Stable and Continuous Production of Amines via Reductive Amination in a Green Switchable Solvent System with Efficient Water Removal

Abstract

We present highly selective Rh-catalyzed reductive amination (RA) with integrated recycling of the homogeneous catalyst via a thermomorphic multiphase system (TMS) in a continuous-flow miniplant. Methanol, as a green alternative, was used as the polar solvent. In continuous operation for over 90 h and without catalyst makeup, the yield of the desired tertiary amine was constantly over 90%. For the first time in RA, additional continuous removal of the coproduct water was successfully implemented via an organic solvent nanofiltration (OSN) membrane, keeping the water concentration constant below 3.1 wt % and showing the highest catalyst rejection (>99.7%). Rhodium loss via the product phase was unprecedentedly small, with only 0.003% per hour, a value never achieved with any known TMS system so far. This value corresponds to a concentration of 33 ppb in the product phase and only 0.6 mg Rh loss per kg of produced amine. The present study gives a very optimistic outlook for the potential of TMS as a catalyst recycling method for continuous chemical processes, given its excellent performance in the most relevant areas, such as reaction yields and catalyst stability/retention, as well as the removal of coproducts.

Continuous Homogeneously Catalyzed Production of Fatty Amines in Microemulsions─Proof of Concept in a Mini-Plant

“…In a comparable continuous operation of the RA in a TMS with the same educt-to-catalyst ratio, a maximum yield of 94% was achieved for 80 h and selectivities up to 96% . By comparison, the yield and selectivity in the current MES investigation are much lower.…”

“…In a comparable continuous operation of the RA in a TMS with the same educt-to-catalyst ratio, a maximum yield of 94% was achieved for 80 h and selectivities up to 96%. 48 By comparison, the yield and selectivity in the current MES investigation are much lower. The employed method of diluting the aldehyde with a cosolvent to suppress the aldol side reaction does not work sufficiently, and other options to boost selectivity should be investigated.…”

“…An equally low leaching rate of 0.003 %/h was only achieved in one suprising recent investigation of the RA in TMS. 48 The TMS currently achieves much higher selectivities and yields for the RA, as discussed in Section 4.2, compared to the MES, while the MES usually achieves better catalyst retention. This shows that the selection of an approach for catalyst retention in homogeneous catalysis is case-specific and requires careful consideration.…”

“…Focusing on the RA, homogeneous synthesis routes for the reaction with milder process conditions − are under investigation. Further investigations into these synthesis routes are being conducted to enable catalyst recycling with liquid multiphase approaches (biphasic, , TMS, , or MES), even in continuous operation …”

“…Further investigations into these synthesis routes are being conducted to enable catalyst recycling with liquid multiphase approaches (biphasic, 40,46 TMS, 32,47 or MES 36 ), even in continuous operation. 48 The main goal of our investigation is to prove the concept that continuous production of fatty amines is also possible in MES. In the current research state of MES, scale-up and continuous operation are the missing links toward industrial application.…”