Intensifying levulinic acid hydrogenation using mechanochemically prepared copper on manganese oxide catalysts

A series of copper nanoparticles supported on manganese oxide octahedral molecular sieves (OMS-2) were prepared using mechanochemical (Ball-Mill) and conventional wet-impregnation (Wet-Imp) methods. All catalysts prepared were thoroughly characterized using ICP-OES elemental analysis, X-ray diffraction (XRD), N2-sorption, H2 temperature programmed reduction (TPR) and transmission electron microscopy (TEM) techniques. The catalyst preparation methods greatly affected the size of the Cu nanoparticles. TEM images showed that 5 wt % Cu/OMS-2 (Ball-Mill) catalyst had a narrow particle size distribution with an average Cu nanoparticle size of 2.1 nm, while the corresponding 5 wt% Cu/OMS-2 catalyst prepared using wet-impregnation method had an average Cu nanoparticle size of 19.2 nm. The structural features of the catalysts were corelated with the catalytic activity using the liquid phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL), as an exemplar process. In LA hydrogenation at 190 ◦C and 20 bar H2 pressure, the ball milled catalysts achieved higher LA conversion, and greater GVL yield, as compared to the corresponding catalysts prepared by wet-impregnation method, reinforcing that Cu nanoparticle size and metal dispersion are important tool to intensify the catalytic activity. For instance, 5 wt% Cu/OMS-2 (Ball-Mill) catalyst achieved almost twice the turnover frequency (TOF), 24.7 h− 1 as compared to the 5 wt% Cu/OMS-2 (Wet-Imp) catalyst, TOF 11.8 h− 1 , under identical reaction conditions. The results of this study demonstrate that ball milling is a superior method for Cu/OMS-2 catalyst preparation than wet impregnation.