Homogeneous esterification of cellulose in room temperature ionic liquids: Homogeneous esterification of cellulose

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ReviewReceived: 12 January 2015 Accepted article published: 10 February 2015 Published online in Wiley Online Library: 2 April 2015

(wileyonlinelibrary.com) DOI 10.1002/pi.4883

Homogeneous esterification of cellulose inroom temperature ionic liquidsJinming Zhang, Weiwei Chen, Ye Feng, Jin Wu, Jian Yu,Jiasong He and Jun Zhang*

Abstract

Homogeneous esterification of cellulose, which can control the molecular structure of the resultant cellulose esters and facilitatethe versatility of cellulose-based materials, has drawn much attention. Recently, the advent of room temperature ionic liquids(ILs) capable of dissolving cellulose has provided a new and versatile platform for the efficient and homogeneous esterification ofcellulose. A variety of conventional, novel and functional cellulose esters have been successfully synthesized in ILs. Meanwhile,development of esterification techniques, utilization of agricultural residues and up-scaling of pilot schemes of esterificationin ILs have been in progress. This review summarizes the advances and developments in homogeneous synthesis of celluloseesters in ILs in recent decades.© 2015 Society of Chemical Industry

Keywords: cellulose ester; ionic liquid; homogeneous esterification; derivatization of cellulose

INTRODUCTIONAs a major constituent in plants, cellulose is an almost inex-haustible renewable resource with fascinating structure andproperties.1 It has excellent mechanical and chemical propertiescombined with the features of a bioresource, e.g. biodegrad-ability, biocompatibility and sustainability. More importantly, itsphysicochemical properties can be dramatically modified throughsubstitution reactions of its hydroxyl groups.2 Cellulose esters ofinorganic and organic acids are pioneer compounds of cellulosechemistry and important cellulose derivatives with a wide rangeof applications in coatings, plastics, films, fibers, explosives, sepa-ration media and cigarette filters, as well as medical applicationssuch as dialysis, tissue and bone engineering, wound dressing andjoint replacement.3,4 However, the presence of numerous hydroxylgroups in cellulose also generates a well-developed intra- andintermolecular hydrogen bonding network, and thus natural cel-lulose is neither meltable nor soluble in conventional solvents. Atpresent, the preparation method for cellulose esters begins witha heterogeneous reaction between cellulose and a large excessof acylation reagent in the presence of pyridine or sulfuric acid ascatalyst. These heterogeneous reactions cause some problems,such as uneven distribution of substituents, side reactions, beingtime-consuming and with considerable degradation of cellulose.5

Furthermore, it is impossible to control directly the synthesisprocess and chemical structure of the resultant cellulose esters.

Currently, homogeneous esterification of cellulose in appropri-ate media, the alternative procedure to heterogeneous processes,is one focus in cellulose chemistry.5 It not only provides oppor-tunities to obtain effective control of the degree of substitution(DS), distribution and uniformity of the functional groups alongcellulose chains, but also creates more options to induce noveland functional groups. Recently, as new kinds of aprotic polar sol-vents, room temperature ionic liquids (ILs), which permit efficientand homogeneous synthesis of cellulose esters in solution, present

a new and versatile platform for cellulose chemists.6 – 8 Almostall conventional cellulose esters and a variety of novel and func-tional cellulose esters, especially partially and regioselectively sub-stituted cellulose esters, have been successfully synthesized usingILs as reaction media. Meanwhile, development of esterificationtechniques, utilization of agricultural residues and up-scaling ofpilot schemes of esterification in ILs have made a lot of progress.

Herein, the progresses in homogeneous esterification of cellu-lose in ILs during recent decades are summarized. The general syn-thesis route and an overview of cellulose esters obtained in ILs areshown in Fig. 1. In addition, the perspective of the application ofILs in the cellulose industry in the future is briefly discussed.

