High pressure direct synthesis of adipic acid from cyclohexene and hydrogen peroxide via capillary microreactors

The direct synthesis of adipic acid from hydrogen peroxide and cyclohexene was investigated in capillary microreactors at high temperature (up to 115°C ) and pressure (up to 70 bar). High temperature was already applied in micro-flow packed-bed reactors for the direct adipic acid synthesis. In our previous work we showed that the process suffered from unavoidable gas generation due to hydrogen peroxide decomposition when working at low pressure. Herein, we used a high pressure strategy to minimize hydrogen peroxide decomposition. Huge hotspots were observed inside a micro-flow packed-bed reactor under high pressure conditions. Capillary microreactors display a better heat transfer efficiency and thus may provide a better alternative for scaling-up. Consequently, capillary microreactors were selected for the reaction process with high pressure. One assisting element is the addition of phosphoric acid which is generally known to reduce the decomposition of H2O2. The use of phosphoric acid had a positive influence on the isolated yield. We could improve the yield further by increased interfacial mass transfer between the organic and aqueous slugs, when increasing the flow rate whilst keeping the same residence time. A further gain was given by using the of 2-stage temperature ramping strategy which we recently introduced for the micro-flow packed bed reactor. Applying all these aspects led to a maximum yield of 59% at 70-115 ˚C and 70 bar. The stabilizing effect of phosphoric acid on H2O2 is more obvious in a the 2-stage temperature ramping scenario as in a single-temperature operation. In addition, channel clogging by adipic acid precipitation in microreactor was observed. Therefore, several useful strategies were proposed to prevent the channel clogging at high temperature and high pressure.