Dicationic ionic liquid: a novel method for improving the isomerization

isomerization degree of n-pentane. Jinshe Chena, b ... n-pentane isomerization, in particular, for the ionic liquids with higher melting points. Moreo...
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Cite This: Energy Fuels 2018, 32, 5518−5526

Dicationic Ionic Liquid: A Novel Method for Improving the Isomerization Degree of n‑Pentane Jinshe Chen,†,‡ Lingbin Yang,†,‡ Wenbo Zhou,‡ Lijun Zhu,*,† Yulu Zhou,† Yuzhi Xiang,† and Daohong Xia*,†,‡ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, People’s Republic of China College of Chemical Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: A series of dicationic ionic liquids were successfully prepared, and they were first used to convert the n-pentane into iso-alkanes as an environmentally safe way to improve the octane number of gasoline. As a novel and green catalyst, the dicationic ionic liquid [tetramethylethylenediamine(EtBr)2]-AlCl3 ([TMEDA(EtBr)2]-AlCl3) exhibits higher catalytic performance in n-pentane isomerization than the traditional monocationic ionic liquid [1-butyl-3-methylimidazolium]Cl-AlCl3 ([BMIM]Cl-AlCl3). And for the n-pentane isomerization reaction catalyzed by [TMEDA(EtBr)2]-AlCl3, the optimal reaction temperature, reaction time, and mass ratio of catalyst to oil were proven to be 100 °C, 3 h, and 1:1, respectively. For dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3, the acid strength of it increases steadily with the increase of AlCl3 mole fractions. And the initiator is conducive to increasing n-pentane conversion, improving the yield of i-C5 and i-C6, and inhibiting the C4 component. Additionally, as the length of substitute alkyl groups of cationic structures in dicationic ionic liquids increases, the catalytic conversion of n-pentane and the yield of iso-alkanes decrease constantly. For novel dicationic ILs, the moderate rising of temperature facilitates improving the catalytic performance of n-pentane isomerization, in particular, for the ionic liquids with higher melting points. Moreover, the mechanism of n-pentane isomerization catalyzed by dicationic ionic liquid was studied, which revealed the reason that [TMEDA(EtBr)2]-AlCl3 IL has better catalytic performance than monocationic ionic liquid and other dicationic ILs.

1. INTRODUCTION In recent years, the isomerization of linear alkanes has attracted great attention as an environmentally safe way to improve the octane number of gasoline.1−4 In particular, the isomerization oil of C5/C6 alkanes has the advantage of a high octane number, low sensibility, and low sulfur content.5,6 Meanwhile, benzene, aromatics, and olefins are barely contained in them, which are environmentally harmful and green.7,8 So far, noble metal bifunctional catalysts have been widely used for C5/C6 alkane isomerization in industrial processes, and their carriers are usually zeolite and chlorided alumina. For zeolite-based catalysts with noble metals, the reaction temperature of them is high in general. For chlorided alumina-based catalysts, although the reaction temperature of them is low relatively, the active components of them are also generally noble metals. Because noble metals were used in them, these catalysts have common drawbacks of high cost and low tolerance to sulfur.9−12 Additionally, the operating cost is high due to the presence of hydrogen in the reaction.3 For the past few years, ionic liquids, which are also called room temperature ionic liquids, have been widely applied in catalysis and organic synthesis as a kind of novel catalyst and green solvent.8,13−16 They have many advantages such as nonflammability, nonvolatility, high thermal stability, easy adjustment of acid properties, etc. Especially acidic ILs are considered to be very prominent novel catalysts for overcoming the handicaps of industrially employed solid catalysts (zeolites, chlorided alumina, etc.). This could be owed to the fact that physical and chemical properties of ILs rely on the constitution © 2018 American Chemical Society

of the cation and anion, which can be regulated easily in a broad, wide range. Acidic ILs have been studied and used in the isomerization of tetrahydrotricyclopentadiene,17−19 rearrangement of polycyclic hydrocarbons,20 coupling and cracking reactions of alkanes,21 alkylation of isobutane with butenes,22−25 and the conversion of glucose.26,27 Ionic liquids have also been used in the isomerization of normal paraffins, and satisfying results were obtained. Liu et al. investigated the n-pentane isomerization catalyzed by chloroaluminate ionic liquid (Et3NHCl-AlCl3), and the results revealed that Et3NHCl-AlCl3 (mole fraction of AlCl3 is 0.67) has good catalytic performance for n-pentane isomerization.8 Wasserscheid and Meyer reported the preparation of a novel class of superacidic ionic liquids ([cation]Cl/AlCl3+ H2SO4), and they were applied to isomerize n-octane effectively under exceptionally mild conditions.28 Moreover, novel bifunctional solid catalysts with ionic liquid layer (SCILL) systems were prepared by Wasserscheid and Meyer, and they were applied in the skeleton isomerization of n-octane in a slurry-phase reaction mode. The bifunctional solid catalysts exhibited remarkable catalytic performance under mild conditions in the presence of hydrogen.13 It was also reported that ionic liquids composed of a nitrogen-containing heterocyclic or aliphatic organic cation and an anion derived from metal halides were used for the isomerization of C5−C8 alkanes at relatively low reaction Received: January 23, 2018 Revised: March 28, 2018 Published: March 29, 2018 5518

