Transesterification of vegetable oils on basic large mesoporous alumina supported alkaline fluorides-Evidences of the nature of the active site and catalytic performances

KF, LiF and CsF/Al₂O₃ catalysts with different loadings from 1 to 20 wt% were prepared using aqueous solutions of the alkaline fluoride compounds by wet impregnation of basic mesoporous MSU-type alumina. The catalysts were activated under Ar at 400 °C for 2 h and monitored by in situ XRD measurements. The catalysts were also characterized using several techniques: N₂ adsorption/desorption isotherms at -196 °C, FTIR, DR-UV-vis, CO₂-TPD, XRD, ²⁷Al CP/MAS NMR. These characterizations led to the conclusion that the deposition of alkaline fluorides on the alumina surface generates fluoroaluminates and aluminate species. The process is definitivated at 400 °C. The fluorine in these structures is less basic than in the parent fluorides, but the oxygen becomes more basic. The catalysts were tested for the transesterification of fatty esters under different experimental conditions using conventional heating, microwave and ultrasound irradiation. Recycling experiments showed that these catalysts are stable for a limited number of cycles.