Characterization of Pore Size Distributions of Mesoporous Materials from Adsorption Isotherms

Enhancing photocatalytic tetracycline degradation through the fabrication of high surface area CeO2 from a cerium-organic framework

Water pollution is a global environmental issue, and the presence of pharmaceutical compounds, such as tetracyclines (TCs), in aquatic ecosystems has raised growing concerns due to the potential risks to both the environment and human health. A high surface area CeO2 was prepared via atmospheric thermal treatment of a metal-organic framework of cerium and benzene-1,3,5-tricarboxylate. The effects of calcination temperature on the morphology, structure, light absorption properties and tetracycline removal efficiency were studied. The best activity of the photocatalysts could be achieved when the heat treatment temperature is 300 °C, which enhances the photocatalytic degradation performance towards tetracycline under visible light. The resulting CeO2 particles have high capacity for adsorbing TCs from aqueous solution: 90 mg g-1 for 60 mg L-1 TCs. As a result, 98% of the initial TC can be removed under simulated sunlight irradiation. The cooperation of moderate defect concentration and disordered structure showed tetracycline removal activity about 10 times higher than the initial Ce-MOF. An embryotoxicity assessment on zebrafish revealed that treatment with CeO2 particles significantly decreased the toxicity of TC solutions.

Porous Fe-Porphyrin as an Efficient Adsorbent for the Removal of Ciprofloxacin from Water

Antibiotics are widely used in medicine, but they are not fully metabolized in the body and can end up in wastewater. Conventional wastewater treatment methods fail to completely remove antibiotic residues, which can then enter rivers and streams. Adsorption is a promising technique for removing antibiotics from wastewater, even at low concentrations. The successful one-pot synthesis of an adsorbent, iron-containing porphyrin-based porous organic polymer (Fe-POP), was achieved through the reaction of pyrrole groups and terephthalaldehyde in the presence of FeCl3. Characterized by a substantial BET surface area of 597 m2 g-1, Fe-POP was systematically investigated for its adsorption potential in the removal of the antibiotic Ciprofloxacin (CIP) from aqueous solutions. By systematic variation of key parameters, including pH, adsorbent loading, and CIP concentration, the adsorption conditions were optimized. Under the optimal conditions at pH = 3, CIP concentration of 5 ppm, and 25 mg of Fe-POP, the maximum adsorption capacity reached an impressive 263 mg g-1. The robust adsorption behavior was elucidated through the fitting of experimental data to the Langmuir adsorption isotherm (R2 = 0.962) and the pseudo-second-order kinetic model (R2 = 0.999) with lower error values. These models suggested that the adsorption process predominantly involved chemical interactions between CIP molecules and the Fe-POP surface. Fe-POP exhibited a robust structure with a high adsorption capacity, showcasing its efficacy in removing CIP contaminants from water. Therefore, Fe-POP can be considered a valuable adsorbent for water treatment applications, specifically for antibiotic removal.

Physiochemical characterization of metal organic framework materials: A mini review

