Identification of the catalytically active component of Cu–Zr–O catalyst for the hydrogenation of levulinic acid to γ-valerolactone†
Cu–ZrO2 catalysts were synthesized by the methanothermal (Me) and oxalate gel precipitation (Og) methods. Detailed characterization of the catalysts synthesized by the Me method shows that these contain only Cu substituted into the tetragonal ZrO2 lattice. For catalysts prepared using the Og method Cu is found not only in the tetragonal ZrO2 lattice but also in the form of CuO particles on the zirconia surface. When these materials were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) it was found that Me materials show no catalytic activity, whereas GVL was formed using Og catalysts. A reduction treatment of the Og catalysts prior to use resulted in a marked increase in the catalytic activity, however, no activity increase was observed when the Me material was exposed to a similar treatment before testing. Based on these results and characterization data, we conclude that the catalytically active component of Cu–ZrO2 catalysts for the hydrogenation of LA is reduced Cu particles dispersed on the catalyst surface with strong interaction with the Cu incorporated zirconia support, while the role of Cu in the zirconia lattice is to improve the adhesion of these particles and maintain their dispersion.