Li Wenshen; Ren Yongqi; Li Jianxiang; Zhao Zhijun; Liu Jie

(1. School of Petrochemical Engineering, Liaoning Petrochemical University, Fushun 113001;2. School of Innovation and Entrepreneurship, Liaoning Petrochemical University, Fushun 113001)

Abstract: An inexpensive coordinated ionic liquid NMP-0.5ZnCl2 was synthesized by reacting N-methyl-pyrrolidone with anhydrous ZnCl2, and its structure was characterized with FT-IR spectroscopy. The performance of IL for removing basic nitrogen compounds from model oil containing quinoline and actual coker diesel was studied. Experimental results showed that the IL, NMP-0.5ZnCl2, exhibited a good denitrogenation performance, which can be attributed to its low viscosity and unoccupied orbitals of Zn ion, while obtaining a 99.68% quinoline removal efficiency under conditions covering a temperature of 50 ℃, an IL/model oil mass ratio of 1:2, and a reaction time of 30 min. In the case of coker diesel, above a 91% basic N-removal efficiency (with N-content reduced from 536 μg/g to 47 μg/g) was realized by the IL after 5-stage extraction. Moreover, the quinoline extraction efficiency could still reach 96.73% during four recycles of the IL.

Key words: coordinated ionic liquid; NMP-0.5ZnCl2; denitrogenation; quinoline; coker diesel

1 Introduction

Denitrogenation of fuel oils has attracted an increasing attention worldwide because of their inhibiting effect on the hydrodesulfurization process and the environmental pollution caused by NOx emission during combustion of fuels[1-4]. Nitrogen compounds (N-compounds) existing in fuel oils are mainly classified as basic N-compounds (such as pyridine and quinoline) and non-basic N-compounds(e.g. carbazole and indole), and the nitrogen compounds may result in poor oxidization stability of fuel oils，while some basic N-compounds can also deactivate hydrogenation catalyst[5-7]. Removal of N-compounds especially basic N-compounds from fuel oils has become a more and more important research subject.Nowadays hydrogenation is considered as the optimum denitrogenation method in the petroleum refining industry[8-9], however, this method is highly expensive,since it needs high operating temperature, pressure,and highly active catalysts. Therefore, alternative denitrogenation techniques, such as adsorption[10], solvent extraction[11]and oxidation[12], have been explored. Among these techniques, extractive denitrogenation based on ionic liquids (ILs) has been extensively studied because of its high extraction efficiency and facile operation[13-14].

The Br?nsted and Lewis acidic ILs were found to be very effective in removing basic N-compounds from fuel oils. For example, imidazolium-based ILs with HSO4-anion or H2PO4-anion can achieve a very high basic N-extraction efficiency of more than 90%[15-18]. Lewis acidic ILs bearing ZnCl2can also act as a better extraction effect for removing N-compounds and particularly basic N-species[19-21]; for example, by using [Bmim]Cl/ZnCl2in extractive denitrogenation of model oil, a 93.8% carbazole removal and a 97.8% pyridine removal efficiencies were realized after only one-stage extraction, and the N-content could not be detected after 2-stage extraction. Generally,the excellent basic N-extraction ability of the Br?nsted and Lewis acidic ILs can be attributed to the complex reaction between H+or the unoccupied orbitals on metal ions and the lone pair electrons of the N atom in the basic N-compound.

However, most reported ionic liquids used in denitrogenation are imidazolium cations with different anions, and their higher price and relatively complex synthesis process make them difficult to realize the large-scale application in industry[22]. Recently, some inexpensive quaternary ammonium-based ILs have been prepared from quaternary ammonium salts and metal halide anions[23], e.g., xEt3NHCl·FeCl3[24],C5H9NO·xFeCl3/ZnCl2[25], and C5H9NO·SnCl2[26].However, the study on their denitrogenation performance was rarely reported. Meanwhile, as a green solvent, pureN-methyl-2-pyrrolidone (NMP, C5H9NO) presented a lower basic N-extraction efficiency[27]. In view of this,N-methyl-2-pyrrolidone-based (C5H9NO or NMPbased) ionic liquid was prepared by reacting NMP with ZnCl2in this work, and its denitrogenation performance was investigated in detail, hereby, quinoline was selected as a representative basic N-compound. The effects of temperature, IL/oil mass ratio, reaction time,and reusability on the denitrogenation process were systematically investigated. Moreover, the prepared ionic liquid was also used in denitrogenation of actual diesel fuel. The work may present a new approach to denitrogenation of fuel oils.

