Analysis and Comparison of Nitrogen Compounds in Different Liquid

Oct 4, 2010 - tracting a great deal of attention for a long time.1,2 However, the coal ..... alumina as a stationary phase and different solvents as a...
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Energy Fuels 2010, 24, 5539–5547 Published on Web 10/04/2010

: DOI:10.1021/ef1007598

Analysis and Comparison of Nitrogen Compounds in Different Liquid Hydrocarbon Streams Derived from Petroleum and Coal Na Li,†,‡ Xiaoliang Ma,*,† Qingfang Zha,‡ and Chunshan Song*,† †

EMS Energy Institute and Department of Energy and Mineral Engineering, The Pennsylvania State University 209 Academic Project Building, University Park, Pennsylvania 16802, United States , and ‡College of Chemistry and Chemical Engineering, China University of Petroleum, Dongying, Shandong 257061, China Received June 17, 2010. Revised Manuscript Received September 7, 2010

Identification and quantification of nitrogen compounds in five typical liquid hydrocarbon streams, including straight run gas oil, light cycle oil, and ultralow sulfur diesel derived from petroleum and coal liquids from the coal pyrolysis and the direct coal liquefaction, have been conducted by a combination of the modified solid phase extraction and gas chromatography with a mass spectrometry detector (GC-MS) and a nitrogen/phosphorus detector (GC-NPD). A simple and efficient method was applied for preisolating and concentrating the nitrogen compounds from the hydrocarbon matrix and coexisting oxygen-containing compounds. The identification and quantification results show that the coal-derived streams not only have much higher nitrogen content than the petroleum-derived ones but also contain quite different types of the nitrogen compounds. The major nitrogen compounds in the coal-derived streams are the basic nitrogen-containing compounds including aniline, quinoline, and their derivatives; while those in the petroleum-derived ones are the neutral nitrogen compounds, such as carbazole and its alkyl substituted derivatives. The implication of the analysis results on the fuel processing and deep denitrogenation is also discussed.

for the reactant approach to the active site.7,8 Many studies have proved that the preremoval of the nitrogen compounds from the liquid hydrocarbon streams can remarkably improve the catalytic performance for HDS, HDA, hydrocracking, and reforming.9-13 The nitrogen compounds existing in the final fuels also cause odor and color and reduce the thermal and oxidative stability of fuels.14 Finally, the presence of the nitrogen compounds in the final fuels increases the emission of nitrogen oxides when the fuels are burned.15 The nitrogen compounds in fuels, the intermediates, and products derived from them in the fuel processor could also poison the catalysts in the fuel processor and fuel cells.16 Thus, one of the major challenges in producing ultraclean fuels is to remove nitrogen from various liquid hydrocarbon streams, especially for ultradeep HDS, HDA, upgrading of the coal liquids derived from the coal pyrolysis and direct coal liquefaction, and production of ultraclean fuel for fuel cell applications. Consequently, a great deal of attention has been paid to the denitrogenation of

1. Introduction There are many organic nitrogen compounds existing in various liquid hydrocarbon streams derived from petroleum, such as straight run gas oil, light cycle oil, and coker gas oil, which are usually used for the diesel pool in refineries. On the other hand, coal liquids as a petroleum substituent for producing liquid hydrocarbon transportation fuels have been attracting a great deal of attention for a long time.1,2 However, the coal liquids derived from direct coal liquefaction or coal pyrolysis have a nitrogen concentration 10 times higher, or even more, than those of the liquid hydrocarbon streams derived from petroleum with a similar boiling range.3,4 It is well-known that the presence of the nitrogen compounds in liquid hydrocarbon streams, even at very low concentration, strongly deactivates the catalysts used in the fuel refining processes, such as hydrodesulfurization (HDS), hydrodearomatization (HDA), hydrocracking, and reforming,5,6 as the coexisting nitrogen compounds and/or their intermediates and products competitively occupy the active sites on the catalysts and thus change the property of the active sites or block the way

(8) Van Looij, F.; van der Laan, P.; Stork, W. H. J.; DiCamillo, D. J.; Swain, J. Appl. Catal., A: Gen. 1998, 170 (1), 1–12. (9) Sano, Y.; Choi, K.-H.; Korai, Y.; Mochida, I. Energy Fuels 2004, 18 (3), 644–651. (10) Song, C. S. Catal. Today 2003, 86 (1-4), 211–263. (11) Zeuthen, P.; Knudsen, K. G.; Whitehurst, D. D. Catal. Today 2001, 65 (2-4), 307–314. (12) Turaga, U. T.; Ma, X.; Song, C. Catal. Today 2003, 86 (1-4), 265–275. (13) Sano, Y.; Choi, K.-H.; Korai, Y.; Mochida, I. Appl. Catal., B: Environ. 2004, 49 (4), 219–225. (14) Wandas, R.; Chrapek, T. Fuel Process. Technol. 2004, 85 (11), 1333–1343. (15) Mao, J.; Pacheco, C. R.; Traficante, D. D.; Rosen, W. Fuel 1995, 74 (6), 880–887. (16) Cheng, X.; Shi, Z.; Glass, N.; Zhang, L.; Zhang, J.; Song, D.; Liu, Z.-S.; Wang, H.; Shen, J. J. Power Sources 2007, 165 (2), 739–756.

