Evaluation of RuxNi1-x/SBA-15 catalysts for depolymerization features of lignin macromolecule into monomeric phenols

The bimetallic catalysts were prepared by a wetness impregnation method with different molar ratios of Ru and Ni (RuxNi1-x, x = 0.2, 0.4, 0.6, 0.8, and 1.0) on mesoporous silica SBA-15. Soda lignin (SL) was directly depolymerized to phenol-rich oil fraction (lignin-oil) over bimetallic catalysts under supercritical ethanol. 2D-HSQC-NMR demonstrated that representative interunit linkages such as β-O-4, β-5, and β-β were considerably degraded during depolymerization process by thermal reaction. Phenol was mainly produced from SL with other phenolic compounds such as guaiacol, 4-ethylphenol, 4-methylguaiacol, 4-ethylguaiacol, and syringol. This study revealed that the chemical properties of lignin-oil were clearly affected by the physicochemical properties of bimetallic catalysts. By increasing the amount of hydrogen binding sites and the acidity of the catalyst, the yields of lignin-oil, total phenols, and phenol increased while that of char decreased. In particular, acidic sites in bimetallic catalysts have a significant effect on the formation of phenol. The yields of total monomeric phenols and phenol were largest with the Ru0.6Ni0.4 catalyst (12.7 wt%, and 4.7 wt%, respectively).