Progress on the pre-treatment of lignocellulosic biomass employing ionic liquids

Renewable and Sustainable Energy Reviews 105 (2019) 268–292

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Progress on the pre-treatment of lignocellulosic biomass employing ionic liquids

T

Pobitra Haldera, Sazal Kundua, Savankumar Patela, Adi Setiawanb, Rob Atkinc, ⁎ Rajarathinam Parthasarthya, Jorge Paz-Ferreiroa, Aravind Surapanenid, Kalpit Shaha, a

Chemical & Environmental Engineering, School of Engineering, RMIT University, Melbourne, Victoria 3000, Australia Mechanical Engineering Department, Faculty of Engineering, Universitas Malikussaleh, Bukit Indah, Lhokseumawe 24352, Indonesia c School of Molecular Sciences, The University of Western Australia, Australia d South East Water, Frankston, Victoria 3199, Australia b

ARTICLE INFO

ABSTRACT

Keywords: Lignocellulosic biomass Delignification Ionic liquid pre-treatment Biofuel Biochemicals

The effective pre-treatment methods are required for the destruction of the complex biomass structure to economically produce high grade fuels and valuable platform chemicals. Ionic liquids have high potential for energy efficient biomass pre-treatment due to their low vapour pressure, emission profile, recyclability and tuneable properties; some ionic liquids can even be prepared from renewable biomass feedstocks. However, a number of issues currently impede the large scale uptake of ionic liquids including their cost of production, detailed understanding the macro, micro and molecular level deconstruction mechanisms which inhibits process optimisation and modelling, and the need for techno-economic astable sessment on large scale trials. So far, laboratory to bench scale IL pre-treatments of various lignocellulosic biomasses were studied by changing various process parameters where the aims were to investigate the biomass dissolution mechanism and understand the pretreatment performance of ILs. This review outlines current research gaps and potential applications for ionic liquids in the destruction of biomass into its components followed by separation of lignin, hemicellulose and cellulose rich fractions.

1. Introduction The production of bioethanol and biodiesel from lignocellulosic biomass for replacement of traditional liquid fuels such as petrol and diesel has received global attention, on account of the possibility of reducing greenhouse gas emissions via use of a renewable feedstock [1–6]. Additionally, interest on the synthesis of green chemicals from the conversion of lignocellulosic biomass is growing very fast [7,8]. Biomass is typically composed of cellulose (40–80%), hemicellulose (15–30%) and lignin (10–25%) [9–11]. Cellulose is a high molecular weight semi-crystalline polysaccharide of β-1,4-glycosidic bonds [12] which is water insoluble [13]. Hemicellulose is a lower molecular weight, amorphous, multicomponent polysaccharide with numerous functional groups including acetyl and methyl, as well as glucuronic, cinnamic and galacturonic acids [13]. Lignin is comprised of aromatic rings in long polymeric chain joined primarily by β-O-4 ether bonds and some C–O and C–C linkages. Guaiacylpropane, syringylpropane and phydroxyphenylpropane are the three building blocks of lignin. The lignin content in biomass depends on the type (i.e. soft, hard etc.) and

⁎

characteristics of biomass fibre [14–16]. Cellulose, hemicellulose and lignin construct a complex and rigid structure which impedes rapid dissolution and decomposition of biomass. Lignin generally acts as the “glue” for binding the fibrous cellulosic components, and is the main barrier on the deconstruction of lignocellulosic biomass and conversion into biofuels and bio-chemicals [11]. However, an efficient biomass pre-treatment increases the cellulose accessible surface area, porosity and reduces its crystallinity which favours the hydrolysis, bio-digestibility and thermochemical conversion into biofuels and bio-chemicals [17–22]. So far, physical, chemical, physicochemical and biological pretreatment methods have been extensively studied for lignocellulosic biomass disintegration [19,23,24]. Fig. 1 classifies the available methods. Physical pre-treatment includes mechanical comminution, nonthermal plasma, ultrasound and microwave pre-treatment. Mechanical comminution disrupts the lignocellulosic structure by various mechanical processes including grinding, milling and chipping resulting physical modification (mainly size reduction) [25]. Non-thermal plasma, ultrasound and microwave pre-treatment processes degrade the

Corresponding author. E-mail address: [email protected] (K. Shah).

https://doi.org/10.1016/j.rser.2019.01.052 Received 4 September 2018; Received in revised form 18 December 2018; Accepted 26 January 2019 1364-0321/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature1

1

H NMR Proton nuclear magnetic resonance [Bmim][OTf] 1-butyl-3-methylimidazolium trifluoromethanesulfonate [Bmim][Me2PO4] 1-butyl-3-methylimidazolium dimethylphosphate [Emim][Me2PO4] 1-ethyl-3-methylimidazolium dimethylphosphate [Bmim][OPr] 1-butyl-3-methylimidazolium propionate [Bmim][OBu] 1-butyl-3-methylimidazolium butyrate [Bmim][OOCCH2CH(OH)COO] 1-butyl-3-methylimidazolium malate [Bmim][OOCCH2CH2COO] 1-butyl-3-methylimidazolium succinate [Bmim][OOCCHCHCOO] 1-butyl-3-methylimidazolium maleate [Bmim][H-C3H2O4] 1-butyl-3-methylimidazolium H-malonate [Bmim][H-OOCCH2CH2COO] 1-butyl-3-methylimidazolium H-succinate [Bmim][H-OOCCHCHCOO] 1-butyl-3-methylimidazolium H-maleate [HOPmim][OAc] 1-hydroxypropyl-3-methylimidazolium acetate [HOPmim][Tf2N] 1-hydroxypropyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [HOEmim][Tf2N] 1-hydroxyethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][BF4] 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][Tf2N] 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [TBA][OH] Tetrabutylammonium hydroxide [Ch][AA] Cholinium amino acid DMA Dimethylacetamide [C4(Mim)2] 1,4-bis(3-methylimidazolium-1-yl) butane [Bmim][FeCl4] 1-butyl-3-methylimidazolium tetrachloroferrate [Smim][Cl] 1-sulfonic acid-3-methylimidazolium chloride [Smim][FeCl4] 1-sulfonic acid-3-methylimidazolium tetrachloroferrate [Bmim][HSO4] 1-butyl-3-methylimidazolium bisulfate [SBmim][HSO4] 1-(4-sulfobutyl)-3-methylimidazolium bisulfate [BSHmim][HSO4] 1-butyl sulfonic acid-3-methylimidazolium hydrogensulfate [Bmim][Cl]-SO3H 1-(4-sulfonic acid) butyl-3-methylimidazolium chloride [C3SO3Hmim][HSO4] 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogensulfate [C3SO3Hmim][PTS] 1-methyl-3-(3-sulfopropyl)-imidazolium paratoluenesulfonate [C3SO3Hmim][Cl] 1-methyl-3-(3- sulfopropyl)-imidazolium chloride [Hmim][Cl] 1-H-3-methylimidazolium chloride [C2OHmim][BF4] 1-hydroxyethyl-3-methylimidazolium tetrafluoroborat MIBK Methyl isobutyl ketone [SO3Hmim][CF3SO3] 1-sulfonic acid-3-methylimidazolium trifluorate [C3SO3Hmim][H2PO4] 1-methyl-3-(3-sulfopropyl)imidazolium dihydrogen phosphate [C3SO3Hmim][CH3SO3] 1-methyl-3-(3-sulfopropyl)imidazolium methanesulfonate [C3SO3Hmim][PhSO3] 1-methyl-3-(3-sulfopropyl)imidazolium phenylsulfonate [C3SO3Hmim][1-NS] 1-methyl-3-(3-sulfopropyl)imidazolium 1naphthalenesulfonate [C4SO3Hmim][HSO4] 1-butyl sulfonic acid-3-methylimidazolium hydrogensulfate [C3SO3HPy][HSO4] N-(3-sulfopropyl)pyridinium hydrogensulfate [C3SO3HN111][HSO4] N,N,N-trimethyl-N-(3-sulfopropyl)ammonium hydrogensulfate [BSmim][HSO4] 1-(4-sulfonic acid) butyl-3-methylimidazolium hydrogensulfate

[Bmim][Cl] 1-butyl-3-methylimidazolium chloride [Amim][Cl] 1-allyl-3-methylimidazolium chloride [Emim][OAc] 1-ethyl-3-methylimidazolium acetate [Bmim][OAc] 1-butyl-3-methylimidazolium acetate [Emim][MeO(H)PO2] 1-ethyl-3-methylimidazolium methylphosphonate [Amim][OFo] 1-allyl-3-methylimidazolium formate TGA Thermogravimetric analysis FTIR Fourier-transform infrared [C2CNBim][Cl] 1-propyronitrile-3-butylimidazolium chloride [C2CNAim][Cl] 1-propyronitrile-3-allylimidazolium chloride [C2CNHEim][Cl] 1-propyronitrile-3–2-hydroxyethyl imidazolium chloride [C2CNBzim][Cl] 1-propyronitrile-3-benzyllimidazolium chloride [Bmim][HSO4] 1-butyl-3-methylimidazolium hydrogen sulphate [Emim][DEP] 1-ethyl-3-methylimidazolium diethyl phosphate [Hmim][Cl] 1-hexyl-3-methylimidazolium chloride [Emim][ABS] 1-ethyl-3-methylimidazolium alkylbenzenesulfonate [Py][OFo] Pyridinium formate [Py][OAc] Pyridinium acetate [Py][OPr] Pyridinium propionate [Ch][OAc] Cholinium acetate [BMPyr][N(CN)2] N-butyl-N-methylpyrrolidinium dicyanamide [BMPyr][Tf2N] N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [P4441][MeSO4] Tributylmethylphosphonium methyl sulphate [P44416][Cl] Tributyl(hexadecyl)phosphonium chloride [P66614][N(CN)2] Trihexyl(tetradecyl)phosphonium dicyanamide [Ch][Lys] Cholinium lysine [Ch][Gly] Cholinium glycine [Ch][Ala] Cholinium alanine [Ch][Ser] Cholinium serine [Ch][Thr] Cholinium threonine [Ch][Met] Cholinium methionine [Ch][Pro] Cholinium proline [Ch][Phe] Cholinium phenylalanine [Bmim][MeSO4] 1-butyl-3-methylimidazolium methylsulfate [Emim][Gly] 1-ethyl-3-methylimidazolium glycine [Mmim][MeSO4] 1,3-dimethylimidazolium methylsulfate [Hmim][CF3SO3] 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [Bmim][Br] 1-Butyl-3-methylimidazolium bromide [Bmim][PF6] 1-Butyl-3-methylimidazolium hexafluorophosphate [Bm2im][BF4] 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [Bmpy][PF6] 1-butyl-4-methylpyridinium hexafluorophosphate [Dmim][MeSO4] 1-Decyl-3-methylimidazolium methylsulfate [Bmim][CF3SO3] 1-butyl-3-methylimidazolium trifluoromethanesulfonate [Bzmim][Cl] 1-benzyl-3-methylimidazolium chloride [Emim][Ace] 1-ethyl-3-methylimidazolium acesulfamate [Bmim][Ace] 1-butyl-3-methylimidazolium acesulfamate [Hmim][OAc] 1-hexyl-3-methylimidazolium acetate [Omim][OAc] 1-octyl-3-methylimidazolium acetate [Bmim][MeSO3] 1-butyl-3-methylimidazolium methanesulfonate 2D NMR Two dimensional nuclear magnetic resonance [Emim][TFA] 1-ethyl-3-methylimidazolium trifluoroacetate [Ch][OPr] Cholinium propionate [Bmim][HSCH2COO] 1-butyl-3-methylimidazolium thioglycollate [Bmim][(C6H5)COO] 1-butyl-3-methylimidazolium benzoate [Bmim][H2NCH2COO] 1-butyl-3-methylimidazolium aminoethanic [Bmim][HOCH2COO] 1-butyl-3-methylimidazolium glycolate [Bmim][CH3CHOHCOO] 1-butyl-3-methylimidazolium lactate [Bmim][N(CN)2] 1-butyl-3-methylimidazolium dicyanamide 269

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[BSmim][CF3SO3] 1-(4-sulfonic acid) butyl-3-methylimidazolium trifluorate [BSmim][OAc] 1-(4-sulfonic acid)-butyl-3-methylimidazolium acetate

[C4(Mim)2][2HSO4] 1,1-Bis(3-methylimidazolium-1-yl) butylene hydrogensulfate [DMA]+ [CH3SO3]- Dimethylacetamide methanesulfonate [NMP]+ [CH3SO3]- N-methyl-2-pyrrolidonium methanesulfonate

Non-thermal plasma Microwave Ultrasound

Physical

Mechanical comminution (Chipping, grinding, milling) Extrusion

Lignocellulosic biomass treatment

Chemical

Dilute acid

Steam explosion

OrganoSolv (Ethanol, methanol, ethylene glycol, glycerol etc.)

Liquid hot water

Alkaline

Ammonia fibre expansion

Physochemical CO2 explosion

Brown fungi White fungi Biological

Ammonia recycle percolation

Soft-rot fungi

Oxidative

Fig. 1. An overview of biomass pre-treatment methods [25–29].

lignocellulosic structure as well as separate lignin and hemicellulose [27–29]. In contrast, chemical pre-treatments uses various chemicals such as dilute acid [30], alkali [31] and organic solvent [32]. OrganoSolv chemical pre-treatment of biomass uses a wide range of solvents such as ethanol [33,34], methanol [35], ethylene glycol [36], glycerol [37], acetic acid [38] and formic acid [39] with or without catalyst for pre-treatment of various biomass feedstock. As the name suggests, physicochemical pre-treatment methods combine chemical and physical approaches. Commonly employed physicochemical pre-treatments include steam explosion [40], carbon dioxide explosion [41], liquid hot water treatment [42], ammonia fibre expansion [43], ammonia recycle percolation [44] and oxidative treatment [45]. For biological pretreatments, various microorganisms such as brown, white and soft-rot fungi are employed [16]. During the biological pre-treatment process, the microorganisms produce enzymes which degrade the lignocellulosic structure. These pre-treatment methods notably alter the chemical and physical structure of lignocellulosic biomass which enhances hydrolysis [46–48]. The successful implementation of the various pretreatment methods at commercial scale is evaluated using the parameters as described in Table 1. However, most pre-treatment methods have only been investigated at laboratory scale; only a few methods are industrially

applicable. In 2009, a demonstration plant based on acid catalysed steam explosion pre-treatment method was commissioned in Salamanca province of Spain with the aim of producing 4000 t of ethanol per year from wheat straw [20]. In the same year, another demonstration scale plant in Kalundborg (Denmark) commenced operations, but used liquid hot water instead of steam to pre-treat straw. In 2011, a demonstration scale acid pre-treatment plant with a production capacity of 4500 t ethanol per year from sugarcane bagasse was established in Örnsköldsvik (Sweden). In addition to these, several industrial scale organic solvent pre-treatment plants were developed in the USA, France, Brazil, Germany and Canada [49]. However, these pre-treatment methods exhibited several shortcomings including environmental concerns, high costs and limited application ranges [19]. In addition, these methods can form inhibitory products (e.g., phenols, furans and carboxylic acids) during pre-treatment process which suppress the hydrolysis or fermentation steps [16,50]. Recently, the utilisation of ionic liquids (ILs) in ligonocellulosic biomass pre-treatment has gained significant attention to the scientific community. ILs are pure salts with low melting points [60,61]. ILs with melting points less ambient temperatures are known as room temperature ionic liquids [62,63]. Most ILs have some exciting solvent properties such as non-flammability, low or negligible vapour pressure, chemical and thermal stability (Fig. 2) [64–67]. The physical and chemical properties of ILs can be tuned for a specific task by varying the cation and anion combinations [68,69]. On account of their low vapour pressure, ILs are considered to be a green alternative to volatile organic solvents [70,71], but many ILs have toxicities similar to structurally similar ionic surfactants if released into the environment. Nonetheless,

1 Note: The cations [Bmim]+, [Emim]+, [Amim]+, [Hmim]+, [Dmim]+, [Omim]+, [Bm2im]+ are also expressed as [C4C1im]+, [C2C1im]+, [(C1═C2)C1im]+, [C6C1im]+, [C10C1im]+, [C8C1im]+, [C4C1C12im]+, respectively.

270

271

alteration

hemicellulose solubility

lignin removal with • Low moderate structure

Alkaline

of seconds to days

room temperature with • Atresidence time in the domain

to minutes

alteration

hemicellulose solubility

lignin removal • Moderate with high structure

accessible surface • High area with low

alteration

hemicellulose solubility

lignin removal • Moderate with high structure

alteration

140–215 °C with a accessible surface • Atresidence • High time of few seconds area with high

Dilute acid

accessible surface • High area lignin removal • Moderate with high structure

150–350 °C with high • Atpressure, typically, 5–20 MPa

hemicellulose solubility

reduction in cellulose • High crystallinity lignin removal with • High high structure alteration

hemicellulose solubility

°C with a flow rate accessible surface • Atof 5140–210 • High mL/min and a residence area with moderate

200 °C with high pressure of accessible surface • At6.90–27.58 • High MPa area with high

reduction in cellulose • High crystallinity lignin removal with • High high structure alteration

hemicellulose solubility

60–100 °C with pressure of accessible surface • At1.72–2.09 • High MPa for 5 min area with moderate

time of 90 min

Economic aspects [19,20,25,26,56,57]

Energy requirement [25,26,58]

Process development [20,25,46,55,57,59]

expensive • Highly recycling system • Efficient requirement

temperature lime method

energy consumption • High • Not used for large-scale plant in case of high

consumption

(continued on next page)

amount of toxic • Low compounds generation inhibition to fermentation • No • Effective for low lignin biomass

amount of toxic • Low compounds generation • Low inhibition to fermentation

amount of toxic • High compounds generation • High inhibition to fermentation

scale

environmental and • High safety concerns on a large

Additional remarks [25,26,52,57]

cost of ammonia at large demonstration in lab concerns with • High • High energy requirement • Successful • Environmental scale however no demonstration/ strong smell of ammonia from commercial plant yet exists commercial plant recycling system • Efficient amount of toxic compounds requirement • Low generation inhibition to fermentation • No for high lignin • Ineffective biomass cost of CO2 inhibition to fermentation • Low • High energy requirement • No extensive feasibility study • No cost of equipment for Ineffective for dry biomass • High • excessive pressure cost of ammonia at large commercial scale plant for concerns with • High • High energy requirement • No • Environmental scale treatment the strong smell of ammonia at commercial plant recycling system • Efficient amount of toxic requirement • Moderate compounds generation inhibition to fermentation • Low for high lignin • Ineffective biomass recycling of chemicals energy demand at amount of toxic • No • Low • Not proven at commercial scale • Low required low temperature compounds generation inhibition to fermentation • Low investment cost • Low effective for high lignin • Less biomass investment cost of High energy consumption Proven at commercial scale High amount of toxic • High • • • anticorrosive equipment development compounds generation chemical cost High inhibition to fermentation • High • recycling system High reaction rate • Efficient • requirement

no chemical requirement

accessible surface power and energy demonstration in lab • High • Highly expensive • High • Successful area consumption however no demonstration/ commercial plant yet exists reduction in cellulose • High crystallinity accessible surface effective for hardwoods and applied at • High • Cost • Low energy consumption • Tested area with high but less effective for softwood commercial scale hemicellulose solubility quantity of chemicals • Limited consumption lignin removal • Moderate with high structure • No recycling cost alteration accessible surface attractive energy and large at lab scale and • High • Economically • High • Tested area with high because of low reactor cost and quantity of water developed at commercial scale

Pre-treatment effects [25,54,55]

Oxidative

Ammonia recycle percolation

CO2 explosion

Ammonia fibre expansion

170–230 °C with pressure • Atgreater than 5 MPa

Liquid hot water

seconds to a few minutes

160–240 °C with • At0.7–4.8 MPa for several

on the required size • Depends of biomass particle

Operating conditions [25,26,41,48,51–53]

Steam explosion

Mechanical comminution

Pre-treatment method

Table 1 Comparative study of currently available options for biomass pre-treatment.

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for high lignin • Effective biomass hazards, • Explosion environmental and health safety

2. IL pre-treatment processes and laboratory to bench scale investigations

concerns

2.1. Applications of pre-treated fractions produced from biomass

• Low energy requirement

effective for large scale • Less application

The recalcitrance of lignocellulosic biomass is a major concern for the sustainable conversion of it into biofuels and chemicals. An effective IL pre-treatment breaks down the crystalline composite structure of lignocellulosic biomass which enables lignin and hemicellulose to be isolated from cellulose rich material (CRM) as shown in Fig. 3. When lignin and hemicellulose are separated, CRM can have a wider range of advanced chemical and energy production applications. The IL fractionation of lignocellulosic structure is considered to be a golden route for the selective production of renewable chemicals from each of the fractions. Typically, lignin contains 61–65% carbon [74] which produces hydrocarbon fuel with a low amount of oxygen compared to cellulose. Lignin is of complex structure and considered to be a renewable source for aromatic building blocks. Depolymerisation of lignin has huge potential to convert it into biofuels (e.g., phenolic oil, syngas etc.), many value-added materials (e.g., bio-plastics, nano-composites, nano-particles, carbon fibre etc.), industrially important chemicals (e.g., benzene, toluene, xylene etc.) and macromolecule (e.g., dispersants, resins, surfactants etc.) as well as aromatic compounds such as vanillin [75–77]. So far, catalytic methods, alkaline hydrolysis, thermochemical processes (e.g., pyrolysis, gasification, liquefaction etc.), chemical oxidation, biochemical and alternative solvent depolymerisation methods have been widely reviewed for the fragmentation of lignin into monomers as well as various chemicals [77]. Recently, the depolymerisation of lignin using ILs has been investigated for the production of guaiacol [78]. An additional application of ILs on the pre-treatment of lignin is the enhancement of the production of phenolic compounds in pyrolysis oil [79]. CRM recovered from the IL pre-treatment of biomass is of porous and low crystalline structure and this recovered material can be converted to bio-ethanol by enzymatic hydrolysis followed by fermentation [80]. CRM is also favourable for the production of biogas and cellulose nano-fibre. Pyrolysis of CRM has enormous potential for the selective production of various platform chemicals such as levoglucosan and furfurals. In addition, the pyrolysis of CRM produces upgraded pyrolysis oil by reducing aldehydes and ketones in oil [81,82]. Acid catalytic hydrolysis of CRM can produce platform chemicals including furfural, 5-hydroxymethylfurfural (5-HMF), levulinic acid (LA), ethyl levulinate, ethyl formate and lactones [8]. However, direct hydrolysis incorporating the acidic ILs is considered to be a promising alternative route for the production of platform chemicals instead of using acidic catalysts [83]. In contrast to cellulose, hemicellulose is chemically heterogeneous in structure. However, similar to cellulose, hemicellulose holds massive potential for the production of fuel grade ethanol [84]. Additionally, under acidic conditions, hemicellulose can be converted to C5 sugars (xylose) and further dehydrated to produce furfurals as platform chemicals that have many applications such as extraction agent, resins, solvents etc. [85].

chemicals requirement • No • No recycling cost

2.2. Biomass fractionation by IL pre-treatment Microbial

°C with low and • Athigh100–250 boiling point solvents OrganoSolv

mild environmental • Atconditions

accessible surface • High area with moderate

hemicellulose solubility

lignin removal with • High high structure alteration reduction in cellulose • High crystallinity

accessible surface commercial cost of • High • High area solvents and catalysts reduction in cellulose recycling system • High • Efficient crystallinity requirement lignin removal with • High high structure alteration

Economic aspects [19,20,25,26,56,57] Pre-treatment effects [25,54,55] Operating conditions [25,26,41,48,51–53] Pre-treatment method

Table 1 (continued)

ILs have high potential for energy efficient biomass pre-treatment due to their low vapour pressure, emission profile, recyclability and tuneable properties; some ionic liquids can even by prepared from renewable biomass feedstocks. Here the state-of-the-art in the field IL pretreatment of lignocellulosic biomass is reviewed.

to fermentation due • Inhibition to presence of solvent friendly • Environment • Low reaction rate

Energy requirement [25,26,58]

Process development [20,25,46,55,57,59]

at commercial scale • High energy consumption • Proven level

Additional remarks [25,26,52,57]

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Separation of lignin from biomass by IL pre-treatment is one pathway to biofuel production. In this process, lignocellulosic biomass is pre-treated with ILs and water with heating and stirring. Then an 272

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Fig. 2. A comparative study of properties of ILs with ideal solvent [67,72,73].

