Reaction mechanism of ionic liquid catalyzed alkylation

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Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

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Reaction mechanism of ionic liquid catalyzed alkylation: Alkylation of 2-butene with deuterated isobutene Ying Liu ∗ , Lihong Wang, Rui Li, Ruisheng Hu School of Chemistry and Chemical Engineering, University of Inner Mongolia, Hohhot, China

a r t i c l e

i n f o

Article history: Received 26 January 2016 Received in revised form 29 April 2016 Accepted 9 May 2016 Available online 10 May 2016 Keywords: Alkylation Isobutane Ionic liquid Reaction mechanism

a b s t r a c t The ionic liquid catalyzed alkylation of 2-butene with deuterated isobutane was studied in a continuous flow equipment. Product analyses with time and deuterated distribution determinations were obtained. It is found that the induction period of ionic liquid alkylation is much shorter than that of sulfuric acid. A considerable difference in isobutane solubility between ionic liquid and sulfuric acid was observed with ionic liquid having a greater tendency to dissolve isobutane at the start-up of alkylation. Deuterated product distributions indicate that trimethylpentane fractions stemmed primarily from the self-alkylation of isobutane, the direct alkylation reaction of C4 hydrocarbons, and the scission of C12 + intermediates. Most dimethylhexanes should come from the direct addition of sec-butyl carbonium ions to 2-butenes. © 2016 Elsevier B.V. All rights reserved.

tert-C4 H9 + + 2-C4 H8 → C8 H17 + (2, 2, 3-TMP+ )

1. Introduction The acid catalyzed alkylation of butenes with isobutanes is an important process in the petrochemical industries. The products from a sulfuric or hydrofluoric acid alkylation reactor consist of a mixture of mono-methyl, di-methyl, and tri-methyl alkanes. Of these, the tri-branched octanes are the most desired ingredients in gasoline because they have high octane numbers. In order to improve the yield and selectivity of isooctanes in alkylate, the alkylation mechanism of protonic acids (i.e., H2 SO4 and HF) has been studied for many years. C4 alkylation reaction is believed to proceed via a carbonium ion mechanism [1–3]. First, rapid protonation of 2-butene to sec-butyl carbonium ion (sec-C4 H9 + ) followed by the hydride transfer between isobutane and sec-C4 H9 + forms tert-butyl carbonium ion (tert-C4 H9 + ) and n-butane. The resulting tert-C4 H9 + reacts rapidly with 2-butene to produce trimethylpentane cation (TMP+ ), which is converted to 2,2,3-trimethylpentane (2,2,3-TMP) via hydride transfer from isobutane. A new tert-butyl carbonium ion is generated during the hydride transfers process and another reaction cycle starts. A general reaction scheme for isobutane/2-butene alkylation can be summarized as follows: 2-C4 H8 + H+ → sec-C4 H9 +

(1)

sec-C4 H9 + + i-C4 H10 → tert-C4 H9 + + n-C4 H10

(2)

∗ Corresponding author. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.molcata.2016.05.005 1381-1169/© 2016 Elsevier B.V. All rights reserved.

+

2, 2, 3-TMP + i-C4 H10 → 2, 2, 3-TMP + tert-C4 H9

(3) +

(4)

Albright [4–6] has proposed a more complete reaction scheme when H2 SO4 is used as a catalyst. The reactions of butyl sulfates as well as the role of acid-soluble hydrocarbons are considered in the global scheme. When hydrofluoric or trifluoromethanesulfonic acid was used to catalyze isobutanes and isobutylenes, a detailed reaction pathway was described by Olah et al. [7,8] Density functional theory and Ab initio method have also been employed to determine the reaction mechanism of C4 alkylation [9,10]. The results of theoretical calculation tended to support the classic mechanism assumption that 2-butene is rapidly protonated to form sec-C4 H9 + , and then reacts with isobutane to form tert-C4 H9 + . The resulting carbonium ion is deprotonated to form isobutene that reacts rapidly with tert-C4 H9 + to form TMP+ . Ionic liquids (ILs) as new catalysts have shown many interesting properties [11,12]. For instance, the most important properties of C4 alkylation may be the acid strength and the solubility of isobutane in the acid [13]. But for ILs, these properties can be readily tuned by using different organic cations or inorganic anions [14–17]. Additionally, many ILs can maintain high catalytic activity during the whole alkylation reaction without carbon deposition problems as compared to solid acid alkylation. Some ILs have been reported for the pilot-scale alkylation test [11]. However, few studies have been devoted to the reaction mechanism of IL catalyzed alkylation reaction. One possible reason may be that the alkylation reaction process is complex because of the simultaneous reactions including olefin polymerization, protonation, and cracking.

