China Petroleum Processing and Petrochemical Technology
Simulation and Optimization
2013, Vol. 15, No. 3, pp 79-85
September 30, 2013
Experimental and Molecular Dynamics Simulations for
Investigating the Effect of Fatty Acid and Its Derivatives
on Low Sulfur Diesel Lubricity
Luo Hui; Fan Weiyu; Li Yang; Zhao Pinhui; Nan Guozhi
(State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555)
Abstract: In this work, fatty acid and its derivatives were adopted as lubricity additives for low sulfur diesel. Tribological
evaluation obtained from the High-Frequency Reciprocating Rig (HFRR) apparatus showed that the lubricating performance
of the additives increased in the following order: stearic acid>glycol monopalmitate>stearyl alcohol>ethyl palmitate>cetyl
ethyl ether. The adsorption behavior of the additives on Fe (110) surface and Fe2O3 (001) surface was investigated by molecular dynamics (MD) simulations to verify their lubricity performance. The results suggested that adsorption energies of
the additives on Fe (110) surface are determined by the van der Waals forces, while adsorptions on Fe2O3 (001) surface are
significantly attributed to the electrostatic attractive forces. Higher values of adsorption energy of the additives on Fe2O3 (001)
surface indicate that the additive has more efficient lubricity enhancing properties.
Key words: MD simulation; adsorption; lubricity additive; fatty acid; low-sulfur diesel
1　Introduction
Due to environmental requirements, increasingly strict
regulations on the sulfur content of diesel fuel were legislated, and petroleum refiners are engaging in production of low sulfur diesel with enthusiasm. However, the
desulfurization process also removes from diesel fraction
the oxygen and nitrogen compounds, which are responsible for the lubricity of diesel fuel[1-3]. Generally, when
the sulfur content of petroleum-based diesel fuel is less
than 500 μg/g, it could produce unacceptable wear [4].
Treating diesel fuel with lubricity additives have been
proposed to compensate for the deterioration in natural
lubricity of diesel and eliminate the excessive wear of
rubbing surface. Oxygen containing compounds such as
long chain fatty acids and their derivatives are superior
lubricity additives because of their polarity-imparting
oxygen atoms[5-9].
Boundary lubrication formed by the polar compounds
plays a key role in the lubrication of fuels[10-11]. Lubricity additives have a high affinity to metallic surfaces and
could form a thin protective metal-metal contact film[12-13].
This film was considered most important to reduce the
number of points at which true metal to metal contact
occurs. Rupture of the lubricant film will lead to deterioration in friction and wear[4], which suggests that the
strength and stability of the film are key factors for diesel lubricity. An order of oxygenated lubricity additives
(COOH>CHO>OH>COOCH3>C=O>C-O-C) has been
obtained from studying various oxygenated 10-carbon-atom (C10) compounds[8]. In the case of the ethers, alcohols
or ethyl acetate, alcohols appear to be better lubricants
than ethers and ethyl acetate[3, 6]. Although there have
been several studies on the lubricity performance of oxygen compounds, the mechanism relating to the influence
of molecular structure on the lubricity performance of the
lubricity additive has rarely been studied.
The interactions between the additives and the metal surface are difficult to measure experimentally. However,
applying the molecular dynamics (MD) simulation would
be a powerful method to investigate the adsorption behavior at the microscopic molecular level and provide useful
information[14-18]. Yanagisawa investigated the adsorption
Recieved date: 2013-04-15; Accepted date: 2013-05-04.
Corresponding Author: Prof. Fan Weiyu, E-mail: fanwyu@
upc.edu.cn.
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China Petroleum Processing and Petrochemical Technology
of typical organic molecules onto Fe (110) surface using density functional theory[19]. The results showed that
acetic acid, dimethyl ether, methylacetate and methanol
did not generate appreciable chemical adsorption on the
iron surface, and such adsorption is attributed to small
physical interactions between a molecule and the iron
surface, such as van der Waals and electrostatic interactions. These physical interactions could be obtained using
molecular simulation method. Khaled used MD simulations to find the most stable adsorption sites for thiourea
derivatives on Fe (110) surface[20]. It suggested that the
efficiency order of thiourea derivatives adopted as corrosion inhibitor could be verified by the binding energies of
these molecules adsorbed on iron surface. Therefore, the
MD simulation could be used to describe the adsorption
behavior of fatty acid and its derivatives on iron surface
and provide further insight into their lubricity efficiency.
