Mild and Highly Selective Preparation of Alkylate Gasoline Promoted by Using Pol

Gao Xu; Yu Fengli; Wang Zhiping; Xie Congxia

(1. State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao Uniνersity of Science and Technology, Qingdao, 266042;2. College of Chemical Engineering, Qingdao Uniνersity of Science and Technology, Qingdao, 266042)

Abstract: A polyetheramine (PEA)-based Brønsted-acidic ionic liquid (IL) was firstly prepared and used to promote the alkylation of isobutane with isobutene catalyzed by tri fluoromethanesulfonic acid (TfOH). PEA-IL not only can resolve the persistent problem of poor solubility of the volatile and refractory reactants, but also can satisfactorily exhibit the separation of the catalyst from product oil for reuse. The PEA-IL/TfOH catalytic system with an adjustable acidity ensures a high alkylate selectivity. Under the conditions covering a VIL/VTfOH ratio of 10:3, a temperature of 25 °C, and a reaction time of 25 min, the C8-product selectivity reaches 86.63%. The PEA-IL/TfOH catalyst can be reused 13 times without a decrease in the catalytic performance. After many operating cycles, the hydrophobic PEA-IL can be easily regenerated by simply adding water. This study provides a green, economic, and highly efficient method for producing high-octane alkylate gasoline.

Key words: alkylation; alkylate gasoline; tri fluoromethanesulfonic acid; polyetheramine-based ionic liquid; isobutene

1 Introduction

The alkylation of isobutane with C4olefins over an acidic catalyst is mainly used for the production of alkylate gasoline, which is an excellent gasoline blending component featuring high-octane value, low Reid vapor pressure, absence of aromatics, sulfur, or nitrogen contents,and good knock resistance[1]. A mixture of highly branchedchain C8alkanes as the primary component of alkylate gasoline is mainly composed of trimethylpentane (TMP)and dimethylhexane (DMH). A high ratio of TMP/DMH provides high-grade alkylate gasoline with a high octane number. The target product of C8alkylate, in particular the TMP product, can be only formed when the acidity of the catalyst is within an optimal range ofH0between -8.1 and -12.7[2-4]. Otherwise, the main side reactions including cracking and polymerization are promoted, resulting in the unexpected C5-7and C9+products.

The industrial preparation of alkylate gasoline mainly involves the alkylation of isobutane catalyzed by traditional inorganic acids including concentrated sulfuric acid (H2SO4) or hydrofluoric acid (HF)[5]. Owing to their unique physical and chemical properties, ionic liquids (ILs) have received considerable attention as environmentally friendly solvents or catalysts in the chemical industry[6-8]. There are nearly 200 kinds of ILs currently being used in the catalyzed alkylation process[9].Numerous Lewis acidic ILs, mainly metal halide type ILs,have been applied for alkylation of isobutane with butene,and excellent catalytic results have been gained[10-12].However, these Lewis acidic ILs have their inherent defects, such as high viscosity, low stability, and poor reusability[13]. Owing to their low acidity, the Brønsted acidic ILs with high stability have been rarely used as the sole catalyst for alkylation[14].

As a part of our ongoing efforts to explore new catalysts for the preparation of alkylate gasoline, our group has made certain improvements[15-17]. Herein, we first report the alkylation of isobutane with isobutene catalyzed by polyetheramine (PEA)-based Brønsted-acidic ILs coupled with tri fluoromethanesulfonic acid (TfOH), which exhibits an excellent catalytic activity and C8alkylate selectivity, in particular TMP selectivity. The newly developed PEA-IL/TfOH catalytic system used for the preparation of alkylate gasoline with high octane number is characterized by mild reaction condition and high recycling efficiency.

2 Experimental

2.1 Reagents and instruments

All chemicals used, including PEA, 1,3-propanesulfonate,TfOH, and dichloromethane, were purchased in advance.The gas mixture composed of isobutane (90.93%) and isobutene (9.07%) under a pressure of 1.0 MPa N2was obtained from the Dalian Special Gas Co., Ltd.Dichloromethane was puri fied before use.

The prepared PEA-IL was characterized using a Nicolet 510P Fourier transform infrared (FT-IR) spectrometer and a Brucker AV500 nuclear magnetic resonance (NMR)instrument. The thermal stability of the IL was detected using a Netzsch-TG209 thermogravimetric analyzer(TGA). The viscosity of IL was measured using a NDJ-5S digital rotational viscometer. The alkylation products were qualitatively analyzed by using a gas chromatograph-mass spectrometer (GC-MS) equipped with a FID detector and a HP-PONA capillary chromatographic column (50 m × 0.2 mm × 0.5 μm), and quantitatively analyzed by using a GC-9790 Plus gas chromatograph (Zhejiang Fuli Analysis Co., Ltd.) equipped with the same capillary chromatographic column.

