Influence of Linker Functionalization and Heating Mode on the Synthesis of UiO-66 MOF for γ-Valerolactone Production

Abstract

This study evaluates the synthesis and catalytic performance of metal-organic frameworks (MOFs) of the UiO-66 type for the production of γ-valerolactone through catalytic transfer hydrogenation. Materials synthesized by the conventional solvothermal method (24 h) using terephthalic acid (M24) and aminoterephthalic acid (N24) were compared against a variant obtained via microwave irradiation in 2 h (M2). Characterization through BET isotherms revealed that the synthesis technique and functionalization drastically affect porosity. The M2 material achieved the highest surface area (1200 m²·g^-1), surpassing M24 (1063 m²·g^-1) and N24 (823 m²·g^-1). Thermogravimetric analysis confirmed that microwave irradiation acts as a tool for generating material defects, increasing the density of missing linkers from 7,48% (M24) to 10,37% (M2). These defects generate coordination vacancies at the zirconium nodes that act as Lewis acid sites, which are fundamental for activating the substrate through the Meerwein-Ponndorf-Verley mechanism. In catalytic tests using isopropanol as a hydrogen donor, the M2 material exhibited superior performance with a conversion of 99,99% and a yield of γ-valerolactone of 85,72%. In contrast, the functionalized material N24 showed the lowest conversion (52,90%) and a marked tendency toward the formation of the intermediate isopropyl levulinate (36,19%). This behavior suggests that the basic sites provided by the amino group in the linker favor the initial esterification, but limit the subsequent reduction to the lactone. It is concluded that microwave-assisted synthesis not only drastically reduces crystallization times but also develops structural defects and the Lewis acidity of UiO-66, maximizing efficiency in the valorization of levulinic acid into γ-valerolactone.