The objective of this work is to investigate the lubricity
evaluation of low sulfur diesel additized with fatty acid
and its derivatives, such as alcohol, ester and ether, by a
high frequency reciprocating rig (HFRR) apparatus. Subsequently, the adhesiveness of fatty acid and its derivatives
on iron and iron oxide surfaces was examined by using the
MD simulations, which could provide an interpretation of
the correlations between the polar groups of lubricity additives and their lubricity enhancing performance.
2013,15(3):79-85
and then distilled in a vacuum evaporator to obtain the
final products.
2.2　Lubricity measurements
A hydrotreated diesel supplied by the Qilu Petrochemical Company was adopted as the test fuel in this study,
with its typical properties listed in Table 1. The sulfur
and nitrogen content of the diesel was determined using
an Antek 9000 sulfur and nitrogen fluorescence analyzer.
Density, viscosity, and distillation range of the diesel sample were determined according to the standard method
GB/T 2540—1981, GB 265—1975, and GB 255—1977,
respectively. Additives were added to the test fuel in a
concentration range from 50 μg/g to 500 μg/g. All lubricity measurements were performed by the High-Frequency
Reciprocating Rig (HFRR) apparatus according to the
ISO-12156 method.
Table 1　Properties of the hydrotreated diesel fuel
Items
Specific gravity
(20 ℃), g/cm3
Sulfur content, μg/g
Data
Items
Data
2
1.84
Nitrogen content, μg/g
2.58
0.771 4 Viscosity (40 ℃), mm /s
9.21
Distillation, ℃
Hydrocarbon group analysis, %
IBP
196
Alkanes
60.4
50%
219
Cycloalkanes
28.7
90%
262
Aromatics
10.9
EP
294
HFRR WSD Value, μm
648
2　Experimental Details and Computational Methods
2.3　MD simulations
2.1　Lubricity additive preparation
During the desulfurization process, the polar compounds,
Five oxygen compounds, such as stearic acid (SA), stearyl alcohol (SAL), ethyl palmitate (EP), glycol monopalmitate (GMP) and cetyl ethyl ether (CEE) were chosen as
lubricity additives. The esters were prepared via reaction
of their corresponding fatty acid with alcohols in the presence of p-toluenesulfonic acid used as catalyst. The fatty
acid and alcohols were mixed at a molar ratio of 1:1 and
dissolved in toluene. The dosage of the catalysts was 1.0%
of the reaction mixture. The reaction mixture was stirred
at 80—160 ℃ for 5h. The reaction process was monitored
through determination of acid value and hydroxy value
of the product. Afterward, the reaction products were
washed with 5.0% of aqueous sodium bicarbonate. The
organic phase was dried over anhydrous sodium sulfate
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such as nitrogen and oxygen compounds would be removed simultaneously, resulting in a diesel with low polar compounds content. It is difficult to form a protective
layer on the metal surface for the low polar components.
Therefore, the lubricity additives, such as SA, SAL, EP,
GMP, or CEE, were adopted as adsorbate. Fe (110) surface and Fe2O3 (001) surface were chosen as substrabte,
and their sizes had dimensions of 3.475 3 nm×3.475 3
nm×1.418 8 nm, and 3.524 5 nm×3.524 5 nm×1.312 5 nm,
respectively. The condensed-phase optimized molecular
potentials for atomistic simulation studies (COMPASS) [21]
forcefield as applied in Accelrys Materials Studio (MS)
5.5 was used for the MD simulations. The atom-based
method with a cutoff of 1.55 nm was applied to compute
Luo Hui, et al. Experimental and Molecular Dynamics Simulations for Investigating the Effect of Fatty Acid and Its Derivatives on Low Sulfur Diesel Lubricity
the non-bonds Van der Waals and Coulomb interactions.
