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Article

Structural Characterization and Aggregation of the Sub-fractions of Preasphaltene from Direct Coal Liquefaction

Zhicai Wang, Zhijun Zhao, Hengfu Shui, Shibiao Ren, ChunxiuPan, Zhiping Lei, Shigang Kang, Yan Ge, and Jingchen Hu

Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 07 Nov 2014

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1

Structural Characterization and Aggregation of the Sub-fractions of 1

Preasphaltene from Direct Coal Liquefaction 2

Zhicai Wang*, Zhijun Zhao, Hengfu Shui, Shibiao Ren, Chunxiu Pan, Zhiping Lei, Shigang Kang, 3

Yan Ge, Jingchen Hu 4

School of Chemistry and Chemical Engineering, Anhui Key laboratory of Clean Coal Conversion 5

& Utilization, Anhui University of Technology, 243002 Ma’anshan, China 6

Abstract 7

Preasphaltene (PA) defined as tetrahydrofuran (THF) soluble and toluene insoluble is an important 8

intermediate product of direct coal liquefaction (DCL). To investigate the structure of PA can not 9

only improve the DCL technology, but also understand the structure of coal. In this paper, two 10

types of PAs from the DCL residue of 6 t/d Shenhua process developing unit (PDU) and the 11

products of a batch hydro-liquefaction in autoclave were first separated into different sub-fractions 12

by the column chromatography. Then the obtained sub-fractions were characterized by Fourier 13

transform infrared spectroscopy (FTIR), ultra-violet-visible spectroscopy (UV) and fluorescence 14

spectroscopy (FL), matrix-assisted laser desorption/ionization time of flight mass spectrometry 15

(MALDI-TOF-MS) and gel permeation chromatography (GPC), respectively. The results indicate 16

that the PA from DCL mainly exists in the form of aggregates. Two different PAs show similar 17

distribution of sub-fractions, but the PA(B) obtained by the batch liquefaction displays larger 18

molecular weight than the PA(A) from Shenhua PDU technology. A molecule of PA contains two 19

or more aromatic nucleus (fluorophors). Sub-fractions I-III consist of smaller fused aromatic 20

nucleus than the other sub-fractions. In general, their intermolecular aggregations increase from 21

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sub-fraction I to VII, and the aggregation of PA(A) sub-fractions is stronger than that of 22

corresponding PA(B) sub-fractions. 23

Keywords: Preasphaltene, Aggregation, Separation, Characterization, Direct hydro-liquefaction 24

* Corresponding author. Tel. :+86 13955530691; fax: +86 0555 2311822

E-mail: zhicaiw@ahut.edu.cn, wzc0101@sina.com

1. 1. 1. 1. IIIIntroductionntroductionntroductionntroduction 25

Recently, the direct coal liquefaction (DCL) has been unprecedented attention in China in order to 26

obtain an affordable, reliable and sustainable alternative of oil to ensure energy security. The 27

practice of Shenhua DCL demonstration plant has showed that there are still some intractable 28

problems to be settled, especially the separation and utilization of liquefaction residue (DCLR).1 29

Beside of minerals, catalyst and unreacted organic substance of coal, the DCLR contains a great 30

deal of heavy products such as heavy oil, asphaltene (AS), preasphaltene (PA) and semicoke 31

(insoluble products).2 There is about 50 % soluble organic in the DCLR from Shenhua DCL 32

plant.1,3

These soluble organics, especially AS and PA, has important influences on the property of 33

DCLR.4 Therefore, their structural characterizations are important to elucidate coal liquefaction 34

mechanisms, to determine the process for solid-liquid separation and to furnish information on the 35

utilization of DCLR.5 36

In previous works, it has been found that the chemical structure of organic components has 37

significant influence on the viscosity of DCLR6,7

and coal derived liquid8. The molecular 38

interactions involving polar functional groups, especially the hydrogen bonding between the acid 39

and base fractions9, are one of the most important factors. Tanaka et al.

10 investigated the DCLR of 40

Illinois No. 6 coal, suggesting that the coal-derived AS (c-AS) generally has higher aromaticity, 41

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less and shorter the substitution of the peripheral carbons, smaller unit molecular weight (MW) 42

than the petroleum-based AS (p-AS). Meanwhile, the c-AS contains more hydroxyl and pyrrolic 43

groups than the p-AS.11

By the characterization of coal liquids from solid solvent-refined coal 44

product (SCR-I) and liquid solvent-refined coal product (SRC-II), Seshadri et al.3 found that the 45

mixture of AS and PA is ‘oligomeric’ in structure, with aromatic clusters linked by carbon bridges 46

with different functional groups, and ester groups. Recently, Gu et al.12,13

investigated the heavy 47

oil and AS fractions from Shenhua DCLR in a 0.1 t/d bench scale unit (BSU). Their average MWs 48

are 339 amu and 1387 amu, respectively. The AS shows larger scale of fused aromatic nucleus and 49

more side chains of methyl, hydroxyl and ether groups than the heavy oil. Zhong et al.14

found that 50

the H/C ratio, aromaticity and condensation degree of aromatic nucleus of the solvent extracts of 51

