c9gc00473d 3499..3535ltu.diva-portal.org/smash/get/diva2:1335165/FULLTEXT01.pdf · aerogels), cellulose (cellulose-based ionic liquids, functional composites, adsorbent materials,

Green Chemistry

TUTORIAL REVIEW

Cite this: Green Chem., 2019, 21,3499

Received 6th February 2019,Accepted 21st May 2019

DOI: 10.1039/c9gc00473d

rsc.li/greenchem

Cascade utilization of lignocellulosic biomass tohigh-value products†

Yanrong Liu, a,b Yi Nie, a Xingmei Lu, a Xiangping Zhang, a Hongyan He, a

Fengjiao Pan, a Le Zhou, a Xue Liu, a Xiaoyan Ji *b and Suojiang Zhang *a

Lignocellulosic biomass is a potential sustainable feedstock to replace fossil fuels. However, the complex

structure of biomass makes it difficult to convert into high-value products. Utilization of lignocellulosic

biomass in a green and effective way is of great significance for sustainable development. Based on the

analysis of different options, we proposed that cascade utilization according to its composition, charac-

teristics, and nature is the best way to utilize the lignocellulosic biomass. To promote the cascade utiliz-

ation of lignocellulosic biomass, this article provides a review of the latest research results from the aspect

of cascade utilization of lignocellulosic biomass covering the whole chain from pretreatment to high-

value products, and the research on the non-conventional pretreatments including microwave irradiation,

supercritical fluids, ultrasonic irradiation, electric field, hydrodynamic cavitation, and ionic liquids are pre-

sented in detail and evaluated by 4 proposed levels, and the newly developed high-value applications

were further overviewed for lignin (carbon/graphene/carbon nano-tubes, dye dispersants, bioplastics, and

aerogels), cellulose (cellulose-based ionic liquids, functional composites, adsorbent materials, carbon,

and aerogels), and hemicellulose (films and pharmaceutical carriers), respectively. Finally, perspectives on

the future research on the cascade utilization of lignocellulosic biomass are highlighted.

1. Introduction

Biomass plays an essential role in energy supply, and approxi-mately 25% of global energy is being supplied by biomassinstead of fossil fuels.1,2 As an alternative to fossil fuels,biomass has the merits of being abundant, cheap and cleanwith high utilization potential, and it has been identified to be

Yanrong Liu

Yanrong Liu is a post-doc in thedivision of Energy Science/Energy Engineering at the LuleåUniversity of Technology inSweden. She was a joint PhDstudent of the TechnicalUniversity of Denmark (DTU)and the Institute of ProcessEngineering (IPE), ChineseAcademy of Sciences (CAS) from2015 to 2018. She received herPhD in Chemical Engineeringfrom DTU in 2018. Her researchinterests mainly focus on

biomass dissolution and property prediction in ionic liquids, andconductive cellulose fiber fabrication using ionic liquids as thesolvent.

Yi Nie

Prof. Yi Nie is a Professor of theInstitute of Process Engineering(IPE), Chinese Academy ofSciences (CAS). She received herPhD in Chemical Technologyfrom the Beijing University ofChemical Technology in 2007.Her research interests mainlyfocus on designing novel ionicliquids to dissolve wool keratin/cellulose and spinning them intofilaments, setting up dry-jet wet-spinning devices, and fabricatingcarbon nano-conductive fiber.

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c9gc00473d

aCAS Key Laboratory of Green Process and Engineering, Beijing Key Laboratory of

Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems,

Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190,

PR China. E-mail: [email protected] Engineering, Division of Energy Science, Luleå University of Technology,

97187 Luleå, Sweden. E-mail: [email protected]

This journal is © The Royal Society of Chemistry 2019 Green Chem., 2019, 21, 3499–3535 | 3499

www.rsc.li/greenchem

http://orcid.org/0000-0003-1394-7925

http://orcid.org/0000-0002-1271-9595

http://orcid.org/0000-0003-4712-656X

http://orcid.org/0000-0002-1431-0873

http://orcid.org/0000-0003-1291-2771

http://orcid.org/0000-0002-4477-9760

http://orcid.org/0000-0002-9647-7924

http://orcid.org/0000-0003-0605-7078

http://orcid.org/0000-0002-0200-9960

http://orcid.org/0000-0002-9397-954X

http://crossmark.crossref.org/dialog/?doi=10.1039/c9gc00473d&domain=pdf&date_stamp=2019-06-26

one of the most important energy sources in the future.3,4

Among various kinds of biomass resources, lignocellulosicbiomass represents a natural renewable chemical feedstockthat can be used to produce high value-added products.1 It hasbeen estimated that the worldwide production of ligno-cellulosic biomass is about 1.3 × 1010 metric tons perannum,5,6 among which China is responsible for 25% of theworld’s cotton production and produces 126 million metrictons of wheat straw. Europe accounts for 64% of the global oatstraw production. The corn stover that accounts for 80% of theagricultural residue is produced in the U.S.A. Brazil produces25% of the world’s sugarcane bagasse.7

Lignocellulosic biomass can be further segmented intodifferent types, such as forestry (birch, eucalyptus, spruce, oak,pine, poplar), forestry waste (the residues of trees and shrubs,sawdust), energy crops (sorghum, miscanthus, kenaf, switch-grass, corn, sugarcane), agricultural residues (corn stover,wheat straw), algae, industrial and domestic waste (wastepaper, fruit or vegetable waste), and any other animal manure(cattle, swine, poultry). These lignocellulosic biomassresources significantly differ in the contents, compositionsand structures of cellulose, hemicellulose, and lignin withcompletely different properties.

Lignocellulosic biomass can be used to obtain electrical/thermal energy or bio-fuel for transportation, or as rawmaterials for the production of chemicals, and their specificapplications continue to grow and are increasing rapidly.Briefly, their utilization can be divided into thermochemical,biochemical, and chemical processes.8,9 In the thermochemi-cal process, lignocellulosic biomass is directly used in thecombustion, pyrolysis, gasification, and hydrothermal techno-logy. Combustion is the simplest technology for convertingbiomass to electricity and thermal energy. This process takesplace at 800–1000 °C with high NOx and SOx emissions,causing environmental pollution,8 and the electrical efficiency

of biomass combustion is only 20–35%.10 Pyrolysis and hydro-thermal technologies are employed to convert lignocellulosicbiomass to bio-oil, bio-char, and gas,11 and generally largeamounts of energy are needed for cracking.12,13 H2, CO, CO2

and CH4 are the main gases produced from the lignocellulosicbiomass gasification.8 After further purification and reform-ing, the syngas can be further converted into fuels for powergeneration, electricity and heat, or chemicals.8,14–16 Althoughbiomass gasification is a low-cost and low-emission techno-logy, the high amount of tar produced in the gasificationprocess and low fuel flexibility cause technical problems for itsapplication.14 In addition, the reforming process also needsenergy, for example, in the tar reforming process, a tempera-ture above 1100 °C is needed,17 and high amounts of waterand energy are also needed in the steam reforming process.18

In a word, the thermochemical process is with low efficienciesand/or high energy-demands for lignocellulosic biomass utiliz-ation,3 and thus the investigation on the high-value utilizationof lignocellulosic biomass in alternative ways has attractedgreat interest, especially in the last decade.

The biochemical process has been proposed as an alterna-tive to the thermochemical process, in which the ligno-cellulosic biomass is broken down into sugars that can thenbe converted into biofuels (gaseous or liquid fuels) and biopro-ducts (alcohols, organic acids, or hydrocarbons) through theuse of microorganisms and enzymes.19 The most popular bio-chemical technologies are anaerobic digestion and fermenta-tion.8 These conversion processes take place under mild reac-tion conditions with low energy utilization, and pretreatmentis used to enhance the hydrolysis for further conversionwithout separation. However, this process takes a long reactiontime of 2 to 4 weeks,20 and, currently, it is still unable to com-pletely utilize the lignocellulosic biomass.21 Moreover, thehigh cost of microorganism cultivation, low hydrolysis ratesand low yields impede its application.20

Xiaoyan Ji

Dr Xiaoyan Ji is a Professor inthe division of Energy Science/Energy Engineering at the LuleåUniversity of Technology inSweden. She received her PhD inChemical Engineering from theNanjing University of ChemicalTechnology in 2000. Her exper-tise is on applied thermo-dynamics focused on fundamen-tal theoretical modelling andexperimental measurements ofcomplex fluids with interfaces.Since 2008, her research has

been focusing on developing novel IL/DES-based technologies forCO2 capture and separation. Up to now, as a first author or maincontributor, she has published 95+ journal articles and 5 books/book chapters.

Suojiang Zhang

Prof. Suojiang Zhang is aProfessor & Director General ofthe Institute of ProcessEngineering (IPE), ChineseAcademy of Sciences (CAS), anacademician of CAS and aFellow of the RSC. He receivedhis PhD in Physical Chemistryfrom Zhejiang University in1994. He currently serves as theEditor-in-Chief of Green Energy& Environment (GEE) and theProcess Engineering Journal ofChina. His research focuses on

ionic liquids, green process engineering, and large-scale energystorage.

Tutorial Review Green Chemistry

3500 | Green Chem., 2019, 21, 3499–3535 This journal is © The Royal Society of Chemistry 2019

In fact, lignocellulosic biomass is composed of cellulose,lignin and hemicellulose, and each component has its ownspecial characteristics. For example, cellulose and hemi-cellulose are the carbohydrate components, and are valuablefeedstocks for bioenergy production, while lignin is an aro-matic polymer consisting of propenyl phenol units, and ispromising for obtaining synthetic aromatic polymers.22 Tomaximize the utilization with the consideration of character-istics of each component, the cascade utilization of ligno-cellulosic biomass should be the best option, where the ligno-cellulosic biomass is first pretreated and then separated intodifferent components and then further used individuallybased on their characteristics to produce high-value products(Scheme 1). Compared with the thermochemical and bio-chemical processes, this process can utilize lignocellulosicbiomass effectively and completely, and the products are morediverse.

To achieve cascade utilization, the separation of ligno-cellulosic biomass to different components is a key step.However, the complex structure of lignin constitutes the mostrecalcitrant component, which makes it the greatest barrier toachieve an economic separation.23,24 To improve the separ-ation performance, pretreatment of lignocellulosic biomasshas been proposed, and its essential role has been high-lighted. Various methods have been reported, and briefly, theycan be classified into two branches: conventional and non-conventional pretreatments. The typical conventional pretreat-ment includes mechanical (grinding, milling, chopping),chemical (alkali, acid, H2O2), organosolv (methanol, ethanol,formic acid, acetic acid), biological (fungi), and thermal(hydrothermal, steam explosion) processes. The non-conven-tional pretreatment can either involve an ultrasonic method orsupercritical fluids, microwave irradiation, electric and/or mag-netic fields, and ionic liquids (ILs).25 Recently, the concept of“Green Chemistry” has gained increasing attention, furtherimposing the development of novel technologies to usebiomass in a more environment-friendly manner, eliminatingwaste and avoiding the use of toxic and hazardous materials. Alot of work has been conducted with rapid progress in the areaof non-conventional pretreatment, making it necessary toupdate the summary of the research status in this aspect.

After pretreatment, the separation can be performed. To thebest of our knowledge, the technology development in the sep-

aration unit is very limited, and currently, the conventionalmethods, such as extraction, regeneration, centrifugation, fil-tration, distillation and drying, are widely used. After the sep-aration, a lot of research work has been conducted to obtainbiochemicals, biofuels and other high-value products fromindividual components based on their characteristics. Forexample, cellulose and hemicellulose have been studied toproduce pulp, biofuel, carbon material, nanofibrous materials,low molecular-weight chemicals (e.g., methane, ethanol, formicacid and acetic acid), and 5-hydroxymethyl furfural. Lignin haspotential to convert to renewable alternatives and synthetic aro-matic polymers (e.g., polyimide, thermoplastics, phenolic resinand composite films), as well as biogas, phenol, furfural,ethanol, succinic acid, and levulinic acid.22 The annual numberof publications based on cellulose applications is about 8000,and that on the lignin applications is more than 3000.26

Considering the importance of biomass utilization forvarious high-value products and the rapid research process inthis area, review articles have been reported based on biomasspretreatment and utilization, for example, the review articlesfor biomass pretreatment concerning (1) oxidative pretreat-ment processes;24 (2) emerging technologies of ultrasound,microwave, gamma ray, electron beam, high hydrostaticpressure and high pressure homogenization;27 (3) hydrodyn-amic cavitation (HC) process;23 (4) chemical, mechanical,thermal, and biological pretreatment;25 (5) plasma pretreat-ment;2 (6) hydrothermal biomass processing;3 (7) subcriticaland supercritical water (SCW) hydrolysis;28 (8) supercriticalwater (SC-H2O) technology;29,30 (9) IL pretreatment.31 Thereview articles on biomass utilization are mainly based on (1)ultrasound-assisted biological conversion of biomass andwaste materials to biofuels;32 (2) lignin conversion to high-value polymeric materials;33 (3) bio-based bioplastics;34 (4)lignin conversion to low-molecular-weight aromatics;35 (5) con-version of cellulose into key platform chemicals;36 (6) convert-ing biomass to biofuels,37,38 renewable chemicals,37,39 and fer-mentable sugars40 in SCW; (7) hydrogen production frombiomass and biomass gasification in SC-H2O;

41,42 (8) ultra-sound enhanced ethanol production from Parthenium hystero-phorus.43 The articles or reports on biomass cascade utilizationare mainly focused on (1) strategies for a more efficientcascade utilization of biomass;44 (2) barriers to cascadingbiomass use and policy responses;45 (3) definitions, policies

Scheme 1 Cascade utilization of lignocellulosic biomass.

Green Chemistry Tutorial Review


and effects on the international trade of woody biomasscascade utilization;46 (4) from theory to practice of increasingresource efficiency by biomass cascade utilization.47 However,to the best of our knowledge, there are no review articles thatsummarize the research work from the angle of cascade utiliz-ation with “green” technologies covering the whole chain frombiomass pretreatment to high-value products.

To fulfil this gap and to promote the cascade utilization oflignocellulosic biomass, this review was to conduct the litera-ture survey and then summarize the research work from theangle of cascade utilization covering the whole chain. Thespecific content was focused on the non-conventional ligno-cellulosic biomass pretreatment methods with 4 proposedevaluation levels, and the high-value products derived fromlignin, hemicellulose, and cellulose. 500+ latest publishedarticles were collected and analyzed and then narrowed downto 300+ relevant papers for further review in order to avoid rep-etition of the published reviews. The potential research taskswere identified and the need for future research was high-lighted. In addition, for the cascade utilization, the structuresand properties of the lignocellulosic biomass as well as thosefor each constituent-component are of importance, and thus, abrief description was provided in the beginning of the reviewas a background.

2. Lignocellulosic biomass and itsstructure

The compositions of lignin, hemicellulose and cellulose inlignocellulosic resources vary for different types of plantresources. Cotton may contain as high as 98% cellulose, andconiferous wood may have approximately 90% cellulose.48

Basically, softwood consists of a higher amount of lignin(25–35%) compared to other types of biomass, while hemi-cellulose is in a relatively greater proportion of 25–35% inhardwood than that in softwood with a value of 20–30%.4,49 Thecompositions of cellulose, lignin and hemicellulose in variouslignocellulosic resources are summarized in Table S1, ESI.†Several reviews have introduced the lignocellulosic biomassstructures. For example, Vanneste et al.2 and Moon et al.50 intro-duced the cellulose structures. Hu et al.51 introduced the mainhemicellulose monomeric sugars as well as the main hemi-cellulose structures in hardwoods and softwoods. Bugg et al.,52

Dutta et al.53 and Mao et al.54 described the structural units andchemical linkages found in lignin. Zakzeski et al.55 indicatedthe representative structures for a hardwood and softwood.Ragauskas et al.56 reported the primary monolignols indifferent types of lignocellulosic biomass resources. Based onthese reports, the detailed structures of cellulose, hemicelluloseand lignin are given as follows:

Cellulose is a linear polymer consisting of several hundredto over ten thousand β-(1 → 4)-linked glucose repeating units,together with numerous intra-molecular (between the samechains) and inter-molecular hydrogen bonds (between theneighboring chains).2,6,57 The inter- and intra-chain hydrogen

bonding network makes cellulose a stable polymer and givescellulose fibrils high axial stiffness. Cellulose chains arearranged in highly ordered crystalline regions as well as amor-phous or disordered regions as illustrated in Fig. 1.50 The coex-istence of ordered and disordered regions makes cellulose in-soluble in water and in the common organic solvents.58

Hemicellulose is a heterogeneous polysaccharide andlocated between the lignin and cellulose fibers, and it has arandom and amorphous structure with low strength. Unlikecellulose, hemicellulose consists of shorter chains with500–3000 sugar units per polymer.51 The structure and poly-meric properties depend on the origin (i.e., plant species), theindustrial processing as well as the isolation and purificationprocedures. Based on the structure of the cell wall, generally,hemicellulose can be further divided into four types: xylan,mannan, β-glucan with mixed linkages, and xyloglucan.59

Among them, xylan is the major component (Fig. 2a). It is

Fig. 1 (a) Molecular structure of a single cellulose chain (n = DP, degreeof polymerization). (b) Schematics of an idealized cellulose microfibril withthe configurations of crystalline and amorphous regions.50

Fig. 2 Typical hemicellulose. (a) Xylan;6 (b) typical hemicellulose ofhardwoods: O-acetyl-4-O-methylglucoronoxylan;51 (c) typical hemi-cellulose of softwoods: O-acetyl-galactoglucomannan.51



usually built up from a β-(1 → 4) linked xylopyranose backbonechain and substituted with sugar units such as α-L-arabino-furanose, O-acetyl groups or 4-O-methyl-glucuronic acid residues,while the occurrence of homoxylan is very rare. Table S2† illus-trates the monosaccharide composition and the type of xylanfor hemicellulose from different biomass resources.60 FromTable S2,† it can be found that agriculture waste such as wheatstraw, rice husk and rye bran has a less complex structure con-taining arabinose substituents. The hemicellulose in hard-wood (e.g., aspen, birch) is mainly composed of 4-O-methyl-glucuronoxylan. In the backbone of hardwood hemicellulose,approximately 10% of xylopyranose is substituted by 4-O-methyl glucuronic acid (Fig. 2b). The main component of soft-wood hemicellulose is O-acetyl-galactoglucomannan, and thebackbone is composed of mannose units combined withglucose units randomly (Fig. 2c). Compared with softwood,hardwood hemicellulose contains acetyl substituents besidesthe methyl-glucuronic acid residues. In addition, xylan in soft-wood carries non-acetyl substituents.51

Lignin is a cross-linked racemic macromolecule and is rela-tively hydrophobic and aromatic. It fills the space between thehemicellulose structures, covering the cellulose skeleton andmaking the lignocellulosic matrix.25 It is a three-dimensionalpolymer without any sugar in it. The basic monomers formingthe repeat units of lignin are p-coumaryl alcohol (H-unit), coni-feryl alcohol (G-unit) and sinapyl alcohol (S-unit), and they areinterconnected heterogeneously via several types of C–C (i.e.,

β-1′, β-5′, β–β′, 5–5′) and C–O (i.e., β-O-4′, α-O-4′, 4-O-5′)linkages.6,26,52 The monolignol composition of the ligninpolymer is dependent upon the cell type, environmental con-ditions and plant species. For example, grass contains all threesubunits, i.e., H/G/S-lignin, with a proportion of 5–33/33–80/20–54. Hardwood contains G- and S-lignin in a roughly equalproportion, whereas softwood is rich in G-lignin with a pro-portion of 95–100.61 The various linkages and model com-pounds are given in Fig. 3, and their structures and contentsof chemical linkages are displayed in Table 1.

