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Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent
Fuel Processing Technology 159 (2017) 27–30
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Short communication
Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent Yu-Gao Wang a,⁎, Ze-Shi Niu a, Jun Shen a,⁎, Lei Bai b, Yan-Xia Niu a, Xian-Yong Wei c, Rui-Feng Li a, Jun Zhang a, Wen-Yi Zou a a b c
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, United States Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
a r t i c l e
i n f o
Article history: Received 23 September 2016 Received in revised form 12 January 2017 Accepted 12 January 2017 Available online xxxx Keywords: Direct coal liquefaction residue CO2-triggered switchable solvent Polycyclic aromatic structures Structural characterization
a b s t r a c t Extracting polycyclic aromatic structures (PASs)-enriched portion from direct coal liquefaction residue (DCLR) with an energy-efficient way is significantly important for the effective utilization of DCLR. In the investigation, dipropylamine (DPA) with weak polarity was used to extract DCLR as a CO2-triggered switchable solvent. When the extract solution was triggered by CO2, the extract automatically precipitated with the yield of 22.8% of DCLR. Furthermore, adding proper amount of NaOH could successfully achieve the recovery of DPA, and the recovered solvent could be reused for extracting DCLR. According to the characterization results of the extract by multiple analytical tools, the extract mainly consisted of PASs with over 4 rings, and the average molecular model of aromatic cluster could be represented by the derivative of coronene with 8 substituent groups. The investigation proved that extraction of PASs-enriched portion using DPA from DCLR was feasible, and tremendous energetic and environmental benefits could be potentially gained given that the separation of extract and solvent as well as solvent recovery could be achieved by the non-thermal CO2-trigger. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Direct coal liquefaction residue (DCLR) is the significant byproduct from the direct coal liquefaction (DCL) process, generally accounting for about 30 wt% of the coal consumption in DCL [1]. The effective utilization of DCLR is paid increasing attention along with the development of DCL. Usually, DCLR is subjected to combustion, gasification, and carbonization to afford heat, hydrogen, and coke, respectively [2]. More importantly, there are abundant polycyclic aromatic structures (PASs) in DCLR, making DCLR a promising precursor for preparing high valueadded carbon materials, such as carbon microfibers [3], mesoporous carbons [4], and hierarchical porous carbons [5]. Therefore, developing convenient and effective means to get the PAS-enriched portion is greatly necessary to realize high value-added utilization of DCLR. Extraction with organic solvents is a common method, in which benzene [6], N,N-dimethylacetamide [7], and coal liquid fraction [8] are usually adopted. Considering that these solvents are volatile, toxic, and environmentally unfriendly, Zhang's group successfully developed various kinds of ionic liquids to extract PAS-enriched portions from DCLR, and the extracts would precipitate from the extract solution by adding some amount of water even hot water of 70 °C as a stripping agent [9, ⁎ Corresponding author. E-mail addresses: [email protected] (Y.-G. Wang), [email protected] (J. Shen).
http://dx.doi.org/10.1016/j.fuproc.2017.01.021 0378-3820/© 2017 Elsevier B.V. All rights reserved.
10]. Thus, the recovery of the ionic liquids inevitably involves distillation of water, however, which obviously is not energy efficient. CO2-triggered switchable solvents, first reported by Jessop’ group [11], can be reversibly converted from a non-ionic form to an ionic form by bubbling and then removing CO2 in the solvent system at room temperature, leading to the solvent recovery under CO2 trigger instead of distillation and evaporation involving thermal processes. These CO2-triggered switchable solvents including amines, amidines, and guanidines can be significantly advantageous for extracting oils from soybean or algae [12–15], because oils are easily released from the extraction solution when the solvent is switched from non/weak-polar form to polar form triggered by CO2, resulting in reducing energy consumption in the separation of oil and the solvent and the recovery of the solvent, as diagramed in Fig. S1 [14]. Herein, we evaluated the application of DPA as the CO2-triggered switchable solvent for extracting DCLR because of its limited volatility, low viscosity, and weak polarity [15], and characterized the extract with elemental analyzer, gas chromatograph/mass spectrometer (GC/MS), solid-stated 13C nuclear magnetic resonance (13C NMR). 2. Materials and methods DCLR was obtained by liquefying Shenmu–Fugu subbituminous coal at 445–455 °C under 18 MPa of hydrogen [16] in Shenhua direct coal liquefaction plant, Erdos, Inner Mongolia, China. Its proximate and