Metal-organic frameworks (MOFs) are promising materials offering exceptional performance across a myriad of applications, attributable to their remarkable physicochemical properties such as regular porosity, crystalline structure, and tailored functional groups. Despite their potential, there is a lack of dedicated reviews that focus on key physicochemical characterizations of MOFs for the beginners and new researchers in the field. This review is written based on our expertise in the synthesis and characterization of MOFs, specifically to provide a right direction for the researcher who is a beginner in this area. In this way, experimental errors can be reduced, and wastage of time and chemicals can be avoided when new researchers conduct a study. In this article, this topic is critically analyzed, and findings and conclusions are presented. We reviewed three well-known XRD techniques, including PXRD, single crystal XRD, and SAXS, which were used for XRD analysis depending on the crystal size and the quality of crystal morphology. The TGA profile was an effective factor for evaluating the quality of the activation process and for ensuring the successful investigation for other characterizations. The BET and pore size were significantly affected by the activation process and selective benzene chain cross-linkers. FTIR is a prominent method that is used to investigate the functional groups on pore surfaces, and this method is successfully used to evaluate the activation process, characterize functionalized MOFs, and estimate their applications. The most significant methods of characterization include the X-ray diffraction, which is utilized for structural identification, and thermogravimetric analysis (TGA), which is used for exploring thermal decomposition. It is important to note that the thermal stability of MOFs is influenced by two main factors: the metal-ligand interaction and the type of functional groups attached to the organic ligand. The textural properties of the MOFs, on the other hand, can be scrutinized through nitrogen adsorption-desorption isotherms experiments at 77 K. However, for smaller pore size, the Argon adsorption-desorption isotherm at 87.3 K is preferred. Furthermore, the CO2 adsorption isotherm at 273 K can be used to measure ultra-micropore sizes and sizes lower than these, which cannot be measured by using the N2 adsorption-desorption isotherm at 77 K. The highest BET was observed in high-valence MOFs that are constructed based on the metal-oxo cluster, which has an excellent ability to control their textural properties. It was found that the synthesis procedure (including the choice of solvent, cross-linker, secondary metal, surface functional groups, and temperature), activation method, and pressure significantly impact the surface area of the MOF and, by extension, its structural integrity. Additionally, Fourier-transform infrared spectroscopy plays a crucial role in identifying active MOF functional groups. Understanding these physicochemical properties and utilizing relevant characterization techniques will enable more precise MOF selection for specific applications.

Preparation of food active packaging materials based on calcium hydroxide and modified porous medium for reducing carbon dioxide and kimchi odor

Carbon dioxide and kimchi odor compounds, formed during fermentation, negatively affect the long-distance distribution of commercial kimchi. To address these issues, in this study, we modified different porous media (activated carbon, bleaching earth, diatomite, and zeolite) using sodium bicarbonate and silver (Ag) ions. Functional sheets were prepared using linear low-density polyethylene, calcium hydroxide, a porous medium, and a blowing agent. Various prepared porous media and sheets were effective in removing acetic acid, sulfur compounds (allyl methyl sulfide, dimethyl disulfide, allyl methyl disulfide, and diallyl disulfide), and carbon dioxide. Porous media with micropores exhibited a sulfur compound removal efficiency of 43.5%-99.4%, while no effect was observed on acetic acid removal. However, porous media with mesopores showed an acetic acid removal efficiency of 42.3%-90.7%, with no reduction in sulfur compounds removal. The impregnation of porous materials with sodium bicarbonate significantly (p < 0.05) enhanced the acetic acid removal activity. Ag modification improved the sulfur compound removal of the mesoporous bleaching earth and diatomite statistically (p < 0.05). Additionally, the incorporation of sodium bicarbonate-impregnated mesoporous media significantly improved carbon dioxide removal, reducing concentrations from 25.97% to 14.27% with respect to the control group. Our functional food packaging materials can solve the current issues in kimchi distribution by removing carbon dioxide and kimchi odor without affecting its quality. PRACTICAL APPLICATION: Food active packaging materials containing calcium hydroxide and modified porous medium are effective in removing carbon dioxide and kimchi odor (acetic acid and sulfur compounds). The removal of carbon dioxide and kimchi odor, which adversely affect the distribution and sale of commercial kimchi, can help solve the current issues with kimchi distribution without affecting its quality.

Effects of Interfacial Molecular Structures on Pressure-Driven Brine Flow in Silica Mesopores

In this work, we conduct molecular dynamics simulations to investigate pressure-driven brine flow in silica mesopores under typical reservoir conditions (323 K and 20 MPa). While surface counterions accumulate strongly in the vicinity of fully deprotonated silica surfaces, water forms multilayer structures due to hydrogen bonding, counterion hydration, and excluded-volume effect. Brine flow behaviors exhibit adsorption, transition, and bulk-like regions in fully deprotonated silica mesopores, while the transition region is negligible in fully protonated ones. In the adsorption region, strong surface hydrogen bonding and a high degree of counterion hydration collectively hinder water mobility. Even without surface hydrogen bonding, persistent ion hydration impedes water flow, leading to the transition region in fully deprotonated silica mesopores and higher viscosity of brine (with 10 wt % NaCl) in the bulk region. This work elucidates the collective effects of surface chemistry and interfacial water structures on brine flow behaviors in silica mesopores from molecular perspectives.

