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We report the adsorption isotherms and the isosteric heats of adsorption of pure methane, ethane, and CO2 and a mixture of methane and CO2 in the periodic mesoporous silica MCM-41 using a multicomponent adsorption calorimeter of the Tian−Calvet type, looking in particular at the degree of heterogeneity in the adsorption of these species. The adsorption of methane and ethane in MCM-41 was found to be essentially homogeneous, while the adsorption of pure CO2 and of CO2 from a CO2/methane mixture was found to be significantly heterogeneous, reflecting the electrostatic interactions between CO2 and the adsorbent.

The isosteric heat of adsorption of methane in an activated carbon adsorbent has been modeled by Monte Carlo simulation, using a pore-size distribution (PSD) to relate simulation results for pores of different sizes to the experimental adsorbent. Excellent fits were obtained between experimental and simulated isosteric heats of adsorption of methane in BPL activated carbon. The PSD was then used to predict the adsorption of methane and ethane in the same carbon adsorbent, with good results. The PSD derived from isosteric heat data was shown to be richer in information than PSDs obtained by the more conventional method of fitting to isotherm data.

We have carried out a percolation analysis of the adsorption of ethane and nitrogen in SBA-2, a structured mesoporous silica consisting of a hexagonal close-packed (hcp) array of spherical cavities connected by cylindrical channels. Our analysis explains the different uptakes of nitrogen and ethane in terms of the greater accessibility of the network to the smaller nitrogen molecule. The analysis also allows us to quantify the connectivity of the SBA-2 pore network. The effective coordination number of the cavities, defined as the average number of channels per cavity that are large enough to allow nitrogen to pass, is 4.9, much less than the theoretical maximum value of 12. Taking into account only the smaller set of channels large enough to admit ethane, the effective coordination number is 1.8, just above the percolation threshold of the network.

We have measured the adsorption of methane and ethane to high pressure on SBA-2, a structured mesoporous silica composed of spheres connected by narrow channels. The experimental data were analyzed by carrying out Monte Carlo simulations of adsorption in pore structure models of different complexity and then adjusting the parameters of the models to match the Monte Carlo results to the experimental data. We found that a model based on single-sized spherical cavities was inadequate and that it is necessary to explicitly account for the interconnecting channels. Further, we found that despite the basic regularity of the SBA-2 structure, it is necessary to allow for a distribution of the sizes of both the cavities and the channels. These size distributions were obtained by fitting the parameters of the model to the experimental adsorption data, revealing detailed structural information not previously known for this material. The channels were found to be 5−15 Å in diameter, while the cavities were 40−50 Å in diameter. There is some evidence that the distribution of channel sizes leads to a percolation effect whereby the pore structure is not equally accessible to all adsorptives.

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.

The adsorption of pure ethane and carbon dioxide, and binary mixtures of these components, in MCM-41 has been studied experimentally and by grand canonical Monte Carlo (GCMC) simulation, at temperatures between 264 and 303 K and pressures up to 3 MPa. The experimental isotherms were measured using a bench-scale, open-flow adsorption/desorption apparatus. The simulations were carried out using three different models for MCM-41 with different degrees of surface heterogeneity. Model 1 has a nearly homogeneous pore surface while model 2, which is derived from a matrix of crystalline α-quartz, has a heterogeneous but nevertheless regular surface. These two models give generally good predictions of the adsorption of ethane in MCM-41, except at low pressures where the surface heterogeneity of MCM-41 dominates the adsorption. Model 3 has an amorphous structure, generated by an energy-minimization procedure; this model gives better predictions for ethane adsorption, especially at low pressures, suggesting that it incorporates a good representation of the heterogeneity of the real MCM-41 material. Excellent predictions of the adsorption of pure carbon dioxide and binary mixtures of ethane and carbon dioxide in MCM-41 are obtained with model 3, further confirming the realism of this model. Long-ranged electrostatic interactions are included for the simulation of carbon dioxide; these interactions, which play an important role, are treated by a simple one-dimensional summation method, which gives an accurate calculation of the potential.