*To whom correspondence should be addressed. E-mail: mxx2@ psu.edu (X.M.); [email protected] (C.S.). (1) Longwell, J. P.; Rubin, E. S.; Wilson, J. Prog. Energy Combust. Sci. 1995, 21, 269–360. (2) Couch, G. IEA Clean Coal Centre-Report, London, 2008. (3) Murti, S. D. S.; Sakanishi, K.; Okuma, O.; Korai, Y.; Mochida, I. Fuel 2002, 81 (17), 2241–2248. (4) Almarri, M.; Ma, X. L.; Song, C. S. Ind. Eng. Chem. Res. 2008, 48 (2), 951–960. (5) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52 (4), 381–495. (6) Furimsky, E.; Massoth, F. E. Catal. Rev.: Sci. Eng. 2005, 47 (3), 297–489. (7) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30 (9), 2021– 2058. r 2010 American Chemical Society

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Gas chromatography coupled with some selective nitrogen detectors, such as nitrogen/phosphorus detector (NPD),24 atomic emission detector (AED),3,25 nitrogen chemiluminescence detector (NCD), 26-29 or mass spectrometer (MS),25,30-35 has been reported for identification and quantification of nitrogen compounds in various liquid hydrocarbon streams. However, for the detailed analysis of the nitrogen compounds in the liquid hydrocarbon streams, a preseparation of nitrogen compounds from huge hydrocarbon matrix and others is usually required prior to the gas chromatographic analysis to reduce the interference from the coexisting hydrocarbons or others. Different methods for preseparating the nitrogen compounds from other compounds have been reported by either liquid-liquid extraction,15,25,29,34,36-39 solid-phase extraction (or liquid chromatography),24,32,35,40-42 or a combination of the liquid-liquid extraction and the solid-phase extraction.3,40,43 The liquid-liquid extraction methods are complex and may cause the significant loss of some types of the nitrogen compounds, such as neutral nitrogen compounds, due to their lower extraction selectivity. The reported liquid-solid extraction methods and the combination of the liquid-liquid extraction and the solid-phase extraction are usually tedious and need two or more steps for efficient isolation, which make the isolation complex and easily cause the loss of some nitrogen compounds. A major objective in this study is to analyze and compare the various nitrogen compounds in some typical liquid hydrocarbon streams derived from petroleum and coal, which can provide valuable information and implication of the nitrogen compounds related to the previous and subsequent fuel processes and in the final applications. The present study also attempted to explore a relatively simple but efficient method

Table 1. Typical Basic and Neutral Nitrogen Compounds Present in Liquid Hydrocarbon Streams

various liquid hydrocarbon streams recently.4,13,17-20 The identification and quantification of the various nitrogen compounds and the clarification of their distribution in different liquid hydrocarbon streams are essential for developing either a novel process or more efficient catalysts or adsorbents for denitrogenation of various liquid hydrocarbon streams and for understanding the mechanism in ultradeep HDS, hydrodenitrogenation (HDN), adsorptive denitrogenation (ADN), and extractive denitrogenation (EDN). In comparison with the identification and quantification of the sulfur compounds in the liquid hydrocarbon streams,21-23 relatively few reports are available in literature on the identification and quantification of the nitrogen compounds in various streams, especially in the ultralow sulfur diesel and the coal liquids from the state-of-the-art direct coal liquefaction process. This is because the relatively lower concentration and complicated structures of the nitrogen compounds in the liquid hydrocarbon streams and the tedious separation and concentration procedures before the analysis by using gas chromatography. In general, the nitrogen compounds in the liquid hydrocarbon streams can been divided into two major groups: nonbasic (neutral) and basic nitrogen compounds, as shown in Table 1. The nonbasic nitrogen compounds usually include pyrrole, indole, carbazole, and their alkylated derivatives. The basic nitrogen compounds usually include amines, aniline, pyridine, quinoline, benzoquinoline, and their alkylated and hydrogenated derivatives. The concentrations of nitrogen compounds in the streams are usually much lower in comparison with sulfur compounds in the petroleum-derived streams, and the presence of huge hydrocarbon matrix and sulfur/oxygen-containing compounds interferes with the analysis of nitrogen compounds. Some approaches in identification and quantification of nitrogen compounds in different liquid hydrocarbon streams have been reported in the literature.

