Simulated Adsorption Isotherms Research Articles

Do Internal and External Surfaces of Metal-Organic Frameworks Have the Same Hydrophobicity? Insights from Molecular Simulations.

Reliable information on the hydrophobicity of porous materials is important in the design of many catalytic and separation processes. In general, hydrophobicity is assessed by measuring the contact angle of water (external surface) or the adsorption isotherm of water (internal surface). However, it is not clear how these different assessments are related. In this paper, molecular dynamics simulations of microscopic water droplets on the external surfaces of metal-organic frameworks are used to investigate the influence of the surface nature and hydrophobicity on the contact angle. The metal-organic frameworks MOF-5 and CAU-10 were modeled with external surfaces of different hydrophobicities, while the internal surface was maintained. It was observed that microscopic droplets orientate their spreading to the nature of the external surfaces. Comparing the simulated contact angles and adsorption isotherms confirms the necessity to distinguish between internal and external hydrophobicity.

Determining the effect of the non-ionic surfactant AEO9 on lignite adsorption and wetting via molecular dynamics (MD) simulation and experiment comparisons

By combining a molecular dynamics (MD) simulation approach with experimental measurements, this study elucidated the molecular interactions between the fatty alcohol polyoxyethylene ether-9 (AEO9; i.e., a surfactant) and lignite. Moreover, the micro-dynamic process of wetting and adsorption of surfactant molecules onto coal dust molecules were also studied. In the wettability test, the evaporation rate of coal decreased significantly after AEO9 addition. The measured contact angle first decreased and then increased with increased AEO9 concentration. A lower contact angle was indicative of better wetting performance. Using the Materials Studio software, the motion process of the water/AEO9/lignite system was simulated. The resulting structural chart indicated that AEO9 can be adequately adsorbed onto the coal surface. By analyzing the changes in the relative concentration curves along the Z-axis and water mobility before and after AEO9 addition, it was found that AEO9 can enhance the hydrophilicity of lignite. The simulated adsorption isotherms and the calculated interaction energy between molecules suggested simultaneous adsorption onto the lignite surface. X-ray photoelectron spectroscopy (XPS) analysis also revealed that the elemental C content of the surface decreased while the O content increased after coating the lignite surface with AEO9. Given that a great number of oxyethylene groups are present in AEO9, we concluded that AEO9 efficiently adsorbed onto the lignite surface. The present study is of great significance to the enhancement of dust suppressant performance and reducing coal dust hazards.

Application of computational chemistry for adsorption studies on metal–organic frameworks used for carbon capture

Abstract Computational chemistry is invaluable in calculating macroscopic and microscopic details of systems application in chemical industries which are involved in carbon capture through precombustion, post-combustion and oxy combustion technologies. This review discusses the role of computational chemistry for adsorption studies of metal–organic frameworks (MOFs) which can be utilized for carbon capture. Principles of quantum mechanics–molecular mechanics are used to devise the electrostatic charges and isotherm parameters on the MOFs. MOFs for carbon capture which can be compatible and which can withstand the severity in chemical industries can be effectively studied using grand canonical Monte Carlo simulation by selecting appropriate force fields. Since flue gases contain a host of other gases in addition to oxides of carbon, capture by MOFs has to be carefully modelled and the software useful for this study are mentioned in this review. The simulated adsorption isotherms should be compared with experimental adsorption isotherms to validate the study. The adsorption model for carbon dioxide adsorption on MOFs is generally reported to be type I reversible isotherm and the kinetics is in good agreement with pseudo-second-order kinetics. Graphical Abstract: Graphical Abstract

Adsorption of arsenic(V) onto single sheet iron oxide: X-ray absorption fine structure and surface complexation

