Gas adsorption in active carbons and the slit-pore model 1: Pure gas adsorption

We describe procedures based on the polydisperse independent ideal slit-pore model, Monte Carlo simulation and density functional theory (a 'slab-DFT') for predicting gas adsorption and adsorption heats in active carbons. A novel feature of this work is the calibration of gas-surface interactions to a high surface area carbon, rather than to a low surface area carbon as in all previous work. Our models are used to predict the adsorption of carbon dioxide, methane, nitrogen, and hydrogen up to 50 bar in several active carbons at a range of near-ambient temperatures based on an analysis of a single 293 K carbon dioxide adsorption isotherm. The results demonstrate that these models are useful for relatively simple gases at near-critical or supercritical temperatures.     

High-pressure adsorption of CO2 on NaY zeolite and model prediction of adsorption isotherms

Supercritical carbon dioxide is an efficient solvent for adsorptive separations because it can potentially be used as both the carrier solvent for adsorption and the desorbent for regeneration. Recent results have demonstrated an anomalous peak or "hump" in the adsorption isotherm near the bulk critical point when the adsorption isotherm is plotted as a function of bulk density. This work presents new data for the adsorption and desorption of carbon dioxide in the near-critical region on a crystalline, well-structured adsorbent (NaY zeolite). The results indicate a strong affinity for CO(2) as well as a significant hump near the critical point. The lattice model previously developed by Aranovich and Donohue is applied to analyze the adsorption.     

Elution dynamics of adsorption of Chladon 113 and methyl bromide on active carbons

The elution dynamics of adsorption of Chladon 113 (at high contents of the adsorptive) and methyl bromide on active carbons with different lengths of the layer was studied. The adsorption isotherms were described by the equation of the theory of volume filling of micropores. The elution curves were calculated using the model of the equilibrium adsorption layer. The effective kinetic constant calculated in the framework of the model is 1.5 cm for Chladon 113 at a linear velocity of vapor-air flow of 1250 cm min⁻¹ and 1.0 cm for methyl bromide at the velocity of 700 cm min⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1353–1355, July, 1998.     

DFT-based prediction of high-pressure H2 adsorption on porous carbons at ambient temperatures from low-pressure adsorption data measured at 77 K

Hydrogen adsorption isotherms were measured both at cryogenic temperatures below 1 atm and at ambient temperature at high pressures, up to 90 atm, on selected porous carbons with various pore structures. The nonlocal density functional theory (NLDFT) model was used to calculate the pore size distributions (PSDs) of the carbons, from H2 adsorption isotherms measured at 77 K, and then to predict H2 adsorption on these carbons at 87 and 298 K. An excellent agreement between the predicted and measured data was obtained. Prior to analyzing the porous carbons, the solid-fluid interaction parameters used in the NLDFT model were derived from H2 adsorption data measured at 77 K on nonporous carbon black. The results show that the NLDFT model with appropriate parameters may be a useful tool for optimizing carbon pore structures and designing adsorption systems for hydrogen storage applications.     

Estimation of micropore size distribution in active carbons from adsorption isotherms of water vapor

A possibility of estimation of the micropore size distribution in the carbon adsorbents with the developed micro-and mesoporous structure by analysis of the adsorption isotherms of water vapors was considered. At saturation water condenses in micropores in a form of a weakly compressed liquid. However, water molecules in micropores are packed not so closely as in the liquid because of steric hindrance. Therefore, the real density of water adsorbed in the micropores is lower than that of water adsorbed on an open surface and lower than the density of the normal liquid. An analysis of the adsorption isotherms of water vapors with account for the both opposite effects on the water density gives reliable data on the micropore sizes of the carbon adsorbents. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 40–43, January, 2007.     

Selective adsorption of refractory sulfur species on active carbons and carbon based CoMo catalyst

Adsorption technique could be a reliable alternative in removing to a certain remarkable extent the sulfur species from the feedstock of petroleum oil. The performance of various carbons on adsorption of model sulfur compounds in a simulated feed solution and the sulfur containing compounds in the real gas oil was evaluated. The adsorption experiments have been carried out in a batch scale at ambient temperature and under the atmospheric pressure. In general, the most refractory sulfur compounds in the hydrotreatment reactions were selectively removed and adsorbed. It was found that the adsorbents affinities to dibenzothiophene and 4,6-dimethyldibenzothiophene were much more favored and pronounced than the aromatic matrices like fluorene, 1-methylnaphthalene and 9-methylanthracene. Among the sulfur species, 4,6-dimethyldibenzothiophene was the highest to be removed in terms of both selectivity and capacity over all the present adsorbents. The studied adsorbents showed significant capacities for the polyaromatic thiophenes. The electronic characteristics seem to play a certain role in such behavior. Regeneration of the used adsorbent was successfully attained either by washing it with toluene or by the release of the adsorbates through heat treatment. A suggested adsorptive removal process of sulfur compounds from petroleum distillate over carbon supported CoMo catalyst was discussed.     

Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).     

Modified langmuir-like model for modeling the adsorption from aqueous solutions by activated carbons

A new equation, a modified Langmuir-like equation (M-LLE), for describing adsorption from solution by activated carbons is proposed for the first time in this work. The M-LLE assumes that there are two types of interactions: (a) specific interactions which are typical, enthalpy-driven interactions and (b) nonspecific interactions driven by the loss of water structuring, upon adsorption (hydrophobic bonding), around the nonpolar parts of the drug. The proposed model was evaluated by studying the adsorption of three drugs: procaine, fluoxetine, and phenobarbital by four different activated carbons under different experimental conditions. As the hydrophobicity of the drug increased, the capacity constant representing the interactions driven by hydrophobic bonding (K(HB), M-LLE equation) increased. Experimental conditions that decrease hydrophobic bonding, such as increased temperature and higher cosolvent concentration, resulted in a decrease in K(HB). Salts that tend to increase water structuring and hydrophobic bonding caused an increase in K(HB). All of these studies support the M-LLE, because they support the notion of hydrophobic-bonding-driven interactions.     

Pore structure and adsorption properties of stone active carbons prepared by physicochemical and chemical activation methods

Active carbons (ACs) with diverse microporous and developed mesoporous structures were prepared by chemical and phisochemical activation methods from walnut shells, fruit stones, and grape seeds. The surface chemistry was studied by chemical titration and spectroscopy in the IR, UV, visible, and near IR regions. The ACs prepared by chemical activation contain carboxyl and phosphate groups, which impart acidic properties to the surface. Basic functional groups are mainly formed on the ACs prepared by physicochemical activation. The AC surface has a complicated chemical composition, which results in high adsorpion activity. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 744–748, May, 2006.     

Steam-carbon gasification catalyzed by calcium: Assessment of the porous structure of active carbons from plum stones and synthetic active carbons

A need for an elaboration of the methods for synthesis and characterization of activated carbons with a requisite porous structure has existed for a long time. One of the methods giving possibility for creating controlled mesopore and micropore structures deals with the steam gasification of various carbon materials. In this work the effects of calcium catalyst on the catalytic steam gasification of active carbons from plum stones and porous polymers are presented. Determination of micropores capacity and specific adsorption in mesopores have been performed by means of the _s method, but adsorption on the heterogeneous solids was described by the integral equation with various local isotherms. This equation has been solved by the regularization method. Based on this method the changes in structural parameters of active carbons depending on the amount of calcium catalyst were estimated.Nomenclature d width of slit-like micropore - F(x) distribution function of the half-width - p vapor pressure of sorbate - p/p _N relative pressure - PSAC Plum Stone Active Carbon - average pore radius, nm - S ₁ relative limit of the validity of experimental point on the adsorption isotherm in the computations by means of regularization method - SAC Synthetic Active Carbon - S _BET specific surface area calculated by means of BET method, m²/g - S _mes mesopore surface area, m²/g - S _mic micropore surface area, m²/g - T absolute temperature, K - V _mes sorption capacity of mesopores, cm³/g - V _mic sorption capacity of micropores, cm³/g - V _p sorption capacity of pores, cm³/g - w/w weight in weight concentration - x half-width of slit-like micropore, nm - x ₁ maximum of half-width of micropore slit, nm - average half-width of slit-like micropore, nm - X _min-X _max integration limits of thex Greek Letters Greek Letters variance of average half-width of slit-like micropore, nm² - local relative filling of micropores - total relative filling of micropores     

Uranyl adsorption on solvated edge surfaces of pyrophyllite: a DFT model study

In a computational study we addressed the adsorption of uranyl UO(2)(2+) on solvated (110) and (010) edge surfaces of pyrophyllite, applying a density functional approach to periodic slab models. We explored bidentate adsorption complexes on various partially deprotonated adsorption sites: octahedral Al(O,OH), tetrahedral Si(O,OH), and mixed AlO-SiO. Aluminol sites were determined to be most favorable on the (110) surface of pyrophyllite, while on the (010) surface mixed AlO-SiO sites are preferred. The structural parameters of all low-energy complexes on both surfaces agree rather well with EXAFS results for the structurally similar mineral montmorillonite. We calculate the average U-O distance to surface and aqua ligand oxygen atoms to increase with the increasing coordination number of uranyl whereas EXAFS results indicate the opposite trend. According to our results, several adsorption species, with different coordination numbers on different edge faces, may coexist on clay minerals. This computational finding rationalizes why earlier spectroscopic studies indicated the existence of more than one adsorption species, whereas a single type of adsorption complex was suggested from most EXAFS results.     

