Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Review
Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges Kirtikumar C. Badgujar, Bhalchandra M. Bhanage ⇑ Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Present review discusses the state of
Lignocellulosic biomass
art for dissolution of lignocellulosic biomass. IL properties for processing of lignocellulosic biomass discussed in detail. Processing parameters are reviewed and discussed for dissolution of biomass. Challenges and opportunities are presented to achieve feasibility and sustainability.
a r t i c l e
Processing parameters Temperature Time
Biomass loading Opportunities….?
Ionic Liquid properties
Cation-anion H-bond basicity (β) Viscosity
Bio-energy & value added products
Melting point Thermal stability Challenges….?
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 24 September 2014 Accepted 26 September 2014 Available online xxxx
The utilisation of non-feed lignocellulosic biomass as a source of renewable bio-energy and synthesis of fine chemical products is necessary for the sustainable development. The methods for the dissolution of lignocellulosic biomass in conventional solvents are complex and tedious due to the complex chemical ultra-structure of biomass. In view of this, recent developments for the use of ionic liquid solvent (IL) has received great attention, as ILs can solubilise such complex biomass and thus provides industrial scale-up potential. In this review, we have discussed the state-of-art for the dissolution of lignocellulosic material in representative ILs. Furthermore, various process parameters and their influence for biomass dissolution were reviewed. In addition to this, overview of challenges and opportunities related to this interesting area is presented. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Lignocellulosic biomass processing Ionic liquids Biomass dissolution factors Bio-energy products Renewable bioresource technology
1. Introduction The worldwide climatic changes and depleting fossil-fuel resources attracted more attention to develop sustainable routes for generation of energy and valuable chemical products (Bhaskar et al., 2011; Sun et al., 2011). Recently United State Department of Energy predicted that, the liquid fuels requirement in the US will increased up to 200% which requires extra 36 bn ⇑ Corresponding author. Tel.: +91 22 3361 2601/2222; fax: +91 22 2414 5614. [email protected],
Regeneration, Transformation Isolation & Purification
Particle size Biomass type
i n f o
E-mail addresses: (B.M. Bhanage).
Dissolution & pre-treatment
[email protected]
gallons of fuel for transportation by 2022 (US Environmental Protection agency). Moreover, the European Union (EU) has set a target to attain 20% renewable energy share by 2020 for future sustainability (Birol, 2010). Thus, it is well-known fact that future energy consumption budget (economy) will be governed by renewable energy sources like wind, tidal, hydro, geothermal, solar and biomass (Birol, 2010; Pu et al., 2007; Brandt et al., 2013). More recently, Pacific Northwest National Laboratory (PNNL) and National Renewable Energy Laboratory (NREL) have recognised 12 speciality platform chemicals which can be obtained from biomass resources (PNNL/NREL, 2004). These 12 platform chemicals and bio-energy obtained from biomass processing can be work
http://dx.doi.org/10.1016/j.biortech.2014.09.138 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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K.C. Badgujar, B.M. Bhanage / Bioresource Technology xxx (2014) xxx–xxx
as building blocks to construct ‘‘Green Bio-based Economy’’ (PNNL/ NREL, 2004). Thus, shifting an attitude of society from non-renewable to renewable (from petroleum-based to biomass-based) resources is generally viewed as solution to develop a ‘‘Green-Sustainable Society’’ (Wang et al., 2012; PNNL/NREL, 2004). Hence, research for generation of biomass derived energy (bio-fuel) and valuable products (platform chemicals) are of great importance in academia and industry (Birol, 2010; Bhaskar et al., 2011). The lignocellulosic biomass is the most abundant basic renewable energy source present on the Earth, which consist of non-feedable plant materials such as woody trees, shrubs and grasses with an approximate worldwide production of around 1.1 1011 tons/ year (Tadesse and Luque, 2011; Sun and Cheng, 2002). It is mainly constituted by three bio-polymeric sub-components namely (i) cellulose (semi-crystalline polysaccharide present about 40–55%) (ii) hemicelluloses (amorphous multicomponent polysaccharide present around 20–30%) and (iii) lignin (amorphous phenyl-propanoid polymer present about 15–20%) (Tadesse and Luque, 2011). The percent compositions for these three components depend on the plant species, geophysical and ecological position (Brandt et al., 2013). The cellulose is a major component of lignocellulosic biomass present in form of micro-fibrils with a radius of 2–3 nm (Tadesse and Luque, 2011). There are thousands of micro-fibrils enclosed within a tough protective sheath of lignin and hemicelluloses to form a plant cell wall of typical dimensions in the range of 10–100 lm. These thousands of plant cell walls are cemented together by lignin-rich region known as the middle lamella (Brandt et al., 2013). The protective sheath and rigid complex structure of lignocellulosic biomass results into stable fabric material which is extremely recalcitrant to chemo- and bio-degradation (Sun et al., 2011; Wang et al., 2012; Li et al., 2010). The lignin component is one of the richest source of important platform organic moieties while, cellulose is an abundant, non-toxic, inexpensive, biocompatible, eco-friendly, and virtual inexhaustible source of the lignocellulosic biomass (Wang et al., 2012; Bhaskar et al., 2011). In context to this, researchers are paying more attention to use of lignocellulosic material as a major raw source to obtain platform chemicals and bio-energy (Li et al., 2011, 2009; Doherty et al., 2011). The dissolution and subsequent disintegration of the cellulose is a basic pre-requisite to produce valuable output of bio-energy and valuable chemicals (Zhao et al., 2009a,b; Tadesse and Luque, 2011). However, dissolution is one of the challenging tasks as it consists of thousands of b-(1–4)-linkages of glucose monomers in a polysaccharide chain, with rigorous inter- and intra-molecular H-bonding (Vitz et al., 2009). There are few aqueous and non-aqueous cellulose solvents which can able to dissolve cellulose like N,N-dimethyl acetamide/lithium chloride; dimethyl sulphoxide/tetra-N-butyl ammonium fluoride; N,N-dimethyl formamide/nitrous tetraoxide etc. (Wang et al., 2012; Novoselov et al., 2007). These solvent showed solubility of cellulose but all of them have drawbacks such as high flammability, high-cost, poisonous gas production, difficulty in solvent recovery, difficulty in down-streaming process, recyclability and insufficient solvating ability (Wang et al., 2012; Novoselov et al., 2007; Liu et al., 2012). However, above listed problems can be solved by use of non-conventional neoteric solvents known as room temperature ionic liquids (ILs) which having melting points less than 373 K (100 °C) (Pinkert et al., 2010; Zhao et al., 2009a). ILs are referring to Newton’s liquid composed of cations (generally organic) as well as anions (organic or inorganic) and they are liquids below the temperature 373 K (Brandt et al., 2013). These ILs having unique physico-chemical properties, such as high solvating ability, non-volatile nature (low vapour pressure), nonflammable properties, better thermal stability, wide electro-chemical range, high ionic conductivity, recyclability, broad liquid range,