30

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

Fig. 1. Continuous flow reactor for C4 alkylation.

The difficulty in detecting the intermediates under the rigorous reaction conditions also leads to the alkylation mechanism of ILs has not been validated [18]. On the other hand, almost all of previous research works on both the alkylation reaction and the role of ILs had been carried out in batch reactors, in which the acid was contacted with the isoparaffin-olefin and the entire mixture analyzed at the end of the reaction. It is almost impossible to detect the intermediates and to study the reaction mechanism by the product distribution over time [19]. There are some experimental results of IL alkylation indicating that the classical reaction mechanism cannot provide a full explanation. For example, when chloroaluminate ILs are used as catalysts as compared to H2 SO4 alkylation, a short contact time between the acid phase and feed is favored [20,21]. If a composite IL [22,23] or a Brønsted-Lewis acidic IL [16] is employed under the optimal operating conditions, the alkylate will contain about 5–10 wt% C5 C7 compounds (light ends) and 5–10 wt% compounds of higher molecular weight (heavy ends). Particularly, the most valuable trimethylpentanes are often greater than 80 wt%. These results from the point of view of product distributions are far superior to those of traditional acids. The aim of this paper is to study the reaction mechanism of IL catalyzed C4 alkylation through experimental measurements of reaction. Two ionic liquids, [BMIm]Cl-AlCl3 and [BMIm]-AlCl3 CuCl, were used as catalysts. The approach adopted in this investigation was to utilize flow equipment, in which the reaction could be followed as a function of time, and the product distributions with time could be obtained. In addition, deuterated isobutane (i-C4 D10 ) was used as an isoparaffin feed and a tracer to react with 2-butenes. By means of analyzing the deuterated product distributions, a relatively easy way was found to investigate the reaction mechanism. A better understanding of the chemistry of C4 alkylation might help explain the formations of trimethylpentanes, light ends, and heavy ends in different ionic liquids. 2. Experimental The chloride salt of the 1-butyl-3-methyl-imidazolium cations ([BMIm]Cl) was obtained from Sigma-Aldrich Chemical Company and dried under vacuum at 100 ◦ C. [BMIm]Cl-AlCl3 ionic liquid (BIL) and [BMIm]Cl-AlCl3 -CuCl (composite ionic liquid, CIL) were prepared and characterized using methods as early described [17]. In this work, the mole ratios of AlCl3 and CuCl to organic salt are 1.6:1 and 0.4:1, respectively. Isobutane and 2-butene were commercial products from China National Petroleum Corporation (CNPC) with 99% purity and used without further purification. Deuterated

isobutane (i-C4 D10 , 98%) was obtained from Cambridge Isotope Laboratories, Inc. Experimental work was carried out in a small continuous flow reactor, as shown in Fig. 1. A typical experiment is performed in the following manner. After purging the entire system with nitrogen, the ionic liquid is charged to the reaction tube (over-all length 12 cm, inside diameter 1.5 cm) to a level about 5 cm above the bottom, so that it completely fills the reaction zone. The feed is a premixed 2-butene/isobutane (or deuterated isobutane) blend with a mole ratio of i-C4 H10 /C4 H8 of approximately 7/1. The feed of hydrocarbon flows from the reactant charge cylinder and is catalyzed by IL in the reaction tube. The hydrocarbon-acid emulsion is disengaged in the top of the reaction tube and the hydrocarbon is depressurized and then vaporized to analyze. The total hydrocarbon sample is periodically analyzed on a gas chromatograph (Hewlett-Packard, 6890 Series II) equipped with a mass spectrometer (Hewlett-Packard, 5972 Series II column). The initial ionic liquid charge remains in the reactor for the duration of the experiment. Studies on the induction period were carried out in the same reactor using method as literature [24] described. Isobutane and 2-butene were all gas-phase feeds. In a typical experiment, the IL/isobutane mixture was put into the reactor at first and the reactor was initially at atmospheric pressure. After mixing to assure saturation of the IL with isobutane, 2-butene was pumped into the reactor at a constant rate (e.g. 10 mL/min). Meanwhile, the consumption of isobutane with time was measured by observing the input of isobutane required to maintain a constant pressure (e.g. 140 kPa, absolute pressure) in the reactor. 3. Results and discussion 3.1. Induction period and the alkylate components with time Stewart and Calkins have measured the induction period of the sulfuric acid alkylation by observing the gaseous isobutane consumption [24]. We determined the induction period of the IL alkylation using the similar method. Fig. 2 shows the results made during a typical experiment. Both IL alkylation processes all have an induction period, during which little or no isobutane is used. However, the induction period of IL alkylation is less than 4 min, which is far shorter than that of H2 SO4 alkylation. Table 1 lists the product distributions at the induction period, the steady state period, and the terminal reaction period, respectively. The BIL and CIL product distributions have a marked difference at the induction period. The CIL alkylate contains more C8 compositions, however, the content of C12 is also higher than that of BIL. During the steady state and

