TABLE 1.
Description of the most relevant pretreatment techniques for assisting biomass conversion processes.
| Process | Description | |
| Biological | Fungi | Lignin biodegradation by laccases and manganese peroxidases. High selectivity at very long pretreatment times |
| Bacteria | Genetically modified organisms that are able to convert biomass into fuels and chemicals (consolidated bioprocessing) | |
| Enzymes | Selective removal of high molar mass components in cellulosic matrices using lipases, lignin-degrading enzymes and hydrolases | |
| Physical | Milling | Reduction of particle size and increase in substrate surface area for biological or chemical conversion processes |
| Microwave | Disruption and swelling facilitating hemicelluloses and lignin removal; heating and reaction times are greatly reduced | |
| Ultrasound | Structure modification by cavitation; bonds in lignin-carbohydrate complexes are cleaved by radical chemistry | |
| Chemical | Concentrated acid hydrolysis | Cellulose swelling and partial hemicellulose hydrolysis; lignin coalescence and condensation |
| Dilute acid hydrolysis | Cellulose accessibility increased by partial hemicellulose removal; lignin coalescence, fragmentation, and condensation | |
| Alkaline extraction | Lignin extraction and partial hydrolysis of aryl-ether bonds, reducing its average molar mass | |
| Oxidation | Delignification with strong oxidants such as hypochlorite, oxygen radicals, and ozone | |
| Ionic liquids | Carbohydrate or lignin extraction due to its high polarity and strong intermolecular interactions with the cellulosic matrix | |
| Supercritical CO2 | Partial acid hydrolysis of hemicelluloses; increase in substrate pore volume and available surface area | |
| Combined | Organosolv | Selective biomass delignification, whose efficiency can be increased by adding an exogenous acid catalyst |
| Kraft pulping | Alkaline delignification of lignocellulose at ∼170°C using aqueous Na2S/NaOH to isolate cellulose fibers (holocellulose) | |
| Sulfite pulping | Acid delignification at around 160°C using sulfite/bisulfite species to isolate almost pure cellulose fibers and lignin as lignosulfonate | |
| SPORL | Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose, developed from sulfite pulping to improve enzymatic hydrolysis | |
| AFEX | Partial removal of hemicelluloses and lignin plus changes in the crystalline state of cellulose (from I to III) | |
| Hydrothermal | Hemicellulose removal and lignin fragmentation and redistribution by autohydrolysis, increasing cellulose accessibility to enzymatic hydrolysis and hemicellulose recovery mostly as water-soluble oligosaccharides | |
| Liquid hot water | ||
| Steam explosion | Acid hydrolysis of hemicelluloses and lignin modification and redistribution; may be assisted by acid or basic catalysts | |
Reprinted from Ramos et al. (2019) with permission from the Bulgarian Chemical Communications (Open Access).