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Y.-G. Wang et al. / Fuel Processing Technology 159 (2017) 27–30
ultimate analyses as well as those of the original coal (Shenmu–Fugu subbituminous coal) and its extraction residue after extracting using DPA were listed in Table S1. DCLR was pulverized to pass through a 100-mesh sieve followed by desiccation in a vacuum oven at 80 °C for 24 h prior to use. DPA used in the experiment was an analytical reagent. As Fig. 1 illustrates, 3 g of DCLR was extracted with 105 mL of DPA under ultrasonic wave irradiation bearing 150 W of ultrasonic power at room temperatures for 45 min. Then, the extract solution and the extraction residue were separated via filtration. For improving the following phase separation and switching efficiency, the extract solution was added an equal volume of H2O creating two phases. Subsequently, CO2 was bubbled into the system, during which the two phases in the system gradually turned into one phase, while some black solids precipitated, i.e., the solid extract from DCLR. When no extract came out, bubbling CO2 was terminated. The extract was obtained by filtration, dried in a vacuum oven, and weighted to afford extract yield. In the reference [15], DPA could be finally recovered by heating the remaining solvent system at 90 °C for 1 h accompanied with bubbling N2, however, which does not seem to be energy-saving. Considering of the huge CO2 emission and the resulting environmental damage, it may be better to store CO2 rather than remove CO2 in the system by heating. In the study, we tried to replace DPA in the forming salt by adding proper amount of NaOH and successfully realized the recovery of DPA, and the fact could be evidenced by GC analysis of the recovered solvent, in which DPA accounted for more than 99% of relative content (RC) as exhibited in Fig. S2, and some heteroatom-containing compounds except DPA and a small amount of arenes were further detected by GC/MS as displayed in Fig. S3 and Table S2. Interestingly, the recovered solvent could be used to extract DCLR with an extract yield approximately equal to the fresh DPA at the same condition. In the following investigation, the recovery of DPA would be improved including
selecting proper base, such as Ca(OH)2 to afford CaCO3 with nanometer size [17] and optimizing DPA recovery condition.
3. Results and discussion The resulting extract yield in the study was 22.8% based on the mass of DCLR, which was similar to the extract yields of DCLR extraction using benzene [6] and some magnetic ionic liquids [9], but much less than those of DCLR extraction with N,N-dimethylacetamide [7] and coal liquid fraction [8]. The difference in the extract yield using different solvents in Table 1 was partly attributed to the extraction condition. In fact, the extract yield would increase to ca. 30% under further optimizations of extraction condition, which was reported elsewhere [18]. Regardless of the extraction condition and extract yield, the extraction solvents and solvent recovery in Table 1 were simply compared. DPA is much less volatile and toxic than benzene. The recoveries of N,Ndimethylacetamide and coal liquid fraction from the extract solution are supposed to be distillated above 160 °C at atmospheric pressure, which obviously are more energy-consuming than the recovery of DPA. The ionic liquids are the promising solvents for extracting DCLR, but they are expensive and their recovery involves the distillation of water. Therefore, the extraction of DCLR using DPA as a CO2-triggered switchable solvent is suitable and promising. The obtained extract was further analyzed by elemental analyzer, GC/MS, and 13C NMR spectrometers to investigate the PASs in the study. Compared with DCLR, the carbon, hydrogen, and nitrogen contents of the extract increased, but the oxygen content decreased. The elemental analyses of the extract in the study and the benzene extract in reference [6] were almost same, while the H/C value of the extract herein was more than that of the extracts with ionic liquids [9,10] and N,N-
Fig. 1. Procedure of DCLR extraction using DPA, separation between extract and DPA, and recovery of DPA.