DCLR from the Shenhua 6 t/d process developing unit (PDU) are 0.99, 0.694 and 0.675, 52

respectively. 53

Compared to heavy oil and AS, PA shows lower solubility and volatility due to its high MW 54

and polarity. Meanwhile, it is also responsible for the high viscosity of the products. On a weight 55

basis, PA produces a viscosity about twice that of AS fraction.4 Therefore, up to now, the structure 56

of PA has remained generally unknown15

, though its chemical composition, such as elemental 57

compositions and functional groups, had been extensively investigated by the elemental analyses, 58

nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy 59

(FTIR), etc.15-19

For example, Yoshida et al.15

studied the formation and chemical structure of PA 60

in coal hydrogenolysis by solid state cross-polarization magic angle spinning 13

C-NMR 61

spectroscopy, and found that there are aromatic rings of from 1 to 3-5 per condensed aromatic ring 62

system. By comparison of the structural features of AS and PA, Baltisberger et al.16

suggested that 63

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PA contains more condensed aromatic molecules, and the majority of the PA lies in a range from 64

600 to 2500 amu. Allen et al.17

thought that the chloroform-soluble PA is composed of structures 65

with MW in the mass range ≈200-800 amu and predominantly mono- and dihydroxy compounds, 66

as well as non-hydroxy compounds contained pyrrolic functionality. Seshadri et al.3 studied 67

various structural features of a mixture of AS and PA by 13

C-NMR and FTIR, and found that part 68

of the carbonyl groups in the heavy fractions is in esters, which cross-link aromatic clusters. In 69

order to enhance the resolution of solid-state NMR, a high-temperature NMR analysis by applying 70

gated decoupling and the distortionless enhancement by polarization transfer (DEPT) pulse 71

sequence was successfully used to investigate the average aromatic structures of PA and AS from 72

the liquefaction of Victorian brown coal.18

73

Since coal derived PA, even AS, is a mixture consisting of a series of similar macromolecular 74

compounds, the real MW and MW distribution (MWD) is crucial to the understanding of PA 75

structure. According to the results of average MWs determined by gel permeation chromatography 76

(GPC) and average aromatic structures characterized by high-temperature NMR, Masuda et al.18

77

suggested that the average molecules in AS carry two aromatic units such as phenanthrene and 78

pyrene rings and in the PA three aromatic units such as two pyrene and phenanthrene rings, which 79

are connected by biaryl, methylene, naphthene or furan-like linkages. However, the MWD of 80

carbonaceous materials derived from petroleum, coal, bitumen, etc. has been a matter of 81

considerable controversy for many years due to their high polarity and dispersity, poor solubility, 82

and strong aggregation (or association).19

For example, field ionization mass spectroscopy (FI-MS) 83

results of petroleum AS have indicated a MW in the range of 700 amu, while the vapor pressure 84

osmometry (VPO) has produced much larger MW for the same AS, e.g., 4000 amu.20

The 85

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aggregation in certain concentration solution should be one of the key factors resulting in 86

significantly different MWs.19

The formation of nano-aggregate and flocculate should be 87

responsible for higher MW obtained by GPC and VPO.21,22

Meanwhile, Champagne et al.23

found 88

in the GPC measurement of bitumen that the structure of molecule beside of MW also obviously 89

influences the retention volume. In fact, the VPO and the freezing point depression methods 90

measured by the colligative property of solution are also irresistibly subjected to intermolecular 91

aggregation (association) of solute. 23

As an example, it is because of the lack of convincing real 92

MWD that the structure model of petroleum AS, which is ‘island’ model or ‘archipelago’ model, is 93

still in controversy.24-27

94

Mass spectroscopy by field desorption ionization (FD) and laser desorption ionization (LDI) 95

were known as a satisfied method to measure the MW of AS,28,29

but the results of laser 96

desorption ionization mass spectrometry (LDI-MS) has also been questioned owing to the effects 97

of self-aggregation in the plume and different ionization efficiencies. Hortal et al.30

found that low 98

laser energies (≤30 µJ/pluse) can avoid the self-aggregation, especially for coal AS where no trace 99

of cluster formation was observed at low laser energies. Meanwhile, a particularly efficient 100

dilution of the analyte in the sample is required in order to obtain reliable MWD for the 101

monomeric species of strongly self-aggregating compounds.29,31

Dilution of the petroleum AS 102

with organic compounds (e.g., matrices) provides a convenient method to circumvent this 103

problem.18,30

Since the process of matrix assisted laser desorption/ionization (MALDI) almost 104

exclusively generates singly charged ions,32

it is always combined with a time-of-flight (TOF) 105

mass analyzer to resolve large heterogeneous molecules.33-35

In addition, fractionation of complex 106

mixtures by solvent separation or planar chromatography has enabled observation of larger MW 107

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materials with greater clarity.35,36

108

Based on above brief review, the structural characterization of the PA in DCLR is very 109

significant and essential for the optimization of Shenhua DCL process and the utilization of DCLR. 110

In our previous work,37

the molecular structure and size of two types of AS and PA from the 111

DCLR of 6 t/d Shenhua PDU and a batch hydro-liquefaction in laboratory were characterized, 112

respectively. It was suggested that the ‘archipelago’ model, in which several aromatic nucleuses 113

link by bridge bonds or hydrogenated aromatic rings, should be valid at least for the PA, and the 114

