New Insights into the Primary Reaction Products of Naomaohu Coal

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New Insights into the Primary Reaction Products of Naomaohu Coal via Breaking Weak Bonds with Supercritical Ethanolysis Shisheng Liang, Yucui Hou, Weize Wu, Li Li, and Shuhang Ren Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01154 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Energy & Fuels

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New Insights into the Primary Reaction Products of Naomaohu

2

Coal via Breaking Weak Bonds with Supercritical Ethanolysis

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Shisheng Lianga, Yucui Houb, Weize Wua,*, Li Lia, Shuhang Rena

4

aState

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Technology, Beijing 100029, China

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b Department

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ABSTRACT:

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Coal is an important energy source in the world, and its chemical structure is the basis

9

of its application, especially for its pyrolysis and liquefaction. Supercritical

10

ethanolysis is a type of chemical extraction which can effectively depolymerize some

11

weak bonds in organic matter. In this work, new insights into the primary products of

12

Naomaohu coal were studied with supercritical ethanolysis. The non-covalent bonds

13

and weak covalent bonds (such as ether bonds and ester bonds) in the coal were

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broken to yield small molecular compounds (SMCs) with a conversion of 70.3% (dry

15

and ash-free base) at 370 °C. SMCs including esters, alcohols, aldehydes, ethers,

16

ketones, hydrocarbons (aromatics and aliphatic hydrocarbons), acids, phenols, and

17

heteroatom compounds were identified quantitively through gas chromatography/

18

mass spectrometry (GC/MS). Fourier transform infrared spectroscopy (FTIR) and 13C

19

nuclear magnetic resonance (NMR) were used to characterize the coal’s structure and

20

its ethanolysis residues. The structure characteristic of the coal was deduced through

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analyzing the SMCs and residues. Interestingly, the SMCs can reflect the primary

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reaction products of the coal during its pyrolysis or liquefaction.

Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

of Chemistry, Taiyuan Normal University, Shanxi 030619, China

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1

ACS Paragon Plus Environment

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1. INTRODUCTION

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Coal resource is abundant in the world. Studying coal structure not only has

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important theoretical significance, but also has important guiding significance for coal

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processing and utilization, such as oxidation1, thermal dissolution2, pyrolysis3,4, and

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liquefaction5-7. More importantly, understanding the structure of coal or the

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composition of its primary reaction products can provide information for its pyrolysis

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and liquefaction reaction, which are two essential ways to obtain valuable chemicals

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and fuel oil from coal. As a basic reaction of coal utilization, pyrolysis can break coal

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structure to form gas, tar, and char at temperatures above 500 °C, while the pyrolysis

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products can reflect some aspects of coal structure8,9. For direct coal liquefaction,

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coal can be converted to liquid fuel by hydrocracking in the presence of hydrogen,

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solvent, and catalyst at high pressures and temperatures. The liquefaction residue,

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heavy intermediates such as asphaltene and preasphaltene, and liquefied oil products

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were analyzed and characterized to deduce some information of coal structure10-12. As

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we know, coal has a three-dimensional structure formed by a number of basic

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structural units connected by chemical bonds. The connection bonds between the

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structural units are achieved through ester bond, ether bond, thioether bond, or Cal−Cal

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bond. However, pyrolysis and liquefaction involve complex free radical reactions,

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including the generation of free radicals (the primary reactions) via cracking the

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covalent bonds and the coupling of free radicals (the second or multiple reactions) to

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form volatiles and char. Due to the interferes of the second reactions, the pyrolysis

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and liquefaction products cannot reflect the primary reaction products during the

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depolymerization process of coal structure. It is essential to select a method to

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effectively crack the weak bonds such as ester and ether bonds, and obtain the

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primary reaction products. 2


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Energy & Fuels

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Supercritical alcoholysis is a type of chemical method which can effectively

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depolymerize some weak bonds in the organic matter of coal or fossil fuel under mild

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reaction conditions2,13-16. For example, Li et al. reported that the maximum yield of

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the ethanol-soluble portion (ESP) of Zhaotong lignite ethanolysis was 64.9% at

53

305 °C, and the ESP included esters, alcohols, acids, and some other compounds,

54

which can reflect the structural characteristics of Zhaotong lignite14. Liu et al.

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conducted the supercritical ethanolysis of Huadian kerogen and found that 87.4% of

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organic matter was converted at 375 °C by breaking the weak bonds to small

57

molecular compounds, including aliphatic acid esters, aliphatic acids, and other

58

compounds, which can deduce the original structure of Huadian kerogen13. Fan et al.

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analyzed the ethanolysis products of Dongming lignite, and found that supercritical

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ethanol not only could depolymerize the macromolecular structure of coal, but also

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could induce the release of O- and N-containing compounds via hydrogen bond

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disruption, alcoholysis and alkylation16.

