Correlation and Calculation of Multicomponent Adsorption Equilibria Data Using a Generalized Adsorption Isotherm

Enhanced by the need for reliable and accurate data of multicomponent gas adsorption equilibria on porous solids like activated carbons or zeolites, a new method to measure and correlate coadsorption equilibria has been developed. This method is a combination of gravimetric or volumetric measurements of the total load of pure or multicomponent adsorbates (Staudt, 1994; Gregg and Sing, 1982) and a correlation and calculation procedure using a new adsorption isotherm (AI) (Keller, 1990). This AI is thermodynamically consistent and describes adsorbates with fractal dimension for single- or multicomponent systems and load dependent adsorption energies. This method allows calculation of partial loads of multicomponent coadsorption equilibria from pure component data and the total loads of the mixture adsorption equilibria. This will be demonstrated for binary and ternary adsorption equilibria of CH₄, C₂H₄ and C₂H₆ on activated carbon (Reich et al., 1980).     

Principal component analysis and multicomponent surface free energy theories

The same underlying mathematical structure characterizes some of the most popular multicomponent models for the prediction of surface free energies and adhesion works. After a brief illustration of the general methods for the computation of liquid and solid components in typical multicomponent theories, it is shown that both model definition and component estimate may take great advantage from application of Principal Component Analysis techniques, owing to the very peculiar structure of adhesion work equations. It is also put into evidence that a problem of scale multiplicity arises as a consequence of the symmetries involved in the model equations for adhesion work and surface free energy. A special discussion is devoted to the specific cases of van Oss–Chaudhury–Good acid–base theory, Qin–Chang model and extended Drago theory, which constitute the most common multicomponent models usually applied in the analysis of adhesion phenomena.      

Determination of the adsorption volume and surface of carbon adsorbents with developed mesoporosity

The isotherms of excess adsorption of CH₄ (atP=0.001–160 MPa), SF₆ (atP=0.001–2.4 MPa), and C₆H₆ (atP=0.0001–0.1 MPa) on carbon adsorbents—microporous carbons CMS and FAS with developed mesoporosity and graphitized soot—were measured in the 298–408 K temperature region. Calculation of the isotherms of absolute adsorption of the total content of these substances requires knowledge of the adsorption volume, which was determined by different methods: by the Dubinin—Radushkevich equation; by the experimental isotherm of excess adsorption and the equation of absolute adsorption; by the method using the intersection of nonlinear isosteres of excess adsorption and isosteres of absolute adsorption; by the comparative plot of values of the excess C₆H₆ adsorption Γ_FAS—Γ_soor; by the method using the difference of molecular radii of adsorptives and the surface of the specific adsorbent; and by the calculation of the adsorption layer thickness using the FHH equation for mesoporous systems. The results of determination of the adsorption volume for microporous systems of these carbons agree well with each other and with the passport data for the adsorbents. Analysis of the results revealed the peculiarity of the sulfur hexafluoride adsorption related to the formation of associates on the surface of the carbon adsorbents. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 688–696, April, 2000.     

Adsorption Calculations Using the Film Model Approximation for Intraparticle Mass Transfer

An approximate rate equation based on a film-model representation of diffusional mass transfer has been developed to describe the kinetics of multicomponent adsorption. The model describes mass transfer as a pseudo-steady state diffusion process through a flat film of thickness equal to one fifth of the particle radius. The flux relationships are integrated across the film yielding analytical expressions for the rate of mass transfer in a multicomponent adsorption system. The usefulness of the film model approximation is tested by carrying out calculations for three different practical adsorption systems: the adsorption of n-pentane and n-heptane mixtures on NaCaA zeolite discussed by Marutovsky and Bülow (1987); the adsorption of air in molecular sieve RS-10 discussed by Farooq et al. (1993); and the separation of air in a kinetically-controlled nitrogen PSA process discussed by Farooq and Ruthven (1990) and Sundaram and Yang (1998). In each case, the film model approximation predicts the expected trends accounting for the coupling of diffusion fluxes in the adsorbed phase.     

