Structural Features and Pyrolysis Behaviors of Extracts from

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Structural features and pyrolysis behaviors of extracts from microwaveassisted extraction of a low rank coal with different solvents Yankun Xiong, Lijun Jin, Yang Li, Zhou Yang, and Haoquan Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03255 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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Energy & Fuels

Structural features and pyrolysis behaviors of extracts from microwaveassisted extraction of a low rank coal with different solvents

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Yankun Xiong, Lijun Jin, Yang Li, Yang Zhou, Haoquan Hu

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State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering,

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School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

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Abstract

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Microwave-assisted extraction (MAE) of a low rank Naomaohu (NMH) coal was

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conducted with two solvents, cyclohexanone (CYC) and tetrahydrofuran (THF), to obtain

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extracts (ECYC and ETHF) and residues (RCYC and RTHF). The parent coal, extract, and residue

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were characterized by ultimate analysis, TG, FT-IR, GPC, 1H NMR and solid state 13C NMR.

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The results showed that MAE process is more efficient than Soxhlet extraction. The solvent

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CYC exhibits higher total extract yield of 8.3 wt.% than THF being 4.7 wt.% during MAE,

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and more efficiency in extracting organic components of NMH coal, especially condensed

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arenes and macromolecular compounds. The average molecular weight of ECYC is higher than

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that of average aromatic cluster of NMH coal. Online pyrolysis-vacuum ultraviolet

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photoionization mass spectrometry was taken to determine the initial pyrolysis products of

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parent coal, RCYC and ECYC. The distribution of initial pyrolysis products suggested that NMH

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coal and ECYC possess similar basic arene structures, and ECYC is rich in macromolecular

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cluster with lots of side chains and bridge bonds. The chemical structure of ECYC could reflect



Corresponding author. Tel/Fax: +86-411-84986157.

E-mail address: [email protected] (Haoquan Hu). 1

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the macromolecular network structure of NMH coal to some extent. This could be an

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effective method to understand the organic structure of coal.

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Keywords: coal; microwave-assisted extraction; cyclohexanone; condensed arenes;

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macromolecular network structure

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1. Introduction

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The extensive use of low rank coal has been limited by its characteristics, such as high

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moisture and oxygen content, and low bulk density1,2. A deep insight into the structure of

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low rank coal will promote its effective utilization. Many efforts were devoted to understand

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the organic structure of coal in the past years on the basis of the products of pyrolysis,

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liquefaction

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thermogravimetric analyzer (TG)7,8, Fourier transform infrared spectroscopy (FT-IR)9,10,

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nuclear magnetic resonance (NMR)11,12, gel permeation chromatography (GPC)13,14, and X-

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ray photoelectron spectroscopy (XPS)15-17 are also introduced into the construction of coal

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structure model. However, there is still no accurate description for its structure because of

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the complexity, heterogeneity and variability.

and/or

extraction3-6.

Some

modern

analysis

instruments,

such

as

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Extraction has been considered as an effective method for understanding the organic

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structure of coal18. The organic matters in coal are comprised of fixed phase and mobile

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phase19, which could be separated by solvent extraction through the breakage of non-covalent

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and/or weak covalent bonds. Characterization of extract and residue could provide some

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valuable information on coal structure. However, due to low extract yield and small

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molecular size, the extract usually lacks the representativeness for macromolecular structure 2


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of coal. Therefore, for acquiring detailed information about the organic structure of coal, it

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is vital to find a suitable solvent and an efficient extraction route to obtain considerable

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amount of extract with more condensed arenes and macromolecular structure.

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Microwave-assisted extraction (MAE) has already exhibited superior capacity to

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separate valuable and/or noteworthy compounds from the sample matrix accompanied by

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less extraction time and organic solvent consumption20. Ge et al. found the increase of pore

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volume and surface area, as well as generation of micropores when three brown coals were

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dealt with microwave irradiation (MWI)21, resulting in easily diffusion of solvent into the

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coal matrix. With the assistance of MAE using different solvents, considerable amount alkane

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hydrocarbons, alkylated aromatic compounds, even polycyclic aromatic hydrocarbons were

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successfully separated from coal and detected22,23. Various organic solvents, such as

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tetrahydrofuran (THF)24, acetone25, pyridine26, organic amines19, N-methyl-2-pyrrolidinone

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27, 28,

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the organic matters from coals. Notably, although some of them demonstrate an excellent

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extraction performance for bituminous coals32, most of the solvents have low extract yield

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for low rank coals, because they are unapplicable to these coals and extraction system

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employed is not efficient enough6. Cyclohexanone (CYC), as an electron-donor solvent,

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exhibits good compatibility with low rank coal33,34. Besides, CYC could easily diffuse into

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the coal inner regions due to its condensed plane structure and shows high swelling ability34.

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Tian et al.35 investigated the extraction behaviors of low rank coals in CYC and found that

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about 15.65% and 20.54% yield were obtained for Chifeng and Hami coals, respectively.

dimethyl sulphoxide26, dimethylformamide29 and ionic liquids30,31 were used to extract

3


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However, up to now, few studies were reported on the extraction performance of low rank

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coal in CYC with assistance of MWI.

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The decomposition of basic structure of coal usually occurs during pyrolysis. Some

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insight related to coal structure could be acquired by the analysis of pyrolysis products. Many

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coal structural models36,37 have been proposed based on valuable information obtained from

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coal pyrolysis. Nevertheless, the key is to acquire the initial pyrolysis products. Recently, an

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online pyrolysis-vacuum ultraviolet photoionization mass spectrometry (Py-VUVPI-MS)

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system was developed in our group and the pyrolysis behaviors and structure characteristics

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of coals could be investigated for its superior ability to realize the detection for most initial

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pyrolysis products38-41.

