Sebastião M.P. Lucena

Unusual adsorption site behavior in PCN-14 metal-organic framework predicted from Monte Carlo simulation.

The adsorption equilibrium of methane in PCN-14 was simulated by the Monte Carlo technique in the grand canonical ensemble. A new force field was proposed for the methane/PCN-14 system, and the temperature dependence of the molecular siting was investigated. A detailed study of the statistics of the center of mass and potential energy showed a surprising site behavior with no energy barriers between weak and strong sites, allowing open metal sites to guide methane molecules to other neighboring sites. Moreover, this study showed that a model assuming weakly adsorbing open metal clusters in PCN-14, densely populated only at low temperatures (below 150 K), can explain published experimental data. These results also explain previously observed discrepancies between neutron diffraction experiments and Monte Carlo simulations.

Adsorption of Anionic and Cationic Dyes from Aqueous Solution on Non-Calcined Mg-Al Layered Double Hydroxide: Experimental and Theoretical Study

Non-calcined Mg-Al layered double hydroxide (LDH) with Mg/Al molar ratio of 3:1 was synthesized using the co-precipitation method. Sorption of anionic (acid blue 25 - AB25, reactive blue 4 - RB4), and cationic (methylene blue - MB) dyes by Mg-Al LDH form aqueous solution was investigated. The effect of solution pH, initial concentration, and contact time were investigated by batch adsorption experiments. The adsorbed amount increases with decrease in pH solution for AB25 and RB4. The cationic dye (MB) solution was insensitive to pH variation and also exhibited a low performance in the kinetic equilibrium studies. While anionic dyes were almost completely extracted from the solution, 90% of the methylene blue remained in solution. The equilibrium data were well described using the Langmuir-Freundlich model for RB4, AB25, and MB dyes with maximum adsorption capacity of 328.90, 246.10, and 43.48 mg/g, respectively. Finally, the mechanism of adsorption involving the dyes and LDH was evaluated using the Monte Carlo approach in the NVT ensemble. The results suggest that molecular simulation can be used to preview quantitatively the dye uptake. Supplemental materials are available for this article. Go to the publishers online edition of Separation Science & Technology to view the supplemental file.

Easily tunable parameterization of a force field for gas adsorption on FAU zeolites

Considering the great economic and environmental interests in the capture and separation of CO2 and the wide availability of faujasites zeolites (FAU), we propose a set of parameters based on classical force fields that has good transferability among Na-FAU sieves and CO2. In addition to CO2, the parameterization strategy was tested for H2S, O2, N2 and CH4 gases. For these gases, the force field adequately predicts the adsorption isotherms at low pressure. The force field was also tested for N2 in the FAU framework with different monovalent and divalent cations, resulting in quantitative agreement for monovalent cations and qualitative agreement for divalent cations. The good tradeoff between the reliability and ease of implementation will enable rapid evaluation of the adsorption properties of gaseous mixtures of industrial relevance. The reasoning of the re-parameterization strategy is also discussed in detail.

Pore wall thickness and interpore influence on adsorption of alkanes in carbons using explicit pore models

In modeling of activated carbons, the pores are often assumed to be slit-shaped formed of a constant number of graphene layers. X-ray diffraction studies show that micropores are formed between stacks of different numbers of graphene layers. In this study, we investigate, through the grand canonical Monte Carlo method, the influence on the adsorbed alkanes densities of pore walls with different graphene layers thickness and the related interpore adsorbate interaction when the pore wall has only one graphene layer. All studies of thickness and interpore interaction to date were performed using the Steele 10-4-3 potential model. Instead of Steele model, we propose explicit models made up of graphene layers of discrete carbon atoms. We also investigated the sensitivity of the system to the cut-off and solid-fluid parameter. With our explicit model we found that the influence of the number of carbon layers is not significant for n>2 as previously observed by Steele model—DFT studies. The system was also insensitive to cut-off and well deep parameter variations. A new pore model with an extra dummy graphene wall was proposed to investigate the interpore interaction. The interpore interaction study with the alkanes series C1 to C4 shows that the retention capacity of heavier alkanes is the same whether for activated carbons with few layers (stronger interpore interaction) as for carbons with two or more layers (stronger solid-fluid interaction) assuming negligible surface mediation. The explicit models proposed can be successfully used in the elaboration of virtual porous carbon models to reproduce wall thickness and interpore adsorbate interactions phenomena.

