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Adsorption: Journal of the International Adsorption Society (v.13, #5-6)

On the physical adsorption of gases on carbon materials from molecular simulation by G. R. Birkett; D. D. Do (pp. 407-424).

In this paper we present a series of work covering a range of aspects relating molecular simulation to experiment. The importance of surface mediation type effects to the adsorption of simple and complex gases is demonstrated. Coupled with the adsorption of simple gases is their projection area when used for surface area determination. The pressure dependence of a projection area is demonstrated for argon at 77 and 87.3 K. A simple model is used to account for the degree of graphitisation of a surface is demonstrated and used to account for the isosteric heat behaviour of non-graphitised carbon blacks. Turning from surfaces to porous solids, an alternative treatment of experiment data (either sub or super critical) is presented that avoids the ambiguity of excess amounts adsorbed. Using this method one is able to obtain pore size distributions and amounts adsorbed without relying on such things as helium expansion volumes. Since this type of method is usually applied to composite solids we also demonstrate the correct method for calculating the heat of adsorption using independent sets of simulations. The final topic covered in this paper is an example of the information that can be gained from the heat capacity of an adsorbed phase.

Keywords: Molecular simulation; Carbon adsorbents; Surface interaction; Pore size distribution

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment by Francisco R. Hung; Supriyo Bhattacharya; Benoit Coasne; Matthias Thommes; Keith E. Gubbins (pp. 425-437).

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 Å, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.

Keywords: Templated mesoporous silica materials; MCM-41; SBA-15; Gas adsorption; Molecular simulation; Monte Carlo

Adsorption of n-alkanes in faujasite zeolites: molecular simulation study and experimental measurements by A. Wender; A. Barreau; C. Lefebvre; A. Di Lella; A. Boutin; P. Ungerer; A. H. Fuchs (pp. 439-451).

We report an application of a previously developed force field for adsorption of hydrocarbons in silicalite (Pascual, P., et al. in Phys. Chem. Chem. Phys. 5:3684–3693, 2003), to the case of the linear alkane-sodium faujasite systems. In order to extend this force field from siliceous to cationic zeolites, we propose to take into account the polarization part of the zeolite-molecule interaction energy. A first order polarization term is explicitly considered for this purpose, using standard molecular polarizabilities. Polarization appears to amount to 30–40% of the zeolite-alkane interaction energy, as a consequence of the strong electric field created by the sodium cation distribution and negatively charged framework. This approach is compared with experimental adsorption isotherms of ethane, propane, n-octane and n-decane in NaY from the literature and with original measurements of n-butane isotherms in NaY obtained by thermal gravimetry. Henry constants and heats of adsorption at zero coverage of n-alkanes (n=6–10) are also compared with experimental measurements. Although no specific parameter has been calibrated for extending the force field, the general agreement between simulation results and experiments is satisfactory. Cation redistribution upon alkane adsorption is not observed in these simulations.

Keywords: Adsorption; Alkanes; Zeolites; Faujasites; Grand Canonical Monte Carlo Simulation

CO₂ adsorption in LiY and NaY at high temperature: molecular simulations compared to experiments by Guillaume Maurin; Youssef Belmabkhout; Gerhard Pirngruber; Lucia Gaberova; Philip Llewellyn (pp. 453-460).

Grand Canonical Monte Carlo simulations combined with adsorption measurements have been carried out to gain further insight into the CO₂ adsorption process at the microscopic scale in both LiY and NaY faujasites at various temperatures. A new Li⁺−CO₂ force field derived by ab initio calculations was validated by a reasonable agreement between the simulated isotherms and those obtained by experiments in a wide range of temperature (from 323 K to 473 K). In addition, the microscopic mechanisms of CO₂ adsorption in both systems, consistent with the trends observed for the simulated differential enthalpies of adsorption as a function of the loading, were proposed. It was observed that two different types of adsorption behaviour exist for NaY and LiY at 323 K and 373 K, mainly caused by the significant more exposed position of the SII Na⁺ from the six-ring plane of the supercage compared to those occupied by the SII Li⁺, whereas at higher temperature, both faujasites exhibit the same flat profile for the differential enthalpy of adsorption as a function of loading.