DOI: 10.1021/acs.energyfuels.8b00267 Energy Fuels 2018, 32, 5518−5526

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Energy & Fuels temperatures.29 As mentioned above, the ionic liquids currently being studied in n-alkane isomerization are mainly focused on monocationic ionic liquids (MILs); however, relatively few active sites in monocationic ionic liquids may limit the improvement of their catalytic activity. Dicationic ionic liquids (DILs) as a kind of novel catalytic material have emerged recently, which have two cationics connected with each other by linking groups. Due to unique structures and more active sites in DILs, they have been used in reaction media,30,31 separation technologies,32−34 material preparation,35,36 and especially in catalysis reactions.37−41 But there are few studies reported on dicationic ionic liquids for conversing n-alkanes into iso-alkanes, which should be studied in depth. Considering the urgency of developing novel and green catalysts, in this paper, a series of dicationic ionic liquids were successfully synthesized, and they were used in converting npentane for the first time. The catalytic performance of dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3 was compared with that of monocationic ionic liquids [BMIM]Cl-AlCl3, and reaction conditions for [TMEDA(EtBr)2]-AlCl3 were investigated to obtain the best reaction conditions and the reaction mechanism. At the same time, the effects of cationic structures on catalytic activity of dicationic ionic liquids were studied. On the basis of those results, the mechanism of n-pentane isomerization catalyzed by dicationic ionic liquids was proposed, and it is expected to develop a novel and green catalyst with good catalytic performance to convert n-pentane into iso-alkanes in an environmentally safe way.

[TMEDA(1-bromononane)2]-AlCl3, and [TMEDA(1-bromohexadecane)2]-AlCl3, were prepared, and they were represented by DE, DB, DH, DN, and DS, respectively. The DE dicationic ionic liquid was prepared as follows. A total of 50 mL of ethyl alcohol, 10 mL of tetramethylethylenediamine, and 20 mL of bromoethane were added into a round-bottomed flask, and the mixture was heated under a reflux condenser at 80 °C for 48 h, stirring constantly. After the product was filtered and dried under a vacuum for 2 h at 50 °C to remove residual solvents and bromoethane, the white precipitate (quaternary ammonium salt bromide) was collected. Finally, the quaternary ammonium salt (QAS) was mixed sufficiently with a certain amount of anhydrous AlCl3 at 90 °C for 12 h.43 The DE dicationic ionic liquid was synthesized according to the mechanism shown below (Scheme 2).

Scheme 2. Synthesis of DE Dicationic Ionic Liquid [TMEDA(bromoethane)2]-AlCl3

In order to study the structures of dicationic ionic liquids on the catalytic performance, four other kinds of novel dicationic ionic liquids containing different cationic structures were also prepared. Because of similar preparation conditions and procedures for them, the preparation of DB dicationic ionic liquid can be taken as an example. First, 0.64 mL of acetonitrile, 2.5 mL of 1-bromobutane, and 0.42 mL of tetramethylethylenediamine were added into a round-bottomed flask, and the mixture was heated under a reflux condenser for 3 h, stirring constantly. After that, 4 mL of acetonitrile was added into the mixture, and it was cooled to ambient temperature. Then, a white precipitate (quaternary ammonium salt bromide) began to appear. Second, the white precipitate was washed several times with acetonitrile and dried under a vacuum for 3 h at ambient temperature, and the quaternary ammonium salt was obtained. Last, the quaternary ammonium salt and anhydrous AlCl3 were added into a flask according to a molar ratio of 1:4. The reactants were fully stirred, and they were heated at 90 °C for 12 h. Then, DB dicationic ionic liquid was prepared. The synthesis of quaternary ammonium salt bromide for DH dicationic ionic liquid was by the reaction of tetramethylethylenediamine with 1-bromohexane, and the synthesis of quaternary ammonium salt bromide for DN and DS dicationic ionic liquids was by the reaction of tetramethylethylenediamine with 1-bromononane and 1-bromohexadecane, respectively. And other preparation conditions and procedures were followed by the method adopted in the preparation of DB dicationic ionic liquid. In addition, the four kinds of dicationic ionic liquids were synthesized according to the mechanism shown in the Supporting Information (Scheme S1). In the synthesis of dicationic ionic liquids, the molar fraction x of AlCl3 is calculated by x = n(AlCl3)/[n(AlCl3) + n(tetramethylethylenediamine)]. Where n(AlCl3) and n(etramethylethylenediamine) represent the molar amounts of AlCl3 and tetramethylethylenediamine, respectively. And in Scheme 2, m was 1, 2, or 3. 2.3. Characterization and Isomerization Experiments. A Nicolet 6700 FTIR Fourier was used to record the transform infrared spectra, and pyridine or acetonitrile was used as a probe molecule to characterize the acid property of samples, according to a reported procedure.23,44−47 1 H NMR was used for the analysis of the quaternary ammonium salt bromides for dicationic ionic liquids. The instrument was a Bruker AV400 NMR, and the solvent adopted was D2O.

