Mixed Oxides: Role of Washing and Residual Ions in Transesterification Reactions

The mixed oxides (MOs) serve as catalysts for many reactions such as transesterification, transformation of ethanol to butanol, catalytic cracking, or dehydrogenation reactions. MOs are usually synthesized from hydrotalcites, which are often prepared by the coprecipitation method. However, some chemicals can remain after coprecipitation and influence the properties of MOs, including subsequent applications. The novelty lies in investigating how the residual chemicals affect the properties of hydrotalcites, MOs, and the transesterification reaction (conducted in both one- and two-step processes). Mg-Al and Mg-Fe hydrotalcites were synthesized from chloride and nitrate salts via coprecipitation with NaOH, followed by washing with varying amounts of redistilled water, resulting in variations in the sodium ion content (more water, less sodium ions). All materials were characterized by many analytical methods such as X-ray diffraction, metal determination, scanning electron microscopy, textural properties, and basicity determination. MOs synthesized from chlorides contained stable NaCl, which is not catalytically active, and blocked the pores, leading to a reduced surface area and, consequently, a lower transesterification yield. In contrast, MOs prepared from nitrates contained unstable NaNO₃, which decomposed during calcination and, upon exposure to water, formed basic species (NaOH) that promoted transesterification. Therefore, the effect of residual sodium varies depending on the material precursors. This understanding helps us to improve the synthesis of hydrotalcites and mixed oxides.