Gas adsorption in active carbons and the slit-pore model 1: Pure gas adsorption

We describe procedures based on the polydisperse independent ideal slit-pore model, Monte Carlo simulation and density functional theory (a 'slab-DFT') for predicting gas adsorption and adsorption heats in active carbons. A novel feature of this work is the calibration of gas-surface interactions to a high surface area carbon, rather than to a low surface area carbon as in all previous work. Our models are used to predict the adsorption of carbon dioxide, methane, nitrogen, and hydrogen up to 50 bar in several active carbons at a range of near-ambient temperatures based on an analysis of a single 293 K carbon dioxide adsorption isotherm. The results demonstrate that these models are useful for relatively simple gases at near-critical or supercritical temperatures.     

Examining the accuracy of ideal adsorbed solution theory without curve-fitting using transition matrix Monte Carlo simulations

Ideal adsorbed solution theory (IAST) is a well-known approach to predicting multicomponent adsorption isotherms in microporous materials from experimental or simulation data for single-component adsorption. A limitation in practical applications of IAST is that useful calculations often require extrapolation of fitted single-component isotherms beyond the range for which data are available. We introduce a molecular simulation approach in which the intrinsic accuracy of IAST can be examined in a context that avoids any need to perform curve fitting with single-component data. Our approach is based on using transition matrix Monte Carlo to define single-component adsorption isotherms for arbitrary bulk-phase pressures from a single simulation. We apply our approach to several light gas mixtures in silica zeolites and a carbon nanotube to examine the intrinsic accuracy of IAST for these model systems.     

Adsorption equilibrium of binary methane/ethane mixtures in BPL activated carbon: isotherms and calorimetric heats of adsorption

The adsorption of pure methane and ethane in BPL activated carbon has been measured at temperatures between 264 and 373 K and at pressures up to 3.3 MPa with a bench-scale high-pressure open-flow apparatus. The same apparatus was used to measure the adsorption of binary methane/ethane mixtures in BPL at 301.4 K and at pressures up to 2.6 MPa. Thermodynamic consistency tests demonstrate that the data are thermodynamically consistent. In contrast to two sets of data previously published, we found that the adsorption of binary methane/ethane in BPL behaves ideally (in the sense of obeying ideal adsorbed solution theory, IAST) throughout the pressure and gas-phase composition range studied. A Tian-Calvet type microcalorimeter was used to measure low-pressure isotherms, the isosteric heats of adsorption of pure methane and ethane in BPL activated carbon, and the individual heats of adsorption in binary mixtures, at 297 K and at pressures up to 100 kPa. The mixture heats of adsorption were consistent with IAST.     

Large-scale computational screening of zeolites for ethane/ethene separation

Large-scale computational screening of thirty thousand zeolite structures was conducted to find optimal structures for separation of ethane/ethene mixtures. Efficient grand canonical Monte Carlo (GCMC) simulations were performed with graphics processing units (GPUs) to obtain pure component adsorption isotherms for both ethane and ethene. We have utilized the ideal adsorbed solution theory (IAST) to obtain the mixture isotherms, which were used to evaluate the performance of each zeolite structure based on its working capacity and selectivity. In our analysis, we have determined that specific arrangements of zeolite framework atoms create sites for the preferential adsorption of ethane over ethene. The majority of optimum separation materials can be identified by utilizing this knowledge and screening structures for the presence of this feature will enable the efficient selection of promising candidate materials for ethane/ethene separation prior to performing molecular simulations.     

Molecular simulation investigation into the performance of Cu-BTC metal-organic frameworks for carbon dioxide-methane separations

We report a molecular simulation study for Cu-BTC metal-organic frameworks as carbon dioxide-methane separation devices. For this study we have computed adsorption and diffusion of methane and carbon dioxide in the structure, both as pure components and mixtures over the full range of bulk gas compositions. From the single component isotherms, mixture adsorption is predicted using the ideal adsorbed solution theory. These predictions are in very good agreement with our computed mixture isotherms and with previously reported data. Adsorption and diffusion selectivities and preferential sitings are also discussed with the aim to provide new molecular level information for all studied systems.     

