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Adamantane-based Cation and [MF] Anion Synergisticly Enhanced Catalytic Performance of Sulfuric Acid for Isobutane Alkylation Liuyang Wang, Guoying Zhao, Xiao-Qian Yao, Baozeng Ren, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01192 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Industrial & Engineering Chemistry Research

Adamantane-based Cation and [MFn]- Anion Synergisticly Enhanced Catalytic Performance of Sulfuric Acid for Isobutane Alkylation Liuyang Wang, †,‡ Guoying Zhao, ‡ Xiaoqian Yao, ‡ Baozeng Ren,*, † Suojiang Zhang*,‡

†School of Chemical Engineering and Energy, Zhengzhou University，450001, Zhengzhou, People’s Republic of China

‡ Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China

1

ACS Paragon Plus Environment


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Abstract: A series of adamantane-based ionic liquids (ADM-ILs) with [MFn ]- anions were synthesized as co-catalyst for the alkylation of isobutane and butene. By systematically tuning the structure of cation and anion and their combination, the optimized ionic liquids―ADM-C12 -SbF6 exhibit significant enhancement on the C8 selectivities( especially trimethylpentanes(TMPs)), the research octane number (RON) of the alkylate products and the lifetime of sulfuric acid. The selectivity of TMPs was improved from 81.9% to 84.5% and the alkylate RON from 96.6 to 98.6 with the addition of ADM-ILs. In addition, the lifetime of ADM-ILs/H2 SO 4 system was increased twice that of using H2 SO4 alone. Based on experimental measurements and DFT calculation, all these enhancements were attributed to the multifunctio ns cooperatively integrated into the task-specific ADM-ILs, such as surfactant actionimproving interfacial properties of acid/hydrocarbon biphases, buffer action-stabiliz ing the acidity change during the reaction process, and hydride donor action-accelera ting the H- transfer rate which promoted the production of TMPs. This study is beneficial to improve the isobutane alkylation process catalyzed by concentrated sulfuric acid.

1. INTRODUCTION Alkylate oil, idea blending components for gasoline pool, has attracted much more attention because of stringent fuel standards and environmental protection regulations. 1 Alkylate are commercially produced using two dominant alkylation technologies that use sulfuric acid (H2 SO4 ) or hydrofluoric acid (HF) as catalyst at present.2 However, HF technology is prohibited to issue new production licenses in some regions due to the extremely volatility and toxicity of HF. In contrast, H2 SO4 is relatively safe, and resource-abundant as well as inexpensive. In China, nearly 84% of the new alkylatio n units in recent five years adopt H2 SO 4 technology. Even so, H2 SO 4 alkylatio n 2


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technology causes much concern owing to the considerable acid consumption , high cost for acid regeneration, and severe environment pollution. 3 Much attention has been paid to developing environmentally friendly, economic-efficient and energy-effic ie nt (3E) alkylation technologies. Among various advanced technologies, the newly developed ones based on ionic liquids or zeolites catalysts exhibit great potential in alkylation technology innovation.

4-10

However, their commercialization is slow due to

some bottleneck problems such as the high cost of ionic liquids and rapid deactivatio n of zeolite.11 Therefore, in short term, it is probably a quicker, more economical and effective way to alleviate the environment impact of alkylation units by improving the current sulfuric acid technology. Several improved sulfuric acid technologies have been developed in recent years. CDAlky technology employed low temperature (-3℃) and decreased inert alkane concentration of feedstock to minimize side reactions.12 In addition, the UOP technology improved Stratco reactors13 and Refining Hydrocarbon Technologies (RHT) designed14 the novel eductor mixing device to enhance the acid/hydrocarbon dispersion and temperature control so as to improve the alkylate quality. In another way, many institutes and companies have developed several kinds of additives, based on one or two physical and chemical properties of alkylation process, to enhance the catalytic performance of sulfuric acid. For example, alkylation generally occurred at or at least near the interface of acid/hydrocarbon dispersion when using sulfuric acid as catalyst and C 4 as feedstock.