2. EXPERIMENTAL SECTION 2.1. Materials. Anhydrous AlCl3, tetramethylethylenediamine, bromoethane, 1-chlorobutane, 1-bromobutane, 1,2-dibromoethane, npentane, 1-bromohexane, 1-bromononane, and 1-bromohexadecane were purchased from Sinopharm Chemical Reagent Co. Ltd., and the purity of them was all Analytical Reagent (AR). [BMIM]Cl (1-butyl-3methylimidazolium chloride, CP) was supplied by Shanghai Cheng Jie Chemical Co. Ltd. 2.2. Preparation of Ionic Liquid Catalysts. 2.1.1. Preparation of the Monocationic Ionic Liquid. The monocationic ionic liquid [BMIM]Cl-AlCl3 was prepared by mixing [BMIM]Cl and anhydrous AlCl3.42 A certain amount of [BMIM]Cl was added into a roundbottomed flask, and then some n-heptane was added to prevent [BMIM]Cl to be oxidized. Anhydrous AlCl3 was further added into the flask quickly, and the mixture was stirred at room temperature for 0.5 h. After that, the mixture was heated at 80 °C for 2 h. In the end, the monocationic ionic liquid was separated with a separatory funnel and sealed. And the scheme for the synthesis of monocationic ionic liquid is shown below (Scheme 1).

Scheme 1. Synthesis of Monocationic Ionic Liquid

In the synthesis of monocationic ionic liquid, the molar fraction x of AlCl3 is simply calculated by x = (AlCl3)/[n(AlCl3) + n([BMIM]Cl)]. Where n(AlCl3) and n([BMIM]Cl) represent the molar amount of the substances AlCl3 and [BMIM]Cl, respectively. And in Scheme 1, m was 1, 2, or 3. 2.1.2. Preparation of the Dicationic Ionic Liquids. In order to study the catalytic performance and mechanism of n-pentane isomerization catalyzed by dicationic ionic liquids, five kinds of dicationic ionic liquid, [TMEDA(bromoethane)2]-AlCl3, [TMEDA(1bromobutane) 2 ]-AlCl 3 , [TMEDA(1-bromohexane) 2 ]-AlCl 3 , 5519

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Energy & Fuels The n-pentane isomerization was carried out in an autoclave (Figure 1). At first, ionic liquid catalyst, n-pentane, and initiators were added

Figure 2. FT-IR spectra of [BMIM]Cl-AlCl3 IL using acetonitrile as a probe. (a) Acetonitrile, (b) x = 0.6, (c) x = 0.63, (d) x = 0.67, (e) x = 0.70, (f) x = 0.73.

Figure 1. Apparatus for n-pentane isomerization. (1) Constant pressure gas, (2) valve, (3) stirrer, (4) autoclave, (5) reactant and product, (6) ionic liquid, and (7) constant temperature water. into the autoclave, and then the nitrogen or hydrogen was filled into it to replace the air inside. After that, the valve for high-pressure nitrogen or hydrogen was opened, and the pressure was adjusted to 0.5 MPa and maintained in order to keep the reactants and products in the liquid phase. After checking the air tightness, the autoclave was heated up to the desired temperature, and the time meter was started simultaneously. The autoclave was cooled after the reaction, and a gas sample was collected by the drainage method. Meanwhile, the liquid sample was collected, and the analysis methods of liquid products were carried out according to the literature.48,49 The conversion of n-pentane (X) was calculated according to the following equation:

