Zhihui Sun*,Weihong Zhang

School of Chemistry and Chemical Engineering,Xianyang Normal University,Xianyang 712000,China

1.Introduction

With the reduction of crude oil in the world,seeking for alternative source of energy is presently one of the most effective ways to solve the energy crisis[1].Coal tar,as a byproduct of coal pyrolisis,is seen as liquid fuel feedstock.Hydrogenation plays an important role in coal tar upgrading in recent years[2-7].However,compared with high-temperature coal tar,low and middle-temperature coal tar has been expected to be ideal hydrogenation processing feed stock due to its lower density and higher content of aliphatic hydrocarbons and other characteristics.

The distillation residues of coal tar are obtained from distillation of coal tar[8].Compared with raw material,they contain higher contents of heteroatoms(such as O,N and S)and metal,and are more complex in chemical structures.Therefore it is more difficult to make distillation residues of coal tar light during the process of hydrogenation.Further research on their compositions and molecular structures is important in upgrading coal tar.Nevertheless,it is difficult to better characterize them because of their abundant compounds,complex structures and broader molecular weight distributions.

In general,it is an effective approach to separate coal tar and their derivatives into different classes and then to analyze them individually.According to literatures[9-18],there were many methods for separating coal tar pitch,such as solvent extraction,distillation,planar chromatography and size exclusion chromatography.The main purposes of hydrogenation of coal tar are aromatics saturation,ring opening of cycloalkanes and long-chain alkanes broken and reduction in molecular weight.In other words,the distributions of saturate,aromatic,resin and asphaltene compounds of coal tar will change greatly through hydrogenation.Therefore,the conventional saturate-aromatic-resin-asphaltene(SARA)method was selected to be the preferred separation approach based on its hydrogenation mechanism.Moreover,for a deeper knowledge of how the group composition changes for coaltar under different hydrogenation conditions,it is necessary to analyze the molecular structure of individual fraction.

According to reference materials,coaltarpitch has been characterized by using different techniques such as Fourier transform infrared spectroscopy,NMR,elemental analysis,molecular weight analysis and so on[10-11,13,15-16].Among them,as the methods for the determination of molecular weight,mass spectrometry(MS),vapor pressure osmometry(VPO)and gel permeation chromatography(GPC)have been usually applied to measure average molecular weights of coal tar fractions[13,16,19].The widely used VPO technique is simple and easy to operate.However,a higher molecular weight is always obtained by using this method because of aggregate formation.In general,each method has its own limitations.On the one hand,considering almost no association existing in the saturate fraction,VPO method was selected to determine their molecular weights.On the other hand,the molecular weights of aromatic,resin and asphaltene fractions were measured by using GPC.As a note,the molecular weights determined are not absolute values and they can be only used for comparison between different fractions.

Above all,as different research purposes,both in sample separation method and characterization techniques are different.In this paper,the distillation residues of middle-temperature coal tar(DRMCT)were separated into four main fractions(saturate,aromatic,resin and asphaltene)using the combination of solvent extraction and column chromatography separation[19,20].In addition,the collected fractions were then investigated by means of elemental analysis,FTIR,combined with proton nuclear magnetic resonance(1H NMR)analysis and molecular weights.

2.Experimental

2.1.Sample

A middle-temperature coal tar from a coking plant of Shanbei(northern Shaanxi Province),a byproduct of brown coal using Middle Temperature Pyrolysis Process,was used as raw material.DRMCT with a total yield of 43%were obtained by distillation of the coal tar to 653 K and their main properties are presented in Table 1.

Table 1Chemical analysis of DRMCT

2.2.Isolation of DRMCT

DRMCT were separated into saturate,aromatic,resin,and asphaltene fractions.Fig.1 illustrates the separation process in a simplified sequence flow diagram.