Carboxylated chitosan-mediated improved efficacy of mesoporous silica nanoparticle-based targeted drug delivery system for breast cancer therapy

Nanoparticle-based targeting of overexpressed cell-surface receptors is a promising strategy that provides precise delivery of drugs to cancer cells. In the present study, we developed highly reproducible and monodispersed, chitosan-coated (pH-responsive), doxorubicin-loaded, aptamer-mesoporous silica nanoparticle (MSN) bioconjugates for actively targeting breast cancer cells harboring overexpression of EGF receptors (EGFR/HER2). The developed targeted MSNs demonstrated higher uptake and cytotoxicity of triple negative and HER2 positive breast cancer cells when compared to non-targeted MSNs. The chitosan coating imparted pH-responsiveness and endo/lysosomal escape ability to MSNs, which augmented cytosolic delivery of an anticancer drug. Partial carboxylation of chitosan coated on MSNs allowed for a greater release of drug in a shorter duration of time while retaining pH-responsiveness and endo/lysosomal escape ability. Overall, the coating of carboxylated-chitosan over MSNs enabled tunable drug release kinetics, conjugation of aptamers (targeting agents), and endo/lysosomal escape which together significantly enhanced the efficacy of the developed drug delivery system.

Silica particles with fluorescein-labelled cores for evaluating accessibility through fluorescence quenching by copper

Core-shell particles with fluorescent cores were synthesised by growing silica shells on fluorescein-labelled Stöber-type particles. The porosity of the shell could be altered in a subsequent pseudomorphic transformation step, without affecting the particle size and shape. These core-shell particles constitute a platform for the evaluation of pore connectivity and core accessibility by observing the effect of a quencher on the fluorescence signal emitted by the fluorescein-labelled cores. In combination with argon sorption measurements, quenching experiments with copper provided valuable information on the porosity generated during the shell formation process. It was further observed that the introduction of well-defined mesopores by pseudomorphic transformation in the presence of a structure-directing agent reduces the core accessibility. This led to the conclusion that the analysis by conventional gas sorption methods paints an incomplete picture of the mesoporous structure, in particular with regard to pores that do not offer an unobstructed path from the external particle surface to the core.

Salt of Organosilicon Framework as a Novel Emulsifier for Various Water-Oil Biphasic Systems and a Catalyst for Dibromination of Olefins in an Aqueous Medium

Pickering emulsifiers are significant for organic reactions in an aqueous medium because they have the ability of emulsifying water-oil biphasic systems. For this reason, 2,5-bis[(E)-2-(triethoxysilyl)vinyl]pyridine [BTOSVP] containing a pyridine bridging group was selected as a precursor to prepare a novel salt of organosilicon framework (SOF), an amphiphilic mesoporous pyridine hydrobromide nanosphere. We first synthesized a mesoporous organosilicon framework made up of organic groups containing vinyl groups, pyridine groups, and so forth. Then, hydrobromic acid was added to protonate the pyridine groups in the mesoporous organosilicon framework. Eventually, pyridine hydrobromide salt was formed on the surfaces of channels, and the SOF was successfully prepared for the first time. Pyridine hydrobromide salt can be ionized in water into protonated pyridine cations located on the SOF surfaces and free Br-anions swimming around the protonated pyridine cations because of the electrostatic interaction. In the water-oil biphasic systems, hydrophilicity of SOF originates from the protonated pyridine cations and the lipophilicity of SOF comes from organic groups in the framework; thus, this new kind of SOF can be used as a new generation of solid Pickering emulsifiers. Most importantly, the mesoporous SOF nanosphere can also be used as a catalyst for significantly improved dibromination of olefins in an aqueous medium.

