Qian Liu,Xianglan Zhang,*,Wei Li

1School of Chemical and Environmental Engineering,China University of Mining and Technology,Beijing 100083,China

2 School of Chemical Engineering,Beijing Institute of Petro-Chemical Technology,Beijing 102600,China

Keywords:Extraction M-cresol Ionic liquids Separation COSMO-RS Hydrogen bond

ABSTRACT Low temperature coal tar contained a large amount of phenols,aromatic hydrocarbons and alkanes;the separation of phenols from coal tar has a great significance to the deep processing of coal tar.In this work,the separation of m-cresol from cumene and n-heptane by liquid-liquid extraction using ionic liquids(ILs)as extractants was studied.The suitable ILs were screened by conductor-like screening model for real solvents (COSMO-RS)model and the liquid-liquid phase equilibrium(LLE)experiments were to verify the accuracy of the screening results.The extraction conditions such as extraction time,extraction temperature and mass ratio of ILs to model oils were evaluated.An internal mechanism of the m-cresol extract by ILs was revealed by COSMO-RS calculation and FT-IR.The results showed that the selected ILs can extract m-cresol effectively from cumene and nheptane,1-ethyl-3-methylimidazolium acetate(emimCH3COO)was the best extraction solvent.A hydrogen bond between anion of ILs and phenolic hydroxyl groups was observed.M-cresol in model oils could be extracted with extraction efficiencies up to 98.85% at an emimCH3COO:model oils mass ratio of 0.5 and 298.15 K,emimCH3COO could be regenerated and reused for 4 cycles without obvious decreases in extraction efficiency and extractant mass.

1.Introduction

Phenolic compounds (such as phenol,cresol,etc.)are important chemical raw materials and organic synthetic intermediates.They are widely used in the synthesis of phenolic resin,caprolactam,plant protectant,plastic protectant,preservative,lubricant,rubber antiager,antioxidant and all kinds of additives[1,2].These compounds are mainly derived from coal tar,coal liquefied oil and petroleum.In particular,the content of phenolic compounds in low temperature coal tar can reach 20%-30% [3],the existence of phenolic compounds affects the storage stability of the coal tar and increases hydrogen consumption in the following oil refining process[4-6].So it is necessary to extract and separate phenolic compounds from coal tar.

The traditional method of extracting phenols from low temperature coal tar is mainly alkali washing.In this method,strong alkaline aqueous solutions (such as NaOH)and mineral acids (such as H2SO4or H2CO3)are used[7,8].This method not only consumes a large amount of alkaline solution,but also produces a large amount of phenol containing wastewater.After the separation of phenolic compounds,the phenolic compounds in phenol waste water are reclaimed.The process is complicated and uneconomical.Other methods to separate phenols from oils have been investigated,such as methanol-mediated water extraction,aqueous Na2CO3solution extraction and aqueous salt solution [9,10].However,these methods use water as a solvent and also produce a large amount of phenol containing wastewater.Therefore,these results require the development of a separation method that uses environment-friendly methods.

In recent years,a new green solvent,ILs have many advantages,such as low vapor pressure,non-toxic,good thermal stability and structure design.It has a great advantage in extraction and separation process as extraction solvent.There are growing interests using ILs as extractants to separate phenolic compounds from oils.For instance,Hou et al.[11]separated phenol from a model oil of hexane with a high extraction efficiency of phenol by using imidazolium-based ILs,and 1-butyl-3-methylimidazolium chloride (bmimCl)was used to extract phenols from real coal liquefaction oil,the extraction efficiency of phenols could reach 90%.As the same time,a new type of solvents called deep eutectic solvents(DES),which have extremely similar properties to ILs,they are also called ionic liquid analogues,has applied widely in separation[12].Pang et al.[13]found that choline chloride can form DES with phenols to separate phenols from model oils and recycle many times.Lin et al.[14]studied the LLE for the ternary mixture of phenol+toluene +quaternary ammonium salts(QAS)at 298.15 K,313.15 K and 338.15 K under atmospheric pressure,and discussed the effects of temperature,type and dosage of QAS on the equilibrium.Guo et al.[15]studied different QAS to separate phenols from model oils,and discussed the effects of different anions and cations on the extraction efficiency of phenol.Yao et al.[16]studied that betaine and L-carnitine could form DES with phenol,and the DES were insoluble in model oils,FT-IR showed that betaine and L-carnitine formed DES with phenol through hydrogen bonding.There are no Cl−in betaine and L-carnitine,and they greatly reduce the corrosion of steel equipment[17].However,the research on separating phenols from oils by using ILs is limited to the experiment.Through the combination of different anions and cations,there are ten million kinds of ILs in theory.Therefore,it is significant to screen out the suitable ILs[18,19].