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

31

Volume delivered, mL

Terminal reaction period 300

S 300

250

250

B

200

S

Steady state period B

200

150

150

100

[BMIm]Cl-AlCl3-CuCl

100

50

[BMIm]Cl-AlCl3

50

Induction period 0 0

2

4

6

8 10 12 14 16 18

0 0

2

4

Time, min

6

8 10 12 14 16 18

Time, min

Fig. 2. Rate of consumption of isobutane (S) gas produced by the addition of 2-butene (B) 2-butene gas at a constant rate, in ILs.

Table 1 Comparison of alkylates produced at different periods.a [BMIm]Cl-AlCl3

[BMIm]Cl-AlCl3 -CuCl

Reaction time (min)

1

2

6

13

1

2

6

13

Isoparaffins (wt%) C5 C7 C8 = Trimethylpentanes Dimethylhexanes Other C8 ◦ 2,2,5-trimethylhexanes C12 = and C12 ◦ Other C9+

15.4 16.3 10.4 10.7 4.6 5.6 3.8 33.2

15.2 14.5 12.8 10.8 5.3 5.1 4.9 31.4

14.6 12.9 15.7 9.5 5.9 4.6 9.1 27.7

9.8 6.6 23.9 8.7 4.1 4.9 13.4 28.6

13.5 4.0 23.1 6.4 3.5 3.8 22.1 23.6

10.5 3.9 26.6 7.3 4.5 3.6 20.4 23.2

7.2 4.1 37.3 6.9 4.3 3.9 12.7 23.6

4.5 5.2 47.5 6.4 5.0 5.7 5.1 20.6

IL = 5 mL,

rate

of

TMP

C8

a Reaction conditions: input = 20 mL/ min.

temperature = 25 ◦ C,

2-butene

50

is that a marked decrease of dodecene (C12 = ) and dodecane (C12 ◦ , saturation of C12 = by protonation and hydride transfer) occurred during the CIL alkylation. A higher amount of dodecene is formed at the initial 30 min of continuous reaction. Meanwhile, the amount of octenes (C8 = ) was less than 3 wt%. With the time increasing, the amounts of dodecenes decreased but trimethylpentanes and octenes (C8 = ) increased. Finally, the amounts of dodecenes, trimethylpentanes, and octenes nearly remained constant. By contrast, the octenes and octanes were the main products before 30 min for the BIL systems, but octenes decreased and dodecenes increased with time increasing. Generally, the rate of 2-butene oligomerization is faster than that of alkylation at low isobutane concentration. It is easy to understand C8 = could be transferred to C12 = over time in the BIL system. But for CIL system, it is proposed that 2-butene around Cu ions were firstly polymerized to C12 = , and then C12 ◦ cleaved to C8 = and isobutane. The direct evidence is illustrated in Fig. 6.