(24) M€ uhlen, C. v.; Oliveira, E. C. d.; Morrison, P. D.; Zini, C. A.; Caram~ao, E. B.; Marriott, P. J. J. Sep. Sci. 2007, 30 (18), 3223–3232. (25) Shin, S.; Sakanishi, K.; Mochida, I.; Grudoski, D. A.; Shinn, J. H. Energy Fuels 2000, 14 (3), 539–544. (26) Yang, Y. Chin. J. Chromatogr. 2008, 26 (4), 478–483. (27) Revellin, N.; Dulot, H.; Lopez-Garcia, C.; Baco, F.; Jose, J. Energy Fuels 2005, 19 (6), 2438–2444. (28) Nakajima, N.; Lay, C.; Du, H.; Ring, Z. Energy Fuels 2006, 20 (3), 1111–1117. (29) Adam, F.; Bertoncini, F.; Brodusch, N.; Durand, E.; Thiebaut, D.; Espinat, D.; Hennion, M.-C. J. Chromatogr., A 2007, 1148 (1), 55–64. (30) Ignatiadis, I.; Schmitter, J. M.; Arpino, P. J. Chromatogr., A 1985, 324, 87–111. (31) Mushrush, G. W.; Beal, E. J.; Hardy, D. R.; Hughes, J. M. Fuel Process. Technol. 1999, 61 (3), 197–210. (32) Laredo, G. C.; Leyva, S.; Alvarez, R.; Mares, M. T.; Castillo, J.; Cano, J. L. Fuel 2002, 81 (10), 1341–1350. (33) Cheng, X. G.; Zhao, T.; Fu, X. G.; Hu, Z. D. Fuel Process. Technol. 2004, 85 (13), 1463–1472. (34) Bauserman, J. W.; Nguyen, K. M.; Mushrush, G. W. Pet. Sci. Technol. 2004, 22 (11), 1491–1505. (35) Oliveira, E. C.; Vaz de Campos, M. C.; Rodrigues, M. R.; Perez, V. F.; Melecchi, M. I.; Vale, M. G.; Zini, C. A.; Caramao, E. B. J. Chromatogr., A 2006, 1105 (1-2), 186–90. (36) Adam, F.; Bertoncini, F.; Dartiguelongue, C.; Marchand, K.; Thiebaut, D.; Hennion, M.-C. Fuel 2009, 88 (5), 938–946. (37) Link, D. D.; John, P, B. Energy Fuels 2007, 21 (3), 1575–1581. (38) Burchill, P.; Herod, A. A.; Pritchard, E. J. Chromatogr., A 1982, 246 (2), 271–295. (39) Brown, D.; Earnshaw, D. G.; McDonald, F. R.; Jensen, H. B. Anal. Chem. 2002, 42 (2), 146–151. (40) Schmitter, J. M.; Ignatiadis, I.; Arpino, P.; Guiochon, G. Anal. Chem. 1983, 55 (11), 1685–1688. (41) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Anal. Chem. 1981, 53 (11), 1612–1620. (42) Wiwel, P.; Knudsen, K.; Zeuthen, P.; Whitehurst, D. Ind. Eng. Chem. Res. 2000, 39 (2), 533–540. (43) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N. Fuel 2003, 82 (13), 1621–1631.

(17) Egorova, M.; Prins, R. J. Catal. 2004, 224 (2), 278–287. (18) Kim, J. H.; Ma, X. L.; Zhou, A. N.; Song, C. S. Catal. Today 2006, 111 (1-2), 74–83. (19) Benedik, M. J.; Gibbs, P. R.; Riddle, R. R.; Willson, R. C. Trends Biotechnol. 1998, 16 (9), 390–395. (20) Prins, R.; Egorova, M.; Rothlisberger, A.; Zhao, Y.; Sivasankar, N.; Kukula, P. Catal. Today 2006, 111 (1-2), 84–93. (21) Depauw, G. A.; Froment, G. F. J. Chromatogr., A 1997, 761 (1-2), 231–247. (22) Hua, R.; Li, Y.; Liu, W.; Zheng, J.; Wei, H.; Wang, J.; Lu, X.; Kong, H.; Xu, G. J. Chromatogr., A 2003, 1019 (1-2), 101–109. (23) Ma, X. L.; Sakanishi, K.; Isoda, T.; Mochida, I. Fuel 1997, 76 (4), 329–339.

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Table 2. Composition and Properties of the Five Liquid Hydrocarbon Samples property

LCO

SRGO

ULSD

CDL1a

CDL2b

source C (wt %) H (wt %) N (ppmw) S (ppmw) O (diff.) (wt %) density (15 °C) (g/cm3) boiling point range (°C)

PARC Company 85.62 8.39 598 18 100 4.10 1.05 175-376

Kuwait 83.95 13.05 144 10 328 1.95 0.85 210-360

BP - Fuels Technology 86.80 12.90 12 13 0.3 0.83 166-345

Intertek-PARC 88.85 9.05 980 70 2.0 0.96 150-356

China Coal Research Institute 83.55 10.41 2200 552 5.76 0.90 196-380

a

The coal-derived liquid from hydrotreatment of coal pyrolysis oil. b The coal-derived liquid from the direct coal liquefaction of Heishan coal.

for identifying and quantifying the nitrogen compounds in the liquid hydrocarbon streams. The nitrogen compounds in five typical liquid hydrocarbon streams, including straight run gas oil (SRGO), light cycle oil (LCO), commercial ultralow sulfur diesel (ULSD), and two different coal-derived liquids (CDL1, CDL2) from coal pyrolysis and the state-of-the-art coal liquefaction process, were identified and quantified by a combination of the modified solid-phase extraction preseparation and gas chromatography with a mass spectrometry detector (GC-MS) and a nitrogen/phosphorus detector (GC-NPD). The major nitrogen compounds and their concentrations in different streams were compared and the implication was discussed.

