Hydrothermal Synthesis and Characterization of Bio-Sourced Macroporous Zinc Phosphates Prepared with Casein Protein

The development of an original and simple procedure of hydrothermal porous biosourced zinc phosphates synthesis from casein protein is reported in this study. The synthesis procedure does not require additional phosphorus source and structure-directing agent for macroporosity formation. The formation of zinc phosphates has been investigated as a function of the pH of the starting mixture (4.5–14.0) and of the temperature of calcination (from 150 to 750 °C). A material composed of hopeite (Zn3(PO4)2·4H2O) and casein was obtained after synthesis at pH 4.5 and 100 °C from a mixture of casein and zinc nitrate solutions. Macroporous zinc phosphates composed of α-Zn3(PO4)2 and α-Zn2P2O7 with large porous size distribution (pore diameter between 350 to 1000 nm) were successfully obtained after the complete casein decomposition at 750 °C. Samples were characterized by X-ray powder diffraction, solid-state 31P NMR spectroscopy, thermal analysis, scanning electron microscopy, nitrogen adsorption, and by fluorescence spectroscopy. The macroporous zinc phosphates have a good stability in water for at least 24 h with no detectable change in their structure, porosity, and crystal morphology.