Hydrocarbon in Alkylate, wt%

40 30

=

C12

3.2. C12 + intermediates

[BMIm]Cl-AlCl3-CuCl

20 10 0 30 20

TMP

10

=

C8

C

12

[BMIm]Cl-AlCl3 0 0

20

40

60

80

100

120

140

160

180

Time, min Fig. 3. TMP, C8 = and C12 components vs. continuous reaction time.

the terminal reaction periods, C12 fractions of the CIL alkylate have a tendency to decrease while TMP increasing with time. The change of alkylate components with time was also investigated in the small flow unit using liquid-phase i-C4 H10 /2-C4 H8 feed, and the data are illustrated in Fig. 3. An important difference observed when comparing BIL and CIL product compositions

Table 2 lists the product distributions obtained when deuterated isobutane is alkylated with 2-butene in the presence of ILs. Most researchers have considered that scission of the C12 + cations will lead to the precursors of pentanes, hexanes, and heptanes. In addition, C16 fractions are also a result of the polymerization reaction of C12 + and butenes: C12 + + C4 = → C16 + + i-C4 ◦ →C16 + i-C4 + . However, the deuterated product distribution shows that C12 fractions may be formed through two reaction pathways. Some dodecene and dodecane are found containing less deuterium element, which molecular weight is 168 (C12 H24 ) and 171 (C12 H25 D from (22)), respectively. But many dodecanes with more deuterium were also obtained, and they have higher molecular weight (C12 D10 H16 -180 from (23)). These results suggest that the alkylation between C8 + and 2-butene, as well as the oligomerization of 2-butene produce C12 + intermediates. 3C4 H8 → C12 H24 (168) + H+ → C12 H25 + + i-C4 D10 → C12 H25 (171) + i-C4 D9 +

(22)

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Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

Table 2 GC-Mass spectrometric component analyses of alkylates.a Component

Formula

MW

C4 Isobutane n-Butane

i-C4 D10 C4 H9 D

C5 2-Methylbutane 2-Methylbutane C6 2-Methylpentane 3-Methylpentane C7 2,4Dimethylpentane 2,4Dimethylpentane 2,3Dimethylpentane 2,3Dimethylpentane 2-Methylhexane C8 = (Octenes) C8 (Trimethylpentanes) 2,2,4-TMP 2,2,3-TMP 2,3,4-TMP

[BMIm]Cl-AlCl3

[BMIm]Cl-AlCl3 -CuCl

68 59

0.54 0.23

0.48 0.19

C5 H12 C5 H8 D4

72 76

0.01 4.95

0.83 0.21

C6 H6 D8 C6 H14

94 86

2.61 8.77

0.56 5.85

C7 H10 D6

106

4.59

0.89

C7 H16

100

0.07

3.77

C7 H10 D6

106

0.32

0.3

C7 H16

100

0.01

0.37

C7 H16 C8 H16

100

1.97 5.66

1.28 4.89

C8 H18 C8 H18 C8 H18

114 114 114

1.03 0.48 0.31

0.99 0.33 0.19

C8 H16 D2 C8 H9 D9 C8 H8 D10 C8 H5 D13 C8 D18

116 123 124 127 132

3.17 6.3 13.43 0.2 8.21

14.2 5.1 10.8 0.3 14.4

C8 H18 C8 H18 C8 H18

114 114 114

2.71 3.28 4.71

0.65 1.03 1.42

C8 H8 D10

124

1.1

0.23

C9 H20 C9 H20 C9 H10 D10

128 128 138

0.13 0.71 2.73

0.32 2.47 3.12

C10 H10 D12 C11 H8 D16

154 172

3.96 3.84

3.98 5.44

C12 H24

168

0.82

0.76

C12 H25 D C12 D10 H16

171 180

1.51 11.64

13.37 1.28

C8 (Deuterated TMP)

C8 (Dimethylhexanes) 2,3-DMH 2,4-DMH 2,5-DMH C8 (Deuterated DMH) C9 (Trimethylhexanes) 2,2,4-TMH 2,2,5-TMH C9 (Deuterated TMH) Deuterated Decane Deuterated Undecane Dodecenes Deuterated dodecanes

a

Alkylate sample of continuous reaction at 30 min, i-C4 D10 and 2-butene feed, I/O = 7:1, temperature = 25 ◦ C.

are formed by the oligomerization of non-deuterated 2-butene when CIL is used as the catalyst (22).

i-C4 D9 + + C4 H8 → i-C8 D9 H8 +

3.3. The formation of light ends i-C8 D9 H8 + + C4 H8 → C12 D9 H16 + + i-C4 D10 → C12 D10 H16 (180) + i-C4 D9 +

(23)

