Modeling the effect of structural changes during dynamic separation processes on MOFs

A model able to describe the effect of structural changes in the adsorbent or adsorbed phase during the dynamic (breakthrough) separation of mixtures on metal-organic frameworks (MOFs) is presented. The methodology is exemplified for a few pertinent case studies: the separation of xylene isomers and ethylbenzene on the flexible MOF MIL-53 and the rigid MOF MIL-47. At low pressures, no preferential adsorption of any component occurs on both MOFs. Contrarily, at higher pressures separation of ethylbenzene (EB) from o-xylene (oX) occurs on MIL-53 as a result of the breathing phenomenon within the MIL-53 structure. The increase in selectivity, starting from the gate-opening pressure, could be modeled by using a pressure-dependent saturation capacity for the most strongly adsorbed component oX. In the separation of m-xylene (mX) from p-xylene (pX) on the rigid MOF MIL-47, separation at higher pressures is a result of preferential stacking of pX. Here, the selectivity increases once the adsorption of pX switches from a single to a double file adsorption. By implementing a loading dependent adsorption constant for pX, the different unconventional breakthrough profiles and the observed selectivity profile on MIL-47 can be simulated. A similar methodology was used for the separation of EB from pX on MIL-47, where the separation is a result from steric constraints imposed onto the adsorption of EB.     

Selective adsorption and separation of ortho-substituted alkylaromatics with the microporous aluminum terephthalate MIL-53

The metal-organic framework MIL-53(Al) was tested for selective adsorption and separation of xylenes and ethylbenzene, ethyltoluenes, and cymenes using batch, pulse chromatographic, and breakthrough experiments. In all conditions tested, MIL-53 has the largest affinity for the ortho-isomer among each group of alkylaromatic compounds. Separations of the ortho-compounds from the other isomers can be realized using a column packed with MIL-53 crystallites. As evidenced by Rietveld refinements, specific interactions of the xylenes with the pore walls of MIL-53 determine selectivity. In comparison with the structurally similar metal-organic framework MIL-47, the selectivities among alkylaromatics found for MIL-53 are different. Separation of ethyltoluene and cymene isomers is more effective on MIL-53 than on MIL-47; the pores of MIL-53 seem to be a more suitable environment for hosting the larger ethyltoluene and cymene isomers than those of MIL-47.     

A rationale for the large breathing of the porous aluminum terephthalate (MIL-53) upon hydration

Aluminum 1,4-benzenedicarboxylate Al(OH)[O(2)C-C(6)H(4)-CO(2)]. [HO(2)C-C(6)H(4)-CO(2)H](0.70) or MIL-53 as (Al) has been hydrothermally synthesized by heating a mixture of aluminum nitrate, 1,4-benzenedicarboxylic acid, and water, for three days at 220 degrees C. Its 3 D framework is built up of infinite trans chains of corner-sharing AlO(4)(OH)(2) octahedra. The chains are interconnected by the 1,4-benzenedicarboxylate groups, creating 1 D rhombic-shaped tunnels. Disordered 1,4-benzenedicarboxylic acid molecules are trapped inside these tunnels. Their evacuation upon heating, between 275 and 420 degrees C, leads to a nanoporous open-framework (MIL-53 ht (Al) or Al(OH)[O(2)C-C(6)H(4)-CO(2)]) with empty pores of diameter 8.5 A. This solid exhibits a Langmuir surface area of 1590(1) m(2)g(-1) together with a remarkable thermal stability, since it starts to decompose only at 500 degrees C. At room temperature, the solid reversibly absorbs water in its tunnels, causing a very large breathing effect and shrinkage of the pores. Analysis of the hydration process by solid-state NMR ((1)H, (13)C, (27)Al) has clearly indicated that the trapped water molecules interact with the carboxylate groups through hydrogen bonds, but do not affect the hydroxyl species bridging the aluminum atoms. The hydrogen bonds between water and the oxygen atoms of the framework are responsible for the contraction of the rhombic channels. The structures of the three forms have been determined by means of powder X-ray diffraction analysis. Crystal data for MIL-53 as (Al) are as follows: orthorhombic system, Pnma (no. 62), a = 17.129(2), b = 6.628(1), c = 12.182(1) A; for MIL-53 ht (Al), orthorhombic system, Imma (no. 74), a = 6.608(1), b = 16.675(3), c = 12.813(2) A; for MIL-53 lt (Al), monoclinic system, Cc (no. 9), a = 19.513(2), b = 7.612(1), c = 6.576(1) A, beta = 104.24(1) degrees.     