Hollow Silica Cubes with Customizable Porosity

Hollow silica cubes were synthesized by a deposition of a thin silica shell onto micrometer-sized hematite cubes. Ordered mesopores with well-defined pore diameters of 2.8 nm and 3.8 nm were introduced into the silica shell by means of pseudomorphic transformation after removal of the hematite core. The particles retained their cubic morphology upon pseudomorphic transformation, allowing for the preparation of close-packed layers of the hollow mesoporous silica cubes by drop-casting and the visualization of the hollow core by focused ion beam scanning electron microscopy.

Combining cobalt ferrite and graphite with cellulose nanocrystals for magnetically active and electrically conducting mesoporous nanohybrids

Free-standing mesoporous membranes based on cellulose nanocrystals (CNCs) are fabricated upon the incorporation of cobalt ferrite (CoFe₂O₄) and graphite nanoparticles at concentrations up to 20 wt % through a soft-templating process. Scanning electron microscopy (SEM) and N₂ adsorption-desorption isotherms reveal the development of highly-porous interconnected random 3D structure with surface areas up to 193.9 m² g^-1. Thermogravimetric analysis (TGA) shows an enhanced thermal stability thanks to the formation of a tortuous network limiting the hindrance of degradation by-products. Vibrating sample magnetometer (VSM) reveals a maximum magnetization saturation of 8.77 emu·g^-1 with materials having either ferromagnetic or diamagnetic behaviour upon the incorporation of CoFe₂O₄ and graphite, respectively. Four-point-probe measurements display a maximum electrical conductivity of 9.26 ± 0.04 S·m^-1 when graphite is incorporated into CNCs. A proof of concept for the applicability of synthesized nanohybrids for environmental remediation is provided, presenting the advantage of their easy recovery using external magnetic fields.

A template-free and low temperature method for the synthesis of mesoporous magnesium phosphate with uniform pore structure and high surface area

Mesoporous phosphates are a group of nanostructured materials with promising applications, particularly in biomedicine and catalysis. However, their controlled synthesis via conventional template-based routes presents a number of challenges and limitations. Here, we show how to synthesize a mesoporous magnesium phosphate with a high surface area and a well-defined pore structure through thermal decomposition of a crystalline struvite (MgNH4PO4·6H2O) precursor. In a first step, struvite crystals with various morphologies and sizes, ranging from a few micrometers to several millimeters, had been synthesized from supersaturated aqueous solutions (saturation index (SI) between 0.5 and 4) at ambient pressure and temperature conditions. Afterwards, the crystals were thermally treated at 70-250 °C leading to the release of structurally bound water (H2O) and ammonia (NH3). By combining thermogravimetric analyses (TGA), scanning and transmission electron microscopy (SEM, TEM), N2 sorption analyses and small- and wide-angle X-ray scattering (SAXS/WAXS) we show that this decomposition process results in a pseudomorphic transformation of the original struvite into an amorphous Mg-phosphate. Of particular importance is the fact that the final material is characterized by a very uniform mesoporous structure with 2-5 nm wide pore channels, a large specific surface area of up to 300 m2 g-1 and a total pore volume of up to 0.28 cm3 g-1. Our struvite decomposition method is well controllable and reproducible and can be easily extended to the synthesis of other mesoporous phosphates. In addition, the so produced mesoporous material is a prime candidate for use in biomedical applications considering that magnesium phosphate is a widely used, non-toxic substance that has already shown excellent biocompatibility and biodegradability.

Multilevel Morphology of Complex Nanoporous Materials

This work exploits gas adsorption and small-angle X-ray scattering (SAXS) to determine the morphology of complex nanoporous materials. We resolve multiple classes of porosity including previously undetected large-scale texture that significantly compromises the canonical interpretation of gas adsorption. Specifically, a UVM-7 class mesoporous silica was synthesized that has morphological features on three length scales: macropores due to packing of 150 nm globules, 1.9 nm radius spherical mesopores inside the globules, and >7 nm pockets on and between the globules. The total and external surface areas, as well as the mesopore volume, were determined using gas adsorption (α_s-plot) and SAXS. A new approach was applied to the SAXS data using multilevel fitting to determine the surface areas on multiple length scales. The SAXS analysis code is applicable to any two-phase system and is freely available to the public. The total surface area determined by SAXS was 12% greater than that obtained by gas adsorption. The macropore interfacial area, however, is only 30% of the external surface area determined by the α_s-plot. The overestimation of the external surface area by the α_s-plot method is attributed to capillary condensation in nanoscale surface irregularities. The discrepancy is resolved assuming that the macropore-globule interfaces harbor fractally distributed nooks and crannies, which lead to gas adsorption at pressures above the mesopore filling pressure.