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Naomaohu coal is a kind of sub-bituminous coal of China and has been studied

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from many aspects, such as extraction17,18, pyrolysis19-23, and liquefaction24-27. For

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example, Li et al.17 conducted the degradative solvent extraction of three low-rank

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coals including Naomaohu coal with 1-methylnaphthalene at 350 °C and found that

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using a recycled mixture solvent is feasible for the extraction of low-rank coals. Xu et

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al.19 investigated the pyrolysis of Naomaohu coal with an infrared heating device with

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high heating rates and found that the heating rates and temperature greatly affected

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the yield and composition of volatiles. Hao et al.27 studied the preheating stage of

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direct liquefaction of Hami coal (Naomaohu coal is a kind of Hami coal) in tetralin at

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a temperature range of 200–350 °C and found that the oxygen-containing functional

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groups were reduced after preheating. However, the products of weak bond cleavage 3


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in Naomaohu coal, namely primary reaction products, have not been qualitatively and

75

quantitatively analyzed and studied in published papers. It is important to find a

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proper method to gain insights into primary reaction products, which can not only

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understand the coal structure but also provide information for coal liquefaction and

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pyrolysis. Furthermore, this can give some guidance for the selection of coal for direct

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liquefaction.

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In this work, supercritical ethanolysis was used to break the weak bonds (mainly

81

O-containing functional groups) of Naomaohu coal. The resulted products dissolved

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in ethanol were analyzed and identified with gas chromatography/mass spectrometry

83

(GC/MS). Meanwhile, the raw coal and ethanolysis residues were also characterized

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with

85

spectroscopy (FTIR), and elemental analysis to show the structure change during

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ethanolysis. The weak bonds distribution and the structure information of Naomaohu

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coal were revealed by combining the analysis results of ESPs and ethanolysis

88

residues.

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2. EXPERIMENTAL

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2.1. Materials and methods

13C

nuclear magnetic resonance (13C NMR), Fourier transform infrared

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Naomaohu coal used in this work was supplied from Naomaohu Coal Field,

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Hami, China. The ultimate and proximate analyses of Naomaohu coal were carried

93

out, and the results are shown in Table 1. The Naomaohu coal sample was pulverized

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to pass through a 200 mesh sieve (sieve particle sizes < 74 μm). Ethanol (≥ 99.7%)

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was supplied by Beijing Tongguang Fine Chemicals Co., Ltd., Beijing, China without

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further purification.

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Ethanolysis experiment was conducted in a 50 cm3 high-pressure mechanical 4


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agitation batch reactor supplied by Haian Petroleum Scientific Research Co. Ltd.,

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Jiangsu, China. A coal sample of 1.00 g and 30.0 cm3 ethanol were put into the

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reactor. Then the reactor was sealed and was purged with nitrogen three times to

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remove the inside air and finally 1 MPa nitrogen was left as the protection gas. Then

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it was heated to a desired temperature (320, 340, 360, 370, or 380 °C) in about 30

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minutes and the finally reaction temperature lasted for 2 h. Ethanolysis reactions at

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different temperatures had different final pressures, and the final reaction pressures at

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temperatures of 320, 340, 360, 370, and 380 °C were about 10.2, 10.9, 11.6, 12.2, and

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13.2 MPa, respectively. When the reaction was finished, the reactor was rapidly

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cooled to room temperature in cold water bath. Then the reaction mixture was taken

108

out and separated into a filtrate and a residue through membrane filter with pore size

109

of 0.45 μm. The filtrate was concentrated with a rotary evaporator to afford ESP. For

110

convenience, ESPs obtained at 320 °C, 340 °C, 360 °C, and 370 °C were symbolized

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as ESP320, ESP340, ESP360, and ESP370, respectively. The corresponding residues were

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dried in a vacuum oven at 60.0 °C for 3 h and symbolized as Re320, Re340, Re360, and

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Re370, respectively. The conversion of the coal (C) was calculated according to the

114

equation (1).

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𝐶=

𝑚𝑐𝑜𝑎𝑙 ― 𝑚𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑚𝑐𝑜𝑎𝑙,𝑑𝑎𝑓

× 100%

(1)

116

where mcoal and mresidue refer to the masses of original coal and ethanolysis residue,

117

respectively, and mcoal,daf refers to the dry and ash free mass of original coal. Each

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experiment was repeated at least for two times and the experimental deviations were

119

less than 3%.

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2.2. TGA analysis

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Pyrolysis experiment of Naomaohu coal was conducted in a STA7300 thermal

122

analyzer. About 4.50 mg of Naomaohu coal was put into the ceramic crucible and 5


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heated from room temperature to 900 °C at a heating rate of 10 °C/min with argon as

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carrier gas and the flow rate was 300 mL/min.

