The significance of physical factors on the adsorption of polyaromatic compounds by activated carbons

The adsorption of three synthetic organic compounds (SOCs) (i.e., phenanthrene, biphenyl, and 2-chlorobiphenyl), with similar physicochemical properties but different molecular conformations (i.e., planar and nonplanar), by an activated carbon and an activated carbon fiber was investigated. The physical characteristics of the carbons were obtained from low temperature nitrogen adsorption isotherms using BET, DR, and DFT models. Their surface chemistry was characterized by water vapor adsorption, pH of the point of zero charge, acid/base uptakes, and elemental analysis. The results indicated that adsorbent pore structure characteristics and adsorbate molecular conformation are important in the adsorption of SOCs by porous carbonaceous adsorbents. To predict the adsorption of SOCs by activated carbons, accurate characterization of pore shape and size distribution in primary micropores is important. The results indicated that adsorbate molecules can access and fill more efficiently the slit-shape pores than ellipsoidal pores, whereas the ellipsoidal pores create higher adsorption potential than slit-shape pores. Both molecular conformation and dimensions of adsorbate affect the adsorption. Planar molecules appear to access and pack in slit-shape pores more efficiently as compared to nonplanar molecules. Nonplanar molecular conformation weakens the interactions between adsorbate molecules and carbon surfaces.     

Influence of the composition and temperature of heat treatment of porous glasses on their structure and light transmission in the visible spectral range

The pore structure and light transmission of high-silica porous glasses in the visible spectral range have been investigated as a function of the heat treatment temperature and the composition of the initial two-phase alkali borosilicate glass. The character of light transmission in porous glasses has been analyzed in the framework of the concepts of structural features of their pore space and the processes occurring in the porous glass during heating. It has been demonstrated that an increase in the temperature of heat treatment of porous glasses with different compositions leads to an increase in the pore size and a decrease in their specific surface area (with a nearly constant total porosity), which is associated with the processes of overcondensation of pores due to the rearrangement and the change in the packing density of secondary silica particles. It has been revealed that the introduction of phosphate and fluoride ions into the initial sodium borosilicate glass results in an increase in the light extinction coefficient of porous glasses due to the increase in the sizes of phase-separated inhomogeneity regions in the initial two-phase glasses, the formation of larger pores, and the presence of nanosized microcrystalline phases in porous glasses.     

Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.     

Microporous Substructure of Porous Glasses: Dynamic and Equilibrium Approaches

Porous glasses produced by the leaching of two-phase alkali borosilicate glasses have been studied by dynamic and equilibrium methods for diagnostics of pore morphology. It has been detected the availability of microporous substructure with some kinds of adsorbing micropores of diameter 0.3–2 nm including ultramicropores of molecular size in porous glasses with transporting pores of mean diameter from 4 to 15 nm. The multimodal nanoporous structure of porous glasses detected by kinetic mass spectral method of gas diffusion diagnostics (DD-method) at low temperatures is consistent with the results obtained from analyzing equilibrium desorption isotherms of nitrogen, oxygen and argon at 77.5 K by different calculation techniques including an equilibrium method of gas desorption at low partial pressures (LPED-method). Micropore volume in porous glasses is equal to 6–18% from total pore volume. The dependence of nanoporous morphology of porous glasses on conditions of their production and composition has been established. The diffusion and equilibrium characteristics of different molecules (nitrogen, oxygen, argon) varying in molecule size and quadrupole moment value have been determined for primary and secondary porous substructures of porous glasses at liquid nitrogen temperature at the first time.     

Development of a technique to prepare porous materials from glasses

A new technique to prepare porous materials has been developed for recycling of used glasses. Porous materials were produced from used glass powders by hydrothermal reaction of glass powders with water at low temperatures followed by calcination at high temperatures for expansion. The new technique does not require additional vesicants such as calcium carbonate, silicon carbide, and organic polymers that have been used in the ordinary method. In this process, the water incorporated into the glass structure by hydrothermal treatment acts as a vesicant. When the hydrothermally treated glass was heated at high temperatures around 700 °C, water was released as vapor from the softened glass to form pores. The obtained porous materials had different pore structures that were determined by the sizes of raw glass particles. Crystallization occurred during the calcination of the hydrothermally treated glass powder.     