In this paper we present results from the molecular simulation of water adsorption in slit-shaped activated carbon pores. We calculate adsorption isotherms by grand canonical Monte Carlo (GCMC) simulation, Henry's constants by Monte Carlo integration, and vapor−liquid equilibrium data by the gauge-cell Monte Carlo method, to investigate the chemical heterogeneity of activated carbon adsorbents. Several types of polar oxygen-containing sites are placed on the surface of the carbon with different densities and local distributions, in order to determine the individual effects of each of these factors on the adsorption of water. Our results confirm the role of surface sites in the enhancement of water adsorption. Furthermore, we show that the local distribution of these sites has a strong effect on low-pressure adsorption, while the overall site density affects mainly the vapor−liquid phase transition. The type of oxygen-containing group is shown not to be of critical importance, since more complex groups can effectively be represented by simpler sites. This study forms the basis for the development of a model for activated carbon that is able to represent the chemical heterogeneity of this type of material.

We report experimental measurements of the adsorption of methane and ethane, and binary mixtures of these components, on MCM-41. This adsorbent, which has a simple, regular pore structure and a chemically homogeneous surface, was used as a model system on which to evaluate methods for the prediction of adsorption equilibrium. Two methods were evaluated:  ideal adsorbed solution theory and grand canonical Monte Carlo simulation. Both methods gave accurate predictions of binary adsorption. In addition, the grand canonical Monte Carlo method was able to accurately predict pure-component adsorption.

The aim of this work is to predict the adsorption of pure-component and binary mixtures of methane and carbon dioxide in a specific activated carbon, A35/4, using grand canonical Monte Carlo (GCMC) simulation. Methane is modeled as a one-center Lennard-Jones (LJ) fluid and carbon dioxide as a two-center LJ plus point quadrupole fluid. Experimental adsorption data for the system have been obtained with a new flow desorption apparatus. The pore size distribution (PSD) for the carbon was determined from both of the experimental CH4 and CO2 isotherms at 293 K. To extract numerically the PSD, GCMC-simulated isotherms for both pure components in slit-shaped pores ranging from 5.7 to 72.2 Å were used. Using only pure experimental CO2 isotherm data, it was not possible to determine a PSD that allowed a reasonable prediction of the pure methane adsorption. However, with both experimental data sets for the pure components, it was possible to derive a PSD that allowed both experimental pure-component isotherms to be fitted. With this PSD and the simulated adsorption densities in single pores, it was possible to predict in good agreement with experiment (i) the adsorption of binary mixtures of CO2 and CH4 and (ii) the adsorption of both pure components at higher temperatures. However, the model was unable to reproduce precisely the experimental pressure dependence of the CO2 selectivity.

The coordination number of the pore network of a mesoporous solid can be determined from its nitrogen adsorption and desorption isotherms, using an analysis method based on percolation theory. However, measurement of these isotherms is time-consuming and problematical very close to the bulk condensation pressure of nitrogen. In this paper, we demonstrate that mercury intrusion and nitrogen desorption share the same mechanism so that the mercury intrusion curve can be used in place of the nitrogen desorption isotherm in the connectivity analysis.

The ability to determine whether a pore size distribution can be fitted to a given set of experimental data is relevant to the development of pore network models of the internal structure of activated carbons and other amorphous adsorbents. A closely related problem that arises when pore size distributions can be fitted to a given set of data is to determine suitable methods to calculate reliable, stable, and representative pore size distributions. This paper addresses both of these issues by presenting a procedure to determine whether a pore size distribution based on a specified model pore geometry can be fitted to a set of experimental data and then using this procedure, in conjunction with regularization techniques that stabilize the calculations, to determine statistically significant pore size distributions. This approach has been demonstrated using the adsorption of methane at 308 K onto BPL-activated carbon as a case study.

Nitrogen sorption is a commonly used experimental technique for the characterization of solids that are primarily mesoporous. The standard nitrogen adsorption experiment can also detect pores at the lower end of the macropore range, up to perhaps 3000 or 4000 Å. However, some solids of industrial importance contain macropores beyond the size range probed by the standard nitrogen adsorption experiment. In this work, a modified experimental method is used to obtain a more complete characterization of the solid. The experiment involves the bulk condensation of nitrogen in the sample tube after completing the adsorption isotherm and before measurement of the desorption isotherm. Modifications to existing pore size distribution and connectivity analyses are proposed to incorporate data from the experimental desorption isotherms. These modified analyses enable an improved estimate for the pore size distribution and a lower bound on the mean coordination number of the pore network to be obtained for these samples.