Due to the relatively complex composition of coal tar,in order to screen out suitable ILs to extract the phenols from coal tar,a simple design method was adopted[20].m-cresol,cumene and n-heptane were used as mainly representative components in low temperature coal tar.COSMO-RS[21-24]model was adopted to calculate the activity coefficient,and the LLE experiment was carried out to select the ILs.In addition,the distribution coefficient and the selectivity were calculated according to the LLE data and used as the standard to evaluate the separation efficiency.The experimental data were verified by Othmer-Tobias,Bachman and Hand equation[25,26].The mechanism of the extraction process was explored by COSMO-RS calculation and FT-IR.Moreover,the effects of the structure of ILs and extraction conditions,such as extraction time,extraction temperature and mass ratio were studied.Based on the analysis of the above results,the suitable IL to extract m-cresol from model oil was selected.The recycling performance of the IL was also studied.

2.Material and Methods

2.1.COSMO-RS calculation details

There are generally two steps in the COSMO-RS prediction procedure[27],namely(1)the quantum chemical COSMO computation for the molecular species involved,and(2)the COSMO-RS statistical thermodynamic treatment.A standard COSMO-RS prediction only requires the screening charge density(SCD)information of the interested compounds.In this work,the Turbomole[28]software was adopted to the SCD at the BP-TZVP level.The main steps of calculation are as follows:the structure was drawn in the COSMOthermX software and optimized by density functional theory(DFT)[29],and then the optimal structures under ideal gas condition and COSMO files were obtained.By means of quantum chemistry calculation,the σ-profile of m-cresol,cumene,nheptane,20 anions and 10 cations were obtained.The detailed information about the 20 anions and 10 cations is provided in Tables S1 and S2,respectively.An IL was treated as a mixture of ions.In this work,anions and cations are treated as individual species with equal molar fractions[30].

Then,the activity coefficient of a solute i in a solvent S(γiS)was calculated on the basis of the difference between the chemical potential of the solute in the solvent μiSand in the pure solute μii,which was calculated according to Eq.(1).The infinite dilution activity coefficients of mcresol,cumene,and n-heptane in the ILs were calculated by COSMOthermX (version:BP_TZVP_C30_1501),which were used as the criterion for measuring the performance of extraction solvent.The solute distribution coefficient at infinite dilution,which revealed the extraction capacity of the solvent,can be calculated in terms of the infinite dilution activity coefficientof the solute in the solvent,which was calculated according to Eq.(2),where the subscript i represented cumene,n-heptane or m-cresol.The selectivity coefficient provided a useful index for selection of suitable IL,which was defined as the ratio of the composition of m-cresol in the extractive phase to that of cumene or n-heptane in the raffinate phase.The selectivity coefficient at infinite dilution[31]was calculated according to Eq.(3).In Eq.(3),stand for the infniite dilution activity coeffciients of cumene,n-heptane and m-cresol in the IL solvent,respectively.The performance index at infinite dilutionexpressed the comprehensive extraction capacity of the IL [20,27],which was calculated according to Eq.(4).

2.2.Chemical materials

1-Ethyl-3-methylimidazolium acetate (emimCH3COO,99%),1-butyl-3-methylimidazole acetate (bmimCH3COO,99%),1-butyl-3-methylimidazolium chloride (bmimCl,99%),1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4,99%),1-butyl-3-methylimidazolium hexafluorophosphate(bmimPF6,99%),m-cresol(99%),cumene(99%)and isopropyl ether(99%)were purchased from Shanghai Macklin Biochemical Co.,Ltd.,China.Absolute ethyl alcohol(99.7%)and n-heptane(99.5%)were purchased from Beijing Chemical Factory,Beijing,China.Acetone with a purity of ＞99.5%was purchased from Sinopharm Chemical Reagent Co.,Ltd.,Beijing,China.All the chemicals in the experiments were used without further purification.