Improvement in the Adsorption of Anionic and Cationic Dyes from Aqueous Solutions: A Comparative Study using Aluminium Pillared Clays and Activated Carbon

The aim of this work was to evaluate the adsorption properties of anionic dye Reactive Black 5 (RB5) and cationic dye Methylene Blue (MB) from salted aqueous solution using natural clay, aluminum pillared clay (Al-PILC), and activated carbon. The textural properties of the materials were obtained by N2 adsorption at 77 K and the structural properties of natural and pillared clays were determined by X-ray diffraction. The effect of pH, contact time, initial concentration of dye, and influence of the addition of NaCl were evaluated by batch adsorption. Adsorption isotherms of Al-PILC, in different salt concentration were compared with natural clay and activated carbon. The adsorption isotherms were well fitted by the Langmuir and Langmuir-Freundlich models. The process of pillaring only improved the adsorption of the anionic dye RB5. Depending on the system adsorbent/adsorbate analyzed, the salt concentration can either help or hinder dye adsorption. We found that a special morphology formed during the process of pillaring greatly increased adsorption of the MB cationic dye in the range of high salt concentrations. This unexpected result may help in developing new pillarization strategies to treat effluents with high salt content.

Sensitivity to guest–host force fields in adsorption equilibrium of cyclic hydrocarbons in one-dimensional molecular sieve

We have investigated the effect of different guest–host force fields (united atom, UA; all-atom, AA; and anisotropic united atom, AUA) on the adsorption of cyclic hydrocarbon molecules in AlPO4-5 molecular sieve. Grand canonical Monte Carlo (GCMC) simulations of benzene, xylene and cyclohexane adsorption and positioning are carried out. The results suggest that the force field choice affects considerably the positioning of benzene and cyclohexane molecules while xylenes presented a smaller sensibility. The force fields difference of sensibilities among cyclical molecules seems to be associated with the critical geometric condition identified by Yashonath and Santikary denominated levitation effect. In this condition, the net forces on the guest due to the host is at a minimum, the levitation ratio (γ) is close to unity and we have a weakly bound guest that can be easily moved from its balance position. Unlike xylene, that is only slightly affected by the choice of the force field, benzene and cyclohexane molecules in AlPO4-5 have such dimensions that result in levitation ratio (γ) close to unity. These results point out to the care that should be taken in the choice of the force field when the guest–host size ratio is near the value defined for the levitation effect.

Adsorption equilibrium in one-dimensional molecular sieve: a study of force fields effect on linear alkanes molecules

The effect of different force fields (united atom and all-atom) has been investigated on the adsorption of linear hydrocarbon molecules in AlPO4-5 molecular sieve using grand canonical Monte Carlo (GCMC) simulations of n-hexane, n-pentane and n-butane. Adsorption isotherms and structural analysis were performed for linear alkanes and compared with reported experimental data. We show that the modulation between wide and narrow regions in AlPO4-5 and the strong interaction between terminal methyl groups and oxygen atoms of the wide regions drive position and adsorption sites. Discrepancies in the positioning of n-pentane molecules suggest that the force field choice affects considerably n-pentane while n-hexane and n-butane presented lower sensibility. The force fields difference of sensibilities among the linear molecules seems to be associated with the critical geometric condition identified by Yashonath and Santikary and denominated ‘levitation effect’. The molecules of n-pentane in AlPO4-5 have dimensions that result in a levitation ratio (γ) close to unity. As previously shown for cyclic molecules, proper care should be taken in the choice of the force field when the guest–host size ratio of linear molecules is near the value defined for the levitation effect.