Hyperactive catalytic species in the liquid phase from nanoparticles or immobilized molecular species – HYPERCAT

A wide variety of precatalysts will be synthesized, either based on nanoparticles or on transition metal complexes. They will then be tested and selected for Suzuki coupling and alkene hydrosilylation reactions. To quantify the level of leaching and identify which species are responsible for the catalytic activity, ICP/MS analyses will be performed on samples withdrawn at specific locations and moments in specific reactors (e.g. compartmented reactor). Besides the synthesis of the pre-catalysts library, the choice of the reactors is thus also a pillar of the project, allowing to decouple the contribution of adsorbed catalytic species and that of species in solution.
As a fall-back solution, in the case where active species are not at the ppb level but ppm, solutions can be found to confine the noble metal in the reactor, not by classical immobilization methods, but by using an appropriate reactor taking profit from the well-known release-and-catch (R&C) mechanism.
At the end of HYPERCAT project, it is expected to have a better knowledge of both Suzuki and alkene hydrosilylation reactions, to build a model allowing to discriminate the contribution of heterogeneous active species and species in solution during a catalytic reaction, and to have a better knowledge of the capabilities of a Reverse-flow reactor to run reactions with pre-catalysts presenting a release-and-catch behavior.

A variety of complexes of palladium with biscarbene, bispyridine and bisphosphine ligands were synthesized, characterized and tested in a batch reactor. Their activity was compared to that of the commonly used Herrmann-Beller palladacycle. The best complexes were structurally modified to allow grafting on TiO2. The activity of the latter catalysts was evaluated and compared to that of the homogeneous homologues. In some cases, a different behaviour (activity/stability) was observed, which could lead to presume an active role of the support either in terms of stabilization of active species, or in terms of substrate co-activation.
In addition, Pd nanoparticles, as well as bimetallic PdSn and PdCo nanoparticles have been prepared. Characterizations are in progress, as well as the first tests in Suzuki coupling reaction of iodoacetophenone with phenylboronic acid. Pt nanoparticles have also been synthesized for the hydrosilylation reaction and Pt3Sn nanoparticles are also being studied.
Continuous reactor experiments have so far used commercial solid catalysts (pending the supply of catalysts from the partners). For all the catalysts tested (Pd(0) supported on silica, alumina or carbon, and Pd(II) supported on silica), it was shown that the catalytic activity obtained during the coupling reaction of halogeno-acetophenones with phenylboronic acid was mainly due to Pd metal leaching. Moreover, even if we have only indirect arguments for the moment, nothing comes against the hypothesis showing that 100% of the activity would be due to Pd in solution and that the catalytic active species would be common to the different solids tested. These results were obtained using a «split flow reactor«, a very simple test developed at the LGPC laboratory.

Continuation of catalyst synthesis :
- immobilization/grafting of Pd complexes
- Synthesis of Pt, Pd, and bimetallic PtX or PdX nanoparticles, and characterization
- immobilization of nanoparticles on solid supports
- Catalytic tests in batch reactor for the Suzuki-Miyaura reaction in the case of Pd and for the hydrosilylation of alkenes in the case of Pt
- testing a wide variety of solid catalysts (commercial, then those synthesized by the project partners) in the «split-flow« reactor to demonstrate more directly, without the use of a model underpinning simplifying assumptions, our hypothesis on the nature and generation of active species.

Bourouina, A.; Meille, V., de Bellefon, C. About Solid Phase vs. Liquid Phase in Suzuki-Miyaura Reaction, Catalysts, 2019, 9, 60.

Bourouina, A, PhD-thesis from Université Claude Bernard Lyon I, N°ordre : 2019LYSE1258, “Desperately Seeking For The Catalytic Species In Suzuki-Miyaura Reaction”

The Suzuki-Miyaura reaction (Suzuki) suffers from serious drawbacks for a wide industrial application. The first is the fate of the catalysts. The Suzuki reaction indeed operates in the presence of Pd, in the form of molecular complexes, colloidal or supported Pd nanoparticles. Health, environmental and economic (HEE) issues are thus being addressed: 1) the Pd content in the product must comply with very strict regulations - For instance, the residual palladium in the active pharmaceutical ingredient (API) must be below below 10 ppm but regulations will certainly decrease the level below 1 ppm in the future -, 2) Pd is a heavy metal that must be contained within the production unit and 3) palladium is a very expensive metal which must be recovered and recycled.
The picture for the other target reaction, i.e. alkene hydrosilylation is very much the same: it is also widely used in industry (silicone market), and also suffers from a major drawback related to the use of a soluble platinum complex. Platinum remains in the reaction products at too high levels, thus leading to very high costs, Pt resource depletion and imposing a downstream adsorption on charcoal for some products.
In both reactions, there is however a small window left open: in the case where traces, i.e. “homeopathic”, amount of metallic species could efficiently catalyse the reactions, it could be acceptable to fulfill the HEE requirements, i.e. yield of desired product > 95%, precious metal content in the outlet process flux below 1 ppm, TOF > 100000/h. The window is small but real. The available literature provides examples where traces of Pd, as low as 10 ppb, can effectively catalyse the Suzuki reaction. The bottleneck is actually to broaden the scope of such “homeopathic” catalytic species and to make them available on demand.
Thus, the objective of the HYPERCAT project is to find new and generic ways to generate very active catalytic species from Pd and Pt.
To reach this goal, our hypothesis, based on a careful analysis of miscellaneous literature reports,  is that the metal precursor must be conditioned in a specific environment in order to generate hyperactive species. It is not expected that all the pre-catalysts leading to ppb levels of metal in solution will lead to hyperactice species.
A wide variety of precatalysts will be synthesized, either based on nanoparticles or on transition metal complexes. They will then be tested and selected for Suzuki coupling and alkene hydrosilylation reactions. To quantify the level of leaching and identify which species are responsible for the catalytic activity, ICP/MS analyses will be performed on samples withdrawn at specific locations and moments in specific reactors (e.g. compartmented reactor). Besides the synthesis of the pre-catalysts library, the choice of the reactors is thus also a pillar of the project, allowing to decouple the contribution of adsorbed catalytic species and that of species in solution.
As a fall-back solution, in the case where active species are not at the ppb level but ppm, solutions can be found to confine the noble metal in the reactor, not by classical immobilization methods, but by using an appropriate reactor taking profit from the well-known release-and-catch (R&C) mechanism.
At the end of HYPERCAT project, it is expected to have a better knowledge of both Suzuki and alkene hydrosilylation reactions, to build a model allowing to discriminate the contribution of heterogeneous active species and species in solution during a catalytic reaction, and to have a better knowledge of the capabilities of a Reverse-flow reactor to run reactions with pre-catalysts presenting a release-and-catch behavior.