CELLULOSE ALIPHATIC ESTERSAn initial success of homogeneous esterification of cellulosein ILs was reported in 2004.9 In 1-allyl-3-methylimidazoliumchloride (AmimCl), acetylation of cellulose with acetic anhy-dride was accomplished without using any catalysts, andcellulose acetates (CAs) with a wide range of DS values from0.94 to 2.74 were obtained using only one step. It was easy tocontrol the DS value by stoichiometric method, temperatureand reaction time, especially the latter two factors. The syn-thesized CAs were soluble in common organic solvents such

∗ Correspondence to: Jun Zhang, Beijing National Laboratory for Molec-ular Sciences, CAS Key Laboratory of Engineering Plastics, Institute ofChemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China.E-mail: [email protected]

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory ofEngineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS),Beijing 100190, China

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www.soci.org J Zhang et al.

Dr Jinming Zhang is an associate pro-fessor of polymer science at the Labo-ratory of Engineering Plastics, Instituteof Chemistry, Chinese Academy of Sci-ences (ICCAS). He received his PhD atICCAS in 2010. His areas of research arefunctional materials based on naturalpolysaccharides, novel polysaccharidederivatives and green synthesis meth-ods with ionic liquids.

Weiwei Chen received her bachelordegree from the College of Chemistryand Materials Science, Northwest Uni-versity in 2010. Since 2011, she hasbeen studying for a PhD under thedirection of Prof. Jun Zhang at the Insti-tute of Chemistry, Chinese Academy ofSciences. Her current research concernsthe development of chiral recognitionmaterials using polysaccharides.

Ye Feng is a PhD candidate at the Insti-tute of Chemistry, Chinese Academyof Sciences (ICCAS). Her research inter-ests are focused on cellulose-basedfunctional materials. She received herundergraduate education at HebeiUniversity, before moving to ICCASto work with Professor Jun Zhangon her PhD project, concentratingon cellulose-based gas separationmembranes.

Dr Jin Wu is an associate professor ofpolymer science at the Laboratory ofEngineering Plastics, Institute of Chem-istry, Chinese Academy of Sciences(ICCAS). He received his PhD at ICCAS in2006. His areas of research are synthesisof new ionic liquids, physical processesof natural polysaccharides with ionicliquids and high-performance fibersand films.

Dr Jian Yu is an associate profes-sor of polymer science at the Lab-oratory of Engineering Plastics, Insti-tute of Chemistry, Chinese Academyof Sciences (ICCAS). He received hisPhD at ICCAS in 1999. His researchinterests include polymer crystalliza-tion and microcellular foaming undersupercritical CO2, regenerated naturalpolymer membranes with ionic liquidsand cellulose-based aerogels.

as dimethylsulfoxide (DMSO), acetone and chloroform, dependingon the DS values. Compared with the heterogeneous process, thishomogeneous process possessed several obvious advantages,

Dr Jiasong He is a professor at the Insti-tute of Chemistry, Chinese Academyof Sciences. His research is mainly onpolymeric materials and fundamen-tal aspects involved, with over 180peer-reviewed journal articles andbook chapters. He was the recipientof the Paul J. Flory Polymer ResearchPrize in 2008. He has recently actedas Associate Editor, Polymer Interna-tional; Titular Member, International Union of Pure and AppliedChemistry Polymer Division (IUPAC PD); and co-chairman ofthe Subcommittee on Structure and Properties of CommercialPolymers, IUPAC PD.

Dr Jun Zhang is a full professor ofpolymer science at the Laboratoryof Engineering Plastics, Institute ofChemistry, Chinese Academy of Sci-ences. His research interests includeprocessing and functionalization ofnatural polymers, physics and chem-istry of cellulose, ionic liquids andtheir applications in polymer materialsand high-performance polymers andpolymer composites.

