Jianying Dai,Yaqin Sun,Zhilong Xiu*

School of Bioengineering,Dalian University of Technology,Dalian 116024,China

ABSTRACT Ionic liquids (ILs) are known as green solvents,and have been widely used in the dissolution and transformation of biopolymers,the extraction of bioactive compounds and metal ions,and the capture of SO2or CO2.However,less attention was given to the separation of bio-based chemicals,such as diols and organic acids.Bio-based chemicals can be efficiently separated by organic solvent-based salting-out extraction (SOE) from fermentation broths,while organic solvents are normally unfriendly to environment and process safety in commercialized production due to their toxicity or/and flammability.In recent years,the IL-based SOE system has been explored in the separation of bio-based chemicals as an alternative of organic solvent-based SOE system.In this review,the progress of IL-based SOE of biobased chemicals has been summarized,including the effect of ILs structure on the formation of aqueous two phases,and the influences of ILs structure and concentration,temperature and pH on the partition behaviors of target products and ILs as well as removal of impurities.Most of bio-based chemicals could be distributed into the IL-rich phase with high recovery,while the partition behaviors of bio-based chemicals are sometimes different from that in organic solvent-based SOE systems.Although the results of ILbased SOE are promising,further studies are still required in the increased selectivity of target products over by-products,recovery and recycling of ILs,and the separation between ILs and bio-based chemicals.Additionally,three kinds of integrated bioprocesses would be developed on basis of utilization of ILs as extractant for SOE,catalyst for condensation reaction and solvent for pretreatment of lignocellulose.

Keywords:Downstream processing Bioseparation Ionic liquids Salting-out extraction Partition behavior Bio-based chemicals

1.Introduction

The motivation of developing technologies to replace fossilbased resources with renewable biomass resources leads to increased interest in the biological production of bio-based chemicals,such as alcohols (ethanol,butanol,1,3-propanediol,and 2,3-butanediol),organic acids (acetic,lactic,butyric,and succinic acids),and ketones (acetone,acetoin) [1–5].However,most of the products are non-competitive compared with petroleum derivatives due to the high production cost.In order to overcome this shortcoming,researchers have studied and enhanced the production process from various points,including utilization of cheap raw materials [1,3,6],development of fermentation strategies [7–10],strain enhancement or consortia utilization [2,5,11–15],and exploiting new separation techniques [16–20].

Generally,separation is the bottleneck step in industrial production of bio-based chemicals,which occupies more than 50% of the production cost due to their high affinity to water.Many separation techniques have been tried in the separation of bio-based chemicals,such as membrane separation,chromatography,evaporation,and extraction[21–23].Out of all these methods,extraction has several advantages,such as easy scale-up and low energy consumption,therefore,it is prospective for industrialization of biobased chemicals.However,simple solvent extraction is not applicable because of the low extraction efficiency.Novel extraction techniques have been developed to improve this process,including salting-out extraction (SOE) [23–26],sugaring-out extraction [17–19],and reactive extraction [27–29].High recovery of target product was obtained when SOE or reactive extraction was applied.Reactive extraction possesses complicated operation steps and potential reaction of aldehyde with by-products,whereas SOE is much easier to operate and scale-up.Major challenge using this technique is the problem associated with the safe handling of organic solvents due to their toxicity or/and flammability.In order to lower the risk to process safety and reduce the environmental impacts,a promising alternative — ionic liquids (ILs) has been explored in the separation process [30].

Ionic liquids are salts and exist as liquid at low temperatures.As shown in Table 1,typical cations of ILs include imidazolium,pyridinium,piperdinium,alkylammoium,phosphonium,etc.,while anions are more diverse,including organic and inorganic anion,such as halides,carboxylate,[CF3SO3]-,[SCN]-,[CH3SO4]-,[BF4]-,and [N(CN)2]-.ILs have some unique physical properties,such as negligible vapor pressure,high thermal stability,so ILs are also known as green solvents and designers’ solvents due to their manipulated composition of cations and anions.Therefore,the potential applications of ILs stimulate the broad investigations of ILs and their solutions [31–34].Another alternative of organic solvents is deep eutectic solvents (DESs),a homogeneous mixture of two or more constituents at certain composition ratio.DESs are formed through hydrogen bonding interaction and have many similar properties with ILs,so DESs are now widely recognized as IL analogues [35].