15

surfactants,

ammonium salt16 , n-dodecylamine 1 7 ,

such as sulfolane

aminobenzoic acid

18

quaternary

So series of cation, anion or neutral

and so on, have been developed to accelerate the alkylatio n

reaction kinetics and improve the alkylate quality by improving interfacial properties. 3



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In addition, the intermolecular hydride transfer reaction is rate-determining for isobutane alkylation catalyzed by Brønsted acid, which referred to the H- transferred from isobutane to C 8 + to form trimethylpentane (Scheme 1) . 19,20 Some researchers have improved reaction performances by accelerating the rate of hydride transfer step using hydride donors such as adamantane, pinene, and their derivatives with at least one unoccupied bridgehead carbon.21,22 Moreover, it is well accepted that the acidity and composition of the catalyst affect the protonation of the olefin and the rate of hydride transfer.22-24 The easy reduction of sulfuric acid’s acidity which is due to the poisoning of the side products and the dilution of the inner impurities entrained from the feeds causes the short lifetime of sulfuric acid and a lot of spent acid during reaction process. Therefore, Zhao and her colleagues

25

introduced the ionic liquid [Bmim][SbF 6 ] as

buffer agent to inhibit the acidity reduction and tune the micro-structure and composition of the catalyst system, etc., which prolong at least 2 times the lifetime of sulfuric acid used alone. However, challenges remain to develop multifunctio na l additives which could act as surfactant, hydride donor and also buffer agent to significantly enhance the overall alkylation performance. Due to their unique properties, ionic liquids have exhibited enormous applicatio ns in many fields such as catalysis, separation, material science and electrochemis tr y, etc.17-35 . The outstanding advantages lie in the ion designability, diversity and cooperativity. It offers an opportunity to design the ionic liquids co-catalyst from single function to multiple and cooperative functionalities for isobutane alkylation. Therefore, we have synthesized specifically a series of adamantane based ionic liquids (ADM-ILs), 4


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which integrated surfactant action, acidity buffer, and hydride donor together to enhance the catalytic performance of sulfuric acid in this study. The anion types, alkyl chain length of adamantane–based cation, addition amount of ADM-ILs and operation conditions have been systematically optimized and maximize their enhancements on the alkylation performance. In addition, recycling times and impurities tolerance of the ADM-ILs/H2 SO 4 catalytic systems have been investigated to test their stability during reaction. Moreover, the effects of the ADM-ILs additives on the acidities, the interfac ia l tensions and reaction mechanism of the catalytic systems have been studied with experiment and calculation method. All these studies are beneficial to developing efficient additives that improve the sulfuric acid alkylation technologies. 2. EXPERIMENTAL SECTION 2.1 Materials and Methods All chemicals (AR grade) were purchased commercially and used as received unless otherwise noted. Sulfuric acid, isopropyl alcohol, formic acid, formaldehyde and other reagents were purchased from Beijing Chemical Works, China. NaSbF 6 , LiNTf2 , KPF6 , and 1-adamantanamine (1) were purchased from Aladdin Industrial Corporation. [Bmim][SbF6 ] was purchased from Linzhou Keneng Material Technology Co., Ltd. Isobutane (98.63% in wt. %) was purchased from Shandong Yuean Chemical Industry Co., Ltd., and C 4 industrial MTBE tail gas from Beijing HaiRui Tongda Gas Technology Co., Ltd., the compositions of which were shown in supporting informa tio n (Table S1-S2). The feed gas of isobutane and butene with a mole ratio of 10:1 was prepared in lab, the composition of which were shown in Table 1 (standard deviatio n 5



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see Table S3). 1H

NMR and

13 C

NMR spectra were measured with a Bruker Avance Ⅲ (600

MHz) spectrometer. Mass spectra (MS) in ESI mode were recorded on a Bruker microTOF-QⅡ spectra spectrometer. C, H, N contents (wt. %) were determined using a Vario EL cube element analyzer (EA). Interfacial tension was measured with an automatic tension meter (Powereach JK99C1). Chemical oxygen demand (COD) was determined by the COD detector (Lian-hua Tech.Co.,Ltd. (5B-3C(V8)). 2.2 Ionic Liquid Synthesis The synthetic route

