Interest of the Precatalyst Design for Olefin Metathesis Operating in a Discontinuous Nanofiltration Membrane Reactor

This study aimed at evaluating the impact of the structure of several new olefin metathesis homogeneous catalysts on the performances of a membrane reactor running in a discontinuous mode and equipped with a nanofiltration membrane that was stable in toluene. A set of tailor-made ruthenium-based precatalysts were prepared with a first objective of enhancing the retention of the precatalyst, that is the stable source of the active catalyst, by organic solvent nanofiltration using a commercial polyimide membrane (Starmem122). These prototype precatalysts were designed taking into account both the molecular weight and the physicochemical characteristics allowing up to 99.6% retention of the precatalyst in toluene. The new precatalysts were then engaged in a model ring-closing metathesis reaction in the membrane reactor. Results, expressed as the precatalyst apparent turnover number, showed significant differences according to the selected precatalyst, underlining that the membrane reactor advantages and limitations were closely linked to the intrinsic activity of the catalyst. In addition to the retention of the precatalyst by the membrane, a major parameter was the percentage of the precatalyst really activated during the first load of the substrate since that controls the residual amount of precatalyst to be engaged in the following reaction cycle. The main consequence was the proposal of different running modes consisting of a cascade of synthesis in batch mode and separation by the membrane or a membrane reactor process.