High-Efficiency Extraction and Modification on the Coal Liquefaction

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High-efficiency extraction and modification on coal liquefaction residue using supercritical fluid with different types of solvents Xiongchao Lin, Shouyi Li, Fenghua Guo, Guangce Jiang, Xujun Chen, and Yonggang Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00326 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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High-efficiency extraction and modification on coal liquefaction residue using supercritical fluid with different types of solvents Xiongchao Lina*, Shouyi Lia, Fenghua Guoa, Guangce Jianga, Xujun Chenb, Yonggang Wanga a.

School of Chemical & Environmental Engineering, China University of Mining and Technology

(Beijing), D11 Xueyuan Road, Haidian District, Beijing 100083, P.R. China. b.

Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia

* Correspondence author: X.C. Lin, D11 Xueyuan Road, Haidian District, Beijing 100083, P.R. China. Tel: +86-10-6233-1048; E-mail address: [email protected]

Abstract: This study aims to systematically illustrate the mechanism of supercritical fluid extraction (SFE) and modification on coal liquefaction residue (CLR), and to identify the evolution and characteristics of mesophase produced from the carbonization of SFE extracts. Results show that the extraction performance of SFE and the properties of mesophase precursor were strongly dependent on the selection of operating conditions and solvents. The SFE process using acetone and iso-propanol presented excellent extraction performance owning to the effect of solvent polarity on the degradation or supercritical reaction, achieving their respective CLR extraction yields of 45.85 wt% and 30.12 wt%; while it attained an extraction yield of 53.78 wt% when using benzene, benefiting from its strong affinity to condensed aromatic hydrocarbons. More practically, the QI (quinoline-insoluble) decreased from 48.84 wt% to 1.13 wt% after SFE processing, which significantly upgraded the quality of mesophase precursor. To an extent, the supercritical acetone exhibited strong reaction activity during extraction as its extract contained higher amount of HS (hexane-soluble) fraction, which could optimize the molecular weight distribution of mesophase precursor. The well developed bulk mesophase in the carbonized SFE extracts was remarkably improved compared to the raw CLR. Presumably, the SFE extract was favorable to forming 100 % mesophase, where dominated flow textures were observed. 1

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Keywords: Coal Direct Liquefaction Residue, Supercritical Fluid Extraction, Quinoline-insoluble Removal, Mesophase Precursor

1. Introduction A commercial-scale plant (first-stage 24,000 bbl/day) for synthetic fuel production by direct coal liquefaction (DCL) is currently on stream in Inner Mongolia, China. However, its undesirable byproduct, vast quantities (ca. 0.5 million t/a) of coal liquefaction residue (CLR), has been a very serious problem confronting the industry. On average, the yield of CLR can be as high as 20–30 wt% in terms of the raw coal consumed during DCL [1]; therefore, converting the CLR to relatively light but high-value products is generally supposed to be the critical step in the overall process. Meanwhile, growing concerns about the disposal of CLR and its adverse environmental impact should be properly addressed as well. In light of economic and environmental considerations, necessary work about the recovery of high-value products from CLR needs to be duly done. CLR mainly contains inorganic matters deriving from coal and catalysts used as raw materials, and organic matters such as remaining coal fraction, heavy oils, pre-asphaltenes and asphaltenes [2]. Many studies have been sought to convert CLR into various high value-added products, such as lighter oil from catalytic hydroprocessing, flexicoking and pyrolysis [3, 4], or hydrogen-rich syngas from gasification and dynacracking [5]. Obtaining carbon materials with excellent mesophase features from pitch precursors has long been considered as an ideal goal in the area of materials research [6, 7]. Indeed, effort has been made to prepare high-performance carbon materials using CLR as a precursor, preferring its unique properties and special features (such as being much denser and heavier with high asphaltenes) [8-12]. Technically, the pretreatment and modification of raw CLR prior to its application are necessary. The selection of these processes, however, depends on the style of feeding and the subsequent utilization of their products. High viscosity and high solid content of CLR severely restrict its upgrading progress. Currently, the sole method adopted is to dissolve the CLR in specific solvents and then filter the mixture with the aid of pressing and heating for the purpose of removing the undesirable substances [13-16]. However, this method encounters 2


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many serious issues, such as the subsequent solvents separation and recovery; on the other hand, the vast amounts of ultra-fine solid particles in CLR are likely to plug the filter cake, retarding the filtration rate. More importantly, the original structure of CLR with highly condensed aromatic hydrocarbon is practically unsuitable for direct utilization with some special purposes; therefore, structural modification is acutely needed. Accordingly, novel approaches for efficiently upgrading of CLR need full consideration. Owning to its excellent extraction performance, supercritical fluid extraction (SFE) is nowadays of significant interest to supplement the conventional separation process. It exhibits unique properties by forming intermediate phases between gas and liquid, thus presenting lower viscosities, higher mass transport co-efficiency and diffusivity, as well as pressure-dependent dissolving capacity [17]. Actually, its application to extract heavy compounds has been widely investigated [18]; nevertheless, its application in practical refining of a heavy pitch-like material via a multi-stage process is currently rare, especially for CLR featuring quite high molecular weight and vast solid particles [19]. The homogeneous supercritical fluid, in theory, is able to penetrate into the compacted structures of CLR and selectively dissolve specific organic compositions, while its physicochemical properties, e.g., diffusivity and polarity, can be continuously regulated by adjusting the pressure and temperature in progress [20]. Moreover, the SFE process could contribute controllable reaction [20], which would benefit the flexible modification on the molecular structure of CLR. These unique properties, in combination with the high solubility of solvents, make SFE an ideal candidate for extracting and modifying CLR. In addition, many studies deemed the CLR as an excellent mesophase precursor; however, so far the features of mesophase from the CLR are rarely referred [21]. The mesophase is the intermediated that would essentially determine the quality of the resultant products, through its chemical and physical transformation. The properties of mesophase have strong relation with the composition of constituent molecules and are governed by its thermotropic nature [22]. Thus, the feature understanding of mesophase is a key to find ways to optimize the processing condition. 3


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To complement our previous work [23], this study aims to evaluate the efficiency of SFE process on purification/refining of CLR and the mechanism of supercritical reaction on molecular structural modification, particularly on heavier components. Further, the evolution of mesophase during carbonization of SFE extracts as well as the relationship between the microstructural anisotropy and their molecular structure was tentatively investigated. Notably, the yield of high value product and the feature of mesophase precursor from CLR through SFE were appreciable in this respect.

