In this work, we explore the ability of an inhomogeneous integral equation approach to provide a full three dimensional description of simple fluids under conditions of confinement in porous media. Explicitly, we will consider the case of argon adsorbed into silicalite-1, silicalite-2, and an all-silica analogue of faujasite, with a porous structure composed of linear (and zig-zag in the case of silicalite-1) channels of 5-8 Å diameter. The equation is based on the three dimensional Ornstein-Zernike approximation proposed by Beglov and Roux [J. Chem. Phys. 103, 360 (1995)] in combination with the use of an approximate fluid-fluid direct correlation function furnished by the replica Ornstein-Zernike equation with a hypernetted chain closure. Comparison with the results of grand canonical Monte Carlo/molecular dynamics simulations evidences that the theory provides an accurate description for the three dimensional density distribution of the adsorbed fluid, both at the level of density profiles and bidimensional density maps across representative sections of the porous material. In the case of very tight confinement (silicalite-1 and silicalite-2), solutions at low temperatures could not be found due to convergence difficulties, but for faujasite, which presents substantially larger channels, temperatures as low as 77 K are accessible to the integral equation. The overall results indicate that the theoretical approximation can be an excellent tool to characterize the microscopic adsorption behavior of porous materials. C 2015 AIP Publishing LLC. [http://dx
In this work, the adsorption of toluene on puresilica MEL zeolite is investigated using a variety of experimental and simulation methods. First, we measured the volumetric and calorimetric isotherms at 301 K and at 315 K. The volumetric adsorption isotherm presents a substep at a loading of roughly 4 molecules per unit cell that shifts to higher pressures at higher temperatures and that coincides with a sudden increase in the isosteric heat of adsorption. Grand canonical Monte Carlo simulations reveal that the substep at half load is caused by the adsorption of toluene molecules at different energetic sites within the porous network. According to this, toluene molecules occupy first the intersections, and once all intersections are filled, additional toluene molecules place themselves within the channels. The structure of the adsorbate/adsorbent system was further investigated by performing X-ray powder diffraction experiments of the zeolite at three different loads: empty, at half load (before the substep), and at high load (after the substep). Numerous new low intensity peaks and splittings of existing peaks at the empty and half-loaded diffractograms appear in the diffraction pattern of the high load sample. Atomic structural models compatible with the experimental spectra were obtained by performing N-reverse Monte Carlo simulations. Whereas at half load a good fit of the experimental diffraction pattern can be obtained using the rigid zeolite approximation, at high load, this is only possible when the flexibility of the zeolite is incorporated. In this structural model, the channel cross sections are deformed from a nearly circular shape in the empty zeolite to a more elliptical shape in the case of the high load zeolite.
An extension of the well established Reverse Monte Carlo (RMC) method for modeling systems under close confinement has been developed. The method overcomes limitations induced by close confinement in systems such as fluids adsorbed in microporous materials. As a test of the method, we investigate a model system of 36 Ar adsorbed into two zeolites with significantly different pore sizes: Silicalite-I (a pure silica form of ZSM-5 zeolite, characterized by relatively narrow channels forming a 3D network) at partial and full loadings and siliceous Faujasite (which exhibits relatively wide channels and large cavities). The model systems are simulated using Grand Canonical Monte Carlo and, in each case, its structure factor is used as input for the proposed method, which shows a rapid convergence and yields an adsorbate microscopic structure in good agreement with that of the model system, even to the level of three body correlations, when these are induced by the confining media. The application to experimental systems is straightforward incorporating factors such as the experimental resolution and appropriate q-sampling, along the lines of previous experiences of RMC modeling of powder diffraction data including Bragg and diffuse scattering.
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