Syngas-Based Upgrading of Pyrolysis Oil over Spent Hydrotreatment Catalysts: Effects of CO/CO ₂ Cofeeding and Dilution

Hydrotreating is essential to stabilize pyrolysis oils rich in reactive aromatic olefins such as styrene. This work evaluates the partial hydrogenation of refuse-derived-fuel (RDF) pyrolysis oil using syngas-like H2/N2/CO/CO2 mixtures instead of pure hydrogen, together with a spent sulfided Ni–Mo/Al2O3 refinery catalyst. The individual effects of H2 dilution and the presence of CO and CO2 were systematically analyzed under fixed-bed conditions at 210 °C and 3.5 MPa. Dilution experiments reveal that at least 50% H2 is required to maintain high initial activity and prevent oligomerization. CO2 (≤20%) markedly improves olefin conversion and catalyst stability, attributed to electronic promotion of Ni–Mo–S sites and partial inhibition of coke precursor formation. In contrast, CO inhibits hydrogenation through competitive adsorption on metallic sites. A unified kinetic model accurately predicts olefin conversion and alkyl aromatic selectivity for all gas compositions, integrating deactivation and competitive adsorption effects. The results obtained with binary gaseous mixtures remained valid when testing a realistic gas mixture representative of RDF pyrolysis (50% H2, 30% N2, 20% CO), confirming the proposed hydrogenation mechanism and the kinetic model. These results demonstrate a viable circular strategy in which pyrolysis-derived gases, or gases with similar composition produced from other thermal processes, can serve as reducing agents for stabilizing pyrolysis oils using discarded refinery catalysts.