Deep oxidation desulfurization with a new imidazole-type acidic ionic liquid polymer

Abstract

A new solid acidic ionic liquid polymer (PIL) has been synthesized through the copolymerization of acidic ionic liquid oligomers and divinylbenzene (DVB). Its oxidation activities were investigated through oxidizing benzothiophene. The results showed that the PIL was very efficient for oxidizing benzothiophene with its oxidation capacity reaching 95.5% at 323 K for 20 min. The oxidation activities were quite high so that the reactions can occur simultaneously under very mild conditions. The PIL exhibits the advantages of high activities and high stability.

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1. Introduction

New environmental regulations regarding the sulfur content in traditional fuels have motivated researchers to develop efficient processes for producing cleaner fuels. The removal of sulfur containing compounds from diesel is currently achieved by hydrodesulfurization (HDS). The current HDS technology can desulfurize saturated compounds on an industrial scale but is less effective for aromatic thiophenes and thiophene derivatives because of its steric hindrance. On the other hand, the removal of these compounds by HDS requires: (1) high temperature, (2) bigger reactor size, (3) the capacity of catalyst.^1–11 Meanwhile the efficiency of HDS decreases substantially when used to produce ultralow-sulfur traditional fuels.^12,13 Therefore, many new approaches based either on the adsorption or on the oxidation of sulfur-containing compounds have been proposed.^14,15 Among them, reactive oxidation desulfurization is considered to be a promising approach for deep desulfurization.¹⁶

Ionic liquids (ILs) have received extensive attention because of their special properties.¹⁷ Typically, ILs are composed of organic imidazolium cations and inorganic anions.^18,19 Many efficient procedures for chemical synthesis using ILs as extractant or adsorbent have been reported.^20–23 The ILs also have some drawbacks such as limited solubility with some organic compounds, especially the polar molecules, which not only makes extractant or adsorbent loss, but also add the purification difficulty. Furthermore, the high viscosity increased the mass transfer resistance and limited their industrial application. Many efforts were made to solve the problems including adjusting the molecular structure. Various supports and linkages are employed for the purpose. Polystyrene owned the aromatic rings and double bonds are used as the support. Various acidic ionic liquids were immobilization on polystyrene using different coupling agents and showed high activities.^24–26 However, expensive and toxic reagents are used for the immobilization, which further add extractant or adsorbent cost. Therefore, the stability of new supported ILs should be concerned.

In this work, a new solid acidic polymeric ionic liquid prepared from a Brønsted acidic ionic liquid and divinylbenzene (DVB) is presented (Fig. 1). DVB was directly copolymerized with the IL monomer, thereby avoiding the use of expensive coupling reagents and reducing the cost. To form ion clusters and improve the ion interactions, the IL is polymerized first to form oligomers. Then, the oligomers were copolymerized with DVB. Poly divinylbenzene (PDVB) provides a high hydrophobic Brunauer–Emmett–Teller (BET) surface area.²⁷ A new efficient oxidizing benzothiophene using the solid acidic ionic liquid polymer (PIL) is developed. The results showed that the new PIL is very efficient for the reactions, with oxidation capacity greater than 95% under mild conditions.

Fig. 1 Synthesis route of the new acidic ionic liquid polymer.

2. Results and discussion

2.1. Characterization of acidic ionic liquid and acidic ionic liquid polymer

The BET surface of the PIL is 323 m² g⁻¹ (Fig. 2). The PIL material exhibited many disordered nanopores with non-uniform pore sizes ranging from 1.7 to 300 nm, in accordance with the BET surface analysis.

Fig. 2 (a) Nitrogen adsorption isotherm of PIL at 78 K (b) The pore size distribution derived from the BJH method.

Scanning electron microscopy (SEM) analysis (FEI INSPECT S50) was used to capture and determine the morphologies of the crystalline PIL. The PIL material observed comprise irregular pore with a particle size of about 0.5–1.0 μm (Fig. 3). This structure is quite similar to that of PDVB obtained from solvothermal polymerization.²⁸ The IL monomer contained three terminal double bonds and could participate in the cross-linking of DVB, resulting in a highly porous structure. The small particles were interconnected together, lacking an obvious boundary.

Fig. 3 SEM of the PIL.