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To investigate the extraction mechanism and obtain more amounts of soluble organic

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matters from coal, especially macromolecular structure, in this work, a low rank coal, from

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Naomaohu coal mine in Xinjiang, China (abbreviated as NMH) was chosen to study for its

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huge reserves and extensive uses. The extraction behaviors in CYC and THF solvents with

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assistance of MWI were investigated, and the chemical properties of parent coal and its

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extraction products were characterized by ultimate analysis, TG, FT-IR, GPC, 1H NMR and

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solid state 13C NMR. The pyrolysis performances of parent coal, extraction residue (RCYC)

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and extract (ECYC) were studied by Py-VUVPI-MS. This work will shed light on the MAE

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performance of CYC and the way to study macromolecular network structure of coal.

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2. Experimental section

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2.1 Materials

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NMH coal, used in this work, was pulverized to below 160 mesh and dried at 80 °C in

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vacuum oven for 24 h before experiments. Table 1 lists the proximate and ultimate analyses

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of NMH coal and extraction products. The solvents CYC (99.5%, AR, boiling point 155 oC)

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and THF (99.0%, AR, boiling point 66 oC) employed in this study were purchased from

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Damao Chemical Reagent Factory (Tianjin, China) and further purified by distillation before

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use. THF (HPLC grade, 99.9%) was purchased from Oceanpak Alexative Chemical., Ltd

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(Sweden) for GPC analysis.

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2.2 Microwave-assisted extraction

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MAE was conducted on a microwave extraction apparatus (MAS-Ⅱ, Sineo Microwave

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Chemistry Science and Technology Ltd.), equipped with a water-cooled reflux condenser.

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About 5 g coal sample and 100 mL solvent were added in a glass flask, and then MAE was

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performed at a temperature close to the boiling point of solvent for a certain time. Magnetic

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stirring was used for ensuring thorough mixture of coal with solvent.

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The mixture was cooled to room temperature after extraction and separated by filtration.

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The resultant residues (RCYC and RTHF) were first washed with corresponding fresh solvent

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until received the colorless supernatant (3-5 times). Then RCYC was washed with acetone and

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water each three times, and dried in vacuum oven at 80 oC for 48 h, while RTHF was washed

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with water three times. CYC and THF in filtrate were removed through rotary evaporation

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and drying oven, separately, to obtain extract (ECYC and ETHF). To ensure repeatability, the 5


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same experiment was performed at least three times. The extract yield used in this work was

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averaged and determined by following formula: Extract yield (wt.%,daf) =

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coal(g) ― residue(g) × 100% coal(g,daf)

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where, coal (g, daf) is the weight of coal sample in dry ash-free basis.

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2.3 Soxhlet extraction

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A Soxhlet extractor connected to a glass flask and water-cooled condenser was used in

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this work. Firstly, about 5 g coal sample and 100 ml solvent were added into the extractor

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and glass flask respectively. Then, Soxhlet extraction was conducted for 6 h. Finally,

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extraction residue and extract were dealt same as MAE process, as well as the calculation of

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extract yield.

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2.4 Py-VUVPI-MS

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The pyrolysis of NMH coal, ECYC and RCYC was conducted in an online Py-VUVPI-MS

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apparatus and the detailed procedure has been given in our previous publication42. Briefly,

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the apparatus involves three chambers, which are used for pyrolysis, ionization and TOF-MS

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detection, respectively. The pressure of pyrolysis and TOF-MS detection zone was generally

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kept at about 10−4 and 10−5 Pa, respectively. In a typical experiment, the sample (about 10

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mg) was heated from room temperature to 850 °C at a heating rate of 5 °C/min through an

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electrical resistance. Because of their lower ionization energy, most volatile products would

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generate corresponding molecular ions and be detected in TOF-MS chamber when ionized

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by vacuum UV light (10.6 eV). The mass spectra were recorded per second with a data

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acquisition card. Resolution is more than 2000, and 100 μg/m3 of sensitivity can be reached 6


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for organic volatile gases.

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2.5 Characterizations

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FT-IR were used to measure functional groups of NMH coal, extracts and residues on a

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Bruker EQUINOX55 spectrometer. Samples were mixed with KBr in a mass ratio of 1/100

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and pressed in the form of pellet for measurement.

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A Waters 1525 liquid chromatography, equipped with a Waters 2487 ultraviolet-visible

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detector (UV/vis) and three Waters styragel columns (7.8 mm × 300 mm), was employed to

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measure GPC curve of extracts. THF (HPLC grade) was used for eluent at 1 mL/min flow

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rate. The detection was performed at 20 oC with a wavelength of 254 nm.

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The characterization of carbon types in samples were performed by solid state 13C NMR

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at 100.6 MHz on a Bruker Avance Ⅲ 600 MHZ Wide Bore spectrometer. Chemical shift of

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13C

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Hz in a zirconia rotor. The cross-polarization experiments were performed with a 3 s recycle

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delay time and 3 ms contact time.

spectrum was calibrated by tetramethylsilane (TMS). The dry sample was spun at 9000

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The extracts were analyzed by 1H NMR spectrometer (Bruker Avance II 400) at room

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temperature using deuterated tetrahydrofuran (C4D8O) as solvent. The samples were

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dissolved in C4D8O at a concentration of 80 mg mL−1 and TMS was selected as an internal

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reference.

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Mettler Toledo TGA/SDTA851e analyzer was employed for TG analysis. About 15 mg

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sample was heated from 25 oC to 800 oC under 10 oC/min after being placed in a ceramic

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crucible, and N2 was used for carrier gas. Only 5 mg extract sample was used in TG analysis 7


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due to its high volatilization.