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2.3. GC/MS analysis

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The ESP was injected neatly into an Agilent 7890B-5977A (GC/MS) system

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equipped with a HP-5 capillary column (30.0 m × 250 μm × 0.25 μm) for quantitative

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product analysis. High purity helium was used as carrier gas and the flow rate was 1

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ml/min. The heating program was set as follows: initial temperature was 60 °C and

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kept for 3 min, then the temperature rose to 150 °C at the rate of 10 °C/min, and

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150 °C was kept for 3 min. After that, the temperature started from 150 °C to 290 °C

132

at the rate of 10 °C/min and the final temperature was kept for 1 min. The mass

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spectra of the measured compounds were compared with the standard spectrogram in

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the NIST11 library, and the molecular structure of the compounds was determined

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according to the matching degree. The relative content of various compounds in ESP

136

was determined by area normalization method.

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2.4. 13C NMR analysis

138

Structural parameters of various carbons in Naomaohu coal and ethanolysis 13C

139

residues were obtained by

140

spectrometer with a

141

cross-polarization contact time were set as 7 s and 2 ms, respectively. PeakFit

142

software was used to fit the 13C NMR data.

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2.5. FTIR analysis

13C

NMR, which was performed in a Bruker AV-300

frequency of 67.8 MHz. Cyclic delay time and

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FTIR was used to analyze the functional groups in Naomaohu coal and

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ethanolysis residues. The apparatus was Thermo Scientific Nicolet 6700 spectrometer

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with a 4 cm−1 resolution. Samples were prepared through KBr pressing method with 1 6


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mg sample and 100 mg KBr and scanned in the range of 400−4000 cm−1.

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2.6. Elemental analysis

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Elemental analysis was carried out using an Elementar Vario MICRO cube in the

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mode of CHNS.

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3. RESULTS AND DISCUSSION

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3.1. TGA analysis of Naomaohu coal

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Differential thermogravimetric (DTG) curve of each coal can reflect the

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temperature range required for the rupture of different covalent bonds in the coal28,29.

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In this work, the DTG curve of the coal was fitted with six sub-curves through

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PeakFit software to reveal the information of covalent bonds in Naomaohu coal. The

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results are shown in Figure 1. The attribution of peaks is listed in Table 2. Peak 1 with

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a peak temperature of 205 °C is generated from the decarboxylation and release of

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bonded water. According to the previous study29, peak 2 with a peak temperature of

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361 °C is assigned to the crack of weak bonds such as Cal−O, Cal−N, and Cal−S. The

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assignment of peak 3 with a peak temperature of 439 °C mainly contains the rupture

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of Cal−Cal bonds. The full width at half maximum for peak 3 was 65 °C, which means

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that Cal−Cal bonds in Naomaohu coal start to crack at temperatures above 400 °C.

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Peak 4 with a peak temperature of 519 °C can be assigned to the cleavage Car−O and

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Car−Cal bonds. CO2 released from the decomposition of carbonates in Naomaohu coal

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leads to the formation of peak 5 with a peak temperature of 628 °C. With the increase

167

of pyrolysis temperature, the condensation of aromatic rings occurs, and hydrogen is

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released, and this leads to the formation of peak 6 with a peak temperature of 741 °C.

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3.2. Conversion of Naomaohu coal with ethanolysis at different temperatures 7


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Previous reports have shown that reaction temperature was the critical

171

influencing factor for ethanolysis reaction13,30. In this work, the ethanolysis

172

experiment was conducted at different temperatures from 320 °C to 380 °C, and the

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corresponding conversion was shown in Figure 2. From Figure 2, it can be seen that

174

the conversion of Naomaohu coal starts to increase significantly from 320 °C to

175

360 °C, then the rise speed slows down from 360 °C to 370 °C. When the temperature

176

is further increased to 380 °C, the conversion hardly changes, suggesting the

177

decomposition of weak bonds is finished31. To obtain the highest yield of small

178

molecular compounds (SMCs) via weak bonds cleavage and reduce the possibility of

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slight decomposition of SMCs due to high temperature, 370 °C was selected as the

180

optimal ethanolysis temperature. Based on the previous analysis, ethanolysis

181

experiments with temperatures higher than 380 °C were not conducted because high

182

temperatures above 400 °C may lead to severe pyrolysis reaction with the cleavage of

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Cal−Cal bonds or Cal−H bonds (as discussed in the above subsection) and the second

184

reactions may occur, which will prevent the identification and characterization of

185

primary reaction products. Combined the above TGA analysis results and ethanolysis

186

experiment results, the maximum conversion was 70.3% (dry and ash-free base) at

187

370 °C for 2 h, where the weak covalent bonds were broken and SMCs were formed

188

as ESP. After the reaction, the gas products were about 0.24 MPa with a volume of 15

189

cm3 at 20 °C. The gas products which are likely generated from decomposition of

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some oxygen functional groups can be ignored for their small amounts compared with

191

the ESP13,32.