Infrared spectrometer (AVATAR 370 from Thermo Nicolet) was used to analyze the PIL at 700–4000 cm⁻¹ (Fig. 4). The IR spectrum of the PIL showed the sulfonic acid group absorbability at 1071 cm⁻¹, which confirmed the acid groups. FT-IR spectrum also showed that the PIL contained resident functionalities. The peak 1175 cm⁻¹ represents C=C. The bands at 1445 cm⁻¹ and 1387 cm⁻¹ are formation vibration of –N–C–. The peak 1575 cm⁻¹ represents –C=N– stretching vibration of the imidazole ring. The C–H stretching vibration is observed at 2932 cm⁻¹. After desulfurization, the PIL also remains the structure.²⁸

Fig. 4 IR spectrum of the PIL ((a): after desulfurization (b) before desulfurization).

2.2. The oxidation activities

The amount of PIL and H₂O₂ had a great influence on catalytic oxidative desulfurization of benzothiophene (BT). The desulfurization system containing PIL and H₂O₂ was used to investigate the effect of different amounts of catalyst. As shown in Fig. 5(a), when the amount of PIL was increased from 20 mg to 80 mg, the sulfur removal increased remarkably from 38.8% to 95.5%, but the increase was slow from 95.5% in 80 mg to 96.7% in 100 mg. The result indicated that the amount of catalyst had a significant effect on removal of BT in the desulfurization reaction.

Fig. 5 (a) Effect of the amount of PIL and H₂O₂ on catalytic oxidative desulfurization of BT (b) effect of temperature and time on catalytic oxidative desulfurization of BT.

In order to study the influence of the amount of H₂O₂ on catalytic oxidative desulfurization of BT, various H₂O₂/S (O/S) molar ratios were investigated in Fig. 5(a). The sulfur removal increased with H₂O₂/S molar ratio up to H₂O₂/S = 5 : 1 and then slightly decreased beyond this value. According to the stoichiometry of the reaction, 1 mol of BT would consume 2 mol of H₂O₂. However, the optimal value of O/S = 5 was higher than the stoichiometric value of O/S = 2. When the O/S molar ratio increased from 1 : 1 to 5 : 1, the removal of BT from the model oil increased from 50.2% to 95.5%. Thus, the O/S molar ratio 5 : 1 was the optimal choice to achieve deep desulfurization.

In order to evaluate the role of the temperature and time on the reaction efficiency, the oxidation of BT was performed in catalytic oxidative desulfurization of BT system from 303 K to 333 K. Fig. 5(b) illustrates oxidative desulfurization of BT on PIL at different temperatures and times. The capacity catalytic oxidative desulfurization of BT at 303 K is 63.1% for 20 min and increases to 95.5% at 323 K for 20 min. But higher temperature meant more decomposition of H₂O₂, which led to lower sulfur removal (Fig. 5(b)). When the temperature was increased to 333 K, sulfur removal deceased. For the reaction time, sulfur removal increased rapidly at the initial stage but slowly from 95.5% in 20 min to 95.8% in 25 min. Thus, it was suitable to carry out the desulfurization reaction at 323 K in 20 min.

As seen in Fig. 6, in the catalytic oxidation process, BT was adsorbed into the surface of PIL, simultaneously was oxidized to its sulfoxide and sulfone, so a continuous decrease in the content of BT in n-heptane was observed during this process. As is well known, H₂O₂ is a strong oxidant in the acidic medium.²⁹ In this process, BT was oxidized to corresponding sulfone by H₂O₂ catalyzed by Brønsted acidic PIL.³⁰

Fig. 6 The oxidation reaction of BT in an oil–PIL–H₂O₂ system.

2.3. Effect of recycling

The reusability of the PIL catalyst was investigated on the removal of BT in catalytic oxidative desulfurization of BT system, as show Fig. 7. When the reaction was finished, the upper layer oil was decanted and recharged with fresh oil. The sulfur removal was 92.6% after the fourth time recycle. So it may lead to the slight decrease of catalytic activity in the recycles.

Fig. 7 Effect of recycling. Conditions: T = 323 K, m(PIL) = 80 mg, t = 20 min, O/S = 5/1 (molar ratio), BT (S: 2000 ppm) in n-heptane.

2.4. Comparison of the catalytic activities with different catalysts

Comparison of the catalytic activities of PIL with those of other acid catalysts is carried out (Table 1). All of the reactions are carried out under same reaction conditions for each acid catalyst. This study indicated that the new PIL is the most efficient catalyst.