Adsorption onto two-dimensional nanosheet materials offers new possibilities for fast and efficient removal of contaminants from waters. Here, the adsorption of As(V) to a new type of iron oxides – single sheet iron oxide (SSI) – has been studied as a function of time, loading and pH. Adsorption of As(V) onto SSI was very fast compared to other iron oxides, with 80% of total As(V) adsorbed within 10 min. Examination by extended X-ray absorption fine structure analysis showed that As(V) forms a bidentate inner-sphere surface complex with SSI. Arsenic(V) adsorption isotherms and adsorption envelopes were well described using a 1-pK Basic Stern surface complexation model involving protonated (Fe2O2AsO2H−) and unprotonated (Fe2O2AsO2−2) inner-sphere surface complexes. The surface complexation constants for As(V) binding to SSI is similar to constants found for goethite and ferrihydrite. Simulated adsorption isotherms for intermediate As(V) concentrations also demonstrate that SSI is performing equally well as goethite and ferrihydrite based on surface area normalized adsorption capacities. The binding affinities at micromolar to submicromolar As(V) solution concentrations are similar for SSI and ferrihydrite. SSI has interesting potential as a stable, high-affinity sorbent for use in applications where efficient and fast removal is required.

Computing the temperature dependence of adsorption selectivity in porous solids

We describe a method that computes gas adsorption equilibria in porous solids, at arrays of temperature and mixture compositions. The method uses an initial dataset of single component adsorption isotherms, obtained either from experiment or grand canonical Monte Carlo simulations. It utilizes the Clausius Clapeyron equation to expand the number of the available isotherms for the adsorbate components at a specified sequence of temperatures. It employs the ideal adsorbed solution theory (IAST) on the expanded data for the complete range of mole fractions in the mixture. We show case the method considering experimental isotherms of CO2 and CH4 adsorption at near ambient temperatures, for an activated carbon cloth, an ordered mesoporous carbon and on the alpha phase of magnesium formate. In the same context, we produce a set of simulated adsorption isotherms for a zeolitic imidazolate framework, namely ZIF69. The adsorbate mixture capacities of CO2 and CH4 on ZIF69, predicted by the IAST method are validated with explicit mixture adsorption simulations performed at the same conditions of interest. The method is appropriate for fast screening the selectivity performance of porous solids aiming to minimize the effort regarding experiments for multicomponent physisorption.

Hydrogen Adsorption in a Zeolitic Imidazolate Framework with lta Topology

The adsorption of H2 in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn2+ ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H2 adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H2 adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (Qst) value for H2 in ZIF-76 was determined to be 7.7 kJ mol–1, which is comparable to that for other ZIFs and is fairly high for a material that does...

Adsorptive separation of carbon dioxide from flue gas using mesoporous MCM-41: A molecular simulation study

Realistic molecular models of MCM-41 have been developed and used for studying the separation of carbon dioxide from flue gas mixtures using grand canonical Monte Carlo simulations. The simulated X-ray diffraction pattern and surface area of the models are in good agreement with experimental results reported in literature. Adsorption of pure carbon dioxide was studied on the three different models at two different temperatures, 273.2 K and 303.2 K. Isosteric heats of adsorption of CO2 calculated from the simulations were in the range 20–25 kJ/mol, which matches well with reported experimental values. The simulated CO2 adsorption isotherms showed good agreement with experimental isotherms at both the temperatures for two of the models, which were selected for further mixture adsorption studies. Binary CO2/N2 adsorption simulations were performed at different bulk gas compositions, and the selectivities of CO2 over N2 were observed to be in the range 4-10. Further studies on adsorption of ternary and quaternary bulk gas mixtures containing water vapor and O2 in addition to CO2 and N2 did not reveal any significant effect on CO2 adsorption and CO2-N2 selectivity.