2.3.Experimental processes

In order to further evaluate the separation efficiency of the ILs in practice,LLE experiments were conducted and carried out at 298.15 K controlled by the water bath under atmospheric pressure.The experimental method was described in literature[20,32,33],and the reliability of the experimental system has been evaluated[34].In this paper,a certain amount(10.0 g ILs+8.0 g cumene+2.0 g n-heptane)of the liquid mixture was sealed in a bottle,then adding different qualities of mcresol(such as 1.0 g,1.5 g,2.0 g,2.5 g,3.0 g,3.5 g and 4.0 g)to the bottle and stirred with a magnetic stirrer for at least 1 h and stood for 1 h.Afterwards,samples were taken from the oil and IL phases with syringes and analyzed by gas chromatography(GC)to determine their compositions using an external standard method.Acetone was used as external standard,absolute ethyl alcohol was used to dilute the samples and acetone to maintain a homogeneous mixture.Due to ILs have negligible vapor pressure,they cannot be analyzed by GC.Therefore,the IL composition was determined by a mass balance of the measured mass fractions of the organics [35].The analysis was performed at least two times for each sample.In the process,the error bar of the liquid phase composition was calculated in TableS3.

In the extraction experiments,the mixtures of m-cresol(40 wt%),cumene(40 wt%)and n-heptane(20 wt%)were used as model oils.A certain amount of model oil and IL with a specific mass ratio were sealed in a bottle,then the following operation was the same as the LLE experiments.One change was that the oil and IL phases needed to separate by a separating funnel,and the mass of the IL phase was weighed.The experimental apparatus of LLE and extraction experiments was shown in Fig.1.

Fig.1.Schematic diagram of LLE and extraction experiments.1.thermometer;2.circulating water chamber;3.equilibrium cell chamber;4.circulating water inlet;5.condenser;6.circulating water outlet;7.light phase port;8.heavy phase port;9.magnetic stirrer.

To recycle the ILs,it is necessary to regenerate them from the mixture of IL and m-cresol.In this work,we used isopropyl ether as an anti-solvent to regenerate IL and recover m-cresol from the mixture of IL and m-cresol.IL of 5.0 g was added to 10.0 g model oil at 298.15 K.Then the mixture was stirred with a magnetic stirrer for 30 min and allowed to stand for 30 min.The upper and lower phases were separated by a separating funnel,the mixture of IL and m-cresol in lower phase.Then 15.0 g of isopropyl ether was added as an anti-solvent to the lower phase,then the mixture was stirred with a magnetic stirrer for 1 h and allowed to stand for 1 h.The IL was separated in the new lower phase and the mass of it was weighed.The experimental apparatus was shown in Fig.2.

Fig.2.Schematic diagram of regenerating IL.1.organic phase;2.IL phase.

2.4.Definitions

Distribution coefficient(Dm-cresol),selectivity coefficient(Sm-cresol)and extraction efficiency(EEm-cresol)of m-cresol were calculated according to Eqs.(5)-(7):

where w1andare the mass fractions of cumene in the oil phase and the IL phase,respectively,w2andare the mass fractions of n-heptane in the oil phase and the IL phase,respectively,w3andare the mass fractions of m-cresol in the oil phase and the IL phase,respectively,mILand mOare the mass of IL phase and model oil,respectively,and wO3is the mass fraction of m-cresol in model oil.In this paper,we defined 1,2,3 and 4 of subscripts represented cumene,n-heptane,m-cresol and ILs,respectively.

2.5.Analysis methods

The contents of materials were analyzed using a GC(GC-SP3420,Beifei-Ruili)equipped with a capillary column KB-WAX (50 m ×0.25 mm×0.25 μm)and a flame ionization detector(FID).The temperature program of the GC was started at 353.15 K,settled for 2 min,and then increased at a rate of 15 K·min−1until the temperature reached 473.15 K and settled for 14 min.The temperature of the injection port and FID were 493.15 K.The extraction mechanism and the recycled IL were measured using Fourier transform infrared spectrometer(FT-IR,T27-Hyperion,Bruker,Germany).The recycled IL was also measured using1H NMR(700 MHz,DMSO-d6,AVANCE III,Bruker,Switzerland).

