Three different types of heterocycle of nitrogen-containing alkaline ionic liquids treatment of acid oil to remove naphthenic acids

Article abstract of DOI:10.1016/j.cattod.2012.07.023

Ionic liquids had enormous potential for industrial use as environmental friendly chemicals. A green and effective deacidification method was found based on ionic liquids. N-alkylpyridinium bromide ([APy]Br), 1-alkyl-3- methylimidazolium bromide ([AMIm]Br) and 1-alkyl-3-methylimidazolium imidazolide ([AMIm]Im) were used to remove naphthenic acids from acid oil. The performance of deacidification followed the order [APy]Br < [AMIm]Br < [AMIm]Im. The stronger alkalinity of ionic liquids was, the higher deacidification would be. The growth of alkyl chain length was also beneficial to get high deacidification. The deacidification mechanism was that liquid clathrate could form due to interaction between the ionic liquids and π-bond in naphthenic acids through π-π interaction. When the reagent/oil ratio of [OMIm]Im (1-octyl-3-methylimidazolium imidazolide)/oil was 0.008, the deacidification rate could be reached 100%. The preliminary results reveal that the process consumes less energy, saves time and produces less pollution to environment.

Full text of DOI:10.1016/j.cattod.2012.07.023

Catalysis Today 212 (2013) 180–185
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Three different types of heterocycle of nitrogen-containing alkaline ionic
liquids treatment of acid oil to remove naphthenic acids
Jiyun Duan, Yu Sun, Li Shi∗
The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ionic liquids had enormous potential for industrial use as environmental friendly chemicals. A green
and effective deacidification method was found based on ionic liquids. N-alkylpyridinium bromide
([APy]Br), 1-alkyl-3-methylimidazolium bromide ([AMIm]Br) and 1-alkyl-3-methylimidazolium imi-
dazolide ([AMIm]Im) were used to remove naphthenic acids from acid oil. The performance of
deacidification followed the order [APy]Br < [AMIm]Br < [AMIm]Im. The stronger alkalinity of ionic liq-
uids was, the higher deacidification would be. The growth of alkyl chain length was also beneficial to get
high deacidification. The deacidification mechanism was that liquid clathrate could form due to inter-
action between the ionic liquids and ␲-bond in naphthenic acids through ␲–␲ interaction. When the
reagent/oil ratio of [OMIm]Im (1-octyl-3-methylimidazolium imidazolide)/oil was 0.008, the deacidifi-
cation rate could be reached 100%. The preliminary results reveal that the process consumes less energy,
saves time and produces less pollution to environment.
Received 12 January 2012
Received in revised form 2 June 2012
Accepted 26 July 2012
Available online 18 September 2012
Keywords:
Ionic liquids
Naphthenic acid
Deacidification
1-Alkyl-3-methylimidazolium bromide
N-alkylpyridinium bromide
1-Alkyl-3-methylimidazolium imidazolide
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
to solve these problems, to exploit the environmental friendly tech-
nology have been of keen interest in the development of chemical
In recent years, the world’s high-acid crude oil production
increases by 0.3% per year [1]. Crude oils are considered highly
acidic if their total acid number (TAN) exceeds 0.5 mg KOH/g by
titration. The acid number of oil is mainly caused by naphthenic
acid. The term “naphthenic acids” is used to account for all car-
boxylic acids present in crude oil, including acyclic and aromatic
acids, which are complicated mixtures. Aromatic rings or fused
aromatics are usually present in high-molecular-weight acids [2,3].
The presence of naphthenic acid is harmful for the production
equipment, storage and transport facilities, and can reduce the ser-
vice performance of oil [4]. On the other hand, pure naphthenic
aids are important raw material in the chemical industry, which
can be applied as a timber antiseptic, a paint drying reagent, an
additive in petroleum, and so on [5]. The methods that are cur-
rently used to remove the acid from oil include soda wash refining,
distillate hydrorefining, ammonia refining, solvent refining, and
adsorption refining [6]. However, there are many disadvantages
to these methods. The soda wash refining method badly pollutes
the environment, and wastes oil. The expenditure of distillate in
hydrorefining is very high, the process is very complicated and the
naphthenic acid cannot be recovered. The high-energy consump-
tion and environmental pollution are common problems. In order
industry.