3. Green and efficient pretreatmentprocess

Pretreatment is often needed to change the individual com-ponent content or their structure for achieving an efficient sep-aration. Conventional pretreatment methods have been widelyused but with different challenges. For example, the mechani-cal (or physical) pretreatment method can control the particlesize and make it easy for material-handling, but the highenergy utilization makes it prohibitively expensive for large-scale implementation.62 An alternative is the chemical methodthat can be either acid-, alkali- or oxidation-based. The acid-based pretreatment is mainly on converting hemicellulose tomonosaccharides, whereas the alkali- and oxidation-basedmethods are to remove lignin and hemicellulose. Comparedwith the acid pretreatment, the alkaline method results in lesssugar degradation while the oxidative method can achievefewer by-products. However, all these chemicals are corrosiveand toxic, and have high cost.27 In the organosolv process, theinternal lignin and hemicellulose bonds are broken down,increasing the surface area, while the organic solvents areassociated with low-boiling-point, flammability, volatility, andhigh cost.63 In the biological hydrolysis, the yield is relativelylow along with a long duration without a catalyst. The hydro-thermal process is an approach in which the water in theliquid or vapor phase is used to pretreat lignocellulosicbiomass without any catalyst and corrosion problem. Steamexplosion is a successful option that involves heating ligno-cellulose with superheated steam followed by a sudden decom-pression.13 However, high processing temperature results inhigh water consumption and large energy inputs for hydro-thermal and steam explosion processes.

Fig. 3 Schematic depiction of lignin with various linkages and modelcompounds. Models: (A) phenol and methoxy functionalities, (B) aβ-O-4’ linkage, (C) a 5–5’ linkage, (D) a propyl side chain, and (E) abenzylic group.53

Table 1 Structures and contents of chemical linkages found in lignin54

Nameβ-Aryl ether(β-O-4′)

Di-arylpropane

Biphenyl(5–5′)

Diaryl ether(4-O-5′)

Phenylcoumarane(β-5′)

Spirodienone(β-1′/α-O-4′)

Pinoresinol(β–β′)

Structure

Hardwood (%) 60 1–7 3–9 6–9 3–11 3–5 3–12Softwood (%) 45–50 1–9 20–25 4–7 9–12 2 2–6



To cope with the challenges of conventional pretreatmentmethods, non-conventional technologies have been proposed,such as ultrasound, supercritical fluids, microwave irradiation,electric and/or magnetic fields, HC and ILs. Review articleshave been published for these emerging non-conventional pre-treatment technologies. Therefore, in this section, only therecent research work on unconventional technologies of themicrowave irradiation, supercritical fluids, ultrasonicirradiation, electric field, HC, and ILs has been surveyed andreviewed.

Based on a comparative survey and analysis of the experi-mental results, literature publications including 20+ publi-cations for microwave irradiation pretreatment since 2014, 60+publications for supercritical fluids since 2011, 100+ publi-cations of ultrasonic irradiation since 2013, 15+ publicationsof HC since 2015, 20+ publications of electric field since 2015and 100+ publications of ILs since 2017 were first chosen.Afterward, the contents of lignin, hemicellulose and celluloseafter pretreatment were evaluated, summarized and compared.Overall, the content variation depends on the pretreatmentmethods, and it can decrease, increase or show no effect. Tofurther quantify, considering that the average influence onthe lignin content is about 50% (mainly decrease), 4 levelswere used to identify the effects of the pretreatment on thelignin, hemicellulose and cellulose contents quantitatively,based on the chosen 300+ literature publications. Theresults are summarized in tables, where the slight- or noeffect is marked as “—”, and the effect below 35% is markedas “L1”, from “35 to 60%” is marked as “L2”, and morethan 60% is marked as “L3”. The best recovery or extractionyield of these three compounds is also given in the tablesfor each pretreatment method. The main contributions ofeach method were briefly introduced in the followingsubsections.

3.1 Microwave irradiation

Over the past 30 years, microwave irradiation has been studiedto pretreat different lignocellulosic biomass resources.Compared with conventional heating, microwave irradiationhas significant merits: (1) fast heat-transfer and short reaction-time, (2) selective and uniform volumetric heating perform-ance, (3) easy operation and high energy-efficiency, and (4) lowdegradation or low formation of by-products. Moreover, themicrowave hydrothermal pretreatment removes more acetylgroups in hemicellulose.64 For microwave irradiation, themain drawback is that the composition, geometry and sizeaffect the distribution of microwave power, which may lead toa local overheating phenomenon.65 According to the analysisof the studied articles, the microwave assisted pretreatmenttechnologies can be classified into five main groups: micro-wave-water, microwave-acid, microwave-alkali, microwave-deepeutectic solvent (DES), and microwave-ILs (Table 2). With themicrowave-water pretreatment, the contents of cellulose, hemi-cellulose and lignin are slightly increased or decreased, andlignin can be removed depending on the feedstock and pre-treatment conditions. It has been demonstrated that themicrowave-water pretreatment can only slightly dissolve othercomponents rather than cellulose, hemicellulose andlignin.65,66

With the microwave-acid pretreatment, hemicellulose andlignin are removed, leaving cellulose in the solid residues. Thecellulose content reaches 76.1% after pretreatment and separ-ation. Sometimes, this method does not result in a consider-able removal of hemicellulose and lignin (e.g., from beechwood) but causes the destruction of the biomass structure.The structure destruction can enhance further conversion.67

The effect of the type of acid was studied systematically byAvelino et al. based on the lignin extraction from coconut, and

Table 2 Microwave assisted solvents for the pretreatment of lignocellulosic biomass

Method Biomass Cellulose Hemicellulose Lignin Main changes in biomass after pretreatment

Microwave-water

Corn straw and ricehusk,65 catalpasawdust,66

paddy straw,75

— — — Slightly increases or decreases the contents of cellulose,hemicellulose and lignin.

rapeseed straw76 / / L2 Delignification yield is 38.4%.Microwave-acid

Beech wood,67

moso bamboo,77

coconut shells68

L2 L1 L2 orL3

Microwave-formic acid decreases the hemicellulose and lignincontents and increases the cellulose content. Lignin removal capacityof microwave-H2SO4 is stronger than microwave-HCOOH.13 The ligninextraction yields’ order is microwave-H2SO4 (56.5%) > microwave-HCl(54.4%) > microwave-CH3COOH (9.2%).68

Microwave-alkali

Catalpa sawdust66 L1 L1 L1 Decreases the lignin and hemicellulose contents and increases thecellulose recovery. Microwave-Ca(OH)2 is stronger than microwave-NaOH.

Paddy straw75 L2 L2 L3 Cellulose increases from 43.6 to 69.2%. Hemicellulose decreasesfrom 23.8 to 12.8%. Lignin decreases from 6.0 to 2.4%.

Microwave-DES

Wood70 / / L3 80% lignin is extracted after 3 min pretreatment. The purity ofthe extracted lignin is up to 96%.

Microwave-ILs

Kraft lignin74 / / L2 42% Kraft lignin is dissolved in protic ILs with multiaromaticimidazolium cations.

“—”: slight or no effect; “L1”: the effect below 35%; “L2”: the effect from 35 to 60%; “L3”: the effect more than 60%. “/” indicates no mention.Effect means a decrease or an increase in the contents of lignin, hemicellulose and cellulose after pretreatment.



the result indicated that the microwave-H2SO4 pretreatmenthas a higher lignin extraction of 56.6%, followed by –HCl of54.4% and –CH3COOH of 9.2%. CH3COOH as a catalyst showspoor ability to produce H3O

+ ions and results in low yield,which are responsible for the protonation of ether groups pre-sented in lignin.68

The effects of microwave assisted alkali-pretreatment withalkalis such as NaOH and Ca(OH)2 were studied by Jin et al.66

The contents of both hemicellulose and lignin are decreaseddue to their solubility in the solvents. For example, the contentof lignin in catalpa sawdust decreased from 18.95 to 17.09%and that for hemicellulose decreased from 17.21 to 15.82%,while that for cellulose it increased from 50.87 to 55.78% afterthe microwave-NaOH pretreatment. For microwave-Ca(OH)2pretreatment, the contents of lignin and hemicellulosedecreased to 16.77 and 14.71%, respectively, while the cell-ulose recovery was up to 56.28%. More serious erosion on thecatalpa sawdust surface was observed with SEM after themicrowave-Ca(OH)2 pretreatment, indicating that microwave-Ca(OH)2 is more promising than microwave-NaOH.

DESs have the characteristics of both ILs and organic sol-vents, and they have emerged and widely accepted as a newgeneration of green solvents with the merits of easy and cheapfabrication and no need for purification.69,70 DESs showeffects on delignification.71,72 45% delignification wasachieved by microwave irradiation with a three-componentDES (lactic acid/glycerol/choline chloride) after treating wheatstraw.73 DESs can thus be used as a hydrolytic medium forlignocellulose pretreatment because of the partial dissolutionof lignin. Choline chloride (ChCl) was selected to couple withamines, carboxylic acids and alcohols to obtain three kinds ofDESs. All of them showed good solubilities of xylan and ligninbut poor solubility of microcrystalline cellulose (MCC). Aftermicrowave-DES treatment for 3 min and separation, theextracted lignin showed a low molecular weight of 913 and isconsisted of lignin oligomers with a high purity of 96%. Theexplanation is based on the fact that the chloride ions breakthe intramolecular hydrogen-bond network, and the oxalicacid contributes to dissolving lignin fractions and polysacchar-ides. In addition, it was found that the microwave radiationcan maximize the ionic characteristics and increase the mole-cular polarity of DESs, and both the processing time and temp-erature can be decreased.66 The same phenomenon can befound in the process of microwave assisted ILs as another kindof green solvent. Based on the experimental results of the con-ventional and microwave-IL heating, the amount of dissolvedlignin is virtually the same with both heating sources, whilethe dissolution rate is significantly faster under the microwaveirradiation (about 5 min) compared to the conventionalheating with over 8 h.74

3.2 Supercritical fluids

Supercritical fluids possess favorable properties such as lowviscosity, high diffusivity, high dissolving power and lowsurface tension, enhancing mass transfer of solutes inside asolid matrix. The supercritical fluids used to pretreat ligno-

cellulosic biomass include supercritical CO2 (SC-CO2), subcriti-cal and supercritical water (SCW), supercritical H2O (SC-H2O),supercritical ethanol (SC-ethanol), or the mixtures of SC-CO2

with co-solvents, such as H2O, ethanol and ILs under theirsupercritical conditions.

SC-CO2 has been of interest in recent years because it is alow-cost reagent with a low critical temperature and pressureas well as low viscosity, non-toxicity, recyclability and environ-mental friendliness.78 During the pretreatment, the CO2 mole-cules penetrate into the micropores of lignocellulosic biomass,causing a physical change and destroying the structure by arapid release of CO2.

79 Meanwhile, the dissolution of CO2 intothe water in the lignocellulosic biomass forms carbonic acidin situ, acting as a reagent of hemicellulose hydrolysis andthen breaking the hydrogen bond between cellulose, hemicel-luloses and lignin. In addition, the water in the lignocellulosicbiomass helps to enlarge its micropores, allowing CO2 mole-cules to penetrate deeper during the explosive release of CO2

pressure,80 while ethanol only behaves as a good polar solventin the pretreatment.78 The CO2 can be removed in the depres-surized process to avoid further environment and corrosionproblems.81,82 Water under sub- and supercritical or supercriti-cal conditions behaves totally different from that underambient conditions, and it was evidenced as a good mediumfor lignocellulosic biomass based on the followingadvantages:37,38,40,83 (1) wet biomass can be directly used asfeedstock, (2) free water can be removed in the liquid statewithout vaporization, (3) rapid reaction speed (i.e., severalseconds) can be achieved, and (4) low dielectric constant andhigh ion products (i.e., H+ and OH−) behave as non-polar sol-vents that can dissolve and degrade a variety of lignocellulosicbiomass resources even without any additives. SCW can beused either as a pretreatment for the selective fractionation ofcellulose, hemicellulose and lignin or as a reaction medium todirectly hydrolyze the lignocellulosic biomass into sugars.40

However, the costs are high for the supercritical pretreatmentdue to the high pressures and the high energy required for theapplication.28,84

The capacity of supercritical fluids for pretreating ligno-cellulosic biomass is summarized in Table 3. SC-CO2 was usedto pretreat the empty fruit bunch80,85 and bagasse.86 The resultshowed that the contents of lignin, hemicellulose and cell-ulose only slightly change after pretreatment; however, thesugar yield increases. It has been evidenced that the mixturesof SC-CO2 and SC-H2O or SC-ethanol have a stronger effectthan SC-CO2. As shown in Table 3, when the co-solvent ofSC-CO2/H2O/ethanol was employed to pretreat biomass, forexample, rice husk,84 corn stover,78 sugar cane bagasse andPinus taeda wood chips,87 the delignification reached 93.1%for wood chips.87 Lü et al. studied SC-CO2/H2O, SC-CO2/ethanol and SC-CO2/H2O/ethanol, individually and as mix-tures, based on corn stover.78 For SC-CO2/H2O, the fiber-surface of corn stover becomes porous and forms visiblecracks, meanwhile, a lot of small particles deposit on the fibersurface. However, such surface deposition cannot be observedfor SC-CO2/ethanol, which may be attributed to the good solu-



bilities of lignin and hemicellulose in ethanol. For SC-CO2/H2O/ethanol, the fibers crimp and fold noticeably and becomeloose and rough, and the surface area of fibers increases sig-nificantly. These results suggest that SC-CO2/H2O/ethanol candisrupt the compact matrix structure of corn stover, makingthe cellulose fibers highly exposed. The cellulose content afterSC-CO2/H2O/ethanol pretreatment reaches 74%; however, thehemicellulose and lignin contents are only 9% and 6%,respectively. The cellulose content after SC-CO2/H2O/ethanolpretreatment is richer than those with SC-CO2/H2O (38%) andSC-CO2/ethanol (43%). An innovative method for the pretreat-ment of sugarcane bagasse using SC-CO2/H2O2 under mildconditions was proposed.88 This method was found to besuperior to the individual pretreatment with SC-CO2, ultra-sonic irradiation or H2O2, and the sequential combination ofSC-CO2 and ultrasound results in almost the double yields of

cellulose and hemicellulose. It was stated that during the pre-treatment, the hydroxyl radicals (HO) and superoxide anionradicals (O2) produced by the decomposition of H2O2 in thealkali can oxidize and degrade lignin, and thus the pretreat-ment with SC-CO2/H2O2 achieves relatively high amounts ofcellulose and hemicellulose.

The poplar and pitch pine woods were pretreated with SCWby Kim et al.89 As shown in Table 3, cellulose/hemicellulosewas easily hydrolyzed during SCW pretreatment into mono-meric sugars with the total yield of 7.3% and 8.2% for poplarwood and pitch pine wood, respectively. In addition, the effectof HCl on the yield of monomeric sugars was also investigatedduring SCW pretreatment. It was found that the total yield ofmonomeric sugars was increased about 3-fold to 23.0% forpoplar wood and 25.1% for pitch pine wood, respectively, inthe presence of 0.05% (v/v) of HCl. Holm oak wood was pre-

Table 3 Supercritical fluids for the pretreatment of lignocellulosic biomass


SC-CO2 Empty fruit bunch,80,85

bagasse86— L1 — Slightly increases cellulose or hemicellulose.