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3.3. GC/MS analysis of ESPs

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GC/MS has been widely used in identifying compounds of coal extract and

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thermal dissolution33-36. The total ion chromatogram (TIC) of ESP320, ESP340, ESP360, 8


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and ESP370 are shown in Figures S1-S4 in the Supporting Information for comparison.

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Through GC/MS analysis and the comparison with mass spectrogram in the NIST11

197

library, all detectable compounds originated from Naomaohu coal in all 4 ESPs were

198

identified. The corresponding component information of ESP320, ESP340, ESP360, and

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ESP370 through GC/MS analysis is shown in Tables S1-S4 in the Supporting

200

Information. Through the comparison of compositions of ESP320, ESP340, and ESP360

201

with those of ESP370, it can be found that the compositions of latter cover those of

202

former. This means that more components were formed after the cleavage of weak

203

bonds with the increase of temperature (or the concentrations of some former

204

components were increased). Through above analysis, ESP370 not only has the highest

205

yield but also has the most abundance components, so ESP 370 was chosen as the key

206

point to be studied in detail to show the main primary reaction products via weak

207

bonds cleavage of Naomaohu coal.

208

As shown in Table S4, total 95 compounds originated from raw coal in ESP370

209

were identified. The information for the 95 compounds such as structural formula and

210

name are shown in Table S4 in detail. These compounds can be divided into esters,

211

alcohols, aldehydes, ethers, ketones, hydrocarbons (aromatics and aliphatic

212

hydrocarbons), acids, phenols, and heteroatom compounds. Relative contents of the 9

213

group components mentioned above, which are obtained by area normalization

214

method, are shown in Figure 3. From the figure, it can be seen that esters, alcohols,

215

and aldehydes have high relative contents with 28.6%, 27.6%, and 19.3%,

216

respectively.

217

Esters in ESP mainly contain fatty acid ethyl ester and fatty acid methyl ester.

218

The former should be derived from the esterification of carboxylic acid in Naomaohu

219

coal with ethanol or the trans-esterification of esters originally existing in Naomaohu 9


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220

coal with ethanol. The latter with a small content of 3.16% properly exists in

221

Naomaohu coal in a free state, such as 6, 9, 12, 15-docosatetraenoic acid, methyl

222

ester, 4,7-octadecadiynoic acid, methyl ester. Ethanolysis reaction paths including

223

esterification can be seen in the published work2. The relationship between carbon

224

number in esters and acids (carbon number in ethanol was not included in fatty acid

225

ethyl esters) and the relative contents of esters and acids are shown in Figure 4. As

226

shown in Figure 4, the carbon numbers of these esters and acids range from 4 to 24,

227

including n-alkanoic acid esters, unsaturated alkanoic acid esters, and small amounts

228

of naphthenic acid esters. It is noteworthy that some short-chain aliphatic acids like C4

229

and C5 are found as liquid products, while they usually exist as gas products after

230

pyrolysis37,38. Meanwhile, the relative contents of n-hexanoic acid ethyl ester and

231

hexenoic acid ethyl ester were obviously higher than other esters, and the results were

232

consistent with the esters distribution in ESP of Zhaotong lignite ethanolysis39, which

233

implies that there are some similarities in the parent structure of Naomaohu coal and

234

Zhaotong lignite.

235

As shown in Table S4, there are 24 alcohols detected in the ESP, including

236

n-alkanoic alcohols, unsaturated alkanoic alcohols, aromatic alcohols, and naphthenic

237

alcohols. Low carbon alcohols with carbon number below 10 were the major

238

components in alcohols, especially, 1-methoxy-2-propanol with a quite high relative

239

content of 14.1%. The reason for the dissolution of alcohols is that ethanol destroys

240

the hydrogen bonds between the alcohols inherent and the large molecular skeleton of

241

Naomaohu coal or the transesterification between the esters of Naomaohu coal and

242

the ethanol. Aldehydes and ketones were also detected in the ESP with a total relative

243

content of 22.6%, particularly, 3-methyl-pentanal has a high relative content of

244

17.9%. Aromatics and olefins in the ESP have a total relative content of 5.40%. 10


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Olefins may originate from decarboxylation reaction of unsaturated alkanoic acids

246

and esters in Naomaohu coal during coal maturation process40. The existence of

247

aromatics in the ESP implies that some compounds with single or double rings, which

248

are connected to solid aromatic clusters with weak bonds, dissolved in ethanol after

249

ethanolysis, and this can be validated from the analysis of ethanolysis residues in the

250

next section.

251

The relative contents of ethers and phenols are 7.01% and 1.90%, respectively.