Very large breathing effect in the first nanoporous chromium(III)-based solids: MIL-53 or Cr(III)(OH) x [O(2)C-C(6)H(4)-CO(2)] x [HO(2)C-C(6)H(4)-CO(2)H](x) x H(2)O(y)

The first three-dimensional chromium(III) dicarboxylate, MIL-53as or Cr(III)(OH) x [O(2)C-C(6)H(4)-CO(2)].[HO(2)C-C(6)H(4)-CO(2)H](0.75), has been obtained under hydrothermal conditions (as: as-synthesized). The free acid can be removed by calcination giving the resulting solid, MIL-53ht or Cr(III)(OH) x [O(2)C-C(6)H(4)-CO(2)]. At room temperature, MIL-53ht adsorbs atmospheric water immediately to give Cr(III)(OH) x [O(2)C-C(6)H(4)-CO(2)] x H(2)O or MIL-53lt (lt: low-temperature form, ht: high-temperature form). Both structures, which have been determined by using X-ray powder diffraction data, are built up from chains of chromium(III) octahedra linked through terephthalate dianions. This creates a three-dimensional structure with an array of one-dimensional large pore channels filled with free disordered terephthalic molecules (MIL-53as) or water molecules (MIL-53lt); when the free molecules are removed, this leads to a nanoporous solid (MIL-53ht) with a Langmuir surface area over 1500 m(2)/g. The transition between the hydrated form (MIL-53lt) and the anhydrous solid (MIL-53ht) is fully reversible and followed by a very high breathing effect (more than 5 A), the pores being clipped in the presence of water molecules (MIL-53lt) and reopened when the channels are empty (MIL-53ht). The thermal behavior of the two solids has been investigated using TGA and X-ray thermodiffractometry. The sorption properties of MIL-53lt have also been studied using several organic solvents. Finally, magnetism measurements performed on MIL-53as and MIL-53lt revealed that these two phases are antiferromagnetic with Néel temperatures T(N) of 65 and 55 K, respectively. Crystal data for MIL-53as is as follows: orthorhombic space group Pnam with a = 17.340(1) A, b = 12.178(1) A, c = 6.822(1) A, and Z = 4. Crystal data for MIL-53ht is as follows: orthorhombic space group Imcm with a = 16.733(1) A, b = 13.038(1) A, c = 6.812(1) A, and Z = 4. Crystal data for MIL-53lt is as follows: monoclinic space group C2/c with a = 19.685(4) A, b = 7.849(1) A, c = 6.782(1) A, beta = 104.90(1) degrees, and Z = 4.     

Adsorption of CO2 in metal organic frameworks of different metal centres: Grand Canonical Monte Carlo simulations compared to experiments

A Grand Canonical Monte Carlo study has been performed in order to compare the different CO₂ adsorption mechanisms between two members of the MIL-n family of hybrid metal-organic framework materials. The MIL-53 (Al) and MIL-47 (V) systems were considered. The results obtained confirm that there is a structural interchange between a large pore and narrow pore forms of MIL-53 (Al), not seen with the MIL-47 (V) material, which is a consequence of the presence of μ ₂-OH groups. The interactions between the CO₂ molecules and these μ ₂ OH groups mainly govern the adsorption mechanism in this MIL-53 (Al) material. The subsequent breaking of these adsorption geometries after the adsorbate loading increases past the point where no more preferred adsorption sites are available, are proposed as key features of the breathing phenomenon. After this, any new adsorbates introduced into the MIL-53 (Al) large pore structure experience a homogeneous adsorption environment with no preferential adsorption sites in a similar way to what occurs in MIL-47 (V).     