PA exhibits stronger aggregation than the AS. In present work, the PAs were further separated into 115

several sub-fractions by the column chromatography in order to investigate their structures and 116

aggregations in detail. All sub-fractions were characterized by FTIR, ultra-violet-visible 117

spectroscopy (UV), fluorescence spectroscopy (FL), GPC and matrix-assisted laser 118

desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) techniques and their 119

aggregations were also discussed. 120

2. 2. 2. 2. ExperimentExperimentExperimentExperiment 121

2.1 2.1 2.1 2.1 MMMMaterials aterials aterials aterials 122

In present work, two types of PA used as sample. PA(A) was separated from the DCLR in 6 t/d 123

Shenhua PDU by solvent extraction, and PA(B) was prepared by a conventional direct liquefaction 124

in a batch autoclave with Shenhua coal as feed. The original DCLR is a solid block at room 125

temperature. Its true density and softening point are 1.59 g/cm3 and 148

oC, respectively. A 126

detailed procedure of hydro-liquefaction and separation can be seen in our previous works.37,38

127

Ultimate and proximate analyses of PA(A) and PA(B) are listed in Table 2. All reagents were 128

commercially available without further purification. 129

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2.2 2.2 2.2 2.2 Separation of PASeparation of PASeparation of PASeparation of PAssss 130

Two PAs were further fractionated by the column chromatography, respectively, in which 200 131

mesh silica was used as adsorbent. A typical procedure is following, 1.0 g dried PA was dissolved 132

in THF and mixed with 10 g of neutral silica gel under ultrasonic irradiation for 30 min. 133

Subsequently, tetrahydrofuran (THF) solvent was removed by rotary evaporation, and the silica 134

gel sample obtained was dried in a vacuum at 80 oC for 24 h. The silica gel adsorbed PA was 135

loaded onto a 2.0 cm × 50 cm neutral silica gel column activated at 260 oC. Toluene, toluene/THF 136

(19:1, volume), toluene/THF (4:1, volume), toluene/THF (1:4, volume), THF and methanol 137

solvent were used as eluent in turn, so that the components of PA could gradually be eluted by 138

their polarity as much as possible. After eluting the column with a particular solvent, the solvent 139

was removed by rotary evaporation, and the eluted sub-faction was dried in a vacuum at 80 oC for 140

24 h. All separated sub-factions, which were defined sequentially as sub-fraction I, II, III, IV, V 141

and VI, were weighted and analyzed. Finally, the silica gel on the top of column was exhaustively 142

extracted by THF in a Soxhlet extractor to afford the residue fraction on eluted column (as 143

sub-fraction VII). 144

2222.3 .3 .3 .3 Characterization of Characterization of Characterization of Characterization of SSSSubububub----fractionsfractionsfractionsfractions 145

FTIR spectra were obtained on a Nicolet 6700 FTIR spectrometer at ambient temperature. A 146

normal KBr disk technique was used with a ratio of sample/KBr, 10 mg/200 mg. The spectra were 147

recorded at a resolution of 4 cm-1

. The element analysis was carried out by Elementar Vario EL III. 148

Mass spectra of sub-fractions were determined using a Bruker Biflex III matrix assisted laser 149

desorption/ionization time of flight (MALDI-TOF) mass spectrometer equipped with a 337 nm 150

nitrogen laser. Both matrice 4-hydroxy-α-cyanocinnamic acid and sample were dissolved in 1:1 151

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(v/v) acetonitrile : water with 1% trifuoroacetic acid. And 0.5 µL of this mixture solution was 152

placed on a metal sample plate and air-dried at ambient temperature. Mass spectra were acquired 153

in positive linear mode and using an acceleration voltage of 19 kV. External mass calibration was 154

performed using a standard peptide mixture. Spectra were obtained by setting the laser power 155

close to the thresold of ionization and generally 100 pulses were acquired and averaged. 156

Sub-fractions solutions were used to measure their absorption spectra, fluorescent spectra and 157

GPC curve. These solutions were prepared by dissolution of the dried solid samples in 158

tetrahydrofuran (THF) or dichloromethane (DCM), and then were left in ultrasonic bath for 30 159

min to assure complete dissolution. Finally, the samples were diluted to certain concentration and 160

stood overnight before determination. 161

Absorption spectra of samples were taken using a diode array spectrometer (Lambda 35 162

UV-vis) of 1 nm spectral resolutions. All measurements were made at room temperature. Quartz 163

sample cell with 1 cm optical path was used. Fluorescence spectrum was recorded on a Hitachi 164

F-4600 spectrophotometer with 150 W Xenon lamp as the excitation source. Fluorescence 165

measurement was made using a classic optic mount at 90° signal observation. Emission and 166

excitation slits were set at 5 nm. The scanning speed was kept constant (1200 nm/min). The 167

spectral measurement at room temperature was made with the use of quartz cell of 1 cm path 168

length. 169

A Shimadzu LC-20AT liquid chromatography, equipped with a SPD-20A UV/vis detector, 170

was used to measure the GPC curve. The GPC column was a Shim-pack GPC-8025. THF was 171

selected as eluent at 1 mL/min flow rate. The detective wavelength was set at 254 nm and the 172

column temperature was controlled at 20 oC. The data were processed using calibration curves 173

