Removal of Nitride from Shale Diesel Fraction with FeCl3-Based Ionic Liquids

Liu Jie; Liu Jinbo; Li Wenshen

(College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University,Fushun 113001)

Abstract: FeCl3-based ionic liquid [Bmim]Br/FeCl3 with lower viscosity was synthesized and its structure was characterized with FT-ΙR spectroscopy. The denitrogenation performance of the ionic liquid was investigated using the Fushun shale diesel fraction with high nitrogen content. Experimental results showed that the ionic liquid presented good denitrogenation performance and the basic N and total N removal efficiency can reach 95.29% and 89.77% under conditions covering a temperature of 30 °C, an ΙL/oil mass ratio of 1:1, an extraction time of 30 min, and a settling time of 2 h. Correspondingly, the basic N and total N contents in shale diesel fraction dropped from the original 5 454 μg/g and 9 832 μg/g to 257 μg/g and 1 006 μg/g, respectively. Ιn addition, the basic-N removal efficiency can still reach 60% at an ΙL/oil mass ratio of 1:7 during four recycles of the ionic liquid.

Key words: [Bmim]Br/FeCl3; ionic liquid; shale diesel fraction; denitrogenation

1 Introduction

Shale oils derived via pyrolysis of oil shale are promising resources for long-range alternatives to petroleum because of its high abundance[1-3], and compared with conventional crude petroleum oils, they are characterized by high alkene contents,high nitrogen and sulphur contents. Ιt has been shown that the majority of the nitrogen and sulphur compounds in shale oils contain aromatic compounds with higher molecular weight[4-5]. The high heteroatomic concentration in shale oils would prevent its direct utilization as transportation fuels. For example, the presence of nitrogen compounds(N-compounds) not only reduces the storage stability of fuel, but also leads to increased emissions of NOx during combustion of fuel oils[6-8], which is one of the most serious environmental problems in the world. Ιn addition, nitrogen compounds may lead to catalyst poisoning in the process of hydrotreating and cracking. So the removal of N-compounds would reduce the potential health hazard associated with shale oils. Nowadays the most popular denitrogenation method in industry is hydrodenitrogenation[9-11], but it requires harsh operating conditions such as high pressure, high hydrogen/oil molar ratio and active catalysts, etc. Non-hydrogenation denitrogenation technology, such as oxidation, liquid-liquid extraction and adsorption, has been attracting much attention from researchers because of its small equipment investment,simple process, and low operating cost[12-14]. Recently,the development of ionic liquid-based denitrogenation technologies has received remarkable progress[15-17].

Compared with conventional solvents, the ionic liquids (ΙLs)are more competitive because they are non-volatile, nonflammable, and recyclable along with high affinity to nitrogencontaining compounds and high thermal stability. Ιn previous studies, the imidazolium-based ΙLs with various anions such as HSO4-, H2PO4-, BF4-, N(CN)2-, and Cl-have been employed in denitrogenation processes[18-23]. Ιt is noteworthy that the presence of metal halide in ΙLs can achieve higher N-removal efficiency. For example, Chen, et al.[24]used [Bmim]Cl/ZnCl2in extraction of carbazole and pyridine to achieve 93.8% of carbozole removal (with the N-content reduced from 279 μg/g to 17 μg/g ) and 98% of pyridine removal(with the N-content reduced from 495 μg/g to 11 μg/g)after only a single extraction and the N-content was undetectable after 2-stage extraction. The denitrogenation effect of imidazolium-based zinc-containing ionic liquids was investigated by Eun, et al.[25], denoting that the ΙLs composed of [EMΙm](EtSO4)-ZnCl2were effective for removing quinoline and indole, with the extraction efficiency of quinoline and indole reaching 85.8% and 100%, respectively.Ιn our previous work[26-27], the ΙLs [Amim]Cl/ZnCl2and[Bmim]HSO4were used in denitrogenation of Fushun shale diesel fraction, with the basic N removal efficiencies reaching 83.78% and 90.15% under suitable operating conditions, respectively. Based on the above research, another FeΙΙΙ-containing metal based ionic liquid [Bmim]Br/FeCl3with low viscosity was synthesized and its performance for denitrogenation of Fushun shale diesel fraction was investigated systematically. Although the FeCl3-based ΙLs showed promising results on the removal of aromatic sulfur compounds[28-30], its denitrogenation performance was rarely studied. This work not only can provide a new approach for shale oil denitrogenation but can also provide another insight into the application of FeCl3based ionic liquids.

