Spent Coffee Grounds-Templated Magnetic Nanocatalysts for Mild Oxidations

Environmental harmful effects of spent coffee in stream waters pushed our research group to find new applications for this waste material. A new family of magnetic nanocatalysts was synthesized based on spent coffee grounds and ammonium iron(III) citrate as iron precursor via solvent-free mechanochemical milling followed by calcination at different temperatures. X-ray diffraction data together with magnetic susceptibility measurements showed the unique properties of these materials, including the unprecedented presence of a maghemite phase at calcination temperatures as high as 800 degrees C. The work is completed with the help of a multiethnic characterization approach based on data obtained by N-2 physisorption, transmission electronic microscopy (TEM)-high-resolution TEM, X-ray photoelectronic spectroscopy, and thermogravimetric analysis. Considering the advantages related to magnetic features, mostly associated with the simple recovery and reuse, materials were tested in the catalytic oxidation of isoeugenol towards vanillin under both, conventional heating and microwave-assisted conditions. Remarkably, a clear enhancement in the catalytic behavior was observed by using microwave irradiation. The results evidenced that hematite content could be a decisive factor to control the activity and selectivity of the reaction.