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.
A novel metal-organic framework (MOF) material, flake-like NH 2 -MIL-53(Cr), was prepared by a simple reflux method and then characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The as-prepared MOF was found to be an effective material for the selective electrochemical detection of microgram levels of Pb 2+ in aqueous solution. Detection was performed by square wave anodic stripping voltammetry. The NH 2 -MIL-53(Cr)-modified electrode showed excellent selectivity and stability for Pb 2+ detection. The electrochemical properties and applications of the modified electrode were studied. Operational parameters such as pH, deposition potential, and deposition time, were also optimized to detect trace amounts of metal ions. This method offers a new strategy for heavy metal detection using MOF materials.
Selective adsorption and separation of three xylene isomers of seven stable metal-organic frameworks (MOFs) were investigated, where MIL-53(Cr) manifested the highest adsorption capacity and apparent preference for o -xylene. Further detailed studies with MIL-53(Cr) showed that pseudo-second-order model fit best with adsorption kinetics of xylene isomers, and the isotherms were perfectly fitted by Langmuir isotherm model. The adsorption was proved to be a quick process, and both the changes of entropy and enthalpy contributed to the o -xylene selectivity, but which mainly resulted from the entropy effect. The adsorption capacity of preferable isomer o -xylene, on account of better packing efficiency, was record-breaking 8.19 mmol/g, and selectivity of OX/PX, OX/MX and MX/PX were 13.75, 8.01 and 1.72, respectively. Both the adsorption capacity and OX selectivity on MIL-53(Cr) in liquid phase adsorption and breakthrough experiments were super high, which discriminated it from zeolites and other materials reported so far. Combined with extremely short equilibrium time and high stability, MIL-53(Cr) can be a promising adsorbent for separation of xylenes in industrial process.
MIL-53(Cr) is a well-known and important metal-organic framework material. However, the long synthesis time of MIL-53(Cr) restricts its applications. Herein, we report a facile method to kinetically modulate the crystallization route and decrease the synthesis time of 72 to only 8 h by simply increasing the usage of HF.
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