Fig. 3. Schematic illustration of pre-treatment of complex lignocellulosic biomass structure integrated with biorefinery concept.

anti-solvent such as acetone-water mixture [86,87], deionised water [88], alcohol [89] or dilute acid [90] is added to precipitate cellulose from the IL-biomass solution. The anti-solvent can then be recovered by evaporation as depicted in Fig. 4. Once the anti-solvent is removed, lignin can be separated from the solution by filtration. So far, the dissolution of lignocellulosic biomass has been demonstrated using various ILs including [Bmim][Cl] [91], [Amim][Cl] [92], [Emim][OAc] [93], [Bmim][OAc] [94], [Emim][MeO(H)PO2] [95] and [Amim][OFo] [96]. In addition to biomass pre-treatment, ILs have been used to treat coal [97–100]. Coal contains high amounts of carbon and aromatic rings, and the linkage between their monomeric units is similar to lignin. TGA and FTIR analyses reveal ILs are able to alter the thermo-physical state of the coal. Characterisation of recovered CRM and lignin is necessary to understand the effect of IL pre-treatment on the treated materials, and to ascertain the suitability of the treated materials for applications; high thermal stability and crystallinity can limit the wide range of applications. Table 2 shows the characteristics of biomass pre-treated using different ILs. It can be shown from Table 2 that IL pre-treatment of lignocellulosic biomass decreases the crystallinity and increases porosity/accessible surface area of recovered CRM compared to untreated biomass. This leads to the enhancement of the rate of enzymatic hydrolysis. The thermal stability of recovered CRM is reduced compared to untreated biomass. This is due to the deconstruction of lignocellulosic structure during pre-treatment that leads to the formation of low molecular weight materials in recovered CRM [101]. Similar phenomena is observed for lignin. Lignin thermal stability is reduced compared to commercial lignin due to cleavage of β-O-4 bonds during IL pre-treatment [102]. Conversely, a few papers report increased CRM crystallinity and thermal stability following IL pre-treatment [103,104],

due to removal of amorphous hemicellulose and lignin. 2.3. Screening of ILs in laboratory studies Numerous laboratory scale studies have focused on the synthesis of ILs with various cations and anions and their pre-treatment performance under different treatment conditions (Table 3). A comparative study of four nitrile based ILs, [C2CNBim][Cl], [C2CNAim][Cl], [C2CNHEim][Cl] and [C2CNBzim][Cl], for lignin extraction from bamboo at 120 °C for 24 h has been reported, with [C2CNBzim][Cl] showing the highest lignin extraction efficiency (53%) [87]. The performance of imidazolium-based ILs, [Amim][OAc], [Bmim][OAc], [Bmim][Cl] and [Bmim][HSO4] for enzymatic saccharification of Norway spruce [123] revealed [Bmim][HSO4] was most effective for dissolving lignin in high water content. The pre-treatment characteristics of both softwood (Pinus radiata) and hardwood (Eucalyptus globulus) were investigated in chloride and acetate based ILs under microwave heating between 140 and 170 °C for 1 h and lignin was regenerated through precipitation using methanol as anti-solvent [105]. The pre-treatment of Eucalyptus cell with [Bmim][OAc] followed by treatment with NaOH was found to boost the lignin separation and increase the glucose production up to 90.53% [88]. This is due to the liberation of more reactive sites and accessible surface area in the biomass sample during the IL pre-treatment. Additionally, almost 2.18% and 12.33% of xylan removals were observed for [Bmim][Cl] and [Bmim][OAc] pre-treatments, respectively. The pre-treatment of oil palm frond biomass (OPFB) with [Emim][DEP] followed by enzymatic delignification reduced the lignin and hemicellulose contents in the treated OPFB from 24% to 8.5% and 20–12.1%, 20% to 12.1%, respectively [110]. In addition, [Emim][OAc] was found to separate 273

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small amount of water separated 58% of lignin from corn stover with an IL recovery of 89% [132]. The rapid mixing of precipitating agents such as alcohol or water with [Emim][OAc] pre-treated biomass was found to enhance the cellulose precipitation. A range of novel ILs have been prepared from renewable biomaterials like choline [142]. These ILs have higher biodegradability and lower toxicity compared to traditional ILs, e.g. those containing imidazolium or pyridinium as cations. With the aim of understanding the dissolution of kraft lignin, three protic ILs, [Py][OFo], [Py][OAc] and [Py][OPr] were synthesised [143]. These protic ILs, synthesised from one-step reaction of a low cost acid (acetic acid) and base reagents (amine), can overcome the problems of traditional ILs [136]. More than 70% of lignin solubility was found for [Py][OFo] at 75 °C within 1 h treatment. Hydrogen sulphate ([HSO4]−) based protic ILs, produced from sulphuric acid and simple amines, is competitive with other ILs in terms of cost, thermal stability and saccharification yields [144]. Glass et al. studied six non-imidazolium ILs, [Ch][OAc], [BMPyr][N(CN)2], [BMPyr][Tf2N], [P4441][MeSO4], [P44416][Cl] and [P66614][N(CN)2], for their capacity to dissolve lignin from mixed pine pulp at 90 °C [145]. The authors concluded that the ILs containing ammonium, phosphonium or pyrrolidinium cations were extremely effective for wood dissolution compared to imidazolium ILs. In another study, Hou et al. synthesised eight choline based renewable ILs for the pre-treatment of rice straw [104]. When rice straw was pre-treated with [Ch][Gly] at 90 °C for 24 h, lignin and xylan were recovered with the maximum values of 60.4% and 55%, respectively. Table 4 summarises the lignin extraction data of various binary solvents. A number of aprotic polar solvents including dimethylsulfoxide (DMSO), N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMAc), 1, 3-dimethyl-2-imidazolidinone (DMI) were incorporated as co-solvents with ILs for lignocellulosic decomposition [146]. Several researchers investigated the biomass decomposition mechanism in different binary solvents such as [Bmim][OAc]/DMAc [147], [Bmim][OAc]/DMF [148] and [Bmim][OAc]/DMSO [149] at very low temperature with no heat incorporation. Xu et al. synthesised [Bmim][OAc]/DMAc from [Bmim][OAc] and DMAc and investigated the effects of molar ratio of the IL and co-solvent on cellulose decomposition at 25 °C [147]. The results indicated that the cellulose dissolved more rapidly in [Bmim][OAc]/DMAc than in [Bmim][OAc] only. However, the decomposition rate of cellulose in [Bmim][OAc]/ DMAc decreased with the increased IL/co-solvent molar ratio above a certain value due to dilution of IL cation and anion. The effect on the addition of water on the efficacy of ILs for lignin dissolution is variable. The effectiveness of [Emim][OAc] for lignin dissolution increases for IL concentrations to 65% [150] but decreases thereafter, perhaps due to higher viscosity limiting mass transport rates. Several previous works reported that the addition of water (20–85 wt%) with [Emim][OAc] showed very similar effect on lignin separation when compared to biomass pre-treatment with IL in absence of water [139,151–153]. Those works also found that the reduction of cellulose crystallinity is linked to the transformation of Cellulose I to Cellulose II. Wei et al. reported that [Bmim][Cl], mixed with 20 wt% water, was capable to dissolve a maximum of 29.1 wt% legume straw at 150 °C during 2 h pre-treatment which is almost three times higher than that of [Bmim][Cl] only pre-treatment [154]. Wang et al. also analysed the effect of different cations and anions for lignin solubility in dialkylimidazolium based IL-water mixtures [155] and found water aided pretreatment. However, Brandt et al. observed reductions in pre-treatment performance of ILs containing [MeOSO3] anion when water was added [129]. Binary combination of IL-water effectively reduces the treatment cost as well as decreases the viscosity of ILs [156]. Xu et al. and Zhao et al. studied the influence of different co-solvents (DMSO, DMF, DMAc) in binary solvent systems [159,160]. The cellulose solubility in these solvents was [Bmim][OAc]/DMSO > [ Bmim][OAc]/DMF > [Bmim][OAc]/DMAc. This order largely depends on the dipole moment of the aprotic solvents [160]. The addition of

Biomass

Water

Crashing and drying Biomass + Ionic liquid + Water

Ionic liquid Anti-solvent to cellulose

Heating and stirring Anti-solvent + Biomass + Ionic liquid + Water

Cellulose rich material

Stirring Anti-solvent

Evaporation n

Cellulose precipitation Lignin + Water + Ionic liquid + Hemicellulose + Anti-solvent

Water

Evaporation n

Filtration, washing and drying Lignin

Lignin precipitation Ionic liquid + Hemicellulose +Water

Filtration, washing and drying

Anti-solvent to hemicellulose Ionic liquid + Hemicellulose

IL regeneration

Hemicellulose

Hemicellulose precipitation

Filtration, washing and drying

Fig. 4. Schematic representation of biomass dissolution and fractionation processes using ILs and the regeneration of ILs.

lignin from the cellulosic structure of lignocellulosic wood biomass when pre-treated at 80 °C for 1 h [103]. Zhang et al. pre-treated corn stover using [Bmim][OAc] at 50–110 °C for 6 h [124]. In that case, the pre-treatment at 110 °C for 6 h removed the maximum amount of lignin (31.47%) and xylan (43.95%). In contrast, 76% hemicellulose was recovered when corn stover was pre-treated with [Emim][OAc] at 160 °C for 3 h [58]. Mohtar et al. studied the fractionation of oil palm empty fruit bunch using [Bmim][Cl] at 100 °C for 8 h under N2 condition and obtained approximately 16.82% lignin and 27.17% hemicellulose [125]. Lignin was produced experimentally from palm empty fruit bunch (EFB) via liquefaction with [Bmim][Cl], and a mathematical model was developed to predict the lignin production [135]. Pre-treatment of rice hulls at 110 °C for 8 h with [Emim][OAc] dissolved 100% of lignin, however, [Amim][Cl] and [Hmim][Cl] were ineffective in dissolving lignin under the same operating conditions [140]. Zavrel et al. investigated the quasi-continuous dissolution profiles of twenty one ILs; however, only six completely deconstructed the lignocellulosic biomass, with [Emim][OAc] the most efficient [141]. Switchgrass pretreatment with [Emim][OAc] removed more lignin but less hemicellulose compared to dilute acid treatment [111]. In this pre-treatment, the recovery values of lignin and xylan were 69.2% and 62.6%, respectively. Tan et al. presented a comparative study involving lignin materials obtained from biomass pre-treated with [Emim][ABS] and organic solvents [116]. In addition, almost 53.7% xylan was removed during the IL pre-treatment of biomass for 2 h at 190 °C. Infrared spectra showed IL treated lignin had a small band at 1168 cm−1 and a large band at 680 cm−1. These signatures were completely absent in solvent treated lignin. The IL treated lignin decomposed at about 200 °C. [Emim][OAc] in a solvent mixture of acetone, 2-propanol, and a 274

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Table 2 Highlights of IL pre-treatments effects. Biomass

Softwood Cypress wood chips Pinus radiata Southern yellow pine Pine Pinus radiata Hardwood Maple wood flour Poplar wood Eucalyptus Red oak Maple wood flour Oil palm Eucalyptus Eucalyptus Herbaceous Bamboo Switchgrass Corn stover Rice straw Bamboo Rice straw Rice straw Wheat straw Pine straw Alfalfa Flax fiber Sugarcane plant waste Rice straw Switchgrass Agave bagasse Energy cane bagasse Bagasse Sugarcane bagasse Corncob

Pre-treatment process characteristics

Change in recovered CRM relative to untreated biomass

Change in lignin relative to commercial lignin

Reference

Cellulose dissolution

Lignin separation

Cellulose crystallinity

Porosity/ surface area

Cellulose thermal stability

Hydrolysis Thermal stability

✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓

↑ × ↓ ↓ ≈

≠ × ≠ × ×

↑ × × ↓ ↓

× × × × ×

× ↓ × × ↓

[103] [105] [94] [106] [107]

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

↓ ↓ × ↓ ↓ × ↓ ↓

× ↑ × ≠ × ↑ ↑ ×

× ≈ × × × ↑ × ↓

↑ × × × ↑ ↑ ↑ ×

× × ↓ × × × × ×

[108] [109] [105] [94] [89] [110] [88] [106]

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓

✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ × ✓ ✓

≈ ↓ ↓ ↓ ↓ ↓ ↑ × × × × × ↓ ↓ ↓ ↓ ↓ × ×

× ↑ × ↑ ≠ ≠ ↑ × × × × × × × ↑ × × × ×

× × × × × × × × × × × × × ↓ ↓ × × × ×

× ↑ ↑ ↑ × × ↑ × × × × × ↑ × ↑ ↑ ↑ ↑ ×

↓ × × × × × × ↓ ↓ ↓ ↓ ↓ × × × × × × ↓

[87] [111] [112] [113] [90] [114] [104] [115] [115] [115] [115] [116] [117] [106] [118] [119] [120] [121] [122]

✓ Investigated; × Not investigated; ↑ Increased; ↓ Decreased; ≈ Unchanged; ≠ Changed

DMSO to [Emim][OAc] reduced the viscosity of [Emim][OAc] during the pre-treatment process and facilitated the enzymatic digestion of eucalyptus wood [161]. Xu et al. used four solvent systems, [Emim] [OAc]/DMSO, [Bmim][OAc]/DMSO, [Hmim][OAc]/DMSO and [Omim][OAc]/DMSO to investigate the effect of cation alkyl chain length on biomass dissolution [162]. Increasing the alkyl chain length enhanced cellulose dissolution up to C4, due to the formation of higher number of hydrogen bonds. Further increasing the alkyl chain length impedes hydrogen bond formation and reduces cellulose dissolution. Chang et al. showed that pre-treatment of rice straw in the presence of 1% sodium dodecyl sulphate and 1% cetyltrimethylammonium bromide in [Bmim][Cl] increases the lignin extraction to 49% and 34% respectively compared to 25% in the absence of surfactant [113]. Sun et al. presented a comparative study of structure and physicochemical properties of lignin extracted from eucalyptus using ILs/organic solvent mixture and alkaline ethanol solvent [163] and found the co-solvent increased the lignin extraction efficiency. The ILs/organic solvent mixture efficiency decreased in the order IL/toluene > IL/dioxane > IL/ethyl acetate > IL/DMAc > only IL. It can be concluded from the previous works that the pre-treatments of lignocellulosic biomass is both biomass specific as well as IL specific. The addition of various aprotic solvents with IL can significantly lower the viscosity of IL. This approach lowers the cost of ILs and in most cases, increases the effectiveness of the process. In contrast to aprotic solvents, there is a contradiction in the literature on the role of water of IL-water solvent in the efficiency of pre-treatment.

2.4. Bench / intermediate scale study with ILs Large-scale application of ILs for biomass pre-treatment and biorefining has not been studied extensively. Even when examined collectively, the data is insufficient for prediction of the operational parameters and the possible problems associated with full scale industrial application. Attempts have been made to develop bench to medium scale reactors for biomass treatment with different ILs. Table 5 shows a summary of select bench scale biomass pre-treatment studies. Li et al. treated switchgrass with [Bmim][OAc] in a 600 fold scale unit (6 L vs. 0.01 L) at 15% solid loading [165]. Later the authors hydrolysed the recovered CRM with the aim of producing sugar employing a 150-fold scale unit (1.5 L vs. 0.01 L) with a solid loading of 10%. Larger scale pre-treatment resulted in higher recovery of CRM yields with values of 55.3% and 49.3% for 6 L and 0.01 L units, respectively. In that study, the scaled-up pre-treatment recovered approximately 42.6% xylan from switchgrass. However, lower enzymatic xylan digestibility was observed for 6 L scale pre-treatment. This was because of limited enzyme accessibility and formation of end products during saccharification. A rheological study of switchgrass/water mixtures after pre-treatment indicated a decrease in viscosity during largescale processing. However, the need for a large volume of precipitating solvent is the foremost limiting factor for large-scale implementation [165]. Li et al. employed a 30-fold bench scale (6 L vs. 0.2 L) setup and compared results to laboratory scale testing for mixed agricultural wastes and mixed municipal solid wastes at 10% solid loading with the 275

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Table 3 Biomass pre-treatment processes found from previous studies and the lignin yields of those processes. Feed material

ILs

Anti-solvent

Softwood Southern yellow pine

[Emim][OAc]

Southern yellow pine

[Emim][OAc]

Southern yellow pine Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine kraft pulp Pine sapwood

[Emim][OAc] [Bmim][Cl] [Mmim][MeSO4] [Mmim][MeSO4] [Mmim][MeSO4] [Hmim][CF3SO3] [Hmim][CF3SO3] [Hmim][CF3SO3] [Bmim][MeSO4] [Bmim][MeSO4] [Bmim][MeSO4] [Bmim][Br] [Bmim][PF6] [Bm2im][BF4] [Bmpy][PF6] [Bmim][HSO4] (20% water) [Bmim][OAc] (20% water) [Emim][Ace] [Bmim][Ace] [Emim][OAc]

Acetone/water mixture of 1:1 Acetone/water mixture of 1:1

Pine sapwood Pinus radiata Pinus radiata Norway Spruce Hardwood Oil palm biomass

[Bmim][Cl]

Maple Wood flour Eucalyptus Eucalyptus Maple wood flour Maple wood flour Maple wood flour Maple wood flour Maple wood flour Maple wood flour Maple wood flour Red oak

[Emim][OAc] [Bmim][Cl] [Bmim][OAc] [Emim][OAc] [Emim][OAc] [Emim][OAc] [Bmim][OAc] [Bmim][OAc] [Bmim][OAc] [Bmim][MeSO4] [Emim][OAc]

Willow

[Bmim][HSO4] (20% water) [Dmim][MeSO4] [Mmim][MeSO4] [Bmim][CF3SO3] [Emim][OAc] [Amim][Cl] [Bmim][Cl] [Bzmim][Cl] [Bmim][OAc]

Rubber wood Maple Wood flour Maple Wood flour Maple Wood flour Maple Wood flour Maple Wood flour Maple Wood flour Poplar Herbaceous Bamboo

[C2CNAim][Cl]

Bamboo

[C2CNBim][Cl]

Bamboo

[C2CN HEim][Cl]

Bamboo

[C2CN Bzim][Cl]

Bamboo

[Bmim][Cl]

Corn stover

[Emim][OAc]

Wheat straw

[Emim][OAc]

Switchgrass Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw

[Emim][OAc] [Ch][Lys] [Ch][Gly] [Ch][Ala] [Ch][Ser] [Ch][Thr] [Ch][Met]

Feed size (µm)

Temperature (ºC)

Time (h)

Lignin yield (%)

Reference

Microwave heating

4 min

68.65

[126]

0.125–0.250 mm

110

46

26.1

[94]

< 0.125 mm

30 min

22

49.4 13.9 g/L 74 g/L 344 g/L 74.2 g/L Undissolved 275 g/L < 10 g/L 62 g/L 312 g/L 61.8 g/L 17.5 g/L Undissolved 14.5 g/L Undissolved 81.7

[127] [128] [128] [128] [128] [128] [128] [128] [128] [128] [128] [128] [128] [128] [128] [129]

Methanol

0.18–0.85 mm

175 75 Room temp. 50 25 Room temp. 70 50 Room temp. 50 25 75 70 – 120 70 – 120 70 – 120 120

Methanol

0.18–0.85 mm

120

22

30.7

[129]

Acetone Acetone Deionised water

0.1-0.5 0.1-0.5 0.7-1.7 mm

100 100 100

2 2 6

43 38 31.7

[107] [107] [130]

Acetone/water mixture of 1:1 0.1 N NaOH Deionised water Deionised water

< 250

110

8 under N2 condition

54

[86]

250 80–100 mesh 80–100 mesh

70 under N2 30 min 30 min 6 12 24 6 12 24 12 25

> 85 7.5 16.97 25 35 49 26 32 37 19 34.9

[89] [88] [88] [108] [108] [108] [108] [108] [108] [108] [94]

85

[129]

13.03 0.8 g/kg 0.5 g/kg 4.4 g/kg 5.2 g/kg 3.2 g/kg 1.9 g/kg 31.90

[131] [89] [89] [89] [89] [89] [89] [109]

