Gas Adsorption Properties of the Chromium-Based Metal Organic Framework MIL-101

Abstract

MIL-101, a chromium-based metal organic framework, is known to adsorb large amounts of green house gases such as CO₂ and CH₄. Measurement, analysis, and modeling of the pure gas adsorption isotherms of desired gases are necessary for any attempt to use this framework for separation/storage applications. In an attempt to understand adsorption characteristics of this framework, pure gas adsorption properties of CO₂ and CH₄ along with C₃H₈, SF₆, and Ar were measured at three temperatures 283, 319, and 351 K using a standard gravimetric method. The adsorbates were chosen based on their physical characteristics such as polarizability and quadrupole moment. Dual site Langmuir (DSL) isotherm proved to be useful for modeling adsorption of gases on this type of materials that are known to have heterogeneity. Active metal centers and sites inside the pores of supertetrahedra act as major locations for adsorption. Analysis of enthalpy of adsorption using the DSL model revealed that, for all gases, it initially decreases with loading and remains constant thereafter. For all gases considered, the enthalpies of adsorption were found to be lower than those on purely siliceous zeolite such as silicalite, suggesting that only moderate interaction exists between the gas and the MIL-101 framework. The enthalpy of adsorption at zero coverage and the logarithm of Henry’s constant were found to be linear functions of polarizability of the adsorbate.