Both ILs and DESs have been widely used as solvent in the dissolution and transformation of biopolymers,extraction of bioactive compounds and metals as well as capture of SO2or CO2[32,33,35–37].In the production of bio-based chemicals,ILs were mainly used in the pretreatment of lignocellulose [38]and separation process by SOE based on hydrophilic ILs [30,39–43]or extraction with hydrophobic ILs [44,45],while DESs were mainly used in the pretreatment of lignocellulose [46,47].

In this review,the progress of SOE of bio-based chemicals using ILs would be introduced,including phase diagram,the partition behaviors of ILs and target products,and removal of impurities.The research showed that the extraction of bio-based chemicals by IL-based SOE systems was efficient at an appropriate condition,whereas the partition behaviors of products were different sometimes from those in organic solvent-based SOE systems,indicating the complexity of ILs-based SOE.Therefore,much more effort should be made to develop this technique.

2.Types and Properties of Ionic Liquid-based Aqueous Two-Phase Systems

Hydrophilic ILs can form aqueous two phases with organic and inorganic salts,amino acids and carbohydrates [34,48].Two types of aqueous two-phase system (ATPS),i.e.IL-salt and IL-glucose,were tried in the separation of bio-based chemicals [30,39–41,43].A typical phase diagram of ATPS is composed of three zones(Fig.1):liquid–liquid-solid ternary phase,liquid–liquid binary phase and homogeneous liquid phase,in which liquid–liquid binary phase is used for SOE.Generally,the top phase is rich in ILs,while the bottom is rich in salt or sugar.However,there is exception,in which the IL-rich phase sometimes existed as the bottom phase because the densities of ILs are greater than water.For example,[C4C1Im][BF4]was mainly distributed into the bottom phase and glucose in the top phase when glucose concentration was not greater than 22 wt% in a ATPS formed by 25 wt%[C4C1Im][BF4]and glucose [41].

The phase equilibrium of IL-based ATPS is influenced by the structures of ILs and various types of salting-out agents,as well as temperature and pH,which have been well reviewed in literature [34,48].In general,ILs with lower hydrophilicity or polarity showed a higher tendency toward salting-out from aqueous solution.For example,the overall order of cations for ATPS forming ability was[P4444]+>[N4444]+>[C4Py]+>[C4C1Im]+in concentrated aqueous solution of potassium phosphate,the ability of anions for ATPS formation was [CF3SO3]->[HSO4]->[N(CN)2]->[CF3CO2]->Br->Cl-[48],and the phase forming ability of the combination was[C4C1Im][CF3SO3]=[N4,1PI][CF3SO3]≈[N4,1Mo][CF3SO3]>[C4-C1Im][N(CN)2]>[C4C1Im][SCN]≈[C2C1Im][CF3SO3]≈[C2O1C1Im][CF3SO3]>[C4C1Im][CH3SO4][30].

The formation of aqueous two phases is resulted from the competition between salt ions and IL ions for the formation of hydration complex.Therefore,the Kamlet–Taft parameters,hydrogenbond donating ability (acidity) (α) and hydrogen-bond accepting ability (basicity) (β),are usually used to reflect the phase forming ability of cations and anions of ILs[49].Anions have stronger effect on phase formation than cation,and anions with lower β are easy to be salted-out to form two phases[30,48].This tendency is commonly applicable for the ILs which share a same cation with different anions.When α and β are used to evaluate the phase forming abilities of ILs containing different core cations,the applicability depends on the structure of cores.For example,both [P4444]+and[N4444]+don’t have hydrogen bond donating groups,so same α values and β values were determined when anions were same,e.g.α of 0.88 and β of 1.04 for [P4444][Ala]and [N4444][Ala],and α of 0.96 and β of 1.46 for [P4444][Val]and [N4444][Val][49].In this situation,all the Kamlet–Taft parameters cannot differentiate the phase forming ability.The calculated octanol/water partition coefficient ClogP shows the polarity difference,in which [P4444]+has a larger ClogP than [N4444]+,indicating a weaker polarity and stronger ability to form ATPS.

