Catalytic depolymerization of lignin macromolecule to alkylated phenols over various metal catalysts in supercritical tert-butanol

Three technical lignins (Asian lignin (AL), Organosolv lignin (OL), and Inbicon lignin (IL)) were selectively degraded to alkylated phenols (AP) over Pt/C, Pd/C, Ru/C, and Ni/C in supercritical t-BuOH. AP and other phenolics present in the lignin depolymerized oil fraction (lignin-oil) were subjected to several chemical analyses. GC/MS analysis showed AL yielded a maximum of 1.40 wt% of 4-ethylphenol and 4-ethylguaiacol, while OL produced 1.89, 1.11, and 1.43 wt% of syringol, 4-methylsyringol and 4-propylsyringol, respectively under Pt/C condition. Relatively small amounts of AP were obtained from IL because it contained not only lignin moiety but also carbohydrate, ash, and other components. Meanwhile, the average molecular weights (Mw) of the lignin-oils were significantly lower than those of the lignins, clear evidence of lignin depolymerization. Moreover, elemental analysis indicated hydrodeoxygenation mainly occurred during lignin depolymerization. With increasing catalyst amount (Pt/C), total monomeric phenol yields decreased due to side reaction, but some kind of AP such as 4-ethylphenol and 4-ethylguaiacol showed increase trend with increasing catalyst amount.