Incorporation of metallocenes into the channel structured Metal-Organic Frameworks MIL-53(Al) and MIL-47(V)

A selection of metallocene inclusion compounds with channel structured MOFs (MOF = Metal-Organic Framework) were obtained via solvent-fee adsorption of the metallocenes from the gas-phase. The adsorbate structures ferrocene(0.5)@MIL-53(Al) (MIL-53(Al) = [Al(OH)(bdc)](n) with bdc = 1,4-terephthalate), ferrocene(0.25)@MIL-47(V) (MIL-47(V) = [V(O)(bdc)](n)), cobaltocene(0.25)@MIL-53(Al), cobaltocene(0.5)@MIL-47(V), 1-formylferrocene(0.33)@MIL-53(Al), 1,1'dimethylferrocene(0.33)@MIL-53(Al), 1,1'-diformylferrocene(0.5)@MIL-53(Al) were determined from powder X-ray diffraction data and were analyzed concerning the packing and orientation of the guest species. The packing of the ferrocene guest molecules inside MIL-47(V) is significantly different compared to MIL-53(Al) due to the lower breathing effect and weaker hydrogen bonds between the guest molecules and the host network in the case of MIL-47(V). The orientation of the metallocene molecule is also influenced by the substituents (CH(3) and CHO) at the cyclopentadienyl ring and the interaction with the bridging OH group of MIL-53(Al). The inclusion of redox active cobaltocene into MIL-47(V) leads to the formation of a charge transfer compound with a negatively charged framework. The reduction of the vanadium centers is stoichiometric. The resulting material is a mixed valence compound with a V(3+)/V(4+) ratio of 1:1. The new compounds were characterized via thermal gravimetric analysis, infrared spectroscopy, solid state NMR, and differential pulse voltammetry. Both systems are 1D-channel pore structures. The metallocene adsorbate induced breathing effect of MIL-53(Al) is more pronounced compared to MIL-47(V), this can be explained by the different bridging groups between the MO(6) clusters.     

Evidence of CO(2) molecule acting as an electron acceptor on a nanoporous metal-organic-framework MIL-53 or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2))

The adsorption mode of CO(2) at low coverage in the nanoporous metal benzenedicarboxylate MIL-53(Cr) or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2)) has been identified using IR spectroscopy; the red shift of the nu(3) band and the splitting of the nu(2) mode of CO(2) in addition to the shifts of the nu(OH) and delta(OH) bands of the MIL-53(Cr) hydroxyl groups provide evidence that CO(2) interacts with the oxygen atoms of framework OH groups as an electron-acceptor via its carbon atom; this is the first example of such an interaction between CO(2) and bridged OH groups in a solid.     

Remarkable adsorptive performance of a metal-organic framework, vanadium-benzenedicarboxylate (MIL-47), for benzothiophene

Liquid-phase adsorption of benzothiophene over isotypic MOFs such as MIL-47 and MIL-53(Al, Cr) has shown that a metal ion of a MOF-type material has a dominant role in adsorptive desulfurization and MIL-47 has a remarkable performance.     

