LAUSR

The binary system of metal oxides is significantly considered due to improving the weakness of each metal cation by supporting other metal cations. Although SiO 2 –TiO 2 is a conventional binary system which usually fabricated by the sol–gel method, the comprehensive study requires on their structure to achieve the suitable properties. Therefore, a series of SO 4 2− /SiO 2 –TiO 2 was synthesized by combining sol–gel and impregnation methods. The effects of Si/Ti molar ratio (0.7, 0.8, 0.9, and 1), and nitric acid concentration as complex agent (2, 4, and 6 M) in the sol–gel method, and sulfate concentration (0.5, 1, and 2 M) in the impregnation method were evaluated. The results revealed that Si‒O‒Ti bands were fully formed and the high amount of nitric acid concentration has an insignificant effect on the silica–titania framework structure. After loading a sulfate group, TiOSO 4 was formed as a highly active structure, leading to increasing the acidity of the sample. However, this structure was transformed to form Ti 2 (SO 4 ) 3 at higher sulfate concentration that has an insignificant influence on the activity of the catalyst. The optimum sample (Si/Ti ratio of 0.8, the nitric acid concentration of 4 M, and sulfate concentration of 1 M) presented a high activity in the esterification reaction. 94.1% of oleic acid was converted at the conditions of 120 °C, 3 wt% of catalyst, methanol/FFA molar ratio of 9, and 4 h of reaction time. Although the catalytic activity was dropped around 9% for the second use, the catalyst preserved its activity for the next five times (i.e., 10% reduction in conversion) that can prove its ability for industrial application.