Slightly decreases lignin.SC-CO2/H2O Corn stover78 L1 L1 — Decreases the cellulose and hemicellulose.

Enhances the hydrolysis of hemicellulose.SC-CO2/ethanol Corn stover78 L3 L3 L2 Increases cellulose. Good solubilities of

hemicellulose and lignin.SC-CO2/H2O/ethanol Corn stover,78,103 poplar

chips104L3 L3 L3 A strong increase of cellulose (from 41 to 74%)

and removal of the hemicellulose (from 32 to9%).78 The lignin removal yield reaches 90%.103

SC-CO2/H2O2 Sugarcane bagasse88 — — — Superior to the individual pretreatment withSC-CO2, ultrasonic irradiation, or H2O2 and thesequential combination of SC-CO2 and ultrasonicirradiation.

SC-CO2/ethanol-BmimAc

Sugarcane bagasse105 / / L1 The delignification is 41.9%.

SCW Poplar and pitch pine wood,89

holm oak wood90/ / / Cellulose/hemicellulose was easily hydrolyzed

into monomeric sugars with the total yield of7.3% and 8.2% of poplar and pitch pine,respectively.89 A sugar yield of 75% was reached.90

SCW/0.05% HCl (v/v) Poplar and pitch pine wood89 / / / Total yield of the monomeric sugars is 23.0% forpoplar wood and 25.1% for pitch pine wood,respectively, in the presence of 0.05% (v/v) ofHCl.89

SC-H2O Nannochloropsis microalgae,94

sugar beet pulp,95 Quercusmongolica96

/ / / 80.3% C element was acquired.94 The highestyields of 71% C5 and 61% C6 sugars wereobtained.95 27.8% sugar yield was achieved.96

SC-H2O/Pt/C Nannochloropsis microalgae94 / / / 80.9% C element was achieved.SC-H2O/H2SO4(0.05%)

Quercus mongolica96 / / / The sugar yield is 49.8%.

SC-CO2/SC-H2O Pocidonia oceanica residues97 / / / 14.1 g L−1 reducing sugar was obtained.SC-H2O/Ru/MCM-48 Sugar beet pulp98 / / / 15% yield of hexitols was achieved.SC-H2O/KOH Sugarcane bagasse99 L3 L3 L3 Sugarcane bagasse completes gasification.

75.6 mol kg−1 H2 was achieved.SC-ethanol Rice stalk100 L1 L1 — Decreases hemicellulose from 22.97 to 15.44%.

Cellulose increases from 35.88 to 41.59%. Slightlyincreases lignin. 55.03% bio-oil yield wasachieved.

SC-ethanol/subcritical CO2 orSC-CO2

Rice straw101,106 / / / 47.78% bio-oil was achieved.

SC-ethanol/Fenton’sreagent

Sorghum bagasse102 / / / 75.8% phenolic oil was produced.

“—”: slight or no effect; “L1”: the effect below 35%; “L2”: the effect from 35 to 60%; “L3”: the effect more than 60%. “/” indicates no mention.Effect means a decrease or an increase in the contents of lignin, hemicellulose and cellulose after pretreatment. BmimAc: 1-butyl-3-methylimidazolium acetate.



treated with SCW.90 Cellulose and hemicellulose were solubil-ized and partially hydrolyzed, and a sugar yield of 75% wasreached, mainly composed of xylose and glucose oligomersand smaller amounts of other chemicals (e.g., acetic acid or5-HMF).

SC-H2O has been successfully used to yield sugar, bio-chemical and biofuel from biomass at a significantly largescale. For example, Renmatix commercially produces a varietyof products from the fermentable sugars derived from the reac-tion of non-food biomass with SC-H2O via its Plantroseprocess.91 ARA has developed SC-H2O-based cleanup and con-version technologies to produce biofuel and biochemical frombiomass for over a decade.92 Licella Holdings and Canfor Pulpcompanies constructed a large biorefinery to convert thewoody waste to biocrude oil by SC-H2O.

93 These technologiesbreak down a wide range of non-food biomass in seconds,while other processes take days (e.g., enzymatic hydrolysis).The SC-H2O with and without catalysts can be used to pretreatlignocellulosic biomass (Table 3). SC-H2O was used to pretreatNannochloropsis microalgae,94 sugar beet pulp95 and Quercusmongolica,96 showing that SC-H2O pretreatment allowed aselective and simultaneous recovery of both cellulose andhemicellulose fractions as C5 and C6 sugars, and obtained alignin-like solid fraction.95 Jeong et al. used SC-H2O andSC-H2O/H2SO4 (0.05%) as the media to pretreat Quercus mongo-lica individually.96 27.8% sugar yield was achieved for SC-H2Opretreatment, while it is 49.8% for SC-H2O/H2SO4. SC-CO2/SC-H2O was used to pretreat Pocidonia oceanica residues.97 Theresult indicates that SC-CO2 can be used as a pretreatmentmethod for increasing the availability of hemicelluloses andcelluloses. Under the optimized conditions, 14.1 g L−1 redu-cing sugar was obtained by consecutive SC-CO2/SC-H2O hydro-lysis. Romero et al. pretreated sugar beet pulp with two cata-lysts of Ru/MCM-48 and Ru/C using SC-H2O.

98 The preparedcatalyst of Ru/MCM-48 showed better behavior than the com-mercial Ru/C, and 15% yield of hexitols was achieved afterSC-CO2/Ru/MCM-48 pretreatment. Sheikhdavoodi et al.studied sugarcane bagasse gasification in SC-H2O/KOH.99

Sugarcane bagasse was completely gasified and 75.6 mol kg−1

hydrogen was achieved.Li et al. produced bio-oil from rice stalk by supercritical

ethanol (SC-ethanol) liquefaction (Table 3).100 It was indicatedthat hemicellulose was the main reaction compound duringSC-ethanol pretreatment and a maximum value of 55.03% bio-oil was achieved from rice stalk. SC-ethanol/subcritical CO2

and subcritical CO2/subcritical H2O were used to produce bio-oil from rice straw.101 It was found that the bio-oil yield of SC-ethanol/subcritical CO2 (47.78%) is higher than subcriticalCO2/subcritical H2O (23.59%). In SC-ethanol/subcritical CO2,SC-ethanol donates hydrogen, promoting the reaction ofhydroxyl-alkylation. While in subcritical CO2/subcritical H2O,CO2 promotes the carbonyl reaction. Sorghum bagasse wasmodified with Fenton’s reagent and then selectively depoly-merized in SC-ethanol,102 and a maximum yield of 75.8% phe-nolic oil was acquired. The result indicated that the Fenton’sreagent modification not only increases the yields of phenolic

monomers, particularly ethyl-p-coumarate and ethyl-ferulate,but also enhances the hydrolysis.

3.3 Ultrasonic irradiation

Ultrasonic irradiation is a novel technology that can reduce thepretreatment time and decrease the energy utilization. It wasreported that the use of ultrasonic irradiation causes thedegradation of the cell wall and ruptures the bonds that bindlignin, hemicellulose and cellulose.107 In addition, it has beenevidenced that the ultrasonic assisted chemical contributes toboth external surface destruction and internal disruption oflignocellulose. The mechanism is mainly based on three steps:(1) rapid dissolution of lignin on the surface of lignocellulose bythe chemical with the assistance of ultrasonic waves; (2) ultra-sonic waves generate micro-bubble cavitation that providesenergy to the surface of lignocellulose, mechanically breakingdown the external structure and facilitating the penetration ofions into internal areas; and (3) dissociated ions permeate andinteract with the cellulose chains, breaking down the inter-molecular hydrogen bonds and disrupting the associationsbetween cellulose and hemicellulose or lignin.108 During theultrasonic pretreatment, the ultrasonic energy is finally dissi-pated as heat in the liquid medium, and the induced high temp-erature can lead to problems for the ultrasonic technology.109

Ultrasonic irradiation can be an efficient method to pretreatlignocellulosic biomass and enhance biomass utilization. Thepretreatment capacity with ultrasonic waves was analyzed andidentified and is summarized in Table 4. According to the col-lected references, the ultrasonic pretreatment mainly focuseson 8 types: ultrasonic, ultrasonic-water, -organosolv, -acid,-alkali, -ILs, -enzyme, and -TiO2. Considering there are manypublished studies, Table 4 provides only the types of ultrasonicpretreatments, the types of biomass resources treated and thesignificant results. A more specific description is given asfollows.

The ultrasonic assisted water is used to pretreat Agave dur-angensis leaves. The results indicated that both the lignin andhemicellulose contents are significantly increased after pre-treatment, and the hemicellulose content is increased moreobviously than lignin.110 It was suggested that the effect ofultrasonic waves is exerted on the release of lignin and hemi-cellulose by the hydrolysis of the linkages between lignin andthe carbohydrate.111–113

Zhang et al. used ultrasonic waves and sulfuric acid to pre-treat corn stover and sorghum stalk in two steps.114 For pre-treatment with only ultrasonic radiation, no significant differ-ence in the lignin content was observed, indicating that ultra-sonic radiation itself cannot alter the chemical composition ofbiomass. When ultrasonic radiation was combined with diluteH2SO4 to pretreat the same biomass, the percentage of xylanwas reduced significantly from 24 to 2%. This illustrated thatthe ultrasonic assisted acid has a strong effect on the hemi-cellulose removal. According to the previous studies, HCl ismore effective for lignin removal than H3PO4, HNO3 andH2SO4.

115 The ultrasonic assisted HCl was further used to pre-treat two Pennisetum samples at 100 W, 353 K with a duty cycle



of 70%. Compared with that without ultrasonic assisted pre-treatment, the delignification was increased from 33 to 80.4%for deenanath grass and from 33.8 to 82.1% for hybrid napiergrass.

Ultrasonic radiation plus alkali can result in relatively highlignin removal. It was stated that the ultrasonic assisted alkali

pretreatment has a significant effect on the cleavage of etherlinkages between lignin and hemicellulose from the cellwall,116 or it can destroy the lignocellulose structure and helpthe alkali diffuse into the internal areas of lignocellulose toaccelerate the degradation of lignin. 1.7–2.0-fold enhancementin the delignification extent was achieved by ultrasonic

Table 4 Ultrasonic for the pretreatment of lignocellulosic biomass

Pretreatment Biomass Main changes in biomass after pretreatment

Ultrasonic radiation Corn stover and sorghum stalk,114 excess sludge,133

grape134Slightly increases or decreases the contents of cellulose,hemicellulose and lignin. Ultrasonic radiation damages thelignocellulose structure, but does not alter the chemicalcomposition of biomass much.

Ultrasonic-H2O Agave durangensis leaves,76 sugarcane bagasse,116 grassclipping,120 birch sawdust,131 rice straw,132 sugar beetshreds,135

Slightly increases or decreases the contents of cellulose,hemicellulose and lignin, mainly increases the contents ofthese three compounds.

rapeseed straw,76 wheat straw,108 napier grass,126

exhausted olive pomace136Strong removal of 41.5% lignin.76

Ultrasonic-organosolvEthanol Rapeseed straw122 Strongly removes 73% lignin.γ-Valerolactone Wheat straw73 27% delignification.Ultrasonic-acidH2SO4 Corn stover and sorghum stalk,114 coffee silverskin137 Lignin increases from 16.6 to 32.3% for corn stover and

19.5 to 33.2% for sorghum stalk.114 A slight increase of cell-ulose and hemicellulose.137

Exhausted olive pomace136 Strongly removes 18% cellulose, 76% hemicellulose and14% lignin.

H2SO4-ethanol Wood123 Strongly increases cellulose from 46.08 to 88.43%.Hemicellulose decreases from 34.67 to 11.01%, andcellulose decreases from 19.09 to 0.4%.

H2SO4-ethanol/H2O2-NaOH

Wood123 Strongly increases cellulose from 46.08 to 93.18%.Hemicellulose decreases from 34.67 to 4.91%, and lignindecreases from 19.09 to 0.1%.

HCl Deenanath grass and hybrid napier grass115 82% lignin is removed.H3PO4 Rice straw138 Lignin slightly increases from 15.1 to 19.5%.Fenton’s reagent Rice straw,132 corncob139 Slightly increases cellulose. Moderately decreases

hemicellulose.132 Removes 80% lignin.139

Ultrasonic-alkaliCa(OH)2 Grass clipping120 Strongly increases cellulose. Moderately decreases

hemicellulose. Slightly decreases light.NaOH Grass clipping,120 sugarcane bagasse,116,140,141 corn

stalks,119,142 groundnut shells, coconut coir andpistachio shells,117 sunn hemp,118 wheat straw,108 hybridnapier grass and deenanath grass,143 rice straw,138,144,145

Saccharum spontaneum, Lantana camara, Eichhorniacrassipes and Mikania micrantha,146 cane bagasse,147

rapeseed hulls pulps,148 reed,149 rapeseed straw122

Mainly increases cellulose content up to 96.7%.142

Decreases hemicellulose content. Removes lignin up to89.3%.

KOH Peanut shells150 Lignin extraction rate is 12.57%.TBAH Wheat straw108 Slightly removes cellulose (2%). Removes hemicellulose

(28.1%) and lignin (69.5%).Aqueous ammonia Corn stover, corn cob and sorghum stalk121 Mainly increases cellulose content, decreases hemicellulose

and lignin contents.Ultrasonic-saltSDS Fruit and vegetable waste125 Strongly removes lignin from 16.7 to 4.7%.KMnO4 Spent coffee waste130 Cellulose increases 46%, hemicellulose increases 75%, and

lignin increases 98%.Ultrasonic-ILsBmimCl Water hyacinth stems and leaves127 Cellulose increases from 19.63 to 33.1%, hemicellulose

decreases from 33.63 to 30.5%, lignin decreases from 26.13to 20.53%.

SDS-BmimCl Water hyacinth stems and leaves127 Cellulose increases from 19.63 to 38.2%, hemicellulosedecreases from 33.63 to 30.8%, lignin decreases from 26.13to 16.2%.

Ultrasonic-enzymeFungal Myrotheciumverrucaria

Birch sawdust131 Cellulose decreases from 35 to 27.87%. Lignin decreasesfrom 19.91 to 6.38%.

Laccase Napier grass126 75% lignin is removed.Ultrasonic-TiO2 Corncob139 50.31% lignin is removed.



assisted NaOH for the studied feedstocks compared with thatof NaOH only under the conditions of 20 kHz, 100 W, 70 minand 1 mol L−1 NaOH.117 Manasa et al. also used ultrasonicassisted NaOH to pretreat sunn hemp and obtain a cellulose-rich material (68%). The result indicated that the ultrasonicassisted alkali pretreatment has a better effect on the hemi-cellulose and lignin degradation than that of alkali pretreat-ment.118 Dong et al. investigated the effects of single-frequencyand dual-frequency ultrasonic pretreatment combined with 2%NaOH solution for corn stalks, and it was found that morelignin, hemicellulose and cellulose were removed in the dual-frequency ultrasonic pretreatment, especially for cellulose.119

For ultrasonic assisted alkali pretreatment, the capacities oflignin removal follow the order ultrasonic-Ca(OH)2 > ultrasonic-NaOH,120 and ultrasonic-tetra-n-butylammonium hydroxide(TBAH) > ultrasonic-NaOH108 > ultrasonic-aqueous ammonia.121

In addition, organosolv (ethanol) pretreatment under harshconditions (200 °C, 1% H2SO4 and a duration of 80 min)shows better delignification performances than those withsoda treatment.122 The hybrid ultrasonic-H2SO4–ethanol andultrasonic-H2SO4 + ethanol–H2O2 + NaOH were studied by Liet al.,123 and it was found that lignin was removed almost com-pletely from wood together with good solubility of hemi-cellulose. Finally, the cellulose-rich materials with values of88.43% for ultrasonic-H2SO4–ethanol and 93.18% for ultra-sonic-H2SO4 + ethanol–H2O2 + NaOH were acquired.

Sodium dodecyl sulphate (SDS) is an anionic surfactant.The sonication effect in the liquid medium can be augmentedusing SDS in an ultrasonic field.124 Shanthi et al.125 measuredthe lignin removal rate using SDS combined with ultrasonicradiation for fruit and vegetable waste. The study showed that72% lignin was removed after SDS-ultrasonic pretreatmentunder the conditions of 25 kHz and 90 W, whereas only 18%lignin was removed using ultrasonic pretreatment alone(24 kHz and 100 W) for napier grass.126 The better result forSDS is not only due to the increase of the cavitation effect ofultrasonication by the surfactant but also due to the delignifi-cation of biomass by cleaving the linkages between the ligninand cellulose complex. For ultrasonic assisted SDS-1-butyl-3-methylimidazolium chloride (BmimCl) pretreatment, it wasalso found that the addition of surfactants to the pulp as thedelignifying agents not only reduces the surface tension of themedium and enhances the cavitation, which may break downthe a-O-4 and b-O-4 linkages between the cellulose and lignin,but also decreases the amount of lignin deposited on thebiomass, which further induces the lignin removal.127–129 Basedon the available studies, the lignin removal capacity of ultra-sonic-SDS is higher than that with ultrasonic-KMnO4.