Keywords: CO₂ ; LiY; NaY; Adsorption mechanism; Isotherm; Enthalpy of adsorption; High temperature; GCMC simulations; Interatomic potentials; Adsorption measurements

Adsorption of CO₂ in metal organic frameworks of different metal centres: Grand Canonical Monte Carlo simulations compared to experiments by Naseem A. Ramsahye; Guillaume Maurin; Sandrine Bourrelly; Philip L. Llewellyn; Thomas Devic; Christian Serre; Thierry Loiseau; Gerard Ferey (pp. 461-467).

A Grand Canonical Monte Carlo study has been performed in order to compare the different CO₂ adsorption mechanisms between two members of the MIL-n family of hybrid metal-organic framework materials. The MIL-53 (Al) and MIL-47 (V) systems were considered. The results obtained confirm that there is a structural interchange between a large pore and narrow pore forms of MIL-53 (Al), not seen with the MIL-47 (V) material, which is a consequence of the presence of μ ₂-OH groups. The interactions between the CO₂ molecules and these μ ₂ OH groups mainly govern the adsorption mechanism in this MIL-53 (Al) material. The subsequent breaking of these adsorption geometries after the adsorbate loading increases past the point where no more preferred adsorption sites are available, are proposed as key features of the breathing phenomenon. After this, any new adsorbates introduced into the MIL-53 (Al) large pore structure experience a homogeneous adsorption environment with no preferential adsorption sites in a similar way to what occurs in MIL-47 (V).

Keywords: CO₂, adsorption; Metal-organic frameworks; Grand Canonical Monte Carlo; Microcalorimetry; Isotherm; Differential adsorption enthalpy, adsorption mechanism, breathing effect

A computational study of CO₂, N₂, and CH₄ adsorption in zeolites by E. García-Pérez; J. B. Parra; C. O. Ania; A. García-Sánchez; J. M. van Baten; R. Krishna; D. Dubbeldam; S. Calero (pp. 469-476).

The adsorption properties of CO₂, N₂ and CH₄ in all-silica zeolites were studied using molecular simulations. Adsorption isotherms for single components in MFI were both measured and computed showing good agreement. In addition simulations in other all silica structures were performed for a wide range of pressures and temperatures and for single components as well as binary and ternary mixtures with varying bulk compositions. The adsorption selectivity was analyzed for mixtures with bulk composition of 50:50 CO₂/CH₄, 50:50 CO₂/N₂, 10:90 CO₂/N₂ and 5:90:5 CO₂/N₂/CH₄ in MFI, MOR, ISV, ITE, CHA and DDR showing high selectivity of adsorption of CO₂ over N₂ and CH₄ that varies with the type of crystal and with the mixture bulk composition.

Keywords: Adsorption isotherms; Zeolites; Separations; Carbon dioxide; Methane; Nitrogen

Monte Carlo and energy minimization studies of binary xylene adsorption in AEL and AFI networks by S. Mardônio P. Lucena; Randall Q. Snurr; Célio L. Cavalcante Jr. (pp. 477-484).

Adsorption of binary xylene mixtures in AEL and AFI networks was investigated using normal and biased GCMC simulations. Preferential o-xylene adsorption was evidenced in the simulations, as previously reported in single-component experimental data. In contrast to the FAU and MFI sieves, the AEL and AFI networks exhibit surprising azeotropic behavior. The selectivity switches from o-xylene to p-xylene at a gas phase mole fraction of ca. 0.5. Energy minimization was performed in the AlPO₄-11 molecular sieve to determine the energy differences between the adsorption sites. The minimization study showed that AlPO₄-11 has small adsorption energy differences between sites. The azeotropic behavior of the AEL and AFI networks can be explained using the two patch model proposed by Do and Do (Adsorption 5:319–329, 1999).