In IL-based ATPS,both solvent and salting-out agent are salts,thus pH plays an important role due to its effect on the ionized state of salts and ILs.A common phenomenon is that ATPS is readily formed in the composition of ILs and alkaline salts,while much harder with acidic and neutral salts[50,51].When the pH value of the system was decreased,the binodal curve upward shifted,leading to an increase of the area of homogenous phase [52].

Temperature can affect the solubility of ILs and salts,and the interaction among ILs,salts and water.With temperature increasing,the two-phase area decreased,and more ILs and salts are needed for phase separation.The variation of two-phase area depends on the characteristics of ILs and salts.For example,the upward shift of binodal curve was very obvious when temperature increased every10°C from 15°C to 65°C in the system of[C4C1Im]Br and tri-sodium citrate,while the shift in[C4C1Im]Cl-K2HPO4could be omitted when temperature increased from 20°C to 35°C[43,48].

Table 1 Name and respective acronym of the cation–anion combinations in ionic liquids

Fig.1.Schematic phase diagram composed of ionic liquid,salt and water.

The overview of SOE of bio-based chemicals by ILs was shown in Fig.2.In the SOE of bio-based chemicals,some important parameters are widely used to describe the distribution behavior and evaluate the extraction efficiency,including partition/distribution coefficient(K),recovery(Y),phase ratio(R)and selectivity(Sj).Kis usually defined as the concentration(or mass fraction)ratio of target product or IL (KIL) in the top phase to that in the bottom phase,describing the distribution behavior of the compound within the two phases[40–43,53].Yis the mass ratio of compound in the top phase to the total mass of this compound,exhibiting the extraction efficiency of target product from fermentation broth by SOE or the recovery of IL(YIL)[41–43].Ris the volume ratio of top phase to bottom phase,indicating the water content of top phase,which influences the energy consumption of following process for product concentration [41–43].Sjis the ratio of partition coefficient of target product to that of compoundj,indicating which one is preferably extracted to the top phase between target product and compoundj[42,43,53].

3.Application of ILs-based SOE in the Separation of Bio-Chemicals from Fermentation Broths

3.1.Bio-diols

Diols such as 1,3-propanediol(1,3-PD)and 2,3-butanediol(2,3-BD)are highly hydrophilic and their properties are closer to water,so their separation from fermentation broth is more challenging.When hydrophobic ILs were used to extract diol without addition of salt,the values ofKwere less than 1 [44].When hydrophilic ILs were used to separate diols by SOE,most of diols were preferred to distribute into the IL-rich phase(Table 2).On the basis of recovery,IL-based SOE is promising in diol separation.

Generally,diols take part in the formation of ATPS during SOE process,so a downward shift of binodal curve was observed,indicating less IL was required to form two phases at the same concentration of salt [23,43].The existence of diol is beneficial for diol extraction.The higher the concentration of diol,the higher the value ofK.For example,1,3-PD mass fraction increased from 0.01 to 0.07 at the mixing point,Kincreased from 4 to 10 [53].

In most cases,Kof diols increases with increasing concentration of salt [42,43].However,there are exceptions.When [C4C1Im]Cl-K2HPO4was used to extract 2,3-BD,Kincreased with increasing concentration of K2HPO4at lower concentration of IL,whileKdecreased with increased K2HPO4when [C4C1Im]Cl content was 30 wt% [43].Rdecreased when salt concentration increased[39,42,43].As a result ofRandK,Yincreased with increasing salt in the cases of 2,3-BD extraction by [C2C1Im][CF3SO3]-K2HPO4,10 wt% [C4C1Im]Cl-K2HPO4,and [EOA][Bu]-K3PO4systems [42,43],while decreased in the cases of 2,3-BD extraction by 15 wt%–30 wt% [C4C1Im]Cl-K2HPO4,and 1,3-PD extraction by [N2,1Pip]Br-K2HPO4[39,43].

