Table 1.

Summary and Characteristics of Lignin Depolymerization Strategies

Depolymerization	Catalyst	Additives	Solvents	T (°C)	PH2 (bar)	Selectivity	Yield
Reductive
Mild hydroprocessing (MHD)	Nobel metal, base metal, mixed metal	H3PO4, HCl, MClx, NaOH, KOH, Na2CO3	H2O, MeOH, EtOH, iPrOH, dioxane, tetrahydrofuran, or solvent mixture	130–390	10–100	High toward methoxyphenols or catechols	Moderate <20wt%
Harsh hydroprocessing (HHD)	Nobel metal, base metal		Mostly solventless, MeOH, 1-methylnaphthalene	320–450	35–100	Low toward phenol, methylated phenols, and phenols with long alkyl chains	Moderate <30wt%
Bifunctional hydroprocessing (BHD)	Nobel metal, base metal		H2O, MeOH, tetrahydrofuran, heptane, methyl cyclohexane, dodecane, hexadecane	150–320	20–70	High toward cycloalkanes C6–C18	High <50%–70%
Liquid phase reforming (LPRD)	Nobel metal, base metal	H-zeolites, nafion SAC-13, H3PO4, heteropolyacid, NaOH	H2O, formic acid, MeOH, EtOH, iPrOH, tetralin, glycerol	150–400	Liquid phase	Very low toward a broad range of compounds	High 20%–86%

Depolymerization	Catalyst	Oxidants	Solvents	T (°C)	PO2 (bar)	Selectivity	Yield
Oxidative
Alkaline oxidation to phenolics (AlOF)	Soluble catalyst, solid catalyst	O2	NaOH in H2O, MeOH, EtOH, dioxane, tetrahydrofuran, KOH in water	120–190	2–14	High toward phenolic aldehydes such as vanillin	Low <10%–20%
Acidic (AcOF) and pH-neutral (NOF) lignin oxidation to phenolics	Soluble catalyst, solid catalyst	O2, H2O2, peracetic acid	H2O, MeOH, acetic acid, methyl isobutyl ketone, ionic liquid	60–210	5–30	Moderate toward phenols	Low <10–20%
Lignin oxidation to non-phenolic carboxylic acids (OCA)	Solid catalyst	O2, H2O2	Mainly liquid phase H2O (neutral), acidic (H2SO4 or acetate buffer), alkaline (NaOH)	60–225 (liquid phase), 327–377 (gas phase)		High toward carboxylic acids (formic, acetic, succinic, oxalic, and malonic acids)	High 10%–60%

Depolymerization	Catalyst	Additives	Solvents	T (°C)	Selectivity	Yield
Base- & Acid-Catalyzed
Base-catalyzed depolymerization (BCD)	Soluble base (mostly NaOH) or solid base		H2O (mostly), MeOH, EtOH, iPrOH, tetrahydrofuran, 3-methyl-3-pentanol	240–330	Moderate, methoxyphenols (T < 300), catechol (T > 300)	<10%–20%
Acid-catalyzed depolymerization (ACD)	Lewis acid, solid or soluble Brønsted acid	[Ir(cod)Cl]2/PPh3, [Rh(cod)Cl]2/dppp	H2O, MeOH, EtOH, iPrOH, 1-BuOH, ethylene glycol, dioxane, octane, formic acid	140–400	Low with wide array of products, methoxyphenols (T < 300), catechol (T > 300)	<20%–60%

Depolymerization	Catalyst	Solvents	T (°C)	Selectivity	Yield	Note
Solvolytic & Thermal
Solvolytic (SLD)		Water, MeOH, EtOH, iPrOH, 1-BuOH, tetrahydrofuran, acetone, octane, dihydroanthracene, tetralin, naphthalene, solvent mixture	250–450	Low	<10%–20%	Hydrogen-donating solvents are mostly used
Fast pyrolysis (FPD)			400–800	Low	<20%	Under inert atmosphere
Catalytic fast pyrolysis (CFP)	In situ or ex situ: silica/alumina, zeolites, metal on zeolite oxides		500–700	High toward deoxygenated aromatics, benzene, toluene, xylene, and naphthalene	<30%	Under inert atmosphere

Depolymerization	Catalyst 1st Stage	Catalyst 2nd Stage	T (°C)	Selectivity	Yield	Note
Two-Stage
Benzylic alcohol oxidation and depolymerization (BAOD)	4-acetoamide-TEMPO/HNO3/HCl, DDQ/tBuONO, [4-AcNH-TEMPO]BF4-mediated, NHPI/2,6-lutidine-mediated	Aqueous formic acid/sodium formate	110	Phenolic diketones (syringyl- and guaiacyl-1,2-propanedione), aldehydes (syringaldehyde and vanillin), and acids (syringic, vanillic, and p-hydroxybenzoic acid)	52%	Under aerobic oxidation or electrocatalytic oxidation
Benzylic alcohol methylation and depolymerization (BAMD)	Al2(SO4)3	Pd/C, liquid phase reforming in methanol/formic acid	280	Low	17%	Under microwave radiation and methanol solvent