Y.-G. Wang et al. / Fuel Processing Technology 159 (2017) 27–30
29
Table 1 Yield, ultimate analyses, and several structural parameters obtained by 13C NMR analysis of extracts from DCLR with different solvents. Solvent
DPA Benzene N,N-dimethylacetamide Coal liquid fractionb 1-Butyl-1-methylpyrrolium [FeCl4] 1-Butyl-3-methylimidazolium [FeCl4] N-butylpyridium [FeCl4] N-methylmidazolium benzoate Trietyhylammoium formate 3-Methypridinium formate 3-Methypridinium benzoate a b c d
Yield (wt%)
22.8 23.8 62.2 50.0 11.2 22.8 24.5 29.4 10.0 32.3 33.8
Ultimate analysis (wt%)
H/C
C
H
N
O
87.11 87.19 88.74 90.63 82.62 86.51 88.71 88.77 90.09 90.75 91.55
6.32 6.53 4.69 7.32 5.35 5.46 5.2 5.71 6.08 5.78 5.25
1.63 2.08 1.72 1.19 1.79 1.87 1.28 1.13 1.1 1.07 0.83
4.51 4.17 4.68 – 5.79 5.51 4.29 4.31 2.62 2.29 2.3
0.87 0.90 0.63 0.97 0.78 0.76 0.70 0.77 0.81 0.76 0.69
Structural parameters
Reference
fa
Ra
0.65 –a 0.76 –a 0.72 0.74 0.75 0.67 0.64 0.66 0.69
7 –a –a –a 7 8 7 7 7 7 6
This study [6] [7] [8] [9]c
[10]d
No corresponding data are obtained by 13C NMR. Coal liquid fraction was distillated ranging from 220 to 260 °C from coal liquid form of Shengli Lignite. The used solvents are magnetic ionic liquids. The used solvents are protic ionic liquids.
dimethylacetamide, but less than that of the extract with coal liquid fraction, as summarized in Table 1. A total of 41 compounds were identified in the extract with GC/MS, consisting of 27 arenes with total RC of 82.23%, 7 n-alkanes with total RC of 6.36%, and 7 other species (OSs) with total RC of 11.41%, as displayed in Fig. S4 and Tables S3–S4 (see the Supporting information). Arenes dominated in the extract, which were mainly composed of nonsubstituted arenes with 2–7 rings as well as some substituted arenes and a small amount of hydroarenes. Among these GC/MS-detectable components, benzo[ghi]perylene (Peak 34) was the most abundant with the RC of 20.83%, whose absolute content was 1.01 mg·g− 1 in the extract by external standard method in GC analysis. As shown in Table S3, the compounds with RC beyond 4% were polycyclic aromatic compounds with more than 4 rings, indicating that the average number of aromatic rings in the GC/MS-detectable portion was over 4. The detected hydroarenes were likely to result from the hydrogenation of the existing arenes in Shenmu–Fugu subbituminous coal during its DCL, since no hydroarenes were detected in the extracts of the coal [19]. Methyl is the main substituted group, and monomethylarenes (Peaks 14, 22, and 37) were the dominant substituted arenes. These nalkanes consecutively ranged from C20 to C26, which may be from either desorption of adsorbed hydrocarbons trapped in the macro molecular portion or the rupture of side chains on PASs during DCL. Among the OSs, benzo[pqr]tetraphen-6-ylmethanol (Peak 31) is the richest, whose similar compound was identified in the extract of DCLR with petroleum [20]. Hexadecahydropyrene, possibly forming by complete hydrogenation of pyrene, further proved that hydroarenes were generated from the hydrogenation of the arenes. Compared with GC/MS only analyzing those thermal stable and volatile species, 13C NMR can provide the comprehensive structural information on the sample bulk. As Fig. S5 shows, the aromatic band in the range of 90–165 ppm was predominant in the 13C NMR spectrum, indicating that the extract was abundant in PASs, in agreement with the GC/ MS analysis. By curve fitting, the band was further separated into 5 individual peaks (see in Fig. S5 and Table S5), representing different aromatic carbon types. Furthermore, several structural parameters were obtained for fully understanding PASs in the extract as displayed in Table S6 on the basis of the data in Table S5 [21–23]. The aromaticity index (fa) was 0.65, indicating that there were 65 aromatic carbons per 100 carbon atoms in the extract. The molar content of aromatic bridgehead carbon (xb) is an important parameter to estimate the aromatic cluster size. On the basis of xb, the average number of carbon atoms per aromatic cluster Ca in the extract could be deduced as 23.58 by calculation method as reported in the literature. Correspondingly, the average number of aromatic nucleus (Ra) was ca. 7, implying that the molecular model of average aromatic cluster may be present as
