Thermal Liquefaction of Lignin to Aromatics: Efficiency, Selectivity, and Product Analysis

This perspective addresses efficiency and selectivity of high-temperature lignin liquefaction processes conducted in various reaction media as sub-and supercritical fluids. The challenges in efficient and selective production of high-value organic monomers from lignin are reviewed critically, along with analytical protocols essential for their accurate recovery after lignin degradation. The current approaches targeting the formation of phenolic monomers from lignin are discussed in terms of their repolymerization, a process that decreases the reaction selectivity and yield of the dominant phenolic monomers. The potential to solve this grand challenge is analyzed in terms of acid and/or protic cosolvent application, reduction of the reaction temperature, quenching of the reactive lignin depolymerization intermediates, and presence of heterogeneous catalysts, such as zeolites, metals, and metal oxides, sulfides, and phosphides.