Scalable and green juglone synthesis via heterogeneous photocatalysis in a photomicroreactor†

The selection of green and scalable heterogeneous photocatalysis is always a great dilemma for the chemistry and chemical engineering communities. In this report, we resolved this vital conundrum by synergistically examining the scalability and green potential of heterogeneous photocatalysis for juglone synthesis. We scaled up juglone productivity and the space–time yield to 15 g per day and 43.87 g L⁻¹ h⁻¹, respectively, via heterogeneous photocatalysis with a high-power LED as a light source and Amb–m-TcPP as a photocatalyst, which was fabricated by incorporating meso-tetracarboxyphenylporphyrine (m-TcPP) over the polystyrene ion-exchange resin Amberlyst-15. Amb–m-TcPP showed excellent recyclability in multiple cycles and exhibited high photostability for 12 h continuous operation without m-TcPP leaching with a turnover frequency of 409.7 h⁻¹. The apparent rate constants were higher than those in previously employed heterogeneous photocatalysis, and intrinsic rate constants of intermediate steps were evaluated using transition state theory with quantum mechanical models using DFT simulations, which was useful for gaining mechanistic insights into this photooxidation process. Finally, heterogeneous DHN photooxidation was confirmed to be well aligned with green chemistry principles, and E factors were found to be under an acceptable range. Thus, this heterogeneous DHN photooxidation can be regarded as a green and sustainable synthesis route for multigram juglone production.