段婵

Polyurethane open cell foams coated by polydopamine as structured supports for nickel--‐photoredox dual catalysis

Context and objectives.
Nickel's chemistry is currently experiencing a renewal. In addition to economical and ecological benefits, nickel indeed presents some advantageous characteristics when compared to its widely used 3d counterpart, palladium – easier oxidative addition, slower β--‐H--‐elimination, and multiple oxidation state easy accessible – that make of it much more than a low cost ‘erzatz’,1,2 in particular in the case of visible--‐light mediated nickel--‐photoredox dual catalysis.

This latter domain, actually, is booming since seminal works3 demonstrated that single--‐electron transfers or transmetallations mediated by a photocatalyst (PC) could be intercepted by Ni(II) complexes to generate Ni(III) intermediates from which easy reductive elimination of the desired cross--‐coupled product could occur. Thanks to this new paradigm, cross--‐couplings deemed to be difficult such as Csp3–Csp2 and Csp3–Csp3 couplings,4 or previously inaccessible nickel--‐catalysed C–O couplings,5 can now be readily achieved under mild conditions.

Major limitations, however, come from (i) the high cost of the PC, which is typically a ruthenium or iridium polypyridyl complex, and (ii) the often high molar extinction coefficients of the PC that hamper light penetration into the solution and thus causes dramatically reduced reaction rates when scaling--‐up reactions in traditional batch reactors due to the low surface area to volume ratio of the latter.6

Possible answers to these limitations go through the immobilisation of the PC (and the Ni catalyst) on a solid sup--‐ port to develop easily recoverable and reusable heterogeneous visible light PCs, and/or the use of continuous flow reactors. In this context, the objective of this PhD is to immobilize the PC and the Ni catalyst on a support that is ideally structured to allow both their easy recovery and reuse, and their use under continuous flow conditions for challenging couplings (Fig. 1). State of the art. Continuous processes based on Structured Catalytic Supports (SCS) are widely used in industry. Indeed SCS allow an important surface over volume ratio, a small pressure loss, efficient mass transfers, an intimate mixing of the reagents, and an easy separation of the catalyst from the products.7

In this context, we have developed the use of polyurethane open cell foams (PUF) as SCS. These inexpensive foams present the structural and transport properties of traditional SCS, with the advantage of being easily engineered because of their lightweight and high mechanical flexibility. Inspired by a biomimetic approach,8 we have shown that the surface of this polymeric structured material can be efficiently coated with an adhesive layer and further functionalized with either inorganic particles or molecular compounds.9,10 The process relies on catechol chemistry and consists in coating a PUF with a layer of polydopamine (PDA) by simple immersion in a buffered aqueous solution of dopamine (Fig. 2).  

Thanks to the adherence properties of the catechol units,8 metallic or metal oxide (nano)particles can be easily grafted all over the surface of the foam by simple dip--‐coating.9a In particular, by using TiO2 nanoparticles, a flexible structured--‐supported PC, PUF@PDA@TiO2, has been designed to degrade a variety of air and water pollutants, under batch or flow conditions.10a Thus, thanks to the easiness of engineering of our SCS and to its large pores that easily let light and fluids go through, PUF@PDA@TiO2 rolls have been used in a flow reactor equipped with a UVc lamp under continuous conditions for several weeks for the degradation of water pollutants without any loss of activity (Fig. 3).

Moreover, thanks to the presence of the hydroxyl groups of the catechol units, the covalent grafting of molecular catalysts bearing a group that can form covalent bonds with them, such as an alkoxysilane, an amine or a thiol group has also been achieved (Fig. 4).9a

PhD objectives. Considering these precedents, a first objective of the PhD will be to establish a proof of concept with an Ir--‐ or Ru--‐ based PC. For that purpose, a suitably modified polypyridyl Ir or Ru complex will be covalently anchored on PUF@PDA, and the resulting structured catalytic material will be tested in a model PC reaction. Several pitfalls may be encountered at this stage, and it will be necessary to establish the robustness of the anchoring, the chemical resistance of the support and the reusability of the catalyst.

Next the covalent grafting, on a single foam, of both a PC and a nickel cross--‐coupling catalyst will be investigated. The resulting PUF@PDA@Ni--‐PC structured material will be studied in a model nickel--‐photoredox--‐catalyzed reaction. At this stage, it will be of main interest to establish whether the PC and the nickel catalyst will be close enough to ensure efficient electron transfers. Further developments will involve optimisation and scale--‐up in continuous flow photoreactors, as well as extensions to original Ni catalysts and PCs.

Work environment: The PhD will be conducted in the team of Applied Organometallic Chemistry of the LIMA (UMR CNRS 7042) at the Ecole européenne de Chimie, Polymères et Matériaux (ECPM) of the Université de Strasbourg. Strong interactions with the LAGEPP (UMR CNRS 5007) at the University of Lyon I, and the Institut Charles Sadron (UPR CNRS 022) in Strasbourg will allow the PhD candidate to develop additional competences in Chemical Engineering and Surface Sciences.

Starting date: October 2019 --‐
Duration: 36 months --‐
Net salary: ca. 1400