Characterization of Middle-Temperature Gasification Coal Tar. Part 2

May 9, 2012 - Pilot-plant study of upgrading of medium and low-temperature coal tar to clean liquid fuels. Rui Wang , Donghui Ci , Xin Cui , Yu Bai , ...
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Characterization of Middle-Temperature Gasification Coal Tar. Part 2: Neutral Fraction by Extrography Followed by Gas Chromatography− Mass Spectrometry and Electrospray Ionization Coupled with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Haiyang Long,† Quan Shi,*,† Na Pan,† Yahe Zhang,† Dechun Cui,† Keng H. Chung,‡ Suoqi Zhao,† and Chunming Xu*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China Well Resources Incorporated, 3919-149A Street, Edmonton, Alberta T6R 1J8, Canada

‡

S Supporting Information *

ABSTRACT: A commercial lignite gasification-derived middle-temperature coal tar (MTCT) was subjected to acid−base extraction to obtain acidic, basic, and neutral fractions. The neutral fraction was characterized by mass spectrometry (MS) for hydrocarbon-group-type analysis and further fractionated by extrography into six subfractions, which were characterized by gas chromatography−mass spectrometry (GC−MS). Saturate, aromatic, and resin fractions of the neutral fraction accounted for 16.4, 47.6, and 36.0 wt %, respectively. The GC−MS analysis showed that the first neutral subfraction (15.7 wt %) contained alkanes, alkenes, and cycloalkanes; the second subfraction (52.0 wt %) contained 1−6-ring aromatics; the third subfraction (4.6 wt %) contained neutral nitrogen compounds, such as indoles, carbazoles, and benzocarbazoles; the fourth subfraction (8.2 wt %) contained neutral polar compounds, such as C8−C28 alkyl nitriles and aliphatic and aromatic ketones, such as 4-, 5-, and 6-ketones and phenyl ketones, derived from a series of propiophenone to decanophenone; the fifth subfraction (14.9 wt %) contained 2-ketones and aromatic ketones, such as acetophenones, indanones, and acetonaphthones; and most of the sixth subfraction (1.3 wt %) cannot be eluted from GC. Electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to analyze the third neutral subfraction, which was enriched with neutral nitrogen compounds. In addition to indoles, carbazoles, and benzocarbazoles, FT-ICR MS analysis showed that dibenzocarbazoles and tribenzocarbazoles with various carbon numbers were present in the third neutral subfraction.

1. INTRODUCTION Middle-temperature coal tar (MTCT) is a byproduct from coal carbonization and gasification. The composition of MTCT is similar to that of the low-temperature coal tar (LTCT) but distinctly different from that of the high-temperature coal tar (HTCT). From a broad perspective, LTCT and MTCT are commonly referred to as LTCT, because of their similar molecular compositions. Coal-derived chemicals play an important role in Chinese industry. The production of LTCT in China was about 2 million tons in 2008.1 In recent years, tremendous attention has been devoted to find a better use of LTCT. Hence, a more in-depth understanding of the molecular composition of LTCT is vital to technology development. The LTCT is a complex mixture of heteroatom compounds. It contains monohydric phenols,2,3 dihydric phenols,4 basic nitrogen compounds,5−9 and more than 60% neutral compounds, which originated from coal tar.10,11 Previous characterization work showed the presence of aliphatic12−14 and aromatic15−18 hydrocarbons in neutral compounds found in coal tar. However, only a few neutral heteroatom compounds9,19,20 were identified in LTCT. In fact, the molecular compositions of neutral heteroatom compunds in coal tar have not been thoroughly defined. Although infrared (IR) spectroscopy10,21 and nuclear magnetic resonance (NMR)22 were used to analyze LTCT, they were inadequate to show the molecular © 2012 American Chemical Society

compositions of neutral heteroatom compounds. Other analytical tools, such as gas chromatography (GC)19,20,23 and GC−mass spectroscopy (GC−MS)22,24 could provide molecular composition information, but the chromatographic peaks of various compounds overlapped because of the complexity and structural similarity of the components in coal tar. Therefore, an effective separation method is an essential sample pretreatment step prior to characterization of neutral heteroatom compounds in LTCT. Extrography has been used in fractionating complex heteroatom compounds in coal-derived streams25−29 and crude oil.30 In a recent paper,31 a MTCT sample was separated to acidic, basic, and neutral fractions and the MTCT basic nitrogen compounds were characterized by positive-ion electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In this paper, extrography was used to fractionate the neutral fraction of MTCT into various subfractions. The MTCT neutral subfractions were subjected to GC−MS and FT-ICR MS analyses for neutral nitrogen compounds. Received: December 27, 2011 Revised: April 30, 2012 Published: May 9, 2012 3424

