Conversion of Lignin with Supercritical Water−Phenol Mixtures

TLDR

Lignin decomposition was studied in supercritical water at 673 K with and without phenol, varying water density to observe changes in THF‑insoluble yield and THF‑soluble molecular‑weight distribution. Higher water density increased lignin conversion, while phenol addition lowered TIS yield, shifted TS products to lower molecular weight, produced alkylphenols, and by reacting with reactive sites it suppressed cross‑linking and promoted decomposition to lower‑molecular‑weight compounds.

Abstract

The decomposition of lignin was examined in supercritical water with and without phenol at 673 K. In the absence of phenol, the yield of tetrahydrofuran (THF)-insoluble (TIS) products decreased and the molecular weight distribution of THF-soluble (TS) products shifted toward lower molecular weights as the water density increased. The increase in water density enhanced the lignin conversion. In the presence of phenol, the yield of TIS products was lower and the molecular weight distribution of TS products shifted toward lower molecular weights than those in the absence of phenol. Some alkylphenols were obtained only in the presence of phenols, because of the reaction of phenol with the decomposition products. These results show that the reaction of phenol with reactive sites occurred in supercritical water and suppressed cross-linking reactions among reactive sites of large fragments. This promoted the decomposition of lignin to lower-molecular-weight compounds.