Tailoring Lewis Acid Properties of Metal–Organic Framework Nodes via Anion Post-Replacement for Gas Adsorption and Separation

Metal–organic frameworks (MOFs) with open metal sites (OMSs) have many applications from sensing, catalysis, to gas adsorption and separation. The OMSs have Lewis acid nature, which are capable of accepting electron density from guest molecules, thus enabling selective gas adsorption. The Lewis acid strength of OMSs plays a pivotal role in the adsorption properties of MOFs. In this work, we report a general method capable of adjusting the Lewis acid strength of OMSs in a cationic framework MIL-101. Relying on a simple anion post-replacement, the chemical environment around the metal centers can be regulated effectively, thus achieving the purpose of the Lewis acid strength adjustment. The Lewis acid properties of OMSs have been studied using different characterization techniques, including Fourier transform infrared, X-ray photoelectron spectroscopy, ultraviolet–visible, and in situ IR spectroscopic analysis, combined with a molecular electrostatic potential calculation. The effect of the Lewis acid strength of the OMSs for gas adsorption and separation was validated by adsorption experiments. Gas sorption isotherms reveal that the modified materials show reverse N2, CH4 selectivity with pristine MIL-101-NO3, which was mainly attributed to the different sensitivity of N2 and CH4 to the open Cr sites, as evidenced by density functional theory calculations. Furthermore, the modified material, MIL-101-Cl, exhibited by far the highest N2O adsorption capacity (136 cm3 g–1 at 1.0 bar and 298 K) and benchmark CO2 uptake. Our method will facilitate further deliberate postsynthetic modifications of other MOFs with OMSs to improve them for applications in chemical separation processes.