Immobilized p-Sulfonic Acid Calix[4]Arene on Silica for Glycerol Acetalization: Influence of Preparation Method on Catalyst Microstructure and Catalytic Properties

Abstract

In this investigation, we synthesized magnetic nanoparticles coated with silica and functionalized with p-sulfonic acid calix[4]arene (CX4), to be used as heterogeneous acid catalysts for acetal synthesis. We evaluated the catalytic efficiency of these nanoparticles for acetal synthesis using various aldehydes and glycerol under microwave irradiation at atmospheric pressure, with and without solvents. Two distinct synthesis methods, Pechini (Fe/pch/Si/CX4) and precipitation (Fe/ppt/Si/CX4), resulted in variations in the physical and chemical properties of the solids produced. Structural characterization involved thermogravimetry analysis (TG), X-ray diffraction (XRD), Mössbauer spectroscopy, vibrating sample magnetometry, scanning and transmission electron microscopy (SEM and TEM), infrared spectroscopy (FTIR), and titration for acidic site quantification. Fe/ppt/Si/CX4 exhibited goethite phases, while Fe/pch/Si/CX4 showed a substantial proportion of maghemite. Both catalysts demonstrated significant activity in acetal synthesis, achieving approximately 80% conversion without solvents. Notably, Fe/ppt/Si/CX4 exhibited higher activity likely due to its structural properties and crystalline phases of the synthesized iron oxide, suggesting a composition difference from the Pechini method.