Layered Double

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Research on the synthesis of ionic liquids/layered double hydroxides intercalation composites and their application on removal of naphthenic acid from oil Xiangsheng Shao, Guanhao Liu, Jingyi Yang, and Xinru Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01879 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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Research on the synthesis of ionic liquids/layered double hydroxides intercalation composites and their application on removal of naphthenic acid from oil XiangSheng Shao, Guanhao Liu, Jingyi Yang*, Xinru Xu The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China * Corresponding author. Tel.: +86 021 64252160; fax: +86 021 64252160 E-mail address: [email protected] (Jingyi Yang)

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Abstract Three kinds of imidazolium ionic liquids were synthesized and the [Bmim][CH3COO] was selected due to its higher deacidification rate at lower cost. Then the synthetic ionic liquid was immobilized by layered double hydroxides, and the composites were characterized by Fourier transform infrared (FT-IR) spectroscopy, proton nuclear magnetic resonance (1H NMR), carbon-13 nuclear magnetic resonance (13C NMR), electrospray ionization mass spectrometry (ESI-MS), X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and N2 adsorption-desorption isotherms. The results showed that [Bmim][CH3COO] synthesized by intercalation method exhibited the better deacidification property compared to dipping method. When [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) synthesized by intercalation method was used as reagent, the deacidification rate reached to 97.61%. And the optimum reaction conditions were at the reagent/oil mass ratio of 0.08, the reaction temperature of 313 K and the reaction time of 1 h. Moreover, the composites still had an excellent performance on deacidification after being repeatedly used for six times. Keywords: ionic liquid; layered double hydroxides; intercalation method; deacidification

2


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1. Introduction In recent years, with the continuous development and utilization of crude oil resources, the exploitation of heavy crude oil has also increased, and its density, viscosity and acid number have also been increasing

[1]

. In general, it is considered to be high acid

crude oil when the acid number exceeds 1.0 mg KOH/g. The acidic substances in high acid crude oil mainly include naphthenic acid, fatty acid, inorganic acid, aromatic acid, phenol and mercaptan, and naphthenic acid has the highest content, accounting for about 90%

[2,3]

. High acid crude oil has a negative influence on the operation and

reliability of refineries, such as corrosion, scaling, catalyst poisoning and environmental pollution

[4]

. However, pure naphthenic acids are important raw

material in the chemical industry, which can be applied as a paint drying reagent, an additive in petroleum, and so on [5]. At present, the conventional deacidification methods can be divided into physical separation methods and chemical conversion methods. The physical separation methods directly separate the naphthenic acid from crude oil without destroying the chemical state of the naphthenic acid, and then recycle it. The adsorption separation is one of the most widely used methods [6]. It is based on the principle of “like dissolves like”, and chooses the appropriate solvent to extract petroleum acids according to the difference between the polarity of petroleum acids and hydrocarbons. However, the cost of solvent recovery is expensive, and the solvent also dissolves some polycyclic aromatic hydrocarbons in the oil which can affect the purity of petroleum acids and the yield of petroleum products. The principle of chemical conversion methods is to destroy the carboxyl groups by chemical reactions, and it will completely destroy the structure of petroleum acids

[7-10]

. At present, the acid-base neutralization method is

widely used, and its principle is to use the alkali react with petroleum acids. However, the addition of alkali will lead to the formation of emulsions of crude oil and water which results in the low separation efficiency and the high energy consumption. These problems limit the industrialization of the process. 3


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In recent years, ionic liquids (ILs) have attracted wide attention due to the mild reaction conditions, the high selectivity and the simple operation, and it is mainly used in the extraction, catalysis, electrochemistry and desulphurization

[11-15]

. At

present, ionic liquids are gradually used to remove naphthenic acids from high acid crude oil [16-18]. Shi et al. [19] revealed that the optimal content of 2-methylimidazole in ethanol was 20% (w/w) and the optimal extraction time was 10 min, with the reagent/oil ratio being 0.4:1 (w/w), and the acid-removal rate could reach up to 67.0%. Sun et al

[20]

found that

when the reagent/oil mass ratio of [OMIm]Im-oil was 0.008, and the naphthenic acids conversion could be achieved 100%. However, high viscosity and its secondary pollution of crude oil have greatly weakened the practical application of ionic liquids. If ionic liquids were immobilized and used as an adsorbent, these defects could be overcome. Recently, layered double hydroxides (LDHs) have become an important kind of adsorbents owing to its efficient adsorption and reusability. In brief, LDHs has the general formula [M1−xIIMxIII (OH)2]x+[An−]x/n·mH2O, where MII and MIII stand for the divalent and trivalent cation

