Molecular Dynamics Analysis on Hydrogenation Process of Asphaltene Aggregate for Medium and Low Temperature Coal Tar
Xiaokang Han University of Architecture and Technology Wenzhou Yan University of Architecture and Technology Xu Liu Northwest University Dong Li ( [email protected] ) Northwest University https://orcid.org/0000-0002-4578-0595 Yonghong Zhu Northwest University
Research Article
Keywords: Meddle-low temperature coal tar, Asphaltene aggregate, Hydrogenation, Molecular dynamics, Materials Studio
Posted Date: March 22nd, 2021
DOI: https://doi.org/10.21203/rs.3.rs-304675/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License
1 Molecular dynamics analysis on hydrogenation process of asphaltene aggregate for
2 medium and low temperature coal tar
3 Xiaokang Han1,2, Wenzhou Yan1, Xu Liu3,4, Dong Li*3,4, Yonghong Zhu3,4
4
5 1School of Management, Xi’an University of Architecture and Technology, Xi’an 710055, People’s
6 Republic of China
7 2Project Control Department, Hualu Engineering & Technology Co., Ltd., Xi’an 710065, People’s
8 Republic of China
9 3School of Chemical Engineering, Northwest University, Xi’an 710069, People’s Republic of China
10 4The Research Center of Chemical Engineering Applying Technology for Resource of Shaanxi, Xi’an
11 710069, People’s Republic of China
12
13 Corresponding author: Dong Li
14 ORCID: 0000-0002-4578-0595
15 Telephones: +86-029-88373425
16 Fax: +86-029-88305825
17 Email: [email protected]
18
19 Abstract
20 In this paper, the average molecular structure models of middle-low temperature coal tar (MLCT)
21 asphaltene before and after hydrogenation were obtained by 1H-NMR characterization. Then the
22 aggregation structure models of MLCT asphaltenes before and after hydrogenation was constructed in
23 Materials Studio 8.0 software. The comparative analysis of structural characteristics between original
24 and hydrogenated asphaltenes were carried out. The final configuration, molecular rotation radius and
1
25 interaction energy of asphaltene aggerates before and after hydrogenation were analyzed by molecular
26 dynamic simulation in different solvents.
27 The conclusions are as follows: (1) The MLCT asphaltenes exist in the form of asphaltene aggregates
28 before and after hydrogenation. After structural optimization, the original asphaltene molecules become
29 2-3 asphaltene molecules with approximately parallel, orderly arrangement and polycyclic aromatic
30 hydrocarbons (PAH) stacking aggregate structure. The molecular accumulation number of its aggregate
31 is less than that of petroleum asphaltene, and more complex nano-aggregates are formed by self-assembly
32 among multiple aggregates. The accumulation number of asphaltene molecules after MLCT
33 hydrogenation are less than that of original asphaltene. (2) The total interaction between MLCT primary
34 asphaltene and hydrogenated asphaltene mainly comes from the intermolecular Van der Waals force.
35 Compared with original asphaltene, the van der Waals energy and electrostatic energy of hydrogenated
36 asphaltene are smaller and more affected by temperature. This shows that the intermolecular interaction
37 of hydrogenated asphaltene is weak and the structure of the aggregate is unstable. (3) Aromatic
38 compounds, strong basic compounds and strong polar compounds have good dissociation effect on
39 MLCT asphaltenes and can effectively inhibit the aggregation and asphaltenes.
40 Keywords: Meddle-low temperature coal tar, Asphaltene aggregate, Hydrogenation, Molecular
41 dynamics, Materials Studio
42 1 Introduction
43 Medium and low temperature coal tar is a by-product in the process of coal gasification, production of
44 semi-coke and upgrading of low-rank coal (Sun et al. 2015; Zhu et al. 2015). Asphaltene of middle-low
45 temperature coal tar refers to the substances that are insoluble in n-alkanes with low carbon number but
46 soluble in aromatics. Its structural unit takes the condensed aromatic ring system as the core, with a
47 number of naphthenic rings and alkyl side chains, as well as sulfur, nitrogen, oxygen and various metal
48 heteroatoms. It’s similar to crude oil that asphaltene is the most complex and difficult removal component
49 in coal tar, which usually accounts for 15-30wt% of coal tar. Coal tar asphaltene has the following
2
50 characteristics compared with crude oil asphaltene (Zhu et al. 2016; Berrgmaan et al. 2003; Sabbah et al.
51 2011):
52 I: The aromatic carbon ratio (fa) of coal tar asphaltene is about 0.7, the aromatic ring condensation
53 parameter (HAu/CA) is about 0.5, which are higher than that of all kinds of crude oil asphaltenes;
54 II: The heteroatom (O+N+S) content of coal tar asphaltene is about 10wt%, which is higher than
55 that of all kinds of crude oil asphaltenes;
56 III: The hydrogen-carbon atomic ratio of coal tar asphaltene is less than 1 and the ratio of aromatic
57 core carbon number to aromatic ring peripheral carbon number is less than 1.5;
58 IV: The molecular weight of coal tar asphaltene is about 500 to 600 which is lower than that of
59 ordinary crude oil asphaltene;
60 V: The alkyl side chain of condensed aromatic ring of coal tar asphaltene is mainly methyl and
61 ethyl, which is shorter than that of general petroleum asphaltene.
