How Does Competition between Anionic Pollutants Affect Adsorption onto Mg–Al Layered Double Hydroxide? Three Competition Schemes

Competitive adsorption of three anionic orange-type dyes (i.e., methyl orange, orange II, orange G) and three inorganic anions (HPO₄^2–, SO₄^2–, CrO₄^2–) onto Mg–Al layered double hydroxide (LDH) from bisolute aqueous solutions has been studied in view of potential application of the sorption-based technology in dye removal from textile wastewater effluents. The individual solute affinities for the LDH sample were inferred from the calorimetry measurements of the cumulative enthalpy of displacement in single-solute systems, thus leading to the following affinity order: CrO₄^2– ≈ SO₄^2– < OG^2– ≈ HPO₄^2– < OII^– ≪ MO^–. The individual and composite adsorption isotherms from bisolute aqueous solutions were determined by a solution depletion technique and supplemented by XRD studies on the well-defined adsorption systems corresponding to selected points on isotherms. Based on the shape of the individual adsorption isotherms for a dye and inorganic anion, it was possible to distinguish three different competition schemes: when the reduction in adsorption extent concerns mainly one of the solutes (Type I and Type III) or when this reduction applies to both solutes (Type II). The order of decreasing effectiveness of the inorganic anion in depressing the retention capacity of Mg–Al–LDH–NO₃ toward a given dye, <i>on a per-equivalent basis</i>, was as follows: HPO₄^2– > SO₄^2– ≈ CrO₄^2–, methyl orange; HPO₄^2– > CrO₄^2– > SO₄^2–, orange II; HPO₄^2– ≈ SO₄^2– > CrO₄^2–, orange G

Study of Adsorption and Intercalation of Orange-Type Dyes into Mg–Al Layered Double Hydroxide

In the context of depollution and textile wastewater treatment, the sorption-based processes are good candidates to achieve the efficient removal of such toxics substances as dyes. In the present study, the exchange–adsorption from aqueous solutions of three azoic dyes, Methyl Orange (MO), Orange II (OII), and Orange G (OG), onto Mg–Al–LDH–NO₃ layered double hydroxides (LDH, molar Mg:Al ratio of 2) was investigated through monitoring all retained and removed species in combination with direct calorimetry and X-ray diffraction measurements. Kinetic curves, determined for several initial concentrations of the three dyes, indicated that the process was fast (between 60 and 100 min) and followed the pseudo-second order model in line with the passage of the removed dye through a chemisorption stage, thus constituting the rate-limiting step. Dye adsorption isotherms (H2-type) showed some differences in the maximum adsorption quantity (5.5 mmol g^–1, MO; 2.7 mmol g^–1, OII; 1.7 mmol g^–1, OG), consistent with anionic exchange capacity and adsorption on the external surface, depending on the cross-sectional area of the dye species and with their hydrophobic–hydrophilic character. The uptake of sodium cations as a function of the dye type and the surface coverage ratio pointed that the counterions can either stay in solution or be adsorbed to neutralize the free –SO₃^– moieties or other anionic species in the interlayer space. The cumulative enthalpy of displacement was negative in conformity with the exothermic character of the overall process. The intercalation of dye anions into the interlayer space of LDH materials led to its expansion with various distances being dependent both on the dye type and on the overall exchange balance. The latter included also the desorption of nitrates as well as the presence of carbonate species within the interlayer space, due to exchange in open systems exposed to the ambient atmosphere

A Joint Experimental/Computational Exploration of the Dynamics of Confined Water/Zr-Based MOFs Systems

A joint modeling (molecular dynamics simulations)/​experimental (broadband dielectric spectroscopy) approach was conducted to investigate the water adsorption in the UiO-66­(Zr) MOF, and its functionalized versions bearing acidic polar groups (−COOH or 2-COOH per linker). It was first pointed out that the proton conduction measured at room temperature increases with (i) the water uptake and (ii) the concentration of the free acidic carboxylic functions. This trend was further analyzed in light of the preferential arrangements of water within the pores of each MOF as elucidated by molecular dynamics simulations. Indeed, it was revealed that the guest molecules preferentially (i) form interconnected clusters within the UiO-66­(Zr)­s cages and generate a H-bond network responsible for the proton propagation and (ii) strongly interact with the −COOH grafted functions, resulting in the creation of additional charge carriers in the case of the hydrated functionalized solids. Broadband dielectric spectroscopy shed light on how these water configurations impact the local dynamics of both the water molecules and the MOF frameworks. The dielectric relaxation investigation evidenced the existence of one or two relaxation processes, depending on the nature of the UiO-66­(Zr) framework and its hydration level. Compared to the dielectric behavior of water confined in a large variety of media, it was thus concluded that the fastest process corresponds to the dynamics of the water molecules forming clusters, while the slowest process is due to the concerted local motion of water/ligand entities