Estimation of the desorption energy of dichloromethane and water in MIL-53 by DSC and ab-initio calculations

Desorption energies of dichloromethane(CH_2Cl_2) and water(H_2O) in a metal-organic framework, MIL-53(Al), were investigated by the combination of experimental(differential scanning calorimeter, DSC) and computational(ab-initio calculations) methods. The differences of desorption energy and natural log of the frequency factor of CH_2Cl_2 and H_2O in MIL-53(Al) were analyzed by a thermo active process using DSC measurements. The interaction energy of guest molecules with MIL-53(Al), which corresponds to the desorption in the thermal active process, was explored using ab-initio calculation. As a result of the difference in the interaction energies of H_2O and CH_2Cl_2 in MIL-53(Al), the site near the μ_2-OH groups has two potential wells. Both experimentally and computationally, MIL-53 presents the preferential adsorption of CH_2Cl_2 than H_2O.     

Chemical and thermal stability of isotypic metal-organic frameworks: effect of metal ions

Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal-oxygen bond in common oxides like Al(2)O(3), Cr(2)O(3), and V(2)O(5). In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.     

Pore-filling-dependent selectivity effects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47

Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal-organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 degrees C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 degrees C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70-150 degrees C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.     

Study of selective adsorption of aromatic compounds from solutions by the flexible MIL-53(Al) metal-organic framework

The dynamic method under conditions close to equilibrium was applied to study the liquid-phase adsorption in the Henry region for a series of aromatic compounds on the MIL-53(Al) metal-organic framework at different temperatures. The interpretation of the obtained experimental adsorption data was based on the TOPOS analysis of the structure of the cavities in the MIL-53(Al) framework using the Voronoi—Dirichlet polyhedra concept. It is shown that the adsorption activity of the investigated material under the liquid-phase conditions is governed by a possible expansion of the channels and cavities in the structure and by a breathing effect of the structure caused by the temperature variation. The selectivity of adsorption shown by MIL-53(Al) for a series of the studied compounds is due to the adsorbate—adsorbent π—π-interaction and hydrogen bonding of adsorbate molecules with Brönsted acid sites of the metal-organic framework. High adsorption selectivity of the MIL-53(Al) framework were found for compounds differed in the number of aromatic rings in the molecule and the presence of the methyl substituent, as well as for aromatic hydrocarbons and their sulfur-containing heterocyclic analogs.     

金属-有机骨架对苯二甲酸酯-铝吸附水中酚类化合物动力学和热力学

以吸附等温线、动力学和热力学等方法研究了金属-有机骨架对苯二甲酸酯-铝[MIL-53(Al),MIL:Materials of Institut Lavoisier]对水中邻硝基苯酚、苯酚和邻苯二酚的吸附行为。 MIL-53(Al)对上述酚类化合物的吸附符合准二级吸附动力学模型,且包含表面吸附和孔内扩散两个过程。 吸附热力学结果表明,MIL-53(Al)对酚类化合物的吸附是自发的,且为吸热和熵增加过程。 在40 ℃条件下,MIL-53(Al)对邻硝基苯酚、苯酚和邻苯二酚的吸附量分别为78.6、30.5和16.5 mg/g。     

High-performance liquid chromatographic separation of position isomers using metal-organic framework MIL-53(Al) as the stationary phase

Metal-organic framework MIL-53(Al) was explored as the stationary phase for high-performance liquid chromatographic separation of position isomers using a binary and/or polar mobile phase. Baseline separations of xylene, dichlorobenzene, chlorotoluene and nitrophenol isomers were achieved on the slurry-packed MIL-53(Al) column with high resolution and good precision. The effects of mobile phase composition, injected sample mass and temperature were investigated. The separation of xylene, dichlorobenzene, chlorotoluene and nitrophenol isomers on MIL-53(Al) were controlled by entropy change.     

Conformational analysis of 1,2-dichloroethane adsorbed in metal-organic frameworks

This paper describes the conformational analysis of 1,2-dichloroethane adsorbed into three different metal-organic frameworks, MIL-53(Al), MIL-68(In), MIL-53-NH₂(Al), by using FT-Raman spectroscopy in combination with powder XRD and TGA. For non-polar frameworks, the main guest-host interactions are van der Waal interactions between the CH bonds of 1,2-dichloroethane (DCE) and the π system of terephthalate ligands. The polar framework of MIL-53-NH₂ is able to stabilize the gauche conformation of DCE at room temperature. The conformational enthalpy of each system was determined through variable temperature FT-Raman spectroscopy. Furthermore, the line-width of the Raman bands provides information regarding the molecular motion of the halocarbons at various temperatures inside the framework.     