26-28

and the structures of adamantane-based ionic liquids

(ADM-ILs) are shown in Scheme 2. Typically, 1-adamantanamine (1) was firstly methylated using excess formic acid and formaldehyde to produce N-(1-adamantyl)- N, N-dimethylamine (2, ADM). Then the ADM reacted with alkyl bromide (C n H2n+1 Br (n=4, 8, 12, 16)) to produce the corresponding N-(1-adamantyl)-N, N, N-dimethylalk ylammoniun bromide and finally to the desired ADM-ILs by anion metathesis. Synthesis of N-(1-adamantyl)-N, N-dimethylamine (ADM) 1-adamantine (1) (0.1mol) in isopropyl alcohol (100mL) was warmed to 35℃ with stirring in a three-necked round-bottomed flask. Then formic acid (0.5mol) was dripped in within 40 min via a constant pressure funnel. The mixture was then heated to 50℃. After that formaldehyde was added dropwise with vigorous stirring over a period of 4.5h. Then the mixture was stirred for 21h at 80℃. After cooling to room temperature, the mixture was alkalized with 25% aqueous NaOH until the pH was 13. Then upper layer was extracted with hexane three times. The solvent was distilled off 6


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by rotary evaporator, and the crude product was obtained. The obtained crude product was mixed with ethyl ether (30mL), successively with acetic anhydride (0.1mol) and then refluxed for 2h. 30mL of 10% hydrochloric acid was added into the mixture with a constant pressure funnel and stirred for another 2h. After cooling to room temperature, the mixture was transferred into a separatory funne l and alkalized with 25% aqueous NaOH until the pH was 13. Then the upper layer was extracted with hexane three times. Finally, N-(1-adamantyl)-N, N-dimethylamine was obtained after the removal of solvent and dried overnight at 80 ℃ under vacuum before analysis. N-(1-adamantyl)-N, N-dimethylamine (ADM, 2): The yield was 95%, colorless transparent crystal, 1 H NMR (DMSO, 600MHz, 20℃, δ ppm): 1.531~1.625 (m, 12H, 6CH2 ), 2.016 (s, 3H, 3CH), 2.129 (s, 6H, 2CH3 ). ESI-MS: m/z (+) found 180.1739, calculated 180.1747. Elemental analysis for CHN (%), analysis (calculated): C 81.38(80.38), H 11.04(11.81), N 7.59 (7.81) Synthesis of N-(1-adamantyl)-N,

N, N-dimethylalkyl-1-ammonium bromide s

(ADM-Cn -Br) N-(1-adamantyl)-N,N-dimethylamine (2) (0.07mol) was dissolved in 115mL of isopropyl alcohol and warmed to 45℃. Cn H2n+1 Br (~2 eq. n=4, 8, 12, 16) was added with stirring over a period of 45min. Then the mixture was refluxed for 28h. After cooling, the solvent was distilled off. The crude products were washed twice with excess ethyl ether and dried under vacuum for 24h before analysis. N-(1-adamantyl)-N, N, N-dimethylbutyl-1-ammonium bromide (ADM-C4 -Br, 3): 7



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The yield was 74%, white crystal,

1H

NMR(DMSO, 600MHz, 20 ℃ , δ ppm):

0.922~0.976 (t, 3H, CH3 ), 1.282~1.359 (m, 2H, CH2 ), 1.581~1.655 (m, 12H, 6CH2 ), 1.988~2.062 (d, 2H, CH2 ), 2.219 (s, 3H, 3CH), 2.811 (s, 6H, 2CH3 ), 3.105~3.170 (m, 2H, CH2 ). ESI-MS: m/z (+) found 236.2373, calculated 236.2373. N-(1-adamantyl)-N, N, N-dimethyloctyl-1-ammonium bromide (ADM-C8 -Br, 4): The yield was 60%, white crystal,

1H

NMR(DMSO, 600MHz, 20 ℃ , δ ppm):

0.851~0.884 (t, 3H, CH3 ), 1.214~1.352 (m, 12H, 6CH2 ), 1.546~1.737 (m, 10H, 5CH2 ), 1.988~2.043 (m, 4H, 2CH2 ), 2.218 (s, 3H, 3CH), 2.790~2.819 (s, 6H, 2CH3 ) ESI-MS: m/z (+) found 292.2995, calculated 292.2999. N-(1-adamantyl)-N, N, N-dimethyldodecyl-1-ammonium bromide (ADM-C12 -Br, 5): The yield was 45%, white crystal,

1H

NMR(DMSO, 600MHz, 20℃, δ ppm):