Grand canonical Monte Carlo simulations of pore structure influence on methane adsorption in micro-porous carbons with applications to coal and shale systems

Coal and shale are strong heterogeneous anisotropic media involving nanoscale pore size and variance of microstructure. The complexity of methane adsorption is expressed both in diverse chemical properties and confined pore structures. In this study, Grand canonical Monte Carlo simulations were carried out to assess the influence of pore structure on methane adsorption at temperature 318 K, 333 K and pressure up to 20 MPa. The pore radii of physical carbon-based model range from 0.55 nm to 1.15 nm at the step of 0.1 nm. Simulated results indicate that the excess adsorption isotherms and maximum excess adsorption density are notably different for different pore structures. The triangle pore exhibits largest value of maximum excess adsorption density followed by the slit pore, circle pore and square pore. The maximum excess adsorption density is larger than 6 × 103 mol/m3 at simulated temperatures for triangle pore with pore radius less than 1 nm. The excess adsorption amount first increases with the increase of pressure and then decreases when the pressure is larger than 7.5 MPa for slit pore and 5 MPa for the circle pore, triangle pore and square pore. The excess adsorption amount for circle pore and square pore drops down to negative value when the pressure is larger than 12.5 MPa while the excess adsorption amount stays above zero across simulated pressure for the slit pore and triangle pore. The adsorption isotherms of micro-porous carbons were obtained by superposition of simulated adsorption isotherms based on the pore size distribution and were compared with coal samples experimental data gathered from the same temperature. The experimental isotherm is more close to slit pore excess isotherm and predicted excess isotherms based on circle pore and square pore under-estimate excess adsorption capacity.

Adsorption and Depletion Regimes of a Nonionic Surfactant in Hydrophilic Mesopores: An Experimental and Simulation Study.

Adsorption and aggregation of nonionic surfactants at oxide surfaces has been studied extensively in the past, but only for concentrations below and near the critical micelle concentration. Here we report an adsorption study of a short-chain surfactant (C6E3) in porous silica glass of different pore sizes (7.5 to 50 nm), covering a wide composition range up to 50 wt % in a temperature range from 20 °C to the LCST. Aggregative adsorption is observed at low concentrations, but the excess concentration of C6E3 in the pores decreases and approaches zero at higher bulk concentrations. Strong depletion of surfactant (corresponding to enrichment of water in the pores) is observed in materials with wide pores at high bulk concentrations. We propose an explanation for the observed pore-size dependence of the azeotropic point. Mesoscale simulations based on dissipative particle dynamics (DPD) were performed to reveal the structural origin of this transition from the adsorption to the depletion regime. The simulated adsorption isotherms reproduce the behavior found in the 7.5 nm pores. The calculated bead density profiles indicate that the repulsive interaction of surfactant head groups causes a depletion of surfactant in the region around the corona of the surface micelles.

On the evolution of the heat spike in the isosteric heat versus loading for argon adsorption on graphite-A new molecular model for graphite & reconciliation between experiment and computer simulation

We have carried out an extensive computer simulation of argon adsorption on graphite at temperatures in the range 40 K–100 K, using a new molecular model for graphite, and compared our simulation results with new high-resolution experimental data. The new model accounts for: (1) The energetic corrugation of the graphene surface. (2) The smaller collision diameter of the carbon atoms (0.28 nm) in the outermost graphene layer compared to 0.34 nm in the lower layers. (3) The increase in the interaction energy well depth between argon and the carbon atoms of the outermost layer. (4) The closer spacing between the first and second layers compared to that between the inner layers. The simulated adsorption isotherms and isosteric heats give an improved description of the experimental data, especially in capturing the low temperature transition from a 2D liquid to a solid-like adsorbate and subsequently to an incommensurate solid, and of the variation in these properties with respect to temperature. Analysis of the isosteric heat versus loading reveals details of the rearrangement of the adsorbed molecules during the layer transition.

