Table 1 Mechanism of action, advantages and disadvantages of different pretreatment methods

Mechanical extrusion

Reducing the particle size and space structure of biomass

∙ No inhibitor formation

∙ Environmental friendliness

∙ Easy control

∙ High energy

[16, 17]

Milling/grinding

Increasing specific surface area and reducing the crystallinity of cellulose

∙ No inhibitor formation like HMF and levulinic acid

∙ High effectiveness for enzymatic hydrolysis

∙ High energy

∙ Effect is limited when no chemical agents are used

[17,18,19]

Microwave pretreatment

The expansion of biomass via rapid and volumetric heating

∙ Short reaction time

∙ Easy operation

∙ Minimum generation of byproducts

∙ High cost

∙ Effect is limited when no other pretreatment are used

[16, 20]

Ultrasound

Cleaving the α-O-4 and β-O-4 linkages in lignin

∙ Reducing pretreatment time and enzyme consumption

∙ Effect is limited when no other pretreatment are used

[16, 21]

Acid

Reducing the crystallinity of cellulose, releasing oligomers and carbohydrates

∙ Simple method

∙ No thermal energy demand

∙ Produce inhibitors

∙ Corrosive properties

∙ Environmental concerns

[22, 23]

Alkali

Removing lignin and part of the hemicellulose, and reducing cellulose crystallinity

∙ Efficiency in obtaining cellulose pulp

∙ Low energy consumption

∙ Formation of irrecoverable salts

∙ Toxic compounds generation

[22, 24]

Ionic liquids

Reducing cellulose crystallinity and partial removing hemicellulose and lignin

∙ Less energy

∙ Easy to operate

∙ High cost of recovery and recycling

∙ Toxic compounds generation

[16, 17, 25]

Organic solvent

Solubilizing hemicellulose and extracting lignin

∙ High penetration efficiency

∙ Recycling and reuse

∙ Expensive investments,

∙ Environmentally unfriendly

∙ High inhibitory products

[16, 26]

Deep eutectic solvents

Removing lignin and hemicelluloses

∙ Green solvent

∙ Highly biodegradable

∙ High pretreatment temperatures

∙ Instability

[16, 20, 25]

Oxidative pretreatment

Reducing the crystallinity of cellulose

∙ Environmentally friendly

∙ Low toxic compounds generation

∙ Mild conditions

∙ High cost

[20, 23, 27]

Biological pretreatment

Decomposing lignin and hemicellulose

∙ Mild conditions

∙ Low power consumption

∙ Low efficiency

[21, 28]

Steam explosion

Lignin softening and particle size reduction

∙ Low requirement of hazardous chemicals

∙ High sugar recovery

∙ Produce inhibitors

∙ High energy

∙ High pressure

[16, 29, 30]

Ammonia Fiber Expansion

Reducing the crystallinity of cellulose and removing lignin

∙ High efficiency and selectivity for reaction with lignin

∙ Low efficiency for softwood

∙ Cost of ammonia

[16, 20, 31]