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Adsorption: Journal of the International Adsorption Society (v.5, #1)
Cluster-Mediated Water Adsorption on Carbon Nanopores by K. Kaneko; Y. Hanzawa; T. Iiyama; T. Kanda; T. Suzuki (pp. 7-13).
The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.
Keywords: water cluster; water adsorption; hysteresis; electron radial distribution function; activated carbon fibers; carbon aerogel
Adsorption Characteristics in Reversed-Phase Liquid Chromatography Using Ethanol/Water Mixed Solvent by Kanji Miyabe; Shigeya Takeuchi; Yoshisato Tezuka (pp. 15-24).
Adsorption characteristics were studied in a reversed-phase liquid chromatography consisting of an octadecylsilyl (ODS)-silica gel and ethanol/water mixture (70/30, v/v), and were compared with corresponding results obtained by using methanol/water and acetonitrile/water mixtures (70/30, v/v) as mobile phase. Similar tendencies were observed for some adsorption characteristics in the three chromatographic systems. However, the magnitude of the characteristics was not entirely identical in the three systems. Surface diffusion was dominant for intraparticle diffusion in the ODS-silica gel particles irrespective of the type of the organic modifiers in mobile phases. A few correlations were confirmed with regard to surface diffusion, i.e., an enthalpy-entropy compensation and a linear free-energy relation. The analogous correlations on surface diffusion phenomena suggest the similarity in the mechanism of surface diffusion in the three chromatographic systems.
Keywords: liquid chromatography; reversed phase; mass transfer; surface diffusion; organic modifier
Sorption Equilibria and Kinetics of Hydrocarbons onto Activated Carbon Samples Having Different Micropore Size Distributions by K. Wang; D.D. Do (pp. 25-37).
This paper deals with the prediction of adsorption equilibrium and kinetics of hydrocarbons onto activated carbon samples having different micropore size distribution (MPSD). The microporous structure of activated carbon is characterised by the distribution of slit-shaped micropores, which is assumed to be the sole source of surface heterogeneity. The interaction between adsorbate molecule and pore walls is described by the Lennard-Jones potential theory. Different adsorbates have access to different pore size range of activated carbon due to the size exclusion, a phenomenon could have a significant influence on both multicomponent equilibria and kinetics. Activated carbons with three different MPSDs are studied with ethane and propane as the two model adsorbates. The Heterogeneous Macropore Surface Diffusion model (HMSD) is employed to simulate adsorption kinetics. The simulation results show that the MPSD is an important factor affecting both the multicomponent equilibria and kinetics.
Keywords: micropore size distribution; activated carbon; adsorption; desorption; equilibrium; kinetics
Modification of Surface and Structural Properties of Ordered Mesoporous Silicates by Mietek Jaroniec; Michal Kruk; Christopher P. Jaroniec; Abdelhamid Sayari (pp. 39-45).
Two simple modification methods for ordered mesoporous silicas were examined and compared. MCM-41 molecular sieve was physically coated with 4′-cyano-4-biphenyl [4(4-pentenyloxy)]benzoate (CBPB) and chemically modified using trimethylethoxysilane. The structural and surface properties of the obtained materials were characterized using elemental analysis, thermogravimetry and nitrogen adsorption over a wide pressure range.It was shown that the pore size of the MCM-41 material was not decreased significantly after the coating procedure, even for high loadings of CBPB. Moreover, low pressure adsorption measurements indicated that significant fractions of the MCM-41 surface were not covered by CBPB, even for high CBPB loadings, which suggests that the attained coverage may be very nonuniform. The chemical bonding procedure led to a marked decrease in the pore size and change of surface properties.It was demonstrated that nitrogen adsorption measurements provide a means of a thorough characterization of modified MCM-41 materials, allowing to estimate the surface area, pore volume and pore size distribution. Moreover, low pressure adsorption data can be used to qualitatively or semiquantitatively assess the surface coverage of the coated/bonded organic groups, which may be used to estimate the uniformity of the coverage and therefore, the usefulness of the modification procedure.