2 Experimental

2.1 Materials

NMP (AR grade), anhydrous ZnCl2(AR grade), quinoline(99.5%),n-dodecane (99%), toluene (99.5%) and carbon tetrachloride (99.5%) were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China), and were used as received without further purification. Actual coker diesel with a basic nitrogen content of 536 μg/g was obtained from the Fushun Petrochemical Company, PetroChina.

2.2 Experiment apparatus

The experimental devices included: a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific); a TSN-5000 series fluorescence nitrogen/sulfur analyzer (Jiangfen Electroanalytical Co., Ltd., China); a constant temperature magnetic heating stirrer DF-101S (Gongyi City Instrument Co., Ltd, China); an automatic potentiometric titrator ZD-2(A) (Shanghai Dapu Instruments Co.,Ltd., China); a vacuum oven ZK-82J (Shanghai Experimental Instrument Factory, China); an electronic balance FA2104N (0.0001 g, Shanghai Jingke Scientific Instruments Co., Ltd., China); and a TG/DTA SDT Q600 analyzer (American TA Instruments).

2.3 Preparation of model oil

Model oil (～500 μg/g of N) containing quinoline was prepared by dissolving 0.4628 g of quinoline in 100 g ofn-dodecane/toluene mixture (with a toluene mass fraction of 0.2).

2.4 Preparation of ionic liquid

NMP and ZnCl2were mixed at a certain molar ratio (1:0.1,0.2, 0.3, 0.4, and 0.5, respectively) in a round-bottomed flask. The mixture became a transparent liquid under vigorously stirring with a magnetic stirrer at a temperature of 80 ℃ for 2 h, which can be called coordinated ionic liquid[28], herein expressed as NMP-xZnCl2(x=0.1, 0.2, 0.3,0.4, 0.5). It was found in the experiment that a clear solution could not be obtained at room temperature when the molar ratio of ZnCl2/NMP was higher than 0.5. The structures of these ILs were identified by FTIR spectrometry. The possible synthesis process of IL is shown in Figure 1.

Figure 1 Synthesis of NMP-xZnCl2

2.5 Denitrogenation experiment procedure and N-content analysis

In a typical experiment, the model oil and IL were placed in a 50-mL beaker and were magnetically stirred at a specified temperature. After the extraction was completed in a specified time, the reaction mixture was settled for 60 min for phase separation. The basic nitrogen content in the upper oil phase was analyzed on a TSN-5000 series fluorescence nitrogen/sulfur analyzer equipped with a liquid auto-sampler.Basic N-extraction efficiencyE(%) is determined according to the following formula, where Ciand Cfare the initial and final basic nitrogen contents in fuel oil.

3 Results and Discussion

3.1 FT IR analysis

To observe the interaction between NMP and ZnCl2, FTIR spectra of IL NMP-0.5ZnCl2and pure NMP are shown in Figure 2. It was reported that the formation of oxygento-metal coordination bond would result in a shift of the band of C=O to a lower frequency[25-26]. It was clear that the band at 1674 cm-1for C=O in NMP was shifted to 1624 cm-1in NMP-0.5ZnCl2, showing the formation of coordination bond between NMP molecule and the Zn2+ion.

Figure 2 FTIR spectra of IL NMP-0.5ZnCl2(a) and pure NMP (b)

3.2 TG analysis

To determine the thermal stability of the IL, the TG curve of NMP-0.5ZnCl2is shown in Figure 3, along with the curve of pure NMP for comparison.

Figure 3 TG curves of NMP and NMP-0.5ZnCl2 IL

As shown in Figure 3, the mass of IL was reduced by 5%at 125 ℃, whereas pure NMP was almost evaporated entirely at this temperature. Hence, the addition of ZnCl2improved the thermal stability of NMP greatly[26], verifying further the formation of coordination bond between oxygen atom in NMP molecule and the Zn2+ion. The higher thermal stability of IL NMP-0.5ZnCl2is beneficial to denitrogenation recation and the later recycle of IL.

