Investigation of the Air Separation Properties of Zeolites Types A,X and Y by Monte Carlo Simulations

Abstract

The air separation properties of zeolite types A, X, and Y have been studied using grand canonical Monte Carlo simulations of nitrogen, oxygen, and argon adsorbed in these zeolite lattices. Nitrogen is adsorbed preferentially due to the quadrupole-ion electrostatic interactions with the extra framework cations. The localization of adsorption sites for nitrogen near cations and the more diffuse distributions of oxygen and argon within zeolite cavities are clearly illustrated. Predicted nitrogen/oxygen selectivity for 5A from simulations is in good agreement with that determined experimentally. The effect of the calcium-sodium ion exchange on the predicted nitrogen/oxygen selectivity is examined, and is shown to be sensitive to the magnitude of the charges assigned to the extra framework cations.     

Editorial

Abstract

Grand canonical molecular dynamics (GCMD) simulations are used to study the adsorption and desorption of Lennard-Jones nitrogen in three slit pore junction models of microporous graphite. These networks consist of two narrow pores separated by a wider (cavity) pore. We report results for cases where the narrow pore has a width of only two or three molecular diameters. Using the GCMD technique, a novel freezing transition is observed which results in pore blocking in the narrow pores of the network, which are less than 1 nm wide. This freezing results from the adsorption energy barrier at the junction between the narrow and wider pores. This type of pore blocking could account for the apparent increase in pore volume with increasing temperature that has been experimentally observed in microporous graphite systems. For networks in which the narrower pores are somewhat larger, with a width of 1.28 nm, this pore blocking effect is much reduced, and adsorbate molecules enter and fill the central cavity. In such cases, however, desorption is incomplete, some residual adsorbate remaining in the central cavity even at the lowest pressures.     

Adsorption of Binary Mixtures in a Zeolite Micropore

Abstract

We investigate the selective adsorption of xenon, argon, and methane in zeolite NaA by applying the grand canonical ensemble Monte Carlo simulation technique to an adsorbed binary mixture and to two reference systems: i) an adsorbed single component system and ii) a bulk mixture. We define and calculate selectivities and excess densities due to i) mixing and ii) adsorption in terms of differences between the binary adsorbed system and these reference systems. We observe that xenon selectively adsorbs in both xenon-argon and xenon-methane mixtures at low chemical potential (low pressure) due to its greater energetic interaction with the zeolite. However, a reversal in selectivity occurs at higher chemical potential in both of these mixtures. This is due in large part to the greater efficiency in which the smaller component “packs” in the pore as compared to the bulk. We show that the crossover in selectivity occurs at a lower chemical potential for a mixture where one component can occupy regions of the porespace inaccessible to the other. We suggest that this crossover in selectivity may be a general feature of microporous adsorption.     

In vitro evaluation of the capacity of zeolite and bentonite to adsorb aflatoxin B1 in simulated gastrointestinal fluids

Anin vitro study using single concentration and isotherm adsorption was carried out to evaluate the capacity of Vietnamese produced zeolite and bentonite to adsorb aflatoxin B₁ (AFB₁) in simulated gastrointestinal fluids (SGFs), and a commercial sorbent hydrated sodium calcium aluminosilicate (HSCAS) was used as reference. In this study, AFB₁ solution was mixed with sorbents (0.3, 0.4 and 0.5% w/v) in SGFs at pH 3 and pH 7 and shaken for 8 h, centrifuged and the supernatant measured by Vicam fluorometer. Adsorption of AFB₁ onto zeolite and bentonite varied according to the pH of SGFs and was lower than HSCAS. Linearity between the increased amount of AFB₁ adsorbed on sorbents and the decrease of sorbent concentration was observed for bentonite and HSCAS, except for zeolite in SGFs at pH 7. The observed maximum amounts of AFB₁ adsorbed on bentonite and HSCAS were 1.54 and 1.56 mg/g, respectively. The adsorption capacities of bentonite and HSCAS for AFB₁ were 12.7 and 13.1 mg/g, respectively, from fitting the data to the Freundlich isotherm equation. Improvement in processing and purification for bentonite is needed to enhance the surface area, which would probably result in better adsorptive capacity for this sorbent.     