and better dissolving capacity etc. (Bourbigou et al., 2010). Therefore, ILs are extensively employed in variety of fields such as synthesis, electro-chemistry, homo-, heterogeneous catalysis, extraction process, bio-catalysis, engineering fluid mechanics and polymer chemistry etc. (Wang et al., 2012; Fort et al., 2007). In addition to this, vast number of ILs can be synthesized by minor modification or combination of cations and anions, so called them as ‘‘Designer Solvents’’ (Pinkert et al., 2010; Vitz et al., 2009). These excellent properties of IL make them an ideal and promising solvent which can replace traditional cellulose solvents (Swatloski et al., 2002; Bourbigou et al., 2010; Sun et al., 2011). The Scopus literature survey showed that biomass processing is an area of great interest while numbers of publication in the field of biomass processing are continuously increasing since 2009. However, limited numbers of reviews are available discussing the dissolution of lignocellulosic biomass (Fig. 1). Hence in present review, we have discussed fundamental physico-chemical properties of IL and process parameters which affect dissolution of lignocellulosic biomass. This review also assesses various difficulties, opportunities/challenges linked with biomass processing in IL such as: economical viability, scale-up and sustainability.
2. History A French professor of agriculture chemistry Anselme Payen (in 1839) extracted some white pasty material (free of lignin) from timber and called it as ‘‘cellulose’’. The word cellulose was derived from the Latin word ‘‘Cellula’’ means ‘‘Cell’’ (Skoracki, 2011). In 1846, Cross F. Schönbein made an attempt to dissolve nitrate derivative of cellulose in mixture of alcohol and ether (Skoracki, 2011). Charles Cross (in 1892) invented that cotton or woody cellulose mass could be dissolved in form of cellulose xanthate by treatment with base and CS2 (Skoracki, 2011). In 1934, Graenacher discovered the use of organic N-alkylpyridinium chlorides salts as low melting solvents for the dissolution of cellulose. However, theses solvents are not considered as a true IL since all of these were melted at 20–30 K (Graenacher, 1934). Hence, to modify the melting point they added co-solvents such as dimethyl sulfoxide of N,N-dimethyl formamide (Linko et al., 1977). These interesting history and practical observations of the Graenacher’s results gave scope and attracted significant attention of researchers for the dissolution of lignocellulosic biomass materials in IL.
3. A debate on cellulose dissolution mechanism in IL The interaction of IL cation and anion with cellulosic biomass is of great interest and challenging area of research. Hence, many researchers effort to study the cellulose dissolution mechanism in IL using various advance techniques such as 13C-NMR, 35/37ClNMR spectroscopy and computational modelling studies (Pinkert et al., 2010). It is well known fact that, the dissolution is a physical solvent–solute interaction which can be governed by the nature of both components of IL and physical factors (viscosity of IL and processing temperature). The cellulosic biomass dissolution is subject of controversial debates as various researchers claimed different theories depending on experimental or simulation studies (Pinkert et al., 2010; Lu et al., 2014). Several studies proposed that the nature of IL anion and its H-bond basicity is responsible for the dissolution of cellulosic biomass in IL (Swatloski et al., 2002; Moulthrop et al., 2005; Youngs et al., 2006). 1. In 2005, Moulthrop et al. performed 13C and 35/37Cl NMR relaxation measurements to know the cellulose dissolution process in [Bmim][Cl] (Please see Supplementary information for the
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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Fig. 1. Current status of biomass related research survey from Scopus with respects to annual publication and type of document (A) No. of publication vs. year of publication, for given queries indicated in legend. (B) Document type vs. number of document from year January 1995 to July 2014, for given queries indicated in legend.
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nomenclature and structure of IL). They concluded that anion [Cl] interact stoichiometrically with the hydroxyl group (–OH) of cellulose and forms an electron donor-electron acceptor complex (EDA-complex) (Moulthrop et al., 2005). This EDA-complex results into breaking of H-bonding and causes subsequent dissolution of cellulosic material. Youngs et al. (2006) confirmed H-bonding between the IL anion and –OH group of cellulose via molecular dynamic modelling. In this study they observed that lignin comes in contact with the solvent IL and both the components (i.e. cellulose and lignin) can be dissolved. Novoselov research group verified destruction of the intermolecular H-bonding between cellulose chains and formation of salvation complex in between solvent IL anion and –OH of cellobiose (Novoselov et al., 2007). Thus, Moulthrop et al. (2005), Youngs et al. (2006) and Novoselov et al. (2007) showed that IL anions play an important role in dissolution of cellulose. Heinze group practically observed disappearing of a carbon signal of glucose unit after dissolution in IL [Emim][OAc]; which may be attributed due to covalent bond formation between the C1 carbon of glucose (monomer of cellulose) unit and C2 carbon of imidazolium ring (Heinze et al., 2005, 2008). Later on, Heinze’s group results were also supported by Ebner research group using fluorescence and 13C labelling studies which concluded that heterocyclic carbene of dialkyl imidazolium cations and C2-proton play a vital role in cellulose dissolution. Based on Heinz and Ebner’s result, it was concluded that IL cation plays a major role in dissolution of cellulosic biomass (Heinze et al., 2005; Ebner et al., 2008).