Acetone/water mixture of 1:1 Methanol

0.125–0.250 mm

90 120 120 90 90 90 90 90 90 90 110

0.18–0.85 mm

120

22

Methanol 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH Acetone

0.2 mm 250 250 250 250 250 250 2 mm

100 80 80 80 80 80 80 130

2 24 24 24 24 24 24 12

Acetone/water mixture of 7:3 Acetone/water mixture of 7:3 Acetone/water mixture of 7:3 Acetone/water mixture of 7:3 Acetone/water mixture of 7:3 Acetone/water mixture of 1:1 Acetone/water mixture of 9:1 Deionised water 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH 0.1 N NaOH

125

120

24

47

[87]

125

120

24

41

[87]

125

120

24

44

[87]

125

120

24

53

[87]

125

120

24

38

[87]

140

3

58

[132]

< 0.5 mm

120

2

87

[133]

50–400 50–400 50–400 50–400 50–400 50–400

160 oil bath 90 90 90 90 90 90

3 24 24 24 24 24 24

69.2 60.4 59.9 58.3 54.7 53.1 55.2

[111] [104] [104] [104] [104] [104] [104]

under under under under under under

N2 N2 N2 N2 N2 N2

(continued on next page) 276

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Table 3 (continued) Feed material

ILs

Anti-solvent

Feed size (µm)

Temperature (ºC)

Time (h)

Lignin yield (%)

Reference

Rice straw Rice straw Energy cane bagasse Bamboo

[Ch][Pro] [Ch][Phe] [Emim][OAc] [Emim][Gly]

50–400 50–400 125 μm to 1 mm

90 90 120 120

24 24 30 min 8

52.9 41.5 32 85.3

[104] [104] [134] [90]

Sugarcane plant Bagasse Sugarcane plant Bagasse Sugarcane plant Bagasse Sugarcane plant Bagasse Sugarcane plant Bagasse Bagasse Palm empty fruit bunch

[Emim][ABS]

0.1 N NaOH 0.1 N NaOH Deionised water Acetone/water mixture of 7:3

2–3 mm

190

90 min

97

[116]

[Emim][ABS]

2–3 mm

190

60 min

96

[116]

[Emim][ABS]

2–3 mm

190

30 min

67

[116]

[Emim][ABS]

2–3 mm

170

2

67

[116]

[Emim][ABS]

2–3 mm

180

2

78

[116]

[Emim][OAc] [Bmim][Cl]-liquifaction

< 0.25 mm 0.4–1 mm

175 150.5

60.3 26.6

[127] [135]

Oil palm frond

[Emim][DEP]

0.25–0.5 mm

90 oil bath

10 min 151 min with H2SO4 as catalyst 4

45.8

[110]

Corn stover Triticale straw Switchgrass Triticale straw Triticale straw

Protic IL [Emim][OAc] [Emim][OAc] [Emim][OAc] [Emim][OAc] (50% water) [Bmim][MeSO4] (20% water) [Bmim][HSO4] (20% water) [Emim][OAc] [Emim][OAc] [Emim][OAc] [Emim][OAc] [Emim][OAc] [Bmim][OAc]

0.5 mm 0.5 mm

90 150 120 150 150

24 2 with N2 3 90 min 90 min

75 52.7 34 76.3 62.9

[136] [137] [138] [139] [139]

Methanol

0.18–0.85 mm

120

2

27

[129]

Methanol

0.18–0.85 mm

120

22

93

[129]

1.168 - 1.651 mm 1.168 - 1.651 mm 1.168 - 1.651 mm 0.25 mm 10 mm 0.3–0.45 mm

90 110 110 125 160 100

4 4 8 1 3 6

21 47 100 44 84.2 31.47

[140] [140] [140] [112] [58] [124]

Miscanthus giganteus Miscanthus giganteus Rice hull Rice hull Rice hull Corn stover Corn stover Corn stover

Acetone/water mixture of 1:1 Acetone/water mixture of 1:1 0.1 N NaOH

Deionised Deionised Deionised Deionised Deionised

0.5mm

water water water water water

Table 4 IL based binary solvents employed for biomass pre-treatment process. Feed material Softwood Pinus radiata Hardwood Eucalyptus Eucalyptus Eucalyptus Eucalyptus Herbaceous Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Corncob Corncob Corncob Corncob Corncob Corncob a b

ILs/co-solvents

Anti-solvent

Feed size (mm)

Temperature (ºC)

Time (h)

Lignin extraction (%)

Reference

[Bmim][Ace]/DMSO

Acetone

0.1-0.5

100

2

58

[107]

[Emim][OAc]-DMSO [Emim][OAc]-DMAc [Emim][OAc]-Dioxane [Emim][OAc]-Methanol

Acetone Acetone Acetone Acetone

[Bmim][Cl]-DMSO (1:1, v/v) [Bmim][Cl]-DMSO (1:4, v/v) [Bmim][Cl]-DMA (1:1, v/v) [Bmim][Cl]-DMA (1:4, v/v) [Bmim][Cl]-DMF (1:1, v/v) [Bmim][Cl]-DMF (1:4, v/v) [Emim][OAc]-DMSO (1:1, v/v) [Emim][OAc]-DMSO (1:4, v/v) [Emim][OAc]-DMA (1:1, v/v) [Emim][OAc]-DMA (1:4, v/v) [Emim][OAc]-DMF (1:1, v/v) [Emim][OAc]-DMF (1:4, v/v) [Emim]OAc/H2O (3:7, w/v) [Emim]OAc/DMF (3 :7, w/v) [Emim]OAc/DMSO (3:7, w/v) [Emim]OAc/DMAc (3:7, w/v) [Emim]OAc/DMF (6 :4, v/v) [Emim]OAc/DMSO (6:4, v/v)

Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Acetone/water Distilled water Distilled water Distilled water Distilled water Acetone/water Acetone/water

40-60 40-60 40-60 40-60 (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1, (1:1,

v/v) v/v) v/v) v/v) v/v) v/v) v/v) v/v) v/v) v/v) v/v) v/v)

(1:1, v/v) (1:1, v/v)

mesh mesh mesh mesh

0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 0.125–0.250 80–120 mesh 80–120 mesh 80–120 mesh 80–120 mesh < 150 μm < 150 μm

heating is carried out in 50 w microwave. lignin percentage is in terms of total biomass weight. 277

b

120 120 120 120

3 3 3 3

8.6 11.8b 14.1b 3b

[157] [157] [157] [157]

80 80 80 80 80 80 80 80 80 80 80 80 110 110 110 110 120 120

2mina 2mina 4mina 5mina 3mina 5mina 2mina 2mina 3mina 3mina 3mina 2mina 5 5 5 5 24 24

22b 32b 8b 4b 12b 6b 36b 40b 11b 7b 18b 14b 8.72b 4.04b 9.78b 7.24b 77.16 82.58

[117] [117] [117] [117] [117] [117] [117] [117] [117] [117] [117] [117] [122] [122] [122] [122] [158] [158]

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Table 5 Biomass pre-treatment scale-up investigation. Biomass feedstock

ILs

Scale-up volume

Operating conditions

Reactor

Lignin recovery (%)

Reference

Corn stover/Non-recyclable paper blend

[Emim][Cl] [Emim][Cl] [Emim][Cl] [Bmim][Cl] [Bmim][Cl] [Bmim][Cl] [Bmim][Cl] [Emim][OAc] [Emim][OAc]

600 fold (6 L vs. 0.01 L)

Parr vessel, model: 4555–58

30 fold (6 L vs. 0.2 L)

140 °C; 2 h

Parr reactor, model: 4558

100 kg

120 °C; 3 h

Pressurized batch reactor

46.2 66.4 23.9 20.3 42.3 49.5a 76.7b 58.68 52.2 65.9 59.9 16 22 25

[164]

600 fold (6 L vs. 0.01 L) 30 fold (6 L vs. 0.2 L)

4 × 4 matrix; 120 °C; 2 h 4 × 4 matrix; 140 °C; 2 h 4 × 4 matrix; 160 °C; 2 h 4 × 4 matrix; 120 °C; 2 h 4 × 4 matrix; 140 °C; 2 h 4 × 4 matrix; 160 °C; 2 h 4 × 4 matrix; 160 °C; 2 h 2 mm screen; 160 °C; 3 h 140 °C; 1 h

Switchgrass Switchgrass Eucalyptus Switchgrass/Eucalyptus mixture Municipal solid waste/Corn stover blend Agave bagasse a b

[Bmim][Cl] [Emim][Cl] [Emim][OAc]

Parr reactor, model: 4558 Parr reactor, model: 4558

[165] [166] [167] [168]

10% solid loading. 15% solid loading.

aim of optimising the large-scale process parameters [166,167]. The overall xylan and glucan yields were observed to be 62.8% and 99.7% respectively from the scaled-up pre-treatment of mixed feedstock [166]. Encouragingly, these investigations demonstrated that the outcomes of bench-scale treatment is comparable with small-scale experiments. Liang et al. studied the deconstruction of sixteen municipal solid wastes and biomass blends using different ratios of IL and organic material at millilitre scale employing [Emim][Cl] and [Bmim][Cl] followed by enzymatic hydrolysis [164]. A blended sample (Corn stover: Non-recyclable paper, 8:2) was chosen for 6 L scale-up study to evaluate scalability. The results indicated that the scaled-up study produced lower glucose recovery (58% vs. 65%) and xylose yield (35% vs. 91%) compared to the small-scale under the same pre-treatment conditions. This was due to differences in the level of blending, which ultimately affected the reaction rate [164]. Gardner et al. used a bench scale (6 L) reactor for pre-treatment of switchgrass, eucalyptus and a mixed feedstock using [Emim][OAc] at less than 140 °C and 1 h reaction time. [169]. Pre-treated feedstocks were then hydrolysed at 50 °C for 72 h in a 2 L continuously stirred reactor, and the data statistically validated using Pearson's product-moment coefficient. The lignin yield was strongly correlated with energy density. So far, the understanding of heat and mass transfer in large scale solid pre-treatment and rheological behaviour of mixed feedstock is not well explored. Therefore, detail investigations are required to understand the scale-up issues that can be applied in reactor design as well as process development.

maximum TRS of 99% at 100 °C for 1 h. The production of TRS from the direct hydrolysis of MCC using 1-propyl sulfonic acid-2-phenyl imidazoline hydrogensulfate was found to be increased up to the pre-treatment temperature of 100 °C [173]. Any further increase in pre-treatment temperature decreased the production of TRS. This was mainly because of the conversion of TRS to HMF at higher temperatures. The TRS yield was also observed to be reduced at high IL loadings (more than 0.2 g IL in 0.1 g MCC) and this may be explained by the catalytic dehydration of TRS. Jiang et al. studied the kinetics of [BSHmim] [HSO4] hydrolysis of cellulose and found that the β-1,4 glycosidic bonds of cellulose degraded to glucose in the acidic environment and then converted to HMF [174]. It was also noticed that the higher pretreatment time at a certain temperature enhances the production of HMF instead of TRS from cellulose. Khan et al. investigated three different acidic ILs for the one-pot hydrolysis of four different lignocellulosic biomass such as rice husk, rubber wood, bamboo and palm oil frond aiming to produce LA [184]. Among those ILs, [C4(Mim)2][(2HSO4)(H2SO4)4] yielded the highest LA (47.52%) from bamboo biomass pre-treated at 110 °C for 1 h. Tiong et al. studied the effects of reaction time and IL loading on the production of 5-HMF and LA from the direct hydrolysis of oil palm empty fruit bunch and mesocarp fibre using Brønsted Lewis acidic IL [187]. The authors reported the decrease in the conversion of 5-HMF to LA at high IL loading and this was may be due to the fact that 5-HMF is highly unstable and it condenses easily. The one-pot hydrolysis was carried out at 160 °C for 5 h. Further, the LA was converted to ethyl levulinate (EL) in presence of excess ethanol through esterification at 90 °C for 10 h. Fig. 5 shows the schematic of cellulose hydrolysis using acidic ILs for the production of platform chemicals such as 5-HMF, LA and EL. Hu et al. hydrolysed soybean straw and corn straw using functional acidic ILs under ultrasound heating and conventional oil bath heating and found higher TRS yield in the case of ultrasound heating [189]. Moreover, the addition of water up to a certain weight ratio (water: biomass= 5:1) increases TRS yield and any further addition of water suppresses the production of TRS. Amarasekara and Shanbhag performed hydrolysis of switchgrass biomass using 1-(alkylsulfonic)-3methylimidazolium chloride [190]. In this case, water bath heating resulted in higher TRS yield when compared to microwave heating. Li et al. claimed the decrease of TRS yield at longer reaction time during the hydrolysis of corn stalk using the strong acidic ILs such as [Bmim] [HSO4] and [SBmim][HSO4] [191]. This was because of the higher degradation rate of TRS compared to the depolymerisation rate of the polysaccharides in the strong acidic environment at longer reaction time. Matsagar et al. investigated the production of C5 sugars and direct production of furfural from bagasse, rice husk, wheat straw, cotton stalk, corn cob and jute employing [C3SO3Hmim][HSO4] [188]. The

2.5. Direct hydrolysis of biomass with ILs Recently, the utilisation of Brønsted acidic ILs for the dissolution and hydrolysis of lignocellulosic biomass has gained attention. The acidic properties of ILs favour the one-pot production of furan based chemicals and various acids. In the one-pot hydrolysis, IL pre-treatment and hydrolysis are carried out in a single step avoiding the separation of IL between pre-treatment and hydrolysis [170]. Table 6 summarises the application of acidic ILs for the production of various platform chemicals. Ramli and Amin obtained LA and 5-HMF with maximum values of 68% and 18% respectively from the one-pot pre-treatment of glucose using [SMIM][FeCl4] at 150 °C for 4 h [83]. In a different study, total reducing sugar (TRS) was produced from the direct hydrolysis of microcrystalline cellulose (MCC) using functionalised Brønsted acidic ILs and the study showed a maximum yield of 85% TRS when cellulose was pre-treated with [Bmim][Cl]-SO3H at 100 °C for 90 min [171]. Liu et al. compared the performance of 1-methylimidazole, 1-vinylimidazole and triethylamine based SO3H functionalised ILs for the direct hydrolysis of MCC [172]. The authors found that the triethylamine based IL yielded a 278

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Table 6 Production of various platform chemicals from biomass/cellulose/hemicellulose using acidic ILs. Feed material

Glucose Glucose Glucose Glucose Glucose Glucose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Microcrystalline cellulose Hemicellulose Hemicellulose Hemicellulose Hemicellulose Hemicellulose Hemicellulose Hemicellulose Hemicellulose Xylan Xylan Xylan Xylan Xylan Xylan Xylan Xylan Xylan Rubber wood Rubber wood Rubber wood Palm frond Rice husk Bamboo Bamboo Bamboo Bamboo Bamboo Bamboo Bamboo Bamboo Bamboo Rice straw Rice straw Rice straw Rice straw Rice straw Rice straw Red nut sedge tuber Red nut sedge shoot Indian doab Marijuana Water spinach Water hyacinth Datura Yellow dock Dodder Pigweed Gajar ghas root Gajar ghas shoot Spiny pigweed Foxtail straw Wild elephant foot yam Cycus leaf Red nut sedge shoot Dodder

IL

[Smim][FeCl4] [Smim][Cl] [Bmim][FeCl4] [C2OHmim][BF4] [Emim][HSO4] [SO3Hmim][CF3SO3] [C3SO3Hmim][H2PO4] [C3SO3Hmim][CH3SO3] [C3SO3Hmim][HSO4] [C4SO3Hmim][HSO4] [C3SO3HPy][HSO4] [C3SO3HN111][HSO4] [C3SO3Hmim][HSO4] [BSmim]CF3SO3 [BSmim]HSO4 [BSmim]OAc [C4SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [Ch-SO4H][CF3SO3] [Ch-SO4H][HSO4] [Ch-SO4H][TsO] [Ch-SO4H][CF3SO3] [Ch-SO4H][HSO4] [Ch-SO4H][TsO] [Ch-SO4H][CF3SO3] [Ch-SO4H][HSO4] [Ch-SO4H][TsO] [C4(Mim)2][(2HSO4)(H2SO4)0] [C4(Mim)2][(2HSO4)(H2SO4)2] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C4(Mim)2][(2HSO4)(H2SO4)4] [C3SO3Hmim][PhSO3] [C3SO3Hmim][1-NS] [C3SO3Hmim][Cl] [C3SO3Hmim][H2PO4] [C3SO3Hmim][CH3SO3] [C3SO3Hmim][HSO4] [DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][DMA]+[CH3SO3][NMP]+[CH3SO3][NMP]+[CH3SO3]-

Co-solvent

Water Water Water DMSO MIBK Water Water Water Water Water Water Water Water Water Water [Emim][OAc] Water + toluene Water + toluene Water + toluene Water Water Water Water + p-xylen Water + MIBK 1,4-dioxane 1,4-dioxane 1,4-dioxane Toluene Toluene Toluene γ-Valerolactone γ-Valerolactone γ-Valerolactone Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl DMA–LiCl

Pre-treatment conditions

Products yield (%)

Temperature (ºC)

Time (min.)

Furfural

150 150 150 180 100 120 160 160 160 160 160 160 160 120 120 120 160 160 170 180 160 160 160 170 170 120 120 120 140 140 140 170 170 170 100 100 100 100 100 100 110 110 110 110 110 110 110 110 180 180 180 180 180 180 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

240 240 240 60 240 360 30 30 30 30 30 30 240 120 120 120 210 240 240 240 60 240 360 240 240 360 360 360 240 240 240 60 60 60 120 120 120 120 120 120 60 2.5 5 10 15 30 45 75 90 90 90 90 90 90 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

35 62 64 10 21 17 49 52 82 74.8 66.5 46.3 28.3 21.8 58.5 54.2 41.8

Reference

HMF

LA

17.7 6.3 48.3 67.3 9 75.1

67.8 25.8 22.4

34.9

33 35 28 13 11 19 12 30 32 27 37 23 20 58 29 33 32 26

2.8 3.5 36.3 44.5 41.4 40.5 43.2 53.7 45.1 39.4 27.6

24.6 26.7 30 27.61 34.48 35.02 47.52 3 8.5 12.92 19.06 28.86 38.85 47.67 15.7 12.2 20.9 10.7 15.1 17.1

[83] [83] [83] [175] [176] [177] [178] [178] [178] [178] [178] [178] [179] [180] [180] [180] [181] [182] [182] [182] [182] [182] [182] [182] [182] [183] [183] [183] [183] [183] [183] [183] [183] [183] [184] [184] [184] [184] [184] [184] [184] [184] [184] [184] [184] [184] [184] [184] [185] [185] [185] [185] [185] [185] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186] [186]

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Table 6 (continued) Feed material

Gajar ghas root Foxtail straw Cycus leaf Miscanthus Miscanthus Miscanthus Oil palm empty fruit bunch Oil palm mesocarp fiber Rice husk Bagasse Jute Wheat straw Corn cob

IL

[NMP]+[CH3SO3][NMP]+[CH3SO3][NMP]+[CH3SO3][Bmim][HSO4] [Bmim][MeSO3] [Bmim][OTf] [Hmim][HSO4] [Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4] [C3SO3Hmim][HSO4]

Co-solvent

DMA–LiCl DMA–LiCl DMA–LiCl

Water Water Water Water Water Water Water

+ + + + +

toluene toluene toluene toluene toluene

yield percentages of C5 sugars were between 67% and 93% and furfural was between 76% and 86%. These yield values were higher than that of conventional solid heterogeneous catalyst. Kuroda et al. obtained high glucose yields (30–40%) from the hydrolysis of bagasse, eucalyptus and Japanese cedar using microwave assisted acidic IL pre-treatment compared to almost no glucose yield with H2SO4 catalyst and conventional heating [192]. When compared to cellulose and lignocellulosic biomass, the onepot hydrolysis of hemicellulose using acidic ILs has been investigated by limited researchers. For instance, Matsagar and Dhepe synthesised Brønsted acidic ILs such as [C3SO3Hmim][HSO4], [C3SO3Hmim][PTS] and [C3SO3Hmim][Cl] for the one-pot hydrolysis of hardwood hemicellulose [193]. When the hardwood was pre-treated at 160 °C for 1 h, the maximum yield of xylose (C5 sugars) was found to be 87%. Any further increase in reaction time reduces the sugar yield and converts the sugar to furfural because of dehydration reaction under the acidic environment. During the pre-treatment of wheat straw, hydrolysis of hemicellulose to produce C5 sugar was observed. In addition, further dehydration of C5 sugar was found which yielded furfural and hydroxymethylfurfural due to the acidic catalytic properties of [Bmim] [HSO4] [194]. Da Costa Lopes et al. optimised the process conditions for wheat straw pre-treatment and hydrolysis of hemicellulose using [Emim][HSO4]-H2O mixture [195]. At the optimised process conditions, a maximum yield of 80.5% pentoses (xylose and arabinose) was obtained in that study. The hydrolysis of hemicellulose was also studied by Carvalho et al. for the production of xylose and the conversion of xylose to furfural using [Bmim][HSO4] [196]. The effect of reaction

Pre-treatment conditions

Products yield (%)

Temperature (ºC)

Time (min.)

Furfural

120 120 120 120 120 120 160 160 170 170 170 170 170

2 2 2 1320 1320 1320 180 180 180 180 180 180 180

33 13 34 > 80 73 86 78 76

HMF

Reference LA

35 52 33

8.4 9.3

[186] [186] [186] [129] [129] [129] [187] [187] [188] [188] [188] [188] [188]

temperature was more profound on both xylose and furfural productions when compared to the effect of pre-treatment time. Cox and Ekerdt investigated the depolymerisation of oak wood lignin using [Hmim][Cl] at 110–150 °C and found that the lignin hydrolysis occurs by the cleavage of β-O-4 ether linkage under acidic condition leading to the production of guaiacol and other fragmented monomers [197]. Similar findings were reported by Jia et al. and Cox et al. from the depolymerisation study of model lignin using [Hmim][Cl] and [Bmim] [HSO4], respectively [78,198]. 3. Biomass–IL interactions and dissolution kinetics 3.1. Biomass dissolution mechanism in ILs During lignocellulosic biomass pre-treatment, the lignin is typically dissolved in ILs by the deconstruction of lignocellulosic structure through the splitting of inter-unit lignin linkages and by the formation strong ion-dipole bonds [152]. Lignin consists of seven different types of linkage bonds such as β-O-4, α-O-4, β-5, 5-5, 4-O-5, β-1 and β-β. However, the β-O-4 bond accounts for between 50% and 60% of total linkages. During the cleavage of β-O-4 bond, an intermediate β-1 interlinkage is formed before further degradation, whereas β-5 is converted into stilbenes [199,200]. Stilbene is a comparatively unreactive and colourless compound initially present in lignin. Stilbene is insoluble in water and has mainly two isomers [201]. The cleavage of these interunit linkages enhances porosity and reduces crystallinity. Cox et al. reported cleavage of β-O-4 aryl ether bond was assisted by the

Fig. 5. Conversion of cellulose to 5-HMF, levulinic acid and ethyl levulinate using acidic ILs [187].

