Adsorption in micropores (nanopores): a computer appraisal of the Dubinin equations

New method to determine surface area and its energy distribution for nonporous solids: a computer simulation and experimental study

We present in this article a new method to determine the "geometrical" surface area of nonporous solids. This method is based on the total number of molecules dosed into the adsorption cell and a knowledge of the distribution of molecules between the gas phase and the surface phase. By matching this experimental amount with the corresponding theoretical equation, we can derive not only the surface area but also its energy distribution and the void volume of the adsorption cell. The method avoids the limitations of other methods presented in the literature. The BET method, for example, involves unrealistic assumptions and necessitates the choice of a molecular projection area. Our method does not suffer from these assumptions or limitations and is self-consistent, from the measurement of adsorption data to the final analysis of the surface area. The novelties of the method are the following: (i) it is valid over the complete range of reduced pressure, (ii) it does not require a molecular projection area, (iii) beside the total surface area, we also derive its energy distribution, and (iv) the helium expansion method (or any equivalent method) is not required to determine the void volume.

Investigating cluster formation in adsorption of CO2, CH4, and Ar in zeolites and metal organic frameworks at subcritical temperatures

The critical temperatures, T(c), of CO(2), CH(4), and Ar are 304 K, 191 K, and 151 K, respectively. This paper highlights some unusual characteristics of adsorption and diffusion of these molecules in microporous structures such as zeolites and metal organic frameworks at temperatures T < T(c). Published experimental adsorption data for T < T(c) show that the isotherms invariably display stepped characteristics. The inverse thermodynamic factor 1/Gamma(i) identical with d ln c(i)/d ln f(i) exceeds unity for a range of fugacities f(i) and molar concentrations c(i) within the pore corresponding to the steep portion of the isotherm. With the aid of Monte Carlo simulations of isotherms for different temperatures T < T(c) in a variety of zeolites (AFI, MTW, FAU, NaY, MFI, and MOR), metal-organic frameworks (IRMOF-1, CuBTC, MIL-47 (V), and MIL-53 (Cr)), and covalent-organic frameworks (COF-102, and COF-108), we investigate the conditions required for 1/Gamma(i) > 1. For each of the three species investigated, data on pore concentrations c(i) at any given temperature below T(c) fall within the binodal region for the bulk fluid phase. We present evidence to suggest that, in the concentration ranges for which 1/Gamma(i) > 1, clustering of molecules occurs. The extent of clustering is enhanced as T falls increasingly below T(c). Furthermore, molecular dynamics simulations of diffusion demonstrate that the concentration dependence of the diffusivities is markedly influenced in the regions where 1/Gamma(i) > 1. In regions where molecular clustering occurs, the Fick diffusivity shows, in some cases, a decreasing trend with concentration.