Thermodynamics of adsorption of aromatic compounds from non-aqueous solutions by MIL-53(Al) metal-organic framework

The thermodynamics of adsorption of mono-, di-, and tricyclic aromatic compounds by MIL-53(Al) metal-organic framework from their solutions in MeCN, MeOH and n-C₆H₁₄–PrⁱOH was studied for the first time. It was found that the adsorption of the test substances from solutions in MeCN and MeOH is characterized by positive values of enthalpy and entropy changes, and the adsorption from n-C₆H₁₄–PrⁱOH medium is characterized by negative enthalpy and entropy changes. Upon adsorption by MIL-53(Al) framework from polar media, aromatic compounds were proposed to transfer from the liquid phase with a higher degree of association into the solvent medium with a lower degree of association, molecules of which are disordered due to the strong interaction with the hydrophobic walls of the framework pores. It was concluded that the driving force of adsorption by MIL-53(Al) from MeCN and MeOH is increase in entropy of the system, while the factor of adsorption from n-C₆H₁₄–PrⁱOH medium is decrease in enthalpy of the adsorption system. The compensation effect in liquid-phase adsorption of aromatic compounds by MIL-53(Al) framework was discovered. The effect of the liquid phase nature on selectivity of adsorption from solutions onto investigated metal-organic framework was demonstrated.     

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.     

Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework

Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions and a flexible cyclic polyaminocarboxylate. We have demonstrated, for the first time, that a porous rigid framework can be obtained by starting from a flexible amine building block. Sorption measurements revealed that the material exhibited a high surface area (135 m(2) g(-1)) and was the best compromise between capacity and selectivity for CO(2) over CO, CH(4), N(2) and O(2); as such it is a promising new selective adsorbent for CO(2) capture.     

The effects of electronic polarization on water adsorption in metal-organic frameworks: H(2)O in MIL-53(Cr)

The effects of electronic polarization on the adsorption of water in the MIL-53(Cr) metal-organic framework are investigated using molecular dynamics simulations. For this purpose a fully polarizable force field for MIL-53(Cr) was developed which is compatible with the ab initio-based TTM3-F water model. The analysis of the spatial distributions of the water molecules within the MIL-53(Cr) nanopores calculated as a function of loading indicates that polarization effects play an important role in the formation of hydrogen bonds between the water molecules and the hydroxyl groups of the framework. As a result, large qualitative differences are found between the radial distribution functions calculated with non-polarizable and polarizable force fields. The present analysis suggests that polarization effects can significantly impact molecular adsorption in metal-organic frameworks under hydrated conditions.     

五乙烯六胺改性金属有机骨架材料MIL-101(Cr)对CO_2的吸附性能

采用溶剂热法合成金属有机骨架材料MIL-101(Cr),用回流法将五乙烯六胺(PEHA)负载到MIL-101(Cr)孔道中的不饱和金属位点上,使用扫描电镜、粉末X射线衍射、氮气物理吸附、元素分析和傅里叶变换红外光谱等表征手段考察材料的结构和形貌,测试氨基改性前后的MIL-101(Cr)在25℃、不同压力下对CO_2的吸附效果。结果表明,负载0.24 m L五乙烯六胺后的MIL-101(Cr)对CO_2的吸附效果最好,在25℃、常压下对CO_2的饱和吸附量可达58.944 mg/g,相比未负载五乙烯六胺的MIL-101(Cr)吸附量(CO_2饱和吸附量为44.208 mg/g)增加了33%。随着吸附压力的增加,MIL-101(Cr)和0.24PEHAM IL-101(Cr)对CO_2的饱和吸附量逐渐增加,当吸附压力为1.1 MPa时,两者的吸附量分别为1 147.59和1 256.74 mg/g,表明该类材料在高压下对CO_2有着良好的吸附效果。     

Adsorption separation of CO2 and N2 on MIL-101 metal-organic framework and activated carbon

In this work, the CO₂ and N₂ adsorption properties of MIL-101 metal-organic framework (MOF) and activated carbon (AC) were investigated using a standard gravimetric method within the pressure range of 0–30 bar and at four different temperatures (298, 308, 318 and 328 K). The dual-site Langmuir–Freundlich (DSLF) model was used to describe the CO₂ adsorption behaviors on these two adsorbents. The diffusion coefficients and activation energy E _a for diffusion of CO₂ in the MIL-101 and AC samples were estimated separately. Results showed that the isosteric heat of CO₂ adsorption on the MIL-101 at zero loading was much higher than that on the AC due to a much stronger interaction between CO₂ molecule and the unsaturated metal sites Cr³⁺ on MIL-101. Meanwhile, the dramatically decreased isosteric heats of CO₂ adsorption on MIL-101 indicated a more heterogeneous surface of MIL-101. Furthermore, the adsorption kinetic behaviors of CO₂ on the two samples can be well described by the micropore diffusion model. With the increase of temperature, the diffusion coefficients of CO₂ in the two samples both increased. The activation energy E _a for diffusion of CO₂ in MIL-101 was slightly lower than that in AC, suggesting that MIL-101 was much favorable for the CO₂ adsorption. The CO₂/N₂ selectivities on MIL-101 and AC were separately estimated to be 13.7 and 9.2 using Henry law constant, which were much higher than those on other MOFs.     