Control of the coordination status of the open metal sites in metal-organic frameworks for high performance separation of polar compounds

Metal-organic frameworks (MOFs) with open metal sites have great potential for enhancing adsorption separation of the molecules with different polarities. However, the elution and separation of polar compounds on such MOFs packed columns using nonpolar solvents is difficult due to too strong interaction between polar compounds and the open metal sites. Here, we report the control of the coordination status of the open metal sites in MOFs by adjusting the content of methanol (MeOH) in the mobile phase for fast and high-resolution separation of polar compounds. To this end, high-performance liquid chromatographic separation of nitroaniline, aminophenol and naphthol isomers, sulfadimidine, and sulfanilamide on the column packed with MIL-101(Cr) possessing open metal sites was performed. The interaction between the open metal sites of MIL-101(Cr) and the polar analytes was adjusted by adding an appropriate amount of MeOH to the mobile phase to achieve the effective separation of the polar analytes due to the competition of MeOH with the analytes for the open metal sites. Fourier transform infrared spectra and X-ray photoelectron spectra confirmed the interaction between MeOH and the open metal sites of MIL-101(Cr). Thermodynamic parameters were measured to evaluate the effect of the content of MeOH in the mobile phase on the separation of polar analytes on MIL-101(Cr) packed column. This approach provides reproducible and high performance separation of polar compounds on the open metal sites-containing MOFs.     

Hydrogen adsorption in the nanoporous metal-benzenedicarboxylate M(OH)(O2C-C6H4-CO2) (M = Al3+, Cr3+), MIL-53

Hydrogen adsorption has been studied in the nanoporous metal-benzenedicarboxylate M(OH)(O2C-C6H4-CO2) (M = Al3+, Cr3+); these solids show a hydrogen storage capacity of 3.8 and 3.1 wt.% respectively when loaded at 77 K under 1.6 MPa.     

VIII(OH)[O2C-C6H4-CO2].(HO2C-C6H4-CO2H)x(DMF)y(H2O)z(or MIL-68), a new vanadocarboxylate with a large pore hybrid topology: reticular synthesis with infinite inorganic building blocks?

V(III)(OH)[O(2)C-C(6)H(4)-CO(2)].(HO(2)C-C(6)H(4)-O(2)H)(x)(DMF)(y)(H(2)O)(z) or MIL-68 was solvothermally synthesised in a non-aqueous medium. Its structure, built up from octahedral chains connected by terephthalate linkers, exhibits large hexagonal channels containing different occluded moieties. Their irreversible removal releases a specific surface area of 603(22) m(2).g(-1)(BET).     

Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H′-octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB), into the pores of the well-known metal-organic frameworks MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF-AZB_0.34@MIL-53(Al), a structural model based on high-resolution synchrotron powder diffraction data was developed and the host-guest and guest-guest interactions were elucidated from this model. These interactions of O−H⋅⋅⋅F and π⋅⋅⋅π type were confirmed by significant shifts of the O−H frequencies in loaded and unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably, all of the synthesized F-AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo-isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.     

Different adsorption behaviors of methane and carbon dioxide in the isotypic nanoporous metal terephthalates MIL-53 and MIL-47

A distinct step in the isotherm occurs during the adsorption of CO2 on MIL-53 at 304 K. Such behavior is neither observed during the adsorption of CH4 on MIL-53 nor during the adsorption on the isostructural MIL-47. This phenomenon seems to be due to a different mechanism than that of previous adsorption steps on MOF samples. It is suggested that a breathing behavior is induced in MIL-53 during CO2 adsorption.