Computer simulations of the structure and properties of organic pillared MFI zeolite catalyst

Multifunctional porous hybrid materials have recently received a lot of attentions because they combine the advantages of incorporating both inorganic oxides and organic species into the structure to solve the limitation of sole component material. This paper reports a type of model of organic pillared MFI zeolite catalyst, in which the full-atom mimetic model is constructed by using molecular modeling technique. Dreiding force field is used to calculate the interactions between sorbate–sorbate and sorbate–sorbent in the organic pillared MFI zeolite. The optimized model is characterized by calculating low-angle and high-angle X-ray diffraction patterns. Besides, the adsorption behavior of benzene and toluene is also investigated through the mimetic model. Simulated adsorption isotherms of benzene and toluene are agreement with the experimental data at low pressure. Density distributions of benzene and toluene show that toluene adsorbs preferentially in the region of interlayer space of lamellar MFI zeolite while benzene tends to adsorb in the area of micropores at low pressure. With increasing pressure, the guest molecules gradually fill in the area of micropores and mesopores of the organic pillared MFI zeolite. Moreover, the developed strategy in the present work can be applied to build other model for organic pillared zeolite catalyst by arranging different zeolite frameworks or organic species.

Atomistic models of hydroxylated, ethoxylated and methylated silica surfaces and nitrogen adsorption isotherms: A molecular dynamics approach

In this paper, we have developed realistic models of hydroxylated, ethoxylated and methylated silica surfaces. They represent several sol-gel materials, among which a functionalized methylated silica used for chemical sensor applications. The functional groups were tuned in order to match the experimental data available. The creation of the silica models implies after the cutting of a bulk of amorphous silica, to add covalent OH, –OCH2-CH3, or -CH3 functions at the surface while keeping the ionic characteristic of the core. In total, five models of silica surface were built. The number OH groups for the hydroxylated silica were adjusted to match the Zhuravlev constant (αOH = 4.56 OH nm−2) and the repartition of the different types of silanols was found to be in good agreement with previous work with 80% of SiOH, 19% of Si(OH)2 and 1% Si(OH)3. In addition, the proportions of vicinal and isolated hydroxyls were distinguished. The non-functionalized (Silica-OEt/OHCEA) and functionnalized silica (Silica-Me2/OHCEA) contain respectively 2 OEt nm−2 and 3 OH nm−2and 3 SiMe2 nm−2 and 2 OH nm−2. Further, we have simulated nitrogen adsorption isotherms using Molecular Dynamics. Contribution of the functionalization to the surface behavior was shown not to be significative. The utilization of the CP and VSH models yields similar results. The isotherms were analyzed in terms of relaxation functions. The N2 adsorption and desorption were predicted to occur through first order mechanisms with an adsorption rate constant of 0.031 ps−1. Agreement between the simulated curves and experimental BET results is quite remarkable. This work would contribute to the development of chemical sensors as such surfaces will allow accurate investigations of the interaction of target molecules at a molecular level.

An atomistic model of a disordered nanoporous solid: Interplay between Monte Carlo simulations and gas adsorption experiments

A combination of physisorption measurements and theoretical simulations was used to derive a plausible model for an amorphous nanoporous material, prepared by Friedel-Crafts alkylation of tetraphenylethene (TPM), leading to a crosslinked polymer of TPM connected by methylene bridges. The model was refined with a trial-and-error procedure, by comparing the experimental and simulated gas adsorption isotherms, which were analysed by QSDFT approach to obtain the details of the porous structure. The adsorption of both nitrogen at 77 K and CO2 at 273 K was considered, the latter to describe the narrowest pores with greater accuracy. The best model was selected in order to reproduce the pore size distribution of the real material over a wide range of pore diameters, from 5 to 80 Å. The model was then verified by simulating the adsorption of methane and carbon dioxide, obtaining a satisfactory agreement with the experimental uptakes. The resulting model can be fruitfully used to predict the adsorption isotherms of various gases, and the effect of chemical functionalizations or other post-synthesis treatments.