(∑ Ai − An‐C5)/∑ Ai

X (%) = 100

Figure 3. FT-IR spectra of DE IL using pyridine as a probe. (a) Pure pyridine, (b) pyridine-DE IL.

formation of Brønsted acid sites could be due to the combination of [AlCl4]− or [Al2Cl7]− with trace amounts of water, which exist in reagents and air. Meanwhile, a strong band also appears at approximately 1450 cm−1, which is ascribed to the chemisorption of pyridine on Lewis acid sites.50 The results confirmed that the dicationic ionic liquid has both Brønsted and Lewis acid sites. Figure 4 shows the FT-IR spectra of DE IL using acetonitrile as a probe. The vibration stretching absorbance peaks at 2305 and 2335 cm−1 are used to gauge the degree of Lewis acidity.19 It can be seen that the FT-IR result of DE dicationic ionic liquid was similar to that of monocationic ionic liquid. The intensity of the absorptions at 2305 and 2335 cm−1 increases

(1)

where Ai and An‑C5 are the corrected chromatographic area for a particular compound and for residual n-pentane. The selectivity (Si) and yield (Yi) of a particular product were calculated according to eqs 2 and 3, respectively. Si (%) = 100Ai /

(∑ Ai − An‐C5)

Yi (%) = X × Si/100

(2) (3)

3. RESULTS AND DISCUSSION 3.1. Monocationic Ionic Liquid [BMIM]Cl-AlCl3 and Its Characterization. Figure 2 shows the FT-IR spectra of [BMIM]Cl-AlCl3 IL using acetonitrile as a probe. It can be seen that the intensities of the absorptions at 2305 and 2335 cm−1 increase steadily with the increase of the AlCl3 mole fraction, which suggests that the acid strength increases with the increase of the AlCl3 mole fraction.8 3.2. Dicationic Ionic Liquids and Their Characterization. 3.2.1. DE Dicationic Ionic Liquid [TMEDA(EtBr)2]AlCl3 and Its Characterization. The dicationic ionic liquid was mixed well with pyridine according to the volume ratio of 1:3, and then the prepared mixture was injected into the CaF2 liquid cells. The FT-IR spectra of pyridine and the DE dicationic ionic liquid using pyridine as a probe in the wavenumber range 1400−1650 cm−1 are shown in Figure 3. The FT-IR spectra of DE dicationic ionic liquid using pyridine as a probe exhibit one band at around 1540 cm−1, which is attributed to the chemisorption of pyridine on Brønsted acid sites. The

Figure 4. FT-IR spectra of DE IL using acetonitrile as probe. (a) Acetonitrile; (b) x = 0.75, (c) x = 0.80, (d) x = 0.83. 5520

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Energy & Fuels steadily with the increase of AlCl3 mole fractions. As previously reported, the types of anions in the IL catalysts are closely related to the mole fraction of AlCl3, and with x beyond 0.67, some [Al3Cl10]− anions with stronger acidity began to form.8,17,51 This suggests that the acid strength increases with the AlCl3 mole fraction due to the increasing concentrations of [Al2Cl7]− and [Al3Cl10]− in IL,8,18,51 the characteristic of which would have an important influence on catalytic activity of DE IL with different AlCl3 mole fractions. 3.2.2. DB, DH, DN, and DS Dicationic Ionic Liquids and Their Characterizations. 1H NMR was used to confirm the structures of the quaternary ammonium salts. D2O was used as a solvent to determine the quaternary ammonium salt for DB and DH; meanwhile, CDCl3 was used as a solvent to determine the quaternary ammonium salt for DN and DS. The results of 1H NMR analysis of the quaternary ammonium salts are shown in Table 1 and Figure S1. It can be seen that the structures of quaternary ammonium salts are well confirmed by the 1H NMR results.

Figure 6. FT-IR spectra of different dicationic ILs (x = 0.80) using acetonitrile as probe. (a) Acetonitrile, (b) DS IL, (c) DN IL, (d) DH IL, (e) DB IL, (f) DE IL.

acetonitrile, the absorption peaks of different dicationic ILs appear at a wavenumber of about 2305 and 2330 cm−1, indicating that DE, DB, DH, DN, and DS dicationic ionic liquids all have Lewis acidity. With the increase of the alkyl chains of cationic structures in dicationic ionic liquids, the electron-donating ability of substitute alkyl groups increases, which weakens the electron deficiency of N atoms and decreases the acid strength of acid sites in the end. Therefore, it could be seen that the intensities of the absorptions gradually decrease at 2305 and 2335 cm−1 for the five kinds of dicationic ionic liquids. 3.3. Comparison of the Catalytic Activity of Dicationic Ionic Liquid [TMEDA(EtBr)2]-AlCl3 (DE) and Monocationic Ionic Liquid [BMIM]Cl-AlCl3 for n-Pentane Isomerization. Figure 7 shows a comparison of the catalytic activity of DE ionic liquid and [BMIM]Cl-AlCl3 under the same reaction

