Magnetic Response of Paramagnetic Metal–Organic Frameworks Under an External Magnetic Field and Its Application for Sorption Quantification

Strong external magnetic fields can keep paramagnetic porous particles static without falling down while immersed in a liquid medium, and the strength of the required magnetic field, controlled indirectly by the distance to a permanent magnet, correlates with the mass of adsorbate captured, giving rise to a new technique for quantifying adsorption processes in a liquid medium.

It has long been known that paramagnetic materials are weakly attracted by external magnetic fields but this attraction has been little studied for porous paramagnetic materials whose composition depends on adsorption processes. In this work, how paramagnetic metal–organic frameworks (MOF) particles can be displaced and even kept stationary by strong external magnetic fields without falling down while immersed in a liquid medium is shown. The magnetic field required for this behavior is specific to each MOF but it also changes with each adsorption process in which the composition, and hence, the mass accumulation in the pores, varies. Therefore, the threshold at which this stasis phenomenon occurs can be correlated to the mass of adsorbate trapped by the porous paramagnetic MOFs. Based on this phenomenon, a simple device has been constructed that allows the determination of the amount of mass trapped, modulating the magnetic field with the variation of the distance between the sample holder containing the paramagnetic MOF particles and a permanent magnet.

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