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obtained with narrow-dispersity polystyrene standards. 174

3. 3. 3. 3. RRRResults and discussionesults and discussionesults and discussionesults and discussion 175

3.1 3.1 3.1 3.1 SeSeSeSeparation of PAparation of PAparation of PAparation of PAssss andandandand EEEElemental lemental lemental lemental CCCCompositionsompositionsompositionsompositions of of of of SSSSubububub----fractionsfractionsfractionsfractions 176

As listed in Table 1, a certain amount of toluene eluate (sub-fraction I) could be obtained from two 177

types of PAs, though PA is a mixture of toluene insolubles. Different preparation procedures of PA 178

samples (see Figure I, Supporting Information) may responsible for the formation of sub-fraction I. 179

Since PA(A) was obtained as toluene insoluble/THF soluble from DCLR by solvent extraction 180

with n-hexane, toluene and THF in turn, some toluene soluble (11.6 %) could not be extracted by 181

toluene solvent due to be embedded and/or aggregated with heavy products such as PA. 182

However, PA(B) was separated from n-hexane insoluble (a mixture of PA and AS, separated from 183

THF soluble by n-hexane solvent 38

), a little toluene soluble (4.1 %) was remained PA due to the 184

disaggregation of previous THF extraction. Addition of a small amount of THF in toluene 185

(toluene : THF=19 : 1, volume) could further elute sub-fraction II. The yields of sub-fraction IV 186

and III, which eluted by toluene/THF=1 : 4, and 4 : 1, are the highest, and the second highest in 187

every PA, respectively. After THF eluted, the cumulative yields of sub-fractions I~V are 75.1 % 188

and 70.9 % for PA(A) and PA(B), respectively. The yields of sub-fraction VI eluted by methanol 189

are very low for two types of PAs. After all elution carried out, 10.1 % and 9.1 % yields of 190

sub-fraction VII were extracted from the silica gel loading PA(A) and PA(B) samples by THF 191

soxhet extraction, respectively. Finally, the total separation recoveries of PA(A) and PA(B) are 192

81.8 % and 86.0 %, respectively. Similar to the result reported by Shui et al., 39

the separation of 193

the weak polar components (such as sub-fraction I and II) from PA by disaggregation could result 194

in the formation of the strong polar components, which are insoluble in THF and/or irreversibly 195

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adsorb at silica gel to reduce the separation recovery. The more the sub-fraction I yields, the lower 196

the separation recovery. Therefore, it can be speculated that PA should be a kind of aggregate, and 197

the disaggregation of PA occurred in the process of chromatograph separation. Moreover, Table 1 198

also shows that the distribution of PA(A) sub-fractions is similar to that of PA(B) sub-fractions. 199

As shown in Table 2, for the sub-fractions of PA(A), Hdaf% and H/C (mole ratio) decrease, and 200

Ndaf% increases successively from sub-fraction I to V, indicating that toluene solvent only elutes 201

the weak components with high H/C, and THF solvent promotes the elution of the components 202

with low H/C and the compounds containing nitrogen. Surprisingly, THF solvent dose not show 203

obvious preference for the elution of components containing oxygen. Compared with sub-fractions 204

I~V, sub-fractions VI and VII show significantly different element compositions, in which very 205

high Odaf% and low Cdaf% can be observed. As a result, H/C and O/C ratios in the two 206

sub-fractions are very higher than those in other sub-fractions, suggesting that the sub-fractions VI 207

and VII mainly consist of polar compounds containing oxygen. For sub-fractions of PA(B), similar 208

changes of elemental compositions to those of PA(A) can be found, except for the gradual increase 209

of Odaf% and O/C ratio from sub-fraction I to V. In addition, the sub-fraction V of PA(B) has 210

remarkable difference from sub-fractions I~IV. It shows high Hdaf% and Odaf%, but low Cdaf%. Its 211

H/C and O/C ratios are similar to those of sub-fractions VI and VII. Based on the elemental 212

compositions of sub-fractions, it was suggested that the sub-fraction I and II should be weak polar 213

components, and the sub-fraction VI and VII (containing the sub-fraction V of PA(B)) be strong 214

polar components. THF could rupture the non-covalent interactions between compounds 215

containing nitrogen. 216

3.2 3.2 3.2 3.2 FTIRFTIRFTIRFTIR, , , , UVUVUVUV----visvisvisvis and and and and FFFFluorescence luorescence luorescence luorescence SSSSpepepepectra of PA ctra of PA ctra of PA ctra of PA SSSSubububub----fractionsfractionsfractionsfractions 217

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As Figure 1 shows, all sub-fractions display a strong and broad OH peak with a maximum at 3441 218

cm-1

and 3435 cm-1

for PA(A) and PA(B), respectively, indicating that these sub-fractions contain 219

plenty of OH group bonded by hydrogen bonds. Different sub-fractions from the same PA have 220

similar distribution of OH groups. The relative strength of aliphatic C-H peaks (2920 cm-1

and 221

2850 cm-1

) changes consistently with its H/C ratio shown in Table 2. These peaks in sub-fraction I 222

and VII are the strongest and second strongest, respectively. The peaks of carbonyl groups are 223

complex. For sub-fractions of PA(A), two obvious peaks at 1770 cm-1

and 1720 cm-1

can be 224

attributed to the carbonyl groups of phenolic ester and aromatic acid ester, respectively. For the 225

sub-fractions of PA(B), there is also the carbonyl peak of aromatic acid ester near to 1717 cm-1

