Renewable Aromatics from Kraft Lignin with Molybdenum-Based Catalysts

The catalytic depolymerization of Kraft lignin in supercritical ethanol was explored in the presence of Mo2C- and MoS2-based catalysts. At 280 °C, Mo2C and Mo2C/Al2O3 afforded aromatic yields of 425 and 419 mg/g lignin, respectively: amongst the highest yields reported to date. Ionic-liquid-assisted delamination of MoS2 resulted in highly active catalysts, capable of quantitative conversion of lignin at the expense of aromatic yield (ca 186 mg/g lignin). Across all the catalysts studied, between 0.04 wt% and 0.38 wt% of molybdenum leached into solution under supercritical conditions, according to ICP analyses (corresponding to 27–570 µg of molybdenum in the reaction supernatant). A small contribution to the molybdenum in solution comes from the reactor itself (Hastelloy C contains 16 wt% Mo). Analysis of a depolymerization performed with fresh Kraft lignin and the soluble portion of the reaction mixture from a previous reactor run indicated that the leached species were neither active enough to afford the high conversions observed, nor selective enough to give high yields of aromatic products. In conjunction with the ICP data and differential chemoselectivities of the Mo2C- and MoS2-based catalysts, these results suggest that the bulk of the catalysis is heterogeneous.