Accelerated H2 activation over Pt/M-ZrO2 for the reductive amination of levulinic acid esters under benign conditions

Abstract

Catalytic reductive amination of biomass-derived levulinic acid and its esters to pyrrolidones, having numerous applications in the pharmaceutical and agrochemical fields, is an attractive approach. A robust Pt-impregnated monoclinic ZrO₂ (Pt/M-ZrO₂) catalyst is developed for the reductive amination of methyl levulinate (MLevu) with octyl amine (OA) in the presence of H₂, obtaining a quantitative yield of N-octyl-5-methyl-2-pyrrolidones (OMP) at ambient temperature. Adversely, Pt-impregnated tetragonal ZrO₂ (Pt/T-ZrO₂) displays an 18-fold lower activity than Pt/M-ZrO₂ in terms of turnover frequency at similar conversion. Transmission electron microscopy with energy dispersive X-ray (TEM-EDX) spectroscopy suggests higher Pt dispersion in Pt/M-ZrO₂, having a 15-fold higher surface area than Pt/T-ZrO₂, as evidenced by N₂-sorption analysis. Pt/M-ZrO₂ contains 15 and 2-fold more acidic and basic sites than Pt/T-ZrO₂, respectively, which helps to activate the CN bond of the formed intermediate and H_2, thus facilitates hydrogenation to form OMP selectively. Further, the poisoning studies reveal that both acidic and basic sites in Pt/M-ZrO₂ play a vital role in enhancing the yield of OMP, indicating the formation of a frustrated Lewis pair (FLP) that facilitates the reaction at room temperature. Moreover, H₂-temperature programmed desorption (H₂-TPD) evidently supports that Pt/M-ZrO₂ possesses a low-temperature desorption peak (50–100 °C), which is absent in the case of Pt/T-ZrO₂, indicating high H₂ activation and hydrogenation ability at room temperature which significantly contributes to hydrogenation of the formed imine intermediate to OMP. Further, Pt/M-ZrO₂ is recyclable for at least three times with no significant changes in the conversion and yield.