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A machine learning (ML) methodology that uses a histogram of interaction energies has been applied to predict gas adsorption in metal–organic frameworks (MOFs) using results from atomistic grand canonical Monte Carlo (GCMC) simulations as training and test data. In this work, the method is first extended to binary mixtures of spherical species, in particular, Xe and Kr. In addition, it is shown that single-component adsorption of ethane and propane can be predicted in good agreement with GCMC simulation using a histogram of the adsorption energies felt by a methyl probe in conjunction with the random forest ML method. The results for propane can be improved by including a small number of MOF textural properties as descriptors. We also discuss the most significant features, which provides physical insight into the most beneficial adsorption energy sites for a given application. [ABSTRACT FROM AUTHOR]

Knowledge about the interfacial properties of water/oil mixtures is important for the petrochemical industry and for understanding detergency and hydrophobic effects. Here, we probe the liquid/vapour interface of water/n-hexane mixtures using configurational-bias Monte Carlo simulations in the osmotic Gibbs ensemble. We study the effect of n-hexane at several partial pressures ranging from 25% to 95% of its saturated vapour pressure and observe that the surface tension decreases with increasing n-hexane pressure. Additionally, we analyse the simulation trajectories to provide molecular-level insights on the spatial distribution of n-hexane and the structure of the interface. The n-hexane molecules strongly adsorb from the vapour phase onto the liquid interface with a preferentially parallel orientation with respect to the interface. The surface excess, from the Gibbs adsorption isotherm equation, is calculated and used to systematically define the domain of adsorbed n-hexane. Integrating over this gives the free energy of adsorption of n-hexane, which is highly favourable, varying from to  kJ/mol as the partial pressure of n-hexane is increased. The enrichment of n-hexane molecules on the interface yields a positive deviation from Henry's law at higher partial pressures, providing evidence for favourable adsorbate-adsorbate interactions. [ABSTRACT FROM AUTHOR]

Raw natural gas is a complex mixture comprising methane, ethane, other hydrocarbons, hydrogen sulfide, carbon dioxide, nitrogen, and water. For sour gas fields, selective and energy-efficient removal of H2S is one of the crucial challenges facing the natural-gas industry. Separation using nanoporous materials, such as zeolites, can be an alternative to energy-intensive amine-based absorption processes. Herein, the adsorption of binary H2S/CH4 and H2S/C2H6 mixtures in the all-silica forms of 386 zeolitic frameworks is investigated using Monte Carlo simulations. Adsorption of a five-component mixture is utilized to evaluate the performance of the 16 most promising materials under close-to-real conditions. It is found that depending on the fractions of CH4, C2H6, and CO2, different sorbents allow for optimal H2S removal and hydrocarbon recovery. [ABSTRACT FROM AUTHOR]

Hydrophobic zeolites, including Ti- and Sn-beta, have been found to adsorb and isomerize glucose into fructose. An experimental question has been the significance of the entropic contribution to the free energy of transfer of glucose from solution to zeolite. We here perform Gibbs ensemble Monte Carlo calculations to quantify the enthalpy, entropy, and free energy of transfer of glucose from the aqueous phase to the zeolite phase. We find that the entropic contribution is large and positive, nearly compensating for an unfavorable enthalpy of transfer in all-silica zeolite beta. A significant component of the positive entropy of transfer from the aqueous phase to zeolite is the unstructuring of first coordination shell waters around glucose as it leaves the solution. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3523-3529, 2013 [ABSTRACT FROM AUTHOR]

Abstract: Configurational-bias Monte Carlo simulations in the Gibbs ensemble (CBMC-GE) are used to investigate the adsorption of both linear and branched alkanes (ethane, propane, n-butane, and 2-methylpropane) from dilute solutions in liquid methane onto a carbon slit pore at T =160K and at either the saturation pressure or p ext =100atm. Thermodynamic properties (adsorption isotherms, selectivities, and Henry's law constants) and structural properties (density and orientational distributions) are presented. Both the Henry's law constants and the separation factors depend exponentially on the number of carbon atoms for the linear alkanes, whereas chain branching and higher pressure lead to a reduction of these properties. The solute density profiles show oscillatory behavior along the surface normal, and peaks in the number density are correlated with a preference for parallel orientations. The CBMC-GE approach allows for the efficient calculation of these selective adsorption phenomena, and data for multiple solutes (in the dilute regime) can be extracted from a single simulation. [Copyright &y& Elsevier]

Abstract: Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C18 stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile. [Copyright &y& Elsevier]