[21]

, respectively, An− is the interlayer anion which is

located in the interlayer and the lamellar surface. As mentioned above, ionic liquids can be well supported on LDHs, and ILs/LDHs can be used in the printing and dyeing industry [22], biodiesel production [23], etc. However, to the best of our knowledge, the application of ILs/LDHs on removal of naphthenic acid from oil has not been reported in the literature. In this paper, three kinds of imidazolium ionic liquids were synthesized. Then the synthetic ionic liquid was immobilized on layered double hydroxides by dipping method or intercalation method, and the deacidification rate was determined respectively. In addition, the optimization of reaction conditions and the recycling performance were carried out, and the deacidification mechanism was also discussed.

2. Experimental Section 4


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2.1. Chemical Reagents 1-Methylimidazole (99%), n-butyl bromide (98%) and naphthenic acid (pract.) were purchased from Aladdin Industrial Corporation (Shanghai, P.R. China). Ethanol (99.5%), ethanol (95%), dichloromethane (99.5%), ethyl acetate (99.5%), n-heptane (97%), methanol (99.5%), potassium acetate (99%), potassium formate (98%), potassium propionate (98%), potassium hydroxide (85%), sodium hydroxide (96%), sodium carbonate (99.8%), nitric acid (65~68%), aluminium nitrate nonahydrate (99%), magnesium nitrate hexahydrate (99%) and calcium nitrate tetrahydrate (99%) were purchased from Shanghai Titan Scientific Co., Ltd (Shanghai, P.R. China).

2.2. Synthesis of ionic liquids At first, 0.1 mol 1-methylimidazole was added in a glass reactor with stirring in nitrogen atmosphere. When the liquid reached 343 K, 0.12 mol n-butyl bromide was added into the reactor dropwise. After stirring for 24 h at 343 K, the yellowish viscous liquid ([Bmim]Br) was obtained. Then the liquid was washed several times with ethyl acetate and dried in the vacuum oven at 353 K for 6 h. 0.06 mol potassium acetate was added into methanol to form the transparent solution, and the solution was mixed with 0.05 mol [Bmim]Br in nitrogen atmosphere. After stirring for 24 h at 298 K, the target product [Bmim][CH3COO] was obtained by filtering, washing and drying. [Bmim][HCOO] and [Bmim][C2H5COO] were synthesized by the same process. 2.3. Synthesis of Mg0.5Ca2.5Al1 The Mg0.5Ca2.5Al1 was synthesized by the solution A containing Mg(NO3)2, Ca(NO3)2 and Al(NO3)3 and solution B including NaOH and Na2CO3 which were mixed in a flask at pH 10 and 343 K for 1 h with stirring tempestuously. The molar ratio of Mg2+/Ca2+/ Al3+ was 0.5 : 2.5 : 1.0. Then the obtained precipitate was aged at 343 K for 24 h. Finally, the product Mg0.5Ca2.5Al1 was obtained by filtering, washing and drying.

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2.4. Synthesis of ILs/Mg0.5Ca2.5Al1 (D) by dipping method The [Bmim][CH3COO] (1.50 g) and Mg0.5Ca2.5Al1 (4.50 g) were mixed in 50 mL toluene solution. The mixture was stirred at room temperature for 24 h in nitrogen atmosphere. After filtration, the mixture was washed with ethyl acetate several times and

dried

at

353

K

in

vacuum

oven

for

12

h

to

obtain

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D). 2.5. Synthesis of ILs/Mg0.5Ca2.5Al1 (I) by intercalation method The [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) was synthesized by the solution A containing Mg(NO3)2, Ca(NO3)2 and Al(NO3)3, solution B including NaOH and Na2CO3 and [Bmim][CH3COO] which were mixed in a flask at pH 10 and 343 K for 1 h with stirring tempestuously. The molar ratio of Mg2+/Ca2+/ Al3+ was 0.5 : 2.5 : 1.0 and the ionic liquid accounted for 25% of the total mass of the product. Then the obtained precipitate was aged at 343 K for 24 h. Finally, the product [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) was obtained by filtering, washing and drying. 2.6. Characterization Fourier transform infrared (FT-IR) spectroscopy was measured by Nicolet 6700 using anhydrous KBr as dispersing agent and recorded in a range of 4000-400 cm-1. Proton nuclear magnetic resonance (1H NMR) was determined by Bruker Avance 500 spectrometer using tetramethylsilane as internal standard and chloroform-d as the solvent, and carbon-13 nuclear magnetic resonance (13C NMR) was determined by Bruker Avance 500 spectrometer using deuteroxide as the solvent. Electrospray ionization mass spectrometry (ESI-MS) was measured by Waters Xevo G2-XS Tof using methanol as solvent and obtained in positive mode and negative mode. X-ray diffraction (XRD) was performed on Rigaku D/max 2550 VB/PC operating with CuKα radiation (λ=1.5406 Å), at 100 mA, 45 kV, 2θ scanning angle range from 5° to 75° and a step of 0.02 deg/s. Scanning electron microscope (SEM) images were conducted on a field-emission microscope (JEOL, JSM-6360LV) operating at 15 kV. 6