Luminescent thermometer based on Eu3+ /Tb3+ -organic-functionalized mesoporous silica

In this work we investigate a mesoporous silica (MS) decorated with dipyridyl-pyridazine (dppz) ligands and further grafted with a mixture of Eu³⁺ /Tb³⁺ ions (28.45%:71.55%), which was investigated as a potential thermometer in the 10-360 K temperature range. The MS material was prepared employing a hetero Diels-Alder reaction: 3,6-di(2-pyridyl)-1,2,4,5-tetrazine was reacted with the double bonds of vinyl-silica (vSilica) followed by an oxidation procedure. We explore using the dppz-vSilica material to obtain visible emitting luminescent materials and for obtaining a luminescent thermometer when grafted with Eu³⁺ /Tb³⁺ ions. For the dppz-vSilica@Eu,Tb material absolute sensitivity S_a of 0.011 K^-1 (210 K) and relative sensitivity S_r of 1.32 %K^-1 (260 K) were calculated showing good sensing capability of the material. Upon temperature change from 10 K to 360 K the emission color of the material changed gradually from yellow to red.

Highly ordered periodic mesoporous organosilica nanoparticles with controllable pore structures

A general synthetic procedure for highly ordered and well-dispersed periodic mesoporous organosilica (PMO) nanoparticles is reported based on a single cationic surfactant cetyltrimethylammonium bromide (CTAB) and simple silica sources with organic bridging groups via an ammonia-catalyzed sol-gel reaction. By changing the bridging group in the silica sources, the pore structures of the as-made particles with three-dimensional hexagonal (P6(3)/mmc), cubic (Pm3n), two-dimensional hexagonal (P6mm), and wormlike structure were evidenced by powder X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The size range of the nanoparticles can be adjusted from 30 nm to 500 nm by variation of the ammonia concentration or the co-solvent content of the reaction medium. The PMO nanoparticles with high concentration of organic groups in the framework offered good thermal stability, good dispersion in low polarity solvent and high adsorption of small hydrophobic molecules. Finally, the dye functionalized PMO nanoparticles show low cytotoxicity and excellent cell permeability, which offers great potential for biomedical applications.

Preparation of trimethylchlorosilane-modified acid vermiculites for removing diethyl phthalate from water

A hybrid organic-inorganic material based on vermiculite was prepared to remove diethyl phthalate (DEP) from aqueous solution. Natural vermiculite was activated with HCl to improve the specific surface area and was then modified by silanization using trimethylchlorosilane. Organovermiculite prepared by ion exchange with hexadecyl trimethylammonium bromide (HDTMAB) was also tested for comparison. The leaching of 2 mol L(-1) HCl at 80°C increased the specific surface area of vermiculite from 14.4 to 500.0m(2)g(-1), and the average pore-diameter was decreased from 7.90 nm to 2.75 nm. Fourier transform infrared spectroscopy (FTIR) spectra indicated that trimethysilyl groups were grafted covalently on the surface of acid vermiculites. The specific surface area of trimethylchlorosilane-modified acid vermiculites (TMAVs) (355.4 m(2) g(-1)) was much larger than that of organovermiculite (6.0 m(2) g(-1)). The isotherm adsorption experiments of DEP showed that TMAVs exhibited linear isotherms, suggesting that the uptake of DEP was controlled by partitioning mechanism. The maximal partition coefficient (K(d)) of TMAVs was 3.1 times higher than that of organovermiculite, implying that TMAVs had stronger organic affinity than organovermiculite. The results demonstrate that the adsorption capacity and mechanism of organoclays were controlled by the specific surface area and organic loading, whereas the length of alkyl chain of organic modifier was not the key factor.