Monte Carlo simulation of mixed lennard-jones nonionic surfactant adsorption at the liquid/vapor interface

New Monte Carlo simulations are presented for nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent specifically investigating the roles of tail attraction and binary mixtures of different tail lengths. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential. Adjacent atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method. We generate thermodynamic adsorption and surface-tension isotherms and compare results quantitatively to single-surfactant adsorption (Langmuir, 2007, 23, 1835). Surfactant tail groups with attractive interaction lead to cooperative adsorption at high surface coverage and higher maximum adsorption at the interface than those without. Moreover, adsorption and surface-tension isotherms with and without tail attraction are identical at low concentrations, deviating only near maximum coverage. Simulated binary mixtures of surfactants with differing lengths give intermediate behavior between that of the corresponding single-surfactant adsorption and surface-tension isotherms both with and without tail attraction. We successfully predict simulated mixture results with the thermodynamically consistent ideal adsorbed solution (IAS) theory for binary mixtures of unequal-sized surfactants using only the simulations from the single surfactants. Ultimately, we establish that a coarse-grained LJ surfactant system is useful for understanding actual surfactant systems when tail attraction is important and for unequal-sized mixtures of amphiphiles.     

高比表面活性碳微球分离H2中少量CO2

以实验数据为依据, 结合双Langmuir模型研究了用高比表面活性碳微球材料分离H2中少量CO2的行为. 在实验中, 用高精度的IGA-003重力吸附仪测定了温度为298、273 和268 K, 压力在0-1.8 MPa范围内CO2、H2及n(CO2):n(H2)=1:9混合物在活性碳微球中的吸附等温线. 比较不同吸附模型的计算结果与实验数据, 结果表明, 双Langmuir模型与实验结果拟合得较好; 而且通过结合理想吸附溶液理论, 该模型可以准确地计算不同的混合物体系(包括H2-CO2体系)的吸附量和吸附选择性. 利用该模型求解了不同温度下各组分的分吸附量, 得到了CO2的吸附选择性;在268 K和1.7 MPa下, CO2的吸附选择性可达到73.4, 表明活性碳微球是一种优秀的吸附H2中少量CO2的材料.     

Carbon dioxide-methane mixture adsorption on activated carbon

In this work, we report new experimental data of pure and binary adsorption equilibria of carbon dioxide and methane on the activated carbon RB2 at 273 and 298 K. The pressure range studied were 0–3.5 MPa for pure gases and 0–0.1 MPa for mixtures. The combination of the generalized Dubinin model to describe the pure CO₂ and CH₄ isotherms with the IAST (Ideal Adsorbed Solution Theory) for the mixtures provide a method for the calculation of the binary adsorption equilibria. This formulation predicts with acceptable accuracy the binary adsorption data and can easily be integrated in general dynamic simulation of PSA (pressure swing adsorption process) adsorption columns. It involves only three parameters, independent of the temperature, and directly determined with only one adsorption isotherm of CO₂.     

Adsorption equilibria of binary gas mixtures on graphitized carbon black

Adsorption equilibria for binary gas mixtures (methane-carbon dioxide, methane-ethane, and carbon dioxide-ethane) on the graphitized carbon black STH-2 were measured by the open flow method at 293.2 K. The experimental pressure range was (0 to 1.6) MPa. The extended Langmuir (EL) model and the ideal adsorption solution theory (IAST) have been adopted to predict the equilibria of binary gas mixtures. The results indicate that gas mixtures adsorbed on the homogeneous surface of STH-2 exhibit the nonideal behavior, which is mainly induced by adsorbate-adsorbate interactions. The real adsorption solution theory (RAST) has been used to analyze the property of the adsorbed mixtures. The activity coefficients have been correlated with the Wilson equation. The investigation demonstrates that the nonideality of adsorbed phase is completely dissimilar with the bulk liquid phase. The adsorption of the heavier component would benefit the adsorption of the lighter component.     

Determination of the micropore volume distribution function of activated carbons by gas adsorption

A new method for the determination of the micropore volume distribution function of activated carbons is presented. It is based on the treatment of pure gas adsorption isotherms by a theoretical model derived from the Hill-de Boer theory. Adsorption data (isotherms and heat curves) for carbon dioxide, ethane and ethylene on activated carbon (F30/470 CHEMVIRON CARBON) have been provided by a thermobalance coupled to a calorimeter (TG-DSC 111 SETARAM) at different temperatures (233, 273, 303 and 323 K) for pressures up to 100 kPa. Adsorption isotherms of carbon dioxide and ethane at 303 and 323 K have been used for the determination of the micropore volume distribution function of the activated carbon of interest. The knowledge of its structure has then allowed the simulation of adsorption isotherms and heats for the same adsorbates at the same temperatures as those experimentally studied. Similar calculations have been conducted for ethylene. Whatever the adsorbate (carbon dioxide and ethane used for the determination of the micropore volume distribution function or ethylene), the mean deviation between experimental and calculated isotherms does not exceed 4% at quasicritical and supercritical temperatures (303 and 323 K). In the same temperature conditions, discrepancies between calculation and experiment reach about 10% for adsorption heats. For both isotherms and heats, large discrepancies appear at low temperature (233 and 273 K). This method allows the determination of the micropore volume distribution function of activated carbons. The validity of the results is insured using several isotherms of several adsorbates and taking into account the calorimetric effect of the phenomenon. That is the reason why this method can also be seen as a new possible model for pure gas adsorption data prediction. This paper also presents a brief summary of the state of the art in this field.     