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Figure 1. Extrography scheme for the neutral fraction. The picture in the top left and top right were taken before and after solvent eluting, respectively. Pictures at the bottom were subfractions obtained from the extrography without removing solvent. used as the carrier gas with a flow rate of 1 mL/min. The ion source temperature was maintained at 250 °C, and ionizing voltage was 70 eV. 2.4. Hydrocarbon-Group-Type Analysis. The neutral fraction (150 mg) of MTCT was separated into three fractions by a solid extraction column packed with 2 g of silicon gel. Normal pentane, n-pentane/dichloromethane (1:1, v/v), and dichloromethane were used to elute saturates, aromatics, and resin, respectively. The solvent was delivered by a microdistillate device. The yields of subfractions were determined by weight. An Agilent 6890GC coupled with 5973 mass spectrometry (MS) was used for the hydrocarbon-group-type analysis. A 30 m × 0.25 mm blank fused silica capillary column was used to transfer the hydrocarbons into MS. The GC injector temperature was at 330 °C, and the oven was kept at 60 °C for 1 min, programmed to 330 °C at a rate of 30 °C/min, and then held constant for 5 min. The transfer line was at 300 °C. The MS ionizing voltage was 70 eV, and the mass range was 35−600 with a 0.5 s scan period. The mass spectra of all species, except the solvent, were co-added and derived an average spectrum for hydrocarbon-group-type calculation according to American Society for Testing and Materials (ASTM) D2786 and D3239 methods for saturates and aromatics, respectively. 2.5. FT-ICR MS Analysis. Bruker apex-ultra FT-ICR MS was used to analyze the third MTCT neutral subfraction (F-III). The MS analyzer was equipped with a 9.4 T superconducting magnet. The sample was dissolved in methanol/toluene (3:1), and then the sample solution was diluted with a solvent mixture of toluene/methanol (1:1) to 0.2 mg/mL. The sample solution was infused by an Apollo electrospray source at 180 μL/h using a syringe pump. The ESI source was operated in negativeion mode. The emitter voltage was 3.5 kV. The capillary column front end voltage was 4.0 kV. The capillary column end voltage was −320 V. The ion accumulated time was 0.01 s, and the time-of-flight window was 0.9 ms. The mass range was set at m/z 100−600. The data size was set to 4 M words. A total of 64 scans were accumulated. Methodologies for FT-ICR MS mass calibration, data acquisition, and processing were reported elsewhere.32,33

2. EXPERIMENTAL SECTION 2.1. Materials. A MTCT sample was obtained from a commercial Lurgi lignite gasification plant. Light distillates contained a large amount of benzene, toluene, and small-molecular polar compounds.31 These compounds have a major impact on the polarity of eluting solvent, which is detrimental to the extrography separation. In addition, light distillates vaporize during the solvent removal step and could potentially enhance the vapor carryover of the heavier components, leading to yield loss. Hence, the light distillates were removed by distillation before acidic and basic extraction. The topped MTCT was extracted with 3 M sodium hydroxide solution to remove tar acids and 6 M hydrochloric acid solution to remove tar bases. The remaining MTCT neutral fraction was obtained. The yield of the MTCT neutral fraction was 69.9 wt % of the residue.31 2.2. Extrography. Silica gel was purified by Soxhlet extraction with chloroform for 24 h. The purified silica gel was activated by heating at 120 °C for 5 h, followed by deactivation by adding 2 wt % water. A total of 1 g of MTCT neutral fraction was dissolved in 70 mL of dichloromethane (CH2Cl2), and then 20 g of deactivated silica gel was added. The diluted MTCT neutral fraction and deactivated silica gel were homogeneously mixed and then dried under nitrogen at 50 °C. The remaining mixture sample was placed in a glass column, which was prepacked with 35 g of activated silica gel (120 °C for 5 h). To secure the mixture sample in a packed column, 2 g of deactivated silica gel was placed on the top of the mixture. The LTCT neutral fraction was subjected to extrographic fractionation, as shown in Figure 1. The six MTCT neutral subfractions (F-I−F-VI) were obtained by sequential solvent elution using cyclohexane, cyclohexane/toluene (1:1, v/v), toluene/CH2Cl2 (1:1, v/v), CH2Cl2, ethyl ether/methanol (9:1, v/v), and methanol, respectively. The solvent in each extragraphic subfraction was removed by a rotary evaporator. 2.3. GC−MS Analysis. The GC−MS analyses of MTCT neutral subfractions were performed using Thermo-Finnigan Trace DSQ GC−MS equipped with a HP-5 MS column (30 m × 0.25 mm × 0.25 μm). The GC oven was held at 60 °C for 10 min, increased to 300 °C at a rate of 8 °C/min, and then held isothermal for 20 min. The injector and transfer line temperatures were held at 300 and 250 °C, respectively. Helium was