2.2 Preparation of PEA-IL

PEA-IL was synthesized using the one-pot method with PEA, 1,3-propanesulfonate, and TfOH serving as the starting materials. One-equivalent PEA (D-230, D-400, or D-2000) was first dissolved in 30 mL of dichloromethane. Under continuous stirring, twoequivalent 1,3-propanesulfonate was slowly added dropwise into the flask. Subsequently, the mixture was stirred for 2 h at 50 °C under condensation re flux. Next,two-equivalent TfOH was slowly added dropwise to the formed solution. The reaction was maintained for 4 h at 50 °C. After removing the solvent through reduced pressure distillation, the remaining oily liquid was desiccated to yield the target PEA-IL.

As regards the typical PEA-IL prepared with D-2000, the viscosity and density at 20 °C are 831.5 cP and 1.1856 g/mL,respectively. The typical IR and1H NMR spectral data are shown as follows. IR (KBr):ν= 3 490.58 cm-1(-NH2),2 871.35 cm-1(C-H), 1 455.26 cm-1(S=O), 1 293.64 cm-1(C-C), 1 244.20 cm-1(C-N), 1 107.75 cm-1(C-O-C),1 030.46 cm-1(S-O), 639.04 (-CH2).1H NMR: (500 MHz,DMSO):δ=1.02 (O-C-CH3), 1.04-1.15 (N-C-CH3), 1.95(C-CH2-C), 2.68 (N-CH2), 3.05 (S-CH2), 3.27-3.46 (NCH, O-CH), 3.47-3.66 (O-CH2). Because the degree of polymerization (n≈33) is high, the active hydrogen atoms are not shown. The thermal decomposition temperature is higher than 150 °C.

2.3 Preparation of alkylate gasoline

The alkylation reactions were conducted in a 75-mL miniature high-pressure reaction kettle provided with mechanical mixing device. Firstly, 10 mL of the prepared PEA-IL and a certain volume of TfOH were added to the kettle. After sealing the kettle, the internal atmosphere was replaced with a nitrogen stream four times under a pressure of 4 MPa. After the inside pressure was reduced to atmospheric pressure, the mixture was first stirred for 5 min to fully mix the PEA-IL and TfOH. Next, a certain volume of feed gas was filled into the kettle.Subsequently, the reactions were conducted at the desired temperature for a specified time under a certain stirring rate. After the reaction terminated, the kettle was cooled down. The remaining gas in the kettle was collected, and the isobutene conversion was determined by GC. After a pressure relief, the upper alkylate product was removed and washed with a saturated sodium bicarbonate solution three times. After drying, the alkylate product was analyzed both qualitatively and quantitatively. A lowerphase PEA-IL functioning as the catalyst did not require processing and could be directly reused.

3 Results and Discussion

3.1 Synthesis of PEA-based IL

PEA is an amino-terminated polyoxypropylene. By using each end of the amino group, two sulfonic groups (-SO3H)can be introduced to form the Brønsted-acidic PEAIL through the one-pot synthesis, as shown in Scheme 1. The easily obtained PEA versions, including D-230,D-400, and D-2000, were selected as the materials used to synthesize the corresponding ILs. The color of three types of ILs with an increasing molecular mass turned respectively from opaque dark red to transparent deep red, and finally transparent golden yellow, It is found that the higher the degree of polymerization of PEA-IL, the lower the viscosity is, which is opposite to the previously reported results for polyether-based ILs[16].

Scheme 1 Synthesis of PEA-based acidic IL

3.2 Catalytic alkylation

The effects of different catalysts on the alkylation reaction of isobutane with isobutene are shown in Table 1. Despite the use of PEA-IL with two sulfonic groups, the acidity (H0≈ -5) could not reach the requirement of alkylation (entry 1). Because little product oil was formed, it could not be gathered and analyzed. TfOH as the strongest organic acid is typically used to replace traditional inorganic acids to improve certain reaction processes. The TfOH-catalyzed alkylation can achieve an 100% of isobutene conversion, albeit with a low C8selectivity and TMP/DMH owing to a higher acidity ofH0= -14.1 (entry 2).The PEA-IL coupled with TfOH with a tunable acidity can improve the C8selectivity and TMP/DMH yield.In addition, importantly, the TfOH with PEA-IL can be separated well from the product oil formed to improve the recycling efficiency of the catalyst. Upon comparing the entries 3-5, the alkylation process using the D-2000-based IL can obtain the best C8selectivity and TMP/DMH ratio. ILs originating from D-230 and D-400 with a high viscosity require a higher reaction temperature, resulting in a low C8selectivity.