0.821~0.889 (t, 3H, CH3 ), 1.185~1.361 (m, 12H, 6CH2 ), 1.539~1.725 (m, 12H, 6CH2 ), 1.835 (s, 2H, CH2 ), 1.992~2.052(m, 6H, 3CH2 ), 2.217 (s, 3H, 3CH), 2.809 (s, 6H, 2CH3 ), 3.104~3.162 (m, 2H, CH2 ). ESI-MS: m/z (+) found 348.3614, calculated 348.3625. N-(1-adamantyl)-N, N, N-dimethylhexadecyl-1-ammonium bromide (ADM-C16 Br, 6): The yield was 55%, white crystal, 1 H NMR(DMSO, 600MHz, 20℃, δ ppm): 0.836~0.886 (t, 3H, CH3 ), 1.190~1.360 (m, 22H, 11CH2 ), 1.581~1.728 (m, 12H, 6CH2 ), 1.992~2.060 (m, 6H, 3CH2 ), 2.223 (s, 3H, 3CH), 2.808 (s, 6H, 2CH3 ), 3.092~3.174 (m, 2H, CH2 ). ESI-MS: m/z (+) found 404.4220, calculated 404.4251. Synthesis of N-(1-adamantyl)-N, N, N-dimethylalkyl-1-ammonium ionic liquids (ADM-ILs) 8


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ADM-ILs were synthesized by ion metathesis of the corresponding bromide salts (1 g) with 1.05-1.2 equivalents of NaSbF6 /LiNTf2 /KPF6 in dichloromethane. After stirring for 28h under room temperature, the product was filtered and washed with water several times until there was no faint yellow precipitate in aqueous phase using the silver nitrate titration method and ionic liquids 7-18 were dried under vacuum for 24h before analysis. ADM-Cn - SbF6 /NTf2 /PF6 were 7-18 for short in the following. The NMR information of ionic liquids 7-18 was detailed in supporting information. N-(1-adamantyl)-N, N, N-dimethylbutyl-1-ammonium hexafluoroantion (ADMC4 -SbF6 , 7): The yield was 80.2%, colorless transparent liquid. ESI-MS: m/z (+) found 236.2371, calculated 236.2373; m/z (-) found 234.8884, calculated 234.8942. N-(1-adamantyl)-N, N, N-dimethyloctyl-1-ammonium hexafluoroantion (ADMC8 -SbF6 , 8): The yield was 80.9%, colorless transparent liquid. ESI-MS: m/z (+) found 292.2996, calculated 292.2999; m/z (-) found 234.8903, calculated 234.8942. N-(1-adamantyl)-N,

N,

N-dimethyldodecyl-1-ammonium

hexafluoroantion

(ADM-C12 -SbF6 , 9): The yield was 81.2%, colorless transparent liquid. ESI-MS: m/z (+) found 348.3613, calculated 348.3625; m/z (-) found 234.8918, calculated 234.8942. N-(1-adamantyl)-N,

N, N-dimethylhexadecyl-1-ammonium

hexafluoroantion

(ADM-C16 -SbF6 , 10): The yield was 84.0%, colorless transparent liquid. ESI-MS: m/z (+) found (%): 404.4219, calculated 404.4251; m/z (-) found 234.8917, calculated 234.8942. N-(1-adamantyl)-N,

N, N-dimethylbutyl-1-ammonium

bis [(trifluoromethy l)

sulfonyl] imide (ADM-C4 -NTf2 , 11): The yield was 78.4%, colorless transparent 9



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liquid. ESI-MS: m/z (+) found 236.2373, calculated 236.2373; m/z (-) found 279.9245, calculated 279.9167. N-(1-adamantyl)-N,

N, N-dimethyloctyl-1-ammonium

bis [(trifluoromethy l)

sulfonyl] imide (ADM-C8 -NTf2 , 12): The yield was 79.5%, colorless transparent liquid. ESI-MS: m/z (+) found 292.2982, calculated 292.2999; m/z (-) found 279.9280, calculated 279.9167. N-(1-adamantyl)-N, N, N-dimethyldodecyl-1-ammonium bis [(trifluoromethy l) sulfonyl] imide (ADM-C12 -NTf2 , 13):

The yield was 79.8%, colorless transparent

liquid. ESI-MS: m/z (+) found 348.3603, calculated 348.3625, 100; m/z (-) found 279.9297, calculated 279.9167. N-(1-adamantyl)-N, N, N-dimethylhexadecyl-1-ammonium bis [(trifluoromethy l) sulfonyl] imide (ADM-C16 -NTf2 , 14): The yield was 81.4%, colorless transparent liquid. ESI-MS: m/z (+) found 404.4222, calculated 404.4251; m/z (-) found 279.9302, calculated 27 9.9167. N-(1-adamantyl)-N,

