Preparation of Brönsted-Lewis dual acidic catalyst Ce-HPW-F and its simultaneous catalytic esterification and transesterification of oleic acid and castor oil with methanol to synthesize biodiesel

A series of Brönsted-Lewis dual acidic catalyst Ce(x)-HPW-F(x = 1/3, 2/3, 1, 4/3, x is the molar ratio of Ce to HPW) were synthesized by introducing Ce3+ and F- into 12-tungstophosphoric acid (H3PW12O40, HPW) with Keggin structure. The structure, morphology, valence state, elemental composition, acid site density and type of Ce-HPW-F catalyst were analyzed by FT-IR, XRD, BET, SEM, XPS, NH3-TPD, and Py-IR. The catalytic activities and stabilities of the obtained catalysts were studied in the esterification and transesterification reactions of oleic acid and castor oil (Simulated acidified oil, SAO). The Ce(1/3)-HPW-F has presented optimum catalytic activity. A yield of fatty acid methyl esters (FAME) 93.0 % was obtained at the 18:1 methanol to SAO molar ratio, 4 wt% catalyst, 110°C reaction temperature and reaction time of 90 min. The high catalytic activity of Ce(1/3)-HPW-F could be attributed to the strong Brönsted and Lewis acidity. The Lewis acidity was generated from the partial exchange of Ce3+ with the proton (H+) in HPW and increased through the formation of Ce-F bond between the strongly electronegative F- and Ce3+. Meanwhile, in the highly polar medium (methanol), the binding force between O and H+ was reduced due to the negative charge dispersion of the outer oxygen in the phosphotungstic acid anion and the polarization of W=Od formed by the terminal O and W. This may lead to the stepwise dissociation of H+ in HPW that had not been exchanged by Ce3+. However, F- could form the hydrogen bond with H+ to avoid the occurrence of the above phenomenon. After reusing Ce(1/3)-HPW-F catalyst for four times, the total yield of FAME could still achieve more than 75 %.