Dicationic Ionic Liquid: A Novel Method for Improving the

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Catalysis and Kinetics

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 Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00267 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Dicationic ionic liquid: a novel method for improving the isomerization degree of n-pentane Jinshe Chena, b, Lingbin Yanga, b, Wenbo Zhoub, Lijun Zhu*a, Yulu Zhoua, Yuzhi Xianga, and Daohong Xia*a, b a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

Qingdao 266580, People’s Republic of China. b

College of Chemical Engineering, China University of Petroleum, Qingdao 266580,

People’s Republic of China. Abstract: A series of dicationic ionic liquids were successfully prepared and they were firstly used to converse the n-pentane into iso-alkanes as an environmental safety 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

mono-cationic

ionic

liquid

[1-Butyl-3-methylimidazolium]Cl-AlCl3 ([BMIM]Cl-AlCl3). And for 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 increase n-pentane conversion, improve the yield of i-C5 and i-C6 and inhibit C4 ∗

Corresponding author. E-mail address: [email protected] (D.H. Xia), [email protected](L.J. Zhu) Tel.: +86 532 86981869; fax: +86 532 86981787.

ACS Paragon Plus Environment

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component. Additionally, as the increase of the length of substitute alkyl groups of cationic structures in dicationic ionic liquids, 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 mono-cationic ionic liquid and other dicationic ILs. keywords: n-pentane isomerization; dicationic ionic liquids; cationic structures; reaction conditions; reaction mechanism. 1. Introduction In recent years, the isomerization of linear alkanes has attracted great attention as an environmental safety way to improve the octane number of gasoline.1-4 In particular, the isomerization oil of C5/C6 alkanes has an advantage of high octane number, low sensibility, and low sulfur content.5-6 Meanwhile, benzene, aromatics, and olefins are barely contained in them, which are environmental harmful and green.7-8 So far, noble metal bifunctional catalysts have been widely used for C5/C6 alkane isomerization in industrial process, 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 the noble metals were used in them, these catalysts have common drawbacks


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of high cost and low tolerance to sulfur. 9-12 Besides, the operating cost is high due to the presence of hydrogen in 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 catalysts and green solvents.8,

13-16

They have many advantages such as

nonflammability, nonvolatility, high thermal stability and easy adjustment of acid property, etc. Especially acidic ILs, are considered to be a very foreground novel catalysts to overcome the handicaps of industrially employed solid catalysts (zeolites and chlorided alumina etc.). This could be owed to the fact that physical and chemical properties of ILs rely on the constitution 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. Z.C. 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 P. Wasserscheid et al. reported that 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


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Layer (SCILL) systems were prepared by P. Wasserscheid et al. and they were applied in the skeleton isomerization of n-octane in a slurry-phase reaction mode, and 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 temperature.29 As mentioned above, the ionic liquids currently being studied in n-alkanes isomerization are mainly focused on mono-cationic ionic liquids (MILs), however, relatively few active sites in mono-cationic 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 medium,30-31 separation technologies, 32-34 material preparation35-36 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 catalyst, in this paper, a series of dicationic ionic liquids were successfully synthesized and they were used in conversing n-pentane for the first time. The catalytic performance of dicationic ionic liquid [TMEDA(EtBr)2]-AlCl3 was compared with that of mono-cationic 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


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same time, the effects of cationic structures on catalytic activity of dicationic ionic liquids were studied. Based on 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 converse n-pentane into iso-alkanes as an environmental safety way. 2. Experimental 2.1. Materials Anhydrous AlCl3, tetramethylethylenediamine, bromoethane, 1-chlorobutane, 1-bromobutane, 1,2-dibromoethane, n-pentane, 1-bromohexane, 1-bromononane, 1-bromohexadecane were purchased from Sinopharm Chemical Reagent Co.Ltd and the

purity

of

them

was

all

analytical

Reagent(AR).