Effect of the Structure of Benzo- and Dibenzocrown Ethers on Their Reaction with the Aerosil Surface

The adsorption of 4-substituted benzo- and 5,5′-disubstituted dibenzocrown ethers from benzene by aerosil A-300 was studied. The ethers included a total of 45 compounds with diphenyl oxide, diphenyl, and diphenyl sulfide fragments. Analysis of the Henry coefficients (K_H) and the extent of desorption, the IR spectra of the adsorbed compounds, and the relationship between their structure and adsorption capacity indicated that all structural fragments of the crown ethers studied interact to some extent with aerosil surface fragments. The number of oxygen atoms in the polyether fragment and the size of the macrocycle are the predominant factors. A large, nonadditive increase in K_H is observed in going from pentadentate to hexadentate crown ethers. __________ Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 41, No. 4, pp. 247–251, July–August, 2005.     

Multicomponent Adsorption of Pesticides onto Activated Carbon Fibers

The adsorption equilibria of pesticides and metabolites (atrazine, deethylatrazine, deisopropylatrazine and simazine) are studied onto activated carbon fibers –ACF– with a broad pore size distribution (32% mesopore volume, 68% micropore volume). Mono-and multi-component isotherms have been determined for low concentrations, from 0.23×10⁻⁶ to 9.52×10⁻⁶ mol L⁻¹. Single solute isotherms, modeled by Freundlich and Langmuir models, tend to prove the influence of the adsorbate's solubility in the adsorption capacity of activated carbon fibers. Binary solute isotherms confirm the strong influence of pesticide solubility on the competitive adsorption mechanism: the competition is higher in the case of adsorbates of different solubilities (atrazine and DEA or DIA for example). Multicomponent experimental data were modeled by extended Langmuir-based equations and the Ideal Adsorbed Solution theory. Whereas the first ones failed to model accurately binary adsorption due to restrictive hypothesis, the IAS model showed a good agreement between experimental and predicted data. It emphasised also the difficulty in satisfying the hypothesis of the model in the case of highly adsorbed compounds. Finally, the simultaneous adsorption of atrazine and NOM (in a natural water, DOC = 18.2 mg L⁻¹) shows no adsorption competition effects between natural organic matter and atrazine. This is due to the presence of secondary micropores (0.8–2 nm) and mesopores in the ACF, which limit a pore blockage phenomenon by NOM.     

Dynamic electrochemical impedance spectroscopy of the adsorption and relaxation of polyaniline chains during potentiodynamic redox transformations

A new approach to an equivalent circuit analysis of impedance spectra of a conducting polyaniline (PAni) layer is presented. Film properties are often modeled by constant-phase elements (CPE), which are closer to experimental results than capacitors. We want to take an insight into the CPE. We propose a novel equivalent circuit based on the Frumkin-Melik-Gaikazyan adsorption model, as we suppose that the PAni layer molecules may behave like other adsorbing and relaxing organic compounds. Measurements are performed using recently developed experimental technique—dynamic electrochemical impedance spectroscopy. Obtained spectra are analyzed using the proposed equivalent circuits. Characteristics of several parameters are analyzed and discussed. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 9, pp. 1111–1119. The text was submitted by the authors in English.     

Equilibrium adsorption of binary mixtures of gases by zeolites and the state of the adsorption phase

A method for analyzing the state of the adsorption phase was developed on the basis of statistical thermodynamics for the case of equilibrium adsorption of binary gaseous mixtures. The procedure for treating experimental data to determine the Helmholtz energy and other thermodynamic functions of a mixture of molecules occluded within zeolite cavities was proposed. A measure of ideal behavior of a mixture of a small number of molecules in the micropore was formulated; in the asymptotic limit such a behavior leads to the Raoult law and to assumption of the validity of the Raoult law when moving along the line of constant value of the Gibbs integral in the ideal adsorption solution theory. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1927–1932, October, 1998.     

Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar,Non-Porous Naphthalenediimide-Template on HOPG

The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC₃₃-NDI-uC₃₃ , a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene ( PyrOR ) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC₃₃-NDI-uC₃₃ template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC₃₃-NDI-uC₃₃ monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.     

