Theoretical Investigations of Structural Changes in Bimetallic MIL-53(Al/Cr)MOF Under Different States and Liquids.

Abstract

MIL-53(Al/Cr) can exhibit pore expansion or contraction in response to different external stimuli and liquid environments. These macroscopic structural transitions are governed by the local rhombic distortion of the Cr³⁺ coordination octahedron. Based on the experimentally measured EPR spectra, the structural distortion characteristics of MIL-53(Al/Cr) are quantitatively studied in this work by using the perturbation formulas of the spin Hamiltonian parameters (SHPs) for 3d³ ions in rhombically distorted octahedra. The microstructure features under different states or liquids (specifically the bond-length distortion ΔR (0.0529-0.0671 Å) and bond-angle distortion δ (7.99°-10.14°) of the Cr³⁺ site, which exhibit a cooperative variation pattern characterized by an increase in ΔR accompanied by a decrease in δ during the narrow-pore to large-pore transition and the reverse trend upon the opposite transition) are studied. Based on this structural information, the relationship between the large-pore and narrow-pore breathing states and the microstructure is further clarified, thus elucidating the impact of the state and liquid on the macroscopic structure of the material.