6. In context to this, NMR spectroscopic study performed by Zhang et al. (2010) suggested that formation of H-bonding is a major cause of cellulose dissolution in which both IL cation and anion are involved. They showed that [OAc] anion preferred to form H-bonds with H-atoms of cellulose-OH group, while C2 aromatic protons of cation [Emim] leads to form H-bond with oxygen atoms of cellulose-OH (Zhang et al., 2010). 7. Zhao et al. (2012) performed molecular dynamic study to determine the dissolution mechanism and they found that electronwithdrawing group in alkyl side chain of cation enhances the cellulose dissolution. Moreover, they also concluded that heterocyclic aromatic cation ring also affect the dissolution process as, cellulose showed better solubility in [3MBpy][Cl] than [Bmim][Cl] (Zhao et al., 2012). Thus, Zhao et al. (2012) stated that IL cationic structure predicts interaction of IL anion with cellulose as, cation [3MBpy] allowed more interaction of [Cl] with cellulose and offered better dissolution in IL [3MBpy][Cl]. 8. More recently, Lu et al. (2014) proposed that acidic protons of heterocyclic rings of cations are necessary for dissolution of cellulose in ILs. These acidic protons can form H-bonding with –OH and etheric oxygen of cellulose chain which leads to increase cellulose solubility (Lu et al., 2014). Herein, Zhao et al. (2012) and Lu et al. (2014) showed that dissolution process mainly governed by nature of cation. Thus, various studies reported diverse observations for cellulose dissolution. However determination of exact role of IL and interaction of IL components for cellulose dissolution is still part of debate which needs comprehensive study with experimental evidences. In
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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K.C. Badgujar, B.M. Bhanage / Bioresource Technology xxx (2014) xxx–xxx
our opinion, it was seen that both components of IL (anion and cation) are actively involves in dissolution mechanism. 4. Properties of ionic liquids and processing parameters that influences biomass dissolution 4.1. Effect of IL anion The mechanistic studies indicated anion’s contribution to from H-bond and subsequent cellulose dissolution in IL. It was showed that anions (such as [OAc], [OFo] and [Cl] etc.) with good H-bond basicity possess superior ability to dissolve cellulose (Table 1, entries 1–55) (Vitz et al., 2009). Contrary to this low-basicity anions, like [DCA] and [Tos] are not effective for cellulose dissolution (Zhao et al., 2009a,b; Xu et al., 2010). Furthermore, ILs containing non-coordinating anions, such as [BF4] and [PF6] are too sluggish to dissolve cellulose (Table 1, entries 28–34) (Lee et al., 2009). Larger size of anion having low H-bond formation ability hence dissolution ability decreases with increase in the size (Vitz et al., 2009). Fukaya et al. (2008) proposed trend of cellulose dissolution in phosphate or phosphonate based ILs as: [Emim][(MeO)2 PO2] < [Emim][(MeO)MePO2] < [Emim][(MeO)(H)PO2] at 45–65 °C. In contrast to this, [Emim][Cl] melted at 85 °C and hence does not show any cellulose dissolution at 45 °C (Fukaya et al., 2008). The influence of anion for lignocellulosic biomass dissolutions exclusively governed by the H-bond basicity, since IL anions play a key role in the disruption of inter- and intramolecular H-bonds of cellulose (Pu et al., 2007; Lee et al., 2009). Xu et al. (2010) observed following sequence for b-value and cellulose solubility as: [OAc] > (HSCH2COO) > [OFo] > [OBz] > H2NCH2COO > HOCH2 COO > CH3CHOHCOO > [DCA] (Table 1, entries 35–40). Thus, Xu et al. (2010) demonstrated two important results (i) cellulose dissolving ability may be predicted by H-bond basicity (b-value), (ii) multiple (second) hydrogen donating ability of anion may leads to affect b-value of anions like [H2NCH2COO], [HOCH2COO] and [CH3CHOHCOO] which lowers their dissolution ability. Nature of amino-acid anion influences the dissolution of lignin, as lignin showed better solubility results than cellulose in amino-acid based IL (Table 1, entries 42–55) (Liu et al., 2012; Hamada et al., 2013). The role of anion is vital however, many researchers claimed that cation also contributed for cellulose dissolution. (Zhang et al., 2010; Zhao et al., 2012; Lu et al., 2014). 4.2. Effect of IL cation The experimental, mechanistic and computational data proposed that cation of IL can significantly affect solubility of cellulose (Erdmenger et al., 2007; Heinze et al., 2005). The effect of cation for cellulose dissolution is controlled by (i) nature of functional group, (ii) cation size and (iii) attached side chain moiety. A variety of IL functionality like choline, imidazolium, pyrrolidinium, pyridinium, ammonium and phosphonium based cations along with alkyl, etheric or allylic side chain were reported for dissolution of lignocellulosic biomass by various researchers (Zhao et al., 2012, 2008; Lu et al., 2014; Vitz et al., 2009). 4.2.1. Effect of functional groups present on cation The aromatic imidazolium and pyridinium based ILs have delocalised p–p electron aromatic character and they are most widely studied for cellulose dissolution (Heinze et al., 2005; Lu et al., 2014). Heinze et al. (2005) showed that heterocyclic aromatic functionality like pyridinium [3MBpy] and imidazolium [Bmim] based cation showed better dissolution of cellulose than non-heterocyclic benzyl ammonium based cations [BDTAC] (Table 1, entries 56–59). Furthermore, non-aromatic cyclic cations based