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3 Results and discussion

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3.1 Extract yield

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The extract yield of MAE and Soxhlet extraction at different conditions are displayed

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in Figure 1. It can be found that the extraction yield reaches about 5.1 wt.% at 15 min during

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MAE with CYC and then increases with prolonging the time. And increase of yield is not

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very obvious after extraction time exceed 45 min. Furthermore, the extraction time of

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microwave-assisted extraction of coal in literatures was always less than 1h24,43. So an

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extraction time of 45 min was employed during MAE process of this work. Moreover, as a

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contrast, Soxhlet extraction was also conducted in this work. It is easy to be found that the

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extraction yield of MAE process within 45 min is higher than that of Soxhlet extraction

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within 6 h. Therefore, MAE process is more efficient than Soxhlet extraction, which has also

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been demonstrated by Rajendra et al.22. Notably, the total extract yield is 8.3 wt.% and 4.7

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wt.% during MAE process with CYC and THF, respectively. The reason for higher yield

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with CYC may be ascribed to its easily disruption of hydrogen bonds, leading to a more

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chance to extract the soluble matters and have a good compatibility with low rank coals33.

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3.2 Comparison of MAE with different solvent

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3.2.1. TG/DTG analysis of NMH coal and extraction product

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Figure 2 displays the TG and DTG curves of raw coal, its extracts and residues. Slight

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difference between parent coal and extraction residues below 200 oC can be observed,

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resulting from the different amount of water evaporation. The mass loss is in the order of 8


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NMH coal > RTHF > RCYC when temperature ranges from 350 to 800 oC, suggesting that more

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volatile matters can be extracted by CYC during MAE, which is in accordance with the

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different extract yields of two solvents. Li et al.44 investigated extraction behavior of

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Zhaotong lignite in different solvents. They found that more organic matter was extracted

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from Zhaotong lignite in methanol than in cyclohexane. As a result, the mass loss of methanol

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extraction residue is much less than that of cyclohexane extraction residue during TG analysis.

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During MAE process, there are some extracts which have been separated from the coal matrix

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but still remained in the micropores of extraction residues. The significant weigh loss of

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extraction residues at the temperature slight higher than 200 oC in DTG may be ascribed to

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the release of these extracts during thermal degradation. Furthermore, a similar phenomenon

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was also found in literature45. Different from raw coal and the residues, both extracts obtained

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by THF and CYC solvents have an extra peak at about 300 oC in DTG curves. And the

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temperature with maximum mass loss rate of two extracts is at about 300 oC, remarkably

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lower than those of parent coal and residues. The total mass loss of ECYC and ETHF, exceeds

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those of parent coal and residues at end temperature of 800 oC. All these results demonstrate

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that extracts are mainly composed of light components compared to parent coal and residues.

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These components easily escape and/or decompose during thermal degradation. ECYC has less

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total mass loss compared with ETHF, indicating that ECYC possess more macromolecular

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structure that easily forms coke or char during pyrolysis, which can be further confirmed by

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GPC analysis.

9


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3.2.2. GPC analysis of the extract

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Figure 3 illustrates the molecular weight (MW) distributions of two extracts by GPC

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analysis. A molecular mass scale based on a calibration by polystyrene standards is given in

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Figure 3. It’s noted that the detected molecules are mainly benzene-related compounds due

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to the detection wavelength of 254 nm. Two GPC curves of extracts display a broad peak

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with a long-tail on lower mass side. Compared with ETHF, ECYC displays a distinct bimodal

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distribution in GPC spectrum and has a tendency to migrate to high mass side, meaning that

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ECYC has larger MW than ETHF. To evaluate the MW distributions quantitatively, Gaussian

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peak fitting method was used46. The spectra of two samples were divided into four individual

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peaks via curve fitting, corresponding to four molecular weight regions (80-340 amu, 340-

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690 amu, 690-1870 amu, 1870-19500 amu), respectively, and the results are displayed in

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Figure 4. The relative content derived from the curve fitting analysis and average molecular

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weight are shown in Table 2. Obviously, the weight-average molecular weight (Mw) of ECYC

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is larger than that of ETHF, indicating that abundant macromolecular clusters with more fused

200

aromatic rings or side chains and bridge bonds are extracted. The comparison of the relative

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content of different molecular weight between two samples can further confirmed it. It is

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analyzed that CYC, with high swelling power to low rank coal and condensed plane structure,

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could easily diffuse into the coal inner regions34, which increasing the chance for extraction

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of macromolecular cluster. Meanwhile, the π-π interactions between condensed arenes and

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solvent could promote its extraction from coal during MAE10,43.

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3.2.3. Structural features of NMH coal, extract and residue from FT-IR 10


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Figure 5 displays the FT-IR spectra of NMH coal, its extracts and residues by MAE with

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CYC and THF. The absorptions of aliphatic moieties in extracts around 2920, 2850 and 1374

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cm-1 are remarkably stronger than those in parent coal and residues, implying that aliphatic

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species in NMH coal can be extracted easily. Compared with NMH coal and residues, a clear

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absorption band around 3418 cm-1 (assigned to hydroxyl groups) can be found in the spectra

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of two extracts, manifesting that strong interactions exist in solvents and -OH groups in NMH

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coal. In comparison, ECYC exhibits stronger absorptions in this band than ETHF, as a result of

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its superior ability to disrupt hydrogen bonds and loosen the coal structure. As seen from

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Figure 5, two extracts have very strong absorptions in 1708 cm-1 and 1294-1048 cm-1 regions

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(assigned to carbonyl groups and C-O groups, respectively), indicating the existence of large

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amount of oxygen-containing species. The absorbance near 807 and 725 cm-1 (assigned to

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bending vibration of aromatic bonds) in coal and residues are not obvious compared with

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extracts owing to the interference of mineral matters45.

