A metal-organic framework with highly polar pore surfaces: selective CO2 adsorption and guest-dependent on/off emission properties

A 3D porous Zn(II) metal-organic framework {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)(H(2)O)]·4H(2)O} (1; H(2)dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H(4)dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn(2)(H(2)dht)(dht)(0.5)(azpy)(0.5)]} (1') provides 52.7% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated Zn(II) sites and pendant hydroxyl groups of H(2)dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO(2) over other gases, such as N(2), H(2), O(2) and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (λ(max)≈530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H(2)dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH···OCO hydrogen-bonding interaction in the H(2)dht linker in 1'/1.     

X-ray diffraction structure analysis of MCM-48 mesoporous silica

The structure of MCM-48 mesoporous silicate materials has been fully characterized from X-ray diffraction data by applying recently developed methods of mesostructure analysis and full-profile refinement. The pore wall thickness of both as-made and calcined MCM-48 was determined with high precision to be 8.0(1) Angstrom. No regular variations of the wall thickness were detected, but its density was found to be ca. 10% higher in the low-curvature regions. The surfactant density in the pores was assessed around 0.6-0.7 g/cm(3) and was found to have a distinct minimum in the pore center similar to that detected previously in MCM-41. A new extended model function of the density distribution in MCM-48 is proposed on the basis of the structural features that were revealed.     

Study of hexane adsorption in nanoporous MCM-41 silica

We study here the adsorption of hexane on nanoporous MCM-41 silica at 303,313, and 323 K, for various pore diameters between 2.40 and 4.24 nm. Adsorption equilibria, measured thermogravimetrically, show that all the isotherms, that are somewhat akin to those of type V, exhibit remarkably sharp capillary adsorption phase transition steps and are reversible. The position of the phase transition step gradually shifts from low to high relative pressure with an increase in the temperature as well as the pore sizes. The isosteric heats of adsorption derived from the equilibrium information using the Clapeyron equation reveal a gradual decrease with increasing adsorbed amount because of the surface heterogeneity but approach a constant value near the phase transition. A decrease in the pore size results in an increase in the isosteric heat of adsorption because of the increased dispersion forces. A simple strategy, based on the Broekhoff and De Boer adsorption theory, successfully interprets the hexane adsorption isotherms for the different pore size MCM-41 samples. The parameters of an empirical expression, used to represent the potential of interaction between the adsorbate and adsorbent, are obtained by fitting the monolayer region prior to capillary condensation and the experimental phase transition simultaneously, for some pore sizes. Subsequently, the parameters are used to predict the adsorption isotherm on other pore size samples, which showed good agreement with experimental data.     

Facile assembly of mesoporous silica nanoparticles with hierarchical pore structure for CO2 capture

In the work, we propose an efficient one-pot approach for synthesis of a new type of mesoporous silica nanoparticles (MSNs). That can be successfully realized by using tetraethylorthosilicate (TEOS) and N-[3-(trimethoxysilyl)propyl]ethylenediamine (TSD) as the silica precursors and cetyltrimethylammonium bromide (CTAB) as the structure-directing agent through a facile assembly process. The as-synthesized MSNs possess a spherical morphology with about 230 nm, a relatively high surface area of 133 m²/g, and a hierarchical pore size distribution. When applied as the sorbents, the amine-functioned MSNs demonstrate the enhanced adsorption capacity for CO₂ capture (at 1 bar, 15 vol% CO₂, up to 80.5 mg/g at 75 °C), high selectivity, and good cycling durability, benefiting from the suitable modification of polyethyleneimine.     

