Oxidation of aliphatic alcohols and benzyl alcohol by H2O2 under the hydrothermal conditions in the presence of solid-state catalysts using batch and flow reactors

Oxidation combining three key technologies-reaction with H2O2, hydrothermal processing, and solid-state catalysis-is attractive for the development of environmentally friendly processes. However, the combination of these technologies has not been extensively investigated. We evaluated the potential for environmentally friendly oxidation of alcohols using H2O2 under hydrothermal conditions in the presence of several types of solid-state catalysts. The reactions were investigated using both a conventional batch reactor and a hydrothermal micro-flow reactor (HMFR), which was originally developed by our group for monitoring reactions with time scales within 0.002-200 s at temperatures of up to 400 °C. The oxidation of methanol, ethanol, propanol, and butanol to carboxylic acids using H2O2 did not readily proceed at 120 °C, unlike the oxidation of formaldehyde to formic acid. The catalytic effect of tungstate in the oxidation was observed, and the observation of near infrared (NIR) spectra at 2050-2040 nm indicated that zeolite possesses a catalytic property in the oxidation of formaldehyde to formic acid. Conversely, H2O2 was effective for oxidation of benzyl alcohol to benzaldehyde and benzoic acid at 120 °C and higher temperatures. The selective catalytic properties of tungstate and platinum group metal-deposited catalysts were observed in the oxidation of benzyl alcohol to benzaldehyde. The oxidation reaction was successfully detected using an HMFR with a reaction time of 0.64 s at 300 °C. This study demonstrates the potential and usefulness of the environmentally friendly combination of H2O2, hydrothermal conditions, and solid-state catalysis for the oxidation of aromatic and aliphatic alcohols.