In situ incorporation of nickel nanoparticles into the mesopores of MCM-41 by manipulation of solvent-solute interaction and its activity toward adsorptive desulfurization of gas oil

In this contribution, different amounts of nickel were incorporated into the mesopores of MCM-41 via an in situ approach. A hydrophobic nickel precursor was incorporated into the nanochannels of mesoporous silica by manipulation of solvent-solute interaction. The synthesized material was characterized using X-ray diffraction, nitrogen physisorption, temperature programmed reduction, and transmission electron microscopy. The results implicate the formation of MCM-41 with well-ordered hexagonal structure and establish also the presence of nickel nanoparticles inside the nanochannels of mesoporous silica. Adsorptive desulfurization of gas oil was conducted using the nickel-incorporated MCM-41 samples. The effects of nickel concentration, temperature of process and feed flow rate on the desulfurization process were examined. The MCM-41 containing 6 wt.% of nickel had both the highest breakthrough sulfur adsorption capacity and total sulfur adsorption capacity, which were 0.69 and 1.67 mg sulfur/g adsorbent, respectively. The breakthrough sulfur adsorption capacity was almost regained after reductive regeneration of spent adsorbent. The obtained results suggest that the method applied for the synthesis of Niy/MCM resulted in formation of well-dispersed, accessible and small nickel nanoparticles incorporated into the pores of MCM-41 which might be an advantage for adsorption of refractory sulfur compounds from low sulfur gas oil.

A Model for the Structure of MCM-41 Incorporating Surface Roughness

The surface area of MCM-41 mesoporous silica, estimated by several models in the literature, is significantly less than the value derived from BET analysis of nitrogen adsorption at 77.4 K. In the past, the difference has been attributed to several reasons including the errors involved in the BET analysis of the multilayer-capillary condensation region and the heterogeneity of the walls. In the present work, we present an alternate model of MCM-41 based on molecular simulations that gives surface area values that are in closer agreement to those determined by experiment. The model incorporates bulk heterogeneity of the material, surface hydroxyls, and most importantly, physical deformations or indentations of the pore surface. The model predicts small-angle X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) results that are consistent with experimental data as well as surface areas and pore volumes that compare favorably with published experimental results. The simulation results are consistent with the hypothesis that the interstitial space in MCM-41 is relatively amorphous despite the regular arrangement of the mesopores. The surface roughness associated with the amorphous structure increases the surface area beyond the nominal value produced by assuming smooth cylindrical pores.

Knowledge-based reconstruction of random porous media

An evolutionary optimization technique is used to reconstruct digitized material models of 300(3) nm3 size for mesoporous two-phase systems. The models are adapted to the two-point probability (TPP) and to a volume-based pore-size distribution (PSD) which were derived from SANS and adsorption experiments and which carry statistical information about morphology and topology of the pore system. To avoid extreme update-costs, the bulk of mutations are assessed by means of a suitable approximation of the PSD; it is demonstrated that a sporadic insertion of the PSD suffices to drive the algorithm towards satisfactory models in acceptable time. Our approach is knowledge-based in the sense that (i) the mutations are restricted to expedient exchanges of phase-voxels by a heuristic rule, and (ii) the sporadic calculation of the PSD from the current state of the model, in essence, provides an efficient self-control for the evolutionary process. We applied the method to reconstruct periodic models of the xerogel Gelsil 200. Such reconstructs of real mesoporous solids could be utilized, for instance, to verify theories of adsorption and capillary condensation.