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3.2.4. Structural features of NMH coal and extract from 13C NMR

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Solid state 13C NMR analysis of parent coal and its extracts were carried out to deeply

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understand the various types of carbon and the spectra are shown in Figure 6. To acquire the

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carbon distributions in the samples, curve fitting of each spectrum was conducted43,47,48 and

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the results are given in Table 3. The detailed curve fitting parameters are listed in Supporting

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Information (Table 1S). According to curve fitting results, the structural parameters of carbon

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skeleton were calculated and summarized in Table 4. As expected, aliphatic carbons are most

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abundant, accounting for 52.1% of total carbon atoms in NMH coal while aromatic carbons 11


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just account for 36.0%. Furthermore, among aliphatic carbon, methylene carbons are

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dominant. As for aromatic carbons, the substituted carbons are abundant.

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Extraction influences the distribution of carbon types. As Table 4 listed, the aromaticity

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(fa) of ECYC and ETHF is lower than that of NMH, suggesting that two extracts contain less

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aromatic carbon atoms compared with parent coal. Moreover, CYC could extract more

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amount aromatic species than THF. The number of aromatic cluster rings can be evaluated

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by molar fraction of aromatic bridgehead carbon (Xb)48. The Xb of ECYC is higher than that of

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parent coal and ETHF, revealing that more condensed arenes were extracted from NMH by

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CYC. Several studies23,49 also proved that condensed arenes can be extracted efficiently by

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MAE. In addition, the ratio of carbonyl carbon (faC) for ECYC and ETHF are 4.8% and 4.2%

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respectively, manifesting that some carbonyl group-containing species are also extracted,

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which is coincided with the absorbance band around 1710 cm−1 in FT-IR spectra.

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The detailed structure information of NMH coal can be further derived from the

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parameters in Table 4. The Xb of NMH coal is 0.19, close to the value of naphthalene (0.20),

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implying that each cluster possesses two aromatic rings on average in NMH coal. The

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substituted degree (δ) of aromatic ring is 0.66, indicating that there are four (6×0.66)

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substituents on each aromatic on average. To further evaluate the aromatic cluster size and

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the extent of bridging between clusters, some structural parameters deduced from the results

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in Table 3 and 4 are presented in Table 5. The calculation is according to the method of

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literature48. In brief, the number of aromatic carbons per cluster (Ca), number of peripheral

248

carbons per cluster (Cp), and number of aromatic rings per cluster (Ra) were obtained from 12


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the value of Xb. And then the number of total carbons per cluster (Ccl), average number of

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attachments per cluster ( σ +1), total molecular weight of a cluster (MW’), and average

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molecular weight of the cluster attachments (Matt) can be calculated by following formulas.

252

𝐶cl = 𝐶a/𝑓a

253

σ + 1 = 𝐶a × 𝛿

254

𝑀W ′ =

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𝑀att =

(1) (2)

𝐶cl × 12.01

(3)

𝐶%/100

𝑀W′ ― [𝐶a ― (σ + 1) + 𝐶a × 12.01] σ+1

(4)

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The results show that the NMH coal has a cluster size of about 28 carbon atoms. Each

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aromatic cluster possesses two rings on average with Matt being 52 amu. In addition, MW for

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NMH coal is 466 amu, which is smaller than that of extracts, especially ECYC. The similar

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aromatic structure parameters, including Ca, Cp, Ra and σ+1, suggest that the basic arene

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structure of ECYC could reflect the aromatic cluster structure of NMH coal. The different

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molecular weight between parent coal and extract results from their different substituted side

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chains.

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3.2.5. 1H NMR of the extracts

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The 1H NMR spectra of ECYC and ETHF are displayed in Figure 7. The spectra are consist

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of aromatic (HAr) and aliphatic protons (HR), corresponding to the regions of 6.3-9.3 and

266

0.5-6.3 ppm, respectively. Moreover, as listed in Table 6, HAr and HR can be further divided

267

according to literature50 and relative content of different protons were calculated. The results

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show that the proportion of HAr is lower than that of HR for ECYC and ETHF, indicating that

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the extracts from MAE process with two solvents involve more aliphatic species. Combined 13


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with the 13C NMR of two extracts, it can be inferred that the aromatic nuclei in extracts were

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connected by lots of side chains and bridge bonds, resulting in less aromatic protons, which

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is accordant with the results presented by Zhang et al.23. They investigated the extracts

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obtained by MAE of three low rank coals with THF and found that extracts are mainly

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comprised of highly branched aliphatic hydrocarbons and para-alkyl substituted arenes with

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1~2 rings. Compared with ETHF, ECYC has higher fa obtained from 13C NMR and less content

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of HAr, implying that it contains more arenes connected by lots of side chains and bridge

277

bonds, which is in accordance with the results derived from the GPC analysis.

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As for aliphatic protons, H and H are dominant for two extracts, suggesting the

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existence of branched alkyl side chains in the  and  position to the aromatic rings, such as

280

methyl, methylene, and/or methine. In addition, two samples shows a multiple peaks in 1.0-

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2.0 ppm, indicating the existence of both paraffinic and naphthenic methylene structures51.

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Additionally, a weak peak can be observed at 5.7 ppm, indicating that there exists some

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phenolic containing species in ECYC. The above results suggest that more organic matters can

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be extracted by CYC than THF during MAE process.