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Rearrangement to Hibbert’s ketones

With coordinating anion Enol Ether

Guaiacol

(A) Without coordinating anion Vinyl Ether

Guaiacol

R= H (GG), CH3 (VG); X−= IL anion

(B)

Ionic liquid

Fig. 6. Schematic representation of reaction mechanism of (A) β-O-4 aryl ether bond cleavage in acidic IL [198] and (B) cellulose dissolution in imidazolium based IL [204].

hydrogen bond originated by IL-water interactions (acidic mono alkylimidazolium ILs) as illustrated by Fig. 6. Moreover, the 2D NMR study conducted by Çetinkol et al. also indicated the decrease of β-O-4 aryl ether linkage during biomass dissolution in ILs [202]. Cammarata et al. reported an α, β-dehydration reaction mechanism for disintegration of lignocellulosic biomass in ILs [203]. According to a molecular dynamics simulation, the interaction between ILs and biomass largely depends on the intrinsic solubility of ILs [205]. Lignin recovery performance of ILs is dependent on the solvation parameters such as Kamlet−Taft, Abraham, Hildebrand and Hansen solubility model and the solvation power of ILs can be described by these solvation parameters. The Kamlet−Taft model is widely used for the prediction of three empirical polarity parameters of ILs in biomass pre-treatment. These are hydrogen bond acidity ( ), hydrogen bond basicity ( ) and dipolarity/polarisability ( *). The value of is primarily dominated by the nature of cation and the value of is affected by the nature of anion, while the * value is impacted by both the cation and anion. The Kamlet−Taft parameters are estimated from UV–VIS spectroscopy in the presence of N,N-diethyl-4-nitroaniline (DENA) and 4-nitroaniline (4-NA) by using [206]:

*=(

DENA

= [E t ×30

27.52)/ 3.183

(1)

14.6×( * 0.23) 30.31]/16.5

(2)

= [(1.035×

DENA)

4NA + 2.64]/2.8

(3)

where E t is the empirical polarity, DENA is the wavelength corresponding to the absorbance of DENA and 4NA is the wavelength corresponding to the absorbance of 4-NA. Several researchers investigated the impacts of the polarity parameters on biomass decomposition. Muhammad et al. concluded that more aromatic cations component and anions with higher hydrogen bond basicity lead to the enhancement of biomass decomposition [90]. The researchers found that [Emim][Gly] is more suitable for lignin separation because of its high hydrogen bonding capacity (β = 1.19) compared to [Emim][TFA] (β = 0.233) and [Ch][OPr] (β = 0.98). The weak electron withdrawing group (–NH2) of [Emim][Gly] has insignificant impact on hydrogen bonding with biomass while the strong electron withdrawing group (-F) of [Emim][TFA] has major impact on hydrogen bonding, which limits its decomposition effectiveness. Doherty et al. investigated the relationship among , and * of [Emim] [OAc], [Bmim][OAc] and [Bmim][MeSO4] and their effect on lignin recovery from maple wood flour. The ILs containing [OAc] , which has the largest hydrogen bond basicity ( = 1) value, extracted 32% more lignin [108]. Table 7 summarises the Kamlet-Traft parameters of various ILs. The Abraham solvation parameter of ILs can be defined by the following empirical equation [207]: 281

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[213,215]. Weerachanchai et al. investigated the effects of IL types, temperature and concentration of DMA on the Hildebrand solubility parameter of ten ILs employing intrinsic viscosity [216]. The authors reported that the anions played a strong effect on the Hildebrand solubility parameter. The Hildebrand solubility parameter decreased with the increase in temperature from 25 to 60 °C and the maximum solubility parameter was observed at 60% DMA concentration. Wong et al. also noticed the decrease in Hildebrand solubility parameter of amidium ILs when the reaction temperature was increased from 25 to 60 °C [217]. In some cases, several ILs may have very similar Hildebrand solubility parameters; however, the solvation power of those ILs may vary [218]. In those cases, Hansen parameters can be incorporated to distinguish the solvation power of the ILs. Agata and Yamamoto investigated the effects of cations and anions on the Hansen solubility parameters and suggested that the parameters are capable to predict the dissolution compatibility of a solute in ILs [218]. For instance, the total solubility parameter of microcrystalline cellulose is very close to that of [Bmim][Cl] compared to [Bmim][BF4] and [Bmim][PF6] [219] indicating the better solubility of microcrystalline cellulose in [Bmim] [Cl]. This estimation is in accordance with the previous experimental results [91]. The Hansen solubility parameters are significantly affected by the nature of cations and anions. The values of these parameters decrease with the increase in temperature from 25 to 60 °C [213]. In the case of acidic ILs, Hammett acidity function estimates the acidity level or catalytic performance of the ILs during direct hydrolysis. The Hammett acidity function (Ho) of ILs can be expressed as follows [184].

Table 7 Values of Kamlet−Taft parameter of various ILs used in biomass pre-treatment. ILs

α

β

π*

Reference

[Bmim][OTf] [Bmim][OTf] [Bmim][N(CN)2] [Bmim][MeSO4] [Bmim][Cl] [Bmim][Cl], 75 ºC [Bmim][Me2PO4] [Bmim][Me2PO4], 8% wt [Emim][Me2PO4] [Bmim][OAc] [Bmim][OAc] [Bmim][OAc], 90 ºC [Bmim][OAc]+5% H2O], 90 ºC [Bmim][OAc]+10% H2O], 90 ºC [Bmim][OAc], 90 ºC [Bmim][OAc]+5% H2O], 90 ºC [Bmim][OAc]+10% H2O], 90 ºC [Bmim][MeSO4], 90 ºC [Bmim][MeSO4]+5% H2O], 90 ºC [Bmim][MeSO4]+10% H2O], 90 ºC 25%[Bmim][OAc]/75%[Bmim][MeSO4], 90 ºC 50%[Bmim][OAc]/50%[Bmim][MeSO4], 90 ºC 75%[Bmim][OAc]/25%[Bmim][MeSO4], 90 ºC [Bmim][OAc] [Bmim][OPr] [Bmim][OBu] [Bmim][HOCH2COO] [Bmim][OOCCH2CH(OH)COO] [Bmim][OOCCH2CH2COO] [Bmim][OOCCHCHCOO] [Bmim][H-C3H2O4] [Bmim][H-OOCCH2CH2COO] [Bmim][H-OOCCHCHCOO] [HOPmim][OAc] [HOPmim][Tf2N] [HOEmim][Tf2N] [Bmim][BF4] [Bmim][PF6] [Bmim][OTf] [Bmim][Tf2N]

0.634 0.63 0.543 0.545 0.83 0.44 0.452 0.44 0.51 0.470 0.43 0.57 0.53 0.55 0.53 0.50 0.56 0.55 0.61 0.60 0.43 0.48 0.51 0.44 0.41 0.39 0.34 0.38 0.36 0.32 0.51 0.91 1.14 0.63 0.63 0.62 0.62

0.483 0.46 0.596 0.672 0.83 0.84 1.118 1.00 1.00 1.201 1.05 1.18 1.07 0.98 1.06 0.96 0.91 0.60 0.58 0.57 0.84 0.98 1.04 1.05 1.16 1.23 0.87 1.00 1.08 1.02 0.71 0.82 0.62 0.99 0.24 0.28 0.38 0.21 0.46 0.24

0.974 1.01 1.052 1.046 1.03 1.14 0.970 0.97 1.06 0.971 1.04 0.89 0.93 0.94 0.97 1.02 0.99 1.00 0.99 1.03 0.97 0.95 0.95 1.04 0.94 0.92 1.12 1.10 1.09 1.11 1.03 1.07 1.08 1.08 1.06 1.08 1.05 1.03 1.00 0.98

[208] [209] [208] [208] [208] [210] [208] [208] [211] [208] [212] [108] [108] [108] [108] [108] [108] [108] [108] [108] [108] [108] [108] [212] [212] [212] [212] [212] [212] [212] [212] [212] [212] [212] [212] [212] [209] [209] [209] [209]

logk = c + rR2 + s H 2 ,

H 2

+a

H 2

+b

H 2

Ho = pKa [I ] + log

and are the solute dipolarity, solute hydrogen bond where acidity and hydrogen bond basicity, respectively. s, a and b denotes the dipolarity, hydrogen bond acidity and hydrogen bond basicity, respectively. R2 indicates excess of molar refraction, c is a constant, L describes solute-solvent partition coefficient, l is the dispersion force and logk measures the relative retention time. Hildebrand solubility parameter (δ) of ILs can be defined as the square root of the cohesion energy (CE) [213,214]. CE is the amount of energy required to completely disrupt the interactions of per unit volume of molecules. Hildebrand solubility parameter can be further expressed in terms of three Hansen solubility parameters as detailed below.

=

CE =

H 2

H 2

Hv

RT V

=

D

2

+

H

2

+

P

2

(6)

where pKa [I] is the pKa value of indicator with respect to an aqueous solution. [I] and [IH+] indicates the molar concentration of un-protonated and protonated form of the indicator in the solvent, respectively. The value of Ho or catalytic activity of ILs is influenced by both cations and anions [182]. Suzuki et al. established a correlation between the Ho values of Brønsted acidic ILs in aqueous medium and their catalytic performance. They found that Ho value must be less than 1.5 to perform the IL as acidic catalyst which increases with the increase in alkyl side chain in IL cation [220]. Fig. 7 shows the catalytic evaluation of some acidic ILs. Khan et al. estimated Ho of three acidic ILs when producing LA and found that the yield of LA is directly influenced by the value of Ho [184]. The researchers observed that [C4(Mim)2][(2HSO4)(H2SO4)4] is more efficient for the production of LA because of its lower value of Ho compared to [C4(Mim)2][(2HSO4)(H2SO4)0] and [C4(Mim)2] [(2HSO4)(H2SO4)2] (Fig. 7). When studying the catalytic hydrolysis of cellulose using heteropolyacid (HPA) IL, Sun et al. found that the lower value of Ho enhances the catalytic effect of ILs [221]. Ramli and Amin synthesised three acidic functionalised ILs namely, [BMIM][FeCl4], [SMIM][Cl] and [SMIM][FeCl4] and explored the effect of Ho on 5-HMF and LA yields [83]. Similar to the findings of Khan et al., the authors concluded that the lower the value of Ho, the higher the production rates of 5-HMF and LA. Parveen et al. studied [BMIM][Cl] based functionalised Brønsted acidic ILs and reported that the values of Ho can be described in the following order: [BMIM][Cl]-SO3H < [BMIM][Cl]COOH < [BMIM][Cl]-OH [171]. The highest TRS production capability of [BMIM][Cl]-SO3H among the three ILs studied can be recognised by this relationship. Anions of ILs play the most important role for cellulose decomposition and lignin separation. For instance, [Bmim][Cl] has higher cellulose solubility compared to [Hmim][Cl] and [Omim][Cl] because the Cl ion concentration in [Bmim][Cl] is higher [222]. In contrast, KIlpeläinen et al. reported that both cations and anions of ILs played significant roles on the dissolution of lignocellulosic biomass constituents [223]. Holmes et al. observed that the anions are more important for reducing lignin molecular weight than the cations [144].

(4)

+ llogL

[I ] [IH+]

(5)

where Hv , R, T and V are the molar enthalpy of vaporisation, gas constant, temperature and molar volume, respectively. D , H and P are the dispersion, hydrogen bond and polar interactions parameters, respectively. The solubility performance of ILs is influenced by the values of the Hildebrand solubility parameter; for example, higher values of this parameter indicate the greater polarity of ILs [213]. Solubility measurement, swelling, osmotic pressure and intrinsic viscosity are most commonly used for the estimation of Hildebrand solubility parameter 282

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chain length of cations on the biomass dissolution did not generate a same conclusion. Therefore, further work is needed in this domain to understand the reasons behind these conflicting findings.

4.0 3.5

Hemmett acidity

3.0

3.2. Reaction kinetics and molecular modelling

2.5 2.0

The reaction kinetics of biomass pre-treatment using ILs has not been investigated in a great detail. A limited number of simulation methods has been used to investigate the molecular dynamics and kinetics of biomass decomposition with ILs [112,229–231]. During the pre-treatment process, the molecular level interactions between anions and cations with biomass has been studied using the atoms in molecules (AIM) theory, density functional theory (DFT), Wiberg bond index (WBI) analysis and natural bond orbital (NBO) analysis [229,230]. The authors concluded that the intermolecular hydrogen bonding interactions within lignin and anion of IL dominantly affect the dissolution of lignin in IL. Moreover, Zhu et al. described the molecular mechanism of β-O-4 type lignin dissolution in [Amim][Cl] using combined quantum chemistry calculation and molecular dynamics simulation [232]. The molecular modelling indicated the disruption of intramolecular O H OH bond in the β-O-4 lignin because of both cationic and anionic effects. Parthasarathi et al. studied the molecular level forces and optimised the molecular geometry of [TBA][OH] employing hybrid density functional theory (Hybrid DFT) for the decomposition of Switchgrass [233]. The molecular electrostatic potential map of the IL indicated the strong interactions of IL with biomass. However, the IL showed more affinity to lignin rather than cellulose. The overall reaction rate was mainly controlled by the heat and mass transfer rates during the pre-treatment process. The mass transfer rate (m ) increases significantly with turbulence and can be expressed by the following equation [234]:

1.5 1.0 0.5

[B m im ][ [S [Sm FeC m l [B im im] 4 ] [B m ][ [C m im Fe l] i C m ] [ B ] [C l m [ C l]- 4 ] i l [C m][ ]-C OH O [C h-S Cl]- O [ C h - O SO H h- SO 4 H 3 H [C SO 4 ][T [C 3 S 4 H H][ sO 3 S O ][C HS ] O 3H F O 3H m 4] [C mi im]3 SO m [1 3 ] [D 3 SO ][H -N S M 3 [C [ A +Hm 2 PO ] 3 S NM ] [C im 4 ] O P H ][C [C 3 H ] + l 3 S m [ C 3 SO ] [C O3 im] H3 3 ] S [ H 3 S m CH O O im 3 3 H ][ 3 SO ] m Ph 3 [C 4 S [BS im] SO ] O [C [C 3 H mi [HS 3 ] m O 4 [C (Mi [C 3 SO mim ][O 4 ] 3 m S 4 (M ) O 3 H ][ A [C im 2 ][( 3 H Py HS c] 4 (M 2) ] 2H N1 ][H O4 ] im [(2 SO 11 ][ SO H 4 H 4 2) ][ SO )(H SO ] (2 4 2 S H )(H O 4 ] SO 2 S 4 0) [B 4 )(H O4 ] Sm 2 S 2) ] im O ][H 4 4) SO ] 4]

0.0

Fig. 7. Hemmett acidity values of several acidic ILs [83,171,178,180,183–186].

For [Bmim], increasing carboxylate anions alkyl chain length positively affects biomass fractionation [224]. The interaction of lignin with unsaturated bonds, π-π in the nitrile based ILs enhances lignin extraction [87]. ILs containing good hydrogen bond acceptor anions such as [OAc] , [OFo] , [DEP ] and [Cl] exhibits superior capability for lignin separation from lignocellulosic structure [225]. Xu et al. synthesised a number of ILs such as [Bmim][OAc], [Bmim][HSCH2COO], [Bmim] [OFo], [Bmim][(C6H5)COO], [Bmim][H2NCH2COO], [Bmim] [HOCH2COO], [Bmim][N(CN)2] and [Bmim][CH3CHOHCOO] combining [Bmim]+ cation with Brønsted basic anions [226]. 1H NMR analysis revealed the hydrogen bond accepting ability of anion enhances the rate of cellulose decomposition. A controversial result was found for cellulose decomposition using ILs with low basicity anions, for example tetrafluoroborate (BF 4 ) and hexafluorophosphate (BF6 ) [91]. Pulp cellulose was found to be insoluble in [Bmim][BF4] and [Bmim][BF6]. For lignin dissolution in IL-water mixtures, the addition of water reduces the number of hydrogen bonds between the cations and anions of ILs. Later lignin dissolves in the binary solvents because of new hydrogen bonds between anion and lignin as well as π-π interaction between cation and benzene ring of lignin [227]. Additionally, increasing the cation alkyl chain length stimulates the formation of hydrogen bonds which enhances the biomass fractionation. However, there is a maximum value of chain length above which the decomposition of biomass reduces because of the steric impediment [162]. A contradiction with this understanding was reported by other researchers [91,204]. The later studies described that the increase of alkyl chain length in imidazolium cation decreases biomass dissolution. Food-additive derived and task specific ILs, synthesised by Pinkert et al., were employed to extract lignin from biomass [107]. The lignin extraction efficiency was about 43% at 100 °C for 2 h heating and it increased up to 60% by the addition of DMSO. From a Nuclear Magnetic Resonance (NMR) spectrometry analysis, Besombes et al. concluded that the intramolecular hydrogen-bonding interactions in guaiacyl β-O4 lignin model compound was an important factor for lignin dissolution [228]. Ji et al. examined the dissolution mechanism of a lignin model compound, 1-(4-methoxyphenyl)-2-methoxyethanol (LigOH) with [Amim][Cl] and indicated that hydrogen bond interaction between the IL and lignin caused the lignin dissolution [229]. Wang et al. claimed hydrogen bonds and stacking as the dominant factors for lignin separation [230]. The presence of aprotic polar solvents such as DMSO, DMA and DMF causes the release of free [OAc]− anions and [Bmim]+ cations from the dissociation of [Bmim][OAc] [159,160]. The large number of free anions and cations enhances biomass decomposition. However, the role of cations on IL-biomass interactions is still unclear. For instance, previous investigations of the effects of increasing alkyl

m = km (Ci

(7)

C)A

where km is the mass transfer coefficient, Ci is the interfacial concentration, C is the concentration of solution and A is the interfacial 6n area. The interfacial area ( A = dv ) can be expressed as the ratio of volume fraction of solids in the mixture (n v ) and the Sauter mean solid diameter (d ) [235]. The mass transfer of a solid-liquid agitated closed system can be predicted in terms of the correlation of Reynolds number (NRe =

km D

ND2 ) µ

=

and Schmidt number (NSc =

(NRe )q (NSc ) s

µ

) [236]:

(8)

where is the density of the mixture, N is the stirring speed, D is the diameter of the agitator, µ is the viscosity of the solution and is the diffusivity of solute. The values of the and q for a turbine agitator are 0.00058 and 1.4 at NRe < 7400 , both 0.62 at NRe > 7400 respectively. The values of the and q are 0.0043 and 1.0 for propeller agitators with a Reynolds number range of 3300–33,000. The value of Schmidt number exponent s can be taken as 0.5 for any Reynolds number [236]. Eta et al. investigated the fractionation of birch chips into cellulose, hemicellulose and lignin using switchable ionic liquids (SWILs) employing both loop reactor and batch reactor. SWILs are solvent mixtures that can be transformed between ionic and non-ionic liquids by the removal or addition of an acid gas such as CO2 and SO2 [237,238]. The mass transfer was considered as the result of particle transport and analysed by Fick's law as follows [239].

kB T

= 6 µ

(

3Wm 4 NA

)

1/3

(9)

where kB is the Boltzmann constant, T is the temperature of solution, Wm is the molecular weight of solute and NA is the Avogadro number. Anugwom et al. optimised the differential mass transfer equation using Simplex and Levenberg Marquardt methods [240]. 283

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Shill et al. studied lignocellulosic biomass deconstruction into its three major components. The authors described the reaction kinetics of biomass deconstruction using Arrhenius temperature dependent equation [241]. The heat transfer in an agitated reactor can be expressed in terms of 2 cp µ hD Reynolds number ( ND ), Prandtl number k , Nusselt number k

and viscosity ratio (

µ µ ). µw

( )

Designer software [244]. This model uses a detailed design of the process and incorporates process pathway including feedstock preparation, biomass pre-treatment, IL recovery, and enzymatic hydrolysis. However, it excluded the consideration of feedstock logistics. In a different study, a process model was developed for a IL pre-treatment plant for the conversion of lignocellulosic biomass into ethanol in absence of enzyme using SuperPro Designer software [245]. In addition, Parthasarathi et al. developed a process model taking into consideration of several parameters including the reactor design and the energy intensity of IL using the same software [233]. The model showed a pathway to reduce the pre-treatment energy cost by 75%. Brandt-Talbot et al. investigated the possible economic routes for the IonoSolv fractionation of Miscanthus x giganteus biomass using low cost IL, triethylammonium hydrogen sulphate [246]. The process schematic of the same is presented in Fig. 8. The process optimisation carried out indicated 85% delignification with 80% lignin recovery and 80% glucose yield. It was identified that the higher IL recycling rate (99%) remarkably reduces the levelised glucose production cost which makes the IonoSolv fractionation process economically favourable [246]. Sen et al. developed a process model for large-scale production of sugar from IL pre-treatment of corn stover biomass. The modelling was based on experimental studies and a simulated moving bed (SMB) separation technique [247]. This modelling was based on the embodiment of knowledge from a previous literature [248]. The decrystallisation unit was designed with 150 min for first hydrolysis and 180 min for second hydrolysis, and SMB separation technology was devised based on the 98% IL recovery capacity. The researchers identified the cost of IL as the major hindrance for commercial implementation. In order to lower the IL consumption, the authors suggested dilute acid hydrolysis first, followed by IL hydrolysis. Fu and Mazza optimised the process parameters (reaction time, temperature and IL concentration) for wheat straw pre-treatment with [Emim][OAc] using response surface methodology for future scaling-up or commercial application [249]. The embodiments were analysed based on the following secondorder polynomial model using SAS and Design-Expert statistical software.

