Selective depolymerization of lignin into oxygen-containing aromatics by novel RuW/TiO2 catalyst

Catalytic hydrogenolysis of dealkaline lignin represented a promising route to produce valuable chemicals. In this work, a novel bifunctional ruthenium-tungsten/titanium dioxide (RuW/TiO 2 ) catalyst was developed, in which RuW served as the active components for the activation of hydrogen donor and β-O-4 in the substrate, while titanium dioxide (TiO 2 ) acted as the support to tune the electronic structure of RuW alloy. The prepared catalyst was applied to convert dealkaline lignin into oxygen-containing valuable chemicals using methanol as both the solvent and hydrogen source. The reaction results of dealkaline lignin model compounds showed that the selective cleavage of the β-O-4 linkage in lignin model compounds could be achieved under the synergistic role of the catalyst and methanol. The conversion of 2-phenoxy-1-acetophenone was 99.9 %, and yield of phenol reached up to 97.8 % at 185 °C. Under dealkaline lignin experimental conditions, the presence of a gaseous hydrogen source significantly improved catalyst performance, achieving a high depolymerization degree of 99.8 % and the yields of aromatic monomers 25.1 %, oxygenated compounds 41.2 % and hydrocarbon compounds 1.3 % under the optimal conditions, which fulfilled the efficient conversion of dealkaline lignin. The complete conversion of dealkaline lignin facilitated the recycling of the catalyst, and the RuW/TiO 2 catalyst can be reused for more than five consecutive cycles without any significant loss of efficiency. The catalyst prepared in this study demonstrated a higher dealkaline lignin depolymerization degree (99.8 %) and total monomer yields (82.8 %) compared to other analogous catalysts under similar reaction conditions. Mechanistic analysis showed that RuW formed an alloy that activated methanol or hydrogen (H 2 ) to generate active H atoms. Density Functional Theory (DFT) calculation revealed that RuW effectively activated the β-O-4 ketone intermediate and exhibited a lower activation energy for C β -O bond cleavage. The RuW/TiO 2 catalyst efficiently catalyzed the direct C α -C β bond cleavage to remove the methyl structure and form an ester group on the aromatic ring. The high efficiency, facile recycling process, and excellent reusability of the catalyst rendered it competitive and potential for the conversion of dealkaline lignin into oxygen-containing aromatics. The RuW/TiO 2 catalyst can achieve the complete and selective conversion of lignin, yielding up to 82.8 % oxygen-containing chemicals, of which 62.9 % are oxygen-containing aromatic compounds. • RuW/TiO 2 can fulfill selective conversion of lignin to oxygen-containing chemicals. • Methanol acts as the solvent and hydrogen donor. • XANES indicated Ru, W and TiO 2 interacted in the form of W-Ru, W-O-Ru, and Ru-O-Ti. • DFT shows RuW can activate the β-O-4 ketone intermediate.