Adsorption of Pure Gases and Mixtures on Porous Solids up to High Pressures

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. An overview about adsorption measurements of pure gases H₂, He, O₂, N₂, Ar, CO₂, CO, CH₄, C₂H₄ and C₂H₆ and some of their mixtures in the pressure range vacuum < p < 50 MPa at different temperatures 10^∘C–70^∘C were investigated. Also a thermodynamic formalism based on a modified van Ness method and on a new 3 parameter Isotherm equation (3-PIG) to describe the excess amount adsorbed was developed. Results are shown and discussed. Dedication to the memory of W. Schirmer.     

Adsorption equilibria of CO/H2 with a magnetic suspension balance: Purely gravimetric measurement

Adsorption equilibria of pure gases and binary gas mixtures can be measured with a new magnetic suspension balance. For this measurement no additional concentration measurement is required, neither for the gas phase nor for the adsorbed phase. The new instrument allows gravimetric adsorption measurements in a wide range of pressure (UHV...50 MPa) and temperature (210 K...570 K) to be performed. The experimental procedure and the instrument set up is fairly easy and can be compared to pure gas adsorption experiments. The new instrument and experimental procedure have been tested by performing coadsorption measurements with CO/H₂ mixtures on a commercial 5A zeolite.This revised version was published online in November 2005 with corrections to the Cover Date.     

Pure and Binary Gas Adsorption Equilibria and Kinetics of Methane and Nitrogen on 4A Zeolite by Isotope Exchange Technique

The Isotope Exchange Technique (IET) was used to simultaneously measure pure and binary gas adsorption equilibria and kinetics (self-diffusivities) of CH₄ and N₂ on pelletized 4A zeolite. The experiment was carried out isothermally without disturbing the adsorbed phase. CH₄ was selectively adsorbed over N₂ by the zeolite because of its higher polarizability. The multi-site Langmuir model described the pure gas and binary adsorption equilibria fairly well at three different temperatures. The selectivity of adsorption of CH₄ over N₂ increased with increasing pressure at constant gas phase composition and temperature. This curious behavior was caused by the differences in the sizes of the adsorbates. The diffusion of CH₄ and N₂ into the zeolite was an activated process and the Fickian diffusion model described the uptake of both pure gases and their mixtures. The self-diffusivity of N₂ was an order of magnitude larger than that for CH₄. The pure gas self-diffusivities for both components were constants over a large range of surface coverages (0 < < 0.5). The self-diffusivities of CH₄ and N₂ from their binary mixtures were not affected by the presence of each other, compared to their pure gas self-diffusivities at identical surface coverages.     

Measurement of Gas Mixture Adsorption Equilibria of Natural Gas Compounds on Microporous Sorbents

Physisorption equilibria of multicomponent gases on microporous solids like zeolites or activated carbons are considered. In view of lack of reliable and simple methods to calculate mixture adsorption isotherms from pure component data, experiments are still indispensable. An overview of classical and new methods to measure multicomponent gas adsorption equilibria is given. Some of the basic concepts like the Gibbs excess mass and the absolute mass adsorbed underlying these methods are discussed. Experimental data and a class of new adsorption isotherms for inhomogeneous microporous adsorbents of fractal dimension will be given in another subsequent paper (ADSO 635-98) by the same group of authors.     

载铜活性炭吸附一氧化碳的密度泛函理论计算

应用密度泛函理论和相对论有效核势方法, 用C₁₆H₁₀, C₁₃H₉, C₁₂H₁₂原子簇模型模拟活性炭表面, 计算得到了CO在载铜活性炭上的吸附位、吸附构型和吸附能. 研究表明: 载铜活性炭吸附CO的过程, 本质上是Cu(I)通过σ-π配键与CO络合, 形成Cu—C键的过程. 载铜活性炭对CO的络合吸附能在50～60 kJ/mol之间, 远大于活性炭对CO的物理吸附能(9.15 kJ/mol), 因而络合吸附更稳定, 选择性也更高. Cu(I)选择吸附在活性炭表面的顶位和桥位, 一个Cu(I)至多可以吸附一个到两个CO分子, 但吸附一个CO比吸附两个CO稳定.     