The effect of ILs on the distribution of diols is more complicated,which is related with both structure and concentration of IL.The distribution of diols depends on the polarity or ability of hydrogen-bond formation of the cations and anions.In general,theKincreases with increasing polarity of the cations [30,39].Compared with cations,anions dominate theKmagnitude of diols[30].The general rule is that the larger the β value,the larger theKvalue.However,sometimes there is deviation.For example,the β value of [C4C1Im][N(CN)2]is less than that of [C4C1Im][SCN],but theKvalues of 1,3-PD in IL-phosphate were in the same range[30].

Fig.2.Overview of salting-out extraction of bio-based chemicals by ionic liquids.

Table 2 Salting-out extraction of bio-based chemicals by different IL-salt system

Three phenomena ofKchange were observed with the variation of IL concentrations.One was that increasedKwas obtained with increased IL concentration at the same concentration of salt,such as 2,3-BD in the system of [C2C1Im][CF3SO3]-K2HPO4[43],while the opposite was obtained in the system of [EOA][Bu]-K3PO4,in whichKdecreased with increasing concentration of IL [42].The third phenomenon was that largerKwas obtained under higher IL concentration when the salt concentration was low,while at high salt concentration largerKwas obtained under lower IL concentration,e.g.2,3-BD in system of [C4C1Im]Cl-K2HPO4[43].As a consequence,the highestYwas obtained at high concentrations of IL and salt in the first case,while detailed SOE conditions have to be explored to obtain highYin other cases.

Mülleret alexamined the temperature effect on 1,3-PD distribution from 32 to 42 °C using [C4C1Im][CF3SO3]-phosphate,and no significant difference ofKwas found [53].Similar result was also obtained in the investigation of temperature effect on phase diagrams of hydroxylammonium ILs and K3PO4(25–45 °C),[C4C1Im]Cl-K2HPO4and [C2C1Im][CF3SO3]-K2HPO4(20–35 °C)[42,43].Such a result was preferred,because most of the fermentation was carried out in the range of 30–37 °C.Once it was ready,the fermentation broth could be used for separation without considering temperature effect on separation efficiency.

Diols cannot be ionized,however,research showed that theKvalue of 1,3-PD increased constantly with increased pH of the[C4C1Im][CF3SO3]-phosphate system [53].The main reason is that pH changes the ionized state of salts and ILs,thus affects the interaction between diols,ILs and salts,and finally the distribution of diols.Moreover,pH influences the distribution of the ionized byproducts,such as acetic acid,butyric acid and lactic acid,thus the selectivity was also influenced.Therefore,pH is also an important factor influencing the separation of diols.

3.2.Bio-organic acids

In conventional extraction of bio-organic acids,large amines or organophosphorus compounds are used to separate organic acids from fermentation broths,while the strong affinity between solvent and organic acids enhances the extraction efficiency but makes the regeneration become problematic [56].As an excellent solvent superior to these conventional solvents,the extraction of organic acids by ILs gained much interest in recent years,and most of the works focused on the design of hydrophobic ILs to extract organic acid on the basis of coordination mechanismviaHbonding with undissociated acid or ion exchange mechanism at basic pH [55,57–60].Although these tuned hydrophobic ILs can extract organic acids from fermentation broths,a higher recovery of organic acids could be obtained with the addition of salts due to the salting-out effect [55].

Because the dissociation of organic acids in solution is strongly dependent on pH changes,so the distribution of organic acids in the two phases was influenced by the pH of solution.When lactic acid was extracted by[C10C1Im][Sac],it was found that the pH variation had a great influence on the recovery of lactic acid but a slight effect on the phase ratio,and a pH value below pKa was preferred for higher lactic acid distribution [55].In the intermediate pH range (coexisting in dual acid/conjugate base form),theKof organic acids decreased with increasing the pH of solution,while remained unchanged in the extremely high pH-range where the acid was existed in conjugate base form [55].