such as simple, catalyst-free, rapid, DS value-controllable andsolvent recyclable. Subsequently, a series of homogeneous syn-theses of cellulose aliphatic esters in ILs were demonstrated.Heinze’s group10 – 13 synthesized almost all cellulose aliphaticesters with monofunctional groups in good yield, such as cel-lulose acetate, propionate, butyrate, pentanoate, hexanoateand laurate, in five ILs, namely 1-butyl-3-methylimidazoliumchloride (BmimCl), 1-ethyl-3-methylimidazolium chloride(EmimCl), 1-butyl-2,3-dimethylimidazolium chloride (BdmimCl),1-allyl-2,3-dimethylimidazolium bromide (AdmimBr) and1-butyl-3-methylpyridinium chloride (BmpyCl). All reactions werecarried out under mild conditions, with low excess of reagent andin a short reaction time. It was also pointed out that, in ILs, acylchloride was more efficient than acid anhydride, and the reactivityof cellulose in different ILs was increased in the order: BmpyCl <BmimCl < AdmimBr < BdmimCl < EmimCl. King et al.14 and Huanget al.15 introduced two long-chain fatty acids into cellulose chainsby homogeneous esterification in AmimCl and BmimCl, respec-tively. The resultant cellulose decanoate and stearate with DSabove 2.0 exhibited good solubility in nonpolar organic solvents,such as chloroform, toluene and hexane.

Among the cellulose aliphatic esters, the synthesis of cellulosepropionate (CP) and cellulose butyrate (CB) with relatively highDS is very difficult without a catalyst in ILs. For example, themaximum DS values of CP and CB obtained in BmimCl were 0.9and 0.4.13 Our group found that 4-dimethylaminopyridine (DMAP)was an effective catalyst for both propionylation and butyrylationof cellulose in AmimCl.16 At 30 ∘C, CP and CB with DS from 0.89 to2.89 were synthesized within only 30 min. The conversion of acidanhydrides was as high as 90%, even 96%. Recently, Yang et al.17

used a mixture of a novel CO2-based reversible IL as the solventof cellulose. The rapid and efficient syntheses of CA, CP and CBwere successfully accomplished under mild conditions without theaddition of other catalysts.

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Homogeneous esterification of cellulose www.soci.org

Figure 1. General synthesis route and molecular structure of cellulose esters homogeneously prepared in ILs.

In addition to cellulose aliphatic esters with monofunctionalgroups, cellulose mixed esters containing two kinds of estergroups have also been prepared in ILs. The mixed acylation of cel-lulose using AmimCl as reaction medium was investigated. Cellu-lose acetate butyrate (CAB) with butyrate content of 6–47 wt %was obtained using a one-pot process without any catalysts.18

Different addition order, ratio of anhydrides and reaction condi-tions had a significant impact on the content and distributionof substituents. Also in AmimCl, we obtained efficiently threekinds of cellulose mixed esters, namely CAB,19 cellulose adaman-tate acetate (CAdAc)20 and cellulose acetate diphenyl phosphate,21

using DMAP or pyridine as catalyst. Compared with CA, CAB andCAdAc exhibited better solubility, gas permeability and formationof flexible films.

Microwave-assisted organic synthesis is a ‘green’ and efficienttechnology for accelerating the course of many organic reactions,producing high yields and selectivity, lower quantities of sideproducts and, consequently, easier work-up and purification ofproducts. Abbott et al.22 demonstrated the microwave-assisteddissolution of cellulose, and then efficient acetylation of cel-lulose by conventional heating in a Lewis acidic IL based oncholine chloride and zinc chloride. El Seoud’s group23,24 gave a

first report on the use of low-energy microwave heating for bothdissolution and subsequent esterification of cellulose in an IL,1-allyl-3-butylimidazolium chloride. Five cellulose aliphatic esterswith monofunctional groups and four mixed esters were preparedin the absence of catalyst both rapidly and conveniently. Com-pared with conventional heating, microwave irradiation resultedin considerable reduction in dissolution and reaction times, andenhancement in conversions.