130

Besides, the ultrasonic assisted fungal or Fenton’s reagent alsoshows promising lignin removal performance. When couplingultrasonic treatment with fungal treatment to pretreat birchsawdust, the lignin removal rate is about 4.94-fold for ultrasonictreatment and 1.49-fold for fungal treatment.131 Meanwhile,Fenton’s reagent may preferentially degrade hemicellulose andlignin in biomass, while ultrasonic treatment can degrade cell-ulose, hemicellulose and lignin simultaneously.132

3.4 Electric field

Electric field technologies mainly focused on the electromag-netic field,151 pulsed electric field,24,152 electric field,153 highvoltage electrical discharges,122 electron scattering,154 electro-lysis,155 micro-voltage,156 pulsed electric fields and high-voltage electrical discharges have been studied.148,157 Pulsedelectric field (PEF) pretreatment is based on the electropora-tion phenomena, including electromechanical compressionand electric field-induced tension, where an external electricfield is used to induce the critical electrical potential acrossthe cell membrane.158 The biological membrane is disruptedand the semi-permeability is lost, allowing the passage ofintracellular compounds to the surrounding solution.159 PEFcauses lethal damage to cells or induces sublethal stress bytransient permeabilization of cell membranes and electrophor-etic movement of charged species between cellular compart-ments.160 The size and number of pores of the cell membraneare related to the strength of the applied electric field, and themembrane porosity is proportional to the strength of theapplied electric field.158 The characteristic of PEF treatmentmakes it possible to extract various compounds from biomasswithout modifying the chemical composition of the treatedsamples.134 Ammar et al. indicated that the PEF pretreatmentdoes not affect the chemical composition of grape pomace,but it leads to a drastic increase in permeability due to theappearance of pores in the cell membranes that can releasesome components to the extracellular medium.161 The lipidyield after the PEF pretreatment and extraction with ethanolcan be four times higher than that for untreated cells.162

However, there are several disadvantages of PEF, such as thehigh maintenance costs, high operation temperature, and thedecomposition of fragile compounds.152

The function of high voltage electrical discharges (HVED) isthe electrical breakdown in water. The electrical dischargesresult in a generation of hot and localized plasmas that emithigh-intensity ultraviolet (UV) light. Shock waves are producedduring this process and hydroxyl radicals are generated duringwater photo-dissociation.148 All these processes damage thecellular structure and release biomolecules from the cytoplasmof cells after the fragmentation of the particles.163 Brahimet al. reported that HVED induced a partial degradation of cell-ulose. HVED-soda pretreatment was 4–8 fold more effectivethan the blank experiment. Under the harsh conditions of200 °C, 1% H2SO4, 80% duration time and with HVED-ethanol, 70% lignin can be removed from rapeseed straw,more effective compared with HVED-soda (58%).122 A micro-electric field was also used to pretreat cow dung at 200 °C with0.25 V cathode voltage, and 57.32% cellulose and 56.60%lignin can be removed from cow dung. Compared with thatwithout voltage under the same conditions, the biogas pro-duction of cow dung and the methane content are increased.This can be explained by two reasons: one is that the hydro-thermal pretreatment can remove hemicellulose and part oflignin; the other is that the microbes stimulated by thecathode voltage in the fermentation become more active.



Therefore, the micro-voltage and hydrothermal pretreatmentcan effectively improve the lignocellulose degradation ratio.156

3.5 Hydrodynamic cavitation (HC)

HC has been reported as a highly energy-efficient and suitablemethod for large scale applications of biomass pretreat-ment.164 The cavitation phenomenon can be defined as theformation and subsequent growth and collapse of micro-bubbles due to the changes in pressure at constant tempera-ture. Compared with the acoustic cavitation, several additionaladvantages have been identified for HC, such as simplificationand efficiency besides the flexibility to vary the cavitationintensity by using different device configurations and adjust-ing the operation parameters.165 It was reported that HC issensitive to temperature, and the suitable HC operation temp-erature is only from 30 to 45 °C. A higher temperature willdecrease the intensity of cavitation and reduce the cavitationefficiency.23

The use of HC for the pretreatment of lignocellulosicbiomass was first reported by Kim et al. in the study of reedpretreatment to produce ethanol using orifice plates.149 Sincethen, various kinds of biomass resources, such as cornstover,166 sugarcane bagasse141,167 and wheat straw, werestudied for ethanol and biogas production. All these studiesare based on the orifice plates or venturi systems, except thelast one168 that used rotational equipment (Fig. 4). As shownin Fig. 4, these systems are composed of a pump, a reservoirtank, pipes, valves and a cavitation device, plus pressure andtemperature indicators. Based on the available investigations,the orifice plate can be highlighted and is associated with thenon-recirculation of solids for avoiding the clogging problems,the high pressure decreases and consequently the loss of thehigh cavitation intensity.8

The mechanism of HC for accelerating delignification ismost likely attributed to the cavitation from physical andchemical effects. The collapse of micro-bubble cavities gener-ates enormous destructive forces to dissociate water molecules,and thus the generation of radicals like HO•, HOO• and O2

• oxi-dizes the lignin molecules to low molecular weight organicproducts and even carbon dioxide (see Fig. 4).141,169 Theefficiency of HC gets affected by the parameters such as: (1)the configuration of the HC system, (2) the process para-meters, (3) the characteristics of biomass, and (4) the presenceof a chemical catalyst.

The results of lignocellulosic biomass pretreatment withHC are summarized in Table 5. From Table 5, it can be foundthe main effect of HC pretreatment is the removal of ligninand hemicellulose, increasing the cellulose content except forHC-NaOH-H2O2. The number of holes and the diameter of theorifice plate affect the lignin removal efficiency.170 ForHC-NaOH pretreatment, the orifice plate with 27 holes and1 mm D (diameter) is more efficient to remove lignin fromsugarcane bagasse compared with that with 16 holes and1 mm D, and this observation is consistent with the results byThangavelu et al. with HC-enzyme pretreatment.170 Comparedwith different alkali-catalyzed pretreatments for sugarcanebagasse with HC (16 holes, 1 mm D) listed in Table 5, thelignin removal efficiency follows the order NaOH-H2O2 > KOH> NaOH > Na2CO3 > Ca(OH)2. In addition, temperature has aneffect on energy release during the collapse of micro-bubbles,141 while the pressure mainly affects the cavitationintensity.171 The overall investigation of the above-mentionedfour parameters can help in achieving an efficient pretreat-ment with HC.

3.6 Ionic liquids (ILs)

Recently, ILs and IL-based solvents have emerged as aneffective system for the pretreatment of biomass. Ever sinceSwatloski et al. discovered in 2002 that an IL (BmimCl) is ableto dissolve cellulose,175 numerous studies on the processing ofcellulose, lignin or various kinds of biomass with ILs havebeen investigated. As ILs are potentially attractive “green”solvents,176,177 the merits of biomass-pretreatment with ILscan be summarized as follows: (1) ILs can be modified freelyby the combination of various cations and anions to be suit-able for biomass dissolution, (2) ILs show high dissolutionrates for large biopolymers, (3) the biomass hydrolysis processcan be improved after IL pretreatment, and (4) ILs and the dis-solved biomass can be easily recovered after thepretreatment.178,179 However, the main drawback of ILs is thehigh cost.

It has been evidenced that the ILs with the cations namelyimidazolium, pyridinium, pyrrolidonium, morpholinium,ammonium and cholinium and the anions that can formstrong hydrogen bonds with hydroxyl groups, e.g., Ac−,HCOO−, HSO4

−, etc. are promising for pretreating ligno-cellulosic biomass. The studied ILs can be grouped into fivetypes: (1) dissolving lignin, cellulose and hemicellulose simul-

Fig. 4 Schematic diagram of different hydrodynamic cavitation systems.8,141



taneously; (2) selectively dissolving lignin and hemicellulosebut not cellulose; (3) dissolving hemicellulose but neither cell-ulose nor lignin; (4) dissolving lignin but neither cellulose norhemicellulose; (5) not dissolving lignin, hemicellulose andcellulose at all.180 The removal of lignin with ILs results fromits depolymerization through the homolytic cleavage of α-O-4′and β-O-4′ bonds, releasing aromatic compounds and enablingthe lignin to be solubilized.181 Based on the survey and ana-lysis of recently published work, the most significant perform-ance for each IL or IL plus co-solvent pretreatment is summar-ized in Table 6, in which the references in each line are relatedto the order of combined cations and anions or co-solvents.The more specific description of Table 6 is as follows.

EmimAc is frequently used. It shows that 95% cellulosicmaterials can be recovered from chestnut shells.182 50–55%lignin can be removed from pine wood183 and 56% from cornstover.184 The combination of co-solvents or microwave withEmimAc has been widely studied. It was indicated thatEmimAc-acid can remove hemicellulose and lignin,185 EmimAc-alkali has the potential to remove hemicellulose and lignin andincrease the cellulose content,186 and EmimAc-organosolv canbe used to remove lignin and increase the hemicellulose andcellulose contents.187–189 However, EmimAc-microwave can beused to increase the lignin content.190,191

ILs can be hydrophilic and hydrophobic. Williams et al. pre-treated pine with a wide array of ILs and confirmed that thehydrophilic ILs with polar protic anions work better forbiomass deconstruction than the hydrophobic ILs. Meanwhile,for most herbaceous feedstocks, a hydrophilic IL (EmimAc)increases the sugar content and decreases the lignin content,while a hydrophobic IL (BmimPF6) decreases the sugar contentand increases the lignin content, and the increased lignincontent can be up to 189%.184 For the hydrophobic ILs with

the anions such as Cl−, mesy−, or OH−, the lignin removalcapacities follow the order Hpy+ > Hmim+ > Hnmp+,192 BHEM+

> 2-HTEAF+ > DMEA+,193 and TEA+ > Hbim+ > Hmim+,194,195

respectively. For the ILs with the same cation of TEA+, thelignin removal capacities are in the order HSO4

− (82%) >MeSO3

− (20%).194,195 Four kinds of ammonium-based ILs withthe same cation namely diisopropylethylammonium (DIPEA+)but different anions such as Ac−, propanoate (P−), octanoate(O−) and benzoate (B−) were investigated for the lignin extrac-tion from coffee husk.196 The result showed that the ligninextraction yield was affected by the pretreatment temperaturesand the choice of anions. At high extraction temperatures, theyield of lignin isolation increases for the studied ILs. Forexample, the lignin extraction yield at 80 °C is 24.3% forDIPEAAc, while it is 71.2% at 120 °C. Meanwhile, with theincrease of the chain length or decrease of the acidity of thealiphatic anions, the dissolution and the extracted lignin yieldfrom coffee husk are decreased, and the lignin extractioncapacities at 120 °C follow the order DIPEAAc > DIPEAP >DIPEAO > DIPEAB.

The sole pretreatment method may have limitations inefficiency due to the multi-scale biomass recalcitrance. Asystem combined alcohol solution with the catalystBmimHSO4 can effectively fractionate coir and poplar intohigh-quality cellulose-rich material and high-yield lignin with adelignification rate up to 98%. The catalyst exhibited a signifi-cant enhancement in the lignin extraction with a nearly 6-foldincrease.197 The study on the ultrasonic and microwave assistedTBAOH showed that microwaves have a stronger effect thanultrasound waves.198,199 This is because the excellent micro-wave-absorbing properties result in vibrating TBA+ with adrastic interaction with lignin to cause the violent deconstruc-tion. In addition, hemicellulose can be removed almost comple-

Table 5 HC for the pretreatment of lignocellulosic biomass


HC-alkali Orifice plate(16 holes, 1 mm D)NaOH–H2O2 Sugarcane

bagasse172L1 L1 L3 Increases cellulose and hemicellulose. Lignin removal:

62.83%.NaOH Sugarcane

bagasse173,174L1 L1 L2 Cellulose increases: less than 20%, hemicellulose

removal: 21.98%, and lignin removal: 45.57%.Na2CO3 Sugarcane

bagasse174— L1 L1 A slight increase of cellulose, hemicellulose removal:

10.5%, and lignin removal: 23.07%.KOH Sugarcane

bagasse174L1 L1 L2 Cellulose increases from 40.15 to 47.69%,

hemicellulose removal: 19.81%, and lignin removal:48.31%.

Ca(OH)2 Sugarcanebagasse174

— — L1 A slight increase of cellulose. A slight decrease ofhemicellulose. Lignin removal: 14.24%.

HC-NaOH Orifice plate(27 holes, 1 mm D)

Sugarcanebagasse141

/ / L3 Lignin removal: 60.4%.

Reed149 / / L1 Lignin removal: 24.5%.HC-enzyme Orifice plate(9 holes and 2 mm D)

Corncob170 / / L2 Lignin removal: 47.44%.

HC-enzyme Orifice plate(4 holes and 3 mm D)

Corncob170 / / L2 Lignin removal: 35.91%.

“—”: slight or no effect; “L1”: the effect below 35%; “L2”: the effect from 35 to 60%; “L3”: the effect more than 60%. “/” indicates no mention.Effect means a decrease or an increase in the contents of lignin, hemicellulose and cellulose after pretreatment. D is the diameter.



Table 6 ILs and IL based co-solvents or methods for the pretreatment of lignocellulosic biomass

Cations Anions Biomass Main findings after pretreatment

Emim+ Ac−, HSO4−,

MeOHPO2−, BF4

−, Cl−,DEP−

Corn stover184 and chestnut shells,182 wheat straw,200,210 sprucesawdust and oak sawdust,211 cedar powder,212 coconut shells,213

lignocellulose214

1. Lignin removal: EmimAc (56%), EmimHSO4 (39%),EmimMeOHPO2 (20%), EmimBF4 (0.68%).2. Hemicellulose removal: EmimHSO4 (≈ 100%), EmimAc(<10%), EmimMeOHPO2 (—).3. Cellulose recovery: EmimAc (95%), EmimHSO4 (60.6%),EmimMeOHPO2 (—).4. Lignin recovery: 4.3%.5. EmimDep enhances the butanol production.

Bmim+ Ac−, Cl−, PF6−, Ace− Coconut shell,213 coconut213 and chestnut shell,182 corn stover,184

Tamarix austromongolica1811. Lignin recovery: BmimAc (8.6%), BmimCl (4.6%), BmimAce(—), BmimPF6 (189% increase).2. Hemicellulose removal: BmimAce from 19.14 to 1.24%.3. Cellulose recovery: BmimCl (75%), BmimAce (71.34%).

Amim+ HCOO−, Cl− Hybrid poplar,215 lignocellulose214 1. AmimHCOO: Almost no effect on the biomass componentcontent.2. AmimCl: Enhances the butanol production.

Hmim+, Hpy+, Hnmp+ Cl− Poplar192 1. Lignin extraction: HpyCl (61%) > HmimCl (60.4%) > HnmpCl(47.9%).2. Cellulose recovery: HmimCl (79.3%) > HnmpCl (68%) >HpyCl (59.7%).

DMEA+, BHEM+,2-HTEAF+

mesy− Corn straw193 1. Cellulose and hemicellulose increase, lignin decreases withthe order of BHEMmesy > 2-HTEAFmesy > DMEAmesy.2. Cellulose recovery up to 91.81% by BHEMmesy pretreatment.

A1+ A−, P−, Ac−, F−, C−, G− Oil palm216 Cellulose recovery: A1P (65%) > A1Ac (62%) > A1F (58%) > A1C(53%) > A1G (52%) > A1A (48%).

DIPEA+ Ac−, P−, O−, B− Coffee husk196 Lignin extract yields: DIPEAAc (71.2%) > DIPEAP (63.7%) >DIPEAO (54.2%) > DIPEAB (46.2%).

Ch+ Ac− and Bu−, Arg−,Lys−, Orn−

Empty fruit bunch,208 rice straw,204 switchgrass,203 sugarcanebagasse207

1. Cellulose recovery: ChAc (32.6%), ChBu (26.2%).2. ChArg, removal: 46% lignin.3. Lignin removal: ChAc (from 23.6 to 16.8%), ChBu (from 23.6to 18.1%), ChLys (from 21 to 10.7%), ChOrn (from 21.2 to8.3%).

FurEt2NH+,

VanEt2NH+,

p-AnisEt2NH+

H2PO4− Switchgrass, poplar, eucalyptus, corn stover, pine, rice straw209 Lignin removal: p-AnisEt2NHH2PO4 (43%), FurEt2NHH2PO4

(20%), VanEt2NHH2PO4 (3.9%).

ILs Co-solvent Biomass Main findings after pretreatment

EmimAc HCl, H2SO4, NaOH, gamma-valer-olactone, cyclohexane-Triton,DMSO

Poplar wood,185 bamboo powder,217 corn straw,186 cornstover,187 rice straw,188 switchgrass189

1. EmimAc-HCl: Hemicellulose recovery reaches91.15%.2. EmimAc-NaOH: Strongly removes lignin andincreases cellulose than other five solvents.3. EmimAc-cyclohexane-Triton: Better to removelignin and increase hemicellulose and cellulosecontents.4. EmimAc-DMSO: Slightly increases cellulose anddecreases lignin.

BmimCl NaOH Sunflower stalks218 The saccharification efficiency: 37.9%.AmimCl Methanol, ethanol Wood powder201 Hemicellulose is almost totally removed. Also

removes hemicellulose and cellulose.

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Table 6 (Contd.)

ILs Co-solvent Biomass Main findings after pretreatment

BmimHSO4 Glycol-H2O, glycerol-H2O Coir and poplar,197 coir202 1. BmimHSO4-glycol-H2O: Lignin is removed 98% forpoplar.2. BmimHSO4-glycerol-H2O: Cellulose is recovered90.8%. Hemicellulose is removed 75.4%. Lignin isremoved 34.5%.

AmmorphAc DMSO Rice straw219 Maximum 21% of the lignin is removed.BmimBF4, HmimHSO4/HbimHSO4/TEAMeSO3, TEAHSO4, TBAOH,N2220HSO4

H2O Corn stalk,220 cotton stalks,195 miscanthus × gigan-teus,194 eucalyptus sawdust,198 oil palm empty fruitbunches and coconut husk221

1. BmimBF4-water: Slightly affects the contents ofcellulose and lignin.2. Lignin removal: HmimHSO4-H2O > HbimHSO4-H2O > TEAMeSO3-H2O.3. TEAHSO4-H2O: Over 90% hemicellulose isremoved and maximum 82% lignin is removed.4. TBAOH-H2O: Increases cellulose, decreases hemi-cellulose and lignin.5. N2220HSO4-H2O: Lignin removal from 32.8 to12.8%.