2. Experimental 2.1 Materials The raw CLR from the commercialized direct coal liquefaction plant of Shenhua Group in Erdos, Inner Mongolia was analyzed and summarized in Table 1. The CLR contained 77.50 % C and 4.95 % H (C/H=1.30). Its ash content (note that the inorganic minerals were calculated after completely burning the CLR in the atmosphere, thus the compositions would be oxidized form rather than their original ones) was 15.28 wt%. The aromaticity fraction (fa=Car/CTotal) 0.9, which is the percentage of aromatic carbon in the total carbon, can be easily obtained according to the

13

C

NMR data. Its softening point ranged at 166 ± 3oC. The solvents used in this study were n-pentane, benzene, acetone and iso-propanol (all in analytical pure) with their supercritical conditions being at: 196.4 oC and 3.37 MPa, 289.2 oC and 4.91 MPa, 235.5 oC and 4.72 MPa, 234.9 oC and 4.76 MPa, respectively. These solvents varying from straight-chain paraffin to aromatic hydrocarbon, carbonyl compound and hydroxyl compound were supposed to exhibit diverse extraction capacities. The supercritical conditions of selected solvents were relatively moderate, and their boiling points were adequately low, ensuring their prime recovery by simply distillation. 2.2 Experimental apparatus and procedure The schematic diagram of the flow SFE system consists of solvent supply, extraction and solvent recovery units, as shown in Figure 1. 4


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To begin the extraction process approximately 100 g CLR was put into the SFE kettle, followed by the pre-heated solvent pumped at the rate of 4.5 L/h. Once the pressure inside the SFE kettle reached the setting value, the throttle was opened, allowing solvent and extraction product to steadily flow into a sample collecting pot. The extraction pressure was controlled by a dynamic stability control system, keeping the balance between inflow and outflow of the solvent. The solvent then was circulated back to the solvent tank after vaporization in the collecting pot and cooling down through the condensation system. The extraction temperature (temperature in SFE kettle) verified by two thermocouples and the temperature of column (T1, T2 and T3) were set at target temperature (T) to avoid the back mixing of product gathered in the collecting pot. This cascade control system (Ta: inner temperature of furnace, Tb: temperature of furnace wall) could accurately control the extraction temperature with the deviation being less than 2 oC, while the deviation of extraction pressure ranging from 3.5 to 9.0 MPa was smaller than 0.1 MPa. A series of experiments were conducted to extend our previous study [23] to investigate the effect of extraction conditions on the product yield and purity. The extractions were carried out at five sets of operating conditions, 4.7 MPa, 5.7 MPa, 6.7 MPa, 7.7 MPa, and 8.7 MPa for each solvent aforementioned at their supercritical temperatures. The cumulative extraction yields were achieved by (weight of extract)/(weight of feedstock)×100% after completely separating solvent with feedstock, and the extracts from supercritical n-pentane, acetone, iso-propanol, and benzene were hereafter abbreviated to be SFE-nP, SFE-A, SFE-P, SFE-B, respectively. Further, the extracted samples were carbonized and then analyzed for mesophase features. Approximately 15 g of the sample weighed in an aluminum foil tube (diameter 20 mm, height 150 mm) was carbonized in a stainless steel tube bomb by rapidly inserting it into a furnace at the prescribed temperature. The heating rate was ca. 50 oC/min from ambient temperature to 400 oC and to 463 oC (actual temperature), respectively, and then held for 6 hours. The carbonization pressure was 1 bar adjusted by the initial nitrogen pressure and purging gas (N2) through a control valve during the carbonization. The carbonized product after going through a series of the prescribed 5


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heating times was quenched in atmosphere, and then the lump sample was obtained in the foil. 2.3 Sample characterization The samples prepared from SFE were quantitatively analyzed for the elements of carbon, nitrogen, hydrogen, and sulfur with the automatic elemental analyzer Vario EL III (Elementar Analysen systeme Comp., Hanau, Germany). Each sample was measured for three times to avoid the deviation. The solubilities of samples were examined by the Soxhlet extraction using hexane, toluene, tetrahydrofuran (THF), and quinoline as solvents. Powdered sample (ca. 5 g) was put into an extraction thimble and then extracted with 150 ml of each solvent during 24 h refluxing in a Soxhlet apparatus to completely separate the soluble and insoluble fractions. The solvent was, thereafter, evaporated under reduced pressure (rotary evaporator, Shanghai Yarong Co. Ltd.). QI (quinoline-insoluble) particles separated from CLR were extracted by hot quinoline (75 oC) using ultrasonic bath. Approximately 10 g of sample after being ground and sieved to < 0.4 mm was blended with 25 ml quinoline and then subjected an ultrasonication for 30 min. The suspension was hot filtered using a No. 5 porous ceramic plate, and then dried in a furnace at 110 °C until its weight l no longer changed. The gel permeation chromatography (GPC) instrument (Waters 515-2410, America), equipped with 515-LC pump and a 2410 type refractive index detector was employed to measure the molecular distribution of sample. The standards used were Polymer Lab narrow polystyrenes standards (162 D, 615 D, 1270 D, and 3360 D purchased from the Alfa Aesar Co. Ltd) while the mobile phase was THF at a rate of 1.0 ml/min. The chromatography columns were Styragel HR2-3-4E in series with the concentration and the injection volume of sample being 2 mg/ml and 50 mL, respectively. The temperature of the detector was set at 30 ± 2 °C. Following the supercritical extraction, high performance liquid chromatography (HPLC) was used to quantitatively determine the changes of aromatic compounds through external calibration with no less than six concentrations (i.e., 2, 4, 6, 8, 10, 20 µg/ml) of PAHs (poly-aromatic 6