3. RESULTS AND DISCUSSION 3.1. MTCT Neutral Fraction and Its Subfractions. Table 1 shows the hydrocarbon-group-type analysis of the bulk MTCT neutral fraction by the ASTM D3239 and D2786 methods. 3425

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topped MTCT. The sum of yields of F-I and F-II subfractions amounted to 70 wt % of the total LTCT neutral fraction. The remaining four extrographic subfractions (F-III−F-VI) accounted for 30% of the total LTCT neutral fraction, which was in-line with the amount of resins determined by the standard ASTM methods. All of the subfractions were subjected to GC−MS analysis. The yield of F-VI was the lowest (1.6 wt %) among all of the MTCT subfractions. F-VI did not have distinct chromatographic peaks. A small amount of hydrocarbon residue may be present in F-VI, but most of these compounds cannot be eluted from GC. 3.2. MTCT Subfraction F-I. Figure 2 shows the total ion and mass chromatograms of the F-I subfraction. Mass chromatograms m/z 85 and 97 showed the distribution of n-alkanes and n-alkenes, respectively. Normal alkanes with 10−30 carbons were abundant in F-I. Pristane and phytane, which are commonly found in petroleum and coal extracts,31 were not abundant in F-I. This suggested that pristane and phytane were from the lignite pyrolysis reactions. Cyclic alkanes, such as hopanes and steranes, were not detected by GC−MS. The hydrocarbon-group-type analysis by MS analysis also showed that cyclic alkanes accounted for a small fraction of the saturate hydrocarbons. 3.3. LTCT Subfraction F-II. Figure 3 shows the total ion chromatogram of the F-II subfraction. Figure 4 shows the mass chromatograms of alkyl benzenes, naphthalenes, phenanthrenes, and chrysenes. Figures 3 and 4 indicate the enrichment of unsubstituted aromatics and 1−6-ring aromatics in F-II, in which 2−3-ring aromatics were dominant (see Figure 4). The alkyl chains of aromatic hydrocarbons were relatively short, except for alkybenzenes. Although the amount of alkybenzenes was low, they had a broad carbon number distribution (C10−C25; see Figure 4). The results showed that F-II contained abundant retene, which is a common species used as a biomarker for resenes from higher plants.34 Dibenzofurans (C0−C5) are oxygen-containing heteroatom hydrocarbons, which were identified in the F-II fraction and exhibited a relatively high abundance. The F-II fraction accounted for 52 wt % of the neutral fraction, which was the sum of yields of all aromatics determined by MS, as listed in Table 1. 3.4. LTCT Subfraction F-III. Figure 5 shows the total ion chromatogram of the F-III subfraction. Carbazoles were dominant in this fraction. Carbazole was more abundant than its homologues. The three isomers of benzocarbazoles identified were benzo[a]carbazole, benzo[b]carbazole, and benzo[c]carbazole. Benzo[b]carbazole was more abundant among the three isomers. The presence of abundant benzo[b]carbazole in LTCT was distinctly different from that in petroleum crude, which has a trace quantity of benzo[b]carbazole.35 Indolic compounds were much less abundant than carbazoles and benzocarbazoles. The low amount of indolic compounds in MTCT was also different from the products derived from thermally cracked or catalytically cracked heavy petroleum, in which the amount of indolic compounds is much higher.36 The GC−MS and FT-ICR MS analyses showed that the chromatogram peaks “A” and “B” in Figure 5 were oxygen-containing compounds with molecular formulas of C20H28O and C20H30O. Because these oxygen-containing compounds were ionized in negative mode, they were either alcohol or phenolic compounds. However, the structures of these compounds cannot be identified solely by mass spectra. Figure 6 shows the iso-abundance map of double bond equivalents (DBE) as a function of the carbon number for the N1 class species in the F-III subfraction. The N1 class species