N,

N-dimethylbutyl-1-ammonium

hexafluorophosphate

(ADM-C4 -PF6 , 15): The yield was 65.0%, colorless transparent liquid. ESI-MS: m/z (+) found 236.2372, calculated 236.2373; m/z (-) found 144.9670, calculated 144.9642. N-(1-adamantyl)-N,

N,

N-dimethyloctyl-1-ammonium

hexafluorophosphate


N,

N-dimethyldodecylammonium 10


hexafluorophosphate

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N, N-dimethylhexadecylammonium

hexafluorophosphate

(ADM-C16 -PF6 , 18): The yield was 69.7%, colorless transparent liquid. ESI-MS: m/z (+) found 404.4221, calculated 404.425; m/z (-) found 144.9667, calculated 144.9642. 2.3 Acidity Determination The Hammett acidity of catalyst system was determined by

13 C

NMR according

to the literatures.25,29,30 In a typical procedure, specific amount of indictor (mesityl oxide, α, β-unsaturated ketone) was added to the catalyst to make three samples with varied concentration from 0.05 to 0.8 mol/L. Subsequently, the sample was loaded into a NMR tube with a capillary tube containing tetramethylsine as an internal standard. The

13 C

NMR spectra were acquired at 300K on a Bruker Avance Ⅲ 600 MHz

spectrometer. The chemical shift differences between C β and Cα (Δδ) in the mesityloxide molecular at three varied concentrations were used to determine the one in the finite dilution (Δδ 0 ) by exploration. The Hammett acidity (H0 ) could be calculated through the equations as reported in literatures.

31,32

2.4 Alkylation The alkylation of isobutane with butene was carried out in a 200mL stirred batch autoclave equipped with a cooling groove as shown in Figure 1 (DC 2006-Ⅱ, ethanol as circulating chiller). In a typical procedure, the reactor was charged with a given amount of catalyst, pressured with nitrogen to 0.4MPa. When equilibrated to the desired temperature, the reactor stirred at 1000 r/min was charged with a previously prepared 11



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isobutane/butene mixture (50 mL, mole ratio of isobutane to butene was 10:1)) via a double-acting piston pump at given flow rate. After 10 min reaction time and 5 min static separation, the autoclave was depressurized slowly and the off-gas which was analyzed using SP 6890 gas chromatography. Then the product and catalyst were decanted from the reactor, the alkylate was separated from the acid and washed several times with NaHCO 3 saturated solution and dried over anhydrous MgSO 4 before analysis. The lifetime test experiments were done as the following procedures: after the reaction, the alkylate was pushed out from the outlet pipe for further treatment and analysis. Then the autoclave was pressured with nitrogen to 0.4MPa and charged with C4 feedstocks again for the next cycle. The C4 feedstocks and the off-gas were analyzed

using

SP 6890 gas

chromatograph equipped with a capillary column (FP-PLOT Al2 O3 S, 50m×0.53mm) and a flame ionization detector. The alkylate products were analyzed via a gas chromatograph (SHIMADZU GC 2014) equipped with a capillary column (DB-Petorl, 100m×0.25mm) and a flame ionization detector.

25,30

The research octane number

(RON) of alkylate was calculated according to the method detailed previously.

33

In this work, standard uncertainties u of alkylation variables are u(T) = 0.1 °C, u(p) = 0.01 MPa, and u(t) = 0.1 min, respectively. RON, weight fraction of alkylate distribution, and acid strength are expressed with the average value of multip le measurements (triplicate or more) within the experimental error ±1.0%. And deviatio n

12


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values in the tables and figures shown are determined by the ultimate errors of the inherent and standard deviation (SD). 3 RESULTS AND DISCUSSION Based on the previous research in our group,

