Manipulated adsorption of C8 aromatics in MIL-53(Cr) through pre-adsorbing water molecules

The flexible metal-organic framework MIL-53(Cr) has been demonstrated to be an efficient adsorbent for liquid phase adsorption and separation, while the effect of this attractive “breathing effect” on its performance remains uncertain. In this research, by increasing the amount of introduced water, MIL-53(Cr) first shrank into a narrow pore form through bound water and then reopened due to the introduction of free water. Using MIL-53(Cr) with diverse amounts of water as an adsorbent, the liquid-phase adsorption and separation of C8 aromatics, including o -xylene (OX), p -xylene (PX), m -xylene (MX) and ethylbenzene (EB), was manipulated by the pre-adsorbed water in MIL-53(Cr). Pre-adsorbing water in MIL-53(Cr) induced a preference reversal between MX and PX, where dry samples preferentially adsorbed MX, while high-water-addition samples preferred PX instead. In addition, the reopened MIL-53(Cr) with high water addition hardly adsorbed EB, thereby increasing the selectivity of xylene isomers to EB. A longer equilibrium time was observed on MIL-53(Cr) with high water addition, during which the adsorption preference shifted from OX to MX to PX. Further molecular simulation provided a microscopic explanation that water molecules in the pores tended to package around MX and EB, hindering their interaction with frameworks or other adsorbates.