Adsorption of normal and branched paraffins in faujasite zeolites NaY, HY, Pt/NaY and USY

The effect of chain length and branching of paraffins (from C6 to C12) on adsorption and diffusion in zeolites NaY, Pt/NaY, HY and USY has been investigated using the chromatographic method at 275–400°C. The Henry constants of the paraffins increase exponentially with the chain length (with a factor two per extra carbon group), the heats of adsorption increase with circa 7 kJ/mol per extra carbon group. Multicomponent sorption experiments reveal that longer chains are adsorbed preferentially over shorter chains, even at higher loadings. The multicomponent adsorption can be reasonably well described by an extended Langmuir adsorption isotherm, in which the stronger adsorption of the longer chains is reflected by their higher Henry constants. The molecular shape and zeolite type within this FAU group has only a small influence on the adsorption properties. Mass transfer in the pellets as used in catalytic conditions seems to be limited by macropore diffusion, rather than by micropore diffusion, which cannot be measured with the chromatographic method. Increasing the Si/Al-ratio of the zeolite reduces the adsorption capacity, but does not influence the relative adsorption properties.     

Comparative analysis of methods for determination of structural characteristics of carbon adsorbents

Experimental data on the equilibrium adsorption of sulfur hexafluoride, methane, carbon dioxide, and benzene on carbon adsorbents of different porosity obtained in a wide pressure range at 298–408 K were analyzed. The adsorption volumes, surface areas, and sizes of slit-shaped pores of the carbons were determined using several independent methods. A method for determination of the adsorption volume from the experimental isotherm of excessive adsorption of gases and the total content equation was proposed. The resulting values are similar to the adsorption volumes calculated from the data for vapors. A new method for the calculation of the adsorbent surface area is described. The method is based on the dependence of the adsorption volume of adsorbent pores on the effective size of adsorbate molecules. A possibility to determine the average size of narrow slit-shaped carbon pores from the difference of the initial heats of adsorption of the gas under study on the carbon black and porous carbon adsorbent is considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2219–2227, October, 2005.     

Synthesis of silica adsorbent and its selective separation for flavone

One kind of built-in silica adsorbent, which has high adsorption selectivity to rutin, was synthesized using molecular imprinting technology by the following steps: synthesis of precursor from the reaction between water soluble rutin (as template molecule) and the functional monomer chloropropyltriethoxysilane, co-hydrolysis of the precursor and tetraethoxysilane (TEOS), sol-gel aging process, and removal of template molecules. The results of adsorption experiment show that this adsorbent has a high adsorption capacity for rutin, and good adsorption selectivity towards rutin even under the interference of a flavone with a similar structure. TEM photos suggest that nanocaves corresponding to rutin were formed inside the adsorbent while FTIR spectra indicate that new bond was generated during the recognition process. __________ Translated from Journal of Tianjin University, 2007, 40(4): 411–415 [译自: 天津大学学报]     

Features of the adsorption of hydrogen by various types of microporous materials

A comparative investigation was made of the adsorption of N₂ and H₂ by silicate and phosphate zeolites (Zt) and carbon materials with micropores of various dimensions, shapes, and volumes. It was found that the adsorption of hydrogen has significant sensitivity to the shape of the pores, the chemical composition of the zeolite framework, and the nature of the surface of the adsorbents. In particular, the adsorption is increased in the presence of constrictions (the cationic forms of zeolite LTA) or channels (MFI) with sizes close to hydrogen molecules in the micropores and the ions of divalent metals included isomorphously in the framework of the aluminophosphates. There is also increased affinity to the carbon surface. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 67–73, March–April, 2008.     

Multicomponent analyses of chiral samples by use of regression analysis of UV–visible spectra of cyclodextrin guest–host complexes

