Ultraslow Dynamics of a Framework Linker in MIL-53 (Al) as a Sensor for Different Isomers of Xylene

• Published in: Journal of physical chemistry. C 2016-09, Vol.120 (38), p.21704-21709
• Main Authors: Khudozhitkov, Alexander E, Jobic, Hervé, Freude, Dieter, Haase, Juergen, Kolokolov, Daniil I, Stepanov, Alexander G
• Format: Article
• Language: English
• Subjects: Catalysis; Chemical Sciences; Environment and Society; Environmental Sciences
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b08114

MIL-53 (Al) is an important example of metal–organic frameworks (MOFs) with a flexible framework capable to efficiently separate ortho and para isomers of xylene at moderate temperatures. The MIL-53 MOF contains mobile terephthalate phenylene fragments that can be used as a dynamic probe to investigate the guest–host interactions and the origin of the separation selectivity. Here 2H NMR spin alignment echo technique for the first time was applied to probe ultraslow structural mobility (0.1–1 kHz) in MOFs materials, with particular application to MIL-53­(Al) saturated with ortho or para isomers of xylene. A specific influence of different isomers of xylene adsorbed in the MOF pores on the rotation of the phenylenes in MIL-53 for the temperature range with proved separation selectivity (T < 393 K) is shown. It has been established that the rotation of phenylene fragments is sensitive to the type of xylene isomer. The phenylenes’ rotation performs 1 order of magnitude slower in the presence of o-xylene (k ortho = 70 s–1) compared to the same rotation in the presence of the other isomer, p-xylene (k para = 800 s–1) at T ∼ 373 K. This is rationalized by a stronger interaction of the ortho isomer with the linker than the para isomer. This finding offers an understanding of the molecular mechanism of p- and o-xylene separation by MIL-53: stronger interaction of o-xylene with organic linker compared to p-xylene provides separation of these isomers on MIL-53.