( )

The heat transfer behaviour in a reactor with

turbine agitator and anchor agitator can be described by the Eqs. (8) and (9) respectively [242].

hD = 0.76 k

hD =K k

ND 2 µ

ND2 µ

a

2/3

Cp µ

1/3

k

cv µ k

1/3

µ µw

µ µw

0.24

(10)

0.18

(11)

where h is the heat transfer coefficient, k is the thermal conductivity of mixture inside the reactor, µ w is the viscosity of mixture at heat transfer surface temperature, and c v is the specific heat of the mixture. At 10 < NRe <300 , the values of K and a are 1.0 and 0.5 whereas, at 300 < NRe <40000 the values of these constant are 0.36 and 0.67, respectively. High biomass loading reduces the heat and mass transfer during the pre-treatment process due to higher viscosity. However, a suitable reactor design can induce shear thinning and enhance reaction kinetics [243]. The heat and mass transfer mechanisms of IL pre-treatment processes at high solid loadings are not fully understood; hence comprehensive studies are required on the development of heat and mass transfer phenomenon at high solid loadings. 4. IL pre-treatment process integration and techno-economic assessment 4.1. Pre-treatment process development and process modelling The optimisation of process parameters and effective design of process units are prerequisites for the development of a large-scale biomass pre-treatment plant. Baral and Shah simulated a process for a full-scale biomass hydrolysis pre-treatment plant employing SuperPro

3

Y=

0

+

3 i Xi

i=1

+

2 ii Xi

2

3

+

i=1

ij Xi Xj i = 1 j = i +1

Volatiles and water distillation

Lignocellulose transport Volatiles

Conveyor

Wet IL liquor Lignin precipitation Water + residual IL

Lignocellulose

Storage

Water Lignin + IL

IL + 20% water solution

Pretreatment reactor

Water + residual IL

Pulp + IL

Pulp washing Slurry

Wet pulp Enzymatic saccharification

Slurry

Lignin washing Slurry pump Lignin + residual IL Wet lignin

Lignin boiler

Filtration

IL liquor

Slurry pump Filtration Fig. 8. Process flow diagram of IL-based lignocellulosic biomass pre-treatment followed by enzymatic saccharification [246]. 284

(12)

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where Y symbolises the fraction of lignin recovery from the washed solid, ij indicates the two-factor interaction coefficient and X indicates the operation variables; 0 , i and ii represents the constant, linear and quadratic coefficient, respectively. Gogoi and Hazarika optimised the process parameters for the recovery of lignin from the IL pre-treated lignocellulosic biomass using nanofiltration membranes [114]. A simple mathematical model was established for lignin dissolution by alkanol amine based SWIL for three different reactors [240]. The model was analysed and optimised using Modest™ software and it was found that the SpinChem® rotating bed reactor was the most promising reactor in term of lignin dissolution when compared to the non-stirring batch reactor and the loop reactor. While there are a few computational model already developed, further work is required in this area. The computational model can be developed job specific considering vital parameters such as the solid loading, heat and mass transfer barrier, required level of agitation as well as the level of feedstock supply.

tf

QHeating = m

W=

(13)

where Mr is the total mass of substances inside the reactor, MB is the mass of biomass inflow, MI is the mass of IL inflow, Mw is the mass of water inflow and Mt is the mass of treated products outflow. In a number of studies, mass balances were carried out for the pretreatment of biomass with ILs followed by hydrolysis for bench-scale reactors [164,167]. These studies provide an overview on the product distribution in a commercial scale plant. The results of mass balance from earlier studies indicate that a small quantity of materials is lost during pre-treatment process [251,252]. For instance, the mass balance for a commercial scale fermentation of biomass with IL showed approximately 16.4% sugar loss from the system [244]. Brandt-Talbot et al. found 10–20% mass loss in their mass balance [246], but the amount lignin produced was too low to quantify, which was not accounted in the mass balance. In addition, a large quantity of hemicellulose was unaccounted. The non-accounted mass of lignin and hemicellulose had substantial impact on mass balance. The values of the energy required for biomass pre-treatment and energy produced from biomass are important for the process to be feasible. The performance of the pre-treatment process is often expressed in terms of energy efficiency which can be estimated from the production of value-added materials per unit energy consumption. The energy balance of a biomass pre-treatment reactor can be completed by assuming a closed system, where the change in internal energy (ΔU) is attributed by the supplied thermal energy (QHeating ) and work done on the reactor (W) in the form of mechanical work (in this case, by means of agitating the mixture). The energy needed for biomass pre-treatment (EP ) is the internal energy change of the mixture sample which can be expressed by Eq. (11) [253]. Therefore, the overall energy requirement for biomass pre-treatment (EOP ) is estimated from summation of energy consumptions for milling (size-reduction) of biomass (EM ) and energy for pre-treatment (EP ). It has been reported that the energy consumption for disk milling varies from 0.54 to 2.88 MJ/kg [254].

U = QHeating + W

(14)

EOP = EM + QHeating + W

(15)

EP =

N 3D5Np/ g

(17)

where , N, D and g indicate the density of mixture, speed of stirring, length of the stirrer or diameter of the agitator and gravitational acceleration, respectively. Np is the power number which depends on the Reynolds number (NRe = ND 2 / µ ) and is determined form the Np versus NRe curve. A comparative study of rice husk decomposition via three ILs provided the insight in terms of energy requirement [256]. The thermal energy necessary for the pre-treatment process using [Bmim][Cl] was approximately 8.12 MJ/kg. The corresponding energy values for [Emim][OAc] and [Emim][DEP] were 9.91 MJ/kg and 10.33 MJ/kg, respectively. There is very limited literature available for comparing the energy requirement of the biomass-IL pre-treatment process with other methods. The pre-treatment of 1 kg oil palm frond residue by [Bmim] [Cl] and [Emim][DEP] requires approximately 3.27 kWh and 2.46 kWh energy, respectively. These values are higher than the energy required for alkali, dilute acid and hot water pre-treatments [257]. In contrast to oil palm pre-treatment process, the pre-treatment of sugarcane bagasse by [Emim][OAc] consumes approximately 55% lower energy compared to acidic and alkaline pre-treatments [255]. Proper understanding on the mass balance during biomass pre-treatment and the estimation of energy required for the process are critical on the development of energy efficient commercial scale pre-treatment processes. However, detail studies related to material balance and energy requirement for large-scale reactors are very limited in the literature.

Analysis of mass and energy balance is essential to develop the correlation with lignin yield and energy output. The mass balance during the pre-treatment process can be understood from the following equation [250].

Mt

(16)

where m is the total mass of the mixture, ti is the initial temperature, t f is the pre-treatment temperature of the mixture and Cv is the specific heat of the mixture. The work done can be calculated in terms of the power required to drive the stirrer and expressed as follows [242]:

4.2. Mass and energy balance studies

d (Mr ) = MB + MI + Mw dt

Cv dT ti

4.3. Economical perspectives The cost of ILs for biomass delignification is a concern for the development for large scale IL-based industry. Although ILs are expensive, they can be recycled and reused. Thus the cost of biomass pre-treatment by IL can be reduced and the process becomes significantly cost-effective [168]. In the last few decades, recycling methods were developed and studied at the lab-scale setup [258]. The recycling and reuse of eight ILs containing cholinium as cation and amino acids as anions ([Ch][AA]) was examined, with a slight drop in lignin extraction efficiency (from 58.8% to 52.5%, after recycling the IL five times) [104]. A different study showed that the recycling of [Amim][Cl] and [Bmim] [OAc] reduces the cellulose digestibility slightly; however, recycling the IL still reduces the overall processing cost [259]. Konda et al. conducted a detailed techno-economic analysis considering waterwashing as well as one-pot processes for the production of biofuel from IL pre-treated biomass [260], and demonstrated that the ethanol production cost can be reduced to $3/gallon (a price which is competitive to the market price) by introducing a few advancements including the lignin valorisation. This study mainly focused on the operating cost, including the raw material cost, facility dependent cost and utility cost. In contrast, Sen et al. developed a techno-economic model for 10 t/h biomass pre-treatment plant comprising both the capital costs of each unit and operating costs including raw material cost, maintenance cost and waste disposal cost [247]. The total cost of the biomass delignification plant (CT ) can be approximated by adding the capital cost (CCapex ) and the plant operation cost (COpex ) as follows:

CT = CCapex + COpex

The heat required for the pre-treatment process can be estimated employing the following equation [255].

(18)

The following equations can be employed for estimating cost of a 285

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large-scale plant employing the capacities of the base and scale-up plant [261,262].

CCapex = C0 (M / Mo) k

5.1. Establishment of scale-up rules for large scale operations The study of both hydrodynamics and reaction kinetics at scale and in dynamic operational conditions are extremely important for the development of large scale IL pre-treatment operations. There are very limited studies performed on the hydrodynamics and reaction kinetics for ILs. Research on IL pre-treatment reactor design and optimisation is non-existent in the literature. The use of computational fluid dynamics (CFD) is also very limited in this area. Normally, dimensional analyses are performed to establish scale-up rules. Dimensional analysis is a mathematical method which involves mass, length and time for establishing scale-up rules. The work on dimensional analysis is also nonexistent in the literature. It is therefore suggested that the knowledge obtained from lab and bench scales should be used to establish scale-up rules and study hydrodynamics and reaction kinetics at large scale.

(19)

COpex = CP + CD + CF = CPo (M /Mo )m + CDo (M / Mo)n + CFo (M / Mo) (20) where CP is the sum of plant's total operation cost, CD is the biomass delivery cost, CF is the biomass purchasing cost at farm, M is the installed capacity of the plant, Mo is the base size plant capacity and the cost components with subscript o represent the cost at capacity Mo . The value of dimensionless scale factors k, m and n are considered 0.6–0.8, 0.9–1 and 1.5, respectively [261,263]. Additionally, the equipment cost (CE ) can be estimated using the standard power law expression.

CE = a + bH r

(21)

5.2. Studies related to cost reduction in the IL pre-treatment

where a and b are the cost constant, H is the size parameter and r is the exponent for the equipment [264]. Baral and Shah developed a techno-economic model for a commercial biomass treatment plant with IL using SuperPro Designer software [244]. The sensitivity analysis for capital cost and operating cost was conducted by varying the cost of IL, recovery rate of IL, recovery rate of waste and severity factor. The reaction severity incorporates a few elements including residence time, temperature and catalyst loading. The study showed that the severity factor is the most important process parameter. The outcome of the model leads to a conclusion that the process with a production capacity of 113 million litres per year can be economically competitive for a few conditions including the IL recovery of more than 97%, more than 90% waste heat recovery and the IL cost of less than $1 per kg. However, these values will require further optimisation in order to overcome the bottleneck (i.e. high cost of IL and high energy requirement) associated with the commercialisation of the process. Klein-Marcuschamer et al. demonstrated that the minimum ethanol selling price (MESP) is sensitive to the IL cost, IL recovery and IL loading [265]. The authors also showed that the reduction of IL cost is the most significant for process cost minimisation and the price of MESP can be decreased to $1.50 per gallon by generating revenue generation from the sale of lignin. A similar tech-economic model showed IL cost of $1.25 per kg for the MESP considering a relatively low IL cost for a plant with 25 year lifetime [144]. A recent study suggested synthesis of inexpensive ILs in bulk scale from cheap raw materials rather than focusing on effective recycling [266]. This report says the production cost of triethylammonium hydrogen sulphate and 1-methylimidazolium hydrogen sulphate can be as low as $1.24 per kg and $2.96–5.88 per kg, respectively and these prices are competitive to the prices of organic solvents. Although the costs of these ILs are lucrative, their delignification performance must be comprehensively investigated before they can be considered for commercial scale plants. Additionally, efficient recycling methods of ILs are required to be developed for removing impurities and maintaining satisfactory performance of the recovered ILs.

The cost of IL synthesis is a major constraint for industrial implementation of this technology. Therefore, future focus should be to identify ways to synthesise low cost ILs. It has been noted that reuse of ILs reduces the processing cost; however, reuse leads to a significant reduction of its solvation power over cyclic operation. Therefore, focus should be on underpinning the fundamentals of cyclic reduction in solvation power and development of an efficient strategy to minimise it. Also, future work should focus on the regeneration/reactivation of degraded ILs or identification of secondary applications, which otherwise might create the biggest concern, that of spent ILs disposal. Another way to bring down the cost of ILs is by blending with solvents including other ILs and water. However, their pre-treatment performance, thermal stability, recyclability and degradation behaviour at high temperature are required to be considered. Studies on blends are limited and should be a focus in the future. The effect of water addition to the IL pre-treatment is also contradictory in the literature. Some studies reported the very similar effect with or without addition of water while others described a slight reduction in lignin separation efficiency. Considering only a slight reduction in the performance of aqueous ILs, the direct use of recycled aqueous ILs can be considered more effective. However, more studies are required to support this claim. The regeneration of ILs after biomass pre-treatments may not be able to remove various impurities such as halide, residual solvents and water in ILs [270]. The impurities and moisture content in ILs can significantly affect the solvation properties of ILs as well as the lignocellulosic biomass dissolution efficiency [258,271]. So far, pervaporation, electrodialysis, vacuum membrane distillation, adsorption, extraction, membrane separation and nanofiltration have been widely investigated for the regeneration of ILs in laboratory scale [272]. However, most of these methods are capital and energy intensive and therefore requires further improvements. 5.3. Studies on the hydrolysis of lignocellulosic biomass

5. Current challenges and future directions

The direct hydrolysis of lignocellulosic biomass employing acidic ILs instead of using catalysts with ILs is also considered to be the emerging route for the conversion of lignocellulosic biomass into different biofuels and bio-chemicals. However, the acidic IL pre-treatment process is associated with a numbers of critical issues. For instance, the development of separation processes on achieving high purity products (particularly in the case of platform chemicals), low catalytic potential of ILs and lack of detailed understanding on acidic IL-biomass interactions require to be addressed prior to commercial scale development.

The laboratory and bench-scale experimental investigations on IL pre-treatment have been extensively reported in the literature. The reaction mechanisms, optimum process conditions and dissolution (intrinsic) kinetics have been obtained for numerous IL systems [80,101,267–269]. Excluding a few contradictions reported in the literature, by and large role of cations and anions as well as the effects of several process parameters such as temperature, residence time, stirring speed and IL:biomass ratio have been established. The major challenges in the IL pre-treatment methods that need to be resolved for ILs to be commercially viable in biomass processing are highlighted below with some recommendations that provide future research directions.

5.4. Rheological studies of ILs, IL-Water and IL-Solvent mixtures The work on investigating viscosity and rheology of cellulose 286

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dissolved in IL is very limited in the literature. It is important to study rheological properties to understand mass and heat transfer mechanisms and identify ways to improve process efficiency and IL recovery. Therefore, future work should more focus on measuring oscillatory and steady state shear behaviour of ILs, IL-Water and IL-Solvent mixtures for delignification studies. The effects of shear rate, temperature and concentration on viscosity should be examined in detail. The understanding on Newtonian, non-Newtonian and pseudo-plastic behaviour of ILs or their mixtures at different operating conditions (i.e., temperature, stirring speed, residence time, IL: water: solvent: biomass ratio) should be thoroughly investigated in detail.

number of future research scopes in this area is summarised below. Issues

Knowledge gaps to be addressed

High ILs cost

1.) Development of efficient recycling and reuse technique for large volume handling, 2.) Development of low cost ILs, 3.) Feasibility of production of ILs at large scale, 4.) Improve recyclability, 5.) Regeneration/reactivation of degraded ILs and 6.) Identification of secondary applications of spent ILs. Identifying interaction mechanisms for blending of feedstock or ILs and solvents or all of them. Process integration to bring down energy and material costs. 1.) Modelling of mass and heat transfer 2.) Molecular dynamics, 3.) Derivation of detailed reaction kinetics and 4.) Reactor design and establishment of scale-up rules. Understanding the change in structural, thermal and chemical properties and deformation and flow of ILs during biomass pre-treatment.

Blends Process development

5.5. Development of IL property database for process modelling tools such as ASPEN Plus and techno-economic assessment

Process and reactor design, modelling and optimisation

IL pre-treatment based process modelling studies in ASPEN Plus are not very well reported in the literature. The process modelling is an important tool to map energy and material requirements and identify optimum ways to integrate various process streams. One of the major bottlenecks here is the lack of property data compilation or generation for IL systems and their incorporation in the ASPEN platform. It is understood that data compilation or generation for all ILs and their mixtures is not feasible as they can be synthesised by numerous combinations of cations and anions. A previous study suggests that their numbers can be in the domain of trillions [273]. A manageable solution in this case can be the implementation of computer-aided design [274,275] in pre-screening ILs that helps in identifying ILs with the desired properties and then generate missing data or compile them for their incorporation in the ASPEN plus. Further to this, development of accurate cost models, based on IL pre-treatment reactor design and process modelling, is also extremely important which can help performing in-depth techno-economic assessments for evaluating the commercial feasibility of various IL pre-treatment processes.

Rheological study

Acknowledgements The first and third authors greatfully acknowledge the financial supports provided by the School of Engineering, RMIT University in the form of postgraduate scholarships. References [1] Patel A, Sindhu DK, Arora N, Singh RP, Pruthi V, Pruthi PA. Biodiesel production from non-edible lignocellulosic biomass of Cassia fistula L. fruit pulp using oleaginous yeast Rhodosporidium kratochvilovae HIMPA1. Bioresour Technol 2015;197:91–8. https://doi.org/10.1016/j.biortech.2015.08.039. [2] Joshi B, Bhatt MR, Sharma D, Joshi J, Malla R, Sreerama L. Lignocellulosic ethanol production: Current practices and recent developments. Biotechnol Mol Biol Rev 2011;6:172–82. [3] Binod P, Sindhu R, Singhania RR, Vikram S, Devi L, Nagalakshmi S, et al. Bioethanol production from rice straw: an overview. Bioresour Technol 2010;101:4767–74. https://doi.org/10.1016/j.biortech.2009.10.079. [4] Yousuf A. Biodiesel from lignocellulosic biomass – prospects and challenges. Waste Manag 2012;32:2061–7. https://doi.org/10.1016/j.wasman.2012.03.008. [5] Schmer MR, Vogel KP, Mitchell RB, Perrin RK. Net energy of cellulosic ethanol from switchgrass. Proc Natl Acad Sci USA 2008;105:464–9. https://doi.org/10. 1073/pnas.0704767105. [6] Ch AK, el, Ch G, rasekhar, Radhika K, Ravinder R, et al. Bioconversion of pentose sugars into ethanol: a review and future directions. Biotechnol Mol Biol Rev 2006;6:8–20. [7] Kabir G, Hameed BH. Recent progress on catalytic pyrolysis of lignocellulosic biomass to high-grade bio-oil and bio-chemicals. Renew Sustain Energy Rev 2017;70:945–67. https://doi.org/10.1016/j.rser.2016.12.001. [8] Kang S, Fu J, Zhang G. From lignocellulosic biomass to levulinic acid: a review on acid-catalyzed hydrolysis. Renew Sustain Energy Rev 2018;94:340–62. https://doi. org/10.1016/j.rser.2018.06.016. [9] Singh YD, Mahanta P, Bora U. Comprehensive characterization of lignocellulosic biomass through proximate, ultimate and compositional analysis for bioenergy production. Renew Energy 2017;103:490–500. https://doi.org/10.1016/j.renene. 2016.11.039. [10] Kassaye S, Pant KK, Jain S. Hydrolysis of cellulosic bamboo biomass into reducing sugars via a combined alkaline solution and ionic liquid pretreament steps. Renew Energy 2017;104:177–84. https://doi.org/10.1016/j.renene.2016.12.033. [11] Yin C. Microwave-assisted pyrolysis of biomass for liquid biofuels production. Bioresour Technol 2012;120:273–84. https://doi.org/10.1016/j.biortech.2012.06. 016. [12] Kan X, Yao Z, Zhang J, Tong YW, Yang W, Dai Y, et al. Energy performance of an integrated bio-and-thermal hybrid system for lignocellulosic biomass waste treatment. Bioresour Technol 2017;228:77–88. https://doi.org/10.1016/j. biortech.2016.12.064. [13] Brandt A, Gräsvik J, Hallett JP, Welton T. Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem 2013;15:550–83. https://doi.org/10.1039/ c2gc36364j. [14] Singh R, Shukla A, Tiwari S, Srivastava M. A review on delignification of lignocellulosic biomass for enhancement of ethanol production potential. Renew Sustain Energy Rev 2014;32:713–28. https://doi.org/10.1016/j.rser.2014.01.051. [15] Mendu V, Harman-Ware AE, Crocker M, Jae J, Stork J, Morton S, et al. Identification and thermochemical analysis of high-lignin feedstocks for biofuel and biochemical production. Biotechnol Biofuels 2011;4:43. https://doi.org/10. 1186/1754-6834-4-43.