Ionic liquid (IL), as a new green chemistry reagent, is typi-
cally nonvolatile, nonflammable, and thermally stable. In recent
years, significant progress has been made in the application of
room temperature ionic liquids, such as extractions, purifica-
tion, electrochemistry, and catalysis [7–12]. Yu et al. [13] found
that a low-viscosity ionic liquid of 1-ethyl-3-methylimidazolium
dicyanamide was capable of effectively extracting thiophene and
dibenzothiophene from oils, and the concept process was designed.
Zhang et al. [14] revealed that 1-alkyl-3-methylimidazolium
tetrafluoroborate and hexafluorophosphate and trimethylamine
hydrochloride showed remarkable selectivity for the absorption of
aromatic S- and N-containing molecules from transportation fuels.
Meindelsma et al. [15] observed that ionic liquids were suitable for
the separation of aromatic hydrocarbons from mixtures of aromatic
and aliphatic hydrocarbons. Butylpyridinium bromochloroalumi-
nate was successfully used to remove of trace olefins from
aromatics in our earlier research work [16]. In this paper, three
different kinds of nitrogen-containing alkaline ionic liquids, N-
alkylpyridinium bromide ([APy]Br), 1-alkyl-3-methylimidazolium
bromide ([AMIm]Br) and 1-alkyl-3-methylimidazolium imida-
zolide ([AMIm]Im) were used to remove naphthenic acids from
acid oil. The different alkalinity of these ionic liquids and the struc-
ture and size of the cations were investigated on the effects of
deacidification. Electrospray ionization mass spectrometry (ESI-
MS), at the electron energy of 70 eV, was used to shed light on the
∗
Corresponding author. Tel.: +86 21 64252274.
E-mail address: yyshi@ecust.edu.cn (L. Shi).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.07.023

J. Duan et al. / Catalysis Today 212 (2013) 180–185
181
Table 1
The properties of white oils.
Items
Properties
Kinematic viscosity (40 ◦C) (mm2 s−1
Flash point (◦C)
)
10.93
162
−40
+30
0
Pour point (◦C)
Color (Saybolt number)
Acid value (mg KOH g−1
)
Mechanical impurities (%)
Water content (%)
Alkanes
0
0
99.69
0.31
1.4629
<1
aromatics(ultraviolet absorbance 260–420 nm) cm
Refractive index (20 ◦C)
Arsenic content (␮g g−1
Lead content (␮g g−1
)
)
<1
deacidification mechanism from the molecular viewpoint. The
combination of these three different ILs provides a novel and green
method of deacidification, as well as a guide for further synthesis
and application of ILs.
Fig. 1. FT-IR of [BPy]Br.
2. Experimental
in a flask equipped with a magnetic stirrer. Equimolar amount
of imidazole was added at ambient temperature. After half
an hour, equal mole of 1-butyl-3-methylimidazolium bromide
([BMIm]Br) was added into the mixtures. And an appropriate
amount of anhydrous ether was needed 3 h later. The mixtures were
stirred overnight at ambient temperature. After filtration, rotary
evaporation, washed with anhydrous ether to give products 1-
butyl-3-methylimidazolium imidazolide ([BMIm]Im). [EMIm]Im,
[HMIm]Im, [OMIm]Im were prepared at the similar way. The struc-
ture of these ILs, as shown in Fig. 4, has been identified by FT-IR,
and the FT-IR data agree with ILs structures. Taking [BMIm]Im as
an example, its FT-IR data is shown in Fig. 3.