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hydrocarbons) to cover the range measured in the various samples. Standards and CLR solutions were analyzed by a LC (SHIMADZU LC-20A) with injection volume being 10 µL and the detection was carried out at 254 nm (SHIMADZU UV-detector). The column used to separate molecules was a PAH (5 µm, 4.6 mm×250 mm, SUPELCOSIL LC-PAH). A gradient elution was applied with acetonitrile/water at a flow rate of 1.5 ml/min, as following: from 0 to 20 min, the concentration of acetonitrile phase would change from 25 % to 50 %, and then to 100% during the following 5 min. After 30 min, it would decrease from 100 % to 25 % in 10 min and then to 0 % at 45 min. Scanning electron microscopy (SEM) (ZEISS EVO18) with field emission (FE) model was used to examine the QI samples. X-ray diffraction (XRD) analysis was carried out using a Rigaku RINT ultimate-III powder diffractometer with a graphite monochromator, a NaI(Tl) detector, and CuKa radiation. The XRD scans were performed between 5 and 90 oC, with a step of 0.02 oC/s. Fourier Transform Infrared Spectroscopy (FT-IR) analyses for the raw and as-prepared samples were conducted on a FT/IR-615 JASCO (JASCO, ltd. Japan) spectrometer using transmission mode. The measurement detail can be found in previous study [24]. A PerkinElmer Pyris 1 thermogravimetric analyzer (TGA) was employed to measure the decomposing behavior of samples. Approximately 8-10 mg of each sample was heated to 1000 °C at an interval of 10 oC/min in nitrogen atmosphere. The particle size distribution and average particle size of the QIs were analyzed using Mastersizer (Malvern Instrument, UK) fitted with Scirocco 2000 unit. The equipment was equipped with laser beam to detect the individual particles. 13

C-NMR experiments were performed using a 400 MHz Brüker NMR Spectrometer, with

TMS acting as internal standard and CDCl3 as the solvent of extracted samples, and chemical shifts were reported in ppm. 2.4 Optical microscopy of carbonized coke 7


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Representative samples of carbonized coke embedded in an epoxy resin, were successively ground with two grades of silicon carbide paper (600 and 1200 grit) and then polished with two grades of alumina (0.3 and 0.05 µm) using an automatic grinding–polishing machine. The optical texture of the carbonized cokes was studied by a reflected-light microscope equipped with a polarizer and a 1-λ retarder plate for generating interference colors. Representative photographs of polished samples were taken using Montage method with objectives of ×50 and ×200.

3. Results and Discussion 3.1 Extraction yield The extraction yield was profoundly affected by the operating conditions and solvents. As shown in Figure 2, the extraction yields using benzene were the highest under all operating conditions, while n-pentane achieved the least. Furthermore, the extraction using n-pentane, an alkane with lesser polarity and lower density, was ineffective at 4.7-7.7 MPa; even, the extraction yield was only 6.23 wt% at 8.7 MPa. N-pentane solvent was normally used to extract the deasphalted oil stream from the bitumen feed, leaving an insoluble, heavy-end fraction known as asphaltenes [25]. Despite the fact that the extraction yield was strongly related with the chain length of the alkane solvent used [26], the low extraction yield achieved by n-pentane, indicated the low content of light oil and olefins in CLR; otherwise, they would be easily extracted by supercritical n-pentane and thus resulted in a higher extraction yield. The application of acetone as a supercritical fluid is not common but has been studied in a few purposes, e.g., degradation of polymers [27] and dehydration of carbohydrates [28]. Particularly, acetone presented excellent extraction performance on CLR under its supercritical conditions. The yields of SFE-A gradually increased with the increasing of operation pressure at its critical temperature. It is thought that under supercritical conditions, the miscibility of acetone with CLR increased substantially and the extra extraction pressure was also essential to increase the density of supercritical solvent [29], both helping improve the extraction capacity. The highest extraction yield using acetone solvent reached up to 45.85 wt%, probably due to its strong polarity force, which, in 8


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theory, could significantly accelerate the dissolving in the similar material structure [30]. In addition, the extraction results implied that supercritical acetone may work as solvent and/or reactant (acted as hydrogen donor) [27]; therefore, the high extraction yield might partly be attributed to its reaction with CLR. The supercritical iso-propanol was usually used in solvolysis processes for effective decomposition of the cross-linked structure of polymer (e.g., epoxy resins) [31], as it could play a significant role in cleaving the chemical bonds of the polymer structure, and thus, offered a new attractive alternative route for extraction of CLR. The yields of SFE-P varied from 20.21 wt% (at 5.7 MPa) to 30.12 wt% (at 8.7 MPa), lower than that of acetone under the same extraction pressures though their properties were similar. The hydrogen bonding force of iso-propanol was stronger than that of acetone, so it could significantly restrain the dissolution of non-polar substances in CLR. Even if the supercritical iso-propanol was able to be partially oxidized to acetone and acted as a hydrogen donor [31], the excessive hydrogenation was less likely to happen, thus slightly impacting the extraction yields. Initially, benzene was selected as solvent because of its relative inert reactive activity and strong solubility to condensed aromatic hydrocarbons during supercritical extraction [32]. The supercritical benzene logically presented the best extraction capacity among all solvents used in this study. Generally, the extraction yields increased with increasing of extraction pressure and decreasing of extraction temperature while the other was constant. Though higher temperature might enhance the diffusion of solvent, the softening point of raw CLR was only 166 ± 3oC and it would pyrolyze at around 350 oC, i.e., higher extraction temperature of benzene would principally lower the phase density rather than enhance its mass flow. Therefore, the extraction temperature was unlikely to be the dominate factor affecting the extraction yield. The yields of SFE-B gradually rose from 41.10 wt% to 53.78 wt%, as the extraction pressure rose from 4.7 MPa to 8.7 MPa. It is widely known that different from homogeneous fluid, local density in homogeneities would affect the polarity and hydrogen bonding on the density augmentation in supercritical fluids, thus 9


