Homogenous Catalysis for the Sustainable Valorization of Biomass

Being the only sustainable hydrocarbon-based source, biomass plays an important role in our energy landscape and has the potential to have a negative carbon footprint. Gaining access to sustainable fuels and chemicals through biomass valorization can be considered as a formula for a swift green transition. Herein, I developed a benign method for converting biomass waste material to fuels and chemicals through the interplay between organometallic pincer catalyst and a Brønsted acid.
Homogeneous transition metal-based Ru-PNP complexes are well known for (de)hydrogenation reactions for small molecule transformation. However, PNP pincer complexes have never been reported for its activity under acidic conditions and here comes the novelty of my PhD project. I demonstrated that the right combination of Ru-PNP catalyst and a Brønsted acid is able to transform the biomass feedstock directly to GVL (gamma-valerolactone). GVL holds highly promising potential as a future biomass-derived sustainable fuel and feed compound in many chemical industries. It is therefore extremely attractive to directly convert biomass to GVL. However, such a transformation is elusive owing to the high complexity and relatively inert nature of biomass. In concise, low loadings of Ru-MACHO-BH allows for valorizing woody and starchy biomasses to GVL under mild conditions (30 bar H₂, 140 °C, H₃PO₄(aq), 24-72 hours) with yields ranging from 16-26 wt%. No chemical treatment or isolation is required for this game-changing method and will represent the state-of-the-art for this transformation or for any transformations of biomass derived substrates for that matter. In addition to the raw biomass feedstock, we also investigated cellulose and hemicellulose as substrates to obtain insight into the underlying mechanisms and performances of the two main reaction pathways for the biomass.
Synthesis, characterization and some catalytic activities of a novel Ru-CNC pincer complex have also been demonstrated in the thesis as well. Exchanging the phosphorus arms with N-heterocyclic carbenes results in a transition metal that is significantly more electron-rich to the extent that the metal-hydrido complex can deliver its hydrogen even under acidic conditions. Based on the preliminary results, this scenario opens for carrying out hydrodeoxygenation (HDO), CO₂ hydrogenation and several other transformations of potential substrates.