Due to the influence of pH on the dissociation of organic acids,two types of salts were efficient in the SOE of organic acids,neutral salt and alkaline salt.The addition of a neutral salt such as MgSO4or(NH4)2SO4was preferred because of the little change of existing acid form of organic acids,while the addition of alkaline salt caused the dissociation of acid to its conjugate base form.For example,when MgSO4was added into the solution of lactic acid for SOE by [C8C1Im][Sac],initial pH decreased from 2.04 to 1.51 and the equilibrium pH of the bottom aqueous phase decreased from 3.16 to 2.62,thus lactic acid remained in un-dissociated form[55].As a result,the existence of 500 g·L-1MgSO4in lactic acid solution increased the recovery of lactic acid of 15% compared to the “zero”source [55].Similar phenomenon was also observed in the SOE of succinic acid by [C6C1Im]Br,in which high recovery was obtained with the addition of (NH4)2SO4(Y,85.5%) and K2CO3(Y,82.8%) [40].

At a specified pH where organic acids exist only in acid or conjugate base form,the distribution of organic acids depends on no pH but salt used.For example,the salts of K2HPO4,K3PO4and K2CO3are alkaline,and theClgPvalues of K2HPO4and K3PO4obtained from ChemBioDraw are identical(-2.174),while the values of lgSare different,in whichSis the solubility.Therefore,lgSis used to represent the properties of salt in water,and the sequence of lgSwas K2HPO4(1.959)>K3PO4(1.822)>K2CO3(1.246)(Chem-BioDraw).When 20 wt%salt solution was used,theKvalue of succinic acid was increased with decreasing lgSin the SOE of succinic acid by[C6C1Im]Br,i.e.K2HPO4(0.45)

The characteristics of ILs also affect the distribution and extraction efficiency of organic acids.It seemed that little difference was observed in the extraction efficiency of succinic acid by piperidinium,imidazolium and pyrrolonidium ILs,while the ability of piperidinium ILs in the formation of aqueous two phases was higher than that of pyrrolidinium and imidazolium ILs in the presence of K2HPO4[54].The difference in phase forming ability could be well explained by theClgPvalues,while the extraction was more complicated.The highestYvalues of succinic acid using these ILs were around 95% in the range of 20–50 g·L-1succinic acid,while the highestYwas obtained at different succinic acid concentration,e.g.50 g·L-1for [N2,1Pip]Br,20 g·L-1for [N2,1PI]Br and 30 g·L-1for [C2C1Im]Br,indicating the complexity of the interactions in the SOE system.If same core structure of cation was used,more succinic acid was recovered when ILs with shorter alkyl chain were used [54].

The concentration of organic acid affects its own distribution,but the influence differs in ILs.For example,theKdecreased with increasing concentration of succinic acid in the systems of [Nn,1-Pip]Br-K2HPO4[54]and 40 wt% [C6C1Im]Br-10 wt% (NH4)2SO4[41].With increasing concentration of succinic acid,the recovery decreased in the systems of 40 wt% [C6C1Im]Br-10 wt% (NH4)2SO4[41]and [N2,1PI]Br-K2HPO4[54],changed little in the system[C2C1Im]Br/[C4C1Im]Br-K2HPO4,while increased in the systems of[N2,1Pip]Br-K2HPO4and [N4,1Pip]Br-K2HPO4[54].

Unlike the conventional extraction of organic acids by large amines or organophosphorus compounds,it seems that the reextraction is not problematic in the IL-based SOE.In the SOE of lactic acid by [C8C1Im][Sac]-MgSO4,95% of lactic acid was reextracted from the IL-rich phase by means of K2HPO4-stripping solution [55];71% of succinic acid was re-extracted from[C6C1Im]Br-rich phase by NaOH solution,which might be improved by optimizing the processes of extraction and crystallization [40].

3.3.Bio-ketones

Acetone and acetoin are the main bio-ketones produced by fermentation.As one of the products in ABE (acetone-butanolethanol) fermentation,less attention was paid on the separation of acetone from ABE fermentation broth,where the emphasis was the separation of bio-butanol.The three components have a polarity difference,so some ILs can separate them in different efficiency.For example,under optimized conditionN,N-bis(2-hydroxy propyl)octan-1-aminium hydroxide imidazole extracted 78%butanol,34% acetone and 6% ethanol [45].