Guest-host encapsulation of microporous zeolites in ordered mesoporous materials by molecular simulations

The present work provides the first study of ordered mesoporous materials SBA-15 coated with microporous zeolites ZSM-5 using molecular simulations. Several model structures with characteristics such as periodic arrangement of mesopores, randomly arranged micropores, surface hydroxyls and bulk deformations of SBA-15 were used. Cartesian coordinates of ZSM-5 unit lattice were obtained from the literature and the 100 face of H-ZSM-5 unit cell was then placed on the surface of SBA-15 and the entire structure was equilibrated to obtain final configuration. The resulting structure was characterized using simulated small angle and wide angle X-ray diffraction, Connolly surface area (to compare BET area), accessible pore volume for nitrogen molecules (to compare with t-plot volume of micro and mesopores) and methane adsorption at 303 K. The orientation of ZSM-5 on the SBA-15 had no effect on the surface area, pore volume or adsorption capacity. In order to find out if the addition of microporous ZSM-5 should increase the total methane adsorption capacity due to addition of micropores, we studied adsorption on bare and coated SBA-15. However, total adsorption capacity was found to decrease, while the number of methane molecules adsorbed per unit cell of the SBA-15 structure increased. An existing experimental method (J. Am. Chem. Soc., 2004, 126, 14324) of the synthesizing hybrid ZSM-5/SBA-15 structure was studied using accessible micropore volume (by t-plot). It was found that the procedure made all the micropores inaccessible. A modification of the method or use of other host materials is suggested to use the benefits of narrow micropore distribution in ZSM-5.

Modeling of Adsorption and Nucleation in Infinite Cylindrical Pores by Two-Dimensional Density Functional Theory

In this paper, we present an analysis of argon adsorption in cylindrical pores having amorphous silica structure by means of a nonlocal density functional theory (NLDFT). In the modeling, we account for the radial and longitudinal density distributions, which allow us to consider the interface between the liquidlike and vaporlike fluids separated by a hemispherical meniscus in the canonical ensemble. The Helmholtz free energy of the meniscus was determined as a function of pore diameter. The canonical NLDFT simulations show the details of density rearrangement at the vaporlike and liquidlike spinodal points. The limits of stability of the smallest bridge and the smallest bubble were also determined with the canonical NLDFT. The energy of nucleation as a function of the bulk pressure and the pore diameter was determined with the grand canonical NLDFT using an additional external potential field. It was shown that the experimentally observed reversibility of argon adsorption isotherms at its boiling point up to the pore diameter of 4 nm is possible if the potential barrier of 22kT is overcome due to density fluctuations.

Improvement of the Derjaguin-Broekhoff-de Boer theory for capillary condensation/evaporation of nitrogen in mesoporous systems and its implications for pore size analysis of MCM-41 silicas and related materials

In this work, we propose an improvement of the classical Derjaguin-Broekhoff-de Boer (DBdB) theory for capillary condensation/evaporation in mesoporous systems. The primary idea of this improvement is to employ the Gibbs-Tolman-Koenig-Buff equation to predict the surface tension changes in mesopores. In addition, the statistical film thickness (so-called t-curve) evaluated accurately on the basis of the adsorption isotherms measured for the MCM-41 materials is used instead of the originally proposed t-curve (to take into account the excess of the chemical potential due to the surface forces). It is shown that the aforementioned modifications of the original DBdB theory have significant implications for the pore size analysis of mesoporous solids. To verify our improvement of the DBdB pore size analysis method (IDBdB), a series of the calcined MCM-41 samples, which are well-defined materials with hexagonally ordered cylindrical mesopores, were used for the evaluation of the pore size distributions. The correlation of the IDBdB method with the empirically calibrated Kruk-Jaroniec-Sayari (KJS) relationship is very good in the range of small mesopores. So, a major advantage of the IDBdB method is its applicability for small mesopores as well as for the mesopore range beyond that established by the KJS calibration, i.e., for mesopore radii greater than approximately 4.5 nm. The comparison of the IDBdB results with experimental data reported by Kruk and Jaroniec for capillary condensation/evaporation as well as with the results from nonlocal density functional theory developed by Neimark et al. clearly justifies our approach. Note that the proposed improvement of the classical DBdB method preserves its original simplicity and simultaneously ensures a significant improvement of the pore size analysis, which is confirmed by the independent estimation of the mean pore size by the powder X-ray diffraction method.