IL, such as pyrrolidinium or piperidinium are failed to dissolve cellulose which concludes that aromatic functionality promotes dissolution phenomenon (Table 1, entries 64–73) (Lu et al., 2014). The introduction of small chain allyl functionality to aromatic imidazolium based ring showed higher dissolution as compared to regular alkyl (methyl/ethyl/butyl) group. This might be attributed due to the lower melting point and lower viscosity of the allyl based ILs (Zavrel et al., 2009; Fukaya et al., 2006). Multiple (double) bond of allyl group induces strong polarity which is essential to create polarisation interface with cellulose molecules for dissolution (Zavrel et al., 2009). Similar to allyl group, –C„N group also showed better dissolution ability because of smaller size and electron withdrawing nature of –C„N group (Table 1, entries 77–82) (Lateef et al., 2009; Zhao et al., 2012). In addition to this, cations with alkyloxy-alkyl (ether) functionality at side chain gave interesting results for cellulose dissolution. Since ether functionality incorporates lower viscosity and lower melting point, which favours better solvation and dissolution (Table 1; entries 83–88) (Zhao et al., 2008). Zhao et al. (2008) proposed alkoxy-alkyl ammonium based IL cation also showed moderate solubility like analogous imidazolium based IL (Table 1, entry 83, 84). The presence of –OH group in alkyl side chain (hydroxyl-alkyl) causes decrease in cellulose solubilisation, as – OH groups competes for H-bond formation with cellulose (Zhao et al., 2008). Thus, alkyloxy-alkyl side chain IL [Me(OEt)3-EtIm][OAc] showed better cellulose solubility than analogous hydroxyl-alkyl [H(OEt)3-Me-Im][OAc] side chain IL (Lu et al., 2014; Zhao et al., 2008). More recently Lu et al. (2014) showed that alkoxyalkyl functionality contributes moderate solubility than hydroxyl-alkyl group due to more electron donating nature of –OCH3 than –OH groups (Table 1, entries 88, 89) (Lu et al., 2014). Interestingly, choline [Ch] and bulkier di-aza-biocyclo [DBUCn] based cation IL showed excellent solubility for lignocellulosic biomass (Table 1; entries 48–55, 124–126) (Liu et al., 2012; Diop et al., 2013). Zhao et al. (2008) observed that IL having cation [Bu4N], [Bu4P], [P66614] and [Amm110] showed lower cellulose solubility due to bulkiness of cations, which may obstruct the H-bond formation (Table 1, entries 90–93). Thus from the above study it is observed that (i) presence of N-heterocyclic aromatic functionality with acidic protons and (ii) side chain with electron withdrawing group in cation provided better solubility of lignocellulosic biomass in IL; however size of cation or side chain may affect solubilisation of lignocellulosic material. 4.2.2. Effect of size of cation and side chain moiety The increase in chain length showed increase in bulkiness and viscosity of IL which causes reduction of dissolution ability. Hence, Erdmenger et al. (2007), Vitz et al. (2009) and Zhao et al. (2012) found trend of solubility as: [Bmim][Cl] > [Hexmim][Cl] > [Omim] [Cl] > [Dmim][Cl] (Table 1, entries 101–117). Moreover similar type of trend was observed to Diop et al. (2013) as: [DBUC4] [Cl] > [DBUC6][Cl] > [DBUC8][Cl] for lignin dissolution. The presence of long chain in [Amim110] based cation of IL showed lesser solubility of cellulose because of higher viscosity and steric reason (Table 1, entries 91, 95, 100) (Zhao et al., 2008). Recently, Lu et al. (2014) proposed that larger size of cations interrupt polarizability and H-bonds formation between anion and hydroxyl group of cellulose which decreases cellulose solubility (Table 1, entries 64–73). Interestingly, Vitz et al. (2009) explained an odd–even effect with alkyl-chain length of chloride based ILs (Table 1, entries 113–118). They demonstrated that alkyl-chain length with even number of carbon atom is more effective for cellulose dissolution than odd number of carbon atom (Vitz et al., 2009). Conversely, odd–even effect for alkyl chain length was not observed to Padmanabhan et al. (2011) and Erdmenger et al. (2007). Thus, in conclusion (i) smaller size of aromatic heterocyclic cation and
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
5
K.C. Badgujar, B.M. Bhanage / Bioresource Technology xxx (2014) xxx–xxx Table 1 Influence of IL cation and anion for the dissolution of lignocellulosic biomass. No.
IL
Biomass material
Dissolution; temp °C; (Time)
Refs.
Comment: influence of anion with [Amim] based cation on cellulose dissolution (entries 1, 2) 1 [Amim][Cl] MCC 4 wt%; 85 2 [Amim][OFo] MCC 22 wt%; 85
Fukaya et al. (2006) Fukaya et al. (2006)
Comment: influence of halide based anions for dissolution of soft kraft lignin (entries 3, 4) 3 [Bmim][Cl] Soft kraft lignin 4 [Bmim][Br] Soft kraft lignin
Pu et al. (2007) Pu et al. (2007)
13.9 g/L; 75 17.5 g/L; 75
Comment: influence of phosphate and phosphonate based anions for dissolution of MCC (entries 5–7) 5 [Emim][(MeO)(H)PO2] MCC 10 wt%; 45; 30 min 6 [Emim][(MeO)MePO2] MCC 6 wt%; 45; 30 min MCC 2 wt%; 45; 30 min 7 [Emim][(MeO)2PO2]
Fukaya et al. (2008) Fukaya et al. (2008) Fukaya et al. (2008)
Comment: Zhao et al. proposed effect of various anions for avicel cellulose dissolution (entries 8–15) 8 [Bmim][Cl] Avcel 10 wt%; 110 9 [Bmim][OFo] Avcel 8 wt%; 110 10 [Bmim][DCA] Avcel 1 wt%; 110 11 [Bmim][NTf2] Avcel 100 g/kg; 90; (24 h) 29 [Bmim][BF4] Kraft lignin 40 g/kg; 90; 24 h 30 [Bmim][PF6] Kraft lignin 1 g/kg; 90; (24 h) 31 [Bmim][Cl] maple wood flour >30 g/kg; 80; (24 h) 32 [Bmim][BF4] maple wood flour
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Review
Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges Kirtikumar C. Badgujar, Bhalchandra M. Bhanage ⇑ Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Present review discusses the state of
Lignocellulosic biomass
art for dissolution of lignocellulosic biomass. IL properties for processing of lignocellulosic biomass discussed in detail. Processing parameters are reviewed and discussed for dissolution of biomass. Challenges and opportunities are presented to achieve feasibility and sustainability.
a r t i c l e
Processing parameters Temperature Time
Biomass loading Opportunities….?