coronene, and the number of substituent aromatic carbons was ca. 8, suggesting there were about 8 aromatic carbons substitute by aliphatic chain or oxygen-containing functional groups per aromatic cluster. Therefore, the derivative of coronene with 8 substituent groups could be used to represent the average molecular model of aromatic cluster in the extract, indicating that PAs with more than 7 aromatic rings existed in the extract, which could not be detected by GC/MS. As Table 1 exhibits, the extract in the study had the similar aromaticity with the extracts using protic ionic liquids [10] but owned the lower aromaticity than the extracts using magnetic ionic liquids [9], while the values of Ra in these extracts were approximately equal to each other, demonstrating the average aromatic cluster in these extracts had similar condensed aromatic skeleton with 6–8 rings. 4. Conclusions DPA was used to extract DCLR as a CO2-triggered switchable solvent, and the extract could be released from the extract solution when the solvent system was triggered by CO2. Moreover, DPA could be successfully recovered by adding NaOH. The result proved that the application of DPA in extracting DCLR and the recovery of DPA without heating were feasible. According to GC/MS analysis, the condensed arenes with more than 4 rings are the main components in the extract, and benzo[ghi]perylene had the highest RC of 20.83%. The 13C NMR analysis suggested the extract was dominated by PASs. Furthermore, it could be concluded that the number of aromatic rings per aromatic cluster was ca. 7, maybe corresponding to coronene, and each aromatic cluster had ca. 8 substituent groups on average. The investigation provides an energy-saving way to get PASs-enriched portions from DCLR. Nomenclature C NMR 13C nuclear magnetic resonance DCL direct coal liquefaction DCLR direct coal liquefaction residue DPA dipropylamine FTIR Fourier transform infrared GC/MS gas chromatograph/mass spectrometer OSs other species PASs polycyclic aromatic structures RC relative content
13
Acknowledgements This work is supported by the NSFC-Shanxi joint fund for coal-based low carbon (Grant No. U1610223) and Scientific and Technological Project of Shanxi Province, China (Grant Nos. 20150313014-4 and
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2014011012-3). The authors sincerely thank Dr. Du Ying in University of Twente for her warm and kind help on the CO2-triggered switchable solvents. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.fuproc.2017.01.021. References [1] J. Li, J. Yang, Z. Liu, Hydrogenation of heavy liquids from a direct coal liquefaction residue for improved oil yield, Fuel Process. Technol. 90 (2009) 490–495. [2] W.J. Luo, L XZ, S YH, J.P. Fu, Research progress on utilization of coal liquefaction residue, Math. Rev. 27 (2013) 15–157 (Chinese journal). [3] Y. Zhou, N. Xiao, J. Qiu, Y. Sun, T. Sun, Z. Zhao, et al., Preparation of carbon microfibers from coal liquefaction residue, Fuel 87 (2008) 3474–3476. [4] J. Zhang, L. Jin, S. Zhu, H. Hu, Preparation of mesoporous activated carbons from coal liquefaction residue for methane decomposition, J. Nat. Gas Chem. 21 (2012) 759–766. [5] J. Zhang, L. Jin, J. Cheng, H. Hu, Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes, Carbon 55 (2013) 221–232. [6] X.H. Gu, S.D. Shi, M. Zhou, Study on the molecular structure of asphaltene fraction from the Shenhua coa l direct liquefaction residue, J. China Coal Soc. 31 (2006) 785–789 (Chinese journal). [7] J.L. Zhong, W.B. Li, S.D. Shi, X.S. Zhu, Solvent extraction research on organic matter in direct coal liquefaction residue, J. China Coal Soc. 37 (2012) 316–322 (Chinese journal). [8] Y. Sheng, The research of coal liquefaction of residue solvent extraction and the characteristics of the extract, Chin. Coal Res. Inst. (2009) (Chinese master dissertation). [9] J. Wang, H. Yao, Y. Nie, L. Bai, X. Zhang, J. Li, Application of iron-containing magnetic ionic liquids in extraction process of coal direct liquefaction residues, Ind. Eng. Chem. Res. 51 (2012) 3776–3782.