285

3.3 Initial pyrolysis products analysis by Py-VUVPI–MS

286

Because CYC exhibits superior extraction efficiency than THF, the RCYC and ECYC

287

samples were employed to investigate the pyrolysis behaviors using Py-VUVPI-MS. Due to

288

the characteristics of Py-VUVPI-MS, such as soft ionization and low system pressure, it is

289

reasonable to regard the detected compounds as initial pyrolysis products of coal38,39. Figure

290

8 illustrates the mass spectra of initial pyrolysis products of NMH coal, RCYC and ECYC. 14


Page 14 of 37

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Moreover, as Table 7 listed, the main organic compounds were assigned according to MW39,41

292

and roughly classified into four categories: alkenes, oxygen-containing compounds, sulfur-

293

containing compounds, and arenes. The release temperature of most initial pyrolysis products

294

from three samples is at over 300 oC. As Figure 8 exhibited, the predominant pyrolysis

295

products of three samples are generally similar, implying that they share the similar basic

296

structures. In addition, H2S (m/z 34) and CH3SH (m/z 48) from decomposition of thioesters

297

bonds connected with aromatic and hydroaromatic units, are observed in NMH coal and ECYC,

298

indicating that some sulfur-containing compounds exist in mobile phase of coal. Notably,

299

owing to the lower ionization potential than VUV lamp, H2S (10.46 eV) and CH3SH (9.45

300

eV) can be ionized easily.

301

To further understand the difference among pyrolytic products, for each sample, the

302

mass spectrum peak area of each compound was compared with that of C3H6, i.e., the relative

303

content of C3H6 is set as 1.0, which has been used in other work39,41,52. Figure 9 presents the

304

relative content of dominate pyrolysis products from NMH coal, RCYC and ECYC. It can be

305

seen that the relative contents of alkenes in NMH coal are high compared with other low rank

306

coals investigated by our previous work41. This manifests that macromolecular structure in

307

NMH coal involves more side chains and bridge bonds, which is in accordance with the

308

results of 13C NMR. Abundant amount of alkenes in ECYC mean that the molecular clusters

309

from mobile phase of coal possess more side chains and bridge bonds too. Higher content

310

ratio of n-alkyl dihydroxybenzene to n-alkyl phenol in ECYC than NMH coal indicates that

311

OH-containing compounds could be easily extracted by CYC, which is accordant with the 15


Energy & Fuels 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 58 59 60

312

absorbance band around 3418 cm−1 in FT-IR spectrum. Compared with parent coal, ECYC has

313

relative high content of n-alkyl phenanthrene, especially m/z 192, suggesting that both small

314

molecule and condensed arenes existed in mobile phase of NMH coal could be extracted by

315

CYC. These results indicate the similar basic structure of NMH coal and ECYC, such as cluster

316

size of aromatic units, and ECYC is rich in macromolecular cluster with lots of side chains and

317

bridge bonds. This phenomenon can be explained by the synergistic effect among substituted

318

arenes, CYC and MWI. For its better dielectric properties, substituted arenes can be extracted

319

from coal with the assistance of MWI43. Furthermore, swelling capacity of CYC could be

320

strengthened by MWI, so that substituted arenes disperse to the solvent successfully. As a

321

result, the polarity of mixed solution increases, resulting in better absorption of MWI for the

322

mixed solution in turn. In addition, due to the complex structure of coal, it is appropriate to

323

consider ECYC as the sample to study the pyrolysis mechanism of coal by using Py-VUVPI-

324

MS.

325

4 Conclusions

326

Solvent has influence on the NMH extraction. About 8.3 wt.% and 4.7 wt.% total extract

327

yields can be obtained by MAE with CYC and THF as solvent, respectively. Both extracts

328

contain plenty of aliphatic species but fewer aromatic and carbonyl group-containing species

329

compared with raw coal, suggesting that aliphatic species are easily extracted by the solvent

330

with MAE. Compared to THF, CYC can extract the organic components of NMH coal more

331

efficiently, especially condensed arenes and macromolecular structure due to its π electrons,

332

swelling power to low rank coal, and synergistic effect with MWI. The average molecular 16


Page 16 of 37

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weight of ECYC is larger than that of average aromatic cluster of NMH parent coal, but very

334

similar basic structure of arenes. Py-VUVPI-MS results show that NMH coal and ECYC

335

possess similar basic structures and macromolecular clusters with lots of side chains and

336

bridge bonds. These results suggest that the chemical structure of ECYC could reflect the

337

macromolecular network structure of NMH coal to some extent.

338

Acknowledgments

339

The work was financially supported by the National Key Research and Development

340

Program of China (2016YFB0600301).