Keywords: mesoporous molecular sieves; chemical modification; coating; nitrogen adsorption; structural characterization of adsorbents
Influence of Moderate Temperature Treatment under Oxidation Atmosphere on Acidic Site Distribution of γ-Alumina by Zheng-Ming Wang; Makoto Yamaguti; Ichiro Goto; Mikio Kumagai (pp. 47-55).
In order to elucidate the role of acidity in reaction of selective NO reduction by hydrocarbons in oxygen-rich circumstances, a commercial γ-alumina was treated under atmosphere at moderate temperature range and the acidic sites of treated alumina were determined by differential NH3 adsorption heat at 473 K. The alumina has little impurity of Na, Fe, Si. The BET surface area and average pore width are 350 m2/g and 4 nm respectively. Air treatment at moderate temperature has no serious effect on the total amount of acidic sites above 70 kJ/mol, but influences the acidic site distribution at the initial and the tail energy sides. Treatment at higher temperature gives rise to a higher initial adsorption heat due to activation of alumina surface. Disappearance of carbon-contained species and appearance of distinct surface hydroxyl groups under air treatment above 673 K are plausibly observed by FT-IR. Degree of dehydration has a tremendous effect on development of Brönsted acidic sites. The relationship of changes in acidic site distribution at moderate temperature and De-NOx activity at the same temperature was discussed.
Study of Zeolite Molecular Sieves for Production of Oxygen by Using Pressure Swing Adsorption by S. Yoshida; N. Ogawa; K. Kamioka; S. Hirano; T. Mori (pp. 57-61).
Adsorption of nitrogen on Li or Ca exchanged Faujasite at various Si/Al mole ratios (1.00, 1.23, and 1.69) and exchange levels (0 to nearly 100%) has been measured at 298 K. Nitrogen capacities were compared with the positions of Li and Ca. It has been demonstrated that Li at site III and Ca at site II adsorb nitrogen, and nitrogen capacities depend on Li per unit cell in Li-Faujasite and Ca per unit cell in Ca-Faujasite regardless of Si/Al.
Keywords: Faujasite; nitrogen adsorption; positions of cations
Fabrication of Hard Dextran DEAE: Adsorption Equilibria of BSA by Hiroyuki Yoshida; Koji Fujita; Keiko Kudo (pp. 63-71).
A hard dextran-DEAE ion exchanger (Hard Dextran DEAE) was developed. It is hard and keeps good properties of dextran-DEAE for protein separation. It is not compressed in a column and can be used in much wider range of flow rate in the column than the commercial hard gel, DEAE Sepharose Fast Flow made of agarose. The saturation capacity of BSA on Hard Dextran DEAE is about 1.7 times of that on DEAE Sepharose Fast Flow at pH 6.9. Equilibrium isotherm for adsorption of BSA depends on pH considerably. When pH ≥ 5.5, the equilibrium isotherm is correlated by the Langmuir equation. When pH ≤ 5.05, the isotherm is correlated by the Freundlich equation. The higher the concentration of NaCl is, the smaller the amount of BSA adsorbed. When the concentration of NaCl is higher than 100 mol m−3 at pH 6.9 and 50 mol m−3 at pH 4.8, BSA was not adsorbed on the resin. This may suggest that BSA is adsorbed by electrostatic attraction. About 100 mol m−3 NaCl aqueous solution can be used as an eluant of proteins.
Keywords: composite resin; hard dextran; protein; BSA; equilibria
A FAD-SMT Model of Large Scale Liquid Adsorption Chromatography by Zhong Li; Yuechun Zhao; Wenli Shu; Qingsheng Lin; Zhenhua Ye (pp. 73-79).
