vapors.

The most contracted cerium(iv)-based metal-organic framework (MOF) with fcu topology incorporating an alkyne-based linker, namely acetylenedicarboxylate (ADC), was synthesized under green conditions in water at room temperature and thoroughly characterized. The structure of this new MOF, denoted as Ce-HHU-1, was determined from powder X-ray diffraction data and Rietveld refinement and is made up of octahedral [Ce6O4(OH)4]12+ clusters, each of which is connected to other inorganic units by twelve ADC linkers to give a porous network with fcu topology analogous with UiO-66. The permanent microporosity of Ce-HHU-1 was confirmed by nitrogen sorption, meanwhile its high hydrophilicity was displayed by a type I water vapor sorption isotherm. The adsorption of CO2 in Ce-HHU-1 features a remarkably high zero-coverage isosteric heat of adsorption of 47 kJ mol-1, attributed to the presence of the -C[triple bond, length as m-dash]C- triple-bond in the framework. The latter also allows for Ce-HHU-1 to capture and irreversibly chemisorb Br2 vapors, as well as both chemi- and physisorb I2 vapors in an effective manner, making this material potentially applicable for air cleaning from toxic halogen vapors.

Acetylenedicarboxylate and In Situ Generated Chlorofumarate-Based Hafnium(IV)-Metal-Organic Frameworks: Synthesis, Structure, and Sorption Properties.

New acetylenedicarboxylate (ADC) and chlorofumarate (Fum-Cl) based hafnium-metal-organic frameworks have been synthesized by alternatively reacting acetylenedicarboxylic acid in DMF or water with appropriate hafnium salt, in the presence of acetic acid modulator. The two materials of respective ideal formulas [Hf6O4(OH)4(ADC)6] (Hf-HHU-1) and [Hf6O4(OH)4(Fum-Cl)6] (Hf-HHU-2) have been structurally characterized by powder X-ray diffraction to be UiO-66 isostructural, consisting of octahedral [Hf6O4(OH)4]12+ secondary building units each connected to other units by 12 ADC or Fum-Cl linkers into a microporous network with fcu topology. This structure was confirmed by Rietveld refinement. Hf-HHU-2 is formed by in situ hydrochlorination of acetylenedicarboxylic acid to chlorofumarate. Its presence has been determined by combined Raman spectroscopy, solid-state NMR, scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopies. Hf-HHU-1 and Hf-HHU-2 exhibit very high hydrophilicity as revealed by their water sorption profiles, meanwhile Hf-HHU-2 adsorbs CO2 with an isosteric heat of 39 kJ mol-1. Hf-HHU-2 also adsorbs molecular iodine vapor exclusively as polyiodide anions due to grafted chloro-functions on the pores surface. It has been observed that defective nanodomains with reo tolopology can be introduced in the structure of Hf-HHU-2 by variation of the linker to metal-salt molar ratio.