Quantitative evaluation of the responses of a gravimetric gas sensor based on mesoporous functionalized silica: Application to 2,4-DNT and TNT detection

The literature reports several attempts to predict the responses of gas sensors using as only input the molecular structures of the analyte and sensing material. However, these are restricted to relatively simple and homogeneous materials. The present paper reports for the first time the semiquantitative estimation of the sensing responses of quartz crystal microbalance sensors based on a complex material, taking relative humidity into account. The sensing material is methylated mesoporous silica, and the analytes are vapors of the nitroaromatic compounds 2,4-dinitrotoluene (2,4-DNT) and trinitrotoluene (TNT). Our approach is based on molecular dynamics simulations. The amount of vapor (water, 2,4-DNT or TNT) adsorbed on the sensing surfaces is obtained as a function of their partial pressure by simulating adsorption isotherms at high temperature and extrapolating the results to ambient conditions. Furthermore, a correction factor is introduced to account for the role of residual water within the mesoporous silica. The order of magnitude of the simulated sensing responses to 2,4-DNT and TNT is found to be consistent with experimental data.

Accurate Evaluation of the Dispersion Energy in the Simulation of Gas Adsorption into Porous Zeolites.

The force fields used to simulate the gas adsorption in porous materials are strongly dominated by the van der Waals (vdW) terms. Here we discuss the delicate problem to estimate these terms accurately, analyzing the effect of different models. To this end, we simulated the physisorption of CH4, CO2, and Ar into various Al-free microporous zeolites (ITQ-29, SSZ-13, and silicalite-1), comparing the theoretical results with accurate experimental isotherms. The vdW terms in the force fields were parametrized against the free gas densities and high-level quantum mechanical (QM) calculations, comparing different methods to evaluate the dispersion energies. In particular, MP2 and DFT with semiempirical corrections, with suitable basis sets, were chosen to approximate the best QM calculations; either Lennard-Jones or Morse expressions were used to include the vdW terms in the force fields. The comparison of the simulated and experimental isotherms revealed that a strong interplay exists between the definition of the dispersion energies and the functional form used in the force field; these results are fairly general and reproducible, at least for the systems considered here. On this basis, the reliability of different models can be discussed, and a recipe can be provided to obtain accurate simulated adsorption isotherms.

Molecular simulations of adsorption and separation of acetylene and methane and their binary mixture on MOF-5, HKUST-1 and MOF-505 metal–organic frameworks

In this work, the adsorption of acetylene and its binary mixture with methane on MOF-5, HKUST-1 and MOF-505 was studied using Grand Canonical Monte Carlo molecular simulations. The preferred adsorption sites of acetylene and methane molecules into metal–organic frameworks (MOFs) were investigated. The simulated adsorption isotherms of acetylene on MOF-5 and MOF-505 agreed well with the experimental ones without any reparameterisation of the potential parameters but for HKUST-1 the interaction parameters of the acetylene and copper ion were reparameterised. Comparisons of the calculated adsorption isotherms of acetylene in the studied MOFs showed that the MOF-5 had the lowest adsorption capacity. Our results revealed that guest molecules were most adsorbed on the entrance windows of the octagon pore of HKUST-1, while the preferred adsorption sites were large pores and on the metal ion cluster of MOF-505 and MOF-5, respectively. Adsorption of binary mixtures of methane and acetylene on MOF-5, HKUST-1 and MOF-505 revealed that acetylene adsorption is higher than that of methane. Finally, the results showed that C2H2/CH4 selectivity values on HKUST-1 are significantly higher than on MOF-505 and MOF-5. The preferred adsorption sites of acetylene and methane in an equimolar binary mixture were calculated and discussed.

Adsorption of Ar and SO2 on graphitized carbon black: The importance of functional groups

Grand Canonical Monte Carlo (GCMC) simulation was used to determine the isotherms and isosteric heats of argon and the strongly polar molecule, sulphur dioxide (SO2), adsorbed at 78 and 273 K on a graphitized thermal carbon black (GTCB) surface with functional groups. The functional group, was modelled as oxygen atoms bonded to a C-atom in the graphene surface, since these have been shown to be retained after thermal treatment of GTCB. The simulated adsorption isotherms and isosteric heats of argon and SO2 were compared with the experimental data. It is shown that, while functional groups do not affect the adsorption of argon, adsorption of SO2 is very sensitive to their concentration, especially at low loadings, where the adsorption is dominated by the electrostatic interaction between SO2 and the functional group. This is confirmed by analysis of the various contributions to the isosteric heat: (1) fluid-functional group interactions, (2) fluid-basal plane interactions, and (3) fluid–fluid interactions. Finally, we investigated the orientation of SO2 in the first and second layers depends on loading as well as on the distance of the molecule from the surface.

