Separation/recycling methods for homogeneous transition metal catalysts in continuous flow

Catalytic processes are of paramount importance in the chemical industry. Homogeneous catalysts are of great interest for synthesizing fine-chemical/specialty chemical/pharmaceutical products for their advantages of high activity and selectivity. However, their separation from the product remains a challenge. Transition metals and especially platinum group metals are used extensively as catalysts. They are expensive and there are strict regulations on the permitted levels of these metals in pharmaceutical products. Therefore, their effective separation is required. Moreover, we are running short of these and many other valuable metals. This puts more emphasis on the need to separate these homogeneous metal catalysts in their active form and recycle them back to the reactor. This review aims to provide the reader with an overview of the current literature on the separation/recycling methods of homogeneous transition metal catalysts in continuous flow. These include heterogenization, scavenging, using biphasic systems and organic solvent nanofiltration. There are numerous successful demonstrations on the laboratory scale and recently also on the industrial scale.