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.

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Selective liquefaction of lignin is important for synthesis of value-added phenolic monomers contributing to green chemistry and sustainable energy applications. In the present study, a synergistic effect of a supercritical carbon dioxide (scCO2) acidic catalyst in combination with a heterogeneous metal oxide catalyst, specifically nickel oxide (NiO) or ceria-doped scandia-stabilized zirconia (CeScSZ), in sub- or supercritical water (sbcr/scH2O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO2-assisted hydrothermal process in the temperature range of 100–400 °C resulted in highly selective synthesis of the phenolic monomers. On the basis of the total organic carbon (TOC) and gas chromatography–mass spectroscopy (GC–MS) analysis, it is evident that the scCO2 catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO2 and heterogeneous NiO catalysts resulted in the highest total relative yield of the phenolic monomers, reaching ∼97% at 200 °C. It is confirmed that the scCO2-assisted hydrothermal depolymerization of the alkali lignin in the presence of both scCO2 and a heterogeneous catalyst is superior to that of the each of these catalysts alone and in comparison of the systems where scCO2 is replaced by inert nitrogen.