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.     

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.     

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).     

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.     

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。     

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.     

乙二胺改性轻金属铝-金属有机骨架材料用于CO2/CH4分离

胺类分子在CO₂的捕获中可以起到选择性提升的作用,本文选择小尺寸的乙二胺分子对具有不饱和金属位点的轻金属铝基金属有机骨架（Al-MOFs）材料MIL-100Al进行改性,利用XRD、N₂吸附和FT-IR等对改性材料的结构进行表征,测试了不同浓度的乙二胺改性的MIL-100Al对CO₂和CH₄吸附性能。结果表明,与原始的MIL-100Al材料相比,改性后的材料对CO₂吸附量有明显提高,CH₄的吸附量却降低,从而进一步提高了材料的CO₂/CH₄吸附选择性,提升了吸附分离的效果。     

乙二胺改性轻金属铝-金属有机骨架材料用于CO_2/CH_4分离

胺类分子在CO_2的捕获中可以起到选择性提升的作用,本文选择小尺寸的乙二胺分子对具有不饱和金属位点的轻金属铝基金属有机骨架(Al-MOFs)材料MIL-100Al进行改性,利用XRD、N2吸附和FT-IR等对改性材料的结构进行表征,测试了不同浓度的乙二胺改性的MIL-100Al对CO_2和CH4吸附性能。结果表明,与原始的MIL-100Al材料相比,改性后的材料对CO_2吸附量有明显提高,CH4的吸附量却降低,从而进一步提高了材料的CO_2/CH4吸附选择性,提升了吸附分离的效果。     

含钛低聚物合成高结晶度的MIL-125显著提高二甲苯异构体吸附分离选择性

二甲苯作为石油化工行业中的基础原料,其应用价值很大且需求量逐年增加,因此二甲苯分离提纯的研究具有重要理论意义和工业应用价值.同时,二甲苯异构体分离被认为是改变世界的七大分离难题之一.金属有机骨架材料MIL-125是一种典型的含钛金属有机骨架材料,被广泛应用于光催化氧化和吸附分离中.目前传统金属有机骨架材料MIL-125的合成方法需要严格的无水操作环境以及无水级的超纯试剂.由于合成过程中钛源极易水解,很难控制其水解速率.本研究组提出了一种新的MIL-125的合成方法,该法引入了耐水解的含钛无机-有机杂化低聚物,并且可以稳定合成得到高结晶度且形貌规整的MIL-125.由于MIL-125材料本身在二甲苯异构体分离领域展现出的较大优势及潜力,将新方法合成的MIL-125进行了二甲苯分离性能评价,并对其选择性显著增强的原因和不同溶剂体系下分离性能的影响进行了探讨.为了考察新方法合成的MIL-125在液相中二甲苯的分离性能.在25℃的条件下进行了二甲苯异构体的单组分液相吸附实验.单组分等温吸附曲线结果表明,pX(对二甲苯)吸附量最多,其次是oX(间二甲苯)和mX(邻二甲苯),表明MIL-125吸附剂对pX的吸附能力最强,而对mX和oX的吸附能力相对较低.同时双组分竞争吸附实验得到pX/mX的竞争选择性高达13.6,pX/oX的选择性为8左右,这种选择性优于大多数对位选择性的多孔吸附剂材料.为了在更接近工业应用的条件下评估吸附剂的动态分离性能,在造粒后的MIL-125装填的吸附柱中进行了双组分等摩尔pX/mX穿透实验,穿透曲线显示了对pX和mX的理想分离效果,并再次证实该吸附剂对pX/mX具有较高的分离选择性.穿透实验后,用溶剂洗脱吸附柱,由于洗脱过程中mX的快速解吸,pX的纯度高达99.5％.在液相吸附实验和穿透实验中溶剂的选择会影响吸附剂骨架结构对二甲苯异构体分子的分离效果.因此本文通过实验和理论计算结合探讨了不同溶剂种类对MIL-125分离性能的影响.正庚烷和均三甲苯与二甲苯分子的微量热结果表明,二甲苯分子与均三甲苯的相互作用更弱,说明二甲苯分子与骨架的结合作用更强.使用DFT理论计算了不同溶剂体系下二甲苯分子与吸附剂间的相互作用能.在两种类型的溶剂中,pX与MIL-125骨架的相互作用能均低于mX,表明MIL-125更优先吸附pX.值得注意的是,当使用均三甲苯作为溶剂时,骨架与pX/mX之间相互作用能的差异更加明显,从而导致更明显的分离效果.由于规则的晶体形貌和高结晶度,改进方法合成的MIL-125比目前的对位选择性吸附剂具有更高的pX/mX选择性.进一步的量热实验和理论计算证实了不同类型的溶剂对于二甲苯吸附分离的影响.此外,该方法解决了钛源的易水解问题,并通过引入耐水解的含钛低聚物简化了MIL-125的合成条件,这将有助于进一步将MIL-125应用于二甲苯异构体的分离,同时也拓宽了其他新型含钛材料的应用领域.     