ChOrn FeCl2 Sugarcane bagasse207 Cellulose recovery 53.5%. Hemicellulose and ligninare decreased from 26.5 to 11.3% and 21.2 to 8.5%,respectively.

DMSO: Dimethyl sulfoxide; 2-HTEAFmesy: Tris(2-hydroxyethyl)ammonium methanesulfonate; A1A: N,N-diethyl-N,N-dimethylammonium ascorbate; A1Ac: N,N-diethyl-N,N-dimethylammonium acetate; A1C: N,N-diethyl-N,N-dimethylammonium citrate; A1F: N,N-diethyl-N,N-dimethylammonium formate; A1G: N,N-diethyl-N,N-dimethylammonium gluconate;A1P: N,N-diethyl-N,N-dimethylammonium phosphate; AmimCl: 1-allyl-3-methylimidazolium chloride; AmimHCOO: 1-allyl-3-methylimidazolium formate; AmmorphAc: N-allyl-N-methylmorpholinium acetate; BHEMmesy: 2-hydroxy-N-(2-hydroxyethyl)-N-methylethanaminium methanesulfonate; BmimAce: 1-butyl-3-methylimidazolium acesulfamate; BmimBF4: 1-butyl-3-methylimidazolium tetrafluoroborate; BmimHSO4: 1-butyl-3-methylimidazolium hydrogen sulfate; BmimPF6: 1-butyl-3-methylimidazolium hexafluorophosphate; ChAc: cholinium acetate;ChArg: cholinium arginate; ChBu: choline butanoate; ChLys: cholinium lysinate; ChOrn: cholinium ornithine; DIPEAAc: diisopropylethylammonium acetate; DIPEAB: diisopropyl-ethylammonium benzoate; DIPEAO: diisopropylethylammonium octanoate; DIPEAP: diisopropylethylammonium propanoate; DMEAmesy: 2-hydroxy-N,N-dimethylethanaminium methane-sulfonate; EmimAc: 1-ethyl-3-methylimidazolium acetate; EmimBF4: 1-ethyl-3-methylimidazolium tetrafluoroborate; EmimCl: 1-ethyl-3-methylimidazolium chloride; EmimDEP: 1-ethyl-3-methylimidazolium diethyl phosphate; EmimHSO4: 1-ethyl-3-methylimidazolium hydrogen sulfate; EmimMeOHPO2: 1-ethyl-3-methylimidazolium methylphosphonate; FurEt2NHH2PO4:N-ethyl-N-(furan-2-ylmethyl)ethanamine dihydrogen phosphate; HbimHSO4: 1-butylimidazolium hydrogen sulfate; HmimCl: 1-H-3-methylimidazolium chloride; HmimHSO4: 1-methyl-imidazolium hydrogen sulfate; HnmpCl: N-methyl-2-pyrrolidonium chloride; HpyCl: pyridinium chloride; N2220HSO4: triethylammonium hydrogen sulfate; p-AnisEt2NHH2PO4: N-ethyl-N-(4-methoxybenzyl)ethanamine dihydrogen phosphate; TBAOH: tetrabutylammonium hydroxide; TEAHSO4: triethylammonium hydrogen sulfate; TEAMeSO3: triethylammonium methanesulfo-nate; VanEt2NHH2PO4: 4-((diethylamino)methyl)-2-methoxyphenol dihydrogen phosphate.

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tely with EmimHSO4200 and AmimCl-methanol pretreatment,201

and the hemicellulose removal capacities for different solventsfollow the order BmimHSO4/glycerol/H2O (94.2%)202 > EmimAc-HCl (87–91.15%)185 > BmimHSO4/glycerol/H2O (75.4%).202

Certain ILs have shown effectiveness in biomass pretreat-ment and delignification; however, most of them are not bio-compatible and exhibit toxicity to the hydrolytic enzymes andmicrobes used in the biomass conversion and require anextensive amount of water to remove ILs after pretreatment.203

In recent years, the ILs derived from sustainable sources havebeen reported, and some of them have shown impressiveefficiencies and lower toxicity. It was found that approximately46% lignin in the native rice straw was fractionated after beingpretreated with ChArg,204 and the crystallinity of celluloseand lignin can be destroyed with ChAc205 and ChLys.203,206

ChOrn207 can be used to selectively dissolve lignin or hemi-cellulose and lignin from switchgrass or sugarcane bagasse,and the lignin removal capacity of ChOrn is higher than thatof ChLys. ChOrn/FeCl2 solution was used to pretreat sugarcanebagasse. The research showed that: (1) the FeCl2 with a con-centration higher than 2% (w/v) can significantly increase theremoval rate of hemicellulose; (2) the extraction of hemi-cellulose can be up to 57.3% at a concentration of 4% (w/v)FeCl2, providing a further increase of enzymatic digestibilityup to 90.1%; (3) the good delignification capacity of ChOrn/FeCl2 reflects the values of recovered lignin from sugarcanebagasse.207 ChAc and ChBu can also be used to pretreat emptyfruit bunches. The result illustrated that ChAc is stronger thanChBu for removing lignin and increasing the cellulosecontent.208 Socha et al. investigated the ILs derived from ligninand hemicellulose.209 Switchgrass, poplar, eucalyptus, cornstover, pine, and rice straw were used as feedstocks to preparefurfural, vanillin and p-anisaldehyde, and then applied to syn-thesize dihydrogen phosphate ILs. The results showed thatthese ILs can be used to remove lignin from biomass in theorder p-AnisEt2NHH2PO4 (43%) > FurEt2NHH2PO4 (20%) >VanEt2NHH2PO4 (3.9%).

4. Biomass renewal utilization process

The main components of biomass are lignin, hemicelluloseand cellulose. Lignin can be employed for power and heat gene-ration in the paper and pulp industries as well as for other lowmolecular chemicals and derived-products. Hemicellulose con-sists of complex C5 and C6 polysaccharides, and it is valuablefor the production of important chemical intermediates.Cellulose has a ribbon-like flat structure containing manyglucose molecules, and it can be utilized for producing valuablelow molecular chemicals, obtaining other important derivedproducts and depolymerizing into biofuels.222

4.1 Lignin utilization

Various chemicals can be acquired from lignin, such as pheno-lic compounds, guaiacol, vanillin, formic acid, bioethanol, etc.Considering that the reviews with the focus on fuels and

chemicals from lignin have been reported recently,223 the workon the low molecular chemicals from lignin was excluded inthis review, instead, the overall situation on carbon materials,dye dispersants, bioplastics and aerogels was reviewed becauseof their high values.

4.1.1 Carbon/graphene/carbon nanotubes. Lignin is onekind of attractive carbon precursor because of its abundance,high carbon content, low cost and renewablility.224 To date,lignin-derived carbon materials, such as hard carbon, porouscarbon, activated carbon, lignin-based carbon nano-composites, bio-char pellets, graphene, carbon nanotubes,etc., have been reported as remarkable application candidatesfor energy storage,225,226 lithium-ion or lithium–sulfur bat-teries, supercapacitors and photocatalysts to realize the high-value utilization of lignin.

Carbon. Yang et al.224 successfully grafted melamine intolignin to prepare nitrogen doped hard carbon with the aid of acatalyst (Ni(NO3)2·6H2O) at 1000 °C as an anode material inlithium-ion batteries. XRD and TEM analyses demonstratedthe co-existence of graphitic and amorphous structures in thishard carbon. The prepared anode exhibits higher reversiblecapacity (345 mA h g−1 at 0.1 A g−1), higher rate capability(145 mA h g−1 at 5 A g−1) and excellent cycling stability thanthe samples without nitrogen-doping and catalysis, owing tothe synergistic effect of nitrogen doping, graphitic structureand the amorphous structure. A new route to prepare hierarch-ical porous carbon derived from lignin using KOH as both anactivating agent and a template was studied by Zhang et al.227

The obtained hierarchical porous carbon is composed of aunique 3D macroporous network with mesopores and micro-pores decorated on the carbon walls. This unique hierarchicalporous structure provides high active surfaces for adsorptionand storage of lithium ions as well as fast paths for the trans-portation of lithium ions (Fig. 5). The developed carbon wasused as the anode material of a lithium ion battery, and it dis-played a stable and high capacity (470 mA h g−1) after 400 gal-vanostatic charge–discharge cycles at a current density of200 mA g−1. For the application of lithium–sulfur batteries, alignin derived macro-/micro-porous carbon with oxygen-con-taining functional groups on the outside surface was preparedby a one-step carbonization/activation method and then com-bined with the sulfur loading at different durations (6 h and10 h).228 The result revealed that the specific surface area ofthe porous carbon is 1211.6 m2 g−1 and the porous volume is0.59 cm3 g−1. After 10 h sulfur loading, the composite deliversa high discharge capacity of 1241.0 mA h g−1 in the 2nd cycleand maintains a specific value of 791.6 mA h g−1 until the100th cycle. This performance is much better compared withthat of 6 h sulfur loading. An interconnected hierarchicalporous nitrogen–carbon (HPNC) from lignin-derived bypro-ducts was investigated for the application of super-capacitors.229 The constructed porous carbon displayed bowl-like nanostructures with hierarchical pore distributions and alarge specific surface area of 2218 m2 g−1 through the com-bined hydrothermal treatment and activation. The HPNC-based supercapacitor has an excellent specific capacitance of



312 F g−1 at 1 A g−1 and 81% retention at 80 A g−1 as well as anexcellent cycle life of 98% initial capacitance after 20 000cycles at 10 A g−1 in 6 M KOH aqueous electrolyte. When usingthe IL of EmimBF4 as an electrolyte, the energy density of thesupercapacitor was enhanced to 44.7 W h kg−1 at an ultrahighpower density of 73.1 kW kg−1.

Wang et al. used a one-pot in situ method to prepare lignin-based carbon/ZnO (LC/ZnO) nanocomposites as a photo-catalyst for methyl orange.230 The synthesis pathway for LC/ZnO nanocomposites is illustrated in Fig. 6. It showed that theBET surface area of the LC/ZnO (139.53 m2 g−1) is much largerthan that of pure ZnO (47.68 m2 g−1). LC/ZnO (≈ 6.38% lignin)photodegraded 98.9% of methyl orange under simulated solarlight in 30 min and showed a 5-fold enhancement of photo-catalytic activity over pure ZnO nanoparticles. In addition, theprepared lignin-based carbon has great potential to replace theexpensive graphene in the field of photocatalysis.

Graphene. Graphene has gained tremendous attention inmany fields because of its unique two-dimensional (2D)network structure with the merits of exceptionally high carriermobility, low density, extremely large specific surface area,considerable mechanical strength and outstanding anticorro-sion properties.231 There are two methods for the preparationof graphene at a large scale, i.e., oxidation–reduction andchemical vapor deposition (CVD). CVD meets the requirementsof both large-scale and high-quality, however, the transfer ofgraphene is difficult. The solid carbon sources (e.g., derivedfrom lignin) with metal catalysts were, thus, used to preparegraphene, in which less resource is needed and the operationis easier compared with the two available methods.232

Yan et al. investigated graphene fabrication with differentcarrier gases by using Kraft lignin as the carbon source withiron(III) as the catalyst at 1000 °C.233 It was found that the gra-phitization degree of Kraft lignin not only depends on thetemperature but also the types of carrier gases during the heattreatment. Methane and natural gas as the carrier gases seemto accelerate the formation of multilayer graphene materialswith a range of 2–30 layers, and hydrogen and carbon dioxidehave an etching effect on the solid carbon species during thecatalytic graphitization process, while the multilayer graphene-encapsulated iron nanoparticles are the main products in thecase of argon as the carrier gas. The iron powders were used asthe catalyst to prepare graphene from Kraft lignin at 1000 °Cas reported by Liu et al.232 The result indicated that with aratio of carbon source to iron of 3 : 1, the graphene fold struc-ture was obtained after thermal treatment for 90 min. The gra-phene quantum dots (GQDs), which are highly desirable fore.g., bioimaging, biosensors, photocatalysis, solar cells andlight-emitting diodes, can also be derived from lignin. Dinget al. reported a two-step method to prepare single-crystallineGQDs from alkali lignin.234 Ultrasonication was introduced todisperse the lignin in nitric acid aqueous solution, and thenthe dispersion was subsequently transferred into an autoclaveand heated to 180 °C and kept for 12 h. The product wasfinally obtained by filtering through a 0.22 μm microporousmembrane. Notably, the prepared single-crystalline GQDspresent a hexagonal honeycomb graphene network and theyare 1–3 atomic layers thick. The prepared GQDs have the fea-tures of bright fluorescence, up-conversion properties, long-term photostability, high-water solubility and biocompatibility,making them potentially excellent nanoprobes for multicolourbioimaging. In summary, the utilization of renewable biomassresources paves the way for green, low-cost and large-scale pro-duction of high quality graphene and allows for the develop-ment of sustainable applications.

Carbon nano-tubes. Carbon nano-tubes (CNTs) have attracteda great deal of interest due to their remarkable carrier mobilityof 1000–4000 cm2 V−1, high Young’s modulus of 0.27–1.25TPa, excellent electrical conductivity of 104–105 S cm−1 andthermal conductivity of 3000–6600 W m−1 K−1 as well as highthermal stability up to 700 °C in air.235 They have desirableapplications in electrochemical and biomedical fields. Onecheap and easy method to prepare CNTs from lignin was pro-

Fig. 5 Formation process of the lignin derived hierarchical porousstructure and lithium ion storage in it.227

Fig. 6 LC/ZnO nanocomposite synthesis pathway (left) and a possible mechanism for the degradation of methyl orange by the LC/ZnO nano-composites (right). AL is alkali lignin, and QL is quaternized lignin.230



posed by Liu et al.232 In this study, Kraft lignin is the carbonsource and iron powders are the catalysts. CNTs can beobtained after thermal treatment for 105 min at 1000 °C.Shi et al.236 studied the microwave-enhanced pyrolysis methodto treat gumwood at 500 °C with silicon carbide as a microwaveabsorber to produce CNTs that are formed on the surface ofbio-char (Fig. 7). These CNTs are grown from the pyrolyzedround particles without the use of specific catalysts, substratesand carrier gases.

4.1.2 Dye dispersants. Dye has been widely applied inindustries including textile, rubber, paper, plastic and leather.It has been estimated that over 10 000 types of commercialdyes with 7 × 105 t per year are being consumed.237 Dyes canbe classified into various types, such as direct dyes, reactivedyes, disperse dyes, vat dyes, sulfur dyes, basic dyes, acid dyesand solvent dyes,238 and all of them are difficult to disperse inwater without dispersants. Dye dispersants play an importantrole in the dyeing process. As early as 1909, studies found thatthe lignosulfonate in the waste liquor of a sulfite process couldbe used as auxiliaries in the processing of dye up-take.239

Since then, lignin-based dye dispersants have been attractinggreat attention with significant contributions to improving thedispersion performance, and this is because such dispersantshave good thermal stability and are environment-friendly andrenewable.

The lignin-based dye dispersants are mainly synthesizedfrom lignosulfonate and alkali lignin that are the byproductsfrom the lignin industry.240 Qiu et al.238 and Zhang et al.241

indicated that the dark colour of the industrial lignin is themain obstacle for it being used as high value-added dyestuff

dispersants. A UV/H2O2 whitening process was proposed todecolorize the sulfonated alkali lignin (SAL) by Qin et al. and thecolour was faded by 50%.238 After whitening, the SAL was notonly practically depolymerized but also disaggregated. It wasfound that the cleavage of phenolic hydroxyl and methoxylgroups as well as the weakening of π–π interaction in SAL contrib-ute to its decolorization, and the colour of the polyester fiberstained with the modified light-coloured SAL (LSAL) is almostwhite, while that stained with SAL is earthy-yellow (Fig. 8).

Four lignin-based dye dispersants were studied by Qinet al.,242 i.e., sodium lignosulfonate (NaSL), naphthalene sulfo-nate formaldehyde condensates (NSFC), SAL and hydroxypro-pyl sulfonated alkaline lignin (HSAL). The result illustratedthat HSAL with the lightest colour is the best one with a dyeuptake of 85.3%. The effect of the average molecular weight ofHSAL was further studied by Qin et al.,243 and the HSAL with amolecular weight of 11 020 Da has a better effect on the disper-sive ability, higher temperature stability and higher dye uptakecompared with other HSALs and the sodium naphthalene sulfo-nic (SNF) acid formaldehyde condensation. The study on theadsorption mechanism indicated that the fiber is expanded andmany voids are generated during dyeing (Fig. 8). The disper-sants with small size and high content of OHphen groups, suchas AL or HSAL (8750 Da), are more easily embedded into fibervoids, increasing the adsorption amount. The dispersant with ahigh molecular weight, such as the HSAL (14 830 Da), has ahigh adsorption amount but low fiber staining because the lowOHphen content makes a weak hydrophobic and H-bondinginteraction between the fiber and dispersant. In the study byQin et al.,238 SNF has a minimum adsorption amount due to alinear molecular structure with a main chain of the aromaticring that is difficult to embed into the fiber voids (Fig. 9).

