Investigation of the Role of Egg Membrane in CaO Synthesis and Methods for Stable Composites Syntheses

Upon thermal decomposition, eggshells are a reliable source of high purity CaO. Usually, the membrane sheath(s) accompanying the shells are not recognized. Although CaO is famous for its transesterification catalytic activity, upon catalysis, it is converted to another active catalyst (calcium diglyceroxide) which has limited potential for use in a typical industrial reactor. This study investigates the influence of egg membrane in the production of CaO from eggshells and several techniques for producing chemically stable calcium-based transesterification catalysts. CaO was produced from eggshells with and without membrane inclusion. In view of improving the stability of CaO catalyst, CaO–MgO–Al₂O₃ composite catalysts were prepared via wet mixing of oxides, co-precipitation, mechanical alloying, and wet ball milling. The synthesized materials were, respectively, characterized and used as catalysts in the methanolysis of beef tallow. Inclusion of egg membrane increased the BET surface of eggshells both before (from 0.32 to 2.98 m²/g) and after (from 0.58 to 3.61 m²/g) calcination but slightly reduced the catalytic activity of synthesized CaO from 96.78% to 95.82% beef tallow methyl ester (BTME) yield. Egg membrane was found responsible for the conventional characteristic morphology of eggshell-derived CaO. While co-precipitation gave the most stable composite with 90.12% BTME yield, stable mixed oxide phases were identified in all the composite catalysts.