[BMIM]Cl

(1-Butyl-3-methylimidazolium chloride, CP) was supplied by Shanghai Cheng Jie Chemical Co.Ltd. 2.2. Preparation of ionic liquid catalysts 2.1.1. Preparation of the mono-cationic ionic liquid The mono-cationic 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 round-bottomed 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 0.5h, after that, the mixture was heated at 80℃ for 2h. In the end, the mono-cationic ionic liquid was separated with a separatory


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funnel and sealed. And the scheme for synthesis of mono-cationic ionic liquid is shown below (Scheme 1). N

AlCl3

N Cl

80℃, 2h

N

N [AlmCl3m+1]

Scheme 1. Synthesis of mono-cationic ionic liquid. In the synthesis of mono-cationic 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 substance of AlCl3 and [BMIM]Cl, respectively. And in the 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(1-bromohexane)2]-AlCl3,

[TMEDA(1-bromobutane)2]-AlCl3, [TMEDA(1-bromononane)2]-AlCl3,

[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. 50mL ethyl alcohol, 10mL tetramethylethylenediamine

and

20ml

bromoethane

were

added

into

a

round-bottomed flask, and the mixture was heated under a reflux condenser at 80℃ for 48 h, stirring constantly. After the product was filtered and dried under vacuum for 2h at 50℃ 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


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AlCl3 at 90℃ for 12h.43 The DE dicationic ionic liquid was synthesized according to the mechanism showed below (Scheme 2). C2H5OH N

N

+ 2CH3CH2Br

N

N

48h, 80℃

Br Br

[AlmCl3mBr] N

N

AlCl3 Br

Br

90℃ , 12h

N

N

[Al4-mCl3(4-m)Br]

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, other four kinds of novel dicationic ionic liquids containing different cationic structures were also prepared. Because of similar preparation conditions and procedures for them, take the preparation of DB dicationic ionic liquid as an example. Firstly,

0.64mL

acetonitrile,

2.5mL

1-bromobutane

and

0.42mL

tetramethylethylenediamine were added into a round-bottomed flask, and the mixture was heated under a reflux condenser for 3h, stirring constantly. After that, 4mL acetonitrile was added into the mixture and they were cooled to ambient temperature, and then white precipitate (quaternary ammonium salt bromide) began to appear. Secondly, the white precipitate was washed several times with acetonitrile and dried under vacuum for 3h at ambient temperature, and the quaternary ammonium salt was obtained. Lastly, the quaternary ammonium salt and anhydrous AlCl3 were added into a flask according to the molar ratio of 1:4. The reactants were fully stirred, and they were heated at 90℃ for 12h, and then DB dicationic ionic liquid was prepared.


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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 showed 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 amount of substance of AlCl3 and tetramethylethylenediamine, respectively. And in the 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 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 Bruker AV400 NMR and the solvent adopted was D2O. The n-pentane isomerization was carried out in an autoclave (Fig. 1). At first, ionic liquid catalyst, n-pentane and initiators were added into the autoclave, and then the nitrogen or hydrogen was filled into it to replace the air inside. After that, the valve


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for high-pressure nitrogen or hydrogen was opened, and the pressure was adjusted to 0.5MPa and maintained in order to keep the reactants and products in 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 reaction and 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

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. The conversion of n-pentane (X) was calculated according to the following equation: X(%)= 100 (∑Ai-An-C5)/∑Ai

(1)

where Ai and An-C5 are corrected chromatographic area for particular compound and for residual n-pentane. The selectivity (Si) and yield (Yi) to particular product were calculated according to Eqs. (2) and (3), respectively.