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N2 adsorption-desorption isotherms of reagents were detected by ASAP 2010 N to estimate specific pore size and surface area at the pressure of 3 MPa. The weight of reagents for characterization was about 0.15 g. Energy dispersive spectrometer (EDS) was carried on Falion 60S instrument to obtain surface elemental compositions of the products. The basic strength of catalysts was qualitatively assessed using Hammett indicators by matching color change. The used Hammett indicators included bromthymol blue, phenolphthalein, 2,4-dinitroaniline and 4-nitroaniline(H_=18.4). The basicity of catalysts was measured using Hammett indicator-benzene-carboxylic acid titration [24].

2.7. Preparation of Model Acid Oil The model 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 of oil was around 1.20 mg KOH/g.

2.8. Deacidification Process The oil was put into an Erlenmeyer flask equipped with a magnetic stirrer and heated to a certain temperature. Then ILs/Mg0.5Ca2.5Al1 was added by a certain mass ratio, and the mixture was stirred at the constant temperature for a period of time. After the reaction, the mixture was filtrated by a vacuum pump to separate solid phase and liquid phase. The liquid phase was determined the acid number, and the solid phase was recycled by washing, filtering and drying.

2.9. Analytical Methods In this paper, the determination of TAN in oil was routinely carried out by a standard method in the oil industry using American Society for Testing and Materials (ASTM) D664, and the formula of the deacidification rate was as follows:

R=

X0 − XS × 100% X0

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where R is the deacidification rate; X0 is the acid number of original model acid oil; and XS is the acid number after reaction. 3. Results and Discussion 3.1. Characterization of ionic liquids The FT-IR patterns of [Bmim][HCOO], [Bmim][CH3COO] and [Bmim][C2H5COO] were shown in Fig. 1. The broad band at 3400 cm-1 could be assigned to the -OH stretching vibration of hydroxyl group. The band at 3150 and 3100 cm-1 could be ascribed to the C-H stretching vibration of the imidazole ring. The bands at 3000-2700 cm-1 could be ascribed to the stretching vibration of saturated C-H bonds in branch chain. The bands at 1650, 1450 and 1160 cm-1 could be corresponded to the vibration of the imidazole ring, and the bands around 1570 and 1410 cm-1 attributed to the -COO- stretching vibration. Thus, the basic skeleton of the ionic liquids could be confirmed from the FT-IR patterns. The 1H NMR patterns of [Bmim][HCOO], [Bmim][CH3COO] and [Bmim][C2H5COO] were shown in Fig. 2. Spectroscopic analysis of [Bmim][HCOO]: 1H NMR (500 MHz, Chloroform-d) δ=7.06 (s, 1H), 6.61 (s, 1H), 6.32 (d, J = 7.5 Hz, 1H), 6.22 (d, J = 7.5 Hz, 1H), 4.22 (t, J = 7.7 Hz, 2H), 3.72 (s, 3H), 1.88 (p, J = 7.8 Hz, 2H), 1.38 (h, J = 7.9 Hz, 2H), 0.98 (t, J = 8.0 Hz, 3H). Spectroscopic analysis of [Bmim][CH3COO]: 1

H NMR (500 MHz, Chloroform-d) δ=7.06 (s, 1H), 6.32 (d, J = 7.5 Hz, 1H), 6.22 (d,

J = 7.5 Hz, 1H), 4.22 (t, J = 7.7 Hz, 2H), 3.73 (s, 3H), 2.52 (s, 3H), 1.88 (p, J = 7.8 Hz, 2H), 1.38 (h, J = 7.9 Hz, 2H), 0.98 (t, J = 8.0 Hz, 3H). Spectroscopic analysis of [Bmim][C2H5COO]: 1H NMR (500 MHz, Chloroform-d) δ=7.06 (s, 1H), 6.32 (d, J = 7.5 Hz, 1H), 6.22 (d, J = 7.5 Hz, 1H), 4.22 (t, J = 7.7 Hz, 2H), 3.72 (s, 3H), 2.52 (s, 2H), 1.88 (p, J = 7.8 Hz, 2H), 1.38 (h, J = 7.9 Hz, 2H), 0.98 (t, J = 8.0 Hz, 6H). As shown in Fig. 2, the chemical shifts of hydrogen on the imidazole ring were in the low field, and the peaks were at 7.06, 6.32 and 6.22 respectively. The chemical shifts of methyl and methylene linked to the imidazole ring markedly increased, and the peaks