表面活性剂修饰对MCM-41负载离子液体吸附CO2影响的研究

以表面活性剂修饰的MCM-41为载体,采用浸渍法制备了负载离子液体[NH2p-mim][PF6]的二氧化碳吸附剂,考察了表面活性剂对离子液体在MCM-41上分散的影响以及所导致的CO2吸附性能的变化.利用红外光谱(FT-IR),X-射线衍射(XRD),高分辨透射电子显微镜(HRTEM),热重分析(TG)技术对所合成的负载型离子液体吸附剂进行了表征研究,并与其吸附CO2的性能变化、离子液体与表面活性剂相互作用方式等因素进行了关联.结果表明:MCM-41负载离子液体后对CO2的吸附性能略有提高,而经表面活性剂修饰的MCM-41负载离子液体后,对CO2的吸附容量较载体本身提高了2.5倍.这一方面是因为表面活性剂胶束改善了MCM-41上离子液体的分散性,另一方面是表面活性剂胶束对离子液体分子上电荷分布的影响,导致离子液体内部阴阳离子之间的相互作用减弱,从而引起离子液体中-NH2上N原子电子云密度增大,使其与CO2作用更容易.CO2在经表面活性剂修饰后的MCM-41负载离子液体[NH2p-mim][PF6]吸附剂上的吸附受扩散控制,其吸附-脱附CO2所需能量较小,经过5次吸附-脱附循环后,其吸附性能仍保持稳定.热重分析结果表明,经表面活性剂修饰后的MCM-41负载离子液体吸附剂在100℃下氮气气氛再生时不会发生性质改变.     

Baeyer-Villiger oxidation of cyclic ketones over iron-containing mesoporous MCM-48 silica materials

Fe-MCM-48 mesoporous material was found to be a highly active catalyst for the Baeyer-Villiger oxidation of several cyclic ketones. The catalyst could be reused several times without any loss of activity.     

Gas adsorption in mesoporous micelle-templated silicas: MCM-41, MCM-48, and SBA-15

This paper reports a molecular simulation and experimental study on the adsorption and condensation of simple fluids in mesoporous micelle-templated silicas MCM-41, MCM-48, and SBA-15. MCM-41 is described as a regular cylindrical silica nanopore, while SBA-15 is assumed to be made up of cylindrical nanopores that are connected through lateral channels. The 3D-connected topology of MCM-48 is described using a gyroid periodic minimal surface. Argon adsorption at 77 K is calculated for the three materials using Grand Canonical Monte Carlo simulations. Qualitative comparison with experiments for nitrogen adsorption in mesoporous micelle-templated silicas is made. The adsorption isotherm for SBA-15 resembles that for MCM-41. In particular, capillary condensation and evaporation are not affected by the presence of the connecting lateral channels. In contrast, the argon adsorption isotherm for MCM-48 departs from that for MCM-41 having the same pore size. While condensation in MCM-41 is a one-step process, filling of MCM-48 involves two successive jumps in the adsorbed amounts which correspond to condensation in different domains of the porosity. The condensation pressure for MCM-48 is larger than that for MCM-41. We attribute this result to the morphology of the MCM-48 surface (made up of both concave and convex regions) that differs from that for MCM-41 (concave only). Our results suggest that the pore connectivity affects pore filling when the size of the connections is comparable to that of the nanopores.     

MCM-41, MOF and UiO-67/MCM-41 adsorbents for pre-combustion CO2 capture by PSA: adsorption equilibria

Three different adsorbent materials, which are promising for pre-combustion CO₂ capture by a PSA (Pressure Swing Adsorption) process, are synthesized, pelletized and characterized. These materials are USO-2-Ni metal organic framework (MOF), mesoporous silica MCM-41 and a mixed material consisting of UiO-67 MOF bound with MCM-41. On these materials, equilibrium adsorption isotherms of CO₂ and H₂ are measured at different temperatures (25–140?°C) in a wide pressure range (up to 15?MPa). From the experimental data the parameters of different isotherm equations (Langmuir, Sips and Quadratic) are determined, together with the isosteric heats of adsorption. Binary adsorption of CO₂/H₂ mixtures on USO-2-Ni MOF is additionally measured and compared to predicted values using IAST (Ideal Adsorbed Solution Theory) showing a good agreement. The potential of the materials for the application of interest is evaluated by looking at their cyclic working capacity and compared to those of a commercial activated carbon. From this evaluation especially the USO-2-Ni MOF adsorbent looks promising compared to the commercial activated carbon. For the other two materials a smaller improvement, which is limited to lower temperatures, is expected.     

