Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

Direct measurements of pore fluid density by vibrating tube densimetry

The densities of pore-confined fluids were measured for the first time by means of vibrating tube densimetry (VTD). A custom-built high-pressure, high-temperature vibrating tube densimeter was used to measure the densities of propane at subcritical and supercritical temperatures (between 35 and 97 °C) and carbon dioxide at supercritical temperatures (between 32 and 50 °C) saturating hydrophobic silica aerogel (0.2 g/cm(3), 90% porosity) synthesized inside Hastelloy U-tubes. Additionally, supercritical isotherms of excess adsorption for CO(2) and the same porous solid were measured gravimetrically using a precise magnetically coupled microbalance. Pore fluid densities and total adsorption isotherms increased monotonically with increasing density of the bulk fluid, in contrast to excess adsorption isotherms, which reached a maximum and then decreased toward zero or negative values above the critical density of the bulk fluid. The isotherms of confined fluid density and excess adsorption obtained by VTD contain additional information. For instance, the maxima of excess adsorption occur below the critical density of the bulk fluid at the beginning of the plateau region in the total adsorption, marking the end of the transition of pore fluid to a denser, liquidlike pore phase. Compression of the confined fluid significantly beyond the density of the bulk fluid at the same temperature was observed even at subcritical temperatures. The effect of pore confinement on the liquid-vapor critical temperature of propane was less than ~1.7 K. The results for propane and carbon dioxide showed similarity in the sense of the principle of corresponding states. Good quantitative agreement was obtained between excess adsorption isotherms determined from VTD total adsorption results and those measured gravimetrically at the same temperature, confirming the validity of the vibrating tube measurements. Thus, it is demonstrated that vibrating tube densimetry is a novel experimental approach capable of providing directly the average density of pore-confined fluids, and hence complementary to the conventional gravimetric or volumetric/piezometric adsorption techniques, which yield the excess adsorption (the Gibbsian surface excess).     

Structural characteristics of silica sonogels prepared with additions of isopropyl alcohol

Wet silica gels with approximately 1.4 x 10(-3) mol SiO2/cm3 and approximately 92 vol % liquid phase were obtained from sonohydrolysis of tetraethoxysilane (TEOS) with different additions of isopropyl alcohol (IPA). The IPA/TEOS molar ratio R was changed from 0 to 4. Aerogels were obtained by supercritical CO2 extraction. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The wet gels exhibit mass fractal structure with fractal dimension increasing from D approximately 2.10 to D approximately 2.22, characteristic length xi decreasing from approximately 9.5 to approximately 6.9 nm, as R increases from 0 to 4, and an estimated characteristic length for the primary silica particles lower than approximately 0.3 nm. The supercritical process apparently eliminates a fraction of the porosity, increasing the mass fractal dimension and shortening the fractality domain in the mesopore region. The fundamental role of isopropyl alcohol on the structure of the resulting aerogels is to decrease the porosity and the pore mean size as R changes from pure TEOS to R = 4. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure, with correlated mass fractal dimension Dm approximately 2.7 and surface fractal dimension Ds approximately 2.3, as inferred from SAXS and nitrogen adsorption data.     

Gibbs ensemble Monte Carlo simulation of supercritical CO2 adsorption on NaA and NaX zeolites

Adsorption of supercritical carbon dioxide on two kinds of zeolites with identical chemical composition but different pore structure (NaA and NaX) was studied using the Gibbs ensemble Monte Carlo simulation. The model frameworks for the two zeolites with SiAl ratio being unity have been chosen as the solid structures in the simulation. The adsorption behaviors of supercritical CO2 on the NaA and NaX zeolites, based on the adsorption isotherms and isosteric heats of adsorption, were discussed in detail and were compared with the available experimental results. A good agreement between the simulated and experimental results is obtained for both the adsorbed amount and the bulk phase density. The intermediate configurational snapshots and the radial distribution functions between zeolite and adsorbed CO2 molecules were collected in order to investigate the preferable adsorption locations and the confined structure behavior of CO2. The structure behaviors of the adsorbed CO2 molecules show various performances, as compared with the bulk phase, due to the confined effect in the zeolite pores.     

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.     