Table 1. 1H NMR Spectra of Different Quaternary Ammonium Salts (QAS) quaternary ammonium salts (QAS) for DB for DH for DN for DS

1

H NMR

δ 3.90 (s, 4H), 3.47−3.34 (m, 4H), 3.19 (s, 12H), 1.77 (dt, J = 12.2, 7.8 Hz, 4H), 1.38 (d, J = 7.4 Hz, 4H), 0.94 (t, J = 7.4 Hz, 6H). δ 3.89 (s, 4H), 3.48−3.34 (m, 4H), 3.19 (s, 12H), 1.78 (s, 4H), 1.32 (s, 12H), 0.85 (s, 6H). δ 4.78 (s, 4H), 3.80−3.63 (m, 4H), 3.51 (s, 12H), 1.81 (s, 4H), 1.32 (d, J = 46.1 Hz, 24H), 0.88 (t, J = 6.7 Hz, 6H). δ 4.79 (s, 4H), 3.69 (dd, J = 21.8, 12.6 Hz, 1H), 3.51 (s, 12H), 1.78 (s, 4H), 1.31 (d, J = 50.2 Hz, 52H), 0.88 (t, J = 6.8 Hz, 6H).

Figure 5 shows the FT-IR spectra of quaternary ammonium salts for novel dicationic ionic liquids, and similar IR spectra of

Figure 5. FT-IR spectral of quaternary ammonium salts (QAS).

them were observed. The band located at approximately 3445 cm−1 corresponds to O−H stretching vibrations, which forms due to the existence of trace amounts of water adsorbed in quaternary ammonium salts. And three bands at around 2958, 2828, and 2860 cm−1 are assigned to C−H stretching vibrations, and the band at 1468 cm−1 indicates the existence of C−N stretching vibrations in quaternary ammonium salts. The FT-IR spectra of CH3CN and dicationic ionic liquids are shown in Figure 6. Compared to the absorption peaks of pure

Figure 7. (A) The conversion of n-pentane and (B) the selectivity to isomers for DE ionic liquid and [BMIM]Cl-AlCl3. Reaction conditions: speed = 600 r/min, T = 100 °C, P = 0.5 MPa, reaction time = 4 h, Vn‑butyl_chloride/Vn‑pentane = 0.08, mass ratio of catalyst to oil = 1:1, x = 0.80. 5521

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increase of n-butyl chloride. Simultaneously, the selectivity to iso-alkanes has a similar tendency to that for DE ionic liquid. In addition, when no initiator was added in the reaction, the n-pentane conversion and the yield of iso-alkanes for DE IL were 82.3% and 67.8%, respectively. Correspondingly, the npentane conversion and the yield of iso-alkanes for monocationic ionic liquid [BMIM]Cl-AlCl3 were 10.2% and 9.4%, respectively, which strongly confirmed that more acid sites could promote the formation of more carbenium ions and improve the catalytic efficiency for n-pentane isomerization greatly in the end. Furthermore, when the same content of initiator was added in the reaction, the n-pentane conversion and the yield of isoalkanes for DE IL were also higher than that for monocationic ionic liquid [BMIM]Cl-AlCl3, also suggesting that the catalytic efficiency for n-pentane isomerization could be improved greatly due to the existence of two adjacent acid sites in DE IL. In order to study further the effect of initiator on n-pentane isomerization (Vn‑butyl_chloride/Vn‑pentane = 0.08), the distribution of product catalyzed by DE ionic liquid is listed in Figure 8B. The result indicates that the yield of i-C5 and i-C6 increases significantly after adding initiator and the yield of i-C4 decreases to a great degree, and other products’ content changes little. After the addition of n-butyl chloride, the isomerization process is accelerated, and the cracking reaction is inhibited at the same time, which is in favor of improving the yield of i-C5 and i-C6 and reducing the yield of the C4 component. Figure 9A presents the effect of reaction temperature on npentane isomerization, from which it is obviously seen that npentane conversion increases gradually as the temperature increases from 60 to 100 °C and continues to increase slightly

