Ruthenium Single Atom Anchored in Polyoxometalate for C–O Bond Breaking of Lignin

Lignin is an aromatic polymer that makes up 30% of wood biomass and is also the largest source of renewable aromatics. The monomer units in lignin are mainly bound by C–O bonds, and the 4-O-5 bond has the largest bond energy and therefore is the most difficult to break. Catalytic hydrodeoxygenation is considered as the most promising method for lignin increment, which can convert lignin and its derivatives into valuable chemicals and fuels. In this work, three single-atom ruthenium catalysts were designed to achieve the efficient breaking of C–O bonds. The 4-O-5 model substrate, diphenyl ether, can be completely converted into cyclohexane and cyclohexanol at 160 °C, H2 2 MPa, and 3 h with isopropanol as the solvent. HAADF-STEM and CO-DRIFTS show that Ru is in monatomic environments, which makes H2 dissociate via heterolysis. NH3-DRIFTS confirms that the catalyst has abundant Brönsted acid sites, which favor C–O bond cleavage. Through isotopic tracer experiments, hydrogen as the main hydrogen source and isopropyl alcohol as the secondary hydrogen source participated in the formation of cyclohexane, and the addition of water maintained the cyclic stability of the catalyst; XPS, EPR, and hydrogen overflow experiments demonstrate that the addition of a small amount of water in the isopropyl alcohol solvent could maintain the Brönsted acid strength of the catalyst. Moreover, the reaction pathway studies via the H–D exchange experiment show that a benzene ring near the catalyst is hydrogenated, and the hydrogen source is provided by hydrogen and isopropyl alcohol. Then the C–O bond is broken, and cyclohexane and phenol are formed. Finally, phenol is hydrogenated to cyclohexanol with H2 as the hydrogen source. In addition, other carbon–oxygen bond types, including β-O-4-type and α-O-4-type model substrates and native birch lignin, are all converted with high selectivity over this kind of catalyst.