341

References

342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365

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(29) Klotzkin, M. P., Solvent treatment of coals: 1. Effects on microporosity at ambient temperature. Fuel 1985, 64, (8), 1092-1096. (30) Jin, L.; Qi, Y.; Chaffee, A. L., Effect of temperature on the solubility of Victorian brown coal in the ionic liquid DIMCARB. Fuel 2018, 216, 752-759. (31) Lei, Z. P.; Cheng, L. L.; Zhang, S. F.; Zhang, Y. Q.; Shui, H. F.; Ren, S. B.; Wang, Z. C., Dissolution performance of coals in ionic liquid 1-butyl-3-methyl-imidazolium chloride. Fuel Process. Technol. 2015, 129, 222-226. (32) Mathews, J. P.; Burgessclifford, C.; Painter, P., Interactions of Illinois No. 6 Bituminous Coal with Solvents: A Review of Solvent Swelling and Extraction Literature. Energy Fuels 2015, 29, (3), 12791294. (33) Tian, B.; Qiao, Y. Y.; Tian, Y. Y.; Xie, K. C.; Liu, Q.; Zhou, H. F., FTIR study on structural changes of different–rank coals caused by single/multiple extraction with cyclohexanone and NMP/CS2 mixed solvent. Fuel Process. Technol. 2016, 154, 210-218. (34) Shin, Y.-J.; Shen, Y.-H., Preparation of coal slurry with organic solvents. Chemosphere 2007, 68, (2), 389-393. (35) Tian, B.; Qiao, Y.; Liu, Q.; Li, D.; Tian, Y., Structural features and thermal degradation behaviors of extracts obtained by heat reflux extraction of low rank coals with cyclohexanone. J. Anal. Appl. Pyrolysis 2017, 124, 266-275. (36) Hüttinger, K. J.; Michenfelder, A. W., Molecular structure of a brown coal. Fuel 1987, 66, (8), 1164-1165. (37) Nomura, M.; Artok, L.; Murata, S.; Yamamoto, A.; Hama, H.; Gao, H.; Kidena, K., Structural Evaluation of Zao Zhuang Coal. Energy Fuels 1998, 12, (12), 512-523. (38) Li, G.; Zhang, S.-Y.; Jin, L.-J.; Tang, Z.-C.; Hu, H.-Q., In-situ analysis of volatile products from lignite pyrolysis with pyrolysis-vacuum ultraviolet photoionization and electron impact mass spectrometry. Fuel Process. Technol. 2015, 133, 232-236. (39) Shi, Z.; Jin, L.; Zhou, Y.; Li, Y.; Hu, H., Online analysis of initial volatile products of Shenhua coal and its macerals with pyrolysis vacuum ultraviolet photoionization mass spectrometry. Fuel Process. Technol. 2017, 163, 67-74. (40) Shi, Z.; Jin, L.; Zhou, Y.; Li, H.; Li, Y.; Hu, H., In-situ analysis of catalytic pyrolysis of Baiyinhua coal with pyrolysis time-of-flight mass spectrometry. Fuel 2018, 227, 386-393. (41) Shi, L.; Wang, X.; Zhang, S.; Wu, X.; Yuan, L.; Tang, Z., A new in-situ pyrolytic time-of-flight mass spectrometer instrument for study on coal pyrolysis. J. Anal. Appl. Pyrolysis 2016, 117, 347-353. (42) Yang, Z.; Li, G.; Jin, L.; Zhou, J.; Wang, J.; Li, Y.; Hu, H., In Situ Analysis of Catalytic Effect of Calcium Nitrate on Shenmu Coal Pyrolysis with Pyrolysis Vacuum Ultraviolet Photoionization Mass Spectrometry. Energy Fuels 2018, 32, 1061-1069. (43) Yu, X.-Y.; Wei, X.-Y.; Li, Z.-K.; Chen, Y.; Zong, Z.-M.; Ma, F.-Y.; Liu, J.-M., Comparison of three methods for extracting Liuhuanggou bituminous coal. Fuel 2017, 210, 290-297. (44) Li, Z.-K.; Wei, X.-Y.; Yan, H.-L.; Yu, X.-Y.; Zong, Z.-M., Characterization of soluble portions from thermal dissolution of Zhaotong lignite in cyclohexane and methanol. Fuel Process. Technol. 2016, 151, 131-138. (45) Tian, B.; Qiao, Y. Y.; Tian, Y. Y.; Xie, K. C.; Li, D. W., Effect of heat reflux extraction on the structure and composition of a high-volatile bituminous coal. Appl. Therm. Eng. 2016, 109, 560-568. 19


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(46) Kawate, T.; Gouaux, E., Fluorescence-detection size-exclusion chromatography for precrystallization screening of integral membrane proteins. Structure 2006, 14, (4), 673-681. (47) Yang, F.; Hou, Y.; Wu, W.; Niu, M.; Ren, S.; Wang, Q., A new insight into the structure of Huolinhe lignite based on the yields of benzene carboxylic acids. Fuel 2017, 189, 408-418. (48) Mark S. Solum, R. J. P., and David M. Grant, 13C Solid-state NMR of Argonne premium coals. Energy Fuels 1989, 3, 187-193. (49) Zhang, L.; Hu, S.; Chen, Q.; Han, H.; Xiao, L.; Xu, J.; Jiang, L.; Xu, K.; Su, S.; Wang, Y.; Xiang, J., Identification of the structural characteristics of the asphaltenes in the tetrahydrofuran-microwaveextracted portions from two Chinese coals. Fuel Process. Technol. 2017, 160, 86-92. (50) Zou, L.; Jin, L.; Wang, X.; Hu, H., Pyrolysis of Huolinhe lignite extract by in-situ pyrolysis-time of flight mass spectrometry. Fuel Process. Technol. 2015, 135, 52-59. (51) Bartz, K. W.; Chamberlain, N. F., Determining Structure of Paraffinic Chains by NMR. Anal. Chem. 1964, 36, (11), 2151-2158. (52) Jia, L.; Weng, J.; Wang, Y.; Sun, S.; Zhou, Z.; Qi, F., Online analysis of volatile products from bituminous coal pyrolysis with synchrotron vacuum ultraviolet photoionization mass spectrometry. Energy Fuels 2013, 27, (2), 694-701.

466 467

20


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468

Table 1. Proximate and ultimate analyses of NMH coal and extraction products Sample

469

Proximate analysis (wt.%)

Ultimate analysis (wt.%, daf)

Mad

Ad

Vdaf

C

H

N

S

O*

NMH

3.54

5.22

52.23

71.64

6.01

0.85

0.42

21.08

RCYC

1.96

5.51

51.35

72.46

5.99

0.85

0.41

20.29

RTHF

2.36

5.39

51.70

71.80

5.93

0.84

0.40

21.06

ECYC

-

-

-

70.84

11.25

0.22

0.10

17.59

ETHF

-

-

-

74.27

11.95

0.33

0.09

13.36

*By difference

470

21


Energy & Fuels 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 58 59 60

471

Table 2. Average molecular weight (MW) and relative content with different MW of extracts Average MW Sample

472 473

Page 22 of 37

Mn (amu)

Relative content (%)

Mw (amu)

19500-1870

1870-690

690-340

340-80

(amu)

(amu)

(amu)

(amu)

ECYC

786

1963

27.55

41.34

25.98

5.12

ETHF

645

1218

14.88

36.94

41.57

6.61

Mn: number-average molecular weight; Mw: weight-average molecular weight

22


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Table 3. Distribution of carbon types in NMH and its extracts Carbon type

