Acid properties of Sn-SBA-15 and Sn-SBA-15-PrSO3H materials and their role on the esterification of oleic acid

To modulate the acid properties of Sn-SBA-15-PrSO3H materials, the synthesis of heterogeneous Sn-SBA-15 catalysts modified with –PrSO3H groups was performed using the sol-gel method. This method enabled the integration of Sn in the structure of SBA-15, which was achieved by replacing Si+4 with Sn+4 ions, or by generating small amounts of SnO2 nanoparticles in octahedral coordination on the surface of the material. Incorporation of the –PrSO3H groups into the Sn-SBA-15 catalysts results in a significant increase in Brönsted and, to a lesser extent, Lewis acid sites. The relative concentration of Brönsted to Lewis acid sites was approximately 4:1. This improvement in the acid properties of the catalysts, led to an increase in the reaction rate for the esterification of oleic acid with methanol, achieving conversions that reach 96% at 100 °C after 2 h of reaction. The most favorable synthesis conditions were observed when the highest concentration of acid was used in the synthesis gel (1.1 M). This is due to the higher production of Sn+4 tetrahedral species in the SBA-15 structure and better integration of the -PrSO3H groups into the materials, which are mainly related to Brönsted acid sites. Finally, a linear relationship was established between the concentration of Brönsted acid sites in the catalyst and the catalytic activity for the oleic acid esterification and a second-order reversible model satisfactorily reproduces the experimental kinetic data.