Effects of site occupancy, cation relocation, and pore geometry on adsorption kinetics in ETS-4

Strontium-exchanged titanium silicate, Sr-ETS-4, is a new molecular sieve with promises for exciting applications in gas separation. The literature mentions poor thermal stability and low adsorption capacity as the drawbacks of the as-synthesized Na-ETS-4 and therefore the need for cation exchange. Upon ion exchange, Sr-ETS-4 shows appreciable kinetic selectivity between methane and nitrogen, which can be improved further by controlled dehydration. In this study, we trace the changes at the molecular level behind this improvement. By combining distributed information in the literature, it is shown that the governing factors are changes in pore geometry with progressive dehydration, changes in ion occupancy, and relocation of cations due to Sr exchange.     

Modeling gas adsorption and transport in small-pore titanium silicates

Engelhard titanium silicate, ETS-4, is a promising new adsorbent for size-selective separation of mixtures of small gases, a leading industrially important example of which is methane-nitrogen separation. Single component equilibrium and kinetics of oxygen, nitrogen, and methane adsorption in Na-ETS-4 and cation-exchanged Sr-ETS-4, measured in an earlier study over a wide range of temperatures and pressures, are analyzed in this study. The adsorbent crystals were synthesized and pelletized under pressure (without any binder), thus giving rise to a bidispersed pore structure with controlling resistance in the micropores. Change in equilibrium and kinetics of adsorption of the aforementioned gases in Sr-ETS-4 due to pore shrinkage with progressively increasing dehydration temperature has also been investigated. Differential uptakes have been measured at various levels of adsorbate loading, which has allowed the elucidation of the nature of concentration dependence of micropore diffusivity. Both homogeneous and heterogeneous models are examined on the equilibrium data, while a bidispersed pore diffusion model is able to capture the differential uptakes very well. On the basis of chemical potential gradient as the driving force for diffusion, the impact of isotherm models on the concentration dependence of micropore diffusivity is also analyzed. It is shown that pore tailoring at the molecular scale by dehydration can improve the kinetic selectivity of nitrogen over methane in Sr-ETS-4 to a promising level. The models investigated are evaluated to identify essential details necessary to reliably simulate a methane-nitrogen separation process using the promising new Sr-ETS-4 adsorbent.     

Adsorption and diffusion parameters of methane and nitrogen on microwave-synthesized ETS-4

ETS-4 is a titanium silicate developed by Engelhard Corporation, which possesses a small pore network, the size of which can be reduced by heat treatment to improve its kinetic selectivity in nitrogen/methane separation. Most of the reported studies about ETS-4 employ crystals synthesized with conventional heating. Furthermore, information available on the adsorption properties of ETS-4, especially the diffusion properties, is scarce. In this work, Na-ETS-4 crystals have been synthesized by microwave heating and have been exchanged with strontium to obtain Sr-ETS-4 using also microwave heating. This method for obtaining the strontium form of ETS-4 has not been reported before. Both materials have been dehydrated to reduce their pore size. The adsorption and diffusion parameters of nitrogen and methane on these materials, which have not been measured for microwave-synthesized ETS-4 up to the present date, have been estimated by modeling the desorption breakthrough curves of both gases using a fixed bed of ETS-4 crystals. The kinetic selectivity of nitrogen over methane at 298 K of microwave-synthesized Sr-ETS-4 is 26. This value is higher than the maxima reported in the literature for this material.     

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.     

Measurement of Excess Functions of Binary Gas Mixtures Adsorbed in Zeolites by Adsorption Calorimetry

Adsorption equilibria and heats of adsorption were measured for mixtures of ethylene and ethane on NaX at 298 K. The pure-component isosteric heat of adsorption of ethane increases with loading due to gas-gas interactions; the heat of adsorption of ethylene is approximately constant with loading because of a balance between cooperative interactions and gas-solid energetic heterogeneity. This mixture, which is nearly ideal on carbon, exhibits moderate negative deviations from ideality on NaX. The nonideality is explained by a difference in the polarities of the molecules: ethylene has a quadrupole moment but ethane is nonpolar. The infinite-dilution activity coefficients are unity in the Henry's law region and decrease exponentially to a value of 0.56 at high loading. Regular-solution theory fails to agree with experiment. All three excess functions (free energy, enthalpy, and entropy) are negative; thus, activity coefficients are less than unity and the enthalpy of mixing in the adsorbed phase is exothermic. These results are consistent with an adsorbed solution in which the molecules are segregated into regions of different composition.     

