Solid deformation induced by the adsorption of methane and methanol under sub- and supercritical conditions

Effect of Adsorbent Properties on Adsorption-Induced Deformation

Adsorption-induced adsorbent deformation is of fundamental importance to geoscientists and engineers. To gain insight into the deformation behaviors of different materials, we presented grand canonical Monte Carlo (GCMC) simulations of methane adsorption-induced deformation in slit pores. Adsorption isotherms and deformation behaviors of the pores were obtained for adsorbents with variations in solid density and affinity. The results showed that the adsorption-induced deformation depends on adsorbate loading, pore width, solid density, and affinity. The deformation at a given adsorption loading could be comparable between different solid densities or affinities because solid density or affinity is related to the solvation pressure as the driving force behind the deformation and also the resistance of the deformation. The interaction of these two effects controls the deformation behavior. We expect that our results will help to understand the adsorption-induced deformation in solids with heterogeneous properties and estimate deformation using the gas adsorption data.

Heat of adsorption, adsorption stress, and optimal storage of methane in slit and cylindrical carbon pores predicted by classical density functional theory

Temperature, pressure and pore-size dependences of the heat of adsorption, adsorption stress, and adsorption capacity of methane in simple models of slit and cylindrical carbon pores are studied using classical density functional theory (CDFT) and grand-canonical Monte-Carlo (MC) simulation. Studied properties depend nontrivially on the bulk pressure and the size of the pores. Heat of adsorption increases with loading, but only for sufficiently narrow pores. While the increase is advantageous for gas storage applications, it is less significant for cylindrical pores than for slits. Adsorption stress and the average adsorbed fluid density show oscillatory dependence on the pore size and increase with bulk pressure. Slit pores exhibit larger amplitude of oscillations of the normal adsorption stress with pore size increase than cylindrical pores. However, the increase of the magnitude of the adsorption stress with bulk pressure increase is more significant for cylindrical than for slit pores. Adsorption stress appears to be negative for a wide range of pore sizes and external conditions. The pore size dependence of the average delivered density of the gas is analyzed and the optimal pore sizes for storage applications are estimated. The optimal width of slit pore appears to be almost independent of storage pressure at room temperature and pressures above 10 bar. Similarly to the case of slit pores, the optimal radius of cylindrical pores does not exhibit much dependence on the storage pressure above 15 bar. Both optimal width and optimal radii of slit and cylindrical pores increase as the temperature decreases. A comparison of the results of CDFT theory and MC simulations reveals subtle but important differences in the underlying fluid models employed by the approaches. The differences in the high-pressure behaviour between the hard-sphere 2-Yukawa and Lennard-Jones models of methane, employed by the CDFT and MC approaches, respectively, result in an overestimation of the heat of adsorption by the CDFT theory at higher loadings. However, both adsorption stress and adsorption capacity appear to be much less sensitive to the differences between the models and demonstrate excellent agreement between the theory and the computer experiment.

Adsorption of n-pentane on mesoporous silica and adsorbent deformation

Development of quantitative theory of adsorption-induced deformation is important, e.g., for enhanced coalbed methane recovery by CO2 injection. It is also promising for the interpretation of experimental measurements of elastic properties of porous solids. We study deformation of mesoporous silica by n-pentane adsorption. The shape of experimental strain isotherms for this system differs from the shape predicted by thermodynamic theory of adsorption-induced deformation. We show that this difference can be attributed to the difference of disjoining pressure isotherm, responsible for the solid-fluid interactions. We suggest the disjoining pressure isotherm suitable for n-pentane adsorption on silica and derive the parameters for this isotherm from experimental data of n-pentane adsorption on nonporous silica. We use this isotherm in the formalism of macroscopic theory of adsorption-induced deformation of mesoporous materials, thus extending this theory for the case of weak solid-fluid interactions. We employ the extended theory to calculate solvation pressure and strain isotherms for SBA-15 and MCM-41 silica and compare it with experimental data obtained from small-angle X-ray scattering. Theoretical predictions for MCM-41 are in good agreement with the experiment, but for SBA-15 they are only qualitative. This deviation suggests that the elastic modulus of SBA-15 may change during pore filling.

Microscopic configurations of methanol molecules in graphitic slit micropores: a computer simulation study

We report a detailed computer simulation study of methanol adsorption in graphitic slit-like micropores to investigate the effects of temperature and pore size on the adsorptive capacity and the configurations of methanol molecules in a confined space. Simulation results show that in the temperature range studied (273-422 K) the amount adsorbed increases gradually with pressure in 0.65 and 0.8 nm pores (where only one molecular layer can be accommodated), while for pores having widths greater than 1.0 nm the adsorption isotherms exhibit a sharp jump at low temperatures which becomes gradual as temperature is increased above the critical pore temperature, which increases with pore width. For a given pore size, the pressure at which a large uptake of adsorption occurs, increases and the excess amount adsorbed, decreases with temperature. The interaction between adsorbate molecules and a pore was studied via the solvation pressure, which exhibits oscillations with pore size. The peaks of this oscillation correspond to pores that have an integer number of layers of methanol molecules. At low loadings snapshots showed methanol molecules in isolated clusters of four or five molecules which maximise the hydrogen bonding within each cluster, in the same way as they do on an open surface. At high loadings, the isolated cluster configuration changes to molecular chains in small pores (0.65 and 0.8 nm), which become more distorted by inter-layer interactions in larger pores. The positions of the first peaks of the O-O and O-H radial distributions for the confined methanol are the same as those for bulk liquid for all pore sizes. However, for confined methanol in pores with an integer number of molecular layers, the amplitude of the first peaks of the O-O and O-H radial distributions are higher than for the bulk liquid, and the positions of the second peaks are slightly shifted to the left. In the incommensurate pores the amplitude of the first peaks and the positions of the second peaks of the O-O and O-H radial distributions are similar to those of liquid methanol. Our simulation results agree well with the experimental results of Ohkubo et al.