Table 1 Effects of the different catalysts on the alkylation

3.3 Solubilization performance of PEA-IL

The alkane/ole fin ratio (I/O) is an extremely important index in an alkylation reaction. I/O will directly affect the selectivity of alkylate. The alkane gas such as isobutane is very poorly soluble in an acidic catalysis system; meanwhile, the olefine such as isobutene can be more easily dissolved in the same system.When the I/O cannot reach a certain required level,copolymerization of the ole fin over an acidic catalyst can directly take place to obtain heavy components,resulting in a low C8selectivity. Moreover, the heavy components produced are easily dissolved in the catalyst to cause its reduced recycling efficiency.Therefore, the alkylation process is usually conducted at a high I/O such as 50/1 or 20/1[18].

In the novel catalytic system, the alkylation reaction is smoothly carried out at an extremely low I/O of 10/1,owing to the excellent solubility of stubborn isobutane in PEA-IL. Microscopic images of bubbles taken with a laser scanning confocal microscope are shown in Figure 1. The existence of numerous bubbles after the reaction clearly shows that isobutane is well dissolved in the PEA-IL. Because isobutene is mostly consumed during the reaction, the remaining bubbles are mostly excessive isobutane. In addition, many bubbles are still found after 24 h, which further con firms that isobutane can stably exist in the IL. The higher viscosity of IL in comparison with an organic solvent affects the transmission of light, and thus the bubbles do not appear to be in stereo.

Figure 1 Microscopic images of bubbles of the reactant gas in the PEA-IL

3.4 Optimization of reaction conditions

To obtain the best-quality alkylate gasoline, various catalytic reaction conditions were optimized using the single factor tests combined with orthogonal experiments. The acidity of the catalyst and the reaction temperature are typically the main factors influencing the alkylation reaction. Two single-factor tests onVIL:VTfOHand the reaction temperature were first carried out to roughly determine the scope of the orthogonal experiments, with the results shown in Figures 2 and 3. Hereby,VIL:VTfOHwas chosen as 5:1―5:2, and the reaction temperature was set at 10°C―30°C.Subsequently, an orthogonal experiment with five factors and five levels was designed, and five factors coveredVIL:VTfOH, the reaction temperature, the reaction time, the pressure, and the stirring rate, respectively.

Figure 2 Effects of VIL:VTfOH on the alkylation reaction

Based on the above experiments, the optimal reaction conditions were obtained as follows:VIL:VTfOH= 10:3,T=25 °C,SR= 1 000 r/min,t= 25 min, andP= atmospheric pressure. Under the optimal conditions, the isobutene conversion is 98.21%, and the C8-product selectivity reaches 86.63% with a TMP/DMH ratio of 42.7.According to the method for determining the acidity using13C NMR[19-20], the acidity of the best catalyst (VIL:VTfOH=10:3) isH0= -9.74.

Figure 3 Effects of reaction temperature on the alkylation reaction

3.5 Alkylation mechanism

Isobutane alkylation with isobutene takes place over a series of consecutive and simultaneous processes[21-22].In the presence of Brønsted acid, isobutene is first protonized to generatetert-butyl carbocation, which can subsequently react with another isobutene molecule to form 2,2,4-trimethyl pentane carbocation (TMP+). The following reaction of TMP+is extremely important to the selectivity of the product. If it reacts with isobutane through a negative hydrogen transfer, the target product of the TMP is obtained with a newt-butyl carbocation,which then enters the next circle. This is the ideal way to obtain high-octane alkylate gasoline, as shown in Scheme 2. However, TMP+can also easily react with isobutene to form a polymer, which would subsequently split into small molecules including DMH. The carbocation intermediates and products formed during the entire reaction process also undergo isomerization. The C8molecules formed might be disproportionated under low acidity. Alkylation is accompanied with polymerization,cracking reaction, isomerization, and disproportionation.To improve the TMP selectivity, it is crucial to control the acidity of the catalytic system and the reaction temperature. Higher acidity and higher temperature are favorable to isomerization and polymerization. A sufficient blending of isobutane and isobutene can prevent the polymerization of isobutene.