Acetoin is the precursor of 2,3-BD,and both of them were detected in the fermentation broth,whether in the process of acetoin fermentation or 2,3-BD fermentation.The polarity of acetoin is a little less than that of 2,3-BD,so the SOE system for 2,3-BD separation could be applied in the separation of acetoin [23].With acetoin as the target product,SOE was explored using hydroxylammonium ILs as extractants[42].With ethanolammonium as cation,the IL with butyrate anion showed higher extraction efficiency than acetate and propionate.The increased concentration of[EOA][Bu]caused a decreasedKof acetoin,thus recovery decreased.The highestKof acetoin (40.54) was obtained at 6 wt% [EOA][Bu]-38 wt% K3PO4,where the recovery of acetoin,2,3-BD and IL was 92.7%,76% and 86%,respectively.The efficient extraction resulted from the hydrogen-bonding interaction between acetoin and O-H,N-H,-COO-of hydroxylammonium IL [42].

The above summary of published works demonstrated the complexity of SOE by ILs.In these works,β values of ILs were used to describe the effect of IL structures on the distribution of biobased chemicals,and the β values of ILs used in SOE were in the range of 0.5–0.9.However,part of β values are not available due to the diversity of ILs,whereasClgPcould be calculated by Chem-BioDraw.Therefore,ClgPof the ions in acid form or conjugate acid form are used in Fig.3.As shown in Fig.3,anions with samller ClogP or cations with larger ClogP are usually beneficial for the recovery of bio-based chemicals when ILs with same cation or same anion and core cation are used,and theClgPvalues of cations or anions are in the range of-4 to 1.Compared with ILs with high hydrophilicity or hydrophobicity,ILs with medium polarity are more applicable in the SOE due to combination of phase forming ability and extraction efficiency.

4.Comparison between IL-based and Organic Solvent-based SOE

The pursuit of more environmentally friendly solvent pushes the trials of replacing organic solvents with ILs.As shown in Table 2,ILs demonstrated potential in the separation of biobased chemicals.If only comparing the value ofK,it seems hydrophilic organic solvents are more preferred to SOE of diols,ketone and undissociated organic acids due to the interaction of hydrogen bonding,while hydrophilic ILs are preferred to SOE of dissociated organic acids due to the electrostatic interactions between ions.For example,in the SOE of succinic acid,Kobtained from 1-propanol (1.76) was larger than that from [C6C1Im]Br (1.06) when 200 g·L-1(NH4)2SO4was added into the aqueous solution,while smallerKwas obtained with the addition of 200 g·L-1K2CO3(0.01vs2.07) [40];in the SOE of 2,3-BD,theKfrom alcohol was a little larger than that from ILs when K2HPO4was used in SOE[24,43].However,Kwas not the only parameter to evaluate the separation method.By optimizing the composition of SOE system,similarYof target product could be obtained from IL-based and organic solvent-based SOE.Therefore,the followed operations,the separation of target product from ILs in the top phase and the recovery and recycling of ILs and salts,also play important roles in the application of IL-based SOE.

In the study of organic solvent-based SOE,the widely used salts were (NH4)2SO4and K2HPO4and their recovery and recycling of salts were well studied,in which over 90% salt was recovered by pH adjustment and solvent precipitation [61–63].Because the recovered salt contained residual impurities which were different from strains,the effect of salt recycling on product recovery was different and complicated.For example,in the treatment of acetoin fermentation broth produced bySerratia marcescens,acetoin recovery changed from 95.1%to 93.4%when the phosphate was reused 3 times [61],while acetoin recovery decreased 9% when phosphate was reused once in the treatment of acetoin fermentation broth fromB.subtilis[64].The recovery and reuse of Na2CO3were more environmentally friendly,which were integrated with fermentation process.In the SOE of 1,3-PD from fermentation broths by ethanol-Na2CO3,part of bottom phase was used to adjust pH during fermentation where 1,3-PD production was increased 16%,or reacted with CO2to produce NaHCO3which could reduce CO2emission in exhaust gas [65].