62 For a long time, many scholars have studied the structural unit model of asphaltene, but because of the
63 complexity of the molecular structure of asphaltene, the molecular structure model of asphaltene is still
64 controversial (Karimi et al. 2011; Wargadalam et al. 2002). At present, it is considered that the molecular
65 structure unit models of asphaltenes are mainly "group island type", "continental type" and "mixed type"
66 (Fernando et al. 2013, Gray et al. 2011). Schuler et al. (2015, 2017, 2020) demonstrated the first direct
67 observation of wide diversity of asphaltene molecular motifs with peri- and cata-condensed polycyclic
68 aromatic hydrocarbons (PAHs). Various types of coal-derived asphaltene structures were obtained by the
69 atomic force microscopy. A large amount of resin and asphaltene in heavy oil make it form a relatively
70 stable colloidal dispersion system. The stability of colloidal dispersion system is closely related to the
71 aromatic ring, aliphatic chain, heteroatomic structure, temperature, pressure, solvent and other external
72 conditions of asphaltene (Xu et al. 2016). The change of the above influencing factors will destroy the
73 stability of the colloidal system, resulting in the existence of asphaltene in the form of aggregation and
74 precipitation (Jian et al. 2014). As asphaltene aggregation and sedimentation will cause many problems,
75 such as tar fluidity reduction, pipeline blockage, catalyst deactivation and so on, many scholars have
76 used different methods to study the behavior of asphaltene aggregation.
77 Dickie et al. (1967) proposed the “Yen” model of asphaltene colloid aggregation for the first time. The
3
78 “Yen” model explains the aggregation mechanism of asphaltenes from molecular state to cluster state,
79 that is, a single aromatic layer can be stacked to form a PAH complex, which is further associated into a
80 micelle, and when the concentration of asphaltene is high enough, the micelles associate with each other
81 to form aggregates. The Yen model provides an important theoretical basis for the measurement and
82 analysis of phase separation asphaltenes. With the deepening of asphaltene research and the development
83 of analytical technology, Mullins et al. (2010) proposed an improved “Yen-Mullins” model. The basic
84 characteristics of the “Yen-Mullins” model are as follows: the asphaltene is mainly "continental"
85 molecular structure, and the asphaltene is distributed in the form of monomer molecules, nano-aggregates,
86 clusters and flocculationn in the solution. "Yen-Mullins" model proposed that the π-π force between
87 aromatic rings is the dominant force for asphaltene stacking to form molecular clusters. However, it's
88 important to note that Redelius et al. (2000, 2007) and Acevedo et al. (2010) investigated the physical
89 chemistry properties of asphaltenes or bitumen using solubility parameters of asphaltenes and their
90 fractions. Redelius et al. (2000) proposed a new bitumen model showing that the stability of bitumen
91 depends on the mutual solubility of all components. This statement is contradictory to the “old” ideas of
92 bitumen consisting of a dispersion of asphaltene micelles. The results indicated that the polar interactions
93 and hydrogen bonding are extremely important for the solubility properties, stability of bitumen and
94 mechanical properties of bitumen. In addition, Bake et al. studied three different source asphaltene
95 including immature source roc, petroleum, and coal to obtain their chemical compositions, molecular
96 architectures and structure-solubility relationships. It showed that π-π stacking is strongest in planar
97 asphaltenes with high aromatic content, whereas steric disruption is larger in less planarasphaltenes with
98 higher aliphatic content. Furthermore, Eyssautier et al. (2011 and 2012) studied the asphaltene
99 nanoaggregate with extensive experiment using the small angle neutron and X-ray scattering (SANS,
100 SAXS) measurements. One of the important finding is that the asphaltene nanoaggregate can be
101 represented by a dense aromatic core (PAH stacking in the middle) and a shell highly concentrated in
102 aliphatic carbon, which were best described by a disk of total radius 32 Å with 30% polydispersity and
103 a height of 6.7 Å. A hierarchical aggregation model, from free monomers to nanoaggregates and then
104 assemble into high aggregation number fractal clusters, was identified as a multi-scale organization by
105 Eyssautier et al. (2011).
4
106 The principle of the molecular dynamics (MD) method is to use the finite difference method to accurately
107 solve the Newtonian equations of motion of the simulated system in order to calculate the conformational
108 and wave changes of all particles in the system at the same time. MD simulation can not only predict and
109 determine the properties and structural characteristics of various systems, such as intermolecular
110 interactions, coordination characteristics of molecules in solution and so on. In recent years, MD
111 simulation has become an important tool for the study of asphaltene aggregation, which has been widely
112 used to study the structure of asphaltene association of heavy oil and made some progress.
