Synthesis, Characterisation and Catalytic Activity of Sol-Gel Derived Silica-Mineral Acids

Sol-gel silica-supported mineral acids (namely sol-gel perchloric acid (slg HClO4), sulfuric acid (slg H2SO4) and phosphoric acid (slg H3PO4), prepared by incorporating mineral acids molecules into silica via sol-gel technique, were used as an efficient heterogeneous catalyst for the direct addition reaction of oleic acid (OA) to form estolides compound. The reactions were carried out under vacuum (2 mBar) for 10 hours at 70°C under solvent-less conditions. Liquid chromatography-mass spectrometry time of flight (LC-MS ToF) of reaction products results showed chromatographic peaks for the presence of two new estolide compounds, oleic-oleic monoestolide acid (m/z 563.51, as [M--), and oleic-oleic diestolide acid (m/z 845.77 [M--). The HClO sol-gel catalyst turned out to be the best catalyst, achieving a final conversion of 76.4% with 83.4% selectivity to oleic-oleic monoestolide acid (OOM) and 16.6% selectivity to oleic-oleic diestolide acid (OOD). The activity and selectivity of the mineral acid sol-gel catalysts have been investigated and compared with homogeneous HClO4. The samples were characterised by XRD, TPD-NH3 and XPS.