3.Results and Discussion

3.1.COSMO-RS calculation results

Fig.3 showed the m-cresol distribution coefficients in different ILs at the infinite dilution condition.For most of the investigated cations,the m-cresol distribution coefficients of ILs with anions CH3COO−and Cl−were generally higher than ILs with other anions,particularly with CH3COO−.This is because anions CH3COO−and Cl−and m-cresol all have strong polarity,there is a strong interaction between them and m-cresol.Compared with ILs with F-containing anions,such asand,anions containing a Cl atom or a carboxyl group have more delocalized electrons than F-containing anions.Anions with more delocalized electrons show weaker interactions with cations,which boosts the interaction of the IL with m-cresol[27].Other anions such asand,due to they have a larger volume,detrimental to contact with m-cresol,so their m-cresol distribution coefficients are lower than that of other anions.

Fig.4 showed the σ-profiles for m-cresol,cumene,n-heptane and anions (CH3COO−,Cl−,In Fig.4,the two vertical dashed lines were the locations of the cut-off values for the hydrogen bond(HB)donor(σ ＜−0.82 e·nm−2)and acceptor(σ ＞0.82 e·nm−2)[36,37].The importance of this cut-off value lies were in the fact that profile lying in the left side of σ=−0.82 e·nm−2will have high hydrogen bond donor ability and right side of σ=0.82 e·nm−2will have high hydrogen bond acceptor ability.For the three organic compounds,the main peak of cumene and n-heptane was in the −0.82 e·nm−2＜σ ＜0.82 e·nm−2which indicated non-polar region,while the peak of m-cresol was at the σ ＞0.82 e·nm−2and σ ＜−0.82 e·nm−2that indicated polar region.For four anions,all of them had a strong peak at σ＞0.82 e·nm−2,indicating that the anions had a strong polarity,they can form hydrogen bond with m-cresol.Combined of the respective peak positions of the four anions,their polar orders were:CH3COO−＞Cl−＞.With the increase of the polarity of anions,the hydrogen bond interaction between ILs and m-cresol increased,which was agreement with the distribution coefficient values of ILs in Fig.1.

Fig.3.m-Cresol distribution coefficient of ILs composed of 20 different anions and 10 different cations(Tables S1 and S2)predicted by the COSMO-RS method at the infinite dilution condition(1.Cl−,2.Br−,3.,4.,5.CH3COO−,6.CF3COO−,7.(CF3SO2)2N−,8. , 10. SCN−, 11. , 12. , 13. , 14. ,15.,16.,17.,18.,,20.B().

Fig.4.The σ-profiles form-cresol,cumene,n-heptane and anions (CH3COO−,Cl−,BF4 −,PF6 −).

Fig.5.m-Cresol selectivity coefficient in cumene(a)and n-heptane(b)of ILs composed of 20 different anions and 10 different cations predicted by the COSMO-RS method at the infinite dilution condition.

Fig.5 showed the selectivity coefficients of the m-cresol-cumene and m-cresol-n-heptane in different ILs at the infinite dilution condition,respectively.For most of the investigated cations,the change regulation about m-cresol selectivity coefficients in cumene and n-heptane of ILs with anions were the same as the m-cresol distribution coefficients.For most of the anions,imidazolium ILs had larger selectivity coefficients than other ILs,m-cresol selectivity coefficients gradually decrease with the cation side chain length,i.e.,emim+＞bmim+＞hmim+＞omim+.This is because the length of alkyl on the cation of ILs may influence the π-π conjugation of the cation and m-cresol.An increase in the length of the alkyl on the cation of ILs may decrease the interaction of ILs with m-cresol and increase the interaction of ILs with cumene and n-heptane,resulting in a decrease of the m-cresol selectivity coefficients.This is consistent with the reported experimental results where bmimCl,hmimCl and omimCl were studied as solvent for the separation of phenol from hexane[11].And due to imidazolium ILs were easy to synthesize and their chemical properties were stable,emim+and bmim+were suitable cations which separated m-cresol from cumene and n-heptane.In addition,the m-cresol selectivity coefficients in nheptane of ILs were generally higher than in cumene.Both of the mcresol and cumene contain aromatic rings,cumene could also be combined with ILs by π-π conjugation,it makes them more difficult to separate.Corderí et al.[35]also reported that emimMSO4had a higher affinity for toluene than the alkanes.

Fig.6 showed the σ-profiles for m-cresol,cumene,n-heptane and cations(emim+,bmim+,hmim+,omim+).The σ-profiles of the four cations were similar in nature.All cations showed a peak at the outer most position in the negative direction.It showed that a very small fraction of the profile lies in the donating region for the cations,so the hydrogen bond interaction of the cations with m-cresol was negligible.And it is clear that the σ-profiles of cations and m-cresol were complimentary which indicated high miscibility through CH-π bonding.