Capillary condensation and adsorption of binary mixtures

The adsorption of equimolar binary mixtures of hydrogen-carbon dioxide, hydrogen-methane, and methane-carbon dioxide in porous material models is determined by grand canonical Monte Carlo simulations. The material models have an adsorbent surface similar to that of nanofibers with a herringbone structure. Our main result, which is relevant for hydrogen purification and carbon dioxide capture, is that the adsorption selectivities calculated for the mixtures can differ significantly from those deduced from simulations of the adsorption of pure gases, in particular, when one of the adsorbed gases presents a capillary condensation induced by confinement within the pore network. A comparison of our data is also made with theoretical models used in the literature for predicting the properties of the mixture adsorption.     

二氧化碳/甲烷/氮气二元混合物在有序介孔碳材料CMK-3 中的吸附和分离

采用分子模拟与吸附理论研究了天然气成分在有序介孔碳材料CMK-3上的吸附和分离.巨正则系综蒙特卡罗(GCMC)模拟表明,CH4和CO2气体的较优存储条件分别为208 K、4 MPa和298 K、6 MPa,其最大超额吸附量分别为10.07和14.85 mmol· g-1.基于双位Langmuir-Freundlich (DSLF)模型,使用理想吸附溶液理论(IAST)预测了不同二元混合物在CMK-3中的分离行为,发现吸附选择性Sco2/CH4与ScH4/N2比较接近,在298 K和4 MPa下约等于3,而N2-CO2体系中的CO2吸附选择性较高,可达到7.5,说明CMK-3是一种适合吸附和分离天然气组分的碳材料.     

Effects of molecular siting and adsorbent heterogeneity on the ideality of adsorption equilibria

The ideal adsorbed solution (IAS) theory is the benchmark for the prediction of mixed-gas adsorption equilibria from pure-component isotherms. In this work, we use atomistic grand canonical Monte Carlo simulations to test the effects of molecular siting and adsorbent energetic heterogeneity on the applicability of the IAS theory. Pure-component isotherms generated by atomistic simulation are used to predict binary isobaric isotherms using the IAS theory. These predicted isotherms are compared with those obtained by a full atomistic simulation of the binary mixture. Binary mixtures of argon, methane, and CF4 in silicalite are found to obey IAS theory, while benzene/methane and cyclohexane/methane in silicalite are nonideal. The mixture of argon and CF4 is ideal despite the large difference in the sizes of the two species. This contradicts previous hypotheses in the literature, which state that mixtures of species of unequal size do not adsorb ideally. The nonideal behavior of the benzene/methane and cyclohexane/methane systems occurs because of adsorbent heterogeneity in these systems, which depends on both sorbent and sorbate. In addition, we use a lattice gas model with parameters derived from atomistic simulation to demonstrate analytically that a sufficiently energetically heterogeneous adsorbent will result in the breakdown of IAS theory even in the absence of interactions between sorbates.     

Prediction of high-pressure adsorption equilibrium of supercritical gases using density functional theory

In this paper, we present the results of the prediction of the high-pressure adsorption equilibrium of supercritical gases (Ar, N2, CH4, and CO2) on various activated carbons (BPL, PCB, and Norit R1 extra) at various temperatures using a density-functional-theory-based finite wall thickness (FWT) model. Pore size distribution results of the carbons are taken from our recent previous work,(1,2) using this approach for characterization. To validate the model, isotherms calculated from the density functional theory (DFT) approach are comprehensively verified against those determined by grand canonical Monte Carlo (GCMC) simulation, before the theoretical adsorption isotherms of these investigated carbons calculated by the model are compared with the experimental adsorption measurements of the carbons. We illustrate the accuracy and consistency of the FWT model for the prediction of adsorption isotherms of the all investigated gases. The pore network connectivity problem occurring in the examined carbons is also discussed, and on the basis of the success of the predictions assuming a similar pore size distribution for accessible and inaccessible regions, it is suggested that this is largely related to the disordered nature of the carbon.     