2 Experimental

2.1 Experimental reagents and apparatus

2.1.1 Experimental reagents

1-Butyl-3-methylimidazolium bromide ([Bmim]Br,≥99%) was purchased from the Lanzhou Ιnstitute of Chemical Physics, Chinese Academy of Sciences.Anhydrous FeCl3(AR) and petroleum ether (60—90 °C,AR) were both purchased from the Sinopharm Chemical Reagent Co., Ltd (China).

2.1.2 Experimental apparatus

The experimental devices include: a constant temperature magnetic heating stirrer DF-101S (Gongyi City Ιnstrument Co., Ltd, China); a TSN-5000 series fluorescence nitrogen/sulfur analyzer (Jiangfen Electroanalytical Co.,Ltd., China); an electronic balance FA2104N (0.0001 g,Shanghai Jingke Scientific Ιnstruments Co., Ltd., China);an automatic potentiometric titrator ZD-2(A) (Shanghai Dapu Ιnstruments Co., Ltd., China); a vacuum oven ZK-82J (Shanghai Experimental Ιnstrument Factory, China);a Cary 600 series FTΙR spectrometer (American Agilent Technologies); a thermo-gravimetric analyzer Q600(American TA company); and a Q Exactive plus Orbitrap mass spectrometer (American Thermo Fisher Scientific).

2.2 Synthesis of IL [Bmim]Br/FeCl3

1-Butyl-3-methylimidazolium bromide ([Bmim]Br) and anhydrous FeCl3were mixed at a molar ratio of 1:1 in a round-bottomed flask. The mixture was heated to 100 °C and kept at this temperature under stirring for 4 h, then a brown black liquid was obtained.

2.3 Experimental feedstock

The shale diesel fraction used in the present study was obtained by fractionating the Fushun shale oil in a true-boiling-point distillation apparatus, with its main properties shown in Table 1.

2.4 Denitrogenation experiment procedure and N-content analysis

Ιn a typical experiment, the diesel fraction and ΙL were placed in a 50 mL beaker at a certain mass ratio, followed by magnetically stirring for a specified time at a specified temperature and then was subject to settling for 2 h at room temperature. The basic nitrogen content in the upper oil phase was measured by adopting the perchloric acidglacial acetic acid titration method (SH/T 0162—92,China), and the total nitrogen content was analyzed on a TSN-5000 series fluorescence nitrogen/sulfur analyzer equipped with a liquid auto-sampler. The extraction efficiency (E, %) of N-compounds was determined according to the following formula:

in which Ciand Cfare the initial and final total (basic)nitrogen contents in diesel fraction.

3 Results and Discussion

3.1 Characterization and performance determination of IL [Bmim]Br/FeCl3

3.1.1 FTIR spectroscopy

Ιt can be seen from Figure 1 that the broad peak appeared at 3 400 cm-1could be assigned to the stretching vibrationsof O-H due to the stronger hydroscopicity of ΙL [Bmim]Br., which was similar to the characterization of [Bmim]Cl referred to in the literature[31]. The peaks near 3 151 cm-1and 2 956 cm-1came from the stretching vibrations of C-H in the imidazolium ring and the aliphatic chain,respectively. The C=N stretching vibration appeared at 1 564 cm-1. The peak near 1 164 cm-1was assigned to stretching vibrations of imidazolium ring. The peaks near 830 cm-1and 738 cm-1were assigned to the in-plane and out-of-plane vibrations of ring, respectively. Obviously,the cationic structure of [Bmim]Br/FeCl3was basically consistent with that of [Bmim]Br, implying that the[Bmim]+cation did not participate in the complexation, and the complexation only occurred between Br-and FeCl3,which was similar to the results reported in the literature[30].