Fig.6.The σ-profiles for m-cresol,cumene,n-heptane and cations (emim+,bmim+,hmim+,omim+).

In order to evaluate the overall extraction efficiency of ILs,their performance index at infinite dilution was analyzed.Based on the above results,the PI∞of the emim-based and bmim-based ILs paired with the anions of,andwere checked for the separation of m-cresol and cumene,m-cresol and n-heptane.The results were presented in Fig.7.As shown in Fig.7,the anions had obviously influence on the separation for the mixture of m-cresol and cumene or nheptane compared with the slight effects of cations.Meanwhile,the selectivity coefficients of the all emim-based and bmim-based ILs were greater than 1 in Fig.5,which indicated that the above ILs were effective in the separation of m-cresol and cumene or n-heptane.Due to m-cresol had stronger hydrogen-bond donor ability,it could combine with those anions with the hydrogen bond acceptor abilities through hydrogen bonding.Therefore,the ILs containing anions such as CH3COO−,Cl−,andwere effective in extracting m-cresol from cumene and n-heptane.In addition,the order of the PI∞of these anions was as follow:,which was agreement with the results of screening charge density as shown in Fig.4.

Based on the above discussion of the effect of cation and anion types on the separation of m-cresol from cumene and n-heptane,five imidazolium-based ILs:bmimCH3COO,bmimCl,bmimBF4,bmimPF6and emimCH3COO were selected as solvents for the liquid-liquid equilibrium experimental verification in the following work.

3.2.Experimental of liquid-liquid phase equilibrium

Fig.7.M-cresol performance index in cumene(a)and n-heptane(b)of ILs composed of 20 different anions and emim+,bmim+cations predicted by the COSMO-RS method at the infinite dilution condition.

In order to separate m-cresol from the mixture of cumene and nheptane,according to the results of IL solvent screening in the COSMO-RS,the liquid-liquid equilibrium data for the quaternary mixture of cumene+n-heptane+m-cresol+emimCH3COO/bmimCH3COO/bmimCl/bmimBF4/bmimPF6were measured at 298.15 K under atmospheric pressure.The experimental data and the quaternary phase diagram for the mixture of cumene-n-heptane-m-cresol-ILs were shown in Table S3 and Fig.8,respectively.From the LLE diagram,five ILs could form big two-phase area with m-cresol,which were considered as the appropriate solvents used to extract m-cresol from cumene and nheptane.It is clear that m-cresol showed a much higher solubility in the IL phase than cumene and n-heptane.In addition,the miscibility gap was very broad at the low m-cresol content region,which indicated that ILs were very suitable to separate m-cresol at low concentration from cumene and n-heptane.

Fig.8.Quaternary phase diagram for Cumene-n-heptane-m-cresol-ILs at 298.15 K,(a)emimCH3COO,(b)bmimCH3COO,(c)bmimCl,(d)bmimBF4,(e)bmimPF6.

In order to further compare the separation effects of different ILs,the m-cresol distribution coefficient(a)and selectivity coefficient(b)were calculated and shown in Fig.9.From Fig.9a,emimCH3COO had a much higher m-cresol distribution coefficient than the other four ILs.In addition,its m-cresol distribution coefficient decreased at increasing mcresol concentration in the IL phase.From Fig.9b,the m-cresol selectivity coefficient of all five ILs also decreased as the m-cresol concentration in the IL phase increased.EmimCH3COO showed the largest m-cresol selectivity coefficient over the whole m-cresol concentration region,and it was about 4-6 times higher than the m-cresol selectivity coefficient of bmimPF6.The LLE experiment data confirmed that emimCH3COO in fact had a high extraction efficiency at low m-cresol concentration,indicating its great potential for the deep separation of m-cresol from cumene and n-heptane.Based on the above research,the order of the extraction ability of the ILs was emimCH3COO/bmimCH3COO＞bmimCl ＞bmimBF4＞bmimPF6,which was agreement with the screening results of COSMO-RS model.This is consistent with the reported experimental results where bmimCl,bmimBF4and bmimPF6were studied as solvent for the separation of phenol from hexane[11].These findings suggested that the COSMO-RS model was an effective method for screening of ILs without the need for extensive experiments.