2.1. Materials
N-methylimidazole (density, 1.03 g/ml, purity >99%) was pur-
chased from Shanghai Chemical Reagent Co., Ltd., China. Pridine
(density, 0.98 g/ml, purity ≥99.5%), imidazole (density, 1.03 g/ml,
purity ≥99.5%), 1-bromoethane (BP, 311 K, density, 1.46 g/ml,
purity, CP), 1-bromobutane (BP, 375 K, density, 1.28 g/ml, purity,
CP), 1-bromohexane (BP, 429 K, density, 1.16 g/ml, purity, CP),
1-bromooctane (BP, 472 K, density, 1.11 g/ml, purity, CP) were
purchased from Shanghai Ling Feng Chemical Reagent Company,
China. Methanol (AR), ether (AR), naphthenic acid (CP) and sodium
hydroxide (AR) were purchased from Sinopharm Chemical Reagent
Company, Shanghai, China. White oil, whose properties are shown
in Table 1, was purchased from Sinopec Shanghai GaoQiao Petro-
chemical Corporation.
2.3. Preparation of model acid oil
The acid oil (referred to as oil below) used in this paper was
prepared by adding a certain amount of naphthenic acid to white
oil until the acid number to be a certain value.
2.2. Synthesis of ionic liquids
N-butylpyridinium bromide ([BPy]Br) was prepared as follows:
1-bromobutane was placed in a three-neck round-bottom flask
attached to a reflux condenser and equipped with a magnetic agi-
tator and a glass thermometer, and pridine was added using a
dropping funnel at the temperature of 333–338 K. After 12 h of reac-
tion, the mixtures were allowed to cool down to room temperature,
and the solvent was removed through distillation under vacuum,
with N-butylpyridinium bromide ([BPy]Br) remaining in the flask.
[EPy]Br, [HPy]Br, [OPy]Br were synthesized at the same way. The
structure of these ILs, as shown in Fig. 4, has been identified by FT-
IR, and the FT-IR data agree with ILs structures. Taking [BPy]Br as
an example, its FT-IR data is shown in Fig. 1.
2.4. Deacidification process
Oil and ionic liquids were put into an Erlenmeyer flask equipped
with a magnetic stirrer. The mixtures were stirred at a constant
temperature during the process. After the reaction, the mixtures
1-Butyl-3-methylimidazolium bromide ([BMIm]Br) was pre-
pared as follows: N-methylimidazole was placed in a three-neck
round-bottom flask attached to a reflux condenser. 1-Bromobutane
was added using a dropping funnel at 343 K in the flask. The reactant
mixtures were then cautiously heated to reflux for hours until the
mixtures became yellowish. The solvent was removed through dis-
tillation in vacuum, left ionic liquids 1-butyl-3-methylimidazolium
bromide in the flask. [EMIm]Br, [HMIm]Br, [OMIm]Br were synthe-
sized at the same way. The structure of these ILs, as shown in Fig. 4,
has been identified by FT-IR, and the FT-IR data agree with ILs struc-
tures. Taking [BMIm]Br as an example, its FT-IR data is shown in
Fig. 2.
1-Butyl-3-methylimidazolium imidazolide ([BMIm]Im) was
prepared as follows: methanol solution of NaOH was placed
Fig. 2. FT-IR of [BMIm]Br.

182
J. Duan et al. / Catalysis Today 212 (2013) 180–185
Table 2
Effect of alkalinity of ionic liquids on deacidification: T = 323 K, t = 2 h.
Ionic liquid
The reagent/oil ratio
Deacidification
rate (%)
[EPy]Br
[BPy]Br
[HPy]Br
[OPy]Br
[EMIm]Br
[BMIm]Br
[HMIm]Br
[OMIm]Br
[EMIm]Im
[BMIm]Im
[HMIm]Im
[OMIm]Im
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.016
0.012
0.010
0.008
3.47
27.76
55.17
64.44
26.76
34.54
63.24
93.22
100.00
100.00
100.00
100.00
the longer the alkyl chain length was, the higher the deacidification
rate would be, which would be discussed below.
Fig. 3. FT-IR of [BMIm]Im.
Pyridine and imidazole both contain six ␲-electrons. Their elec-
tronic structures are in agreement with Hückel’s (4n + 2) ␲-electron
rule, so we can judge that pyridine and imidazole are aromatic
substances. The nitrogen atom of imidazole is sp2 hybridization
and provides two electrons to form the large ␲-bond, whereas all
of the ␲-electrons of pyridine are provided by the double bond.