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influencing the intermolecular interactions of CLR. 3.2 Effect of SFE on the impurity in CLR The removal of impurities in CLR is a critical step to obtain high quality mesophase precursor. During the SFE of CLR, apart from the refined product in liquid phase at elevated temperatures, solid materials such as undissolved coal macerals and minerals, and “reactor solids” also need to be separated. The raw CLR contained quite high proportion of undesirable fractions. The QI, which was the major impurity in the CLR, mainly made up of inorganic materials, unreacted coal, pyrolytic carbons and highly condensed aromatic hydrocarbons. Extraction with quinoline has been generally used to determine the amount of inorganic substances and β-resign in the coal tar pitch [33]. However, the results described in Figure 3a indicated that the residues from those extracts might not be composed entirely of anisotropic mesophase-derived semi-coke, but presumably, more or less, contained materials with too high molecular weight to be solved. Except for the ash in QI, the supercritical benzene apparently extracted most high-molecular-weight components from CLR, implying that the QI content in SFE-B was the highest among all extracts (Figure 3a). Probably, the SFE-A, composing of lighter compounds partially deriving from the refining or reaction of CLR with supercritical acetone, was supposed to carry much less QI. The ash content in SFE-nP was the lowest due to its weakest extraction capacity and relatively low extraction yield. On the contrary, the ash content in SFE-B was the highest, owning to the fierce entrainment during the extraction. Interestingly, the SFE-A contained much less ash than that of SFE-B at similar extraction yield, meaning that the process of refining or upgrading could substantially convert the CLR to lighter compositions during supercritical extraction. The iso-propanol extraction had lower yield but higher ash content, which further implied that the refining of CLR during acetone extraction was much better than that of iso-propanol. As shown in Figure 3b, the ash contents of raw and extracted CLR showed similar trends with the QI contents. The ash content of extracted CLRs drastically declined from 15.28 wt% of raw 10


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CLR to 0.02 wt%, 0.05 wt%, 0.06 wt% and 0.17 wt% of SFE-nP, SFE-A, SFE-P and SFE-B, respectively. The main inorganic materials in CLR were protogenetic minerals in coal and added catalyst (Fe-S and/or FeOOH in nano size [2]) dispersively distributing in the CLR. Despite the supercritical fluid has higher density than the one in normal status (i.e., standard temperature and pressure), the dominated inorganic substances, such as silica, alumina, iron oxides and calcium oxide, which generally exhibited higher cluster density and poor compatibility with organic components, were easily separated. Additionally, the separation could be efficiently enhanced by the fractionating tower of SFE equipment. The major minerals in CLR QI were quartz, calcite, kaolinite and pyrrhotite, as shown in Figure 4. The coal tar derived QI after hot quinoline extraction and heat filtration principally formed spherical particles, as depicted in Figure 5a. The QI from raw CLR presented much larger blocks with irregular profile (Figure 5b), and it had a normal distribution particle size in the range of 1 to 100 µm, much larger than that of coal tar QI whose peak value was ca. 1 µm (as shown in Figure S1a and b). Since the QI with larger particle size would be harmful to form high quality mesophase, therefore, the QI in raw CLR need be removed. The QIs in supercritical-fluid-extracted CLR were similar with that of coal tar QI, shaping as partially spherical granule, as shown in Figure 5c; few inorganic substances were brought out from the raw CLR due to the lower density of supercritical fluids, and their particle sizes were smaller than that of raw CLR (Figure S1c-e). 3.3 Variation of SFE extracts The compounds extracted by supercritical fluid had different elemental distributions and solubilities, as summarized in Table 1 and Figure 6. Figure 6a demonstrated that the raw CLR contained less of hexane soluble (HS, 3.25 wt%), toluene insoluble and tetrahedrofuran soluble (TI-THFS, 0.42 wt%), and tetrahedrofuran insoluble and quinoline soluble (THFI-QS, 1.91 wt%), while was rich in QI (48.84 wt%). In contrast, the SFE extracts contained high amounts of HS (34.89 wt%, 34.41wt%, and 25.02 wt% for SFE-A, SFE-P, 11


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and SFE-B extracts, respectively), and hexane insoluble and toluene soluble (HI-TS,54.47 wt%, 57.08 wt%, and 65.42 wt% for SFE-A, SFE-P, and SFE-B extracts). The TI-QSes (including TI-THFS and THFI-QS) were 9.51 wt%, 5.55 wt%, and 5.96 wt% in SFE-A, SFE-P, and SFE-B extracts; and especially, the QIs decreased to 1.13 wt%, 2.96 wt%, and 3.60 wt%, accordingly. Initially, the SFE processing was expected to physically separate the inorganic matters and recover the organic compounds avoiding any further reaction. As shown in Figure 6b, the relative solubility recalculated by normalizing the fractions excluding QI was likely to illustrate the changes of fraction after the SFE processing. The relative proportion of HS sharply increased from 6.35 wt% of raw CLR to 35.29 wt%, 35.46 wt%, and 25.95 wt% in SFE-A, SFE-P, and SFE-B extracts. More interestingly, the major fraction of HI-TS in CLR (89.09 wt%) significantly decreased to 55.09 wt%, 58.82 wt%, and 67.86 wt% in SFE-A, SFE-P, and SFE-B extracts. The solvents with different characterizations might exhibit selective extracting performance on the CLR fractions. In addition, the different proportions of fractions in SFE extracts suggested that the supercritical reaction should have more significant impacts on the compositional variation. Moreover, after direct liquefaction of coal, most of the sulfur including those previously existed in the added catalyst was concentrated in CLR, leading to its higher content of sulfide. It was worth to note that the sulfur content slumped after the SFE extraction, implying the desulfurization of SFE reaction (Table 1). For a deeper understanding of the reaction mechanism, combined analyses were carried out to illuminate the reason why supercritical organic solvents favored the compositional variation. The applied methods seem to give a good hint for the explanation of the reaction mechanism. Since the CLR and SFE extracts were composed of chemical species with different aromaticities, functional groups and heteroatoms; their analyses were quite difficult and time-consuming due to the complexity and diversity. HPLC was employed to identify the distribution of aromatics in CLR extracts, as described in Figure 7 with a series of poly aromatic hydrocarbons varying from 2 to 6 fused-rings acting as standard materials. Compound identification 12