Figure 1. Scheme of preseparation and concentration of nitrogen compounds from petroleum-derived samples (PDS).

2. Experimental Section 2.1. Liquid Hydrocarbon Streams and Chemicals. Five typical liquid hydrocarbon streams, including SRGO, LCO, ULSD, CDL1, and CDL2, were used in this study. The SRGO with a boiling point range of 210-360 °C and N content of 144 ppm (ppmw) was from a Kuwait crude oil. The LCO (from fluid catalytic cracking of petroleum) with a boiling point range of 175-376 °C and N content of 598 ppmw was provided by PARC Company. The commercial ULSD was provided by BP-Fuels Technology with a boiling point range of 166-345 °C and N content of 12 ppmw. CDL1 with a boiling point range of 150-356 °C and N content of 980 ppmw was provided by Intertek-PARC, which was from the hydrotreatment of coal pyrolysis oil (from high temperature carbonization of coal) provided by Koppers Chemical. CDL2 with a boiling point range of 196-380 °C and N content of 2200 ppmw was a mild distillate from the direct liquefaction of Heishan coal (longflame coal) in a 0.12 ton/day continuous coal liquefaction unit at 455 °C, 19 MPa and the average retention time of 1.68 h in the presence of the Fe-based catalyst.44 This mild distillate was provided by the China Coal Research Institute. The five liquid hydrocarbon samples were from quite different sources but had a similar boiling point range. The detailed composition and properties of the five samples are listed in Table 2. The standard samples for identification including aniline, quinoline, 1,2,3,4-tetrahydroquinoline, 5,6,7,8-tetrahydroquinoline, pyridine, acridine, indole, and carbazole were purchased from Aldrich Chemical Co. All the HPLC grade solvents, including benzene, chloroform, and methanol, were also purchased from Aldrich Chemical Co. Neutral alumina ordered from Aldrich Chemical Co. was used as a stationary phase in the column separation and was activated at 110 °C for 12 h before use. 2.2. Preseparation of Nitrogen Compounds. In order to better identify the nitrogen compounds in various liquid hydrocarbon samples by GC-MS, the nitrogen compounds in the samples were first separated by using a separation column with neutral alumina as a stationary phase and different solvents as a mobile phase in turn. A diagram of the preseparation procedure is shown in Figure 1 for the petroleum-derived samples and in

Figure 2. Scheme of preseparation and concentration of nitrogen compounds from coal-derived samples.

Figure 2 for the coal-derived liquids. For the preseparation of the petroleum-derived samples, approximately 100 g of the activated neutral alumina was packed in the glass column and about 4 g of the sample was placed on the top of the column. The sample was eluted first with 250 mL of benzene for the nonpolar fraction and then with 250 mL of a benzene-methanol mixture with a volume ratio of 1:1 for the polar fraction. The solvent in the collected eluate was removed by evaporation, and the nonpolar and polar fractions were weighed. For the preseparation of the coal-derived liquids, since the polar fraction obtained according to the method shown in Figure 1 contains not only nitrogen compounds but also many oxygen-containing compounds that interfere significantly with the identification of nitrogen compounds by using GC-MS, a modified column separation using different solvents as the mobile phases was conducted, as shown in Figure 2. For a typical preseparation, about 4 g of oil sample was placed on the top of the column with 100 g of the activated neutral alumina. The sample was eluted with 250 mL of benzene, 1000 mL of chloroform, 500 mL of chloroform, and 500 mL of methanol in turn, and the corresponding eluates were collected. After removal of the solvent from the eluates by evaporation, the nonpolar fraction (NPF), polar fraction-I (PF-I), polar fraction-II (PF-II), and polar fraction-III (PF-III) were obtained. The preseparation effect was further examined by using a gas chromatograph (Varian CP 3800 gas chromatograph) with a capillary column (30 m length, 0.25 mm internal diameter, and 0.25 mm film thickness) coupled with a flame ionization detector (GC-FID) and a nitrogen-phosphorus detector (GC-NPD). The column temperature was programmed at 40 °C for 4 min, from 40 to 290 °C at a rate of 3 °C/min, and at 290 °C for 5 min. The injector and detector temperatures were set at 290 °C. 2.3. Identification. The identification of the nitrogen compounds in each fraction was carried out by the gas chromatography-mass

(44) Wu, X. Z.; Li, K. J.; Li, W. B. Coal Convers. 2009, 32 (1), 40–42.