[10] L. Bai, Y. Nie, Y. Li, H. Dong, X. Zhang, Protic ionic liquids extract asphaltenes from direct coal liquefaction residue at room temperature, Fuel Process. Technol. 108 (2013) 94–100. [11] P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L. Liotta, Green chemistry: reversible nonpolar-to-polar solvent, Nature 436 (2005) 1102. [12] A.R. Boyd, P. Champagne, P.J. McGinn, K.M. MacDougall, J.E. Melanson, P.G. Jessop, Switchable hydrophilicity solvents for lipid extraction from microalgae for biofuel production, Bioresour. Technol. 118 (2012) 628–632. [13] C. Samorì, D.L. Barreiro, R. Vet, L. Pezzolesi, D.W. Brilman, P. Galletti, et al., Effective lipid extraction from algae cultures using switchable solvents, Green Chem. 15 (2013) 353–356. [14] P.G. Jessop, S.M. Mercer, D.J. Heldebrant, CO2-triggered switchable solvents, surfactants, and other materials, Energy Environ. Sci. 5 (2012) 7240–7253. [15] Y. Du, B. Schuur, C. Samorì, E. Tagliavini, D.W.F. Brilman, Secondary amines as switchable solvents for lipid extraction from non-broken microalgae, Bioresour. Technol. 149 (2013) 253–260. [16] Y. Zhang, Shenhua coal conversion technology and industry development, Proceedings of Global Climate and Energy Project International Workshop on Exploring the Opportunities for Research to Integrate Advanced Coal Technologies with CO2 Capture and Storage in China Palo Alto, Stanford University, 2005. [17] T. Zhao, B. Guo, F. Zhang, F. Sha, Q. Li, J. Zhang, Morphology control in the synthesis of CaCO3 microspheres with a novel CO2-storage material, ACS Appl. Mater. Interfaces 7 (2015) 15918–15927. [18] Y.G. Wang, Z.S. Niu, J. Shen, Y.X. Niu, G. Liu, Q.T. Sheng, Energy sources, Energy Sources, Part A 39 (2016) 83–89. [19] Y.G. Huang, Z.M. Zong, Z.S. Yao, Y.X. Zheng, J. Mou, G.F. Liu, et al., Ruthenium ion-catalyzed oxidation of Shenfu coal and its residues, Energy Fuel 22 (2008) 1799–1806. [20] P. Li, Z.M. Zong, F.J. Liu, Y.G. Wang, X.Y. Wei, X. Fan, et al., Sequential extraction and characterization of liquefaction residue from Shenmu–Fugu subbituminous coal, Fuel Process. Technol. 136 (2015) 1–7. [21] M.S. Solum, R.J. Pugmire, D.M. Grant, Carbon-13 solid-state NMR of Argonne-premium coals, Energy Fuel 3 (1989) 187–193. [22] M.S. Solum, A.F. Sarofim, R.J. Pugmire, T.H. Fletcher, H. Zhang, 13C NMR analysis of soot produced from model compounds and a coal, Energy Fuel 15 (2001) 961–971. [23] L. Qian, S.Z. Sun, D. Wang, H.R. Guo, H.H. Xu, M JQ, et al., The 13C NMR measurements of two types of lignite and the CPD simulation of lignite rapid pyrolysis at high temperature, J. China Coal Soc. 38 (2013) 455–460 (Chinese journal).
Contents lists available at ScienceDirect
Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Short communication
Extraction of direct coal liquefaction residue using dipropylamine as a CO2-triggered switchable solvent Yu-Gao Wang a,⁎, Ze-Shi Niu a, Jun Shen a,⁎, Lei Bai b, Yan-Xia Niu a, Xian-Yong Wei c, Rui-Feng Li a, Jun Zhang a, Wen-Yi Zou a a b c
College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China Department of Chemical and Biomedical Engineering, West Virginia University, Morgantown, WV 26506, United States Key Laboratory of Coal Processing and Efficient Utilization, Ministry of Education, China University of Mining & Technology, Xuzhou 221116, Jiangsu, China
a r t i c l e
i n f o
Article history: Received 23 September 2016 Received in revised form 12 January 2017 Accepted 12 January 2017 Available online xxxx Keywords: Direct coal liquefaction residue CO2-triggered switchable solvent Polycyclic aromatic structures Structural characterization
a b s t r a c t Extracting polycyclic aromatic structures (PASs)-enriched portion from direct coal liquefaction residue (DCLR) with an energy-efficient way is significantly important for the effective utilization of DCLR. In the investigation, dipropylamine (DPA) with weak polarity was used to extract DCLR as a CO2-triggered switchable solvent. When the extract solution was triggered by CO2, the extract automatically precipitated with the yield of 22.8% of DCLR. Furthermore, adding proper amount of NaOH could successfully achieve the recovery of DPA, and the recovered solvent could be reused for extracting DCLR. According to the characterization results of the extract by multiple analytical tools, the extract mainly consisted of PASs with over 4 rings, and the average molecular model of aromatic cluster could be represented by the derivative of coronene with 8 substituent groups. The investigation proved that extraction of PASs-enriched portion using DPA from DCLR was feasible, and tremendous energetic and environmental benefits could be potentially gained given that the separation of extract and solvent as well as solvent recovery could be achieved by the non-thermal CO2-trigger. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Direct coal liquefaction residue (DCLR) is the significant byproduct from the direct coal liquefaction (DCL) process, generally accounting for about 30 wt% of the coal consumption in DCL [1]. The effective utilization of DCLR is paid increasing attention along with the development of DCL. Usually, DCLR is subjected to combustion, gasification, and carbonization to afford heat, hydrogen, and coke, respectively [2]. More importantly, there are abundant polycyclic aromatic structures (PASs) in DCLR, making DCLR a promising precursor for preparing high valueadded carbon materials, such as carbon microfibers [3], mesoporous carbons [4], and hierarchical porous carbons [5]. Therefore, developing convenient and effective means to get the PAS-enriched portion is greatly necessary to realize high value-added utilization of DCLR. Extraction with organic solvents is a common method, in which benzene [6], N,N-dimethylacetamide [7], and coal liquid fraction [8] are usually adopted. Considering that these solvents are volatile, toxic, and environmentally unfriendly, Zhang's group successfully developed various kinds of ionic liquids to extract PAS-enriched portions from DCLR, and the extracts would precipitate from the extract solution by adding some amount of water even hot water of 70 °C as a stripping agent [9, ⁎ Corresponding author. E-mail addresses: [email protected] (Y.-G. Wang), [email protected] (J. Shen).
http://dx.doi.org/10.1016/j.fuproc.2017.01.021 0378-3820/© 2017 Elsevier B.V. All rights reserved.
10]. Thus, the recovery of the ionic liquids inevitably involves distillation of water, however, which obviously is not energy efficient. CO2-triggered switchable solvents, first reported by Jessop’ group [11], can be reversibly converted from a non-ionic form to an ionic form by bubbling and then removing CO2 in the solvent system at room temperature, leading to the solvent recovery under CO2 trigger instead of distillation and evaporation involving thermal processes. These CO2-triggered switchable solvents including amines, amidines, and guanidines can be significantly advantageous for extracting oils from soybean or algae [12–15], because oils are easily released from the extraction solution when the solvent is switched from non/weak-polar form to polar form triggered by CO2, resulting in reducing energy consumption in the separation of oil and the solvent and the recovery of the solvent, as diagramed in Fig. S1 [14]. Herein, we evaluated the application of DPA as the CO2-triggered switchable solvent for extracting DCLR because of its limited volatility, low viscosity, and weak polarity [15], and characterized the extract with elemental analyzer, gas chromatograph/mass spectrometer (GC/MS), solid-stated 13C nuclear magnetic resonance (13C NMR). 2. Materials and methods DCLR was obtained by liquefying Shenmu–Fugu subbituminous coal at 445–455 °C under 18 MPa of hydrogen [16] in Shenhua direct coal liquefaction plant, Erdos, Inner Mongolia, China. Its proximate and
28
Y.-G. Wang et al. / Fuel Processing Technology 159 (2017) 27–30
ultimate analyses as well as those of the original coal (Shenmu–Fugu subbituminous coal) and its extraction residue after extracting using DPA were listed in Table S1. DCLR was pulverized to pass through a 100-mesh sieve followed by desiccation in a vacuum oven at 80 °C for 24 h prior to use. DPA used in the experiment was an analytical reagent. As Fig. 1 illustrates, 3 g of DCLR was extracted with 105 mL of DPA under ultrasonic wave irradiation bearing 150 W of ultrasonic power at room temperatures for 45 min. Then, the extract solution and the extraction residue were separated via filtration. For improving the following phase separation and switching efficiency, the extract solution was added an equal volume of H2O creating two phases. Subsequently, CO2 was bubbled into the system, during which the two phases in the system gradually turned into one phase, while some black solids precipitated, i.e., the solid extract from DCLR. When no extract came out, bubbling CO2 was terminated. The extract was obtained by filtration, dried in a vacuum oven, and weighted to afford extract yield. In the reference [15], DPA could be finally recovered by heating the remaining solvent system at 90 °C for 1 h accompanied with bubbling N2, however, which does not seem to be energy-saving. Considering of the huge CO2 emission and the resulting environmental damage, it may be better to store CO2 rather than remove CO2 in the system by heating. In the study, we tried to replace DPA in the forming salt by adding proper amount of NaOH and successfully realized the recovery of DPA, and the fact could be evidenced by GC analysis of the recovered solvent, in which DPA accounted for more than 99% of relative content (RC) as exhibited in Fig. S2, and some heteroatom-containing compounds except DPA and a small amount of arenes were further detected by GC/MS as displayed in Fig. S3 and Table S2. Interestingly, the recovered solvent could be used to extract DCLR with an extract yield approximately equal to the fresh DPA at the same condition. In the following investigation, the recovery of DPA would be improved including
selecting proper base, such as Ca(OH)2 to afford CaCO3 with nanometer size [17] and optimizing DPA recovery condition.