8


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were at 3.73 and 4.22. However, the chemical shifts of methyl and methylene on the side chain which were not directly linked to the imidazole ring, were in the high field. The peaks of methyl and methylene on the anion was similar to those on the side chain, hence they could not be clearly separated or even coincident. The hydrogen in formate ion was active, and its chemical shift was in the low field. The peaks of hydrogen in acetate ion and propionate ion which were directly linked to the carboxyl group were in the high field, and the peak of methyl in propionate ion which was not directly linked to the carboxyl group was at 0.98. The

13

C

NMR

patterns

of

[Bmim][HCOO],

[Bmim][CH3COO]

and

[Bmim][C2H5COO] were shown in Fig. 3. Spectroscopic analysis of [Bmim][HCOO]: 13

C NMR (500 MHz, Deuteroxide) δ=166.35, 138.50, 124.94, 120.24, 50.20, 36.75,

32.55, 20.00, 13.62. Spectroscopic analysis of [Bmim][CH3COO]:

13

C NMR (500

MHz, Deuteroxide) δ=177.14, 138.50, 124.94, 120.24, 50.20, 36.75, 32.55, 21.83, 20.00, 13.62. Spectroscopic analysis of [Bmim][C2H5COO]:

13

C NMR (500 MHz,

Deuteroxide) δ=180.76, 138.50, 124.94, 120.24, 50.20, 36.75, 32.55, 27.46, 20.00, 13.62, 8.71. As shown in Fig. 3, the chemical shifts of carbon on the imidazole ring were at 138.50, 124.94 and 120.24 respectively, and the chemical shifts of methyl and methylene linked to the imidazole ring were at 36.75 and 50.20. However, the chemical shifts of methyl and methylene on the side chain which were not directly linked to the imidazole ring, were at 13.62, 20.00 and 32.55. Moreover, the peaks of carbon in acetate ion and propionate ion which were directly linked to the carboxyl group were at 21.83 and 27.46 respectively, and the peak of methyl in propionate ion was at 8.71. The chemical shifts of carbon in the carboxyl group were at 166.35, 177.14 and 180.76 respectively owing to the effect of side chain. The ESI-MS patterns of [Bmim][HCOO], [Bmim][CH3COO] and [Bmim][C2H5COO] were shown in Fig. 4. In ESI (+), the characteristic signals were detected corresponding to the cation [Bmim]+ at m/z 139. In ESI (-), we could observe anions [HCOO]- at m/z 45, [CH3COO]- at m/z 59 and [C2H5COO]- at m/z 73. The structure

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of prepared ionic liquids could be further confirmed by the analysis of ESI-MS patterns above.

3.2. Characterization of [Bmim][CH3COO]/Mg0.5Ca2.5Al1 The FT-IR patterns of Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Fig. 5. For Mg0.5Ca2.5Al1 sample, the broad band at 3470 cm-1 could be assigned to the -OH stretching vibration of hydroxyl group. The band at 1620 cm-1 could be ascribed to the -OH bending vibration of crystal water. The band at 1380 cm-1 could be attributed to the C-O asymmetric stretching vibration of carbonate. After the immobilization, the Mg0.5Ca2.5Al1 characteristic bands appeared in the spectra of [Bmim][CH3COO]/Mg0.5Ca2.5Al1. For [Bmim][CH3COO]/Mg0.5Ca2.5Al1 synthesized by the dipping or intercalation method, the bands around 2960, 2930 and 2870 cm-1 could be ascribed to stretching vibration of saturated C-H bonds in branch chain. The bands around 1450 and 1160 cm-1 corresponded to C=N bond and C-C bond of the imidazole ring. The XRD patterns of Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Fig. 6. As shown in Fig. 6, the diffraction peaks of pure layered double hydroxides (Mg0.5Ca2.5Al1) were sharp and symmetrical, and the crystal plane (003), (006), (009), (015), (018), (110) and (113) indicated that the compound had the typical layered structure of LDHs. The XRD patterns of layered double hydroxides modified by ionic liquids presented that they still had the good crystal morphology. The SEM images of Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Fig. 7. As shown in Fig. 7, the compounds had the clear layered structure before and after modification. For [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), the ionic liquids broadened the original layers after being intercalated into LDHs which caused a modest distortion. And the increase of interlayer space was more favorable for the adsorption of naphthenic acid. The EDS patterns of Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Fig. 8. As shown in Fig. 8, there were some significant differences between 10