Ion transport in the microporous titanosilicate ETS-10

Impedance spectroscopy was used to investigate ion transport in the microporous crystalline framework titanosilicate ETS-10 in the frequency range from 1 Hz to 10 MHz. These data were compared to measured data from the microporous aluminosilicate zeolite X. Na-ETS-10 was found to have a lower activation energy for ion conduction than that of NaX, 58.5 kJ/mol compared to 66.8 kJ/mol. However, the dc conductivity and ion hopping rate for Na-ETS-10 were also lower than NaX. This was found to be due to the smaller entropy contribution in Na-ETS-10 because of its high cation site occupancy. This was verified by ion exchanging Na(+) with Cu(2+) in both microporous frameworks. This exchange decreases the cation site occupancy and reduces correlation effects. The exchanged Cu-ETS-10 was found to have both lower activation energy and higher ionic conductivity than CuX. Zeolite X has the highest ion conductivity among the zeolites, and thus the data shown here indicate that ETS-10 has more facile transport of higher valence cations which may be important for ion-exchange, environmental remediation of radionucleotides, and nanofabrication.     

Characterization of surface energetic heterogeneity of pure and poly (acrylic acid)‐adsorbed carbon nanotubes by deconvoluting the nitrogen adsorption isotherm

The surface energetic heterogeneity of pure and poly (acrylic acid) (PAA)‐adsorbed carbon nanotubes (CNTs) were studied by a nitrogen probe adsorption technique in a wide range of pressures. The adsorption energy distributions (AEDs) were calculated from the low‐pressure data of isotherms by deconvoluting the low‐pressure experimental nitrogen adsorption isotherms. The surface of pure CNTs is heterogeneous as its AED presents four peaks at 42, 52, 57 and 78 K. It is observed that the AED of CNTs can be evidently modified by PAA adsorption. While the PAA adsorption amount increases, the high‐energy peaks at 52, 57 and 78 K gradually weaken and diminish at last, whereas the low ones such as at 42 K strengthen and new peaks arise at 27 and 32 K. It is proposed that PAA molecules prefer interacting with and screening the higher energetic sites to the lower ones. It will facilitate the understanding of the polymer adsorption on energetic heterogeneity surfaces. Copyright © 2006 John Wiley & Sons, Ltd.     

n- and isoalkane adsorption mechanisms on zeolite MCM-22

Low-coverage adsorption properties (Henry constants, adsorption enthalpy, and entropy) of linear and branched alkanes (C3-C8) on zeolite MCM-22 were determined using the chromatographic technique at temperatures between 420 and 540 K. It was found that adsorption enthalpy and entropy of linear alkanes vary in a nonmonotonic way with carbon number. The adsorption behavior of alkanes was rationalized on the basis of the pore geometry. Short molecules prefer to reside in the pockets of the MCM-22 supercage, where they maximize energetic interaction with the zeolite. Longer molecules reside in the larger central part of the supercage. For carbon numbers up to six, singly branched alkanes are selectively adsorbed over their linear counterparts. This preference originates from the entropic advantage of singly branched molecules inside MCM-22 supercages, where these species have high rotational freedom because of their small length.     

Synthesis, characterization and metal adsorption properties of the new ion exchanger polymer 3-n-propyl(4-methylpyridinium) silsesquioxane chloride

The preparation and anion exchange properties of 3-n-propyl(4-methylpyridinium) silsesquioxane chloride polymer are described. This new polymer was prepared by the sol-gel processing method and is designated as SiPic+Cl-. It is insoluble in water and showed an anion exchange capacity of 1.46x10(-3) mol g-1. The adsorption isotherms of ZnCl2, CdCl2 and HgCl2 were determined from aqueous solutions and the adsorption equilibria simulations fit the model of fixed bidentate centers with the absence of lateral interactions and energetic heterogeneity between them. The metal ions diffuse into the solid solution interface and are dominantly present as MCl2-(4) species for Zn(II), MCl(2-)4 and MCl-3 species for Cd(II) and MCl-3 species for Hg(II).     