Molecular dynamics simulation study: The decryption of bi and tri aromatics behavior with NaX zeolite

Molecular dynamics simulations have been carried out to provide an atomic description of the behavior of naphthalene and anthracene in NaX zeolite. The force field parameters, which were selected in this process, were chosen carefully to examine dependence of the self-diffusion coefficient of sorbates over a wide range of loading, temperature, and pressure. The simulated adsorption isotherm and calculated adsorption energies at low concentration were found to be in a reasonable qualitative and quantitative agreement with the corresponding scarce experimental data which can evaluate the effectiveness of proposed calculation method and force field parameters. The simulations provided new insights into the simulated concentration dependence of the atomic behavior of bi and tri aromatics inside NaXmicropores. Collective effects of the mutual interactions of sorbates molecules competing for the most preferable sites at the supercages (SCs) were found to be key factors responsible for the observed behavior of the adsorption isotherms, heat of adsorption, activation energy, and self-diffusivity. All involved calculations were performed in time period 6ns and repeated calculations have been done at least two times to confirm the results and use the average values, which made the results being reliable.

On the explanation of hysteresis in the adsorption of ammonia on graphitized thermal carbon black.

We present a Monte Carlo simulation and experimental study of ammonia adsorption on graphitized thermal carbon black. Our new molecular model for the adsorbent is composed of basal plane graphene surfaces with ultrafine pores grafted with hydroxyl groups at the junctions between graphene layers. The simulated adsorption isotherms and isosteric heats are in good agreement with the experimental data of Holmes and Beebe, and the simulations reproduce the unusual experimental hysteresis of ammonia adsorption on an open graphite surface for the first time in the literature. The detailed mechanisms of adsorption and desorption, and the origin of hysteresis, are investigated by the microscopic analysis of the adsorbate structures to show that restructuring occurs during adsorption. The main results from this work are: (i) at the triple point, ammonia adsorbs preferentially around the functional groups to form clusters in the ultrafine pores and spills-over onto the basal plane as the loading is increased; followed by a 2D condensation on the graphite surface to form a bilayer adsorbate; (ii) at the boiling point, adsorption occurs on the basal plane due to the increasing importance of thermal fluctuations (an entropic effect); (iii) the isosteric heat is very high at zero loading due to the strong interaction between ammonia and the functional groups, decreases steeply when the functional group is saturated, and eventually reaches the heat of condensation as the fluid-fluid interaction increases.

Structure, dynamics and stability of water/scCO2/mineral interfaces from ab initio molecular dynamics simulations.

The boundary layer at solid-liquid interfaces is a unique reaction environment that poses significant scientific challenges to characterize and understand by experimentation alone. Using ab initio molecular dynamics (AIMD) methods, we report on the structure and dynamics of boundary layer formation, cation mobilization and carbonation under geologic carbon sequestration scenarios (T = 323 K and P = 90 bar) on a prototypical anorthite (001) surface. At low coverage, water film formation is enthalpically favored, but entropically hindered. Simulated adsorption isotherms show that a water monolayer will form even at the low water concentrations of water-saturated scCO2. Carbonation reactions readily occur at electron-rich terminal Oxygen sites adjacent to cation vacancies that readily form in the presence of a water monolayer. These results point to a carbonation mechanism that does not require prior carbonic acid formation in the bulk liquid. This work also highlights the modern capabilities of theoretical methods to address structure and reactivity at interfaces of high chemical complexity.