In order to check the thermodynamic consistency of the quaternary experimental data,Bachman,Hand and Othmer-Tobias equations were used to correlate the data of LLE of ILs-m-cresol-cumene-n-heptane at 298.15 K,respectively.The expressions of three equations were as following Eqs.(8)-(10):

where w1,w2and w3are the mass fractions of cumene,n-heptane,and m-cresol in the oil phase,respectively,w3′and w4′are the mass fractions of m-cresol and IL in the IL phase,respectively.The equation parameters and corresponding linear regression coefficients R2were shown in Table 1 and the straight lines plotted by Eqs.(8)-(10)were shown in Fig.10.In Table 1,the R2of each equation was close to 0.99,which indicated that reliability of the experimental LLE data.

Fig.9.Experimental m-cresol distribution coefficient(a)and selectivity coefficient(b)for the quaternary systems Cumene-n-heptane-m-cresol-ILs at 298.15 K.

3.3.Effect of extraction conditions

3.3.1.Effect of extraction time

Extraction time was an important parameter for the extraction separation,so it was investigated and the result was shown in Fig.11.The extraction time was investigated from 5 min to 60 min,and emimCH3COO,bmimCl and bmimPF6were used for research.For the three kinds of ILs,the m-cresol extraction efficiencies increased quickly at 10 min,then became constant as the extraction time continually increasing.Taking emimCH3COO as an example,the m-cresol extraction efficiency was 98.65% when the extraction time was 10 min,which the value was much better than bmimPF6.From the experiment,we found that the time taken to reach a complete equilibrium of the extraction was generally 10 min.In order to ensure a sufficient extraction process,the extraction time of 30 min was chosen in the following work.

3.3.2.Effect of the extraction temperature

The extraction temperature was investigated from 298.15 K to 338.15 K.The extraction process was performed with IL(emimCH3COO,bmimCl,bmimPF6)to model oil mass ratio of 1:2 for 30 min.The results were shown in Fig.12.With the increase of temperature,the m-cresol extraction efficiency decreased slightly with emimCH3COO and bmimCl,but decreased sharply with bmimPF6.This is because the solubility of mcresol in cumene and n-heptane increases with increasing temperature[38].As indicated in Fig.4,there are stronger hydrogen bond interactions between m-cresol and the IL of emimCH3COO and bmimCl.The hydrogen bond is much stronger than the molecular interactions between m-cresol and cumene or n-heptane.At the investigated temperature range,temperature has a small effect on the hydrogen bond between ILs and m-cresol[39].But for bmimPF6,there are weaker hydrogen bond interactions and mainly van der Waals interactions between m-cresol and bmimPF6,temperature has a great influence on the extraction effect of bmimPF6.This is consistent with the reported experimental results where N8881Cl and hmimBF4were studied as solvent for the separation of methacrylic acid from diluted aqueous solution [40].In summary,increasing temperature is unfavorable for extraction separation,and room temperature is suitable for the separation.The extraction temperature of 298.15 K was chosen in the following work.

3.3.3.Effect of the mass ratio of IL to model oil

The extraction efficiencies of m-cresol in the IL-oil quarter phase system for different mass ratio of ILs to model oil were shown in Fig.13.For bmimPF6,the m-cresol extraction efficiency increased from 28.52%to 82.05% alone with mass ratio changed from 0.1 to 2.EmimCH3COO showed good m-cresol extraction efficiency even at less mass ratio,which could achieve 85.76%when the mass ratio of emimCH3COO to model oil was 0.1.The extraction efficiency of m-cresol for bmimCl was similar to emimCH3COO at the same mass ratio,because they both have stronger hydrogen bond interactions ability with m-cresol.It could be seen that the m-cresol extraction efficiency increased with the increase of mass ratio of IL to model oil for the three kinds of ILs.This is consistent with the reported experimental results where betaine and L-carnitine were studied as solvent for the separation of phenol from toluene[16].In a word,emimCH3COO was the best extractant in the three kinds of ILs.Due to the highly cost of ILs,high m-cresol extraction efficiency required as small an amount of IL as possible,the mass ratio of IL to model oil of 0.5 was suitable for the separation.

Table 1 Fitting parameters of Bachman,Hand and Othmer-Tobias equations

Fig.10.Bachman(a),Hand(b)and Othmer-Tobias(c)plots of the quaternary systems Cumene-n-heptane-m-cresol-ILs.