In general, the efficient acylation of cellulose can be achievedwhen acyl chloride and acid anhydride are used as the acyla-tion reagents. The reaction of cellulose dissolved in ILs withcarboxylic acid does not yield cellulose esters. Dorn et al.25 usedN,N′-carbonyldiimidazole to activate carboxylic acids and synthe-sized successfully cellulose 3,6,9-trioxadecanoic acid ester and3,6-dioxaheptanoic acid ester with high DS in BmimCl. Granströmet al.26 also reported that, using N,N′-carbonyldiimidazole and1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide hydrochlorideto activate carboxylic acids in situ, cellulose stearate was obtainedreadily in AmimCl. The p-toluenesulfonyl group is commonly usedas a leaving group in nucleophilic substitution reactions, makingit a practical intermediate for subsequent cellulose modificationreactions. The homogeneous reaction of cellulose with lactams in

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the presence of p-toluenesulfonic acid chloride was achieved inBmimCl to obtain water-soluble cellulose esters bearing cationicamino functions.27 Depending on the reaction conditions, novelcellulose esters with DS ranging from 0.12 to 1.17 could beprepared.

Zhao et al.28 performed enzymatic transesterifications of methylmethacrylate with cellulose. By catalytic transesterification in amixture of BmimCl and co-solvents, Schenzel et al.29 employed1,5,7-triazabicyclo[4.4.0]dec-5-ene as catalyst and efficientlyconverted cellulose into cellulose esters. CB and cellulose10-undecenoate were prepared with DS in the range 0.2–0.7.Additionally, cellulose 10-undecenoate was modified further byemploying two thiols for efficient thiol–ene addition reactions.

The introduction of ester groups into cellulose can endowcellulose-based materials with novel functionalities. For example,after introducing the bulky adamantane group into cellulose, weprepared cellulose-based gas separation membrane materials, ofwhich gas permeabilities were significantly enhanced with goodideal permselectivity for several gas pairs, such as O2/N2, CO2/N2,CO2/CH4 and CO2/CO.20 After introducing the acrylate group andsubsequently grafting some IL, novel cellulose-based membraneswere synthesized in AmimCl with both high permeability andpermselectivity for CO2.30

Promisingly, cellulose-based macro-initiators, including cel-lulose 2-bromoisobutyrylate,31 – 33 2-bromopropionate34,35 andchloroacetate,36 were successfully synthesized by direct homoge-neous acylation of cellulose in AmimCl, BmimCl and zinc-basedILs. Subsequently, well-defined cellulose-graft-poly(methylmethacrylate),31,34,36 cellulose-graft-polystyrene,31 cellulose-graft-poly(methyl methacrylate)-block-polystyrene,31 cellulose-graft-poly(N-isopropylacrylamide) and cellulose-graft-polyisoprene32

were synthesized in a grafting-from approach by atom transferradical polymerization, and cellulose-graft-poly(N,N-diethylacry-lamide)35 and cellulose-graft-poly(N-isopropylacrylamide)35 byreversible addition–fragmentation chain transfer polymerization.The obtained cellulose-graft-copolymers had grafted polymerchains with well-controlled molar mass and dispersity, and dis-played self-assembly behavior and thermoresponsive property.

Some pioneer exploration of up-scaling of homogeneous ester-ification of cellulose in ILs has been carried out. The feasibilityof homogeneous acetylation of cellulose was proved at relativelyhigh cellulose mass concentrations (8–12 wt%).37 Without usingany catalysts, CA with DS in the range 0.4–3.0 was synthesizedin AmimCl. Kosan et al.38 investigated a one-pot process for theproduction of CA fibers with controllable DS values and theirproperties by employing a vertical kneader system. Zhang et al.39

reported chemical modification of cellulose by in situ reactiveextrusion with several chemicals, such as urea, maleic anhydrideand so on, in a co-rotating twin-screw extruder.

CELLULOSE AROMATIC ESTERSIn the case of homogeneous synthesis of cellulose aromatic esters,phenyl isocyanate was used as acylation reagent in BmimCl forthe first time by Heinze and co-workers.11,12 The carbanilationof not only cotton pulp but also bacterial cellulose with signifi-cantly high degree of polymerization of 6500 was achieved underhomogeneous and mild reaction conditions. The well-solublecellulose phenylcarbamates were chosen to determinate theirmolar mass by gel permeation chromatography, confirming thatno significant degradation of polymer chains occurred duringthe dissolution and reaction with phenyl isocyanate. And the

products may expect to find use for chiral separation. Recently,Liu et al.40,41 prepared, in AmimCl, fully and regioselectively substi-tuted cellulose 3,5-dimethylphenylcarbamates, and subsequentlycoated and immobilized them on silica gel to separate success-fully some racemic pesticides. In addition, by means of the linker1,3-phenylene diisocyanate, Bagheri et al.42 obtained polyami-doamine dendrimers covalently bonded to cellulose in BmimCl.The formation of cellulose dendrimer exhibited a good ability forenzyme immobilization.