25

acid/hydrocarbon was fixed at

40/50(V/V), stirring speed at 1000r/min in this study. Feed speed and on-start temperature were optimized at 100mL/h and 1.5 ℃ that detailed in supporting information (Table S4, Figures S2-S3). 3.1 Effect of the ADM-ILs and its Addition Amounts Generally, surfactants with the alkyl chain longer than 12 can significa ntly improve the biphasic interfacial properties in study. 37,38 Hence ADM-C12 -SbF6 was synthesized and mixed with sulfuric acid to catalyze the isobutane/butene alkylatio n. Results are shown in Table 2 (SD see Table S5). With the addition of ADM-IL in H2 SO4, the desired TMPs selectivity especially that with high RON was significantly improved and the production of undesired C 8 components (DMHs) inhibited. As shown in Figure 2, the selectivity of 2, 2, 3-TMP and 2, 3, 3-TMP were 5 wt.% and 22.7 wt.% respectively for H2 SO 4 /ADM-ILs system, higher than the ones for H2 SO 4 /BmimSbF 6 system and around 1.5 times that of H2 SO 4 system. Thus the TMP/DMH and RON for H2 SO 4 /ADM-ILs system were increased to 21.3 from 10.8 and 98.6 from 96.6 respectively compared with that using H2 SO4 alone. Furthermore, the effect of the addition amount of ADM-C12 -SbF6 on the alkylation performance was investigated and shown in Table 2. It indicated that a maximum of TMP selectivity of 80.06 wt. % and RON of 98.6, were obtained with the addition of 0.5wt.% ADM-C12 -SbF6 . Further increment of ADM-C12 -SbF6 addition amounts resulted in the decreased acid strength of catalyst system(Figure S4), decreased TMPs selectivity, increased the heavy ends 13



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(C9+) and thus worsen the alkylate quality. Therefore, the addition amount of ADM-IL co-catalyst was fixed at 0.5wt. % in the following studies. One of the reasons for the significant promotion of TMPs production is probably because the ADM-ILs as hydride donor involved the hydride transfer steps(shown in Scheme 3), reduced the reaction barrier and accelerated the alkylation kinetics, which determined the alkylates distribution. The DFT calculations were also carried out to probe the action of ADM-IL in the alkylation mechanism. All calculations were carried out using the Gaussian 09 program. The B3LYP/6-311++G(d,p) method has been used for structure optimizations, and subsequent frequency calculations at the same level verify the optimized structures to be ground states without imaginary frequenc ies (NImag = 0). In order to simplify the calculation, the computations were performed to investigate the interaction and possible reactions between adamantane and TMP +, and between adamantane cation and isobutane. It was found that there was a stable structure for the coordination adamantane with TMP+ via hydrogen bonds by calculations. The interaction will lead to the formation of special intermediate in Figure 3 with only 1.98 kcal/mol energy release, and subsequently breaking of the Ct-H bond will only need 5.51 kcal/mol dissociation energy. Based on calculations, it was found that the TMP+ did not take the isomerization reaction but directly reacted with adamantane, then a

lower-energy active intermediate (

) was produced, followed by the target

product TMP and adamantane carbenium ion (

).

14


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Similar process was found for the interaction of adamantane carbenium ion with isobutane by calculations showed in Figure 4. The transformation of hydrogen from isobutane to intermediate is with advantage of energy of 2.19 kcal/mol. The following bond breakage to form adamantane is with a little higher energy cost of 10.95 kcal/mol. According to the calculations, adamantane carbenium ion could be easily transferred to adamantane for the next cycle so that the alkylation reaction with ADM-ILs could be carried out with high activity. These results indicated that adamantane intermed ia te played an important role in influencing the reaction and energy process in whole catalysis cycle. 3.2 Effect of Alkyl Chains in Cations and Anions The effect of the alkyl chain length of cation (C n , n=4, 8, 12, 16) and anion types (SbF6 -, NTf2 -, PF6 -) on alkylation performance were investigated via alkylation reaction and interfacial tension measurements. The results of alkylation reaction in Table 3 (SD see Table S6) showed that the additives with C 12 alkyl chain exhibited the maximum enhancement for all the studied ADM-ILs with the same anion. It suggests that increasing the chain length appropriately did good to the reaction, but further would have the opposite effect. Then, the isopentane was used to substitute isobutane and the 2,2,4-TMP for alkylate oil to measure the biphasic interfacial tension between the investigated catalytic systems and isopentane or 2,2,4-TMP. The interfacial tension values and the corresponding standard deviations were shown in Table S7 in supporting information.