Computational Simulations for the Assessment of the Mechanical Properties of Glass with Controlled Porosity

Porous glass with closed controlled porosity is used as a model system in order to numerically assess the effect of pores on the macroscopic mechanical and fracture behavior of brittle solids. A computational code called OOF, which converts digitalized two-dimensional (2-D) images of materials microstructures into finite element meshes, is adopted, so that the effect of 2-D microstructural features (e.g. pore size and shape) on the global mechanical response of the material can be determined. Firstly, microstructures of porous glass bodies containing isolated pores were considered. These specimens were numerically investigated in terms of fracture initiation and propagation: the numerical model predicted that larger pores initiate fracture, in agreement with experimental results. Then, the effect of porosity on the elastic and fracture properties was thoroughly investigated by means of model two-dimensional microstructures consisting of selected area fractions of pores (equivalent to pore volume fractions in three dimensions) and with prescribed pore shape, orientation and dimensions. In particular, the effect of pore dimension and shape was studied, finding that the critical stress for crack initiation scales with pore dimension and aspect ratio, i.e. oblate and larger pores oriented perpendicularly to the stress direction cause a higher reduction of strength of the specimen. Finally, several 2-D microstructures characterized by different values of area fraction of pores of the same shape were investigated, in order to determine the variation of elastic properties and the fracture response of porous glasses with pore content. The study confirms the suitability of the 2-D OOF code to investigate the mechanical and fracture behavior of porous materials. Issues regarding the limitation of the model due to its 2-D character are also discussed where appropriate.     

碱处理Beta分子筛吸附脱硫动力学

当吸附质尺寸与分子筛的孔结构尺寸相当时才能进入分子筛的内部,分子筛的孔道尺寸是影响吸附性能的重要因素之一,NaOH溶液处理可实现分子筛孔径的调变,以改善分子筛对吸附质的选择性和扩散传质能力。通过对碱处理后的Beta分子筛进行BET、TEM、FT-IR、XRD和NH₃-TPD表征,以考察碱处理对Beta分子筛晶体结构、表面酸性和孔结构参数的影响。碱处理Beta分子筛吸附脱硫的结果显示,碱处理后Beta分子筛吸附量和吸附速率明显增加,尤其动力学尺寸较大的二苯并噻吩吸附量达到了5.06 mg·g^-1。吸附动力学研究显示,该吸附过程符合准二级动力学方程,碱处理后分子筛吸附速率常数和粒子内扩散速率常数明显增大,有效地改善了硫化合物在孔道内扩散传质性能,脱硫效率增加。     

Effects of compression on the textural properties of porous solids

This work deals with the analysis of the effects of compression in the textural properties, the crystallinity and the packing density of porous solids. It has been found that compression produces the decrease of both, the pore volume and the interparticle voids of porous solids. The reduction of the pore volume depends on the mechanical strength of the material. Activated carbons and inorganic porous oxides with a high mechanical strength, show a relatively low reduction of their pore volume, while porous solids in which the pore walls are constituted by organic frameworks, like MOF-5, have a lower mechanical strength and thus, their porous texture is largely affected by compression. In general terms, an increasing compressive pressure produces the removal of pores in the following sequence: mesopores, broad micropores and narrow micropores. Thus, compression of porous materials could be considered as a procedure to tailor the pore volume and the pore size distribution of porous solids. The results obtained in this work allow affirming that, in the preparation of adsorbent materials, together with the development of a high specific pore volume or a specific surface area, the behaviour upon compactatation must, undoubtedly, be taken into account.     

Application of partial sintering of waste glasses for preparation of porous glass bodies

Porous glasses were prepared by partial sintering of waste glasses. Polyvinyl alcohol was added as a binder to the glass powder and the mixture was un-axially cold pressed under two different forces, followed by sintering at 700 °C. The effects of thermal history, particle size of glass powder, binder content and applied pressing forces on pore size and total porosity of fabricated porous glasses were investigated and final products with the porosity of 15–32 % were prepared. The average pore size of the specimens was determined using mercury porosimetry. The morphology of the porous glasses was observed by scanning electron microscopy. These produced porous glasses can be used for selective and accurate filtration.     