conditions. It is evident in Figure 7A that the conversion of npentane for DE ionic liquid could reach 87.8%, which was about 18% higher than that for [BMIM]Cl-AlCl3. Figure 7B shows that the selectivity to i-C6 for DE ionic liquid was higher than that for [BMIM]Cl-AlCl3, and the selectivity to i-C5 and n-C6 for DE ionic liquid was also a little higher than that for [BMIM]Cl-AlCl3. Additionally, the selectivity to i-C4 for DE ionic liquid was much lower than that for [BMIM]Cl-AlCl3, and the low level of i-C4 cracked components is conducive to improving the quality of the product, indicating that DE ionic liquid possesses better catalytic performance for isomerization compared with monocationic ionic liquid. As shown in Scheme 2, there are two adjacent acid sites in DE IL, but only one acid site exists in monocationic ionic liquid [BMIM]Cl-AlCl3 (Scheme 1). More acid sites could promote the formation of more carbenium ions and improve the catalytic efficiency for n-pentane isomerization greatly, which is probably the reason why DE IL shows better catalytic performance than [BMIM]Cl-AlCl3. 3.4. Effect of Reaction Conditions on the Catalytic Performance of DE Ionic Liquid. Figure 8A displays the

Figure 8. Effect of n-butyl chloride (as initiator) content on (A) npentane isomerization and (B) the distribution of product for DE ionic liquid.

effect of initiator content on n-pentane isomerization for DE ionic liquid, and other reaction conditions are also conducted according to the conditions in section 3.3. It could be seen that by increasing the ratio of Vn‑butyl_chloride/Vn‑pentane, the n-pentane conversion increases step by step and the selectivity to isoalkanes stays almost the same. Meanwhile, the yield of isoalkanes shows a similar tendency with the n-pentane conversion as the ratio of Vn‑butyl_chloride/Vn‑pentane increases. To compare the difference of the effect of initiator content on DE ionic liquid and monocationic ionic liquid, the effect of initiator content on n-pentane isomerization for monocationic ionic liquid is listed in Figure S2. From Figure S2, it could be seen that the n-pentane conversion and the yield of iso-alkanes all increase at first and then keep almost the same with the

Figure 9. Effect of (A) reaction temperature and (B) reaction time on n-pentane isomerization. 5522

DOI: 10.1021/acs.energyfuels.8b00267 Energy Fuels 2018, 32, 5518−5526

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Energy & Fuels after the reaction temperature reaches 100 °C. Generally speaking, the viscosities of ionic liquid decrease with the increase of temperature, which is in favor of increasing uniformity of the mixture of ionic liquid and n-pentane. Meanwhile by increasing reaction temperature, the reaction rate of n-pentane isomerization increases step by step, which results in increasing n-pentane conversion. On the other hand, the increase of reaction temperature would also aggravate the cracking reactions of n-pentane and reduce the selectivity to iso-alkanes. As can be seen from Figure 9A, the selectivity to iso-alkanes decreases slightly. On account of the results of npentane conversion and selectivity to iso-alkanes, the yield of iso-alkanes increases first from 60 to 100 °C and then stays almost the same as the temperature continues to increase. Figure 9B shows the effect of reaction time on n-pentane isomerization; it can be seen that the n-pentane conversion, the selectivity to iso-alkanes, and the yield of iso-alkanes all increased first and remained almost the same afterward. When the reaction time was 3 h, the conversion of n-pentane, the selectivity to iso-alkanes, and the yield of iso-alkanes could reach 88.0%, 83.2%, and 73.2%, respectively. As reaction time continues to be prolonged, they stay almost the same, indicating that the isomerization reaction could reach equilibrium in a short time and DE ionic liquid has good catalytic performance. As shown in Figure 10A, n-pentane conversion gradually increased as the mass ratio of catalyst to oil increased from 1:2 to 1:1, and there was only a slight increase for n-pentane conversion when the mass ratio of catalyst to oil increased from 1:1 to 4:3. Meanwhile, the selectivity to iso-alkanes decreased slightly due to the cracking reaction for iso-alkanes happening

