Ultra-Slow Dynamics of Framework Linker in MIL-53 (Al) as a Sensor for Different Isomers of Xylene
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MIL-53 (Al) is an important example of the metal?organic frameworks (MOFs) with a flexible framework capable to efficiently separate ortho and para isomers of xylene at moderate temperatures. The batch adsorption and breakthrough experiments have demonstrated that the o-/p-xylenes separation factor was ~ 3.5 at saturation conditions and T = 298 K. The MIL-53 MOF contains mobile terephthalate phenylene fragments that can be used as dynamical probe to investigate the guest-host interactions and the origin of the separation selectivity. 2H NMR spin alignment echo technique was applied for the first time to identify and characterize an ultra-slow motion (0.1-1 kHz) in MOFs.
Xylene isomers influence the mobility of the linkers in a different way causing drastic discrepancy in its rates of motion. It has been established that phenylenes rotation perform one order of magnitude slower in the presence of o-xylene (kortho = 70 s-1) compared to the same rotation in the presence of the other isomer, p-xylene (kpara = 800 s-1) at T ~ 373 K. This is explained by the stronger interaction of ortho isomer with the linker than para isomer. However, this difference in rotation rates is irregular and depends on the temperature.