It is noteworthy that the formation of C12 + intermediates in BIL and CIL has obvious differences. A relatively high quantity of C12 D10 H16 (180) is detected, indicating the secondary alkylation between isooctyl ion and 2-butene in BIL should be the main pathway of the formation of C12 + intermediates (23). In CIL system, most of dodecanes are found with lower molecular weight (C12 H25 D171). The only reasonable explanation is that the C12 intermediates

As Table 2 listed, the deuterium elements were detected in the fragments of C5 C7 , and the saturated C5 C7 hydrocarbons were the primary products of light ends. Deuterated C5 C7 products would be attributed to the scission of deuterated C12 + intermediates. The precursors of pentanes, hexanes, and heptanes were generated by the following reaction: C12 + → C5 + + C7 =

(5)

C12 + → C5 = + C7 +

(6)

C12 + → C6 + + C6 =

(7)

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

Saturation of the ionic fragments by hydride transfer and of the olefins by protonation would increase these low boiling saturated hydrocarbons observed in the continuous experiments. If a C12 + fragment containing a uniform distribution of deuterium were to split into C5 + and C7 + fragments, the amount of deuterated pentanes would be the same as that of deuterated hexanes. The product of BIL alkylation provides a better chance to observe this process for its C12 components (C12 D10 H16 ) containing more deuterium elements. As data listed in Table 2, the total amount of deuterated C5 fragments is 4.95 wt%, which is almost equal to the value of deuterated C7 components 4.92 wt% (2,4dimethylpentane + 2,3-dimethylpentane). The agreement between theoretical and experimental values for BIL system again indicates that the C5 - and C7 - fractions come from scission of a C12 -intermediate. However, most C5 C7 fractions of the CIL alkylate do not contain deuterium element. These hydrocarbons would be a result from the protonation of dodecene (C12 H25 + ). It is an indication that 2-butenes in the CIL system prefer to form dodecenes by oligomerization reaction even at high isobutane concentration (I/O = 7:1). 3.4. Heavy ends Most heavy ends (i.e., C9+ isoparaffins) are produced by alkylation-type reactions of butenes in the acid phase. For instance, the addition of a 2-butene molecule to a C5 + carbocation can form a trimethylhexane (2,2,4- or 2,2,5-TMH). Decane and undecane would come from the alkylation of 2-butene with C6 + and C7 + fragments, respectively. A general reaction scheme for 2-butene alkylation can be summarized as Eq. (24). i-Cn + + C4 H8 → i-Cn+4 H2n+9 + + i-C4 H10 → i-Cn+4 H2n+10 + i-C4 H9 +

(n = 5-7)

(24)

Dodecanes undoubtedly arise from saturation of the C12 + intermediate by hydride transfer with isobutane. In the presence of CIL systems, dodecenes and dodecanes would also be formed by the oligomerization reaction of 2-butenes as Eq. (22), for most of them with less deuterium element (Table 2). Insufficient material with molecular weight higher than dodecane was present in the IL alkylates, so that it was impossible to obtain analyses on these fractions.

33

or 2-C4 H8 + tert-C4 D9 + → 2, 2, 3-TMP+ + H− → 2, 2, 3-TMP(C8 H9 D9 -123)

(10)

(3) 2-butene is isomerized to isobutylene followed by the alkylation reaction with isobutane. 2-C4 H8 → i-C4 H8

(11)

i-C4 H8 + i-C4 H10 → 2, 2, 4-TMP or i-C4 H8 + i-C4 D10 → 2, 2, 4-TMP(C8 H8 D10 -124)

(12)

(4) Isomerization of dimethylhexanes. Deuterated TMP distribution data in Table 2 indicate that selfalkylation reactions are important in the IL systems, although self-alkylation is not so in H2 SO4 [25]. Over 8 wt% TMP-132 (molecular weight = 132) are observed in the two IL alkylates, suggesting that these trimethylpentanes should come from the deuterated isobutane through self-alkylation (Reactions (8) and (9)): i-C4 D8 + iC4 D10 → 2,2,4-TMP (C8 D18 -132). This may be due to the higher solubility of isobutane with respect to butenes in IL than in H2 SO4 . Reactions (10)–(12) might predominate for the formation of trimethylpentanes in the BIL system, because a large proportion of trimethylpentanes in the alkylate is TMP-123 (from Reaction (10)) and TMP-124 (from Reaction (12)). Additionally, if more 2-butene transferred to isobutylene, more TMP-124 would be obtained through Reactions (11) and (12). Thus, the amounts of TMP-123 and TMP-124 also indicate the extent of isomerization of 2-butene to isobutylene. In both IL systems, the amount of TMP-123 is significantly lower than that of TMP-124, indicating that 2-butene is more likely to form isobutylene before the alkylation reaction. The CIL catalyst yielded significantly less TMP-123 and TMP-124 as compared to BIL. However, TMP-116 fraction accounts for about 30% of total trimethylpentanes. The most plausible explanation for these data involves rapid oligomerization of the non-deuterated 2-butene followed by cracking and hydrogen transfer reactions that produce trimethylpentanes with less deuterium. A mechanistic scheme for this is presented in Reactions (13) through (16). 3C4 H8 → C12 H24 + H+ → C12 H25 +