5.6. Studies on the integration of ILs with other pre-treatment methods The combination of two or three pre-treatment methods can be considered for the commercial scale process development. An IL pretreatment method can be integrated with physical and non-IL chemical pre-treatment methods. An integrated method can overcome many economical, environmental and technological issues of single pretreatment method. An example of this can be the integration of IL pretreatment with microwave or ultrasound methods instead of conventional heating, dilute acid or alkali treatments and steam explosion. However, a detailed understanding on the process development of integrated methods is scarce in the literature and this knowledge gap should be fulfilled prior to any large-scale application. 6. Conclusion Ionic liquids are promising green solvents with a number of unique properties. A wide numbers of cations and anions have been already investigated in IL pre-treatment of biomass as can be found in the literature. The developed knowledge can be utilised for further investigations. The pre-treatment of biomass using ILs provides several advantages over organic solvent pre-treatment. In spite of their various benefits, a few drawbacks can be noted including their high viscosity and high cost. These limitations raise the question in their applicability in the commercial scale. Thus systematic studies are required on biomass pre-treatment with ILs to explore the possible pathways; so that ILs can be applied in biomass pre-treatment in an economically viable way. The literature on biomass pre-treatment using ILs is extensive. However, the studies conducted are mainly on laboratory scales while bench and pilot scale investigations are still very limited. Therefore, large scale trial investigations are required to overcome the challenges of commercial scale application of IL in the pre-treatment of biomass. A 287

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P. Halder et al. [16] Kumar AK, Sharma S. Recent updates on different methods of pretreatment of lignocellulosic feedstocks: a review. Bioresour Bioprocess 2017;4:7. https://doi. org/10.1186/s40643-017-0137-9. [17] Taherzadeh MJ, Karimi K. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. Int J Mol Sci 2008;9:1621–51. https:// doi.org/10.3390/ijms9091621. [18] Mirahmadi K, Kabir MM, Jeihanipour A, Karimi K, Taherzadeh M. Alkaline pretreatment of spruce and birch to improve bioethanol and biogas production. BioResources 2010;5:928–38. https://doi.org/10.15376/biores.5.2.928-938. [19] Brodeur G, Yau E, Badal K, Collier J, Ramachandran KB, Ramakrishnan S. Chemical and physicochemical pretreatment of lignocellulosic biomass: a review. Enzym Res 2011;2011:787532. https://doi.org/10.4061/2011/787532. [20] Harmsen P, Huijgen W, Bermúdez López L, Bakker R. Literature review of physical and chemical pretreatment processes for lignocellulosic biomass; 2010. [21] Tarves PC, Serapiglia MJ, Mullen CA, Boateng AA, Volk TA. Effects of hot water extraction pretreatment on pyrolysis of shrub willow. Biomass- Bioenergy 2017;107:299–304. https://doi.org/10.1016/j.biombioe.2017.10.024. [22] Zhurinsh A, Dobele G, Jurkjane V, Meile K, Volperts A, Plavniece A. Impact of hot water pretreatment temperature on the pyrolysis of birch wood. J Anal Appl Pyrolysis 2017;124:515–22. https://doi.org/10.1016/j.jaap.2017.01.030. [23] Mood SH, Golfeshan AH, Tabatabaei M, Jouzani GS, Najafi GH, Gholami M, et al. Lignocellulosic biomass to bioethanol, a comprehensive review with a focus on pretreatment. Renew Sustain Energy Rev 2013;27:77–93. [24] Chiaramonti D, Prussi M, Ferrero S, Oriani L, Ottonello P, Torre P, et al. Review of pretreatment processes for lignocellulosic ethanol production, and development of an innovative method. Biomass- Bioenergy 2012;46:25–35. https://doi.org/10. 1016/j.biombioe.2012.04.020. [25] Alvira P, Tomás-Pejó E, Ballesteros M, Negro MJ. Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: a review. Bioresour Technol 2010;101:4851–61. https://doi.org/10.1016/j.biortech.2009. 11.093. [26] Agbor VB, Cicek N, Sparling R, Berlin A, Levin DB. Biomass pretreatment: fundamentals toward application. Biotechnol Adv 2011;29:675–85. https://doi.org/ 10.1016/j.biotechadv.2011.05.005. [27] Jérôme F, Chatel G, De Oliveira Vigier K. Depolymerization of cellulose to processable glucans by non-thermal technologies. Green Chem 2016;18:3903–13. https://doi.org/10.1039/c6gc00814c. [28] Vanneste J, Ennaert T, Vanhulsel A, Sels B. Unconventional pretreatment of lignocellulose with low-temperature plasma. ChemSusChem 2017;10:14–31. https:// doi.org/10.1002/cssc.201601381. [29] Chen H, Liu J, Chang X, Chen D, Xue Y, Liu P, et al. A review on the pretreatment of lignocellulose for high-value chemicals. Fuel Process Technol 2017;160:196–206. https://doi.org/10.1016/j.fuproc.2016.12.007. [30] Lloyd TA, Wyman CE. Combined sugar yields for dilute sulfuric acid pretreatment of corn stover followed by enzymatic hydrolysis of the remaining solids. Bioresour Technol 2005;96:1967–77. https://doi.org/10.1016/j.biortech.2005.01.011. [31] Kaar WE, Holtzapple MT. Using lime pretreatment to facilitate the enzymic hydrolysis of corn stover. Biomass- Bioenergy 2000;18:189–99. https://doi.org/10. 1016/S0961-9534(99)00091-4. [32] Holtzapple MT, Humphrey AE. The effect of organosolv pretreatment on the enzymatic hydrolysis of poplar. Biotechnol Bioeng 1984;26:670–6. https://doi.org/ 10.1002/bit.260260706. [33] Wildschut J, Smit AT, Reith JH, Huijgen WJJ. Ethanol-based organosolv fractionation of wheat straw for the production of lignin and enzymatically digestible cellulose. Bioresour Technol 2013;135:58–66. https://doi.org/10.1016/j.biortech. 2012.10.050. [34] Mesa L, González E, Ruiz E, Romero I, Cara C, Felissia F, et al. Preliminary evaluation of organosolv pre-treatment of sugar cane bagasse for glucose production: application of 23 experimental design. Appl Energy 2010;87:109–14. https://doi. org/10.1016/j.apenergy.2009.07.016. [35] Shimizu K, Usami K. Enzymatic hydrolysis of wood. III. Pretreatment of woods with acidic methanol-water mixture. AGRIS Journal 1978;24:632–7. [36] Alriols MG, Tejado A, Blanco M, Mondragon I, Labidi J. Agricultural palm oil tree residues as raw material for cellulose, lignin and hemicelluloses production by ethylene glycol pulping process. Chem Eng J 2009;148:106–14. https://doi.org/ 10.1016/j.cej.2008.08.008. [37] Sun F, Chen H. Enhanced enzymatic hydrolysis of wheat straw by aqueous glycerol pretreatment. Bioresour Technol 2008;99:6156–61. https://doi.org/10.1016/j. biortech.2007.12.027. [38] Sun XF, Sun RC, Tomkinson J, Baird MS. Degradation of wheat straw lignin and hemicellulosic polymers by a totally chlorine-free method. Polym Degrad Stab 2004;83:47–57. https://doi.org/10.1016/S0141-3910(03)00205-2. [39] Wang K, Bauer S, Sun R. Structural transformation of miscanthus × giganteus lignin fractionated under mild formosolv, basic organosolv, and cellulolytic enzyme conditions. J Agric Food Chem 2012;60:144–52. https://doi.org/10.1021/ jf2037399. [40] Cara C, Ruiz E, Ballesteros M, Manzanares P, Negro MJ, Castro E. Production of fuel ethanol from steam-explosion pretreated olive tree pruning. Fuel 2008;87:692–700. https://doi.org/10.1016/j.fuel.2007.05.008. [41] Zheng Y, Lin H-M, Tsao GT. Pretreatment for cellulose hydrolysis by carbon dioxide explosion. Biotechnol Prog 1998;14:890–6. https://doi.org/10.1021/ bp980087g. [42] Mosier N, Hendrickson R, Ho N, Sedlak M, Ladisch MR. Optimization of pH controlled liquid hot water pretreatment of corn stover. Bioresour Technol 2005;96:1986–93. https://doi.org/10.1016/j.biortech.2005.01.013. [43] Balan V, Bals B, Chundawat SPS, Marshall D, Dale BE. Lignocellulosic biomass

[44] [45]

[46]

[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58]

[59] [60] [61] [62] [63] [64] [65]

[66]

[67] [68] [69]

[70]

288

pretreatment using AFEX. Methods Mol Biol 2009;581:61–77. https://doi.org/10. 1007/978-1-60761-214-8_5. Iyer PV, Wu Z-W, Kim SB, Lee YY. Ammonia recycled percolation process for pretreatment of herbaceous biomass. Appl Biochem Biotechnol 1996;57–58:121–32. https://doi.org/10.1007/BF02941693. Njoku SI, Uellendahl H, Ahring BK. Comparing oxidative and dilute acid wet explosion pretreatment of cocksfoot grass at high dry matter concentration for cellulosic ethanol production. Energy Sci Eng 2013;1:89–98. https://doi.org/10. 1002/ese3.11. Galbe M, Zacchi G. Pretreatment of lignocellulosic materials for efficient bioethanol production. In: Olsson L, editor. Biofuels. Advances in biochemical engineering/biotechnology 108. Berlin, Heidelberg: Springer Berlin Heidelberg; 2007. p. 41–65. https://doi.org/10.1007/10_2007_070. Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind Eng Chem Res 2009;48:3713–29. https://doi.org/10.1021/ie801542g. Hendriks ATWM, Zeeman G. Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresour Technol 2009;100:10–8. https://doi.org/10.1016/ j.biortech.2008.05.027. Zhang K, Pei Z, Wang D. Organic solvent pretreatment of lignocellulosic biomass for biofuels and biochemicals: a review. Bioresour Technol 2016;199:21–33. https://doi.org/10.1016/j.biortech.2015.08.102. Capolupo L, Faraco V. Green methods of lignocellulose pretreatment for biorefinery development. Appl Microbiol Biotechnol 2016:9451–67. https://doi.org/ 10.1007/s00253-016-7884-y. Sánchez Ó, Cardona C. Trends in biotechnological production of fuel ethanol from different feedstocks. Bioresour Technol 2008;99:5270–95. https://doi.org/10. 1016/J.BIORTECH.2007.11.013. Sun Y, Cheng J. Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresour Technol 2002;83:1–11. https://doi.org/10.1016/S09608524(01)00212-7. Jørgensen H, Kristensen JB, Felby C. Enzymatic conversion of lignocellulose into fermentable sugars: challenges and opportunities. Biofuels, Bioprod Bioref 2007;1:119–34. https://doi.org/10.1002/bbb.4. Singh LK, Chaudhary G. Advances in biofeedstocks and biofuels. Volume 1, Biofeedstocks and their processing. 2017. Mosier N, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M, et al. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour Technol 2005;96:673–86. https://doi.org/10.1016/j.biortech.2004.06.025. Clark TA, Mackie KL. Steam explosion of the softwood Pinus radiata with sulphur dioxide addition. I. Process optimisation. J Wood Chem Technol 1987;7:373–403. https://doi.org/10.1080/02773818708085275. Maurya DP, Singla A, Negi S. An overview of key pretreatment processes for biological conversion of lignocellulosic biomass to bioethanol. 3 Biotech 2015;5:597–609. https://doi.org/10.1007/s13205-015-0279-4. Li C, Cheng G, Balan V, Kent MS, Ong M, Chundawat SPS, et al. Influence of physico-chemical changes on enzymatic digestibility of ionic liquid and AFEX pretreated corn stover. Bioresour Technol 2011;102:6928–36. https://doi.org/10. 1016/j.biortech.2011.04.005. Mosier NS, Hendrickson R, Brewer M, Ho N, Sedlak M, Dreshel R, et al. Industrial scale-up of pH-controlled liquid hot water pretreatment of corn fiber for fuel ethanol production. Appl Biochem Biotechnol 2005;125:77–97. Wang H, Gurau G, Rogers RD, Rogers RD, Mirska I, Jodynis-Liebert J, et al. Ionic liquid processing of cellulose. Chem Soc Rev 2012;41:1519. https://doi.org/10. 1039/c2cs15311d. Marsh K, Boxall J, Lichtenthaler R. Room temperature ionic liquids and their mixtures—a review. Fluid Phase Equilib 2004;219:93–8. https://doi.org/10.1016/ j.fluid.2004.02.003. Zhu S, Wu Y, Chen Q, Yu Z, Wang C, Jin S, et al. Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chem 2006;8:325. https://doi. org/10.1039/b601395c. Cull SG, Holbrey JD, Vargas-Mora V, Seddon KR, Lye GJ. Room-temperature ionic liquids as replacements for organic solvents in multiphase bioprocess operations. Biotechnol Bioeng 2000;69:227–33. Diallo A-O, Fayet G, Len C, Marlair G. Evaluation of heats of combustion of ionic liquids through use of existing and purpose-built models. Ind Eng Chem Res 2012;51:3149–56. https://doi.org/10.1021/ie2023788. Fox EB, Smith LT, Williamson TK, Kendrick SE. Aging effects on the properties of imidazolium‑, quaternary ammonium‑, pyridinium‑, and pyrrolidinium-based ionic liquids used in fuel and energy production. Energy Fuels 2013;27:6355–61. https://doi.org/10.1021/ef401148j. Li G, Xue Z, Cao B, Yan C, Mu T. Preparation and properties of C=X (X: O, N, S) based distillable ionic liquids and their application for rare earth separation. ACS Sustain Chem Eng 2016;4:6258–62. https://doi.org/10.1021/acssuschemeng. 6b01984. McCrary PD, Chatel G, Alaniz SA, Cojocaru OA, Beasley PA, Flores LA, et al. Evaluating ionic liquids as hypergolic fuels: exploring reactivity from molecular structure. Energy Fuels 2014;28:3460–73. https://doi.org/10.1021/ef500264z. Ye C, Liu W, Chen Y, Yu L, Dilling WL. Room-temperature ionic liquids: a novel versatile lubricant. Chem Commun 2001;35:2244–5. https://doi.org/10.1039/ b106935g. Rodriguez Castillo AS, Guihéneuf S, Biard P-F, Paquin L, Amrane A, Couvert A. Physicochemical properties of some hydrophobic room-temperature ionic liquids applied to volatile organic compounds biodegradation processes. J Chem Technol Biotechnol 2017. https://doi.org/10.1002/jctb.5343. Mallakpour S, Dinari M. Ionic liquids as green solvents: progress and prospects.

Renewable and Sustainable Energy Reviews 105 (2019) 268–292

P. Halder et al.

[71] [72] [73] [74] [75] [76] [77] [78]

[79] [80]

[81]

[82] [83] [84] [85]

[86]

[87]

[88]

[89] [90]

[91] [92] [93]

[94]

[95]

Green Solvents II, Dordrecht. Netherlands: Springer; 2012. p. 1–32. https://doi. org/10.1007/978-94-007-2891-2_1. Nelson WM. Are ionic liquids green solvents? ACS Symposium Series American Chemical Society; 2002. p. 30–41. https://doi.org/10.1021/bk-2002-0818.ch003. Soto A. Ionic liquids for extraction processes in refinery-related applications. Ion Liq Better Sep Process 2016:39–55. https://doi.org/10.1007/978-3-662-48520-0. Zhang Q, Shreeve JM. Energetic ionic liquids as explosives and propellant fuels: a new journey of ionic liquid chemistry. Chem Rev 2014;114:10527–74. https://doi. org/10.1021/cr500364t. Gellerstedt G, Sjöholm E, Brodin I. The wood-based biorefinery: a source of carbon fiber? Open Agric J 2010;4:119–24. https://doi.org/10.2174/ 1874331501004010119. Norgren M, Edlund H. Lignin: recent advances and emerging applications. Curr Opin Colloid Interface Sci 2014;19:409–16. https://doi.org/10.1016/j.cocis.2014. 08.004. Ragauskas AJ, Beckham GT, Biddy MJ, Chandra R, Chen F, Davis MF, et al. Lignin valorization: improving lignin processing in the biorefinery. Science (80-) 2014;344:1246843. https://doi.org/10.1126/science.1246843. Sun Z, Fridrich B, De Santi A, Elangovan S, Barta K. Bright side of lignin depolymerization: toward new platform chemicals. Chem Rev 2018;118:614–78. https:// doi.org/10.1021/acs.chemrev.7b00588. Jia S, Cox BJ, Guo X, Zhang ZC, Ekerdt JG. Cleaving the β-O-4 bonds of lignin model compounds in an acidic ionic liquid, 1-H-3-methylimidazolium chloride: an optional strategy for the degradation of lignin. ChemSusChem 2010;3:1078–84. https://doi.org/10.1002/cssc.201000112. Lei ZP, Hu ZQ, Shui HF, Ren SB, Wang ZC, Kang SG, et al. Pyrolysis of lignin following ionic liquid pretreatment at low temperature. Fuel Process Technol 2015;138:612–5. https://doi.org/10.1016/j.fuproc.2015.06.049. Elgharbawy AA, Alam MZ, Moniruzzaman M, Goto M. Ionic liquid pretreatment as emerging approaches for enhanced enzymatic hydrolysis of lignocellulosic biomass. Biochem Eng J 2016;109:252–67. https://doi.org/10.1016/j.bej.2016.01. 021. Zheng A, Chen T, Sun J, Jiang L, Wu J, Zhao Z, et al. Toward fast pyrolysis-based biorefinery: selective production of platform chemicals from biomass by organosolv fractionation coupled with fast pyrolysis. ACS Sustain Chem Eng 2017;5:6507–16. https://doi.org/10.1021/acssuschemeng.7b00622. Muhammad N, Omar WN, Man Z, Bustam MA, Rafiq S, Uemura Y. Effect of ionic liquid treatment on pyrolysis products from bamboo. Ind Eng Chem Res 2012;51:2280–9. https://doi.org/10.1021/ie2014313. Ramli NAS, Amin NAS. A new functionalized ionic liquid for efficient glucose conversion to 5-hydroxymethyl furfural and levulinic acid. J Mol Catal A Chem 2015;407:113–21. https://doi.org/10.1016/j.cej.2017.10.112. Gírio FM, Fonseca C, Carvalheiro F, Duarte LC, Marques S, Bogel-Łukasik R. Hemicelluloses for fuel ethanol: a review. Bioresour Technol 2010;101:4775–800. https://doi.org/10.1016/j.biortech.2010.01.088. Peleteiro S, Da Costa, Lopes AM, Garrote G, Parajó JC, Bogel-Łukasik R. Simple and efficient furfural production from xylose in media containing 1-butyl-3-methylimidazolium hydrogen sulfate. Ind Eng Chem Res 2015;54:8368–73. https://doi. org/10.1021/acs.iecr.5b01771. Mohtar S, Tengku Malim Busu T, Md Noor A, Shaari N, Yusoff N, Bustam Khalil MA, et al. Extraction and characterization of lignin from oil palm biomass via ionic liquid dissolution and non-toxic aluminium potassium sulfate dodecahydrate precipitation processes. Bioresour Technol 2015;192:212–8. https://doi.org/10. 1016/j.biortech.2015.05.029. Muhammad N, Man Z, Bustam MA, Mutalib MIA, Rafiq S. Investigations of novel nitrile-based ionic liquids as pre-treatment solvent for extraction of lignin from bamboo biomass. J Ind Eng Chem 2013;19:207–14. https://doi.org/10.1016/j. jiec.2012.08.003. Li H-Y, Chen X, Wang C-Z, Sun S-N, Sun R-C. Evaluation of the two-step treatment with ionic liquids and alkali for enhancing enzymatic hydrolysis of eucalyptus: chemical and anatomical changes. Biotechnol Biofuels 2016;9:166. https://doi. org/10.1186/s13068-016-0578-y. Lee SH, Doherty TV, Linhardt RJ, Dordick JS. Ionic liquid-mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis. Biotechnol Bioeng 2009;102:1368–76. https://doi.org/10.1002/bit.22179. Muhammad N, Man Z, Bustam MA, Mutalib MIA, Wilfred CD, Rafiq S. Dissolution and delignification of bamboo biomass using amino acid-based ionic liquid. Appl Biochem Biotechnol 2011;165:998–1009. https://doi.org/10.1007/s12010-0119315-y. Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellulose with ionic liquids. J Am Chem Soc 2002;124:4974–5. https://doi.org/10.1021/JA025790M. Zhang H, Wu J, Zhang J, He J. 1-allyl-3-methylimidazolium chloride room temperature ionic liquid: a new and powerful nonderivatizing solvent for cellulose. Macromolecules 2005;38:8272–7. https://doi.org/10.1021/MA0505676. Tan X, Li X, Chen L, Xie F, Wasserscheid P, Leipertz A, et al. Solubility of starch and microcrystalline cellulose in 1-ethyl-3-methylimidazolium acetate ionic liquid and solution rheological properties. Phys Chem Chem Phys 2016;18:27584–93. https://doi.org/10.1039/C6CP04426C. Sun N, Rahman M, Qin Y, Maxim ML, Rodríguez H, Rogers RD. Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3-methylimidazolium acetate. Green Chem 2009;11:646–55. https://doi.org/10.1039/ b822702k. Husson E, Buchoux S, Avondo C, Cailleu D, Djellab K, Gosselin I, et al. Enzymatic hydrolysis of ionic liquid-pretreated celluloses: contribution of CP-MAS 13C NMR and SEM. Bioresour Technol 2011;102:7335–42. https://doi.org/10.1016/J. BIORTECH.2011.04.097.

