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.     

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).     

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.     

基于MIL⁃53(Al)的碳纳米管复合材料的制备及其用于超级电容器的性能

基于金属有机框架材料MIL?53(Al),制备出多孔碳原位生长碳纳米管(CNTs)的碳复合材料(C?MIL?53(Al)和C?Co@MIL?53(Al))。得益于MIL?53(Al)和CNTs的复合结构以及CoF2的形成,C?Co@MIL?53(Al)复合材料作为超级电容器电极时,在0.5 A·g^-1电流密度下展现出了高比电容(240.1 F·g^-1),并且经过2000次充放电循环后表现出良好的循环稳定性。     

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.     

Bimetallic RuM (M=Co,Ni) Alloy NPs Supported on MIL-110(Al): Synergetic Catalysis in Hydrolytic Dehydrogenation of Ammonia Borane

By adjusting various Ru/M (M=Co, Ni) molar ratios, a series of highly dispersed bimetallic RuM alloy nanoparticles (NPs) anchored on MIL-110(Al) have been successfully prepared via a conventional impregnation-reduction method. And they are first used as heterogeneous catalysts for the dehydrogenation reaction of AB at room temperature. The results reveal that the as-prepared Ru₁Co₁@MIL-110 and Ru₁Ni₁@MIL-110 exhibit the highest catalytic activities in different RuCo and RuNi molar ratios, respectively. It is worthy of note that the turnover frequency (TOF) values of Ru₁Co₁@MIL-110 and Ru₁Ni₁@MIL-110 catalysts reached 488.1 and 417.1 mol H₂ min^-1 (mol Ru)^-1 and the activation energies (E_a) are 31.7 and 36.0 kJ/mol, respectively. The superior catalytic performance is attributed to the bimetallic synergistic action between Ru and M, uniform distribution of metal NPs as well as bi-functional effect between RuM alloy NPs and MIL-110. Moreover, these catalysts exhibit favorable stability after 5 consecutive cycles for the hydrolysis of AB.     

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.     

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.     

金属有机骨架材料MIL-53(Al)负载钴催化剂的CO催化氧化反应性能

采用溶剂法合成了热稳定性高的金属有机骨架材料MIL-53(Al)(MIL:Materials of Institut Lavoisier),用此材料为载体负载钴催化剂用于CO的催化氧化反应,并与Al2O3负载的钴催化剂进行了对比.采用热重-差热扫描量热(TG-DSC)、傅里叶变换红外(FTIR)光谱、X射线衍射(XRD)、N2物理吸附-脱附、透射电子显微镜(TEM)、氢气程序升温还原(H2-TPR)等方法对催化剂的结构性质进行了表征.TG和N2物理吸附-脱附结果表明,载体MIL-53(Al)有好的稳定性和高的比表面积;XRD以及TEM结果表明Co/MIL-53(Al)上负载的Co3O4颗粒粒径(平均约为5.03 nm)明显小于Al2O3上Co3O4颗粒粒径(平均约为7.83 nm).MIL-53(Al)的三维多孔结构中分布均匀的位点能很好地分散固定Co3O4颗粒,高度分散的Co3O4颗粒有利于CO的催化氧化反应.H2-TPR实验发现Co/MIL(Al)催化剂的还原温度低于Co/Al2O3催化剂的还原温度,低的还原温度表现为高的催化氧化活性.CO催化氧化结果表明,MIL-53(Al)负载钴催化剂的催化活性明显高于Al2O3负载钴催化剂,MIL-53(Al)负载钴催化剂在160°C时使CO氧化的转化率达到98%,到180°C时CO则完全转化,催化剂的结构在催化反应过程中保持稳定.     