Influence of supercritical drying fluids on structures and properties of low-density Cu-doped SiO2 composite aerogels

The Cu-doped SiO₂ composite aerogels were successfully prepared by sol–gel process and subsequently supercritical drying with ethanol and CO₂. The Cu-doped SiO₂ composite aerogels had porous texture, low density (<100 mg cm^?3) and high specific surface area (>800 m² g^?1), which were investigated by FESEM and nitrogen adsorption desorption porosimetry. The FTIR spectra of the aerogels showed that the ethanol-dried aerogels had been modified by ethyl while the corresponding CO₂-dried aerogels had more Si–OH groups. The phase structure and thermal stability were investigated by XRD and TGA, respectively. Due to the reducibility of ethanol, the copper was crystalline in ethanol-dried sample. The Cu-doped SiO₂ composite aerogels dried with supercritical ethanol had larger pore diameter and better thermal stability under 400 °C in comparison with CO₂-dried composite aerogels. The structures and properties of Cu-doped SiO₂ composite aerogels are obviously affected by supercritical drying conditions. The effect research could instruct the synthesis of different state of Cu in composite aerogels.     

Near-critical CO2 in mesoporous silica studied by in situ FTIR spectroscopy

Attenuated total reflection Fourier transform infrared spectroscopy was used to correlate the band shift of the nu2 vibrational band of carbon dioxide with the density of the fluid. Upon adsorption of CO2 on mesoporous silica and a nonporous SiO2 film, additional bands were detected due to interactions of CO2 with SiO2. Near the saturation pressure for the porous samples, the absorbance of the nu2 band increased strongly, which was concluded to be caused by liquidlike CO2 inside the pores. Integration of single-beam-sample-reference spectra between bulk CO2 and CO2 adsorbing on the mesoporous silica coated on one part of the internal reflection element revealed excess adsorption type isotherms with sharp maxima at 21 degrees C. A flatter curve shape could be observed at 25 degrees C, which allowed estimating the pore critical temperature. Moreover, the density of the fluid inside and outside the pores could be compared. Over the investigated ranges of pressure, temperature, and pore size, the results evidenced that the CO2 density was always higher in the silica pores than in the bulk, even under supercritical conditions. This has important consequences on the pressure dependence of dissolution power and diffusivity of fluids in mesoporous solids. An overview is given on the influences of fluid phase behavior in the bulk and in the pores at various conditions on solubility and diffusivity.     

SiO2气凝胶的非超临界干燥法制备及其形成过程

通过对正硅酸乙酯的两步水解-缩聚反应速率的调控，使生成的醇凝胶具有比较完整的网络结构，配合乙醇溶剂替换和正硅酸乙酯乙醇溶液浸泡和陈化，改善和增强凝胶的结构和强度，在分级干燥下实现了SiO2气凝胶的非超临界干燥制备，并采用SEM、TEM、TG-DTA、XRD和吸附-脱附技术等手段对所得气凝胶样品进行表征.结果表明, 该气凝胶是由粒径约10 nm均匀球状纳米粒子构成的具有连续网络结构的低密度多孔材料，密度为200～400 kg•m－3，孔径分布在10～30 nm范围内，孔隙率约为91％，比表面高达625.65 m2•g－1.外观及微观构造与应用超临界干燥制得的气凝胶完全一致.调节反应体系中各组分的配比以及控制两步水解-缩聚过程中酸与碱的加入量可以获得不同密度的块状SiO2气凝胶.     

溶胶－凝胶法制备超细SiO_2──制备参数对气凝胶织构性质的影响

ＳｉＯ_２气凝胶超细粉由ＴＥＯＳ（正硅酸乙酯）在乙醇溶液中水解聚合后经超临界流体干燥（ＳＣＦＤ）制得。本文考察了ＴＥＯＳ的浓度、水和ＴＥＯＳ的摩尔比以及焙烧温度对超细粉织构性能的影响。结果表明，超细ＳｉＯ_２的比表面积、总孔容、孔分布、最可几孔径及表现堆密度均随制备参数而改变。而以ＳｉＯ_２气凝胶超细粉体为基本载体材料的Ｃｏ／ＺｒＯ_２－ＳｉＯ_２催化剂显示出高的ＣＯ＋Ｈ_２合成重质烃的活性和选择性。     

Investigation of confined ionic liquid in nanostructured materials by a combination of SANS, contrast-matching SANS, and nitrogen adsorption

Small-angle neutron scattering (SANS), contrast-matching SANS, and nitrogen adsorption have been utilized to investigate the confined ionic liquid (IL) [bmim][PF(6)] phase in ordered mesoporous silica MCM-41 and SBA-15. The results suggest that the pores of SBA-15 are completely filled with IL whereas a small fraction of the pore volume, the pore "core", of MCM-41 is empty. The contrast-matching SANS measurements confirm the enhanced solubility of water in IL. In addition, they provide strong evidence that water does not enter the empty pore core of MCM-41, possibly because of the preferred orientation of the IL molecules in the adsorbed layer.     