252

The dissolution of some phenolic compounds may be due to the destruction of the

253

hydrogen bonds between phenolic compounds and carboxyl groups or hydroxyl

254

groups in the macromolecules skeleton of Naomaohu coal. Meanwhile, ethanol can

255

destroy the Cal−O bonds in coal to produce phenolic compounds. In addition, two

256

nitrogenous compounds were found in the ESP. However, sulfur-containing

257

compounds were not detected in ESP.

258

To further quantitatively analyze the ethanolysis products, hexanoic acid ethyl

259

ester was used as the standard substance with a known concentration. The total mass

260

yield of 95 compounds was 52.7% (daf, water and gas were not included). The

261

deviation between mass conversion (70.3%) and total mass yield may originate from

262

the loss of water and gas, or analysis error of GC/MS, such as the restrictions on

263

compound retrieval, the loss of compounds below C4 (solvent delay effect) or the loss

264

of compounds with high molecular weight which cannot be detected by GC/MS

265

because they are difficult to be vaporized. However, these compounds detected by

266

GC/MS can reflect the main structural characteristics of raw coal although there are

267

some deviations.

268

As mentioned above, 70.3% of Naomaohu coal was degraded into SMCs and

269

these SMCs can be dissolved in ethanol. However, the oil yield with coal 11


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low-temperature pyrolysis at 450–650 °C reaches a maximum at 20.0%

271

solid products are char, semi-coke, and coke (all smokeless solid fuels). At the initial

272

stage of pyrolysis, weak bonds are first broken, then big molecules and SMCs can be

273

yielded as states of radicals. The big molecules are difficult to be accurately detected

274

and identified with GC/MS for their high boiling point and high polarity, but the

275

SMCs can be detected and identified with GC/MS, as discussed above. With

276

increasing the reaction time under pyrolysis condition, these SMCs obviously undergo

277

further condensation reactions, which lead to the formation of gas and char, thus

278

giving a low oil yield. In direct coal liquefaction, big molecules and SMCs are also

279

yielded at the initial stage of pyrolysis via breaking weak bonds. These big molecules

280

and SMCs are hydrogenated by hydrogen donor solvent or hydrogen, which can

281

prevent most of them from condensation reactions or secondary reactions. However,

282

the severe reaction condition of direct coal liquefaction makes it also happen for the

283

secondary reactions of the products from weak bond cleavages and strong covalent

284

bond cleavages, such as Cal−Cal and Cal−Car. So it is difficult to distinguish the

285

primary reaction products and the secondary reaction products. But, supercritical

286

ethanolysis can selectively break the weak bonds in coal structure under relatively

287

mild conditions. As shown in Figures 3 and 4, these SMCs formed with supercritical

288

ethanolysis are easily converted to liquid products by hydrogenation, which suggests

289

that Naomaohu coal is favorable to liquefaction. In the next section, 13C NMR, FTIR,

290

and elemental analysis were used to analysis the ethanolysis residues of Naomaohu

291

coal to evaluate the rupture of weak bonds during the ethanolysis process.

292

3.4. Analysis of carbon skeleton structures of ethanolysis residues by 13C NMR

293 294

13C

and the

NMR analysis technique has been widely used in characterizing the carbon

skeleton structure of fossil fuel such as coal and oil shale42-44. After ethanolysis, 12


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SMCs are extracted into the ethanol solvent while the residues cannot be dissolved in

296

the ethanol. It is necessary to acquire the structure information of Naomaohu coal and

297

ethanolysis residues to know the variety of weak bonds in Naomaohu coal when

298

extracted at different temperatures. Figure 5 shows the

299

fitting curves of Naomaohu raw coal, Re320, Re340, Re360, and Re370. The carbon types

300

in Naomaohu coal and residues are ascribed to aliphatic (0−90 ppm), aromatic

301

(90−165 ppm), and carbonyl carbon species (165−220 ppm) according to different

302

ranges of chemical shift. The carbon skeleton structures of the Naomaohu coal and

303

residues can be better understood by the calculation of the structural parameters, such

304

as the aromatic carbon ratio (far), the aliphatic carbon ratio (fal), the average methylene

305

chain length (Cn), the aromatic ring substitution degree (δ), and the ratio of aromatic

306

bridge carbon (Xb). The detailed fitting results of different carbon types are listed in

307

Table S5. Several significant structure parameters have been calculated according to

308

Table S5 and are listed in Table 3.