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Figure 4. GC-FID and GC-NPD chromatograms of CDL2-PF.

indicate that the measured peak area for each nitrogen compound in GC-NPD analysis of the samples is directly proportional to its nitrogen concentration, if ignoring the interference from the coeffluent hydrocarbons. Thus, the nitrogen concentration (Ci) corresponding to each nitrogen compound can be estimated according to the following equation: Ci ¼ Ctotal Ai =ΣA

Figure 3. GC-NPD chromatograms of LCO and LCO-PF and GC-FID chromatogram of LCO-PF. (The nitrogen compounds corresponding to the peaks are listed in Table 3.)

spectrometry analysis. The GC-MS analyzer consisted of a Shimadzu GC-17A gas chromatograph coupled with a Shimadzu QP-5000 mass spectrometer. The gas chromatograph was fitted with a fused silica capillary column (Rxi-5 ms, 30 m  0.25 mm i.d.  0.25 μm film thickness) purchased from Restek. The ultrahigh-purity helium was used as a carrier gas at a flow rate of 1 mL/min. The column temperature program was the same as that for the GC-FID analysis. The injector and detector temperatures were set at 290 °C. The sample injection volume was 1 μL, and the split ratio was 15:1. The mass spectrometer was operated in the electron impact mode using an ionization energy of 70 eV. Identification was conducted by a similarity search of the spectra of the compound from the NIST 107 mass spectral library. Identification of some nitrogen compounds, such as indole, quinoline, 1,2,3,4-tetrahydroquinoline, 5,6,7,8-tetrahydroquinoline, and carbazole was further confirmed by the standard samples. Major alkyl carbazoles were identified by comparison of the relative retention times with the ones in the literature using similar analysis conditions.11,33,42 2.4. Quantification. The nitrogen-phosphorus detector (NPD) has been widely used in the selective analysis of the nitrogen-containing compounds in the hydrocarbon matrix,24,45 as the responsibility of NPD for nitrogen is approximately 100 000 times higher than that of the normal hydrocarbons, and NPD is much cheaper and easier to be maintained than the other nitrogen-selected detectors, such as NCD and AED. In order to confirm that the signal generated in the GC-NPD analyzer is linear in the nitrogen concentration range of the analyzed samples, a series of standard samples, which contained six nitrogen compounds (aniline, quinoline, pyridine, acridine, indole, and carbazole) in toluene with the nitrogen concentration ranging from 2 to 500 ppmw for each compound were prepared. The standard samples were analyzed by the GC-NPD analyzer, and the response signal was correlated with the known nitrogen concentration of the corresponding compound. The results show that the responsibility of the GC-NPD analyzer is linear for each nitrogen compound in the nitrogen concentration range of 2-500 ppmw with a R2 value higher than 0.99, and the response factor for nitrogen in the different nitrogen compounds is almost the same. The results

where Ai is the peak area corresponding to the N-containing compound i, ΣA is total area of peaks, and Ctotal is the total nitrogen concentration of the sample. The total nitrogen concentration of the sample was measured by using the Antek 9000 series nitrogen analyzer with a measurement limitation of 0.5 ppmw.

3. Results and Discussion 3.1. Isolation of Nitrogen Compounds. In order to avoid the interference of the coexisting hydrocarbons and others with identification of the nitrogen compounds by GC-MS, each petroleum-derived sample was first separated into the nonpolar and polar fractions according to the procedure shown in Figure 1. The total nitrogen content of the polar and nonpolar fractions was analyzed by using the total nitrogen analyzer. According to the nitrogen balance analysis for the preseparation of the LCO, more than 90% of the nitrogen compounds in the LCO were recovered and more than 98% of the recovered nitrogen compounds were in the polar fraction (LCO-PF). It indicates that the most of the nitrogen compounds were concentrated into LCO-PF. GC-NPD chromatograms of the LCO and LCO-PF and a GC-FID chromatogram of LCO-PF are shown in Figure 3. By comparison of the GC-NPD chromatogram of LCO and GC-NPD chromatogram of LCO-PF, for almost each peak in the GC-NPD chromatogram of LCO, a corresponding peak with the same retention time and similar relative peak intensity was found in the GC-NPD chromatogram of LCO-PF, implying that the concentrated nitrogen compounds in LCO-PF included almost all the nitrogen compounds in the LCO. By comparison of the GC-NPD and GC-FID chromatograms of LCO-PF, as shown in Figure 3, the essential peaks in the GC-FID were found to correspond to the peaks in GC-NPD, indicating that major compounds in LOC-PF were the nitrogen-containing compounds. The results verified that the simple column preseparation used in this study was efficient for isolating the

(45) Lancas, F. M.; Barbirato, M. A. Pet. Sci. Technol. 1994, 12 (3), 507–518.