, 226

but no any carbonyl peak of phenolic ester. In addition, a weak peak at 1741 cm-1

attributed to the 227

carbonyl of aliphatic acid ester can be observed in some sub-fractions of PA(B). It suggested that 228

the PA(A) contains the ester of aromatic acid and the phenolic ester, but the PA(B) mainly contains 229

the ester of aromatic acid. These ester groups as one of cross-linkings of aromatic clusters were 230

thought as a normal structural feature of coal and coal-derived products.40

The stretching vibration 231

peak of aromatic ring, which is observed in the ranges of 1606-1635 cm-1

, is very strong and broad 232

in all sub-fractions, suggesting that all sub-fractions contain complex aromatic fragments 233

consisting of fused and substituent aromatic rings. The peaks at 1385 cm-1

and near to 1457 cm-1

234

attributed to the in-plane bending vibration peak of CH3 and CH2 indicate that the content of the 235

methylene group in the sub-fractions of PA(A) is more than that in the sub-fractions of PA(B). In 236

addition, a series of weak peaks in the range of 750~850 cm-1

appeared in the sub-fractions I-IV of 237

PA(A) may suggest that these sub-fractions should contain the fused aromatic rings with less 238

substitutent and more Car-H than other sub-fractions.3 239

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Figure 2 provides UV-absorbance spectra of sub-fractions of PA(A) and PA(B) in DCM 240

solvent, which normalized at 260 nm, and the inset shows the absolute spectra of 10 mg/L 241

solutions. There are essentially uniform normalized spectra that display a strong terminal 242

absorption band at less than 400 nm and a broad tailed band in range of 400-700 nm, suggesting 243

that all sub-fractions contain a relatively abundance of naphthalene ring fragments and a few of 244

multi-ring aromatic fragments. For the sub-fractions of PA(A), the relative absorbance of 245

sub-fractions I-III between 260 nm and 340 nm is higher than those of other sub-fractions, but the 246

results are the opposite above 340 nm except of sub-fraction VI. In addition, the sub-fraction I 247

displays some resolved additional peaks at 310 nm, 325 nm, 345 nm, 411 nm, 440 nm, etc., and 248

the sub-fraction VI almost shows the lowest absorbance in rang of 260-700 nm compared with 249

other sub-fractions. It indicated that the percent of naphthalene ring fragments in sub-fractions 250

I-III is higher than that in sub-fractions IV, V and VII, and the percent of fused ring fragments in 251

sub-fraction VI is low. Thus, the sub-fraction I should have simpler compositions than other 252

sub-fractions. For the sub-fractions of PA(B), there are similar results to those observed in the 253

sub-fractions of PA(A), but these differences between sub-fractions of PA(B) are smaller than 254

those of PA(A). Further, it can be seen from the insets in Figure 2 that the sub-fraction III of PA(A) 255

and sub-fraction II-IV of PA(B) display obviously higher absolute absorbance than other 256

sub-fractions, respectively, suggesting that these sub-fractions contain more fused aromatic 257

structures. Therefore, we further speculated that more fused aromatic rings in sub-fractions of 258

PA(A) should have been partly hydrogenated compared to those in sub-fractions of PA(B). 259

FL is an important technique to characterize the size distribution of fused aromatic fragments, 260

but also to provide the information of aggregation between aromatic fluorophors.41-43

Although 261

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2~3 rings fused aromatic fragments, such as naphthalene, anthracene and phenanthrene rings, are 262

prevalent in PA and its sub-fractions according to their absorbance spectra, the emission spectra 263

shown in Figure 3 display exclusively a non-structural broad band without any fluorescent of 264

monomers (the emission of locally excited state)44

, suggesting that there should be complex 265

photophysical processes such as excimer/exciplex formations, intramolecular charger-transfer, and 266

electronic energy transfer.45

However, compared with the emission spectra of polymer labeled by 267

aryl group42

and diarylalkane43

, in which more or less emission of single aryl can be observed 268

beside the emission of excimer, there are no any emission of single fluorophor in the fluorescence 269

spectra of PA and its sub-fractions. It indicates that there are very strong interactions between 270

fluorophors of PA and its sub-fractions due to small steric hindrance and high local concentration 271

of fluorophors. Taking into account very dilute solution (10 mg/L) used in the present work, the 272

intramolecular interactions between fluorophors predominated in above photophysical processes, 273

speculating that a molecule of PA and its sub-fractions should contain two or more fluorophors. 274

By comparison of the emission spectra of different sub-fractions as shown in Figure 3, It can 275

be found that sub-fractions I-III show shorter emission wavelength and higher quantum efficiency 276

than sub-fractions of IV, V and VII. The sub-fraction VI of PA(B) shows short emission 277

wavelength and low quantum efficiency, though the sub-fraction VI of PA(A) is similar to 278

sub-fraction V and VII of PA(A). So we thought that the sub-fractions IV, V and VII show stronger 279

aggregations than the former sub-fractions I-III. It can also be supported by comparing the 280

quantum efficiency with the absorptivity, which are not relevant. Further, as shown in Figure 4, the 281

synthetic spectra of two PAs obviously show shorter wavelength of peak and higher intensity than 282

their corresponding measured spectra, suggesting that there should also be some interactions 283