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Si(%)= 100 Ai/(∑Ai-An-C5)

(2)

Yi(%)= X×Si/100

(3)

3. Results and discussion 3.1. Mono-cationic ionic liquid [BMIM]Cl-AlCl3 and its characterization

Figure 2. FT-IR spectra of [BMIM]Cl-AlCl3 IL using acetonitrile as probe. (a) acetonitrile, (b) x=0.6, (c) x=0.63, (d) x=0.67, (e) x=0.70, (f) x=0.73. Fig. 2 shows the FT-IR spectra of [BMIM]Cl-AlCl3 IL using acetonitrile as probe. It can be seen that the intensities of the absorptions at 2305 and 2335 cm–1 increase steadily with the increase of AlCl3 mole fraction, which suggests that the acid strength increases with the increase of 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


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Figure 3. FT-IR spectra of DE IL using pyridine as probe. (a) pure pyridine；(b) pyridine-DE IL. 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 probe in the wavenumber range 1400-1650 cm-1 are shown in Fig. 3, respectively. The FT-IR spectra of DE dicationic ionic liquid using pyridine as probe exhibits one band at around 1540 cm-1, which is attributed to the chemisorption of pyridine on Brønsted acid sites. The formation of Brønsted acid sites could be due to the combination of [AlCl4]- or [Al2Cl7]- with trace amounts of water which exists 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.


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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. Fig. 4 shows the FT-IR spectra of DE IL using acetonitrile as 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 with that of mono-cationic ionic liquid. The intensity of the absorptions at 2305 and 2335 cm–1 increases 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 concentration 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 1

H-NMR was used to confirm the structures of the quaternary ammonium salts.

D2O was used as solvent to determine the quaternary ammonium salt for DB and DH,


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meanwhile, CDCl3 was used as 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 Table1 and Fig. S1. It can be seen that the structures of quaternary ammonium salts are well confirmed by the 1H-NMR results. Table 1 1

H-NMR spectra of different quaternary ammonium salts (QAS) Quaternary ammonium salts (QAS)

1

H-NMR

for DB

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).

for DH

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).

for DN

for DS

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).

QAS for DS

product 4

QAS for DN

product 3

QAS for DH

product 2

QAS for DB

3500

3000

2500

2000

product 1

1500

1000

wavenumbers/cm-1

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


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Fig. 5 shows the FT-IR spectral of quaternary ammonium salts for novel dicationic ionic liquids and similar IR spectra of 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 cm-1，2828 cm-1 and 2860 cm-1 are assigned to C-H stretching vibrations, and the band at 1468cm-1 indicates the existence of C-N stretching vibrations in quaternary ammonium salts.

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 The FT-IR spectra of CH3CN and dicationic ionic liquids are shown in Fig. 6. Compared the absorption peaks of pure 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 weaken the electron-deficiency of N atoms and decrease the acid strength of acid sites in the end.


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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 mono-cationic ionic liquid [BMIM]Cl-AlCl3 for n-pentane isomerization

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= 600r/min, T= 100℃, P= 0.5 MPa, Reaction time= 4h, Vn-butyl chloride/Vn-pentane=0.08, Mass ratio of catalyst to oil=1:1, x=0.80.

Figure 7 shows that the comparison of the catalytic activity of DE ionic liquid and [BMIM]Cl-AlCl3 under the same reaction conditions. It is evident in Fig. 7A that the conversion of n-pentane for DE ionic liquid could reach 87.8%, which was higher about 18% than that for [BMIM]Cl-AlCl3. Fig. 7B shows that the selectivity to i-C6 for DE ionic liquid was higher than that for [BMIM]Cl-AlCl3, and the selectivity to


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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 improve the quality of product, indicating that DE ionic liquid possesses better catalytic performance for isomerization compared with mono-cationic ionic liquid. As shown in Scheme 2, there are two adjacent acid sites in DE IL, but only one acid site exists in mono-cationic ionic liquid [BMIM]Cl-AlCl3( Scheme 1).The 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


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Figure 8. Effect of n-butyl chloride (as initiator) content on (A) n-pentane isomerization and (B) the distribution of product for DE ionic liquid. Fig. 8A displays the effect of initiator content on n-pentane isomerization for DE ionic liquid and other reaction conditions are also conducted according to the conditions in 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 iso-alkanes keeps almost the same. Meanwhile, the yield of iso-alkanes shows 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 mono-cationic ionic liquid, the effect of initiator content on n-pentane isomerization for mono-cationic ionic liquid is listed in Fig. S2. From Fig. S2, it could be seen that the n-pentane conversion and the yield of iso-alkanes all increase at first and then


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keep almost the same with the increase of n-butyl chloride. Simultaneously, the selectivity to iso-alkanes has the similar tendency with 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 n-pentane conversion and the yield of iso-alkanes for mono-cationic 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 iso-alkanes for DE IL were also higher than that for mono-cationic 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 Fig. 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 in 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 C4 component.