(13)

3.5. The formation of trimethylpentanes

C12 H25 → i-C4 H9 + C8 H16

(14)

Trimethylpentanes (TMP) are the most desired isooctane product because of their high research octane numbers. Although the TMP selectivity of CIL is much greater than that of BIL, there are few profound discussion and research on this difference. Table 2 also lists the product distribution of trimethylpentanes for both ILs with deuterated isobutane as the reactant, which would provide insight into the formation mechanism of TMP using different ILs. According to the carbonium ion mechanism, four reaction pathways lead to forming trimethylpentanes: (1) Self-alkylation of isobutanes, the reaction involves the transfer of hydrogen from isobutane to 2-butene and then alkylation of the resulting isobutylene with isobutane.

i-C4 D10 + C8 H16 → i-C8 H16 D2 (TMP-116) + i-C4 D8

(15)

i-C4 D10 + i-C4 D8 → i-C8 D18 (TMP-132)

(16)

+

i-C4 H10 + C4 H8 → i-C4 H8 + n-C4 H10

(8)

i-C4 H8 + i-C4 H10 → 2, 2, 4-TMP

(9)

(2) Isobutane is directly alkylated with 2-butene. C4 H8 + tert-C4 H9 + → 2, 2, 3-TMP+

+

As compared to the BIL alkylate, more TMP-132 was obtained when CIL was employed as catalyst. These extra TMP-132 compositions should be attributed to Raction (16). Thus, Reactions (13)–(16) could be considered as another pathway of isobutane self-alkylation to form TMP. 3.6. The formation of dimethylhexanes When 2-butene is alkylated with sec-butyl carbonium ions, the expected primary products would be 3,4-dimethylhexane (3,4-DMH, Reaction (17)). Clearly, other paths such as the rearrangement of a trimethylpentyl cation (Reaction (18)) and cracking of C12 + (Reaction (19)) must also contribute to dimethylhexanes’ formation. 2-C4 H8 + sec-C4 + → 3, 4-DMH+

34

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

reactions. Dimethylhexanes should be formed at the initial period of alkylation. It is possible to conclude that Reaction (17) is responsible for the formation of most dimethylhexanes. In the IL systems, sec-butyl carbonium ions (sec-C4 H9 + ) are believed to be the primary intermediates to form dimethylhexanes.

12 10 Dimethylhexane in Alkylate, wt%

8

DMH-114 DMH-124

6 4

[BMIm]Cl-AlCl3

2

3.7. The protonation of 2-butene

0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

2-C4 H8 + sec-C4 D9 + → 3, 4-DMH+ + H− → DMH-114(C8 H18 ) (17)

According to the aforementioned mechanism, sec-butyl carbonium ions would be the main source of DMH for the IL alkylation process. It is necessary to learn more information about the formation of sec-butyl cations. In fact, tert-butyl cations are the principal cations during the whole C4 alkylation reaction, because tert-butyl cations are easily formed through hydride transfer reaction of isobutanes. But the product distribution with time suggests that most DMH are formed in the initial step of the alkylation reaction. DMH mainly comes from the alkylation of butenes with sec-butyl carbonium ions in the ILs. Few studies have discussed the formation of sec-butyl ions in Lewis acids. If deuterated isobutanes are used as isoparaffin feed, the following reactions seem suggest that n-butane with molecular weight 59 could indicate the formation of sec-C4 H9 + in the Lewis acidic IL.

