Low dimensional Zr-based catalysts for the effective upgrading of ethyl levulinate to γ-valerolactone

Novel low-dimensional zirconium-based silica nanostructures (i.e. hollow nanotubes and porous nanospheres) were synthesized and successfully tested as catalysts for the conversion of ethyl levulinate to ɣ-valerolactone. The nature of the active sites and the role of the Lewis/Brønsted (L/B) acid ratio was studied in-depth. To investigate the impact of the morphology on the catalytic activity, two silica nanostructures were employed: 1D hollow nanotubes and 0D porous nanospheres. Two approaches were considered: co-synthesis and post functionalization resulting respectively in tetrahedral insertion in the SiO2 framework and fine dispersion at the silica surface of Zr-based species. The activity of the solids was correlated with their structural and textural properties as well as with the L/B ratio through a plethora of characterization techniques including electron microscopy, X-ray photoelectron spectroscopy, FT-IR and solid-state NMR spectroscopy. Hollow nanotubes displayed enhanced catalytic performances also in comparison with other solids reported in the literature. This improved activity was ascribed to the highly accessible structure and to the favorable Lewis/Brønsted acid ratio unveiled via a combination of FT-IR and solid-state NMR characterization techniques using different probe molecules. Moreover, their stability over multiple catalytic cycles was proved and a low E-factor was obtained.