Role of exchangeable cations on geometrical and energetic surface heterogeneity of kaolinites

Textural and energetic proprieties of kaolinite were studied by low-pressure argon adsorption at 77 K. The heterogeneity of four kaolinites (two low-defect and two high-defect samples) modified on their surface by cation exchange with Li+, Na+, or K+ was studied by DIS analysis of the derivative argon adsorption isotherms. The comparison between the derivative adsorption isotherms shows that the nature of the surface cation influences the adsorption phenomena on edge and basal faces. In the case of basal faces, two adsorption domains are observed: for the first one, argon adsorption is slightly sensitive to the nature of the surface cation; for the second one, argon adsorption energy depends on the nature of surface cation suggesting their presence on theoretically uncharged basal faces. This study also shows that the shape of elementary particles, as derived from basal and edge surface areas, changes with the nature of cation. This anomalous result is due to the decrease of edge surface area with increasing the size of the cation. This surface cation dependence can be accounted for the area occupied by the edge surface cations in the first argon monolayer.     

On the applicability of Arrhenius plot methods to determine surface energetic heterogeneity of adsorbents and catalysts surfaces from experimental TPD spectra

Recovering adsorption energy distribution from experimental data belongs to most difficult problems of adsorption science. In the case when thermodesorption data are used as a source of information, that difficult problem is overcome by the common use of the Arrhenius plot methods. So, we decided to carry out an extensive model investigation to show, how reliable information concerning the surface energetic heterogeneity is obtained by using the Arrhenius plot methods. Like in our previous publications we have used the Statistical Rate Theory of Interfacial Transport to describe the adsorption/desorption kinetics. Our model investigations showed, that the Arrhenius plot methods, cannot provide reliable information about the surface energetic heterogeneity. Moreover, for strongly heterogeneous surfaces a linear relationship exists between the logarithm of the pre-exponential constant and the adsorption energy, for certain adsorption coverages. That kind of compensation effect has, so far, been ascribed to interactions between the adsorbed molecules. The failure of the popular Arrhenius plot method puts, as an urgent agenda, the development of reliable methods for recovering adsorption energy distribution from the thermodesorption data.     

Free energy, energy, and entropy of swelling in Cs-, Na-, and Sr-montmorillonite clays

A Monte Carlo method for grand canonical and grand isoshear ensemble simulations has been used to characterize the free energy, energy, and entropy of clay mineral swelling. The Monte Carlo approach was found to be more efficient at simulating water content fluctuations in the highly constrained clay environment than a previously developed molecular dynamics method. Swelling thermodynamics calculated for Cs-, Na-, and Sr-montmorillonite clays indicate a strong dependence of swelling on the interlayer ion identity, in agreement with various experimental measurements. The Sr clay swells most readily, and both the Na and Sr clays prefer expanded states (two-layer hydrate or greater) when in contact with bulk water. In contrast, swelling is inhibited in the Cs clay. Differences in swelling behavior are traced directly to the tendency of the different ions to hydrate. The swelling free energies are decomposed into their energetic and entropic components, revealing an overall energetic driving force for the swelling phenomena. Entropic effects provide a smaller, mediating role in the swelling processes. The results provide a unique molecular perspective on experimentally well-characterized swelling phenomena.     

ADSORPTION OF BENZOIC ACID AND P-NITROBENZOIC ACID ON A NEW HYPERCROSSLINKED PHENOL GROUP PST ADSORBENT

1.INTRODUCTIONThepolymericadsorbentAmberliteXAD-4isconsideredoneofthemostsuitablepolymericadsorbentsforremovingphenoliccompoundsfromwaterstreams[1,2]becauseitischemicallystable,notsolubleinsolventsandmoreselectiveforaromaticringsduetoitshydrophobicproperties.However,methanol,acetoneoracetonitrile,hastobeusedtoenhancethesurfacecontactbetweenadsorbentandthesolute.DavankovandTsyurupadescribedanewseriesofadsorbents[3,4].Thistechniqueyieldedpolystyrenesorbentsofunusualhypercrosslinkedstructu…     