TMP+ → DMH+

2-C4 H8 + H+ → sec-C4 H9 +

DMH-114 DMH-124 [BMIm]Cl-AlCl3-CuCl

20

40

60

80

100

120

140

160

180

Time, min Fig. 4. The amounts of dimethylhexanes with time.

or

(18)

C12 + → DMH + i-C4 +

sec-C4 H9 + i-C4 D10 → n-C4 H9 D(59) + i-C4 D9

(19)

A higher concentration of isobutane on the acid might increase the selectivity to TMP and decrease the length of the induction period [1]. Thus, a test on isobutane solubility was carried out in the reactor to investigate the participation of isobutane at the start-up of alkylation reaction. The mixture of i-C4 D10 , i-C4 H10 , and 2-butene (molar ratio = 1:10:1) was used as a liquid-phase feed for the alkylation reaction. The total hydrocarbon sample, the product with an excess of isobutane feed, is periodically analyzed on a mass spectrometer. The mass spectrum of initial i-C4 D10 /i-C4 H10 mixture is shown in Fig. 5(a), and the peak at m/z = 41.0 shows a 97% relative abundance due to the presence of i-C4 D10 . When i-C4 D10 was dissolved in the acid phase over time and reacted with 2butene, the isobutane sample from the reactor outlet would contain less i-C4 D10 . This process could decrease the relative abundance of isobutane isotope at m/z = 41.0. The relativity abundance of peak at

(25)

43.1

Relative Abundance, %

80

i-C4H10

60

41.0

40 20 0 100

27.1 39.2

57.1

15.1 41.0

80

43.1

60

i-C4H10 / i-C4D10

40

27.1 39.2

20 0

105.0

85.1

72.4

57.1 77.1

15.1 20

30

40

50

60

(a) m/z

70

80

94.9 90

100

(21)

3.8. The participation of isobutane and reaction mechanism

As Fig. 4 shown, no obvious fluctuations of the amounts of DMH were observed in both IL systems after 10 min of the continuous 100

+

Data in Table 2 shows that about 0.2 wt% n-C4 H9 D was detected during the first 30 min of continuous reaction. This result could be considered as the evidence that the protonation of butenes did occur in the Lewis acids.

The formation of dimethylhexanes may in fact proceed via several routes at once. In the previous studies, it is not clear which pathway is the most important for IL systems. DMH distribution data in Table 2 would help study more details of the formation of DMH fractions. Because C12 + intermediates in BIL systems were mainly generated from the deuterated isobutane through (23). If the scission of C12 + were the main source of DMH, the dimethylhexane would contain more deuterium or has a higher molecular weight. When BIL was employed, however, the amount of DMH-114 (C8 H18 , from Reaction (17)) in Table 2 is found much higher than that of DMH124 (C8 H8 D10 , from TMP–124 → DMH-124), 10.7 wt% vs. 1.1 wt%. Similarly, in the CIL system only less than 0.3 wt% of the product are DMH with deuterium (DMH-124), in contrast with 3.1 wt% DMH without deuterium (DMH-114). Obviously, (25) would not form most BIL dimethylhexanes after saturation of the isoolefins produced in the cracking reaction or after hydride transfer to the i-C8 + released in the fragmentation. i-C12 + → i-C8 = + i-C4 + ori-C12 + → i-C4 = + i-C8 +

(20)

+

110

Relative abundance of m/z = 41.0, %

0

100

[BMIm]Cl-AlCl3-CuCl [BMIm]Cl-AlCl3

90

H2SO4

80 70 60 50 40 0

20

40

60

80 100 120 140 160 180 200

(b) Continous reaction time, min

Fig. 5. Solubility of i-C4 D10 /i-C4 H10 mixture: (a) mass spectra of i-C4 D10 /i-C4 H10 mixture, (b) relative abundance at m/z = 41.0 with time.

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

=

Deuterated C8 and C12, wt%

C8-112 25

TMP-116 TMP-124 TMP-132 C12-171

20 15 10 5 0

10

20

30

40

50

60

Time, min Fig. 6. Deuterated TMP and C12 components vs. continuous reaction time.