Ionic Liquid properties
Cation-anion H-bond basicity (β) Viscosity
Bio-energy & value added products
Melting point Thermal stability Challenges….?
a b s t r a c t
Article history: Received 29 July 2014 Received in revised form 24 September 2014 Accepted 26 September 2014 Available online xxxx
The utilisation of non-feed lignocellulosic biomass as a source of renewable bio-energy and synthesis of fine chemical products is necessary for the sustainable development. The methods for the dissolution of lignocellulosic biomass in conventional solvents are complex and tedious due to the complex chemical ultra-structure of biomass. In view of this, recent developments for the use of ionic liquid solvent (IL) has received great attention, as ILs can solubilise such complex biomass and thus provides industrial scale-up potential. In this review, we have discussed the state-of-art for the dissolution of lignocellulosic material in representative ILs. Furthermore, various process parameters and their influence for biomass dissolution were reviewed. In addition to this, overview of challenges and opportunities related to this interesting area is presented. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Lignocellulosic biomass processing Ionic liquids Biomass dissolution factors Bio-energy products Renewable bioresource technology
1. Introduction The worldwide climatic changes and depleting fossil-fuel resources attracted more attention to develop sustainable routes for generation of energy and valuable chemical products (Bhaskar et al., 2011; Sun et al., 2011). Recently United State Department of Energy predicted that, the liquid fuels requirement in the US will increased up to 200% which requires extra 36 bn ⇑ Corresponding author. Tel.: +91 22 3361 2601/2222; fax: +91 22 2414 5614. [email protected],
Regeneration, Transformation Isolation & Purification
Particle size Biomass type
i n f o
E-mail addresses: (B.M. Bhanage).
Dissolution & pre-treatment
[email protected]
gallons of fuel for transportation by 2022 (US Environmental Protection agency). Moreover, the European Union (EU) has set a target to attain 20% renewable energy share by 2020 for future sustainability (Birol, 2010). Thus, it is well-known fact that future energy consumption budget (economy) will be governed by renewable energy sources like wind, tidal, hydro, geothermal, solar and biomass (Birol, 2010; Pu et al., 2007; Brandt et al., 2013). More recently, Pacific Northwest National Laboratory (PNNL) and National Renewable Energy Laboratory (NREL) have recognised 12 speciality platform chemicals which can be obtained from biomass resources (PNNL/NREL, 2004). These 12 platform chemicals and bio-energy obtained from biomass processing can be work
http://dx.doi.org/10.1016/j.biortech.2014.09.138 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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as building blocks to construct ‘‘Green Bio-based Economy’’ (PNNL/ NREL, 2004). Thus, shifting an attitude of society from non-renewable to renewable (from petroleum-based to biomass-based) resources is generally viewed as solution to develop a ‘‘Green-Sustainable Society’’ (Wang et al., 2012; PNNL/NREL, 2004). Hence, research for generation of biomass derived energy (bio-fuel) and valuable products (platform chemicals) are of great importance in academia and industry (Birol, 2010; Bhaskar et al., 2011). The lignocellulosic biomass is the most abundant basic renewable energy source present on the Earth, which consist of non-feedable plant materials such as woody trees, shrubs and grasses with an approximate worldwide production of around 1.1 1011 tons/ year (Tadesse and Luque, 2011; Sun and Cheng, 2002). It is mainly constituted by three bio-polymeric sub-components namely (i) cellulose (semi-crystalline polysaccharide present about 40–55%) (ii) hemicelluloses (amorphous multicomponent polysaccharide present around 20–30%) and (iii) lignin (amorphous phenyl-propanoid polymer present about 15–20%) (Tadesse and Luque, 2011). The percent compositions for these three components depend on the plant species, geophysical and ecological position (Brandt et al., 2013). The cellulose is a major component of lignocellulosic biomass present in form of micro-fibrils with a radius of 2–3 nm (Tadesse and Luque, 2011). There are thousands of micro-fibrils enclosed within a tough protective sheath of lignin and hemicelluloses to form a plant cell wall of typical dimensions in the range of 10–100 lm. These thousands of plant cell walls are cemented together by lignin-rich region known as the middle lamella (Brandt et al., 2013). The protective sheath and rigid complex structure of lignocellulosic biomass results into stable fabric material which is extremely recalcitrant to chemo- and bio-degradation (Sun et al., 2011; Wang et al., 2012; Li et al., 2010). The lignin component is one of the richest source of important platform organic moieties while, cellulose is an abundant, non-toxic, inexpensive, biocompatible, eco-friendly, and virtual inexhaustible source of the lignocellulosic biomass (Wang et al., 2012; Bhaskar et al., 2011). In context to this, researchers are paying more attention to use of lignocellulosic material as a major raw source to obtain platform chemicals and bio-energy (Li et al., 2011, 2009; Doherty et al., 2011). The dissolution and subsequent disintegration of the cellulose is a basic pre-requisite to produce valuable output of bio-energy and valuable chemicals (Zhao et al., 2009a,b; Tadesse and Luque, 2011). However, dissolution is one of the challenging tasks as it consists of thousands of b-(1–4)-linkages of glucose monomers in a polysaccharide chain, with rigorous inter- and intra-molecular H-bonding (Vitz et al., 2009). There are few aqueous and non-aqueous cellulose solvents which can able to dissolve cellulose like N,N-dimethyl acetamide/lithium chloride; dimethyl sulphoxide/tetra-N-butyl ammonium fluoride; N,N-dimethyl formamide/nitrous tetraoxide etc. (Wang et al., 2012; Novoselov et al., 2007). These solvent showed solubility of cellulose but all of them have drawbacks such as high flammability, high-cost, poisonous gas production, difficulty in solvent recovery, difficulty in down-streaming process, recyclability and insufficient solvating ability (Wang et al., 2012; Novoselov et al., 2007; Liu et al., 2012). However, above listed problems can be solved by use of non-conventional neoteric solvents known as room temperature ionic liquids (ILs) which having melting points less than 373 K (100 °C) (Pinkert et al., 2010; Zhao et al., 2009a). ILs are referring to Newton’s liquid composed of cations (generally organic) as well as anions (organic or inorganic) and they are liquids below the temperature 373 K (Brandt et al., 2013). These ILs having unique physico-chemical properties, such as high solvating ability, non-volatile nature (low vapour pressure), nonflammable properties, better thermal stability, wide electro-chemical range, high ionic conductivity, recyclability, broad liquid range,
