Zhihao Ma ,Xianyong Wei,2, *,Mingyao Zhou,Guanghui Liu,Fangjing Liu,Zhongqiu Liu,Xinyue Yu,Zhimin Zong

1 Key Laboratory of Coal Processing and Efficient Utilization,Ministry of Education,China University of Mining&Technology,Xuzhou 221116,China

2 State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering,Ningxia University,Yinchuan 750021,China

Keywords:Anthracene slag Ultrasonic-assisted extraction Medium pressure liquid chromatography Intermolecular interactions

ABSTRACT Ultrasonic-assisted extraction(UAE)combined with medium pressure liquid chromatography(MPLC)was designed for carbazole separation from anthracene slag(AS).The effects of liquid/solid ratio,temperature,and extraction times on carbazole separation were investigated.When using CCl4and ethyl acetate as extraction solvents and combining with MPLC,carbazole recovery and purity are 75.1%and 95.4%,respectively.The mechanism for carbazole separation were presumed by examining intermolecular interactions such as N-H…π,π-π,and C-Cl…π interactions.These results demonstrate that UAE/MPLC has a considerable potential as a green and promising strategy for separating and purifying carbazole and other chemicals from AS.

1.Introduction

As one of nitrogen-containing aromatics in anthracene slag(AS),a fraction of coal tar,carbazole is a value-added compound(VAC).Because of their special rigid fused ring structures,carbazole and its derivatives have been widely used in dyes[1,2],electron-donating materials[3],photosensitive materials[4],special inks,medicines[5-7],and pesticides[8].However,commercial carbazole cannot meet the urgent market demand.Since carbazole contents in coal tar and crude anthracene are only 1.5%and 2.5%,respectively[9],effectively separating carbazole from coal tar and crude anthracene faces a great challenge.AS in the waste discharges from anthracene oil processing industry contains many VACs,especially carbazole.Unfriendly disposal of AS produced in the traditional distillation resulted in serious water and soil pollution.Therefore,insight into reasonable approach for AS utilization,especially enriching VACs from AS,is necessary.

Conventional methods for separating carbazole and its derivatives include chemical and physical methods[10,11].Although ca.67.9%of carbazole recovery and ca.99.0% of carbazole purity were obtained from crude anthracene with chlorobenzene[12],the carbazole recovery still needs to be further improved.In fact,chemical methods for separating carbazole tends to be replaced by physical methods due to a large amount of industrial wastewater generated.Since various aromatics,such as anthracene and phenanthrene,have adjacent boiling points,separating pure aromatics from a complex mixture by distillation is difficult[12].As a new separation technique,supercritical fluid extraction combines some features of distillation and liquid-liquid extraction,but it requires harsh conditions and thereby is not conducive to further industrialization[11,13,14].High-purity chemicals can be obtained via the zone melting technique,which is complicated to operate and often requires strict temperature control to prevent crystal agglomeration and block pipes[15,16].The emulsion membrane separation method combining with extraction has better target compound selectivity,but it is not widely applied due to the poor stability of this system[17,18].Therefore,developing a convenient and effective approach is crucially important for carbazole separation.

Solvent extraction was used for separating carbazole[12,19].The absolute content(AC)and recovery of carbazole isolated with CCl4and ethyl acetate(EA)are 87.4%and 92.1%,respectively[20].However,improvement of the AC and recovery was seldom reported.Although many investigations focus on“l(fā)ike dissolves like”,the mechanism for extracting carbazole is not well understood.

Herein,a simple and reliable method was designed to separate and purify carbazole from AS.The experiment parameters,such as liquid/solid ratio,temperatures,and extraction times(ETs),were examined and the possible separation mechanisms were proposed.

2.Experimental

2.1.Materials

AS was provided by Shanxi Hongte Coal Chemical Co.,Ltd.and pulverized to pass through a 80-mesh sieve(particle size≤178 μm).Table 1 lists some analytical results of AS.The particle size of silica gel was selected in the range of 48-74μm.All the solvents,including nhexane,EA,and CCl4,are commercially purchased.

2.2.Experimental procedure

As shown in Fig.1,30 g AS was sequentially and exhaustively extracted with a certain amount of CCl4,EA,and CCl4by ultrasonicassisted extraction(UAE).Then,the liquid phase was separated by filtration and evaporated to afford extracts 1-3(E1-E3).The corresponding extraction residues were named as ER1-ER3.Finally,the fractions(Fn)in ER3were isolated through medium pressure liquid chromatography(MPLC)with different volume ratios of n-hexane to EA.Each carbazole recovery(CRi)was calculated as mass ratio of carbazole(mC)to each sample(mi,where i denote AS,ER1,E2,or ER3)on a dry and ashfree base,i.e.,CRi=mC/mi,daf.