Sorption Equilibria and Kinetics of Hydrocarbons onto Activated Carbon Samples Having Different Micropore Size Distributions

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.     

Thermodynamic characteristics of hydrocarbon and chloroalkane adsorption by polymethylsilsesquioxane

Adsorption of n-alkanes (C₇H₁₆–C₁₁H₂₄), cyclohexane, benzene, CHCl₃, CCl₄, and ClCH₂CH₂Cl by polymethylsilsesquioxane [MeSi(OH)_0.4O_1.3] _n was investigated by inverse gas chromatography in the Henry region. The adsorption isotherms are linear for all adsorbates. The standard changes in the thermodynamic parameters of adsorption were determined. The absence of specific selectivity with respect to aliphatic, cyclic, or aromatic hydrocarbons or chloroalkanes indicates that the adsorbent is chemically homogeneous.     

Adsorption of CO on oxygen preadsorbed neutral and charged gas phase Pd4 clusters: A density functional study

We present the results of a density functional calculation on adsorption of O₂, CO, and their coadsorption at various sites of neutral, cationic, and anionic Pd₄ clusters. For all the clusters, the dissociative adsorption of oxygen sitting on Pd bridge sites is found to be preferable. Both O₂ and CO binding energies are found to be higher for the anionic Pd₄ cluster followed by cationic and neutral cluster. However, binding energies of O₂ or CO in the coadsorption complexes follow the trend: anionic > neutral > cationic. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Adsorption of nitrogen, methane, carbon monoxide, and their binary mixtures on aluminophosphate molecular sieves

Experimental isotherms describing the adsorption of pure N₂, CH₄ and CO in AlPO₄-11, AlPO₄-17, and AlPO₄-18 were determined using the volumetric method at 40°C and at 23°C (AlPO₄-11 only) over a pressure range up to 123 kPa, and subsequently fitted with the Langmuir or Freundlich equations, as well as the Flory-Huggins Vacancy Solution Theory equation. The capacities for the adsorbates investigated were found to depend on the geometry of the sieve pore size, as well as the molecular dimensions and the polority of the adsorbate involved. At 40°C and over the investigated pressure range, AlPO₄-11 and AlPO₄-17 adsorbed pure CH₄ in the highest amounts, while AlPO₄-18 had a slightly higher capacity for pure CO. The model parameters obtained by fitting the experimental pure-component isotherms permitted the prediction of binary adsorption information for the CO−N₂, CH₄−CO, and CH₄−N₂ gas mixtures at 101.3 kPa total pressure, using the Extended Langmuir Model, the Ideal Adsorbed Solution Theory, and/or the Flory-Huggins Vacancy Solution Theory for mixtures. An explanation of the behaviour predicted by each model for each adsorption system is attempted.     

High Pressure Adsorption Data of Methane, Nitrogen, Carbon Dioxide and their Binary and Ternary Mixtures on Activated Carbon

Adsorption equilibria of the gases CH₄, N₂, and CO₂ and their binary and ternary mixtures on activated carbon Norit R1 Extra have been measured in the pressure range 0 P 6 MPa at T = 298 K. Pure gas adsorption equilibria were measured gravimetrically. Coadsorption data of the three binary mixtures CH₄/N₂, CH₄/CO₂, and CO₂/N₂ were obtained by the volume-gravimetric method. Isotherms of five ternary mixtures CH₄/CO₂/N₂ were measured using the volumetric-chromatographic method. First, we present in a short overview the method and procedure of measurement. In a second part, the measured data of pressures, surface excess amounts adsorbed and absolute amounts adsorbed are presented and analyzed. In the last part of the paper the resulting pure gas adsorption data are correlated using a generalized dual-site Langmuir isotherm. Mixture adsorption can be predicted by this model using only pure component parameters with fair accuracy. Results are presented and discussed in several tables and figures.     