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was accomplished by comparing the retention times of peaks in those samples with that of PAH standards conducted under the same conditions. Several peaks were observed over the retention time of indeno (1, 2, 3-cd) pyrene, indicating that the substances with more than 6 fused-rings might exist in the SFE extracts. The SFE-B extract mainly contained 3, 4, and 6 fused-rings PAHs. In contrast, much more peaks were found in the SFE-A and SFE-P extracts (Figure 7a) than that of supercritical benzene extraction (Figure 7b and c), especially strong peaks before the main peaks at the retention time of ca. 5 min in SFE-A, which might be ascribed to the more intense supercritical reaction between acetone and iso-propanol. The substituted complexes or the derivatives generated during SFE would modify the original molecular structures of CLR and might create more favorable characters for the formation of mesophase. The molecular weight distribution was examined by GPC, as shown in Figure 8. The molecular distribution of SFE-P and SFE-B extracts were roughly in parallel with two slightly distinguishing ranges, revealing the relative similar compositions in these two extracts. Comparatively, the curve of SFE-A extract had two shoulder peaks that shifted toward to the smaller molecular weight region, which was in consistency with the HPLC analysis, again, showing that the supercritical reaction of acetone was more intense. The 13C NMR spectra of raw CLR and SFE extracts are shown in Figure 9. The aromatic and aliphatic carbon regions as summarized in Table 2 provided the qualitative information of different types of carbons [34-36]. The aliphatic carbon region (11–70 ppm) was divided into six pieces according to different types of carbons (see Figure 9). An intense peak at 77 ppm, which was ascribed to the solvent peak of CDCl3, should be ruled out when structural parameters were calculated. The spectrum of raw CLR was quite simple presenting less aliphatic carbon peak, and a sole peak in region 3 probably represented CH2 in the cycloalkanes. For the SFE extracts, the aliphatic carbon of CH3 at the γ position or ethyl groups at region 1 (11-15 ppm), and CH3 directly bonded to the aromatic ring including naphthenic and/or hydroaromatic rings in region 2 (17-24ppm) were detected. Large amounts of CH2 peaks in the region 3 (23-34 ppm) were also found in these 13


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extracts, especially in the SFE-A extract. Essentially, during supercritical extraction some free radicals like CH3 would generate from the cleaving of fused ring, which possibly increase the number of carbon in alkyl-substituents. The peaks of bridge/hydroaromatic structures (methylene carbons in α position bonding to two aromatic rings) in region 4 (34-49 ppm) appeared in SFE-A extract, indicating obvious cracking of condensed aromatic rings. Peaks in region 5 were the hydroxyl carbon CH2-OH, hydroxyl-ether carbon, or CH, which stand for the formation of hydroxyl-substituted aromatic compounds. While the region 6 presented carbonyl carbon, or alkynes carbon ≡CH, which may derive from the cracking and isomerization of fused rings. Based on the NMR analysis, the conclusion can be drawn that SFE-A extraction had more aliphatic carbon structures than raw CLR, SFE-P and SFE-B, and the carbon chain length of aliphatics and/or aliphatic side chains in SFE-A was the longest. Presumably, the condensed poly hydrocarbons in CLR were modified by the SFE processing, especially upgraded by SFE-A. The aromatic region, 100-157 ppm, was divided into seven pieces according to the types of carbons and the assignments of different types of aromatic carbons are shown in Table 2. Peak regions 9 (122-124 ppm) and 10 (124-133 ppm) were dominated in all samples. These regions included both protonated and pericondensed or quaternary aromatic carbons. Peaks in the regions of 7 and 8 representing protonated aromatic carbons had a relatively high proportion in total aromatic carbons. In addition, a peak in region 13 in the SFE-A extraction was ascribed to be aryl, while a phenolic hydroxyl group was thought to be caused by the hydroxylation reaction of supercritical acetone. The fa of raw CLR was the highest, 0.90. The fa of SFE-P and SFE-B extracts was 0.85 and 0.88 respectively, slightly lower than that of raw CLR. Particularly, the SFE-A extract achieved the lowest fa (0.73), mainly due to the aliphatic side chains and/or aliphatics generated during supercritical extraction; likely, the supercritical reactions accelerated the cracking of CLR. By identifying the FT-IR profiles of the samples, the features of functional groups in each sample can be further clarified, as shown in Figure 10. Band assignments are shown in Table 3 [24]. 14


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Stronger peaks of CH, CH2 and CH3 functional groups were observed in the SFE extracts comparing to that of raw CLR, which was in agreement with the NMR analysis (see the intensity of the peaks in the 2850-3039 cm-1 region). The large amounts of inorganic matters in raw CLR could severely shield the IR signal; whereas, the signal would change to be more apparent after extraction; on the other hand, the SFE processing led to slight pyrolysis and cracking of aromatic rings and thus producing some extra aliphatic groups. Particularly, the IR absorption of the conjugated aromatic nucleus (C=O) at ca. 1696 cm-1 in the carbonyl-group-containing moieties demonstrated strong vibration, implying the generation of C=O in SFE-A through the supercritical reaction of acetone. The C=C IR absorption bands at 1600 cm-1, which should be placed in the aromatic, were clearly distinguished. Besides, IR absorption band at around1450 cm-1and 1376 cm-1 corresponding to the CH3, CH2 groups in aliphatic chains were enhanced after SFE extraction. These bands with relatively high strength in the macromolecules possibly prevented the bonds from cleaving. Simultaneously, peaks at the region of 747-1260 cm-1 representing the CH bending vibration on the aromatic were once identified in the SFE extracts, which were relative stable during the SFE processing. Nevertheless, such aliphatic and/or aliphatic side chains in CLR extracts would cleave severely during carbonization and accelerate the generation of gaseous matters in the mesophase. 3.4 Mesophase features of carbonized CLR and SFE extracts In this research, the amount of optical textures was used to compare the tendencies during coking of various solvent-refined CLR products. Solid lump cokes produced from raw CLR and SFE extracts exhibited optical anisotropy with different contents of flow texture and various degrees of orientation. The raw CLR carried a considerable amount of asphaltene and pre-asphaltene, which were the major origin for mesophase and had high reactivity ascribing to the large molecular weight of the aromatic nuclei, abundant alkyl side chains, and micelle agglomeration. However, the excessive amount of QI in CLR severely restricted the forming of mesophase, especially when asphealtene and pre-asphaltene molecules lost their alkyl side chains during carbonization. Moreover, the period 15


