Superacidity in Zr(iv)/Ce(iii) MOF-808: unlocking biodiesel production from microalgae lipids at reduced temperatures

Abstract

To manage production costs and lower the conversion temperature of microalgal lipids, this study engineered superacid sites in MOF-808, facilitating lipid transformation at markedly reduced temperatures. Through simple substitution and activation steps, secondary building units formed by Zr(IV) and Ce(III) were assembled with benzene-1,3,5-tricarboxylate ligands, resulting in a unique superacid structure. This superacid structure has a Hammett acidity value of less than −14.5 and a deprotonation energy of 1016.25 kJ mol⁻¹, both lower than those of sulfuric acid. Density functional theory and solid-state NMR analyses confirmed the superacidity arises from a weakly constrained superacid proton within a bridging structure, formed by two methanol molecules attracted to adjacent Ce and Zr atoms. Variable temperature infrared spectroscopy demonstrated the low-temperature activation of methanol, with the optimal reaction temperature reduced from 200 °C to 150 °C and pressure reduced by 2952 kPa. Unlike traditional solid superacids, this methanol-based superacid configuration prevents the loss of superacid sites, maintaining over 97% efficiency after five cycles.