Cellulose benzoate, in addition to its use as a thermoplasticcellulose-based material, also exhibits high chiral recognitionability and has been extensively used as a chromatographic chiralstationary phase for the separation of enantiomers in HPLC. Aseries of benzoyl chlorides with different para-substituents onphenyl moieties were used to study homogeneous benzoyla-tion of cellulose.43 In AmimCl, benzoylation of cellulose withbenzoyl chlorides was readily carried out under mild conditionswithout any catalyst. Cellulose benzoates with a controllableDS in the range from about 0.8 to 3.0 were accessible byaltering the reaction conditions. The electronic modificationalso resulted in prominent changes in reaction rates. Prelim-inary chromatographic experiments showed that cellulosebenzoates with DS from 2 to 3 exhibited good enantiorecog-nition abilities. Surprisingly, the benzoylation of cellulose inAmimCl exhibited exclusively O-6 regioselectivity. Subsequently,some regioselectively modified cellulose mixed esters, suchas cellulose 2,3-bis(4-nitrobenzoate)-6-benzoate and cellulose2,3-bis(4-chlorobenzoate)-6-benzoate, were synthesized directlyby adding acylation reagents twice in one pot. Xu et al.44 trieddirect esterification of cellulose with three sterically demandingacylating reagents (pivaloyl chloride, adamantoyl chloride, and2,4,6-trimethylbenzoyl chloride) in AmimCl at the lowest practicalreaction temperature. However, the product analysis indicatedthat whereas O-6 substitution was consistently preferred, noconditions could be identified in which it was exclusive; O-2/3substitution was always observed at significant levels even atDS below 1.0. Furthermore, Zoia et al.45 utilized the benzoyla-tion reaction in AmimCl to accurately determine the molecularweight distribution of whole lignocellulosic materials using gelpermeation chromatography.

In addition, Heinze’s group synthesized, in the presence ofpyridine, cellulose furoate46 and phenyl carbonate47 with DS inthe range 0.4–3.0 in the homogeneous acylation of cellulosewith 2-furoyl chloride and phenyl chloroformate, respectively.Degradation of cellulose almost could be ignored when usingpyridine. A homogeneous synthesis in AmimCl of cellulosenaphthoate and 9-anthracenecarboxylate with DS in the range0.85–2.61 was reported.48 The novel cellulose esters exhibitedstrong fluorescence property and improved solubility in someorganic solvents. Especially, cellulose naphthoates exhibited goodmembrane-forming ability. Using a solution-casting method, ahighly transparent and flexible membrane was obtained, as shownin Fig. 2(A).

Due to well-developed hydrogen bonding networks, natu-ral cellulose is unmeltable and extremely difficult to dissolve,which limits its utilization with conventional melt-processing andsolution-processing. Recently, a short and bulky diphenylphos-phate substituent was introduced into cellulose to synthesizenovel and thermoplastic cellulose esters.21 Via injection moldingand hot compression molding techniques, highly transparent filmand disc-like specimens were successfully prepared without anyexternal plasticizers, as shown in Fig. 2(B).


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(A)

(B)

(C)

Figure 2. Functional materials of cellulose esters. (A) Fluorescence spectra of the cellulose naphthoate (CNp)/tetrahydrofuran solution (concentrationof CN: 100 mg L−1) and photograph of CNp film obtained by solution casting.48 Copyright 2013 Acta Polymerica Sinica. (B) Synthesis and processing ofcellulose acetate–diphenylphosphate mixed esters (C-A-Dp) in AmimCl.21 Copyright 2014 Springer Science+Business Media, Dordrecht. (C) PEC capsulesprepared from cellulose sulfates and enzyme activity test performed with encapsulated glucose oxidase.50 Copyright 2009 American Chemical Society.

