Visible-light photooxidation of benzene to phenol in continuous-flow microreactors

Abstract

• Phenol was efficiently prepared via the benzene photooxidation in a microreactor. • The influencing factors were optimized to achieve a high phenol yield. • The photomicroreactor with LEDs greatly shortened the reaction time and saved energy. • A kinetic model was obtained with the photooxidation mechanism understanding. In this work, phenol was synthesized by the photooxidation of benzene in the continuous-flow microreactor with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as the oxidant and water as the oxygen source under 450 nm LEDs irradiation. The effects of the molar ratio of H 2 O to benzene, the molar ratio of DDQ to benzene, the concentration of benzene, the residence time, solvent type and reaction environment on the reaction performance were systematically studied. Under the optimized condition, the yield of phenol reached a high value (i.e., 94%) and the selectivity was over 99%. The photomicroreactor with the blue-LEDs as an eco-friendly light source significantly reduced the reaction time, and largely improved the light utilization efficiency and process efficiency for the synthesis of phenol from benzene compared to the batch reactor. Moreover, the reaction kinetic model was established based on a plug flow model, and the reaction order with respect to benzene and DDQ was found to be two and one in this photooxidation process, respectively. Such establishment of a reaction kinetics was beneficial for further understanding the photooxidation mechanism. This work demonstrates that the application of microreactors in combination with LEDs provides a promising and efficient strategy for the photooxidation of benzene to phenol and for the deep understanding on its photooxidation mechanism.