All the energy minimization steps were performed by
the Smart Minimizer algorithm, and the convergence criterion adopted for the value of maximum force was 102
kcal/(mol·nm) for Steepest Descent, 1 kcal/(mol·nm) for
Conjugate Gradient, and 10-6 kcal/(mol·nm) for NewtonRaphson Gradient. The optimized models are shown in
Figure 1.
Figure 1　The optimized models
(a)—stearic acid (SA); (b)—stearyl alcohol (SAL); (c)—ethyl palmitate
(EP); (d)—glycol monopalmitate (GMP); (e)—cetyl ethyl ether (CEE);
(f)—Fe (110) surface, (g)—Fe2O3 (001) surface
Adsorption Locator package included in MS software
was utilized to identify preferential binding sites for an
adsorbate molecule on the surfaces. Simulated annealing using the Metropolis Monte Carlo method has been
implemented to calculate the adsorption density and the
binding energy. Possible adsorption configurations have
been sampled by carrying out Monte Carlo searches of the
configuration space of the additives on the iron surface
system as the temperature is gradually decreased. In this
study, the automated temperature control was adopted,
and 100 temperature cycles were employed for each run.
The binding energy of adsorbates adsorbed on the surfaces is calculated as follows:
Ebinding=Etotal–(Esurface+Eadsorption)
(1)
where Etotal is the total energy of the surface and adsorbate, Esurface is the single-point energy of the surface, and
Eadsorbate is the single-point energy of adsorbate molecular.
3　Results and Discussion
regularly with an increasing concentration of these additives. There is a specific influence of the polar groups
of the additives on their lubricating performance, and an
order of enhancing lubricity (SA>GMP>SAL>EP>CEE)
is obtained.
Figure 2　Effects of fatty acid and its derivatives on the
lubrication properties of low-sulfur diesel fuel
■—Cetyl ethyl ether; ●—Ethyl palmitate; ▲—Stearyl alcohol;
▼—Glycol monopalmitate;
—Stearic acid
3.2　The additives adsorbed on Fe (110) surface
Early studies have indicated that the adsorption forces of
long-chain alkyl fatty acids, alcohols and esters on metal
surfaces were largely physical[22], no evidence regarding
the reaction of these organic compounds with zinc, iron,
platinum, or gold was identified, although adsorption occurred. In the current study, to find out the preferential
adsorption sites of the studied compounds on the iron surface and iron oxide surface, the adsorption behavior was
investigated theoretically by MD simulation methods.
The most suitable adsorption configurations for the oxygen compounds on Fe (110) surface are shown in Figure
3. It can be seen from Figure 3 that the oxygen-containing
group and the carbon chain of the studied compounds remain almost parallel to Fe (110) surface. The adsorption
density of the oxygen compounds on the Fe (110) surface is illustrated in Figure 4. High values of adsorption
density indicate that the oxygen compounds are likely to
adsorb on the iron surface and form stable films to protect
3.1　Lubrication properties
iron from excessive wear.
The results of the HFRR analysis of fatty acid and its
derivatives are shown in Figure 2. It can be seen from
Figure 2 that the WSD of the base diesel fuel decreased
The outputs and descriptors obtained from the adsorption locator module are listed in Table 2. The total energy
(Etotal) of the substrate-adsorbate configuration is defined
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China Petroleum Processing and Petrochemical Technology
Figure 3　The most suitable configuration of the additives
adsorbed on Fe (110) surface
2013,15(3):79-85
as the sum of the energies of the adsorbate components
and the adsorption energy (Eadsorption). The substrate energy
is taken as zero in adsorption locator module. Eadsorption
reports the energy released (or required) when the relaxed
adsorbate component is adsorbed on the substrate and it
is the sum of the rigid adsorption energy (ERigid adsorption)
and the deformation energy (Edeformation) for the adsorbate
components. ERigid adsorption is defined as the energy released
(or required) when the unrelaxed adsorbate components
(i.e., before the geometry optimization step) are adsorbed
on the substrate. EDeformation reports the energy released
when the adsorbed adsorbate components are relaxed on
Figure 4　The adsorption density field of the additives adsorbed on Fe (110) surface
Table 2　Results obtained from the Mont Carlo simulation for adsorption of the additives on Fe (110) surface
Additive
ETotal,kJ/mol
EAdsorption,kJ/mol
ERigid adsorption, kJ/mol
EDeformation, kJ/mol
dEad/dNi, kJ/mol
SA
SAL
EP
GMP
CEE
-1 151.76
-1 028.11
-1 016.04
-1 030.33
-988.76
-840.55
-818.30
-837.56
-864.45
-816.91
-829.23
-816.03
-832.44
-853.17
-815.73
-11.32
-2.27
-5.11
-11.28
-1.18
-840.55
-818.30
-837.56
-864.45
-816.91
the substrate surface. dEad/dNi is defined as the energy
of substrate-adsorbate configurations where one of the
adsorbate components has been removed. The binding
energy (Ebinding) presented in Table 2 is calculated from
Eq. (1), which consists of two parts: the van der Waals
part and the electrostatic part.
