Catalytic conversion of lignin into biofuels by task-specific poly(ionic liquids)-supported metallic nanoparticles – CATAPILS

Homo- and co-polymers based on the imidazolium cation were prepared by free radical polymerization of N-vinyl imidazolium monomers and characterized by size exclusion chromatography and nuclear magnetic resonance. Corresponding nano-catalytic systems were synthesized following a direct pathway, involving a polyol process, that is by heating an alcoholic solution containing the poly(imidazolium)s stabilizer and a metallic salt of ruthenium or rhodium. Under those conditions, catalytically active NPs were formed and completely characterized by transmission electron microscopy, wide-angle XRay diffraction and light scattering. Xray photoelectron spectrometry also allowed us to probe the influence of the poly(imidazolium) stabilizer (especially the counter-anions) over the surface properties. Last, the as-prepared catalysts were mainly investigated in the hydrogenation of unsaturated substrates, as well as in hydrodeoxygenation reactions.

Two main results have been generated in this project; we have evidenced that the nature of the poly(imidazolium) stabilizer counter-anions influences the catalytic properties (activity and selectivity), but also the shape of the nanoparticles in the case of rhodium. For the first time, we could evidence that the catalytic properties of the targeted nano-catalysts could be reversibly switched, simply by exchanging PILs counter-anions. Chiral anions have also been introduced to develop novel asymmetric nano-catalysts.

Thanks to their peculiar properties, poly(imidazolium)s-based stabilizers allow the preparation of highly active and robust catalytic systems, notably for the hydrogenation of various unsaturated substrates. Remarkably and unexpectedly, the catalytic properties of those nano-catalysts can reversibly be tuned by simple anion exchange. The scope and limitations of the present methodology, in terms of substrates and counter-anions, deserve further investigations, especially on lignin itself. Asymetric catalysis involving chiral anions also require further studies to be performed.

Three publications are expected from the major results obtained in this work. (see above). The part dealing with the influence of the counter-anions over the catalytic properties of the as-prepared catalysts has been submitted to Angewandte Chemie. We are writing a second publication concerning the influence of those anions over the shape of Rh NPs although some structural characterization is missing. Some catalytic experiments are also required before the work on chiral NPs can be submitted.

Conversion of biomass, in particular lignin, into valuable renewable energy is one of the next great challenges, given the depletion of fossil-based ressources. The CATAPILS project deals with the catalytic conversion of lignin into biofuels and simple aromatic compounds by an innovative hybrid system composed of metallic nanoparticles stabilized by polymerized ionic liquids (PILs). Compared to traditional homogeneous and heterogeneous catalysts, the “semi-heterogeneous” PIL-NP catalyst will combine advantages of both systems: catalytic activity, modularity, selectivity and recyclability. In order to transform lignin into bio-fuels or simple aromatic compounds, oxygen-containing functions have to be removed, while aromatic residues have to be reduced. The development of an efficient catalyst for reactions such as, hydrogenation, hydrogenolysis, dehydroxylation or hydrodeoxygenation is one of the key aspects of this project. These reactions will be first studied on model compounds of lignin, featuring characteristic functionnal groups (aromatic, phenol, methoxyphenyl…). Noble (Pd, Rh, Au…) and non-noble (Ni, Co…) mono- and bi-metallic NPs, stabilized by the PIL will be prepared and investigated in hydro(deoxy)genation of model substrates. While catalysis results from NPs activity, the PIL component has also essential functions, including NP stabilization and solubilizing properties. Catalytic activity and selectivity of some reactions should be easily tuned by changing the nature (poly(cations), anions…), the structure (homo, statistical or block copolymers), or the different functions of the PIL (amide, imidazole, acide de Bronsted…). Multi-functional catalysts may also result from the association of a functional PIL and a NP, enabling complex reaction, such as hydrodeoxygenation, to be performed. In the last step of this project, catalytic transformation of bio-oils resulting from the cracking of lignin, or lignin itself, into bio-fuels or simple aromatic compounds, will be studied.