Thus, imidazole has an aromatic ring of electron-rich, while pyri-
dine has an electron-deficient ring. Consequently, [APy]Br and
[AMIm]Br with the same anions, and their electron cloud den-
sity followed the order [APy]Br < [AMIm]Br. So [AMIm]Br had
stronger alkalinity than [APy]Br. On the other hand, [AMIm]Im
contained imidazolium rings both in cations and anions, the
electron cloud density of imidazolium anion would be further
strengthened, so imidazolium anion had greater electron density
than bromide anion, and had stronger alkalinity than [AMIm]Br.
Thus [AMIm]Im had the strongest alkalinity, and the order was
[APy]Br < [AMIm]Br < [AMIm]Im. The conclusion was that the ionic
liquid would have better performance on deacidification with
stronger alkalinity. Additionally, according to acid–base ionization
theory, for the ionic liquids containing the same cation, the value
of pKb of anion reflects the strength of alkalinity of ionic liquids,
basically, the smaller value of pKb, the stronger alkalinity of ionic
liquids. The value of pKb of imidazole anion is −0.5 [18], just slightly
larger than hydroxyl (pKb = −1.7). Nevertheless imidazole anion has
a bit weaker alkalinity than hydroxyl, the thermal stability of the
imidazole anion much stronger than hydroxyl. So [AMIm]Im had
excellent performance on deacidification.
were put aside for 2 h at room temperature to gravity separate the
ionic liquids with the acid compounds from the oil.
2.5. Determination of the TAN of oil
In this paper, the determination of TAN in oil was routinely
carried out by a standard method in the oil industry using Ameri-
can Society for Testing and Materials (ASTM) D664, the formula of
deacidification rate was as follows:
X0 − Xs
R =
× 100%
(1)
X0
where R is the deacidification rate; X0 is the acid number of original
distillate oil; and Xs is the acid number after reaction.
3. Results and discussion
3.1. Effect of alkalinity of ionic liquids
The strength of alkalinity of ionic liquids had a great influence
in the deacidification. To study the relationship between the two,
[APy]Br, [AMIm]Br and [AMIm]Im were mixed with the oil. The
effect of alkalinity of ILs is shown in Table 2.
It could be seen that these three different kinds of ionic
liquids had significantly different performance on deacidifi-
cation, and the performance of deacidification followed the
order [APy]Br < [AMIm]Br < [AMIm]Im. [AMIm]Im had outstanding
capability, with the smallest reagent/oil ratio of 0.008, the deacid-
ification rate of 100% would be achieved, which was much better
than our previously obtained results, with the reagent/oil ratio of
[BMIM]Br–AlCl3/oil of 0.04, and the reaction time of 4 h, the deacid-
ification just reached up to 75.9% [17]. In each kind of ionic liquid,
3.2. Effect of different size of cations
To study the influence of different length of alkyl chain of
ionic liquids on the deacidification rate, [EMIm]Im, [BMIm]Im,
[HMIm]Im and [OMIm]Im were mixed with the oil. The effect of
different lengths of alkyl chain is shown in Table 3.
Fig. 4. Chemical structures of ILs.

J. Duan et al. / Catalysis Today 212 (2013) 180–185
183
Table 3
Effect of size of cations of ionic liquids on deacidification: T = 323 K, t = 2 h.
The reagent/oil ratio
[EMIm]Im (%)
[BMIm]Im (%)
[HMIm]Im (%)
[OMIm]Im (%)
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.020
0.024
80.66
–
87.24
–
91.91
96.82
100
100
100
82.63
–
93.84
96.46
100
100
100
100
100
83.43
–
92.88
98.08
100
100
100
100
100
100
100
99.44
100
100
100
100
100
100
As long as the ratio of reagent/oil was big enough, deacidification
rate could be reached 100% in the role of each IL. While with the
growth of carbon chain length, less quality of reagent needed to
achieve the highest deacidification rate. [OMIm]Im required for the
smallest ratio of reagent/oil of 0.008, [HMIm]Im, [BMIm]Im and
[EMIm]Im needed 0.01, 0.012, 0.016, respectively.
reagent/oil ratio. This may be related to the viscosity-temperature
nature of crude oil. With the increase of temperature, the viscos-
ity of the oil was reduced, which was beneficial to mix the ionic
liquids and oil. When the temperature was more than 323 K, the
effect of the temperature on the viscosity became smaller and the
deacidification rate tended to be a constant value.