13C

NMR spectra and the

309

As shown in Table 3, far and fal of Naomaohu raw coal are 48.0% and 42.5%,

310

respectively. In other words, there are 48 aromatic carbons and 43 aliphatic carbons

311

per 100 carbon atoms in Naomaohu coal. After ethanolysis at 370 °C, fal decreases

312

from 42.5% to 33.9%, while far increases from 48.0% to 64.4%. When weak bonds

313

such as O−C=O or Cal−O are broken, some new alcohol or ether compounds can be

314

produced which can dissolve in ethanol. Meanwhile, some alcohols compounds exist

315

in Naomaohu coal with hydrogen bonds and some aliphatic compounds that exist in

316

free-state can also dissolve in ethanol through ethanolysis. However, the aromatic

317

clusters connected with each other with Cal-Cal cannot be broken and left in the

318

residues. As a result, these aspects make fal decrease while far increase.

319

As shown in the Table S5, average methylene chain length Cn = fal2 / farS (fal2 13


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

320

refers to the methylene carbon ratio, while farS refers to the aromatic branched carbon

321

ratio). Previous studies show that Caryl−O band is harder to be broken and Cal−O bond

322

is easier to be broken for its smaller bond energy at the range of 150–230 kJ/mol29.

323

When Car−O−Cal and Cal−O−Cal bands are broken, the new Cal−O−ethyl, Car–OH, and

324

Cal–OH bands are formed with the ethoxyl groups and hydrogen supplied by ethanol.

325

Meanwhile, some new formed compounds with CH2 group can dissolve in ethanol,

326

which leads to the decrease of fal2, while the aromatic clusters connected with Cal-Cal

327

bonds cannot be broken and left in the residues as mentioned above, leading to the

328

rise of farS values. Therefore, the Cn value decreases from 2.35 to 0.865.

329

Xb is used to determine the condensation degree of aromatic structures in coal.

330

As shown in Table 3, the Xb values of Naomaohu raw coal is 0.219, which is close to

331

the Xb value of naphthalene (Xb = 0.200), indicating that the average number of

332

aromatic rings in aromatic clusters of Naomaohu coal is 2. The results show that the

333

Xb values of ethanolysis residues increase with the increase of ethanolysis reaction

334

temperature and the Xb value for Re370 is 0.352, which implies that aromatic clusters

335

with high condensation degree cannot be dissolved in the ethanol solvent while some

336

SMCs including single-ring aromatic compounds are soluble after ethanolysis as

337

shown in Table S4. The δ values for Naomaohu raw coal and Re370 are 0.458 and

338

0.335, respectively. The decrease of δ is due to the decarboxylation reaction and the

339

dissolution of small aromatic compounds with substituent groups. Interestingly, the

340

molar percent of oxygen-aromatic carbon (farO) is decreased after ethanolysis (from

341

14.8% of raw coal to 9.02% of Re370) while the molar percent of aromatic branched

342

carbon (farS) is increased from 7.20% of raw coal to 12.6% of Re370. Some phenolic

343

compounds formed during ethanolysis can be dissolved in ethanol, which can lead to

344

decreasing farO of the residue. However, small amounts of phenolic compounds were 14


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Energy & Fuels

345

detected with GC/MS, and this may be explained by the following two reasons. On

346

the one hand, high boiling point or high polarity of compounds such as catechol,

347

indophenol, and naphthol make the volatility of these phenolic compounds lower. On

348

the other hand, small amounts of these volatilized phenolic compounds are more

349

difficult to be detected with GC/MS in the substrate of volatile compounds39.

350

Meanwhile, aromatic clusters connected by methylene chains, cannot be broken with

351

ethanolysis because of the strong Cal−Cal bond with a high bond energy of 250-320

352

kJ/mol29, resulting in the increase of farS in ethanolysis residues.

353

3.5. Functional groups analysis of ethanolysis residues by FTIR

354

13C

NMR focuses on the analysis of carbon skeleton structure, while FTIR

355

focuses on the study of the existence of functional groups according to the vibration

356

frequency. Combining these two analysis technologies can more comprehensively

357

obtain the structural characteristics of Naomaohu coal and ethanolysis residues. FTIR

358

spectra of Naomaohu raw coal, Re320, Re340, Re360, and Re370 are shown in Figure S5.

359

The connection between aliphatic and cyclic fragments in Naomaohu coal is usually

360

through ether, ester, thioether and Cal−Cal bond. In the FTIR spectra, the wavenumber

361

range of 1800~1000 cm−1 belongs to the absorption vibration region of

362

oxygen-containing functional group (OCFG). There are usually four types of OCFGs

363

(C−O−C, C−OH, C=O, and O−C=O) in coal. In order to clearly understand the

364

change of weak bonds at different temperatures during ethanolysis (OCFGs were

365

analyzed in detail for their dominant content), infrared spectra region of 1800−1000

366

cm−1 was fitted with 18 sub-curves as shown in Figure S6. The assignment of these

367

curves was determined according to the method provided in the published work43 and

368

the detailed fitting results are shown in Table S6. The total relative content of four

369

OCFGs is set as 100% and the relative content change of these functional groups in 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

370

raw coal and residues at different ethanolysis temperatures is displayed in Figure 6.

