Acidity and catalytic performance of Yb-doped SO42−/Zr in comparison with SO42−/Zr catalysts synthesized via different preparatory conditions for biodiesel production

• Amount and dispersion of active sites play significant role in facilitating higher conversion.
• SZ doped with ytterbium showed superior acidity and catalytic performance over SZ catalysts.
• Hydrophobic surface eliminates water from the tetrahedral intermediate to form ester during esterification.
• Doping SZ with ytterbium stabilizes the mesostructured channels and crystallographic phase.
• Acidity, SBET, mesoporous channels and tetragonal phase of zirconia facilitate selectivity.
• Moderate to high concentration of strong acidic sites favored appreciable reaction rates.

Highly efficient, robust and mesoporous sulfated zirconia, SZ catalysts were prepared by co-precipitation and incipient-wetness routes with sufficient and in-excess acid. The study investigated the effect of pH, precursor type, and concentration, on SZ and compared their acidity and performance with ytterbium-doped SZ toward transesterification. N2 sorption, elemental analysis, XRD, and ammonia temperature-programmed desorption (NH3-TPD) revealed the properties of the catalytic materials. The specific surface area increases independently of the presence of Ti up to 60.33 m2/g, with increasing pH while pore size of the tetragonal crystallites decreases from 37.04 to 6.83 nm. Soaking the material in sufficient 0.5-M H2SO4 produced 37.04 nm SBET in contrast to 21.21 nm from soaking in excess. Double sulfation ensured sulfate incorporation while moderate precursor amounts enhanced activity of the materials. Interestingly, despite low specific surface area, which was due to short aging period, large mesoporosity, high amount, and dispersion of active sites on Yb-doped SZ, SZr-Ti-Yb-500-4/s ensured remarkable activity. The catalyst converted ca. 99% of used frying oil containing ca. 48 wt. % FFA. Consequently, this highlights the prospect of producing biodiesel at lower cost especially with current dwindling price of crude oil.

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