once more with an excessively high mass ratio of catalyst to oil for DE ionic liquid. Additionally, the yield of iso-alkanes increased first and then decreased on account of the above two factors (the conversion of n-pentane and selectivity to isoalkanes), and the highest yield could be obtained when the mass ratio of catalyst to oil was 1:1. Figure 10B shows the effect of AlCl3 content on n-pentane isomerization, and other reaction conditions are conducted according to the conditions in section 3.3. It can be seen that the n-pentane conversion increased gradually when the AlCl3 content increased from 0.67 and 0.80. By further increasing the AlCl3 content, the n-pentane conversion remained almost stable. Meanwhile, the selectivity to iso-alkanes increased first and then decreased, but there was small change in it. The reason for it is that the acid strength increases by increasing AlCl3 content, which could lead to the gradual increase of npentane conversion.3,4 However, when the AlCl3 content was above 0.75, excessively strong acid produced by AlCl3 would enhance the cracking of n-pentane and reduce the selectivity to iso-alkanes. And it could also be seen that the yield of isoalkanes increased first and then decreased. 3.5. Effect of Cationic Structures for Dicationic Ionic Liquid on n-Pentane Isomerization. In order to study the effect of the cationic structures on n-pentane isomerization and the reaction mechanism of isomerization for dicationic ionic liquids, a series of novel dicationic ionic liquids with different cationic structures have been prepared (DB IL, DH IL, DN IL, and DS IL), and the results of the catalytic isomerization performance of them are displayed in Figure 11.

Figure 11. Isomerization of n-pentane for different dicationic ionic liquids. Reaction conditions: speed = 600 r/min, T = 100 °C, P = 0.5 MPa, reaction time = 4 h, Vn‑butyl_chloride/Vn‑pentane = 0.08, the mass ratio of catalyst to oil = 1:1.

As Figure 11 shows, as the length of substitute alkyl groups in dicationic ionic liquids increases, n-pentane conversion and the yield of iso-alkanes decrease constantly. For these dicationic ionic liquids with different structures, with the increase of the length of substitute alkyl groups, the acid strength of these dicationic ionic liquids constantly decreased, which was confirmed by FT-IR (Figure 6). The decrease of acid strength could reduce the rate of formation of carbenium ions and decrease the catalytic performance. At the same time, by increasing the length of substitute alkyl groups, the steric hindrance effect for dicationic ILs interacting with an initiator also increases, inhibiting the formation of carbenium ions from the initiator and leading to a decrease of catalyst efficiency. Additionally, as the length of substitute alkyl groups

Figure 10. Effect of (A) mass ratio of catalyst to oil and (B) AlCl3 content on n-pentane isomerization. 5523

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Energy & Fuels of cationic structures increases, the molecular masses of dicationic ILs increase, which causes the viscosities of dicationic ILs to increase continuously. The increasing of viscosities would decrease the mixability of ionic liquids with n-pentane and ultimately reduce n-pentane conversion and the yield of iso-alkanes. Figure 12 displays the effect of an initiator on n-pentane isomerization for different novel dicationic ILs. After adding the

Figure 13. Effect of reaction temperature on (A) the n-pentane conversion and (B) the yield of iso-alkanes. Reaction conditions: speed = 600 r/min, P = 0.5 MPa, reaction time = 4 h, Vn‑butyl_chloride/Vn‑pentane = 0.08, the mass ratio of catalyst to oil = 1:1.

melting points and lower viscosities, the yields of iso-alkanes decrease slightly with increasing the reaction temperature since the cracking reaction of n-pentane was aggravated. 3.6. Reaction Mechanism Catalyzed by Dicationic Ionic Liquids. On the basis of the above results, the reaction mechanism of n-pentane isomerization for dicationic IL catalysts is proposed (Scheme 3). The mechanism involves (I) one dicationic ionic liquid molecule with two acid sites reacting with two n-butyl chloride molecules (initiator) to form two n-C4 carbenium ions (Scheme 3, step 1), (II) transfer of a hydrogen negative ion from n-pentane and the formation of C5 carbenium ions (Scheme 3, step 2), (III) isomerization of C5 carbenium ions into i-C5 carbenium ions (Scheme 3, step 3), (IV) transfer of a hydrogen negative ion from another npentane to form new C5 carbenium ions and isopentane (Scheme 3, step 4), (V) transfer of hydrogen ions and formation of i-C5 pentene by deprotonation (Scheme 3, step 5), and (VI) the reaction of C5 carbenium ions and i-C5 pentene to form i-C6 hexane (Scheme 3, step 6). Compared with traditional monocationic ionic liquid ([BMIM]Cl-AlCl3), DE IL shows better catalytic performance, and the reason could be ascribed to the following factors. First, DE ionic liquid has two acid sites, possessing more acid sites than monocationic ionic liquid with only one acid site, which is conducive to forming more carbenium ions from an initiator. Second, two acid sites are adjacent in DE ionic liquid, providing a good synergistic effect with each other, which is crucial for improving the catalytic efficiency to complete the isomerization process of n-pentane. Finally, the acid strength of acid sites in DE ionic liquid should be stronger than that in [BMIM]ClAlCl3, making the generation of carbenium ions easier.

