Selective oxidation of alcohols to aldehydes or ketones is a fundamental process in organic chemistry and the development of green and sustainable transformations are strongly required. Indeed, the traditional reaction requires large amounts of solvent and oxidant, causing serious environmental issues. Recently, several efforts have been made to transform the reaction into a more sustainable process. Herein, we focused on the development of Ru-based catalysts deposited over ceria-zirconia (CZRu) or alumina (AlRu) supports for the solvent free oxidation of benzyl alcohol using air as oxidant, thereby meeting atom economy and environmental requirements.
The materials were extensively characterized and, in addition to their activity, selectivity, and reusability, the environmental sustainability of the process was assessed according to green chemistry metrics.
XPS characterization of the material reveals that, for AlRu the formation of metallic Ru on the support plays a key role in the catalytic activity, while, when ruthenium is supported over ceria-zirconia, the formation of RuO2 on the surface is necessary to achieve higher catalytic activity. For CZRu the activity is strongly related to the reducibility of the material and to the close interaction between ceria-zirconia and RuO2, with the likely formation of Ru-O-Ce arrangements. The greater mobility of oxygen due to the formation of bridging oxygens in Ru-O-Ce and to the formation of superoxide species (O2-) over Ce3+ sites, significantly boosts the selective oxidation of alcohols.
CZRu and AlRu are promising materials with remarkable alcohol conversion at 90 °C (more than 60%) and complete selectivity in a very sustainable process (without a solvent and with air as oxidant), as indicated by the very low E-factor value. Overall, the formulations are among the most active reported so far under solvent free conditions and molecular oxygen. In addition, AlRu is a very stable formulation and maintains high activity after recycling.