DRMCT and then-pentane solvent with a about 1:30 volume ratio were mixed.The mixture was in an ultrasonic bath at 313 K for 2 h in order to ensure the sample completely dissolving.Then,the resulting solution was cooled and filtered through a 3 μm quantitative filter for obtaining the asphaltene fraction.Next,the asphaltene fraction was further purified with hot toluene.At the same time,the toluene insoluble material was obtained.The solvent toluene was removed under vacuum condition and then the asphaltene fraction was dried in a vacuum drying oven at 353 K.

Subsequently,the pentane soluble solution was furtherseparated into saturate,aromatic,and resin fractions by using column chromatography separation method.The operation steps were as follows.A 12-mminternal-diameter and 70-cm-long glass column was used and packed with moderate active neutral alumina.After prewetting the alumina column with pentane,the concentrated pentane soluble solution(about 10 ml)was adsorbed onto the alumina.Then 80 ml of pentane and 80 ml of toluene were used to elute the saturate and aromatic fractions fromthe alumina column,respectively.Atlast,about80 mlmixture of toluene and ethanol(1:1 volume ratio)was used to elute the resin fraction.

2.3.Analytical chemistry

Elemental analysis(C,H,S,N)of all the samples was determined by means of a VarioEL III analyzer with a combustion method.

FTIR spectral measurements for the asphaltene,saturate,aromatic and resin fractions from DRMCT were carried out on a Bruker Equinox-55 spectrophotometer(KBr pellets).All the fractions were scanned in the range 400-4000 cm-1.

The molecular weight of the saturate fraction was measured by using K7000 vapor pressure osmometer(VPO)(KNAUER).Toluene was selected to be the solvent and at an operating temperature of 353 K[21].

The molecular weights of the aromatic,resin and asphaltene fractions were determined by gel permeation chromatography(GPC,America,UltiMate3000).HPLC-grade THF was used as the mobile phase and the flow rate was 1 ml·min-1.A range of standard polystyrene samples were used to calibrate the column.

1H NMR spectra of all the samples were recorded on a Varian-FT-80A spectrometer.The analysis was performed at a 1 H resonance frequency of 400 MHz.DMSO-d6 was used as solvent for asphaltene fraction and solvent CDCl3forsaturate,aromatic and resin fractions.Tetramethylsilane(TMS)was used as an internal standard for all samples.

3.Results and Discussion

The photos of saturate,aromatic,resin eluents and asphaltene fraction from DRMCT are shown in Fig.2.The DRMCT showed the following compositions:22.5%saturate,24.3%aromatic,26.5%resin,16.9%asphaltene and 1.8%toluene insoluble.From the data it can be concluded that the overall recovery of oil products is 92.0%of the total sample mass.The loss was attributed to evaporation of saturates and aromatics in the process of evaporating off the solvents such as pentane and toluene[22].Besides,there were also a small amount of strong polar materials absorbed onto the alumina which were not eluted completely.Akmazet al.[23]used the same method to separate Batiraman heavy crude oil into such four fractions(SARA),and the contents of heavier components such as resin and asphaltene fractions were 27%and 28%,respectively.Compare with the crude oil,DRMCT were lighter because they contained lower amount of the two fractions.

Fig.1.Scheme of isolation of DRMCT.

Fig.2.The photos of saturate,aromatic,resin eluents and asphaltene fraction from DRMCT.

3.1.Chemical analysis

The results of the elemental analysis and molecular weights for the asphaltene,saturate,aromatic and resin fractions from DRMCT are listed in Table 2.The number average molecular weights(Mn)were determined by using VPO method for saturate fraction and GPC method for aromatic,resin and asphaltene fractions,respectively.The results,as shown in Table 2,indicate that the asphaltene fraction contains the highest molecular weight,while the aromatic fraction has the lowest molecular weight.However,according to Akmaz study[23],the asphaltene and resin fractions from crude oil had similar molecular weight values.

