Abstract
Solution-phase catalysis using molecular transition metal complexes is an extremely powerful tool for chemical synthesis and a key technology for sustainable manufacturing. However, as the reaction complexity and thermal sensitivity of the catalytic system increase, engineering challenges associated with product separation and catalyst recovery can override the value of the product. This persistent downstream issue often renders industrial exploitation of homogeneous catalysis uneconomical despite impressive batch performance of the catalyst. In this regard, continuous-flow systems that allow steady-state homogeneous turnover in a stationary liquid phase while at the same time effecting integrated product separation at mild process temperatures represent a particularly attractive scenario. While continuous-flow processing is a standard procedure for large volume manufacturing, capitalizing on its potential in the realm of the molecular complexity of organic synthesis is still an emerging area that requires innovative solutions. Here we highlight some recent developments which have succeeded in realizing such systems by the combination of near- and supercritical fluids with homogeneous catalysts in supported liquid phases. The cases discussed exemplify how all three levels of continuous-flow homogeneous catalysis (catalyst system, separation strategy, process scheme) must be matched to locate viable process conditions.
Highlights
Defined transition metal complexes [1] are able to catalyse a large variety of reductive, redox-neutral and oxidative processes, often with high rates and astonishing selectivities [2] including asymmetric control [3]
We provide a brief overview of the development of supported liquid phases (SLPs) catalysts and highlight some characteristics and applications, focusing on promising recent developments based on supported ionic liquids (ILs) phase catalysts with near- and supercritical fluids (SCFs)
From the examples discussed in this contribution it becomes evident that SLPs represent a versatile and promising approach to using molecular catalysts in continuous flow
Summary
Defined transition metal complexes [1] are able to catalyse a large variety of reductive, redox-neutral and oxidative processes, often with high rates and astonishing selectivities [2] including asymmetric control [3]. If a compatible process scheme can be designed around this, an integrated continuous-flow process may be realized (figure 1) [12,13] This resolves the issue of catalyst recovery and affords a number of well-known engineering advantages, including intensified space–time yields (STYs), reduced waste production and enhanced process control through more effective heat management and automation [14,15]. This challenge needs to be addressed in a multi-scale approach considering all conceptual levels from the molecular catalyst to the process [12]. Many of these catalytic systems have stable long-term activity, high selectivity and very low leaching rate, and, represent a robust and competitive technology for chemical manufacturing
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