371

It can be concluded from Figure 6 that the O−C=O functional group with peaks

372

at around 1710 cm−1 was broken after ethanolysis and its relative content decreased

373

from 13.9% of raw coal to 2.40% of Re370. Meanwhile, the peaks near 1650 cm−1 can

374

be assigned to C=O functional group and its relative content decreased from 13.3% of

375

raw coal to 2.40% of Re370 because of cleavage reaction during ethanolysis. The break

376

of O−C=O and C=O can yield some fragments and these fragments can undergo

377

esterification or trans-esterification with ethanol, resulting in SMCs which can be

378

dissolved in the ethanol. The existence of many fatty acid ethyl esters in ESP implies

379

that aliphatic species are mainly connected to the macromolecular framework of

380

Naomaohu coal with O−C=O and C=O bonds. The relative content of C−O−C and

381

C−OH in residues has a rising trend with the increase of ethanolysis temperature.

382

Attack of nucleophilic oxygen atom in ethanol can lead to the cleavage of Cacyl−O and

383

Cal−O. Meanwhile, ethanol can provide the ethoxy group and hydrogen, thus leading

384

to the formation of new C−OH and C−O−C bonds. This explains why the relative

385

content of C−O−C and C−OH in the residues increases. As shown in Table S4, ethers,

386

alcohols, and phenols are also found in ESP. Through the above analysis, we can

387

conclude that almost all OCFGs are broken during ethanolysis.

388

3.6. Elemental analysis of ethanolysis residues

389

To show the absolute change of ethanolysis residues, the elemental analysis of

390

ethanolysis residues has also been conducted and the results are shown in Table 4.

391

The H/C ratios were calculated according to the elemental analysis results. From

392

Table 4, it can be seen that the H/C ratios of Re320 and Re340 are above 1 because of

393

the formation of new C−O−ethyl and C−OH due to ethanolysis reaction explained in

394

the above content. With the increase of reaction temperature, more hydrogen-rich 16


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Energy & Fuels

395

components were dissolved in ethanol, thus leading to the decrease of H/C, which is

396

consistent with the 13C NMR analysis for decreasing fal of ethanolysis residues.

397

4. CONCLUSION

398

In this work, a novel way was provided to gain insights into the primary reaction

399

products of Naomaohu coal via breaking weak bonds with supercritical ethanolysis.

400

The ethanolysis of Naomaohu coal was conducted at different temperatures from 320

401

to 380 °C for 2 h. The maximum conversion of Naomaohu coal is 70.3% at 370 °C.

402

The non-covalent bonds and weak covalent bonds (such as ether bonds and ester

403

bonds) in Naomaohu coal were broken by ethanolysis to obtain SMCs as the primary

404

reaction products. SMCs can be divided into esters, alcohols, aldehydes, ethers,

405

ketones, hydrocarbons (aromatics and aliphatic hydrocarbons), acids, phenols, and

406

heteroatom compounds.

407

analyze the structural characteristics of Naomaohu raw coal and ethanolysis residues.

408

Compared raw coal and Re370, fal and Cn decrease while far increases. FTIR analysis

409

implies that almost all OCFGs were broken during ethanolysis.

410

ACKNOWLEDGMENTS

411

13

C NMR, FTIR, and elemental analysis have been used to

We thank Professors Zhenyu Liu and Qingya Liu for their help. The project is

412

financially

supported

413

(2017YFB0602401).

414

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by

the

National

Key

R&D

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coals by the mixture of low molecular weight extract and solvent as recycled solvent.

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extractable compounds on the structure and pyrolysis behaviours of two Xinjiang

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coal. J. Anal. Appl. Pyrol. 2018, 133, 128-135.

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alkali sodium on the catalytic performance of red mud during coal pyrolysis. Fuel

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Catalytic upgrading of volatiles from coal pyrolysis over sulfated carbon-based

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relationship between benzene carboxylic acids from coal via selective oxidation and

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List of Table Captions

550 551

Table 1. Proximate and ultimate analyses of Naomaohu coal.

552

Table 2. The fitting peaks attribution of DTG curve of Naomaohu coal.

553

Table 3. 13 C NMR structure parameters of Naomaohu coal and ethanolysis residues.

554

Table 4. Elemental analysis of raw coal and ethanolysis residues.

555

List of Figure Captions

556

Figure 1. DTG profiles of Naomaohu coal and the fitting by 6 sub-curves through

557

multiple Gaussian functions.

558

Figure 2. Effect of temperature on the conversion of Naomaohu coal. Conditions:

559

coal, 1.00 g; ethanol, 30.0 cm3; reaction time, 2 h.

560

Figure 3. Compounds distribution of ESP370.