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Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1. Compared to EDS results, we could observe that N element appeared and the content of C element increased in the pattern of [Bmim][CH3COO]/Mg0.5Ca2.5Al1. These changes could be attributed to the presence of imidazole ring and carbon chain. The

N2

adsorption-desorption

isotherms

of

Mg0.5Ca2.5Al1

and

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Table 1 which presented the BET surface area and pore size characterization. The [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) exhibited the larger surface area (83.51 m2/g) and pore size (29.06 nm), and the [Bmim][CH3COO]/Mg0.5Ca2.5Al1

(D)

showed

the

opposite

the

Textural

characterization. The basic strengths of Mg0.5Ca2.5Al1 and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 were shown in Table 2. The Mg0.5Ca2.5Al1 exhibited weak basic strength (15.0-18.4), and the basic strength of [Bmim][CH3COO]/Mg0.5Ca2.5Al1 synthesized by different methods were both in the range of 18.4-26.5 which were stronger than Mg0.5Ca2.5Al1. It indicated that the basic sites of [Bmim][CH3COO] could be grafted on the Mg0.5Ca2.5Al1 by the dipping or intercalation method. 3.3. Effect of Anion Structure of Ionic Liquids on Deacidification Rate To study the effect of different anion structures of ionic liquids on the deacidification rate, the ionic liquids were mixed with the oil when the reaction temperature was 313 K, the reaction time was 1 h and the reagent/oil mass ratio was 0.005. As shown in Fig. 9, different anion structures had different deacidification rates, and conformed to the following order: [Bmim][C2H5COO] > [Bmim][CH3COO] > [Bmim][HCOO]. According to the chemical acid-base principle, the stronger the acidity is, the weaker the basicity of the corresponding conjugate base is. The order of acidity was as follows: HCOOH > CH3COOH > C2H5COOH, hence the order of alkalinity was: [Bmim][C2H5COO] > [Bmim][CH3COO] > [Bmim][HCOO]. As discussed above, the strong alkalinity of ionic liquids was beneficial for increasing the deacidification rate. As a consequence, taking the cost into account, we selected [Bmim][CH3COO] as the 11


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target reactant.

3.4. Effect of Immobilized Method on Deacidification Rate To study the effect of different immobilized methods on the deacidification rate, the [Bmim][CH3COO]/Mg0.5Ca2.5Al1 synthesized by the dipping method or intercalation method was mixed with the oil when the reaction temperature was 313 K, the reaction time was 1 h and the reagent/oil mass ratio was 0.02. As shown in Fig. 10, when the reagent was Mg0.5Ca2.5Al1, the deacidification rate was only 9.20%, and the deacidification rate was higher after being immobilized. This was due to the high viscosity of ionic liquids, and the immobilized ionic liquids were well overcome. We could also find that the deacidification rate of intercalation method was higher than that of dipping method. As shown in Table 1, [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) synthesized by intercalation method exhibited the larger surface area (83.51 m2/g) and pore size (29.06 nm) which could improve the adsorption property. Hence the intercalation method was chosen as the immobilization method.

3.5. Effect of Reaction Conditions on Deacidification Rate The effect of reaction conditions including the reagent/oil mass ratio, the reaction temperature and the reaction time were investigated. When the reagent was [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), the reaction temperature was 313 K and the reaction time was 1 h, the effect of reagent/oil mass ratio on the deacidification rate was tested. As shown in Fig. 11, the reagent/oil mass ratio was an important factor affecting the deacidification rate. When the reagent/oil mass ratio was less than 0.08, the deacidification rate increased rapidly. This could be explained that the probability of effective collision and combination between the naphthenic acid and ionic liquids in composites increased as the reagent/oil mass ratio increased. When the reagent/oil mass ratio was more than 0.08, the increment of the deacidification rate was not obvious. Hence 0.08 was considered as the optimal reagent/oil mass ratio according to make the deacidification rate maximal at a 12