Argon and nitrogen adsorption in disordered nanoporous carbons: simulation and experiment

We report experimental measurements of the isosteric heats of adsorption for argon and nitrogen in two microporous saccharose-based carbons, using a Tian-Calvet microcalorimeter. These data are used to test recently developed molecular models of these carbons, obtained by a constrained reverse Monte Carlo method. Grand canonical Monte Carlo simulation is used to calculate the adsorption isotherms and isosteric heats for these systems, and the results for the latter are compared to the experimental data. For argon, excellent quantitative agreement is obtained over the entire range of pore filling. In the case of nitrogen, very good agreement is obtained over the range of coverage 0.25 < or = gamma/gamma 0 < or = 0.85, but discrepancies are observed at lower and higher coverages. The discrepancy at low coverage may be due to the presence of oxygenated groups on the pore surfaces, which are not taken into account in the model. The differences at high coverage are believed to arise from the presence of a few mesopores, which again are not included in the model. Pair correlation functions (argon-carbon and argon-argon) are determined from the simulations and are discussed as a function of pore filling. Snapshots of the simulations are presented and provide a picture of the pore filling process.     

The interaction of pyridine with the surface of active carbons

The adsorption and desorption of pyridine on the surface of granulated active carbons was studied. It was shown that pyridine was extracted from aqueous solutions because of physical adsorption and specific adsorption with the formation of H-bonds with oxygen-containing surface groups. The desorption of pyridine at temperatures up to 400°C did not cause its complete removal from the surface of adsorbents. The fraction of pyridine remaining on the surface was proportional to the amount of oxygen-containing surface groups of active carbons of the acid type.     

用活性炭及部分热解碳材料进行的质子催化(英文)

The development of environmentally friendly solid acid catalysts is a priority task. Highly oxidized activated carbon and their ion-substituted (saline) forms are effective proton transfer catalysts in esterification, hydrolysis, and dehydration, and thus are promising candidates as solid acid cata-lysts. Computations by the ab initio method indicated the cause for the enchanced acidity of the carboxylic groups attached to the surface of highly oxidized carbon. The synthesis of phosphorilated carbon was considered, and the proton transfer reactions catalyzed by them in recent studies were analyzed. The development of an amorphous carbon acid catalyst comprising polycyclic carbonaceous (graphene) sheets with –SO3H, –COOH and phenolic type OH-groups was carried out. These new catalysts were synthesized by partial pyrolysis and subsequent sulfonation of carbohydrates, polymers, and other organic compounds. Their high catalytic activities in proton transfere reactions including the processing of bio-based raw materials was demonsrated.     

Thermodynamical model and prediction of gas/solid adsorption isotherms

A thermodynamic model of gas/solid adsorption has been constructed from two elements. One of those is the original Gibbs equation. The second is functions psi(theta) or psi(P) calculable from measured isotherms. The model provides the possibility of calculating the relative change in free energy of the surface, and based on the model, implicit isotherm equations of general validity and in integral form can be derived. The prediction of isotherms can be made based on characteristic adsorption functions (CAFs). The CAFs concentrate in one function all measured isotherms having the same change in relative free energy of the surface. From CAFs any isotherm can be predicted if one measured point is known or one required datum of the isotherm can be defined. The maximum average deviation between the measured adsorbed amounts and those calculated from the CAFs is +/-10%. The CAFs are very sensitive to the internal structure of adsorbents (micro-, meso-, and macropores and nanostructures). It is the goal of future investigations to determine the exact connections related to the CAFs and to the structure of adsorbents.     

Numerical analysis of nitrogen adsorption isotherms on active carbons by an employment of the new LBET class models

The reported research concerns properties of the new LBET class models designed to describe the heterogeneous adsorption on microporous carbonaceous materials. In particular, the new adsorption models were used for the analysis of the microporous structure of two active carbons on the basis of nitrogen adsorption isotherms. This paper gives more information on the properties of the proposed identification technique.