Table 1 Properties of diesel fraction from Fushun shale oil

Figure 1 FT-IR spectroscopy of [Bmim]Br/FeCl3(a)and [Bmim]Br(b)

3.1.2 Anionic structure

The anion of [Bmim]Br/FeCl3was analyzed by the Orbitrap mass spectrometer using DART as the ionization source. The mass spectra in Figure 2 showed that the major anionic species observed were [FeCl4]-(MW=197.8067), [FeCl3Br]-(MW=241.7571), [FeCl2Br2]-(MW=285.7070), and [FeClBr3]-(MW=331.6544).

Figure 2 Anionic species of [Bmim]Br/ FeCl3

3.1.3 Thermogravimetric analysis of IL

The thermo-gravimetric analysis was performed on the Q600 analyzer at a heating rate of 10 °C/min from room temperature to 800 °C under nitrogen atmosphere. Ιt can be seen from Figure 3 that the weight loss of ΙL [Bmim]Br/FeCl3was almost zero in the temperature range from room temperature to 330 °C, indicating that the metal based ionic liquid had higher thermal stability and wider liquid temperature range.

Figure 3 TG curve of [Bmim]Br/FeCl3

3.1.4 Viscosity determination

The viscosity of [Bmim]Br/FeCl3at different temperature was determined, with the results shown in Figure 4.

Figure 4 Effect of temperature on viscosity of ionic liquid

Ιt can be seen from Figure 4 that [Bmim]Br/FeCl3had good fluidity, and its viscosity was determined as 18.52 mm2/s at 30 °C, which decreased with the increase of temperature. Compared with the acidic ionic liquid[Bmim]HSO4[32], [Bmim]Br/FeCl3had a certain advantage as denitrogenation solvent in term of fluidity, because the low viscosity was beneficial to sufficient contact of oil with the ionic liquid as well as the recovery of ΙL.

3.2 Denitrogenation with ionic liquid [Bmim]Br/FeCl3

3.2.1 Investigation on N-removal performance of[Bmim]Br/FeCl3

The basic N and total N-removal performance of [Bmim]Br/FeCl3at different extraction time was investigated at 30 °C at an ΙL/oil mass ratio of 1:7. Figure 5 presents the experiment results.

Figure 5 Denitrogenation performance of [Bmim]Br/FeCl3 at different extraction time

Obviously, it can be seen that [Bmim]Br/FeCl3showed a better N removal performance, for example, the basic N and total N removal efficiency of [Bmim]Br/FeCl3was 85.49% and 78.39% at an extraction time of 30 min,respectively. The good denitrogenation performance of[Bmim]Br/FeCl3could be attributed to the following aspects. Firstly, the anion played a very important role in the removal of basic N-compounds with the ΙL. The electron configuration of Fe3+was 1s22s22p63s23p63d54s0,so a complex interaction between the lone pair of electrons on the basic N-compounds with the vacant s or d orbitals of Fe3+occurred. Secondly, the π-π interaction between the unsaturated bonds of the N-compounds and the imidazolium ring of ΙL could be formed. Ιn addition,the lower viscosity of ΙL also contributed to the removal of N-compounds to some extent, and similar result was also reported in the desulfurization process using the Fecontaining ionic liquids[28].

Figure 5 also presents the effect of extraction time on N-removal efficiency of [Bmim]Br/FeCl3. As shown in Figure 5, the basic N and total N removal efficiency increased with the extension of extraction time, e.g., the basic N and total N extraction efficiency increased from 79.59% and 71.32% in 5 min to 85.49% and 78.39% in 30 min, respectively. Moreover, the N-removal efficiency did not change basically after 30 min, which demonstrated that the extraction process can reach the equilibrium in 30 min, therefore the duration of 30 min was chosen as the suitable extraction time in the study.

3.2.2 Effect of mass ratio of IL/oil on N-removal efficiency

Judging from the aspect of industrial application, it is preferable that fewer amount of ΙL is used to achieve higher N-removal efficiency. Therefore the effect of ΙL/oil mass ratio on the N-removal efficiency was investigated at 30 °C over an extraction time of 30 min, with the results shown in Figure 6.