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minimal reagent/oil mass ratio. When the reagent was [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), the reagent/oil mass ratio was 0.08 and the reaction time was 1 h, the effect of reaction temperature on the deacidification rate was tested. As shown in Fig. 12, both high temperatures and low temperatures had a great effect on the deacidification rate. It could be seen that the deacidification rate increased in the temperature range of 293 K to 313 K and decreased when the reaction temperature increased above 313 K. With the increase of reaction temperature, the naphthenic acid and [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) were heated to make the activity increase and the molecular motion accelerate, hence the probability of collision increased which would promote the reaction and increase the deacidification rate. However, the stability of the resultant was poor, and it would decompose at higher temperatures. Hence, we could infer that the optimal reaction temperature was 313 K. When the reagent was [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), the reagent/oil mass ratio was 0.08 and the reaction temperature was 313 K, the effect of reaction time on the deacidification rate was tested. As shown in Fig. 13, the deacidification rate increased with the reaction time increasing. The reaction could be divided into three stages. In the first stage, the reaction happened quickly and the deacidification rate was over 90% within 0.75 h. In the second stage, the reaction turned slow and the deacidification rate reached to 97.61% in 1 h. In the last stage, the reaction reached the equilibrium after 1 h and the deacidification rate did not increase perceptibly when the reaction time continued to extend. Hence, we chose 1 h as the optimal reaction time.

3.6. Effect of Recycling Times on Deacidification Rate The reusability of [Bmim][CH3COO]/Mg0.5Ca2.5Al1 synthesized by the dipping or intercalation method was also evaluated under the optimized conditions as following: the reagent/oil mass ratio was 0.08, the reaction temperature was 313 K and the reaction time was 1 h. As shown in Fig. 14, the deacidification rate decreased gradually in the same conditions with the increase of recycling times. For 13


Energy & Fuels

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[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), the deacidification rate decreased slowly and was still more than 90% at the first three times, and it still had strong activity after being repeatedly used for six times. However, when [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D) was chosen as the reagent, the deacidification rate decreased rapidly. The result showed that [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) has the good reuse ability, and the ionic liquid immobilized by the intercalation method was more stable on removal of naphthenic acid from oil.

3.7. Discuss the Deacidification Mechanism To study the deacidification mechanism, Fourier infrared spectrum (FT-IR) was used to characterize the composites ([Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)) before and after reaction. In Fig. 15, we could see that there was not any change occurred in the main peaks. Moreover, we purified the n-heptane which was used to wash [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) and characterized the extract by FT-IR (Fig. 16). As shown in Fig. 16, the main peaks of the extract were extremely similar to those of naphthenic acid. Thus, we could judge that there were not significant differences in structure of naphthenic acid and the [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) before and after reaction. In the case of the composites [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I), there were two reasons for the removal of naphthenic acid from the model oil. On the one hand, owing to the alkalinity of ionic liquid, it could react with naphthenic acid and neutralize the acid number of the model oil, and improve the deacidification rate. On the other hand, the unsaturated bonds between the imidazolium cation and the alkoxy of naphthenic acid could remove naphthenic acid molecules from the model oil by the π-π interaction to achieve the effect of deacidification [25]. The following experimental results could be explained: (1) the stronger alkalinity the ionic liquid had which means of great electron density, the better the effect of deacidification would be due to the stronger π-π interaction between the imidazolium cation and the alkoxy of naphthenic acid. (2) the composites still showed good performance on deacidification 14


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Energy & Fuels

in the following repetitive reactions after washing with n-heptane. We could infer that the reaction is dominated by π-π interaction and the neutralization reaction rarely occurred. (3) High temperature was not conducive to the adsorption process, hence deacidification rate would decrease at high temperature. Generally, the experimental phenomena could be explained well by the inferred reaction mechanism.

4. Conclusions The ionic liquids/layered double hydroxides composites had excellent performance on removal of naphthenic acid from oil with low reagent/oil mass ratio. The stronger alkalinity of the anion of ionic liquids was, the higher deacidification rate was obtained. Taking the cost into account, we selected [Bmim][CH3COO] as the target reactant. Compared to dipping method, the [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) synthesized by intercalation method showed a better effect on deacidification due to the larger surface area, pore size and basicity. Appropriate reagent/oil mass ratio, reaction temperature and reaction time were conducive to reaching the maximum deacidification rate at the minimum cost. The deacidification mechanism was likely relevant to the π-π interaction between the imidazolium cation and the alkoxy of naphthenic acid. At last, when [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) was used as reagent and the reaction conditions were at the reagent/oil mass ratio of 0.08, the reaction temperature of 313 K and the reaction time of 1 h, the deacidification rate could reach 97.61%. It could make the continuous reaction possible as a result of the reusability of [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I). In addition, we used diesel oil to test its practicability in the deacidification of practical oil. Under the optimum reaction conditions, the composites ([Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)) was reacted with diesel oil, and its deacidification rate reached to 95.37%. Compared with the deacidification rate for model oil (97.61%), the deacidification performance was almost unchanged. The preliminary results revealed that the product was easier to be separated and the process was feasible and environment-friendly. In a word, it was a

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green and effective process to remove naphthenic acid from acid oil by using immobilized ionic liquids which were synthesized by intercalation method.