金属有机骨架材料MIL-53(Al)负载纳米Pd颗粒：一种CO氧化的活性催化剂

通过等体积浸渍法制备了金属有机骨架材料MIL-53(Al) (MIL：Materials of Institut Lavoisier)负载纳米Pd催化剂. 采用X射线衍射(XRD)、透射电子显微镜(TEM)和X射线光电子能谱(XPS)等手段对催化剂的结构进行了表征. 催化剂在反应前后XRD衍射峰保持不变,说明载体MIL-53(Al)具有良好的稳定性. 采用TEM对催化剂进行表征,结果表明,MIL-53(Al)的多孔晶体结构有助于形成高度分散的纳米Pd颗粒,样品2.7% (w) Pd/MIL-53 中Pd颗粒的平均粒径为2.21 nm. 该催化剂在CO氧化反应中表现出较高的催化活性,115 ℃达到完全转化. 同时催化剂可循环使用,多次反应后催化活性和催化剂结构都保持稳定.     

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.     

Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of Metal-Organic Framework: A Double Solvents Approach

Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal-organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a "double solvents" method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation.     

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.     

苯部分加氢制环己烯新型Ru-B/MOF催化剂

制备了多种金属-有机骨架(MOF)材料,采用浸渍-化学还原法制备了非晶态Ru-B/MOF催化剂,考察了它们在苯部分加氢反应中的催化性能.催化性能评价结果表明,这些催化剂的初始反应速率(r0)顺序为Ru-B/MIL-53(Al)Ru-B/MIL-53(Al)-NH2Ru-B/UIO-66(Zr)Ru-B/UIO-66(Zr)-NH2Ru-B/MIL-53(Cr)Ru-B/MIL-101(Cr)Ru-B/MIL-100(Fe),环己烯初始选择性(S0)顺序为Ru-B/MIL-53(Al)≈Ru-B/MIL-53(Cr)Ru-B/UIO-66(Zr)-NH2Ru-B/MIL-101(Cr)Ru-B/MIL-53(Al)-NH2Ru-B/UIO-66(Zr)≈Ru-B/MIL-100(Fe).催化性能最好的Ru-B/MIL-53(Al)催化剂上的r0和S0分别为23 mmol·min-1·g-1和72%.采用多种手段,对催化性能差异最为显著的Ru-B/MIL-53(Al)和Ru-B/MIL-100(Fe)催化剂的物理化学性质进行了表征.发现MIL-53(Al)载体能够更好地分散Ru-B纳米粒子,粒子的平均尺寸为3.2 nm,而MIL-100(Fe)载体上Ru-B纳米粒子团聚严重,粒径达46.6 nm.更小的粒径不仅能够提供更多的活性位,而且也有利于环己烯选择性的提高.对Ru-B/MIL-53(Al)催化剂的反应条件进行了优化,在180°C和5 MPa的H2压力下,环己烯得率可达24%,展示了MOF材料用作苯部分加氢催化剂载体的良好前景.     