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of the solidification, which might determine the final feature of anisotropic texture, may locate after the cleaving of side chains. The textures of coke from the bottom, middle, and top of the tube bomb (along the vertical direction) were observed (as shown in Figure 11 and Figure S2). The mesophase formation of raw CLR was quite incomplete; nevertheless, some small anisotropic spheres spread all over the carbonized coke from bottom to the upper along the vertical orientation; as a consequence, the mosaic texture was dominated in the coke. The chemical reactions involved during carbonization were dealkylation of alkyl aromatics, dehydrogenation of naphthenes, polymerization and condensation of aromatics. The TGA analysis (Figure 12) showed that the maximum cleaving of alkyl side chains took place at around 350-400 oC; however, since the raw CLR had very high molecular weight and condensed structure, its viscosity was higher at lower temperature of 400 oC than at 463 oC; consequently, pyrolysis gas would seal and accumulate in the bulk coke and hence generate larger pore structure rather than release out. Because of its abundant of alkyl side chains, the SFE-A extract possessed the highest weight loss indicating the more severe decomposing of molecular, which was consistency with the analyses of NMR and FT-IR. Furthermore, the decomposition rate would obviously influence on the formation of mesophase, thus the carbonization temperature should be optimized according to the DTG analysis. The well-developed bulk mesophase in the SFE-A extract after carbonization was significantly improved in comparison with the raw CLR, as shown in Figure 13 and Figure S3. The SFE-A extract was favorable of forming 100 % mesophase under optimum carbonization condition. There was less mosaic coke in the lump coke, while many anisotropic spheres with disorientated and short flow textures were observed in the solidified coke prepared at 400 oC. The laminar structure of coke from SFE-A extract should be smaller, because the molecular of SFE-A extract was modified and changed to be less condensed after supercritical reaction. Virtually, the short flow textures was usually caused by the low viscosity of liquid crystal or intermediates in the period of carbonization, which was believed to potentially form better and larger scale oriented flow textures [37]. The lump coke prepared at 463 oC from the SFE-A extraction demonstrated more axially oriented flow 16


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textures (as shown in Figure S3). The mesophase with short flow textures easily merged at higher temperature. The laminar structures became larger after carbonization at higher temperature, and were rearranged with the extrusion caused by pores growth (see region b in Figure S3). The coke prepared at 463 oC exhibited anisotropic texture with excellent unit size, but still there were some random orientated parts in the major area (see region b in Figure S3). Such regional textures of mesophase had little relation with the properties of precursor and it can be optimized by adjusting the carbonization condition, i.e., the heat treatment temperature, holding time, as well as the gas evolution. The cokes prepared from SFE-P extracts exhibited well anisotropic textures in the whole region of the bulk. The SFE-P extraction possessed relatively higher molecular weight with larger laminar structures, which would form aligned texture at lower temperature. Regional area of lump coke, which mainly emerged nearby the bottom section (Figure 14a), exhibited random orientation due to the insufficient gas flow force. To an extent, the growth of mesophase microstructure evidently tended to be parallel with the pore edge, as shown in Figure 14b and c. The mesophase features of cokes prepared at 400 oC and 463 oC were similar, whereas, the pore size of coke prepared at 463 oC was larger, indicating distinct impacts of gas evolution during the mesophase development (as shown in Figure S4). Clearly, the mesophase features of SFE-B extraction were quite similar with that of SFE-P extraction, as presented in the Figure 15 and Figure S5, which was in good agreement with the molecular structure analysis. Mosaic textures barely appeared in such cokes, whereas the oriented flow texture can be optimized under more appropriate condition. The apparent pore sizes of cokes prepared at lower temperature were smaller with those prepared at higher temperature. A vast gas evolution during the carbonization was believed to create both cracks and pores. Though the weight loss and gas evolution principally concentrated in 350-450 oC (Figure 12), the liquid crystal or intermediates had lower viscosity in the period of lower temperature carbonization, thus easily shrunk and aggregated to form relative compacted bulk 17


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structure. In contrast, higher temperature accelerated the polymerization reaction, which would form higher viscosity mesophase and generate mild hydrogen, resulting in more serious bubbling and then more pores. Nevertheless, the pore size distribution can be controlled or modified at the calcination and coking stage [38].

4. Conclusions The CLR featuring high proportions of solids, mineral matter, heteroatoms and pre-asphaltenes and asphaltenes from the commercialized direct coal liquefaction plant was tentatively processed by SFE using different types of solvents. The evolutions as well as the features of mesophase after carbonization of SFE extracts were systematically investigated. The following conclusion can be drawn based on the experimental data. 1) The SFE performance was strongly related with the selection of operating conditions and solvents. The extraction yield was highly dependent on the chain length of the alkane solvent used, therefore, the SFE using lesser polarity and lower density solvent (i.e., n-pentane) was ineffective at lower pressure. While the SFE process using acetone and iso-propanol exhibited excellent extraction performance. The benzene featuring strong affinity with condensed aromatic hydrocarbons presented best extraction capacity among all solvents used in this study, with the extraction yield reaching up to 53.78 wt%. 2) The SFE process showed much better mass transfer and separation capacity than conventional liquid-liquid extraction, hereby, remarkably reducing the QI and ash contents. The supercritical acetone demonstrated high reaction activity during extraction, that is, molecular structure of SFE-A was rich in alkyl side chains with lower molecular weight. Besides, the HS fraction in SFE-A was highest among all extracts. 3) The well-developed bulk mesophase in the SFE extracts after carbonization was substantially improved comparing with that of raw CLR. Probably, the SFE extract was inclined to generate 100 % mesophase, in which dominated flow textures were observed. Experimentally, the features of liquid crystal or intermediates during carbonization implied that the precursors from 18


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CLR could form better and larger scale oriented flow textures by optimizing carbonization conditions. The highly efficient extraction using SFE would be benefit to extent the utilization of direct coal liquefaction residual in a large scale; and thus is expected be able to convert the industrial waste to high value products after optimizing the process.

Supporting Information Particle size distribution of QIs (Figure S1); Montage photographs and enlarged photomicrographs of the coke lump produced at 463 oC under 1 bar from the raw CLR (Figure S2), SFE-A (Figure S3), SFE-P (Figure S4) and SFE-B (Figure S5).

Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Funds of China (No. 21406261) and the Joint Funds of the National Natural Science Foundation of China (Grant U1261213).