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Figure 9. Effect of (A) reaction temperature and (B) reaction time on n-pentane isomerization. Fig. 9A presents the effect of reaction temperature on n-pentane isomerization, from which it is obviously seen that n-pentane conversion increases gradually as the temperature increases from 60 to 100℃ and continues to increase slightly after the reaction temperature reaches 100℃. Generally speaking, the viscosities of ionic liquid decreases 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 result in increasing n-pentane conversion. On 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 Fig. 9A, the selectivity to iso-alkanes


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decreases slightly. On account of the results of n-pentane conversion and selectivity to iso-alkanes, the yield of iso-alkanes increases firstly from 60 to 100℃and then keeps almost the same as the temperature continues to increase. Fig. 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 afterwards. When the reaction time was 3h, 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 prolong, they maintain almost the same, indicating that the isomerization reaction could reach equilibrium in a short time and DE ionic liquid has good catalytic performance.


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Figure 10. Effect of (A) mass ratio of catalyst to oil and (B) AlCl3 content on n-pentane isomerization. As shown in Fig. 10A, n-pentane conversion gradually increased as the mass ratio of catalyst to oil increases 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 increases from 1:1 to 4:3. Meanwhile, the selectivity to iso-alkanes decreased slightly due to that cracking reaction for iso-alkanes happened once more with excessively high mass ratio of catalyst to oil for DE ionic liquid. Besides, the yield of iso-alkanes increased firstly and then decreased on account of above two factors (the conversion of n-pentane and selectivity to iso-alkanes), and the highest yield could be obtained when the mass ratio of catalyst to oil was 1:1.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 10B shows the effect of AlCl3 content on n-pentane isomerization and other reaction conditions are conducted according to the conditions in 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 maintained almost stable. Meanwhile, the selectivity to iso-alkanes increased first and then decreased, but there was small change in it. What’s the reason lies that the acid strength increases by increasing AlCl3 content, which could lead to the gradual increase of n-pentane 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 iso-alkanes increased firstly 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 Fig. 11.


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Energy & Fuels

Figure 11. Isomerization of n-pentane for different dicationic ionic liquids. Reaction conditions: Speed= 600r/min, T= 100℃, P= 0.5 MPa, Reaction time= 4h, Vn-butyl chloride /Vn-pentane= 0.08, the mass ratio of catalyst to oil= 1:1.

As Fig. 11 shows, as the increase of the length of substitute alkyl groups in dicationic ionic liquids, n-pentane conversion and the yield of iso-alkanes decreased 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 were confirmed by FT-IR(Fig. 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 initiator also increases, inhibiting the formation of carbenium ions from initiator and leading to the decrease of catalyst efficiency. Additionally, as the length of substitute alkyl groups of cationic structures increases, the molecular masses of dicationic ILs increase, which causes the viscosities of dicationic ILs to increase continuously. The increasing of


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

viscosities would decrease the mixability of ionic liquids with n-pentane and ultimately reduced n-pentane conversion and yield of iso-alkanes.

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

Fig. 12 displays the effect of initiator on n-pentane isomerization for different novel dicationic ILs. After adding the initiator in 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 initiator, both the n-pentane conversion and yield of iso-alkanes were higher 10% than that without adding initiator.


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Energy & Fuels

After initiators were added in reaction, they were in favor of producing carbenium ions, which could improve the catalytic performance of ionic liquids.

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

To investigate the effect of reaction temperature on n-pentane isomerization for novel dicationic ILs, the isomerization reaction was conducted at 100℃ and 120℃ respectively and the comparison results are shown in Figure 13. It was clear that n-pentane conversion for novel dicationic ILs all increased by raising reaction temperature(Fig.13A), and especially for DH, DN and DS dicationic IL, the n-pentane conversion for them at 120℃ was higher above 10.0% than that for them at 100℃.


