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

Abstract

The critical temperatures, T_c, of CO₂, CH₄ 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/Γ_i ≡ d ln ci/d ln f_i exceeds unity for a range of fugacities f_i and molar concentrations ci 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/Γ_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/Γ_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/Γ_i > 1. In regions where molecular clustering occurs, the Fick diffusivity shows, in some cases, a decreasing trend with concentration.