Pradip Chowdhury

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.

In this work, a complex dielectric transformation of UV-vis diffuse reflectance spectra is proposed to estimate the optical band-gap energies of an array of materials classified as semi-conductors, conductors and insulators and the results are compared with the more common Kubelka-Munk (K-M) transformation. The results show a close match between the proposed method and the Tauc model based on the K-M transformation within ca. 0.16-7.07% variation. The proposed method based on the well-established dielectric transformation is unique in a way to estimate band-gap energy when there remain unresolved or multiple absorption peaks in the diffuse reflectance spectra. Importantly, the complex dielectric transformation method also distinguishes the class of the materials which is of paramount importance to validate and substantiate the band-gap energy values.