Influence of pyrolysis temperature on the adsorptive properties of adsorbents produced from novolac and biomass

Mixtures of novolac resin and olive stone biomass are cured and pyrolyzed at different temperatures to yield carbonaceous adsorbents. The weight losses and the shrinkages taking place in the carbonization process increase up to ca. 600°C. The pyrolysis residues are investigated with respect to their ability to adsorb toluene and cyclohexane from the vapor phase. Toluene is adsorbed on all adsorbents stronger than cyclohexane. The overall diffusion coefficient D of the adsorption process, the specific surface area, and the pore volume of the adsobents are calculated. The adsorptive properties of the products are interpreted taking into account the size and shape of the pores, the polarity of the adsorbate, and the dispersion forces between adsorbent and adsorbate. Slit-shaped pores, which are characteristic of carbon molecular sieves, are likely to be present in the adsorbents which are pyrolyzed at higher temperatures and particularly at 1000°C. This adsorbent shows the highest adsorption concerning the equilibrium uptake, specific surface area, and pore volume of toluene and cyclohexane. However, the diffusion coefficient D is lower as more micropores are present. © 1995 John Wiley & Sons, Inc.     

Microporosity of Porous Glasses: New Investigation Techniques

Two new methods are developed for investigating the microporosity of porous materials. The first method, namely, diffusion diagnostics, is based on a computer simulation of the kinetics of gas desorption from a porous material into high vacuum. In this method, the kinetics of gas desorption is measured using mass spectrometry. Another method is based on the analysis of the equilibrium isotherms of gas desorption at low pressures. The microporous and mesoporous substructures of the porous glasses prepared by leaching of two-phase alkali borosilicate glasses in a 3 M HCl solution at 100°C are investigated using both the new methods and the classical adsorption methods. A correlation between the porous structure of glasses and their synthesis and phase separation conditions is revealed. It is demonstrated that porous glasses have a through mesoporous substructure with mean pore diameters ranging from 4 to 15 nm and a polymodal microporous substructure with pore sizes ranging from 0.35 to 2 nm. This microporous substructure is formed by ultramicropores of molecular sizes and also medium- and large-sized micropores. It is found that porous glasses contain bottle-shaped pores with sizes corresponding to the micropore range. An increase in the temperature of heat treatment of glasses from 550 to 700°C and an increase in the boron oxide content in the silica phase lead to an increase in the volume of micropores in porous glasses.     

The adsorption characteristics of polyvinylidene chloride carbon

A very pure form of polyvinylidene chloride has been prepared and then carbonised in an atmosphere of nitrogen. The resultant carbon has been examined and its porous structure investigated by the adsorption of a series of adsorbates of differing molecular size. The results indicate that the total volume of adsorbate taken up is not a factor which can be related to the relative size of the pores and the adsorbates. However the effective cross-sectional areas of the large adsorbate molecules do appear to be anomalous probably due to the pore structure preventing closest possible packing of the adsorbate molecules.     

New adsorption-condensation theory for adsorption of vapors on porous activated carbons

A new analytical model to describe equilibrium adsorption of condensable vapors on porous adsorbents is developed. It accounts for the heterogeneous pore structure of the adsorbent, adsorption in the micropores by a pore filling mechanism and adsorption and condensation in the macropores. A gamma-type pore size distribution function is used. Langmuir-type adsorption equations are used to describe both micropore filling and adsorption on the macropore walls. The vapor condensation in the pores is described by the Kelvin equation. The model is successfully tested using isotherm data for adsorption of various condensable vapors on different porous activated carbons and charcoals. All three types (I, IV and V) of adsorption isotherms by the Brunauer classification which are depicted by the porous adsorbents can be described by the model.     

Nonexponential Dynamics in Porous Glasses

The dielectric relaxation properties of porous glasses prepared from sodium borosilicate glasses are studied by dielectric spectroscopy over a wide range of frequencies (20 Hz–1 MHz) and temperatures (–100–300°C). The dielectric behavior reflecting the geometric disorder is analyzed within the models describing the non-Debye slowly damped dynamics. It is found that the dielectric response is very sensitive to microstructural and mesostructural features of the porous matrix and the properties of a material filling pores. The response contains information on the dynamics of water molecules in pores, which accounts for the interaction of these molecules with the pore surface.     

Evaluation of effective diffusivity of adsorbate in the absence of adsorption and surface diffusion

A method is suggested for the evaluation of effective diffusivity of adsorbate in the absence of adsorption and surface diffusion, the precise value of which is required for estimating the effective surface diffusivity and the extent of influence of adsorption on the overall mass flux in a porous solid. The method does not require the knowledge of tortuosity factor, the accurate values of average pore radius for micro, transitional and macro pores, or the porosities corresponding to micro and macro pores, which are required in case of diffusion in the transitional region and in the solids containing bimodal pore size distribution.     