4.1.3 Bioplastics. Lignin-based recyclable plastics havebeen paid great attention as an active field in recent years.They can compete with the petroleum-based plastics in bothcost and performance.244 Lignin has the potential to be aremarkable candidate mostly because of its glass transitiontemperatures (Tg) (normally in the range from 90 to 170 °C)245

as well as its renewable and thermoplastic characteristics.Usually, two branches of lignin can be used, one is as a low-cost additive to improve the thermal and mechanical pro-perties of bioplastics; the other is to synthesize bioplasticsfrom lignin.

Fig. 7 SEM micrograph of CNTs formed on the surface of bio-char(left). The micrograph of the CNTs pyrolyzed from gumwood (right).236

Fig. 8 SAL whitening and fiber staining.238



Kraft lignin was used as an additive to modify the pro-perties of wheat gluten bioplastics.246 The addition of 30 or50 wt% Kraft lignin increased the extrusion dye temperaturefrom 80 to 110 °C, the addition of 10–50 wt% Kraft ligninimproved the extrusion ability of wheat gluten, and theaddition of 10–30 wt% Kraft lignin improved the mechanicalproperties and reduced the water adsorption of wheat glutenbioplastics. Two types of lignin obtained from softwood andhardwood were used to manufacture polylactic acid (PLA)-lignin bioplastics.247 According to the analysis results, theaddition of lignin increases the thermal stability, mechanicalproperties, water sorption capacity and weathering ability ofthe PLA-lignin bioplastics.

The content of lignin is in the range of 30–45 wt% in blackliquor. Ház et al. prepared lignin from black liquor and studiedits properties.248 The result showed that the lignin isolated fromthe black liquor has great potential to be used as a componentfor new bioplastic preparation. Efforts were also made to intro-duce hardwood Kraft pulp in plastic composites such as poly-propylene with the aim of producing bio-based materials withdesirable mechanical characteristics.249 It was found that 10%lignin in the composite decreases the flexural strength(30.57 MPa) while 30% lignin increases the flexural strength(40.37 MPa), meanwhile, the degradation temperature increasesfrom 360.16 to 415.15 °C with increasing lignin content.

Lignin contains various functional groups, such as aliphaticand phenolic hydroxyl, methoxyl, carbonyl and carboxyl moi-eties. These reactive groups have effectively enabled the prepa-ration of lignin-derived bioplastics. The lignin modified by thefractionation with solvents and the crosslinking reaction withformaldehyde was used to develop lignin-polybutadiene ther-moplastics.250 The crosslinking increases the molecular weight(Mn = 31 000 g mol−1) whereas that of the as-received lignin isonly 1840 g mol−1. Tuning the molecular weight of ligninenables a successful preparation of novel lignin-derived ther-moplastics. The use of high molecular weight lignin enhancesthe shear modulus due to an efficient network formation oftelechelic polybutadiene bridges.

Poly(ε-caprolactone) (PCL) is one of the main biodegradablepolymers in the current and emerging bioplastics markets.

The renewable high-performance polyurethane bioplasticsderived from lignin-poly(ε-caprolactone) were investigated byZhang et al.251 The synthesis route of Lignin-PCL Polyurethane(LPU) is illustrated in Fig. 10. The mechanical properties of aLPU plastic can be effectively controlled by adjusting thelignin content, the ratio of –NCO/–OH and the molecular weightof PCL. The properties of LPU are also dependent ontheir network architecture. The LPU plastic has a betterthermal stability compared with the purified lignin. The tensilestrength, breaking elongation and tear strength of the LPUfilm can reach 19.35 MPa, 188.36% and 38.94 kN m−1, respect-ively, when the lignin content is up to 37.3%. Moreover,this plastic is very stable at 340.8 °C and presents excellentsolvent-resistance.

A series of renewable triphenylmethane-type polyphenols(TPs) were synthesized from lignin-derived guaiacols (methyl-guaiacol and propylguaiacol) and aldehydes (4-hydroxybenz-aldehyde, vanillin, and syringaldehyde) (Fig. 11).252 The networkexhibits an excellent glassy modulus (12.3 GPa) and a glasstransition temperature (167 °C). Such study widens the syn-thesis route of fully bio-based polyphenols, which can yieldrenewable thermoplastics with excellent properties.

Fig. 9 Adsorption model of dispersants on fiber.243

Fig. 10 The synthesis route of the Lignin-PCL Polyurethane (LPU).251



4.1.4 Aerogels. Aerogels are a class of open-cell and meso-porous foams with special physical and chemical properties.Up to 99.9% ultra-porosities, extremely large pore-volumes orinternal void spaces, exceptional low densities and very highsurface area-to-mass ratios are the features that make aerogelsunique among other solid materials. In addition, the poroustexture of aerogels can be easily controlled by adjusting thefabrication conditions during the sol–gel synthesis and the

subsequent drying steps. Moreover, aerogels can be fabricatedin various forms, such as monoliths, particles, powders, com-posite blankets and thin films, being another major interest incommercial utilizations.253

Aerogels are formed by controlling the extraction of liquidfrom the capillary voids of wet gels or hydrogels using thesupercritical fluid extraction drying technique. The major chal-lenges are the cost ineffective larger-scale production, expen-sive and toxic precursors as well as the time-consuming pro-duction processes. To reduce the cost, several strategies havebeen proposed to simplify the manufacturing processes, forexample, freeze-drying and ambient pressure drying as thealternatives to the supercritical drying.

Recently, the novel precursor, biomass, has attracted greatinterest in terms of reducing the cost for manufacturing aerogelsbecause of its inexpensive, nontoxic, and renewable nature.254

The lignin-based aerogels have a unique three-dimensionalnetwork structure and excellent properties, such as lightweight,low density and high specific surface area, making them attrac-tive materials for applications in adsorbents,255 electromagneticinterference shielding,231 supercapacitors,256,257 supercapacitorelectrodes,258 scaffolds,259 etc. The properties and applications ofthe lignin or lignin modified aerogels are given in Table 7, and amore detailed description is given as follows.

Fig. 11 Synthesis route of fully renewable triphenylmethane-type poly-phenols from lignin-derived aldehydes and para-substitutedguaiacols.252

Table 7 The properties and applications of lignin or lignin modified aerogels

Precursor Method Application Properties Ref.

Corncob lignin/graphene oxide

Hydrothermal: 180 °C, 12 h; freeze-drying: −60 °C, 12 h

Adsorbent Density: 3.0 mg cm−3; 255specific surface area: 270 m2 g−1;porosity: 95.4%

Alkali lignin/graphene oxide

Freeze-drying: −80 °C, 24 h; carbonization:5 °C min−1 to 900 °C and heldat 900 °C for 2 h

Electromagneticinterference shielding

Density: 2.0–8.0 mg cm−3; 231electromagnetic interference shieldingeffectiveness: 21.3–49.2 dB;surface-specific shielding effectiveness:53 250 dB cm2 g−1

Alkali lignin/CuCl2·6H2O

Microwave: 10 W, 10 min; pre-frozen:12 h; Freeze-drying: −50 °C, 2 days;carbonization: 3 °C min−1 to 800 °C andheld at 800 °C for 2 h

Supercapacitor Specific surface area: 899 m2 g−1; 256pore volume: 0.63 m3 g−1;specific capacitance: 257.65 F g−1;porosity: 47.6%

Kraft lignin Gelation and curing: 85 °C, 5 days;solvent exchange: 45 °C, 72 h;carbonization: 1100 °C

Supercapacitor Specific surface area: 120.56 m2 g−1; 257pore volume: 0.45 m3 g−1

Organosolvlignin

Gelation and curing: 85 °C, 5 days;solvent exchange: 45 °C, 72 h;carbonization: 1100 °C

Supercapacitor Specific surface area: 19.20 m2 g−1 257Pore volume: 0.0073 m3 g−1

Crop wastelignin

Curing: 80 °C, 9 h; ambient drying:50 °C, 1 day; carbonization: 3 °C min−1

to 1050 °C and held at 1050 °C for 3 h

Supercapacitorelectrode

Specific surface area: 779 m2 g−1; 258pore volume: 0.48 m3 g−1;specific capacitance: 142.8 F g−1;porosity: 60.4%

Lignin (90%)and cellulose(10%)

Dissolution: BmimCl, 85 °C, 3 h;regeneration: water, >6 h; solvent exchange:ethanol and tert-butyl alcohol bath, >6 h;freeze-drying: −50 °C, 48 h

Sound-adsorptionand thermalinsulation

Density: 0.08 g cm−3; 259specific surface area: 58.2 m2 g−1;pore volume: 0.116 m3 g−1;porosity: 94%;Young’s modulus: 5.9 MPa;sound adsorption: 125–1000 Hz increase,1000–4000 Hz decrease;thermal conductivity: 0.138 W m−1 K−1

Wheat strawlignin/sodiumalginate

Gelation: 4.5 ± 0.5 MPa, 24 h;solvent exchange: ethanol bath; supercriticaldrying: CO2, 12 ± 1 MPa

Scaffold Density: 0.03–0.07 g cm−3; 260specific surface area: 564 m2 g−1;pore volume: 7.2 cm3 g−1;porosity: 31.3 ± 1.9%;Young’s modulus: 0.38 ± 0.05 MPa



Chen et al. prepared a lignin-modified graphene aerogelfrom the precursor of corncob lignin and graphene oxide. A3D framework porous structure was acquired by the freeze-drying method. This prepared aerogel has a specific surfacearea of 270 m2 g−1 and 95.4% porosity, showing remarkableproperties as an adsorbent. The adsorption capacity can reach350 times its own weight when using it for the adsorption ofpetroleum oils or toxic solvents such as toluene, chloroformand carbon tetrachloride. This result is superior to that of agraphene aerogel and among the highest in the reportedadsorbents.255 Alkali lignin and graphene oxide were used toprepare an ultralight and highly elastic reduced grapheneoxide and lignin-derived carbon composite aerogel.231 Thisaerogel has aligned micron-sized pores and cell walls with theapplication of electromagnetic interference shielding. A smallfraction of lignin-derived carbon cell walls enhances the inter-facial polarization effect, greatly boosting the wave adsorptioncapability of the cell walls. The electromagnetic interferenceshielding effectiveness for the studied aerogel is 21.3–49.2 dB,and the surface-specific shielding effectiveness is 53 250 dBcm2 g−1. This result is better than other reported carbon- andmetal-based shields. Alkali lignin/CuCl2·6H2O,

256 Kraft andorganosolv lignin257 as well as crop waste lignin258 were usedto prepare aerogels for the application of supercapacitors. Itwas indicated that the Cu-doped lignin-based carbon aerogelshave a larger specific surface area of 899 m2 g−1 than otherthree aerogels and exhibit a specific capacitance of 257.65F g−1. The specific capacitance remains 95% after 2000 cyclesat the current density of 20 A g−1. These findings show a poten-tial application of carbon aerogels in energy storage. Inaddition, Wang et al. fabricated a lignin aerogel for the appli-cation of sound-adsorption and thermal insulation.259 Lignin

(90%) and cellulose (10%) were dissolved in ILs and simplyregenerated in deionized water, causing the assembly of cell-ulose and lignin at the micro- and nanoscale and even at themolecular level. The resulting lignin aerogels exhibit Young’smodulus up to 5.9 MPa, highly efficient sound-adsorption andexcellent thermal insulation (0.138 W m−1 K−1). Moreover,lignin shows potential for application in scaffolds. The aerogelfabricated from wheat straw lignin and sodium alginate bysupercritical drying was studied by Quraishi et al.260 Theresults illustrated that the alginate-lignin aerogels are non-cytotoxic, making them attractive as the candidates for a widerange of applications including tissue engineering and regen-erative medicine.

4.2 Cellulose utilization

This section provides several high value-added applications ofcellulose mostly based on bio-derived ILs from cellulose, func-tional cellulose composites as well as adsorbent materials,carbon and aerogels prepared from cellulose.

4.2.1 Cellulose-based ionic liquids. Cellulose is a versatileand low-cost sustainable material that undergoes chemicalchanges primarily due to its hydroxyl groups.261 It was alsofound that cellulose imparts surface characteristics and thenincreases total sorbent uptake with a 4 to 8-fold improvementin uptake per gram IL for the solid ILs. This offers the poten-tial to drastically reduce the amount of IL required to separatea given gas volume.262 Bernard et al. studied the CO2 capturepotential by the modified cellulose based ILs. These modifiedcellulose based ILs were prepared in two stages. The first stagewas the modification with citric acid, and the second one wasthe attachment of the cationic moiety (Fig. 12 left). Among thestudied ILs, CL-TBA is the best one for CO2 sorption in this

Fig. 12 Ionic compounds obtained from cellulose modification: (a) CL-CA; (b) CL-BMPYRR; (c) CL-BMIM; (d) CL-TBP; (e) CL-TBA. BMPYRR:1-butyl-1-methylpyrrolidinium chloride. TBPB: tetrabutylphosphonium bromide. TBAB: tetrabutylammonium bromide. DMF: N,N-dimethylformamide.261



study with 71 mgCO2 g−1 at 3 MPa and 25 °C. At low pressures,

this compound presents a high CO2 sorption value (44 mg CO2

per g at 0.1 MPa). It was explained that the weakly coordinatedcounter-cations of the modified cellulose stabilize the systemand maintain the CO2 binding sites of the anion via partiallyor fully non-occupied sites, improving the CO2-philicity of theionic compound.261 Later, using the cellulose extracted fromrice husk, the cationic cellulosic poly(ionic liquids) (PILs) werefurther studied for CO2 capture by Bernard et al.263 Fig. 13depicts the cations namely imidazolium and ammonium com-bined with different counter anions (Cl−, BF4

−, PF6− or bis(tri-

fluoromethanesulfonyl)imide TF2N−) to fabricate cellulose

based ILs. The synthesized PILs have a high CO2 sorptioncapacity that is directly influenced by the anion. At 3 MPaof CO2 partial pressure, the sorption capacity follows theorder Cl− < BF4

− < TF2N− < PF6

−. The best capture value wasfound for the PIL CelEt3NPF6 of 38 mg g−1 at 0.1 MPa and168 mg g−1 at 3 MPa at 25 °C. The recycling experiments ofCO2 capture demonstrated a good reusability of the synthesizedPILs for CO2 sorption.

Cellulose-based ILs were synthesized and used as adsor-bents for highly selective recovery of gold reported by Guoet al.264 In this study, diethylaminoethyl cellulose was used asthe main raw material. The nitrogen atom of the diethyl-

aminoethyl cellulose grafting carboxymethyl group generatesthe cellulose-based IL carboxymethyldiethylammoniumethylcellulose (CMDEAEC) (Fig. 14), which possesses a nanoscaleparticle size and good swelling ability. The result indicatedthat CMDEAEC exhibits a maximum Au(III) adsorption of15 mg g−1 according to the Langmuir model. Additionally,CMDEAEC exhibits a fast adsorption rate, and the adsorptionequilibrium time is 2 h. Regeneration studies reveal the goodpotential of CMDEAEC for reuse and show only a 4.1% loss inadsorption after 7 cycles of adsorption–desorption.

4.2.2 Functional composites. Biological composites areefficient and multi-functional materials, achieving optimalperformance through a complex structural hierarchy and arelatively sparse choice of chemical constituents.265 Cellulosecarries abundant hydroxyl groups, which can interact with theoxygen-containing groups. Meanwhile, the hydrogen bondinteractions and the stirring force synergy result in efficientmixing of cellulose and the selected compounds.266 Using cell-ulose in composites is not only beneficial in an ecological waybut also with technical and economic benefits.267

Cellulose based functional composites have attracted manyinteresting investigations with the applications of catalysts,lithium ion batteries, antibacterial materials, drug carriers,scaffolds, energy storage, etc. The precursors of these func-tional composites are mainly the cellulose combined withmetals, metal oxides or carbon nanomaterials. In recent years,the precursors, composites, applications and the properties ofthe cellulose-based composites have been investigated and aresummarized in Table 8. A ZnO nanosheet-regenerated cell-ulose film (ZNSRC) was studied by Zhou et al. to be used inphotocatalysts.268 The SEM result showed that the ZnOnanosheets were dispersed on both the surface and inside ofthe ZNSRC. The best photocatalytic activity of ZNSRC is that1.53 × 10−5 mol methyl orange was completely degraded in50 min by using 1 g catalyst, which indicated that ZNSRC canbe very useful for degrading organic dye wastewater. Inaddition, the cellulose composite catalyst of graphene oxide/cellulose microspheres-NH2@Fe3O4-H3PW12O40 prepared fromcotton material was applied in the transesterification ofhighly-acidic Pistacia chinensis seed oil to biodiesel. Under theoptimal reaction conditions (80 °C, methanol/oil molar ratio12 : 1, catalyst 15 wt% (w/w of oil) for 8 h), a 94% biodieselyield was achieved.266 Nanofibrillated cellulose was used toprepare a LiFePO4/graphene/nanofibrillated cellulose compo-site. This composite shows potential as the lithium ion batteryelectrode. Compared with the pure LiFePO4 electrode, the

Fig. 14 Synthesis scheme of carboxymethyl-diethylammoniumethylcellulose (CMDEAEC).264

Fig. 13 Synthesis steps and proposed structures of the obtainedmaterials: (a) 6-chloro-6-deoxycellulose (CDC); (b) cellulosic poly(ILs).263



resulting composite (LiFePO4/graphene/nanofibrillated cell-ulose) electrode shows excellent mechanical flexibility and pos-sesses an enhanced initial discharge capacity of 151 mA h g−1.Moreover, the composite electrode can endure the bendingtests up to 1000 times.269

Microcrystalline cellulose functional composites have wideapplications in catalysts, antibacterials and scaffolds. It wasreported that the immobilized EstH/Fe3O4-microcrystallinecellulose is more temperature-stable with a prolonged half-lifeand higher storage stability compared to the EstH free form.270

For the antibacterial function, 97% and 98% reductions werefound in the growth of antibiotic resistant bacteria such asvancomycin resistant Enterococcus faecalis and methicillinresistant Staphylococcus aureus using microcrystalline cell-ulose/wool keratin/Ag0 nanoparticles. This will promotechronic wound healing.271 For the application of drug release,it was reported that ethyl cellulose-MgHPO4 as a drug carriercan release up to 87% of its total loaded drug over a 90 minperiod.272

4.2.3 Adsorbent materials. Cellulose has strong sorptionpower thereby making it a suitable adsorbent either in naturalor modified forms. Celluloses have been used as adsorbentsfor water, oil, organic solvents, metal ions, dyes, drugs, etc.275

A few examples of the types and the capacities of the cellulosesthat were recently published for the sorption of water, oil andorganic solvents and for the removal of metal ions, dyes anddrugs are presented in Table 9, and some have been discussedfurther.