Table 2Elemental analysis and molecular weights of fractions from DRMCT

For the saturate and aromatic fractions,the data of the elemental analysis(listed in Table 2)indicate that their main elements include carbon and hydrogen,their total content is up to 99.3%and 97.0%,respectively.The H/C ratio for the four fractions is in the following order:saturate＞aromatic＞resin＞asphaltene,suggesting decreasing hydrogen efficiency in this sequence.Maya crude oil showed the same trend in Walteret al.[19].Nevertheless,according to Akmaz[23]study,the aromatic fraction contained lower H/C ratio than resin fraction isolated from Batiraman crude oil.The H/C ratio of the saturate fraction from DRMCT is a somewhat high 1.907.This is expected because the saturate fraction contains high amount of methylene groups with a H/C ratio of 2.0.The result of the H/C ratio analysis also indicates that the asphaltene fraction contains the most condensed aromatic ring.The aromatic,resin and asphaltene fractions from DRMCT contain lower H/C ratio than the three fractions from crude oil[23].

It is worth noting that the saturate fraction contains barely any content of heteroatom,such as oxygen,nitrogen and sulfur.For the aromatic and resin fractions,the values of heteroatom content are 2.96%and 10.28%,respectively.However,in contrast to them,the asphaltene fraction contains the highest value(about 16.6%).It reveals that the heteroatoms(S,N and O)in DRMCT mostly exist in aromatics and rarely in par affinic chains,which is consistent with other studies[19,23].According to the data listed in Table 2,the three fractions(aromatic,resin and asphaltene)from DRMCT contain lower total content of sulfur,but higher oxygen than other fractions from Batiraman crude oils[19,23].The contents of sulfur,nitrogen and oxygen in the aromatic,resin and asphaltene three fractions from DRMCT increase with increasing the molecular weights of the three fractions,whereas the content of sulfur in the three fractions from Batiraman crude oil showed no trend.

The chemical analysis reveals that the asphaltene fraction from DRMCT is the most complexity and heaviest.Moreover,in order to better understand the structure of each fraction,a series of methods were applied to further analyze them.

3.2.FTIR structural characterization

The functional groups present in the four fractions from DRMCT were determined by means of FTIR spectroscopy in order to obtain deeper knowledge of differences between them.Fig.3 presents the FTIR spectrum of the saturate,aromatic,resin and asphaltene fractions from DRMCT.According to Akmaz'results in research works[23],it was pointed out that all the fractions from crude oil displayed strong adsorption bands related to aliphatic C-H bonds.However,the FTIR spectrum(Fig.3)reveals that only the saturate fraction exhibits strong absorption peaks at 2919,2850 and 1463,1386 cm-1,which are due to stretching of aliphatic C-H bonds and angle deformation vibrations of C-CH3and-CH2-,respectively[24].Also,a strong absorption band at 727 cm-1related to alkyl chains-(CH2)n-(n≥4),is present in the saturate fraction.The result is in accord with its highest H/C atomic ratio(1.907).On the contrary,the aromatic,resin and asphaltene fractions displayed low absorption at such peaks.

Fig.3.FT-IR spectrum of the four fractions from DRMCT.

For the aromatic,resin and asphaltene fractions from DRMCT,all the spectra show the absorption bands around 1600 cm-1(corresponding to carbon-carbon double bond stretching vibrations of aromatic rings)and the bands in the region 900-700 cm-1(related to aromatic,outof-plane,C-H bending)[10]are observed.However,almost no such absorption bands are presentin the saturate fraction.The results further indicate that the saturate fraction of DRMCT is composed primarily of aliphatic hydrocarbons.