and better dissolving capacity etc. (Bourbigou et al., 2010). Therefore, ILs are extensively employed in variety of fields such as synthesis, electro-chemistry, homo-, heterogeneous catalysis, extraction process, bio-catalysis, engineering fluid mechanics and polymer chemistry etc. (Wang et al., 2012; Fort et al., 2007). In addition to this, vast number of ILs can be synthesized by minor modification or combination of cations and anions, so called them as ‘‘Designer Solvents’’ (Pinkert et al., 2010; Vitz et al., 2009). These excellent properties of IL make them an ideal and promising solvent which can replace traditional cellulose solvents (Swatloski et al., 2002; Bourbigou et al., 2010; Sun et al., 2011). The Scopus literature survey showed that biomass processing is an area of great interest while numbers of publication in the field of biomass processing are continuously increasing since 2009. However, limited numbers of reviews are available discussing the dissolution of lignocellulosic biomass (Fig. 1). Hence in present review, we have discussed fundamental physico-chemical properties of IL and process parameters which affect dissolution of lignocellulosic biomass. This review also assesses various difficulties, opportunities/challenges linked with biomass processing in IL such as: economical viability, scale-up and sustainability.
2. History A French professor of agriculture chemistry Anselme Payen (in 1839) extracted some white pasty material (free of lignin) from timber and called it as ‘‘cellulose’’. The word cellulose was derived from the Latin word ‘‘Cellula’’ means ‘‘Cell’’ (Skoracki, 2011). In 1846, Cross F. Schönbein made an attempt to dissolve nitrate derivative of cellulose in mixture of alcohol and ether (Skoracki, 2011). Charles Cross (in 1892) invented that cotton or woody cellulose mass could be dissolved in form of cellulose xanthate by treatment with base and CS2 (Skoracki, 2011). In 1934, Graenacher discovered the use of organic N-alkylpyridinium chlorides salts as low melting solvents for the dissolution of cellulose. However, theses solvents are not considered as a true IL since all of these were melted at 20–30 K (Graenacher, 1934). Hence, to modify the melting point they added co-solvents such as dimethyl sulfoxide of N,N-dimethyl formamide (Linko et al., 1977). These interesting history and practical observations of the Graenacher’s results gave scope and attracted significant attention of researchers for the dissolution of lignocellulosic biomass materials in IL.
3. A debate on cellulose dissolution mechanism in IL The interaction of IL cation and anion with cellulosic biomass is of great interest and challenging area of research. Hence, many researchers effort to study the cellulose dissolution mechanism in IL using various advance techniques such as 13C-NMR, 35/37ClNMR spectroscopy and computational modelling studies (Pinkert et al., 2010). It is well known fact that, the dissolution is a physical solvent–solute interaction which can be governed by the nature of both components of IL and physical factors (viscosity of IL and processing temperature). The cellulosic biomass dissolution is subject of controversial debates as various researchers claimed different theories depending on experimental or simulation studies (Pinkert et al., 2010; Lu et al., 2014). Several studies proposed that the nature of IL anion and its H-bond basicity is responsible for the dissolution of cellulosic biomass in IL (Swatloski et al., 2002; Moulthrop et al., 2005; Youngs et al., 2006). 1. In 2005, Moulthrop et al. performed 13C and 35/37Cl NMR relaxation measurements to know the cellulose dissolution process in [Bmim][Cl] (Please see Supplementary information for the
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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Fig. 1. Current status of biomass related research survey from Scopus with respects to annual publication and type of document (A) No. of publication vs. year of publication, for given queries indicated in legend. (B) Document type vs. number of document from year January 1995 to July 2014, for given queries indicated in legend.
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nomenclature and structure of IL). They concluded that anion [Cl] interact stoichiometrically with the hydroxyl group (–OH) of cellulose and forms an electron donor-electron acceptor complex (EDA-complex) (Moulthrop et al., 2005). This EDA-complex results into breaking of H-bonding and causes subsequent dissolution of cellulosic material. Youngs et al. (2006) confirmed H-bonding between the IL anion and –OH group of cellulose via molecular dynamic modelling. In this study they observed that lignin comes in contact with the solvent IL and both the components (i.e. cellulose and lignin) can be dissolved. Novoselov research group verified destruction of the intermolecular H-bonding between cellulose chains and formation of salvation complex in between solvent IL anion and –OH of cellobiose (Novoselov et al., 2007). Thus, Moulthrop et al. (2005), Youngs et al. (2006) and Novoselov et al. (2007) showed that IL anions play an important role in dissolution of cellulose. Heinze group practically observed disappearing of a carbon signal of glucose unit after dissolution in IL [Emim][OAc]; which may be attributed due to covalent bond formation between the C1 carbon of glucose (monomer of cellulose) unit and C2 carbon of imidazolium ring (Heinze et al., 2005, 2008). Later on, Heinze’s group results were also supported by Ebner research group using fluorescence and 13C labelling studies which concluded that heterocyclic carbene of dialkyl imidazolium cations and C2-proton play a vital role in cellulose dissolution. Based on Heinz and Ebner’s result, it was concluded that IL cation plays a major role in dissolution of cellulosic biomass (Heinze et al., 2005; Ebner et al., 2008).