Keywords: Aluminophosphate; Xylenes; Simulation; Monte Carlo; Sorption; Binary

Molecular simulation of the adsorption and structure of benzene confined in mesoporous silicas by Benoit Coasne; Christiane Alba-Simionesco; Fabrice Audonnet; Gilberte Dosseh; Keith E. Gubbins (pp. 485-490).

Grand Canonical Monte Carlo simulations are used to study the adsorption of benzene at 298 K in an atomistic cylindrical silica nanopore of a diameter 3.6 nm. The adsorption involves a transition from a partially filled pore (a two layers thick film at the pore surface) to a completely filled pore configuration. Strong layering of the benzene molecules at the pore surface is observed. It is found that the layering decays as the distance to the pore surface increases. The position of the peaks for the density of the C, H atoms and the center of mass of the molecules shows that benzene molecules prefer an orientation in which their ring is perpendicular to the pore surface. This result is corroborated by calculating orientational order parameters and examining the distribution of the distances between the H and C atoms of the benzene molecules and the H and O atoms of the silica substrate.

Keywords: Benzene; Capillary condensation; Monte Carlo simulation; Nanoporous material

Kinetic Monte Carlo study of binary diffusion in silicalite by N. Laloué; C. Laroche; H. Jobic; A. Méthivier (pp. 491-500).

We report a Kinetic Monte Carlo (KMC) study of the diffusion of linear n-hexane (nC6) and 2,2-dimethylbutane (22DMB) mixture in zeolite silicalite. We first investigated the loading dependences of single component self- and corrected diffusivities of nC6 at 300 K. Anisotropic transition rates are implemented to account for the distribution of the molecules within the zeolite framework. Repulsive guest-guest interactions are modeled using the parameter introduced by Reed and Ehrlich (Surf. Sci. 102:588–601, 1981). The results are in good agreement with recent experimental Quasi Elastic Neutron Scattering data of Jobic et al. (J. Phys. Chem. B 110:2195–2201, 2006), although the influence of the adsorption isotherm inflection is not reproduced. The binary diffusion study of nC6/22DMB mixtures was performed by implementing the nC6 transition rates used for the single component study while 22DMB molecules propagate via intersection-intersection hops. This KMC model allows for different saturation capacities and accounts for interactions between molecules by introducing f ij parameters. Results show the large impact of guest-guest interactions between nC6 and 22DMB on both self- and corrected diffusivities of the two components. Molecule-size effects are found to be predominant near 22DMB saturation capacity. Acceleration/deceleration effects already described in the literature are confirmed.

Keywords: Molecular modeling; Separation; Zeolite; Kinetics; Diffusion coefficient

Quantum effect induced kinetic molecular sieving of hydrogen and deuterium in microporous materials by A. V. Anil Kumar; Hervé Jobic; Suresh K. Bhatia (pp. 501-508).

We report here our investigations using Monte Carlo and molecular dynamics (MD) simulations, as well as quasi-elastic neutron scattering experiments, to study the adsorption and diffusion of H₂ and D₂ in zeolite Rho. In the simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. At low temperatures, we observe a reversal of kinetic molecular sieving in which D₂ diffuses faster than H₂. Based on fits of bulk data, we suggest new set of potential parameters for hydrogen, with the Feynman-Hibbs variational approach used for quantum corrections. The transport properties obtained from MD simulations are in excellent agreement with the experimental results, with both showing significant quantum effects on the transport at low temperature. The MD simulation results on two different structures of zeolite Rho clearly demonstrate that the quantum effect is very sensitive to pore size. High transport flux selectivity is noted at low temperatures, suggesting feasibility of kinetic isotope separation.