Further analyze the substituents on the aromatic rings,it is found that three main adsorption peaks near 880,815 and 750 cm-1are present in the asphaltene,aromatic and resin fractions.The peak near 880 cm-1is related to penta-substituted aromatic rings containing isolated C-H bonds[9].The two adsorption bands at about 815 and 750 cm-1are corresponding to systems containing 1,4-subsituted and 1,2-disubstituted aromatics,respectively[25].As seen in Fig.3,the aromatic fraction is characterized by the lowest degree of substitution(the most strong adsorption bands at 879,816 and 758 cm-1).For the resin fraction,two strong adsorption bands at 817 and 752 cm-1are observed.On the contrary,the asphaltene fraction shows only one distinctpeak at820 cm-1in this region.The results suggest that the asphaltene fraction is featured by the highestdegree of substitution compared with otherfractions,which is consistent with the H/C ratio analysis.

The absorption peaks at 1300-1100 cm-1,related to C-O bonding stretching in alcohols,phenols and ethers compounds[1,25]are observed in the spectra of aromatic,resin and asphaltene fractions.By comparing the intensity of the peaks,it seems that the asphaltene fraction contains more of the C-O groups from phenols compounds than the aromatic and resin fractions.The result is consistent with their oxygen content analysis.However,a weak band at 1705 cm-1corresponding to C═O stretching vibrations from carboxylic acids,ketones and aldehydes compounds[26]is observed in the spectrum of aromatic fraction,rather than in the spectra of the resin and asphaltene fractions.

The strong adsorption at 3367 cm-1,which is the characteristic band for self-associated OH hydrogen bond[27]is observed in the spectrum of resin.However,compared with resin fraction,the asphaltene fraction shows a broader peak in the region 3100-3400 cm-1,it demonstrates that the asphaltene fraction contains a stronger hydrogen bond,which may be attributed to its higher oxygen content.

3.3.1H NMR

Fig.4 shows the1H NMR spectra of the saturate,aromatic,resin and asphaltene fractions from DRMCT.According to the literature[24],the distribution of each kind of hydrogen for the four fractions was measured as presented in Table 3.As expected,the saturate fraction contains a small amount of aromatic hydrogen.Besides,the value ofHβin saturate fraction is high up to 69.9%,showing that a high amount of hydrogen in β or further position to aromatic ring are in the form of-CH2and-CH-,or in the form of-CH-attached to methyl groups[19].The result is in accord with the IR analysis that the saturate fraction is rich long alkyl chains.The values ofHα,represent the branching degree of aromatic rings,are in the following sequence:saturate＜aromatic＜resin＜asphaltene.The lowestHαvalue of 10.3%for the saturate fraction is possibly attributed to its lack of aromatic structure.Moreover,the saturate fraction contains the highest amount of γ-hydrogens,in the position of at least three carbons away from aromatic ring.But for the asphaltene fraction,theHγvalue of 8.10%is the lowest.Combined with the highest value ofHA37.2%,it can be inferred that methyl groups are the main alkylside chains in the asphaltene fraction.

Fig.4.1H NMR spectra of the four fractions from DRMCT.(A.Saturate,B.Aromatic,C.Resin,D.Asphaltene).

Table 3Contents of each type of hydrogen in the fractions from DRMCT(wt%)

3.4.Average structural parameter(ASP)calculations

The concept of“average structuralparameter(ASP)”has been widely used to characterize the structures of some complexity of coal,petroleum and their derived materials.The average structural parameters of the four fractions from DRMCT have been calculated by using the improved Brown-Ladner(B-L)method[21],basing on the data of1H NMR,elemental analysis,FTIR combined with number average molecular weight(Mn).The data are listed in Table 4.

Table 4Results of ASP of fractions from DRMCT①

Aromaticity factor(fA)is defined as the atomic ratio of aromatic carbons to total carbons and the value offAis to some extent a measure of the degree of condensation of a molecule.ThefAvalues of the asphaltene,aromatic and resin fractions are much higher and the data are 0.75,0.68 and 0.70,respectively,while the saturate fraction contains significantly lower value 0.13.The results show that the asphaltene fraction is more condensed than other three fractions,according with the elemental analysis result that it has the lowest ratio of H/C.As for the aromaticity factor,compared with the fractions from crude oils[23,29,30],the saturate fraction from DRMCT has similar value,whereas the aromatic,resin and asphaltene fractions all have higher values.The comparison results are consistent with H/C ratio analyses as mentioned above.