Scheme 2 Ideal preparation method of high-octane alkylate gasoline

3.6 Recycling of catalyst

After the reaction, the alkylate product is phase-separated well from the PEA-IL with TfOH, which can be easily reused without treatment. The PEA-IL/TfOH catalyst exhibits an excellent recycling efficiency, as shown in Figure 4. Under the optimized reaction conditions, the high conversion and C8selectivity were retained after 13 cycles. Owing to the excellent solubility of isobutane in PEA-IL, the residual isobutane in the IL slightly increases the isobutene conversion and C8selectivity during the first few cycles. Starting from the 14thcycle,the conversion and C8selectivity apparently decrease;meanwhile, the heavy components rapidly increase. The acid soluble oil (ASO) in the IL gradually increases with the number of recycling, resulting in a decrease in the acidity of the catalytic system. The ASO formed contains a large amount of unsaturated hydrocarbon compounds,which can easily react with isobutene and generate more heavy components, leading to a decline in the isobutene conversion and C8selectivity. However, the TMP/DMH ratio basically remains unchanged during the recycling.Because PEA-IL could be phase-separated well from the alkylate oil after a reaction, only a 2.8% loss of PEA-IL was detected after 16 cycles.

Figure 4 Recycling efficiency of the catalyst

3.7 Regeneration of PEA-IL

Because polyetheramine D-2000 is a hydrophobic molecule, the corresponding PEA-IL is also hydrophobic,which differs from the polyether-based IL. Based on the hydrophobicity of the PEA-IL, it can be regenerated after many cycles using water rather than the selected organic solvent applied in a common back extraction method.As shown in Figure 5, PEA-IL after 16 cycles became a deep brown liquid from an initially golden yellow liquid owing to the presence of ASO. Upon stirring the mixture of PEA-IL and water, an over flow of numerous bubbles occurs. These bubbles are unreacted isobutane that is well dissolved in the PEA-IL. An emulsion is formed, and boiling the mixture can promote the separation process.Subsequently, the ASO rises upward, and the PEA-IL gradually sinks to the bottom. Finally, the PEA-IL can be well separated from the ASO. The regenerated PEA-IL is reused in the recycle of catalytic system. Based on the comparison with the reaction over a fresh catalyst, a high isobutene conversion is maintained, and the C8selectivity shows a slight decrease owing to the slight residual ASO concentration in the regenerated IL.

3.8 Comparison with other catalysts

Table 2 lists the alkylation of isobutane with butene catalyzed by various catalysts. In comparison with the commercialized concentrated H2SO4and the developed Lewis-acidic IL catalysts (entry 1 and entries 2-4), the new PEA-IL/TfOH catalyst has obvious advantages in C8-alkylate selectivity and TMP/DMH (entry 10). Although some trace additives such as CuCl, ZnCl2, or H2O can improve the TMP/DMH ratio,the Lewis-acidic IL has an inherent shortcoming in terms of stability[23-25]. The Brønsted-Lewis acidic IL can also achieve a high TMP/DMH ratio, albeit with a low conversion (entry 5). In comperisin with the composite catalysts composed of Brønsted-acidic IL and the strong acid H2SO4or TfOH (entries 6-10), the new PEA-IL/TfOH catalyst exhibits a highest catalytic activity under a relatively low content of the strong acid. In addition, the added TfOH with PEA-IL can be reused with a high efficiency. Moreover, it should be noted that the simple synthesis and environmentally friendly regeneration of PEA-IL, as well as the mild catalytic conditions, make this new catalytic system more suitable for the industrialized production of high-octane alkylate gasoline.

Figure 5 Regeneration of PEA-IL

Table 2 Alkylation of isobutane with butene catalyzed using various catalysts

4 Conclusions

The Brønsted-acidic PEA-IL with two sulfonic groups was prepared using the one-spot synthesis by applying the amino-terminated polyoxypropylene,1,3-propanesulfonate, and TfOH as the starting materials.PEA-IL coupled with TfOH was applied for the first time to catalyze the alkylation of isobutane with isobutene.Volatile and refractory reactants, in particular isobutane,can be well dissolved in hydrophobic PEA-IL, which allows the alkylation to be carried out under a low alkane/ole fin ratio. PEA-IL not only can ensure a smooth reaction, but also can guarantee a good separation of the PEA-IL/TfOH catalyst and the product oil formed thereby. The PEA-IL/TfOH catalyst with an adjustable acidity can ensure a high C8-alkylate selectivity. The bestVIL:VTfOHratio was identified to be 10:3 withH0=-9.74. At normal temperature and atmospheric pressure,the isobutene conversion is 98.21%, and the C8-product selectivity reaches 86.63% with a TMP/DMH ratio of 42.7. The PEA-IL/TfOH catalyst can be reused 13 times without a decrease in the catalytic performance. The reused PEA-IL can be regenerated by using water. The mild, green, economic, and highly efficient PEA-IL/TfOH catalytic system shows the potential for producing highoctane alkylate gasoline.

Acknowledgements:This work was financially supported by the National Natural Science Foundation of China(Nos. 21476120, 21805158), and the Shandong Province Priority Development Plan of China (Nos. 2017GGX40107,2019GGX102021).