Benzene, Toluene, and Xylene Transport through UiO-66: Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved nonthermal separation techniques and materials. Because of their high surface areas, tunable structures, and chemical stabilities, metal–organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10^–8 and 10^–12 cm² s^–1, where the rate follows the trend: o-xylene < m-xylene < p-xylene. Corresponding activation energies of diffusion (E_diff) were determined to be 44 kJ mol^–1 for the xylene isomers and 34 kJ mol^–1 for both benzene and toluene, with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the μ₃-OH groups located in the tetrahedral cavities of UiO-66, which indicates that BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provides rich insight for the design of next-generation MOFs for cost-effective separation processes.