Water Vapor Uptake and Microwave Heating of Magnetite / Aluminum Fumarate Composites

This study aims to obtain new desiccant materials based on magnetic nanoparticles and metal-organic framework (MOF) that are regenerable by microwave heating.

Among the nanoparticles/MOF composites, the magnetic ones are especially interesting as sorbents with stimuli-triggered release of adsorbate and separation/concentration in an external magnetic field.

The composites were obtained by adding pre-synthesized magnetite nanoparticles to the solution of fumaric acid used to synthesize the porous aluminum fumarate (Al-fum) matrix. The mass fraction of Fe₃O₄ in the composites was evaluated by an o-phenanthroline-based assay, and their structure was characterized by XRD and FTIR. The magnetic properties and humidity-dependent water vapor uptake were measured.

The broad X-ray diffraction signals of magnetite, decreased Curie temperature, and increased water uptake by the composites with a low magnetite content reflect the escalation of microdefects at the interphase boundary. Higher temperatures and pressures were reached in a shorter period of time upon the microwave heating of water-enriched powders and aqueous dispersions of Fe₃O₄/Al-fum composites as compared with pure water.

The absorption of microwave radiation by Al-fum saturated with water is improved to a degree that allows one to overcome the decrease in dielectric losses and tangent of pure water with increasing temperature. The presence of magnetite in the amount of 0.04 g g^-1 or more accelerates the heating of aqueous dispersions of the composites to the same extent as pure magnetite nanoparticles.