Design of Optimized Catalysts for Biomass Upgrading using a Theoretical/Experimental Approach

Lignin pyrolysis produces oxygenated bio-oils with poor fuel efficiency. Hence, they are upgraded through hydrodeoxygenation (HDO) leading to the production of oxygen-free molecules and water as by-product. Our objective is to elaborate new catalysts for HDO by combining Density Functional Theory (DFT) calculations with experimental studies. HDO includes two deoxygenation routes: hydrogenation of aromatic ring before C-O cleavage (HYD) or direct C-O cleavage - Direct DeOxygenation (DDO). Fe@silica catalysts show a good activity and selectivity toward aromatics. Thus, our work focused on designing such catalysts that promote DDO in order to increase aromatics production under lower H2 pressure. The adsorption energies of phenol and inhibitors (CO and water) over silica surfaces, having various silanol densities and types, were computed by DFT. Three interaction modes were investigated: “perpendicular O-int”, “flat π-int”, and “flat O-int”. For amorphous silica, the highest adsorption energies were found for the “flat O-int” mode, and a specific interaction of 120 kJ/mol (with a C-Si bond and phenol deformation) was observed for surfaces with a silanol density between 2 and 3.3 OH/nm2. CO competitive adsorption is negligible for all silica surfaces, which make them more attractive than conventional sulfide catalysts. Hence, these results motivated the synthesis of silica-supported catalysts with a silanol density between 2 and 4 OH/nm2. Single iron atom catalysts (SACs) supported on silica were elaborated using non-ionic/metallic surfactants. Iron distribution within mixed P123/CTAF micelles, used as templates, allows the fine dispersion of those atoms within silica mesopores. The lack of iron clusters was confirmed by synchrotrons PDF, STEM mapping, magnetic and NMR measurements. DFT+U calculations confirmed that those atoms are predominately present as high spin Fe(III). This method increases the number of active sites, which improves the catalytic performance. However, the results of catalytic tests were unsatisfactory due to the difficulty of Fe(III) reduction. Metallic (Fe & Cu) and bimetallic (Fe-Cu) catalysts, synthesized by simple impregnation or through the co-precipitation method with thermal decomposition of urea, were tested for guaiacol HDO conversion. Results proved that bimetallic Fe-Cu have a better performance (90% conversion, 70% selectivity) than Fe-based catalysts since Cu incorporation facilitates the reduction of Fe(III).