41.0 might indicate the changes of isobutane solubility in the acid phase. As Fig. 5 (b) shown, under the identical operation conditions, the relative abundance of isobutane isotope at m/z = 41.0 in both ILs decreased quickly. To the contrary, in sulfuric acid the deuterated isobutane were consumed completely needing more time. It suggests that the solubility of isobutane in acid is higher for IL than for sulfuric acid. This in turn decreases the length of the induction period. In the alkylation reaction of 2-butene with isobutane, the participation of isobutane is the most important factor as butene is much more active. In order to investigate the participation of isobutane and 2-butene in the continuous alkylation reaction, the changes of deuterated C8 fractions with time were illustrated in Fig. 6 by using i-C4 D10 /2-C4 H8 as feed. Deuterated C8 products would indicate the participation of isobutane or 2-butene in the alkylation reaction. As mentioned before, C8 = fractions mainly come from the dimerization of 2-C4 H8 . TMP-124 is the product of primary reaction i-C4 D10 + C8 H16 → iC8 H16 D2 (Reaction (15)), and TMP-132 is generated by the self-alkylation i-C4 D10 + i-C4 D8 → i-C8 D18 (Reaction (16)). As Fig. 6 shown, the alkylation of 2-butene with deuterated isobutane catalyzed by the CIL gave about 27 wt% C12 products at the first 5 min. Nevertheless, these products could be significantly decreased with time through the scission of C12 + (Reactions (13)–(16)). The increase of TMP-116 with time indicates that the alkylation in CIL has such way to form TMP as compared to the BIL (Reactions (1)–(4)). 2-butene is readily to be adsorbed by IL in the presence of

35

Cu ions or CuAlCl4 , which would lead to a relatively high amount of C12 fractions in alkylate at the start-up of alkylation. However, this disadvantage could be weakened by increasing the mole ratio of isobutane to 2-butene as the previous study discussed [23]. Moreover, many C12 fractions could be cracked to form TMP with time, as can be seen in Figs. 3 and 6. In addition, the amount of DMH in the CIL alkylate is lower than that of BIL, which is also a result of the ␲-bond complexation of Cuolefin. If a large number of 2-butenes are adsorbed by the CIL at the start-up of the alkylation process, the possibility of 2-butene reacting with sec-C4 + (Reaction (17), 2-butene + sec-C4 + → 3,4-DMH+ ) is small. Thus, less DMH be produced in CIL systems. Due to the higher solubility of isobutane in the IL, hydrogen transfer reactions between C8 + ions and isobutane are relatively enhanced with respect to H2 SO4 . It increases the selectivity to TMP and allows shorter induction period and shorter contact time to be used in the alkylation reactor. The complexation of Cu-olefin enhances the hydrogen transfer reactions between TMP+ ions and isobutane, promotes the transformation from C12 to TMP, and inhibits the formation of DMH. These effects finally improve the quality of CIL alkylate. A general reaction scheme for the CIL alkylation with time is illustrated in Fig. 7.

4. Conclusion The following conclusions can be drawn from our experimental results: (1) By means of the relative abundance of deuterated isobutane at m/z = 41.0, the induction periods of isobutane/2-butene alkylation are obtained. Due to a high solubility of isobutane in the acid phase, the induction period of IL alkylation is shorter than that of H2 SO4 . (2) The product distribution of i-C4 D10 /2-C4 H8 alkylation is obtained in the continuous flow equipment. The formation of C12 + intermediates in BIL and CIL has obvious differences. For the CIL catalyst, C12 fractions are primarily generated by olefin polymerization reactions, while the secondary alkylation of 2butene with C8 + cations is the main source of C12 + intermediates for BIL system. (3) Most light ends should be attributed to the ␤-scission of C12 + cations, while heavy ends are coming from the reaction of 2butene with C5 C7 alkanes. (4) In IL alkylation, the self-alkylation of isobutanes and the scission of C12 + all can produce trimethylpentane fractions. C12 + inter-

Fig. 7. Scheme of reaction mechanism for the CIL alkylation.

36

Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 421 (2016) 29–36

mediates play an import role for forming trimethylpentanes when CIL is used. (5) 2-butenes are easily protonated in the presence of ILs. The signal of forming sec-C4 H9 + , n-C4 H9 D, is observed during the protonation of 2-butenes. Dimethylhexanes are mainly formed by the alkylation of sec- C4 H9 + with 2-butene. Acknowledgement We thank the National Natural Science Foundation of China for financial support (No. 21266015, No. 20806036). References [1] [2] [3] [4] [5] [6] [7] [8]

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