Untangling the Active Sites in Exposed Crystal Facet of Zirconium Oxide for Selective Hydrogenation of Bioaldehydes

The present study reports the influence of crystal phase, facets, and the active sites of zirconium oxide (ZrO2) on the conversion of bio-aldehydes to corresponding alcohols in isopropanol under mild reaction condition. Various ZrO2-based catalysts, having a different composition of monoclinic and tetragonal crystal phases, are successfully prepared in the presence of a base via the solvothermal process. From the detailed characterizations through XRD, TEM, CO2-TPD, XPS, AES, BET and poisoning studies, M-ZrO2-U-N, synthesized using zirconium oxynitrate and urea as precursor and precipitant, respectively, in water, possesses 100% monoclinic crystal phase with a maximum amount of exposed (-111) facet and surface oxygen concentration along with the highest number of basic sites. The catalytic study on the transformation of furfural (FFA) to furfuryl alcohol (FOH) reveals that M-ZrO2-U-N exhibits the best efficiency with a nearly quantitative yield of FOH. On the other hand, T-ZrO2-U-N, synthesized using zirconium oxynitrate and urea as precursor and precipitant, respectively, in methanol, is found to have 94.4% tetragonal phase and 2.2-fold lower basic sites in comparison with that of M-ZrO2-U-N. The catalytic result with T-ZrO2-U-N displays the lowest activity in terms of FOH yield (8.1%). According to comparative and systematic catalytic studies with the various ZrO2 catalysts having a different amount of tetragonal and monoclinic phases, the ZrO2 having more monoclinic phase with more exposed (-111) facet, basic sites, surface oxygen species and surface area is found to be crucial for the FFA conversion to FOH with high selectivity. M-ZrO2-U-N is found to be stable, recyclable and also shows excellent activity towards the transformation of other bio-aldehydes and ketones to their corresponding alcohols.