Simultaneous catalytic de-polymerization and hydrodeoxygenation of lignin in water/formic acid media with Rh/Al2O3, Ru/Al2O3 and Pd/Al2O3 as bifunctional catalysts

The catalytic solvolysis of 3 lignins of different sources in a formic acid/water media using bifunctional Ru/Al2O3, Rh/Al2O3, Pd/Al2O3 catalysts was explored in a batch set-up at different temperatures and reaction times (340-380 °C and 2-6 h, respectively). Blank experiments using only gamma-alumina as catalysts and non-catalyzed experiments were also performed and compared with the supported catalysts results. All the supported catalysts significantly improved the oil yields on a lignin basis, with yields up to 91.5 wt% using the Ru catalyst. The main components phenol, cresol, guaiacol, methylguaiacol, catechol, ethylcatechol, syringol and o-vanillin are found in different concentrations depending on the catalytic system. The stable Lewis acidity in the alumina support has been found to be active in terms of de-polymerization of lignin, leading to lower average molecular weight oils. In addition, it was found that alumina plays a significant role in the re-polymerization mechanism of the monomers. The effect of the type of lignin on the final oil and solid yields was also established, demonstrating that lignins produced by basic pretreatment of biomass do not show significant increase in oil yield when catalysts on an acid support like alumina are used. The interpretation is that acid conditions are needed for efficient de-polymerisation of the lignin.