Structure and Transport Properties of Nanostructured Materials

In the present manuscript, we have presented the simulation of nanoporous aluminum oxide using a molecular-dynamics approach with recently developed dynamic charge transfer potential using serial/parallel programming techniques (Streitz and Mintmire Phys. Rev. B 1994, 50, 11996). The structures resembling recently invented ordered nanoporous crystalline material, MCM-41/SBA-15 (Kresge et al. Nature 1992, 359, 710), and inverted porous solids (hollow nanospheres) with up to 10 000 atoms were fabricated and studied in the present work. These materials have been used for separation of gases and catalysis. On several occasions including the design of the reactor, the knowledge of surface diffusion is necessary. In the present work, a new method for estimating surface transport of gases based on a hybrid Monte Carlo method with unbiased random walk of tracer atom on the pore surface has been introduced. The nonoverlapping packings used in the present work were fabricated using an algorithm of very slowly settling rigid spheres from a dilute suspension into a randomly packed bed. The algorithm was modified to obtain unimodal, homogeneous Gaussian and segregated bimodal porous solids. The porosity of these solids was varied by densification using an arbitrary function or by coarsening from a highly densified pellet. The surface tortuosity for the densified solids indicated an inverted bell shape curve consistent with the fact that at very high porosities there is a reduction in the connectivity while at low porosities the pores become inaccessible or dead-end. The first passage time distribution approach was found to be more efficient in terms of computation time (fewer tracer atoms needed for the linearity of Einstein's plot). Results by hybrid discrete-continuum simulations were close to the discrete simulations for a boundary layer thickness of 5lambda.

Equilibrium Adsorption in Cylindrical Mesopores: A Modified Broekhoff and de Boer Theory versus Density Functional Theory

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.

Nanocrystalline Iron Oxide Aerogels as Mesoporous Magnetic Architectures

We have developed crystalline nanoarchitectures of iron oxide that exhibit superparamagnetic behavior while still retaining the desirable bicontinuous pore-solid networks and monolithic nature of an aerogel. Iron oxide aerogels are initially produced in an X-ray-amorphous, high-surface-area form, by adapting recently established sol-gel methods using Fe(III) salts and epoxide-based proton scavengers. Controlled temperature/atmosphere treatments convert the as-prepared iron oxide aerogels into nanocrystalline forms with the inverse spinel structure. As a function of the bathing gas, treatment temperature, and treatment history, these nanocrystalline forms can be reversibly tuned to predominantly exhibit either Fe(3)O(4) (magnetite) or gamma-Fe(2)O(3) (maghemite) phases, as verified by electron microscopy, X-ray and electron diffraction, microprobe Raman spectroscopy, and magnetic analysis. Peak deconvolution of the Raman-active Fe-O bands yields valuable information on the local structure and vacancy content of the various aerogel forms, and facilitates the differentiation of Fe(3)O(4) and gamma-Fe(2)O(3) components, which are difficult to assign using only diffraction methods. These nanocrystalline, magnetic forms retain the inherent characteristics of aerogels, including high surface area (>140 m(2) g(-1)), through-connected porosity concentrated in the mesopore size range (2-50 nm), and nanoscale particle sizes (7-18 nm). On the basis of this synthetic and processing protocol, we produce multifunctional nanostructured materials with effective control of the pore-solid architecture, the nanocrystalline phase, and subsequent magnetic properties.

Quantitative information on pore size distribution from the tangents of comparison plots

The comparison plot obtained from the nitrogen adsorption data has a similar shape to that of the curve of accumulating pore volume of a solid. The intrinsic nature of this relation is discussed. It is known that the derivatives of the accumulating pore volume with respect to the pore size are the pore size distribution (PSD) of the solid. Thus, the tangent curve of the comparison plot can display, at least qualitatively, the PSD of a solid, over a wide range of pore sizes (from approximately 1 to 50 nm) because the comparison plot is applicable to both micropores and mesopores. Quantitative pore structure information can be derived from the comparison plots by establishing a relationship between the t value and the pore size from the samples with uniform pore structure and known pore sizes, such as MCM-41 and alumina pillared clay samples. A calculation procedure to derive quantitative PSD from the comparison plots is suggested, giving reasonable results. This study proposes concise and reliable methods based on the comparison plots to derive information on pore structure in porous solids.

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.