Fig.11.The extraction efficiencies of m-cresol for bmimPF6,bmimCl,and emimCH3COO at different extraction times.Extraction conditions:extraction temperature was 298.15 K,mass ratio of IL to model oil was 1:2.

3.4.Mechanism of the separation

3.4.1.Interactions on IL-m-cresol

The intermolecular interaction between ILs in Fig.8 and m-cresol was investigated under COSMO-RS model.The interaction of molecules in COSMO-RS was divided into three parts:electrostatic contact interaction energy(EMF),hydrogen bond interaction energy(EHB)and van der Waals interaction energy(EvdW)[21,40,41].The three interaction energies of m-cresol with different ILs were calculated by COSMOthermX and shown in Table 2.It could be seen that the EMFand EvdWvalues for different m-cresol-IL complexes were close to each other and the EHBvalues were highly different for m-cresol-IL complexes.This showed the hydrogen bond interaction played a very important role between IL and m-cresol.In addition,the intensity of hydrogen bond interaction of ILs followed the order:emimCH3COO/bmimCH3COO ＞ bmimCl＞bmimBF4＞bmimPF6,which were agreement with the experiment distribution coefficients of m-cresol in these ILs.For emimCH3COO,bmimCH3COO and bmimCl,the mainly interaction between ILs and mcresol was hydrogen bond interaction;for bmimBF4and bmimPF6,the mainly interaction between ILs and m-cresol was van der Waals interaction.

Fig.12.The extraction efficiencies of m-cresol for bmimPF6,bmimCl and emimCH3COO at different temperatures.Extraction conditions:extraction time was 30 min,mass ratio of IL to model was 1:2.

Fig.13.The extraction efficiencies of m-cresol for bmimPF6,bmimCl and emimCH3COO at different mass ratio of IL to model oil.Extraction conditions:extraction time was 30 min,extraction temperature was 298.15 K.

3.4.2.FT-IR analysis

The ILs could extract m-cresol from the cumene and n-heptane because the interactions between the ILs and m-cresol were stronger than the interactions between m-cresol and cumene or n-heptane.Based on the above research,hydrogen bond could be formed easily between m-cresol and the ILs with CH3COO−,Cl−,andanion.FTIR is usually used to analyze how hydrogen bond is formed in the extraction process[42].Hydrogen bond has a considerable influence on the v-OH stretching vibration in m-cresol,which is found at wave numbers between 3200 and 3500 cm−1[16].The FT-IR spectra of m-cresol,emimCH3COO,bmimCl and two complexes were shown in Fig.14.Taking FT-IR spectra of m-cresol,emimCH3COO and the complex m-cresolemimCH3COO as an example,the v-OH stretching vibration of m-cresol was observed at 3329 cm−1,which was shifted to 3156 cm−1when the complex was formed.The anion CH3COO−makes a portion of the electron cloud of the oxygen atoms on CH3COO−transfer to the hydrogen bond,resulting in the FT-IR spectrum of m-cresol in complex undergoing a change in vibrational state[43].The shift of the v-OH stretching vibration showed the existence of hydrogen bond between m-cresol and emimCH3COO.The v-COO−stretching vibration of emimCH3COO and v-Cl−of bmimCl occurred at 1582 cm−1and 1632 cm−1,which shifted to 1489 cm−1and 1589 cm−1in the complexes,respectively.These changes also proved the existence of hydrogen bond[44].The solubility of m-cresol in the oil phase is greatly decreased because of the formation of the hydrogen bond with ILs,thereby achieving the separation of m-cresol.

Fig.14.(a)FT-IR of emimCH3COO,m-cresol and formed complex,and(b)FT-IR of bmimCl,m-cresol and formed complex.

