Size-dependent catalytic performance of ruthenium nanoparticles in the hydrogenolysis of a β-O-4 lignin model compound

One-pot depolymerization of lignin to well-defined chemicals and their further deoxygenation to arenes are extremely attractive. Based on our previous results that Nb2O5 species can promote the cleavage of C-O bonds and the synergistic effect between Ru and Nb2O5 species led to the direct transformation of lignin into arenes over a Ru/Nb2O5 catalyst [Nat. Commun., 2017, 8, 161041], the importance and influence of ruthenium particle size need to be further studied. Herein, we report the different performances of Ru/NbOPO4 catalysts with various Ru particle sizes in the hydrogenolysis of a lignin model compound. This study shows that the catalytic activities and the product distributions strongly depend on the size of Ru nanoparticles (NPs): Ru NPs with smaller diameters (1.2 and 1.8 nm) have a higher activity for the cleavage of C-aliphatic-O and C-aromatic-O bonds, whereas Ru NPs with larger diameters (2.5 and 5.3 nm) show a much lower activity for C-O cleavage, especially for the C-aromatic-O bond, resulting in totally different product distributions. Density functional theory (DFT) calculations revealed that the presence of step and edge sites in smaller Ru particles can significantly lower the barriers for both C-aromatic-O and C-aliphatic-O bond cleavage. This study provides a new way for catalyst design towards highly efficient and selective hydrogenolysis of lignin to useful aromatic chemicals.