Conversion of levulinic acid into chemicals: Synthesis of biomass derived levulinate esters over Zr-containing MOFs

Zr-containing metal–organic frameworks (MOFs) formed by either terephthalate (UiO-66) or 2-aminoterephthalate ligands (UiO-66–NH2) are active and stable catalysts for the acid catalyzed esterification of levulinic acid with EtOH, n-BuOH and long-chain biomass derived alcohols, with activities comparable (in some cases superior) to other solid acid catalysts previously reported. The effect of functional group substitution at the ligand benzene ring, alcohol chain length, particle size and contents of defects on the catalytic activity of the MOFs are studied in detail. In UiO-66–NH2, a dual acid–base activation mechanism is proposed, in which levulinic acid is activated on Zr sites and the alcohol at the amino groups of the ligand. Large variations of the catalytic activity from batch to batch suggest that the active sites are located at defect positions associated to ligand deficiency of the solid. Particle size seems to have a minor impact only in UiO-66–NH2, in which diffusion of levulinic acid is somehow hindered due to the amino groups present in the linkers.