Adsorption properties and surface chemistry of MgO

The adsorption properties of MgO, which is used as a sorbent and catalyst support, were studied using gas chromatography. The test absorbents used were n-alkanes (which show only nonspecific dispersion interactions when physisorbed on any adsorbent) and adsorbates whose molecules are capable of specific interactions with the surface reactive sites of MgO. Adsorption isotherms were measured for CHCl₃, CH₃NO₂, CH₃CN, (CH₃)₂CO, CH₃COOC₂H₅, and (C₂H₅)₂O on MgO at 50–100°C. Differential molar enthalpy changes (?ΔH), equal to molar heats of adsorption, were determined. For polar adsorbates, contributions from dispersive and specific interactions into ?ΔH were determined. The electron-acceptor and electron-donor abilities of the MgO surface were estimated.     

Adsorption Mechanism of Acetylene Hydrogenation

First‐principles calculations are carried out to examine the adsorption of acetylene over the Pd (111) surface. A hydrogen adsorption system is initially investigated to confirm the reliability of the selected calculation method. Adsorption energies, Mulliken‐populations, overlap populations, charge density, and projected density of states (PDOS) are then calculated in the optimised acetylene adsorption system. Results show that C₂H₂ molecule tends to adsorb through the threefold parallel‐bridge configuration that is computed to be the most stable. In this structure, the distance of the C? H bond is calculated to be 1.09 Å, and the C‐C‐H bond angle is 128°. The distance of the C? C bond in acetylene is 1.36 Å, increasing from 1.21 Å in the gas phase. Moreover, the C? C bond overlap population decreases from 1.98 to 1.38, revealing that the carbon configuration in C₂H₂ rehybridises from sp to sp² and beyond. The obtained results are compared with available experimental studies on acetylene hydrogenation on single‐metal surfaces. The PDOS study indicates that a carbonaceous layer may generate on the metal surface during acetylene adsorption. The carbonaceous layer can affect the adsorption and reaction of acetylene, thereby inactivating the metal surface. Our experiments also show that Pd exhibits high catalytic activity.     

Adsorption of a four-component gas mixture on zeolite NaX

Adsorption of N₂, CH₄, C₂H₆, C₃H₈, and their mixture on zeolite NaX was studied by the volumetric method under static conditions at 278 K in the pressure range from 0.1 to 0.8 MPa. Compressibility factors were calculated in order to take into account the nonideal character of the gas phase. Adsorption isotherms of individual gases and partial isotherms were obtained. The adsorption properties of gases in the adsorption of a mixture and its components were compared. The selectivity coefficient of adsorption of propane in the N₂-CH₄-C₂H₆-C₃H₈-NaX system was calculated, and its dependence on the total pressure was determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 839–841, April, 1996.     

Carbon dioxide-methane mixture adsorption on activated carbon

In this work, we report new experimental data of pure and binary adsorption equilibria of carbon dioxide and methane on the activated carbon RB2 at 273 and 298 K. The pressure range studied were 0–3.5 MPa for pure gases and 0–0.1 MPa for mixtures. The combination of the generalized Dubinin model to describe the pure CO₂ and CH₄ isotherms with the IAST (Ideal Adsorbed Solution Theory) for the mixtures provide a method for the calculation of the binary adsorption equilibria. This formulation predicts with acceptable accuracy the binary adsorption data and can easily be integrated in general dynamic simulation of PSA (pressure swing adsorption process) adsorption columns. It involves only three parameters, independent of the temperature, and directly determined with only one adsorption isotherm of CO₂.     

Adsorption of Nitrogen on Silica Gel Over a Large Range of Temperatures

To study the mechanism of physical adsorption of supercritical gases, the adsorption equilibria of N₂ on silica gel for 103–298 K using 20 K increments and pressures up to 10 MPa were measured. A transition of the adsorption mechanism was proven on crossing the critical temperature, but the transition way observed is different from that observed with activated carbon. This causes a difference in the locations of the linear section of the n- _g isotherm at the near-critical temperature. Although the isotherm type is different on silica gel and on activated carbon in the sub-critical region, all isotherms in the supercritical region can be well modeled by a single model. It leads to the argument that the adsorption mechanism of supercritical gases is identical no matter what kind of adsorbent is used.     