Theoretical Study of the TiO2 and MgO Surface Acidity and the Adsorption of Acids and Bases

We present ab-initio periodic Hartree–Fock calculations (crystal program) of small molecules on TiO₂ and MgO. The adsorption of the molecules may be molecular or dissociative. This depends on their acid and basic properties in the gas phase. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages: thus, the dissociation of MeOH leads to a preferential basic cleavage (the fragment HO– is adsorbed on a Ti⁺⁴ ion and the fragment Me⁺ is adsorbed on a O²– ion of the oxide). The opposite result is obtained with MeSH. Another important factor is the adsorbate–adsorbate interaction: favorable cases are a sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface so that their mutual orientation resembles the geometry of the CO₂ dimer. With respect to water adsorption, MgO appears to be a basic oxide. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs. basic behaviour but in terms of hard and soft acidity. The MgO surface is a 'soft' acidic surface that reacts preferentially with the soft base, CO₂.     

Simulation of Surface Excess Concentrations for a Binary Hydrocarbon Mixture on Graphite

We perform Molecular Dynamics simulations of thin hydrocarbon films adsorbed on the basal plane of graphite to determine structural and thermodynamic properties. Specifically we study the behaviour of liquid benzene/n-heptane mixtures. The intra adsorbate and the adsorbate-substrate interactions are described using a phenomenological force field whose careful parameterization will be reported in a following paper. The foremost quantity we calculate is the adsorption isotherm, i.e. the surface excess concentration as a function of the benzene bulk mole fraction at T = 283 K, which is in quite reasonable agreement with the experiment. Along with the isotherm we compare the surface induced ordering of the two components in terms of order parameter profiles.     

Molecular interactions of alcohols with zeolite BEA and MOR frameworks

Zeolites can adsorb small organic molecules such as alcohols from a fermentation broth. Also in the zeolite-catalyzed conversion of alcohols to biofuels, biochemicals, or gasoline, adsorption is the first step. Several studies have investigated the adsorption of alcohols in different zeolites experimentally, but computational investigations in this field have mostly been restricted to zeolite MFI. In this study, the adsorption of C1–C4 alcohols in BEA and MOR was investigated using density functional theory (DFT). Calculated adsorption geometries and the corresponding energies of the designed cluster models were comparable to periodic calculations, and the adsorption energies were in the same range as the corresponding computational and experimental values reported in the literature for zeolite MFI. Thus, BEA and MOR may be good adsorption materials for alcohols in the field of downstream processing and catalysis. Aside from the DFT calculations, adsorption isotherms were determined experimentally in this study from aqueous solutions. For BEA, the adsorption of significant amounts of alcohol from aqueous solution was observed experimentally. In contrast, MOR was loaded with only a very small amount of alcohol. Although differences were found between the affinities obtained from gas-phase DFT calculations and those observed experimentally in aqueous solution, the computational data presented here represent molecular level information on the geometries and energies of C1–C4 alcohols adsorbed in zeolites BEA and MOR. This knowledge should prove very useful in the design of zeolite materials intended for use in adsorption and catalytic processes, as it allows adsorption behavior to be predicted via judiciously designed computational models.     

Transport of nitrate and ammonium ions in a sandy loam soil treated with potassium zeolite – Evaluating equilibrium and non-equilibrium equations

The increased use of nitrogen fertilizers in agriculture would cause migration of nitrogen to surface and groundwater; accordingly, would lead to water resources contamination. The objective of this study is to investigate the effect of potassium zeolite on nitrate and ammonium ions sorption and retention in a saturated sandy loam soil in a laboratory condition. The study was conducted as a completely randomized block design with four treatments of 0, 2, 4 and 8 g zeolite per kilogram soil and three replications. Ammonium nitrate fertilizer with concentration of 10 g l⁻¹ was added to soil columns and then leaching was performed. The results show that increasing potassium zeolite to soil causes reduction to the mobility of both nitrate and ammonium and enhancement of the retention of ions in soil. Ions leaching were simulated with convection–dispersion-equation (CDE) and mobile–immobile model (MIM) using HYDRUS-1D code. The results indicate that ammonium ion sorption by soil followed the Freundlich isotherm model. Absorption isotherms and dispersion (D_e) coefficient were determined through the inverse modeling for both ions. Based on the results, optimized values of Freundlich isotherm were much less than the observed amounts. This shows that the HYDRUS-1D model underestimated the adsorption parameters to predict the ammonium ion mobility in soil macropores. Since soil has been disturbed, the prediction of CDE model was equal to MIM model approximately. Both models showed that as the amount of applied zeolite increases, the dispersion (D_e) coefficient of nitrate and ammonium ions in the soil increases.     