Diffusion of Binary CO₂/CH₄ Mixtures in the MIL-47(V) and MIL-53(Cr) Metal–Organic Framework Type Solids: A Combination of Neutron Scattering Measurements and Molecular Dynamics Simulations

The dynamics of CO₂ and CH₄ in a mixture of different compositions has been explored in two metal–organic frameworks, namely, MIL-47­(V) and MIL-53­(Cr), by combining molecular dynamics (MD) simulations and quasi-elastic neutron scattering (QENS) measurements. The experimental and simulated self-diffusion coefficient (<i>D</i>_s) values for CH₄ are in very good agreement in the whole range of the CO₂ explored loadings. It is clearly stated that CH₄ which shows a fast diffusivity at low loading becomes significantly slower in both metal–organic frameworks (MOFs) when CO₂ molecules are introduced within the porosities of these materials. Further, compared to its behavior in a single component, CH₄ tends to diffuse slightly faster in the presence of CO₂. The MD simulations revealed that this speeding up is concomitant with a mutual speeding up or a slowing down of the slower CO₂ molecules in MIL-47­(V) and MIL-53­(Cr), respectively. Analysis of the MD trajectories emphasizes that both gases in the mixture follow individually a 1D-type diffusion mechanism in both MOFs, where the CO₂ molecules diffuse close to the pore wall while the motions of CH₄ are restricted in the central region of the tunnel

Adsorption of Benzene in the Cation-Containing MOFs MIL-141

The adsorption of benzene in the cation-containing metal–organic framework (MOF) MIL-141­(Cs) was explored by manometry measurements coupled with Monte Carlo simulations. This joint experimental/modeling approach demonstrates that this solid shows a high affinity for benzene that does not result from a direct interaction between the guest molecules and the Cs⁺ cations, in contrast to what is commonly observed in zeolites. This behavior was attributed to the high degree of confinement of Cs⁺, which prevents any cation detrapping upon adsorption, as revealed by dielectric relaxation spectroscopy and molecular dynamics simulations. This peculiar adsorption behavior is further discussed in relation to that of other alkali extraframework cations including Li⁺, Na⁺, K⁺, and Rb⁺

How Linker’s Modification Controls Swelling Properties of Highly Flexible Iron(III) Dicarboxylates MIL-88

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4′-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups

How Linker’s Modification Controls Swelling Properties of Highly Flexible Iron(III) Dicarboxylates MIL-88

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4′-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups

How Linker’s Modification Controls Swelling Properties of Highly Flexible Iron(III) Dicarboxylates MIL-88

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4′-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups

How Linker’s Modification Controls Swelling Properties of Highly Flexible Iron(III) Dicarboxylates MIL-88

A series of organically modified iron(III) terephthalate MIL-88B and iron(III) 4,4′-biphenyl dicarboxylate MIL-88D flexible solids have been synthesized and characterized through a combination of X-ray diffraction, IR spectroscopy, and thermal analysis (MIL stands for Material from Institut Lavoisier). The swelling amplitude of the highly flexible MOFs tuned by introducing functional groups onto the phenyl rings shows a clear dependence on the steric hindrance and on the number of groups per aromatic ring. For instance, while the introduction of four methyl groups per spacer in dried MIL-88B results in a large permanent porosity, introducing two or four methyl groups in MIL-88D allows an easier pore opening in the presence of liquids without drastically decreasing the swelling magnitude. The influence of the degree of saturation of the metal center and the nature of the solvent on the swelling is also discussed. Finally, a computationally assisted structure determination has led to a proposal of plausible structures for the closed (dried) and open forms of modified MIL-88B and MIL-88D and to evaluation of their framework energies subject to the nature of the functional groups