The interfacial tension differences

of the isopentane-H2 SO 4 and

isopentane-H2 SO4 /ADM-IL, and the 2,2,4-TMP-H2 SO 4 and 2,2,4-TMP-H2 SO4 /ADM15



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IL were shown in Figure 5 (the absolute values and SD see Table S7). Results showed that increasing the alkyl chain of ADM-IL reduced the interfacial tension for all the investigated catalytic systems, isopentane and 2,2,4-TMP. But the interfacial tension and its reduction rate with increasing the alkyl chain were very different for the ADMIL with varied anions. For example, The interfacial tension between the H 2 SO 4 /ADMC8 -SbF6 system and isopentane or 2,2,4-TMP was 13.61 mN·m-1 and 19.64 mN·m- 1 respectively. In contrast, the interfacial tension was 12.73 mN·m-1 and 13.04 mN·m- 1 for H2 SO 4 , and 10.99 mN·m-1 , 12.14 mN·m-1 for H2 SO4 /ADM-C8 -PF6 system. In addition, the ADM-C8 -SbF6 served as emulsion breaker as it worsened the interfac ia l properties between hydrocarbon and the acid phase while the ADM-C8 -PF6 improved the biphasic interfacial properties acting as surfactant. And the ADM-C12 -SbF6 exhibited the desired properties which decreased the interfacial tension of reactant-acid phase and increased that of product-acid phase. Thus, it is beneficial to the formatio n of acid-hydrocarbon emulsion and accelerate the separation of isooctane from the acid phase, which contribute to the enhancement of the alkylation performance and in agreement with the alkylation reaction results. Moreover, the solubility of the chosen additives in the oil is another important factor, which affects the quality of the alkylate product and the stability of the catalytic system. We have used ESI-MS (cation mode) to determine the solubility of ADM-C12 -SbF6 in oil phase in the presence of only ADMIL or H2 SO4 /0.5wt.% ADM-IL, which were both under the detection limit. It indicated that the ADM-C12 -SbF6 was not easily entrained in the oil and had little effect on the oil products. The results were consistent with our previous work. 25 16


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3.3 Reusability of the ADM-IL/H2 SO4 system The reusabilities of the selected catalyst systems including H2 SO4 /ADM-C12 -SbF6, H2 SO 4 /ADM-C12 -NTf2 and H2 SO4 /ADM-C12 -PF6 were investigated as to the C8 selectivity and RON of the alkylate compared with that of H2 SO4 alone system. For all the above catalytic systems, recycling experiments were stopped when the C8 and RON of the alkylate products plunged with the except of H2 SO 4 /ADM-C12 -SbF6 whose C8 and RON reached nearly the same with the final one as H2 SO4 catalytic system. As shown in Figure 6 and Figure S5, the results indicated that the additive ADM-C12 -SbF6 exhibited the best enhancement on the catalytic performance of H2 SO4 . At least 58 recycling runs could be carried out without loss activity for H2 SO 4 /ADM-C12 -SbF6, 27 runs for H2 SO4 /ADM-C12 -NTf2 and 43 for H2 SO 4 /ADM-C12 -PF6 . In contrast, the catalyst of sulfuric acid only could be reused for 29 runs, which was in well agreement with the previous work 25 . 3.4 Effect of H2 O and ASO Impurities Water and conjunct polymers(acid soluble oil, ASO) are the most notorious impurities for alkylation, which decreased the acidity significantly of sulfuric acid and affected its catalytic activity12 . So the effect of H2 O, ASO on the acidity and catalytic performances were compared for the H2 SO4 and H2 SO4 /ADM-C12 -SbF6 catalysts. As shown in Figures 7-8, the results indicated that the addition of ADM-C12 -SbF6 in H2 SO4 slowed down the acidity reduction of the catalytic system with increased H2 O and ASO content. With the 9 wt. % ASO content, the acid strength maintained around -10.0 in H0 scale for H2 SO 4 /ADM-C12 -SbF6 catalyst system while only 1 wt. % ASO content 17



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for H2 SO 4 . The ASO poisoning effect was significantly inhibited with the addition of ADM-C12 -SbF6 , which exhibit its outstanding buffer action as reported in literature 2 5 . These results probably explained the excellent recycling stability of H2 SO4 /ADM-C12SbF6 catalyst system. The effects of H2 O and ASO content on the catalytic performances were shown in Tables 4-5 (SD see Tables S8-9) respectively. The water content of the catalytic system should be controlled lower than 1wt. %. Otherwise, the presence of water decreased the desired TMPs selectivity,

especially for the

H2 SO 4 /ADM-C12 -SbF6 catalyst system. When present in relatively lower amounts

Adamantane-Based Cation and [MFn ... - ACS Publications

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