The study of water sorption with bacterial cellulose graftingacrylic acid in the presence of N,N′-methylenebisacrylamide asa crosslinker and ammonium persulfate as an initiator wasconducted by Luo et al.276 The results showed that themaximum water adsorbency is 322 ± 23 g/g-distilled-water. Theadsorption capacity decreases with the increase in the concen-tration of salt. The pH value has a significant influence on thewater adsorption performance, and the adsorbency increasesas the pH is increased from 3.5 to 6.0, decreases at pH 7.0,

increases again at pH 8.0, and then continues to decrease untilthe end. The increase of temperature results in a decrease ofthe water retention rate.

Oil-based effluent is categorized into two distinct types: freeoil and emulsified oil. Free oil can be separated easily by gravi-tation and skimming off; however, it is difficult to remove theemulsified oil since it is more stable in the aqueous phase. Ithas been found that the functional groups on the surface ofcellulose are efficient for oil adsorption.277 One kind of super-hydrophobic 3D porous ethyl cellulose adsorbent was devel-oped for the removal of organic solvents and oils from water,such as n-hexane, cyclohexane, heptane, octane, petroleumether, diesel oil, dimethyl silicone oil, soybean oil, diethylether and gasoline.278 The results indicated that the adsorp-tion capacity of the modified cellulose adsorbent depends notonly on the density but also on the viscosity and surfacetension. For the organics with low viscosity such as n-hexane,cyclohexane, petroleum ether, diesel oil, diethyl ether andgasoline, the modified cellulose needs only about 16 secondsto reach the adsorption equilibrium, while it takes about 26seconds in the oils with high viscosity, such as dimethyl sili-cone oil and soybean oil. Owing to the low movement velocityof oil molecules with high viscosity in the pore channels of themodified cellulose, the solvation of the sponge network andswelling in the oil are relatively slower compared with the lowviscous oil.

Heavy metals are the most hazardous contaminants in thechemical-intensive industries. These heavy metals can beeasily adsorbed by living organisms. Usually, the cellulosesurface needs to be modified in order to function as anefficient adsorbent.277 It was reported that the content andstructure of the amine groups (NH2– or NH–) within the mem-branes affect the removal of Cr(VI) metal ions.279 An advancedgreen carboxymethyl cellulose-based adsorbent was preparedby the inclusion of thiosemicarbazide for the adsorption ofCu(II) ions from the aqueous solutions. The mechanistic studyof Cu(II) adsorption through XPS and FTIR analyses indicated

Table 8 The precursors, composites, applications and properties of cellulose based composites

Cellulose Composites Application Results Ref.

Wood pulpcellulose fiber

ZnO nanosheet-regeneratedcellulose film

Photocatalyst Completely degraded 1.53 × 10−5 mol methyl orangein 50 min by using 1 g catalyst.

268

Cotton Graphene oxide/cellulosemicrospheres-NH2@Fe3O4-H3PW12O40

Catalyst Achieved 94% biodiesel yield. 266

Nanofibrillatedcellulose

Ag@nanofibrillated cellulose Catalyst Rate constant: 46.6 × 10−3 s−1 for the reduction of4-nitrophenol.

273

Microcrystallinecellulose

EstH/Fe3O4-cellulose Catalyst Enzyme kinetics: 51.14 μM min−1. 270Rate constant: 520 s−1.

Nanofibrillatedcellulose

LiFePO4/graphene/nanofibrillatedcellulose

Lithium ionbattery electrode

Discharge capacity: 151 mA h g−1. 269Bending test: more than 1000 times.

Ethyl cellulose Ethyl cellulose-MgHPO4 Drug release Released 87% of total loaded drug over 90 min 272Microcrystallinecellulose

Cellulose/wool keratin/Ag0

nanoparticlesAntibacterial 97% and 98% reduction in the growth of antibiotic

resistant bacteria, promotion in chronic woundhealing application.

271

Microcrystallinecellulose

Cellulose-graft-polyacrylamide/nano-hydroxyapatite

Scaffold Compressive strength: 4.80 MPa Elastic modulus: 0.29GPa.

274

Porosity: 47.37%.



that the high adsorption is attributed to the presence of theelectron donor groups such as nitrogen, sulfur and oxygenwith high affinity for the formation of the complex with Cu(II)ions.280 The investigation of Barsbay showed that the enrich-ment of O–CvO, C–O and C–N groups on the cellulose surfaceis beneficial for the adsorptions of Cu(II), Pb(II) and Cd(II).281

While for the Cs+ adsorption, the remarkable groups on thecellulose surface are NH2, O–CvO and OH groups.282

Moreover, a low-cost cellulose-based IL that possesses a nano-scale particle size and good swelling ability was developed toincrease the contact area for Au(III) sorption.264 The cottonfabric as a green biomass resource was used as a carbon pre-

cursor, and an iron precursor was implanted to create meso-pores through a catalytic graphitization reaction. The porestructure of the nanocomposites was tuned by adjusting theloadings of the iron precursor, and the embedded iron carbidenanoparticles served as an active component for magnetic sep-aration after adsorption.283 The results showed a selectiveadsorption of different metal ions with Cr(VI) > Zn(II) > Ni(II) >Cu(II) > Pb(II), and the adsorption capacities of metal ionsfollow the order Cr(VI) > Pb(II) > Cu(II) > Ni(II) > Zn(II).

Dye pollutants occur in many industrial waste-waters pro-duced by a variety of industries, such as textiles, leather, cos-metics, paper, plastics and rubber. These dyes are harmful for

Table 9 The types of adsorbates, adsorbates, cellulose based adsorbents and the adsorption capacities

Types ofadsorbates Adsorbates Cellulose based adsorbent Adsorption capacity

Water Water Bacterial cellulose grafting acrylic acid 322 ± 23 g g−1 distilled water276

Bagasse cellulose grafting acrylic acid 236–325 absorbency ratios in distilledwater287

Oil Dimethylsilicone oil, soybeanoil, diesel oil, gasoline

Ethyl cellulose/Ag/long-chain alkanethiols Around 90% (v/v)278

Diesel fuel, diesel oil slick, cornoil, corn oil slick

Wheat straw cellulose fiber 22–26 g g−1 (ref. 288)

Organicsolvents

n-Hexane, cyclohexane,petroleum ether, diethyl ether

Ethyl cellulose/Ag/long-chain alkanethiols Around 90% (v/v)278

Trichloroacetic acid Cellulose adsorbent with amide and sulphinategroups

The pseudo-first-order rate constant fordegradation is (0.93 ± 0.12) h−1(ref. 289)

Metal ions Cr(VI) Cellulose acetate/Ti-tetraethylenepentaminemembrane

99.8%279

Amino-functionalized cellulose 32.5 mg g−1(ref. 290)Cu(II) Thiosemicarbazide-modified carboxymethyl

cellulose144.92 mg g−1(ref. 280)

Iminodiacetic acid modified cellulose 69.6 mg g−1(ref. 281)Cellulose acetate modified with zwitterion/graphene oxide

32 mg g−1(ref. 291)

Electrospun cellulose acetate/TiO2 23 mg g−1(ref. 292)Pb(II) Iminodiacetic acid modified cellulose 52.0 mg g−1(ref. 281)

Carboxylated cellulose nanocrystal-sodiumalginate

338.98 mg g−1(ref. 293)

Cellulose acetate modified with zwitterion/graphene oxide

27.6 mg g−1(ref. 291)

Electrospun cellulose acetate/TiO2 25 mg g−1(ref. 292)Cd(II) Iminodiacetic acid modified cellulose 53.4 mg g−1(ref. 281)Cs+ Carboxymethyl cellulose/Fe-aminoclay/polyhedral

oligomeric silsesquioxane composite152 mg g−1(ref. 282)

Carboxymethyl cellulose sodium/Prussian blue-lanthanum

35.22 mg g−1(ref. 294)

Au(III) Cellulose-based IL (carboxymethyl-diethylammoniumethyl)

15 mg g−1(ref. 264)

Hg(II) Stearic acid modified cellulose 178 mg g−1(ref. 295)Dye Methylene blue Carboxymethyl cellulose/Fe-aminoclay/polyhedral

oligomeric silsesquioxane composite438 mg g−1(ref. 282)

Carbonylated Cellulose/microfibrillated cellulosespheres

303 mg g−1(ref. 296)

Graphene/2,2,6,6-tetramethyl-1-piperidinyloxy(TEMPO)-oxidized cellulose

227.27 mg g−1(ref. 285)

Tannin based phenolic resin and carboxymethylcellulose with epichlorohydrin

1300 mg g−1(ref. 284)

Chrysoidine G Carboxymethyl cellulose/Fe-aminoclay/polyhedraloligomeric silsesquioxane composite

791 mg g−1(ref. 282)

Crystal violet Cellulose from the leaves of Artocarpusodoratissimus

239 mg g−1(ref. 286)

Methyl violet Cellulose from the leaves of Artocarpusodoratissimus

187 mg g−1(ref. 286)

Congo red Cellulose impregnated bentonite; 43.47 mol g−1(ref. 297)Cellulose impregnated kaolinite 500 mol g−1(ref. 298)



living organisms even with short periods of exposure, makingit very important to remove dyes from waste water.284 In dyeremoval, the operation conditions are different consideringthe dye characteristics, and the particle size of the used cell-ulose is much larger compared with those used for theremoval of heavy metals and other ions. Typically, the inter-action between the adsorbent and the dye particle is categor-ized as physisorption with van der Waals attraction.277 Toimprove the performance, a hybrid monolith of grapheneoxide and nanocellulose fiber with a robust interconnectednetwork was fabricated via an ecofriendly self-assemblymethod. The pore morphology and surface chemistry (such ashydrophilicity and surface charge density) of the monolithwere improved, and the specific surface area and mechanicalproperties were enhanced about 4- and 5-fold, respectively,compared with the unmodified monolith. It was proposed thatthe strong chemical interaction, mainly hydrogen bonding, isthe primary driving force for the formation of a hybrid mono-lith.285 The combination of phenolic resin based on tanninand CMC can also effectively improve the adsorption perform-ance of resin with methylene blue due to a synergistic effectbetween the phenolic hydroxyls of the tannin and the carboxylgroups of the CMC.284 Their adsorption process involves intra-particle diffusion, electrostatic attraction and hydrogenbonding interactions, and thus it depends on the pH andionic strength of the medium.286

4.2.4 Carbon. Carbon dots (CDs) can be easily passivatedto achieve specific functionalities. They have some interestingproperties such as high specific surface area, good biocompat-ibility, low cost and fluorescence properties.299 These pro-perties make them remarkable materials in different areassuch as optoelectronic devices, biosensors, photocatalysis,solar cells and bioimaging (e.g., in immunofluorescence).

The use of biomass to produce CDs can represent theadvantages related to the high availability and low cost of thefinal material, in the case that the synthetic strategy does notrequire expensive material or elaborate experimental set-ups.Eucalyptus Kraft wood pulp cellulose was used as the precur-sor of CD through a simple method using sulfuric and nitricacid.300 The diameter of the quasi-spherical biomass-derivedCD ranged from 1 to 3 nm with an average size of 2 nm, and alarge number of different oxygen groups (mainly carboxylicacids) were detected on the surface. Considering that CD wasobtained with a low cost method and that the used renewableprecursor is highly abundant, this method can represent animportant strategy to reduce the final cost for the productionof carbon nanoparticles with potential applications in theareas such as biomedicine and photocatalysis.

A novel mechanochemical method was first developed tosynthesize carbon nanodots (CNDs) or carbon nano-onions(CNOs) through the high-pressure homogenization of cellulosepowders. The CNDs (less than 5 nm in size) showed sphericaland amorphous morphology, and the CNOs (10–50 nm in size)presented a polyhedral shape, an onion-like outer lattice struc-ture and a graphene-like interlattice spacing of 0.36 nm. CNOsshowed blue emissions, moderate dispersibility in aqueous

media and high cell viability, enabling efficient fluorescenceimaging of cellular media.301

In general, hard carbon is composed of two characteristicdomains: stacked hexagonally bound carbon sheets and micro-pores (nanosized pores), i.e., interstitial spaces of the stackedcarbons. Some biowastes have been used as the startingmaterials for one-step pyrolysis at 700–1500 °C to synthesizehard carbons with a capacity of 300 mA h g−1 or even higher.Since sucrose contains cellulose and lignin, their chemicallymodified precursors are pyrolyzed to tailor high-energy hardcarbons, demonstrating 250–330 mA h g−1 reversible capacity.The cellulose-derived hard carbons show a high capacity, andthe pretreatment can introduce cross-linkages. It was foundthat the optimization facilitates the reversible capacities of 353and 290 mA h g−1 in Na and K cells, respectively.302

Activated carbon has been used as an industrial material,such as an adsorbent. The carbon material directly derived fromcellulose is a non-graphitizable carbon and can be easily acti-vated. However, the yield is low because the chemical structure ofcelluloses contains many O atoms. Improving the yield is impor-tant for the preparation of activated carbon from cellulose inorder to decrease the production cost. It has been reported thatthe addition of a flame retardant, e.g., guanidine phosphate, isan effective option. By the addition, the yield can be increasedfrom ca. 12% to ca. 22%.303 Melamine sulfate is one of thechemicals having a flame-retardant effect on the cellulose, and itwas added to improve the yield of CO2 activation reaction viadoping the dual heteroelements. The addition of melaminesulfate results in the suppression of the Brunauer–Emmett–Teller(BET) specific surface area. For the CO2-activated sample with30 wt%-added melamine sulfate, the BET is 578 m2 g−1.304

Graphene, a single layer of carbon atoms arranged in a hex-agonal lattice, has attracted a lot of well-deserved attentiondue to its high surface area and excellent electrochemical pro-perties. However, graphene is easy to aggregate and has poorvolumetric performance when used as an electrode material.Hence, the dense graphene-like porous carbon (GPC) with ahierarchical pore structure was fabricated from the sheet cell-ulose made by milling the bleached Kraft pulp. The prepa-ration procedure of the graphene-like porous carbon is givenin Fig. 15, and the features of GPC are depicted in Fig. 16. The

Fig. 15 Schematic illustration of preparing graphene-like porouscarbon (GPC). PTFE: polytetrafluoroethylene.305



results indicated that the unique pore structure leads to anincrease of specific surface area up to 2045 m2 g−1, providingabundant storage sites and channels for electrolyte species.The gravimetric (353 F g−1) and volumetric (309.7 F cm−3)specific capacitances of the prepared GPC were obtained in the6 mol L−1 KOH aqueous electrolyte with a current density of 1A g−1, and the gravimetric and volumetric specific energydensity of the prepared symmetrical supercapacitor with thisGPC electrode was 120.1 W h kg−1 and 80.4 W h L−1, respect-ively, in an IL (BmimBF4) mixed with acetonitrile electrolyte.These excellent performances make this material potentiallyuseful for diverse energy storage devices.305

4.2.5 Aerogels. Cellulose aerogel is a porous solid material.It is generally prepared in three steps: dissolving/dispersingcellulose or cellulose derivatives, forming cellulose gel by thesol–gel process, and drying cellulose gel while basically retain-ing its 3D porous structure.306 As the newly-synthesized third-generation aerogels, cellulose aerogels not only have numerousexcellent properties as the traditional silica aerogels andpolymer-based aerogels (such as high porosity (84.0–99.9%),high specific surface area (10–1000 m2 g−1), low-sonic velocity,low light refractive index, low dielectric constant and lowthermal conductivity (<0.025 W (m K)−1), but also have otherunique characteristics inherited from their feedstock (i.e., cell-ulose) including biocompatibility, biodegradability and hydro-philicity.307 In addition, the following advantages can beachieved by using cellulose as the precursor for the prepa-ration of aerogels. (1) The reserve of cellulose raw material isinexhaustible and renewable; (2) the cellulose chain is rich inhydroxyl groups, so that no cross-linking agent is needed inthe aerogel preparation process. A stable three-dimensional

(3D) network structure can be obtained by intramolecular andintermolecular physical cross-linking of hydrogen bonds, thusmaking the aerogel preparation process quite simple; (3) thechemical modification of cellulose to improve the mechanicalstrength and structural characteristics of cellulose aerogels isrelatively easy to accomplish.306

Cellulose aerogels can be divided into three kinds: (1)natural cellulose aerogels, (2) regenerated cellulose aerogels,and (3) cellulose derivative aerogels. The natural cellulose aero-gels include nanocellulose aerogels and bacterial celluloseaerogels. Nanocellulose aerogels are prepared by ultrasonic ormechanical methods to disperse nanocellulose in water, fol-lowed by subsequent drying with or without solvent exchange.Bacterial cellulose is collected from static bacterial culturesand has a natural 3D network gel structure. After the removalof bacteria and other impurities and subsequent drying, cell-ulose aerogels can be obtained. From Table 10, it can be foundthat the natural cellulose aerogels have outstanding propertiesfor applications in thermal insulation, and electrode and dye/oil/water separation. For example, the thermal conductivity ofa bacterial cellulose-silica aerogel is down to 29.2 mW(m K)−1,308 the oil/water separation efficiency is more than99% by cellulose nanofiber (CNF)-chitosan aerogels,309 andthe capacitance of a MnOxNy/rGO/CNF aerogel as an electrodecan reach 455.8 F g−1.310

Synthesis of aerogels is time-consuming because of theslow process of solvent exchange. Some cellulose derivativesare soluble in water and typical organic solvents.306 Forexample, CMC and hydroxypropyl methylcellulose (HPMC) aresoluble in water, triacetyl cellulose (TAC) is soluble in dioxane/isopropanol, ethyl cellulose (EC) is soluble in dichloro-methane, and cellulose acetate (CA) is soluble in acetone.Since acetone and some other organic solvents are soluble inSC-CO2, the time-consuming solvent exchange process can beomitted, thus improving the efficiency of aerogel synthesis.