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existing between different sub-fractions. Clearly, the interactions between fluorophors (fused 284

aromatic nucleus) in PA and its sub-fractions are very complex, and dominate their FL spectra. 285

The mechanism of PA and its sub-fractions is still under investigation. 286

3.3 3.3 3.3 3.3 MALDIMALDIMALDIMALDI----TOF/MS TOF/MS TOF/MS TOF/MS SSSSpectra of pectra of pectra of pectra of SSSSubububub----fractions and fractions and fractions and fractions and IIIIts ts ts ts MMMMolecular olecular olecular olecular WWWWeight eight eight eight DDDDistributionsistributionsistributionsistributions 287

In order to obtain the true MWD of PA sub-fractions without the interfering signal from 288

clusters, their MALDI-TOF-MS spectra are shown in Figure 5. According to the suggestion of 289

Hortal et al.,31

self-aggregates of AS formed in the LDI plume should show an additional broad 290

band at high mass tail of distribution. However, there is not any additional broad band observed in 291

the mass spectra of sub-fractions I-IV from PA(A) and PA(B), except that there is seemingly a 292

very weak broad band in sub-fractions VII from PA(A) and PA(B) and sub-fraction V from PA(A), 293

indicating that the MALDI-TOF-MS spectra of these sub-fractions, at least sub-fractions I-IV, 294

have no interfering signal from aggregates, and should be near to true MWDs of sub-fractions. 295

For the sub-fractions of PA(A), it can be seen from Figure 5 that the mass spectra of the 296

sub-fraction I-III display a relatively narrow MWD centered at about 450 amu, which slightly 297

shifts to high mass from the sub-fractions I to III. Meanwhile, their spectra also gradually widen, 298

especially the upper limits of mass greatly extend to high mass from the sub-fractions I to III. In 299

contrast to the mass spectra of the sub-fractions I-III, those of the sub-fractions IV and V occur 300

obviously a shift to high mass value, especially the sub-fraction V with a upper-limit of mass 301

above 1800 amu. Above results suggest that increasing the ratio of THF in eluent can promote the 302

elution of large molecular components, though the separation selectivity of column 303

chromatography by the scale of component is poor. Except for the large molecular components, 304

the addition of THF in eluent also eluted some small molecular components between 200 amu and 305

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400 amu, which may be polar components or disassociated components. The sub-fraction VI 306

displays a very weak continuous spectrum from 190 amu to 1800 amu, which slowly decreasing 307

signal intensity with the increasing m/z, except for several specific ions. It was speculated that the 308

eluted components (sub-fraction VI) by methanol should be mainly a few of strong polar 309

compounds, and the continuous spectra may attribute to the residual components that should be 310

eluted by THF and its mixture solvents. Further, the sub-fraction VII displays the widest MWD 311

centering at 640 amu than other sub-fractions. Even its relative intensity at 1800 amu still exceeds 312

0.1. So the sub-fraction VII mainly consists of large molecular components. 313

For the sub-fractions of PA(B), it can be observed that other sub-fractions except for the 314

sub-fraction IV display similar MWDs to those of PA(A). In comparison to corresponding 315

sub-fractions of PA(A), the sub-fractions from PA(B) show higher MW and broader MWD, 316

especially higher abundance in the side of high mass, except of sub-fraction V and VI. Based on 317

abovementioned results, it was suggested that the PA(B) possesses larger MW than PA(A) due to 318

weak cracking reaction in the process of the batch hydro-liquefaction. Meanwhile, it was also 319

speculated that the sub-fractions of PA(B) should have weaker intermolecular aggregation, so that 320

its can be disaggregated and eluted by weaker polar eluent than those of PA(A). 321

3.4 3.4 3.4 3.4 GPC GPC GPC GPC CCCCurves ofurves ofurves ofurves of SSSSubububub----fractions and fractions and fractions and fractions and IIIIts ts ts ts AAAAggregationggregationggregationggregation 322

Since GPC curve mainly reflects the MW and MWD of aggregates formed by sample molecules in 323

solution, this work try to investigating the aggregation of PA sub-fractions by comparing the 324

results of MALDI-TOF-MS to those of GPC. According to our previous works,38

100 mg/L 325

solution was used to determine the GPC curve in order to ensure the formation aggregates. It can 326

be obviously found in Figure 6 that the MWs of two types of PA sub-fractions increase in 327

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sequence, and their MWDs also widen gradually from sub-fraction I to VII except sub-fraction VI. 328

Meanwhile, the sub-fractions of PA(A) show higher MW than the corresponding sub-fractions of 329

PA(B) except sub-fraction I, and the MW difference between corresponding sub-fractions of PA(A) 330

and PA(B) also increases gradually from sub-fraction II to VII. Further, the parameters of their 331

