High Phenol Yields from Catalytic Hydropyrolysis of Lignin and Phenolic-Rich Raffinate

In this work, fast pyrolysis of woody biomass for levoglucosan production was combined with reductive thermocatalytic valorization of its two main side streams, being (i) pyrolytic lignin and (ii) a phenolic-rich raffinate. Pyrolytic lignin is the precipitate, obtained upon cold water precipitation of pyrolysis oil. The phenolic-rich raffinate is the organic phase that originates from subsequent solvent extraction of the former water and levoglucosan-rich stream. This organic raffinate contains water-soluble phenolic monomers and is as such an underexplored (co-)feed for hydropyrolysis. Hydropyrolysis was performed of (i) pyrolytic lignin, (ii) pyrolysis oil-derived phenolics from the organic raffinate, (iii) organosolv lignin as model technical lignin, and a combination of pyrolytic lignin and pyrolysis oil-derived phenolics. The effect of hydrogen pressure (4 and 30 bar) and the presence of a Pd/C catalyst on the product slate and yields were assessed. Without catalytic vapor-phase upgrading, many different low-concentration phenolics were obtained, with a total yield of ca. 10 wt % (for pyrolytic lignin) to 13 wt % (for pyrolysis oil-derived phenolics). The product slate positively decomplexified upon catalytic vapor-phase upgrading. The yield in phenolics decreased modestly to 7–10 wt %, mostly being phenol, cresol, and xylenol. Pyrolysis oil-derived phenolics had for these conditions the highest yield in phenolics; its application as co-feed thus always goes in tandem with a yield increase in phenolics. It was also calculated that conversion of all quantified monomers to either phenol or catechol, yielded ca. 5–6 wt % phenol or 5–7 wt % catechol. From pine wood to phenolics, the yield varied between 1.9 (catalytic hydropyrolysis) and 2.3 wt% (non-catalytic). Overall, this work demonstrates value creation from overlooked side streams of the pyrolysis refinery.