As discussed above, strong alkalinity of ILs were beneficial for
deacidification. Although the alkyl chain length change had no
significant effect on polarity of the ionic liquid [19], it affected
the strength of alkalinity of ionic liquids. This is because that
alkyl chain was electrondonating group, the longer the alkyl chain
length was, the stronger the ability of electrondonating was, so the
stronger the alkalinity of ionic liquids would be, and the less qual-
ity of reagent was needed to reach the same deacidification rate.
[OMIm]Im had obviously stronger alkalinity because of its long
enough alkyl chain, and had the best performance on deacidifi-
cation. From Tables 1 and 2 we could see that this phenomenon
existed in each kind of IL, and the effect became not significant
in strong alkaline ILs. Thus, increasing the length of alkyl chain
was beneficial in deacidification, while this effect depended on the
strength of alkalinity of ionic liquid itself. With weak alkalinity of
ionic liquid, the long alkyl chain would increase the deacidification
obviously.
The important thing should pay attention to was that every
curve had a turning point temperature, after which the deacidifica-
tion rate increased sharply. The reason may be that naphthenic acid
in nonpolar liquids formed hydrogen bonds and built dimmers. For
the successful extraction and binding to ionic liquids. These dim-
mers must be broken down with the consumption of some energy.
Once the reaction was induced, it could be carried on effectively.
Although the reaction temperature increased slightly from 313 K to
323 K, the deacidification rate had a qualitative leap. On the other
hand, at high enough temperature, almost 100% deacidification rate
could be reached with low reagent/oil ratio, such as 0.006, 0.004.
From the points of saving energy and maximizing activities of ionic
liquids, the optimal reaction temperature was 323 K.
3.4. Effect of different acid number of oil
More naphthenic acid were added to white oil to make its total
acid number (TAN) higher. The reactions operated under the con-
ditions of the temperature of 323 K, the treating time of 2 h and
reagent/oil ratio of 0.008, was used to know the performance of
ionic liquid [OMIm]Im on treating the oil with different acid num-
ber, especially high total acid number of oil.
From Fig. 6 we could know that the ionic liquids with long alkyl
chain had good performance on the treatment of high total acid
number oil. It was found that the deacidification rates were 100%
when the TAN was changed from 0.21 mg KOH/g to 1.29 mg KOH/g
3.3. Effect of temperature on deacidification rate
When the ionic liquid was [OMIm]Im, the reaction time was
2 h, the resting time was 2 h, and the reagent/oil ratio were 0.004,
0.006, 0.008, respectively, the effect of reaction temperature on the
deacidification was tested.
It could be observed from Fig. 5 that the deacidification rate
increased with the reaction temperature increasing under every
Fig. 5. Effect of reaction temperature.
Fig. 6. Effect of different acid number of oil.

184
J. Duan et al. / Catalysis Today 212 (2013) 180–185
using [OMIm]Im. When the TAN was larger, the deacidification rate,
more than 90%, decreased slightly. Because of the unchanged ratio
of ILs/oil and the increase of TAN, the handling capacity of ionic
liquid was dropped, which leaded to decline in the rate of deacid-
ification. So we concluded that the ionic liquid [OMIm]Im was the
optimized ionic liquid, and could be used to remove naphthenic
acid in the oil with high TAN.
3.5. Repetitive reaction of ionic liquids
The reaction performances of repetitive reaction of ionic liquid
[OMIm]Im were supposed to know the possibility of continuous
reaction. After reaction, the white oil was separated from the mix-
tures and the ionic liquid was left in the flask. Put fresh white oil in
the same flask and reacted for another time, repeated many times.
