Tuning zirconia-supported metal catalysts for selective one-step hydrogenation of levoglucosenone
Levoglucosenone, directly produced from cellulose-containing residual biomass via pyrolysis treatments, is believed to be a promising bio-renewable platform for both fine and commodity chemicals. In this work, the possibilities given by tuneable catalysts based on Pd and Pt supported on metallic oxides to produce the desired product in the one-pot hydrogenation of levoglucosenone are evaluated. Particularly, the excellent catalytic performance of Pd/ZrO2 and Pt/ZrO2 type materials for the synthesis of dihydrolevoglucosenone (or Cyrene) and levoglucosanol, respectively, during the mild hydrogenation of levoglucosenone is demonstrated. In the Cyrene synthesis, the Pd/t-ZrO2 material showed the best catalytic activity compared to other Pd-supported on metallic oxides. This catalyst achieved nearly 95% yields of Cyrene by working under mild reaction conditions, with very low catalyst loadings (≈3 wt%) and using water as the solvent. On the other hand, the one-pot hydrogenation of levoglucosenone to levoglucosanol is reported for the first time with a Pt-based heterogeneous catalyst (Pt/ZrO2-mix, yield ≈90%), by working at low temperatures and mild H2 pressures with water as the solvent. Comparison of the results attained with other Pt-supported metallic oxides let us to conclude that the metal crystal facets (specifically the 100 facet) play an important role in the hydrogenation process to give levoglucosanol selectively. In addition, the stability and re-usability of both catalysts under operational conditions are also evaluated. Finally, catalytic tests including the use of crude bio-liquids obtained from cellulose-rich biomass pyrolysis and containing ≈66 wt% of levoglucosenone are also assayed, thus demonstrating the possibility of scaling-up the process over these metals supported on zirconia catalysts.