113 At present, the research on asphaltene aggregate by MD is mainly in the following three aspects: (1) The
114 effect of alkyl side chains on aggregation. In the MD simulation of the asphaltene structure model,
115 Pacheco-Sanchez et. al (2004) observed that the asphaltene aggregation structure is consistent with the
116 “Yen” model, that is, there are three geometric configurations of Face-to-Face, offset π-stacked and T-
117 shaped between the asphaltene aggregate. The alkyl side chain plays a role in reducing the size of
118 asphaltene aggregates. Jian et al. (2014) found that the longer alkyl side chain is not conducive to the
119 accumulation of aromatic core of asphaltenes, but is conducive to the accumulation of asphaltenes. (2)
120 The effect of temperature and solvent on the asphaltene aggregates. Zhang et al. (2007) found that the
121 distance of asphaltene dimer decreased with the increase of temperature. The simulation of asphaltene
122 binder in different solvents by Carauta et al. (2005) showed that the aromatic nuclear packing distance
123 of asphaltene association in poor solvents such as heptane is smaller than that in good solvents such as
124 toluene. (3) The interaction between asphaltene and other components, such as resin, water, inhibitor and
125 so on. Alvarez et al. (2006) obtained the interaction potential curves of asphaltene-asphaltene, asphaltene-
126 resin and resin-resin systems by simulation. At present, it is considered that the aggregate structure of
127 asphaltene is electrostatic interaction, hydrogen bond, van der Waals interaction and other forces (Coata
128 et al. 2012; Lima et al. 2017).
129 In this paper, the average molecular structure model of LCT asphaltene before and after hydrogenation
130 was obtained by MLCT asphaltene characterization. Then the aggregate structure models of asphaltene
131 before and after hydrogenation were constructed in Materials Studio 8.0 software. The optimal
132 conformation was obtained by the kinetic optimization. Also, the structural characteristics of original and
133 hydrogenated asphaltene were compared. Then the effects of different solvents on the structural
5
134 characteristics, aggregation state and packing characteristics of the asphaltene aggregate were explored
135 by MD simulation of the asphaltene aggregate before and after hydrogenation in different solvents. This
136 helped to analyze the final configuration and molecular rotation radius of the asphaltene aggregate from
137 the point of view of structure. Finally, the interaction energy of asphaltene aggregate was analyzed from
138 the point of view of energy to explore the dominant force leading to asphaltene aggregate and the effect
139 of MLCT hydrogenation on asphaltene aggregate structure.
140 2 Average molecular structure model of coal tar asphaltene
141 The 1H-NMR spectrum of MLCT asphaltene before and after hydrogenation were shown in Fig. 1. The
142 detailed hydrogenation process was consistent with previous studies published by our research group (Li
143 et al. 2017). According to the 1H-NMR spectrum, hydrogen was divided into different types according
144 to chemical shift and integrated (Lalvani et al. 1991). The results are normalized and shown in Table 1.
Original asphaltene
Hydrogenated asphaltene
12 10 8 6 4 2 0 -2 145 Chemical shift/ppm
146 Fig. 1 The 1H-NMR spectrum of LCT original and hydrogenated asphaltenes
147 Table1 Hydrogen atoms content of original and hydrogenated coal tar asphaltenes
Asphaltene Chemical shift /10-6 Symbol Attribution Original Hydrogenated
γ position of aromatic side chain 0.5–1.0 Hγ 0.0793 0.0318 and further hydrogen
6
0.1405 1.0–2.0 Hβ β hydrogen of aromatic side chain 0.2879
0.5167 2.0–4.0 Hα α hydrogen of aromatic side chain 0.3045
0.3283 0.3110 6.0–9.0 HA Aromatic hydrogen
148 Based on the data in Table 1, the average asphaltene structural parameters before and after LCT
149 hydrogenation were calculated by Brown-Ladner formula (Gu et al. 2006). The results are shown in
150 Table 2.
151 Table2 The calculation results of average structural parameters of original
152 and hydrogenated coal tar asphaltenes
Original hydrogenated Parameters Symbol asphaltene asphaltene
Aromaticity fA 0.737 0.727
Hydrogen substitution rate around aromatic ring σ 0.454 0.317
Condensation degree parameters of aromatic ring HAU/CA 0.531 0.629
Total atomic number CT 35.56 29.66
Number of aromatic carbon atoms CA 26.21 21.57
Total number of hydrogen atoms HT 29.81 25.12
Number of saturated carbon atoms Cs 9.35 8.09
Carbon atom numbers in aromatic rings Ci 12.28 7.27
Carbon atoms numbers in periphery Cap 13.93 14.30 of the aromatic ring
α carbon atom numbers in aromatic ring system Cα 4.41 6.49
Aromatic ring number RA 7.14 5.64
Total number of rings Rt 8.96 8.32
Naphthenic ring number Rn 1.82 2.68
Carbon atom numbers in alkyl side chain n 2.73 2.18
153 Based on the calculation and analysis of the above data, the average molecular structure of MLCT
154 original and hydrogenated asphaltene were calculated and speculated as shown in Fig. 2. This original
7
155 asphaltene molecular structure is similar to the results of Schuler et al. (2015) obtained by atomic force
156 microscopy, which further explains the rationality of this asphaltene structures.