The typical results are presented in Fig. 7
As shown in Fig. 7, at the reaction temperature of 323 K,
[OMIm]Im with reagent/oil ratio of 0.008 could keep the deacid-
ification rate of 100% at the first five reactions. After that, the
deacidification rate began to decline in the same conditions. The
rates were 59.36% at the eighth time and 27.27% at the ninth
time, it seems that the ionic liquid deactivated time after time
and there was no activity after the tenth reaction. Finally we
choose [OMIm]Im to remove the naphthenic acids, and at the first
seven reactions more than 80% of the deacidification rate could
Fig. 7. Repetitive reaction of ionic liquids.
Fig. 8. The cations and anions of ionic liquids before and after reactions.

J. Duan et al. / Catalysis Today 212 (2013) 180–185
185
be achieved, with the 0.008 reagent/oil ratio and at 323 K reaction
deacidification
following
the
order
temperature.
[APy]Br < [AMIm]Br < [AMIm]Im. The stronger alkalinity of
ionic liquids was, the higher deacidification would be. With
the increasing length of alkyl chain, each kind of ionic liquids
would have better performance of deacidification. As using
[AMIm]Im, the performance of deacidification followed the order
[EMIm]Im < [BMIm]Im < [HMIm]Im < [OMIm]Im. High reaction
temperature could decrease the reagent/oil ratio to reach maxi-
mum deacidification rate with each ionic liquid. The deacidification
mechanism was discussed using ESI-MS. Liquid clathrate could
form through ␲–␲ interaction between naphthenic acid with
unsaturated ␲ bond and the ILs with heterocyclic ␲ bond. Finally,
at the temperature of 323 K, the reagent/oil ratio of [OMIm]Im-to-
oil of 0.008, the reaction time of 2 h, the deacidification rate could
reach 100%, which had been greatly improved compared with
our previous work. It was possible to make continuous reaction
a reality because of the reusability of [OMIm]Im. The preliminary
results revealed that the process was feasible, product separates
easier and produces less pollution to environment. Using ionic
liquids to remove naphthenic acids from acid oil would become a
green and effective method.
3.6. Discuss the deacidification mechanism
To study the deacidification mechanism, electrospray ionization
mass spectrometry (ESI-MS) was used to characterize the ionic liq-
uids before and after reactions. From Fig. 8, we could see that there
was not any change with cations before and after reactions. The
peaks of anions increased through the process, while no change
occurred in main peaks (Fig. 8). Thus, we judged that there are not
significant differences in structure of ionic liquid before and after
reactions.
Holbrey et al. [20] indicated that liquid clathrate could form due
to the ␲–␲ interaction. The mechanism for removal naphthenic
acid could be explained as a possible ␲–␲ interaction between
naphthenic acid with unsaturated ␲ bond and the ILs with het-
erocyclic ␲ bond, thus naphthenic acid was entrapped to clathrate
structure of ILs to achieve the removal of naphthenic acid. The fol-
lowing experimental results could be explained by this: (1) the
larger the heterocyclic ␲-electron density of ionic liquid was, that
is, the stronger the ionic liquid alkalinity, the better the effect of
deacidification would be; (2) the longer alkyl chain ionic liquids
could make its alkalinity enhanced. On the other hand, it made the
space of clathrate structure bigger, which contributed to the storage
capacity of ILs, thus the deacidification rate increased and (3) in the
experiment without any regeneration operations, under the condi-
tion of less than 100% deacidification rate, the ILs still showed good
performance on deacidification in the next repetitive reactions.
The reason was that each ILs molecule could accommodate more
than one naphthenic acid molecule. When the ratio of naphthenic
acid decreased to a certain value in the oil, due to the surround-
ing of alkane molecules, which made naphthenic acid difficult to
enter the clathrate structure of ILs, then the naphthenic acid could
not be removed completely. However, naphthenic acid molecular
ratio increased after adding fresh acid oil, which made them break
into the surrounding of alkane molecules. Then ILs showed good
performance again until the clathrate structure was filled up. So
the phenomenon of experiments could be explained well by this
mechanism.
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Imidazolium imidazolide-based ionic liquids were found to be
most effective for removal naphthenic acids from acid oil with
low reagent/oil ratio. Pyridinium-based ionic liquids, idazolium-
based ionic liquids and imidazolium imidazolide-based ionic
liquids with different alkalinity had different performance on