Non-hydrolytic sol-gel processing of chloride precursors loaded at forsterite stoichiometry

This paper for the first time investigates the sol-gel reaction of magnesium chloride and silicon tetrachloride, directed at the forsterite (Mg2SiO4) stoichiometry, using dry ethanol and glacial acetic acid as the solvent and chelating agent, respectively. The synthesized particles before and after calcination were characterized by transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy mapping, X-ray diffraction and Fourier transform infrared spectroscopy. According to the results, the calcined nanoparticles showed a magnesia/forsterite structure along with silicon depletion, despite loading the forsterite stoichiometry. On the other hand, silicon-acetoxy bonds were detected in the xerogel (before calcination) as a result of the chelation reaction, albeit with a relatively uniform distribution of the essential elements. Since no non-alcoholyzed silicon-chlorine bond was detected in the xerogel, the development of the calcined structure was explained by the deficient sol-gel condensation and subsequent evaporation of silicon tetraacetate. It was inferred that the excessive amount of hydrogen chloride as the coproduct of the ethanolysis and chelation reactions of the precursors inhibits the condensation step, as confirmed by a supplementary test in an exaggerated circumstance. In conclusion, the silicon-containing species acts like a limiting reagent in the sol-gel condensation of forsterite using the chloride precursors and acetic acid chelator, in spite of loading the related stoichiometry.