Nitration of benzene in a continuous-flow microreactor integrated with microfiber-structured Nafion/SiO₂ solid acid catalyst

We prepared novel monolithic microfiber-structured Nafion/SiO₂ solid acid catalysts with 60%-75% (φ, volume fraction) void volume by coating Nafion/SiO₂ sol-gel onto a sinter-locked stainless steel (SS-316L) microfibrous network. We characterized the microstructured Nafion/SiO₂ catalysts using Fourier transform-infrared spectrometry (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and NH₃-chemisorption. Results show that the sol-gel technology results in highly dispersed Nafion particles which significantly increase the number of accessible acid sites. The Nafion/SiO₂ catalyst coating layer is made up of 200-400 nm particles and looks porous. The catalytic performance of the microstructured Nafion/SiO₂ catalysts was tested using the nitration of benzene in a continuous-flow microreactor that integrated heat-exchange, mixture, and catalytic reactions. The best catalyst had a loading capacity of 36.3% (w, mass fraction) for the Nafion/SiO₂ composite in which the Nafion content was 20% (w). At a reaction temperature of 75 °C, this optimized microstructured solid acid catalyst resulted in a benzene conversion of 44.7% with a nitrobenzene selectivity of 99.9%. At an equivalent conversion level, the activity per acid site of the microstructured Nafion/SiO₂ catalyst is nearly 600 times as that of liquid sulfuric acid.