As for the values ofHau/Car,which represent the H/C ratio of the hypothetical unsubstituted aromatic ring system,the four fractions are listed in order of value:resin＜asphaltene＜aromatic＜saturate.The saturate fraction possesses the highest value ofHau/Car(1.12),showing that for the saturate fraction the number of unsubstituted aromatic rings is more than the numbers for the other three fractions.For the value of σ,represents the degree of the substitution of aromatic rings[13],the saturate fraction possesses the highest value(0.69),followed by resin(0.37),asphaltene(0.33)and aromatic(0.29).

The numbers of paraffinic carbons permolecule for the four fractions vary from 0.33 to 25.00.The lowest value ofCpfor the asphaltene fraction is 0.33,showing thatmethylis the mostsubstituent group in aromatic rings.But for the saturate fraction,this value is up to 25.00,indicating that most amount of carbons are mainly in long-chain alkanes.For the aromatic and resin fractions,the values ofCpare similar.

In order to further understand the structures of the four fractions,the distributions of two types of rings(aromatic(RAr)and naphthenic rings(Rn))are analyzed.For each of the aromatic,resin and asphaltene fractions,RAris more thanRn.Furthermore,for the resin and asphaltene fractions,it seems that they have a similar aromatic ring structure(number of aromatic rings 6.21 and 6.04,respectively).However,the resin fraction seems to have a less naphthenic structure than the asphaltene fraction(number of naphthenic rings 1.63versus3.14).The result suggests that the asphaltene fraction from DRMCT with higher molecular weight contains more heterocyclic and cycloparaffinic rings rather than condensed aromatic rings.

3.5.Hypothetical molecular structure model

The hypothetical molecular structure models for the four fractions from DRMCT were constructed on the basis of the average structure parameters combined with FTIR analysis.However,the saturate fraction can't be constructed based simply on such analysis results.In general,saturates are composed of paraffin and cycloparaffin.Therefore,the model of saturate fraction was obtained assuming it was formed from one naphthenic ring and a few alkyl side chains.Fig.5 presents the hypothetical average molecular structural models for the asphaltene,saturate,aromatic and resin fractions,respectively.Besides,the molecular formulas of the fractions were also obtained based on the hypothetical molecularstructure models.The models give a visual representation of the compositions and structures of the four fractions.

Fig.5.Hypothetical average structures for the four fractions from DRMCT.

4.Conclusions

It was an effective way to separate DRMCT into asphaltene,saturate,aromatic and resin fractions by using solvent extraction combined with column chromatography separation.The yields for the four fractions were as follows:saturate 22.5%,aromatic 24.3%,resin 26.5%and asphaltene fraction 16.9%.The compositions and structures of the different fractions have been further characterized by making use of elemental analysis,FTIR,1H NMR and molecular weights.

In the present work it was found that the saturate fraction from DRMCT was rich long alkyl chains and almost no heteroatom.The molecular weights for the aroamatic,resin and asphaltene fractions were in the following order:asphaltene＞resin＞aromatic.The contents of heteroatom(especially oxygen)and the aromaticity degree in these fractions increase with increasing the molecular weight.

The importantdifferences between the resin and asphaltene fractions from DRMCT and conventional crude oils were that,the fractions from DRMCT contained lowermolecularweight,lowercontentofaliphatic hydrocarbons and higher aromaticity degree.It was also found that the asphaltene fraction from DRMCT,which contained higher molecular weight,had a similar aromatic ring structure with the resin fraction.The result revealed that the heaviest and the most complex components from DRMCT were not always composed of more condensed aromatic rings,butsome heterocyclic compounds and cycloparaf finic rings.Moreover,the less condensed aromatic ring for the asphaltene fraction from DRMCT was attributed to DRMCT's source,low rank coal.

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