[96] Soudham VP, Raut DG, Anugwom I, Brandberg T, Larsson C, Mikkola J-P. Coupled enzymatic hydrolysis and ethanol fermentation: ionic liquid pretreatment for enhanced yields. Biotechnol Biofuels 2015;8:1–13. https://doi.org/10.1186/s13068015-0310-3. [97] Shah K, Atkin R, Stanger R, Wall T, Moghtaderi B. Interactions between vitrinite and inertinite-rich coals and the ionic liquid – [bmim][Cl]. Fuel 2014;119:214–8. https://doi.org/10.1016/j.fuel.2013.11.038. [98] Cummings J, Kundu S, Tremain P, Moghtaderi B, Atkin R, Shah K. Investigations into physicochemical changes in thermal coals during low-temperature ionic liquid treatment. Energy Fuels 2015;29:7080–8. https://doi.org/10.1021/acs. energyfuels.5b01824. [99] Cummings J, Tremain P, Shah K, Heldt E, Moghtaderi B, Atkin R, et al. Modification of lignites via low temperature ionic liquid treatment. Fuel Process Technol 2017;155:51–8. https://doi.org/10.1016/j.fuproc.2016.02.040. [100] To TQ, Shah K, Tremain P, Simmons BA, Moghtaderi B, Atkin R. Treatment of lignite and thermal coal with low cost amino acid based ionic liquid-water mixtures. Fuel 2017;202:296–306. https://doi.org/10.1016/j.fuel.2017.04.051. [101] Hou Q, Ju M, Li W, Liu L, Chen Y, Yang Q, et al. Pretreatment of lignocellulosic biomass with ionic liquids and ionic liquid-based solvent systems. Molecules 2017;22:1–24. https://doi.org/10.3390/molecules22030490. [102] Wen J, Yuan T, Sun S, Xu F, Sun R. Understanding the chemical transformations of lignin during ionic liquid pretreatment. Green Chem 2014;6:181–90. https://doi. org/10.1039/c3gc41752b. [103] Moniruzzaman M, Ono T. Ionic liquid assisted enzymatic delignification of wood biomass: a new green and efficient approach for isolating of cellulose fibers. Biochem Eng J 2012;60:156–60. https://doi.org/10.1016/j.bej.2011.11.001. [104] Hou XD, Smith TJ, Li N, Zong MH. Novel renewable ionic liquids as highly effective solvents for pretreatment of rice straw biomass by selective removal of lignin. Biotechnol Bioeng 2012;109:2484–93. https://doi.org/10.1002/bit.24522. [105] Casas A, Oliet M, Alonso MV, Rodríguez F. Dissolution of Pinus radiata and Eucalyptus globulus woods in ionic liquids under microwave radiation: lignin regeneration and characterization. Sep Purif Technol 2012;97:115–22. https:// doi.org/10.1016/j.seppur.2011.12.032. [106] Singh S, Varanasi P, Singh P, Adams PD, Auer M, Simmons BA. Understanding the impact of ionic liquid pretreatment on cellulose and lignin via thermochemical analysis. Biomass- Bioenergy 2013;54:276–83. https://doi.org/10.1016/j. biombioe.2013.02.035. [107] Pinkert A, Goeke DF, Marsh KN, Pang S, Melnichenko YB, Simmons BA, et al. Extracting wood lignin without dissolving or degrading cellulose: investigations on the use of food additive-derived ionic liquids. Green Chem 2011;13:3124–36. https://doi.org/10.1039/c1gc15671c. [108] Doherty TV, Mora-Pale M, Foley SE, Linhardt RJ, Dordick JS, Person VN, et al. Ionic liquid solvent properties as predictors of lignocellulose pretreatment efficacy. Green Chem 2010;12:1967–75. https://doi.org/10.1039/c0gc00206b. [109] Thanh VoH, Soo Kim C, Sung Ahn B, Sik Kim H, Lee H. Study on dissolution and regeneration of poplar wood in imidazolium-based ionic liquids. J Wood Chem Technol 2011;31:89–102. https://doi.org/10.1080/02773813.2010.486463. [110] Financie R, Moniruzzaman M, Uemura Y. Enhanced enzymatic delignification of oil palm biomass with ionic liquid pretreatment. Biochem Eng J 2016;110:1–7. https://doi.org/10.1016/j.bej.2016.02.008. [111] Li C, Knierim B, Manisseri C, Arora R, Scheller HV, Auer M, et al. Comparison of dilute acid and ionic liquid pretreatment of switchgrass: biomass recalcitrance, delignification and enzymatic saccharification. Bioresour Technol 2010;101:4900–6. https://doi.org/10.1016/j.biortech.2009.10.066. [112] Wu H, Mora-Pale M, Miao J, Doherty TV, Linhardt RJ, Dordick JS. Facile pretreatment of lignocellulosic biomass at high loadings in room temperature ionic liquids. Biotechnol Bioeng 2011;108:2865–75. https://doi.org/10.1002/bit. 23266. [113] Chang K-L, Chen X-M, Han Y-J, Wang X-Q, Potprommanee L, Ning X, et al. Synergistic effects of surfactant-assisted ionic liquid pretreatment rice straw. Bioresour Technol 2016;214:371–5. https://doi.org/10.1016/j.biortech.2016.04. 113. [114] Gogoi G, Hazarika S. Coupling of ionic liquid treatment and membrane filtration for recovery of lignin from lignocellulosic biomass. Sep Purif Technol 2017;173:113–20. https://doi.org/10.1016/j.seppur.2016.09.019. [115] Watkins D, Nuruddin M, Hosur M, Tcherbi-Narteh A, Jeelani S. Extraction and characterization of lignin from different biomass resources. J Mater Res Technol 2015;4:26–32. https://doi.org/10.1016/j.jmrt.2014.10.009. [116] Tan SSY, MacFarlane DR, Upfal J, Edye LA, Doherty WOS, Patti AF, et al. Extraction of lignin from lignocellulose at atmospheric pressure using alkylbenzenesulfonate ionic liquid. Green Chem 2009;11:339. https://doi.org/10.1039/ b815310h. [117] Mai NL, Ha SH, Koo Y. Efficient pretreatment of lignocellulose in ionic liquids/cosolvent for enzymatic hydrolysis enhancement into fermentable sugars. Process Biochem 2014;49:1144–51. https://doi.org/10.1016/j.procbio.2014.03.024. [118] Perez-Pimienta JA, Lopez-Ortega MG, Chavez-Carvayar JA, Varanasi P, Stavila V, Cheng G, et al. Characterization of agave bagasse as a function of ionic liquid pretreatment. Biomass- Bioenergy 2015;75:180–8. https://doi.org/10.1016/j. biombioe.2015.02.026. [119] Qiu Z, Aita GM. Pretreatment of energy cane bagasse with recycled ionic liquid for enzymatic hydrolysis. Bioresour Technol 2013;129:532–7. https://doi.org/10. 1016/j.biortech.2012.11.062. [120] Ninomiya K, Kohori A, Tatsumi M, Osawa K, Endo T, Kakuchi R, et al. Ionic liquid/ ultrasound pretreatment and in situ enzymatic saccharification of bagasse using biocompatible cholinium ionic liquid. Bioresour Technol 2015;176:169–74. https://doi.org/10.1016/j.biortech.2014.11.038.

289

Renewable and Sustainable Energy Reviews 105 (2019) 268–292

P. Halder et al. [121] Karatzos SK, Edye LA, Doherty WOS. Sugarcane bagasse pretreatment using three imidazolium-based ionic liquids; mass balances and enzyme kinetics. Biotechnol Biofuels 2012;5:62. https://doi.org/10.1186/1754-6834-5-62. [122] Sun SN, Li MF, Yuan TQ, Xu F, Sun RC. Effect of ionic liquid/organic solvent pretreatment on the enzymatic hydrolysis of corncob for bioethanol production. Part 1: structural characterization of the lignins. Ind Crops Prod 2013;43:570–7. https://doi.org/10.1016/j.indcrop.2012.07.074. [123] Gräsvik J, Winestrand S, Normark M, Jönsson LJ, Mikkola J-P. Evaluation of four ionic liquids for pretreatment of lignocellulosic biomass. BMC Biotechnol 2014;14:34. https://doi.org/10.1186/1472-6750-14-34. [124] Zhang J, Zhang X, Li C, Zhang W, Zhang J, Zhang R, et al. A comparative study of enzymatic hydrolysis and thermal degradation of corn stover: understanding biomass pretreatment. RSC Adv 2015;5:36999–7005. https://doi.org/10.1039/ c5ra01930c. [125] Mohtar SS, TNZTM Busu, Noor, Shaari AMM, Mat N, An H. ionic liquid treatment and fractionation of cellulose, hemicellulose and lignin from oil palm empty fruit bunch. Carbohydr Polym 2017;166:291–9. https://doi.org/10.1016/j.carbpol. 2017.02.102. [126] Wang H, Maxim ML, Gurau G, Rogers RD. Microwave-assisted dissolution and delignification of wood in 1-ethyl-3-methylimidazolium acetate. Bioresour Technol 2013;136:739–42. https://doi.org/10.1016/j.biortech.2013.03.064. [127] Li W, Sun N, Stoner B, Jiang X, Lu X, Rogers RD. Rapid dissolution of lignocellulosic biomass in ionic liquids using temperatures above the glass transition of lignin. Green Chem 2011;13:2038. https://doi.org/10.1039/c1gc15522a. [128] Pu Y, Jiang N, Ragauskas AJ. Ionic liquid as a green solvent for lignin. J Wood Chem Technol 2007;27:277–3813. https://doi.org/10.1080/ 02773810701282330. [129] Brandt A, Ray MJ, To TQ, Leak DJ, Murphy RJ, Welton T Ionic. Liquid pretreatment of lignocellulosic biomass with ionic liquid–water mixtures. Green Chem 2011;13:2489. https://doi.org/10.1039/c1gc15374a. [130] Heggset EB, Syverud K, Øyaas K. Novel pretreatment pathways for dissolution of lignocellulosic biomass based on ionic liquid and low temperature alkaline treatment. Biomass- Bioenergy 2016;93:194–200. https://doi.org/10.1016/j. biombioe.2016.07.023. [131] Adlie Shamsuri A, Kuang Abdullah D. Isolation and characterization of lignin from rubber wood in ionic liquid medium. Mod Appl Sci 2010;4:19–27. [132] Dibble DC, Li C, Sun L, George A, Cheng A, Çetinkol ÖP, et al. A facile method for the recovery of ionic liquid and lignin from biomass pretreatment. Green Chem 2011;13:3255. https://doi.org/10.1039/c1gc15111h. [133] Costa Lopes AM, João KG, Rubik DF, Bogel-Łukasik E, Duarte LC, Andreaus J, et al. Pre-treatment of lignocellulosic biomass using ionic liquids: wheat straw fractionation. Bioresour Technol 2013:142. https://doi.org/10.1016/j.biortech.2013.05. 032. [134] Qiu Z, Aita GM, Walker MS. Effect of ionic liquid pretreatment on the chemical composition, structure and enzymatic hydrolysis of energy cane bagasse. Bioresour Technol 2012;117:251–6. https://doi.org/10.1016/j.biortech.2012.04. 070. [135] Sidik DAB, Ngadi N, Amin NAS. Optimization of lignin production from empty fruit bunch via liquefaction with ionic liquid. Bioresour Technol 2013;135:690–6. https://doi.org/10.1016/j.biortech.2012.09.041. [136] Achinivu EC, Howard RM, Li G, Gracz H, Henderson WA. Lignin extraction from biomass with protic ionic liquids. Green Chem 2014;16:1114–9. https://doi.org/ 10.1039/C3GC42306A. [137] Fu D, Mazza G, Tamaki Y. Lignin extraction from straw by ionic liquids and enzymatic hydrolysis of the cellulosic residues. J Agric Food Chem 2010;58:2915–22. https://doi.org/10.1021/jf903616y. [138] Arora R, Manisseri C, Li C, Ong MD, Scheller HV, Vogel K, et al. Monitoring and analyzing process streams towards understanding ionic liquid pretreatment of switchgrass (Panicum virgatum L.). BioEnergy Res 2010;3:134–45. https://doi. org/10.1007/s12155-010-9087-1. [139] Fu D, Mazza G. Aqueous ionic liquid pretreatment of straw. Bioresour Technol 2011;102:7008–11. https://doi.org/10.1016/j.biortech.2011.04.049. [140] Lynam JG, Toufiq Reza M, Vasquez VR, Coronella CJ. Pretreatment of rice hulls by ionic liquid dissolution. Bioresour Technol 2012;114:629–36. https://doi.org/10. 1016/j.biortech.2012.03.004. [141] Zavrel M, Bross D, Funke M, Büchs J, Spiess AC. High-throughput screening for ionic liquids dissolving (ligno-)cellulose. Bioresour Technol 2009;100:2580–7. https://doi.org/10.1016/j.biortech.2008.11.052. [142] Liu Q-P, Hou X-D, Li N, Zong M-H. Ionic liquids from renewable biomaterials: synthesis, characterization and application in the pretreatment of biomass. Green Chem 2012;14:304. https://doi.org/10.1039/c2gc16128a. [143] Rashid T, Kait CF, Regupathi I, Murugesan T. Dissolution of kraft lignin using Protic Ionic Liquids and characterization. Ind Crop Prod 2016;84:284–93. https:// doi.org/10.1016/j.indcrop.2016.02.017. [144] George A, Brandt A, Tran K, Zahari SMSNS, Klein-Marcuschamer D, Sun N, et al. Design of low-cost ionic liquids for lignocellulosic biomass pretreatment. Green Chem 2015;17:1728–34. https://doi.org/10.1039/C4GC01208A. [145] Glas D, Doorslaer C Van, Depuydt D, Liebner F, Rosenau T, Vos DE De. Lignin solubility in non-imidazolium ionic liquids. J Chem Technol Biotechnol 2015;90:1821–6. https://doi.org/10.1002/jctb.4492. [146] Rinaldi R. Instantaneous dissolution of cellulose in organic electrolyte solutions. Chem Commun 2011;47:511–3. https://doi.org/10.1039/c0cc02421j. [147] Xu A, Guo X, Xu R. Understanding the dissolution of cellulose in 1-butyl-3-methylimidazolium acetate + DMAc solvent. Int J Biol Macromol 2015;81:1000–4. https://doi.org/10.1016/j.ijbiomac.2015.09.058. [148] Xu A, Cao L, Wang B. Facile cellulose dissolution without heating in [C4mim]

[149] [150] [151] [152]

[153]

[154] [155] [156] [157]

[158] [159] [160] [161]

[162] [163]

[164]

[165] [166] [167]

[168]

[169] [170] [171] [172] [173] [174]

290

[CH3COO]/DMF solvent. Carbohydr Polym 2015;125:249–54. https://doi.org/10. 1016/j.carbpol.2015.02.045. Xu A, Zhang Y. Insight into dissolution mechanism of cellulose in [C4mim] [CH3COO]/DMSO solvent by 13C NMR spectra. J Mol Struct 2015;1088:101–4. https://doi.org/10.1016/j.molstruc.2015.02.031. Sun Y-C, Xu J-K, Xu F, Sun R-C, Jones GL, Gao H, et al. Dissolution, regeneration and characterisation of formic acid and Alcell lignin in ionic liquid-based systems. RSC Adv 2014;4:2743–55. https://doi.org/10.1039/C3RA46278A. George A, Brandt A, Tran K, Zahari SMSNS, Klein-Marcuschamer D, Sun N, et al. Design of low-cost ionic liquids for lignocellulosic biomass pretreatment. Green Chem 2015;17:1728–34. https://doi.org/10.1039/C4GC01208A. Shi J, Balamurugan K, Parthasarathi R, Sathitsuksanoh N, Zhang S, Stavila V, et al. Understanding the role of water during ionic liquid pretreatment of lignocellulose: co-solvent or anti-solvent? Green Chem 2014;16:3830–40. https://doi.org/10. 1039/c4gc00373j. Yuan X, Singh S, Simmons BA, Cheng G. Biomass pretreatment using dilute aqueous ionic liquid (IL) solutions with dynamically varying IL concentration and its impact on IL recycling. ACS Sustain Chem Eng 2017;5:4408–13. https://doi.org/ 10.1021/acssuschemeng.7b00480. Wei L, Li K, Ma Y, Hou X. Dissolving lignocellulosic biomass in a 1-butyl-3-methylimidazolium chloride-water mixture. Ind Crops Prod 2012;37:227–34. https:// doi.org/10.1109/ICEAA.2016.7731406. Wang Y, Wei L, Li K, Ma Y, Ma N, Ding S, et al. Lignin dissolution in dialkylimidazolium-based ionic liquid-water mixtures. Bioresour Technol 2014;170:499–505. https://doi.org/10.1016/j.biortech.2014.08.020. Hou X-D, Li N, Zong M-H. Significantly enhancing enzymatic hydrolysis of rice straw after pretreatment using renewable ionic liquid–water mixtures. Bioresour Technol 2013;136:469–74. https://doi.org/10.1016/j.biortech.2013.02.118. Sun YC, Xu JK, Wang B, Xu F, Sun RC. Selective separation and recovery of the lignin from lignocellulosic biomass using ionic liquid. In: Proceedings of 55th International Convention on Society of Wood Science and Technology, 2012, p. 27–31. Weerachanchai P, Lee J. Effect of organic solvent in ionic liquid on biomass pretreatment. ACS Sustain Chem Eng 2013;1:894–902. https://doi.org/10.1021/ sc300147f. Zhao Y, Liu X, Wang J, Zhang S. Insight into the cosolvent effect of cellulose dissolution in imidazolium-based ionic liquid systems. J Phys Chem B 2013;117:9042–9. https://doi.org/10.1021/jp4038039. Xu A, Zhang Y, Zhao Y, Wang J. Cellulose dissolution at ambient temperature: role of preferential solvation of cations of ionic liquids by a cosolvent. Carbohydr Polym 2013;92:540–4. https://doi.org/10.1016/j.carbpol.2012.09.028. Wu L, Lee SH, Endo T. Effect of dimethyl sulfoxide on ionic liquid 1-ethyl-3-methylimidazolium acetate pretreatment of eucalyptus wood for enzymatic hydrolysis. Bioresour Technol 2013;140:90–6. https://doi.org/10.1016/j.biortech.2013. 04.072. Xu A, Cao L, Wang B, Ma J. Dissolution behavior of cellulose in IL+DMSO solvent: effect of alkyl length in imidazolium cation on cellulose dissolution. Adv Mater Sci Eng 2015;2015:1–4. https://doi.org/10.1155/2015/406470. Sun YC, Xu JK, Xu F, Sun RC. Efficient separation and physico-chemical characterization of lignin from eucalyptus using ionic liquid-organic solvent and alkaline ethanol solvent. Ind Crops Prod 2013;47:277–85. https://doi.org/10.1016/ j.indcrop.2013.03.025. Liang L, Li C, Xu F, He DQ, Yan J, Luong T, et al. Conversion of cellulose rich municipal solid waste blends using ionic liquids: feedstock convertibility and process scale-up. RSC Adv 2017;7:36585–93. https://doi.org/10.1039/ C7RA06701A. Li C, Tanjore D, He W, Wong J, Gardner JL, Sale KL, et al. Scale-up and evaluation of high solid ionic liquid pretreatment and enzymatic hydrolysis of switchgrass. Biotechnol Biofuels 2013;6:154. https://doi.org/10.1186/1754-6834-6-154. Li C, Tanjore D, He W, Wong J, Gardner JL, Thompson VS, et al. Scale-up of ionic liquid-based fractionation of single and mixed feedstocks. Bioenergy Res 2015;8:982–91. https://doi.org/10.1007/s12155-015-9587-0. Li C, Liang L, Sun N, Thompson VS, Xu F, Narani A, et al. Scale-up and process integration of sugar production by acidolysis of municipal solid waste/corn stover blends in ionic liquids. Biotechnol Biofuels 2017;10:13. https://doi.org/10.1186/ s13068-016-0694-8. Perez-pimienta JA, Flores-gómez CA, Ruiz HA, Sathitsuksanoh N, Balan V, Sousa C, et al. Evaluation of agave bagasse recalcitrance using AFEX TM, autohydrolysis, and ionic liquid pretreatments. Bioresour Technol 2016;211:216–23. https://doi. org/10.1016/j.biortech.2016.03.103. Gardner J, He W, Li C, Wong J, Sale K, Simmons B, et al. Calorimetric evaluation indicates that lignin conversion to advanced biofuels is vital to improving energy yields. RSC Adv 2015;5:51092–101. https://doi.org/10.1039/C5RA01503K. Wahlström RM, Suurnäkki A. Enzymatic hydrolysis of lignocellulosic polysaccharides in the presence of ionic liquids. Green Chem 2015;17:694–714. https://doi.org/10.1039/c4gc01649a. Parveen F, Patra T, Upadhyayula S. Hydrolysis of microcrystalline cellulose using functionalized Bronsted acidic ionic liquids - a comparative study. Carbohydr Polym 2016;135:280–4. https://doi.org/10.1016/j.carbpol.2015.08.039. Liu Y, Xiao W, Xia S, Ma P. SO3H-functionalized acidic ionic liquids as catalysts for the hydrolysis of cellulose. Carbohydr Polym 2013;92:218–22. https://doi.org/10. 1016/j.carbpol.2012.08.095. Zhuo K, Du Q, Bai G, Wang C, Chen Y, Wang J. Hydrolysis of cellulose catalyzed by novel acidic ionic liquids. Carbohydr Polym 2015;115:49–53. https://doi.org/10. 1016/j.carbpol.2014.08.078. Jiang F, Zhu Q, Ma D, Liu X, Han X. Direct conversion and NMR observation of

Renewable and Sustainable Energy Reviews 105 (2019) 268–292

P. Halder et al.

[175]

[176]

[177] [178] [179] [180] [181]

[182] [183] [184]

[185] [186] [187]

[188]

[189] [190]

[191] [192]

[193] [194] [195] [196]

[197]

[198]

[199]