Chitosan-based aerogels with high adsorption performance

New natural polymer-based aerogels, cross-linked chitosan aerogels, were prepared by the sol-gel route with glutaradehyde, glyoxal, and formaldehyde as cross-linkers. The alcogels were dried by supercritical carbon dioxide (CO2) fluid extraction. The resulting materials were characterized using scanning electron microscopy (SEM), nitrogen adsorption/desorption analysis, and Fourier transform infrared (FT-IR) spectroscopy. Furthermore, the adsorption of the anionic surfactant sodium dodecylbenzene-sulfonate (SDBS) from aqueous solution by the materials was investigated. The aerogels exhibit high adsorption capability, can remove SDBS from acidic aqueous solutions, and have potential applications in controlling SDBS pollution.     

Ellipsometric and neutron diffraction study of pentane physisorbed on graphite

High-resolution ellipsometry and neutron diffraction measurements have been used to investigate the structure, growth, and wetting behavior of fluid pentane (n-C(5)H(12)) films adsorbed on graphite substrates. We present isotherms of the thickness of pentane films adsorbed on the basal-plane surfaces of a pyrolytic graphite substrate as a function of the vapor pressure. These isotherms are measured ellipsometrically for temperatures between 130 and 190 K. We also describe neutron diffraction measurements in the temperature range 11-140 K on a deuterated pentane (n-C(5)D(12)) monolayer adsorbed on an exfoliated graphite substrate. Below a temperature of 99 K, the diffraction patterns are consistent with a rectangular centered structure. Above the pentane triple point at 143.5 K, the ellipsometric measurements indicate layer-by-layer adsorption of at least seven fluid pentane layers, each having the same optical thickness. Analysis of the neutron diffraction pattern of a pentane monolayer at a temperature of 130 K is consistent with small clusters having a rectangular-centered structure and an area per molecule of approximately 37 A(2) in coexistence with a fluid monolayer phase. Assuming values of the polarizability tensor from the literature and that the monolayer fluid has the same areal density as that inferred for the coexisting clusters, we calculate an optical thickness of the fluid pentane layers in reasonable agreement with that measured by ellipsometry. We discuss how these results support the previously proposed "footprint reduction" mechanism of alkane monolayer melting. In the hypercritical regime, we show that the layering behavior is consistent with the two-dimensional Ising model and determine the critical temperatures for layers n = 2-5.     

Structural characterization of surfactant aggregates adsorbed in cylindrical silica nanopores

The self-assembly of nonionic surfactants in the cylindrical pores of SBA-15 silica with a pore diameter of 8 nm was studied by small-angle neutron scattering (SANS) at different solvent contrasts. The alkyl ethoxylate surfactants C(10)E(5) and C(12)E(5) exhibit strong aggregative adsorption in the pores as indicated by the sigmoidal shape of the adsorption isotherms. The SANS intensity profiles can be represented by a sum of two terms, one accounting for diffuse scattering from surfactant aggregates in the pores and the other for Bragg scattering from the pore lattice of the silica matrix. The Bragg reflections are analyzed with a form factor model in which the radial density profile of the surfactant in the pore is approximated by a two-step function. Diffuse scattering is represented by a Teubner-Strey-type scattering function which indicates a preferred distance between adsorbed surface aggregates in the pores. Our results suggest that adsorption starts with formation of discrete surface aggregates which increase in number and eventually merge to interconnected patches as the plateau value of the adsorption isotherm is approached. A grossly different behavior, viz. formation of micelles as in solution, is found for the maltoside surfactant C(10)G(2), in agreement with the observed weak adsorption of this surfactant in SBA-15.     

Immobilization of sodium ions on the pore surface of a porous coordination polymer

A porous coordination polymer (PCP) with immobilization of sodium cations on the pore surface has been synthesized, by employing a bifunctional carboxylate/sulfonate ligand, and structurally characterized. The porous framework with 1D channels of the dimension of 4.9 x 4.9 A2 shows high thermal stability ( approximately 330 degrees C), affording Type I adsorption isotherms for CO2, acetone, and benzene. The chemical shift of 13C NMR and characteristic adsorption energy (betaE0) of acetone adsorbed in this compound represent the Lewis acidity of this framework.     

Calorimetric study of phase transitions for octylphenylthiolpentyloxybenzoate in silica aerogels

An a.c. calorimetric study has been carried out on octylphenylthiolpentyloxybenzoate (8S5) in three silica aerogels with mass densities rho 0.08, 0.17 and 0.36g cm . Results for the least porous aerogel (rho 0.36) are completely consistent with those reported previously for 8CB in the same aerogel. Freezing of 8S5 damaged the more porous rho 0.08 and 0.17 aerogels by creating internal voids and cracks that are large compared with the intrinsic pore size. As a result, overlapping sharp bulk-like Cp features and rounded features due to 8S5 in residual aerogel pores were observed, and no quantitative separation of these features could be achieved. 3     

Neutron scattering from water adsorbed on an alumina membrane

The pore structure of an alumina membrane was studied by adsorption in conjunction with small-angle neutron scattering (SANS). The effect of an incomplete contrast matching was investigated and simple formulae allowing the calculation of the amount of water adsorbed and the film areas, at various relative pressures, were derived for the ternary system. In the vicinity of the contrast matching point, however, incomplete contrast matching does not introduce a substantial complication to data analysis. The adsorption isotherm was reconstructed from the SANS data and agrees well with the experimental adsorption isotherm. For the alumina membrane, a coordination number of ≈10 was estimated from the SANS determined areas.     