3.3 Extractive denitrogenation ability of different ILs

Figure 4 shows the influence of ZnCl2content in IL NMP-xZnCl2on the extractive removal of quinoline under conditions covering a temperature of 50 ℃, an IL/oil mass ratio of 1:5, and a reaction time of 30 min.

Figure 4 Extractive denitrogenation of quinoline with IL NMP-xZnCl2

It can be seen from Fig. 4 that the ability of NMP-based ILs for extracting quinoline increased slightly with an increasing ZnCl2, i.e., when ZnCl2/NMP molar ratio was 0.5, 0.4, 0.3, 0.2, and 0.1, respectively, the quinoline removal efficiency reached 98.75%, 98.27%, 96.56%,95.06%, and 94.42%, respectively. In the process of removing basic N-compounds with NMP-xZnCl2, the complex interaction between lone pair of electrons on the N atom of quinoline molecule with vacant 4s orbitals of Zn2+with an electron configuration of 1s22s22p63s23p63d104s0occurred, and the increase in content of ZnCl2in ILs contributed to the interaction, so the basic N-removal efficiency was increased. It was reported in the literature[25]that the solubility of diesel in the modified NMP decreased with an increasing amount of chlorides, so NMP-0.5ZnCl2was selected as the desirable IL for denitrogenation process in this study. The effects of temperature, IL/oil mass ratio,reaction time, and IL reusability on the denitrogenation process were discussed in detail by using quinoline model oil in the following experiments.

3.4 Effect of IL to oil ratio

The IL to oil mass ratios considered in this extraction study were 1:10, 1:7, 1:5, 1:4, 1:3, and 1:2. The results are shown in Figure 5. As it was expected, the N-removal efficiency increased with an increasing IL/oil mass ratio, and the efficiency of 97.69% and 99.68% was realized at an IL/oil mass ratio of 1:10 and 1:2,respectively. Nevertheless, the extent of increase in N-removal efficiency was slight. It was worthy of mentioning that the remaining nitrogen content in refined oil was only 1.6 μg/g at an IL/oil mass ratio of 1:2, which was used successfully during the deep denitrogenation process. Considering the cost of the extraction process, the IL/oil mass ratio was determined as 1:10 to investigate the influence of other conditions on the denitrogenation performance of IL NMP-0.5ZnCl2in the work.

Figure 5 N-removal efficiency and N-content at different IL/oil mass ratios

3.5 Effect of temperature

The effect of temperature on denitrogenation of oil by NMP-0.5ZnCl2is shown in Figure 6. It can be seen from Fig. 6 that the basic N-removal efficiency increased a little with the temperature being raised to 50℃. For example, the N-removal efficiency was increased from 96.88% at 30 ℃ to 97.69% at 50 ℃, and when the temperature was further increased to higher than 50℃, the N-removal efficiency was decreased slightly.The possible reason was that the viscosity of NMP-0.5ZnCl2reduced remarkably when the temperature continued to increase until it reached 50℃ (as shown in Figure 7), which was beneficial to the mixing of ionic liquid and model oil, contributing to enhanced denitrogenation. However, at a temperature of higher than 50 ℃, the effect of viscosity on denitrogenation was not predominant, and the reason for the decrease in N-removal efficiency was probably ascribed to the complexation between basic N-compounds and NMP-0.5ZnCl2which was exothermic, and the elevated temperature was not conducive to supporting the reaction along the positive direction. Considering the proper fluidity of IL and denitrogenation efficiency, the temperature was determined as 50 ℃ in the study.

Figure 6 Effect of temperature on basic N-removal efficiency

Figure 7 Effect of temperature on viscosity of NMP-0.5ZnCl2

3.6 Effect of reaction time

Figure 8 shows the performance of NMP-0.5ZnCl2for denitrogenation of model oil at different reaction time. It can be seen from Figure 8 that the basic N-removal efficiency was increased with the extension of reaction time. For example, the basic N-extraction efficiency was increased from 94.93% in 5 min to 97.69% in 30 min. Moreover, the basic N-removal rate did not change basically after 30 min,which indicated that extraction process reached equilibrium in a shorter time because of faster mass transfer and reaction between NMP-0.5ZnCl2and N-compounds. The reaction time was set at 30 min in the study.

