Boosting the hydrodeoxygenation capacity of MoO_x-based catalyst for efficient single-step conversion of bioglycerol to sustainable propylene

Abstract: Within the current scenario of climate changes and of depletion of petrol-based resources, upgrading bioglycerol-to-propylene (GTP) is of great technological-commercial interests because; i) crude bioglycerol is a by-product of world-scale biodiesel industry and ii) propylene market is mainly suffering from shortage. Thus, the so-called GTP route is currently investigated either through high-temperature processes, multi-steps reactions, tandem catalytic reactors, single-step hydrodeoxygenation. This latter strategy seems to be more suitable for mass production of propylene. However, the used MoO_x-based catalyst is still suffering from deactivation by C_xH_yO_z deposition and oxygen accumulation under H₂O-rich reaction medium. In this work, we propose developments to protect MoO_x species into ordered silica (SBA-15) and to balance their deoxygenation-hydrogenation capacities by doping transition metals (Pd, Ru, Ni, W, Co and V).Catalyst properties such as; chemical/composition states, redox/acid sites, location, and distribution, were studied by complementary techniques: ICP-OES, NH₃-TPD, H₂-O₂-titration, in-situ/Operando XPS and HRTEM-EDX. All the catalysts were tested in using a micro-activity setup (Avantium) at temperature ∼335 ºC, H₂-pressure ∼50 bar, C₃H₈O₃/H₂O ∼10wt.%, H₂/C₃H₈O₃ ratio (mol.) ∼55:1, spaces-velocity ∼5.5 h^-1and time-on-stream ∼10h.As shown in fig. 1, the combination of Ni, Ru, Co or V metal with MoO_x species balances better their deoxygenation-hydrogenation rates, leading to high propylene yield (up to 65%). However, the addition of Pd or W metals shifts the hydrodeoxygenation of glycerol rather to propane yield (up to 23%). This prominent hydrogenation events at full glycerol conversion ∼100% was attributed to the high electronic density of transition metals, mainly inducing H₂-spillover phenomenon. The stability of chosen catalysts; 12.9Mo-1.9Co@SBA-15 and 23.4Mo-3.8Ni@SBA-15, were also tested under more relevant spaces-velocity ∼9.5h^-1, temperature ∼400ºC and time-on-stream ∼27.5h. As shown in Fig.2-3, both catalysts feature stable propylene yield ∼63% and ∼50%, respectively, without detecting any deactivation issues. Thus, modifying the properties of MoO_x-based catalyst allowed to greatly improve their hydrodeoxygenation capacity and stability for efficient single-step GTP reaction. The operating conditions of GTP reaction are also object of detailed optimization study by combining Design of Experiments and high-speed experimental methodologies to set propylene yield at maximal values.