Phosphotungstic acid and propylsulfonic acid bifunctionalized ordered mesoporous silica: A highly efficient and reusable catalysts for esterification of oleic acid

In phosphotungstic acid aqueous solution, the ordered mesoporous silica molecular sieve (HPW/OMS-SO3H) simultaneously functionalized by phosphotungstic acid (HPW) and propyl sulfonic acid groups (-pr-SO3H) was successfully prepared via a one-pot co-condensation method. The skeleton of the materials was formed based on the simultaneous hydrolysis and co-condensation of 3-mercaptopropyltriethoxysilane (MPTES) and tetraethoxysilane (TEOS) using in situ oxidation of the thiol groups to sulfoacid groups with H2O2. The materials with an ordered 2D hexagonal mesoporous structure and a high specific surface area were confirmed by the results of small-angle XRD, N2 adsorption–desorption, and TEM. The spectroscopy of FT-IR, UV–Vis DRS, XPS, and Raman indicated that the active component of HPW and propylsulfonic acid groups successfully immobilized on the OMS support and a strong interaction existed among silicon hydroxyl and propylsulfonic acid groups with HPW. The esterification reaction of oleic acid with methanol to produce biodiesel over prepared catalyst showed excellent catalytic activity due to the synergistic effect between the HPW and propylsulfonic acid groups. The biodiesel yield of 95.3% was obtained by 0.3HPW/OMS-SO3H-5 catalyst under the optimum reaction conditions. The catalytic performance was not reduced significantly even after five cycles reused, because the strong interaction prevented acid-active component loss from the support. The activation energy of the reaction by the 0.3HPW/OMS-SO3H-5 materials was investigated, which obeyed the first-order kinetics.