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Reference [1] Pereira, P.; Marzin, R.; Zacarias, L.; Trosell, I. L.; Hernández, F.; Córdova, J. Aquaconversion TM: new option for residue conversion and heavy oil upgrading. Aquaconversion TM: nueva alternative para la conversión de residuos y mejoramiento de crudos pesados. Vis. Tecnol. 1998, 6, 5-13. [2] Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Tandem mass spectrometric characterization of commercial naphthenic acids and a maya crude oil. Energy Fuels 2002, 16, 1178-1185. [3] Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14, 217-223. [4] Varadaraj, R.; Brons, C. Molecular Origins of heavy crude oil interfacial acitivity Part 2: fundamental interfacial properties of model napthenic acids and napthenic acids separated from heavy crude oils. Energy Fuels 2007, 21, 199-204. [5] Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Determination of naphthenic acids in crude oils using nonaqueous ion exchange solid-phase extraction. J. Anal. Chem. 2001, 73, 703-707. [6] Huang, M. F.; Zhao, S. L.; Li, P.; Huisingh, D. Removal of naphthenic acid by microwave. J. Clean. Prod. 2006, 14, 736-739. [7] Varadaraj, R.; Savage, D. W. Process for neutralization of petroleum acids. US Patent No. 6030523. 2000. [8] Sartori, G.; Savage, D. W.; Ballinger, B.H.; Dalrymple, D. C. Process for decreasing the acidity of crudes using crosslinked polymeric amines. US Patent No. 6121411. 2000. [9] Trachte, K. L.; Robbins, W. K. Process for selectively removing lower molecular weight naphthenic acids from acidic crudes. US Patent No. 5897769. 1999. [10] Grande, K.; Sorlie, C. A Process for removing essentially naphthenic acids from hydrocarbon oil. US Patent No. 6063266. 2000. [11] Seddon, K. R.; Hardacre, C.; Mcauley, B. J. Catalyst comprising indium salt and organic ionic liquid and process for friedel-crafts reactions. US Patent No. 7928031. 2011. [12] Lemos, R. C. B.; Silva, E. B.; Santos, A.; Guimarães, R. C. L.; Ferreira, B. M. S.; Guarnieri,

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R. A. Demulsification of water-in-crude oil emulsions using ionic liquids and microwave irradiation. Energy Fuels 2010, 24, 4439-4444. [13] Xie, W. L.; Hu, L. B.; Yang, X. L. Basic ionic liquid supported on mesoporous SBA-15 silica as an efficient heterogeneous catalyst for biodiesel production. Ind. Eng. Chem. Res. 2015, 54, 1505-1512. [14] Zahoor, U.; Mohamad, A. B.; Zakaria, M. Biodiesel production from waste cooking oil by acidic ionic liquid as a catalyst. Renew. Energy 2015, 77, 521-526. [15] Zhang, L.; Cui, Y. D.; Zhang, C. P.; Guan, G. F. Biodiesel production by esterification of oleic acid over brønsted acidic ionic liquid supported onto Fe-incorporated SBA-15. Ind. Eng. Chem. Res. 2012, 51, 16590-16596. [16] Li, J.; Sun, Y.; Shi, L. Study on removal of naphthenic acids from white oil by [Bmim]Br-AlCl3. China Pet. Process Pe 2010, 12, 46-51. [17] Shah, S. N.; Mutalib, M. I. A.; Pilus, R. B. M.; Lethesh, K. C. Extraction of naphthenic acid from highly acidic oil using hydroxide-based ionic liquids. Energy Fuels 2015, 29, 106-111. [18] Duan, J. Y.; Sun, Y.; Shi, L. Three different types of heterocycle of nitrogen-containing alkaline ionic liquids treatment of acid oil to remove naphthenic acids. Catal. Today 2013, 212, 180-185. [19] Shi, L. J.; Shen, B. X.; Wang, G. Q. Removal of naphthenic acids from Beijiang crude oil by forming ionic liquids. Energy Fuels 2008, 22, 4177-4181. [20] Sun, Y.; Shi, L. Basic ionic liquids with imidazole anion: New reagents to remove naphthenic acids from crude oil with high total acid number. Fuel 2012, 99, 83-87. [21] Guo, Y.; Zhu, Z.; Qiu, Y.; Zhao, J. Enhanced adsorption of acid brown 14 dye on calcined Mg/Fe layered double hydroxide with memory effect. Chem. Eng. J. 2013, 219, 69-77. [22] Zhou, Q.; Chen, F.; Wu, W.; Bu, R.; Li, W.; Yang, F. Reactive orange 5 removal from aqueous solution using hydroxyl ammonium ionic liquids/layered double hydroxides intercalation composites. Chem. Eng. J. 2016, 285, 198-206. [23] Sun, J.; Yang, J. Y.; Li, S. P.; Xu, X. R. Basic ionic liquid immobilized oxides as heterogeneous catalyst for biodiesel synthesis from waste cooking oil. Catal. Commun. 2016, 83,