Tailoring the pore chemistry in porous aromatic frameworks for selective separation of acetylene from ethylene

The separation of acetylene from ethylene is a crucial process in the petrochemical industry, because even traces of acetylene impurities can poison the catalysts of ethylene polymerization. Herein, we synthesize a new family of 3D porous aromatic frameworks (PAFs), non-functionalized PAF-28, carbene-functionalized PAF-28 (cPAF-28) and imidazolium-functionalized PAF-28 (iPAF-28), via Sonogashira coupling reactions. These PAFs show high porosity and good thermal stability. Both cPAF-28 and iPAF-28 are proved to be good candidates for C₂H₂ adsorption, demonstrated by C₂H₂/C₂H₄ selectivity of 12.2 and 15.4, and C₂H₂ capacity of 48 cm³ g⁻¹ and 57 cm³ g⁻¹, which are significantly higher than those of non-functionalized PAF-28 (1.8, 37 cm³ g⁻¹). Furthermore, the cPAF-28 and iPAF-28 display good breakthrough performance and remarkable recyclability for the separation of the C₂H₂/C₂H₄ gas mixture. In addition, the C₂H₂/C₂H₄ adsorption sites are revealed by DFT calculations. This work sheds a new light on gas molecular recognition by tailoring the pore chemistry of PAFs.

A family of PAFs with 3D topology was developed through tailoring the pore structures. By anchoring carbene or imidazolium groups in PAF-28, the selectivity for separating acetylene from ethylene was greatly boosted from 1.8 to 15.4.     

Unconventional, highly selective CO2 adsorption in zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO(2) from N(2), the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO(2) was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, we ascribe the CO(2) over N(2) selectivity to differences in binding sites for the two gases, where the primary CO(2) binding site is located in the center of the 8-membered-ring pore window. This CO(2) binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO(2) over N(2) in SSZ-13 and other zeolites containing 8-ring windows.     

Comparison of nitrogen adsorption at 77 K on non-porous silica and pore wall of MCM-41 materials by means of density functional theory

Nitrogen adsorption on a surface of a non-porous reference material is widely used in the characterization. Traditionally, the enhancement of solid-fluid potential in a porous solid is accounted for by incorporating the surface curvature into the solid-fluid potential of the flat reference surface. However, this calculation procedure has not been justified experimentally. In this paper, we derive the solid-fluid potential of mesoporous MCM-41 solid by using solely the adsorption isotherm of that solid. This solid-fluid potential is then compared with that of the non-porous reference surface. In derivation of the solid-fluid potential for both reference surface and mesoporous MCM-41 silica (diameter ranging from 3 to 6.5 nm) we employ the nonlocal density functional theory developed for amorphous solids. It is found that, to our surprise, the solid-fluid potential of a porous solid is practically the same as that for the reference surface, indicating that there is no enhancement due to surface curvature. This requires further investigations to explain this unusual departure from our conventional wisdom of curvature-induced enhancement. Accepting the curvature-independent solid-fluid potential derived from the non-porous reference surface, we analyze the hysteresis features of a series of MCM-41 samples.     

Dielectric properties and Fourier transform IR analysis of MCM-48, Al-MCM-48 and Ti-MCM-48 mesoporous materials

 Broad-band dielectric spectroscopy was used to investigate the dielectric properties of the meso- porous materials MCM-48, Al-MCM-48 and Ti-MCM-48. The samples were examined in the frequency range from 20 Hz to 1 MHz and in the temperature range from −100 to 250 °C. The dielectric relaxation of the materials has a complex nonexponential behavior with some common features for all the samples. The dielectric spectroscopy and Fourier transform IR measurements identified the relaxation process related to percolation of H⁺ ions associated with silanol groups and water adsorbed in the materials. The non-Debye behavior of the macroscopic dipole correlation functions related to the percolation process allowed us to extract the fractal dimensions of the paths of excitation transfer within the porous medium, and the porosity of each sample was estimated. Received: 7 September 1999 Accepted: 10 December 1999     

Estimation of pore size distribution in MCM-41-type silica using a simple desorption technique

A recently developed dynamic desorption technique is used for obtaining vapor isotherms on porous materials. This gravimetric technique does not require any preliminary calibration and is based on analyzing the kinetics of liquid evaporation from a porous sample under quasi-steady state conditions. The crucial feature of the technique is concerned with the fact that no vapor pressure measurements are necessary. The technique is illustrated by desorption of benzene vapors from mesoporous silica MCM-41. To calculate the pore size distribution, the Derjaguin–Broekhoff–de Boer theory in its combination with the Wheeler model for capillary condensation is used. In the calculations, the reference data on benzene adsorption on a nonporous silica gel from two different sources (published by different authors) are applied. The mean mesopore sizes estimated from desorption isotherms are shown to be in a fair agreement with the calculations through the geometrical method based on the X-ray diffraction data. The dynamic desorption technique can serve as an additional tool for the characterization of a porous media.     