NMH

ECYC

ETHF

Peak position (ppm)

Symbol

NMH

ECYC

ETHF

Molar content %

Aliphatic carbon Aliphatic CH3

14.0

16.1

17.0

falM

7.2

7.4

10.3

11.3

16.6

10.0

Aromatic CH3 α or β methylene attached to aromatic ring and methylene

23.4 29.5 33.5

24.0 32.0 33.3

24.0 31.5 33.3

falA falH

21.1

31.5

31.7

Branched carbon in aliphatic chains

39.2

40.8

39.6

falD

7.2

8.4

11.3

Oxygen-attached aliphatic carbon

50.0 67.7

52.0

52.0

falO

5.3

4.2

6.1

Protonated ortho-oxyaromatic carbon Alkylated ortho-oxyaromatic carbon Protonated aromatic carbon Aromatic bridgehead carbon Alkyl-substituted carbon

104.2 116.4 124.5 130.7 141.5

104.0 117.0 124.5 130.0 141.4

104.0 116.2 124.9 130.7 141.8

faO1 faO2 faH faB faS

2.6 6.4 5.5 6.9 7.6

2.0 6.7 3.2 6.3 5.7

1.9 5.8 4.4 5.8 5.0

Oxygen-attached aromatic carbon

153.0 160.6

154.2

154.8

faO3

7.0

3.2

3.5

faC1

7.2

1.6

1.2

faC2

4.7

3.2

3.0

Aromatic carbon

Carbonyl carbon Carboxyl carbon

183.2 197.9

Carbonyl carbon

211.5

178.1 181.2 201.0 214.2

179.5 181.1 201.0 214.0

475 476

23


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477

Page 24 of 37

Table 4. Structural parameters of NMH coal and its extracts from 13C NMR Structural parameter

Symbol

Aromaticity

fa

Ratio of aliphatic carbon

fal

Ratio of carbonyl carbon

Definition fa = faO1 + faO2 + faH + faB + faS + faO3 fal = falM + falA + falH + falD + falO

NMH

ECYC

ETHF

36.0%

27.1%

26.4%

52.1%

68.1%

69.4%

faC

faC = faC1 + faC2

11.9%

4.8%

4.2%

Xb

Xb = faB / fa

0.19

0.23

0.22

δ

δ = (faO1+ faO2+ faO3+ faS) / fa

0.66

0.65

0.61

Molar fraction of aromatic bridgehead carbon Substitutive degree of aromatic ring

478 479

24


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Energy & Fuels

480

Table 5. Aromatic cluster size of NMH coal and its extracts Symbol

Structural parameter

NMH

ECYC

ETHF

10

10

10

Ca

Number of aromatic carbons per cluster

Ccl

Number of total carbons per cluster

27.8

36.9

37.9

Cp

Number of peripheral carbons per cluster

8.0

8.0

8.0

Ra

Number of aromatic rings per cluster

2.0

2.0

2.0

σ+1

The average number of attachments per cluster

6.6

6.5

6.1

MW’

Total molecular weight of a cluster

466

626

613

Matt

Average molecular weight of the cluster attachments

52

77

80

481 482

25


Energy & Fuels 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 58 59 60

483

Page 26 of 37

Table 6. Proton distribution of extracts by 1H NMR Symbol

Chemical

Proton type

shift (ppm)

Relative amount (%) ECYC

ETHF

HAr

Aromatic protons

6.3-9.3

4.7

8.0

HArU

Uncondensed aromatic protons

6.3-7.2

67.9

35.7

HArC

Condensed aromatic protons

7.2-9.3

32.1

64.3

HR

Aliphatic protons

0.5-6.3

95.3

92.0

H

Aliphatic protons of O-H, CHO and alkene

3.0-6.3

6.0

6.9

H

Aliphatic protons, α to aromatic ring

2.0-3.0

18.2

14.2

H

Aliphatic protons, β to aromatic ring

1.0-2.0

67.2

62.9

H

Aliphatic protons, γ and further to aromatic ring

0.5-1.0

8.6

16.0

484 485

26


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486

Energy & Fuels

Table 7. Mass assignment of initial pyrolysis products from NMH coal, RCYC and ECYC Types Sulfur-containing compounds Alkene

n-Alkyl benzene

n-Alkyl phenol

n-Alkyl dihydroxybenzene

n-Alkyl naphthalene

n-Alkyl naphthol

n-Alkyl phenanthrene

n-Alkyl phenanthrenol/ anthracenol

MW

Compound

Formula

34.08

Hydrogen sulfide

H2S

48.11 42.05 56.06

Methyl mercaptan Propene Butane

CH3HS C3H6 C4H8

70.08

Pentene

C5H10

78.05

Benzene

C6H6

92.06

C1 alkyl benzene

C7H8

106.08

C2 alkyl benzene

C8H10

120.09

C3 alkyl benzene

C9H12

94.04

Phenol

C6H6O

108.06

C1 alkyl phenol

C7H8O

122.07

C2 alkyl phenol

C8H10O

136.09

C3 alkyl phenol

C9H12O

110.04

Dihydroxybenzene

C6H6O2

124.05

C1 alkyl dihydroxybenzene

C7H8O2

138.07


C8H10O2

152.08


C9H12O2

128.06

Naphthalene

C10H8

142.08

C1 alkyl naphthalene

C11H10

156.09


C12H12

170.11


C13H14

144.06

Naphthol

C10H8O

158.07

C1 alkyl naphthol

C11H10O

172.09

C2 alkyl naphthol

C12H12O

186.10

C3 alkyl naphthol

C13H14O

178.08

Phenanthrene/anthracene

C14H10

192.09

C1alkyl phenanthrene/anthracene

C15H12

206.11

C2 alkyl phenanthrene/anthracene

C16H14

220.13

C3 alkyl phenanthrene/anthracene

C17H16

194.07

Phenanthrenol/anthracenol

C14H10O

208.09

C1 alkyl phenanthrenol/anthracenol

C15H12O

222.10


C16H14O

236.12


C17H16O

487

27


Energy & Fuels 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 58 59 60

488 489

Figure captions

490 491

Figure 1. Extract yield of MAE with CYC at different time (a) and comparison of MAE and Soxhlet

492 493

Figure 2. TG (a) and DTG (b) curves of NMH coal, its extracts and residues by MAE with CYC and

494

Figure 3. GPC curves of two extracts

495

Figure 4. GPC curve fitting analysis of ECYC and ETHF.