Monte Carlo simulation on the adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41

The adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41 with heterogeneous and cylindrical pores have been studied by using grand canonical ensemble Monte Carlo simulation. The adsorption isotherm, average potential of adsorbate, isosteric heat of adsorption, and number density of molecules in MCM-41 were calculated. The simulated isotherms were compared with experimental ones. Also, different adsorption behaviors in MCM-41 with pore diameter of 2.2 and 3.2 nm were discussed. The capillary-condensation pressure increased for a given adsorbate with an increase in pore diameter. The average densities of carbon tetrachloride, neopentane, and cyclohexane in the two different pores above the capillary-condensation pressure were smaller than the corresponding liquid densities by about 12%. The adsorbate molecules did not form the multilayer in pore below the capillary-condensation pressure. The number of adsorption layers of molecules was constant in a given pore for the three adsorbates above the capillary-condensation pressure. Carbon tetrachloride molecules in pore were also ordered along the pore axis.     

Simulation of nitrogen sorption processes in materials with cylindrical mesopores: Hysteresis as a thermodynamic and connectivity phenomenon

A numerical simulation of nitrogen sorption in materials with cylindrical mesopores is proposed. Multilayer formation and capillary condensation-evaporation processes are followed by using the Sonwane and Bhatia's molecular-continuum model, although an empirical expression is used to represent the potential interaction between the adsorbate and the adsorbent. The pore structure of the solid is generated by inscribing 3D cylindrical pores in a 2D square lattice. The connectivity effects on the nitrogen isotherms are studied by using percolation theory. The ability to predict the multilayer thickness and the relative pressure at which phase transition takes place is corroborated with data reported in literature, finding good fittings in this work. On the other hand, we report for the first time a study on the effect that both pore mean diameter and connectivity have on the extent of hysteresis.     

Advancing the fabrication of YSZ-inverse photonic glasses for broadband omnidirectional reflector films

A single-step and all-colloidal deposition method to fabricate yttrium-stabilized zirconia (YSZ)-inverse photonic glasses with 3 μm pores was developed. The process is based on electrostatic attraction and repulsion in suspension, controlled by surface charge of polystyrene (PS) microspheres and YSZ nanoparticles, used as pore templates and matrix material, respectively. The pH was used as a tool to change surface charges and particle-particle interactions. Photonic glass films with 3 μm pores yielded broadband omnidirectional reflection over the wavelengths of 1–5 μm, relevant for thermal radiation at temperatures around 1200 °C. These highly porous materials maintained their structural stability and reflectance after being annealed at 1200 °C for 120 h.     

Freeze Casting of Porous Bioactive Glass and Bioceramics

Highly porous network structures of hydroxyapatite, tricalcium phosphates, a bioactive glass as well as their composites have been fabricated using variations of camphene-, glycerol-, and ice-based freeze-casting techniques. The ball-milled slurries containing 10%–60% solid loading were cast at ambient temperature, followed by sublimation at temperatures between −70° and 60°C. The green body was sintered in air to a maximum temperature of 1100°C for 4 h, which produced excellent three-dimensionally (3-D) interconnected structures with open pores. The nature of the pore channels varied from dendritic, columnar, and cellular to mixed geometry, with dense outer shells in some cases, depending on the particular method used. A monotonic increase in porosity with loading was observed with a decrease in the loading volume. Microstructural analysis was used for porous structures to extract information on geometry, porosity, and pore size distribution. This proved particularly useful to assess whether some of the 3-D structures produced by these methods are suitable for tissue engineering applications. Differential thermal analysis–thermogravimetry, scanning electron microscopy, X-ray diffraction, and density were used to characterize precursor powders, slurry, and sintered products.     

Physisorption of nitrogen by porous materials

Nitrogen adsorption at 77 K is widely used for the determination of the surface area and pore size distribution of various porous materials. The first stage in the interpretation of a nitrogen isotherm is the identification of the physisorption mechanism(s): that is monolayer-multilayer adsorption, capillary condensation or micropore filling. The Brunauer-Emmett-Teller (BET) method cannot be used to provide a reliable evaluation of the surface area if the solid contains pores of molecular dimensions (i.e. narrow micropores). Furthermore, with such materials, it is necessary to use different probe molecules to determine the pore size distribution. Capillary condensation is involved as a secondary process in the filling of mesopores (pore width in the range 2–50 nm). Recent work on MCM-41, a model mesoporous adsorbent, has confirmed that capillary condensation-evaporation can occur reversibly in open-ended pores of about 4 nm width.