Table 1. Hydrocarbon-Group-Type Composition of the Neutral Fraction by ASTM D2786 and D3239 hydrocarbon group types

weight percent (wt %)

paraffins 1-ring cycloalkanes 2-ring cycloalkanes 3-ring cycloalkanes total cycloalkanes alkylbenzenes naphthenebenzenes dinaphthenebenzenes total monoaromatics naphthalenes acenaphthenes + dibenzofuran fluorenes total diaromatics phenathrenes naphthenephenathrenes total triaromatics pyrenes chrysenes perylenes 4−5-ring aromatics benzothiophenes dibenzothiophenes total sulfur compounds unidentified aromatics total aromatics resins total

11.9 2.3 1.0 1.2 4.5 4.5 5.7 3.6 13.8 5.8 5.8 4.0 15.6 10.7 1.9 12.6 2.8 0.5 0.1 3.4 1.2 0.5 1.7 0.5 47.6 36.0 100.0

The results showed that the MTCT neutral fraction comprised 11.9 wt % linear hydrocarbons, 4.5 wt % cycloalkanes, 13.8 wt % monoaromatics, 15.6 wt % diaromatics, 12.6 wt % triaromatics, 3.4 wt % 4−5-ring aromatics, and 36 wt % resins. Because the ASTM methods are designed for heavy petroleum distillates, it is assumed that no olefins are present in the test sample. Nevertheless, coal tar is known to contain alkenes and aromatic olefins.18 Hence, it is expected that the data of hydrocarbongroup-type analysis, shown in Table 1, would be skewed from the actual values. For instance, the content of monocyclic alkanes would be high for the contribution from chain alkenes, which have similar mass spectral character. The high amount of resins suggested that the MTCT neutral fraction contained a large amount of polar compounds. The six extrographic subfractions were used to determine the polar compounds in the MTCT neutral fraction. Table 2 shows the Table 2. Yield of Extrography Fractions fraction

solvent

solvent used (mL)

yield (wt %)

F-I F-II F-III F-IV F-V F-VI total

cyclohexane cyclohexane/toluene (1:1, v/v) toluene/dichloromethane (1:1, v/v) dichloromethane ethyl ether/methanol (9:1, v/v) methanol

100 230 100 150 165 100

15.7 52.0 4.6 8.2 14.9 1.3 96.7

yields of extrographic MTCT neutral subfractions, which were calculated by dividing the weight of each fraction by that of 3426

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Figure 2. Total ion and mass chromatograms of the F-I subfraction. Peaks marked with a dot were identified as alkenes.

Figure 3. Total ion chromatogram of the F-II subfraction.

varied over a wide range of DBE values (6−18) and carbon numbers (12−30). The N1 class species with DBE values of 6, 9, 12, 15, and 18 should be indoles, carbazoles, benzocarbazoles, dibenzocarbazoles, and tribenzocarbazoles, respectively. The results of FT-ICR MS analysis for carbazoles and benzocarbazoles were in agreement with those of GC−MS. Furthermore,

the FT-ICR MS analysis showed the presence of a low amount of highly condensed nitrogen compounds in F-III. 3.5. MTCT Subfraction F-IV. Figure 7 shows the total ion mass chromatogram of the F-IV subfraction, indicating the presence of ketones, nitriles, and phenolic compounds. Benzonitriles and naphthonitriles were relatively abundant 3427

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Figure 4. Mass chromatograms of the F-II subfraction.