MW distributions listed in Table 3, show that the number-average molecular weight (Mn), the 332

weight-average molecular weight (Mw) and the polydispersity (d) all increase from sub-fraction I 333

to sub-fraction VII except sub-fraction VI, and the Mn, Mw and d of PA(A) sub-fractions are higher 334

than those of corresponding PA(B) sub-fractions, respectively. Surprisingly, the Mn and Mw of 335

sub-fraction VII of PA(A) are 7231 and 11757 amu, respectively, which are significantly higher 336

than those of PA(B). So above results of GPC analyses suggest that the sub-fractions of PA show 337

various intermolecular aggregation abilities. 338

By comparison of the maxima and peak width at half height (PWH) of MALDI-TOF-MS 339

spectra and GPC curves of sub-fractions listed in Table 4, it can be founded that the maxima of 340

sub-fraction I, whether from PA(A) or from PA(B), measured by MS is near to that by GPC, but 341

the maxima of other fractions measured by GPC are obviously higher than their corresponding 342

values by MS. The difference between two maxima measured by MS and GPC increase gradually 343

from sub-fraction II to VII except sub-fraction V and VI, and these differences of PA(A) 344

sub-fractions are more significant than those of corresponding PA(B) sub-fractions. Meanwhile, 345

the PWH also show similar changes to the maxima. So we thought that the intermolecular 346

aggregation enhances from sub-fraction I to VII, and the one of PA(A) sub-fractions is stronger 347

than that of corresponding PA(B) sub-fractions. The majority of sub-fraction I should be in 348

monomer form in solution, and sub-fraction VII could mainly be in aggregates form. 349

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4. 4. 4. 4. CCCConclusion onclusion onclusion onclusion 350

PA from DCL mainly exists in the form of aggregates, and can be separated into different polar 351

sub-fractions by the column chromatography. Further separation is favorable to the understanding 352

of the structure and aggregation of PA. 353

Two PAs obtained by different DCL technologies show similar distribution of sub-fractions. 354

All sub-fractions contain relatively abundance of naphthalene ring fragments and a few of 355

multi-ring aromatic fragments, but the PA(A) consist of more hydrogenated fused aromatic rings 356

than the PA(B) from. Meanwhile, the PA(B) possesses larger MW than the PA(A). Generally, a 357

molecule of PA contains two or more fluorophors. The ester groups in PA(A) consist of aromatic 358

acid ester and phenolic ester, but those in PA(B) are mainly the aromatic acid esters. Sub-fractions 359

I-III consist of smaller fused aromatic nucleus than the other sub-fractions. The sub-fraction VII, 360

which mainly consists of large molecular components, displays the widest MWD centered at 640 361

amu. 362

There are very strong aggregation between fluorophors of PA and its sub-fractions due to 363

small steric hindrance and high local concentration of fluorophors. Different sub-fractions show 364

various intermolecular aggregation abilities. In general, their intermolecular aggregations increase 365

from sub-fraction I to VII, and the aggregation of PA(A) sub-fractions is stronger than that of 366

corresponding PA(B) sub-fractions. The majority of sub-fraction I is in monomer form in solution, 367

and sub-fraction VII mainly is in aggregates form. 368

Acknowledgment 369

This work was supported by National Basic Research Program of China (973 Program, Grant 370

2011CB201302), the Key Project of Coal Joint Fund from Natural Science Foundation of China 371

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and Shenhua Group Corporation Limited (Grant U1261208), the Natural Scientific Foundation of 372

China (Grants 51174254, 21176001, 21306001, U1361125). This work was subsidized by 373

Strategic Chinese-Japanese Joint Research Program (Grant 2013DFG60060). Authors are also 374

appreciative for the financial support from the Provincial Innovative Group for Processing & 375

Clean Utilization of Coal Resource and the Innovative Group of Anhui University of Technology. 376

Authors thank Prof. Shaoxiang Xiong (Institute of Chemistry, CAS) for determining 377

MALDI-TOF/MS spectra. 378

References 379

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444

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Table 1 Distributions of the sub-fractions and separation recoveries of PAs 445

Sample

Yield / wt %

Recovery /%

I II III IV V VI VII

PA(A) 11.6 8.8 16.1 26.6 7.9 0.7 10.1 81.8

PA(B) 4.1 7.4 18.8 39.7 5.2 1.7 9.1 86.0

446

447

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Table 2 Ultimate analyses of PAs and their sub-fractions 448

Sample

Ultimate analysis / wt %

H/C O/C

N C S H O*

PA(A) 1.14 82.43 0.19 6.32 9.92 0.920 0.091

PA(A)

Sub-fraction

I 0.18 84.87 0.19 7.85 6.91 1.110 0.061

II 0.62 84.64 0.13 6.45 8.16 0.915 0.072

III 1.24 84.39 0.19 6.15 8.03 0.875 0.071

IV 1.43 85.34 0.26 6.07 6.90 0.854 0.061

V 1.41 85.85 0.24 5.85 6.65 0.818 0.058

VI 1.23 61.08 0.33 6.97 30.39 1.369 0.373

VII 0.89 71.19 0.61 7.89 19.42 1.330 0.205

PA(B) 1.70 81.01 0.46 6.13 10.70 0.908 0.099

PA(B)