As reported by Desai, et al.[23], the van der Waals interactions are dominant in adsorption of lone-pair electrons of
oxygen on metal surfaces. The data listed in Table 2 also
indicate that the binding energies between the oxygen
compounds and Fe (110) surface are determined by the
van der Waals forces while the electrostatic energies are
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Ebinding, kJ/mol
van der Waals
Electrostatic
829.35
0
816.20
0
832.49
0
852.72
0
815.77
0
zero. The binding energy and the adsorption energy of additives decrease in the following order: GMP>EP ≈ SA
>SAL ≈ CEE, which can be attributed to the fact that the
magnitude of van der Waals force depends on the relative
molecular mass, and a high mass produces a larger force.
In addition, the order of the binding energy is consistent
with the case of adsorption density as shown in Figure 4.
3.3　T he additives adsorbed on Fe 2O 3 (001)
surface
Because the clean iron surface is an uncharged surface,
the interaction of the molecules with the iron surface is
Luo Hui, et al. Experimental and Molecular Dynamics Simulations for Investigating the Effect of Fatty Acid and Its Derivatives on Low Sulfur Diesel Lubricity
resulted from the van der Waals forces. There are no obvious effects of the functional groups on the adsorption
intensity. Iron could react readily with oxygen to form a
layer of oxide to protect the rest of the metal. The presence of this oxide layer on the iron surface could encourage the adsorption[24].
Figure 5 shows the most suitable adsorption configurations of the oxygen compounds on Fe2O3 (001) surface
obtained from MD simulations. The adsorption density of
the molecules on the Fe2O3 (001) surface is presented in
Figure 6, and it is higher than the one on the Fe (110) surface, which suggests that the adsorbates have a higher af-
Figure 5　The most suitable configuration of the molecules
finity to iron oxide surfaces for forming a protective film.
adsorbed on Fe2O3 (001) surface
Figure 6　The adsorption density field of the additives adsorbed on Fe2O3 (001) surface
Actually, a past experimental study suggested that the extent of irreversible adsorption of polar organic compounds
on steel was dependent on the functional groups and
decreased in the following order: acids, alcohols, esters
[25]
. In particular, the value of adsorption density obtained
from MD simulation decreases in the following order:
SA>GMP ≈SAL>EP>CEE, which is in good agreement
with the experimental results. Therefore, the adsorption
on the oxidized surface should be more prevalent for the
oxygen compounds.