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35-38. [24] Xie, W. L.; Peng, H.; Chen, L. G. Calcined Mg-Al hydrotalcites as solid base catalysts for methanolysis of soybean oil. J. Mol. Catal. A: Chem. 2006, 246, 24-32. [25] Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Liquid clathrate formation in ionic liquid-aromatic mixtures. Chem. Commun. 2003, 4, 476-477.

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Fig. 1. FT-IR patterns of (a) [Bmim][HCOO], (b) [Bmim][CH3COO] and (c) [Bmim][C2H5COO]

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a

b

c

10

8

6

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2

0

δ (ppm)

Fig. 2. 1H NMR patterns of (a) [Bmim][HCOO], (b) [Bmim][CH3COO] and (c) [Bmim][C2H5COO]

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δ (ppm)

Fig. 3. 13C NMR patterns of (a) [Bmim][HCOO], (b) [Bmim][CH3COO] and (c) [Bmim][C2H5COO]

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a %

Relative Abundance

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0 100

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ESI (+)

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m/z

Fig. 4. ESI-MS patterns of [Bmim][HCOO], [Bmim][CH3COO] and [Bmim][C2H5COO] obtained in positive mode (left panels) and negative mode (right panels)

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Fig. 5. FT-IR patterns of (a) Mg0.5Ca2.5Al1, (b) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D) and (c) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

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(113)

(110)

(018)

(015)

(009)

(006)

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(003)

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Fig. 6. XRD patterns of (a) Mg0.5Ca2.5Al1, (b) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D) and (c) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

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Fig. 7. SEM images of (a) Mg0.5Ca2.5Al1, (b) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D) and (c) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

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a

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Fig. 8. EDS patterns of (a) Mg0.5Ca2.5Al1, (b) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D) and (c) [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

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Table 1. The Textural characterization of reagents Samples

BET Surface area (m2/g)

Pore size (nm)

Mg0.5Ca2.5Al1

52.98

17.19

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D)

32.66

12.89

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

83.51

29.06

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Energy & Fuels

Table 2. The basic strength and basicity characterization of reagents Samples

Basic strength (H_)

Basicity (mmol/g)

Mg0.5Ca2.5Al1

15.0-18.4

0.13

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (D)

18.4-26.5

0.98

[Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I)

18.4-26.5

1.32

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70 60

Deacidification Rate (%)

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[Bmim][HCOO]

[Bmim][CH3COO]

[Bmim][C2H5COO]

Ionic Liquid

Fig. 9. Effect of anion structure of ionic liquids on deacidification

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60

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0 Mg0.5Ca2.5Al1

[Bmim][CH3COO] IL/Mg0.5Ca2.5Al1 (D) IL/Mg0.5Ca2.5Al1 (I)

Reagent

Fig. 10. Effect of immobilized method on deacidification

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Reagent/Oil Mass Ratio (w/w)

Fig. 11. Effect of reagent/oil mass ratio on deacidification

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Reaction Temperature (K)

Fig. 12. Effect of reaction temperature on deacidification

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Fig. 13. Effect of reaction time on deacidification

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3.5

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IL/Mg0.5Ca2.5Al1 (I) IL/Mg0.5Ca2.5Al1 (D)


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80

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Fig. 14. Effect of recycling times on deacidification

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before reaction

after reaction

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Wavenumbers (cm-1)

Fig. 15. FT-IR patterns of [Bmim][CH3COO]/Mg0.5Ca2.5Al1 (I) before and after reaction

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a

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Wavenumbers (cm-1)

Fig. 16. FT-IR patterns of (a) naphthenic acid and (b) the extract

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Layered Double

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