Empirical Relationships Describing Energetics of Adsorption at Low Coverages on Microporous Carbons

The fundamental empirical relationships that correlate the adsorption energy with physicochemical parameters of adsorbates are discussed. Based on the experimental data of the adsorption enthalpy of different organic adsorbates on microporous activated carbons some new correlations between adsorption enthalpy and entropy at zero surface coverage and physicochemical parameters of adsorbed molecules are proposed. It is shown that they can be applied for the calculation of carbon porosity. The influence of carbon surface oxidation on its energetic heterogeneity is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Series of Mesoporous Metal-Organic Frameworks with Tunable Windows Sizes and Exceptionally High Ethane over Ethylene Adsorption Selectivity

The NIIC-20 (NIIC stands for Nikolaev Institute of Inorganic Chemistry) is a family of five isostructural metal-organic frameworks (MOFs) based on dodecanuclear wheel-shaped carboxylate building blocks {Zn₁₂(RCOO)₁₂(glycol)₆} (glycol is deprotonated diatomic alcohol: ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol or glycerol), quantitatively crystallized from readily available starting chemicals. The crystal structures contain large mesoporous cages of 25 Å connected through {Zn₁₂} rings, of which inner diameter and chemical nature depend solely on the chosen glycol. The NIIC-20 compounds feature high surface area and rarely observed inversed adsorption affinity for saturated hydrocarbon (ethane) over the unsaturated ones (ethylene, acetylene). The corresponding IAST (Ideal Adsorbed Solution Theory) adsorption selectivity factors reach as much as 15.4 for C₂H₆/C₂H₄ and 10.9 for C₂H₆/C₂H₂ gas mixtures at ambient conditions, exceeding those for any other porous MOF reported so far. The remarkable combination of high adsorption uptakes and high adsorption selectivities makes the NIIC-20 series a new benchmark of porous materials designed for ethylene separation applications.     

Role of Adsorption of Reagents in Reduction of NO by Propene on ZrO2-Based Complex Oxide Systems

We have used IR spectroscopy to study adsorption of NO, propene, and their mixture on Rh-Cr₂O₃/ZrO₂ and Rh-CeO₂/ZrO₂ catalysts at temperatures of 293-623 K. We have established that adsorption and coadsorption of the reagents (NO and C₃H₆) have important differences, depending on the nature of the surface. Weak adsorption interactions of the reaction mixture on Rh-CeO₂/ZrO₂ lead to significantly lower activity of this catalyst in selective catalytic reduction (SCR) of NO by propene.     

ECNU‐36: A Quasi‐Pure Polymorph CH Beta Silicate Composed of Hierarchical Nanosheet Crystals for Effective VOCs Adsorption

A quasi‐pure C_H polymorph of microporous zeolite beta, named ECNU‐36, was obtained as a highly crystalline silicate using 1,5‐bis(tetramethylimidazolium) hydroxide as organic structure‐directing agent (OSDA) in fluoride media. An appropriate concentration of free fluoride in the synthetic mother liquor was crucial to purify the C_H‐phase. The framework structure of ECNU‐36 consists of polymorph C_H (>95 %) and polymorph B, elucidated by a combination of PXRD data, DIFFaX simulation, EDT, and HRTEM techniques. For the first time, the framework structure of beta C_H polymorph was directly confirmed and solved using electron diffraction data. The pure‐silica ECNU‐36 showed an unusual crystal morphology, composed of stacked nanosheets, with typical 17 nm thickness and exposed {100} facets, which exhibited attractive adsorption performance for hydrocarbons and aromatics.     

Adsorption and mobility of molecules of water and organic compounds in carbon adsorbents

The mobility of molecules of water and fluorine-containing organic compounds adsorbed in a mixture on active carbons has been studied by the NMR pulsed field gradient technique (¹H and¹⁹F resonances). Samples with various H₂O/C₆F₆ and H₂O/C₂Cl₃F₃ ratios have been examined. The mobility of components at total fill factors > 0.8 has been shown to decrease in comparison with the adsorption of pure substances while the diffusion activation energy increases. The results are interpreted on the basis of adsorption mechanisms of water and organic compounds on active carbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 56–61, January, 1993.In conclusion the authors would like to express their thanks to the German Research Society for the financial support of this work.