Utilisation of O₂ as oxidant in heterogeneous catalysed oxidations has widespread application in organic synthetic chemistry and replaces atom-inefficient stoichiometric reagents. However, low gas-liquid mass transfer rates force >10-fold gas excess in flow, and the hazards of O₂–organic solvent mixtures limit operation to aqueous systems. We have developed a dissolved-gas strategy that fully decouples the gas-to-liquid mass transfer step from the liquid-to-solid and surface reaction steps, allowing safe introduction of stoichiometric O₂ concentrations directly into a packed bed reactor with no gas phase present. The concept is exemplified by the Pd/C-catalysed oxidation of veratryl alcohol. Dialling dissolved O₂ concentrations independently of substrate and base allowed systematic kinetic and selectivity evaluation: the rate is first order in substrate, half order in O₂, with an activation energy of 52 kJmol−1 and strong sensitivity to base concentration. No hydrogen peroxide was detected under any conditions. A microkinetic mechanism based on dissociative adsorption of O₂ and subsequent formation of Pd–OH reactive intermediates is consistent with all observations. Resistance-in-series analysis shows that the estimated mass transfer coefficients do not exceed those predicted by established packed-bed correlations, and the analysis demonstrates the system operates in the reaction limited regime. Scale-up of safe gas dissolution technology enables continuous reaction at stoichiometric gas loading with no gas-phase inventory, reducing safety risk and catalyst costs, and opening the approach to a broader range of gas-liquid-solid processes.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)