Conversion of levulinic acid to γ-valerolactone over Zr-containing metal-organic frameworks: Evidencing the role of Lewis and Brønsted acid sites

Abstract Zr-containing UiO-66 and MOF-808 are evaluated for converting levulinic acid (LA) into γ-valerolactone (GVL) through various routes: (i) Step-wise esterification of LA to n-butyl levulinate (nBuL) and Meerwein-Ponndorf-Verley (MPV) reduction to GVL; (ii) One-pot two-steps esterification with n-butanol followed by MPV reduction with sec-butanol; and (iii) direct conversion of LA into GVL through a tandem reaction. Selection of this multistep complex reaction evidences the participation of the different acid sites (Lewis or Bronsted) of the material in each individual step: Bronsted-induced acid sites catalyze esterification reaction efficiently, while Lewis acid sites are the preferred sites for the MPV step. Sulfation of MOF-808 is used to enhance the Bronsted acidity of MOF-808, which improves the performance for esterification. However, the sulfate groups introduced are detrimental for the MPV step, since they reduce the intra-pore space available to form the required bulky transition state. These results evidence the need to find the best equilibrium between Bronsted and Lewis acid sites to optimize the outcome of this multistep reaction.