561

Figure 4. The relationship between relative content and the carbon number of these

562

esters and acids in ESP370.

563

Figure 5.

564

coal and ethanolysis residues. (a) Raw coal, (b) ER320, (c) ER340, (d) ER360, (e) ER370.

565

Figure 6. Relative content of OCFGs for Naomaohu coal and ethanolysis residues.

13C

NMR characterization spectra and their fitting curves of Naomaohu

24


Page 24 of 34

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Energy & Fuels

Table 1. Proximate and ultimate analyses of Naomaohu coal. Proximate analysis/wt %

Ultimate analysis/wt%, in daf basis

Mad

Aad

Vdaf

C

H

Oa

N

S

5.59

7.06

51.8

74.3

5.5

18.5

0.9

0.8

ad: air-dry basis; daf: dry-and-ash-free basis. M: moisture; A: ash; V: volatile matter content. a

By difference.

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

Page 26 of 34

Table 2. The fitting peaks attribution of DTG curve of Naomaohu coal. Peak No.

Attribution

Bond energy （kJ/mol）

1

Release of combined water and decarboxylation

2

Peak temperature （°C）

400

741

26


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Energy & Fuels

Table 3. 13 C NMR structure parameters of Naomaohu coal and ethanolysis residues. Sample

far/%

fal/%

faC/%

Xb

Cn

δ

farO/%

farB/%

farS/%

Naomaohu coal

48.0

42.5

9.41

0.219

2.35

0.458

14.8

10.5

7.20

Re320

47.1

45.7

7.24

0.280

1.75

0.414

10.2

13.2

9.29

Re340

51.2

44.6

4.23

0.307

1.67

0.420

10.9

15.7

10.6

Re360

55.9

42.3

1.68

0.320

1.43

0.338

8.00

17.9

10.9

Re370

64.4

33.9

1.71

0.352

0.865

0.335

9.02

22.7

12.6

The related parameters were calculated according to the equation mentioned in the supporting information.

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

Page 28 of 34

Table 4. Elemental analysis of raw coal and ethanolysis residues. Cad

Had

Nad

Sad

H/C

Naomaohu coal

68.7

5.05

0.837

0.794

0.882

Re320

70.0

6.19

0.920

0.353

1.06

Re340

71.0

6.54

0.950

0.672

1.11

Re360

71.6

5.95

0.910

0.571

0.997

Re370

70.6

5.17

0.920

0.994

0.878

ad: air-dry basis

28


Page 29 of 34

0.0 -0.5

DTG(%/min)

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

1

6

5

2

4

Measured Fitted

-1.0

R 2=0.99

-1.5 -2.0 -2.5 3

-3.0 -3.5

200

400

600

800

Temperature/C

Figure 1. DTG profiles of Naomaohu coal and the fitting by 6 sub-curves through multiple Gaussian functions.

29


Energy & Fuels

80 70 60

Conversion/%

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

50 40 30 20 10 0

320 330 340 350 360 370 380 Temperature/C

Figure 2. Effect of temperature on the conversion of Naomaohu coal. Conditions: coal, 1.00 g; ethanol, 30.0 cm3; reaction time, 2 h.

30


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Page 31 of 34

30

Relative content(%)

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

25 20 15 10 5 0 Es

r te

s s s ls rs nes bon cids nol es m o e d r to h hy h e o A t t a a o h E Ke oc c o e P r Al ld er A et yd H H

s

Components

Figure 3. Compounds distribution of ESP370.

31


Energy & Fuels

16 14

Relative content/%

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

12 10 8 6 4 2 0

4

6

8 10 12 14 16 18 20 22 24

Carbon number

Figure 4. The relationship between relative content and the carbon number of these esters and acids in ESP370.

32


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Page 33 of 34

Measured Fitting R 2=0.99

(a)

Measured Fitting

200

150

100

50

150

200

0

Chemical shift/ppm


(c)

100

50

0

Chemical shift/ppm

Intensity/a.u.

200

(b)

Intensity/a.u.

R =0.99

Intensity/a.u.

Intensity/a.u.

2

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

150

150

50


200

0

(d)

150

100

50

Chemical shift/ppm


200

100

Chemical shift/ppm

0

(e)

100

50

Chemical shift/ppm

0

Figure 5. 13C NMR spectra and their fitting curves of Naomaohu coal and ethanolysis residues. (a) Raw coal, (b) ER320, (c) ER340, (d) ER360, (e) ER370.

33


Energy & Fuels

100

Relative content/%

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

OC=O

C=O

Page 34 of 34

COC

COH

80 60 40 20 0

Raw coal

Re320

Re340

Re360

Re370

Figure 6. Relative content of OCFGs for Naomaohu coal and ethanolysis residues.

34


New Insights into the Primary Reaction Products of Naomaohu Coal

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