Post-modification of metal-organic framework for improved CO2 photoreduction efficiency

Utilizing metal-organic frameworks (MOFs) to design photocatalysts for CO₂ reduction catalysts is an excellent idea but currently restricted by the relatively low activity. Enhancing CO₂ affinity and tuning the oxidation state of metal clusters in MOFs might be a solution to improve the catalytic performance. Herein, the Cl-bridge atoms in the metal clusters of a cobalt MOF were easily exchanged with OH^?, which simultaneously oxidized a portion of Co(II) to Co(III) and resulted in a much enhanced photocatalytic activity for CO₂ reduction. In contrast, the original framework does not exhibit such superior activity. Comprehensive characterizations on their physicochemical properties revealed that the introduction of hydroxyl group not only greatly increases the CO₂ affinity but also alters the oxidation state of metal clusters, resulting in significantly improved photocatalytic activities for CO₂ reduction. This work provides important insight into the design of efficient photocatalysts.     

Infrared study of the influence of reducible iron(III) metal sites on the adsorption of CO, CO2, propane, propene and propyne in the mesoporous metal-organic framework MIL-100

The present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate [{Fe(3)O(H(2)O)(2)F(0.81)(OH)(0.19)}{C(6)H(3)(CO(2))(3)}(2)] denoted MIL-100(Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe(III) sample, outgassed at 150 °C, and on the partially reduced Fe(II/III), outgassed at 250 °C. The exposure of MIL-100(Fe) to CO(2), propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron(II) sites despite the weaker Lewis acidity of cus Fe(2+) sites compared to that of Fe(3+) ones, as shown by pyridine adsorption.     

Broadly hysteretic H2 adsorption in the microporous metal-organic framework Co(1,4-benzenedipyrazolate)

Reaction of Co(CF3SO3)2 with the new molecule 1,4-benzenedi(4'-pyrazolyl) (H2BDP) in N,N'-diethylformamide (DEF) at 130 degrees C generates the metal-organic framework Co(BDP).2DEF.H2O (1). X-ray analysis reveals the structure of 1 to contain chains of tetrahedrally ligated Co2+ ions linked through BDP2- ligands to generate a three-dimensional framework with 10 x 10 A2 channels. Thermogravimetric data shows the framework to have a high thermal stability, and complete desolvation occurs upon heating at 170 degrees C under dynamic vacuum for two days to afford 1d. X-ray powder diffraction data indicates that 1d possesses a substantially different structure, but converts back to 1 upon exposure to DEF, consistent with the presence of a flexible framework. Nitrogen adsorption isotherms measured for 1d at 77 and 87 K reveal an unprecedented five-step adsorption process and a Langmuir surface area of 2670 m2/g. In addition, high-pressure H2 adsorption data reveal hysteretic uptake and release, with hysteresis loops of width 1.1, 3.8, 13, and 27 bar that shift to higher pressures as the temperature increases from 50 to 65, 77, and 87 K, respectively. The high H2 uptake capacity of 5.5 excess wt % at 50 K suggests that such materials could potentially find utility for hydrogen storage via a kinetic trapping mechanism. Variable-temperature kinetics measurements have also allowed the first study of H2 diffusion within a metal-organic framework, revealing an energy barrier of 0.62 kJ/mol for H2 diffusing within the pores.     

Argon adsorption on Cu3(benzene-1,3,5-tricarboxylate)2(H2O)3 metal-organic framework

Using volumetric adsorption techniques, we have measured the adsorption of argon on Cu3(BTC)2(H2O)3, (BTC = benzene-1,3,5-tricarboxylate), a microporous metal-organic framework structure, at temperatures between 66 and 143 K. In addition to the experiments, we have used Grand Canonical Monte Carlo simulations to calculate the adsorption isotherm of argon at 87 K. Our experimental and theoretical results are compared to those of previous studies. The experiments were performed using a high density of points, allowing us to obtain, in detail, the isosteric heat's coverage dependence. Our values from the simulations are in reasonable agreement with those obtained in the experiments.     