2.3.Preparative purifications

Preparative purifications were conducted on a Sepacore?MPLC system through a glass column(30 mm×460 mm)packed with silica gel(150 g)at a flow rate of ml·min-1.Solid introduction was performed by a glass pre-column connected to the column.Each sample was directly adsorbed to silica gel as evenly as possible prior to the introduction.Furthermore,n-hexane/EA(v/v=15:1)mixed solvent and EA were chosen as the eluent for the separation.Eluates with the same composition identified by thin layer chromatography were incorporated and concentrated with a rotary evaporator.

2.4.Extract characterization

Enand ERnwere analyzed with an Agilent 7890/5973 gas chromatograph/mass spectrometer(GC/MS).The RC of each component was calculated by area normalization and carbazole was quantified by the external standard curve(Fig.S1).Carbazole content in AS is ca.58.2%.

3.Result and Discussion

3.1.Extraction with CCl4

Carbazole is slightly soluble in CCl4but has good solubility in EA[20].So,we used CCl4to extract non-carbazole compounds(NCCs).As depicted in Figs.2 and S2,the RCs of fluoranthene and pyrene decreased,while that of carbazole increased significantly with increasing CCl4/AS.Meanwhile,increasing CCl4/AS decreased the total concentration(TC)of others from CCl4/AS=2 ml·g-1,as shown in Fig.2.So,we adopted CCl4/AS=2 mL·g-1for the following experiments.

As exhibited in Fig.3,with raising temperature from 30°C to 70°C,the RC of carbazole increased from 9.4%to 16.4%,while the RC of fluoranthene decreased and the RC of pyrene did not change much.Asillustrated in Fig.3,both the RC and concentration of carbazole tend to increase with raising temperature from 30°C.Therefore,the following experiments were conducted at 30°C to reduce carbazole extraction with CCl4.

Table 1 Ultimate analysis and ash yield of AS along with the RC of organic compounds in AS

As shown in Fig.4,there are small changes in carbazole concentration(CC)with ETs,but the TC of others dropped sharply by increasing ETs.However,CC increased significantly with increasing ETs.In addition,the AC of carbazole in ER1is 62.5%,which is ca.4.2%higher than that in AS.So,repeated extraction with CCl4was not used in the following experiments.

3.2.Extraction with EA

The increase in CC is more significant than in the TC of others in E2(Figs.5 and S3),while the RCs of fluoranthene and pyrene decreased gradually with increasing EA/ER1.When EA/ER1=10 ml·g-1,the RC of carbazole is 83.3%,showing that carbazole can be enriched by increasing EA/ER1.In this case,the CRER1was ca.60.5%.

Fig.2.RC and TC variations of the components in E1with CCl4/AS.

Fig.3.RC and concentration variations of the components in E1at different temperatures.

As Fig.6 demonstrates,both the RCs and concentrations of components did not fluctuates significantly in the temperature range of 25-55°C,indicating that carbazole enrichment is not sensitive to temperature.As shown in Fig.S4,the carbazole solubility in EA increased slightly from 3.24 g at 25°C to 3.47 g at 55°C.In light of energy saving,25°C was preferentially considered,at which the AC of carbazole in E2is 77.0%.

Fig.4.Concentration and RC variations of the components in E1with ETs.

Fig.5.RC variation of the main components in E2with EA/ER1.

As depicted in Fig.S5,carbazole purity gradually increased,while the contents of other compounds,including fluoranthene and pyrene,gradually decreased with increasing ETs.As shown in Table 2,the RC and AC of carbazole in the 4th extraction increased by 8%and 19.3%,respectively.After 4 extractions,the total CR obtained by extracting ER1is 99.6%.Therefore,the repeated extraction facilitates enriching carbazole from ER1.

3.3.Removal of NCCs from E2by extraction with CCl4

As shown in Fig.S6,the TC of NCCs in E3significantly decreased with increasing CCl4/E2and the RC of carbazole in E3also increased with increasing CCl4/E2(Fig.7).To reduce the TC of NCCs and carbazole loss,CCl4/E2of 3 was determined as the optimal condition,under which the AC of carbazole and CRE2are 84.14%and 98.54%,respectively.