157 158 Fig. 2 The speculated average structural of original and hydrogenated coal tar asphaltenes
159 3 Simulation flow and parameter setting
160 The dimer composition model of MLCT asphaltene before and after hydrogenation and the composition
161 model of benzene, nitrobenzene, pyridine, quinoline and n-heptane were constructed in the 3D Atomistic
162 Document module in Materials Studio software, which were optimized by the molecular mechanics and
163 molecular dynamics. Then the simulated cell with periodic boundary conditions was constructed in the
164 Amorphous cell molecular mechanics module. The dynamics of the cell box was optimized under the
165 isotherm and isobaric (NPT) system. Finally, the MD simulation lasting 1500 ps was carried out on the
166 asphaltene aggregate before and after hydrogenation in different solvents at 298 K and 598 K,
167 respectively. The pre-1000 ps was used as the equilibrium process and the data of the last 500ps was
168 extracted for analysis and calculation. The temperature and pressure of the simulation process were set
169 to 298.15 K and 0.1 MPa. Compass force field was worked as the simulation force field, the
170 BERENDSEN method controls the simulation pressure, the ANDERSEN method controls the simulation
171 temperature, the GROPUBASED method controls the non-bond energy interaction in the simulation
172 process, the integration time is set to 1500 ps and the integration step is 1fs.
173 4 Results and discussion
174 4.1 Model of asphaltene aggregate
175 Based on the average molecular structure model of MLCT asphaltene before and after hydrogenation
176 obtained in section 2, the composition models of original and hydrogenated asphaltenes were constructed.
8
177 The kinetic optimization process is shown in Fig. 3. The most stable conformations of original and
178 hydrogenated asphaltenes were obtained after MD optimization shown in Fig. 4.
90 Original Asphaltene
60
-1 30
0 240 Hydrogenated Asphaltene
Energy/kcal·mo 200
160
120
80 051015202530354045 179 Optimization step/ps 180 Fig. 3 Dynamic optimization state of LCT original and hydrogenated asphaltenes
181 Eight asphaltene molecules were randomly placed in Materials Studio and optimized by dynamics
182 program in Forcite module to obtain aggregation structure of asphaltene molecules, as shown in Fig. 5.
183 184 Fig. 4 The most stable conformations of original and hydrogenated coal tar asphaltenes
185
186 Fig. 5 Aggregation structural diagram of original and hydrogenation coal tar asphaltenes
9
187 As can be seen from Fig. 4, after structural optimization, the eight original asphaltene molecules
188 randomly placed before the simulation are transformed into a PAH stacking aggregate structure with
189 approximately parallel and orderly arrangement of 2-3 asphaltene molecules, indicating that MLCT
190 asphaltene molecules don’t exist in the form of a single molecule in the heavy oil system, and 2-3
191 asphaltene molecules accumulate in one direction to form small nano-aggregates. Multiple aggregates
192 self-assemble to form more complex nano-aggregates, which extend in different directions. Therefore,
193 in the MLCT colloid, the asphaltene molecules that make up the colloidal core are the aggregate of
194 several asphaltene molecules. The existence form of asphaltene molecules is the aggregation state of
195 microscopic partial order and macroscopic disorder. Therefore, the dimer of MLCT asphaltene was
196 selected as the research object in the follow-up study.
197 Hydrogenated asphaltene molecules shown in Fig. 5. an also form a PAH stacking aggregate structure,
198 which exists in the form of aggregates, but the number of molecules is relatively small. This may be due
199 to the reactions, such as naphthenic ring opening, aromatic ring saturation, alkyl side chain breaking and
200 removal of impurity atoms in the process of hydrogenation, resulting in the weakening of the interaction
201 between asphaltene molecules. In addition, according to the study of Ren et al. (2014), it was found that
202 the aggregate numbers of petroleum asphaltene were 4 to 6, which were slightly higher than that of
203 MLCT asphaltene. This probably because the small molecular weight, the small number of aromatic
204 rings and the small core of condensed aromatic rings of MLCT asphaltene, also the small interaction
205 between aromatic rings and the weak association between molecules.
206 4.2 MD simulation of asphaltene aggregate
207 The dimer composition models of MLCT asphaltene before and after hydrogenation and the composition
208 model of benzene, nitrobenzene, pyridine, quinoline and n-heptane were optimized by molecular
209 mechanics and molecular dynamics. The simulated unit cells with optimized periodic boundary
210 conditions were constructed. The structural models of original and hydrogenated asphaltenes dimer were
211 shown in Fig. 6. The original cell structure diagrams of solvent molecule and asphaltene molecule were
212 shown in Fig. 7 (taking original asphaltene as an example).
10
213 214 Fig. 6 Optimized configuration of original and hydrogenated coal tar asphaltene dimers
215
216 Fig. 7 The periodic protocell structures of asphaltene dimer in benzene, nitrobenzene, pyridine,
217 quinoline and n-heptane
218 As shown in Fig. 6, the optimized asphaltene has a layered structure, which verifies the classical theory
219 obtained from X-ray diffraction experimental data that the asphaltene aromatic sheets were stacked to
220 form a PAH stacking complex, which was further aggregated into micelles. The micelles were further
221 aggregated with each other to form aggregates. In addition, the interlayer spacing of the optimized
222 asphaltene aggregate measured by the measure/charge tool in Materials Studio were 1.99, 1.90 Å (before
223 hydrogenation) and 2.41, 2,32 Å (after hydrogenation), respectively. It can be seen that the relative error
224 between the asphaltene layer spacing measured by measure/charge and the experimental data were less
225 than ±4.8%. The layer spacing of hydrogenated asphaltene is larger than that of original asphaltene.
