SP Process Development researchers have invented new recycling methodology for homogeneous noble metal catalysts

Catalysts based on transition metals such as ruthenium, palladium, rhodium and platinum are used extensively in the fine chemical, agrochemical and pharmaceutical industry due to these metals' unique ability to catalyze important synthetic transformations with high selectivity. Besides all benefits, in particular two major challenges are associated with the use of these metals on industrial scale.

The high cost of the metal leads to process economic limitations with high demands on catalyst stability and ability to recycle the catalyst.

Due to their high toxicity the permitted levels of these metals in drugs and consumer products are strictly regulated and should normally not exceed 10 ppm, and the ability to efficiently remove the metal becomes important.

In addition, several of these catalysts have an unfortunate tendency to precipitate in metallic form on the walls of the reaction vessel. In the pharmaceutical industry in particular, this can be a serious problem since a multi-million dollar production plant will in worst case no longer be approved for drug manufacture due to regulatory rules.

The industry has struggled for a long time with these challenges which in many cases has hampered introduction of efficient catalytic methodology and practical general solutions for sustainable catalyst handling are still lacking in many regards.

Now Dr Per Ryberg and Dr Robert Engqvist at SP Process Development have succeeded in developing a palladium based catalyst system for the synthetically very important Heck reaction, which is both easy to separate from the reaction product and stable enough to allow for reuse in several consecutive batches.

- It’s all about being able to control the oxidation state of the catalysts resting state, by tuning the rate of the individual steps in the catalytic cycle, says Per Ryberg.

A catalytic process proceeds in a stepwise manner via a series of catalytic intermediates with different oxidation states on the metal e.g. 0 and +2 for palladium. Some intermediates are stable while others are highly unstable. Depending on which steps are fast and slow, either stable or unstable intermediates will accumulate in the system thereby influencing the lifetime/durability of the catalyst. Our detailed understanding of the catalytic cycle, generated from advanced mechanistic investigations have enabled us to tune the relative rates of the individual steps and thereby the partitioning between intermediates such that the dominating catalyst species is a stable Pd2+ complex. We have demonstrated that our catalyst can be reused a large number of times without any detectable decrease in catalyst activity. Besides being recyclable the catalyst is funneled to a dormant form at the end of the reaction which is very easy to separate from the reaction product such that only a few ppm palladium remain in the product. Yet another advantage of this new catalyst system is that we have been able to completely suppress the catalysts tendency to deposit on the reactor walls, which otherwise is a particularly pronounced problem in palladium catalyzed reactions.

The present discovery constitutes an important step towards identifying general and sustainable solutions for catalyst handling in the fine chemicals and pharmaceutical industry.

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