cellulose to glucose and 5-hydroxymethylfurfural (HMF) catalyzed by the acidic ionic liquids. J Mol Catal A Chem 2011;334:8–12. https://doi.org/10.1016/j. molcata.2010.10.006. Qu Y, Huang C, Song Y, Zhang J, Chen B. Efficient dehydration of glucose to 5hydroxymethylfurfural catalyzed by the ionic liquid, 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate. Bioresour Technol 2012;121:462–6. https://doi. org/10.1016/j.biortech.2012.06.081. Lima S, Neves P, Antunes M, Pillinger M, Ignatyev N, Valente A. Conversion of mono/di/polysaccharides into furan compounds using 1-alkyl-3-methylimidazolium ionic liquids. Appl Catal A Gen 2009;363:93–9. https://doi.org/10.1016/j. apcata.2009.04.049. Tao F, Zhuang C, Cui Y, Xu J. Dehydration of glucose into 5-hydroxymethylfurfural in SO3H-functionalized ionic liquids. Chin Chem Lett 2014;25:757–61. https://doi.org/10.1016/j.cclet.2014.01.044. Ren H, Zhou Y, Liu L. Selective conversion of cellulose to levulinic acid via microwave-assisted synthesis in ionic liquids. Bioresour Technol 2013;129:616–9. https://doi.org/10.1016/j.biortech.2012.12.132. Ren H, Girisuta B, Zhou Y, Liu L. Selective and recyclable depolymerization of cellulose to levulinic acid catalyzed by acidic ionic liquid. Carbohydr Polym 2015;117:569–76. https://doi.org/10.1016/j.carbpol.2014.09.091. Shen Y, Sun JK, Yi YX, Wang B, Xu F, Sun RC. One-pot synthesis of levulinic acid from cellulose in ionic liquids. Bioresour Technol 2015;192:812–6. https://doi. org/10.1016/j.biortech.2015.05.080. Ding ZD, Shi JC, Xiao JJ, Gu WX, Zheng CG, Wang HJ. Catalytic conversion of cellulose to 5-hydroxymethyl furfural using acidic ionic liquids and co-catalyst. Carbohydr Polym 2012;90:792–8. https://doi.org/10.1016/j.carbpol.2012.05. 083. Matsagar BM, Dhepe PL. Effects of cations, anions and H+ concentration of acidic ionic liquids on the valorization of polysaccharides into furfural. New J Chem 2017;41:6137–44. https://doi.org/10.1039/c7nj00342k. Hui W, Zhou Y, Dong Y, Cao Z-J, He F-Q, Cai M-Z, et al. Efficient hydrolysis of hemicellulose to furfural by novel superacid SO4H-functionalized ionic liquids. Green Energy Environ 2018:1–7. https://doi.org/10.1016/j.gee.2018.06.002. Khan AS, Man Z, Bustam MA, Nasrullah A, Ullah Z, Sarwono A, et al. Efficient conversion of lignocellulosic biomass to levulinic acid using acidic ionic liquids. Carbohydr Polym 2018;181:208–14. https://doi.org/10.1016/j.carbpol.2017.10. 064. Liu L, Li Z, Hou W, Shen H. Direct conversion of lignocellulose to levulinic acid catalyzed by ionic liquid. Carbohydr Polym 2018;181:778–84. https://doi.org/10. 1016/j.carbpol.2017.11.078. Alam MI, De S, Dutta S, Saha B. Solid-acid and ionic-liquid catalyzed one-pot transformation of biorenewable substrates into a platform chemical and a promising biofuel. RSC Adv 2012;2:6890–6. https://doi.org/10.1039/c2ra20574b. Tiong YW, Yap CL, Gan S, Yap WSP. One-pot conversion of oil palm empty fruit bunch and mesocarp fiber biomass to levulinic acid and upgrading to ethyl levulinate via indium trichloride-ionic liquids. J Clean Prod 2017;168:1251–61. https://doi.org/10.1016/j.jclepro.2017.09.050. Matsagar BM, Hossain SA, Islam T, Alamri HR, Alothman ZA, Yamauchi Y, et al. Direct production of furfural in one-pot fashion from raw biomass using Brønsted acidic ionic liquids. Sci Rep 2017;7:4–10. https://doi.org/10.1038/s41598-01713946-4. Hu X, Xiao Y, Niu K, Zhao Y, Zhang B, Hu B. Functional ionic liquids for hydrolysis of lignocellulose. Carbohydr Polym 2013;97:172–6. https://doi.org/10.1016/j. carbpol.2013.04.061. Amarasekara AS, Shanbhag P. Degradation of untreated switchgrass biomass into reducing sugars in 1-(alkylsulfonic)-3-methylimidazolium Brönsted acidic ionic liquid medium under mild conditions. Bioenergy Res 2013;6:719–24. https://doi. org/10.1007/s12155-012-9291-2. Li C, Wang Q, Zhao ZK. Acid in ionic liquid: an efficient system for hydrolysis of lignocellulose. Green Chem 2008;10:177–82. https://doi.org/10.1039/b711512a. Kuroda K, Inoue K, Miyamura K, Takada K, Ninomiya K, Takahashi K. Enhanced hydrolysis of lignocellulosic biomass assisted by a combination of acidic ionic liquids and microwave heating. J Chem Eng Jpn 2016;49:809–13. https://doi.org/ 10.1252/jcej.15we292. Matsagar BM, Dhepe PL. Brönsted acidic ionic liquid-catalyzed conversion of hemicellulose into sugars. Catal Sci Technol 2015;5:531–9. https://doi.org/10. 1039/c4cy01047g. Da Costa Lopes AM, João KG, Bogel-ŁUkasik E, Roseiro LB, Bogel-ŁUkasik R. Pretreatment and fractionation of wheat straw using various ionic liquids. J Agric Food Chem 2013;61:7874–82. https://doi.org/10.1021/jf401980p. Da Costa Lopes AM, Lins RMG, Rebelo RA, Łukasik RM. Biorefinery approach for lignocellulosic biomass valorisation with an acidic ionic liquid. Green Chem 2018;20:4043–57. https://doi.org/10.1039/c8gc01763h. Carvalho AV, Da Costa Lopes AM, Bogel-ŁUkasik R. Relevance of the acidic 1butyl-3-methylimidazolium hydrogen sulphate ionic liquid in the selective catalysis of the biomass hemicellulose fraction. RSC Adv 2015;5:47153–64. https:// doi.org/10.1039/c5ra07159c. Cox BJ, Ekerdt JG. Depolymerization of oak wood lignin under mild conditions using the acidic ionic liquid 1-H-3-methylimidazolium chloride as both solvent and catalyst. Bioresour Technol 2012;118:584–8. https://doi.org/10.1016/j. biortech.2012.05.012. Cox BJ, Jia S, Zhang ZC, Ekerdt JG. Catalytic degradation of lignin model compounds in acidic imidazolium based ionic liquids: Hammett acidity and anion effects. Polym Degrad Stab 2011;96:426–31. https://doi.org/10.1016/j. polymdegradstab.2011.01.011. Kishimoto T, Sano Y. Delignification mechanism during high-boiling solvent

[200] [201] [202] [203] [204] [205] [206]

[207] [208] [209] [210] [211] [212] [213]

[214] [215] [216]

[217] [218] [219] [220]

[221] [222] [223] [224] [225] [226]

[227]

291

pulping. part 1. reaction of guaiacylglycerol-β-guaiacyl ether. Holzforschung 2001;55:611–6. https://doi.org/10.1515/HF.2001.100. Li S, Lundquist K. Acid reactions of lignin models of β-5 type. Holzforschung 1999;53:39–42. https://doi.org/10.1515/HF.1999.007. Likhtenshtein G. Stilbenes preparation and analysis. Stilbenes: applications in chemistry, life sciences and materials science. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA; 2009. p. 1–41. Çetinkol ÖP, Dibble DC, Cheng G, Kent MS, Knierim B, Auer M, et al. Understanding the impact of ionic liquid pretreatment on eucalyptus. Biofuels 2010;1:33–46. https://doi.org/10.4155/bfs.09.5. Cammarata L, Kazarian SG, Salter PA, Welton T, Savelli G, Bunton CA. Molecular states of water in room temperature ionic liquids. Phys Chem Chem Phys 2001;3:5192–200. https://doi.org/10.1039/b106900d. Zhang S, Wang J, Lu X, Zhou Q. Structures and interactions of ionic liquids 151. United Kingdom: Springer; 2014. https://doi.org/10.1007/978-3-642-38619-0. Li Y, Liu X, Zhang S, Yao Y, Yao X, Xu J, et al. Dissolving process of a cellulose bunch in ionic liquids: a molecular dynamics study. Phys Chem Chem Phys 2015;17:17894–905. https://doi.org/10.1039/c5cp02009c. El Seoud OA, da Silva VC, Possidonio S, Casarano R, Arêas EPG, Gimenes P. Microwave-assisted derivatization of cellulose, 2-the surprising effect of the structure of ionic liquids on the dissolution and acylation of the biopolymer. Macromol Chem Phys 2011;212:2541–50. https://doi.org/10.1002/macp. 201100348. Mäki-Arvela P, Anugwom I, Virtanen P, Sjöholm R, Mikkola JP. Dissolution of lignocellulosic materials and its constituents using ionic liquids—a review. Ind Crops Prod 2010;32:175–201. https://doi.org/10.1016/j.indcrop.2010.04.005. Brandt A, Hallett JP, Leak DJ, Murphy RJ, Welton T. The effect of the ionic liquid anion in the pretreatment of pine wood chips. Green Chem 2010;12:672–9. https://doi.org/10.1039/b918787a. Crowhurst L, Mawdsley PR, Perez-Arlandis JM, Salter PA, Welton T. Solvent–solute interactions in ionic liquids. Phys Chem Chem Phys 2003;5:2790–4. https://doi.org/10.1039/B303095D. Fukaya Y, Sugimoto A, Ohno H. Superior solubility of polysaccharides in low viscosity, polar, and halogen-free 1,3-dialkylimidazolium formates. Biomacromolecules 2006;7:3295–7. https://doi.org/10.1021/BM060327D. Fukaya Y, Hayashi K, Wada M, Ohno H, Welton T, Stone MO, et al. Cellulose dissolution with polar ionic liquids under mild conditions: required factors for anions. Green Chem 2008;10:44–6. https://doi.org/10.1039/B713289A. Wu Y, Sasaki T, Kazushi K, Seo T, Sakurai K. Interactions between Sspiropyrans and room-temperature ionic liquids: photochromism and solvatochromism. J Phys Chem B 2008;112:7530–6. https://doi.org/10.1021/jp800957c. Weerachanchai P, Wong Y, Lim KH, Tan TTY, Lee JM. Determination of solubility parameters of ionic liquids and ionic liquid/solvent mixtures from intrinsic viscosity. ChemPhysChem 2014;15:3580–91. https://doi.org/10.1002/cphc. 201402345. Chen X, Yuan C, Wong CKY, Zhang G. Molecular modeling of temperature dependence of solubility parameters for amorphous polymers. J Mol Model 2012;18:2333–41. https://doi.org/10.1007/s00894-011-1249-3. Lee SH, Lee SB. The Hildebrand solubility parameters, cohesive energy densities and internal energies of 1-alkyl-3-methylimidazolium-based room temperature ionic liquids. Chem Commun 2005:3469–71. https://doi.org/10.1039/b503740a. Weerachanchai P, Chen Z, Leong SSJ, Chang MW, Lee JM. Hildebrand solubility parameters of ionic liquids: effects of ionic liquid type, temperature and DMA fraction in ionic liquid. Chem Eng J 2012;213:356–62. https://doi.org/10.1016/j. cej.2012.10.012. Wong Y, Chen ZJ, Tan TTY, Lee JM. Hildebrand solubility parameters of amidium ionic liquids. Ind Eng Chem Res 2015;54:12150–5. https://doi.org/10.1021/acs. iecr.5b02705. Agata Y, Yamamoto H. Determination of Hansen solubility parameters of ionic liquids using double-sphere type of Hansen solubility sphere method. Chem Phys 2018;513:165–73. https://doi.org/10.1016/j.chemphys.2018.04.021. Abbott S, Hansen CM, Yamamoto H. Hansen solubility parameters in practice; 2013. Suzuki S, Takeoka Y, Rikukawa M, Yoshizawa-Fujita M. Brønsted acidic ionic liquids for cellulose hydrolysis in an aqueous medium: structural effects on acidity and glucose yield. RSC Adv 2018;8:14623–32. https://doi.org/10.1039/ c8ra01950a. Sun Z, Cheng M, Li H, Shi T, Yuan M, Wang X, et al. One-pot depolymerization of cellulose into glucose and levulinic acid by heteropolyacid ionic liquid catalysis. RSC Adv 2012;2:9058–65. https://doi.org/10.1039/c2ra01328b. Gu T. Pretreatment of Lignocellulosic Biomass Using Green Ionic Liquids. Green Biomass Pretreat. Biofuels Prod., 2013, p. 107–25. doi:10.1007/978-94-0076052-3. Kilpeläinen I, Xie H, King A, Granstrom M, Heikkinen S, Argyropoulos DS. Dissolution of wood in ionic liquids. J Agric Food Chem 2007;55:9142–8. https:// doi.org/10.1021/jf071692e. Xu A, Zhang Y, Lu W, Yao K, Xu H. Effect of alkyl chain length in anion on dissolution of cellulose in 1-butyl-3-methylimidazolium carboxylate ionic liquids. J Mol Liq 2014;197:211–4. https://doi.org/10.1016/j.molliq.2014.05.018. Wang H, Gurau G, Rogers RD. Ionic liquid processing of cellulose. Chem Soc Rev 2012;41:1519–37. https://doi.org/10.1039/c2cs15311d. Xu A, Wang J, Wang H, Sheldon RA, Rogers RD, Ii T, et al. Effects of anionic structure and lithium salts addition on the dissolution of cellulose in 1-butyl-3methylimidazolium-based ionic liquid solvent systems. Green Chem 2010;12:268–75. https://doi.org/10.1039/B916882F. Yan B, Li K, Wei L, Ma Y, Shao G, Zhao D, et al. Understanding lignin treatment in

Renewable and Sustainable Energy Reviews 105 (2019) 268–292

P. Halder et al.

[228]

[229] [230] [231] [232] [233]

[234] [235] [236] [237] [238] [239] [240]

[241] [242] [243] [244] [245]

[246] [247] [248] [249] [250] [251]

dialkylimidazolium-based ionic liquid-water mixtures. Bioresour Technol 2015;196:509–17. https://doi.org/10.1016/j.biortech.2015.08.005. Besombes S, Utille J-P, Mazeau K, Robert D, Taravel FR. Conformational study of a guaiacylβ-O-4 lignin model compound by NMR. Examination of intramolecular hydrogen bonding interactions and conformational flexibility in solution. Magn Reson Chem 2004;42:337–47. https://doi.org/10.1002/mrc.1317. Ji W, Ding Z, Liu J, Song Q, Xia X, Gao H, et al. Mechanism of lignin dissolution and regeneration in ionic liquid. Energy Fuels 2012;26:6393–403. https://doi.org/ 10.1021/ef301231a. Wang J, Shi X, Du X, Cao W. A DFT investigation on interactions between lignin and ionic liquids. Russ J Phys Chem A 2017;91:1468–73. https://doi.org/10. 1134/S0036024417080155. Liu H, Sale KL, Holmes BM, Simmons BA, Singh S. Understanding the interactions of cellulose with ionic liquids: a molecular dynamics study. J Phys Chem B 2010;114:4293–301. https://doi.org/10.1021/jp9117437. Zhu Y, Yan J, Liu C, Zhang D. Modeling interactions between a β-O-4 type lignin model compound and 1-allyl-3-methylimidazolium chloride ionic liquid. Biopolymers 2017;107:e23022. https://doi.org/10.1002/bip.23022. Parthasarathi R, Sun J, Dutta T, Sun N, Pattathil S, Murthy Konda NVSN, et al. Activation of lignocellulosic biomass for higher sugar yields using aqueous ionic liquid at low severity process conditions. Biotechnol Biofuels 2016;9:160. https:// doi.org/10.1186/s13068-016-0561-7. Johnson AI, Huang C-J. Mass transfer studies in an agitated vessel. AIChE J 1956;2:412–9. https://doi.org/10.1002/aic.690020322. Bong E. Solid-liquid mass transfer in agitated vessels with high solids concentration. RMIT University; 2013. Hyman D. Mixing and agitation. Adv Chem Eng 1962:119–202. Yao J, Lao DB, Sui X, Zhou Y, Nune SK, Ma X, et al. Two coexisting liquid phases in switchable ionic liquids. Phys Chem Chem Phys 2017;19:22627–32. https://doi. org/10.1039/C7CP03754F. Jessop PG, Heldebrant DJ, Li X, Eckert CA, Liotta CL. Green chemistry: reversible nonpolar-to-polar solvent. Nature 2005;436:1102. https://doi.org/10.1038/ 4361102a. Eta V, Anugwom I, Virtanen P, Eränen K, Mäki-Arvela P, Mikkola JP. Loop vs. batch reactor setups in the fractionation of birch chips using switchable ionic liquids. Chem Eng J 2014;238:242–8. https://doi.org/10.1016/j.cej.2013.05.103. Anugwom I, Rujana L, Wärnå J, Hedenström M, Mikkola JP. In quest for the optimal delignification of lignocellulosic biomass using hydrated, SO2 switched DBU MEASIL switchable ionic liquid. Chem Eng J 2016;297:256–64. https://doi.org/ 10.1016/j.cej.2016.03.103. Shill K, Miller K, Clark DS, Blanch HW. A model for optimizing the enzymatic hydrolysis of ionic liquid-pretreated lignocellulose. Bioresour Technol 2012;126:290–7. https://doi.org/10.1016/j.biortech.2012.08.062. McCabe WL, Smith JC, Harriot P. Unit operations of chemical engineering. 5th ed. McGraw-Hill Chemical Engineering Series; 1993. https://doi.org/10.1016/00092509(57)85034-9. Cruz AG, Scullin C, Mu C, Cheng G, Stavila V, Varanasi P, et al. Impact of high biomass loading on ionic liquid pretreatment. Biotechnol Biofuels 2013;6:52. https://doi.org/10.1186/1754-6834-6-52. Baral NR, Shah A. Techno-economic analysis of cellulose dissolving ionic liquid pretreatment of lignocellulosic biomass for fermentable sugars production. Biofuels, Bioprod Bioref 2016;10:70–88. https://doi.org/10.1002/bbb. Oleskowicz-Popiel P, Klein-Marcuschamer D, Simmons BA, Blanch HW. Lignocellulosic ethanol production without enzymes - technoeconomic analysis of ionic liquid pretreatment followed by acidolysis. Bioresour Technol 2014;158:294–9. https://doi.org/10.1016/j.biortech.2014.02.016. Brandt-Talbot A, Gschwend FJV, Fennell PS, Lammens TM, Tan B, Weale J, et al. An economically viable ionic liquid for the fractionation of lignocellulosic biomass. Green Chem 2017;19:3078–102. https://doi.org/10.1039/C7GC00705A. Sen SM, Binder BJ, Raines RT, Maravelias TC. Conversion of biomass to sugars via ionic liquid hydrolysis: process synthesis and economic evaluation. Biofuels, Bioprod Bioref 2012;6:444–52. https://doi.org/10.1002/bbb. Binder JB, Raines RT. Fermentable sugars by chemical hydrolysis of biomass. Proc Natl Acad Sci USA 2010:4516–21. https://doi.org/10.1073/pnas.0912073107. Fu D, Mazza G. Optimization of processing conditions for the pretreatment of wheat straw using aqueous ionic liquid. Bioresour Technol 2011;102:8003–10. https://doi.org/10.1016/j.biortech.2011.06.023. Mihail R, Sin G. Dynamic modeling and validation of a lignocellulosic enzymatic hydrolysis process – a demonstration scale study. Bioresour Technol 2013;150:393–403. https://doi.org/10.1016/j.biortech.2013.10.029. Shi J, Gladden JM, Sathitsuksanoh N, Kambam P, Sandoval L, Mitra D, et al. One-

[252] [253] [254] [255] [256] [257]

[258] [259] [260]

[261] [262] [263] [264] [265] [266] [267] [268] [269] [270] [271]

[272] [273] [274] [275]

292

pot ionic liquid pretreatment and saccharification of switchgrass. Green Chem 2013;15:2579. https://doi.org/10.1039/c3gc40545a. Shi J, Thompson VS, Yancey NA, Stavila V, Simmons BA, Singh S. Impact of mixed feedstocks and feedstock densification on ionic liquid pretreatment efficiency. Biofuels 2013;4:63–72. https://doi.org/10.4155/bfs.12.82. Felder R., Rousseau R. Elementary Principles of Chemical Processes. 2005. doi:10. 1017/CBO9781107415324.004. Zhu JY, Pan XJ. Woody biomass pretreatment for cellulosic ethanol production: technology and energy consumption evaluation. Bioresour Technol 2010;101:4992–5002. https://doi.org/10.1016/j.biortech.2009.11.007. Yoon LW, Ngoh GC, May Chua AS, Hashim MA. Comparison of ionic liquid, acid and alkali pretreatments for sugarcane bagasse enzymatic saccharification. J Chem Technol Biotechnol 2011;86:1342–8. https://doi.org/10.1002/jctb.2651. Ang TN, Yoon LW, Lee KM, Ngoh GC, Chua ASM, Lee MG. Efficiency of ionic liquids in the dissolution of rice husk. BioResources 2011;6:4790–800. https://doi. org/10.15376/biores.6.4.4790-4800. Mahmood H, Moniruzzaman M, Yusup S, Muhammad N, Iqbal T, Akil HM. Ionic liquids pretreatment for fabrication of agro-residue/thermoplastic starch based composites: a comparative study with other pretreatment technologies. J Clean Prod 2017;161:257–66. https://doi.org/10.1016/j.jclepro.2017.05.110. Mai NL, Ahn K, Koo Y. Methods for recovery of ionic liquids—a review. Process Biochem 2014;49:872–81. https://doi.org/10.1016/j.procbio.2014.01.016. Xu J, Liu B, Hou H, Hu J. Pretreatment of eucalyptus with recycled ionic liquids for low-cost biorefinery. Bioresour Technol 2017;234:406–14. https://doi.org/10. 1016/j.biortech.2017.03.081. Konda NM, Shi J, Singh S, Blanch HW, Simmons B a, Klein-Marcuschamer D. Understanding cost drivers and economic potential of two variants of ionic liquid pretreatment for cellulosic biofuel production. Biotechnol Biofuels 2014;7:86. https://doi.org/10.1186/1754-6834-7-86. Wright M, Brown RC. Establishing the optimal sizes of different kinds of biorefineries. Biofuels, Bioprod Bioref 2007;1:191–200. https://doi.org/10.1002/bbb. Jenkins BM. A comment on the optimal sizing of a biomass utilization facility under constant and variable cost scaling. Biomass- Bioenergy 1997;13:1–9. https://doi.org/10.1016/S0961-9534(97)00085-8. Nguyen MH, Prince RGH. A simple rule for bioenergy conversion plant size optimisation: bioethanol from sugar cane and sweet sorghum. Biomass- Bioenergy 1996;10:361–5. https://doi.org/10.1016/0961-9534(96)00003-7. Towler G, Sinnott R. Chemical engineering design, principles, practice, and economics of plant design. Elsevier; 2008. Klein-Marcuschamer D, Simmons BA, Blanch HW. Techno-economic analysis of a lignocellulosic ethanol biorefinery with ionic liquid pre-treatment. Biofuels, Bioprod Bioref 2011;5:562–9. https://doi.org/10.1002/bbb.303. Settanni G, Zhou J, Suo T, Schöttler S, Landfester K, Schmid F, et al. Inexpensive ionic liquids: [HSO4]–based solvent production at bulk scale. Green Chem 2014;16:3098–106. https://doi.org/10.1039/x0xx00000x. Zhang Q, Hu J, Lee DJ. Pretreatment of biomass using ionic liquids: research updates. Renew Energy 2017;111:77–84. https://doi.org/10.1016/j.renene.2017. 03.093. Yoo CG, Pu Y, Ragauskas AJ. Ionic liquids: promising green solvents for lignocellulosic biomass utilization. Curr Opin Green Sustain Chem 2017;5:5–11. https://doi.org/10.1016/j.cogsc.2017.03.003. Badgujar KC, Bhanage BM. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: current status, overview and challenges. Bioresour Technol 2015;178:2–18. https://doi.org/10.1016/j.biortech.2014.09.138. Reddy P. A critical review of ionic liquids for the pretreatment of lignocellulosic biomass. S Afr J Sci 2015;111:1–9. https://doi.org/10.17159/sajs.2015/ 20150083. Liang X, Fu Y, Chang J. Research on the quick and efficient recovery of 1-allyl-3methylimidazolium chloride after biomass pretreatment with ionic liquid-aqueous alcohol system. Bioresour Technol 2017;245:760–7. https://doi.org/10.1016/j. biortech.2017.08.145. Zhou J, Sui H, Jia Z, Yang Z, He L, Li X. Recovery and purification of ionic liquids from solutions: a review. RSC Adv 2018;8:32832–64. https://doi.org/10.1039/ c8ra06384b. Forsyth SA, Pringle JM, MacFarlane DR. Ionic liquids—an overview. Aust J Chem 2004;57:113. https://doi.org/10.1071/CH03231. Karunanithi AT, Mehrkesh A. Computer-aided design of tailor-made ionic liquids. AIChE J 2013;59:4627–40. https://doi.org/10.1002/aic.14228. Mai NL, Koo Y-M. Computer-aided design of ionic liquids for high cellulose dissolution. ACS Sustain Chem Eng 2016;4:541–7. https://doi.org/10.1021/ acssuschemeng.5b00958.