6. In context to this, NMR spectroscopic study performed by Zhang et al. (2010) suggested that formation of H-bonding is a major cause of cellulose dissolution in which both IL cation and anion are involved. They showed that [OAc] anion preferred to form H-bonds with H-atoms of cellulose-OH group, while C2 aromatic protons of cation [Emim] leads to form H-bond with oxygen atoms of cellulose-OH (Zhang et al., 2010). 7. Zhao et al. (2012) performed molecular dynamic study to determine the dissolution mechanism and they found that electronwithdrawing group in alkyl side chain of cation enhances the cellulose dissolution. Moreover, they also concluded that heterocyclic aromatic cation ring also affect the dissolution process as, cellulose showed better solubility in [3MBpy][Cl] than [Bmim][Cl] (Zhao et al., 2012). Thus, Zhao et al. (2012) stated that IL cationic structure predicts interaction of IL anion with cellulose as, cation [3MBpy] allowed more interaction of [Cl] with cellulose and offered better dissolution in IL [3MBpy][Cl]. 8. More recently, Lu et al. (2014) proposed that acidic protons of heterocyclic rings of cations are necessary for dissolution of cellulose in ILs. These acidic protons can form H-bonding with –OH and etheric oxygen of cellulose chain which leads to increase cellulose solubility (Lu et al., 2014). Herein, Zhao et al. (2012) and Lu et al. (2014) showed that dissolution process mainly governed by nature of cation. Thus, various studies reported diverse observations for cellulose dissolution. However determination of exact role of IL and interaction of IL components for cellulose dissolution is still part of debate which needs comprehensive study with experimental evidences. In
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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our opinion, it was seen that both components of IL (anion and cation) are actively involves in dissolution mechanism. 4. Properties of ionic liquids and processing parameters that influences biomass dissolution 4.1. Effect of IL anion The mechanistic studies indicated anion’s contribution to from H-bond and subsequent cellulose dissolution in IL. It was showed that anions (such as [OAc], [OFo] and [Cl] etc.) with good H-bond basicity possess superior ability to dissolve cellulose (Table 1, entries 1–55) (Vitz et al., 2009). Contrary to this low-basicity anions, like [DCA] and [Tos] are not effective for cellulose dissolution (Zhao et al., 2009a,b; Xu et al., 2010). Furthermore, ILs containing non-coordinating anions, such as [BF4] and [PF6] are too sluggish to dissolve cellulose (Table 1, entries 28–34) (Lee et al., 2009). Larger size of anion having low H-bond formation ability hence dissolution ability decreases with increase in the size (Vitz et al., 2009). Fukaya et al. (2008) proposed trend of cellulose dissolution in phosphate or phosphonate based ILs as: [Emim][(MeO)2 PO2] < [Emim][(MeO)MePO2] < [Emim][(MeO)(H)PO2] at 45–65 °C. In contrast to this, [Emim][Cl] melted at 85 °C and hence does not show any cellulose dissolution at 45 °C (Fukaya et al., 2008). The influence of anion for lignocellulosic biomass dissolutions exclusively governed by the H-bond basicity, since IL anions play a key role in the disruption of inter- and intramolecular H-bonds of cellulose (Pu et al., 2007; Lee et al., 2009). Xu et al. (2010) observed following sequence for b-value and cellulose solubility as: [OAc] > (HSCH2COO) > [OFo] > [OBz] > H2NCH2COO > HOCH2 COO > CH3CHOHCOO > [DCA] (Table 1, entries 35–40). Thus, Xu et al. (2010) demonstrated two important results (i) cellulose dissolving ability may be predicted by H-bond basicity (b-value), (ii) multiple (second) hydrogen donating ability of anion may leads to affect b-value of anions like [H2NCH2COO], [HOCH2COO] and [CH3CHOHCOO] which lowers their dissolution ability. Nature of amino-acid anion influences the dissolution of lignin, as lignin showed better solubility results than cellulose in amino-acid based IL (Table 1, entries 42–55) (Liu et al., 2012; Hamada et al., 2013). The role of anion is vital however, many researchers claimed that cation also contributed for cellulose dissolution. (Zhang et al., 2010; Zhao et al., 2012; Lu et al., 2014). 4.2. Effect of IL cation The experimental, mechanistic and computational data proposed that cation of IL can significantly affect solubility of cellulose (Erdmenger et al., 2007; Heinze et al., 2005). The effect of cation for cellulose dissolution is controlled by (i) nature of functional group, (ii) cation size and (iii) attached side chain moiety. A variety of IL functionality like choline, imidazolium, pyrrolidinium, pyridinium, ammonium and phosphonium based cations along with alkyl, etheric or allylic side chain were reported for dissolution of lignocellulosic biomass by various researchers (Zhao et al., 2012, 2008; Lu et al., 2014; Vitz et al., 2009). 4.2.1. Effect of functional groups present on cation The aromatic imidazolium and pyridinium based ILs have delocalised p–p electron aromatic character and they are most widely studied for cellulose dissolution (Heinze et al., 2005; Lu et al., 2014). Heinze et al. (2005) showed that heterocyclic aromatic functionality like pyridinium [3MBpy] and imidazolium [Bmim] based cation showed better dissolution of cellulose than non-heterocyclic benzyl ammonium based cations [BDTAC] (Table 1, entries 56–59). Furthermore, non-aromatic cyclic cations based