References [1] I. Mochida, O. Okuma, S.H. Yoon, Chem Rev, 2014, 114, 1637-1672. [2] S. Khare, M. Dell'Amico, Can J Chem Eng, 2013, 91, 1660-1670. [3] J. Li, J.L. Yang, Z.Y. Liu, Fuel Process Technol, 2009,90, 490-495. [4] X.J. Chu, W. Li, B.Q. Li, H.K. Chen, Fuel, 2008, 87, 211-215. [5] X. Chu, W. Li, H. Chen, Process Saf Environ Prot, 2006, 84, 440-445. [6] A.D. Cato, D.D. Edie, Carbon, 2003,41,1411-1417. [7] J.L. Crespo, A. Arenillas, J.A. Vina, R. Garcia, C.E. Snape, S.R. Moinelo, Carbon, 2004, 42, 2762-2765. [8] Y. Zhou, N. Xiao, J.S. Qiu, Y.F. Sun, T.J. Sun, Z.B. Zhao, Y. Zhang, N. Fuel, 2008, 87, 3474-3476. [9] N. Xiao, Y. Zhou, J.S. Qiu, Z.H. Wang, Fuel, 2010, 89, 1169-1171. [10] J.B. Zhang, L.J. Jin, J. Cheng, H.Q. Hu, Carbon, 2013, 55, 221-232. [11] J.L. Yang, Z.X. Wang, Z.Y. Liu, Y.Z. Zhang, Energ Fuel, 2009, 23, 4717-4722. 19


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[12] R. Garcia, J.L. Crespo, S.C. Martin, C.E. Snape, S.R. Moinelo, Energ Fuel, 2003, 17, 291-301. [13] J.B. Zhang, L.J. Jin, S.B. Liu, Y.X. Xun, H.Q. Hu, Carbon, 2012, 50, 952-959. [14] L. Bai, Y. Nie, J.C. Huang, Y. Li, H.F. Dong, X.P. Zhang, Fuel, 2013, 113, 767-767. [15] Y. Nie, L. Bai, H.F. Dong, X.P. Zhang, S.J. Zhang, Sep Sci Technol, 2012, 47,386-391. [16] Y. Nie, L. Bai, Y. Li, H.F. Dong, X.P. Zhang, S.J. Zhang, Ind Eng Chem Res, 2011, 50,10278-10282. [17] W.E. Rudzinski, T.M. Aminabhavi, Energ Fuel, 2000, 14, 464-475. [18] M.Z. Ozel, K.D. Bartle, Turk J Chem, 2002, 26, 417-424. [19] P.J.S. John R. Kershaw, J Supercriti Fluid, 1994, 6, 155-163. [20] O. Kajimoto, Chem Rev, 1999, 99, 355-390. [21] X.L. Cheng, G.N. Li, Y.L. Peng, S.L. Song, X.X. Shi, J.J. Wu, J.X. Xie, M. Zhou, G.Z. Hu, Chem Tech Fuels Oil, 2012, 48, 349-355. [22] I. Mochida, T. Oyama, Y. Korai, Y. Q. Fei, Fuel 1988, 67, 1171-1181 [23] G.C. Jiang, X.C. Lin, S.J. Zhang, Z.Q. Wang, Y.G. Wang, Q. Chen, Y.F. Zhu, Chem J Chinese U, 2015, 36, 544-550. [24] X.C. Lin, C.H. Wang, K. Ideta, J. Miyawaki, Y. Nishiyama, Y.G. Wang, S.H. Yoon, I. Mochida, , Fuel, 2014,118, 257-264. [25] M. Al-Sabawi, D. Seth, T. de Bruijn, Fuel Process Technol, 2011, 92, 1929-1938. [26] X.Y. Zou, L. Dukhedin-Lalla, X.H. Zhang, J.M. Shaw, Ind Eng Chem Res, 2004, 43, 7103-7112. [27] S.C. Oh, D.I. Han, H. Kwak, S.Y. Bae, K.H. Lee, Polym Degrad Stabil, 2007, 92, 1622-1625. [28] M. Bicker, D. Kaiser, L. Ott, H. Vogel, J Supercrit Fluid, 2005,36, 118-126. [29] S.C. Tucker, Chem Rev, 1999, 99, 391-418. [30] B.B. Jin, P.G. Duan, C.C. Zhang, Y.P. Xu, L. Zhang, F. Wang, Chem Eng J, 2014, 254, 384-392. [31] T. Kamitanaka, T. Matsuda, T. Harada, Tetrahedron Lett, 2003, 44, 4551-4553. [32] D. Dellis, I. Skarmoutsos, J. Samios, J Mol Liq, 2010, 153, 25-30. [33] F.D. Harry Marsh, Margaret Iley, Philip L. Walker Jr, Pyung W. Whang, Fuel, 1973, 52 253-261. [34] P.F. Wang, L.J. Jin, J.H. Liu, S.W. Zhu, H.Q. Hu, Fuel, 2013,104 14-21. [35] C. Diaz, C.G. Blanco, Energ Fuel, 2003, 17, 907-913. 20


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[36] L. Zou, L.J. Jin, Y. Li, S.W. Zhu, H.Q. Hu, J Anal Appl Pyrol, 2015, 112, 113-120. [37] X.L. Cheng, Q.F. Zha, J.T. Zhong, X.J. Yang, Fuel, 2009, 88, 2188-2192. [38] Y. Kawano, T. Fukuda, T. Kawarada, I. Mochida, Y. Korai, Carbon, 2000, 38, 759-765.

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Figure 1 SFE equipment with (1) SFE kettle, (2) fraction column, (3) throttle, (4) collecting pot, (5) condensation system, (6) solvent tank, (7) pump, (8) N2 cylinder, (9) pre-heater, operated at supercritical extraction conditions.

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Figure 2 The supercritical extraction yields of CLR under different conditions.

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Figure 3 The impurities in extracted CLR, (a) QI (note that the QI content of SFE-nP was ignored because of its trace amount), and (b) inorganic substances.

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Figure 4 The XRD pattern of QI in raw CLR.