Keywords: Quantum effect; Kinetic molecular sieving; Isotope separation; Hydrogen adsorption; Molecular dynamics

Pore characterization of assembly-structure controlled single wall carbon nanotube by M. Arai; M. Kanamaru; T. Matsumura; Y. Hattori; S. Utsumi; T. Ohba; H. Tanaka; C. M. Yang; H. Kanoh; F. Okino; H. Touhara; K. Kaneko (pp. 509-514).

Single wall carbon nanotube (SWCNT), which has bundle structure and entangled structure, was untangled and cut by sonication in hydrogen peroxide (H₂O₂) solution. The untangled state of SWCNT was examined by SEM, TEM, Raman spectroscopy and N₂ adsorption. It was confirmed that the surface area of sonicated nanotubes strongly depended on the sonication time. The BET specific surface area (SSA) of nanotubes sonicated for 3 h was maximum. The SSA decreased at 6 h or more of sonication time. These results indicated that the bundle structure was untangled and the cap of SWCNT was opened. Thus, N₂ molecules can access the most efficiently inside of the SWCNT sonicated for 3 h. On the contrary, the sonication treatment for 6 h or more decomposed the nanotubes to produce amorphous carbon, evidenced by TEM and SEM observation; the amorphous carbon blocked the open pore sites such as the internal pore spaces and interstitial pores.

Keywords: Carbon nanotube; Nanopore characterization; Micropore filling; Raman spectroscopy; Molecular simulation

Liquid-phase adsorption experiments on ordered mesoporous silicas by R. Rockmann; G. Kalies; O. Klepel (pp. 515-522).

SBA-15, SBA-16 and MCM-48 silicas with hexagonal $par{6}mm$ , cubic $Imar{3}m$ and cubic $Iaar{3}d$ mesopore structure were synthesized according to known methods reported in literature and characterized by X-ray diffraction and nitrogen adsorption. Liquid-phase adsorption experiments were performed on all three materials. The adsorption behavior of binary liquid model mixtures was studied over the whole concentration range with regard to the separation quality of the solid and the dependence on the chain length of adsorptive molecules. Inverted U-shape isotherms were found indicating that the silicas are highly selective for polar components. So far, an influence of mesopore size or structure on liquid excess isotherms cannot be extracted from the measured data. The present work is a first step to create a data base of liquid-phase adsorption on solids with ordered mesopore structure.

Keywords: Liquid-phase adsorption; Excess isotherms; Binary liquid mixtures; Chain length; Ordered mesoporous silicas; Gas adsorption

Optimization of layered double hydroxide stability and adsorption capacity for anionic surfactants by Natasja Schouten; Louis G. J. van der Ham; Gert-Jan W. Euverink; André B. de Haan (pp. 523-532).

Low cost adsorption technology offers high potential to clean up laundry rinsing water. From an earlier selection of adsorbents (Schouten et al. 2007), layered double hydroxide (LDH) proved to be an interesting material for the removal of anionic surfactant, linear alkyl benzene sulfonate (LAS) which is the main contaminant in rinsing water. The main research question was to identify the effect of process parameters of the LDH synthesis on the stability of the LDH structure and the adsorption capacity of LAS. LDH was synthesized with the co-precipitation method of Reichle (1986); a solution of M²⁺(NO₃)₂ and M³⁺(NO₃)₃ and a second solution of NaOH and Na₂CO₃ were pumped in a beaker and mixed. The precipitate that was formed was allowed to age and was subsequently washed, dried and calcined. The process parameters that were investigated are the concentration of the initial solutions, M²⁺/M³⁺ ratio and type of cations. The crystallinity can be improved by decreasing the concentration of the initial solutions; this also decreases the leaching of M³⁺ from the brucite-like structure into the water. The highest adsorption capacity is obtained for Mg²⁺/Al³⁺ with a ratio 1 and 2 because of the higher charge density compared to ratio 3. Storing the LDH samples in water resulted in a reduction of adsorption capacity and a decrease in surface area and pore volume. Therefore, LDH is not applicable in a small device for long term use in aqueous surroundings. The adsorption capacity can be maintained during storage in a dry N₂ atmosphere.