Fig.3.Effect of anions (A) and cations (B) of ILs on the distribution of bio-based chemicals.The SOE systems were as follows:A,1,3-PD and butyric acid,[C4C1Im][X]-K2HPO4;acetoin,[EOA][X]-K3PO4),where[X]- represents different anions and the acidformof anions in a ClgPincreasing order is CH3 SO4H,HSCN,CF3 SO3H,CH 3COOH,C2 H5COOH,and C3H7COOH;B,1,3-PD,[Y][CF3SO3]-K2HPO4;succinic acid,[Y]Br-K2HPO4,where [Y]+ represents different cations and the cation in a ClgP increasingorder is [C2C1Im]+,[N2,1Pip]+,[C4C1Im]+,[N4,1Pip]+,[EOA]+,[IPOA]+,[N6,1Pip]+ and [N8,1Pip]+.

No literature has reported the recovery and reuse of salts in ILbased SOE of bio-based chemicals,in which the current focus is the separation between product and ILs,and the recycling of ILs.In the recovery of antidepressant from pharmaceutical wastes by [N4444]Cl-or[N4444]Br-K3PO4,Zawadzkiet alproposed the direct reuse of salt in SOE after the removal of high molecular weight excipients by ultrafiltration [66].This idea is hard to conduct in most cases because the composition of fermentation broth is very complicated.The salt recovery method in solvent-based SOE was simple and easy to operate [61–63],however,the use of solvent deviated from the original goal of using IL-based SOE.In addition,the saltrich phase still contains a little ILs,which should be recovered.Therefore,further research on treatment of salt-rich phase is required for practical application.

In the organic solvent-based SOE,such as ethanol [24],isopropanol [26],and acetone [61],the solvent in the top phase can be easily recovered by distillation since they have much lower boiling points than diols or organic acids.However,the distillation of solvent concentrated the impurities and target products simultaneously,which was unfavorable for the followed purification of target products.When ILs were used for SOE,the impurities were remained in IL-phase because the general operation was to remove target product from IL-phase.For example,[C4C1Im][CF3SO3]in the top phase were recovered by vacuum distillation of water and followed gas stripping of 1,3-PD,thus impurities such as organic acids were remained in IL-phase [53].The comparison of 1,3-PD SOE using fresh and recovered[C4C1Im][CF3SO3]showed that recovered IL had no obvious negative effect on the distribution of target product and by-products [53].As for organic acid SOE systems,the organic acid can be separated by backward extraction using K2HPO4aqueous solution [55]or alkaline solution [40]for following purification,thus the ILs were also recovered.

5.Integrated Bioprocesses

SOE is a primary separation method used to remove most of the impurities.Based on SOE,some integrated bioprocesses were proposed to simplify the process and reduce cost,including utilization of IL as extractant and catalyst,comprehensive bioprocessing of lignocellulose reusing IL as solvent and extractant,and utilization of fermented byproduct for synthesis of particular IL.The first example is an integration of SOE and derivative synthesis of acetoin on the basis of hydroxylammonium ILs(Fig.4A)[42].Hydroxylammonium ILs are both efficient extractant for SOE of acetoin and catalyst for the aldol condensation between acetoin and furfural or 5-methylfurfural(5-MF),and the products were C9/C10fuel precursors which could be converted to alkane by hydrodeoxygenation [67].After SOE of acetoin,the acetoin-rich top phase could react with 5-methylfurfural under the catalysis of IL in the same phase,and the reactive C10product was liable to obtain in the top phase after reaction due to its hydrophobicity.At least,this integrated process could omit the separation of acetoin from IL.