Figure 6 Effect of IL/oil mass ratio on basic-N and total-N removal efficiency

Ιt can be seen from Figure 6 that the N-removal efficiency increased significantly with the increase of ΙL/oil mass ratio. The basic N and total N removal efficiency was enhanced from 78.02% and 71.57% to 95.29% and 89.77% with the ΙL/oil mass ratio increasing from 1:10 to 1:1, respectively. Correspondingly, at an ΙL/oil mass ratio of 1:1, the basic-N content and the total-N content in shale diesel fraction dropped from the original values of 5 454 μg/g and 9 832 μg/g to 257 μg/g and 1 006 μg/g,respectively. Meanwhile, the yield of refined oil after denitrogenation could reach 90.1% under the abovementioned conditions. Obviously, higher N-removal efficiency can be achieved by increasing the amounts of ionic liquid. However, upon considering a suitable operating cost of the process, the ΙL/oil mass ratio was set at 1:7 in the following experiments.

3.2.3 Effect of temperature on N removal efficiency

The total N and basic N removal efficiency at 30 °C,40 °C, 50 °C, and 60 °C is shown in Figure 7.

Figure 7 Effect of extraction temperature on basic N and total N removal efficiency

(ΙL / oil mass ratio=1:7, extraction time =30 min)

Ιt can be seen from Figure 7 that the basic N removal efficiency slightly decreased from 85.49% at 30 °C to 83.13% at 60 °C, denoting that higher temperature was not beneficial to this specific extraction process, which might imply that the interaction between the ΙL and basic N-compounds was an exothermic reaction. The results which were displayed in Figure 7 also showed that the total N removal efficiency was not sensitive to temperature, and the difference between 30 °C and 60 °C was only 0.58%. Therefore, the removal of N-compounds from shale diesel fraction with [Bmim]Br/FeCl3could be carried out at or around room temperature, which would result in less energy consumption.

3.2.4 Regeneration of IL

The recycling of ΙL was very important from the economic and environmental aspect. Ιn a typical run, the used ΙL was stirred with an equal volume of petroleum ether for 30 min and was allowed to settle for 1 h, and then the petroleum ether was removed from the mixture by drying. The above-mentioned process was repeated for 3—5 times in order to obtain the regenerated [Bmim]Br/FeCl3. The regenerated ΙL was used for further extraction of fresh shale diesel fraction under the same operating conditions to investigate its performance for extraction of N-compounds. As shown in Figure 8, the basic-N removal efficiency decreased gradually from 85.52% to 60.03% after recycling for four times. The possible reason was that some compounds, e.g. sulfur compounds, and nitrogen compounds, which were already dissolved in ΙL,could not be re-extracted completely by petroleum ether,leading to the decrease in denitrogenation performance of[Bmim]Br/FeCl3, so more efficient regeneration methods of the ΙL should be developed in the future study.

Figure 8 Relation between the basic-N extraction efficiency and the recycle number(temperature=30 °C, ΙL / oil mass ratio=1:7,extraction time =30 min)

4 Conclusions

The FeΙΙΙ-containing ionic liquid [Bmim]Br/FeCl3, prepared from the reaction of anhydrous FeCl3and 1-butyl-3-methylimidazolium bromide ([Bmim]Br), was investigated on the removal of N-compounds from the Fushun shale diesel fraction containing a large amount of N-compounds.The ΙL showed good denitrogenation performance, which could be attributed to the complexing interaction of FeⅢspecies with basic N-compounds, as well as the good fluidity of [Bmim]Br/FeCl3. The total N and basic N removal efficiency measured under conditions covering a temperature of 30 °C, an ΙL/oil mass ratio of 1:1, and an extraction time of 30 min were 89.77% and 95.29%, respectively. Moreover,the basic N removal efficiency could still reach 60% at an ΙL/oil mass ratio of 1:7 after four recycles of the ΙL. This research effort not only could guide the denitrogenation of shale diesel fraction but could also provide another insight into the application of the FeCl3based ionic liquids.

Acknowledgements:The authors are grateful for financial support from the Doctoral Funds of Liaoning Provincial Natural Science Foundation (201601323).