226 4.2.1 MD Analysis from structural angle of asphaltene aggregate
227 (1) Configuration of asphaltene aggregate
228 In different solvents, the kinetically optimized final configurations of asphaltene aggregates extracted
229 from periodic cells were shown in Fig. 8. It can be seen that the face-to-face layered structures of both
11
230 original and hydrogenated asphaltenes have not been completely destroyed in all kinds of solvents at 298
231 K. The structure of a single asphaltene molecule has little change indicating that the structure of MLCT
232 asphaltene is stable at normal temperature. The disbanding effect of various solvents on asphaltene is
233 poor. At high temperature of 598 K, the configurations of asphaltene in different solvents changed
234 obviously. The structure of single asphaltene molecule also changed indicating that the structure of
235 MLCT asphaltene was unstable at 598 K. Some solvents can destroy the intermolecular force so that the
236 asphaltene aggregate can be disbanded. The disbanding effect of coal tar asphaltene aggregate in different
237 solvents was shown in Table 3.
238
239 Fig. 8 The final configurations of asphaltene dimers in benzene, nitrobenzene, solvent-free state,
240 pyridine, quinoline and n-heptane
241 Table3 Aggregate state of coal tar asphaltenes dimers in different solvents
Original asphaltene Hydrogenation asphaltene
Solvent Damage to the asphaltene Molecular Damage to the Molecular
layer structure deformation structure deformation
Completely Benzene Destroyed Bending Small destroyed
Completely Nitrobenzene Completely destroyed Bending Small destroyed
12
Solvent-free Intact None Intact None
Increased spacing, Pyridine Bending Intact Small slightly destroyed
Increased spacing, Quinoline Small Intact Small Destroyed,
n-heptane Intact None Intact None
242 (2) Molecular radius of gyration of asphaltene aggregate
243 The radius of gyration (Rog) represents the extension of the asphaltene molecule in the solvent. The
244 higher the Rog value is, the larger the molecular size is, the more the molecule stretches in the solvent.
245 Because the structural unit of asphaltene is centered on the condensed aromatic ring system with some
246 naphthene and alkyl side chains, the molecular Rog data are calculated based on the condensed aromatic
247 core of asphaltene molecules. The specific calculation formula is as follows (Chen et al. 2007):
248 (1) 2 2 2 (푥푎+푦푏+푧푐 ) 푅표푔 = √{ 푁 } 249 In the formula: xa、yb、zc represent the spatial coordinates of the atom, where the center of mass of the
250 asphaltene molecule as the origin. N represents the total number of atoms in the system. The Rogs of
251 asphaltenes before and after LCT hydrogenation in different solvents were shown in Fig. 9.
Solvent-free Qiunoline Pyridine n-heptane Benzene Nitrobenzene
2.1 2.0 Original 298 K Hydrogenated 298 K 2.0 1.8 1.9 1.6 1.8
1.4 1.7
1.6 1.2
1.5 1.0
2.8 3.0
Rog/nm Original 598 K Hydrogenated 598 K 2.6 2.7
2.4 2.4
2.2 2.1
2.0 1.8
1.8 1.5
1.6 1.2
1.4 0.9 0 300 600 900 1200 1500 0 300 600 900 1200 1500 252 Time/ps
253 Fig. 9 Comparison of Rogs of asphaltene dimers in different solvents
13
254 As shown in Fig. 9, the Rogs of MLCT asphaltene were affected little by temperature in n-heptane solvent,
255 while in other solvents, the Rogs of MLCT asphaltene at 598 K was obviously larger than that at 298 K,
256 showing that temperature has almost no effect on the size of MLCT asphaltene in n-heptane solvent,
257 while in other solvents, the size of MLCT asphaltene is larger at high temperature. This is due to the fact
258 that the solvent dissolves asphaltene molecules more strongly and the alkyl side chain of asphaltene
259 molecules is more extended at high temperature, while n-heptane has almost no dissolution effect on
260 asphaltenes at any temperature. This law is not only consistent with the objective law, which confirms
261 the reliability of the simulation, but also proves that high temperature is beneficial to the disbanding of
262 MLCT asphaltenes from the view of molecular dynamics.
263 It can also be seen from Fig. 9 that the change of Rog of hydrogenation asphaltene was more obvious
264 than that of original asphaltene, which is due to the fact that the condensed aromatic core of
265 hydrogenation asphaltene was smaller than that of original asphaltene and the condensation degree of
266 aromatic ring is lower. The average number of aromatic rings of original asphaltene is 8.533 and the
267 condensation degree parameter of aromatic ring is 0.432, while the average number of aromatic rings of
268 hydrogenation asphaltene is 7.057, the condensation degree parameter of aromatic ring is 0.492.
