Lewis acid-base interactions of oxide support for enhanced C–C coupling and deoxygenation reactions

The transition to sustainable energy systems requires efficient catalysts capable of upgrading biomass into liquid fuels and platform chemicals to meet future energy demands. Metal oxides, as supports in bifunctional catalysts, are pivotal due to their ability to provide active sites, create oxygen vacancy defects, and facilitate electron transfer. While these properties are well-studied in the presence of metal nanoparticles, the intrinsic activity and surface properties of stand-alone oxide supports remain underexplored. This study investigates the role of bare metal oxides (TiO₂, ZrO₂, and Al₂O₃) in the direct vapor upgrading of beechwood biomass vapors via a two-stage process comprising a non-catalytic hydropyrolysis step followed by ex-situ catalytic upgrading. The performance of metal oxides was compared with non-metal oxide such as SiO₂. Through extensive characterization (H₂-TPR, NH₃-TPD, O₂-TPD, CO₂-TPD, BET, XRD, and Pyridine-FTIR), we establish that the combination of high weak acidity, low strong basicity, and reducibility of TiO₂ results in superior catalytic performance. Ex-situ upgrading over TiO₂ achieves the lowest oxygen-to-carbon ratio (O/C = 0.09) in bio-oil, the highest C₂+ fraction (98.7 %), and the largest C8-C16 fraction (49.9 %), while minimizing light molecule formation (16.5 %). Binding energy analyses further reveal that weak adsorption of model compounds (acetone, acetic acid, guaiacol) occurs on the TiO₂ (101) surface compared to other oxide surfaces, highlighting its exceptional properties for deoxygenation and C–C coupling. This work establishes the first comprehensive correlation between the catalytic performance of oxide supports and their surface properties using actual biomass feedstock, thus offering valuable insights for designing advanced catalysts for biomass upgrading.