3. Results and discussion The resulting extract yield in the study was 22.8% based on the mass of DCLR, which was similar to the extract yields of DCLR extraction using benzene [6] and some magnetic ionic liquids [9], but much less than those of DCLR extraction with N,N-dimethylacetamide [7] and coal liquid fraction [8]. The difference in the extract yield using different solvents in Table 1 was partly attributed to the extraction condition. In fact, the extract yield would increase to ca. 30% under further optimizations of extraction condition, which was reported elsewhere [18]. Regardless of the extraction condition and extract yield, the extraction solvents and solvent recovery in Table 1 were simply compared. DPA is much less volatile and toxic than benzene. The recoveries of N,Ndimethylacetamide and coal liquid fraction from the extract solution are supposed to be distillated above 160 °C at atmospheric pressure, which obviously are more energy-consuming than the recovery of DPA. The ionic liquids are the promising solvents for extracting DCLR, but they are expensive and their recovery involves the distillation of water. Therefore, the extraction of DCLR using DPA as a CO2-triggered switchable solvent is suitable and promising. The obtained extract was further analyzed by elemental analyzer, GC/MS, and 13C NMR spectrometers to investigate the PASs in the study. Compared with DCLR, the carbon, hydrogen, and nitrogen contents of the extract increased, but the oxygen content decreased. The elemental analyses of the extract in the study and the benzene extract in reference [6] were almost same, while the H/C value of the extract herein was more than that of the extracts with ionic liquids [9,10] and N,N-
Fig. 1. Procedure of DCLR extraction using DPA, separation between extract and DPA, and recovery of DPA.
Y.-G. Wang et al. / Fuel Processing Technology 159 (2017) 27–30
29
Table 1 Yield, ultimate analyses, and several structural parameters obtained by 13C NMR analysis of extracts from DCLR with different solvents. Solvent
DPA Benzene N,N-dimethylacetamide Coal liquid fractionb 1-Butyl-1-methylpyrrolium [FeCl4] 1-Butyl-3-methylimidazolium [FeCl4] N-butylpyridium [FeCl4] N-methylmidazolium benzoate Trietyhylammoium formate 3-Methypridinium formate 3-Methypridinium benzoate a b c d
Yield (wt%)
22.8 23.8 62.2 50.0 11.2 22.8 24.5 29.4 10.0 32.3 33.8
Ultimate analysis (wt%)
H/C
C
H
N
O
87.11 87.19 88.74 90.63 82.62 86.51 88.71 88.77 90.09 90.75 91.55
6.32 6.53 4.69 7.32 5.35 5.46 5.2 5.71 6.08 5.78 5.25
1.63 2.08 1.72 1.19 1.79 1.87 1.28 1.13 1.1 1.07 0.83
4.51 4.17 4.68 – 5.79 5.51 4.29 4.31 2.62 2.29 2.3
0.87 0.90 0.63 0.97 0.78 0.76 0.70 0.77 0.81 0.76 0.69
Structural parameters
Reference
fa
Ra
0.65 –a 0.76 –a 0.72 0.74 0.75 0.67 0.64 0.66 0.69
7 –a –a –a 7 8 7 7 7 7 6
This study [6] [7] [8] [9]c
[10]d
No corresponding data are obtained by 13C NMR. Coal liquid fraction was distillated ranging from 220 to 260 °C from coal liquid form of Shengli Lignite. The used solvents are magnetic ionic liquids. The used solvents are protic ionic liquids.