Sub-fraction

I 0.28 83.88 0.40 8.92 6.52 1.276 0.058

II 0.74 84.71 0.24 7.40 6.91 1.048 0.061

III 1.69 83.63 0.28 6.00 8.40 0.861 0.075

IV 1.86 80.47 0.31 5.81 11.55 0.866 0.108

V 1.54 70.20 0.31 6.71 21.24 1.147 0.227

VI 1.96 71.19 0.61 5.77 20.47 0.973 0.216

VII 1.35 75.69 0.48 7.46 15.02 1.183 0.149

* by difference 449

450

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Table 3 MW distribution parameters of PA sub-fractions determined by GPC 451

Sub-fraction

PA(A) PA(B)

Mn Mw d Mn Mw d

I 674 786 1.17 598 655 1.11

II 999 1229 1.23 938 1143 1.22

III 1468 1924 1.31 1225 1537 1.26

IV 2166 2998 1.38 1627 2145 1.32

V 3102 4576 1.48 2119 2898 1.37

VI 3454 5232 1.51 1988 2797 1.41

VII 7231 11757 1.63 2799 4001 1.43

452

453

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Table 4 Comparison of MWDs of sub-fractions determined by MALDI-TOF/MS and GPC 454

Sub-fraction

PA(A) PA(B)

Maxima PWH Maxima PWH

MS GPC MS GPC MS GPC MS GPC

I 424 441 105 495 488 403 140 415

II 448 562 175 875 551 604 235 805

III 448 837 200 1395 512 765 370 1100

IV 559 1197 285 2170 586 979 580 1550

V 585 1459 320 3240 - 1140 - 2260

VI - 1360 - 3600 - 1056 - 1960

VII 636 2032 595 7045 682 1245 725 2850

455

456

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4000 3500 3000 1800 1600 1400 1200 1000 800 600

Absorbance

Wavenumber /cm-1

1385

3441

I

II

III

IV

V

VI

VII

1720 PA(A)

752

17703042 1038

1457

1275

1168

2920

1606-1635

4000 3500 3000 1800 1600 1400 1200 1000 800 600

Absorbance

Wavenumber /cm-1

I

II

III

IV

VVI

VII

34351385

1605-16252920

2850 7501050

1717

1741

3131

1278

14591124

1171

PA(B)

457

Figure 1 FTIR spectra of the sub-fractions separated from the PA(A) and PA(B), respectively 458

459

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300 350 400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

Absorbance

Wavelength/nm

345

310

Absorbance

Wavelength /nm

I

II

III

IV

V

VI

VII

PA(A)310

345

340

411

440464

300 350 400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

Absorbance

Wavelength/nm

Absorbance

Wavelength /nm

I

II

III

IV

V

VI

VII

PA(B)

340

PA(B)

460

Figure 2 Absorbance spectra of sub-fractions from PA(A) and PA(B), respectively 461

462

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300 400 500 600 700

0

2000

4000

6000

8000

10000

300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

Fluorescence Intensity (a.u.)

Emission Wavelength (nm)

I

II

III

IV

V

VI

VII

PA

PA-I469

I

II

III

IV

V

VI

VII

PA

Normalized Fluorescence Intensity

507443

300 400 500 600 700

0

2000

4000

6000

8000

300 400 500 600 700

0.0

0.2

0.4

0.6

0.8

1.0

Fluorescence Intensity (a.u.)

Emission Wavelength (nm)

I

II

III

IV

V

VI

VII

PA

PA-II471

I

II

III

IV

V

VI

VII

PA

Normalized Fluorescence Intensity

509452

463

Figure 3 Emission spectra and normalized emission spectra of PAs and their sub-fractions, which 464

were obtained by 10 mg/L solution in THF solvent at EX=300 nm. 465

466

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300 400 500 600 700

0

1000

2000

3000

4000

5000

300 400 500 600 700

Fluorescence Intensity (a.u.)

Emission Wavelength (nm)

Original

Synthetic

PA(A) 469

494

PA-II

Original

Synthetic

468

499

467

Figure 4 Emission spectra of PA(A) and PA(B) determined in THF solvent at EX=300 nm, and 468

their synthetic spectra calculated by corresponding sub-fraction distribution and recoveries. 469

470

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PA(A) PA(B)

471

Figure 5 MALDI-TOF-MS spectra of the sub-fractions of PA(A) and PA(B) 472

473

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1000 10000

0.0

0.2

0.4

0.6

0.8

1.0

I VII

VI

Normalized Intensity

Molecular Weight(amu)

40000

I II III IV V VII

VI

PA(A)

200

1000 10000

0.0

0.2

0.4

0.6

0.8

1.0 I VII

VI

Norm

alized

Intensity

Molecular Weight(amu)

200

I II III IV V VII

VI

PA(B)

474

Figure 6 Normalized GPC curves of the sub-fractions from PA(A) and PA(B), respectively 475

476

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Supporting Information 477

AS+PA

PA(B) AS(B) PA(A)

AS(A) Extraction with THF

Extraction with toluene

Extraction with n-hexane

DCLR

n-hexane soluble n-hexane insoluble

Toluene soluble Toluene insoluble

THF soluble THF insoluble

Extraction with toluene

Extraction with n-hexane

Extraction with THF

Liquefied coal

THF insoluble THF soluble

n-hexane soluble n-hexane insoluble

Toluene soluble Toluene insoluble

478

Figure I Preparation procedures of two types of PA sample 479

Page 32 of 32

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