Synthesis and characterisation of Palladium-containing polymeric colloidal nanoparticles for heterogenized catalysis

Sustainability is an important concern for the development of new organic materials. Synthetic procedures should become simpler, more efficient and less resource-intensive. Furthermore, waste must be avoided as much as possible. Hence, the combination of heterogeneous catalysis and micellar approaches can have a major impact on this field. Micellar catalysis helps in getting rid of organic solvents replacing them with water and in creating nanometric environments having enhanced reagent local concentrations to assure higher reaction rate and selectivity1. Easy recovery, reuse, and lack of toxic ligands make heterogeneous catalysts preferable with respect to homogeneous ones, although several challenges have yet to be faced: heterogeneous catalysts have lower activity, selectivity, and stability with respect to homogeneous ones, beside suffering from metal leaching phenomena. Undoubtedly the need for high surface area, reactant compatibility and homogeneous active sites distribution are the starting point for further studies2. Having all these challenges in mind, we developed a new heterogenized catalyst having π-conjugated organic colloidal nanostructures obtained through micellar polymerization as active sites support. They are designed to be intrinsically stable, water-dispersible and to embed the Pd catalyst involved into their own synthesis so that they could be directly reused as heterogenized catalyst in further micellar cross-coupling reactions. As a proof of concept, we synthetised a first-generation heterogeneous catalyst through deposition of Pd metal nanoparticles (Pd-NPs) on reprecipitated Poly(9,9-dioctyl)fluorene (PFO) nanospheres, then we tested its catalytic activity in in-water micellar Suzuki-Miyaura reactions. The efficiency of such catalyst was compared to that of the benchmark heterogeneous palladium catalyst Pd/C (Evonik hydrogenation catalyst). The initial findings show higher conversion associated to the use of the polymer-supported Pd catalyst. Furthermore, recycling of the catalyst is also possible thanks to the development of a straightforward workup procedure. The second-generation heterogenized catalyst requires the synthesis of conjugated polymer nanoparticles (CPNPs) by direct miniemulsion Suzuki polymerization. Literature examples of CPNPs synthesis always contemplate a purification step, to get rid of surfactants3. However, in this case, their entanglement within the colloidal structure is a crucial advantage: they constitute a template in which the polymeric nano-object can grow having a controlled morphology, and they make it suitable for its final application as catalyst. The synthesis produces dispersed spherical Pd-embedding polymer-surfactant semi-interpenetrated networks (sIPN). Such dispersion was then used as heterogenized catalyst showing the highest conversion rate. Encouraged by preliminary results both catalyst generalities are currently under investigation as well as the fine tuning of the structure of the polymeric support. The final big picture foresees the study and understanding of how the presence of heteroatoms, double bonds and porosity can influence the performance of these new catalyst in order to unveil the active role of the supporting part. References: [1] G. La Sorella, G. Strukul, A. Scarso, Green Chemistry, 2015, 17, 2644-683. [2] F. Poovan, V. Chandrashekha, K. Natte, and J. Rajenahally, Catalysis Science & Technology, 2022 12, 6623-6649. [3] J.M. Behrendt, J. A. E. Guzman, L. Purdie, H. Willcock, J. J. Morrison, A. B. Foster, M. L. Turner, Reactive and Functional Polymers, 2016,107, 69-77.