269 Therefore, the morphological change of the side chain of hydrogenated asphaltene had a greater influence
270 on itself and the change of molecular Rog for hydrogenated asphaltene was more significant.
271 At 298 K, the solubility of asphaltene aggregate was poor and the change of molecular Rog was not
272 obvious. Thus, the molecular Rog of asphaltenes at 598 K were analyzed. The Rogs of original and
273 hydrogenated asphaltene in solvent at 598 K were as follows: Nitrobenzene > Quinoline > Benzene >
274 Pyridine > Solvent-free > n-heptane (Original asphaltene); Benzene > Nitrobenzene > Solvent-free>
275 Quinoline > Pyridine > n-heptane (Hydrogenation asphaltene).
276 4.2.2 MD Analysis from energy angle of asphaltene aggregate
277 In the COMPASS force field, the configuration energy (EC) of the aggregate system consists of two parts:
278 the no-bond energy (Eno-bond) and the bond energy (Einternal). Among them, Einternal is caused by molecular
279 deformation, including bond expansion energy (EB), bond angular energy (EA), bond torsion energy (ET)
280 and bond inversion energy (EI), Eno-bond caused by internal interactions, including van der Waals
14
281 interaction energy (Evdw) and electrostatic interaction energy (Ee). The calculation formulas are as follows
282 (Yue et al. 2016):
283
퐶 푛표−푏표푛푑 푖푛푡푒푟푛푎푙 284 퐸(2) = 퐸 + 퐸
285 (3)
푖푛푡푒푟푛푎푙 퐵 퐴 푇 퐼 286 퐸 = 퐸 + 퐸 + 퐸 + 퐸
푛표−푏표푛푑 푣푑푤 푒 287 퐸(4) = 퐸 + 퐸
288 0 0 (5) 푟푖푗 푟푖푗 9 6 퐸푣푑푤 = ∑푖,푗 휀푖푗[2 (푟푖푗) − 3(푟푖푗) ] 289 (6) ∑푖,푗 푞푖푞푗 푒 휀푖푗푟푖푗 퐸 = 0 290 In the formula, εij is the energy parameter, rij represents the stable distance between atoms, qi and qj
291 represent the atomic charge. Based on the MD simulation of the asphaltene aggregate before and after
292 hydrogenation, the final configuration of asphaltene aggregate was statistically analyzed by using the
293 analysis tool of the Forcite module. The Eno-bond and Einternal of the asphaltene aggregates in different
294 solvents before and after hydrogenation were analyzed at 298 K and 598 K, respectively.
295 (1) Interaction energy
296 The comparisons of Eno-bond of asphaltene aggregates in different solvents were shown in Fig. 10. It can
297 be seen from Fig. 10 that the Evdw and Eno-bond of original and hydrogenated asphaltenes in various solvents
298 had the same trend at 298 K and 598 K, indicating that the total interaction between coal tar asphaltene
299 molecules mainly comes from Evdw. The driving force of asphaltene aggregates before and after
300 hydrogenation was Evdw, which may be due to the ring opening and heteroatom removing reactions during
301 asphaltene hydrogenation process leading to the decrease of asphaltene molecular weight, aromaticity,
302 degree of aromatic ring condensation and the content of heteroatom. However, the structural change of
303 asphaltene was not enough to have an important effect on the aggregate driving force considering of the
304 lack of hydrogenation depth. Li et al. (2007) found that the main driving force of petroleum asphaltene
305 aggregate came from electrostatic interaction, which is due to the large interlayer spacing of petroleum
306 asphaltene nano-aggregates. Thus, the long-range Ee was dominant.
307 It can be seen from Fig. 10 that the electrostatic energy of asphaltene before and after hydrogenation was
15
308 less than 0 under the solvent-free simulation condition. But the absolute values of Evdw and Ee of
309 hydrogenated asphaltenes were smaller than that of original asphaltene, also more affected by
310 temperature. This shows that the intermolecular interaction of hydrogenated asphaltene is weak and the
311 structure of the aggregate is unstable. The Ee of hydrogenated asphaltene was lower than that of original
312 asphaltene. The content of heteroatoms is obviously lower than that of original asphaltene. It is speculated
313 that the existence of heteroatoms may be one of the reasons for the aggregation of asphaltene.
314 There was no significant difference in the interaction energy of asphaltene in various solvents at 298 K,
315 which indicates that the asphaltene aggregate is stable at 298 K and the aggregate structure is not easy to
316 be destroyed. When the temperature increases to 598 K, the absolute values of EC, Evdw and Ee of
317 asphaltene decreased, proofing that the asphaltene aggregate is unstable and the intermolecular
318 interaction force is weakened at high temperature, which is beneficial to molecular disbanding. Different
319 solvents have different effects on the disassembly of asphaltene aggregate. The interaction energy of
320 original asphaltene decreased significantly in the solvents of nitrobenzene, quinoline, benzene and
321 pyridine. The interaction energy of hydrogenated asphaltene decreased significantly in the solvent of
322 nitrobenzene indicating that the aggregate structure of asphaltene was destroyed. This is consistent with
323 the previous configuration analysis of asphaltene in various solvents. Therefore, the disbanding effects
324 of various solvents on asphaltenes were as follows: Nitrobenzene > Quinoline > Benzene > Pyridine >