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Figure 5 The SEM analysis of QI morphologies, (a) coal tar derived QI; (b) raw CLR derived QI; and (c) SFE-B (@8.7MPa) derived QI.

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Figure 6 The solubility of raw CLR and supercritical extracted products, a) feedstock basis absolute solubility; b) normalized relative solubility excluding QI.

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Figure 7 The HPLC spectra of extracts, a) SFE-A (@8.7MPa), b) SFE-P (@8.7MPa), c) SFE-B (@8.7MPa), d) PAHs standards (i.e., a-Naphthalene; b-Acenaphthene; c-Acenaphthylene; d-Fluorene; e-Phenanthrene; f-Anthracene; g-Fluoranthene; h-Pyrene; i-Benzo[a]anthracene; jChrysene;

k-Benzo[b]fluoranthene;

l-Benzo[k]fluoranthene;

m-Benzo[a]pyrene;

n-Dibenzo[a,c]anthracene; o-Benzo[g,h,i]perylene; p-Indeno(1,2,3-cd)pyrene), and e) benzene and acetone solvents.

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Figure 8 GPC profiles of a) SFE-A (@8.7MPa), b) SFE-P (@8.7MPa), and c) SFE-B (@8.7MPa) extracts.

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Figure 9 NMR spectra and molecular models of a) raw CLR, b) SFE-A (@8.7MPa), c) SFE-P (@8.7MPa), and d) SFE-B (@8.7MPa) extracts.

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Figure 10 FT-IR spectra of a) raw CLR, b) SFE-A (@8.7MPa), c) SFE-P (@8.7MPa), and d) SFE-B (@8.7MPa) extracts.

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Figure 11 Montage photographs and enlarged photomicrographs of the coke lump produced at 400 o

C under 1 bar from the raw CLR.

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Figure 12 The TGA analysis of raw CLR and SFE extracts.

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C under 1 bar from the SFE-A extract.

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C under 1 bar from the SFE-P extract.

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Figure 15 Montage photographs and enlarged photomicrographs of the coke lump produced at 400 oC under 1 bar from the SFE-B extract

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Table1 Characterization of feedstock, extracts, and SFE scraps. Properties of samples Sample Raw CLR SFE-A (@8.7MPa) SFE-P(@8.7MPa) SFE-B(@8.7MPa) Carbon (daf.)a (wt%) 77.50 84.94 88.47 87.35 Hydrogen (daf.) (wt%) 4.95 8.41 7.28 7.01 Nitrogen (daf.) (wt%) 1.02 1.66 1.3 1.22 Sulfur (daf.) (wt%) 1.87 0.37 0.19 0.39 Oxygen (daf.)b (wt%) 14.67 4.62 2.76 4.03 C/H 1.30 0.84 1.01 1.04 c fa 0.90 0.73 0.85 0.88 ST(oC)d 166±3 62±3 71±3 77±3 Ash compositions of SFE scraps (wt%)e SiO2 15.58 16.03 16.56 16.64 7.39 7.58 7.68 7.64 Al2O3 39.65 38.84 37.19 38.53 Fe2O3 16.40 16.03 16.26 16.42 CaO 16.15 16.49 18.03 16.37 SO3 1.24 1.26 1.24 1.21 Na2O 1.24 1.26 1.18 1.21 MgO 1.05 1.03 1.00 1.03 TiO2 0.62 0.60 0.57 0.59 SrO MnO 0.39 0.38 0.37 0.38 0.05 0.05 0.05 0.04 K2O 99.77 99.90 99.90 99.91 Total a

Daf.: dry ash free; b Difference; (e.g.V,Zr, Y, Zn, Cu, Rb) were ignored.

c

fa: Fraction of carbon aromaticity;

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d

Softening point;

e

Trace elements

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Table 2 Chemical shift assignments of the peaks in the NMR spectra δ (ppm)

Symbols

1-15

1

17-24

2

23-34

3

34-49

4

50-60 67-70 110-118 118-122 122-124 124-133 133-140 140-147 149-157

5 6 7 8 9 10 11 12 13

Assignments CH3 at the γ or further position of aromatic ring, CH3 in ethyl groups probably in ortho to the Car-OH and methyl groups in γ position to the aromatic ring if they are shielded by two adjacent groups CH3 directly bonded to the aromatic ring including naphthenic and/or hydroaromatic rings, and CH3ortho to Car-OH or CH3 shielded by two adjacent groups or rings All the rest of methylene carbons (CH2 different from Cα) Bridge/hydroaromatic structures (methylene carbons in α position to two aromatic rings) Hydroxyl carbon CH2-OH, or hydroxyl-ether carbon, or CH Carbonyl carbon, or alkynes carbon ≡CH Car-H ortho to Car-OH Car-H para to Car-OH and Car-CH3ortho to Car-OH Car-CH3para to Car-OH Quaternary aromatic carbons and Car-H meta to Car-OH Car-CH3 and CH or CH2para to Car-OH CH or CH2 meta to Car-OH Car-OH

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Table 3 Band assignments of the most prominent peaks in the FT-IR spectra Peak (cm-1) 3440-3320 3050-3030 2950 2920 2870 2850 2300-2360 1720-1690 1670-1600 1450 1380-1375 1300-1100 1051 1036 955 880-840 815 795 780 770-730 695

Assignments -OH stretching vibration Aromatic nucleus CH stretching vibration Aliphatic CH3 asymmetric stretching vibration Aliphatic CH2 asymmetric stretching vibration Aliphatic CH3 symmetric stretching vibration Aliphatic CH2 symmetric stretching vibration O=C=O symmetric stretching vibration Conjugated aromatic C=O Aromatic ring stretching C=O or C=C Aliphatic chains CH3-, CH2Aliphatic chains CH3Hydroxybenzene, ether, C-O, C-C, -OH Aromatic ring C-H bending vibration Aromatic ring stretching vibration or C-O stretching vibration Carboxyl –OH stretching vibration Aromatic nucleus CH, one adjacent H deformation Aromatic nucleus CH, two adjacent H deformation Aromatic CH wag Aromatic CH wag Aromatic nucleus CH, three to four adjacent H deformation Aromatic out-of-plane bending or Alkanes side rings [(CH2)n, n>4]

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High-Efficiency Extraction and Modification on the Coal Liquefaction

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