496

Figure 5. FT-IR spectra of NMH coal, its extracts and residues by MAE with CYC and THF

497 498

Figure 6. 13C NMR spectra and their fitting curves for NMH coal and its extracts by MAE with CYC and

499

Figure 7. 1H NMR spectra of two extracts

500

Figure 8. Mass spectra of initial pyrolysis products from NMH coal, RCYC and ECYC by Py-VUVPI-MS

501

Figure 9. Relative content of dominate initial pyrolysis products from NMH coal, RCYC and ECYC

extraction (b)

THF

THF

502

28


Page 28 of 37

Page 29 of 37

(a) 10

(b)

CYC

8

8

6

6

4 2 0

503

10

Yield (wt.%)

Yield (wt.%)

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 58 59 60

Energy & Fuels

MAE Soxhlet CYC THF

4

THF

2

15

30 45 Time (min)

0

60

45 min

6h

45 min Time

6h

504

Figure 1. Extract yield of MAE with CYC at different time (a) and comparison of MAE and Soxhlet

505

extraction (b)

506

29


Energy & Fuels

100

Rate of weight loss (%/oC)

(a)

80 Weight (wt%)

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 58 59 60

60

NMH RTHF

40

RCYC ETHF

20

ECYC 0

507 508

200

400 600 Temperature (oC)

(b)

0.0 -0.1

NMH RTHF

-0.2

RCYC ETHF

-0.3 -0.4

800

Page 30 of 37

ECYC 0

200

400 600 Temperature (oC)

800

Figure 2. TG (a) and DTG (b) curves of NMH coal, its extracts and residues by MAE with CYC and

509

THF

510

30


80

340

690

1870

Intensity (a.u.)

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 58 59 60

Energy & Fuels

19500

Page 31 of 37

ECYC ETHF

10

511 512

15

20

25

30

Time (min)

Figure 3. GPC curves of two extracts

513

31


35

Energy & Fuels

Raw spectrum Fitted spectrum

ECYC

Intensity (a.u.) 10

514 515

Raw specrum Fitted spectrum

ETHF

Intensity (a.u.)

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 58 59 60

Page 32 of 37

15

20 25 Time (min)

30

35

10

15

20 25 Time (min)

Figure 4. GPC curve fitting analysis of ECYC and ETHF.

516

32


30

35

Page 33 of 37

NMH

Transmittance(a.u.)

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 58 59 60

Energy & Fuels

1708 1604 1450

2920 2850

807 725

RTHF ETHF

3418

1374

RCYC ECYC

4000 3500 3000 2500 2000 1500 1000 500

517 518

Wavenumber (cm-1) Figure 5. FT-IR spectra of NMH coal, its extracts and residues by MAE with CYC and THF

519

33


Energy & Fuels

NMH

Intensity

Raw spectrurm Fitted spectrum

200

150

100

50

0

Chemical shift /ppm

520


Intensity

ECYC

220

200

200

180

160

140

120

100

150 100 Chemical shift /ppm

521

50

0


ETHF

Intensity

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 58 59 60

Page 34 of 37

220

200

522 523 524

200

180

160

140

150

120

100

100

50

0

Chemical shift /ppm

Figure 6. 13C NMR spectra and their fitting curves for NMH coal and its extracts by MAE with CYC and THF

525

34


Page 35 of 37 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 58 59 60

Energy & Fuels

H HArU

ECYC

H

H

H

ETHF

10

526 527

9

8

7

6

5 4  (ppm)

3

2

Figure 7. 1H NMR spectra of two extracts

528

35


1

0

Energy & Fuels

20

40

60

80

100

120

140

160

180

200

n-alkyl benzene

Alkene

220

240

NMH n-alkyl-phenanthrene/ anthracene

H2S

Signal intensity (a.u.)

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 58 59 60

Page 36 of 37

RCYC

n-alkyl phenol n-alkyl naphthalene

n-alkyl-phenanthrenol/ anthracenol

n-alkyl dihydroxybenzene

ECYC n-alkyl naphthol

CH3SH

20

529 530

40

60

80

100

120

140

160

180

200

220

240

m/z

Figure 8. Mass spectra of initial pyrolysis products from NMH coal, RCYC and ECYC by Py-VUVPI-MS

531

36


Page 37 of 37

Alkenes n-alkyl benzene n-alkyl phenol n-alkyl dihydroxy benzene n-alkyl naphthalene n-alkyl naphthol n-alkyl phenanthrene / anthracene n-alkyl phenanthrenol / anthracenol

1.6

NMH coal

0.8

Relative Intensity

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 58 59 60

Energy & Fuels

0.0

RCYC

1.6 94

56

0.8

106

78

122

124 152

128

156

144

172

178

206

194

222

0.0 1.6 0.8

70 92

120

ECYC

192

108

42

110 136

138 142

170

158

186

220

208 236

0.0

m/z

532 533

Figure 9. Relative content of dominate initial pyrolysis products from NMH coal, RCYC and ECYC

534 535 536

37


Structural Features and Pyrolysis Behaviors of Extracts from

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