Semi-analytical mean-field model for predicting breathing in metal–organic frameworks

A new semi-analytical mean-field model is proposed to rationalise breathing of MIL-53 type materials. The model is applied on two case studies, the guest-induced breathing of MIL-53(Cr) with CO₂ and CH₄, and the phase transformations for MIL-53(Al) upon xenon adsorption. Experimentally, MIL-53(Cr) breathes upon CO₂ adsorption, which was not observed for CH₄. This result could be ascribed to the stronger interaction of carbon dioxide with the host matrix. For MIL-53(Al) a phase transition from the large pore phase could be enforced to an intermediate phase with volumes of about 1160–1300 Å³, which corresponds well to the phase observed experimentally upon xenon adsorption. Our thermodynamic model correlates nicely with the adsorption pressure model proposed by Coudert et al. Furthermore the model can predict breathing behaviour of other flexible materials, if the user can determine the free energy of the empty host, the interaction energy between a guest molecule and the host matrix and the pore volume accessible to the guest molecules. This will allow to generate the osmotic potential from which the equilibria can be deduced and the anticipated experimentally observed phase may be predicted.     

A theory of cell hydration governed by adsorption of water on cell proteins rather than by osmotic pressure

It is postulated that cell hydration is governed by adsorption of water on cell proteins in accord with the Bradley adsorption isotherm, and that the action of a solute in the surrounding solution is to lower the vapor pressure of the solution so that cell water adsorption is decreased by moving down the Bradley isotherm. From these concepts, it is derived that cell volume (V) should be related to solute concentration (x) by the equationV=−E log₁₀ x+F whereE andF are constants which are independent of type of solute. For a non-adsorbed solute this agrees well with experimental data. For solutes which are adsorbed by cell proteins, a correction in the above equation may be necessary at higher solute concentrations, which is shown to be compatible with various experimental data. The types of experiments which are generally used to support the osmotic pressure theory of cell hydration agree equally well with the adsorption theory. The virtue of the adsorption theory is that, unlike the osmotic pressure theory of cell swelling, it is compatible with permeability of the cell membrane to solutes, which has been experimentally observed for various solutes. The opinions and conclusions contained in this report are those of the author. They are not to be construed as necessarily reflecting the views or the endorsement of the Navy Department.     

The effects of structural parameters of zeolite on the adsorption of hydrogen: a molecular simulation study

The adsorption isotherm of hydrogen in zeolites FAU, LTA, KFI, RWY, RHO and TSC has been simulated employing grand canonical Monte Carlo procedure for a temperature range of 77 to 95 K and different pressures. The effects of structural composition, unit cell volume, framework density and specific surface area of zeolite on hydrogen adsorption in zeolites were investigated. The results clearly show that the adsorption of hydrogen in zeolites with the same silica density is a function of oxygen density at low pressures, and it is approximately the same at intermediate pressures. Nevertheless, at high pressures, the adsorption of hydrogen is a function of pore diameter for zeolites with same silica density. The effect of specific surface area on the adsorption isotherm of hydrogen on zeolites with approximately the same specific surface area is significant at low and high pressures. The results clearly indicate that the adsorption of hydrogen in RWY zeolite has maximum value at 77 K and at high pressures. The optimum condition of pressure for hydrogen adsorption isotherm in RWY zeolite is determined to be 600 bar. At a temperature of 77 K and a pressure of 600 bar, the adsorption of hydrogen in RWY zeolite is 6.93 wt %.     

Evaluating adsorbed-phase activity coefficient models using a 2D-lattice model

Despite the wide use of the real adsorbed solution theory to predict multicomponent adsorption equilibrium, the models used for the adsorbed phase activity coefficients are usually borrowed from the gas–liquid phase equilibria. In this work, the accuracy of the Wilson and NRTL models for evaluating adsorbed phase activity coefficients is tested using a 2D-lattice model. An accurate model for adsorbed-phase activity coefficients should have no problem in fitting adsorption data obtained using this simple lattice model. The results, however, show that the commonly used Wilson and NRTL models cannot describe the adsorbed phase activity coefficients for slightly non-ideal to strong non-ideal mixtures. Therefore, until new models for adsorbed phase activity coefficients are developed, we should use existing models for liquids with care. In the second part of this work, the use of Monte Carlo simulations on a segregated 2D-lattice model, for predicting adsorption of mixtures is investigated. The segregated model assumes that the competition for adsorption occurs at isolated adsorption sites, and that the molecules from each adsorption site interact with the bulk phase independently. Two binary mixtures in two adsorbent materials were used as case studies for testing the predictions of the segregated 2D-lattice model: the binary system CO₂–N₂ in the hypothetical pure silica zeolite PCOD8200029, with isolated adsorption sites and normal preference for adsorption, and the binary system CO₂–C₃H₈ in pure silica mordenite (MOR), with isolated adsorption sites and inverse site preference. The segregated 2D-lattice model provides accurate predictions for the system CO₂–N₂ in PCOD8200029 but fails in predicting the adsorption behaviour of CO₂–C₃H₈ in pure silica MOR. The predictions of the segregated ideal adsorbed solution theory model are superior to those of the 2D-lattice model.     

