WOx/TiO2-Al2O3 catalyst design via W-O-Ti bond modulation for enhanced lignin valorization

The deliberate engineering of a highly active and robust deoxygenation catalyst is a crucial step in achieving efficient catalytic pyrolysis of lignin for aromatic production. Herein, we have developed a novel mixed oxophilic metal oxides (WOx/TiO2-Al2O3, WTA), tailored specifically for catalytic pyrolysis of lignin, which exhibits competitive performance against the commercial HZSM-5 catalyst. By systematically varying WOx concentration, we explore the relationship between WTA's microstructure and its catalytic performance. As WOx loading increases, the catalyst's coordination structure transitions from (Ti-O)n-W to (Ti-O)2-W(=O)2, significantly enhancing its deoxygenation potential through dehydration reactions with phenolic hydroxyl groups. When the WOx content is optimized to 24 %, the BTX (benzene, toluene, and xylenes) yield reaches 2.10 wt%, representing a 9 % increase compared to the HZSM-5 catalyst under identical reaction conditions, while simultaneously achieving a yield that is 15 times greater than that of direct pyrolysis of lignin. However, further increases in WOx loading shift the coordination structure of WTA to Ti-O-W(=O)2, leading to a subsequent reduction in deoxygenation ability. A positive correlation is observed between the total acid concentration of WTA and its W-O-Ti bond content, while the BTX yield correlates positively with the O-W(=O)2 bond content and weak acid concentration of WTA. Cycling experiments demonstrate that the metamorphosis of the Ti-W bonds in WTA from (Ti-O)2-W(=O)2 to Ti-O-W(=O)2 is responsible for the catalyst deactivation. These pivotal findings provide valuable insights into the rational design of improved catalysts for lignin pyrolysis applications.