Bimetallic Zrx-Aly-KIT-6 modified with sulfate as acidic catalyst for biodiesel production from low-grade acidic oils

For the purpose of fabricating an efficient solid acid catalyst for biodiesel production from low-grade acidic oils, bimetallic silica mesoporous composites with different Zr/Al molar ratios were initially synthesized via a facile one-step hydrothermal method, and then sulfated with sulfuric acid at different concentrations to get a series of sulfated Zrx-Aly-KIT-6 catalysts (denoted as SZA-K, where x/y represents the molar ratio of Zr to Al). The sulfated bimetallic solid catalysts were fully characterized by using XRD, FT-IR, TEM, SEM, EDS, NH3-TPD, and nitrogen porosimetry measurements. Results showed that the remarkable amount of acidic sites in the catalysts was derived from the Zr and Al incorporation and further sulfation, and specifically, the Zr4+ and Al3+ could afford abundant weak acid sites while the inductive effect of SO42− species greatly influences the acidic strength of this solid catalyst. In combination with the enhancement in mass transfer resulting from the inherent interconnected mesoporous structure, the SZA-K catalyst could facilitate the transesterification reaction to take place efficiently. The sulfated Zr5–Al1-KIT-6 catalyst (Zr/Al molar ratio of 5/1, treated using 0.75 mol/L sulfuric acid for 12 h) exhibited enhanced acid strength with larger number of acidic sites (7.81 mmol/g), resulting in a high catalytic performance to the oil transesterification reaction. An oil conversion of 96.3% was attained for the transesterification reaction performed at 120 °C for 7 h by using a methanol/oil molar ratio of 20:1 and a catalyst dosage of 6 wt%. Further, this solid catalyst exhibited better acid and water-resistance capacity with above 80% of oil conversion even with 10% of free fatty acid (FFA) and 2% of moisture present in the low-grade acidic oils. Meanwhile, the good reusability was shown for this catalyst with 80.1% conversion even after reusing for three cycles. The good stability of the solid catalyst was resulted from the strong interactions between the acidic species and the bimetallic composite support.