Extending Ru-BINAP Catalyst Life and Separating Products from Catalyst Using Membrane Recycling

Abstract

Organic solvent nanofiltration (OSN) is a new membrane technology with many applications in pharmaceutical and fine chemicals development and manufacture, from laboratory through production scales. One application of particular industrial relevance is the ability to recover and recycle homogeneous catalysts, in particular asymmetric, organometallic, homogeneous transition metal catalysts. Industrial application of this group of catalysts is often limited due to the cost of applying these catalysts to single reactions and the subsequent product yield losses associated with removal of the catalyst from solution. OSN provides a technique that can maximise the value of the catalyst through catalyst recycle from one reaction batch to the next, whilst minimising the concentration of catalyst present in the reaction product. This contribution describes the development and demonstration of OSN catalyst recycle on an example catalytic system, the homogeneous, asymmetric hydrogenation of dimethyl itaconate (DMI) to dimethyl methylsuccinate (DMMS) with Ru-BINAP. The first step of the development process was to demonstrate that Ru-BINAP was sufficiently stable that the process concept, i.e., catalyst recycling, was feasible. A dilute solution (0.8 wt %) of DMI in methanol with a high catalyst loading (S/C = 500) was used, and 14 successive reactions were carried out with constant reaction rate and without yield or enantiomeric excess (ee) reduction. In industrial practice, the goal is always to operate a reaction at the highest, practical S/C ratio, and thus the next stage of development was to optimise the S/C ratio. The limiting, practical S/C ratio was found to be 7000. The process was then run with the optimised dilute reaction system, which allowed 10 reaction cycles to be carried out with only 20% addition of the initial (reaction 1) mass of catalyst to reactions 2−10 to maintain reaction rate, conversion, and ee. This leads to a 5 times increase in the overall S/C ratio. In addition, this optimisation reduced the amount of metal present in the product solution from 130 μg Ru per gram of product to <6 μg Ru per gram of product. The process was then scaled up to an industrially relevant substrate concentration (20 wt % DMI in methanol), and it was found that both the reaction (in terms of reaction rate, conversion, and yield) and the ability to recycle the catalyst were unaffected by the scale-up in concentration.