ADSORPTION OF BENZOIC ACID AND P-NITROBENZOIC ACID ON A NEW HYPERCROSSLINKED PHENOL GROUP PST ADSORBENT

A comparison of the adsorption of benzoic acid and p-nitrobenzoic acid on the new hypercrosslinked polymeric adsorbent AM-I, with that by macroporous Amberlite XAD-4, including the equilibrium adsorption isotherms, the dynamic adsorption behaviors through column and the adsorption thermodynamics were studied. Results show that Freundlich equation gives a fitting adsorption isotherm. The specific surface of AM-l is only 67% of that of Amberlite XAD-4, but the adsorption capacities on AM-1 are much higher about 125%～166% than that on Amberlite XAD-4,which is contributed to the micropore mechanism and polarity. The negative values of the adsorption enthalpy are indicative of an exothermic process. Enthalpy and free energy changes of adsorption both manifest a physic-sorption process. The negative values of the adsorption entropy indicate that the adsorption is well consistent with the restricted mobilities and the configurations of the adsorbed benzoic acid molecules on the surface of studied adsorbents with superficial heterogeneity. Both adsorbents were used in mini-column experiments for adsorbing benzoic acid expecting to elucidate the higher breakthrough adsorption capacity of the new hypercrosslinked polymeric adsorbent AM-1 as compared with that of Amberlite XAD-4.     

Sequential adsorption of bacteriophage T4 lysozyme stability variants at solid–water interfaces

The sequential adsorption of the wild type T4 lysozyme and one of its structural stability variants was studied, using ellipsometry and ¹²⁵I radioisotope labeling techniques. The mutant lysozyme was produced by substitution of the isoleucine residue at position 3 in the wild type with a tryptophan residue, resulting in a protein with lower structural stability. The mutant protein was more resistant to surfactant-mediated elution, and apparently adsorbed at the interfaces with a greater interfacial area/molecule than the wild typeT4 lysozyme. However, the results of each type of experiment suggested that sequential adsorption and exchange of proteins occurred only in the case of the less stable mutant followed by the wild type. This suggests that, in these exchange reactions, properties of the adsorbing protein (e.g. its ability to adsorb when a relatively small amount of unoccupied area is present) were more important than the apparent binding strength of the adsorbed protein molecules.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

Adsorption Equilibrium of Binary Mixtures in Zeolites and State of Adsorbed Phase

The criterion of ideal behavior of a mixture of a few molecules within a separate zeolite cavity is formulated on the basis of the statistical thermodynamics. The criterion determines the dependence of the Helmholtz free energy, internal energy, and entropy of a molecular aggregate on the ratio of the number of molecules of components 1 and 2. The similarity between this criterion and the criterion of ideal behavior for bulk solutions is shown. Expressions of excess thermodynamic functions of the molecular mixture in a cavity are obtained. The negative magnitude of these excess functions is proposed to be due to rearrangement of molecules under influence of energetic heterogeneity. The calculation procedure of the excess functions has been demonstrated for the system CO₂-C₂ H₆-zeolite NaX, the information of both isotherms and isosteric adsorption heats being used simultaneously. The approach offered allows the state of adsorbed mixture in a separate cavity to be analyzed from pure-component and multicomponent experimental data.     

Adsorption isotherms of 2,2,4-trimethylpentane and toluene vapors on hydrocarbon adsorber and light-off catalyst

Two monolithic hydrocarbon adsorbers and a monolithic light-off catalyst were selected as adsorbents, and the adsorptive capacity of a hydrocarbon for the adsorbents was measured by using a precise volumetric adsorption apparatus. 2,2,4-Trimethylpentane and toluene vapors were chosen as adsorbates. Equilibrium experiments were carried out at three different temperatures of 303.15, 323.15, and 343.15 K. Adsorption data of each hydrocarbon was fitted to the well-known isotherms such as the Langmuir equation and the Freundlich equation. The Freundlich isotherm predicted equilibrium data better than the Langmuir isotherm. Furthermore, the surface energetic heterogeneity of the adsorbents was evaluated using the isosteric heat of adsorption based on Clausius-Clapeyron equation. The surface energetic heterogeneity of the adsorbents depended on the precious metal (PM) loading and H-ZSM5 loading.