Study on the Desorption Process of n-Heptane and Methyl Cyclohexane Using UiO-66 with Hierarchical Pores

UiO-66 (UiO for University of Oslo) is a zirconium-based metal–organic framework with reverse shape selectivity, which gives an alternative way to produce high-purity n-heptane (nHEP) used for the manufacture of high-purity pharmaceuticals. A couple of studies have shown that UiO-66 gives a high selectivity on the separation of n-/iso-alkanes. However, the microporous structure of UiO-66 causes poor mass transport during the desorption process. In this work, hierarchical-pore UiO-66 (H-UiO-66) was synthesized and utilized as an adsorbent of nHEP and methyl cyclohexane (MCH) for systematically studying the desorption process of n-/iso-alkanes. A suite of physical methods, including X-ray diffraction patterns, verified the UiO-66 structures, and high-resolution transmission electron microscopy showed the existence of hierarchical pores. N2 adsorption–desorption isotherms further confirmed the size distribution of hierarchical pores in H-UiO-66. Of particular note, the MCH/nHEP selectivity of H-UiO-66 is similar to that of UiO-66 under the same adsorption conditions, and the desorption process of nHEP/MCH from H-UiO-66 is dramatically enhanced; namely, the desorption rates for nHEP/MCH from H-UiO-66 is enhanced by 30%/23% compared with UiO-66 at most. Moreover, desorption activation energy (Ed) derived from temperature-programmed desorption experiments indicate that the Ed for nHEP/MCH is lower on H-UiO-66; that is, the Ed of MCH on H-UiO-66 is ∼37% lower than that on UiO-66 at most, leading to a milder condition for the desorption process. The introduction of hierarchical structures will be applicable for the optimization of the desorption process during separation on porous materials.