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Figure 5. GC-FID and GC-NPD chromatograms of (a) CDL2-PF-I, (b) CDL2- PF-II, and (c) CDL2-PF-III.

nitrogen compounds from LCO, which greatly benefits the subsequent identification of the nitrogen compounds in the petroleum-derived samples by GC-MS. For the coal-derived liquids (CDL), the same preseparation method was first applied. The GC-NPD and GC-FID chromatograms of CDL2-PF are shown in Figure 4. By comparison of the GC-NPD and GC-FID chromatograms of CDL2-PF, lots of peaks corresponding to the peaks in GC-FID were not found in the GC-NPD. It indicates that many polar compounds in CDL2-PF were the nonnitrogen-containing compounds, the peaks of which could overlap with those of the nitrogen-containing compounds, resulting in interference of the subsequent identification of the nitrogen-containing compounds by GC-MS. This is because CDL usually contains much more polar compounds, which include not only the nitrogen compounds but also the oxygen-containing compounds that coexist in the polar fraction. Consequently, a modified prepreparation method, as shown in Figure 2, was developed and used. CDL1 and CDL2 were, respectively, separated into NPF, PF-I, PF-II, and PF-III. The nitrogen balance analysis of each fraction using the total nitrogen analyzer for CDL2 indicated that more than 80% of the nitrogen compounds were recovered with a recovered nitrogen distribution of 1.6,

12, 83, and 3.4% in CDL2-NPF, CDL2-PF-I, CDL2-PF-II, and CDL2-PF-III fractions, respectively. The major nitrogen compounds were concentrated in CDL2-PF-I and CDL2-PF-II. The GC-NPD and GC-FID chromatograms of CDL2PF-I and CDL2-PF-II are shown in parts a and b of Figure 5, respectively. The comparison of the GC-NPD chromatogram and GC-FID chromatogram of CDL2-PF-I and CDL2-PF-II shows that the essential compounds in CDL2PF-I and CDL2-PF-II were the nitrogen-containing compounds. The GC-NPD and GC-FID chromatograms of CDL2-PF-III are shown in Figure 5c. Very few nitrogen compounds were detected, which is consistent with the nitrogen balance analysis, although there were many polar species in this fraction. The GC-MS analysis of CDL2-PF-III indicates that the major compounds in this fraction were the oxygen-containing compounds, such as alkyl phenols and alkyl indanols. These oxygen-containing compounds could not be washed out from the column by chloroform but by methanol, as they may have higher polarity than the nitrogen compounds. The results indicate that the preseparation method proposed in this study for CDL is very efficient and relatively simpler for concentration of the nitrogen compounds from CDL in comparison to the previously 5543

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Table 3. Identification and Quantification Results of Nitrogen Compounds in Five Liquid Hydrocarbon Streams nitrogen concentration (ppm) peak number

nitrogen compound

TR(min)

molecular weight

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

aniline Cl aniline Cl aniline Cl aniline C2 aniline C2 aniline C2 aniline C2 aniline C2 aniline 5,6,7,8-tetrahydroquinoline quinoline C3 aniline C3 aniline C3 aniline indole C1 quinoline C1 quinoline 1,2,3,4-tetrahydroquinoline CI tetrahydroquinoline CI tetrahydroquinoline Cl indole C1 quinoline Cl indole Cl indole Cl indole Cl indole C2 quinoline C2 quinoline CI tetrahydroquinoline CI tetrahydroquinoline CI tetrahydroquinoline C2 indole C2 indole C2 indole C2 indole C2 indole C3 indole benzoquinoline tetrahydrocarbazole tetrahydrocarbazole carbazole 1-methyl-carbazole 3-methyl-carbazole 2-methyl-carbazole 4-methyl-carbazole 1,8-dimethylcarbazole 1,3 þ 1,6-dimethylcarbazole 1,7-dimethylcarbazole 1,5 þ 1,4-dimethylcarbazole 2,6 þ 3,5 þ 2,7-dimethylcarbazole 1,2- þ 2,4-dimethylcarbazole 2,3-dimethylcarbazole 1,3,5-trimethylcarbazole 1,5,7-trimethylcarbazole 1,3,4 þ 2,4,7-trimethylcarbazole 3,4,6-trimethylcarbazole C3 carbazole

16.84 21.63 21.76 21.99 25.74 26.35 26.61 26.83 27.88 28.88 29.26 30.03 30.65 30.89 32.04 32.66 33.22 33.98 34.8 34.98 35.18 35.36 36.00 36.38 36.45 36.70 37.76 37.99 38.41 38.52 38.74 39.14 39.37 39.48 40.35 40.48 41.88 51.60 51.79 52.16 53.48 55.78 56.75 56.96 57.42 57.76 58.97 59.35 59.54 60.09 60.47 60.95 62.07 62.65 63.11 63.38 65.90

93 107 107 107 121 121 121 121 121 133 129 135 135 135 117 143 143 133 147 147 131 143 131 131 131 131 157 157 147 147 147 145 145 145 145 145 159 179 171 171 167 181 181 181 181 195 195 195 195 195 195 195 209 209 209 209 209