We report the first combined use of analytical spectroscopy, guest–host chemistry, and multivariate regression analysis for determination of enantiometric composition of multicomponent samples of chiral analytes. Sample solutions containing multicomponent analytes of ephedrine, tryptophan, propranolol, and proline of varying enantiomeric composition with beta-cyclodextrin (BCD) or methyl-beta-cyclodextrin (Me-BCD) as chiral host molecules were investigated using ultraviolet (UV)–visible spectroscopy. The interactions of enantiomers of chiral analytes with chiral hosts resulted in the formation of transient diastereomeric inclusion complexes with varying spectral properties. Multivariate analysis using partial-least-square (PLS) regression was used to correlate subtle changes in the UV–visible spectra of the guest–host complexes with the enantiomeric composition of the calibration samples. These PLS regressions were carefully optimized and then used to predict the enantiomeric composition of multicomponent chiral analytes of validation samples. The results of these validation studies demonstrate the predictive ability of the regression models for determination of future enantiomeric composition of samples. The accuracy of the models to correctly predict the enantiomeric composition of samples, evaluated by use of the root mean square percent relative error (RMS%RE) was analyte and chiral host dependent. In general, better prediction of enantiomeric composition of samples and low RMS%RE values were obtained when Me-BCD was used as the chiral host. The analyses procedure reported here is simple, rapid, and inexpensive. In addition, this approach does not require prior separation of chiral analytes, thus reducing analysis time and eliminating the need for expensive chiral columns.     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.     

Enhancement of cyanide photocatalytic degradation using sol–gel ZnO sensitized with cobalt phthalocyanine

In this work, the photodegradation of cyanide in aqueous suspension was used to determine the photocatalytic activity of sol–gel prepared ZnO which was impregnated with the Co (II) phthalocyanine (CoPc), as sensitizer. The prepared catalyst was characterized by Scanning Electron Microscopy (SEM) with Energy Dispersed Spectroscopy (EDS) detector, X-Ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR) and Diffuse UV–Vis Reflectance spectroscopy. Specific surface area was calculated from nitrogen adsorption isotherm using BET method. Compared with commercial ZnO and TiO₂ Degussa P25 photocatalysts, the sol–gel prepared ZnO catalyst sensitized with cobalt phthalocyanine showed the highest activities for degradation of cyanide in aqueous solution under visible light irradiation.     

Adsorption and desorption of macromolecules on solid surfaces studied by on-line size exclusion chromatography 2. Preferential adsorption and exchange processes

Preferential and exchange adsorption of polymers differing in molar mass and/or chemical nature under dynamic conditions were investigated using on-line size-exclusion chromatography (SEC). The sample investigated dissolved in an appropriate solvent was injected into a small adsorption–desorption column packed with nonporous silica. A nonadsorbed or desorbed fraction of the polymer was directed into an SEC column for determination of both the amount and the molecular characteristics. This approach is in many aspects superior to other techniques for studies of polymer adsorption onto solid surfaces due to its low sample and time consumption. At a low degree of surface coverage, adsorption and desorption of macromolecules were rapid and were affected by the rate of supply of macromolecules to the adsorbent surface. The exchange between macromolecules at the stage of surface saturation was found to depend on the mean molar masses of preadsorbed and displacing polymer species and possibly also on the chain flexibility of the macromolecules. It was shown that the preferential adsorption driven by the chain-length difference upon saturation of the adsorbent surface was more noticeable if the preadsorbed macromolecules were smaller. Received: 7 April 1999 Accepted in revised form: 21 July 1999     

Adsorption of reactive brilliant red K-2BP on activated carbon developed from sewage sludge

Activated carbon was prepared from the sewage sludge of municipal wastewater treatment plant by chemical activation (activation reagent is ZnCl₂) and was used for the adsorption of dye (reactive brilliant red K-2BP). The impact of adsorbent amount, adsorption time and pH value on adsorption effect, the adsorption kinetics, and the adsorption thermodynamics were discussed according to batch adsorption tests. The results indicated that the activated carbon developed from sewage sludge (ACSS), which was mesoporous, possessed opened porous structures. The iodine number of the ACSS was 326 mg·g⁻¹. The rate of achievement was 51.31%. The BET surface area was 298 m²·g⁻¹ and the contents of heavy metals in the leachate didn’t exceed the contents limit. The adsorption kinetics of reactive brilliant red K-2BP on the ACSS was accorded with the two-step kinetics rate equation and pseudo-second-order kinetics equation. Compared to the Freundlich isotherm equation, the Langmuir isotherm equation showed better applicability for the adsorption. The adsorption which was favorable was an endothermic (enthalpy ΔH > 0) and spontaneous (free energy ΔG < 0) process and was accompanied by an increase in entropy (ΔS > 0). __________ Translated from Journal of Shandong University (Natural Science), 2007, 42(3): 64–70 [译自: 山东大学学报(理学版)]