Keywords: Layered double hydroxide; Stability; Anionic surfactant; Adsorption capacity

Characterization of gels via solvent desorption measurements by Cedric J. Gommes; Francis Noville; Jean-Paul Pirard (pp. 533-540).

The present paper shows how a standard volumetric adsorption device can be used to measure solvent desorption isotherms on gels. As gels are very soft materials, they shrink significantly during the measurement, and the data have to be analyzed in terms of the mechanical properties of the gel’s skeleton. Methanol desorption isotherms are measured on a series of silica gels, and the results are compared with independent characterizations, notably beam bending.

Keywords: Gels; Desorption; Drying; Plastic deformation; Capillary stress

Theoretical investigation of the adsorption of a binary mixture in a chromatographic column using the nonlinear frequency response technique by Milica Ilić; Menka Petkovska; Andreas Seidel-Morgenstern (pp. 541-567).

The nonlinear frequency response of a chromatographic column for the adsorption of two dissolved components is analyzed using the concept of higher order frequency response functions (FRFs) which is based on the Volterra series and generalized Fourier transform. By applying this concept a nonlinear model of a system is replaced by an infinite series of the FRFs of the first, second, etc. order. The FRFs up to the third order are derived theoretically starting from the equilibrium-dispersive model, which is used for description of a chromatographic column, and applying the harmonic probing method. The functions that relate outlet concentration changes of each component to the corresponding inlet concentration changes are derived. At the inlet of a chromatographic column, it is considered: (a) the concentration change of one of the components keeping the concentration of the other component constant and (b) the concentration change of both components keeping their ratio constant. The FRFs are calculated numerically for different steady-state concentrations and relative mixture compositions. It has been found that, despite certain differences in initial conditions, the FRFs exhibit similar behavior. For higher frequencies, the amplitudes of the FRFs tend to zero and phases to −∞. In the low frequency range, which is of interest for investigation of equilibrium parameters, these functions have similar behavior, but tend to different asymptotic values. Correlations between coefficients of competitive adsorption isotherms, i.e. partial isotherm derivatives, and the derived FRFs are established. This theoretical result offers the potential to use the analysis of the nonlinear frequency response of a chromatographic column for estimation of competitive adsorption isotherms.

Keywords: Binary mixture; Competitive adsorption isotherm; Nonlinear frequency response; Higher order frequency response functions

Breakthrough of natural organic matter from fixed bed adsorbers: investigations based on size-exclusion HPLC by Fusheng Li; Akira Yuasa; Yoshihiro Katamine; Hidenori Tanaka (pp. 569-577).

Fixed bed adsorption experiments were performed using four granular activated carbon (GAC) columns designed by packing two size ranges of pulverized and sieved Filtrasorb 400 (d=0.5-0.59 and 1.0-1.19 mm) to two bed depths (L=10 and 20 cm), respectively. Continuous supplying of river water containing a lower content of natural organic matter (NOM) allowed investigation of the breakthrough of aqueous natural organic matrices assessed with lumped quality indices of total dissolved organic carbon (DOC) and ultraviolet absorbance at 260 nm (UV260). The capability of GAC columns in dealing with sudden rise in the load of influent NOM was also displayed by intermittently adding to the influent river water a peaty field groundwater that contained a higher content of NOM. Besides, assisted by the size-exclusion HPLC (SEHPLC), changes in the apparent molecular weight distribution of NOM along the bed depth of GAC columns were evaluated, and an important finding revealing relatively even adsorption for adsorbable NOM constituents within the entire molecular weight range of 1000-5200 g mol⁻¹ as PSS (polystyrene sulfonates) detected for the river and groundwater NOM was obtained. Furthermore, using a previously proposed hypothetical multi-component approach incorporating the ideal adsorbed solution theory and a plug flow homogeneous surface diffusion model, the observed concentration profiles of the river water NOM were predicted.