CELLULOSE INORGANIC ESTERSCellulose inorganic esters, including cellulose nitrate, sulfateand phosphate, have gained importance in military, medicine,printing and membrane material fields. For example, cellulosesulfates have undergone intense investigation during the pastfew decades, because their water-soluble sodium salts offerexcellent anticoagulant, rheological and gel-forming properties,which has increased their importance as film-forming materi-als, anionic polyelectrolytes and biologically active polymers. Inmedia of ILs/co-solvents, Gericke et al.49 synthesized water-solublecellulose sulfates with a variety of DS values using SO3/pyridine,SO3/dimethylformamide and chlorosulfonic acid as reagents. Thehomogeneously prepared cellulose sulfate was highly suitablefor the formation of polyelectrolyte complex (PEC) capsules, inwhich glucose oxidase was entrapped and retained its activity. Inaddition, they also prepared water-insoluble and IL-soluble cellu-lose sulfates with a low DS (0.16) by adjusting reaction conditions

in EmimAc.50 Then, stable spherical PEC capsules (Fig. 2(C)) withencapsulation of glucose oxidase were obtained in a one-potprocedure. In comparison to PEC capsules of water-soluble cellu-lose sulfates, the mechanical stability of the water-insoluble PECcapsules was improved and the enzyme activity retained. Wanget al.51 also synthesized cellulose sulfates in BmimCl and examinedtheir anticoagulation activity. It was pointed out that the molarmass was a major factor in the anticoagulant properties. Highmolar mass was favorable for in vitro applications, and low molarmass for in vivo applications.

Phosphorus-containing cellulose esters, which are used asvalue-added materials in a variety of biomedical fields, areanother group of important cellulose inorganic esters. Vo et al.52

reported a new method for the preparation of phosphorylatedcellulose, namely the phosphorylation of cellulose could be con-ducted by treating cellulose in an IL, 1,3-dimethylimidazoliummethylphosphite, at elevated temperatures over 120 ∘C. A new


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Figure 3. Scheme for homogeneous esterification of cellulose in ILs and recycling of ILs.8 Copyright 2012 MDPI.

type of water-soluble ionic cellulose was obtained convenientlyin high yields. The degree of phosphorylation on the cellulosechain was between 0.4 and 1.3, depending on the reaction tem-perature and time. With an increasing degree of phosphorylation,water solubility was increased. Moreover, the resultant ionicphosphorylated celluloses had high char ratios of around 40–50%at 500 ∘C, thereby implying their potential as flame-retardantmaterials.

ESTERIFICATION OF CELLULOSE FROMAGRICULTURAL RESIDUELow-cost agricultural residue as a renewable resource is availablein large quantities every year in the world. However, numerousagricultural residues are underutilized. For example, China pro-duces around 700 million tons of agricultural straw annually,most of which is simply burnt in the fields, creating a significantlocal air pollution problem. At present, how to utilize agriculturalresidues efficiently and adequately is a focus of interest in cellulosechemistry.

Cellulose was extracted from cornhusk and then successfullytreated by acetylation in AmimCl.53,54 Without using any catalyst,cornhusk CAs with controllable DS in the range 2.16–2.63 wereobtained in one step. The resultant cornhusk CAs were readilydissolved in some organic solvents, such as acetone and DMSO.Then, cast cornhusk CA films prepared from acetone solutionsexhibited good mechanical properties. Due to the recyclability ofAmimCl after each acetylation, this study provided a technicallyfeasible and environmentally acceptable method for preparing

acetone-soluble cellulose diacetates in one step using relativelycheap cornhusk as cellulose resource. Hu et al.55 synthesized cot-ton stalk cellulose carbamate in 1-ethylpyridinium bromide. Thehighest nitrogen content of esterification is about 8%.