Figure 8 Effect of reaction time on basic N-removal efficiency

3.7 Regeneration of IL

The recycling of IL was very important for industrial application. Recycle experiments were performed using carbon tetrachloride as a back extractant in the work. After denitrogenation reaction, the oil phase was separated from IL by a separating funnel. The IL layer was washed with an equal quantity of carbon tetrachloride for 3―5 times, and was evaporated under vacuum to remove carbon tetrachloride. The regenerated IL was used for further extraction with a fresh charge of the model oil under the same operating conditions. The denitrogenation performance of regenerated NMP-0.5ZnCl2is shown in Figure 9. Obviously, there was a slight decrease in nitrogen removal efficiency with the regenerated NMP-0.5ZnCl2. For example, after four recycles, the quinoline removal efficiency dropped from 99.66% to 96.73%. The results showed IL NMP-0.5ZnCl2had a better recycling performance in the denitrogenation process.

Figure 9 The relation of N-extraction efficiency with recycle number

3.8 Removal of basic N-compounds from coker diesel

Coker diesel with a basic nitrogen content of 536 μg/g was selected as the actual oil to investigate further the denitrogenation performance of NMP-0.5ZnCl2. The basic nitrogen content in the upper oil phase was analyzed by adopting the perchloric acid-glacial acetic acid titration method (SH/T0162-92, China).

At a temperature of 50 ℃, an IL/oil mass ratio of 1/2, and a reaction time of 30 min, the basic N-removal efficiency of IL NMP-0.5ZnCl2for treating coker diesel was 56.93%, which was much lower than that for treating quinoline with an N-removal efficiency of 99.68% under the same extraction conditions. The reason might be that many sulfur, oxygen,and aromatic compounds existed in the actual diesel fraction,which decreased the extraction performance of the IL for removing the basic nitrogen compounds.

Multiple-extractive technique was considered as an useful way to reduce the content of heterocycles in oils, so denitrogenation by multiple extractions for coker diesel with IL NMP-0.5ZnCl2was investigated in the current work. Multistage extraction strategy was applied as follows: After extraction of coker diesel over 30 min, the IL phase was separated from the diesel after settlement for 60 min, then the oil phase was extracted again by fresh IL NMP-0.5ZnCl2under the same conditions. Five stages of extraction were performed in the work, with the results presented in Figure 10. As shown in Fig. 10,the basic N-content in coker diesel dropped considerably from 536 μg/g to 47 μg/g after 5 extraction stages, and the basic N-removal efficiency could reach more than 91%.Therefore, the multiple-extractive technique was effective for significantly reducing N-content in fuel oils.

Figure 10 Denitrogenation of coker diesel by multiple extractions using NMP-0.5ZnCl2

4 Conclusions

In conclusion, the coordinated ionic liquids NMPxZnCl2was prepared for denitrogenation of quinoline and actual diesel. The NMP-0.5ZnCl2demonstrated a good denitrogenation performance for treating quinoline because of complex interaction formed between lone pair of electrons on the N atom of quinoline molecule and vacant orbital of Zn ion, achieving an N-removal efficiency of 99.68% under conditions covering a temperature of 50℃, an IL/oil mass ratio of 1:2, and a reaction time of 30 min. In addition, the basic N-removal efficiency can still reach 96.73% after four recycles of the IL NMP-0.5ZnCl2. The feasibility of NMP-0.5ZnCl2for denitrogenation of actual coker diesel was also investigated. The basic N-content in coker diesel dropped considerably from 536 μg/g to 47 μg/g after 5-stage extraction, and the basic N-removal efficiency could reach more than 91%. The coordinated ionic liquid NMP-0.5ZnCl2is an inexpensive, easily synthesized and excellent extractant for basic N-compounds. This work can offer a new insight into designing ionic liquids with lower cost for the scale-up of denitrogenation of fuel oils.

Acknowledgements:The authors are grateful for financial support from the Doctoral Funds of Liaoning Provincial Natural Science Foundation (201601323) and the Research Startup Foundation of Liaoning Petrochemical University (2019xJJ-006).