Table 1 Comparison of different catalysts

Catalyst	PIL	Monomer^b	PSMIM^c	PSMIMHSO4^d	SO4^2−/ZrO2
a The reaction conditions: T = 323 K, m(PIL) = 80 mg, t = 20 min, O/S = 5/1 (molar ratio), BT (S: 2000 ppm) in n-heptane.b The monomer of PIL.c 1-Methyl-imidazolium-3-propylsulfonate.d 1-Methyl-imidazolium-3-propylsulfonate hydrosulfate.
Remove^a (%)	95.5	84.8	77.6	83.7	81.6

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For the sulfonic acid group functionalized ionic liquid monomer and PSMIMHSO₄, the acid sites interact with reactants.²⁸ At the end of the reaction, the high H₂O content causes to decrease the concentration of H⁺, which decreases the remove of BT. Furthermore, water also adds to the difficulty of separation. SO₄²⁻/ZrO₂, which has a high acidity as solid super acid, also exhibited low activities. The new solid acid produced in this work has a high hydrophobic surface area, and the IL is embedded in its pores, which separated the water easily. Therefore, the acid sites hardly interacted with water. So the new catalyst exhibited much higher activities for oxidation desulfurization. These results clearly show that the new PIL should be one of the best choices for the reactions.

3. Experimental

3.1. Synthesis of the new acidic ionic liquid polymer

1-Vinyl imidazole (0.1 mol), 1,3-propanesulfonate (0.1 mol), and tetrahydrofuran (20 mL) were mixed and stirred magnetically for 24 h at 313 K. Then, white solid zwitterions formed. The white solid zwitterions were filtered and washed repeatedly with diethyl ether. After being dried in a vacuum (383 K, 1.33 Pa), the white solid zwitterions were obtained in good yield. An equimolar amount of concentrated sulfuric acid was added to the obtained zwitterions, and the mixture was stirred for 4 h at 333 K to form the ionic liquid monomer. ¹H NMR (400 MHz, D₂O): δ2.154 (t, 2H, J = 7.2 Hz), 2.759 (t, 2H, J = 7.2 Hz), 4.223 (t, 2H, J = 7.2 Hz), 5.248 (m, 1H, J = 7.2 Hz), 5.603 (m, 1H, J = 6.4 Hz), 6.966 (m, 1H, J = 7.2 Hz), 7.439 (s, 1H), 7.607 (s, 1H), 8.881 (s, 1H). ¹³CNMR (400 MHz, D₂O): δ24.86, 47.14, 48.04, 109.44, 119.63, 122.76, 128.13, 134.56. IR (KBr): 867 cm⁻¹ and 1037 cm⁻¹ (–SO₃H), 1164 cm⁻¹ (C=C), 1558 cm⁻¹ (C–N), 1658 cm⁻¹ (C=N).

Monomer (10 mmol), ethanol (20 mL), and azobisisobutyronitrile (AIBN; 3.2 wt‰ based on monomer) were mixed together to form a solution. After this solution had been stirred at 343 K for 4 h, DVB (10 mmol) and AIBN (3.2 wt‰ based on DVB) were added to the mixture, and the mixture was stirred for another 4 h. Then, the mixture was left to stand for 12 h at 358 K to form a white organic gel. A solid monolith was obtained after the evaporation of ethanol solvent at 338 K. The solid was washed with hot water until no acidity was detected in the filtrate. The new solid acidic ionic liquid polymer was obtained after this solid had been dried in an oven at 383 K overnight.^31,32

3.2. Desulfurization

The benzothiophene (2.0 g) was completely dissolved into n-heptane to prepare 2000 ppm simulated sulfur solution. Material and 5 mL of 2000 ppm benzothiophene (BT) solution were added into a tube. Then H₂O₂ was injected to the tube. The tube was located in a water bath kettle at the desired temperature and stirred at a speed of 10 000 rpm. Then the solution was separated by centrifuge, supernatant was injected into the gas chromatography to check the sulfur residue in the solute. The BT concentration was measured with a FID-GC. The conditions of the gas chromatography: keeping the initial temperature at 373 K for 2 min, then raising the temperature to 473 K at a rate of 5 K min⁻¹ and then maintaining the temperature for 2 min. The injection volume of the sample is 5 μL.

Desulfurization efficiency is obtained by using the internal standard method. The calculation formula is:

where W is the desulfurization rate, C₀ is the initial sulfur content of the oil, C_x is the sulfur content of the oil after desulfurization.

4. Conclusions

The new solid acidic PIL has been synthesized through the copolymerization of acidic ionic liquid oligomers and DVB. PIL showed high activity of oxidation at low temperature with oxidation capacity greater than 95%. High stability, high surface area and high activities are the key features of PIL. Because of its operational simplicity, high oxidation capacity, and mild reaction conditions, the desulfurization process using this oxidant holds great potential for industrial application in the future.

Acknowledgements

This research was sponsored by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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