Figure 5. Total ion chromatogram of the F-III subfraction. The inset mass spectrum corresponds to peak A.

in the F-IV subfraction. The C8−C28 alkyl nitriles were identified by extracting the characteristic mass fragment m/z 110 of alkyl nitriles. Nitriles were reported in shale oil37 and aerosol.38 These compounds unlikely occurred in the geological

environment but were derived from the gasification reactions of fatty acid and ammonia,37 in which ammonia was a component of the coal gasification effluent39 and fatty acids were present in the MTCT. 3428

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phenyl ketones, ranging from propiophenone to decanophenone, were also found in the F-IV subfraction, which were identified by extracting the characteristic mass fragment m/z 105. The characteristics of the equally spaced peak distance of a chromatogram and the mass spectra can also identify it as a straight alkyl chain, instead of an iso-alkyl chain or a multisubstituted group. Some mass spectra of identified compounds were shown in Figure S2 of the Supporting Information. 3.6. MTCT Subfraction F-V. F-V had the highest yield of polar species among the subfractions, which accounted for 14.9 wt % of the neutral fraction. Figure 8 shows the mass chromatogram of the F-V subfraction. Ketones were also dominant in F-V subfractions. However, the molecular structures of ketones in F-V were different from those in F-IV: F-V contained 2-ketones and aromatic ketone, such as acetophenones, indanones, and acetonaphthones, whereas F-IV contained aliphatic 4-, 5-, and 6-ketones. Although ketones were abundant in F-IV, some overlapping of mass chromatogram spectra were likely from hydroxyl compounds, such as alcohols and phenolic compounds. The huge unresolved complex matter (UCM) humps in Figures 7 and 8 showed that certain species in F-IV and F-V subfractions could not be identified by GC analysis. Further investigation of UCM will be carried out by FT-ICR MS analysis and discussed in our future paper, in which a detailed characterization of acidic compounds will be discussed.

Figure 6. Iso-abundance maps of DBE as a function of the carbon number for the N1 class species in the F-III subfraction.

Ketones were the most abundant oxygen compounds found in the F-IV subfraction. Mass chromatogram m/z 58 showed the distribution of aliphatic ketones. The substitute position on the alkyl chain of the carbonyl was not assigned. Multiple compounds were lumped under a chromatogram peak (see Figure S1 of the Supporting Information). Various ketonic compounds co-eluted, and shown in each chromatogram peak in m/z 58 of Figure 7 were 4-, 5-, and 6-ketones. A series of

Figure 7. Total ion and mass chromatograms of the F-IV subfraction. 3429

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Figure 8. Total ion and mass chromatograms of the F-V subfraction.

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4. CONCLUSION Saturate, aromatic, and resin fractions of the neutral fraction from the MTCT accounted for 16.4, 47.6, and 36.0 wt %, respectively. The neutral fraction was eluted to six subfractions by extrography. The first neutral subfraction (15.7 wt %) contained alkanes, alkenes, and cycloalkanes. The second subfraction (52.0 wt %) contained 1−6-ring aromatics. The third subfraction (4.6 wt %) contained neutral nitrogen compounds, such as indoles, carbazoles, and benzocarbazoles. The fourth subfraction (8.2 wt %) contained neutral polar compounds, such as C8−C28 alkyl nitriles and aliphatic and aromatic ketones, such as 4-, 5-, and 6-ketones and phenyl ketones, derived from a series of propiophenone to decanophenone. The fifth subfraction (14.9 wt %) contained 2-ketones and aromatic ketones, such as acetophenones, indanones, and acetonaphthones. Most of the sixth subfraction (1.3 wt %) cannot be eluted from GC. ESI coupled with FT-ICR MS was used to analyze the third neutral subfraction, which was enriched with neutral nitrogen compounds. In addition to indoles, carbazoles, and benzocarbazoles, FT-ICR MS analysis showed that dibenzocarbazoles and tribenzocarbazoles with various carbon numbers were presented in the third neutral subfraction.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-8973-3738 (Q.S.); +86-10-8973-3392 (C.X.). Fax: +86-10-6972-4721 (Q.S.). E-mail: [email protected] (Q.S.); [email protected] (C.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Junhui He, Peidong Wang, and Chao Ma for assisting with the GC and GC−MS analyses. This work was supported by the National Basic Research Program of China (2010CB226901).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Mass chromatogram (m/z 58) of the F-IV subfraction from the MTCT (Figure S1) and mass spectra of identified compounds in F-IV and F-V subfractions (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. 3430

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