Predicting mixture coadsorption in soft porous crystals: experimental and theoretical Study of CO2/CH4 in MIL-53(Al)

We present a synergistic experimental and theoretical study of CO(2)/CH(4) mixture coadsorption in breathing metal-organic framework MIL-53(Al). Mixture adsorption experiments were performed and their results were analyzed by comparing them to predictions made from pure-component adsorption data using the Osmotic Framework Adsorption Solution Theory (OFAST). This analytical model, fully validated for the first time, was then used to predict coadsorption properties as a function of temperature, pressure, and mixture composition. The phase diagrams obtained show a surprising non-monotonic behavior.     

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.     

Selective palladium-loaded MIL-101 catalysts

Palladium nanoparticles (NPs) of different mean particle size have been synthesized in the host structure of the porous coordination polymer (or metal-organic framework: MOF) MIL-101. The metal-organic chemical vapor deposition method was used to load MIL-101 with the Pd precursor complex [(η(5)-C(5)H(5))Pd(η(3)-C(3)H(5))]. Loadings higher than 50 wt.% could be accomplished. Reduction of the Pd precursor complex with H(2) gave rise to Pd NPs inside the MIL-101 (Pd@MIL-101). The reduction conditions, especially the temperature, allows us to make size-conform (size of the Pd NPs correlates with the size of the cavities of the host structure of MIL-101) and undersized Pd NPs. The Pd@MIL-101 samples were characterized by X-ray diffraction, IR spectroscopy, Brauner-Emmett-Teller (BET) analysis, elemental analysis, and transmission electron microscopy (TEM). Catalytic studies, hydrogenation of ketones, were performed with selected Pd@MIL-101 catalysts. Activity, selectivity, and recyclability of the catalyst family are discussed.     

Activation of the metal-organic framework MIL-47 for selective adsorption of xylenes and other difunctionalized aromatics

The capacity and selectivity of the metal-organic framework MIL-47 for liquid phase adsorption are shown to heavily depend on the pretreatment of the material, as illustrated in detail by the particular case of selective xylene adsorption. By totally removing the uncoordinated terephthalic acid from the pores and simultaneously avoiding oxidation to nonporous V(2)O(5), pore volume and uptake of xylenes can be maximized. The presence of uncoordinated terephthalic acid in the pores improves the selectivity between p- and m-xylene. Calcination bed thickness and oven geometry influence the optimal calcination procedure. The physicochemical modifications of MIL-47 during its activation are investigated in detail with XRD, SEM, nitrogen physisorption, TGA and diffuse reflectance UV-Vis spectroscopy. Using optimally pretreated MIL-47 as adsorbent for xylene, ethyltoluene, dichlorobenzene, toluidine or cresol isomers, the para-isomer is in each case preferred over the meta-isomer in pulse chromatographic and batch experiments. The role of stacking in the selective adsorption of these isomers is discussed. In the case of the dichlorobenzenes, the meta- and para-isomers can be separated in a breakthrough experiment with a selectivity of 5.0.     

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.     

Prediction of the conditions for breathing of metal organic framework materials using a combination of X-ray powder diffraction, microcalorimetry, and molecular simulation

The adsorption of C1 to C4 linear hydrocarbons in the flexible metal organic framework MIL-53(Cr) has been followed by adsorption manometry coupled with microcalorimetry and Synchrotron X-ray powder diffraction. This experimental investigation was completed by molecular modeling. In the case of methane, the solid remains rigid whatever the adsorbate amount. However for the C2-C4 series, an increasing flexibility of the structure is observed, which is ascribed first to a breathing of the material from a large pore to a narrow pore form followed by a further expansion at high pressure. The collected thermodynamic and structural information suggests that a minimum adsorption enthalpy of ca. 20 kJ mol (-1) in the initial large pore structure of MIL-53(Cr) is required to induce the structural transition "large to narrow pore". Further, the enthalpy of adsorption can be used to predict the pressure at which the structure reopens. Finally, the magnitude of the breathing can be related to the size of the probe molecule via the van der Waals volume. The above trends have been successfully verified in the case of water and carbon dioxide. This combined experimental and theoretical approach gives the first elements for the prediction of whether or not the MIL53 and similar flexible structures will respond to gas loading and what would be the pressure required and further the amplitude of the induced breathing.