Production of Levulinic Esters by Heterogeneous Catalysis with Zr Metal-Organic Frameworks in Pressure Reactors

Abstract

In the field of biomass valorization, the use of levulinic acid, a platform molecule to obtain products with higher-added value, has attracted research interest in recent years. Therefore, the present work proposes the synthesis of levulinic esters by means of catalytic esterification of levulinic acid with methanol, ethanol, and butanol. For this purpose, metal-organic framework UiO-66-NH2catalysts have been synthesized, with zirconium as the active metal, 2-aminoterephthalic acid as the binding agent, and dimethylformamide as the solvent. In addition, an alternative method to the traditional synthesis is presented, which involves ultrasonic agitation and variations of autoclave time in 24, 18, 12, and 6 h. The catalysts were analyzed by infrared spectroscopy, X-ray diffraction, thermal analyses, acidity measurement with a probe molecule (CO), temperature-programmed desorption of CO2, N2adsorption-desorption, scanning electron microscopy, microwave plasma atomic emission spectroscopy, and X-ray photoelectron spectroscopy. The catalytic reaction was carried out in a pressure reactor. The products were analyzed by gas chromatography and mass spectrometry. Pressure was optimized to improve the catalytic performance. All samples showed a high catalytic yield, greater than 80%. The M-UT/6 catalyst showed a significant decrease in the yield values toward methyl levulinate after consecutive recycles. This is attributed to the leached Zr and lost BDC-NH2ligand, propitiated by the high polarity and small size of methanol molecules. In the case of ethyl and butyl levulinate, it was determined that the M-UT/6 catalyst can be reused for three cycles without a considerable loss of yield (?85-95%).