Keywords: NOM; Molecular weight distribution; Activated carbon; Adsorption; Breakthrough

Electrochemically assisted adsorption/desorption of bentazone on activated carbon cloth by C. O. Ania; F. Béguin (pp. 579-586).

This paper investigates the use of electrochemical techniques for the removal of a common herbicide, bentazone, from water streams using a carbon-based electrode. Activated carbon cloth with high surface area and narrow micropores was used as electrode. For a better understanding of the process, adsorption was investigated under both open circuit and controlled polarization conditions, the latter in anodic and cathodic directions. It was found that anodic polarization enhances the kinetics of adsorption of the herbicide on the carbon cloth, the extent of which is strongly related to the applied current value. At converse, cathodic polarization induces the reversible desorption of the compound. Moreover, in-situ UV spectra recording on the solution did not show any structural change of the herbicide upon polarization, demonstrating the reversibility of the process for the regeneration of the adsorbent and the recovery of the compound. Based on these experiments, a mechanism is proposed to interpret the reversible sorption of bentazone under polarization.

Keywords: Electrosorption; Electrodesorption; Activated carbon cloth; Pesticides; Liquid phase adsorption

Modeling equilibrium and kinetics of metal uptake by algal biomass in continuous stirred and packed bed adsorbers by Vítor J. P. Vilar; Cidália M. S. Botelho; Rui A. R. Boaventura (pp. 587-601).

Physical and chemical characterization of algae Gelidium particles shows a gel structure, with two major binding groups, carboxylic and hydroxyl groups, with an affinity constant distribution for protons, well described by a Quasi-Gaussian distribution suggested by Sips. A continuous model, considering a heterogeneous distribution of the carboxylic groups, determined by potentiometric titration experiments, was able to predict equilibrium data at different pH. The metal uptake capacity decreases with the solution pH, suggesting that competition exists between hydrogen ions, present in high concentrations for low pH values, and metal ions. For high ionic strengths, adsorption sites will be surrounded by counter ions and partially lose their charge, which weakens the contribution of the electrostatic binding and decreases the overall adsorption. A small influence of the temperature in the adsorption process was observed. Batch kinetic experiments were also performed, at different pH values, and results were well fitted by a mass transfer model, considering the intraparticle diffusion resistance given by the linear driving force model (LDF). Continuous stirred adsorber (CSTA) and packed bed column configurations were also tested for metal adsorption. The biosorbent regeneration was achieved by contacting it with strong acid (0.1 M HNO₃). A mass transfer model was applied with success to describe the biosorption/desorption process in CSTA and packed bed column, considering the equilibrium given by the Langmuir equation/mass action law and film and intraparticle diffusion resistances.

Keywords: Biosorption; Desorption; Metals; Modeling; Algae Gelidium sesquipedale

Adsorption and desorption of binary mixtures of copper and mercury ions on natural and crosslinked chitosan membranes by R. S. Vieira; E. Guibal; E. A. Silva; M. M. Beppu (pp. 603-611).

Copper and mercury ion adsorption on chitosan membranes was investigated in batch systems (with both single and binary solutions). The Langmuir model and its extensions (extended Langmuir, Jain-Snoeyink, and Langmuir-Freundlich models) were tested for the modeling of experimental data. Chitosan membranes presented more affinity for Hg ions than for Cu ions. The decrease of the amount of metal adsorbed on natural chitosan in binary systems (compared to single-metal solutions) showed the competition effects between the two metal ions. For glutaraldehyde-crosslinked chitosan and epichlorohydrin-crosslinked chitosan, the mixture effect was present, producing unexpected result such as higher adsorption capacities, when compared to the monocomponent solution of each metal. The desorption of the metals was also investigated, and copper and mercury ions could be selectively recovered using a combined process by using NaCl and H₂SO₄ as eluant.

Keywords: Copper; Mercury; Chitosan; Multicomponent adsorption

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Adsorption: Journal of the International Adsorption Society (v.13, #5-6)

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Adsorption: Journal of the International Adsorption Society (v.13, #5-6)