Effect of the amine type on thermal stability of modified mesoporous silica used for CO2 adsorption

In this study, the preparation by grafting of amino-functionalized SBA-15 molecular sieves was carried out. Amino-functionalized molecular sieves were synthesized using a silane coupling agent and different types of amination reagents which react with modified SBA-15. These composites were characterized by FT-IR spectroscopy, X-ray diffraction at low angles, nitrogen physisorption at 77 K, and evaluated by the adsorption of CO₂ and its temperature-programmed desorption—TPD. Thermal stability was investigated by TGA and DTA methods. In the view of a possible use of these amino-functionalized molecular sieves as sorbents for CO₂ removal, their adsorption–desorption properties towards CO₂ were also investigated by the TPD method. The mass loss of amino-functionalized molecular sieves above 215 °C was due to the oxidation and decomposition of amino propyl functional groups. This means that these composites could be used for adsorption of CO₂ at temperatures below 215 °C. The adsorption of CO₂ and its temperature programmed desorption using thermogravimetry were studied for amino-functionalized molecular sieves at 60 °C. The evolved gases during the adsorption–desorption of CO₂ on amino-functionalized molecular sieves were identified by online mass spectrometry coupled with thermogravimetry. CO₂ adsorption isotherms of functionalized samples at 60 °C showed that both the adsorption capacity (mg CO₂/g adsorbent) and the efficiency of amino groups (mol CO₂/mol NH₂) depend on the type of amination reagents and the amount of organic compound used.

     

Tailoring the selective adsorption sites of NiMoO by Ni particles for biomass upgrading assisted hydrogen production

5-Hydroxymethylfurfural electrooxidation reaction(HMFOR) is a promising route to produce valueadded chemicals from biomass.Since it involves HMF adsorption and C-H/O-H cleavage,understanding the adsorption behavior and catalytic process of organic molecules on catalysts is important.Herein,the selective adsorption sites of NiMoO are tuned by Ni particles for HMFOR-assisted H₂ production.Experimental and theoretical calculation results indicate that the synergistic interaction between ...     

MCM-48分子筛的高效合成途径

以正硅酸乙酯(TEOS)为硅源,十六烷基三甲基溴化铵(CTAB)为模板剂,用水热合成法合成MCM-48,在合成过程中通过降低pH值等方法可以使得MCM-48的产率显著提高,当pH=7时,产率达到97%。XRD和物理吸附表明合成的介孔分子筛具有高比表面积(BET,～1000m^2/g)和规整的孔道结构且孔径集中在2.6nm左右。     

Modelling adsorption of a water molecule into various pore structures of silica gel

Silica gel is widely used in commercial applications as a water adsorbent due to its properties including hydrothermally stable, high water sorption capacity, low regeneration temperature, low cost and wide range of pore diameters. Since the water sorption capacity of silica gel strongly depends on the pore size and structure, which can be controlled during synthesis, this paper study the effect of pore shapes and dimensions of silica gel upon the adsorption of a water molecule aiming at maximising the water sorption capacity. In particular, we consider three types of pore structures, namely cylindrical, square prismatic and conical pores. On using the Lennard-Jones potential and a continuum approximation, we find that the minimum radii for a water molecule to be accepted into cylindrical, square prismatic and conical pores are 4.009, 3.7898 and 4.4575 Å, respectively. For cylindrical and square prismatic pores, the critical radii which maximise the adsorption energy are 4.5189 and 4.1903 Å, respectively. Knowledge of these critical pore sizes may be useful for the manufacturing process of silica gel that will maximise the water sorption capacity.