The vibration shift level of v-OH could be a criterion of the hydrogen bond intensity[45].Then a series of the FT-IR spectra of ILs were investigated,which included emimCH3COO,bmimCH3COO,bmimCl,bmimBF4,and bmimPF6.The spectra of the complexes formed by these ILs and m-cresol were shown in Fig.15.The v-OH stretching vibration of m-cresol was observed at 3329 cm−1,and this peak shifted to 3319 cm−1in the complex spectrum of bmimPF6,and 3314 cm−1of bmimBF4,3252 cm−1of bmimCl,3159 cm−1of bmimCH3COO,and3156 cm−1of emimCH3COO.The larger shift of v-OH represented the stronger hydrogen bond interactions ability.Furthermore,the longer alkyl substituents showed smaller vibration shift and weaker hydrogen bond interactions ability.The vibration shift of v-OH of different ILs showed the different degree of the hydrogen bond intensity.Yang et al.[43]reported that the hydrogen bond interactions between bmimBF4,bmimCl,emimCH3COO and tocopherols by quantum chemical calculations,using Gaussian 03 software.They also found that the strength of intermolecular hydrogen bonds is in the order emimCH3COO ＞bmimCl ＞bmimBF4for different ILs and tocopherols.The strong degree of the hydrogen bond decided the stability of the complex formed by m-cresol and ILs.In other words,along with the increasing of the vibration shift of v-OH,the distribution coefficients of mcresol from model oil by ILs would increase.As a result,the separation effect of m-cresol using emimCH3COO was better than that using other ILs.

Table 2 Calculation of interaction energies between ILs and m-cresol by COSMO-RS

Fig.15.FT-IR spectra of m-cresol and the complex of bmimPF6,bmimBF4,bmimCl,bmimCH3COO and emimCH3COO.

3.5.Recycling of the IL

It is important to regenerate and later recycle ILs from the complex of m-cresol and IL.In this study,the ILs were recovered by separate mcresol from IL(emimCH3COO)phase after the extract through stripping method using isopropyl ether.Isopropyl ether has a strong interaction with m-cresol and emimCH3COO is not soluble in isopropyl ether.The recycled emimCH3COO was used to separate m-cresol at the same experimental conditions for four times,and the results of m-cresol extraction efficiency and the mass of recycled emimCH3COO were shown in Fig.16.The figures showed that the m-cresol extraction efficiency had no obvious decrease and the mass of the recycled emimCH3COO had no obvious loss.Afterwards,fresh emimCH3COO and the recycled emimCH3COO were tested by FT-IR and1H NMR,and its spectra was shown in Fig.17.The figures showed that there were no changes in FT-IR and1H NMR spectra of the fresh emimCH3COO and the recycled emimCH3COO,indicating that emimCH3COO could be completely regenerated and recycled without any change in its structure.

4.Conclusions

Fig.16.The m-cresol extraction efficiency(a)and mass of emimCH3COO on recycling times.Separation conditions:temperature was 298.15 K,mass ratio of emimCH3COO to model oil was 1:2.

In this study,in order to separate m-cresol from the mixture of cumene and n-heptane,the COSMO-RS model was used to select the ILs as extractants.The m-cresol distribution coefficient,selectivity coefficient and performance index at infinite dilution were calculated by using the COSMO-RS model for 200 IL extractants(20 anions and 10 cations).According to the screening results and the analysis of the σprofiles for each component,five imidazolium-based ionic liquids were selected to extract m-cresol.And the liquid-liquid equilibrium data for the quaternary mixture of cumene+n-heptane+m-cresol+emimCH3COO/bmimCH3COO/bmimCl/bmimBF4/bmimPF6were measured.The distribution coefficient and selectivity coefficient were calculated,and the order of the extraction ability of the ILs was emimCH3COO/bmimCH3COO ＞bmimCl ＞bmimBF4＞bmimPF6,which was agreement with the screening results of COSMO-RS model.Based on the above results,emimCH3COO was selected as highly efficiency extractant to separate m-cresol from model oil.The results of extraction experiments showed that the extraction was very fast to reach equilibrium,and increasing temperature is unfavorable for extraction separation.The m-cresol extraction efficiency could reach 98.85% by mass ratio of emimCH3COO:model oil=0.5:1 at 298.15 K.A good extraction performance was attributed to the hydrogen bond interaction between ILs with m-cresol by the experimental and calculation results.Firstly,the COSMO-RS calculation explained the hydrogen bond interaction between the anion of ILs and m-cresol.Then the FT-IR spectra confirmed that the hydrogen bond was formed between ILs and m-cresol.Moreover,it is indicated that the extraction performance of ILs were determined by the strength of hydrogen bond interaction between ILs and m-cresol.With increase the hydrogen bond interaction,the extraction performance of ILs became better.EmimCH3COO could be easily recycled with isopropyl ether and regenerated without decline in mass and extraction efficiency.In summary,emimCH3COO was a potential extractant,which could be used in separation of phenols from coal tar.

Fig.17.FT-IR(a)and 1H NMR(b)of fresh emimCH3COO and regenerated emimCH3COO.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.11.009.