Predicting adsorption over a range of operating conditions and the improvement of the adsorbent itself are two important aspects that arise in the industrial application of adsorption. Both of these aspects can be addressed using molecular simulation techniques in conjunction with an appropriate model of the internal structure of the adsorbent. The internal structure of activated carbons is particularly difficult to model due to the fact that the structure is only locally crystalline and that most of the void volumes within the structure have length scales comparable to small molecules. This paper presents a systematic method to develop suitable models of the internal structure that are based on networks of regularly shaped model pores. Important aspects that are addressed include the realism and consistency of the resulting models. The method is illustrated using the adsorption of pure methane and ethane, and binary mixtures of these components, over a wide range of operating conditions onto four activated carbons.

Adsorption of methane was measured on BPL-6 activated carbon, and simulated using the Grand Canonical Monte Carlo method in 40 slit-shaped pores ranging in width from from 0.63 to 5.72 nm (1.65−15 methane molecular diameters). The simulations were used, in combination with a single experimental isotherm at 308 K, to extract a pore size distribution (PSD) for the carbon. This PSD was then used, in combination with higher-temperature simulations, to predict methane adsorption on the same adsorbent at two higher temperatures. The predicted isotherms show excellent agreement with experiment at 333 and 373 K for pressures of up to 3 MPa. We demonstrate that the sensitivity of the PSD to characterization using a supercritical adsorbate such as methane is bounded by a limiting pore width, which is a function of pressure, temperature, and the adsorbate used. Above this limiting pore width, adsorption may be used to characterize porous solids instead by their surface area. At 308 K, the PSD of micropores can be effectively characterized using methane adsorption at pressures up to 3 MPa. An implementation of our method is available on the World Wide Web.

We discuss the origin of adsorbed-phase nonuniformity and its implications for the prediction of a multicomponent adsorption equilibrium in microporous adsorbents. Current treatments of adsorbed phase nonuniformity are reviewed and a new model, the multispace adsorption model (MSAM), is presented. The MSAM multicomponent predictions have as inputs the pure-species adsorption isotherms and a single parameter characteristic of the adsorbent. We extend the original formulation of the model to include the temperature dependence of the parameters. Our model's performance is compared with that of the other thermodynamic models for multicomponent adsorption using our own experimental results and the literature data.

We have investigated the molecular sieving mechanism in carbon molecular sieves (CMSs), by a combination of molecular dynamics simulations and pore network theory, using the separation of air into oxygen and nitrogen as a prototype. Molecular dynamics simulations of diffusion in individual pores show that the diffusion coefficients for oxygen and nitrogen are strong functions of pore width and that the degree of kinetic separation (expressed as the ratio of the species diffusion coefficients) observed experimentally can be reproduced at the level of individual pores. A critical path analysis of diffusion in the pore network of the CMS shows that this degree of kinetic separation can also be obtained with pore networks having a range of pore sizes, even where the width of the pore size distribution is large.

We carried out Monte Carlo simulations of the adsorption of three species&sbd;CH4, CF4, and SF6&sbd;in model pores of various sizes. By comparing the simulated isotherms, integrated over a pore size distribution, with experimental isotherms for the adsorption of these species on a microporous carbon, estimates of the pore size distribution (PSD) of the carbon were obtained&sbd;one for each adsorptive. Because the adsorptives have different molecular sizes and strengths of interaction with the adsorbent, they probe different ranges of pore size; each adsorptive thus provided a partial PSD. By combining the partial PSDs, we were able to obtain a much more complete PSD than could be obtained with a single adsorptive. Comparison of the PSDs obtained with CF4 and SF6, which substantially overlap, shows molecular sieving. By analyzing these two PSDs, using percolation theory, we extracted an estimate of the connectivity of the pore network, an important determinant of the transport properties of the solid. Our approach is applicable to microporous solids in general.