The second integration is comprehensive bioprocessing of lignocellulose reusing IL as solvent and extractant.The pretreatment of lignocellulose by ILs has been proved to be efficient by many works,and further investigations are still required before the commercialization of this process due to the ILs cost and system complexity associated with recovery and recycling of ILs,biomass separation and downstream processing [68,69].The strategy of simultaneous saccharification and fermentation (SSF) integrated two downstream processes of saccharification and fermentation,thus enhanced the production process.However,the typical operation of extensive washing of biomass to remove residual ILs poses a challenge for the scale-up of any IL pretreatment technology.Using IL-tolerant cellulase and microorganisms would reduce the amount of water for washing and waste disposal associated with washing [70–73].For example,a one-pot,wash-free process was tried by combining IL pretreatment and saccharification into a single vessel,and 81.2%glucose and 87.4%xylose were liberated after hydrolysis for 72 h at 70 °C with cellulase loading of 5.75 mg·g-1biomass at 10% [C2C1Im][CH3COO][70].Using an engineered ILtolerant strain ofE.coli,isopentanol was produced from crude lignocellulose hydrolysate without removal of ILs[73].Until now,no literature has reported that ILs could be used both in the processes of lignocellulose pretreatment and product separation.With further understanding of the relationship between IL structure and its function,it may be possible to design such an IL for comprehensive bioprocessing of lignocellulose used as solvent (IL1) and extractant (IL2) (Fig.4B).

The third integration is utilization of fermented byproduct for synthesis of particular IL/DES.In recent years,some selected natural DESs were also found to be efficient in the pretreatment of lignocellulose [74].For example,a glucose yield of 94.3% was obtained in the enzymatic hydrolysis of pretreatedEucalyptus camaldulensisbiomass by a DES solution (choline chloride:lactic acid=1:10)[75].These DESs are biodegradable and biocompatible with enzyme and miroorganisms,because their constituents are natural products or biomass-derived products,such as choline chloride and lactic acid,amino acid and lactic acid,betaine and lactic acid,choline chloride and glycerol[74–78].Thus,the byproduct of lactic in fermentation from lignocellulose could be used to synthesize lactic acid-derived DESs as shown in Fig.4C,which would supply the DESs loss in lignocellulose pretreatment and reduce the processing cost.Of course,the separation of products might be IL-based SOE as shown in Fig.4B.

Fig.4.Integration processes of bio-based chemical production based on ILs or DESs.A,integration of SOE and derivative synthesis;B,integration of lignocellulose pretreatment and SOE;C,integration of lignocellulose pretreatment and IL/DES synthesis.

6.Conclusions and Future Perspectives

Salting-out extraction of bio-based chemicals was a process of energy saving compared with other separation process.The results of the extraction efficiency of target products by ILbased SOE were promising,indicating it is feasible to replace organic solvent-based SOE system.ILs with medium polarity were preferred in the SOE,and the recovery of bio-based chemicals was commonly greater than 90% in one step under appropriate conditions.In the selection of ILs for SOE,the ions with a ClogP value in the range of -4 to 1 were applicable.By comparing ILs with same cation or same anion and core cation,larger distribution coefficient of target product was obtained when anion with smaller ClogP or cations with larger ClogP were used in SOE.However,the research work of IL-based SOE of bio-based chemicals was still in its early stage,and there is still a lot of work required to do for the practical application of this process.The following issues are suggested.

(1) Increase the extraction selectivity of target products over by-products.The present work showed that the conditions for high recovery of target products sometimes could not remove by-products efficiently,such as organic acids from diol fermentation broth.Therefore,further study is required to design appropriate ILs for target product recovery and byproduct removal.

(2) Efficient separation of target products and ILs.After SOE,the target products were rich in the IL-phase.The diols are highly hydrophilic with high boiling points,different organic acids have different back-extraction methods,while different ILs have different characteristics.So the separation between ILs and target products is more difficult than that between organic solvents and target products.The desired method is to integrate this process with downstream purification or derivative synthesis.Therefore,the methods for the efficient separation of target products and ILs are needed to be developed for the application of IL-based SOE.

(3) Reduce the cost of ILs.Until now,the price of ILs is still expensive,so cheap production process of ILs has to be developed for the commercialization of ILs.The feasible way is to develop efficient synthetic method to reduce operation cost,and use cheap raw materials,such as organic acids by-products in diol fermentation.If these organic acids could be used to prepare ILs during the process of diol separation,or used in crude product form,both the diol separation cost and the IL production cost will be reduced.Another point is the recovery and recycling of ILs.In present work of SOE,less attention was paid on the distribution and recovery of ILs,thus further study is required.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Grant No.21978038).