CDL2 10.9 26.4 8.1 22.5 27.8 29.2 34.1 14.8 17.2 8.4 87.5 71.3 25.0 20.9 75.3 6.1 35.7 66.8 22.3 47.1 32.7 30.1 30.0 23.5 27.1 54.2 30.2 25.7 16.2 13.7 10.9 22.8 8.4 30.2 16.2 29.2 16.5 19.7 8.1 13.7 23.9 23.5 19.0 18.3 13.7

CDL1

LCO

SGRO

ULSD

11.5

1.4

13.2 12.6 12.5 11.0 7.3

1.3 2.3 1.9 1.8 1.0

10.0

1.1

82.2 12.2 250.9 9.8 13.0 11.0 16.2

24.7 16.7 21.5 8.2

38.1 46.7 24.7 34.1 46.7 10.7 39.0 19.9 36.9 33.7 28.7 27.1 26.0 16.2 7.8 24.2

65.3

sum of identified peaks

1280.0

466.3

460.5

78.1

10.7

total N concentration (ppm)

2200.0

980.0

598.0

144.0

12.0

reported method.3 The developed method is not only able to separate the nitrogen compounds from the coexisting hydrocarbons but also able to separate the nitrogen compounds from the coexisting organic oxygen-containing compounds in CDL by a simple solid-phase extraction with different solvents. For facilitating comparison, the GC-NPD chromatograms of CDL2, CDL2-PF-I, and CDL2-PF-II are shown

in Figure 6. It can be seen that for each peak in GC-NPD chromatograms of CDL2, a corresponding peak in GC-NPD chromatogram of CDL2-PF-I or CDL2-PF-II can be found, indicating that almost no type of the nitrogen compounds were lost when using CDL2-PF-I or CDL2-PF-II for identification of the nitrogen compounds in CDL2. Consequently, the identification of the nitrogen compounds in the CDLs can be conducted by the identification of the concentrated nitrogen 5544

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Figure 7. GC-NPD chromatograms of SRGO, LCO, ULSD, CDL1, and CDL2. (The nitrogen compounds corresponding to the peaks are listed in Table 3.) Figure 6. GC-NPD chromatograms of CDL2, CDL2-PF-I, and CDL2-PF-II. (The nitrogen compounds corresponding to the peaks are listed in Table 3.)

The three oil samples from petroleum contained similar nitrogen compounds although the total nitrogen content is quite different. The nitrogen content in LCO was 598 ppmw. The major nitrogen compounds in LCO were carbazole and its alkyl substituted derivatives with the alkyl side-chain carbon numbers ranging from 1 to 3. The alkyl carbazole isomers with different carbon number and position of alkyl groups were identified. In total, 16 alkyl carbazoles were identified in the fuel, in which 4 major nitrogen compounds are carbazole, 1-methylcarbazole, 4-methylcarbazole, and 1,3-dimethylcarbazole/1,6-dimethylcarbazole. The nitrogen content in SGRO was 144 ppmw. Seven nitrogen compounds were identified in it. The major nitrogen compounds in SGRO were alkyl carbazoles with the alkyl side-chain carbon numbers ranging from 2 to 3. Interestingly, the commercial ULSD also contained some alkyl carbazoles. The major alkyl carbazoles were 2,3-dimethylcarbazole, 1,3,5-trimethylcarbazole, and 1,5,7-trimethylcarbazole. Almost no amines, aniline, pyridine, quinoline, hydrogenated quinoline, acridine and their derivatives were detected in the three petroleum-derived samples analyzed in this study. 3.2.2. Identification of Nitrogen Compounds in CoalDerived Liquids. As shown in Figures 6 and 7, major nitrogen compounds identified in CDL2 were aniline, quinoline, and tetrahydroquinoline and their alkyl derivatives. Some indole and carbazole and their alkyl derivatives were also detected in CDL2. Figure 7 also shows the GC-NPD chromatograms of CDL1 with the assigned peak number referring to the compounds listed in Table 3. Similar to CDL2, the major nitrogen compounds in CDL1 are anilines, quinolines, and their derivatives. Few alkyl carbazoles were detected in CDL1. The presence of the partially hydrogenated quinolines, such as 1,2,3,4-tetrahydroquinoline and 5,6,7,8-tetrahydroquinoline and their alkyl derivatives, indicates that the samples have suffered from the hydrotreatment, which is consistent with the sample source. 3.3. Quantification of Nitrogen Compounds. In comparison of the total nitrogen content, the five samples had quite different total nitrogen content, although they had a similar