Adsorption of benzene and propene in zeolite MCM-22: a grand canonical Monte Carlo study

The GCMC (grand canonical Monte Carlo) simulation technique was used to predict the competition adsorption characteristics of benzene and propene in different pore systems of MCM-22. The nine-site model of benzene was used, which proved to be effective and efficient. The zeolite was divided into three adsorption sites following a simulated annealing method. It is found that benzene and propene have the same preferential adsorption site and a similar adsorption order in different sites. Moreover, the pure and mixture isotherms of the three sites are drawn. From the isotherms, we obtained a selectivity reversal of the mixture isotherms of benzene and propene in different sites. It is also noted that the competition adsorption in the three adsorption sites for the two adsorbates can fall into three successive steps and the adsorption order of propene in mixture in these three sites is S3→S1→S2. A new model is presented to predict the benzene and propene adsorption equilibrium in MCM-22. This approach yields better multicomponent equilibrium predictions than ideal adsorbed solution theory (IAST). Isotherms at different mole fraction of benzene in gas phase indicate an advantage to increase the feed radio of benzene and propene. Thus, this work is helpful for a better understanding of the adsorption mechanism of benzene and propene in MCM-22 and hence the relation of the catalytic properties of the zeolite to its structure.     

Modeling adsorption in metal-organic frameworks with open metal sites: propane/propylene separations

We present a new approach for modeling adsorption in metal-organic frameworks (MOFs) with unsaturated metal centers and apply it to the challenging propane/propylene separation in copper(II) benzene-1,3,5-tricarboxylate (CuBTC). We obtain information about the specific interactions between olefins and the open metal sites of the MOF using quantum mechanical density functional theory. A proper consideration of all the relevant contributions to the adsorption energy enables us to extract the component that is due to specific attractive interactions between the π-orbitals of the alkene and the coordinatively unsaturated metal. This component is fitted using a combination of a Morse potential and a power law function and is then included into classical grand canonical Monte Carlo simulations of adsorption. Using this modified potential model, together with a standard Lennard-Jones model, we are able to predict the adsorption of not only propane (where no specific interactions are present), but also of propylene (where specific interactions are dominant). Binary adsorption isotherms for this mixture are in reasonable agreement with ideal adsorbed solution theory predictions. We compare our approach with previous attempts to predict adsorption in MOFs with open metal sites and suggest possible future routes for improving our model.     

Molecular Modeling of Adsorption in Activated Carbon: Comparison of Monte Carlo Simulations with Experiment

We present Mont Carlo computer simulation results for a molecular model of fluids adsorbed in porous carbon materials. The model carbon used is based on the platelet model for carbon of Segarra and Glandt (1994). The model we use has a single basal plane per platelet and the structure is isotropic, disordered, with weak short-range correlations between the platelets. We have performed grand canonical Monte Carlo simulations of the adsorption isotherms for methane, ethane, and their mixtures in this model carbon. We find generally good agreement with experimental and the mixture results are quite accurately described by the ideal adsorbed solution theory. An exception to this is the behavior for the mixture at the highest pressures. In this case the experimental data show significant deviations from ideal adsorbed solution theory and the simulation results.     

Monte Carlo Simulations of Adsorbed Solutions in Heterogeneous Porous Materials

We present results of a Monte Carlo simulation study of binary mixtures of ethane and methane in silica gel. The molecular model treats the adsorbent as a matrix of silica microspheres. The adsorption isotherms, adsorption selectivities and isosteric heats of adsorption have been determined for these systems. The results are compared with predictions from the ideal adsorbed solution (IAS) theory and with experiment. The heats of adsorption are accurately described by the IAS theory. The adsorption isotherms are accurately described by the IAS theory at low bulk pressure but the IAS theory overpredicts the density at high bulk pressure. This latter effect is opposite to that observed in bulk mixtures of this type where nonidealities generally lead to a density increase on mixing. The pressure dependence of the selectivity does not exhibit a maximum at low pressure. We discuss this effect in terms of the adsorbent microstructure.     

DFT-based prediction of high-pressure H2 adsorption on porous carbons at ambient temperatures from low-pressure adsorption data measured at 77 K

Hydrogen adsorption isotherms were measured both at cryogenic temperatures below 1 atm and at ambient temperature at high pressures, up to 90 atm, on selected porous carbons with various pore structures. The nonlocal density functional theory (NLDFT) model was used to calculate the pore size distributions (PSDs) of the carbons, from H2 adsorption isotherms measured at 77 K, and then to predict H2 adsorption on these carbons at 87 and 298 K. An excellent agreement between the predicted and measured data was obtained. Prior to analyzing the porous carbons, the solid-fluid interaction parameters used in the NLDFT model were derived from H2 adsorption data measured at 77 K on nonporous carbon black. The results show that the NLDFT model with appropriate parameters may be a useful tool for optimizing carbon pore structures and designing adsorption systems for hydrogen storage applications.