Adsorption and ordering at the vapor-liquid interfaces of mutually saturated water/1-butanol solutions at a temperature of 298.15 K were investigated using configurational-bias Monte Carlo simulations in the Gibbs ensemble and compared to the surface properties of neat water and 1-butanol liquids. A dense 1-butanol monolayer is observed at the surface of the water-rich phase, which results in a substantial decrease of its surface tension. In contrast, there is no enrichment of water molecules at the surface of the butanol-rich phase, and its surface tension is not significantly changed. Analysis of the interracial structures reveals that these systems exhibit orientational ordering and composition heterogeneity. Analysis of the hydrogen-bonding distributions suggests that the formation of the 1-butanol monolayer is driven by an excellent match between water and the primary alcohol; that is, additional hydrogen bonds are formed between the excess free hydrogens of surface water and the excess hydrogen-bond acceptor sites of 1-butanol. [ABSTRACT FROM AUTHOR]

Examines the molecular simulation of concurrent gas-liquid interfacial adsorption and partitioning in gas-liquid chromatography. Adsorption of polar analytes on nonpolar liquid phases; Effects of n-pentane on adsorption; Separation of the interfacial and bulk regions of the squalene slab.

Configurational-bias Monte Carlo (CBMC) simulations inthe isobaric–isothermalversion of the Gibbs ensemble (GE) were carried out to probe the adsorptionfrom aqueous solutions of methanol and/or ethanol onto silicalite-1.This methodology does require neither specification of the chemicalpotential nor any reference to activity models based on experimentaldata. The CBMC-GE methodology can be applied to the complete rangeof mixture compositions from pure water to pure alcohols and can alsobe used when multiple solute types are present at high concentration.The simulations demonstrate high selectivities for the alcohols (αethanol> αmethanol) almost over the entirecomposition range. The ideal adsorbed solution theory is found tosubstantially underpredict the amount of sorbed water and leads tovery large errors for low alcohol solution concentrations. The simulationsindicate that, at lower loadings, the adsorbed alcohol molecules canserve as seeds for water adsorption but, at higher loadings, alcoholsdisplace water molecules from their preferred region. [ABSTRACT FROM AUTHOR]

Computational high-throughput screening was carried out to assess a large number of experimentally reported metal-organic frameworks (MOFs) and zeolites for their utility in hexane isomer separation. Through the work, we identified many MOFs and zeolites with high selectivity (SL+M > 10) for the group of n-hexane, 2-methylpentane, and 3-methylpentane (linear and monobranched isomers) versus 2,2-dimethylbutane and 2,3-dimethylbutane (dibranched isomers). This group of selective sorbents includes VICDOC (Fe2(BDP)3), a MOF with triangular pores that is known to exhibit high isomer selectivity and capacity. For three of these structures, the adsorption isotherms for a 10-component mixture of hexane and heptane isomers were calculated. Subsequent simulations of column breakthrough curves showed that the DEYVUA MOF exhibits a longer process cycle time than VICDOC MOF or MRE zeolite, which are previously reported, high-performing materials, illustrating the importance of capacity in designing MOFs for practical applications. Among the identified candidates, we synthesized and characterized a MOF in a new copper form with high predicted adsorbent capacity (qL+M > 1.2 mol/L) and moderately high selectivity (SL+M ≈ 10). Finally, we examined the role of pore shape in hexane isomer separations, especially of triangular-shaped pores. We show through the potential energy surface and three-dimensional siting analyses that linear alkanes do not populate the corners of narrow triangular channels and that structures with nontriangular pores can efficiently separate hexane isomers. Detailed thermodynamic analysis illustrates how differences in the free energy of adsorption contribute to shape-selective separation in nanoporous materials. [ABSTRACT FROM AUTHOR]

The aim of this work is to develop enabling tools that can assess the potential of zeolite membranes for industrial applications. The specific objectives are: (1) Develop a detailed mathematical model of a zeolite membrane separation process for accurate performance prediction under industrial conditions, and (2) Perform conceptual process design and techno-economic evaluation of the overall process for an application specific flowsheet. To this end, a detailed mathematical model, based on the real adsorption solution (RAS) theory and the Maxwell-Stefan formulation for transport was developed to describe permeation through a zeolite membrane. Effects like support resistances to transport, use of sweep gas and concentration polarization were included. The model is further applied to study butane isomer separation using a MFI-type zeolite membrane. A comparison of steady state flux and separation factor predicted by the model with the experimentally determined values suggests that the permeation in the real membranes is lower than that for the zeolite crystals. This lower permeance is attributed to the microstructural defects and suggests that there is a considerable scope of improvement in the performance of current state-of-the-art real MFI membranes. It is shown that up to a 10-fold increase in permeance through the membrane is practical, beyond which the external resistance starts to dominate. Furthermore, the detailed zeolite membrane model is integrated into a process level simulation, to evaluate the membrane performance in industrial settings, and a techno-economic analysis is also performed to this end. While single stage membrane process does not achieve the target purity and multi-stage membrane process requires prohibitively large area, a hybrid membrane/distillation process is found to be energy efficient and economically attractive. [ABSTRACT FROM AUTHOR]