compounds in fraction CDL2-PF-I and CDL2-PF-II by GC-MS without the interference of the coexisting hydrocarbons and oxygen-containing compounds. 3.2. Identification of Nitrogen Compounds. The identification of the nitrogen compounds in the polar fractions (LCO-PF, SGRO-PF, and ULSD-PF) from petroleum and the polar fractions (CDL1-PF-I, CDL1-PF-II, CDL2-PF-I, and CDL2-PF-II) from CDLs was conducted by GC-MS analysis. The GC-NPD chromatogram of LCO-PF with the peak numbers referring to the compounds listed in Table 3 is shown in Figure 3. The GC-NPD chromatograms of CDL2-PF-I and CDL2-PF-II with the peak numbers referring to the compounds listed in Table 3 are shown in Figure 6. In the comparison of the GC-NPD chromatograms of CDL2-PF-I and CDL2-PF-II, the major nitrogen compounds in CDL2-PF-I were 1,2,3,4-tetrahydroquinoline, methyl-1,2,3,4-tetrahydroquinoline, and alkyl (C2-C3) indoles, while the major nitrogen compounds in CDL2-PF-II were aniline, quinoline, indole, carbazole, and their alkyl substituted derivatives. The results indicate that indole, alkyl anilines, and alkyl quinolines have higher polarity than 1,2,3,4-tetrahydroquinoline and alkyl indoles and thus were eluted later by chloroform solvent. The effluence order of the nitrogen compounds in the preseparation may depend on three factors: polarity, basicity, and ring number of the nitrogen compounds. It should be mentioned that two strong peaks, which are marked by a and b in GC-NPD of CDL2-PF-II, were observed. On the basis of identification by GC-MS, peak a should be C1-tetrahydroquinoline and peak b should be C1-quinoline. However, no significant peaks corresponding to these two compounds were found in GC-NPD of CDL2. Further study is necessary to clarify it. 3.2.1. Identification of Nitrogen Compounds in PetroleumDerived Samples. For facilitating comparison, the GC-NPD chromatograms of SRGO, LCO, ULSD, CDL1, and CDL2 with the assigned peaks are shown in Figure 7. 5545

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carbazoles have at least one alkyl group at the 1-position of carbazole, such as 1,3(or 1,6)-dimethylcarbazole, 1,2-dimethylcarbazole, 1,3,5-trimethylcarbazole, 1,5,7-trimethylcarbazole, and 1,3,4-trimethylcarbazole. It indicates that the steric hindrance of the methyl groups at the 1-position might be a reason for their low reactivity in the hydrotreating process or the methyl groups at the 1-position reduces the hydrogenation activity of carbazoles. It was noted that the magnitude of the measured nitrogen content in the ULSD is the same as that of the sulfur content. It indicates that the presence of these nitrogen compounds may significantly inhibit the desulfurization of ULSD by the competitive adsorption on the active sides on catalyst or adsorbent, if the further desulfurization of ULSD is required, such as for fuel cell applications. In comparison of the two coal derived liquids, CDL1 contains less nitrogen content (980 ppmw) as CDL1 was the hydrotreated product from a coal pyrolysis oil. The major nitrogen compounds in CDL1 are aniline and its derivatives, and almost no indole and carbazole types of the nitrogen compounds were detected in CDL1. There were also some quinoline derivatives and alkyl indoles in CDL1. CDL2 with 2200 ppmw of nitrogen contains all types of the nitrogen compounds. The concentration of the family groups increases in the order of carbazole group < aniline group < indole group < quinoline group. In comparison between the coal-derived and petroleumderived liquid hydrocarbon streams, the coal-derived streams contain much higher nitrogen than the petroleumderived streams, while the petroleum-derived streams contain much higher sulfur than the coal-derived streams, except ULSD, as ULSD has suffered the ultradeep hydrodesulfurization. Moreover, the nitrogen compounds existing in the coal-derived streams and the petroleum-derived streams are quite different. The major nitrogen compounds in the petroleum-derived liquid hydrocarbon streams are the neutral nitrogen compounds, especially carbazole and its alkyl substituted derivatives, while in the coal-derived liquid hydrocarbon streams are the basic nitrogen compounds, such as aniline, quinoline, and their derivatives. Since alkyl carbazoles and alkyl indoles are neutral and have less polarity than alkyl anilines and alkyl quinolines, they may have less poison to the acidic catalysts but may be more difficult to be removed by hydrotreating, adsorption, or extraction. According to this study, it appears that the currently commercial hydrotreating processes that were developed for petroleum refining with focus on ultradeep desulfurization may not be suitable for refining the coal liquids from the direct coal liquefaction or coal pyrolysis, as the major task in upgrading the coal liquids is denitrogenation instead of desulfurization. Adoption of a selective adsorption process or an extraction process for denitrogenation of the coal liquids may be a better choice, as the nitrogen content in the coal liquids is much higher and the major nitrogen compounds in them are basic and have higher polarity.

Figure 8. Distribution of nitrogen compounds in different liquid hydrocarbon samples.

boiling point range. The nitrogen content increased in the order of ULSD (12 ppmw) < SGRO (144 ppmw) < LCO (598 ppmw) < CDL1 (980 ppmw) < CDL2 (2200 ppmw), while the sulfur content increased in the order of ULSD (15 ppmw) < CDL1 (70 ppmw) < CDL2 (552 ppmw) < SGRO (10328 ppmw)