The outputs and descriptors of the adsorbates adsorbed
on Fe2O3 (001) surface are presented in Table 3. It can be
seen from Table 3 that the binding energies of the oxygen
compounds on Fe2O3 (001) surface are much larger than
in the case of Fe (110) surface, and the higher values of
binding energy indicate that the presence of oxide layer
on the iron surface could encourage the adsorption of the
oxygen compounds on the surface, mainly via electrostatic attractive forces. There is little difference between
the van der Waals forces of the molecules on Fe2O3 (001)
surface and the binding energies are originated mainly by
the electrostatic attractive forces. Therefore, it is evident
Table 3　Results obtained from the Mont Carlo simulation for adsorption of the additives on Fe2O3 (001) surface
Molecule ETotal, kJ/mol EAdsorption, kJ/mol ERigid adsorption, kJ/mol EDeformation, kJ/mol dEad/dNi, kJ/mol
SA
SAL
EP
GMP
CEE
-2 412.68
-2 003.88
-1 998.88
-2 312.37
-1 728.02
-2 101.47
-1 820.47
-1 794.01
-2 146.49
-1 514.61
-2 378.82
-1 952.67
-1 952.61
-2 338.29
-1 622.62
277.35
132.20
158.60
191.80
108.01
-2 101.47
-1 820.47
-1 794.01
-2 146.49
-1 514.61
Ebinding, kJ/mol
van der Waals Electrostatic
59.32
2 319.74
50.58
1 932.41
59.71
1 892.95
65.32
2 272.28
51.57
1 571.55
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China Petroleum Processing and Petrochemical Technology
that ionic interactions of a metal substrate with a polar
molecule due to hydrogen bonding and Debye orientation
forces (electrostatic forces) are considerably stronger than
those based on dipole (van der Waals) forces.
The data listed in Table 3 show that the binding energy of
the adsorbates adsorbed on Fe2O3 (001) surface decreases
in the following order SA>GMP>SAL>EP>CEE. Judging from the polarity of the additives, it is suggested that
the binding energy is confirmed to increase markedly with
the increase in the polarity of functional groups. Further,
the lubricity efficiency of additives is found to follow
the same order of their binding energy on Fe2O3 (001)
surface. This means that the additive with a more polarity
would have a higher affinity to the iron oxide surface and
could improve the lubricity more effectively.
2013,15(3):79-85
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Acknowledgment: This work was financially supported by
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Zhejiang University Tackles the Problem of Homogeneous
Distribution of Heterogeneous Catalyst
Zhejiang University’s New Hecheng R&D Center via the
hydrothermal method for carbonization of glucose serving as the carbon source has manufactured a premium
high-efficiency catalyst supported on nitrogen-containing
nanoscale microspheres to realize in a definite degree the
homogeneous distribution of heterogeneous catalyst to
greatly speed up the reaction rate.
Recently the functionalization of carbon materials has
blasted a new idea for designing heterogeneous catalysts.
The hydrothermal method is a novel method for manufacture of porous carbon-based materials. Preparation
of the topologically adjustable nanoscale microspheres
with particle size less than 100 nm is a tough issue for
hydrothermal method aiming at preparation of carbonbased materials. Mr. Wang Yong and other staffs of New
Hecheng R&D Center by combining the polyionic liquid
serving as the structure adjusting agent with the hydrothermal method have prepared carbon microspheres with
a particle size of around 50 nm to successfully grapple
with the problem for preparing microspherical nanoscale
carbon material (≤100 nm). Afterwards they have tried to
use the nitrogen-containing nano-carbon microspheres as
the support for the nano-palladium particles that can function
as the high-efficiency catalyst to achieve the high-efficiency
catalytic oxidation of C—H and O—H bonds under mild
condition in the presence of air serving as the cheap oxidant.
Successful Application of Novel CTP-VA Type Pd/C Catalyst
Developed by SINOPEC Shanghai Petrochemical Research Institute
Following the successful commercial application of the CTPIV type catalyst for p-phthalic acid hydrotreating, a new generation catalyst for hydrotreating of para-phthalic acid developed
by the SINOPEC Shanghai Petrochemical Research Institute
(SPRI) has been successfully applied in the commissioning of
the 400 kt/a PTA unit at the Shanghai Petrochemical Company.
Since the commissioning of the PTA unit on May 30, 2013,
despite fluctuations of raw para-phthalic acid quality at the
start, the novel catalyst has stood up to the strong impact
of feedstock quality to deliver premium grade PTA at full
load. The preliminary conclusion has shown that the CTPVA type Pd/C catalyst has resolved the issue related with
excessively high catalyst activity at the beginning and instable induction period of product caused by higher content
of impurities. The successful application of the CTP-VA
type Pd/C catalyst at the Shanghai Petrochemical Company
has symbolized that the SPRI has stepped onto a higher
rung in research on the Pd/C catalyst.
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