Facile synthesis of sulfated mesoporous Zr/ZSM-5 with improved Brønsted acidity and superior activity over SZr/Ag, SZr/Ti, and SZr/W in transforming UFO into biodiesel

• Interplay between metal function of oxo-anions sulfated zirconia and pore structure of ZSM-5 increases acidity.
• Hydrophobic surface eliminates water from the tetrahedral intermediate to form ester during esterification.
• Hydrophobic surface also minimizes deactivation by polar species like water and glycerol.
• Acidity, SBET, mesoporous channels and tetragonal phase of zirconia facilitate transesterification.
• Appreciable reaction rates are favored by moderate to high concentration of strong acidic sites.

On the basis of the interplay between the metal function of oxo-anions, and structure of zeolites and its acidic properties, we report an innovative approach for enhancing acidity of sulfated zirconia, SZ. This concerns the superiority of SZ comprised of single-Brønsted acid sites dispersed on ZSM-5 over Ag, Ti and W. The influence of doping ZrO2 on MFI framework of ZSM-5 was studied against other composite catalysts characterized by temperature-programmed desorption of ammonia (NH3-TPD), IR spectra of pyridine adsorption, N2 sorption, powder X-ray diffraction, elemental analysis via FE-SEM and EDX. Results showed uniform pore size, high mesopore volume, high surface area, and acid densities on the catalysts. Despite lower pore size distribution, Zr/ZSM-5 exhibited highest total acidity (0.75 mmol/g), and activity in converting >95% used frying oil (48 wt.%) over SZr/Ag, SZr/Ti, and SZr/W. This is outstanding considering the lower reaction parameter of 5 h, 5:1 methanol-to-oil ratio, and 200 °C compared to prior arts. Evidently, structure and strength of Brønsted acids have direct effect on the catalytic activity of the materials. This study also illustrated prospects of converting wastes into biodiesel, which is important especially against the backdrop of the current plummeting price of Brent crude oil.

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