IL, such as pyrrolidinium or piperidinium are failed to dissolve cellulose which concludes that aromatic functionality promotes dissolution phenomenon (Table 1, entries 64–73) (Lu et al., 2014). The introduction of small chain allyl functionality to aromatic imidazolium based ring showed higher dissolution as compared to regular alkyl (methyl/ethyl/butyl) group. This might be attributed due to the lower melting point and lower viscosity of the allyl based ILs (Zavrel et al., 2009; Fukaya et al., 2006). Multiple (double) bond of allyl group induces strong polarity which is essential to create polarisation interface with cellulose molecules for dissolution (Zavrel et al., 2009). Similar to allyl group, –C„N group also showed better dissolution ability because of smaller size and electron withdrawing nature of –C„N group (Table 1, entries 77–82) (Lateef et al., 2009; Zhao et al., 2012). In addition to this, cations with alkyloxy-alkyl (ether) functionality at side chain gave interesting results for cellulose dissolution. Since ether functionality incorporates lower viscosity and lower melting point, which favours better solvation and dissolution (Table 1; entries 83–88) (Zhao et al., 2008). Zhao et al. (2008) proposed alkoxy-alkyl ammonium based IL cation also showed moderate solubility like analogous imidazolium based IL (Table 1, entry 83, 84). The presence of –OH group in alkyl side chain (hydroxyl-alkyl) causes decrease in cellulose solubilisation, as – OH groups competes for H-bond formation with cellulose (Zhao et al., 2008). Thus, alkyloxy-alkyl side chain IL [Me(OEt)3-EtIm][OAc] showed better cellulose solubility than analogous hydroxyl-alkyl [H(OEt)3-Me-Im][OAc] side chain IL (Lu et al., 2014; Zhao et al., 2008). More recently Lu et al. (2014) showed that alkoxyalkyl functionality contributes moderate solubility than hydroxyl-alkyl group due to more electron donating nature of –OCH3 than –OH groups (Table 1, entries 88, 89) (Lu et al., 2014). Interestingly, choline [Ch] and bulkier di-aza-biocyclo [DBUCn] based cation IL showed excellent solubility for lignocellulosic biomass (Table 1; entries 48–55, 124–126) (Liu et al., 2012; Diop et al., 2013). Zhao et al. (2008) observed that IL having cation [Bu4N], [Bu4P], [P66614] and [Amm110] showed lower cellulose solubility due to bulkiness of cations, which may obstruct the H-bond formation (Table 1, entries 90–93). Thus from the above study it is observed that (i) presence of N-heterocyclic aromatic functionality with acidic protons and (ii) side chain with electron withdrawing group in cation provided better solubility of lignocellulosic biomass in IL; however size of cation or side chain may affect solubilisation of lignocellulosic material. 4.2.2. Effect of size of cation and side chain moiety The increase in chain length showed increase in bulkiness and viscosity of IL which causes reduction of dissolution ability. Hence, Erdmenger et al. (2007), Vitz et al. (2009) and Zhao et al. (2012) found trend of solubility as: [Bmim][Cl] > [Hexmim][Cl] > [Omim] [Cl] > [Dmim][Cl] (Table 1, entries 101–117). Moreover similar type of trend was observed to Diop et al. (2013) as: [DBUC4] [Cl] > [DBUC6][Cl] > [DBUC8][Cl] for lignin dissolution. The presence of long chain in [Amim110] based cation of IL showed lesser solubility of cellulose because of higher viscosity and steric reason (Table 1, entries 91, 95, 100) (Zhao et al., 2008). Recently, Lu et al. (2014) proposed that larger size of cations interrupt polarizability and H-bonds formation between anion and hydroxyl group of cellulose which decreases cellulose solubility (Table 1, entries 64–73). Interestingly, Vitz et al. (2009) explained an odd–even effect with alkyl-chain length of chloride based ILs (Table 1, entries 113–118). They demonstrated that alkyl-chain length with even number of carbon atom is more effective for cellulose dissolution than odd number of carbon atom (Vitz et al., 2009). Conversely, odd–even effect for alkyl chain length was not observed to Padmanabhan et al. (2011) and Erdmenger et al. (2007). Thus, in conclusion (i) smaller size of aromatic heterocyclic cation and
Please cite this article in press as: Badgujar, K.C., Bhanage, B.M. Factors governing dissolution process of lignocellulosic biomass in ionic liquid: Current status, overview and challenges. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.09.138
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K.C. Badgujar, B.M. Bhanage / Bioresource Technology xxx (2014) xxx–xxx Table 1 Influence of IL cation and anion for the dissolution of lignocellulosic biomass. No.
IL
Biomass material
Dissolution; temp °C; (Time)
Refs.
Comment: influence of anion with [Amim] based cation on cellulose dissolution (entries 1, 2) 1 [Amim][Cl] MCC 4 wt%; 85 2 [Amim][OFo] MCC 22 wt%; 85
Fukaya et al. (2006) Fukaya et al. (2006)
Comment: influence of halide based anions for dissolution of soft kraft lignin (entries 3, 4) 3 [Bmim][Cl] Soft kraft lignin 4 [Bmim][Br] Soft kraft lignin
Pu et al. (2007) Pu et al. (2007)
13.9 g/L; 75 17.5 g/L; 75
Comment: influence of phosphate and phosphonate based anions for dissolution of MCC (entries 5–7) 5 [Emim][(MeO)(H)PO2] MCC 10 wt%; 45; 30 min 6 [Emim][(MeO)MePO2] MCC 6 wt%; 45; 30 min MCC 2 wt%; 45; 30 min 7 [Emim][(MeO)2PO2]
Fukaya et al. (2008) Fukaya et al. (2008) Fukaya et al. (2008)
Comment: Zhao et al. proposed effect of various anions for avicel cellulose dissolution (entries 8–15) 8 [Bmim][Cl] Avcel 10 wt%; 110 9 [Bmim][OFo] Avcel 8 wt%; 110 10 [Bmim][DCA] Avcel 1 wt%; 110 11 [Bmim][NTf2] Avcel 100 g/kg; 90; (24 h) 29 [Bmim][BF4] Kraft lignin 40 g/kg; 90; 24 h 30 [Bmim][PF6] Kraft lignin 1 g/kg; 90; (24 h) 31 [Bmim][Cl] maple wood flour >30 g/kg; 80; (24 h) 32 [Bmim][BF4] maple wood flour