Smart Monte Carlo Algorithm for the Adsorption of Molecules at a Surface

Abstract

A modified grand canonical ensemble Monte Carlo (GCMC) technique has been developed to simulate adsorption isotherms for molecules on or near a surface. The speed and accuracy of the simulation is increased by using a non-uniform distribution function, related to the force field exerted by the surface and the current configuration, to generate coordinates for the creation of new particles in the simulation. With this method, isotherms are generated more efficiently than with current techniques in which the creation step relies on a uniform distribution to generate the coordinates of a new molecule. This is shown by comparing the calculation of an isotherm for a simple molecule adsorbed on a graphite substrate from a traditional GCMC simulation with that calculated using this new technique.     

Refolding of lactate dehydrogenase by zeolite beta

We used zeolite beta as an adsorbing matrix to refold recombinant lactate dehydrogenase (LDH) protein collected as an insoluble aggregate from a bacterial expression system. The adsorption isotherm revealed that 1 g of zeolite adsorbed 200 mg of denatured LDH solubilized with a buffer containing 6 M of guanidine hydrochloride. The pH of the buffer had little effect on the adsorption, but this property was abolished by preincubation of the zeolite with polyethylene glycol (PEG) in a weight ratio of 1:10. These data suggest that the adsorption of LDH depends on the hydrophobicity of the zeolite surface, and that the adsorption of PEG to zeolite is sufficient to release LDH from its surface. LDH was thus released by refolding buffer containing PEG and arginine, and soluble LDH was obtained in its active enzymatic form. The addition of arginine dramatically increased the yield of LDH in a dose‐dependent manner. The overall refolding efficiency was optimized to 35%. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Changes in the polyelectrolyte-amphiphile interaction due to helix-coil transition induced by specific counterions or variations in temperature

For the system κ-carrageenan/amitriptyline it is shown that the degree of binding of amitriptyline is closely related to the carrageenan conformation as regulated by the counterions (Na⁺ or K⁺). The adsorption becomes much more pronounced when the carrageenan molecule is in the helix form (counterion K⁺) than when it has a coil conformation (counterion Na⁺). Furthermore, for the helical state the adsorption becomes strongly cooperative. It is also shown experimentally that the release from the adsorbed state has a conversion temperature at about 42°C (helix-coil transition). The effect is also related to the linear charge density. For κ-carrageenan with a higher charge density the adsorption is strong and cooperative both in the presence of Na⁺ and K⁺ ions. © 1996 John Wiley & Sons, Inc.     

Atypical Langmuir adsorption of inhalation anesthetics on phospholipid monolayer at various compressional states: difference between alkane-type and ether-type anesthetics

Adsorption of chloroform, halothane, enflurane and diethyl ether on the air/water interface was compared with adsorption on the dipalmitoylphosphatidylcholine monolayer, spread on the air/water interface, at four compressional states; 88.5, 77.0, 66.5 and 50.5 A2 surface area per phosphatidylcholine molecule. Anesthetics were administered from the gas phase. The affinities of these agents to the phosphatidylcholine monolayer varied according to the state of the monolayer. Chloroform and halothane showed a stronger affinity to the highly compressed phosphatidylcholine monolayer (50.5 A2) than to the expanded monolayer (88.5 A2) or to the air/water interface without the monolayer. Diethyl ether behaved in reverse; a stronger affinity to the expanded monolayer was exhibited than to the compressed monolayer. Enflurane showed the highest affinity to the intermediately compressed monolayer (77.0 A2). The adsorption isotherm of anesthetics to the monolayer was characterized by atypical Langmuir-type, in which available number of binding sites changed when anesthetics were adsorbed. The mode of adsorption onto the monolayer was dissimilar to adsorption onto air/water interface, where adsorption followed the Gibbs surface excess. A theory is presented to explain the above differences. The adsorbed anesthetic molecules do not stick to phosphatidylcholine molecules but penetrate into the monolayer lattice and occupy the phosphatidylcholine sites at the interface. Quantitative agreement between the theory and the experimental data was excellent. For the monolayer at 50.5 A2 compression, the changes in the transfer free energy accompanying the anesthetic adsorption from the gas phase to the monolayer were in the order of chloroform greater than halothane greater than enflurane greater than diethyl ether, in agreement with the clinical potencies.     

