Egg White Assisted Synthesis of Fe-Mn Spinel Oxides: Effects of Egg White Ratio, Oxygen Partial Pressure, and Life Cycle Impacts

Egg white was chosen as a renewable, non-toxic agent for the synthesis of FeMn2O4 spinel pre-catalysts to avoid the use of critical transition metals such as Ni and Co. However, synthesizing phase-pure FeMn2O4 remains challenging due to (i) the requirement of low oxygen partial pressures to counter rapid reoxidation of Mn3O4 in the presence of iron oxides, which can be achieved by the preferred oxidation of the egg white during the calcination, and (ii) the probable formation of Fe3O4 and Mn3O4 during intermediate steps in the reaction, leading to multiphase spinel formation caused by a miscibility gap between the spinels. In contrast, spinels with Ni, Co, Zn, or Al are phase-pure. Egg white has significant environmental impacts in the synthesis of all spinel manganites, as assessed from a life-cycle perspective, which can exceed those of petroleum-based agents such as ethylenediaminetetraacetic acid (EDTA) in most impact categories. Therefore, our results show that the investigated synthesis route is not more sustainable, and we demonstrate that implementing quantitative evaluation of environmental impacts already at an early stage is essential to determine whether a synthesis is truly sustainable.