The paper proposes a FAD-SMT model of large-scale liquid chromatography by which a continuous equation of chromatographic separation is decomposed into a convection dispersion partial differential equation and a set of ordinary differential equations. The numerical method for the FAD-SMT model is established. The stability and the convergence condition of numerical solution, and the choice of time and space interval are discussed. The FAD-SMT model is used to simulate liquid adsorption chromatography and cycling adsorption chromatography. Results show that the elution curves calculated by FAD-SMT model are good agreement with the experimental elution curves of the separation of glucose and fructose, the separation of sucrose and reducing sugar and the separation of mannitol and sorbitol. The result of parameter sensitivity analysis shows that the chromatographic elution curves are more sensitive to the changes of the parameter ai in Langmuir isotherms than to the changes of other parameters in the studied system.
Keywords: large scale chromatography; FAD-SMT model; numerical method
A New Approach to the Theory for Adsorption Isotherms on Heterogeneous Surfaces by Eduardo Bottani; William A. Steele (pp. 81-89).
Simulations of the adsorption of nitrogen on several model heterogeneous surfaces (Cascarini de Torre and Bottani, 1997) are analyzed using an alternative description in which the surface is treated as a collection of supersites, each of which can hold 5–6 molecules in the complete monolayer. The local isotherm that is used to describe the sub-monolayer adsorption on a supersite is here taken to be the truncated virial isotherm. This theory fits well to the simulated data. The advantages and disadvantages of this approach are discussed and it is concluded that the supersite concept is a promising approach to the description of a adsorption on realistic models of heterogeneous surfaces.
Keywords: N2; virial; supersite; local isotherm
Adsorption of Carbon Tetrachloride by 3.4 nm Pore Diameter Siliceous MCM-41: Isotherms and Neutron Diffraction by Peter J. Branton; Philip A. Reynolds; Andrew Studer; Kenneth S.W. Sing; John W. White (pp. 91-96).
Adsorption isotherms of carbon tetrachloride at temperatures between 273 and 323 K have been determined on the pure silica form of MCM-41 of pore diameter ca. 3.4 nm. All isotherms were of Type V, the isotherms at 273, 288 and 303 K showing hysteresis loops, whereas the isotherm at 323 K was completely reversible. Despite the questionable validity of the Kelvin equation when applied to narrow mesopores, changes in the relative pressure positions of capillary condensation and evaporation as a function of the temperature appear to be well described. Neutron diffraction measurements at 200 and 273 K show significant changes in the physical properties of the adsorbed CCl4 in the MCM-41 from those of bulk adsorbate. The results also suggest a highly heterogeneous surface and appear to show some flexibility in the pore walls upon pore filling. The conditions required for first order reversible capillary condensation are discussed.
Keywords: mesoporous silica; adsorption isotherm measurements; capillary condensation; neutron diffraction
Effects of Surface Oxygen of Activated Carbon on Alkaloid Adsorption: A Molecular Dynamics Simulation Study by Dianxia Wang; Akiyoshi Sakoda; Motoyuki Suzuki (pp. 97-108).
The influence of the density and the type of surface oxygen on the adsorption of berberine alkaloid onto activated carbon was investigated using the molecular dynamics simulation method in vacuum. The carbon surface consisted of a basal plane of graphite and surface oxygen groups which were bonded on the graphite plane in a regular square array with various densities. Two types of surface oxygen groups, =O and —OH, were employed. The simulation results showed that the berberine alkaloids were favorable to be adsorbed on the negative charged carbon surfaces. It was indicated that the vdw attraction of the carbon surface to the alkaloid molecule dominates the adsorption only at the lower surface density of oxygen. It is also indicated that a good adsorptive selectivity for a certain berberine alkaloid can be obtained by controlling the density of surface oxygen.The adsorption simulation of berberine alkaloids onto activated carbon in the presence of water was also carried out by using a dome-shape molecular model for presenting the alkaloid/water/carbon system. It was found that the adsorption of berberine alkaloids on the activated carbon which has a higher density of surface oxygen was strongly inhibited by the presence of water.
Keywords: adsorption; molecular simulation; activated carbon; surface oxygen; alkaloids