(384a) Deconvoluting Mass Transfer and Chemical Reaction in Segmented Flow Cu/TEMPO-Catalyzed Aerobic Oxidations

Oxidations of alcohol and aldehyde have been studied extensively over the past few decades, and a wide range of stoichiometric reagents and catalytic methods have been developed. Among them, using molecular oxygen for the oxidation of alcohol is of great interest as oxygen is a cheap oxidant and this reaction only produces water as stoichiometric by-products. However, performing this multiphase exothermic aerobic oxidation reaction in a conventional batch reactor faces several challenges, such as severe heat and mass transfer limitations and safety concerns. To circumvent the drawbacks of conventional batch reactor, micro-/milli-fluidic reactors, with enhanced heat and mass transfer properties, have been extensively studied.^1-3 Even though microreactors provide tremendous acceleration in mass transport, predicting intrinsic reaction kinetics under absolutely no mass transfer limitations is still a challenge especially for intrinsically fast multiphase reactions. Consequently, the observed rate of reaction may still be the rate of mass transfer instead of the intrinsic rate of the reaction. Here, we present a novel methodology to obtain intrinsic kinetic data of an intrinsically fast multiphase reaction by using a triphasic millifluidic reactor. We chose a model reaction, oxidations of benzyl alcohol with oxygen as the near-ideal oxidant and Cu (I)/bpy/NMI/TEMPO as the homogeneous catalyst. A triphasic flow reactor incorporating organic slug and air enveloped by a thin layer of continuous fluorinated oil is developed for this reaction. To capture the observed experimental trend and understand the interaction between mass transfer and intrinsic reaction rate, a simple reactor model is derived. Interestingly, based on the model, the k_eff under the same catalyst concentration with different tube length converges at very short residence times, indicating a convergence to the intrinsic reaction-controlled regime. Using this method, we measure an intrinsic rate constant for aerobic oxidation of benzyl alcohol with 54 mM of Cu/TEMPO of approximately 0.07 m³mol^-1s^-1.