Synthesis and Characterization of Templated Mesoporous Materials Using Molecular Simulation

Abstract

Lattice Monte Carlo simulations are used to calculate equilibrium properties of surfactant-solvent-silica liquid-crystal systems under no-polymerization conditions. The formation of a high-surfactant high-silica concentration phase in equilibrium with a dilute phase is observed when the surfactant-silica interactions are stronger than the surfactant-solvent interactions. Different silica structures that are similar to the M41 family are observed, depending on the overall concentration of the system. The formation of a hexagonal phase is favored at a surfactant/silica ratio of 0.2, whereas a lamellar phase is observed a surfactant/silica ratio of 1.

Argon adsorption properties on a model porous structure of the MCM-41 type prepared using this mimetic simulation protocol are calculated using grand canonical Monte Carlo simulation. Heats of adsorption are calculated from fluctuations in the energy and number of molecules [1] following the work of Nicholson and Parsonage [Computer Simulation and the Statistical Mechanics of Adsorption (Academic Press, London), 1982, p 97 8 pp]. A decrease in the heats of adsorption for coverage less than one statistical monolayer is evidence of surface heterogeneity. The results are in qualitative agreement with experimental measurements for argon on MCM-41.     

On the Adsorption of Human Acidic Proline-rich Proteins (PRP-1 and PRP-3) and Statherin at Solid/liquid Interfaces

The objective of the present study was to investigate the adsorption of PRP-1, PRP-3 and statherin to solid surfaces in terms of dependence on concentration, the presence of electrolyte and surface wettability. Time resolved in situ ellipsometry was used to determine the adsorbed amounts and adsorption rates of pure PRP-1, PRP-3 and statherin onto pure (hydrophilic) and methylated (hydrophobized) silica surfaces. The initial film build-up was fast and plateaus were reached within 10 min at all concentrations for both types of surfaces and all proteins. The observed adsorption and calculated diffusion rates of PRP-1, PRP-3 and statherin, respectively, indicated that the initial adsorption was mass transport controlled at low concentrations. At hydrophobic surfaces, isotherm shapes and adsorbed amounts were similar for PRP-1 and PRP-3, while statherin adsorbed to a higher extent. At hydrophilic surfaces only PRP-1 adsorbed substantially, while for PRP-3 and statherin adsorbed amounts were low. The presence of Ca 2+ ions in the phosphate buffer solution increased the adsorption of statherin and PRP-3 on hydrophobic surfaces, while PRP-1 was unaffected. On hydrophilic surfaces, all three proteins adsorbed in higher amounts in NaCl, compared to CaCl 2 at similar ionic strength. It is concluded that acidic PRPs (PRP-1 and PRP-3) and statherin readily form films on a variety of materials and solution conditions, showing that their functions may be fulfilled under a wide range of conditions.     

Adsorption into the MFI zeolite of aromatic molecule of biological relevance. Investigations by Monte Carlo simulations

Adsorption of paracresol and water into the silicalite-1 (MFI) zeolite has been investigated using canonical and grand-canonical Monte Carlo simulations. The most stable sites of adsorption of paracresol are found to be located at the channel intersections. Grand-canonical simulations have shown that at low loading, water molecules adsorb preferably at the vicinity of paracresol molecules, whereas they are also located in the sinusoidal channels as the loading increases. In order to explain the experimental adsorption isotherm observed for the coadsorption of water and paracresol in the MFI zeolite we propose a new concept of apparent adsorption enthalpy that varies with the concentration of the solution. The mathematical expression for the apparent enthalpy is introduced in an adsorption isotherm model. We shall refer to this theoretical isotherm as a non-langmuirian isotherm. The non-linear expression for the apparent adsorption enthalpy accounts for a variable accessibility of the sites of adsorption with respect to the concentration of the solution. Figure Co-adsorption of paracresol and water in silicalite-1 zeolite and comparison between experimental and modelled adsorption isotherms.