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With the increase of temperature, the viscosities of novel dicationic IL decreased significantly, in particular, for DH, DN and DS ionic liquids with higher melting points, which could increase the uniformity of ionic liquids and n-pentane and improve the catalytic performance of dicationic IL. Subsequently, the yield of iso-alkanes were improved obviously for isomerization of n-pentane for DH, DN and DS ionic liquids(Fig.13B). For DE and DB dicationic ILs with lower melting point 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.

Cl [Al2Cl6Br]

[AlCl3Br]

N C 2H 5

C 2H 5 N

[Al2Cl6Br]

C2 H 5 N

N C2 H 5

+

+

2

2 [AlCl4]

Step 1

[AlCl3Br]

Cl +

Step 2

+

Step 3

+

Step 4

+

+

+

i-C 6

H+

+

Scheme 3. The mechanism of n-pentane isomerization for dicationic ionic liquids. Based on the above results, the reaction mechanism of n-pentane isomerization for dicationic ILs catalyst is proposed (Scheme 3). The mechanism involve (I) one


Step 5 Step 6

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Energy & Fuels

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), (℃) transfer of hydrogen negative ion from n-pentane and formation of C5 carbenium ions(Scheme 3, Step 2), (℃) isomerization of C5 carbenium ions into i-C5 carbenium ions (Scheme 3, Step 3), (℃) transfer of hydrogen negative ion from other n-pentane to form new C5 carbenium ions and isopentane (Scheme 3, Step 4), (℃) transfer of hydrogen ions and formation of i-C5 pentene by deprotonation (Scheme 3, Step 5), (℃) the reaction of C5 carbenium ions and i-C5 pentene to form i-C6 hexane (Scheme 3, Step 6). Compared with traditional mono-cationic 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 mono-cationic ionic liquid with only one acid site, which is conducive to form more carbenium ions from initiator. Second, two acid sites are adjacent in DE ionic liquid, providing 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]Cl-AlCl3, making the generation of carbenium ions easier. Besides, with the increase of the length of substitute alkyl chains of cationic structures in dicationic ILs, the acid strength of acid sites decreases gradually (Fig. 6) and DE IL has the 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 inhibit the formation of carbenium ions


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from initiator and reduce 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. 4. Conclusions A series of dicationic ionic liquids were successfully prepared and they were used to converse 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 mono-cationic 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 mono-cationic ionic liquid, both the acid strength 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 increase n-pentane conversion, improve the yield of i-C5 and i-C6 and inhibit 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.


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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 mono-cationic ionic liquid and other dicationic ILs. Acknowledgements 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). References 1. Chica, A.; Corma, A. J. Catal. 1999, 187, 167-176. 2. Setiabudi, H.D.; Jalil, A.A.; Triwahyono, S.; Kamarudin, N.H.N.; Jusoh, R. Chem. Eng. J. 2013, 217, 300-309. 3. Arribas, M.A.; Márquez, F.; Martı&́nez, A. J. Catal. 2000, 190, 309-319. 4. Liu P.; Wang J.; Zhang X.G.; Wei R.P.; Ren X.Q. Chem. Eng. J. 2009, 148, 184-190. 5. Essayem N.; Taârit Y. B.; Gayraud P. Y.; Sapaly G.; Naccache C. J. Catal. 2001, 204, 157-162. 6. Gestel J. V.; Nghiem V.T.; Guillaume D.; Gilson J.P.; Duchet J.C. J. Catal. 2002, 212, 173-181. 7. Setiabudi H.D.; Jalil A.A.; Triwahyono S. J. Catal. 2012, 294, 128-135. 8. Zhang R.; Meng X.H.; Liu Z.C.; Meng J.Y.; and Xu C.M. Ind. Eng. Chem. Res.


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Dicationic Ionic Liquid: A Novel Method for Improving the

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