Thermodynamic and neutron scattering study of hydrogen adsorption in two mesoporous ordered carbons

Two mesoporous ordered carbon materials (MOCs) have been synthesized from silica templates by using sucrose as the carbon precursor. The textural characterization using Ar, N2, and CO2 adsorption combined with neutron diffraction showed that the two samples exhibit a significant microporous volume close to 0.5 cm3/g and an ordered network of mesopores. For both MCM48 and SBA15 templated carbons, adsorption first proceeds with the filling of micropores and then by the filling of mesopores with an adsorption energy close to the enthalpy of vaporization of bulk hydrogen. The hydrogen isosteric heat of adsorption in the micropores (6-8 kJ/mol) is significantly larger than that on the graphite surface (approximately 4 kJ/mol) but still too small for a reasonable use of these MOCs as hydrogen adsorbents for storage at room temperature. The neutron scattering study showed that the structure at 10 K of the adsorbed deuterium phase is poorly organized; it exhibits short and medium range orders of about 13 angstroms in micropores and about 20 angstroms in mesopores, respectively. The average distance between adsorbed molecules decreases with coverage by about 10%. In the mesopores, the diffracted line is consistent with a pseudohexagonal packing.     

超临界二氧化碳脱附固体吸附剂上卤代烃污染物研究

本文使用自制超临界流体萃取仪，考查了从活性炭、Ｃｈｒｏｍｏｓｏｒｂ １０２、Ｃｈｒｏｍｏｓｏｒｂ１０５、Ｔｅｎａｘ－ＴＡ四种吸附剂上脱附１，２，３－三氯丙烷、１，３二溴丙烷、１，４－二溴丁烷、１，２，４－三氯化苯、间－二溴苯五种组分的性能。证明使用甲醇改性后ＣＯ２萃取，比单纯使用ＣＯ２萃取的效率高得多，萃取活性炭时，组分的回收率可达５０％，萃取另３种吸附剂时，组分的回收率超过８０％，这表明利用某些     

Diffusion and phase equilibria of binary fluids in mesopores

The formation of adsorption hysteresis in mesoporous material with random pore structure may be interrelated with different distributions of the fluid density attained along different paths of the system preparation. To access microscopic details of these distributions, in addition to the main sorptive liquid, distribution of which along the pore space of Vycor porous glass was of interest, a small amount of a probe liquid with a substantially lower vapor pressure has been added. Molecular diffusivities of both liquids then have been traced using pulsed field gradient NMR. Due to different vapor pressures, the two molecular species explore different spaces occupied by the capillary-condensed (accessible for both species) and gaseous (accessible only for the molecules of the main sorptive) phases. Comparative analysis of the diffusion properties obtained at different states along the adsorption isotherm revealed further insight into peculiarities of the fluid distribution and mass transfer of binary fluids in pores.     

Preparation and characterization of ruthenium/carbon aerogel nanocomposites via a supercritical fluid route

Carbon-aerogel-supported ruthenium nanoparticles were synthesized by impregnating carbon aerogels with Ru(acac)3 or Ru(cod)(tmhd)2 from supercritical carbon dioxide (scCO2) solutions, followed by thermal reduction of these precursors. Two different carbon aerogels with pore diameters of 4 and 21 nm were synthesized. The kinetics and the thermodynamics of impregnation of carbon aerogels with the ruthenium coordination complexes were studied. The approach-to-equilibrium data indicated very fast adsorption, and the adsorption isotherms were found to follow the Langmuir model. The impregnated carbon aerogel complexes were reduced thermally at different temperatures between 300 and 1000 degrees C in the presence of nitrogen. The resulting nanocomposites were characterized using transmission electron microscopy (TEM) and hydrogen chemisorption. TEM micrographs showed that the ruthenium nanoparticles were dispersed homogeneously throughout the porous carbon aerogel matrix, and the average sizes obtained under different conditions ranged from 1.7 to 3.8 nm. Once complete decomposition of the precursor had been achieved, the mean size of the ruthenium particles increased with increasing reduction temperature.