High-throughput aided synthesis of the porous metal-organic framework-type aluminum pyromellitate, MIL-121, with extra carboxylic acid functionalization

A new porous metal-organic framework (MOF)-type aluminum pyromellitate (MIL-121 or Al(OH)[H(2)btec]·(guest), (guest = H(2)O, H(4)btec = pyromellitic acid) has been isolated by using a high-throughput synthesis method under hydrothermal conditions. Its structure was determined from powder X-ray diffraction analysis using synchrotron radiation (Soleil, France) and exhibits a network closely related to that of the MIL-53 series. It is a three-dimensional (3D) framework containing one-dimensional (1D) channels delimited by infinite trans-connected aluminum-centered octahedra AlO(4)(OH)(2) linked through the pyromellitate ligand. Here the organic ligand acts as tetradendate linker via two of the carboxylate groups. The two others remain non-bonded in their protonated form, and this constitutes a rare case of the occurrence of both bonding and non-bonding organic functionalities of the MOF family. The non-coordinated -COOH groups points toward the channels to get them an open form configuration. Within the tunnels are located unreacted pyromellitic acid and water species, which are evacuated upon heating, and a porous MIL-121 phase is obtained with a Brunauer-Emmett-Teller (BET) surface area of 162 m(2) g(-1). MIL-121 has been characterized by IR, thermogravimetry (TG) analyses, and solid state NMR spectroscopy employing a couple of two-dimensional (2D) techniques such as (1)H-(1)H SQ-DQ BABA, (1)H-(1)H SQ-SQ RFDR, (27)Al{(1)H} CPHETCOR and (27)Al MQMAS.     

Separation of styrene and ethylbenzene on metal-organic frameworks: analogous structures with different adsorption mechanisms

The metal-organic frameworks MIL-47 (V(IV)O{O(2)C-C(6)H(4)-CO(2)}) and MIL-53(Al) (Al(III)(OH)·{O(2)C-C(6)H(4)-CO(2)}) are capable of separating ethylbenzene and styrene. Both materials adsorb up to 20-24 wt % of both compounds. Despite the fact that they have identical building schemes, the reason for preferential adsorption of styrene compared to ethylbenzene is very different for the two frameworks. For MIL-47, diffraction experiments reveal that styrene is packed inside the pores in a unique, pairwise fashion, resulting in separation factors as high as 4 in favor of styrene. These separation factors are independent of the total amount of adsorbate offered. This is due to co-adsorption of ethylbenzene in the space left available between the packed styrene pairs. The separation is of a non-enthalpic nature. On MIL-53, the origin of the preferential adsorption of styrene is related to differences in enthalpy of adsorption, which are based on different degrees of framework relaxation. The proposed adsorption mechanisms are in line with the influence of temperature on the separation factors derived from pulse chromatography: separation factors are independent of temperature for MIL-47 but vary with temperature for MIL-53. Finally, MIL-53 is also capable of removing typical impurities like o-xylene or toluene from styrene-ethylbenzene mixtures.     

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.     

Conversion of carbon dioxide to propionaldehyde over cobalt and rhodium nanoparticles supported on MIL-53 (Al) metal–organic framework

The two-step conversion of carbon dioxide to propionic acid and propionaldehyde has been studied in the presence of novel catalysts, cobalt and rhodium nanoparticles supported on MIL-53(Al) microporous metal–organic framework. The first step is hydrogenation of carbon dioxide with formation of synthesis gas over cobalt-containing catalyst Co/MIL-53(Al) (500°C, 1 atm), and the second step is continuous (without separation) Rh/MIL-53 (Al)-catalyzed hydroformylation of ethylene with the synthesis gas formed in the first step.     