325 Solvent-free > n-heptane (Original asphaltene); Benzene > Nitrobenzene > Solvent-free> Quinoline >
326 Pyridine > n-heptane (Hydrogenation asphaltene).
16
E E E no-bond vdw e 10 Original 298 K Hydrogenated 298 K
0
-10
-20
-1 -30
-40 10 Original 598 K Hydrogenated 598 K 0
Energy/kacl·mol -10
-20
-30
-40
Benzene Byridine -heptane Benzene Byridine -heptane Quinoline n Quinoline n 327 NitrobenzeneSolvent-free NitrobenzeneSolvent-free
328 Fig. 10 Comparison of non-bond energy of asphaltene aggregates in different solvents
329 Benzene and nitrobenzene solvents can seriously destroy the aggregate structure of original asphaltene
330 and completely break the binding of asphaltene after hydrogenation. This shows that both solvent of
331 benzene and asphaltene form a strong Evdw before and after hydrogenation, so the Evdw within asphaltene
332 molecules is destroyed, indicating that the Evdw is an important driving force for the aggregate and
333 precipitation of asphaltenes.
334 Due to the large electronegativity of -NO3 group in nitrobenzene, the polarity of nitrobenzene is larger.
335 The asphaltene is the most polar component in coal tar system (Sun et al. 2015). According to the similar
336 miscibility principle, dipoles and electrostatic interactions are formed between asphaltene and
337 nitrobenzene molecules. Therefore, the Ee within asphaltene molecules is weakened and the asphaltene
338 aggregate structure is destroyed.
339 Benzene and nitrobenzene solvents can completely dissociate the hydrogenation asphaltene and seriously
340 destroy the aggregate structure of the original asphaltene. However, the damage of nitrobenzene to the
341 aggregate structure of the original asphaltene is more significant than that in the benzene solvent. This is
342 because nitrobenzene has strong polarity and aromaticity. It can not only interact with asphaltenes by
343 Evdw, but also interact with asphaltenes by Ee, which further affects the structure of asphaltene aggregate.
344 This shows that the Ee is one of the reasons leading to the association and settlement process of
345 asphaltenes.
17
346 Quinoline and pyridine solvents can destroy the aggregate structure of original asphaltene, but have little
347 effect on asphaltene aggregate after hydrogenation. It can be explained that the pyridine and quinoline
348 are strongly alkaline solvents (Lu et al. 2008). The content of heteroatoms in original asphaltene is high
349 producing hydrogen-bonded acid, which forms the hydrogen bond with pyridine or quinoline molecules,
350 furtherly destroying the aggregate structure of asphaltene. However, the number of heteroatoms in
351 hydrogenation asphaltene decreased. So, the disbanding effect on hydrogenation asphaltene is not
352 obvious. The n-heptane solvent has no effect on original and hydrogenated asphaltenes because of non-
353 polarity characteristic.
354 To sum up, aromatic compounds, strong basic compounds and strong polar compounds have good
355 dissociation effect on MLCT asphaltenes and can effectively inhibit the aggregation and sedimentation
356 of asphaltenes. Therefore, when designing or synthesizing inhibitors and scavengers for asphaltene
357 sedimentation, we should comprehensively consider the aromaticity, alkalinity and polarity of substances.
358 （2）Molecular deformation energy
359 Einternal reflects the deformation of molecular structure to a certain extent. The Einternal of asphaltenes in
360 solvents before and after MLCT hydrogenation were shown in Figure 9 at 298 K and 598 K. As Fig. 11
361 showed that the Einternal of the original and hydrogenation asphaltene aggregates was greatly affected by
362 temperature. The Einternal of hydrogenation asphaltene was smaller and more affected by temperature.
363 When the temperature increased from 298 K to 598 K, the Einternal of the original asphaltene aggregate
364 increased from 1300 to 1400 kcal·mol-1 and that of the hydrogenated asphaltene aggregate increased
365 from 680 to kcal·mol-1, indicating that temperature has a significant effect on the stability of the
366 asphaltene aggregate, especially has a greater effect on the hydrogenation asphaltene. This is due to the
367 small number of aromatic rings of asphaltene after hydrogenation. The layered structure between
368 asphaltene molecules is unstable and easy to be destroyed.
369 When the temperature is 298 K, the Einternal of asphaltene aggregate before and after hydrogenation
370 changed little in different solvents showing that the structure of asphaltene aggregate is stable and keep
371 small solubility in all kinds of solvents is normal temperature. When the temperature was 598 K, the
372 Einternal and ET of original asphaltene aggregate in nitrobenzene, benzene and pyridine solvents increased
373 obviously demonstrating that the C-C bonds in original asphaltene molecules in nitrobenzene, benzene
18
374 and pyridine solvents are twisted leading to the deformation of asphaltene aggregate. Although the Einternal
375 of hydrogenated asphaltene aggregate were different but with little difference in various solvents showing
376 that the deformation of hydrogenated asphaltene aggregate in solvents is very small.