Regenerated cellulose aerogels are currently being studiedvery extensively. The regeneration of cellulose aerogels hasfour main steps: cellulose dissolution, cellulose regeneration,solvent exchange and drying. Meanwhile, the modificationthat can change the physical and chemical properties of cell-ulose is an important way to functionalize the cellulose aero-gels. The applications of regenerable cellulose aerogels aremainly based on wastewater treatment (i.e., metal or dyeremoval), organic solvent/oil/water separation, and CO2

adsorption. More details of these applications can be found inTable 10.

4.3 Hemicellulose utilization

To the best of our knowledge, the application of hemicelluloseis still limited and much less compared to those of lignin andcellulose, and the review based on the hemicellulose appli-cation is also scarce. In this section, the hemicellulose appli-cation is mainly focused on films and pharmaceutical carriers.

4.3.1 Films. The formation of films from hemicelluloseacetates was reported as early as 1949. The films from therenewable materials have numerous potential applications in

Fig. 16 SEM images of (a) sheet cellulose, (b) GPC-600, and (c) its high-magnification view; TEM images of (d) GPC, (e) layer-like structure of GPCand (f) pore morphology of GPC; porosity analysis of (g) N2 adsorption–de-sorption isotherms of GPCs (inserted plots is N2 adsorption of Graphite (redcurve, precursor is cellulose pulp) and a Graphite (green curve, precursor isbleached Kraft pulp)); (h) pore size distribution of GPC; (i) variation of theBET surface area and particle density against temperature.305



the food and medicinal industries, such as food packaging,wound dressings and drug capsules.326 Hemicellulose-basedfilms have become promising candidates for eco-friendlypackaging applications because of their biodegradability andcost-effectiveness. However, the inherently poor mechanicaland hygroscopic properties of hemicellulose-based filmslargely hinder their potential for target applications.327 Byforming hemicellulose derivatives through functionalizingavailable hydroxyl groups or by forming blend films, the pro-perties such as hydrophilicity and mechanical behavior can bemodified.

An effective and convenient approach to prepare blendfilms with enhanced mechanical properties by the incorpor-ation of CMC into quaternized hemicelluloses (QH) wasstudied by Gao et al.328 The blend film prepared from QH andCMC (1 : 2 m/m) has mechanical properties such as a tensilestrength of 65.2 MPa, while it is only 18.02 MPa for a CMCfilm. This suggested that the addition of CMC enhances themechanical properties by the strong electrostatic interactions,and the hydrogen bondings were enhanced with QH. Inaddition, chitosan was introduced into a QH/montmorillonite(MMT) matrix to produce nanocomposite films with signifi-

cantly enhanced mechanical properties.329 The chitosan dis-turbs the regular arrangement of quaternized hemicellulosewhen incorporating into MMT due to the enhancement ofhydrogen bonding and electrostatic interaction between twocontrarily charged fillers. The QH-MMT-CS films even with asmall amount of CS show an excellent tensile strength (57.8MPa), which is 30.2% higher than the composite filmQH-MMT. One hemicellulose-based eco-friendly packagingwas fabricated by adding nanocrystalline cellulose (NCC) orcationically modified nanocrystalline cellulose (CNCC) tohemicelluloses (HC)/sorbitol (SB). It was found that the tensilestresses of the composite films with 9% NCC and 9% CNCCare 9.18 and 10.44 MPa, respectively, which are increased by14% and 30% in comparison with that of a pure HC/SB film(8.05 MPa). This result strongly supports the conclusion thatthe addition of NCC or CNCC is effective at improving themechanical properties of HC/SB films.327 In addition, poly-acrylamide hemicellulose hybrid films were synthesizedthrough the copolymerization of acrylamide (AM) monomersand hemicellulose initiated by the potassium persulfate/N,N,N′,N′-tetramethylethylenediamine (KPS/TMEDA) redox initiatorsystem with the cross-linker N,N-methylene-bis(acrylamide)

Table 10 The types of cellulose aerogels, aerogels, dry methods, application and adsorption capacities/properties

Types of celluloseaerogels Aerogels Dry methods Application Adsorption capacity/properties Ref.

Natural celluloseaerogels

Cellulose-Fe3O4 Freeze-dried Dye adsorption Adsorbent wavelength for Congo red:497 nm

311

Bacterial cellulose-silica

SC-CO2 Heat insulator Density: 0.066 g cm−3; 308porosity: 92.7%;thermal conductivity: 29.2 mW (mK)−1

MnOxNy/rGO/CNFs Freeze-dried Electrode Capacitance: 455.8 F g−1 310Nanofibrillatedcellulose

Freeze-dried Thermal insulation Thermal conductivity: 25.5 mW (mK)−1

312

CNFs-chitosan Freeze-dried Oil/water separation Separation efficiency: >99% 309Regeneratedcellulose aerogels

MC-DBNHCO2Et SC-CO2 / Specific surface area: 240–340 m2 g−1;densities: 0.04–0.07 g cm−3

313

Electrospun celluloseacetate

Freeze-dried Hydrocarbon–water separationand energy Storage

Density: 1.1 mg cm−3; 314porosity: 99.9%;chloroform: 201–73 g g−1;capacitance: 51.3 mF cm−2

MCC-Ti3C2Tx Freeze-dried Microwave adsorption Minimum reflection loss of −43.4 dBat 11.2 GHz

315

Cotton pulp-GO Freeze-dried Dye Methylene blue: 68 mg g−1 316Cellulose-chitosan Freeze-dried Oil and organic solvent

adsorbentVarious oils and organic solvents:13.77–28.20 g g−1

317

Cellulose derivativeaerogels

TEMPO-CNFs Freeze-dried Wastewater treatment Cu(II) removal: 93.33% 318CMC-rGO Freeze-dried Wastewater treatment Rhodamine B: 186.33 mg g−1 319Cellulose-polyethylenimine

Freeze-dried Wastewater treatment Chromium(VI): 229.1 mg g−1 320

Graphene/CNFs/silica Freeze-dried Organic solvent and oiladsorption

Hexane and chloroform: ≈100% 321Oils: 39–68 times weight gain

Cellulose-DMAEMA Freeze-dried Oil/water separation Water contact angle: 0° 322Hydrophobicity: 130°

CA-SiO2/TiO2 Roomtemperature

Oil/water separation Separation efficiency: 99.99 wt% 323

CNFs-AEAPMDS Freeze-dried CO2 adsorption CO2: 1.78 mmol g−1 324CNFs-APS Freeze-dried CO2 adsorption CO2: 1.91 mmol g−1 325

AEAPMDS: N-(2-Aminoethyl)(3-aminopropyl)methyldimethoxysilane; APS: N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane; CA: celluloseacetate; CMC: carboxymethyl cellulose; CNFs: cellulose nanofibers; DBNHCO2Et: 1,5-diazabicyclo[4.3.0]non-5-enium propionate; PDAMEMA:poly (2-dimethylaminoethyl methacrylate); rGO: reduced graphene oxide; TEMPO: 2,2,6,6-tetramethylpiperidine-1-oxyl.



(MBA) (Fig. 17). All results showed that the prepared filmspossess many advantages, such as excellent water solubility atambient temperature (dissolved in water in 116 seconds), lowoxygen permeability (8.75–22.97 cm3 μm (m2 d kPa)−1), goodmechanical properties (10.7 MPa, strain 120%) and greatrecyclability. This implies that it is an effective way to utilizethe hemicellulose waste liquor, having both great economicand environmental benefits.330

Hemicellulose can also be used to prepare conductive filmscombined with carbon nanotubes (CNTs).331 The CNTs pre-pared by Shao et al. are double-walled carbon nanotubes(DWNTs), and HC is the commercial xylan, an extract frombeech tree. The DWNT-HC conductive solution was dispersedusing the ultrasonic method. The DWNT-HC films were de-posited by either spin casting or drop-dry casting and dis-persed on the pieces of a commercially purchased highlydoped silicon wafer with a 300 nm thick silicon oxide insula-tion layer to form different thickness films. The DWNT-HCcomplex has the advantage of relatively easy dispersal in water.The bulk conductivity of the prepared films is up to 2000 Scm−1, much higher comparable to the pure CNT material. Inthe DC measurements conducted at low temperatures, a rela-tively weakly increasing resistivity of the CNT-HC wasobserved, showing that the material is close to the metal–insu-lator transition.

4.3.2 Pharmaceutical carriers. Environmentally sensitivehydrogels have enormous potential in various applications. Inthe pharmaceutical area, some environmental variables, suchas pH, ionic strength, solvent composition, temperature aswell as electric and magnetic fields need to be considered.Environmental changes in the body may occur, such as low pHand elevated temperatures. Thus, either pH-sensitive and/ortemperature-sensitive hydrogels can be used for site-specificcontrolled drug delivery.332,333 The hydrogels that are respon-sive to specific molecules, such as glucose or antigens, can beused as biosensors and drug delivery systems. Peng et al.334

reported that the xylan-rich hemicellulose hydrogels expandedat high pH, whereas shrinkage occurred at low pH or in saltsolutions as well as in organic solvents. The ionic hydrogelshad high water adsorption capacity and showed rapid and

multiple responses to pH, ions and organic solvents. Furtherstudy illustrated that the xylan-rich hemicellulose hydrogelsexhibited highly efficient regeneration and metal ion recoveryefficiency, and they can be reused without a noticeable loss ofadsorption capacity for Pd2+, Cd2+ and Zn2+ after quite anumber of adsorption/desorption cycles.335

It was reported that the hydrogels based on hemicelluloseof wheat straw can be used as a novel carrier to control drugdelivery.336 The prepared hemicellulose hydrogels are pH-responsive. The swelling kinetics of the hydrogels follows aFickian diffusion process in a medium with a pH of 1.5, andthe water uptake can be controlled collaboratively by hydrogelrelaxation and water diffusion in the media with pH values of7.4 and 10. With acetylsalicylic acid as a model drug, therelease dynamics of the drug-loaded hydrogels shows a zero-order drug release kinetics for 6 h, and the cumulative releaserate of 85% can be achieved. The release of theophylline fromthe hydrogels verified the controlling effect of the hemi-cellulose hydrogels. This study indicates that the hemi-cellulose hydrogels can be used in biomedical fields, especiallyfor controlling the drug release.

A novel photoresponsive hydrogel prepared by free radicalcopolymerization of xylan-type hemicellulose methacrylatewith 4-[(4-acryloyloxyphenyl)azo]benzoic acid (AOPAB) wasinvestigated. It showed multi-responsive behaviors to pH,water/ethanol alternating solutions, and light.337 The swellingratios of the prepared hydrogels in the distilled waterdecreased from 9.8 to 2.2 g g−1 when the AOPAB content wasincreased from 2 to 16%. The hydrogel displayed rapid swell-ing and de-swelling performances in water and ethanol alter-nating solutions. Additionally, under UV irradiation, the trans-conformation of azobenzene in the hydrogel generally con-verted into the cis-conformation and resulted in the hydro-philic/hydrophobic balance variation of the hydrogel.Therefore, the hydrogel loaded with vitamin B12 (VB12) showeda higher drug cumulative release rate under UV irradiationthan that without UV irradiation.

Xylan, a promising hemicellulose, has been reported fordrug delivery. One kind of maleic anhydride-modified xylan(MAHX), a pH- and thermo-responsive hydrogel, was studiedfor acetylsalicylic acid and theophylline release. The cumulat-ive release rate of acetylsalicylic acid for MAHX-based hydro-gels is higher than that of theophylline. Besides, in the gastro-intestinal sustained drug release, the acetylsalicylic acidrelease rate is extremely slow for the initial 3 h in the gastricfluid (24.26%), and then the cumulative release rate reaches90.5% after sustained release for 5 h in the stimulated intesti-nal fluid. Importantly, MAHX-based hydrogels have satisfac-tory biocompatibility with NIH3T3 cells.338

5. Conclusion and perspectives

Lignocellulosic biomass materials are receiving increasingattention as a renewable, economic and abundant alternativeto fossil resources for the production of various high-value pro-

Fig. 17 Proposed synthetic process of the PAM-hemicellulose hybridfilm and the corresponding chemical structure of the product.330



ducts. For the utilization of lignocellulosic biomass, theefficiency highly depends on the types of biomass resourceswith different contents, compositions and structures of cell-ulose, hemicellulose and lignin. The cascade utilization is thebest way because this option considers the composition,characteristics and the nature of cellulose, hemicellulose andlignin.

The fractionation of biomass into its individual com-ponents with pretreatment is the first step to achieve thecascade utilization of biomass. The performance of pretreat-ment depends on the used methods and the subsequent targetproduct, and each method has its own specific disadvantages.This makes it a challenge to choose a universal pretreatmentmethod for all the commercial applications. The conventionaltechniques that have been used in an industrial scale will berestricted due to the strategic requirements of “green andenergy saving”, and therefore development of eco-friendly,economic and time-saving pretreatment technologies isneeded.

Based on the review on the emerging unconventional pre-treatment technologies including microwave irradiation, super-critical fluids, ultrasonic irradiation, electric fields, HC andILs, it was found that: the single pretreatment method has alimited enhancement of the fractionation efficiency due to thecomplex structure of biomass, the technologies assisted withco-solvents, catalysts or novel physical systems can signifi-cantly improve the pretreatment efficiency, microwave-IL is asignificantly fast method to reduce the reaction time fromseveral hours to minutes, and the SC-CO2/H2O/ethanol systemcan be used to acquire cellulose rich materials (up to 70%)and the lignin removal yield reaches 90%. In addition, theultrasonication assisted single ethanosolv pretreatment can beused to obtain cellulose with a high purity up to 95%. Amongthe reviewed techniques, the efficiency of the electric field isthe lowest, and HC has the potential to achieve a large scaleindustrial application to oxidize lignin molecules to low mole-cular weight organic products and even CO2. An IL as a greensolvent shows remarkable properties for biomass separationand can be used to acquire high purity target products (e.g.,95% cellulose recovery) by designing its cation and anion.

The economic feasibility of the emerging pretreatmenttechnologies is limited due to the high investment cost. Theultrasonication, microwave irradiation and electric field pre-treatments need to be further developed.152 Supercriticalfluids, such as supercritical H2O, have been used in the pulpindustry, and CO2 and H2O at high pressures also show highpotential.339 Hydrodynamic cavitation is an energy-efficientand suitable method for large scale applications.164

Developing efficient, low viscosity and inexpensive ILs as wellas the corresponding IL-recycling technologies will acceleratethe application of ILs in pretreatment.

The conversion of lignocellulosic biomass to fuels andchemicals is a promising alternative to replace petroleum as arenewable source. However, most of the proposed processesare currently unable to compete economically with the pet-roleum refineries partially due to the incomplete utilization of

the biomass feedstock. From the aspect of cascade utilizationof lignin, hemicellulose and cellulose, producing high-valueand novel products from each component has been proposedsuch as (1) cellulose: converts into dissolving pulp for fibersand chemicals, (2) hemicellulose: used as a raw material orsynthetic intermediate for bio-based films or pharmaceuticalcarriers, and (3) lignin: converts into carbon foam, bioplasticsor battery anodes. However, it should be pointed out that mostof the research on the lignocellulosic biomass utilization isstill in its infancy, such as the functional materials of gra-phene, CNTs, bioplastics, aerogels, cellulose-based ILs andpharmaceutical carriers. Energy demand, capital cost andprocess efficiency are still the challenges towards the commer-cial scale application of lignocellulosic biomass. In the future,at least, the following aspects need to be considered.

(1) According to the composition, characteristics and thetype of lignocellulosic biomass, integrating the merits ofseveral pretreatment methods can be an optimal option to pre-treat the lignocellulosic biomass.

(2) Classifying the lignocellulosic biomass types in an orderbased on the value of the products, and establishing a modelof cascade utilization to enable a selection of appropriatebiomass for a specific target product in order to improve theefficiency of the lignocellulosic biomass utilization.

(3) Establishing an integrated process with the consider-ation of the complexities of the biomass utilization process,inter-dependence of the pretreatment processes and theeconomy related to the market in order to maximize the con-version of lignocellulosic biomass into high-value products.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 21576262, 21776276, and21606233), the Beijing Municipal Natural Science Foundation(Grant No. 2182070) in China, the Major Program of NationalNatural Science Foundation of China (Grant No. 21890762),the K. C. Wang Education Foundation and the KempeFoundation in Sweden.

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