Figure 12. Effect of n-butyl chloride (initiator) on (A) n-pentane conversion and (B) the yield of iso-alkanes. Reaction conditions: speed = 600 r/min, T = 100 °C, P = 0.5 MPa, reaction time = 4 h, the mass ratio of catalyst to oil = 1:1.

initiator in the reaction, the n-pentane conversion and yield of iso-alkanes all increased, but the extent of the increase of them was different for novel dicationic ILs with different structures. Generally speaking, for the catalytic performance of DB, DH, DN, and DS after adding the initiator, both the n-pentane conversion and yield of iso-alkanes were 10% higher than those without adding an initiator. After initiators were added in reaction, they were in favor of producing carbenium ions, which could improve the catalytic performance of ionic liquids. To investigate the effect of reaction temperature on npentane isomerization for novel dicationic ILs, the isomerization reaction was conducted at 100 and 120 °C, and the comparison results are shown in Figure 13. It was clear that npentane conversion for novel dicationic ILs all increased by raising the reaction temperature (Figure 13A), and especially for DH, DN, and DS dicationic ILs, the n-pentane conversion for them at 120 °C was higher above 10.0% than that for them at 100 °C. With the increase of temperature, the viscosities of novel dicationic ILs decreased significantly, in particular, for DH, DN, and DS ionic liquids with higher melting points, which could increase the uniformity of ionic liquids and npentane and improve the catalytic performance of dicationic IL. Subsequently, the yield of iso-alkanes was improved obviously for isomerization of n-pentane for DH, DN, and DS ionic liquids (Figure 13B). For DE and DB dicationic ILs with lower 5524

DOI: 10.1021/acs.energyfuels.8b00267 Energy Fuels 2018, 32, 5518−5526

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Energy & Fuels Scheme 3. Mechanism of n-Pentane Isomerization for Dicationic Ionic Liquids

Additionally, with the increase of the length of substitute alkyl chains of cationic structures in dicationic ILs, the acid strength of acid sites decreases gradually (Figure 6), and DE IL has stronger acid sites than other dicationic ILs. What’s more, as the length of substitute alkyl groups of cationic structures increases, the steric hindrance effect for dicationic ILs also increases, which inhibits the formation of carbenium ions from the initiator and reduces the catalytic efficiency finally. Simultaneously, as the length of substitute alkyl groups of cationic structures increases, the viscosities of dicationic ILs increase continuously due to the increase of molecular masses, which also leads to the decrease of the mixability of ionic liquids with n-pentane and reduces the catalytic performance of dicationic ILs in the end. Therefore, the catalytic performances of dicationic ILs decrease step by step with the increase of the length of substitute alkyl chains, and DE IL shows better catalytic performance than other dicationic ILs.

The moderate rising of temperature facilitated improving the catalytic performance of dicationic ionic liquids in n-pentane isomerization, in particular for the dicationic ionic liquids with higher melting points. The mechanism of n-pentane isomerization for dicationic ionic liquids suggests that due to more acid sites, weaker steric hindrance effects, and lower viscosity owned in DE IL, it shows better catalytic activity than monocationic ionic liquid and other dicationic ILs.

4. CONCLUSIONS A series of dicationic ionic liquids was successfully prepared, and it was used to convert the n-pentane into iso-alkanes as a novel and green catalyst. The results show that dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3 exhibits better catalytic performance than monocationic ionic liquid [BMIM]Cl-AlCl3. With increasing the length of substitute alkyl groups in dicationic ionic liquid, the catalytic conversion of n-pentane and yield of iso-alkanes decreases constantly in the isomerization process of n-pentane. For dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3 and monocationic ionic liquid, the acid strength of both of them increased steadily with the increase of AlCl3 mole fractions, which could lead to the gradual increase of n-pentane conversion. And the initiator is conducive to increasing npentane conversion, improving the yield of i-C5 and i-C6, and inhibiting the C4 component. For n-pentane isomerization catalyzed by dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3, the optimal reaction temperature, reaction time, and mass ratio of catalyst to oil are 100 °C, 3 h, and 1:1, respectively.

Corresponding Authors

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b00267. Figures S1 and S2 and Scheme S1 (PDF)

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AUTHOR INFORMATION

*Tel.: +86 532 86981869. Fax: +86 532 86981787. E-mail: [email protected]. *Tel.: +86 532 86981869. Fax: +86 532 86981787. E-mail: [email protected]. ORCID

Daohong Xia: 0000-0003-3648-7888 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant U1662115), the China University of Petroleum for Postgraduate Technology Innovation Project (Grants no. YCX2014032), and the Fundamental Research Funds for the Central Universities (No. 18CX02118A).

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