Einternal EB EA ET EI 1500 800 Original 298 K Hydrogenated 298 K
1200 600
900 400
600
-1 200 300
0 0
1800 900 Original 598 K Hydrogenated 598 K
Energy/kacl·mol 1500
1200 600
900
600 300
300
0 0
Benzene Byridine -heptane Benzene Byridine -heptane Quinolinen Quinolinen 377 NitrobenzeneSolvent-free NitrobenzeneSolvent-free 378 Fig. 11 Comparison of bond energy of asphaltene aggregate in different solvents
379 5 Conclusion
380 The average molecular structure models of MLCT asphaltene before and after hydrogenation were
381 obtained by the characterization of the asphaltenes. Also, the aggregation structure models of MLCT
382 asphaltenes before and after hydrogenation were constructed in Materials Studio 8.0 software. The
383 optimal conformation was obtained through the kinetic optimization. The comparative analysis of
384 structural characteristics between original and hydrogenated asphaltenes were carried out. In order to get
385 the effects of different solvents on the structural characteristics, aggregation state, accumulation
386 characteristics and dominant force of asphaltene aggregates, the final configuration, molecular rotation
387 radius and interaction energy of asphaltene aggerates before and after hydrogenation were analyzed by
388 molecular dynamic simulation in different solvents. The conclusions are as follows:
389 (1) The unit structure and aggregate structure models of MLCT asphaltene before and after hydrogenation
390 were constructed and the kinetics optimization and MD simulation were carried out. After structural
391 optimization, the original asphaltene molecules become 2-3 asphaltene molecules with approximately
19
392 parallel, orderly arrangement and PAH stacking aggregate structure, while multiple aggregates self-
393 assemble to form more complex nano-aggregates, extending in different directions. The MLCT
394 asphaltene molecules after hydrogenation also form a PAH stacking association structure, which exists
395 in the form of aggregates, but the stacking number of asphaltene molecules is less than that of original
396 asphaltene. (2) The total interaction between MLCT original and hydrogenated asphaltenes mainly come
397 from the intermolecular van der Waals interaction, while the main driving force of petroleum asphaltene
398 aggregate comes from electrostatic interaction. Compared with original asphaltene, the absolute values
399 of van der Waals energy and electrostatic energy of hydrogenated asphaltene are smaller and more
400 affected by temperature. The intermolecular interaction of hydrogenation asphaltene is weaker and the
401 structure of hydrogenation asphaltene is unstable. (3) The disbanding effect of solvents on asphaltene is
402 as follows: Nitrobenzene > Quinoline > Benzene > Pyridine > Solvent-free > n-heptane (Original
403 asphaltene); Benzene > Nitrobenzene > Solvent-free> Quinoline > Pyridine > n-heptane (Hydrogenation
404 asphaltene). (4) The aromatic compounds, strong basic compounds and strong polar compounds have
405 good dissociation effect on MLCT asphaltenes and can effectively inhibit the aggregation and
406 sedimentation of asphaltenes. Therefore, when designing or synthesizing inhibitors and scavengers for
407 asphaltene sedimentation, it needs comprehensively consider the aromaticity, alkalinity and polarity of
408 substances.
409 The research results can not only provide a theoretical basis for the fluctuation control of product quality,
410 blockage of pipeline heat exchangers and oil storage in the current coal tar processing process, but also
411 provide technical support for continuous and stable operation and macro-control of industrial units.
412 Acknowledgments
413 Declarations
414 Funding: The financial support of this work was provided by the National Natural Science Foundation
415 of China (21978237)
416 Conflicts of interest/Competing interests: N/A
417 Availability of data and material: N/A
418 Code availability: N/A 20
419 Authors' contributions:
420 Xiaokang Han: Writing - Review & Editing, Conceptualization, Methodology, Writing - original draft.
421 Wenzho Yan: Visualization, Investigation, Writing - original draft.
422 Lu Xu: Software, Investigation, Writing - original draft.
423 Dong Li: Validation, Formal analysis.
424 Yonghong Zhu: Resources, Data curation.
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Figures
Figure 1
The 1H-NMR spectrum of LCT original and hydrogenated asphaltenes Figure 2
The speculated average structural of original and hydrogenated coal tar asphaltenes Figure 3
Dynamic optimization state of LCT original and hydrogenated asphaltenes Figure 4
The most stable conformations of original and hydrogenated coal tar asphaltenes Figure 5
Aggregation structural diagram of original and hydrogenation coal tar asphaltenes
Figure 6
Optimized con guration of original and hydrogenated coal tar asphaltene dimers Figure 7
The periodic protocell structures of asphaltene dimer in benzene, nitrobenzene, pyridine, quinoline and n- heptane Figure 8
The nal con gurations of asphaltene dimers in benzene, nitrobenzene, solvent-free state, pyridine, quinoline and n-heptane Figure 9
Comparison of Rogs of asphaltene dimers in different solvents Figure 10
Comparison of non-bond energy of asphaltene aggregates in different solvents Figure 11
Comparison of bond energy of asphaltene aggregate in different solvents