dimethylacetamide, but less than that of the extract with coal liquid fraction, as summarized in Table 1. A total of 41 compounds were identified in the extract with GC/MS, consisting of 27 arenes with total RC of 82.23%, 7 n-alkanes with total RC of 6.36%, and 7 other species (OSs) with total RC of 11.41%, as displayed in Fig. S4 and Tables S3–S4 (see the Supporting information). Arenes dominated in the extract, which were mainly composed of nonsubstituted arenes with 2–7 rings as well as some substituted arenes and a small amount of hydroarenes. Among these GC/MS-detectable components, benzo[ghi]perylene (Peak 34) was the most abundant with the RC of 20.83%, whose absolute content was 1.01 mg·g− 1 in the extract by external standard method in GC analysis. As shown in Table S3, the compounds with RC beyond 4% were polycyclic aromatic compounds with more than 4 rings, indicating that the average number of aromatic rings in the GC/MS-detectable portion was over 4. The detected hydroarenes were likely to result from the hydrogenation of the existing arenes in Shenmu–Fugu subbituminous coal during its DCL, since no hydroarenes were detected in the extracts of the coal [19]. Methyl is the main substituted group, and monomethylarenes (Peaks 14, 22, and 37) were the dominant substituted arenes. These nalkanes consecutively ranged from C20 to C26, which may be from either desorption of adsorbed hydrocarbons trapped in the macro molecular portion or the rupture of side chains on PASs during DCL. Among the OSs, benzo[pqr]tetraphen-6-ylmethanol (Peak 31) is the richest, whose similar compound was identified in the extract of DCLR with petroleum [20]. Hexadecahydropyrene, possibly forming by complete hydrogenation of pyrene, further proved that hydroarenes were generated from the hydrogenation of the arenes. Compared with GC/MS only analyzing those thermal stable and volatile species, 13C NMR can provide the comprehensive structural information on the sample bulk. As Fig. S5 shows, the aromatic band in the range of 90–165 ppm was predominant in the 13C NMR spectrum, indicating that the extract was abundant in PASs, in agreement with the GC/ MS analysis. By curve fitting, the band was further separated into 5 individual peaks (see in Fig. S5 and Table S5), representing different aromatic carbon types. Furthermore, several structural parameters were obtained for fully understanding PASs in the extract as displayed in Table S6 on the basis of the data in Table S5 [21–23]. The aromaticity index (fa) was 0.65, indicating that there were 65 aromatic carbons per 100 carbon atoms in the extract. The molar content of aromatic bridgehead carbon (xb) is an important parameter to estimate the aromatic cluster size. On the basis of xb, the average number of carbon atoms per aromatic cluster Ca in the extract could be deduced as 23.58 by calculation method as reported in the literature. Correspondingly, the average number of aromatic nucleus (Ra) was ca. 7, implying that the molecular model of average aromatic cluster may be present as
coronene, and the number of substituent aromatic carbons was ca. 8, suggesting there were about 8 aromatic carbons substitute by aliphatic chain or oxygen-containing functional groups per aromatic cluster. Therefore, the derivative of coronene with 8 substituent groups could be used to represent the average molecular model of aromatic cluster in the extract, indicating that PAs with more than 7 aromatic rings existed in the extract, which could not be detected by GC/MS. As Table 1 exhibits, the extract in the study had the similar aromaticity with the extracts using protic ionic liquids [10] but owned the lower aromaticity than the extracts using magnetic ionic liquids [9], while the values of Ra in these extracts were approximately equal to each other, demonstrating the average aromatic cluster in these extracts had similar condensed aromatic skeleton with 6–8 rings. 4. Conclusions DPA was used to extract DCLR as a CO2-triggered switchable solvent, and the extract could be released from the extract solution when the solvent system was triggered by CO2. Moreover, DPA could be successfully recovered by adding NaOH. The result proved that the application of DPA in extracting DCLR and the recovery of DPA without heating were feasible. According to GC/MS analysis, the condensed arenes with more than 4 rings are the main components in the extract, and benzo[ghi]perylene had the highest RC of 20.83%. The 13C NMR analysis suggested the extract was dominated by PASs. Furthermore, it could be concluded that the number of aromatic rings per aromatic cluster was ca. 7, maybe corresponding to coronene, and each aromatic cluster had ca. 8 substituent groups on average. The investigation provides an energy-saving way to get PASs-enriched portions from DCLR. Nomenclature C NMR 13C nuclear magnetic resonance DCL direct coal liquefaction DCLR direct coal liquefaction residue DPA dipropylamine FTIR Fourier transform infrared GC/MS gas chromatograph/mass spectrometer OSs other species PASs polycyclic aromatic structures RC relative content
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Acknowledgements This work is supported by the NSFC-Shanxi joint fund for coal-based low carbon (Grant No. U1610223) and Scientific and Technological Project of Shanxi Province, China (Grant Nos. 20150313014-4 and
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