Flow Chemistry of the Kolbe-Schmitt Synthesis from Resorcinol: Process Intensification by Alternative Solvents, New Reagents and Advanced Reactor Engineering

The Kolbe-Schmitt synthesis from resorcinol was exemplarily investigated to figure out the process intensification potential of continuous processing in the milli and micro scale, alone and combined with additional intensification means like alternative solvents, new reagents and an advanced reactor design. The oil bath-heated synthesis was investigated for capillary reactors of different dimensions, using aqueous solutions of KHCO3 and reactive ionic liquids. Already the first case led to space-time yields (STY) of 15,500 kg/(m3h) at 37 % yield. Synthesis with different CO2-donating salts showed that KHCO3 has the highest activity and that hydrogen carbonates are better than carbonates. The replacement of KHCO3 by reactive ionic liquids led to a substantial increase, both in yield (58 %) and STY (69,900 kg/(m3h)). The application of scCO2 did not significantly increase the yield despite an even more substantial change of the reaction medium. Using an electrically heated microstructured reactor resulted in a tenfold higher productivity (0.75 t/a) compared to the capillary reactor and does much better ensure scalability of the reaction.