3.4.Carbazole purification by MPLC

About 2 g R3was further separated by MPLC with the eluent mentioned above.Eluates with a similar composition identified by thin layer chromatography were combined.In total,3 fractions,i.e.,fractions 1-3(F1-F3)were obtained.As shown in Fig.S7,large amounts of fluoranthene and pyrene and small amounts of other components appear in F1.The mass of F2is 1.33 g,while the AC of carbazole and CRER3are 95.4% and 75.1%,respectively,showing that carbazole is effectively enriched into F2by MPLC.Noteworthily,F3is richer in carbazole.Since fluoranthene and pyrene have relatively weak polarities compared to carbazole,their interaction with silica gel is weak.As a result,they tend to be eluted with non-polar mobile phase.

Fig.6.Concentration and RC variations of the components in E2at different temperatures.

Table 2 The total content and total CRER1in each extract

3.5.Separation mechanisms

The better separation of carbazole from AS can be achieved by UAE which can rapidly destroy AS by cavitation effect.Furthermore,strong cavitation can create a continuous stream of high temperatures and pressures derived from macro-turbulences and micro-mixing that are produced by the implosion of cavitation bubbles[21-23].Collisions between solvent molecules and components in AS can provide new contact sites,enhancing mass transfer[24-26].Since carbazole,fluoranthene,and pyrene are embedded in AS,the direct result of reducing AS particle size by UAE facilitates the interaction between the solvent and components in AS and subsequent dissolution of some components in the solvent[27-30].

Intermolecular interactions are commonly found in dynamic macromolecular AS systems,which are manifested through the combination of non-covalent weak interactions between the molecules,including intermolecular hydrogen bonds(IMHBs),π-π stacking,electrostatic interactions,hydrophobic interactions,van der Waals forces,weakly conjugated bonds,and π-electron interactions.As shown in Fig.8,considering the selective removal of NCCs by CCl4,it is inferred that＞CCl…π between CCl4and fluoranthene/pyrene is dominant in the system[31,32],while dissolving carbazole in non-polar solvents,e.g.,cyclohexane,n-hexane,n-heptane,and CCl4,is difficult,because the interaction between carbazole and the solvents is lower than their own interactions.Although a delocalized π-bond formed by the p-π conjugated electron cloud exists in carbazole,fluoranthene,and pyrene,the electron cloud density of the nitrogen atom in carbazole is obviously biased toward the aromatic ring due to its strong electron-withdrawing ability.

Fig.7.RC variation of the main components in E3with CCl4/E2.

Fig.8.Molecular graphs for C-Cl…π fluoranthene and C-Cl…π pyrene complexes.The lines connecting the nuclei are the bond paths.Red dots and yellow dots represent bond critical points and ring critical points,respectively.Green dots indicate positions of the cage critical points.

As shown in Fig.9,there are π-type IMHB and π-π bond between carbazole and EA.In addition to IMHBs＞N-H…OC＜and＞C-H…OC＜,＞N-H…π＜and π-π interactions also exist in this system,which are equally important in the field of supramolecular[33-35].The hydrogen bonds existing in the system could be a crucial reason for extracting carbazole from AS with EA.Similarly,non-oxygen IMHBs,e.g.,＞C-H…π interaction,also exist among different arenes[36-41].IMHBs formed among the fluoranthene or pyrene molecules are generally considered to be a kind of generalized IMHB between the C-H group and the system via the space-formed hyperconjugation system or between the C-H group and the π system to counteract the spatial effect.The solubility of fluoranthene or pyrene decreased in polar solvents but increased in non-polar solvents due to the IMHBs.The formation of the IMHBs and intramolecular hydrogen bonds may favor their dissolution in CCl4.

4.Conclusions

A reasonable and feasible process was proposed to separate and purify carbazole from AS.The optimal AC of carbazole and CR obtained are 95.4%and 75.1%,respectively.The strong cavitation of ultrasonic irradiation not only facilitates solvent permeation into AS,but also favor the release of solutes.＞C-Cl…π interaction between CCl4and arenes facilitates extracting arenes from AS with CCl4,while IMHBs,especially＞NH…π,＞N-H…OC＜,and＞C-H…O-C＜interactions,and π-π interaction between carbazole and EA favor carbazole enrichment by extracting ER1with EA.Carbazole was further purified by MPLC via removal of NCCs with a non-polar solvent and subsequent elution with a polar solvent.

Nomenclature

AC absolute content

AS anthracene slag

CC carbazole concentration

CR carbazole recovery

CCl4carbon tetrachloride

EA ethyl acetate

Enextracts

ERnextraction residues

ETs extraction times

GC/MS gas chromatograph/mass spectrometer

IMHBs intermolecular hydrogen bonds

MPLC medium pressure liquid chromatography

NCCs non-carbazole compounds

RC relative content

TC total concentration

UAE ultrasonic-assisted extraction

VAC value-added compound

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.12.012.

Fig.9.The π-type IMHB and π-π bond between carbazole and EA.