Uptake of liquid alcohols by the flexible Fe(III) metal-organic framework MIL-53 observed by time-resolved in situ X-ray diffraction

A comprehensive, time‐resolved, energy‐dispersive X‐ray diffraction study of the uptake of liquid alcohols (methanol, ethanol, propan‐1‐ol and propan‐2‐ol) by the flexible metal‐organic framework solid MIL‐53(Fe)[H₂O] is reported. In the case of the primary alcohols, a fluorinated version of the MIL‐53(Fe) host (C2/c symmetry V ca. 1000 ų), in which a fraction of framework hydroxides are replaced by fluoride, shows uptake of alcohols to give initially a partially expanded phase (C2/c symmetry, V ca. 1200 ų) followed by an expanded form of the material (either Imcm or Pnam symmetry, V ca. 1600 ų). In the case of methanol–water mixtures, the EDXRD data show that the partially open intermediate phase undergoes volume expansion during its existence, before switching to a fully open structure if concentrated methanol is used; analogous behaviour is seen if the initial guest is propan‐2‐ol, which then is replaced by pyridine, where a continuous shift of Bragg peaks within C2/c symmetry is observed. In contrast to the partially fluorinated materials, the purely hydroxylated host materials show little tendency to stabilise partially open forms of MIL‐53(Fe) with primary alcohols and the kinetics of guest introduction are markedly slower without the framework fluorination: this is exemplified by the exchange of water by propan‐2‐ol, where a partially open C2/c phase is formed in a step‐wise manner. Our study defines the various possible pathways of liquid‐phase uptake of molecular guests by flexible solid MIL‐53(Fe).     

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.     

Selective CO(2) adsorption in a metal-organic framework constructed from an organic ligand with flexible joints

A metal-organic framework (SNU-110) constructed from an organic ligand with flexible joints exhibits selective CO(2) adsorption over N(2), O(2), H(2) and CH(4) gases.     

Twelve-connected porous metal-organic frameworks with high H(2) adsorption

The twelve-connected metal-organic frameworks {[Ni(3)(OH)(L)(3)].n(solv)}(infinity) and {[Fe(3)(O)(L)(3)].n(solv)}(infinity) [LH(2) = pyridine-3,5-bis(phenyl-4-carboxylic acid)] have been prepared and characterised: these materials can be desolvated to form porous materials that show adsorption of H(2) up to 4.15 wt% at 77 K.     

Adsorption properties and structure of CO2 adsorbed on open coordination sites of metal-organic framework Ni2(dhtp) from gas adsorption, IR spectroscopy and X-ray diffraction

The microporous metal-organic framework Ni(2)(dhtp) (H(4)dhtp=2,5-dihydroxyterephthalic acid) shows distinct end-on CO(2) coordination to coordinatively unsaturated nickel sites giving rise to high CO(2) adsorption capacity at sub-atmospheric pressures and ambient temperatures.     

Charge distribution in metal organic framework materials: transferability to a preliminary molecular simulation study of the CO(2) adsorption in the MIL-53 (Al) system

Density functional theory calculations have been performed in order to extract the charge distribution in the aluminium-containing MIL-53 structure, to allow further computational studies of adsorption in these materials. Both cluster and periodic methods have been used and the charges calculated for each atom constituting the organic and inorganic part of the material, were discussed. Preliminary grand canonical Monte Carlo simulations, based on a consistent set of potential parameters and this newly derived charge distribution, predicted for enthalpies of adsorption for CO(2) at low coverage in the "large" and "narrow" pore versions of MIL-53 (Al) to be significantly different. These calculated enthalpies reproduced the two distinct ranges of values observed by microcalorimetry on either side of 6 bars quite well. This agreement between experiment and simulation validated our previous assumption, suggesting a structural switching of the hybrid material during the adsorption process. The microscopic mode of interaction between the hybrid porous framework and the CO(2) adsorption was then carefully analysed in both of the MIL-53 (Al) structures.     

Understanding the preferential adsorption of CO2 over N2 in a flexible metal-organic framework

The unusual uptake behavior and preferential adsorption of CO(2) over N(2) are investigated in a flexible metal-organic framework system, Zn(2)(bdc)(2)(bpee), where bpdc = 4,4'-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO(2) with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO(2) with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO(2). DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der Waals-Density Functional Theory method, showing that the more energetically favorable positions for the CO(2) molecules are closer to the C═C bond of the bpee and the C-C bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guest-host interactions and structural changes in the framework.