Table 25. Overview of Solvolytic Methodologies for the Depolymerization of Lignin with Yields of Phenolic Monomers.

Entry	Feedstock	Medium	Conditions	Oil yield	Composition	Notes	Ref
1	Softwood Kraft lignin (Indulin AT)	10 wt% solution in 5 wt% NaOH (aq)	270–315 °C, continuous-flow reactor with LHSV 1.4–4.0 h–1	Up to 19.1 wt% of a monomer-rich fraction obtained after acidification and extraction	Up to 8.4 wt% identified monomers	Main products: guaiacol and pyrocateechol; also other lignins were studied	(430,431)
2	Olive tree prunings ethanosolv lignin	5 wt% solution in 4 wt% NaOH (aq)	300 °C, 90 MPa, batch reactor, 40 min	20 wt% oil yield after acidification, extraction, and evaporation	Oil contained 10 wt% catechol and 5 wt% methyl catechol	Also neutral water, KOH, LiOH, K2CO3, and Ca(OH)2 were tested, as were other organosolv lignins	(432,433)
3	Kraft lignin	100 mg lignin, 25 mL aqueous 10 mM NaOH	200 °C, batch reactor, 8 h	Not provided	13 wt% monomers	>80% selectivity to guaiacol	(434)
4	Beech wood organosolv lignin	2.5 wt% NaOH (aq)	250–340 °C, 5–20 kg/h plug-flow reactor, 450–900 s residence time	Oil yield up to 80–85 wt% at 250 °C to <60 wt% above 300 °C	Guaiacol, catechol, and syringol up to about 40 wt% in the oil	Catechol component increases at higher T	(435)
5	Spruce wood Kraft lignin	2.5–7.5 wt% lignin and NaOH (aq)	250–300 °C, 5–20 kg/h plug-flow reactor, 450–750 s residence time	Oil yield up to 80–85 wt% at 250 °C to around 60 wt% at 300 °C	Guaiacol and catechol up to about 30 wt% in the oil	Catechol component increases at higher T	(435)
6	Alkali lignin	Lignin:NaX (solid base cat):EtOH:H2O 1:1:40:60	250 °C, batch reactor, 1000 rpm stirring	51 wt% oil yield after acidification, extraction, and solvent evaporation	About 16 wt% of the main identified monomers derived from lignin	Methyl guaiacol, vanillin, and vannilone as main products; also other supported metal catalysts and lignins were tested	(436,437)
7	Wheat straw alkali lignin	1 g lignin, 20 g ethylene glycol, and 10 wt% H2SO4	120 °C, batch microwave reactor, 40 min	97% liquified	Up to 14 wt% identified monophenolics, mainly methyl guaiacol	Showing benefit of microwave irradiation and effect of reaction conditions	(438)
8	Wheat straw soda lignin (Protobind 1000)	Different ratios of lignin, HCOOH, and EtOH	360–400 °C, batch reactor and CSTR, 15–1180 min	Approx. 60–70% liquid yield	Up to around 40 mg/g lignin combined yield of phenol, guaiacol, catechol, and alkylated phenolics	Kinetic model for the formation of main components was demonstrated	(439,440)
9	Norway spruce ethanosolv lignin	Different ratios of lignin, formic acid, and water	320–360 °C batch reactor, 2 h	78–94 wt% based on lignin input	Up to 9.5 wt% identified monomeric products	Main components are guaiacol and methyl/ethyl/propyl guaiacol	(441)
10	Pine wood lignin extracted from black liquor from acid hydrolysis	3 g lignin in 90 mL 1:1:0.4 H2O:EtOH:HCOOH	330 °C, batch reactor, 2 h	64.2 wt%	23.0 wt% monomers based on lignin intake	Mainly 4-n-proply guaiacol (5.5 wt%), methylguaiacol (3.4 wt%), catechol (3.0 wt%), ethylguaiacol (2.7 wt%), and phenol (2.1 wt%)	(442)
11	Softwood Kraft lignin (Indulin AT)	0.5 g lignin, 10 mL EtOH, 10 mL 1,4-dioxane, and 2 mL HCOOH	300 °C, batch reactor, 2 h	About 50 wt%	22.4 wt% phenolic monomers	Mainly 4-n-propyl guaiacol	(443)
12	Norway spruce weak acid–enzymatic hydrolysis lignin	2 g lignin, 3 g HCOOH, 5 g H2O, and 0.2 g cat (Rh, Ru, or Pd supported on alumina)	340–380 °C, batch reactor, 2–6 h	80–91 wt% oil yield (highest for Ru on alumina)	Up to 18.5 wt% identified monomers in the oil	Main product is cresol	(444)
13	Corn stover acetosolv lignin	1 g lignin, 100 mL acetic acid, and 0.1 g Zr-KIT-5 (Lewis acid cat)	250 °C, batch reactor, 1000 rpm stirring, 6 h	43 wt% oil yield based on lignin intake	28 wt% cumulative yield of identified phenolic monomers	Catalysts could be reused in multiple runs; GVL and MeOH/H2O were tested as medium with other lignins	(445−447)
14	Bagasse organosolv lignin	1.75 w/v lignin in MIBK and 1 wt% heterogeneous carbonaceaus solid acid catalyst	350 °C, batch reactor, 3 h	Over 90% lignin conversion	32% identified phenolic monomers, mainly ethyl guaiacol	Also other acidic catalysts and solvents were evaluated	(448)