Sorption Quantification through the Magnetic Response of Paramagnetic Metal-Organic Frameworks (MOFs) to External Magnetic Fields

It has been known for a long time that materials with paramagnetic behavior are weakly attracted to external magnetic fields. However, this phenomenon hasnot received significant attention in the context of porous paramagnetic materials (metal-organic frameworks, MOFs, and supramolecularly assembled metal-organic frameworks, SMOFs), the composition of which depends on adsorption processes. In this sense, paramagnetic MOF particles that are immersed in a liquid medium can be moved, or even kept motionless, by strong external magnetic fields without falling down. The required magnetic field varies for each MOF and with each adsorption process, as the composition– and therefore the mass accumulated in the pores– varies. Consequently, the threshold at which this stasis of the particles happens is correlated with the quantity of adsorbate captured by the paramagnetic porous material. Two devices have been developed : one uses electromagnets, and the other uses permanent magnets as the source of the magnetic field. When electromagnets are used, the applied current can modulate the strength of the magnetic field, whereas when permanent magnets are used, it is modulated by displacing the sample from the magnet. In both cases, the specific instrument can provide a precise determination of the magnetic field strength (H) or distance (D) at which the stasis phenomenon ends. Theoretical analysis and experimental evidence indicate that both parameters correlate with the amount of mass loaded into the adsorbent, enabling its determination. The theoretical background, a detailed description of the devices, and examples of their application are provided.