Liu and co-workers56 – 63 performed a series of experimentsto investigate the esterification reaction of sugarcane bagassecellulose with succinic anhydride and phthalic anhydride inBmimCl, AmimCl and BmimCl/DMSO without catalyst or withDMAP, N-bromosuccinimide or iodine as catalyst. Cellulose esterswere prepared with DS ranging from 0 to 2.54. Wang et al.64

accomplished homogeneous sulfation of bagasse cellulose withchlorosulfonic acid in a mixture of BmimCl and dimethylfor-mamide. The resultant bagasse cellulose sulfates with DS in therange 0.52–2.95 exhibited significant anticoagulation activity.Huang et al.65 prepared two cellulose mixed esters, CAB and cel-lulose acetate propionate, in AmimCl using sugarcane bagasseas raw material. Ma et al.66 investigated the glutarylation of sug-arcane bagasse cellulose in BmimCl by high-intensity ultrasoundirradiation without catalyst.

Xie et al.67 studied chemical modification procedures ofunbleached Norway spruce thermomechanical pulp by homoge-neous acylation and carbanilation in ILs. They found that highlysubstituted wood-based lignocellulosic esters could be obtainedunder mild conditions by reacting wood dissolved in ILs withacetyl chloride, benzoyl chloride and acetic anhydride in thepresence of pyridine.

Wen et al.68 performed homogeneous lauroylation of ball-milledbamboo in BmimCl. After lauroylation, the rough appearanceof bamboo meal changed into a relatively homogeneous and


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smooth surface morphology. Furthermore, the lauroylatedbamboo meal had excellent solubility in chloroform, whichprovided feasibility for electrostatic spinning of modified bamboomeal as a biomaterial for industries.

RECYCLING OF IONIC LIQUIDSAfter esterification of cellulose in ILs, efficient recycling of ILs isan important ecological and economical issue that needs to beaddressed in order to apply these solvents in industrial-scale pro-cesses for profit and sustainablility.8 In general, ILs can easily berecycled after a simple filtration and rotary evaporation process,due to their negligible vapor pressure and good thermostabil-ity. A typical esterification of cellulose and recycling of ILs aredepicted schematically in Fig. 3.8 More importantly, using the recy-cled ILs as reaction media, the resultant cellulose esters had almostthe same DS as those synthesized in fresh ILs.43 However, evap-oration of the non-solvent and volatile impurities might be anenergy-consuming process. And some impurities like carboxylicacids, especially long-chain fatty acids, cannot be removed byevaporation below certain concentration levels.69 Therefore, it isurgently needed to develop new technologies for recycling ILsafter chemical modification of cellulose.

SUMMARY AND PERSPECTIVEIn summary, it has been demonstrated that ILs are powerful andpromising media for homogeneous esterification of cellulose. Avariety of traditional and novel cellulose esters with DS in therange 0–3 can be prepared readily. Just controlling the reactionconditions, such as time, temperature and molar ratio of reactants,permits one to obtain cellulose esters with desired DS values.

Production of cellulose esters by heterogeneous methods hasa long history. Some drawbacks of these methods, such as com-plexity of technological process and difficulty in controlling thechemical structure of products, seem to be impossible to over-come. Compared with traditional heterogeneous cellulose deriva-tization, the homogeneous synthesis of cellulose esters using ILsas media has several obvious advantages, such as high efficiency,high yield, no by-products, easy control of chemical structure ofproducts, simple separation of products and solvents, and easysolvent recovery. Thus, industrial use of ILs in cellulose chemistryis attractive and possible. However, the present investigations arestill in the preliminary stage, and further study is required.

From our viewpoint, the following research areas deserve atten-tion in the near future: (1) development of new and highly efficientsynthesis strategies, approaches and facilities for the reaction sys-tem and good control of process and product; (2) cost-beneficialand cost-effective analyses for whole IL-based cellulose esterifi-cation process; (3) evaluation of physical properties of resultantcellulose esters; (4) design and synthesis of novel and functionalcellulose esters in ILs; and (5) utilization of low-cost agricultural andindustrial cellulose resources.

ACKNOWLEDGEMENTSThis work was supported by the National Science Foundation ofChina (nos. 51103167, 21174151 and 51425307).

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