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⁻¹ 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 (approximately 186 mg g⁻¹ lignin). Across all the catalysts studied, between 0.04 wt % and 0.38 wt % of molybdenum leached into the solution under supercritical conditions, according to inductively coupled plasma (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.