Encapsulation of Microperoxidase-8 into MIL-101(Cr/Fe) Nanoparticles: A New Biocatalyst for the Epoxidation of Styrene

A new biocatalyst MP8@MIL-101(Cr/Fe) was prepared by immobilization of a heme octapeptide, Microperoxidase 8 (MP8) within a mixed metal MOF, MIL-101(Cr/Fe). Both MIL-101(Cr/Fe) and MP8@MIL-101(Cr/Fe) were characterized by PXRD, FTIR spectroscopy and TGA. The catalytic activity of MP8@MIL-101(Cr/Fe) for the oxidation of styrene by H₂O₂ and tBuOOH was then examined under various reaction conditions (nature of the co-solvent and of the oxidant, concentration of the oxidant and of the substrate, time, pH) and compared to that of MP8 alone. Under the best conditions used, MP8@MIL-101(Cr/Fe) was then shown to catalyze the oxidation of styrene about 3 times more efficiently than MP8 alone with approximately 50 % selectivity for styrene oxide.     

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.     

Reversible structural transition in MIL-53 with large temperature hysteresis

The metal-organic framework, MIL-53, can have a structural transition from an open-pored to a closed-pored structure by adsorbing different guest molecules. The aid of guest molecules is believed to be necessary to initiate this "breathing" effect. Using both neutron powder diffraction and inelastic neutron scattering techniques, we find that MIL-53 exhibits a reversible structural transition between an open-pored and a closed-pored structure as a function of temperature without the presence of any guest molecules. Surprisingly, this structural transition shows a significant temperature hysteresis: the transition from the open-pored to closed-pored structure occurs at approximately 125 to 150 K, while the transition from the closed-pored to open-pored structure occurs around 325 to 375 K. To our knowledge, this is first observation of such a large temperature hysteresis of a structural transition in metal-organic frameworks. We also note that the transition from the open to closed structure at low temperature shows very slow kinetics. An ab initio computer simulation is employed to investigate the possible mechanism of the transition.     

Breathing Effect via Solvent Inclusions on the Linker Rotational Dynamics of Functionalized MIL-53

The breathing effects of functionalized MIL-53-X (X=H, CH₃, NH₂, OH, and NO₂) induced by the inclusions of water, methanol, acetone, and N,N-dimethylformamide solvents were comprehensively investigated by solid-state NMR spectroscopy. 2D homo-nuclear correlation NMR provided direct experimental evidence for the host-guest interaction between the guest solvents and the MOF frameworks. The variations of the ¹H and ¹³C NMR chemical shifts in functionalized MIL-53 from the narrow pore phase transitions to large pore forms due to solvent inclusions were clearly identified. The influence of functionalized linkers and their host-guest interactions with the confined solvents on the rotational dynamics of the linkers was examined by separated-local-field MAS NMR experiments in conjunction with DFT theoretical calculations. It is found that the linker rotational dynamics of functionalized MIL-53 in narrow pore form is closely related to the computational rotational energy barrier. The BDC-NO₂ linker of activated MIL-53-NO₂ undergoes relatively faster rotation, whereas the BDC-NH₂ and BDC-OH linkers of activated MIL-53-NH₂ and MIL-53-OH exhibit relatively slower rotation. The host-guest interactions between confined solvents and MIL-53-NO₂, MIL-53-CH₃ would significantly induce an increase of the order parameters of unsubstituted carbon and reduce the rotational frequency of linkers. This study provides a spectroscopic approach for the investigation of linker rotation in functionalized MOFs at natural abundance with solvents inclusions.     

Molecular Mobility of Tert-butyl Alcohol Confined in a Breathing MIL-53 (Al) Metal-Organic Framework

We present a detailed solid-state NMR characterization of the molecular dynamics of tert-butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53(Al). ²⁷Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL-53 material with partially shrunk channels. ²H solid-state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two axes and librations of the molecule as a whole about the axis passing through the TBA C−O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τ_D of about 10⁻⁹ s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τ_l≈10⁻⁶ s at 300 K. Self-diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10⁻¹⁰ m² s⁻¹ at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).     

Engineering MIL-53(Al) flexibility by controlling amino tags

The flexibility of MIL-53(Al) can be tuned by adjusting the ratio of terephthalate and 2-amino terephthalate linkers incorporated into the framework during hydrothermal synthesis.